Solution-Processed Zinc OxideField-Effect Transistors Based onSelf-Assembly of Colloidal NanorodsBaoquan Sun* and Henning Sirringhaus

CaVendish Laboratory, UniVersity of Cambridge, Madingley Road,Cambridge, CB3 0HE, U.K.

Received August 11, 2005; Revised Manuscript Received October 23, 2005

ABSTRACT

Colloidal zinc oxide (ZnO) nanocrystals are attractive candidates for a low-temperature and solution-processible semiconductor for high-performance thin-film field-effect transistors (TFTs). Here we show that by controlling the shape of the nanocrystals from spheres to rods thesemiconducting properties of spin-coated ZnO films can be much improved as a result of increasing particle size and self-alignment of thenanorods along the substrate. Postdeposition hydrothermal growth in an aqueous zinc ion solution has been found to further enhance grainsize and connectivity and improve device performance. TFT devices made from 65-nm-long and 10-nm-wide nanorods deposited by spincoating have been fabricated at moderate temperatures of 230 °C with mobilities of 0.61 cm 2V-1s-1 and on/off ratios of 3 × 105 after postdepositiongrowth, which is comparable to the characteristics of TFTs fabricated by traditional sputtering methods.

There is currently significant interest in realizing high-performance thin-film transistors (TFTs) based on solution-processible semiconducting materials for applications re-quiring low-cost, low-temperature manufacturing on large-area flexible substrates.1,2 Much effort has been devoted tolow-temperature solution processible organic semiconductorsas a potential alternative to traditional inorganic semiconduc-tors. OTFTs with mobilities of 0.01-0.1 cm2/Vs, goodreliability, stability, and device-to-device uniformity havebeen demonstrated.3-8 There are also various approaches torealizing solution-processible inorganic semiconductors,which provide a potential route to significantly highermobilities, but for which control of electronic defect stateswhen processed from solution at low temperatures can bechallenging.9 Inorganic semiconductors might also providea route to high performance n-type TFTs required forcomplementary circuits, which are traditionally difficult torealize with organic TFTs, although much progress has beenmade recently.10,11

A variety of solution-processible inorganic semiconductorsfor TFTs have been reported.9 These include tin(II) iodidebased organic-inorganic hybrids,12 chalcogenide semicon-ductors,13 semiconductor nanowires,14 and nanocrystals.15 Thesemiconductor nanowire/nanocrystal approach is very prom-ising because it allows one to decouple the high-temperaturegrowth/synthesis of the nanowire from the low-temperaturedevice fabrication process and achieve high performance.However, it is a tough challenge to disperse micrometer-

long nanowires in a solution for simple solution-coating orprinting-based deposition. This problem is easier to solvewith smaller colloidal nanocrystals, such as CdSe nanocrys-tals, which can be drop-cast onto a substrate and can remeltto form a uniform film after annealing at 350°C because oflowering of the melting point for these ultra-small nanoc-rystals.16 However, the mobility of a thin film of nanocrystalsis significantly lower than the maximum achievable bulkmobility of the semiconducting material because of grainboundaries in the sintered nanocrystal network. As-preparedCdSe nanocrystal devices show n-type behavior with amobility of 1 cm2V-1s-1 and an on/off ratio of 3.1× 104.Some of the important requirements for using semiconductorcolloidal nanocrystals in this application include a gooddispersing capacity (>50 mg/mL) and adequate stability ofthe dispersion (at least one week). Although there are manyreports to synthesize different kinds of nanocrystals, thereare few colloid nanocrystal systems that can meet theserequirements.17

Zinc oxide (ZnO) is an environmentally friendly transpar-ent semiconductor with a large band gap of 3.37 eV. TFTdevices based on polycrystalline ZnO as active layer havebeen reported with mobilities of around 0.2-3 cm2V-1s-1.18-21

Most fabrication methods use a sputtering process to growZnO films. Solution-processing techniques have also beenused to fabricate ZnO devices but have suffered from poordevice performance20 or the need to use a high annealingtemperature (700°C).21 Here we use spin-coated ZnOnanocrystals films to fabricate high-performance TFT devices* Corresponding author. E-mail: bs309@cam.ac.uk.

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at moderately low temperatures. ZnO nanospheres can bedispersed at high concentration beyond 75 mg/mL for solarcell applications as shown in recent reports.22,23Furthermore,the shape of the nanocrystals can be controlled fromnanosphere to nanorods by adjusting the growth time.24 It ispossible to achieve a nanorod suspension with a concentra-tion of up to 85 mg/mL with good stability by adding a smallamount of alkylamine. We investigate the role of the shapeof the nanocrystal on the colloidal self-assembly of thenanoparticles on the substrate and on the resulting deviceperformance. We also show that the TFT device performanceand mobility can be further improved without sacrificing theon-off ratio by simple hydrothermal growth in an openbeaker with aqueous zinc ion solution to increase the domainsize and minimize the effect of grain boundaries.

Experimental Section. ZnO nanorods are prepared ac-cording to a literature method developed by Pacholski23,24

with some modification. Zinc acetate (Zn(Ac)2, 0.8182 g,4.46 mmol) and 250µL of water was added into a flaskcontaining 42 mL of methanol. The solution was heated to60 °C with magnetic stirring. Potassium hydroxide (KOH,0.4859 g, 7.22 mmol, purity 85%) was dissolved into 23mL of methanol as the stock solution that is dropped intothe flask within 10-15 min. At a constant temperature of60 °C, it takes 2 h and 15 min to obtain 6-nm-diameternanospheres. A small amount of water was found helpful toincrease the ZnO nanocrystal growth rate. To grow thenanorods, the solution is condensed to about 10 mL. Thiswas found helpful before further heating to decrease thegrowth time of the nanorods. Then it is reheated for another5 h before stopping the heating and stirring. The upperfraction of the solution is removed after 30 min. Methanol(50 mL) is added to the solution and stirred for 5 min. Theupper fraction of the solution is discarded again after 30 min.This process is repeated twice. For the second washing, theupper fraction of the solution is taken away after overnightstaying. Finally, 3.3 mL of chloroform and 100µL ofn-butylamine are used to disperse the nanorods. The nanorodconcentration is about 85 mg/mL, and the suspension is stablefor more than 2 weeks. Using the modified method reportedhere, it takes only 5 h toobtain 65-nm-long nanorods insteadof several days as reported in the literature.24

Nanocrystal films and devices are fabricated on SiO2(300nm)/Si substrates with photolithographically patterned in-terdigitated Cr(3 nm)/Au(12 nm) electrodes. The devicestructure is shown in Scheme 1. Before spin-casting the ZnOsolution, the substrate is cleaned in an oxygen plasma at apower of 150 W for 2 min. The film is spin-coated from thefiltered (0.45-µm PTFE filter) ZnO solution with a speed of2000 rpm. Then the devices are annealed at 230°C inN2/H2(V/V, 95:5) for 30 min. For additional hydrothermalgrowth, the substrates are immersed upside down into a glassbeaker filled with an aqueous solution containing zinc nitrate(0.025 M) and ethyldiamine (0.04 M) with slow stirring at90 °C. The devices are taken out after 50 min and rinsedwith deionized water. Finally, the device is annealed at 200°C for 15 min in an N2/H2 atmosphere after drying withnitrogen.

Results and Discussions.Transmission electron micros-copy images of ZnO nanospheres (a) and nanorods (b)synthesized as above are shown in Figure 1. The diameterof the nanospheres is about 6 nm. The nanorods have anaverage width of 10 nm and length of 65 nm. The nanorodlength can be tuned by the reaction time. However, longnanorods (longer than 100 nm) are quite difficult to disperseinto any solution. The UV-Vis absorption spectrum ofnanospheres and nanorods are shown in Figure 1S. Theabsorption peak of the nanorods shifts to a longer wave-length, reflecting the larger diameter of nanorods comparedto that of nanospheres. In the synthesis process, it is criticalto have the correct mole ratio between KOH and Zn(Ac)2.The chemical composition of as-prepared nanorods isdetermined by the initial mole ratio. Variations in stoichi-ometry affect the conductivity of the films and the mobilityand on-off current ratio of the TFTs. The characteristicsfor three TFT devices made from ZnO nanorods synthesizedwith different mole ratios are summarized in Table 1. All ofthe devices exhibit n-type field-effect conduction. Theoptimized mole ratio is 1.62. It is found that the conductivityincreases and the mobility decreases as the stoichiometry isvaried from the experimentally determined optimum moleratio in both directions. The stoichiometry can be character-ized by X-ray diffraction. The (002) diffraction signal of theZnO nanocrystals comes only from zinc atoms in the wurtzitecrystal structure. It has been found that the (002) signal ofthe ZnO nanocrystals in their X-ray diffraction patterns ismaximum if the mole ratio is near to its stoichiometricvalue,25 which means that there will be the lowest concentra-tion of oxygen vacancies in this crystal structure at this ratio.ZnO films containing a low concentration of oxygen vacan-cies should exhibit low conductivity because oxygen vacan-cies behave as deep donors.26 Consistent with this expecta-tion, TFT devices based on this ratio show the lowestconductivity. It is worth mentioning that small variations inthe mole ratio do not appear to have a significant effect onthe shape and size of the nanorods but do affect the TFTdevice performance greatly. We believe that the largedifference of the TFTs’ characteristics originates in smallchanges of the stoichiometry of the ZnO films.

Scheme 1. TFT Device Structure

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To obtain reproducible TFT performance, the formationof high-quality films by techniques such as spin-coating isvery important. We have found that high-quality nanocrystalfilms can be obtained from high-concentration suspensions(>50 mg/mL). The as-prepared ZnO nanocrystals compriseacetate (CH3COO-) ligand groups chelating with zinc atomson the surface of nanocrystals. The ligands are very importantto facilitate the dispersion of the nanocrystals in the solvent.For small nanospheres (approximately 6 nm), it is quite easyto achieve a high-concentration suspension using just theshort acetate ligands. However, the acetate ligands are notsufficient to achieve high-concentration dispersions of thelonger nanorods. Long-chain alkylamines (dodecylamine)have been used as ligands to help ZnO nanocrystal suspen-sion.27 Here we use butylamine as a ligand with a shorterchain and a low boiling point (78°C) instead of dodecyl-amine. When butylamine is added to the suspension, the 10× 65 nm2 nanorods can be dispersed into chloroform withconcentration as high as 90 mg/mL. At the same time, theligand can increase the interaction between nanocrystals andfavor formation of a uniform film.28 For the nanospheres,there is a large number of microcracks in the spin-cast filmsif butylamine is not added to the suspension. Butylaminewas used in all of the spin-cast films reported here unlessstated otherwise. Furthermore, residual ligands can beremoved easily by annealing because of their low boilingpoints. It is found that most ligands composed of mixturesof acetate groups and butylamine groups can be removedeasily by annealing. Fourier transform infrared spectroscopy(FTIR) was used to check the presence of the ligand as shown

in Figure 2S. We do not detect a signal from butylamine,probably because most of the butylamine volatilizes in a shorttime during sample preparation and the small amount ofbutylamine on the surface is difficult to detect by FTIRbecause of the spectral overlap between the characteristicamine absorption and that of residual water. The pres-ence of the acetate groups can be checked by observing the-CdO stretching vibrations in the range of 1600-1300 cm-1

and the-CH3 stretching vibrations in the range of 3000-2800 cm-1. Both signals disappear after annealing if butyl-amine has been added as a ligand. In the presence ofbutylamine, the chelating acetate groups are dissociated fromthe surface after being replaced with butylamine. Theyremain present in the film after spin-coating (Figure 2S.II),but can be removed easily during annealing (Figure 2S.IV).Without butylamine, the acetate groups, which remainchelated on the surface of ZnO nanocrystals after spin-coating, are difficult to remove by annealing (Figure 2S.III).

The as-prepared films are annealed under nitrogen/hy-drogen atmosphere to increase the mobile carrier concentra-tion and field-effect mobility. It has been reported thathydrogen can be incorporated into ZnO films in highconcentration at annealing temperatures of 200°C andbehave as a shallow donor acting as a source of conductiv-ity.26 The characteristics of TFTs made from ZnO nano-spheres (a) and nanorods (b) are shown in Figure 2. BothTFT devices exhibit clean n-type transistor behaviors withlow turn-on voltages|V0| ) 0-8 V and good operatingstabilities. For the device made from 6-nm nanospheres, theon-off ratio is 5 × 103 and the linear and saturated field-effect mobilities are 2.37× 10-4 cm2V-1s-1 and 4.62× 10-4

cm2V-1s-1, respectively. The TFT device performance isimproved significantly when nanorods are used as the activelayer instead of nanospheres. These devices exhibit anon-off ratio of 1.1 × 105 and higher mobility of 0.023cm2V-1s-1 derived from the saturated operating region and0.013 cm2V-1s-1 derived from the linear region. The mobilityis improved by almost two orders from devices made from6-nm nanospheres to those from 65-nm-long nanorods. Theapproximately 10x larger size of the nanorod particles

Figure 1. Transmission electron microscopy images of ZnO nanocrystals. (a) nanosphere with average diameter of 6 nm. (b) Nanorodswith average lengths of 65 nm and diameters of 10 nm.

Table 1. TFT Device Characteristics of As-Deposited ZnONanorod Films, Which Were Synthesized from Different MoleRatios between Potassium Hydroxide and Zinc Acetatea

filmmole ratio

KOH/Zn(Ac)2

µsat

(cm2V-1s-1)on/offratio |V0| (V)

a 1.5 1.9e-4 <2b 1.62 2.3e-2 1.1e+5 8c 1.7 8.2e-3 4.4e+1 9

a Field-effect mobilities,µsat, were derived from the saturated region.|V0| is the turn-on voltage of the TFTs.

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compared to the nanospheres will significantly reduce thenumber of interparticle hopping events that an electron hasto undergo when moving from source to drain electrode. Thiswill result in an increase of the mobility even if the nanorodsare not uniaxially aligned along the direction of current flow.If we assumed somewhat simplistically that the conductionprocess involves many electron hopping steps from onenanocrystal to its neighbors, then for a channel length of 20µm and 6-nm-wide nanospheres an electron requires at least3000 internanocrystal hops to transfer from one electrode tothe other. In contrast, for 10× 65 nm2 nanorods, the numberof hops would be reduced to about 500 (assuming anisotropic orientation of the nanorods in the plane). Therefore,the electron hopping steps to cross the channel should bemuch less in the nanorod TFT than in the in the nanospheredevice. However, another important reason for the improvedperformance of the nanorod device is believed to be relatedto the favorable in-plane self-alignment of neighboringcolloidal nanorods with respect to each other when spin-coated onto the substrate as discussed below (see Figure 4a).

The TFT device performances can be further enhancedby the postdeposition hydrothermal growth step in aqueoussolution. The corresponding TFT device characteristics areshown in Figure 3b. Devices composing of nanorods haveachieved a mobility of 0.61 cm2V-1s-1 and 0.24 cm2V-1s-1

as extracted from the saturated and linear transfer charac-teristics, respetively. The on-off ratio measured between-50V and+60 V is 3× 105. It has been observed that thethreshold voltage shifts to high negative values, which weexplain with a possible overdoping with hydrogen afterpostdeposition annealing. After hydrothermal growth, butbefore annealing in hydrogen atmosphere, the thresholdvoltage is still around zero volts as in spin-coated films andthe on-off ratio is 1.6× 105 (between 0 and 60 V, see Figure3S). The saturation mobility value before annealing is 0.10cm2V-1s-1. The annealing in hydrogen leads to a furtherimprovement of mobility but also to a shift of the thresholdvoltage to large negative values.

The device performance has also been evaluated whenmeasured in air without encapsulation because ZnO is known

to interact with CO2 and other atmospheric species. Beforemeasurements, the device was stored in air for 3 days. Thetransfer characteristics of the device shown in Figure 3b weremeasured in air and are shown in Figure 3d. A mobility of0.23 cm2V-1s-1 and 0.20 cm2V-1s-1 was extracted from thesaturated and linear transfer characteristics, respectively. Theon-off ratio is 1.6× 106. The threshold voltage of devicesmeasured in air tends to be more positive than that of devicesmeasured in nitrogen.

The general device performance is comparable to that ofTFT devices fabricated in the same device structure bysputtering methods (mobility: 1.2 cm2V-1s-1; on/off: 1.6× 106).18 For comparison, single ZnO nanowire transistorswith mobilities of 1-5 cm2V-1s-1 have been reported.29 Forthe solution-based method reported here, the raw materialsand deposition methods are low-cost, and the aqueoushydrothermal growth in an open vessel should be applicableto large-area substrates.

To investigate the relationship between film microstructureand device performance and to identify the mechanisms forthe observed improvements of device performance, we haveperformed atomic force microscopy (AFM) (Figure 4) andscanning-electron microscopy (SEM) (Figure 5). From AFMimages such as Figure 4a and the inset SEM image in Figure5a, it is clear that in as-spun films the nanorods arepreferentially oriented with their long axis in the substrateplane. The interactions between the colloidal nanorods duringsolution growth lead to the formation of small liquidcrystalline-like domains with a size on the order of 100 nmin which the nanorods are oriented parallel to each other(see inset of Figure 5a). Similar colloidal self-organizationinto nematic and smectic-A ordered solids has been reported

Figure 2. Log-linear scale plots of linear (Vd ) 5 V) and saturated-(Vd ) 60 V) transfer characteristics for as made TFT device with(a) 6-nm nanospheres and (b) 10× 65 nm2 nanorods withoutpostdeposition hydrothermal growth measured in nitrogen atmo-sphere. The devices have been annealed at a temperature of 230°C before measuring. The channel length (L) and width (W) are 20µm and 1 cm, respectively. The capacitance value of the gatedielectric is 11.4 nF/cm2.

Figure 3. (a) Transfer characteristics of a device composed ofnanospheres in the linear region (Vd ) 5 V) and saturated region(Vd ) 60 V) after postdeposition hydrothermal growth measuredin nitrogen. (b and c) Transfer and output characteristics of a devicemade from nanorods after postdeposition hydrothermal growthmeasured in nitrogen. (d) Transfer characteristics of the deviceshown in b and c measured in air after it had been stored in air for3 days.

Nano Lett., Vol. 5, No. 12, 2005 2411

for CdSe and BaCrO4 nanorods.28,30-31 Because of this self-alignment of the rods, the probability of encountering high-angle domain boundaries is reduced. We believe that thisoriented in-plane self-assembly of the colloidal nanorods isan important factor contributing to the enhanced mobilityof the as-deposited nanorod films compared to nanospherefilms. The films exhibit good uniformity over large areas asevidenced by large-scale AFM (Figure 4S) and opticalmicroscopy images (Figure 5S), which is important toachieving good device uniformity and reproducibility.

During postdeposition hydrothermal growth, the nanorodsgrow further along theirc axis forming longer rods, as shownin Figure 5c and d. The average final nanorod length is aslong as 300 nm. SEM images of the nanorod film near athin edge (Figure 5c) clearly show that near the interfacewith the substrate the nanorods retain their favorable in-planeorientation during the postdeposition hydrothermal growth,whereas on the surface of the film an increasing number ofnanorods grow preferably normal to the film plane. We alsoobserve an increase in the diameter of the nanorods, whichappears to be occurring mainly because of the fusing ofseveral nanorods (Figure 5d). A similar increase in nanorodsize has also been observed in vertically oriented ZnO arrayswhere microwires are formed by the fusing of manynanowires.32,33 The diameter of individual nanorods alsoincreases slightly to about 15 nm after hydrothermal growth,but by the fusing of several closely packed nanorods thediameter can become as large as∼60 nm. Generally, thefusing process prefers to take place in the densely packedregions of the films in which nanorods are oriented parallelto each other. This increase in nanowire diameter and lengthis responsible for the observed improvement of TFT deviceperformance after postdeposition hydrothermal growth. Herewe use a two-step approach to obtain self-aligned 300-nm-long nanorods. It is quite difficult to achieve this in a singlestep because of the poor dispersion properties of longnanorods or nanowires.

If the film is made from nanospheres and subjected topostdeposition hydrothermal growth, then the TFTs showonly a small improvement of mobility to 0.0024 cm2V-1s-1

and the on-off ratio of 5 × 104. The mobility is more than2 orders of magnitude lower than that of TFTs made fromnanorods by the same fabrication process. During postdepo-sition hydrothermal growth, the as-spin-cast nanosphere-seedfilm can grow into an array of ZnO wires, which are,however, aligned randomly with respect to the substratenormal.32 This is less favorable than the in-plane orientation

Figure 5. Scanning electron microscope images of a ZnO filmafter postdeposition hydrothermal growth: (a) nanorods and (b)nanosphere. The inset in Figure 5a shows a scanning electronmicroscope image of the as-spin-cast nanorod film. Figure c is aplan-view scanning electron microscope image at the edge betweenthe ZnO nanorod film and the bare SiO2 substrate after postdepo-sition hydrothermal growth showing the in-plane orientation of thenanorods in the thin region near the edge. Figure d is a plan-viewimage of a film deposited from a dilute concentration of nanorodsafter postdeposition hydrothermal growth.

Figure 4. Atomic force microscopy topograph of as-spin-coated nanorod (a) and nanosphere (b) films. The scanning range is 500× 500nm2. The z topography scale is indicated. The root-mean-square of the film roughness derived from AFM topography images are 8.3(nanorods) and 1.5 nm (nanospheres), respectively.

2412 Nano Lett., Vol. 5, No. 12, 2005

of nanowires obtained near the substrate interface in filmsdeposited from a nanorod dispersion. The nanosphere filmsexhibit good uniformity, as shown in Figure 4b. Whenimmersed in the aqueous solution, the nanospheres growalong a random direction. Some rods are perpendicular tothe substrate and appear as bright spots of high electrondensity in the SEM image; and some rods are growing at anangle to the substrate normal, achieving a limited length ofabout 50 nm, as shown in Figure 5b. Although this length iscomparable to that of the as-prepared nanorod films (65 nm),the TFTs made from nanosphere films subjected to hydro-thermal growth exhibit about 1 order of magnitude lowermobility than those made from as-prepared nanorod films.This is further evidence that the orientation of the nanorodsis an important factor responsible for the improved perfor-mance of devices made from nanorods.

Conclusions.High-performance n-type TFTs have beenfabricated from colloidal ZnO nanorods deposited by spin-coating. Careful control of stoichiometry during nanocrystalgrowth is crucial for achieving sufficiently low film con-ductivity and high field-effect mobility. As-deposited nano-rods preferentially adopt an in-plane orientation with smalldomains on the order of 100 nm in which rods are alignedparallel to each other. Postdeposition hydrothermal growthleads to an increase of nanorod diameter and length andresults in a significant improvement of device performance.Field-effect mobilities of 0.6 cm2/Vs were achieved in spin-cast ZnO nanorod films subjected to postdeposition hydro-thermal growth. The use of nanorods instead of nanospheresas a seed layer for the hydrothermal growth results in longnanorods oriented preferentially in the plane of the substratenear the interface with the gate dielectric, which is favorablefor the charge transport in a TFT. Making use of the self-assembly processes in colloidal nanocrystals is an attractiveand simple route for controlling the microstructure andelectronic properties of solution-processed semiconductornanocrystal films.

Acknowledgment. We thank Dr. Neil Greenham for useof his synthetic facility and Dr. Peng Wang for helpfuldiscussions. This work was supported by InterdisciplinaryResearch Collaboration (IRC) in Nanotechnology.

Supporting Information Available: UV-vis absorptionand FTIR spectra, large-area topographs and optical micros-copy images of ZnO nanocrystals, and the transfer charac-teristics of ZnO nanorod TFTs after postdeposition hydro-thermal growth without further annealing. This material isavailable free of charge via the Internet at http://pubs.acs.org.

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