electronic transport of n-type cdse quantum dot ﬁlms effect

8/13/2019 Electronic transport of n-type CdSe quantum dot lms Effect

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Electronic transport of n-type CdSe quantum dot films: Effectof film treatment

Dong Yu, Brian L. Wehrenberg, Praket Jha, Jiasen Ma, and Philippe Guyot-Sionnesta

James Frank Institute, University of Chicago, 5640 S. Ellis Avenue, Chicago, Illinois 60637

Received 28 September 2005; accepted 8 March 2006; published online 31 May 2006

Solid films of CdSe colloidal quantum dots are conductive when reduced by electrochemistry. The

dark conductivity and the photoconductivity of these n-type CdSe nanocrystalline solids have beeninvestigated as a function of film treatment. When the films are treated with NaOH, a carriermobility of102 cm2/ V s and a charging/discharging time of10 ms are obtained. Under anexcitation intensity of0.75 mW/ cm2, a significant photoconductivity of103 S cm1 has beenobserved. The high mobility allowed demonstrating the operation of a field effect transistor. 2006American Institute of Physics. DOI:10.1063/1.2192288

I. INTRODUCTION

Semiconductor nanocrystals,1,2 with tunable electronicand optical properties, have shown promises for applicationsin biological imaging,3,4 light emitting devices,5,6 quantum

dots lasers,79

photodetectors,10

and solar cells.11,12

Some of the envisioned optoelectronic device applica-tions using the nanocrystals are based on solid state materialswhere the nanocrystals are assembled into close-packednanocrystalline solids. Although the optical properties ofnanocrystals have been extensively studied, the electronicproperties of these nanocrystalline solids are not as muchstudied. One major reason has been their very poor conduc-tivity. The individual nanocrystals in the solid are indeedseparated by a layer of organic ligands which significantlyinhibit charge transport. For example, for CdSe dots cappedby trioctylphosphine oxide/trioctylphosphine, the separationbetween the dots is reportedly 11 .13 The conductivity of

the nanocrystal thin films is then14 as low as 1014 S cm1.This extremely low conductivity would seem to preclude theuse of such solids in any device requiring some measure ofelectronic transport. While intrinsic nanocrystal films are ex-tremely resistive, it is now known15 that reduction using po-tassium or electrochemistry leads to many orders of magni-tude increase in conductivity. The quantized level structureof the quantum dots also shows up in the conductance with amodulated conductance and mobility as a function of fillingof the quantum states. Furthermore, by treating the films withvarious molecular cross linkers of different lengths and con-

jugation, it was possible to significantly improve the conduc-tivity up to 102 S cm1. Annealing of the nanocrystallinesolids can also reduce the distance between the nanocrystalsand improve the conductivity although the quantized levelstructure might be lost in the process.16 Recently it was re-ported that treating intrinsic films of quantum dots with basessuch as NaOH and 1-butylamine led to orders of magnitudeimprovement of the low voltage photoconductivity.17 Wepresent here a study of how various treatments affect thedark conductivity and photoconductivity of the reduced

films. Also, we report on the demonstration of a field effecttransistor based upon an optimized film preparation giving ahigh mobility.

A. Film treatment and mobility

In the following experiments, CdSe nanocrystals withdiameter 5.3 nm were used. They are synthesized withdimethylcadmium as an organometallic precursor, followingthe literature.18 The nanocrystal thin films are made on aglass substrate previously patterned with interdigitated Ptelectrode. The electrodes consist of 50 pairs of 5 m wide,5 mm long, and 110 nm thick Pt pads with 5 m gap. Thesesubstrates are made by Abtech Scientific. To bind the nano-crystal film to the substrate, the clean substrate is first dippedinto 1% volume ratio 3-aminopropyltriethoxysilane APSacetone solution and then 1% volume ratio 1,6-hexanedithiol HDT acetone solution. The silane end of

APS should bind to the glass with the amino end providing abinding site for the nanocrystals. Similarly, the dithiol is usedto provide a good affinity between the Pt electrodes and thenanocrystals. Without these surface treatments, the nanocrys-tal films occasionally falloff the substrate when dipped intosubsequent solutions. The film itself is made by simply dropcasting a hexane:octane 10:1 solution of the nanocrystalson the substrate. Immediately after drying, the film is thendipped into a cross-linking solution for treatment. The effectof this treatment is to render the films insoluble in their origi-nal solvent and to vastly improve the electrochemical19 andconducting properties. It also leads to shrinkage of the films,consistent with the expected shorter distance between dots.

For example, a heptanediamine linker should reduce the in-terdot distance to8 . While molecules were initially cho-sen to have two binding sites so that they would a priorichemically bond the nanocrystals,19 it has, since then, beenrealized that this is not necessary17 as any treatment that canreplace the original capping molecules with much shortergroups will increase the particle-particle interaction and leadto insoluble films.

The solutions used for cross linking are all in metha-nol which is a solvent for the original caps trioctylphosphine/trioctylphosphine oxide TOP/TOPO. We stud-aElectronic mail: [email protected]

JOURNAL OF APPLIED PHYSICS 99, 104315 2006

0021-8979/2006/9910/104315/7/$23.00 2006 American Institute of Physics99, 104315-1
http://dx.doi.org/10.1063/1.2192288http://dx.doi.org/10.1063/1.2192288http://dx.doi.org/10.1063/1.2192288http://dx.doi.org/10.1063/1.2192288http://dx.doi.org/10.1063/1.2192288http://dx.doi.org/10.1063/1.2192288http://dx.doi.org/10.1063/1.2192288http://dx.doi.org/10.1063/1.2192288


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ied pure methanol, or methanol with 1% volume ratio1,7-heptanediamine, 1% volume ratio 1-butylamine, and0.08MNaOH methanol solution. For all solutions the dip-ping time was kept at 10 min and the treated films werethen baked at 100 C under vacuum for 1 h. This step

removes the water and accelerates the recapping. Subse-quently, the films are moved into a N2 glovebox and rinsedwith methanol to remove the extra ligands. After a seconddrying step at 70 C under N2for 10 min, the films areassembled in the electrochemical cells. With atomic forcemicroscopyAFM, the close packed films look smooth andthe roughness is about the diameter the nanocrystalsFig. 1.The thickness of the films is usually in the range of tens ofmonolayers, monitored by AFM or simply by the opticaldensity. The cyclic voltammetry is taken at 60 C and theoccupation number is monitored by measuring the spectralbleaches of the Sand Pexcitonic transition. Figure 2 showsthe typical cyclic voltammetry of TOPO/TOP capped CdSenanocrystal films with different treatments.

The sample treated with pure methanol shows a largehysteresis in the optical bleach indicating a slow charging/discharging rate and the film remains insulating Fig. 2a.Therefore, although cross linking with pure methanol is suf-ficiently effective to lead to films that present an electrochro-mic response, the response is slow. This is presumably be-cause methanol removes capping molecules withoutreplacing them by a ligand that passivates the exposed Cdsurface sites. This would lead to a large number of trappingsites which also lower the conductivity below the detectablerange.

As reported previously,15 the film treated with 1,7-heptanediamine, for which the amine is a strong ligand to theCd sites, shows a much faster and reversible optical responsewhile the conductance improves drastically with a shoulderappearing at about the half filling of the 1Seshell and the firstwave on the reduction or oxidation curve Fig. 2b.

Although 1-butylamine is monodendate, it leads to fur-ther improvements in the system response,17 and the filmsexhibit fast and reversible electrochemistry while they areabout 100-fold more conductive than with heptanediamine.As observed so far with the more conductive films, the 1Seconductance shoulder is present but less pronounced Fig.2c.

The OH base can also passivate the Cd surface sites,facilitating the removal of the original ligands and presum-ably forming cadmium hydroxide complexes. It is the short-est ligand such that the nanocrystals can get to their closestapproach. In contrast to the amine treated films, the filmtreated with NaOH are as conductive but cannot be charged

completely. The 1Soptical bleach is smaller than 0.005 op-tical density OD, indicating less than 10% of the possiblecharge in the 1Se state, and the 1P bleach is not seen. Thecomplete electrochemical doping of the whole film requireseasy flow of the counterions throughout the film network, inconcert with electron transfer to the nanocrystal films. Sincethe electrochemical current is small Fig. 2d for thesefilms, it suggests that the counterions do not diffuse easily inthese films, possibly because they are too compact. The vis-ible absorption spectra of the nanocrystal thin film afterNaOH treatment still show the excitonic features, thereforethe dots are still distinct Fig. 3a. No obvious change inoptical density further indicates that the nanocrystals are not

FIG. 1. Atomic force microscopy AFMof a typical CdSe nanocrystal thinfilm treated with 1,7-heptanediamine. The size of the picture is 3 m. Thelower plot is the cross section in the middle of the top picture.

FIG. 2. Cyclic voltammetry of TOP/TOPO capped CdSe nanocrystal filmsof5.3 nm diameter with different treatments at 60 C. Solid lines rep-resent conduction currents, dashed lines electrochemical currents, dots 1Sbleaches, and triangles 1P bleach. The first three films estimated by theirmaximum 1Sbleaches are about 12, 15, and 28 layers. The last one issimilar in thickness judged by the optical density before doping. aTreatedwith pure methanol: scan rate of10 mV/S and bias of40 mV. bTreated with 1,7-heptanediamine in methanol solution: scan rate of20 mV/S and bias of5 mV.cTreated with 1-butylamine in methanolsolution: scan rate of10 mV/S and bias10 mV.dTreated with NaOHin methanol solution: scan rate of50 mV/S and bias5 mV.

104315-2 Yu et al. J. Appl. Phys.99, 104315 2006


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significantly washed off by NaOH treatment. The first exci-tonic peak redshifts by about 11 nm. This redshift is a pos-

sible indication of increased interaction between the nano-crystals. Infrared spectra of the hydrocarbon stretching bandsshow that their optical density in the infrared spectrum isreduced by 60% after treatment Fig. 3b. The AFMshows a 30%40% shrinking of the film thickness while thesurface of the film remains smooth Fig. 3c. These evi-dences all concur to show that a significant part of cappingmolecules is removed and support the notion of a very com-pact film.

Temperature dependent conductivity provides furtherevidence of the increased coupling between nanocrystals inthese NaOH treated films. The n-type CdSe nanocrystal thinfilm treated with NaOH follows variable range hopping

VRH theory20

as well as all the samples prepared here.However the T1/2 conductance gives a temperature scaling ofT*2000 K Fig. 4 which is about 2.5 times smaller thanfor heptane diamine treated film. In the VRH model, T* isinversely related to the static dielectric constant and the lo-calization length. A dielectric constant of4 was used todescribe the VRH behavior in a heptanediamine treatedfilm.20 The films treated with NaOH are denser and shouldhave a higher dielectric constant, closer to bulk value of10.2. In addition, T* may also be reduced by a larger lo-calization length of the electron wave function. Whichever ofthese two effects dominate, the strongly reduced T* is indica-tive of a stronger coupling between the nanocrystals.

The charging and discharging of the NaOH treated filmsare very fast. As the electrochemical potential is steppedfrom a potential where the film is insulating to about 0.2 Vabove where the film starts to conduct, both the conductanceand electrochemical current change abruptly Fig. 5.

This behavior is rather well captured by a simple RCcircuit model of the electrochemical gating switching. Theconductance is proportional to the mobility and the chargeQ. It should follow thatG =Q1expt/during charg-ing and G =Qexpt/during discharging, where is theRCswitching time of the gate and film. The charging currentshould also follow I= Q/expt/ with an opposite signfor discharging. Focusing on the first 100 ms after the poten-tial jump, the conductance and the electrochemical currentare fitted by exponentials Fig. 6. The time constants are allaround 10 ms, slightly slower for discharging than chargingand slightly slower for the conductance change than the elec-trochemical current change. The mobility is estimated fromthe conductance and the integrated charge, given by = l2G/Q, where the channel length l is 5 m. For the NaOHtreated film, the mobility is then measured to be 102 cm2/V s. This value is not inconsistent with the esti-mation based upon the cyclic voltammetry Fig. 2dgiven

FIG. 3. The effect of NaOH treatment on the CdSe nanocrystal films of5.3 nm diameter. Solid lines are before treatment and dashed lines areafter treatment with 0.08MNaOH methanol solution. a UV-visible ab-sorption spectrum shows that the first exciton peak redshifts of11 nmwhile the optical density remains the same. bInfrared absorption spectrumindicates a 60% reduction of the optical density at hydrocarbon stretchingbands.cAFM of a thin film on glass before and after treatment. The cut ismade by a razor blade on the right of the graph. The surface of the filmremains smooth while the thickness shrinks by 30% 40%. dA cartoonof how the NaOH treatment may remove the capping molecules and shortenthe interdot distance. FIG. 4. Temperature and electrical field dependent conductances of a CdSe

nanocrystal thin film with diameter of 5.3 nm treated with NaOH. ACon-ductance measured at 0.1 V bias as a function of temperature. The linearfitting gives T*=2.0103 K. B Conductance as a function of electricalfield at different temperatures. The straight line corresponds to E* =6.4107 V/m.



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the poor definition of the electrochemical current, and theabsence of independent confirmation of the charge occupa-tion by the optical bleach data.

For a field effect transistor FET,

21

there is a relationbetween the mobility and the cutoff of the frequency re-sponse3 dB point f, given by f= VG VT/2l

2, whereVG is the gate voltage and VT is threshold voltage. In ourelectrochemical experiment, VG VTis estimated to be about0.2 V. With 102 cm2/V s the cutoff frequency is thenf1.3 kHz corresponding to a switching time constant2f1 of0.12 ms. We note that this is much faster thanthe measured 10 ms. This discrepancy is attributed to thecharging time which is more likely limited by the slowercounterion diffusion time through the film.

With the same pulsed response method, the mobility and

the charging/discharging time constants of CdSe nanocrystalthin films treated with 1,7-heptanediamine and 1-butylaminewere also determined. The result is summarized in Fig. 7a.The mobility and the switching time vary by orders of mag-nitude with different treatments. Only the 1,7-heptanediamine treated sample shows a time constant close to thecalculation based upon the FET relation. The charging timefor films with higher mobility is slower than the limit. As forthe NaOH treated films, we attribute this deviation to theslower diffusion rate of the counterions. This is an intrinsicproblem for an electrochemical cell. To achieve faster re-sponse, all solid state devices such as FET are necessary.

B. Photoconductivity

Photoconductivity has been observed previously in in-trinsic semiconductor nanocrystalline solids22,23 but it has notbeen studied in an n-type system. Since reduced nanocrystalsexhibit vastly increased mobilities, one would expect similarimprovements in the photoconductivity.

To investigate the photoconductivity of the films, we

used the same film preparation on the interdigitated elec-

FIG. 5. Measurement of the response of CdSe nanocrystal thin film treatedwith NaOH during charging and discharging at 60 C. aThe electro-chemical potential is stepped from 0.4 to 1 V. b The conductance ofthe film is measured with a 5 mV bias between two working electrodes. cThe electrochemical current pulses.

FIG. 6. Focus of Fig. 5 around the potential jump. The dashed lines are dataand the solid lines are fitting.aThe charging of the film: the conductanceis fitted by G = G0 1expt/1 with fitting parameters G 0 =4.1 mS and1=16.8 ms; the electrochemical current is fitted by I=I0expt/2+Ibwith I0=9.4 A, 2=9.4 ms, and Ib =1.1 A. b The discharging of thefilm: the conductance is fitted by G =G0expt/1+ Gb with G0=5.4 mS, 1=12.6 ms, and Gb=0.17 mS; the electrochemical current isfitted by I=I0exp t/2+Ib with I0=10 A, 2=8.3 ms, and Ib=0.8 A.

FIG. 7. a The charging time vs mobility for the CdSe nanocrystal thinfilms treated with 1,7-heptanedamine circles, 1-butylaminetriangles, andNaOHsquares. The solid line is = l2/VG VTwith the channel lengthl= 5 m and VG VT=0.2 V. b The photoconductivity vs mobility. Thesolid line corresponds to = enwith n0.01e/dot.



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trodes as presented above. The light source is an incandes-

cent light bulb controlled by a dc power supply. The lightbulb is placed 10 cm away from sample. The light is filteredby a bandpass filter covering 560630 nm, and the excita-tion intensity is measured to be 0.75 mW/ cm2.

We first discuss films treated with 1, 7-heptanediamine.With an applied electrical field of103 V /m, correspondingto a 5 mV bias across the 5 m electrode, the photoconduc-tivity of the insulating intrinsic films is not observable in oursetup. When the films are slightly reduced, the illuminationintroduces an increase in conductivity of the order of 108 S cm1. Assuming the electrons are the majority car-riers and using a mobility of106 cm2/V s as deduced ear-

lier, the electron density is increased by n =/e1017 cm3 or0.01e/dot. From the light intensity and thenanocrystals cross section24 of 31015 cm2, the generationrate G should be 7 excitons/dot s. Thus, the carrier musthave a lifetime =n/G103 s. Similarly long lived delo-calized electron states after photoexcitation have been ob-served in the CdSe nanocrystals capped with p-thiocresoland were attributed to efficient hole traps.25 Although herethe nanocrystals are not capped with thiols, the reducing en-vironment should result in efficient hole traps.

The higher mobility films treated with 1-butylamine andNaOH show much more pronounced photoconductivity. Fig-ure 8 demonstrates the dark conductivity and photoconduc-tivity at different potentials for a film treated by NaOH.Jarosz et al. reported that the intrinsic film treated withNaOH shows17 a photoconductivity of108 S cm1 with anexcitation power of 14 mW/cm2 and an electrical field of4107 V/m. The photoconductivity of the n-type film alsotreated with NaOH is five orders of magnitude larger with103 S cm1 under a much weaker excitation intensityonly 0.75 mW/ cm2and with a much lower electrical fieldof 103 V/m. The difference is because the electron traps,which significantly lower the mobility, are filled by the elec-trochemical doping. The improved photoconductivity offilms with different treatments is summarized in Fig. 7b.

Recombination rates are slowed at low temperature, suchthat the ratio of the photoconductivity to the dark conductiv-ity should increase. This is indeed what an initial set of ex-periments has revealed. To measure the photoconductivity atlow temperature, we made an electrochemical cell that re-mained transparent after freezing. The interdigitated Pt elec-trode is covered by a thin glass slide with a 10 m spacer.It is vertically inserted in a reservoir of the tetrabutylamo-nium perchlorate/propylene carbonate TBAP/PC electro-

lyte solution, which rises into the gap between the electrodeand the glass slide because of the capillary force. At lowtemperature the frozen electrolyte solution in the small gapmakes a transparent glass. On the other hand, the spacing isstill large enough to allow the diffusion of the electrolytesuch that one is still able to perform electrochemistry andcharge the nanocrystals before freezing. To illuminate thesample cell inside the dewar, a small light bulb Magnite3 V0.2 A with measured intensity at the sample of30 mW/cm2 is wrapped around this electrochemical cellwith Teflon tape and the whole cell is inserted into a liquidhelium tank.

Figure 9 shows the photoconductivity of a film treated

FIG. 8. Photoconductivity of NaOH treated CdSe nanocrystal thin film ofdiameter of5.3 nm after excitation at different electrochemical potentials.The light source is a 25 W filament with a filter of 10 cm away from thesample. The light intensity at the sample is 0.75 W/cm2 with wave-lengths of 560630 nm. Circles stand for dark conductivity and squares forconductivity increase after excitation.

FIG. 9. Photoconductivity of the n-doped CdSe nanocrystal thin film at lowtemperatures. a IV curves of the thin film with a fairly high electronicdensity.bIVcurves of the same film with a low electronic density.canddThe plots of a and b in half log scale, respectively. Solid lines aredark current and dashed lines are photocurrent excited with a 3 V 0.2 Alight bulb. The curves with circles are taken at 4.3 K, squares at 10 K, andtriangles at 30 K.eand fTime trace of the normalized conductance of

the film with a low electronic density. The photoinduced conductance re-sponse is at least as fast as the 100 ms response time of the light bulb.



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with NaOH at low temperatures. The ratio of the photocon-ductivity to the dark conductivity is indeed much improvedat the lower temperatures. At 4.3 K, the conductance of thefilm with a low electronic density increases by four orders ofmagnitudeFig. 9dupon illumination.

In summary, n doping allows large increases in mobilityand corresponding increase in photoconductivity while lowertemperatures improve the ratio of the photoconductance to

the dark conductance.

C. Semiconductor nanocrystalline solid field effecttransistor

Electrochemical gating allowed studies of charging withwell-defined potentials that can be to a good accuracy re-ported to an absolute scale, but it certainly does not presentthe convenience of solid state gating. Solid state FET has thegreat advantage of not requiring an electrolyte, it is easilycompatible with a sealed or vacuum environment, the Fermilevel can be adjusted at any temperature, and it does notsuffer from speed limitation associated with the motion ofbulky counterions. However, doping nanocrystals by thesolid state gate method is quite difficult. The electrical fieldrequired to singly charge only one monolayer of nanocrystalsis so large E=/0410

8 V/m if taking SiO2 as thedielectric that it can easily break down the dielectric mate-rial. Given such a small charge density, a high mobility isnecessary. Previous work showed that a polycrystalline CdSethin film FET could be made after sintering the nanocrystalsthin film at high temperature.26 Here, with NaOH treatment,the CdSe nanocrystal thin films have a large enough mobilitythat we have been able to detect conduction in a solid stateFET while the films retain the quantum dot integrity.

Our FET device is made by spin coating a polymethyl-

methacrylate PMMA layer

27

on top of the NaOH treatednanocrystal thin film, then evaporating a gold layer as a gateelectrode. The thickness of PMMA layer is 1 m and it isbaked at 170 C for 30 min under N2 to prevent the oxida-tion of nanocrystals. All the measurements are performed atroom temperature and in a dark N2chamber.

Figure 10 shows the current-voltage characteristics ofthe device. As the gate voltage is set at +10 V, source-drainconductance is up to 0.5 nS Fig. 10b. The interfacecharge density is estimated to be q310

8 C/cm2 usingq =0V/L with a PMMA dielectric constant =4 and athicknessL =1 m. Considering a monolayer of nanocrystalswith 5 nm diameter as an n-type channel, this surface

charge density corresponds to at most 0.05e/dot. Althoughthis is a small degree of charging, the photoconductivity ex-periments described above had showed that even 0.01e/dot can change the conductance by orders of magnitude. Inthis FET device, the mobility is given by =Gl/wq, whereG is the conductance, l is the channel length, and w is thechannel width. Using the estimated q from above, the mo-bility is measured to be very small 2107 cm2/V s, con-siderably smaller than 102 cm2/ V s obtained for the elec-trochemically doped film. This discrepancy can arise from anumber of reasons, such as defects introduced in the devicepreparation, but we believe that it is mostly because many ofthe gate charges fill trap states, such that the fraction of q

that goes into the nanocrystal quantum states is actuallymuch smaller than 1. We verified that a negative gate voltageonly reduces the conductance indicating that p-type conduct-ing channel is not achieved in this device. We also verifiedthat the gate to sourcedraincurrent is small and it can be inpart accounted by the 0.1 nF capacitance caused by thePMMA layer Fig. 10c.

This nanocrystal film FET is different from the conven-tional devices by its memory effect. The source-drain con-ductance is steadily decaying after a gate voltage is applied.This effect can be observed from the hysteresis of the source-drain current Fig. 10b and is more pronounced when amuch higher gate voltage up to 60 V is applied Fig. 11.

With a 60 V gate voltage, the source-drain conductance is upto 20 nS but the hysteresis is also bigger Figs. 11aand11b. The capacitive current remains much smaller than thesource-drain currentFig. 11c. After a large gate voltage isapplied, the threshold voltage above which the film conductsis increased Fig. 11a and the system remembers forhours. Memory effects have been observed for nanocrystalensemble28 and carbon nanotube FET.29 Presumably, thelarge fraction of the charge that does not get into the quan-tum states is instead taken up by traps. These traps areweakly coupled to source and drain, and this leads to a largehysteresis and the memoryeffect. When and if well con-trolled these effects could potentially be used for memory

FIG. 10. Current-voltage characteristics of a CdSe nanocrystal FET device.aDrain-source current as a function of the drain-source voltage for severalvalues of the gate-source voltage. The scan rate is 0.5 V/s and the measure-ment is started right after the gate voltage is applied. The amplifier saturatesat 2 nA.bSource-drain voltage is fixed at 4 V and the gate-drain voltageis scanned 0.1 V/s. c With drain electrode open, gate-source capacitivecurrent as a function of the gate-source voltage. The scan rate is 0.5 V/s.



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applications. We note that the FET could not operate in thep-type regime. This is consistent with the fact that CdSecannot be charged positive with holes stably injected, in con-trast to PbSe.30 This is presumably because, at the surface ofthe CdSe nanocrystals, hole trapping is much more effectivethan electron trapping, and this may be explained by therather oxidizing potentials required for hole injection.

Although the proof of concept of a nanocrystal thin filmFET is therefore given here, the effective mobility achievedare still quite low 2107 cm2/ V s. Higher mobility andbetter device performance should be expected by improvingthe sample preparation, controlling better the nature of thetraps, optimizing the gate structure, or by choosing other

materials with higher mobility, for example PbSe.31

II. CONCLUSION

The post-treatment of drop-cast quantum dot films playsan essential role in their electro-optical properties. With bu-tylamine, rather complete electrochemistry, with reversiblecharging of both S and P states, and good mobilities areobserved. With a NaOH treatment, n-type CdSe nanocrystalthin films show the highest mobility 102 cm2/V s, ob-served at rather low charging level. By stepping the electro-chemical potential, the conduction channel can be turned onand off with a switching time of10 ms. The n-type films

show much improved photoconductivity compared to the in-trinsic films, consistent with the high mobilities that areachieved. Finally, based on this treatment, a CdSe nanocrys-tal thin film field effect transistor is demonstrated. Optoelec-tronic devices based on quantum dot thin films with opti-mized postcasting film treatment will be able to provideimproved performance such as higher mobility and fasterswitching time, while still retaining the unique properties of

nanocrystals.

ACKNOWLEDGMENTS

One of the authors D.Y.was supported by the Univer-sity of ChicagoArgonne National Laboratory Consortiumfor Nanoscience Research. Another authorB.L.W.was sup-ported by the U.S. National Science Foundation NSFunderGrant No. DMR-0407624, and the work was supported bythe University of Chicago MRSEC NSF-DMR under GrantNo. DMR-0213745.

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FIG. 11. Current-voltage characteristics of a CdSe nanocrystal FET deviceat high gate voltage. a Drain-source current as a function of the drain-source voltage for several values of the gate-source voltage. The scan rate is1.5 V/s and the measurement is started right after the gate voltage is ap-plied.bSource-drain voltage is fixed at 4 V and the gate-drain voltage isscanned 1.5 V/s. cWith drain electrode open, gate-source capacitive cur-rent as a function of the gate-source voltage. The scan rate is also 1.5 V/s.


electronic transport of n-type cdse quantum dot ﬁlms effect

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