Research ArticleInvestigation of Microwave Irradiation Procedure forSynthesizing CdSe Quantum Dots

Jacob Strimaitis 1 Taliya Gunawansa1 Sangram Pradhan1 and Messaoud Bahoura12

1Norfolk State University Center for Materials Research 555 Park Ave Norfolk VA 23504 USA2Norfolk State University Engineering Department 555 Park Ave Norfolk VA 23504 USA

Correspondence should be addressed to Jacob Strimaitis jstrimaitisspartansnsuedu

Received 17 October 2019 Revised 11 January 2020 Accepted 20 January 2020 Published 29 February 2020

Academic Editor Francesco Ruffino

Copyright copy 2020 Jacob Strimaitis et al (is is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work isproperly cited

In recent years microwave heating techniques for quantum dot (QD) synthesis have come to supplement the typical hot-injectionmethods In addition to increasing control and replicability microwave synthesis can be up-scaled to industry standards anadvantage that increases its lucrativeness (is study depicts a strategy to take a hot-injection procedure for cadmium selenide(CdSe) QD synthesis that is safe enough for undergraduate research labs and adapt it to an easier more energy-efficient mi-crowave synthesis method Additionally this study details successes in synthesizing these QDs along with some challengeslimitations and peculiarities For future users of this method it is recommended to keep holding temperatures between 170degC and240degC to achieve the highest monodispersity of CdSe QDs while also avoiding confounding effects such as wide-spectrumphotoluminescence and bulk CdSe precipitation

1 Introduction

Quantum dots (QDs) are semiconducting nanoparticleswith intriguing optoelectronic properties brought about bythe quantum confinement effect [1ndash3] Specifically as thesize of the QDs approaches the nanoscale they gain theability to absorb and emit higher-energy radiation than theirbulk material counterparts [3] (ese unique phenomenahave been the impetus behind the inclusion of QDs into thinfilms serving the photovoltaic [4 5] light-emitting diode [6]and other energy utilization industries [7] Despite beingrelatively easy to synthesize with basic laboratory equipmentfound in most undergraduate organic or physical chemistrylabs [8] QDs are still emerging materials that require moreresearch to increase their processing viability and by ex-tension integration in future devices

(e most widely used method for QD synthesis is thehot-injection approach [8ndash12] (is method involves twoessential components a hot reaction flask and an injectionprecursor In the reaction flask a solvent with a high boilingpoint is heated usually to sim200degC or higher In a separate

area precursors with the components of the QDs are heatedor stirred until they are completely dissolved (en theprecursors are quickly injected into the hot reaction flaskcausing an immediate nucleation of QDs via supersatura-tion Over time as the cooled reaction flask heats back upthe nanocrystals grow At pointed intervals throughout thegrowth process aliquots of the reaction flask are taken withsmaller nanoparticles taken early on and larger nano-particles taken later (ese acquired QDs are in colloidalform (ie suspended but neither dissociated nor precipi-tated in the solvent) and can be purified characterized orotherwise manipulated to suit the needs of the researcher

(e microwave irradiation technique by contrast is anewer approach designed to reduce some of the frustrationsand limitations of the older hot-injection method To list afew the hot-injection method suffers from a lack of controlreproducibility and large-scale synthesis whereas the mi-crowave synthesis technique excels on all three counts [13]Regarding control and reproducibility the effects of ambientair conditions and uneven heating within the reaction flaskmake the hot-injection method inferior to the microwave

HindawiAdvances in Materials Science and EngineeringVolume 2020 Article ID 2402930 8 pageshttpsdoiorg10115520202402930

(e microwave method works within a sealed environmentunder temperature ranges that can be precisely reproducedFurthermore the hot-injection method cannot reach thesame level of homogeneity in large-scale operations as it canin small-scale environments conversely microwave syn-theses allow for homogeneity as all the components can beprepared separately and allowed to settle in a mixed batchprior to heating

Owing to its usefulness there have been a variety ofsuccessful attempts to make many types of QDs via themicrowave synthesis method [13ndash20] While the merits andscientific processes of the microwave approach have alreadybeen described in these attempts lesser attention has beengiven to some of the approachrsquos setbacks(e twomain goalsof this investigation are to provide an option for microwavesynthesis that originates from a commonplace hot-injectionmethod and more importantly highlight some of thechallenges limitations and peculiarities that are not nor-mally reported in other microwave synthesis studies[8 12 18 19] Such a problem-centered approach mightbenefit the researcher student or industry professionalexploring the phenomenological limits of QDs

2 Method

21 List of Chemicals Selenium powder (Se 99999 AlfaAesar) tri-n-octylphosphine (TOP min 97 StremChemicals Inc) cadmium acetate hydrate (CdAc2middotxH2Oge9999 Sigma-Aldrich) oleic acid (OA tech 90 AlfaAesar) 1-octadecene (ODE tech 90 Alfa Aesar) toluene(ACS reagent 995 Alfa Aesar) acetone (ACS reagentmin 995 VWR) chloroform (min 998 BDH-VWR)and hexane (n-hexane min 60 C6 isomers min 985BDH-VWR)

22 Precursor Preparations As per the instructions of themethod on which this study is based [8] the Se precursorwas prepared by dissolving 99mg of Se powder in 55mL ofTOP(ough easier to do in a round-bottom flask where thesediments can be continually agitated with the stirring el-ement we were able to dissolve the Se powder in 20mL glassvials by moving the vial (and accompanying stir bar) acrossthe stir plate to break up some of the bulkier chunks Atroom temperature complete dissolution took 1-2 hoursAfter even a few hours of exposure to air Se precursorswould show spots of crystallization on the interior of theglass However this decomposition was likely the oxidationof TOP into tri-n-octylphosphine oxide (TOPO) which is acommon native ligand in QD chemistry and thus not causefor immediate concern [21]

(e Cd precursor was prepared by dissolving 53mg ofCdAc2middotxH2O in a 20mL glass vial containing 06 mL of OAand 55mL of ODE Complete dissolution occurred afterstirring and heating to 130degC which usually took twohours Solutions were cooled back down to room tem-perature before being implemented for synthesis (oughthe Cd precursor did not degrade as quickly on a labo-ratory tabletop as the Se precursor it would still show

signs of degradation in the form of a wispy precipitateafter 1-2 days

All of the QD batches for this report were made within 1-2 days of preparing their precursors and if the precursorswere not clear and colorless on the day of synthesis evenafter reheating they were discarded and prepared againPrecursors made with these procedures can make fivebatches of QDs with room to scale up as needed [8]

23 Microwave Synthesis (e procedure for microwavesynthesis was the same as that of the hot-injection method ina previous report [8] save for the inclusion of a microwave(Anton Paar Monowave 300) as the primary heating elementand the distinction that precursors were mixed into thereaction vial (size G30) at room temperature instead of near200degC Aliquots of the Se precursor (1mL) and Cd precursor(1mL) were added to the 10mL of ODE already in thereaction vial causing turbidity in the reaction solutionwithout color change or precipitation (e reaction vial wasthen sealed with a snap cap and silicone septum into which aglass immersion tube was inserted A ruby thermometerwhich was used to more accurately measure the increasingtemperature of the reaction vial than the ambient temper-ature reading of built-in IR sensors [22] was then insertedinto the immersion tube (e reaction vial along with theruby thermometer and a magnetic stir bar was placed in thereaction cavity for synthesis

Microwave reactors work in three steps heating upholding and cooling down During the heating up stepreactors provide maximum inputted power to heat up thesample to the desired temperature For all the reactions inthis study the option to heat as fast as possible was selectedmeaning that it took anywhere from 2 to 5 minutes to reachthe inputted temperature (e maximum power was set at300W and stirring speed of the magnetic stir bar was set at600 rpm During the holding step reactors fluctuate powerlevels to maintain a desired temperature (e holdingtemperature for each experiment in this study was set be-tween 140degC and 280degC with hold times between 0 and 5minutes (ese important holding parameters are consistentwith other similar QD microwave synthesis procedures[18 20] During the cooling down step reactors blow cool airinto the reaction cavity and expel warm air through anexhaust pipe a process which takes anywhere from 2 to 5minutes to reach the desired temperature (e target cooldown temperature for each experiment was 70degC at whichpoint all stirring and cooling processes stopped and thereaction vial could be removed

To illustrate if one wished to reproduce a batch of CdSeQDs heated to 200degC for 3 minutes using our method onewould input the following parameters for the three afore-mentioned steps for the heating up step one would selectldquoheat as fast as possiblerdquo 300W for the maximum powerand 600 rpm for the magnetic stir bar speed For the holdingstep one would select 200degC for holding temperature 3minutes for hold time and 600 rpm again for the magneticstir bar speed For the final cooling step one would select70degC as the target temperature and 600 rpm again for the

2 Advances in Materials Science and Engineering

magnetic stir bar speed After inserting the reaction vial andruby thermometer one would simply tap the start button tobegin the procedure letting the microwave heat up holdand cool down according to the set parameters (e solutionin the reaction vial at the end would contain the colloidalQDs ready for purification or analysis

24Cleaning Previous findings suggest that cleaning QDs isa viable way to remove unreacted impurities from QD so-lutions [23] Based on a method from another report [8] weattempted to clean the QDs by inserting 2mL of QD so-lution 06mL of OA and either 26mL or 52mL of acetoneinto a sim30mL centrifuge tube which was then spun at4000 rpm for 10min (is procedure with either amount ofacetone however yielded no QD pellets After some trial-and-error it was determined that between 1 and 2mL of QDsolution (though we used 14mL for all experiments afterthis one) in a tube with 28mL of acetone spinning at10000 rpm for 15 minutes was enough to get pellets be-tween 1 and 5mm in diameter (ese pellets also ranged incolor from pale yellow to red-orange depending on theQDs used

After precipitation in the polar solvent via centrifuga-tion supernatants containing excess impurities were dis-carded Pellets were then resuspended in 125mL of thetarget solvent which was either ODE toluene hexane orchloroform [24] Save for unusual exceptions noted inSection 3 resuspensions were transparent and monochro-matic between yellow and red-orange under visible light

3 Results and Discussion

31 PL-BroadeningUV-Vis andSize As seen in Figure 1(a)solutions of QDs immediately after microwave synthesisrange in color from pale yellow to dark red under visiblelight (ese colors are consistent with those from otherreports with hot-injection procedures [8 12] Most QDsyntheses are also transparent though some batches near theupper limit of the microwave reactor appear cloudy with adark red precipitate (presumably bulk CdSe powder)Batches made below 120degC show no color change or lu-minescence under UV light suggesting that there is a lowerlimit of temperature beyond which QD nucleation does notoccur

QDs synthesized within the working range of temper-atures show odd patterns of emission under 365 nmUV light(Figure 1(b)) Instead of fluorescing the usual blue to red[8 12] QDs emit colors ranging from a silver-blue to brightwhite to a peachy orange (ese abnormalities appear in thePL spectra as well (Figure 2(b)) where broad-spectrumemissions dominate for lower temperature synthesis batches(long-lined curves) Batches made between 170degC and 240degChave more defined peaks consistent with other reports[8 12] yet even these QDs show inexplicable broad emissionin the range of 650ndash750 nm

With UV-vis spectroscopy however the QDs display amore common pattern (Figure 2(a)) Absorption peaks fallbetween 430 and 580 nm a range that lies within the visible

light spectrum between green and yellow A representativebatch of QDs also shows the familiar Stokes shift (Figure 3)[8 12 18 19 25] indicating the difference in energy be-tween absorbed photons and those that are emitted afternonradiative relaxation

Transmission electronmicroscope (TEM) images of QDssynthesized in our lab have been published in a previousreport [26] In colloidal form QDs are relatively spherical inshape with an inner core composed of lattice CdSe and anouter shell composed of attached organic ligands [8] (esize of the QDs can be approximated using photo-luminescence peak data and the following Brus equation[1 2]

R2

h2

8Ex

1mlowaste

+1

mlowasth1113888 1113889 (1)

where Ex=E minus Eg (CdSe) Eg (CdSe) = 174 eV mlowaste (CdSe)= 013mo (mo is the mass of free electron) mlowasth (CdSe)= 045mo and E is calculated using the de Broglie relationE= hcλ where h is Planckrsquos constant and λ is the peak PLwavelength of the QDs Table 1 depicts the results of thesecalculations

32 Blue Shift fromCleaning One exciting feature of QDs isthat they appear to blue shift after cleaning (is phe-nomenon is represented in Figure 4 where QDs synthesizedat various hold temperatures appear one color just aftermicrowave synthesis but show another color (specificallyblue-shifted) after cleaning in acetone and resuspension intoluene(is effect is not limited to visible light under a low-powered UV lamp (365 nm) QDs also fluoresce a bluer colorthan their unpurified counterparts

Other groups have cleaned QDs without reporting a blueshift [8 23] (e removal of impurities is supposed to in-crease the signal-to-noise ratio of PL spectra specificallywith the removal of minor peaks [8] and have little effect onthe PL peak location (e fact that our QDs blue-shifted sodramatically by contrast suggests that something unin-tended is happening during cleaning One explanation forthis phenomenon is the removal of neutral coordinatingligands It is unlikely that the ligands are replaced entirely byeither acetone or toluene but there is evidence that suggestssome ligands such as TOP are removed during the cleaningprocess [23] (e decrease in surfactant attachments maydecrease the overall size of the QDs to the point that theyrespond to photonic excitation like smaller nanocrystalsAnother possible explanation is the fragmentation of ag-gregate QDs during the high-speed cleaning process (ereplacement of aggregate QDs (red light) with a largeramount of smaller QDs (blue light) would increase theoverall intensity of bluer light

33 Resuspension Solvent Choice QDs are covered in long-chained ligands to disallow aggregation and enable easysolution processing [8 11 27] According to the principleldquolike dissolves likerdquo the closer the shape and polarity amolecule or surfactant is to its solvent the more likely it is to

Advances in Materials Science and Engineering 3

120140170

200265

380 430 480 530 580 630 680Wavelength (nm)

0

05

1

15

2

25

Abso

rptio

n (A

rb u

nits)

(a)

450 500 550 600 650 700 7500

10

20

30

40

50

60

70

80

90

100

PL em

issio

n (A

rb u

nits)

Wavelength (nm)

200265

120140170

(b)

Figure 2 Comparison of UV-vis absorption (a) and PL emission (λex 400 nm) (b) of QDs synthesized via the microwave method

120degC 140degC 170degC 200degC 265degC

(a)

120degC 140degC 170degC 200degC 265degC

(b)

Figure 1 Array of quantum dots in glass vials immediately after microwave synthesis under visible light (a) and under 365 nmUV light (b)From left to right hold temperatures were 120degC 140degC 170degC 200degC and 265degC with hold time remaining constant at 2min 30 s

400 450 500 550 600 650 7000

02

04

06

08

1

12

14

Nor

mal

ized

inte

nsity

Wavelength (nm)

AbsorptionEmission

Figure 3 Example of Stokes shift of QDs (200degC hold time 2min 30 s) showing that absorption (solid line) occurs at a higher energy thanemission (dotted line) via photoexcitation at 400 nm

4 Advances in Materials Science and Engineering

dissolve in said solvent (is reasoning is what fuels thecommon assumption that QDs can be dissolved in mostnonpolar organic solvents [24] since the long-chained li-gands of QDs are themselves nonpolar Surprisingly thisprinciple does not apply in full to QDs synthesized via thismicrowave method In fact resuspension only works well withtoluene and marginally well with 1-octadecene (Figure 5)whereas resuspension is unsuccessful with nonpolar hexaneand nonpolar chloroform Even if some coloration occurs insolution as is the case for resuspended CdSe QDs in hexanethey still will not fluoresce under UV light

34 Photobrightening and Photobleaching Ambient lightcan play a role in altering the stability of QDs in air ex-emplified with two phenomena known as photobrighteningand photobleaching [6] Photobrightening is the tendencyfor ensembles of QDs to show increasing photo-luminescence behavior under continuous photoexcitation[28] whereas photobleaching is the tendency for QD latticestructures to permanently degrade under prolonged lightexposure and therefore lose all photoluminescence behavior[29] (e QDs synthesized in this study showed evidence ofboth effects

A day after synthesis QDs from one batch (170degC2min 30 s) were dispensed into three plastic spectro-photometer cuvettes wrapped in Parafilm and then placedeither in a dark laboratory drawer on top of a table sim15maway from an external window (double-paned low-etempered glass) or on top of a windowsill sim3 cm awayfrom the same external window (ese positions exemplifylocations where researchers might properly (in drawer) or

improperly (on tabletop or windowsill) store samples afterdaily use Light for this experiment came from two primarysources the fluorescent bulbs on the ceiling and thenatural sunlight from the window (e fluorescent bulbswere 32W T8 bulbs with a color temperature of 6500 Killuminating at an average of 2565 lumens (ere were 24bulbs in total separated into groups of three and dis-tributed across the ceiling of the open lab room into eightlight fixtures with crosshatched aluminum light diffusers(e approximate distance from the QD samples and apoint normal to the ceiling was 175m(e lights remainedon day and night throughout the course of the experi-ment (e sunlight coming from the window changed byday but an eight-year report from the National RenewableEnergy Laboratory indicates that the direct normal solarirradiance on Norfolk VA (USA) varies between 46 and50 kWhm2day [30]

(e QDs initially displayed little difference in photo-luminescence (PL) in terms of intensity shape or location ofpeak emission wavelength as expected with identicalsamples but over the course of eight days a few patternsemerged (Figure 6) (e first is that all samples had an initialincrease in PL over the first couple of days (even the drawersample which was exposed to light for short periods be-tween PL testing) indicating the photobrightening effect(Figure 6(a))(e second is that over the course of the rest ofthe experiment both samples continually exposed to lighteventually experienced a decrease in PL intensity from thestable sim15 PLPL0 with the sample on the windowsilldropping in intensity to the level of background noise by day7 (Figure 6(a)) (is evidence indicates that photobleachingfrom the sun and commercial fluorescent bulbs begins after a

Table 1 Comparison of microwave parameters for synthesis of CdSe QDs and the calculated sizes of those QDs based on estimated peak PLwavelengths

Temperature (degC) Hold time (min) PL wavelength (nm) Calculated radius (nm) Calculated diameter (nm)120 2 30 575 30 60140 2 30 624 39 78170 2 30 523 24 48200 2 30 560 28 56265 2 30 605 35 70

180degC 200degC 200degCin tol

225degC 225degCin tol

180degCin tol

(a)

180degC 200degC 200degCin tol

225degC 225degCin tol

180degCin tol

(b)

Figure 4 Images of QDs under visible light (a) and under 365 nmUV light (b) just before cleaning (left in each pair) and after resuspensionin toluene (right in each pair) (e batches were made with a hold time of 3min and represent the following temperatures 180degC (left pair)200degC (middle pair) and 225degC (right pair)

Advances in Materials Science and Engineering 5

165degC5minin tol

165degC5minin hex

180degC3minin tol

180degC3min

in CHCl3

225degC3minin tol

225degC3min

in ODE

(a)

225degC3minin tol

165degC5minin tol

165degC5minin hex

180degC3minin tol

180degC3min

in CHCl3

225degC3min

in ODE

(b)

Figure 5 Comparison of QDs resuspended in different solvents under visible light (a) and 365 nmUV light (b)(e left pair compares QDsprepared at 165degC for 5min and resuspended in either toluene (left) or hexane (right) (e middle pair compares QDs prepared at 180degC for3min and resuspended in either toluene (left) or chloroform (right) (e right pair compares QDs prepared at 225degC for 3min andresuspended in either toluene (left) or 1-octadecene (right)

3

25

2

15

1

05

0

PLP

L 0

0 2 4 6 8 10Time (days)

DrawerTableSill

(a)

ODE DrawerSill Table

(b)

Figure 6 Continued

6 Advances in Materials Science and Engineering

few days of continuous photoexcitation and dramaticallyreduces the PL intensity of QDs within 5ndash7 days (e dif-ference in the rate of PL intensity decline between thewindowsill sample and the tabletop sample indicates that thesun has a larger influence on photobleaching than ambientfluorescent light though this suggestion requires furthertesting for confirmation (ird the photobleaching effect isvisually apparent (Figures 6(b) and 6(c)) (e synthesizedQDs appeared orange at the beginning of experimentation(Figure 6(b)) but by day 9 (Figure 6(c)) both the windowsillsample and the tabletop sample experienced a noticeableblue shift in color

4 Conclusions

It is possible to synthesize CdSe QDs by using a microwavesynthesis reactor instead of the cumbersome glassware andheating elements designed for a hot-injection method allwhile using the same materials and precursor preparationsof the latter In addition to being a valuable tool in theclassroom for learning about quantum confinement inmaterials this simple ldquopress-and-gordquo microwave techniquecan also be up-scaled to research and development or in-dustry environments where access to a microwave reactor isfeasible However there are challenges and limitations toconsider with this technique such as the blue shifting of theQDs after purification organic solvent immiscibility or PL-broadening of freshly synthesized batches Such oddities arenot reported in most hot-injection or microwave synthesispublications and so deserve exposure

If this microwave technique were refined through repli-cation and more research these peculiarities would dissipateleaving microwave synthesis as a cheap quick and energy-efficient alternative to synthesizing QDs via the hot-injectiontechnique In addition to refinement there is also the potentialto try synthesizing other types of QDs based on hot-injectionmethods For instance one of the best types of QDs for solarcell applications is PbS with efficiencies reaching 12 andbeyond [4] To date there are no readily available microwavesynthesis methods for PbS however there are a number of

hot-injection methods all of which could be adapted to amicrowave heating procedure (e research potential formicrowave synthesis is immense and with time it could cometo be the industry standard for QD synthesis andimplementation

Data Availability

(e data used to support the findings of this study areavailable from the corresponding author upon request

Conflicts of Interest

(e authors declare that there are no conflicts of interestregarding the publication of this paper

Acknowledgments

(e authors would like to thank Travis Greene Mark Swiftand Samantha Koutsares for their technical support re-search assistance editing assistance and encouragement ofideas throughout this process (is work was supported bythe National Science Foundation (CREST grant numberHRD 1547771 CREST grant number HRD 1036494)

References

[1] L Brus ldquoElectronic wave functions in semiconductor clustersexperiment and theoryrdquo 0e Journal of Physical Chemistryvol 90 no 12 pp 2555ndash2560 1986

[2] T Kippeny L A Swafford and S J Rosenthal ldquoSemicon-ductor nanocrystals a powerful visual aid for introducing theparticle in a boxrdquo Journal of Chemical Education vol 79no 9 p 1094 2002

[3] V I Klimov Nanocrystal Quantum Dots CRC Press BocaRaton FL USA 2nd edition 2010

[4] J Xu O Voznyy M Liu et al ldquo2D matrix engineering forhomogeneous quantum dot coupling in photovoltaic solidsrdquoNature Nanotechnology vol 13 no 6 pp 456ndash462 2018

[5] P V Kamat ldquoQuantum dot solar cells Semiconductornanocrystals as light harvestersrdquo 0e Journal of PhysicalChemistry C vol 112 no 48 pp 18737ndash18753 2008

ODE DrawerSill Table

(c)

Figure 6 Plot of time-dependent photoluminescence (PL)initial photoluminescence (PL0) (a) image at t 0 days (b) and image at t 9days (c) for comparative samples of CdSe QD solutions (170degC 2min 30 s) QDs were either left in a closed lab drawer with limited exposureto ambient light (solid line square shape) placed on an open laboratory tabletop with continual exposure to mostly fluorescent light (dashedline triangle shape) or placed on an interior windowsill with frequent exposure to sunlight and fluorescent light (dotted line circle shape)for the course of the experiment In the pictures from left to right the samples are neat 1-octadecene QDs left on the windowsill QDs left onthe tabletop and QDs stored in the drawer respectively

Advances in Materials Science and Engineering 7

[6] Y Shirasaki G J Supran M G Bawendi and V BulovicldquoEmergence of colloidal quantum-dot light-emitting tech-nologiesrdquo Nature Photonics vol 7 no 1 pp 13ndash23 2013

[7] C R Kagan E Lifshitz E H Sargent and D V TalapinldquoBuilding devices from colloidal quantum dotsrdquo Sciencevol 353 no 6302 2016

[8] M L Landry T E Morrell T K Karagounis C-H Hsia andC-Y Wang ldquoSimple syntheses of CdSe quantum dotsrdquoJournal of Chemical Education vol 91 no 2 pp 274ndash2792014

[9] C B Murray D J Norris and M G Bawendi ldquoSynthesis andcharacterization of nearly monodisperse CdE (E sulfur se-lenium tellurium) semiconductor nanocrystallitesrdquo Journal ofthe American Chemical Society vol 115 no 19 pp 8706ndash87151993

[10] M B Mohamed D Tonti A Al-Salman A Chemseddineand M Chergui ldquoSynthesis of high quality zinc blende CdSenanocrystalsrdquo 0e Journal of Physical Chemistry B vol 109no 21 pp 10533ndash10537 2005

[11] G H Carey A L Abdelhady Z Ning S M(on OM Bakrand E H Sargent ldquoColloidal quantum dot solar cellsrdquoChemical Reviews vol 115 no 23 pp 12732ndash12763 2015

[12] K J Nordell E M Boatman and G C Lisensky ldquoA safereasier faster synthesis for CdSe quantum dot nanocrystalsrdquoJournal of Chemical Education vol 82 no 11 p 1697 2005

[13] M Z Hu and T Zhu ldquoSemiconductor nanocrystal quantumdot synthesis approaches towards large-scale industrial pro-duction for energy applicationsrdquo Nanoscale Research Lettersvol 10 no 1 pp 1ndash15 2015

[14] O Palchik R Kerner A Gedanken A M WeissM A Slifkin and V Palchik ldquoMicrowave-assisted polyolmethod for the preparation of CdSe ldquonanoballsrdquordquo Journal ofMaterials Chemistry vol 11 no 3 pp 874ndash878 2001

[15] J Zhu O Palchik S Chen and A Gedanken ldquoMicrowaveassisted preparation of CdSe PbSe and Cu2minus xSe nano-particlesrdquo0e Journal of Physical Chemistry B vol 104 no 31pp 7344ndash7347 2002

[16] D W Ayele H-M Chen W-N Su et al ldquoControlledsynthesis of CdSe quantum dots by a microwave-enhancedprocess a green approach for mass productionrdquo ChemistryndashAEuropean Journal vol 17 no 20 pp 5737ndash5744 2011

[17] M M Moghaddam M Baghbanzadeh A Keilbach andC O Kappe ldquoMicrowave-assisted synthesis of CdSe quantumdots can the electromagnetic field influence the formationand quality of the resulting nanocrystalsrdquo Nanoscale vol 4no 23 pp 7435ndash7442 2012

[18] J A Gerbec D Magana A Washington and G F StrouseldquoMicrowave-enhanced reaction rates for nanoparticle syn-thesisrdquo Journal of the American Chemical Society vol 127no 45 pp 15791ndash15800 2005

[19] A L Washington and G F Strouse ldquoMicrowave synthesis ofCdSe and CdTe nanocrystals in nonabsorbing alkanesrdquoJournal of the American Chemical Society vol 130 no 28pp 8916ndash8922 2008

[20] J Ziegler A Merkulov M Grabolle U Resch-Genger andT Nann ldquoHigh-quality ZnS shells for CdSe nanoparticlesrapid microwave synthesisrdquo Langmuir vol 23 no 14pp 7751ndash7759 2007

[21] D A Hines and P V Kamat ldquoRecent advances in quantumdot surface chemistryrdquo ACS Applied Materials amp Interfacesvol 6 no 5 pp 3041ndash3057 2014

[22] Anton Paar Monowave 300 Microwave Synthesis ReactorOriginal Instruction Manual Anton Paar Graz Austria 2015

[23] A J Morris-Cohen M D Donakowski K E Knowles andE A Weiss ldquo(e effect of a common purification procedureon the chemical composition of the surfaces of CdSe quantumdots synthesized with trioctylphosphine oxiderdquo0e Journal ofPhysical Chemistry C vol 114 no 2 pp 897ndash906 2010

[24] Aldrich Chemistry Lumidottrade FAQ Frequently Asked Questionsabout Using QDNanocrystals Aldrich Chemistry St Louis MOUSA 2019 httpswwwsigmaaldrichcomcontentdamsigma-aldrichdocsSigma-AldrichGeneral_Informationlumidot_faqspdf

[25] D Zhou M Lin Z Chen et al ldquoSimple synthesis of highlyluminescent water-soluble CdTe quantum dots with con-trollable surface functionalityrdquo Chemistry of Materialsvol 23 no 21 pp 4857ndash4862 2011

[26] E Jenrette S K Pradhan G Rutherford J Flowers D Haand A K Pradhan ldquoQuantum-dot-conjugated grapheneoxide as an optical tool for biosensorrdquo Optics Express vol 23no 19 p 25017 2015

[27] E Zillner S Fengler P Niyamakom F Rauscher K Kohlerand T Dittrich ldquoRole of ligand exchange at CdSe quantumdot layers for charge separationrdquo 0e Journal of PhysicalChemistry C vol 116 no 31 pp 16747ndash16754 2012

[28] D B Tice M T Frederick R P H Chang and E A WeissldquoElectron migration limits the rate of photobrightening inthin films of CdSe quantum dots in a dry N2 (g) atmosphererdquo0e Journal of Physical Chemistry C vol 115 no 9pp 3654ndash3662 2011

[29] W G J H M van Sark P L T M Frederix A A BolH C Gerritsen and A Meijerink ldquoBlueing bleaching andblinking of single CdSeZnS quantum dotsrdquo ChemPhysChemvol 3 no 10 pp 871ndash879 2002

[30] N Gilroy Direct Normal Solar Resource of Virginia NationalRenewable Energy Laboratory for the US Department ofEnergy Golden CO USA 2017 httpswwwnrelgovgissolarhtml

8 Advances in Materials Science and Engineering

magnetic stir bar speed After inserting the reaction vial andruby thermometer one would simply tap the start button tobegin the procedure letting the microwave heat up holdand cool down according to the set parameters (e solutionin the reaction vial at the end would contain the colloidalQDs ready for purification or analysis

24Cleaning Previous findings suggest that cleaning QDs isa viable way to remove unreacted impurities from QD so-lutions [23] Based on a method from another report [8] weattempted to clean the QDs by inserting 2mL of QD so-lution 06mL of OA and either 26mL or 52mL of acetoneinto a sim30mL centrifuge tube which was then spun at4000 rpm for 10min (is procedure with either amount ofacetone however yielded no QD pellets After some trial-and-error it was determined that between 1 and 2mL of QDsolution (though we used 14mL for all experiments afterthis one) in a tube with 28mL of acetone spinning at10000 rpm for 15 minutes was enough to get pellets be-tween 1 and 5mm in diameter (ese pellets also ranged incolor from pale yellow to red-orange depending on theQDs used

After precipitation in the polar solvent via centrifuga-tion supernatants containing excess impurities were dis-carded Pellets were then resuspended in 125mL of thetarget solvent which was either ODE toluene hexane orchloroform [24] Save for unusual exceptions noted inSection 3 resuspensions were transparent and monochro-matic between yellow and red-orange under visible light

3 Results and Discussion

31 PL-BroadeningUV-Vis andSize As seen in Figure 1(a)solutions of QDs immediately after microwave synthesisrange in color from pale yellow to dark red under visiblelight (ese colors are consistent with those from otherreports with hot-injection procedures [8 12] Most QDsyntheses are also transparent though some batches near theupper limit of the microwave reactor appear cloudy with adark red precipitate (presumably bulk CdSe powder)Batches made below 120degC show no color change or lu-minescence under UV light suggesting that there is a lowerlimit of temperature beyond which QD nucleation does notoccur

QDs synthesized within the working range of temper-atures show odd patterns of emission under 365 nmUV light(Figure 1(b)) Instead of fluorescing the usual blue to red[8 12] QDs emit colors ranging from a silver-blue to brightwhite to a peachy orange (ese abnormalities appear in thePL spectra as well (Figure 2(b)) where broad-spectrumemissions dominate for lower temperature synthesis batches(long-lined curves) Batches made between 170degC and 240degChave more defined peaks consistent with other reports[8 12] yet even these QDs show inexplicable broad emissionin the range of 650ndash750 nm

With UV-vis spectroscopy however the QDs display amore common pattern (Figure 2(a)) Absorption peaks fallbetween 430 and 580 nm a range that lies within the visible

light spectrum between green and yellow A representativebatch of QDs also shows the familiar Stokes shift (Figure 3)[8 12 18 19 25] indicating the difference in energy be-tween absorbed photons and those that are emitted afternonradiative relaxation

Transmission electronmicroscope (TEM) images of QDssynthesized in our lab have been published in a previousreport [26] In colloidal form QDs are relatively spherical inshape with an inner core composed of lattice CdSe and anouter shell composed of attached organic ligands [8] (esize of the QDs can be approximated using photo-luminescence peak data and the following Brus equation[1 2]

R2

h2

8Ex

1mlowaste

+1

mlowasth1113888 1113889 (1)

where Ex=E minus Eg (CdSe) Eg (CdSe) = 174 eV mlowaste (CdSe)= 013mo (mo is the mass of free electron) mlowasth (CdSe)= 045mo and E is calculated using the de Broglie relationE= hcλ where h is Planckrsquos constant and λ is the peak PLwavelength of the QDs Table 1 depicts the results of thesecalculations

32 Blue Shift fromCleaning One exciting feature of QDs isthat they appear to blue shift after cleaning (is phe-nomenon is represented in Figure 4 where QDs synthesizedat various hold temperatures appear one color just aftermicrowave synthesis but show another color (specificallyblue-shifted) after cleaning in acetone and resuspension intoluene(is effect is not limited to visible light under a low-powered UV lamp (365 nm) QDs also fluoresce a bluer colorthan their unpurified counterparts

Other groups have cleaned QDs without reporting a blueshift [8 23] (e removal of impurities is supposed to in-crease the signal-to-noise ratio of PL spectra specificallywith the removal of minor peaks [8] and have little effect onthe PL peak location (e fact that our QDs blue-shifted sodramatically by contrast suggests that something unin-tended is happening during cleaning One explanation forthis phenomenon is the removal of neutral coordinatingligands It is unlikely that the ligands are replaced entirely byeither acetone or toluene but there is evidence that suggestssome ligands such as TOP are removed during the cleaningprocess [23] (e decrease in surfactant attachments maydecrease the overall size of the QDs to the point that theyrespond to photonic excitation like smaller nanocrystalsAnother possible explanation is the fragmentation of ag-gregate QDs during the high-speed cleaning process (ereplacement of aggregate QDs (red light) with a largeramount of smaller QDs (blue light) would increase theoverall intensity of bluer light

33 Resuspension Solvent Choice QDs are covered in long-chained ligands to disallow aggregation and enable easysolution processing [8 11 27] According to the principleldquolike dissolves likerdquo the closer the shape and polarity amolecule or surfactant is to its solvent the more likely it is to

Advances in Materials Science and Engineering 3

120140170

200265

380 430 480 530 580 630 680Wavelength (nm)

0

05

1

15

2

25

Abso

rptio

n (A

rb u

nits)

(a)

450 500 550 600 650 700 7500

10

20

30

40

50

60

70

80

90

100

PL em

issio

n (A

rb u

nits)

Wavelength (nm)

200265

120140170

(b)

Figure 2 Comparison of UV-vis absorption (a) and PL emission (λex 400 nm) (b) of QDs synthesized via the microwave method

120degC 140degC 170degC 200degC 265degC

(a)

120degC 140degC 170degC 200degC 265degC

(b)

Figure 1 Array of quantum dots in glass vials immediately after microwave synthesis under visible light (a) and under 365 nmUV light (b)From left to right hold temperatures were 120degC 140degC 170degC 200degC and 265degC with hold time remaining constant at 2min 30 s

400 450 500 550 600 650 7000

02

04

06

08

1

12

14

Nor

mal

ized

inte

nsity

Wavelength (nm)

AbsorptionEmission

Figure 3 Example of Stokes shift of QDs (200degC hold time 2min 30 s) showing that absorption (solid line) occurs at a higher energy thanemission (dotted line) via photoexcitation at 400 nm

4 Advances in Materials Science and Engineering

dissolve in said solvent (is reasoning is what fuels thecommon assumption that QDs can be dissolved in mostnonpolar organic solvents [24] since the long-chained li-gands of QDs are themselves nonpolar Surprisingly thisprinciple does not apply in full to QDs synthesized via thismicrowave method In fact resuspension only works well withtoluene and marginally well with 1-octadecene (Figure 5)whereas resuspension is unsuccessful with nonpolar hexaneand nonpolar chloroform Even if some coloration occurs insolution as is the case for resuspended CdSe QDs in hexanethey still will not fluoresce under UV light

34 Photobrightening and Photobleaching Ambient lightcan play a role in altering the stability of QDs in air ex-emplified with two phenomena known as photobrighteningand photobleaching [6] Photobrightening is the tendencyfor ensembles of QDs to show increasing photo-luminescence behavior under continuous photoexcitation[28] whereas photobleaching is the tendency for QD latticestructures to permanently degrade under prolonged lightexposure and therefore lose all photoluminescence behavior[29] (e QDs synthesized in this study showed evidence ofboth effects

A day after synthesis QDs from one batch (170degC2min 30 s) were dispensed into three plastic spectro-photometer cuvettes wrapped in Parafilm and then placedeither in a dark laboratory drawer on top of a table sim15maway from an external window (double-paned low-etempered glass) or on top of a windowsill sim3 cm awayfrom the same external window (ese positions exemplifylocations where researchers might properly (in drawer) or

improperly (on tabletop or windowsill) store samples afterdaily use Light for this experiment came from two primarysources the fluorescent bulbs on the ceiling and thenatural sunlight from the window (e fluorescent bulbswere 32W T8 bulbs with a color temperature of 6500 Killuminating at an average of 2565 lumens (ere were 24bulbs in total separated into groups of three and dis-tributed across the ceiling of the open lab room into eightlight fixtures with crosshatched aluminum light diffusers(e approximate distance from the QD samples and apoint normal to the ceiling was 175m(e lights remainedon day and night throughout the course of the experi-ment (e sunlight coming from the window changed byday but an eight-year report from the National RenewableEnergy Laboratory indicates that the direct normal solarirradiance on Norfolk VA (USA) varies between 46 and50 kWhm2day [30]

(e QDs initially displayed little difference in photo-luminescence (PL) in terms of intensity shape or location ofpeak emission wavelength as expected with identicalsamples but over the course of eight days a few patternsemerged (Figure 6) (e first is that all samples had an initialincrease in PL over the first couple of days (even the drawersample which was exposed to light for short periods be-tween PL testing) indicating the photobrightening effect(Figure 6(a))(e second is that over the course of the rest ofthe experiment both samples continually exposed to lighteventually experienced a decrease in PL intensity from thestable sim15 PLPL0 with the sample on the windowsilldropping in intensity to the level of background noise by day7 (Figure 6(a)) (is evidence indicates that photobleachingfrom the sun and commercial fluorescent bulbs begins after a

Table 1 Comparison of microwave parameters for synthesis of CdSe QDs and the calculated sizes of those QDs based on estimated peak PLwavelengths

Temperature (degC) Hold time (min) PL wavelength (nm) Calculated radius (nm) Calculated diameter (nm)120 2 30 575 30 60140 2 30 624 39 78170 2 30 523 24 48200 2 30 560 28 56265 2 30 605 35 70

180degC 200degC 200degCin tol

225degC 225degCin tol

180degCin tol

(a)

180degC 200degC 200degCin tol

225degC 225degCin tol

180degCin tol

(b)

Figure 4 Images of QDs under visible light (a) and under 365 nmUV light (b) just before cleaning (left in each pair) and after resuspensionin toluene (right in each pair) (e batches were made with a hold time of 3min and represent the following temperatures 180degC (left pair)200degC (middle pair) and 225degC (right pair)

Advances in Materials Science and Engineering 5

165degC5minin tol

165degC5minin hex

180degC3minin tol

180degC3min

in CHCl3

225degC3minin tol

225degC3min

in ODE

(a)

225degC3minin tol

165degC5minin tol

165degC5minin hex

180degC3minin tol

180degC3min

in CHCl3

225degC3min

in ODE

(b)

Figure 5 Comparison of QDs resuspended in different solvents under visible light (a) and 365 nmUV light (b)(e left pair compares QDsprepared at 165degC for 5min and resuspended in either toluene (left) or hexane (right) (e middle pair compares QDs prepared at 180degC for3min and resuspended in either toluene (left) or chloroform (right) (e right pair compares QDs prepared at 225degC for 3min andresuspended in either toluene (left) or 1-octadecene (right)

3

25

2

15

1

05

0

PLP

L 0

0 2 4 6 8 10Time (days)

DrawerTableSill

(a)

ODE DrawerSill Table

(b)

Figure 6 Continued

6 Advances in Materials Science and Engineering

few days of continuous photoexcitation and dramaticallyreduces the PL intensity of QDs within 5ndash7 days (e dif-ference in the rate of PL intensity decline between thewindowsill sample and the tabletop sample indicates that thesun has a larger influence on photobleaching than ambientfluorescent light though this suggestion requires furthertesting for confirmation (ird the photobleaching effect isvisually apparent (Figures 6(b) and 6(c)) (e synthesizedQDs appeared orange at the beginning of experimentation(Figure 6(b)) but by day 9 (Figure 6(c)) both the windowsillsample and the tabletop sample experienced a noticeableblue shift in color

4 Conclusions

It is possible to synthesize CdSe QDs by using a microwavesynthesis reactor instead of the cumbersome glassware andheating elements designed for a hot-injection method allwhile using the same materials and precursor preparationsof the latter In addition to being a valuable tool in theclassroom for learning about quantum confinement inmaterials this simple ldquopress-and-gordquo microwave techniquecan also be up-scaled to research and development or in-dustry environments where access to a microwave reactor isfeasible However there are challenges and limitations toconsider with this technique such as the blue shifting of theQDs after purification organic solvent immiscibility or PL-broadening of freshly synthesized batches Such oddities arenot reported in most hot-injection or microwave synthesispublications and so deserve exposure

If this microwave technique were refined through repli-cation and more research these peculiarities would dissipateleaving microwave synthesis as a cheap quick and energy-efficient alternative to synthesizing QDs via the hot-injectiontechnique In addition to refinement there is also the potentialto try synthesizing other types of QDs based on hot-injectionmethods For instance one of the best types of QDs for solarcell applications is PbS with efficiencies reaching 12 andbeyond [4] To date there are no readily available microwavesynthesis methods for PbS however there are a number of

hot-injection methods all of which could be adapted to amicrowave heating procedure (e research potential formicrowave synthesis is immense and with time it could cometo be the industry standard for QD synthesis andimplementation

Data Availability

(e data used to support the findings of this study areavailable from the corresponding author upon request

Conflicts of Interest

(e authors declare that there are no conflicts of interestregarding the publication of this paper

Acknowledgments

(e authors would like to thank Travis Greene Mark Swiftand Samantha Koutsares for their technical support re-search assistance editing assistance and encouragement ofideas throughout this process (is work was supported bythe National Science Foundation (CREST grant numberHRD 1547771 CREST grant number HRD 1036494)

References

[1] L Brus ldquoElectronic wave functions in semiconductor clustersexperiment and theoryrdquo 0e Journal of Physical Chemistryvol 90 no 12 pp 2555ndash2560 1986

[2] T Kippeny L A Swafford and S J Rosenthal ldquoSemicon-ductor nanocrystals a powerful visual aid for introducing theparticle in a boxrdquo Journal of Chemical Education vol 79no 9 p 1094 2002

[3] V I Klimov Nanocrystal Quantum Dots CRC Press BocaRaton FL USA 2nd edition 2010

[4] J Xu O Voznyy M Liu et al ldquo2D matrix engineering forhomogeneous quantum dot coupling in photovoltaic solidsrdquoNature Nanotechnology vol 13 no 6 pp 456ndash462 2018

[5] P V Kamat ldquoQuantum dot solar cells Semiconductornanocrystals as light harvestersrdquo 0e Journal of PhysicalChemistry C vol 112 no 48 pp 18737ndash18753 2008

ODE DrawerSill Table

(c)

Figure 6 Plot of time-dependent photoluminescence (PL)initial photoluminescence (PL0) (a) image at t 0 days (b) and image at t 9days (c) for comparative samples of CdSe QD solutions (170degC 2min 30 s) QDs were either left in a closed lab drawer with limited exposureto ambient light (solid line square shape) placed on an open laboratory tabletop with continual exposure to mostly fluorescent light (dashedline triangle shape) or placed on an interior windowsill with frequent exposure to sunlight and fluorescent light (dotted line circle shape)for the course of the experiment In the pictures from left to right the samples are neat 1-octadecene QDs left on the windowsill QDs left onthe tabletop and QDs stored in the drawer respectively

Advances in Materials Science and Engineering 7

[6] Y Shirasaki G J Supran M G Bawendi and V BulovicldquoEmergence of colloidal quantum-dot light-emitting tech-nologiesrdquo Nature Photonics vol 7 no 1 pp 13ndash23 2013

[7] C R Kagan E Lifshitz E H Sargent and D V TalapinldquoBuilding devices from colloidal quantum dotsrdquo Sciencevol 353 no 6302 2016

[8] M L Landry T E Morrell T K Karagounis C-H Hsia andC-Y Wang ldquoSimple syntheses of CdSe quantum dotsrdquoJournal of Chemical Education vol 91 no 2 pp 274ndash2792014

[9] C B Murray D J Norris and M G Bawendi ldquoSynthesis andcharacterization of nearly monodisperse CdE (E sulfur se-lenium tellurium) semiconductor nanocrystallitesrdquo Journal ofthe American Chemical Society vol 115 no 19 pp 8706ndash87151993

[10] M B Mohamed D Tonti A Al-Salman A Chemseddineand M Chergui ldquoSynthesis of high quality zinc blende CdSenanocrystalsrdquo 0e Journal of Physical Chemistry B vol 109no 21 pp 10533ndash10537 2005

[11] G H Carey A L Abdelhady Z Ning S M(on OM Bakrand E H Sargent ldquoColloidal quantum dot solar cellsrdquoChemical Reviews vol 115 no 23 pp 12732ndash12763 2015

[12] K J Nordell E M Boatman and G C Lisensky ldquoA safereasier faster synthesis for CdSe quantum dot nanocrystalsrdquoJournal of Chemical Education vol 82 no 11 p 1697 2005

[13] M Z Hu and T Zhu ldquoSemiconductor nanocrystal quantumdot synthesis approaches towards large-scale industrial pro-duction for energy applicationsrdquo Nanoscale Research Lettersvol 10 no 1 pp 1ndash15 2015

[14] O Palchik R Kerner A Gedanken A M WeissM A Slifkin and V Palchik ldquoMicrowave-assisted polyolmethod for the preparation of CdSe ldquonanoballsrdquordquo Journal ofMaterials Chemistry vol 11 no 3 pp 874ndash878 2001

[15] J Zhu O Palchik S Chen and A Gedanken ldquoMicrowaveassisted preparation of CdSe PbSe and Cu2minus xSe nano-particlesrdquo0e Journal of Physical Chemistry B vol 104 no 31pp 7344ndash7347 2002

[16] D W Ayele H-M Chen W-N Su et al ldquoControlledsynthesis of CdSe quantum dots by a microwave-enhancedprocess a green approach for mass productionrdquo ChemistryndashAEuropean Journal vol 17 no 20 pp 5737ndash5744 2011

[17] M M Moghaddam M Baghbanzadeh A Keilbach andC O Kappe ldquoMicrowave-assisted synthesis of CdSe quantumdots can the electromagnetic field influence the formationand quality of the resulting nanocrystalsrdquo Nanoscale vol 4no 23 pp 7435ndash7442 2012

[18] J A Gerbec D Magana A Washington and G F StrouseldquoMicrowave-enhanced reaction rates for nanoparticle syn-thesisrdquo Journal of the American Chemical Society vol 127no 45 pp 15791ndash15800 2005

[19] A L Washington and G F Strouse ldquoMicrowave synthesis ofCdSe and CdTe nanocrystals in nonabsorbing alkanesrdquoJournal of the American Chemical Society vol 130 no 28pp 8916ndash8922 2008

[20] J Ziegler A Merkulov M Grabolle U Resch-Genger andT Nann ldquoHigh-quality ZnS shells for CdSe nanoparticlesrapid microwave synthesisrdquo Langmuir vol 23 no 14pp 7751ndash7759 2007

[21] D A Hines and P V Kamat ldquoRecent advances in quantumdot surface chemistryrdquo ACS Applied Materials amp Interfacesvol 6 no 5 pp 3041ndash3057 2014

[22] Anton Paar Monowave 300 Microwave Synthesis ReactorOriginal Instruction Manual Anton Paar Graz Austria 2015

[23] A J Morris-Cohen M D Donakowski K E Knowles andE A Weiss ldquo(e effect of a common purification procedureon the chemical composition of the surfaces of CdSe quantumdots synthesized with trioctylphosphine oxiderdquo0e Journal ofPhysical Chemistry C vol 114 no 2 pp 897ndash906 2010

[24] Aldrich Chemistry Lumidottrade FAQ Frequently Asked Questionsabout Using QDNanocrystals Aldrich Chemistry St Louis MOUSA 2019 httpswwwsigmaaldrichcomcontentdamsigma-aldrichdocsSigma-AldrichGeneral_Informationlumidot_faqspdf

[25] D Zhou M Lin Z Chen et al ldquoSimple synthesis of highlyluminescent water-soluble CdTe quantum dots with con-trollable surface functionalityrdquo Chemistry of Materialsvol 23 no 21 pp 4857ndash4862 2011

[26] E Jenrette S K Pradhan G Rutherford J Flowers D Haand A K Pradhan ldquoQuantum-dot-conjugated grapheneoxide as an optical tool for biosensorrdquo Optics Express vol 23no 19 p 25017 2015

[27] E Zillner S Fengler P Niyamakom F Rauscher K Kohlerand T Dittrich ldquoRole of ligand exchange at CdSe quantumdot layers for charge separationrdquo 0e Journal of PhysicalChemistry C vol 116 no 31 pp 16747ndash16754 2012

[28] D B Tice M T Frederick R P H Chang and E A WeissldquoElectron migration limits the rate of photobrightening inthin films of CdSe quantum dots in a dry N2 (g) atmosphererdquo0e Journal of Physical Chemistry C vol 115 no 9pp 3654ndash3662 2011

[29] W G J H M van Sark P L T M Frederix A A BolH C Gerritsen and A Meijerink ldquoBlueing bleaching andblinking of single CdSeZnS quantum dotsrdquo ChemPhysChemvol 3 no 10 pp 871ndash879 2002

[30] N Gilroy Direct Normal Solar Resource of Virginia NationalRenewable Energy Laboratory for the US Department ofEnergy Golden CO USA 2017 httpswwwnrelgovgissolarhtml

8 Advances in Materials Science and Engineering

120140170

200265

380 430 480 530 580 630 680Wavelength (nm)

0

05

1

15

2

25

Abso

rptio

n (A

rb u

nits)

(a)

450 500 550 600 650 700 7500

10

20

30

40

50

60

70

80

90

100

PL em

issio

n (A

rb u

nits)

Wavelength (nm)

200265

120140170

(b)

Figure 2 Comparison of UV-vis absorption (a) and PL emission (λex 400 nm) (b) of QDs synthesized via the microwave method

120degC 140degC 170degC 200degC 265degC

(a)

120degC 140degC 170degC 200degC 265degC

(b)

Figure 1 Array of quantum dots in glass vials immediately after microwave synthesis under visible light (a) and under 365 nmUV light (b)From left to right hold temperatures were 120degC 140degC 170degC 200degC and 265degC with hold time remaining constant at 2min 30 s

400 450 500 550 600 650 7000

02

04

06

08

1

12

14

Nor

mal

ized

inte

nsity

Wavelength (nm)

AbsorptionEmission

Figure 3 Example of Stokes shift of QDs (200degC hold time 2min 30 s) showing that absorption (solid line) occurs at a higher energy thanemission (dotted line) via photoexcitation at 400 nm

4 Advances in Materials Science and Engineering

dissolve in said solvent (is reasoning is what fuels thecommon assumption that QDs can be dissolved in mostnonpolar organic solvents [24] since the long-chained li-gands of QDs are themselves nonpolar Surprisingly thisprinciple does not apply in full to QDs synthesized via thismicrowave method In fact resuspension only works well withtoluene and marginally well with 1-octadecene (Figure 5)whereas resuspension is unsuccessful with nonpolar hexaneand nonpolar chloroform Even if some coloration occurs insolution as is the case for resuspended CdSe QDs in hexanethey still will not fluoresce under UV light

34 Photobrightening and Photobleaching Ambient lightcan play a role in altering the stability of QDs in air ex-emplified with two phenomena known as photobrighteningand photobleaching [6] Photobrightening is the tendencyfor ensembles of QDs to show increasing photo-luminescence behavior under continuous photoexcitation[28] whereas photobleaching is the tendency for QD latticestructures to permanently degrade under prolonged lightexposure and therefore lose all photoluminescence behavior[29] (e QDs synthesized in this study showed evidence ofboth effects

A day after synthesis QDs from one batch (170degC2min 30 s) were dispensed into three plastic spectro-photometer cuvettes wrapped in Parafilm and then placedeither in a dark laboratory drawer on top of a table sim15maway from an external window (double-paned low-etempered glass) or on top of a windowsill sim3 cm awayfrom the same external window (ese positions exemplifylocations where researchers might properly (in drawer) or

improperly (on tabletop or windowsill) store samples afterdaily use Light for this experiment came from two primarysources the fluorescent bulbs on the ceiling and thenatural sunlight from the window (e fluorescent bulbswere 32W T8 bulbs with a color temperature of 6500 Killuminating at an average of 2565 lumens (ere were 24bulbs in total separated into groups of three and dis-tributed across the ceiling of the open lab room into eightlight fixtures with crosshatched aluminum light diffusers(e approximate distance from the QD samples and apoint normal to the ceiling was 175m(e lights remainedon day and night throughout the course of the experi-ment (e sunlight coming from the window changed byday but an eight-year report from the National RenewableEnergy Laboratory indicates that the direct normal solarirradiance on Norfolk VA (USA) varies between 46 and50 kWhm2day [30]

(e QDs initially displayed little difference in photo-luminescence (PL) in terms of intensity shape or location ofpeak emission wavelength as expected with identicalsamples but over the course of eight days a few patternsemerged (Figure 6) (e first is that all samples had an initialincrease in PL over the first couple of days (even the drawersample which was exposed to light for short periods be-tween PL testing) indicating the photobrightening effect(Figure 6(a))(e second is that over the course of the rest ofthe experiment both samples continually exposed to lighteventually experienced a decrease in PL intensity from thestable sim15 PLPL0 with the sample on the windowsilldropping in intensity to the level of background noise by day7 (Figure 6(a)) (is evidence indicates that photobleachingfrom the sun and commercial fluorescent bulbs begins after a

Table 1 Comparison of microwave parameters for synthesis of CdSe QDs and the calculated sizes of those QDs based on estimated peak PLwavelengths

Temperature (degC) Hold time (min) PL wavelength (nm) Calculated radius (nm) Calculated diameter (nm)120 2 30 575 30 60140 2 30 624 39 78170 2 30 523 24 48200 2 30 560 28 56265 2 30 605 35 70

180degC 200degC 200degCin tol

225degC 225degCin tol

180degCin tol

(a)

180degC 200degC 200degCin tol

225degC 225degCin tol

180degCin tol

(b)

Figure 4 Images of QDs under visible light (a) and under 365 nmUV light (b) just before cleaning (left in each pair) and after resuspensionin toluene (right in each pair) (e batches were made with a hold time of 3min and represent the following temperatures 180degC (left pair)200degC (middle pair) and 225degC (right pair)

Advances in Materials Science and Engineering 5

165degC5minin tol

165degC5minin hex

180degC3minin tol

180degC3min

in CHCl3

225degC3minin tol

225degC3min

in ODE

(a)

225degC3minin tol

165degC5minin tol

165degC5minin hex

180degC3minin tol

180degC3min

in CHCl3

225degC3min

in ODE

(b)

Figure 5 Comparison of QDs resuspended in different solvents under visible light (a) and 365 nmUV light (b)(e left pair compares QDsprepared at 165degC for 5min and resuspended in either toluene (left) or hexane (right) (e middle pair compares QDs prepared at 180degC for3min and resuspended in either toluene (left) or chloroform (right) (e right pair compares QDs prepared at 225degC for 3min andresuspended in either toluene (left) or 1-octadecene (right)

3

25

2

15

1

05

0

PLP

L 0

0 2 4 6 8 10Time (days)

DrawerTableSill

(a)

ODE DrawerSill Table

(b)

Figure 6 Continued

6 Advances in Materials Science and Engineering

few days of continuous photoexcitation and dramaticallyreduces the PL intensity of QDs within 5ndash7 days (e dif-ference in the rate of PL intensity decline between thewindowsill sample and the tabletop sample indicates that thesun has a larger influence on photobleaching than ambientfluorescent light though this suggestion requires furthertesting for confirmation (ird the photobleaching effect isvisually apparent (Figures 6(b) and 6(c)) (e synthesizedQDs appeared orange at the beginning of experimentation(Figure 6(b)) but by day 9 (Figure 6(c)) both the windowsillsample and the tabletop sample experienced a noticeableblue shift in color

4 Conclusions

It is possible to synthesize CdSe QDs by using a microwavesynthesis reactor instead of the cumbersome glassware andheating elements designed for a hot-injection method allwhile using the same materials and precursor preparationsof the latter In addition to being a valuable tool in theclassroom for learning about quantum confinement inmaterials this simple ldquopress-and-gordquo microwave techniquecan also be up-scaled to research and development or in-dustry environments where access to a microwave reactor isfeasible However there are challenges and limitations toconsider with this technique such as the blue shifting of theQDs after purification organic solvent immiscibility or PL-broadening of freshly synthesized batches Such oddities arenot reported in most hot-injection or microwave synthesispublications and so deserve exposure

If this microwave technique were refined through repli-cation and more research these peculiarities would dissipateleaving microwave synthesis as a cheap quick and energy-efficient alternative to synthesizing QDs via the hot-injectiontechnique In addition to refinement there is also the potentialto try synthesizing other types of QDs based on hot-injectionmethods For instance one of the best types of QDs for solarcell applications is PbS with efficiencies reaching 12 andbeyond [4] To date there are no readily available microwavesynthesis methods for PbS however there are a number of

hot-injection methods all of which could be adapted to amicrowave heating procedure (e research potential formicrowave synthesis is immense and with time it could cometo be the industry standard for QD synthesis andimplementation

Data Availability

(e data used to support the findings of this study areavailable from the corresponding author upon request

Conflicts of Interest

(e authors declare that there are no conflicts of interestregarding the publication of this paper

Acknowledgments

(e authors would like to thank Travis Greene Mark Swiftand Samantha Koutsares for their technical support re-search assistance editing assistance and encouragement ofideas throughout this process (is work was supported bythe National Science Foundation (CREST grant numberHRD 1547771 CREST grant number HRD 1036494)

References

[1] L Brus ldquoElectronic wave functions in semiconductor clustersexperiment and theoryrdquo 0e Journal of Physical Chemistryvol 90 no 12 pp 2555ndash2560 1986

[2] T Kippeny L A Swafford and S J Rosenthal ldquoSemicon-ductor nanocrystals a powerful visual aid for introducing theparticle in a boxrdquo Journal of Chemical Education vol 79no 9 p 1094 2002

[3] V I Klimov Nanocrystal Quantum Dots CRC Press BocaRaton FL USA 2nd edition 2010

[4] J Xu O Voznyy M Liu et al ldquo2D matrix engineering forhomogeneous quantum dot coupling in photovoltaic solidsrdquoNature Nanotechnology vol 13 no 6 pp 456ndash462 2018

[5] P V Kamat ldquoQuantum dot solar cells Semiconductornanocrystals as light harvestersrdquo 0e Journal of PhysicalChemistry C vol 112 no 48 pp 18737ndash18753 2008

ODE DrawerSill Table

(c)

Figure 6 Plot of time-dependent photoluminescence (PL)initial photoluminescence (PL0) (a) image at t 0 days (b) and image at t 9days (c) for comparative samples of CdSe QD solutions (170degC 2min 30 s) QDs were either left in a closed lab drawer with limited exposureto ambient light (solid line square shape) placed on an open laboratory tabletop with continual exposure to mostly fluorescent light (dashedline triangle shape) or placed on an interior windowsill with frequent exposure to sunlight and fluorescent light (dotted line circle shape)for the course of the experiment In the pictures from left to right the samples are neat 1-octadecene QDs left on the windowsill QDs left onthe tabletop and QDs stored in the drawer respectively

Advances in Materials Science and Engineering 7

[6] Y Shirasaki G J Supran M G Bawendi and V BulovicldquoEmergence of colloidal quantum-dot light-emitting tech-nologiesrdquo Nature Photonics vol 7 no 1 pp 13ndash23 2013

[7] C R Kagan E Lifshitz E H Sargent and D V TalapinldquoBuilding devices from colloidal quantum dotsrdquo Sciencevol 353 no 6302 2016

[8] M L Landry T E Morrell T K Karagounis C-H Hsia andC-Y Wang ldquoSimple syntheses of CdSe quantum dotsrdquoJournal of Chemical Education vol 91 no 2 pp 274ndash2792014

[9] C B Murray D J Norris and M G Bawendi ldquoSynthesis andcharacterization of nearly monodisperse CdE (E sulfur se-lenium tellurium) semiconductor nanocrystallitesrdquo Journal ofthe American Chemical Society vol 115 no 19 pp 8706ndash87151993

[10] M B Mohamed D Tonti A Al-Salman A Chemseddineand M Chergui ldquoSynthesis of high quality zinc blende CdSenanocrystalsrdquo 0e Journal of Physical Chemistry B vol 109no 21 pp 10533ndash10537 2005

[11] G H Carey A L Abdelhady Z Ning S M(on OM Bakrand E H Sargent ldquoColloidal quantum dot solar cellsrdquoChemical Reviews vol 115 no 23 pp 12732ndash12763 2015

[12] K J Nordell E M Boatman and G C Lisensky ldquoA safereasier faster synthesis for CdSe quantum dot nanocrystalsrdquoJournal of Chemical Education vol 82 no 11 p 1697 2005

[13] M Z Hu and T Zhu ldquoSemiconductor nanocrystal quantumdot synthesis approaches towards large-scale industrial pro-duction for energy applicationsrdquo Nanoscale Research Lettersvol 10 no 1 pp 1ndash15 2015

[14] O Palchik R Kerner A Gedanken A M WeissM A Slifkin and V Palchik ldquoMicrowave-assisted polyolmethod for the preparation of CdSe ldquonanoballsrdquordquo Journal ofMaterials Chemistry vol 11 no 3 pp 874ndash878 2001

[15] J Zhu O Palchik S Chen and A Gedanken ldquoMicrowaveassisted preparation of CdSe PbSe and Cu2minus xSe nano-particlesrdquo0e Journal of Physical Chemistry B vol 104 no 31pp 7344ndash7347 2002

[16] D W Ayele H-M Chen W-N Su et al ldquoControlledsynthesis of CdSe quantum dots by a microwave-enhancedprocess a green approach for mass productionrdquo ChemistryndashAEuropean Journal vol 17 no 20 pp 5737ndash5744 2011

[17] M M Moghaddam M Baghbanzadeh A Keilbach andC O Kappe ldquoMicrowave-assisted synthesis of CdSe quantumdots can the electromagnetic field influence the formationand quality of the resulting nanocrystalsrdquo Nanoscale vol 4no 23 pp 7435ndash7442 2012

[18] J A Gerbec D Magana A Washington and G F StrouseldquoMicrowave-enhanced reaction rates for nanoparticle syn-thesisrdquo Journal of the American Chemical Society vol 127no 45 pp 15791ndash15800 2005

[19] A L Washington and G F Strouse ldquoMicrowave synthesis ofCdSe and CdTe nanocrystals in nonabsorbing alkanesrdquoJournal of the American Chemical Society vol 130 no 28pp 8916ndash8922 2008

[20] J Ziegler A Merkulov M Grabolle U Resch-Genger andT Nann ldquoHigh-quality ZnS shells for CdSe nanoparticlesrapid microwave synthesisrdquo Langmuir vol 23 no 14pp 7751ndash7759 2007

[21] D A Hines and P V Kamat ldquoRecent advances in quantumdot surface chemistryrdquo ACS Applied Materials amp Interfacesvol 6 no 5 pp 3041ndash3057 2014

[22] Anton Paar Monowave 300 Microwave Synthesis ReactorOriginal Instruction Manual Anton Paar Graz Austria 2015

[23] A J Morris-Cohen M D Donakowski K E Knowles andE A Weiss ldquo(e effect of a common purification procedureon the chemical composition of the surfaces of CdSe quantumdots synthesized with trioctylphosphine oxiderdquo0e Journal ofPhysical Chemistry C vol 114 no 2 pp 897ndash906 2010

[24] Aldrich Chemistry Lumidottrade FAQ Frequently Asked Questionsabout Using QDNanocrystals Aldrich Chemistry St Louis MOUSA 2019 httpswwwsigmaaldrichcomcontentdamsigma-aldrichdocsSigma-AldrichGeneral_Informationlumidot_faqspdf

[25] D Zhou M Lin Z Chen et al ldquoSimple synthesis of highlyluminescent water-soluble CdTe quantum dots with con-trollable surface functionalityrdquo Chemistry of Materialsvol 23 no 21 pp 4857ndash4862 2011

[26] E Jenrette S K Pradhan G Rutherford J Flowers D Haand A K Pradhan ldquoQuantum-dot-conjugated grapheneoxide as an optical tool for biosensorrdquo Optics Express vol 23no 19 p 25017 2015

[27] E Zillner S Fengler P Niyamakom F Rauscher K Kohlerand T Dittrich ldquoRole of ligand exchange at CdSe quantumdot layers for charge separationrdquo 0e Journal of PhysicalChemistry C vol 116 no 31 pp 16747ndash16754 2012

[28] D B Tice M T Frederick R P H Chang and E A WeissldquoElectron migration limits the rate of photobrightening inthin films of CdSe quantum dots in a dry N2 (g) atmosphererdquo0e Journal of Physical Chemistry C vol 115 no 9pp 3654ndash3662 2011

[29] W G J H M van Sark P L T M Frederix A A BolH C Gerritsen and A Meijerink ldquoBlueing bleaching andblinking of single CdSeZnS quantum dotsrdquo ChemPhysChemvol 3 no 10 pp 871ndash879 2002

[30] N Gilroy Direct Normal Solar Resource of Virginia NationalRenewable Energy Laboratory for the US Department ofEnergy Golden CO USA 2017 httpswwwnrelgovgissolarhtml

8 Advances in Materials Science and Engineering

dissolve in said solvent (is reasoning is what fuels thecommon assumption that QDs can be dissolved in mostnonpolar organic solvents [24] since the long-chained li-gands of QDs are themselves nonpolar Surprisingly thisprinciple does not apply in full to QDs synthesized via thismicrowave method In fact resuspension only works well withtoluene and marginally well with 1-octadecene (Figure 5)whereas resuspension is unsuccessful with nonpolar hexaneand nonpolar chloroform Even if some coloration occurs insolution as is the case for resuspended CdSe QDs in hexanethey still will not fluoresce under UV light

34 Photobrightening and Photobleaching Ambient lightcan play a role in altering the stability of QDs in air ex-emplified with two phenomena known as photobrighteningand photobleaching [6] Photobrightening is the tendencyfor ensembles of QDs to show increasing photo-luminescence behavior under continuous photoexcitation[28] whereas photobleaching is the tendency for QD latticestructures to permanently degrade under prolonged lightexposure and therefore lose all photoluminescence behavior[29] (e QDs synthesized in this study showed evidence ofboth effects

A day after synthesis QDs from one batch (170degC2min 30 s) were dispensed into three plastic spectro-photometer cuvettes wrapped in Parafilm and then placedeither in a dark laboratory drawer on top of a table sim15maway from an external window (double-paned low-etempered glass) or on top of a windowsill sim3 cm awayfrom the same external window (ese positions exemplifylocations where researchers might properly (in drawer) or

improperly (on tabletop or windowsill) store samples afterdaily use Light for this experiment came from two primarysources the fluorescent bulbs on the ceiling and thenatural sunlight from the window (e fluorescent bulbswere 32W T8 bulbs with a color temperature of 6500 Killuminating at an average of 2565 lumens (ere were 24bulbs in total separated into groups of three and dis-tributed across the ceiling of the open lab room into eightlight fixtures with crosshatched aluminum light diffusers(e approximate distance from the QD samples and apoint normal to the ceiling was 175m(e lights remainedon day and night throughout the course of the experi-ment (e sunlight coming from the window changed byday but an eight-year report from the National RenewableEnergy Laboratory indicates that the direct normal solarirradiance on Norfolk VA (USA) varies between 46 and50 kWhm2day [30]

(e QDs initially displayed little difference in photo-luminescence (PL) in terms of intensity shape or location ofpeak emission wavelength as expected with identicalsamples but over the course of eight days a few patternsemerged (Figure 6) (e first is that all samples had an initialincrease in PL over the first couple of days (even the drawersample which was exposed to light for short periods be-tween PL testing) indicating the photobrightening effect(Figure 6(a))(e second is that over the course of the rest ofthe experiment both samples continually exposed to lighteventually experienced a decrease in PL intensity from thestable sim15 PLPL0 with the sample on the windowsilldropping in intensity to the level of background noise by day7 (Figure 6(a)) (is evidence indicates that photobleachingfrom the sun and commercial fluorescent bulbs begins after a

Table 1 Comparison of microwave parameters for synthesis of CdSe QDs and the calculated sizes of those QDs based on estimated peak PLwavelengths

Temperature (degC) Hold time (min) PL wavelength (nm) Calculated radius (nm) Calculated diameter (nm)120 2 30 575 30 60140 2 30 624 39 78170 2 30 523 24 48200 2 30 560 28 56265 2 30 605 35 70

180degC 200degC 200degCin tol

225degC 225degCin tol

180degCin tol

(a)

180degC 200degC 200degCin tol

225degC 225degCin tol

180degCin tol

(b)

Figure 4 Images of QDs under visible light (a) and under 365 nmUV light (b) just before cleaning (left in each pair) and after resuspensionin toluene (right in each pair) (e batches were made with a hold time of 3min and represent the following temperatures 180degC (left pair)200degC (middle pair) and 225degC (right pair)

Advances in Materials Science and Engineering 5

165degC5minin tol

165degC5minin hex

180degC3minin tol

180degC3min

in CHCl3

225degC3minin tol

225degC3min

in ODE

(a)

225degC3minin tol

165degC5minin tol

165degC5minin hex

180degC3minin tol

180degC3min

in CHCl3

225degC3min

in ODE

(b)

Figure 5 Comparison of QDs resuspended in different solvents under visible light (a) and 365 nmUV light (b)(e left pair compares QDsprepared at 165degC for 5min and resuspended in either toluene (left) or hexane (right) (e middle pair compares QDs prepared at 180degC for3min and resuspended in either toluene (left) or chloroform (right) (e right pair compares QDs prepared at 225degC for 3min andresuspended in either toluene (left) or 1-octadecene (right)

3

25

2

15

1

05

0

PLP

L 0

0 2 4 6 8 10Time (days)

DrawerTableSill

(a)

ODE DrawerSill Table

(b)

Figure 6 Continued

6 Advances in Materials Science and Engineering

few days of continuous photoexcitation and dramaticallyreduces the PL intensity of QDs within 5ndash7 days (e dif-ference in the rate of PL intensity decline between thewindowsill sample and the tabletop sample indicates that thesun has a larger influence on photobleaching than ambientfluorescent light though this suggestion requires furthertesting for confirmation (ird the photobleaching effect isvisually apparent (Figures 6(b) and 6(c)) (e synthesizedQDs appeared orange at the beginning of experimentation(Figure 6(b)) but by day 9 (Figure 6(c)) both the windowsillsample and the tabletop sample experienced a noticeableblue shift in color

4 Conclusions

It is possible to synthesize CdSe QDs by using a microwavesynthesis reactor instead of the cumbersome glassware andheating elements designed for a hot-injection method allwhile using the same materials and precursor preparationsof the latter In addition to being a valuable tool in theclassroom for learning about quantum confinement inmaterials this simple ldquopress-and-gordquo microwave techniquecan also be up-scaled to research and development or in-dustry environments where access to a microwave reactor isfeasible However there are challenges and limitations toconsider with this technique such as the blue shifting of theQDs after purification organic solvent immiscibility or PL-broadening of freshly synthesized batches Such oddities arenot reported in most hot-injection or microwave synthesispublications and so deserve exposure

If this microwave technique were refined through repli-cation and more research these peculiarities would dissipateleaving microwave synthesis as a cheap quick and energy-efficient alternative to synthesizing QDs via the hot-injectiontechnique In addition to refinement there is also the potentialto try synthesizing other types of QDs based on hot-injectionmethods For instance one of the best types of QDs for solarcell applications is PbS with efficiencies reaching 12 andbeyond [4] To date there are no readily available microwavesynthesis methods for PbS however there are a number of

hot-injection methods all of which could be adapted to amicrowave heating procedure (e research potential formicrowave synthesis is immense and with time it could cometo be the industry standard for QD synthesis andimplementation

Data Availability

(e data used to support the findings of this study areavailable from the corresponding author upon request

Conflicts of Interest

(e authors declare that there are no conflicts of interestregarding the publication of this paper

Acknowledgments

(e authors would like to thank Travis Greene Mark Swiftand Samantha Koutsares for their technical support re-search assistance editing assistance and encouragement ofideas throughout this process (is work was supported bythe National Science Foundation (CREST grant numberHRD 1547771 CREST grant number HRD 1036494)

References

[1] L Brus ldquoElectronic wave functions in semiconductor clustersexperiment and theoryrdquo 0e Journal of Physical Chemistryvol 90 no 12 pp 2555ndash2560 1986

[2] T Kippeny L A Swafford and S J Rosenthal ldquoSemicon-ductor nanocrystals a powerful visual aid for introducing theparticle in a boxrdquo Journal of Chemical Education vol 79no 9 p 1094 2002

[3] V I Klimov Nanocrystal Quantum Dots CRC Press BocaRaton FL USA 2nd edition 2010

[4] J Xu O Voznyy M Liu et al ldquo2D matrix engineering forhomogeneous quantum dot coupling in photovoltaic solidsrdquoNature Nanotechnology vol 13 no 6 pp 456ndash462 2018

[5] P V Kamat ldquoQuantum dot solar cells Semiconductornanocrystals as light harvestersrdquo 0e Journal of PhysicalChemistry C vol 112 no 48 pp 18737ndash18753 2008

ODE DrawerSill Table

(c)

Figure 6 Plot of time-dependent photoluminescence (PL)initial photoluminescence (PL0) (a) image at t 0 days (b) and image at t 9days (c) for comparative samples of CdSe QD solutions (170degC 2min 30 s) QDs were either left in a closed lab drawer with limited exposureto ambient light (solid line square shape) placed on an open laboratory tabletop with continual exposure to mostly fluorescent light (dashedline triangle shape) or placed on an interior windowsill with frequent exposure to sunlight and fluorescent light (dotted line circle shape)for the course of the experiment In the pictures from left to right the samples are neat 1-octadecene QDs left on the windowsill QDs left onthe tabletop and QDs stored in the drawer respectively

Advances in Materials Science and Engineering 7

[6] Y Shirasaki G J Supran M G Bawendi and V BulovicldquoEmergence of colloidal quantum-dot light-emitting tech-nologiesrdquo Nature Photonics vol 7 no 1 pp 13ndash23 2013

[7] C R Kagan E Lifshitz E H Sargent and D V TalapinldquoBuilding devices from colloidal quantum dotsrdquo Sciencevol 353 no 6302 2016

[8] M L Landry T E Morrell T K Karagounis C-H Hsia andC-Y Wang ldquoSimple syntheses of CdSe quantum dotsrdquoJournal of Chemical Education vol 91 no 2 pp 274ndash2792014

[9] C B Murray D J Norris and M G Bawendi ldquoSynthesis andcharacterization of nearly monodisperse CdE (E sulfur se-lenium tellurium) semiconductor nanocrystallitesrdquo Journal ofthe American Chemical Society vol 115 no 19 pp 8706ndash87151993

[10] M B Mohamed D Tonti A Al-Salman A Chemseddineand M Chergui ldquoSynthesis of high quality zinc blende CdSenanocrystalsrdquo 0e Journal of Physical Chemistry B vol 109no 21 pp 10533ndash10537 2005

[11] G H Carey A L Abdelhady Z Ning S M(on OM Bakrand E H Sargent ldquoColloidal quantum dot solar cellsrdquoChemical Reviews vol 115 no 23 pp 12732ndash12763 2015

[12] K J Nordell E M Boatman and G C Lisensky ldquoA safereasier faster synthesis for CdSe quantum dot nanocrystalsrdquoJournal of Chemical Education vol 82 no 11 p 1697 2005

[13] M Z Hu and T Zhu ldquoSemiconductor nanocrystal quantumdot synthesis approaches towards large-scale industrial pro-duction for energy applicationsrdquo Nanoscale Research Lettersvol 10 no 1 pp 1ndash15 2015

[14] O Palchik R Kerner A Gedanken A M WeissM A Slifkin and V Palchik ldquoMicrowave-assisted polyolmethod for the preparation of CdSe ldquonanoballsrdquordquo Journal ofMaterials Chemistry vol 11 no 3 pp 874ndash878 2001

[15] J Zhu O Palchik S Chen and A Gedanken ldquoMicrowaveassisted preparation of CdSe PbSe and Cu2minus xSe nano-particlesrdquo0e Journal of Physical Chemistry B vol 104 no 31pp 7344ndash7347 2002

[16] D W Ayele H-M Chen W-N Su et al ldquoControlledsynthesis of CdSe quantum dots by a microwave-enhancedprocess a green approach for mass productionrdquo ChemistryndashAEuropean Journal vol 17 no 20 pp 5737ndash5744 2011

[17] M M Moghaddam M Baghbanzadeh A Keilbach andC O Kappe ldquoMicrowave-assisted synthesis of CdSe quantumdots can the electromagnetic field influence the formationand quality of the resulting nanocrystalsrdquo Nanoscale vol 4no 23 pp 7435ndash7442 2012

[18] J A Gerbec D Magana A Washington and G F StrouseldquoMicrowave-enhanced reaction rates for nanoparticle syn-thesisrdquo Journal of the American Chemical Society vol 127no 45 pp 15791ndash15800 2005

[19] A L Washington and G F Strouse ldquoMicrowave synthesis ofCdSe and CdTe nanocrystals in nonabsorbing alkanesrdquoJournal of the American Chemical Society vol 130 no 28pp 8916ndash8922 2008

[20] J Ziegler A Merkulov M Grabolle U Resch-Genger andT Nann ldquoHigh-quality ZnS shells for CdSe nanoparticlesrapid microwave synthesisrdquo Langmuir vol 23 no 14pp 7751ndash7759 2007

[21] D A Hines and P V Kamat ldquoRecent advances in quantumdot surface chemistryrdquo ACS Applied Materials amp Interfacesvol 6 no 5 pp 3041ndash3057 2014

[22] Anton Paar Monowave 300 Microwave Synthesis ReactorOriginal Instruction Manual Anton Paar Graz Austria 2015

[23] A J Morris-Cohen M D Donakowski K E Knowles andE A Weiss ldquo(e effect of a common purification procedureon the chemical composition of the surfaces of CdSe quantumdots synthesized with trioctylphosphine oxiderdquo0e Journal ofPhysical Chemistry C vol 114 no 2 pp 897ndash906 2010

[24] Aldrich Chemistry Lumidottrade FAQ Frequently Asked Questionsabout Using QDNanocrystals Aldrich Chemistry St Louis MOUSA 2019 httpswwwsigmaaldrichcomcontentdamsigma-aldrichdocsSigma-AldrichGeneral_Informationlumidot_faqspdf

[25] D Zhou M Lin Z Chen et al ldquoSimple synthesis of highlyluminescent water-soluble CdTe quantum dots with con-trollable surface functionalityrdquo Chemistry of Materialsvol 23 no 21 pp 4857ndash4862 2011

[26] E Jenrette S K Pradhan G Rutherford J Flowers D Haand A K Pradhan ldquoQuantum-dot-conjugated grapheneoxide as an optical tool for biosensorrdquo Optics Express vol 23no 19 p 25017 2015

[27] E Zillner S Fengler P Niyamakom F Rauscher K Kohlerand T Dittrich ldquoRole of ligand exchange at CdSe quantumdot layers for charge separationrdquo 0e Journal of PhysicalChemistry C vol 116 no 31 pp 16747ndash16754 2012

[28] D B Tice M T Frederick R P H Chang and E A WeissldquoElectron migration limits the rate of photobrightening inthin films of CdSe quantum dots in a dry N2 (g) atmosphererdquo0e Journal of Physical Chemistry C vol 115 no 9pp 3654ndash3662 2011

[29] W G J H M van Sark P L T M Frederix A A BolH C Gerritsen and A Meijerink ldquoBlueing bleaching andblinking of single CdSeZnS quantum dotsrdquo ChemPhysChemvol 3 no 10 pp 871ndash879 2002

[30] N Gilroy Direct Normal Solar Resource of Virginia NationalRenewable Energy Laboratory for the US Department ofEnergy Golden CO USA 2017 httpswwwnrelgovgissolarhtml

8 Advances in Materials Science and Engineering

165degC5minin tol

165degC5minin hex

180degC3minin tol

180degC3min

in CHCl3

225degC3minin tol

225degC3min

in ODE

(a)

225degC3minin tol

165degC5minin tol

165degC5minin hex

180degC3minin tol

180degC3min

in CHCl3

225degC3min

in ODE

(b)

Figure 5 Comparison of QDs resuspended in different solvents under visible light (a) and 365 nmUV light (b)(e left pair compares QDsprepared at 165degC for 5min and resuspended in either toluene (left) or hexane (right) (e middle pair compares QDs prepared at 180degC for3min and resuspended in either toluene (left) or chloroform (right) (e right pair compares QDs prepared at 225degC for 3min andresuspended in either toluene (left) or 1-octadecene (right)

3

25

2

15

1

05

0

PLP

L 0

0 2 4 6 8 10Time (days)

DrawerTableSill

(a)

ODE DrawerSill Table

(b)

Figure 6 Continued

6 Advances in Materials Science and Engineering

few days of continuous photoexcitation and dramaticallyreduces the PL intensity of QDs within 5ndash7 days (e dif-ference in the rate of PL intensity decline between thewindowsill sample and the tabletop sample indicates that thesun has a larger influence on photobleaching than ambientfluorescent light though this suggestion requires furthertesting for confirmation (ird the photobleaching effect isvisually apparent (Figures 6(b) and 6(c)) (e synthesizedQDs appeared orange at the beginning of experimentation(Figure 6(b)) but by day 9 (Figure 6(c)) both the windowsillsample and the tabletop sample experienced a noticeableblue shift in color

4 Conclusions

It is possible to synthesize CdSe QDs by using a microwavesynthesis reactor instead of the cumbersome glassware andheating elements designed for a hot-injection method allwhile using the same materials and precursor preparationsof the latter In addition to being a valuable tool in theclassroom for learning about quantum confinement inmaterials this simple ldquopress-and-gordquo microwave techniquecan also be up-scaled to research and development or in-dustry environments where access to a microwave reactor isfeasible However there are challenges and limitations toconsider with this technique such as the blue shifting of theQDs after purification organic solvent immiscibility or PL-broadening of freshly synthesized batches Such oddities arenot reported in most hot-injection or microwave synthesispublications and so deserve exposure

If this microwave technique were refined through repli-cation and more research these peculiarities would dissipateleaving microwave synthesis as a cheap quick and energy-efficient alternative to synthesizing QDs via the hot-injectiontechnique In addition to refinement there is also the potentialto try synthesizing other types of QDs based on hot-injectionmethods For instance one of the best types of QDs for solarcell applications is PbS with efficiencies reaching 12 andbeyond [4] To date there are no readily available microwavesynthesis methods for PbS however there are a number of

hot-injection methods all of which could be adapted to amicrowave heating procedure (e research potential formicrowave synthesis is immense and with time it could cometo be the industry standard for QD synthesis andimplementation

Data Availability

(e data used to support the findings of this study areavailable from the corresponding author upon request

Conflicts of Interest

(e authors declare that there are no conflicts of interestregarding the publication of this paper

Acknowledgments

(e authors would like to thank Travis Greene Mark Swiftand Samantha Koutsares for their technical support re-search assistance editing assistance and encouragement ofideas throughout this process (is work was supported bythe National Science Foundation (CREST grant numberHRD 1547771 CREST grant number HRD 1036494)

References

[1] L Brus ldquoElectronic wave functions in semiconductor clustersexperiment and theoryrdquo 0e Journal of Physical Chemistryvol 90 no 12 pp 2555ndash2560 1986

[2] T Kippeny L A Swafford and S J Rosenthal ldquoSemicon-ductor nanocrystals a powerful visual aid for introducing theparticle in a boxrdquo Journal of Chemical Education vol 79no 9 p 1094 2002

[3] V I Klimov Nanocrystal Quantum Dots CRC Press BocaRaton FL USA 2nd edition 2010

[4] J Xu O Voznyy M Liu et al ldquo2D matrix engineering forhomogeneous quantum dot coupling in photovoltaic solidsrdquoNature Nanotechnology vol 13 no 6 pp 456ndash462 2018

[5] P V Kamat ldquoQuantum dot solar cells Semiconductornanocrystals as light harvestersrdquo 0e Journal of PhysicalChemistry C vol 112 no 48 pp 18737ndash18753 2008

ODE DrawerSill Table

(c)

Figure 6 Plot of time-dependent photoluminescence (PL)initial photoluminescence (PL0) (a) image at t 0 days (b) and image at t 9days (c) for comparative samples of CdSe QD solutions (170degC 2min 30 s) QDs were either left in a closed lab drawer with limited exposureto ambient light (solid line square shape) placed on an open laboratory tabletop with continual exposure to mostly fluorescent light (dashedline triangle shape) or placed on an interior windowsill with frequent exposure to sunlight and fluorescent light (dotted line circle shape)for the course of the experiment In the pictures from left to right the samples are neat 1-octadecene QDs left on the windowsill QDs left onthe tabletop and QDs stored in the drawer respectively

Advances in Materials Science and Engineering 7

[6] Y Shirasaki G J Supran M G Bawendi and V BulovicldquoEmergence of colloidal quantum-dot light-emitting tech-nologiesrdquo Nature Photonics vol 7 no 1 pp 13ndash23 2013

[7] C R Kagan E Lifshitz E H Sargent and D V TalapinldquoBuilding devices from colloidal quantum dotsrdquo Sciencevol 353 no 6302 2016

[8] M L Landry T E Morrell T K Karagounis C-H Hsia andC-Y Wang ldquoSimple syntheses of CdSe quantum dotsrdquoJournal of Chemical Education vol 91 no 2 pp 274ndash2792014

[9] C B Murray D J Norris and M G Bawendi ldquoSynthesis andcharacterization of nearly monodisperse CdE (E sulfur se-lenium tellurium) semiconductor nanocrystallitesrdquo Journal ofthe American Chemical Society vol 115 no 19 pp 8706ndash87151993

[10] M B Mohamed D Tonti A Al-Salman A Chemseddineand M Chergui ldquoSynthesis of high quality zinc blende CdSenanocrystalsrdquo 0e Journal of Physical Chemistry B vol 109no 21 pp 10533ndash10537 2005

[11] G H Carey A L Abdelhady Z Ning S M(on OM Bakrand E H Sargent ldquoColloidal quantum dot solar cellsrdquoChemical Reviews vol 115 no 23 pp 12732ndash12763 2015

[12] K J Nordell E M Boatman and G C Lisensky ldquoA safereasier faster synthesis for CdSe quantum dot nanocrystalsrdquoJournal of Chemical Education vol 82 no 11 p 1697 2005

[13] M Z Hu and T Zhu ldquoSemiconductor nanocrystal quantumdot synthesis approaches towards large-scale industrial pro-duction for energy applicationsrdquo Nanoscale Research Lettersvol 10 no 1 pp 1ndash15 2015

[14] O Palchik R Kerner A Gedanken A M WeissM A Slifkin and V Palchik ldquoMicrowave-assisted polyolmethod for the preparation of CdSe ldquonanoballsrdquordquo Journal ofMaterials Chemistry vol 11 no 3 pp 874ndash878 2001

[15] J Zhu O Palchik S Chen and A Gedanken ldquoMicrowaveassisted preparation of CdSe PbSe and Cu2minus xSe nano-particlesrdquo0e Journal of Physical Chemistry B vol 104 no 31pp 7344ndash7347 2002

[16] D W Ayele H-M Chen W-N Su et al ldquoControlledsynthesis of CdSe quantum dots by a microwave-enhancedprocess a green approach for mass productionrdquo ChemistryndashAEuropean Journal vol 17 no 20 pp 5737ndash5744 2011

[17] M M Moghaddam M Baghbanzadeh A Keilbach andC O Kappe ldquoMicrowave-assisted synthesis of CdSe quantumdots can the electromagnetic field influence the formationand quality of the resulting nanocrystalsrdquo Nanoscale vol 4no 23 pp 7435ndash7442 2012

[18] J A Gerbec D Magana A Washington and G F StrouseldquoMicrowave-enhanced reaction rates for nanoparticle syn-thesisrdquo Journal of the American Chemical Society vol 127no 45 pp 15791ndash15800 2005

[19] A L Washington and G F Strouse ldquoMicrowave synthesis ofCdSe and CdTe nanocrystals in nonabsorbing alkanesrdquoJournal of the American Chemical Society vol 130 no 28pp 8916ndash8922 2008

[20] J Ziegler A Merkulov M Grabolle U Resch-Genger andT Nann ldquoHigh-quality ZnS shells for CdSe nanoparticlesrapid microwave synthesisrdquo Langmuir vol 23 no 14pp 7751ndash7759 2007

[21] D A Hines and P V Kamat ldquoRecent advances in quantumdot surface chemistryrdquo ACS Applied Materials amp Interfacesvol 6 no 5 pp 3041ndash3057 2014

[22] Anton Paar Monowave 300 Microwave Synthesis ReactorOriginal Instruction Manual Anton Paar Graz Austria 2015

[23] A J Morris-Cohen M D Donakowski K E Knowles andE A Weiss ldquo(e effect of a common purification procedureon the chemical composition of the surfaces of CdSe quantumdots synthesized with trioctylphosphine oxiderdquo0e Journal ofPhysical Chemistry C vol 114 no 2 pp 897ndash906 2010

[24] Aldrich Chemistry Lumidottrade FAQ Frequently Asked Questionsabout Using QDNanocrystals Aldrich Chemistry St Louis MOUSA 2019 httpswwwsigmaaldrichcomcontentdamsigma-aldrichdocsSigma-AldrichGeneral_Informationlumidot_faqspdf

[25] D Zhou M Lin Z Chen et al ldquoSimple synthesis of highlyluminescent water-soluble CdTe quantum dots with con-trollable surface functionalityrdquo Chemistry of Materialsvol 23 no 21 pp 4857ndash4862 2011

[26] E Jenrette S K Pradhan G Rutherford J Flowers D Haand A K Pradhan ldquoQuantum-dot-conjugated grapheneoxide as an optical tool for biosensorrdquo Optics Express vol 23no 19 p 25017 2015

[27] E Zillner S Fengler P Niyamakom F Rauscher K Kohlerand T Dittrich ldquoRole of ligand exchange at CdSe quantumdot layers for charge separationrdquo 0e Journal of PhysicalChemistry C vol 116 no 31 pp 16747ndash16754 2012

[28] D B Tice M T Frederick R P H Chang and E A WeissldquoElectron migration limits the rate of photobrightening inthin films of CdSe quantum dots in a dry N2 (g) atmosphererdquo0e Journal of Physical Chemistry C vol 115 no 9pp 3654ndash3662 2011

[29] W G J H M van Sark P L T M Frederix A A BolH C Gerritsen and A Meijerink ldquoBlueing bleaching andblinking of single CdSeZnS quantum dotsrdquo ChemPhysChemvol 3 no 10 pp 871ndash879 2002

[30] N Gilroy Direct Normal Solar Resource of Virginia NationalRenewable Energy Laboratory for the US Department ofEnergy Golden CO USA 2017 httpswwwnrelgovgissolarhtml

8 Advances in Materials Science and Engineering

few days of continuous photoexcitation and dramaticallyreduces the PL intensity of QDs within 5ndash7 days (e dif-ference in the rate of PL intensity decline between thewindowsill sample and the tabletop sample indicates that thesun has a larger influence on photobleaching than ambientfluorescent light though this suggestion requires furthertesting for confirmation (ird the photobleaching effect isvisually apparent (Figures 6(b) and 6(c)) (e synthesizedQDs appeared orange at the beginning of experimentation(Figure 6(b)) but by day 9 (Figure 6(c)) both the windowsillsample and the tabletop sample experienced a noticeableblue shift in color

4 Conclusions

It is possible to synthesize CdSe QDs by using a microwavesynthesis reactor instead of the cumbersome glassware andheating elements designed for a hot-injection method allwhile using the same materials and precursor preparationsof the latter In addition to being a valuable tool in theclassroom for learning about quantum confinement inmaterials this simple ldquopress-and-gordquo microwave techniquecan also be up-scaled to research and development or in-dustry environments where access to a microwave reactor isfeasible However there are challenges and limitations toconsider with this technique such as the blue shifting of theQDs after purification organic solvent immiscibility or PL-broadening of freshly synthesized batches Such oddities arenot reported in most hot-injection or microwave synthesispublications and so deserve exposure

If this microwave technique were refined through repli-cation and more research these peculiarities would dissipateleaving microwave synthesis as a cheap quick and energy-efficient alternative to synthesizing QDs via the hot-injectiontechnique In addition to refinement there is also the potentialto try synthesizing other types of QDs based on hot-injectionmethods For instance one of the best types of QDs for solarcell applications is PbS with efficiencies reaching 12 andbeyond [4] To date there are no readily available microwavesynthesis methods for PbS however there are a number of

hot-injection methods all of which could be adapted to amicrowave heating procedure (e research potential formicrowave synthesis is immense and with time it could cometo be the industry standard for QD synthesis andimplementation

Data Availability

(e data used to support the findings of this study areavailable from the corresponding author upon request

Conflicts of Interest

(e authors declare that there are no conflicts of interestregarding the publication of this paper

Acknowledgments

(e authors would like to thank Travis Greene Mark Swiftand Samantha Koutsares for their technical support re-search assistance editing assistance and encouragement ofideas throughout this process (is work was supported bythe National Science Foundation (CREST grant numberHRD 1547771 CREST grant number HRD 1036494)

References

[1] L Brus ldquoElectronic wave functions in semiconductor clustersexperiment and theoryrdquo 0e Journal of Physical Chemistryvol 90 no 12 pp 2555ndash2560 1986

[2] T Kippeny L A Swafford and S J Rosenthal ldquoSemicon-ductor nanocrystals a powerful visual aid for introducing theparticle in a boxrdquo Journal of Chemical Education vol 79no 9 p 1094 2002

[3] V I Klimov Nanocrystal Quantum Dots CRC Press BocaRaton FL USA 2nd edition 2010

[4] J Xu O Voznyy M Liu et al ldquo2D matrix engineering forhomogeneous quantum dot coupling in photovoltaic solidsrdquoNature Nanotechnology vol 13 no 6 pp 456ndash462 2018

[5] P V Kamat ldquoQuantum dot solar cells Semiconductornanocrystals as light harvestersrdquo 0e Journal of PhysicalChemistry C vol 112 no 48 pp 18737ndash18753 2008

ODE DrawerSill Table

(c)

Figure 6 Plot of time-dependent photoluminescence (PL)initial photoluminescence (PL0) (a) image at t 0 days (b) and image at t 9days (c) for comparative samples of CdSe QD solutions (170degC 2min 30 s) QDs were either left in a closed lab drawer with limited exposureto ambient light (solid line square shape) placed on an open laboratory tabletop with continual exposure to mostly fluorescent light (dashedline triangle shape) or placed on an interior windowsill with frequent exposure to sunlight and fluorescent light (dotted line circle shape)for the course of the experiment In the pictures from left to right the samples are neat 1-octadecene QDs left on the windowsill QDs left onthe tabletop and QDs stored in the drawer respectively

Advances in Materials Science and Engineering 7

[6] Y Shirasaki G J Supran M G Bawendi and V BulovicldquoEmergence of colloidal quantum-dot light-emitting tech-nologiesrdquo Nature Photonics vol 7 no 1 pp 13ndash23 2013

[7] C R Kagan E Lifshitz E H Sargent and D V TalapinldquoBuilding devices from colloidal quantum dotsrdquo Sciencevol 353 no 6302 2016

[8] M L Landry T E Morrell T K Karagounis C-H Hsia andC-Y Wang ldquoSimple syntheses of CdSe quantum dotsrdquoJournal of Chemical Education vol 91 no 2 pp 274ndash2792014

[9] C B Murray D J Norris and M G Bawendi ldquoSynthesis andcharacterization of nearly monodisperse CdE (E sulfur se-lenium tellurium) semiconductor nanocrystallitesrdquo Journal ofthe American Chemical Society vol 115 no 19 pp 8706ndash87151993

[10] M B Mohamed D Tonti A Al-Salman A Chemseddineand M Chergui ldquoSynthesis of high quality zinc blende CdSenanocrystalsrdquo 0e Journal of Physical Chemistry B vol 109no 21 pp 10533ndash10537 2005

[11] G H Carey A L Abdelhady Z Ning S M(on OM Bakrand E H Sargent ldquoColloidal quantum dot solar cellsrdquoChemical Reviews vol 115 no 23 pp 12732ndash12763 2015

[12] K J Nordell E M Boatman and G C Lisensky ldquoA safereasier faster synthesis for CdSe quantum dot nanocrystalsrdquoJournal of Chemical Education vol 82 no 11 p 1697 2005

[13] M Z Hu and T Zhu ldquoSemiconductor nanocrystal quantumdot synthesis approaches towards large-scale industrial pro-duction for energy applicationsrdquo Nanoscale Research Lettersvol 10 no 1 pp 1ndash15 2015

[14] O Palchik R Kerner A Gedanken A M WeissM A Slifkin and V Palchik ldquoMicrowave-assisted polyolmethod for the preparation of CdSe ldquonanoballsrdquordquo Journal ofMaterials Chemistry vol 11 no 3 pp 874ndash878 2001

[15] J Zhu O Palchik S Chen and A Gedanken ldquoMicrowaveassisted preparation of CdSe PbSe and Cu2minus xSe nano-particlesrdquo0e Journal of Physical Chemistry B vol 104 no 31pp 7344ndash7347 2002

[16] D W Ayele H-M Chen W-N Su et al ldquoControlledsynthesis of CdSe quantum dots by a microwave-enhancedprocess a green approach for mass productionrdquo ChemistryndashAEuropean Journal vol 17 no 20 pp 5737ndash5744 2011

[17] M M Moghaddam M Baghbanzadeh A Keilbach andC O Kappe ldquoMicrowave-assisted synthesis of CdSe quantumdots can the electromagnetic field influence the formationand quality of the resulting nanocrystalsrdquo Nanoscale vol 4no 23 pp 7435ndash7442 2012

[18] J A Gerbec D Magana A Washington and G F StrouseldquoMicrowave-enhanced reaction rates for nanoparticle syn-thesisrdquo Journal of the American Chemical Society vol 127no 45 pp 15791ndash15800 2005

[19] A L Washington and G F Strouse ldquoMicrowave synthesis ofCdSe and CdTe nanocrystals in nonabsorbing alkanesrdquoJournal of the American Chemical Society vol 130 no 28pp 8916ndash8922 2008

[20] J Ziegler A Merkulov M Grabolle U Resch-Genger andT Nann ldquoHigh-quality ZnS shells for CdSe nanoparticlesrapid microwave synthesisrdquo Langmuir vol 23 no 14pp 7751ndash7759 2007

[21] D A Hines and P V Kamat ldquoRecent advances in quantumdot surface chemistryrdquo ACS Applied Materials amp Interfacesvol 6 no 5 pp 3041ndash3057 2014

[22] Anton Paar Monowave 300 Microwave Synthesis ReactorOriginal Instruction Manual Anton Paar Graz Austria 2015

[23] A J Morris-Cohen M D Donakowski K E Knowles andE A Weiss ldquo(e effect of a common purification procedureon the chemical composition of the surfaces of CdSe quantumdots synthesized with trioctylphosphine oxiderdquo0e Journal ofPhysical Chemistry C vol 114 no 2 pp 897ndash906 2010

[24] Aldrich Chemistry Lumidottrade FAQ Frequently Asked Questionsabout Using QDNanocrystals Aldrich Chemistry St Louis MOUSA 2019 httpswwwsigmaaldrichcomcontentdamsigma-aldrichdocsSigma-AldrichGeneral_Informationlumidot_faqspdf

[25] D Zhou M Lin Z Chen et al ldquoSimple synthesis of highlyluminescent water-soluble CdTe quantum dots with con-trollable surface functionalityrdquo Chemistry of Materialsvol 23 no 21 pp 4857ndash4862 2011

[26] E Jenrette S K Pradhan G Rutherford J Flowers D Haand A K Pradhan ldquoQuantum-dot-conjugated grapheneoxide as an optical tool for biosensorrdquo Optics Express vol 23no 19 p 25017 2015

[27] E Zillner S Fengler P Niyamakom F Rauscher K Kohlerand T Dittrich ldquoRole of ligand exchange at CdSe quantumdot layers for charge separationrdquo 0e Journal of PhysicalChemistry C vol 116 no 31 pp 16747ndash16754 2012

[28] D B Tice M T Frederick R P H Chang and E A WeissldquoElectron migration limits the rate of photobrightening inthin films of CdSe quantum dots in a dry N2 (g) atmosphererdquo0e Journal of Physical Chemistry C vol 115 no 9pp 3654ndash3662 2011

[29] W G J H M van Sark P L T M Frederix A A BolH C Gerritsen and A Meijerink ldquoBlueing bleaching andblinking of single CdSeZnS quantum dotsrdquo ChemPhysChemvol 3 no 10 pp 871ndash879 2002

[30] N Gilroy Direct Normal Solar Resource of Virginia NationalRenewable Energy Laboratory for the US Department ofEnergy Golden CO USA 2017 httpswwwnrelgovgissolarhtml

8 Advances in Materials Science and Engineering

[6] Y Shirasaki G J Supran M G Bawendi and V BulovicldquoEmergence of colloidal quantum-dot light-emitting tech-nologiesrdquo Nature Photonics vol 7 no 1 pp 13ndash23 2013

[7] C R Kagan E Lifshitz E H Sargent and D V TalapinldquoBuilding devices from colloidal quantum dotsrdquo Sciencevol 353 no 6302 2016

[8] M L Landry T E Morrell T K Karagounis C-H Hsia andC-Y Wang ldquoSimple syntheses of CdSe quantum dotsrdquoJournal of Chemical Education vol 91 no 2 pp 274ndash2792014

[9] C B Murray D J Norris and M G Bawendi ldquoSynthesis andcharacterization of nearly monodisperse CdE (E sulfur se-lenium tellurium) semiconductor nanocrystallitesrdquo Journal ofthe American Chemical Society vol 115 no 19 pp 8706ndash87151993

[10] M B Mohamed D Tonti A Al-Salman A Chemseddineand M Chergui ldquoSynthesis of high quality zinc blende CdSenanocrystalsrdquo 0e Journal of Physical Chemistry B vol 109no 21 pp 10533ndash10537 2005

[11] G H Carey A L Abdelhady Z Ning S M(on OM Bakrand E H Sargent ldquoColloidal quantum dot solar cellsrdquoChemical Reviews vol 115 no 23 pp 12732ndash12763 2015

[12] K J Nordell E M Boatman and G C Lisensky ldquoA safereasier faster synthesis for CdSe quantum dot nanocrystalsrdquoJournal of Chemical Education vol 82 no 11 p 1697 2005

[13] M Z Hu and T Zhu ldquoSemiconductor nanocrystal quantumdot synthesis approaches towards large-scale industrial pro-duction for energy applicationsrdquo Nanoscale Research Lettersvol 10 no 1 pp 1ndash15 2015

[14] O Palchik R Kerner A Gedanken A M WeissM A Slifkin and V Palchik ldquoMicrowave-assisted polyolmethod for the preparation of CdSe ldquonanoballsrdquordquo Journal ofMaterials Chemistry vol 11 no 3 pp 874ndash878 2001

[15] J Zhu O Palchik S Chen and A Gedanken ldquoMicrowaveassisted preparation of CdSe PbSe and Cu2minus xSe nano-particlesrdquo0e Journal of Physical Chemistry B vol 104 no 31pp 7344ndash7347 2002

[16] D W Ayele H-M Chen W-N Su et al ldquoControlledsynthesis of CdSe quantum dots by a microwave-enhancedprocess a green approach for mass productionrdquo ChemistryndashAEuropean Journal vol 17 no 20 pp 5737ndash5744 2011

[17] M M Moghaddam M Baghbanzadeh A Keilbach andC O Kappe ldquoMicrowave-assisted synthesis of CdSe quantumdots can the electromagnetic field influence the formationand quality of the resulting nanocrystalsrdquo Nanoscale vol 4no 23 pp 7435ndash7442 2012

[18] J A Gerbec D Magana A Washington and G F StrouseldquoMicrowave-enhanced reaction rates for nanoparticle syn-thesisrdquo Journal of the American Chemical Society vol 127no 45 pp 15791ndash15800 2005

[19] A L Washington and G F Strouse ldquoMicrowave synthesis ofCdSe and CdTe nanocrystals in nonabsorbing alkanesrdquoJournal of the American Chemical Society vol 130 no 28pp 8916ndash8922 2008

[20] J Ziegler A Merkulov M Grabolle U Resch-Genger andT Nann ldquoHigh-quality ZnS shells for CdSe nanoparticlesrapid microwave synthesisrdquo Langmuir vol 23 no 14pp 7751ndash7759 2007

[21] D A Hines and P V Kamat ldquoRecent advances in quantumdot surface chemistryrdquo ACS Applied Materials amp Interfacesvol 6 no 5 pp 3041ndash3057 2014

[22] Anton Paar Monowave 300 Microwave Synthesis ReactorOriginal Instruction Manual Anton Paar Graz Austria 2015

[23] A J Morris-Cohen M D Donakowski K E Knowles andE A Weiss ldquo(e effect of a common purification procedureon the chemical composition of the surfaces of CdSe quantumdots synthesized with trioctylphosphine oxiderdquo0e Journal ofPhysical Chemistry C vol 114 no 2 pp 897ndash906 2010

[24] Aldrich Chemistry Lumidottrade FAQ Frequently Asked Questionsabout Using QDNanocrystals Aldrich Chemistry St Louis MOUSA 2019 httpswwwsigmaaldrichcomcontentdamsigma-aldrichdocsSigma-AldrichGeneral_Informationlumidot_faqspdf

[25] D Zhou M Lin Z Chen et al ldquoSimple synthesis of highlyluminescent water-soluble CdTe quantum dots with con-trollable surface functionalityrdquo Chemistry of Materialsvol 23 no 21 pp 4857ndash4862 2011

[26] E Jenrette S K Pradhan G Rutherford J Flowers D Haand A K Pradhan ldquoQuantum-dot-conjugated grapheneoxide as an optical tool for biosensorrdquo Optics Express vol 23no 19 p 25017 2015

[27] E Zillner S Fengler P Niyamakom F Rauscher K Kohlerand T Dittrich ldquoRole of ligand exchange at CdSe quantumdot layers for charge separationrdquo 0e Journal of PhysicalChemistry C vol 116 no 31 pp 16747ndash16754 2012

[28] D B Tice M T Frederick R P H Chang and E A WeissldquoElectron migration limits the rate of photobrightening inthin films of CdSe quantum dots in a dry N2 (g) atmosphererdquo0e Journal of Physical Chemistry C vol 115 no 9pp 3654ndash3662 2011

[29] W G J H M van Sark P L T M Frederix A A BolH C Gerritsen and A Meijerink ldquoBlueing bleaching andblinking of single CdSeZnS quantum dotsrdquo ChemPhysChemvol 3 no 10 pp 871ndash879 2002

[30] N Gilroy Direct Normal Solar Resource of Virginia NationalRenewable Energy Laboratory for the US Department ofEnergy Golden CO USA 2017 httpswwwnrelgovgissolarhtml

8 Advances in Materials Science and Engineering