386
Tunable electrogenerated chemiluminescence from CdSe nanocrystals1 Jigang Zhou, Jun Zhu, Jack Brzezinski, and Zhifeng Ding
Abstract: Electrochemical behavior and related optoelectronic properties of CdSe nanocrystals (NCs) in aprotic solutions have been investigated. NCs of 2.50 ± 0.50 nm diameter were synthesized using a modified procedure in which the temperatures at the time of Se precursor injection and NC growth were controlled. The electrochemical band gap was found to agree with those determined by UV–vis absorption spectroscopy and by the tunneling current-voltage spectrum in the literature. Electrogenerated chemiluminescence of the NCs with peak maxima at 1.90 eV (red, 653 nm) and 2.55 eV (blue, 486 nm) can be generated and altered by scanning the voltage between –1.60 and –1.80 V and between –2.00 and –2.20 V, respectively. The results demonstrate the potential capability of the NCs for light emission tuned by the applied potential. Key words: CdSe nanocrystals, electrochemistry, electrogenerated chemiluminescence, UV–vis spectroscopy, photoluminescence. Résumé : On a étudié le comportement électrochimique et les propriétés optoélectroniques apparentés de nanocristaux (NC) de CdSe dans des solutions aprotiques. On a effectué la synthèse des NC d’un diamètre de 2,50 ± 0,50 nm en utilisant une méthode modifiée dans laquelle les températures sont contrôlées tant au moment de l’injection du Se précurseur que de la croissance du NC. On a trouvé que l’intervalle de la bande électrochimique est en accord avec ceux déterminés dans la littérature par la spectroscopie d’absorption UV–visible et par les spectres de l’effet tunnel courant/voltage. La chimiluminescence électrogénérée de NC avec un pic maximal à 1,90 eV (rouge, 653 nm) et 2,55 eV (bleu, 486 nm) peut être générée et modifiée par un balayage du voltage respectivement entre –1,60 et –1,80 V et entre –2,00 et –2,20 V. Les résultats démontrent la capacité potentielle des NC d’émettre de la lumière de longueur d’onde modifiée par le potentiel appliqué. Mots-clés : nanocristaux de CdSe, électrochimie, chimiluminescence électrogénérée, spectroscopie UV–visible, photoluminescence. [Traduit par la Rédaction]
Zhou et al.
391
Introduction Semiconductor nanocrystals (NCs), which have a direct band gap structure, strongly exhibit size dependent electronic, optical, and electrochemical properties (1) when their dimensions are smaller than 10 nm. Recently, active devices have been exploited that combine the diversity of organic materials with the high-performance electronic and optical properties of semiconductor NCs (2, 3). Any desired color emission can be varied by changing the NC size or the lumiReceived 5 October 2008. Accepted 14 November 2008. Published on the NRC Research Press Web site at canjchem.nrc.ca on 5 December 2008. Dedicated to Professor Richard J. Puddephat for his many years of pioneering research in inorganic chemistry. J. Zhou, J. Zhu,2 J. Brzezinski, and Z. Ding.3 Department of Chemistry, The University of Western Ontario, London, Ontario N6A 5B7, Canada 1
This article is part of a Special Issue dedicated to Professor R. Puddephatt. 2 Present address: School of Chemical and Environmental Engineering, Beijing Technology and Business University, Beijing 100037, P.R. China 3
Corresponding author (e-mail:
[email protected]).
Can. J. Chem. 87: 386–391 (2009)
nescent organic materials (4). However, reports concerning the optoelectronic properties of semiconductor NCs remain scarce (5, 6) because of limited investigation methods. Electrogenerated chemiluminescence (ECL) (7–9) of semiconductor NCs (10–15) can reveal information about their structure and about the electroluminescence mechanisms of the future devices. Because of their photon-emitting properties, CdSe NCs are beginning to make an impact in light-emitting diode and photonics technology (2–4). However, light emission is known to be sensitive to the nature of the particle surface due to the large surface area and the possible presence of surface states caused by uncoordinated atoms (14). For instance, the peak position in the ECL spectrum of CdSe NCs capped with trioctyl phosphine oxide (TOPO) was redshifted by hundreds of nanometers compared with the PL spectrum (11), while the ECL spectrum of CdSe NCs passivated with a shell of the wider band gap material, ZnSe, is similar to the PL spectrum (12). Here we describe the ECL of small size CdSe NCs (2.50 ± 0.50 nm) capped with TOPO. Two colours of light emission (blue and red) were observed and could be altered by tuning the applied potential. This new observation may lead to potential tuned LEDs fabricated with the NCs.
doi:10.1139/V08-180
© 2008 NRC Canada
Zhou et al.
Experimental section Synthesis of CdSe NCs CdSe NCs capped with TOPO were synthesized according to procedures reported by Peng’s group (16) and Bard’s group (12), with modifications to the temperatures at the time of the Se precursor injection and NC growth (17). Briefly, a Se solution was prepared by mixing 0.16 g of Se powder, 4 g of trioctylphosphine (TOP), and 80 µL of toluene. Approximately 12 g of technical grade trioctylphosphineoxide (TOPO) and 0.1 g of cadmium acetate hydrate were placed into a 3-neck round-bottomed flask and heated to ~140 °C. After degassing with argon for 1 hour at this temperature, the solution in the flask was heated to 280 °C. At this temperature, the heating mantle was removed immediately, and a cold Se solution was quickly injected into the reaction vessel through a rubber septum. The temperature decreased to 250 °C after the injection was done. With continuous stirring, the temperature further decreased to 180 °C. At this point, the flask was reheated to about 220 °C, and the first aliquot, the sample under investigation in this report, was removed from the flask. The sample solution was cooled to ~50 °C with cold tap water. After addition of 10 mL of CHCl3, the solution was kept in the dark. The NCs were precipitated with a copious amount of methanol and collected by centrifugation and decantation. The collected precipitate was dissolved in CHCl3 and precipitated again with methanol. This cycle was repeated three times. The final product was dissolved in CHCl3. NCs were obtained by evaporating the solvent and drying under vacuum. The resulting NCs were used for the electrochemistry and ECL experiments. All chemicals were purchased from Sigma-Aldrich and used with no further purification. Physical methods A JEOL 2010F high-resolution transmission electron microscope (HRTEM), a Varian Cary 100 UV–vis absorption spectrometer, and a Spex Fluorolog 3–11 fluorimeter were employed to characterize the morphology, electronic structure, and photo-induced luminescence (PL) of the prepared NCs. A specially designed photoelectrochemical cell was used for all the electrochemical and ECL measurements. The cell consisted of a Pt disk working electrode (0.07 cm2), a Pt wire counter electrode, and an Ag wire quasi-reference electrode. CdSe NC solutions were prepared with 0.1 mol/L tetrabutylammonium perchlorate (TBAP) as the supporting electrolyte and with 2 mL CH2Cl2 as the solvent, freshly treated by a solvent purification system. The bottom of the cell has a flat Pyrex window for detection of the light generated from the working electrode (7, 18). The loaded cell was degassed with Ar and sealed with an air-tightening Teflon cap. Differential pulse voltammograms (DPVs) were obtained using an electrochemical workstation, CHI 610A (CH instruments, Austin, Texas). The experimental parameters for DPVs were as follows: 0.05 V pulse height, 60 ms pulse width, 200 ms period, and 0.02 V/s scan rate. Voltammetric ECL curves were obtained using the electrochemical work-
387
station coupled with a photomultiplier tube (PMT, R928, Hamamatsu, Japan) held at –750 V with a high-voltage power supply (7, 18). The ECL generated at the working electrode was collected by the PMT under the flat Pyrex window at the bottom of the cell. The photocurrent from the PMT was transformed into a voltage signal by an electrometer (Model 6517, Keithley, Cleveland, Ohio). This signal, along with the potential and current signals from the electrochemical workstation. was sent through a DAQ board (DAQ 6052E, National Instruments, Austin, Texas) to the computer where the dada acquisition system was controlled by a homemade LabVIEW (National Instruments, Austin, Texas) program. ECL spectra (7, 18) were measured by employing a potential program with the electrochemical workstation. By collecting the ECL light through the Pyrex window of the cell in a spectrograph (Spectra Physics, Cornerstone 260M) and dispersing the emission with a grating, the output was recorded on a back-illuminated charge coupled device (CCD) camera (Andor 420BU), which was electrothermally cooled to below –56 °C. The electrochemical potential was calibrated at the end of each experiment by the addition of ferrocene as an internal standard with the formal potential of Fc+/Fc 0.400 V vs. SHE (19). ECL quantum yields were calculated by comparing the integrated ECL intensities and the current values (charge) of the NCs with the reference Ru(bpy)32+ (ΦECL = 5%) (20). The quantum yield was calculated using the following equation:
[1]
Φ NC
bECL dt ∫ = 100 × b a ∫ Current dt a
NC
bECL dt ∫a b ∫ Current dt a
St
where ΦNC is the quantum yield (%) relative to Ru(bpy)32+, ECL is the ECL intensity, Current is the electrochemical current value, St is the standard, and NC is the nanocrystal sample.
Results and discussions The temperature effect on NC growth We believe that precursor concentration and temperature for injection and NC growth are two important factors that manipulate NC size in general. The NC growth was fast, and it was difficult to obtain desired NC sizes under reported temperature conditions (16). Temperature was therefore chosen as our major parameter to optimize. We decreased the reaction temperature of the Cd precursor solution in a flask by using a cold Se precursor solution and a fast injection speed, as well as by removing the heat mantle under the reaction flask before injection. Growth solution was cooled quickly with an ice bath once the desired size was achieved, which was monitored via absorption wavelength. With this method, we were able to control NC sizes undergoing different growth times. The emission colours of the obtained NCs covered almost the entire visible spectrum when excited by a UV lamp. © 2008 NRC Canada
388
Can. J. Chem. Vol. 87, 2009
Fig. 1. (a) Room temperature absorption and emission spectra of CdSe nanocrystals dispersed in CHCl3. Excitation wavelength: 370 nm. Inset is a typical TEM picture of an individual NC. (b) TEM of NCs.
Characterization of CdSe NCs One batch of small size NCs was selected for study, with the focus on probing their electronic structure by electrochemistry and ECL. As shown in Fig. 1, the NCs were characterized by TEM, high-resolution TEM, UV–vis absorption, and PL spectra (Fig. 1). TEM experiments showed a narrow size distribution, and NC size was determined to be 2.50 ± 0.50 nm, as demonstrated in Fig. 1b. According to the UV–vis absorption spectrum, the diameter of the NCs was estimated at about 2.0 nm (21, 22), and the sharp peak illustrated that the size distribution was narrow. From the PL spectrum, one sharp peak at 2.55 eV and a second broad peak at 2.03 eV were observed. The narrow absorption peak diminishes the possibility of wide size distribution, thus we can attribute the sharp PL peak to the core structure emission and the broad peak to the surface state emission. The NCs in question may produce strong PL emission from both core excitons and surface traps, as depicted by the energy diagram in Scheme 1a. Electrochemistry NC reduction (electron injection, whose mechanism is illustrated in Scheme 1b) in the negative bias region, as shown by the DPV in Fig. 2, was characterized by a doublet followed by a continuous increase in current, with the applied potential becoming more negative. This is indicative of two discrete charge transfers followed by multiple charge transfers. The various peak spacings in the DPV can be attributed to Coulomb charging into higher energy levels (10). The DPV responses in this region reveal information about the conduction levels and look very similar to the tunneling current-voltage spectrum (TCVS) of a similar size CdSe NC at 4.2 K, obtained by Alperson et al. (23). The doublet was attributed to the two consecutive electron injections to the first conduction band, 1Se (s character) in the case of electrochemically prepared NCs (23). The energy difference between the first two peaks, i.e., the Coulomb charging, was 0.37 eV in DPV, which agrees well with the value in the TCVS (0.34 eV). Broader peaks were observed in DPV than those observed in the TCVS. While the CdSe NCs were monodisperse in size, the broadness of the peaks might be due to the surface states, which increased the complexity of the electronic structure. Furthermore, a continuous increase in current, instead of a Coulomb staircase for the electron in-
Scheme 1. Schematic mechanisms and energy diagrams describing (a) PL and (b) ECL occurring in CdSe core (hollow) and surface (shadow). Symbols 䊉 and 䊊 represent electrons and holes.
jections to the second conduction band, was observed in DPV. However, up to six peaks spaced by values close to the Coulomb charging energy in TCVS for a single InAs dot were reported by Banin et al. (24). This might be caused by an overlap of core and shell (surface) electronic structures as well as an interplay between level spacing and single electron tunneling. Alternatively, two oxidation (hole injection) peaks were seen in the positive bias side of the DPV in Fig. 2. These peaks were broader, and the Coulomb charging energy was larger (0.48 eV) than that of the negative side. These results were expected because of the higher density of valence levels and the close proximity between Coulomb charging and level spacing as well as the multiplicity of electronic structures. The electrochemical gap was determined to be 2.34 eV from the separation of the first oxidation and reduction peaks in the DPV (Fig. 2). This is in good agreement with the optical band gaps, 2.50 and 2.06 eV, estimated from the absorption onset and the broad PL peak of the NC, respectively (25). The zero current gap in TCVS, 2.88 to 3.13 eV, also provided a qualitative description of the band gap (23). The electrochemical gap is strongly affected by the presence © 2008 NRC Canada
Zhou et al. Fig. 2. Differential pulse voltammograms (DPVs) of a dichloromethane solution containing 0.1 mol/L tetrabutylammonium perchlorate (TBAP) as supporting electrolyte (dotted line) and 2.50 nm CdSe NCs (40 µmol/L) dispersed in the same solution (solid line) at a 0.07 cm2 Pt disk working electrode. The lower lines are DPVs with a potential scan from positive to negative, while the upper lines from negative to positive. A potential increment of 4 mV, a sample width of 0.0167 s, a pulse period of 0.2 s, a pulse width of 0.05 s, and a pulse amplitude of 50 mV were used.
of surface states that can act as local traps for electrons and holes. Considering the small size of NCs studied here, a large part of the CdSe should be on the surface of the NCs (14, 17), which may be capped with TOPO or possibly a small amount of oxide. Very recently, Bard’s group reported an electrochemical study of CdTe NCs (13). In comparison to CdTe NCs, CdSe NCs were assumed to be less passivated by TOPO in similar preparation conditions (11, 13). The CdSe NCs in question had even richer surface states than those reported previously (11). This is observed in an intense photoluminescence peak at a longer wavelength, i.e., surface state emission. Such evidence suggests that the above observed oxidation and reduction may reflect the properties of the NC surface. Electron and hole injections may first occur on the 1Se and 1VB bands of surface CdSe, respectively (23). However, it is not clear from the DPV where further charge injections occur. In addition, forward and reverse DPV scans are relatively less symmetric than those of Si NCs (10), indicating that charge transfers are not very reversible. Cyclic voltammograms (CVs) of the CdSe NCs dispersed in the same dichloromethane electrolyte solution are demonstrated in Fig. 3. As shown by the DPVs, the redox behavior of the NCs is dependent on the scan direction of the applied potential. The NC anions are more stable than the cations as illustrated by the CV in Fig. 3a: the anions generated at the negative potential can be re-oxidized in the forward scan, while the cations produced at the positive potential can barely be re-reduced in the backward scan. A new peak also appears at 0.11 V, probably due to the new species from the decomposed cations. In addition, the CV in the reversed order of forward and backward scans as above does not show evident reduction peaks and the following oxidation reaction due to the depletion of the NCs in the vicinity of the electrode (Fig. 3b).
389 Fig. 3. Cyclic voltammograms (CVs) of a dichloromethane solution containing 0.1 mol/L tetrabutylammonium perchlorate (TBAP) as supporting electrolyte (dotted line) and 2.50 nm CdSe NCs (40 µmol/L) dispersed in the same solution (solid line) at a 0.07 cm2 Pt disk working electrode. (a) CV with a potential scan from –2.000 to 2.000 and back to –2.000 V. (b) CV with a potential scan from 2.000 to –2.000 and back to 2.000 V. A scan rate of 100 mV/s was used.
Electrogenerated chemiluminescence (ECL) ECL of the same electrolyte solution used for electrochemical experiments was first investigated by recording voltammetric photocurrent with a photomultiplier tube (PMT) (10). ECL was observed under various stimulus conditions. We found that emission was produced in both negative and positive sides when the applied potential was swept between –2.30 and 2.30 V, though the ECL intensity is rather weak. An annihilation mechanism, i.e., electron transfer between reduced and oxidized NCs, was assumed in this light emission process (10, 11). More interestingly, a significant ECL signal was detected as the potential was scanned from 0.00 to –3.00 V, as shown in Fig. 4. The ECL-potential curve showed two overlapping peaks, which may correspond to two emission processes. ECL turn-on potential was at –1.30 V, which is slightly more negative than the first reduction peak in DPV. Stable ECL could be obtained even at pesudo constant potentials on the negative side, e.g., slowly scanning from –1.60 to –1.80 V or from –2.00 to –2.20 V. The second peak on the ECL-potential curve appeared at –2.12 V. Note that in these cases, only the reduced species © 2008 NRC Canada
390
Can. J. Chem. Vol. 87, 2009
Fig. 4. Voltammetric ECL curves of at a 0.07 cm2 Pt disk working electrode immersed in 0.1 mol/L TBAP CH2Cl2 electrolyte solution containing 2.50 nm CdSe NCs (40 µmol/L). Scan rate was at 0.5 V/s. Two cycles are overlapped in the figure.
existed in the solution, and the above electrochemical study showed a higher stability of anions than that of cations. The annihilation leading to light emission in this situation could not occur. Therefore, there must be another oxidant that accepted an electron from the valance band of anion NC radicals to form excited states. Bard and co-workers (13) suggested that similar emission from CdTe NCs might occur through an electron transfer reaction between anion CdTe radicals and electrogenerated oxidant radicals, CH2Cl·, from the dichloromethane used as the solvent. This is the socalled co-reactant emission mechanism (26). This mechanism explains well the ECL signal at more negative potential and is depicted in Scheme 1b, according to our observation. This mechanism is illustrated in Scheme 2. However, our ECL “turn-on voltage” of –1.30 V is not negative enough to generate the oxidant, and similar ECL was observed in dimethylformide (DMF) solution. Nevertheless, it was found that ECL was enhanced several times by introducing air to the electrolyte solution. Similar to the chemiluminescence of certain linear hydrazides, aldehydes, and ketones in dimethyl sulfoxide with potassium tert-butoxide and oxygen (27, 28), ECL might be emitted from the excited states formed by the reaction of the anion radicals with oxygen molecules, − (CdSe)NC + e– → (CdSe) •NC O2 − (CdSe) •NC → (CdSe) *NC
(CdSe) *NC → (CdSe)NC + hν It is unlikely that an oxygen molecule accepts an electron to form a superoxide ion (O2–.), as in the case of hydrazides, aldehydes, and ketones, since the redox potential of O2/O2– (–0.33 V) is too negative. Indeed, this redox couple acted as an electron donor in an alkaline solution to the CdSe NC packed film (6). As demonstrated in Scheme 2, CdSe NCs were recycled with dichloromethane or oxygen molecules acting as a scavenger in this emission mechanism. ECL was therefore very stable with repetitive potential cycling over a long time pe-
Scheme 2. Schematic mechanisms for ECL of the CdSe NCs with CH2Cl2 as the co-reactant.
riod. Nearly constant ECL intensity was also measured periodically over several weeks. The ECL efficiency was found to be half that of Ru(bpy)32+ under the same experimental conditions. ECL spectra are presented in Fig. 5. The integration time for the spectra is 6 min. As the potential was scanned from 0 to –2.20 V, the ECL spectrum, consisting two peaks at 1.90 and 2.55 eV, was very broad and weak. However, strong ECL at 1.90 eV (red, 653 nm) was observed at potentials slowly scanned between –1.60 and –1.80 V. Even stronger ECL consisting of two peaks at 1.90 and 2.55 eV (blue, 486 nm) was emitted at more negative potentials (–2.00 to –2.20 V). We assume that ECL at 1.90 and 2.55 eV correspond to the surface state and core structure emission, respectively. These energies are in good agreement with band gaps from surface and core electronic structures as analyzed from our electrochemical and spectroscopic study above. Illustrative ECL mechanisms for the core and the surface are summarized in Scheme 1b. Emission at 2.55 eV implies that the current rise at very negative potentials in DPV resulted from the electron injections to the core conduction bands. Quantitatively, however, it is not clear why the NCs luminesced at 1.90 eV simultaneously when stimulated with higher energy. It is even more difficult to understand at this point why ECL was only observed at longer wavelength (1.70 eV) for 3.2 nm diameter CdSe NCs prepared using similar methods in a previous report (11). We can speculate that ECL from the core was not favorable in the absence of oxygen molecules in an airtight electrochemical cell. Another speculation is that we might obtain the NCs with different crystallinity from that of the NCs in the previous report (11), which lead to different crystal band structures. Bawendi and co-workers (21) found that simulated X-ray powder diffraction spectra for 3.5 nm diameter spherical nanocrystallites such as zincbende, wurtzite, and wurtzite with one stacking fault are very different. However, more detailed studies are needed to understand the relationship between the nanocrystallinity and the ECL.
Conclusion In addition to size-tunable character, light emission is also potentially tunable for the CdSe NCs under investigation. This new phenomenon is rather remarkable as it provides a © 2008 NRC Canada
Zhou et al. Fig. 5. ECL spectra of 40 µmol/L CdSe NCs in CH2Cl2 containing 0.1 mol/L TBAP at a 0.07 cm2 Pt working electrode, obtained by scanning applied potential between (a) –1.60 and –1.80 V, (b) –2.000 and –2.200 V, and (c) 0.000 and –2.200 V. All potentials are referred to Ag wire. Scan rates for (a), (b), and (c) were at 10, 10, and 500 mV/s. An integration time of 6 min was used to obtain the spectra. Dotted curves represent the responses of corresponding blank supporting electrolyte solutions recorded at the same conditions. Vertical lines are cosmic rays on the CCD camera.
new avenue to tune colours of light emission from CdSe NCs simply by switching applied potentials. An impact to optoelectronics technology such as light-emitting diodes and color-display screens is anticipated.
Acknowledgements We thank Hubert Girault, Bin Su, Xuhui Sun, Xingtai Zhou, Xueliang Sun, Gianluigi Botton, Fred Pearson, John Vanstone, Sherrie McPhee, Mary Lou Hart, and Marty Scheiring for discussions and technique supports. This work
391
was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), Canada Foundation for Innovation, Ontario Innovation Trust, the Premier’s Research Excellence Award, and the University of Western Ontario. ZFD was a visiting professor at the Swiss Federal Institute of Technology in Lausanne (EPFL) in the summer of 2003. JZ was an overseas scholar supported by Beijing City in 2004. JB was an NSERC summer student in 2004.
References 1. A.P. Alivisatos. J. Phys. Chem 100, 13226 (1996). 2. S. Coe, W.-K. Woo, M. Bawendi, and V. Bulovic. Nature, 420, 800 (2002). 3. K. Karrai, R.J. Warburton, C. Schulhauser, A. Hoegele, B. Urbaszek, E.J. McGhee, A.O. Govorov, J.M. Garcia, B.D. Gerardot, and P.M. Petroff. Nature, 427, 135 (2004). 4. V.L. Colvin, M.C. Schlamp, and A.P. Allvisatos. Nature, 370, 354 (1994). 5. C. Wang, M. Shim, and P. Guyot-Sionnest. Science, 291, 2390 (2001). 6. S.K. Poznyak, D.V. Talapin, E.V. Shevchenko, and H. Weller. Nano Lett. 4, 693 (2004). 7. C. Booker, X. Wang, S. Haroun, J. Zhou, M. Jennings, B.L. Pagenkopf, and Z. Ding. Angew. Chem. Int. Ed. 47, 7731 (2008). 8. W. Miao. Chem. Rev. 108, 2506 (2008). 9. M.M. Richter. Chem. Rev. 104, 3003 (2004). 10. Z. Ding, B.M. Quinn, S.K. Haram, L.E. Pell, B.A. Korgel, and A.J. Bard. Science, 296, 1293 (2002). 11. N. Myung, Z. Ding, and A.J. Bard. Nano Lett. 2, 1315 (2002). 12. N. Myung, Y. Bae, and A.J. Bard. Nano Lett. 3, 1053 (2003). 13. Y. Bae, N. Myung, and A.J. Bard. Nano Lett. 4, 1153 (2004). 14. A.J. Bard, Z. Ding, and N. Myung. Structure and bonding (Berlin, Germany), 118, 1 (2005). 15. Y. Bae, D.C. Lee, E.V. Rhogojina, D.C. Jurbergs, B.A. Korgel, and A.J. Bard. Nanotechnology, 17, 3791 (2006). 16. J. Aldana, Y.A. Wang, and X. Peng. J. Am. Chem. Soc. 123, 8844 (2001). 17. J. Zhou, X. Zhou, X. Sun, M. Murphy, F. Heigl, T.-K. Sham, and Z. Ding. Can. J. Chem. 85, 756 (2007). 18. J. Zhou, J. Zhu, and Z. Ding. Proc. Electrochem. Soc. 2005– 04, 129 (2005). 19. D.R. Lide. CRC handbook of chemistry and physics. 84th ed. CRC Press LLC, Boca Raton, Fla., USA. 2003. 20. H.S. White and A.J. Bard. J. Am. Chem. Soc. 104, 6891 (1982). 21. C.B. Murray, D.J. Norris, and M.G. Bawendi. J. Am. Chem. Soc. 115, 8706 (1993). 22. W.W. Yu, L. Qu, W. Guo, and X. Peng. Chem. Mater. 15, 2854 (2003). 23. B. Alperson, I. Rubinstein, G. Hodes, D. Porath, and O. Millo. Appl. Phys. Lett. 75, 1751 (1999). 24. U. Banin, Y. Cao, D. Katz, and O. Millo. Nature, 400, 542 (1999). 25. E. Kucur, J. Riegler, G.A. Urban, and T. Nann. J. Chem. Phys. 119, 2333 (2003). 26. L.R. Faulkner and A.J. Bard. In Electroanalytical chemistry. Vol. 10. Edited by A.J. Bard. Dekker, New York. 1977. p. 1. 27. E. Rapaport, M.W. Cass, and E.H. White. J. Am. Chem. Soc. 94, 3160 (1972). 28. E. Rapaport, M.W. Cass, and E.H. White. J. Am. Chem. Soc. 94, 3153 (1972).
© 2008 NRC Canada
