Cent. Eur. J. Phys. • 6(1) • 2008 • 109-115DOI: 10.2478/s11534-008-0025-1

Central European Journal of Physics

Photochromism and magneto-optic response ofZnO:Mn semiconductor quantum dots fabricated bymicroemulsion route

Research Article

Nirmal Misra1, Mohendra Roy1, Dambarudhar Mohanta12∗, Kishor Kumar Baruah1, AmarjyotiChoudhury13

1 Nanoscience Laboratory, Department of Physics, Tezpur University, P.O. Napaam, Tezpur-784 028, Assam, India

2 Laboratory for Molecular Scale Engineering, Department of Electrical and Computer Engineering, University ofWisconsin-Madison, WI 53706, USA

3 Gauhati University, Gopinath Bordoloi Nagar, Guwahati-781 014, Assam, India

Received 11 August 2007; accepted 26 October 2007

Abstract: ZnO:Mn semiconductor quantum dots were prepared by solution casting led microemulsion route. Quan-tum dots of average size ∼2 nm were noticed in transmission electron micrographs. The present workhighlights colour change phenomena (photochromic effect) of quantum dots while subjected to photonillumination. The magneto-optic measurements e.g. magnetic field (H) vs angle of rotation (θ) showstep like behavior and is ascribed to the quantum confinement effect of diluted magnetic ZnO:Mn nanos-tructures. Further, underlying mechanism responsible for exhibiting photochromism and magneto-opticeffects are also discussed.

PACS (2008): 78.67.Hc, 81.07.Ta, 81.16.Be, 85.70.Sq

Keywords: quantum dots • photochromism • Faraday rotation© Versita Warsaw and Springer-Verlag Berlin Heidelberg.

1. Introduction

Quantum dots (QDs) or nanoparticles represent collectionof atoms which have typical dimensions of the order of 1-100 nm. QDs are attractive as their thermal, electrical,optical and magnetic properties drastically change withthe size [1–3]. Although, properties of nanoparticles arenot yet known to the fullest detail, however, light induced

∗E-mail: best@tezu.ernet.in, mohanta@wisc.edu

colour change phenomena (photochromism) was observedsince the time of Michel Faraday. In recent years, therehas been extensive interest in the photochromic propertiesof polymer systems [4, 5]. On the other hand, when a smallamount of magnetic impurity is introduced into the semi-conductor host, the material displays induced magnetismand hence, named diluted magnetic semiconductor (DMS)system. The DMS quantum dots exhibit novel electri-cal, optical and magnetic properties [6–8] with respect totheir nonmagnetic counter parts. The behaviour of car-rier manipulation is the foundation of today’s informationtechnology in terms of material density and packaging.

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Photochromism and magneto-optic response of ZnO:Mn semiconductor quantum dots fabricated by microemulsion route

The integrated circuit and high speed electronic devices,which are made up of semiconductors have great successin modern technology. However, all electronic materials(semiconductors) are based only on the “charge” prop-erty of the electron. The information storage in terms ofmagnetic recording using “spin” property of the electronhas been subject of recent interest in ferromagnetic QDssystems [9, 10]. In this context, DMS systems would beadvantageous in the sense that in addition to the chargeconfinement and transport, spin transport and spin manip-ulation is highly feasible for storing, writing and retrievinginformation. In the present paper, we highlight fabricationof ZnO:Mn QDs by inexpensive microemulsion technique.Further, photochromism, and magneto-optic properties ofthe synthesized QDs have been investigated at 300 K.

2. Experimental: principles andprocedures

2.1. Microemulsions and reactions

A microemulsion can be defined as a thermodynamicallystable isotropic dispersion of two immiscible liquids con-sisting of nano-size domains of one or both liquids in theother, stabilized by an interfacial film of surface activemolecules. Microemulsions may be classified as water-in-oil (w/o) or oil-in-water (o/w) depending on the dispersedand continuous phases. In both cases, the dispersed phaseconsists of monodispersed droplets in the size range of100-1000 Å. It is known that when microemulsions containcomparable amounts of oil and water, some of these sys-tems may show a bicontinuous structure. Microemulsionswere first introduced by Schulman and his co-workers in1943 [11]. They have explained that the microemulsionswere spontaneously formed with the uptake of water oroil due to the negative transient interfacial tension whichallows the free energy to decrease as the total oil-waterinterfacial area increases. At equilibrium, the oil/waterinterfacial tension becomes zero or a very small positivenumber of the order of 10−2 mN/m.In water-in-oil microemulsions, the aqueous droplets con-tinuously collide, coalesce and break apart resulting ina continuous exchange of the solute content. The colli-sion process depends upon the diffusion of the aqueousdroplets in the continuous media, i.e. oil, while the ex-change process depends on the attractive interactions be-tween the surfactant tails and the rigidity of the interface,as the aqueous droplets approach close to each other. Forreactions in water-in-oil microemulsions involving reac-tant species totally confined within the dispersed waterdroplets, a necessary step prior to their chemical reac-

tion is the exchange of reactants by the coalescence oftwo droplets. When chemical reaction is fast, the overallreaction rate is likely to be controlled by the rate of coa-lescence of droplets. Therefore, properties of the interfacesuch as interfacial rigidity are of major importance. A rel-atively rigid interface decreases the rate of coalescence,and hence leads to a low precipitation rate. On the otherhand, a substantial fluid interface in the microemulsionenhances the rate of precipitation. Thus, by controllingthe structure of the interface, one can change the reac-tion kinetics in microemulsions by an order of magnitude.Conceptually, if one takes two identical water-in-oil mi-croemulsions and dissolves reactants A and B respectivelyin the aqueous phases of these two microemulsions, uponmixing, due to collision and coalescence of the droplets,the reactants A and B come in contact with each otherand form AB precipitate.This precipitate is confined to the interior of the mi-croemulsion droplets and the size and shape of the particleformed reflects the interior kinetics of the droplet. This isone of the principles utilized in producing nanoparticlesusing microemulsions (Fig. 1). However, nanoparticlescan also be produced in microemulsions by adding a re-ducing/ precipitating agent, in the form of a liquid or agas, to a microemulsion containing the primary reactantdissolved in its aqueous core.

Figure 1. Schematic of microemulsion reactions.

2.2. Fabrication process

ZnO:Mn QDs were fabricated by microemulsion methodfollowing previously reported works [12–14]. To preparethe 1st set, 1.7 gm of ZnCl21 and 0.01 gm of MnCl22 areadded in 250 ml of D/D water at moderate stirring. 10 ml

1 >99% pure, Merck Ltd.2 99% pure, S.D.Fine Chem. Ltd.

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Nirmal Misra, Mohendra Roy, Dambarudhar Mohanta, Kishor KumarBaruah, Amarjyoti Choudhury

of the as-prepared solution was taken and then, 13 ml ofn-Heptane3 and 3 ml of Butanol4 were added to it thus,producing a two-phase mixture. In order to make a homo-geneous solution, the surfactant cetyl trimethyl amoniumbromide (CTAB) was used.

In the 2nd set, 10 ml of 0.1M NaOH5, 13 ml of n-Heptaneand 3 ml of Butanol are mixed. Then, CTAB was usedto mix up two phases and hence, a homogeneous solutionwas obtained. Next, the two sets of solutions are mixedand stirred for ∼4 hours followed by reflux in a high speedcentrifuge (10000 rpm, R-24 Remi) for 20 mins. Finally,the gel was washed by a mixture of CH3OH6 and CH3Cl7.The ZnO:Mn QD sample so obtained was kept ready forcharacterization. The microemulsion synthesis procedureis shown in the form of a block diagram (Fig. 2).

2.3. Photochromic effect

In order to study photochromic effect the UV–Visible spec-trophotometer (U-2001, Hitachi) was used. For differ-ent incident light wavelengths, corresponding transmittedwavelengths were measured (Fig. 3). At first, the trans-mittances of the empty cuvette were taken for differentwavelengths of light. Then, the transmission response wasmeasured for the cuvette containing QD-sample. There-fore, resulting transmittance values in the reference modewere recorded. The photochromic effect was studied forthree different monochromatic light beams viz., 543.5 nm,594.1 nm and 632.8 nm, respectively. A scheme of pho-tochromism is shown in Fig. 3.

2.4. Magneto-optic set-up

A red diode laser (Power=2 mW, λ=670 nm) was used asa monochromatic source to study magneto-optic effect. Asshown in Fig. 4, the incident light beam was polarizedby means of a polarizer and allowed to pass through thesample in such a way that the light beam is along thedirection of the magnetic field. The cuvette containingQD sample was kept in between the pole pieces of anelectromagnet and the alignment was done with the helpof mirror adjustments. On passing through the sample thelight beam gets reflected back by a mirror. This emergentlight beam was then analyzed by means of an analyzerand photo-detector set-up.

3 99.5% pure, Merck Ltd.4 99% pure, Merck Ltd.5 99% pure, Merck Ltd.6 99.8% pure, Merck Ltd.7 99% pure, Merck Ltd.

3. Results and discussionThe transmission electron micrographs of the ZnO:MnQDs, shown in Fig. 5, depicts that the synthesized QDshave size range 2-9 nm. It is clear from the figure insetthat most of the QDs are of average size ∼ 2 nm. TheQDs are spherical in shape and isolated from each other.The microemulsion method hes led to unclustered QDswhich is otherwise difficult while adopting conventionalnanostructur synthesis [15, 16].It is known that photochromism is a light sensitive phe-nomenon [17]. The photochromic materials display colourchange phenomena while subjected to incident photons ofdefinite wavelength. When the light source is withdrawn,the sample material reverts back to its original state. Notethat QDs possess tunable energy gap with size variation,known as quantum size effect. In addition, we believe,the presence of transition metal ion e.g. Mn in ZnO host,can actively contribute to photochromism owing to variableoxidation states. When light of particular wavelength isallowed to incident on the QD sample, there is substantialmodification in the electronic structure of the QDs whichcould lead to emergent light of different wavelength. Gen-eration of second and/or third harmonics in nanomaterialsinvolves two/three photon absorption (TPA) and is dueto a purely nonlinear optical phenomena [18, 19]. Mag-netization induced third harmonic generation have beenreported in the recent past [20]. On the other hand, pho-tochromism deals with the transient excitation of electronsand involves change in oxidation state of a particular iontemporarily.In a photochromic material [21], the transmitted wave-length (λt) through the sample is given as

λt = λi/T , (1)

where, λi is the incident wavelength and T is the transmit-tance upon illumination. Variation of transmitted wave-length is observed with respect to incident wavelength.Fig. 6 displays λi vs. T and suggests that transmittanceis suppressed significantly due to Mn doping, by a factorof 6 while Mn-concentration in the QDs varies between0-0.5%. Transmission to some extent, was seen to be de-pendent on the incident wavelength λi. Next, referring toFig. 7 one can say that Mn-doping has substantial ef-fect on photochromism. We speculate that suppression inthe transmittance accounts for the light absorption andthe electron excitation, which ultimately gives out lightof higher wavelength. It was revealed that on increasingMn- concentration, one can produce efficiently red lightout of incident light in the visible region.Magneto-optic effect is related to noticeable optical phe-nomena induced by magnetism. Faraday’s relation as a

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Photochromism and magneto-optic response of ZnO:Mn semiconductor quantum dots fabricated by microemulsion route

Figure 2. Flow chart block diagram for synthesis of ZnO:Mn QDs

Figure 3. Schematic of photochromic effect.

consequence of magneto-optic effect is given by [22]:

θ = vHL, (2)

where H is the external magnetic field, L is the length ofthe specimen; v is the Verdet’s constant. The linear re-sponse between θ and H has been previously reported inZnS:Mn nanoaggregates [23]. The present ZnO:Mn QD-system has isolated QDs and therefore, there might besubstantial variation in effective L which can lead to de-parature from the linear relationship between θ andH.Themagnetic energy experienced by Mn2+ ions in the QD-system is given by:

B = 12 µH

2, (3)

where µ is the permeability of ZnO (µ = µ0µr , µr =28 at300 K). The θ vs. H experimental data for ZnO:Mn QDs(Mn 0.5%) system is shown in Table 1 and Fig. 8. It is ex-pected that owing to the presence of Mn-ions (which haveunpaired electrons in the valence shell) in conventionalZn-sites, Mn2+ ions can experience magnetization alongthe preferred magnetic field direction (Fig. 4b) and there-fore, capable of inducing magnetism in QDs. Of course,

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Nirmal Misra, Mohendra Roy, Dambarudhar Mohanta, Kishor KumarBaruah, Amarjyoti Choudhury

Figure 4. Schematic of a) magneto-optic setup and b) response of Mn2+ ions under magnetic field (full arrow) and magneto-optic field (dottedarrow).

Figure 5. TEM micrograph of the ZnO:Mn quantum dots.

the nature of alignment of the magnetic dipoles dependson the applied field strength. In the absence of mag-netic field, a light beam of comparatively large wavelength(λ=670 nm) can pass through the specimen containing ∼2nm QDs (as if transparent) without any departure fromits original path. Nevertheless, when magnetic energy iscoupled to the incoming optical energy, situation becomescompletely different. It is expected that the magnetic en-ergy coupled with the polarized incident light in the vicin-ity of confined nanostructures, could result in deviation ofemergent light by certain angle along the same plane of

Figure 6. Incident wavelength vs transmittance for various Mn con-centrations in ZnO:Mn.

polarization. Previously, it was claimed that enhancementof θ was due to increased magnetization [24]. Accordingly,it should be worth mentioning here that on increasingmagnetic field, Faraday rotation would increase in viewof strong coupling of magnetic and optical energy. It wasreported that the spatial localization of the optical waveresonant to the microcavity mode leads to the Faradayeffect enhancement observed in magnetophotonic crystals

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Photochromism and magneto-optic response of ZnO:Mn semiconductor quantum dots fabricated by microemulsion route

Figure 7. Incident wavelength vs transmitted wavelength forZnO:Mn QDs with varying Mn concentrations.

Figure 8. Magnetic field vs. Faraday rotation for ZnO:Mn (Mn 0.5%)QDs.

with the magnetic garnet [25, 26] and Co-ferrite [27] spac-ers. In our case, the observed step like behaviour in θvs. H plot is quite interesting and suggests occurrenceof discrete energy states in the said diluted magnetic QDsystem. To check the validity of the steplike response, wehave analyzed the data in order to compute Verdat coef-ficients by considering means of a set of data points of aparticular step and compared with the linear trendline fit.The fitting trace (with zero intercept and within an rmserror ±0.001%) gives out a slope of 0.008 and hence pre-dicts Verdat constant value as large as 8.0x10−3 deg G−1

cm−1. Conversely, mean value calculation from steps sug-gests v values in the range 5.87-12.2x10−3 deg-G−1cm−1

for an optical path length L=1 cm. The order of magni-

tude is close to the values observed in a similar systeme.g. ZnS:Mn [23]. It is possible that there could be mi-nor variation in θ corresponding to every H, the magneticfield but due to the limitation of the analyzer (least count0.1 deg) only significant changes were detectable withabsolute accuracy. Beyond H= 550 G, we notice devi-ation from step like response. We speculate that as wehave used continuous laser light and the Faraday rotationwas recorded at different magnetic fields there might bechances of agglomeration or precipitation in the samplechamber containing QDs. Therefore, decrease in θ shouldbe due to instability in the sample due to limitation of ourexperimental arrangements. Considering the applicationof magneto-optical materials for laser diodes operating atvisible red to IR- region, it is very important to obtain in-formation about the Faraday effect at specific wavelengths.In this context, our QD sample ensure appreciable Fara-day effects which might lead to development of Faradayrotator, isolator and laser diode.

Table 1. Magnetic energy vs angle of rotation ZnO:Mn (Mn 0.5%)QDs.

Sl.No

Field variation(H) [Gauss]

Mean appliedmagnetic energy[Joule]

Rota-tionθ[deg]

Verdet Constant[deg G−1 cm−1]

1 0-200 1.758 x10−9 0 2 250-400 24.72 x10−9 2.2 5.87x10−3

3 450-550 43.95 x10−9 6.1 12.2x10−3

4 600 63.29 x10−9 4.3 7.16x10−3

4. ConclusionsTo summarize, we have produced reasonably good, spher-ical and isolated ZnO:Mn QDs following microemulsiontechnique. Mn-concentration dependent photochromismwas observed in ZnO:Mn systems. An increase in Mn-concentration is found to increase corresponding wave-length of transmission. The magneto-optic effect describesanomaly in the linear response owing to presence of iso-lated QDs in the sample which had led to effective vari-ation in L. Increased values of Verdet coefficient suggestthat there is significant enhancement in the magnetiza-tion. The step-like response is expected due to occurrenceof the available discrete energy states owing to quantumconfinement effects of diluted magnetic QDs. A detailedmechanism to understand step like response is in progress.

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AcknowledgementsThe authors are thankful to the Department of MBBT,T.U. for providing UV-VIS spectrophotometer and RSIC,Shillong for extending TEM facility.

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