Cu Doping in Ligand Free CdS Nanocrystals: Conductivity andElectronic Structure StudyG. Krishnamurthy Grandhi,† K. Swathi,§ K. S. Narayan,*,§ and Ranjani Viswanatha*,†,‡

†New Chemistry Unit, §Chemistry and Physics of Materials Unit, and ‡International Centre for Materials Science, Jawaharlal NehruCentre for Advanced Scientific Research, Jakkur, Bangalore 560064, India

*S Supporting Information

ABSTRACT: Ligand-free Cu-doped CdS nanocrystals (NCs) have been synthesized to elucidatetheir surface electronic structure. The Cu-doped ligand-free NCs unlike their undoped counterpartsare shown to be luminescent. We used this Cu-related emission as a probe to study the nature of thesurface trap states that results in negligible luminescence in the undoped NCs. The concentration ofthe sulfide ligands is shown to play a crucial role in the surface passivation of the NCs. Electricalconductivity of these NCs was also studied, and they were shown to exhibit significant conductivityof ∼10−4 S cm−1. Further we have shown that the electrical conductivity is closely correlated to thesurface charge and hence the trap states of the individual NCs have far-reaching consequences in thedevice optimization.

SECTION: Physical Processes in Nanomaterials and Nanostructures

Research on colloidal nanocrystals (NCs) has taken centerstage for the past several decades mainly due to their

potential as active materials in several applications.1−4

However, in the recent past, it has been noticed that thesenanocrystals are typically poor conductors due to the presenceof an insulating organic capping layer surrounding the NCs.5

This prevents electronic communication between the NCscreating a bottleneck in improving the efficiency of severalapplications. Wide-range of studies are underway to resolve thisproblem without affecting the optoelectronic properties of theindividual NCs that can be altered as a consequence of changedsize, shape and/or crystal structure of these materials. One ofthe first studies in this direction showed that the conductivity ofthese NCs improved by 12 orders of magnitude by replacingthe capped TOPO/TOP with small molecules like pyridine andthe 1,4-phenylenediamine as linker molecules6 or by treatingthe CdSe NC films with NaOH.7 This was later followed up byseveral experimental8 and theoretical9 groups using shortconducting inorganic moieties10−18 to improve and optimizethe conductivity and mobility of these inorganic NCs. Theseimproved conductivities opens up possibilities for applicationslike field effect transistors (FETs) with high on/off ratios,12

photodetectors with very sensitive detectivity,13 boost inefficiency of the solar cells19 and improvement in the solarpower conversion efficiency.20

However, while the exchange of organic capping layer withinorganic moieties have demonstrated excellent improvementsin interparticle communication as observed through con-ductivity and mobility, little is known regarding theconsequences of this exchange on the properties of individualNCs. For example, only recently, it was observed that the

nature of the surface of the NCs,21,22 specifically that of surfacetraps, induced decay of optical properties in the presence ofinorganic ligands. The knowledge of surface trap states is notonly important for optimizing the optical properties, but theireffect on the conductivity of NC films is also well-known.11,23

So, a simple method to study the role of these inorganic ligandsin passivating the surface and stabilizing these NCs would leadto a rational optimization of devices in a multidimensionalspace. Study of the role of passivating ligands, thoughrecognized as highly important, is relatively sparse24,25 andnot well understood across a broad range of ligands and hostmaterials. This is further complicated by the fact that the sameligand performs different functions in different host materials asshown by a recent study.26 Fortunately, with the understandingof the mechanism of Cu emission27 in II−VI semiconductors, ithas been shown in earlier literature26,28 that Cu can be used as ananoprobe to understand the role of ligands in passivating thesurface of the nanocrystals. The mechanism of Cu emission issummarized in Scheme 1. Briefly, the quantum yield (QY) ofthe Cu emission is enhanced with increasing number of thehole traps, and the average lifetime decreases with increasingnumber of electron traps. However, this technique is viable onlyif one observes the Cu emission in these NCs. It is well-knownthat in these ligand-free NCs, though the conductivity goes up,the optical response is negligible. This could be either due to

Received: May 15, 2014Accepted: June 18, 2014Published: June 18, 2014

Letter

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the presence of a large number of electron trap states or holetrap states or both.In this Letter, we discuss the synthesis and characterization of

Cu-doped ligand-free CdS and CdxZn1−xS NCs for the firsttime that also shows a weak Cu related luminescence. Furtherwe also study the role of sulfide ions and the counterions on thesurface of the NCs using ζ-potential, steady state, and time-resolved photoluminescence (TrPL) spectra. While theconductivity of a few materials like PbSe (10−2 S cm−1),10

PbS (8 × 10−4 S cm−1)10 and CdSe (10−2 S cm−1)6,7 are wellstudied, CdS NCs are largely unexplored. We have studied theconductivity of Cu-doped CdS NCs as a function of varyingsulfide ion concentration and correlate the results with thatobtained with the passivation studies. A combination of decentelectrical transport and emission properties is promising forapplications such as light emitting FETs and light emittingdiodes (LEDs). In this regard, we report the highestconductivity so far reported for CdS NC films which is

comparable to PbS10 NC films where the conductivity reaches∼10−4 S cm−1 when they are capped with shorter ligands.Ligand free CdS NCs were synthesized using CdSO4 and

(NH4)2S as precursors and formamide as the solvent. Thedetailed synthesis procedure is discussed in the MethodsSection. Similar to the earlier reported literature,29 we observethat excess sulfide is necessary to prevent the agglomeration ofthe NCs. The absorption and emission spectra show thepresence of a band gap corresponding to the CdS NCs and theabsence of band edge emission (Figure S1a, SI), as observed inearlier literature.29 ζ-potential measurements shown in FigureS1b clearly show a peak at negative voltage that is aconsequence of the negatively charged surface due to thehigh percentage of sulfide ions. The negatively charged surfacecauses electrostatic repulsion between two NCs leading tocolloidal stability of these NCs in highly polar solvents likewater, formamide, and N-methyl formamide. It is evident fromearlier literature29 and the current work that the excess sulfideions play an important role in preventing agglomeration andstabilizing these nanocrystal surfaces. However, the exactmechanism of this stabilization has not yet been understood.In order to understand that, we synthesized (discussed inMethods section) Cu-doped all-inorganic CdS NCs by adding asmall percentage of Cu salt during the synthesis. The presenceof Cu in the NCs was confirmed from elemental analysis usinginductive coupled plasma−optical emission spectroscopy (ICP-OES). These results established that the Cu dopantconcentration is about 0.5% ± 0.2% in these NCs. Detailedvalues for various syntheses are tabulated in Table S1 in theSupporting Information. Typical X-ray diffraction (XRD)pattern of the Cu doped CdS nanocrystals shown in Figure1a along with cubic and wurtzite bulk CdS patterns

Scheme 1. Mechanism of the Cu Emission and theAlignment of the Cu Dopant Level with Respect to the VBand CB of the Host CdS NCs

Figure 1. Characterization of 0.5% Cu doped ligand free CdS NCs. (a) XRD pattern of the NCs in comparison with the bulk CdS. 2θ is thediffraction angle. (b) TEM image of the NCs with an inset of the corresponding high resolution image in the (1 1 1) direction. (c) The normalizedabsorption and steady state PL (λex = 405 nm (3.06 eV)) of the NCs. The inset shows the comparison of the absorption spectrum with the PLEspectra (collected at 1.5 eV, 1.6 and 1.7 eV). (d) TrPL spectrum was collected at Cu emission (1.5 eV) with the λex of 405 nm (3.06 eV).

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demonstrates the formation of zinc-blende CdS NCs. Thetransmission electron microscopy (TEM) image shown inFigure 1b and the high resolution image of the same (inset ofFigure 1b) indicates the formation of 4−5 nm sized particleswith a broad size distribution that is not surprising in theabsence of any ligands. The broad size distribution is furtherdemonstrated by the presence of a broad absorption feature inFigure 1c. The steady state PL (Figure 1c) shows that eventhough the undoped dots were nonluminescent, the Cu dopedNCs indeed have luminescence that is very broad and highlyred-shifted compared to the band edge emission. The origin ofthis emission band has been shown in earlier litera-ture26−28,30−32 to arise from the transition of the electronfrom the conduction band (CB) edge to the Cu d state. Thelong lifetime decay plots obtained from time-dependent PLspectroscopy shown in Figure 1d as well as unchanged PLexcitation data taken at various points on the emission peak(inset to Figure 1c) further confirms that the PL is indeed fromCu emission and not from the surface trap states or due to thebroad size distribution.The most compelling evidence for the origin of this emission

peak arises from the energy tuning of the Cu related emission.Cu related emission arises from the transition of the electronfrom the CB to the Cu d level and can be tuned by eitheraltering the size of the host NCs26,28 or by modifying thecomposition of the material. Due to the ligand-free nature ofthe synthesis, it is nontrivial to control the size of these NCs.Hence we synthesized Cu-doped alloy of CdxZn1−xS NCs in asimilar ligand-free synthesis by the introduction of appropriateamount of ZnSO4 during the synthesis. ICP-OES measure-ments were performed on the purified samples, and we findthat the Zn/Cd ratio incorporated in these NCs was smallerthan their stoichiometric amount as shown in Table S2 in theSupporting Information. However, it is also evident that asubstantial percentage of Zn was incorporated into the NCs

starting from 10% to 85%. The shift in the XRD peaks (shownin Figure 2a) for the various compositions also clearlydemonstrates the formation of CdxZn1−xS alloy NCs. Figure2b shows a TEM image of Cu-doped Cd0.6Zn0.4S with an insetof a high-resolution TEM image revealing the formation ofcrystalline NCs with reasonable size distribution. The selectedarea electron diffraction shown in Supporting InformationFigure S2 further exposes the crystalline nature of the NCs. Themost important point of introducing Zn in CdS was to studythe PL characteristics of these materials and obtain the natureof this emission peak. Figure 2c shows the PL spectra of Cu-doped CdxZn1−xS NCs in formamide. The Cu-related emissionis gradually blue-shifted with increase in the Zn/Cd ratio in thealloy due to the increase in the bandgap of the material withincreasing ZnS nature.31 The corresponding lifetime data arealso shown in Figure 2d shows a long lifetime for this emissionas expected for the Cu related emission. More importantly, ithas been observed earlier that the lifetime of this Cu relatedemission in CdxZn1−xS alloy NCs decreases at higher zinccompositions.31 Here we observe that as the compositionapproaches the ZnS end, the lifetime of Cu related emissionindeed decreases, further proving that this is indeed due to theCu and not due to surface states.However, it is interesting to note that these NCs, similar to

the undoped ligand free CdS NCs29 cannot be stabilized withstoichiometric concentrations of cadmium and sulfide ions. Infact, one requires at least 4 times the sulfide ion concentrationcompared to that of cadmium to stabilize these NCs andremains stable even at higher sulphide ion concentrations.However, if this is indeed true, one would observe that thesurface is negatively charged. The charge on the NC surface canbe directly studied using the ζ-potential measurements and canhave indirect but substantial consequences on the PL intensityof the Cu related emission. The ζ-potential measurements forvarying the Cd to S ion ratio are shown in Figure 3a. Not

Figure 2. Characterization of Cu-doped ligand free alloy CdxZn1−xS NCs. (a) XRD pattern at different proportions of cadmium and zinc incomparison with the bulk data. 2θ is diffraction angle. (b) TEM image of Cu doped Cd0.6Zn0.4S and the corresponding HRTEM image along the (11 1) direction in the inset. (c) The normalized PL spectra (λex = 405 nm (3.06 eV)) corresponding to different ratios of cadmium to zinc in the alloyand (d) their corresponding TrPL spectra collected at PL emission maxima with the λex of 405 nm (3.06 eV). Here, S2−/C2+ = 6 and C2+ = Cd2+ +Zn2+.

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surprisingly, from the figure it is evident that the ζ-potentialbecomes more negative with increase in sulfide ionconcentration from 1:4 to 1:8 and that the charge on thesurface is sufficiently large to prevent agglomeration due toelectrostatic repulsion. However, as we go to higher sulfide ionconcentration of 1:12, we observe that the charge on thesurface decreases. This suggests that the nature of the surfaceturns more negative with increasing sulfide ion concentrationuntil about 1:8 and, when the surface is sufficiently passivated,instead of increasing the charge on the individual NC, forms ashielding layer.33 This shield could eventually block the contactto the NCs, thus affecting the conductivity of the material.Hence it is important to study the exact role of the excessanions in the synthesis of ligand free NCs.An interesting method to study the role of ligands on the

nature of the surface has been recently proposed26,28 by theintroduction of Cu as a nanosensor. This study correlates thesurface properties to the extent of electron and hole traps andhence predicts their consequences on the conductivity of theNCs. Briefly, the Cu-related emission originates from therecombination of the photoexcited electron with the opticallyactive hole present on the Cu site and does not require thephotoexcited hole.27 In fact, due to the long lifetime of the Curelated emission, it can only be observed in the presence of holetraps and the quantum yield (QY) increases with increase inhole traps. Similarly, in the current case, it is interesting to notethat in spite of the fact that undoped particles do not fluoresce,the Cu-doped NCs show a weak fluorescence. The presence ofCu related emission even though the band edge emission is

absent indicates that the excess sulfide ions preferentially createmore hole traps than the electron traps. This is reasonablegiven that the surface of the NCs is negatively charged andpreferentially draws the photoexcited hole toward them. Figure3b shows the variation of the Cu related emission for varioussulfide ion concentrations. It is intriguing to note that the PLpeak is slightly red-shifted with increasing anion ratio. Thiscould arise due to three leading causes or a combination ofthese causes. In the ligand free synthesis, the size of the NC isnot really controlled. So it is possible that there could be someshift due to small changes in size. In addition, it is also knownin the literature that when a negative charge present on the NCsurface, the electron or hole wave function may delocalizeoutside the NC surface.34 This leads to a small red shift in bothabsorption and PL. The third factor relates to the number ofligands or the sulfide ions in the present case attached to theNC. Typical examples in literature35 relating to CdSe with thiolligands have shown that both the absorption and PL gets red-shifted with increasing number of ligands, though the exactmechanism is not clear.However, it has to be noted that the QY of the Cu related

emission is quite low (∼6%) compared to that of oleate cappedCdS NCs (∼40%).26 This points to the fact that a large numberof electron traps are also formed during the synthesis of theseNCs. Since Cu related emission in these doped NCs requiresonly a photogenerated electron, the nonradiative component ofemission decay arises entirely due to the presence of electrontraps and hence the percentage of electron traps in Cu dopedNCs is manifested as the fast component in the lifetimedynamics of the Cu related emission. Consequently, theaverage lifetime can be used as a marker to quantify of electrontraps present in the NCs. A study of the lifetime decay of theCu related emission for various sulfide ion concentrations areshown in Figure 3c. From the figure it is apparent that theaverage lifetime increases until about 1:6 ratio of Cd to S andremains constant thereafter, suggesting that the number ofelectron traps does not decrease beyond the ratio of 1:6.A comprehensive overview of the results can be obtained by

analyzing the surface trap states as seen from TrPL and steadystate fluorescence of the Cu emission and the surface charge asobtained from ζ-potential measurements and contrasting thesame with the conductivity measurements. We observe that aminimum of 1:4 excess negative ions are needed to stabilize thesurface of the NCs. Further, with increase up to 1:6 ratio of Cdto S, we observe a decrease in the electron traps and an increasein hole traps, suggesting that the excess sulfide ions are involvedin passivating and stabilizing the surface. Additional increase inthe sulfide ion concentration results in only a slight increase inthe number of hole traps and then a decrease thereafter with nochange in the amount of electron traps. This suggests that theexcess sulfide ions are now not involved in passivating thesurface and hence forming just a negative ion layer around thesurface of the NCs. Even though the nature of the probe isindirect, the consequences of this study can be directly reflectedin the conductivity of the NCs.Figure 4a shows the I−V characteristics of the NC films with

varying sulfide ion concentration. I(V) measurements weredone using two types of injecting contacts, gold (work functionof ∼5.1 eV) and aluminum (work function of ∼4.2 eV) in theplanar geometry. The conductance in Al contact devices ismarginally higher than the Au contact devices. The electrodematerials were chosen so as to align their work functions withthe band edge positions of the NCs. For example, Al poses no

Figure 3. (a) The ζ-potential curves, (b) absorption normalized steadystate PL spectra (λex = 405 nm (3.06 eV)), and (c) TrPL spectra of0.5% Cu-doped ligand-free CdS NCs with increasing sulfideconcentration (1:4 to 1:16) were collected at Cu emission (1.4 eV)with the λex of 405 nm (3.06 eV).

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barrier for electron transport as it is aligned close to theconduction band of the NCs. This can be seen from the ohmicnature of the I(V) curves for sulfide concentrations up to 1:8.This contrasts with the super linear behavior and lower valuesof current in Au devices where it is injection limited due topossible sources such as misalignment of the electrode workfunction with the NC valence band energy level.The I(V) responses directly measure the conductance of Cu

doped CdS NCs and the variation in conductance for differentS2− to Cd2+ ratios for both types of carrier injection is shown inFigure 4b. Here it has to be noted that the conductance witheither electrodes is higher by nearly 5 orders of magnitudecompared to the oleate-capped CdS NCs (SupportingInformation, Figure S3). This is expected since the oleate-capped NCs have long chain nonconducting molecules

attached to the NC surface, which reduces the electroniccoupling between two NCs leading to lower conductance. Inaddition, though the electron and hole conductance increaseswith increase in S2− to Cd2+ ratio initially, the transportmechanisms are quite different. The presence of excess anion isknown to result in nonstoichiometric QDs and manifests as gapstates close to the valence band of the QD.36 This results in p-doping, which leads to an initial increase in hole conductance asthe sulfide ion concentration is increased. Increase in electronconductance is indicative of the decrease in the number of Cd2+

sites (electron-traps) on the NC surface with increasing anionconcentration. However, it is important to note that changingthe stoichiometry of the sulfide with respect to the cadmiumwill eventually lead to n-type behavior in the semiconductor. Asthe Fermi level is increased and the dopant is introduced, a setof extended band-tail states and deeper localized states arecreated leading to the possibility of Anderson-type transi-tion.37−40 However, the CdS (S2−) (with about 0.5% Cudoping) nanocrystals represent a class of highly disorderedsolids where both the EFermi position and the extent of disorderis controlled by the relative proportions of S2− and Cu content.As the sulfide content is increased, a spread in the nanoparticlesize distribution is observed in the TEM images. In addition, itshould be noted that the Fermi level lies in the localized regionin these systems and hence the transport is dominated byhopping between energy levels at room temperature. Eventhough the Fermi level can move closer to the mobility edge byincreasing the sulfide concentration, the electrostatic repulsiveforces between sulfide ions on neighboring QDs is expected todominate. This in turn localizes the electron wave function asthe distance between two QDs increases, leading to lowerconductance (Figure 4b). Hence it is unlikely to undergoAnderson type transition. Nevertheless, conclusive proof of thesame can be obtained by performing a temperature-dependentelectrical transport study as a function of dopant concentration.In any case, beyond a critical ratio of 1:6 (Cd:S), theconductance drops, possibly due to decreased interparticleconnectivity arising from the increased electrostatic repulsionbetween sulfide ions of neighboring QDs. Also, in the presenceof excess anions and their counterions and specifically in thepresent case of NH4

+, a virtual capping layer can be formedwith a shield of negative ions and their positive counterionsaffecting the conductance.33,41

Also shown in Figure 4b is the increase in PL QY of the Curelated emission and the average lifetime of the same,essentially reflecting the number of hole and electron traps,respectively. From the figure, we can see that the averagelifetime saturation occurs at the same anion concentration asthat of the peak of conductance indicating the decrease innumber of electron traps up to 1:6 ratio of Cd:S. Thisobservation is in concurrence with the earlier conclusionobtained from PL and I(V) data that the excess anions areinvolved in passivating the surface until a concentration of 1:6and thereafter start forming an ion shield around the NCs. Theformation of ion shield is also clearly reflected in the PL QY,though at slightly higher ratio as seen from the figure. Thiscould be reflective of the number of hole traps in the systemand hence could be slightly higher. Thus, we have shown forthe first time from these studies that the surface of the NCs canbe studied using this novel technique of Cu doping and thatthere exists a clear correlation between the surface passivationand transport properties such as conductivity.

Figure 4. (a)The I(V) characteristics of the 0.5% Cu-doped NCs withvarying sulfide ion concentrations with respect to the Al electrode andthe corresponding I−V characteristics with respect to the Au electrode(inset). The schematic of the device used for the measurement of I−Vcharacteristics is also shown in the inset. The inter electrode distancewas kept at 70 μm with the electrode width of 4 mm and the typicalsample thickness of 2 μm. (b) The corresponding variation ofconductance (black spheres and circles) and PL quantum yield (redspheres) for varying S2− to Cd2+ ratio. The variation of the averagelifetime is shown in the inset. (c) Intensity modulated spectralphotocurrent response of a sandwich device structure ITO|PEDOT:PSS|CdS|Al at two bias voltages (+10 V for Cu dopedsamples and +5 V for undoped samples) and modulation frequency of70 Hz, with active layer thickness ∼2 μm. Also shown is theabsorbance of the CdS NCs. Absorption spectrum and PL QY weremeasured in water.

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Further, in order to understand the nature of chargetransport in these NCs, we have performed photoconductivitymeasurements that could give information on the nature ofphoto-generated charge carrier transport. Thick samples wereused to ensure that the response is reflective of thephotogenerated carriers in the bulk and with minimizedinterfacial contribution. A typical photocurrent responseobtained for the device structure ITO|PEDOT−PSS|CdS|Al isshown in Figure 4c. The prominent points to note in thephotocurrent response are the following: A clear photocurrentwith large signal-to-noise ratio (of the order of 20) and aspectral response similar to the absorption profile is observed insamples without Cu. The intensity dependence is clearly linear.This proposes a predominantly n-type transport in this systemobtained from a simple estimate taking into account theabsorption coefficient, the active layer thickness and thedifference in bias polarity.42,43 In the case of Cu-doped CdS,the photocurrent response is not as significant as the undopedsystem. A spectral response of this device appears to suggest anantibatic response with a local maximum at the absorptionedge. This response is suggestive of a possible dominant-holetransport in this system.42−44 However, careful thicknessdependence studies with appropriate buffer layers and electro-des will be necessary to ascertain this fact. Present activities inour laboratory are toward this direction.In conclusion, we have studied the ligand-free or sulfide-

capped CdS as a model system to probe the role of inorganicligands in passivating the surface of NCs by copper doping.Interestingly, we observed that these Cu-doped NCs areluminescent unlike the undoped NCs and that the sulfideligands act as hole trapping agents on the NC surface. Further,we have shown that these NCs form good conducting filmswith conductivities of the order of 10−4 S cm−1. We havestudied the effect of sulfide ion concentration on the electricalproperties of these NC films and found that an optimalconcentration of ligands is required to get the best surfacepassivation that is also directly correlated with the efficiency ofthe electrical devices.

■ METHODS SECTIONMaterials. Cadmium sulfate (3CdSO4·8H2O), zinc sulfate(ZnSO4.7H2O), copper chloride (CuCl2.2H2O) and formamide(AR grade), were purchased from SD Fine chemicals limited.Acetone (AR grade) was purchased from Merck, andammonium sulfide ((NH4)2S, 40−48 wt% solution in water)was purchased from Sigma-Aldrich. All the chemicals were usedwithout any further purification.Synthesis of Cu-Doped Ligand-Free CdS and CdxZn1−xS NCs.

Cu-doped ligand-free CdS and CdxZn1−xS NCs were preparedby modified method reported in literature.29 CdSO4 andZnSO4 totaling to 0.2 mmol with various proportions of Cd2+

and Zn2+ and CuCl2 (1−6 μmol) are degassed in formamide at80 °C for 15 min. After 15 min, (NH4)2S (0.8−1.6 m moles)was added to this mixture and kept at 80 °C for 5 min to giveCu-doped CdS NCs with varying sulfide concentration in theNCs. The samples were washed with the formamide/acetonemixture and then preserved in formamide or water.Characterization. TEM was carried out using Technai F30

UHR version electron microscope, using a field emission gun(FEG) operating at an acceleration voltage of 200 kV. Samplesfor TEM were prepared by adding a solution of thenanocrystals dropwise on a carbon-coated Cu grid. Thesolution was allowed to evaporate, leaving behind the

nanocrystals. X-ray diffraction patterns for the NQDs wererecorded on Bruker D8 Advance diffractometer using Cu−Kα

radiation. Because the diffracted intensities from these nano-crystals are generally weak, all patterns were recorded at a slowscan rate (0.75° per minute) in order to get a high signal-tonoise ratio. The bulk XRD was obtained from the InorganicCrystal Structure Database. Absorption spectra of samples wererecorded using Agilent 8453 UV−visible spectrometer. Steadystate PL spectra were obtained using a 450 W xenon lamp asthe source on the FLSP920 spectrometer, Edinburgh instru-ment, while the lifetime measurements were carried out usingthe EPL-405 ps pulsed diode laser as the excitation source (λex= 405 nm). The absolute PL QY was determined using anintegrating sphere for the samples. The relative concentrationsof Cu, Cd, and Zn in the doped nanocrystals were determinedusing inductively coupled plasma optical emission spectroscopy(ICP-OES). Nanocrystals were washed several times withacetone/formamide mixture to remove excess precursors andthen digested in concentrated HNO3 and diluted with Milliporewater. ICP-OES was carried out using a Perkin−Elmer Optima7000 DV machine. ζ-Potential measurements were carried outusing a Nano ZS (Malvern, UK) employing a 532 nm laser at aback scattering angle of 173°. ζ-Potential measurements weredone in both water and formamide. For the planar devicefabrication, aluminum/gold (with 5 nm chromium adhesionlayer) were coated as electrodes by the standard physical vapordeposition technique at a base pressure of 10−6 mbar onprecleaned glass substrates. The resulting thickness and widthof the electrodes are 100 nm and 4 mm, respectively, separatedby a channel of 70 μm length. For the sandwich structure, a 50nm PEDOT: PSS layer was spin-coated on precleaned ITO-coated glass substrates at 4000 rpm for 1 min and annealed at110 °C for 15 min. In both the device structures, the solutioncontaining quantum dots dispersed in formamide was thendrop casted and heated on a hot plate at 80 °C for 1 h toevaporate the solvent. The sample thickness was maintained at2 μm with a variation of ±0.5 μm between samples. I(V) wasobtained by studying more than five samples for each ratio ofS2−/Cd2+ QDs. I(V) measurements for all the devices weredone in an inert atmosphere in dark using a 4200 semi-conductor parameter analyzer. Photocurrent measurementswere performed using the setup described in the literature.44

■ ASSOCIATED CONTENT

*S Supporting InformationThe absorption spectrum, PL spectrum, and ζ-potential curvefor the undoped CdS, the selected area electron diffractionpattern for the Cu doped ligand free Cd0.6Zn0.4S NCs, the I−Vcurve for the oleate capped CdS NCs, and the elementalanalysis (ICP-OES) data for the ligand-free Cu-doped CdSwith varying sulfide concentration. This material is availablefree of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Authors*E-mail: rv@jncasr.ac.in.*E-mail: narayan@jncasr.ac.in.

NotesThe authors declare no competing financial interest.

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■ ACKNOWLEDGMENTSWe thank JNCASR, Sheikh Saqr Laboratory, and theDepartment of Science and Technology, Government ofIndia, for financial support. G.K.G. thanks CSIR for a researchFellowship. We thank Mr. Krishnachary Salikolimi andProfessor M. Eswaramoorthy for ζ-potential measurements.

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