Photophysical and Redox Properties of Molecule-like CdSe

Apr 26, 2013 - Department of Chemistry and Biochemistry, University of Mississippi, Oxford, Mississippi 38677, United States. •S Supporting Informat...
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Photophysical and Redox Properties of Molecule-like CdSe Nanoclusters Sukanta Dolai,† Amala Dass,‡ and Rajesh Sardar*,† †

Department of Chemistry and Chemical Biology, Indiana University-Purdue University Indianapolis, 402 North Blackford Street, Indianapolis, Indiana 46202, United States ‡ Department of Chemistry and Biochemistry, University of Mississippi, Oxford, Mississippi 38677, United States S Supporting Information *

ABSTRACT: Advancing our understanding of the photophysical and electrochemical properties of semiconductor nanoclusters with a molecule-like HOMO−LUMO energy level will help lead to their application in photovoltaic devices and photocatalysts. Here we describe an approach to the synthesis and isolation of molecule-like CdSe nanoclusters, which displayed sharp transitions at 347 nm (3.57 eV) and 362 nm (3.43 eV) in the optical spectrum with a lower energy band extinction coefficient of ∼121 000 M−1 cm−1. Mass spectrometry showed a single nanocluster molecular weight of 8502. From this mass and various spectroscopic analyses, the nanoclusters are determined to be of the single molecular composition Cd34Se20(SPh)28, which is a new nonstiochiometric nanocluster. Their reversible electrochemical band gap determined in Bu4NPF6/CH3CN was found to be 4.0 V. There was a 0.57 eV Coulombic interaction energy of the electron−hole pair involved. The scan rate dependent electrochemistry suggested diffusion-limited transport of nanoclusters to the electrode. The nanocluster diffusion coefficient (D = 5.4 × 10 −4 cm2/s) in acetonitrile solution was determined from cyclic voltammetry, which suggested Cd34Se20(SPh)28 acts as a multielectron donor or acceptor. We also present a working model of the energy level structure of the newly discovered nanocluster based on its photophysical and redox properties.

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dots and molecular clusters23 with their electronic structure organized into discrete energy levels with HOMO−LUMO energy gaps.23 Despite the immense interest, solution-phase electrochemistry of CdE nanoclusters has been reported to be difficult to perform due to their limited solubility properties in nonaqueous solvent.24−27 Moreover, highly purified and monodispersed nanoclusters are required for such analysis.24−27 In addition, the rate of charge transfer of nanoclusters with well-defined core composition is still not fully understood. These studies are crucial and can provide valuable structural− physicochemical relationships, which allow the fabrication of advanced photovoltaic devices and reversible photocatalysts.28−31 In this paper, we report an efficient synthesis of thiophenolate ligand-protected molecule-like CdSe nanoclusters via a single-source precursor method. The isolated nanoclusters displayed two distinct absorption peaks at 347 and 362 nm and a broad emission band centered at 604 nm covering the entire visible spectrum. Matrix-assisted laser desorption ionization (MALDI) mass spectrometry revealed that the molecular weight of the nanoclusters was 8502, and in conjunction with other spectroscopic characterizations, we have determined their molecular formula to be Cd34Se20(SPh)28. To the best of our knowledge, this is the first example in which the

INTRODUCTION In recent years, significant attention has been given to the preparation and isolation of atomically precise, thermodynamically stable ultrasmall (“magic size”) nanoclusters of metal chalcogenides (CdE; E = S, Se, Te).1−6 These single-sized nanoclusters display unique photophysical7 and physicochemical5,8 properties, and the understanding of such properties is essential for designing advanced optoelectronic and photovoltaic devices.1−3,9−11 Additionally, magic-sized nanoclusters have been used as building blocks for synthesis of one- or twodimensional nanostructures with interesting optical properties.12−17 Despite their wide range of applications, synthesis and isolation of CdE nanoclusters with well-defined chemical composition has remained a major challenge in nanotechnology research. Besides a few single-crystal X-ray diffraction characterizations of thiolate-stabilized CdS nanoclusters,18,19 mass spectrometry20,21 or solution-phase molecular weight measurement techniques6,21 were used to determine the chemical composition. However, most of the reports describing the atomic composition of CdE nanoclusters are based on their unique optical features,1,4,5,12,14−17,21,22 whereas direct correlation of optical and electrochemical properties of nanoclusters with precise atomic composition has never been reported. Therefore, understanding the structural and redox properties of the nanoclusters remains elusive. Semiconductor nanoclusters with an average diameter between 1.0 and 2.5 nm bridge the gap between quantum © XXXX American Chemical Society

Received: April 16, 2013

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Figure 1. (A) Absorption spectra of CdSe nanoclusters in CH3CN (blue), PhCN (red), and DMF (black) and CdSe precursor in CH3CN (green). The peak intensity was adjusted by varying the concentration for clarity. (B) Photoluminescence spectra at equal concentrations of CdSe nanoclusters (blue) and CdSe precursor (green) in CH3CN at 310 nm excitation. All spectra were acquired at room temperature. The CdSe nanoclusters were precipitated out from PhCN by addition of hexane. The precipitate was then collected by centrifugation and washed with hexane. The procedure of dissolving the yellow solid in PhCN followed by centrifugation and a hexane precipitation step was repeated at least five times. The final yellow solid was then dried under vacuum (yield 0.07−0.08 g). Mass Spectrometry. MALDI-TOF mass spectra were acquired using a Bruker Autoflex equipped with a nitrogen laser. DCTB2 was used as the matrix with a ∼1:1000 analyte:matrix ratio. A 5 μL volume of the mixture was applied to the target and air-dried. Electrochemistry. Voltammetry was done with a CH Instruments (Austin, TX) model 760D electrochemical analyzer. Solutions were typically degassed before use and contained 0.1 M Bu4NPF6 and 0.05 mM CdSe nanoclusters in CH3CN. The setup was constructed of a 1.6 mm diameter Pt disk working electrode, a Pt wire counter electrode, and a 0.6 mm diameter Ag wire quasi-reference electrode (QRE). The Pt working electrode was polished with 0.05 μm alumina (Buehler) and cleaned electrochemically by potential cycling in 0.1 M H2SO4. For differential pulse voltammetry, the experimental parameters were potential scan rate 10 mV/s, pulse amplitude 50 mV, pulse width 50 ms, pulse period 200 ms, and sample width 16 ms.

synthesis, isolation, and characterization of Cd-rich stable nanoclusters is reported. The electrochemical band gap of the nanoclusters was determined from cyclic voltammetric (CV) and differential pulse voltammetric (DPV) techniques and found to be 3.96 and 4.0 V, respectively. The large electrochemical band spacing is consistent with a moleculelike HOMO−LUMO energy structure. The electrochemical data also revealed diffusion-controlled transport of the nanoclusters to the electrode, and they behaved as multielectron donors or acceptors.

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EXPERIMENTAL SECTION

Chemicals. Cadmium nitrate tetrahydrate (Cd(NO3)2·4H2O; >98%), tetramethylammonium chloride (Me4NCl; 98%), triethylamine (99%), thiophenol (97%), selenium metal (99.99%), trans-2-[3(4-tert-butylphenyl)-2-methyl-2-propenylidene] malononitrile (DCTB), tetrabutylammonium hexafluorophosphate (Bu4NPF6; >99%), anhydrous acetonitrile (CH3CN; >99.8%), hexanes (95%), diethyl ether (98.5%), and benzonitrile (PhCN; >99%) were purchased from Aldrich and used without any further purification. Methanol (99.98%) was from Fisher Scientific. Ferrocene (Fc; 98%) was obtained from Aldrich and then sublimated and vacuum-dried overnight prior to use as an internal standard. Acetonitrile was dried over 4A molecular sieves and then freshly distilled over CaH2 prior to use. Synthesis and Purification of CdSe Nanoclusters. All reaction and purification steps were performed under controlled lighting using a wavelength-selecting filter that removes UV wavelengths. In a typical synthesis, 0.2 g of (Me4N)4[Cd10Se4(SPh)16] and 10 mL of thiophenol were placed in a two-neck flask. The mixture was deoxygenated via repeated cycles of vacuum and N2 for 1 h and then slowly heated to 150 °C under a positive pressure of N2. After approximately 4 h of heating, the colorless thiol solution became dark yellow with complete disappearance of the solid. Heating was continued at this temperature for another 6 h. The reaction temperature was then brought down to 130 °C, and heating was continued for 12 h, which resulted in a dark yellow solid and faint yellow solution. The reaction mixture was allowed to cool to room temperature, and then the faint yellow solution was removed with a syringe. Dry hexane was added to the solid and stirred for 1 h under N2, and then the solid was collected by centrifugation. The hexane cleaning procedure was repeated at least three times until the NMR spectrum confirmed the complete disappearance of the S−H peak at ∼5.0 ppm. The yellow solid was then dissolved in dry CH3CN and stirred for 30 min followed by centrifugation. The clear yellow solution was then vacuum-dried, redissolved in PhCN, and stirred for 30 min at room temperature. The solution was centrifuged and the undissolved white solid discarded.

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RESULTS AND DISCUSSION In a typical synthesis, 0.2 g of [(Me4N)4{Cd10Se4(SPh)16}]32 and 10 mL of thiophenol were combined in a two-neck flask. The mixture was deoxygenated via repeated cycles of vacuum and N2 for 1 h at room temperature. The mixture was slowly heated to 150 °C, and after approximately 4 h of heating, the colorless thiol solution became dark yellow with complete disappearance of the solid. Heating was continued at this temperature for another 6 h. The reaction temperature was then reduced to 130 °C and maintained for another 12 h, which resulted in a dark yellow solid and faint yellow solution. After removal of the yellow solution, the solid was purified via multiple solvent precipitations. The purified thiophenolatecapped CdSe nanoclusters were readily dispersible in acetonitrile (CH3CN), benzonitrile (PhCN), and N,Ndimethylformamide (DMF). Purified CdSe nanoclusters were analyzed by 1H NMR (SIFigure 1, Supporting Information). Disappearance of the peak at ∼5 ppm indicated successful removal of thiophenols. Additionally, the peak associated with (CH3)4N+ at ∼3.5 ppm also disappeared, which suggests that the purified nanoclusters do not contain any starting materials. The relative broadening of the aryl resonances (7.5−6.4 ppm) in the CdSe B

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nanoclusters in comparison to starting materials can be attributed to a combination of spin−spin relaxation broadening, dipolar broadening, and a distribution of chemical shifts,33,34 which was previously observed for thiophenolate ligandprotected gold nanoclusters.35,36 Thermogravimetric analysis (TGA) of the nanoclusters confirmed a single weight loss indicating decomposition of thiophenolate on the CdSe nanocluster surface. This result was also supported by the Xray photoelectron spectroscopic (XPS) analysis of the CdSe nanoclusters in which no trace of N was observed (SI-Figure 2, Supporting Information). The above spectroscopic characterizations suggest that the purified CdSe nanoclusters did not contain any residual starting materials or other byproducts. Figure 1A shows the absorption spectra of CdSe nanoclusters in different solvents, where two distinct bands located at 347 (3.57 eV) and 362 (3.43 eV) nm were observed. The absorption band edge (valence band edge, VB-edge) was found to be 385 nm (3.22 eV). The lower energy transition has a large absorption cross section with an extinction coefficient (ε) of ∼121 000 M−1cm−1. The position of the absorption peak was insensitive to the solvent polarity. This result indicates that the ground state and the corresponding excited states have negligibly small dipole moments, which has been observed for the Cd32S14(SPh)36·4DMF18 and Cd17S4(SCH2CH2OH)26 nanoclusters.19 Therefore, electronic charge transfer from cadmium to thiophenolate can be eliminated on the basis of the optical properties. In spite of the strong absorption peak at 362 nm, the photoluminescence (PL) spectrum displayed a broad emission band centered at 604 nm covering the entire visible region. The PL peak position was independent of the excitation wavelength. The spectrum resembles either trioctylphosphine oxide-capped37 or a mixture of alkylamine- and arylbenzoic acid-stabilized6,38 CdSe nanoclusters. The broad PL can be attributed to surface-trapped electrons and holes. Furthermore, similar kinds of absorption and emission properties were previously observed for thiolate ligandstabilized CdS nanoclusters with definite chemical composition derived from single-crystal diffraction studies.18,19 Interestingly, the position and shape of the low-energy absorption peak (362 nm) indicate the possible composition of the nanoclusters as (CdSe)19, which displays an absorption peak at 363 nm.16 Previously, mass spectrometric analysis showed that the molecular weight of (CdSe)19 is ∼3600.4,21 To determine the molecular weight, our sample was analyzed by MALDI-TOF-MS using DCTB as a matrix.35,36 Figure 2 shows the MALDI of our purified nanoclusters, which reveals that they have a molecular weight of 8502. This result was confirmed with four different batches. The calculated molecular weight of Cd34Se20(SPh)28 clusters is 8458. There is a difference of 44 between the MALDI and theoretical molecular weights. We believe this difference could be due to adsorption of ions on the nanocluster surface or Cd34Se20(SPh)28 being decorated with solvent molecules.39 However, theoretical modeling could provide detailed structural properties of this new nanocluster and is currently under investigation. Additionally, no peak at a molecular weight of 3400 was observed, which suggests that the purified CdSe nanoclusters did not contain any starting materials, which is also in good agreement with 1H NMR, TGA, and XPS analyses. Our synthesized CdSe nanoclusters display a single mass in MALDI-TOF and also an extremely large valence band (VB) and conduction band (CB) gap, which is consistent with molecule-like HOMO and LUMO electronic structure.

Figure 2. MALDI-TOF-MS of CdSe nanoclusters in positive mode. A DCTB matrix was used to obtain the MALDI spectrum. The inset shows the spectral expansion of the highest intensity peak for clarity.

The large optical band gap of our CdSe nanoclusters suggests that they are Cd-rich. Previously, it was reported that the absorption peak of the CdSe nanoclusters red-shifted as the number of Se atoms in the core increased.40,41 To determine the Cd:Se ratio, samples were analyzed by XPS. The spectra (SI-Figure 3, Supporting Information) show Cd 3d5/2, Cd 3d3/2, and Se 3d peaks centered at 405.2, 411.9, and 54.1 eV, respectively, whereas the corresponding peak positions for starting materials were 401.7, 408.1, and 51.8 eV. The sharp peak in the Cd region and absence of a peak at ∼59 eV indicate that the CdSe nanoclusters did not contain any residual CdOx or SeO2.42 Additionally, the XPS analysis revealed that our CdSe nanocluster was Cd-rich with a corresponding Cd:Se ratio of 1:0.53. The MALDI-TOF and XPS results are significant in light of the chemical composition of CdSe nanoclusters. In the starting materials, the Cd:Se ratio was 1:0.4, which increased to 1:0.58 in our isolated nanoclusters. Therefore, we believe that the starting materials underwent a fragmentation process during the synthesis. Then the reconstruction process toward nanoclusters involved nucleus formation followed by the generation of thermodynamically stable nanoclusters with precise chemical composition via a traditional growth process.43 Previously high-temperature (∼220 °C) decomposition of starting materials in the presence of alkylamine resulted in large CdSe nanoclusters (∼4.5 nm) with a certain degree of dispersity, where the Cd:Se ratio was found to be 1:1,43 which is significantly different from the ratio reported here. In our investigation, formation of single-sized nanoclusters can be rationalized as due to their low-temperature synthesis causing controlled growth from thermodynamically stable nuclei in the presence of the strongly stabilizing ligand thiophenolate. On the basis of the empirical formula provided by Yu et al.,44 the diameter of the nanoclusters would be 1.2 nm for the 362 nm lowest energy absorption peak. A representative HRTEM image is provided in SI-Figure 4 (Supporting Information). However, it is difficult to determine adequate structural information such as the exact size and lattice structure of such ultrasmall nanoclusters by TEM. The determination of the chemical composition of metal chalcogenide nanoclusters remains a great challenge. In the literature very few single-crystal X-ray structures of different C

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thiolate-protected CdS nanoclusters have been reported.6 In recent years, significant research has been conducted on various ligand-protected (CdSe)33,34 nanoclusters,1,3,5 and the composition of such nanoclusters was first determined by MALDITOF-MS analysis along with theoretical modeling.7 However, determination of the molecular weight of long-chain aliphatic amine-, acid-, or phosphine oxide-protected CdSe nanoclusters via MALDI-MS is seemingly challenging. The thiophenolateprotected CdSe nanoclusters presented here have a molecular weight of 8502 as determined by MALDI-MS (Figure 2). In the literature XPS analysis was used for the quantitative determination of the number of atoms and surface ligands in CdSe nanoclusters.40,42,45 We also used the XPS analysis to determine the Cd:Se:S ratio in a nanocluster and found it to be 1:0.58:0.82. This elemental ratio obtained from XPS analysis in the CdSe nanoclusters was confirmed for four different batches with different concentrations. The elemental concentrations of Cd, Se, and S were obtained by integrating the Cd 3d5/2, Se 3d, and S 2p peaks, respectively, and dividing the integrated intensities of each element by their corresponding sensitivity factor. On the basis of the XPS analysis, the molecular formula was determined to be Cd34Se20(SPh)28. This formula is also supported by TGA analysis where only one type of weight loss (35.2%) was observed, which is in agreement with the mass loss (36.1%) detected on the basis of the composition of CdSe nanoclusters mentioned above; see Figure 3. The weight loss

lates per CdSe was found to be 29, which is in close agreement with the number determined from the XPS analysis of 28 thiophenolate ligands per nanocluster and strongly supports the molecular formula mentioned above. The optical band gap (3.43 eV) of Cd34Se20(SPh)28 was correlated with the electrochemical band gap by analyzing the sample via solution-phase electrochemistry. Bard et al. first reported the solution-phase electrochemistry of CdS25 and CdSe26 nanoclusters in DMF and toluene, respectively, and correlated their optical and electrochemical band gaps. Few other reports are available to determine the electrochemical band gap of dissolved nanoclusters in solution.46−49 However, the electrochemical band gap data were inconsistent,50 which was due to (1) decomposition of nanoclusters during the measurements and (2) the low potential window of the solvent/electrolyte system that did not provide accurate values. To the best of our knowledge, electrochemical band gap determination of thiophenolate-stabilized CdSe nanoclusters displaying a large optical band gap has not been performed previously. Cd34Se20(SPh)28 is extremely pure and readily soluble in CH3CN and PhCN. These solvents provide a large potential window (∼5 V),51 which is ideal for performing electrochemical measurements of large band gap nanoclusters. Figure 4A illustrates the CV of the CdSe nanoclusters in Bu4NPF6/CH3CN in which peaks at +1.65 and −2.31 V are assigned to oxidation and reduction waves. These peaks can be correlated to the HOMO and LUMO electron transfers, respectively, which is similar to that found with CdS,25 CdSe,26 or PbS52 reported in the literature. Figure 4B represents room temperature DPV of CdSe nanoclusters, which consists of a large electrochemical potential gap between the first oxidation step (ox1, +1.69 V) and the first reduction step (re1, −2.31 V). The DPV response is analogous to that of thiolate-protected gold nanoclusters shown by Murray and Whetten,53,54 and also silicon quantum dots by Bard.24 Most importantly, for the first time reversible electrochemical addition of electrons into Cd34Se20(SPh)28 was observed by DPV for CdSe nanoclusters. This electrochemical signature is significant in light of discovering new redox properties of semiconductor nanoclusters and finding their potential applications as reversible photocatalysts and photovoltaic and electrochromic devices. Importantly, the electrochemical band gap (ΔEeg) determined from the voltammetric experiment can be correlated with the optical band gap (ΔEog) using the following equation:

Figure 3. Comparative plot of TGA of CdSe nanoclusters (red) and the single-source precursor (blue).

ΔEeg = ΔEog + Je/h

(1)

where Je/h is the total Coulombic interaction energy of the electron and hole pair, often called the electron charging energy.55 Therefore, for a particular size of a nanocluster the electrochemical band gap would be expected to be higher than the optical band gap. The Je/h calculated for Cd34Se20(SPh)28 is 0.57 eV, which is the highest value determined from the comparison of electrochemical and optical band gaps. On the basis of Figures 1 and 4, we can develop a rough energy level diagram for Cd34Se20(SPh)28 as shown in Figure 5. A similar type of diagram was previously shown by Murray and coworkers for thiolate-protected gold nanoparticles53 and Banin et al. for InAs nanoclusters.55 The electrochemical band gaps determined from the first oxidation and reduction peaks from CV and DPV are 3.96 and 4.0 V, respectively. We believe that such a large electrochemical band gap in CdSe nanoclusters has not been reported before in organic solvent/electrolyte medium. However, both oxidation

curve from the CdSe nanoclusters was significantly different from that found with the starting materials, where two different weight losses were observed, first 20.4%, which could be attributed to the (CH3)4N+ moiety, and second 26.6%, which could be attributed to thiophenolate, whereas the single weight loss with a similar onset temperature indicates decomposition of only thiophenolate on the CdSe nanocluster surface. These data are significant and suggest that our isolated CdSe nanoclusters did not contain any residual (CH3)4N+. The molecular formula was further confirmed by 1H NMR using ferrocene (Fc) as an internal standard. The sample in CD3 CN was prepared containing 15.9 and 0.51 mM concentrations of pure Fc and purified CdSe nanoclusters, respectively, for 1H NMR analysis. By comparing the integrated intensity of the number of protons of Fc to that of the phenyl ring in CdSe nanoclusters, the average number of thiophenoD

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Figure 4. (A) CV in a 0.1 M Bu4NPF6/CH3CN solution of 0.1 mM CdSe nanoclusters (red) at a potential scan rate of 0.05 V/s with a 1.6 mm Pt working electrode, Pt wire counter electrode, and Ag wire QRE. (B) DPV of the indicated nanoclusters (red) in a 0.1 M Bu4NPF6/CH3CN solution with the same electrode setup. Blue lines in (A) and (B) represent the 0.1 M Bu4NPF6/CH3CN solution.

the adsorbed film on the electrode surface. Figure 6A illustrates the CV of Cd34Se20(SPh)28 nanoclusters at different scan rates. It was observed that during the repeated electrochemical cycles at high negative potential (>−2.0 V) an additional oxidation peak appeared at +1.3 V (data not shown), which could be due to the oxidation of redox species generated from decomposition of nanoclusters.25 To avoid the fouling on the electrode surface and inaccurate determination of the peak current, we used a −1.8 to +2.4 V potential range and an anodic peak current to determine the charge transfer process. Figure 6B illustrates the linear fit of the anodic peak current against the square root of the scan rate, which suggests the diffusion-limited transport of nanoclusters to the electrode and no electrode-adsorbed film. The diffusion coefficient of the Cd34Se20(SPh)28 nanoclusters was determined by using the Randles−Sevcik equation:51

Figure 5. Schematic energy level diagram for Cd34Se20(SPh)28 nanoclusters. All data were taken from the measurements in CH3CN solution. The image is not to scale.

and reduction peaks were irreversible in CV. Similar electrochemical results were also observed in CV for thiolateprotected CdS25 and PbS52 nanoclusters. The irreversibility of CV peaks can be rationalized as a multielectron-transfer-based electrochemical reaction upon charging of the CdSe core, which results in decomposition of the nanoclusters.24−26,52 This decomposition process could involve redox reactions of the CdSe, which would result in CdSe/Cd0 and CdSe/Se0 as proposed by Haram et al.25 The rate of charge transfer of nanoclusters is important for designing efficient photocatalysts and photovoltaic devices. Therefore, CV was recorded at different scan rates. Besides determining the transport process, scan rate dependent CV provides information about the redox reactions, which could originate from either the electroactive species in the solution or

iP = (2.69 × 10)n3/2AD1/2Cν1/2

(2)

where n is the number of electrons delivered per nanocluster, D is the diffusion coefficient of the nanoclusters, C is their concentration, and ν1/2 is the potential scan rate. Considering that a nanocluster donates a single electron, the D value is calculated to be 5.4 × 10 −4 cm2/s for the Cd34Se20(SPh)28 nanocluster. The diffusion coefficient is significantly higher for a 1.2 nm diameter nanocluster.56 This experimental result suggests that the nanoclusters behave as multielectron donors

Figure 6. (A) Scan rate dependent CV response for the Cd34Se20(SPh)28 nanoclusters. The asterisk indicates O2 incorporation during multiplepotential scanning. (B) Nanocluster oxidation peak current (ipeak) vs square root of the potential scan rate for a solution of Cd34Se20(SPh)28 in acetonitrile. E

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A.D. gratefully acknowledges support from NSF Grant 0903787.

or acceptors, which could originate from the electrons or hole trapping inside the Cd34Se20(SPh)28.25 This multielectron transfer process of semiconductor nanoclusters could potentially trigger their decomposition upon charging during the electrochemical measurements, which we have mentioned previously. The diffusion coefficient (D) was also calculated using the no-slip version of the Stokes−Einstein equation:

D=

kT 6πηrH

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where rH is the nanocluster hydrodynamic radius and η is the viscosity of acetonitrile (0.343 cP). From the empirical formula, the radius of the nanocluster core was found to be 0.6 nm, with an additional 0.6 nm for the thiophenol. Equation 1 predicts a diffusion coefficient of 6.1 × 10 −6 cm2/s. This value is significantly lower than the diffusion coefficient determined from cyclic voltammetry (5.4 × 10 −4 cm2/s). Therefore, it is evident that a multielectron transfer process is involved in the charging of the nanocluster’s core. In summary, a simple synthetic method has been developed for the synthesis of 8502 molecular weight CdSe nanoclusters with a molecular formula of Cd34Se20(SPh)28, which displayed a peak at 362 nm and an ε of ∼121 000 M−1 cm−1. The nanoclusters displayed large optical and electrochemical band gaps with a significant contribution from the Coulombic interaction energy. The scan rate dependent electrochemical studies of the dissolved nanoclusters suggested diffusion-limited mass transport. In addition, kinetic effects were observed as the peak position shifted with the scan rate. Unlike metallic nanoclusters where single-electron charging of the core has been reported,53,57,58 the CdSe nanoclusters act as multielectron donors or acceptors. We believe the decomposition of semiconductor nanoclusters during potential scanning can be avoided by introducing redox electrolytes to trap the electrogenerated holes. We expect that our synthetic approach for the preparation of single molecular composition CdSe nanoclusters will lead to a more detailed understanding of the electrochemical properties of metal chalcogenide nanoclusters and their future applications.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed synthetic procedure, purification of CdSe nanoclusters, instrumental analyses, and additional spectroscopic and microscopic characterizations. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.D. and R.S. acknowledge Indiana University-Purdue University Indianapolis (IUPUI) for financial support. We acknowledge Carrie Donley at the Chapel Hill Analytical and Nanofabrication Laboratory (CHANL) for XPS analysis, Praneeth Nimmala for preliminary data, and Dr. J. Goodpaster and Dr. B. Muhoberac for helpful comments and discussions. F

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