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Quantized electroluminescence from Q–CdS films immersed in aqueous electrolytes
Colloids and Surfaces A: Physicochemical and Engineering Aspects 146 (1999) 293 – 298
Quantized electroluminescence from Q–CdS films immersed in aqueous electrolytes K. Tanno, M. Ashokkumar, P. Mulvaney * Ad6anced Mineral Products Research Centre, School of Chemistry, Uni6ersity of Mebourne, Park6ille, 3052 VIC, Australia Received 6 July 1998; accepted 2 October 1998
Abstract Quantum dot electrodes (QDEs) have been constructed by deposition of Q – CdS particles onto indium – tin oxide substrates. Upon application of cathodic potentials to the electrodes in electrolytes containing persulfate ions (S2O28 − ), light emission is observed. The emission peak depends on the particle size, with electrolyte electroluminescence (EEL) being observed down to 478 nm (blue light) for CdS with a particle size of 3.6 nm. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Blue emission; Quantized electroluminescence; Size quantization; CdS particles; Semiconductor; Quantum dot electrode
1. Introduction Current interest in the emission properties of semiconductors stems from their potential applications in blue LEDs, lasers and optoelectronic switches. To date, blue emission from solid state devices has been achieved using new materials such as GaN. One way to circumvent the search for new materials with good electroluminescence (EL) characteristics is to use size quantization to shift the emission response of more familiar semiconductor materials to higher energies. In this vein, Alivisatos et al. have described thin film devices where the EL spectrum depends on the particle size [1,2]. Using CdSe quantum dots
* Corresponding author. Fax: + 61-3-93446233; e-mail: [email protected].
(QDs), they obtained a shift in emission from red light to orange. The device efficiency is dependent on a hole-conducting polymer. Yang and coworkers reported emission from ZnS nanocrystal films [3]. In this case, the emission was red-shifted from the bulk band gap energy, and was due to activation of a surface recombination centre. Woggon et al. have recently reported EL from CdS particles though no quantization effects were reported [4]. They were able to model the electron transport characteristics of quantum dot electrodes (QDEs) based on the I–V and hn –V response of the films, and confirmed that efficient interparticle electron transport is possible, in the absence of a conducting polymer matrix. In this report we examine an alternative mechanism for obtaining quantized EL based on electrochemical electroluminescence (EEL). EEL was
0927-7757/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 9 8 ) 0 0 8 0 9 - 7
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K. Tanno et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 146 (1999) 293–298
originally observed in CdS electrodes by Gerischer and coworkers over 20 years ago [5]. It requires a potent oxidant to directly inject holes into the valence band, located at about + 1.6 V versus NHE. Ellis and colleagues and Ferrer and Salvador have examined the emission processes at the CdS–water interface in some detail [6,7]. Emission occurs because, under cathodic bias, an n-type electrode material will reduce solution oxidants efficiently. If an intermediate in this redox reaction can inject a hole into the electrode, emission of band gap or trap fluorescence is induced. We previously observed that sintered CdS powder electrodes could be made to emit either green or red EL depending on electrode preparation [8]. By employing Q – CdS particles, it is possible to tune this EEL, and obtain direct emission from the quantized semiconductor exciton states. Using this method, blue EL from quantum dot particles has been obtained for the first time. However, as will be shown, the EEL effect is short-lived in our current QDEs because, under strong cathodic bias, the films peel off the ITO substrate, primarily due to hydrogen evolution which accompanies light emission.
2. Experimental section CdS quantum dots were prepared using the optimized methods of Weller et al. [9,10]. Briefly, Cd(ClO4)2.6H2O (0.02 M, 10 ml) and sodium hexametaphosphate (0.02 M, 10ml, Riedel de Ha¨en), were placed in 1 l of Milli-Q water and degassed with nitrogen for 20 min. The pH was adjusted to values between 8.1 and 9.8 according to the particle size required. Then H2S (4.5 ml) was injected by microsyringe. The vessel was shaken for 2 min and then stirred for 10 min until reaction was complete. The pH of the sol was then raised to 10.5 by dropwise addition of 1 M NaOH. Bandgap fluorescence was activated by gradual addition of 0.1 – 0.5 mM Cd(ClO4)2. Fluorescence spectra were obtained with a Hitachi F-4500 spectrofluorimeter using 2.5 nm slits, and UV-vis spectra with a Hitach U-2000 spectrophotometer. ITO glass was procured from Delta Technology Limited and had a surface resistance
of 100 V. To prepare films, the CdS colloid was first rotary evaporated to reduce the solution volume from 1 l to 100 ml. The CdS was applied by several techniques. Initially we spin coated the colloid solution, but found the films to be inhomogeneous, with thick rinds around the edge of the plates. We also tried using a cationic polymer technique pioneered by Fendler et al. [11] as a means to enhance adsorption. Eventually, we found satisfactory films could be prepared by accurately microsyringing 20 ml of sol onto the film, and air drying at 100°C for 10 min in a conventional oven. This process was repeated to form n-layer films. Electrical contact was made with a copper wire and conducting epoxy resin. The copper wire was placed inside a glass capillary to prevent direct contact between the copper and electrolyte. A cross section through a QDE is illustrated in Fig. 1. EEL measurements were made directly in the Hitachi fluorimeter. A small fluid cell with Suprasil quartz windows was placed in the fluorimeter, and a microreference electrode (Ag AgCl) and mini Pt flag counter electrode inserted. The ITO plate (1 cm2) was placed close to the window; the reference electrode was located about 1–2 mm to the side of the ITO plate to minimize the potential drop in solution. The electrolyte contained sodium persulfate and had a pH of 10.5 in most experiments, adjusted with NaOH. The electrochemical cell was controlled by a MacLab 4e controller and potentiostat [12]. The fluorescence output was recorded directly by the fluorimeter under constant potential conditions. The spectra were recorded at 20nm s − 1 from 450 to 800 nm, with an applied bias of − 2.5 V versus Ag AgCl.
Fig. 1. Cross section through a CdS QDE.
K. Tanno et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 146 (1999) 293–298
295
absorption spectra following film formation have been recorded for CdS films using thiol stabilizers by Weller et al. [14] and by Bard and coworkers [15]. The film absorbance increased linearly with the number of deposition cycles, as shown in Fig. 3a, which indicates that the particle density did not change for successive coatings. The PL spectra of the QD films as a function of the number of coatings are shown in Fig. 3b. The peak emission wavelength shifted by no more than 1 nm for
Fig. 2. (a) Absorption spectra of 0.2 mM HMP stabilized Q – CdS colloids prepared at starting pH values of 9.8, 9.1 and 8.1. (b) PL spectra of HMP stabilized 0.2 mM Q–CdS colloids in solution, with excitation at 400 nm, 5 nm slit width.
3. Results In Fig. 2, we show UV-vis absorption spectra of the colloids used for film preparation. The coating procedure leads to a small red-shift of the particle spectrum, and the exciton shoulder is usually a little weaker in the films than in the colloid spectrum. The effect of particle packing on the exciton absorption band is discussed in terms of effective medium theory elsewhere [13]. The change in the average dielectric function at high particle volume fractions causes a red-shift in the measured band position. Changes in the particle
Fig. 3. (a) Absorbance spectra of the CdS/ITO QDEs as a function of number of deposition cycles. CdS prepared at pH 9.1. (b) PL spectra for the same films. Excitation at 400nm, 5nm slit width.
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K. Tanno et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 146 (1999) 293–298
2–3 min, bubble formation began to cause rapid particle flaking. Consequently, spectra were obtained within the 2 min after inducing accumulation layers within the film. Streckert et al. also reported an initial rapid drop in EL intensity from CdS electrodes when stepped to high cathodic potentials. They attributed this to formation of Cd specks on the electrode surface, which catalysed non-radiative charge carrier recombination.
Fig. 4. Thickness of CdS QDEs as a function of the number of coats, as determined by AFM.
successive coatings. The fluorescence spectra did not change with each coating cycle and also increased almost linearly in intensity, indicating that the deposition procedure is homogeneous, and that the drying process did not alter the particle properties. The film thickness could be readily determined by scratching the film with a scalpel (20 mm wide cuts) and recording the cut profile by AFM. Images revealed the surface to have a roughness of 3–5 nm2 mm − 2. The average thickness of each coating was 1399 5 nm. Usually 4 – 5 coats were applied yielding film thicknesses of around 600– 700 nm. In Fig. 4, the film thickness is plotted as a function of the number of deposition cycles. The films were transparent and showed zero absorbance above 600 nm, after subtraction of the plate absorbance. In Fig. 5, we show the EL spectra obtained from various CdS QDEs. The EL spectra blue shifted as the particle size was decreased. For 3.4 nm particles, a peak at 478 nm was observed, well below the bulk value of 530 nm (green) found for polycrystalline CdS powder electrodes in the same electrolyte [8]. The EEL spectrum obtained from a sintered CdS powder electrode is included for comparison. However, the films were found to discolour rapidly under cathodic biasses. This could be reduced by pre-heating the films, but this in turn resulted in particle growth and red-shifted absorption spectra, indicating particle aggregation. After
4. Discussion Weller and coworkers have shown that careful activation of hexametaphosphate coated CdS particles enables almost complete removal of the red surface state emission and simultaneous enhancement of the exciton emission from CdS crystallites to be achieved [9,10]. For CdS, blue EL is only feasible from excitonic emission and not from
Fig. 5. Electroluminescence spectra from CdS QDEs: (a) peak wavelength =478 nm, CdS preparation pH 9.8; (b) peak wavelength =486 nm, CdS preparation pH 9.1; (c) peak wavelength =516 nm, CdS preparation pH 8.1; (d) CdS powder electrode, peak wavelength =525 nm (green) and 667 nm (red). EEL spectra measured at an applied voltage of −2.5 V versus Ag AgCl for CdS QDEs (a); (b); and (c), and at − 1.2 V versus Ag AgCl for the CdS powder electrode (d). Solutions contained 1 M Na2S2O8, pH 10.5. Intensity scale is arbitrary.
K. Tanno et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 146 (1999) 293–298
trap states. The mechanism for EEL is well established, and may be described by the following reaction sequence: − + CdS ecb + hvb
(1)
− + S2O28 − SO4− +SO24 − ecb
(2)
SO h + SO
(3)
− + ecb + hvb exciton blue light
(4)
h + e red light
(5)
+ vb
− 4
+ vb
− ss
2− 4
In the dry EL effect, hot electrons from the injecting cathode are postulated to cause electronhole generation within the active medium via impact ionization, or through an Auger type energy transfer to the lattice. By comparison, an electrolytic cell set-up does not require hot electrons. The energy required for hole formation within the n-type semiconducting CdS is ‘stored’ within the persulfate radical ion, which can directly inject holes into the valence band. This radical can be generated directly via reaction with conduction electrons arriving at the film surface. As a result, the QDE needs to sustain conduction electron transport to the solution interface, but does not rely on hot electron transport for hole formation. In a bulk semiconductor film, the application of a cathodic bias results in band bending, which serves to accelerate electron flow to the surface. Individual QDs cannot support band bending [16], but the overall film will do so [17]. Under accumulation, electrons arriving at the CdS–H2O interface reduce persulfate ion to persulfate radical anions, which in turn inject holes into the valence band (Eq. (3)). These are immediately annihilated by reaction with conduction electrons, resulting in blue emission (Eq. (4)). The fact that hole injection into the particle results in EL, not anodic corrosion of the particles, clearly indicates that the particles are under accumulation during emission. The efficiency of light emission is strongly dependent on film preparation and drying conditions. Lower DC film resistance generally results in improved EL yield. The weak EL tail at longer wavelengths is attributed to the + recombination of hvb with trapped electrons that have filled the surface states when the CdS film is under accumulation (Eq. (5)). This tail is not
297
evident in the PL spectra which are collected under ‘depletion’ conditions where the surface states are vacant. To determine how critical the stabilizer thickness is, we constructed Q–CdS@SiO2 particle films, which yielded good PL spectra [18]. The QDEs had high ohmic resistance and displayed very low EEL. The silica shell in these films was only 4 nm thick, but this was sufficient to drastically reduce electron transport. An advantage of EEL over solid state EL is that simultaneous electron and hole transport through the QDE is not really required. In the thin film devices conceived by Alivisatos and coworkers, holes enter one side of the film and percolate through to meet electrons injected by the cathode. A hole-conducting polymer is essential for efficient operation. Conversely, in EEL only those particles which can sustain cathodic currents can drive persulfate reduction. Only ‘active’ particles can generate the hole injecting anion, and consequently hole injection is directed precisely to those particles capable of EL. Emission is confined to the surface particles, and good hole transport is not essential. This simplifies film structure. However, the fundamental difficulty in such devices remains. Electron transport between particles implies electronic contact whereas quantized emission requires retention of localized band structures and electronic isolation. For quantized EL emission, individual particle integrity is necessary, but a completely impervious coating such as silica that minimizes coalescence adversely affects electron transport and lowers quantum efficiency.
5. Conclusions We have demonstrated that cathodic potentials can induce electron transfer from ITO to CdS quantum dots, an effect reported previously by Bard using STM [15] and Hodes [19] by photoelectrochemical measurements. Electron transfer from the particles to persulfate results in hole injection and subsequent electroluminescence via exciton formation. The colour can be controlled by varying the particle size. Blue emission from
298
K. Tanno et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 146 (1999) 293–298
Q–CdS particles was observed for the first time. The EEL spectrum is not quite the same as the PL spectrum, but shows a weaker tail at longer wavelengths. This is due to hole annihilation by electrons in filled surface states on the particle surfaces. A serious, but hopefully tractable problem, is that hydrogen evolution causes the films to blister within a few minutes of operation.
Acknowledgements We thank James Chon for assistance with the AFM measurements, Geoff Stevens for valuable discussions and the Advanced Mineral Products Research Centre for financial support.
References [1] V. Colvin, M. Schlamp, A. P. Alivisatos, Nature 370 (1994) 354. [2] B.O. Dabbousi, M.G. Bawendi, O. Onitsaka, M.F. Rubner, Appl. Phys. Lett. 66 (1995) 1316. [3] Y. Yang, S. Xue, S. Liu, J. Huang, J. Shen, Appl. Phys. Lett. 69 (1996) 377.
.
[4] M.V. Artemyev, V. Sperling, U. Woggon, J. Appl. Phys. 81 (1997) 6975. [5] B. Pettinger, H.-R. Scho¨ppel, H. Gerischer, Ber. Bunsenges. Phys. Chem. 80 (1976) 849. [6] H. Streckert, B.R. Karas, D.J. Morano, A.B. Ellis, J. Phys. Chem. 84 (1980) 3232. [7] I.J. Ferrer, P. Salvador, J. Appl. Phys. 66 (1989) 2568. [8] K. Tanno, M. Ashokkumar, D. Simpson, P. Mulvaney, Colloids and Surf. 141 (1998) 143. [9] L. Spanhel, H. Weller, A. Henglein, J. Am. Chem. Soc. 109 (1987) 5649. [10] A. Eychmu¨ller, A. Ha¨sselbart, L. Katsikas, H. Weller, Ber. Bunsenges. Phys. Chem. 95 (1991) 79. [11] J.J. Fendler, in: D. Meisel, P. Kamat (Eds.), Semiconductor nanoclusters -Physical, chemical and catalytic aspects, Elsevier, Amsterdam, 1996, pp.261 – 276. [12] A.D. Instruments P/L Castle Hill, N.S.W. [13] P. Mulvaney, Langmuir (submitted for publication). [14] T. Vossmeyer, L. Katsikas, M. Giersig, I.G. Popovic, K. Diesner, A. Chemseddine, A. Eychmu¨ller, H. Weller, J. Phys. Chem. 98 (1994) 7665. [15] K. Hu, M. Brust, A.J. Bard, Chem. Mater. 10 (1998) 1160. [16] W.J. Albery, P.N. Bartlett, J. Electrochem. Soc. 131 (1984) 315. [17] G. Rothenberger, D. Fitzmaurice, M. Gra¨tzel, J. Phys. Chem. 96 (1992) 5983. [18] M.A. Correa-Duarte, M. Giersig, L.M. Liz-Marza´n, Chem. Phys. Lett. 286 (1998) 497. [19] G. Hodes, I.D.J. Howell, L.M. Peter, J. Electrochem. Soc. 139 (1992) 3136.
Quantized electroluminescence from Q–CdS films immersed in aqueous electrolytes K. Tanno, M. Ashokkumar, P. Mulvaney * Ad6anced Mineral Products Research Centre, School of Chemistry, Uni6ersity of Mebourne, Park6ille, 3052 VIC, Australia Received 6 July 1998; accepted 2 October 1998
Abstract Quantum dot electrodes (QDEs) have been constructed by deposition of Q – CdS particles onto indium – tin oxide substrates. Upon application of cathodic potentials to the electrodes in electrolytes containing persulfate ions (S2O28 − ), light emission is observed. The emission peak depends on the particle size, with electrolyte electroluminescence (EEL) being observed down to 478 nm (blue light) for CdS with a particle size of 3.6 nm. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Blue emission; Quantized electroluminescence; Size quantization; CdS particles; Semiconductor; Quantum dot electrode
1. Introduction Current interest in the emission properties of semiconductors stems from their potential applications in blue LEDs, lasers and optoelectronic switches. To date, blue emission from solid state devices has been achieved using new materials such as GaN. One way to circumvent the search for new materials with good electroluminescence (EL) characteristics is to use size quantization to shift the emission response of more familiar semiconductor materials to higher energies. In this vein, Alivisatos et al. have described thin film devices where the EL spectrum depends on the particle size [1,2]. Using CdSe quantum dots
* Corresponding author. Fax: + 61-3-93446233; e-mail: [email protected].
(QDs), they obtained a shift in emission from red light to orange. The device efficiency is dependent on a hole-conducting polymer. Yang and coworkers reported emission from ZnS nanocrystal films [3]. In this case, the emission was red-shifted from the bulk band gap energy, and was due to activation of a surface recombination centre. Woggon et al. have recently reported EL from CdS particles though no quantization effects were reported [4]. They were able to model the electron transport characteristics of quantum dot electrodes (QDEs) based on the I–V and hn –V response of the films, and confirmed that efficient interparticle electron transport is possible, in the absence of a conducting polymer matrix. In this report we examine an alternative mechanism for obtaining quantized EL based on electrochemical electroluminescence (EEL). EEL was
0927-7757/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 9 8 ) 0 0 8 0 9 - 7
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K. Tanno et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 146 (1999) 293–298
originally observed in CdS electrodes by Gerischer and coworkers over 20 years ago [5]. It requires a potent oxidant to directly inject holes into the valence band, located at about + 1.6 V versus NHE. Ellis and colleagues and Ferrer and Salvador have examined the emission processes at the CdS–water interface in some detail [6,7]. Emission occurs because, under cathodic bias, an n-type electrode material will reduce solution oxidants efficiently. If an intermediate in this redox reaction can inject a hole into the electrode, emission of band gap or trap fluorescence is induced. We previously observed that sintered CdS powder electrodes could be made to emit either green or red EL depending on electrode preparation [8]. By employing Q – CdS particles, it is possible to tune this EEL, and obtain direct emission from the quantized semiconductor exciton states. Using this method, blue EL from quantum dot particles has been obtained for the first time. However, as will be shown, the EEL effect is short-lived in our current QDEs because, under strong cathodic bias, the films peel off the ITO substrate, primarily due to hydrogen evolution which accompanies light emission.
2. Experimental section CdS quantum dots were prepared using the optimized methods of Weller et al. [9,10]. Briefly, Cd(ClO4)2.6H2O (0.02 M, 10 ml) and sodium hexametaphosphate (0.02 M, 10ml, Riedel de Ha¨en), were placed in 1 l of Milli-Q water and degassed with nitrogen for 20 min. The pH was adjusted to values between 8.1 and 9.8 according to the particle size required. Then H2S (4.5 ml) was injected by microsyringe. The vessel was shaken for 2 min and then stirred for 10 min until reaction was complete. The pH of the sol was then raised to 10.5 by dropwise addition of 1 M NaOH. Bandgap fluorescence was activated by gradual addition of 0.1 – 0.5 mM Cd(ClO4)2. Fluorescence spectra were obtained with a Hitachi F-4500 spectrofluorimeter using 2.5 nm slits, and UV-vis spectra with a Hitach U-2000 spectrophotometer. ITO glass was procured from Delta Technology Limited and had a surface resistance
of 100 V. To prepare films, the CdS colloid was first rotary evaporated to reduce the solution volume from 1 l to 100 ml. The CdS was applied by several techniques. Initially we spin coated the colloid solution, but found the films to be inhomogeneous, with thick rinds around the edge of the plates. We also tried using a cationic polymer technique pioneered by Fendler et al. [11] as a means to enhance adsorption. Eventually, we found satisfactory films could be prepared by accurately microsyringing 20 ml of sol onto the film, and air drying at 100°C for 10 min in a conventional oven. This process was repeated to form n-layer films. Electrical contact was made with a copper wire and conducting epoxy resin. The copper wire was placed inside a glass capillary to prevent direct contact between the copper and electrolyte. A cross section through a QDE is illustrated in Fig. 1. EEL measurements were made directly in the Hitachi fluorimeter. A small fluid cell with Suprasil quartz windows was placed in the fluorimeter, and a microreference electrode (Ag AgCl) and mini Pt flag counter electrode inserted. The ITO plate (1 cm2) was placed close to the window; the reference electrode was located about 1–2 mm to the side of the ITO plate to minimize the potential drop in solution. The electrolyte contained sodium persulfate and had a pH of 10.5 in most experiments, adjusted with NaOH. The electrochemical cell was controlled by a MacLab 4e controller and potentiostat [12]. The fluorescence output was recorded directly by the fluorimeter under constant potential conditions. The spectra were recorded at 20nm s − 1 from 450 to 800 nm, with an applied bias of − 2.5 V versus Ag AgCl.
Fig. 1. Cross section through a CdS QDE.
K. Tanno et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 146 (1999) 293–298
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absorption spectra following film formation have been recorded for CdS films using thiol stabilizers by Weller et al. [14] and by Bard and coworkers [15]. The film absorbance increased linearly with the number of deposition cycles, as shown in Fig. 3a, which indicates that the particle density did not change for successive coatings. The PL spectra of the QD films as a function of the number of coatings are shown in Fig. 3b. The peak emission wavelength shifted by no more than 1 nm for
Fig. 2. (a) Absorption spectra of 0.2 mM HMP stabilized Q – CdS colloids prepared at starting pH values of 9.8, 9.1 and 8.1. (b) PL spectra of HMP stabilized 0.2 mM Q–CdS colloids in solution, with excitation at 400 nm, 5 nm slit width.
3. Results In Fig. 2, we show UV-vis absorption spectra of the colloids used for film preparation. The coating procedure leads to a small red-shift of the particle spectrum, and the exciton shoulder is usually a little weaker in the films than in the colloid spectrum. The effect of particle packing on the exciton absorption band is discussed in terms of effective medium theory elsewhere [13]. The change in the average dielectric function at high particle volume fractions causes a red-shift in the measured band position. Changes in the particle
Fig. 3. (a) Absorbance spectra of the CdS/ITO QDEs as a function of number of deposition cycles. CdS prepared at pH 9.1. (b) PL spectra for the same films. Excitation at 400nm, 5nm slit width.
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K. Tanno et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 146 (1999) 293–298
2–3 min, bubble formation began to cause rapid particle flaking. Consequently, spectra were obtained within the 2 min after inducing accumulation layers within the film. Streckert et al. also reported an initial rapid drop in EL intensity from CdS electrodes when stepped to high cathodic potentials. They attributed this to formation of Cd specks on the electrode surface, which catalysed non-radiative charge carrier recombination.
Fig. 4. Thickness of CdS QDEs as a function of the number of coats, as determined by AFM.
successive coatings. The fluorescence spectra did not change with each coating cycle and also increased almost linearly in intensity, indicating that the deposition procedure is homogeneous, and that the drying process did not alter the particle properties. The film thickness could be readily determined by scratching the film with a scalpel (20 mm wide cuts) and recording the cut profile by AFM. Images revealed the surface to have a roughness of 3–5 nm2 mm − 2. The average thickness of each coating was 1399 5 nm. Usually 4 – 5 coats were applied yielding film thicknesses of around 600– 700 nm. In Fig. 4, the film thickness is plotted as a function of the number of deposition cycles. The films were transparent and showed zero absorbance above 600 nm, after subtraction of the plate absorbance. In Fig. 5, we show the EL spectra obtained from various CdS QDEs. The EL spectra blue shifted as the particle size was decreased. For 3.4 nm particles, a peak at 478 nm was observed, well below the bulk value of 530 nm (green) found for polycrystalline CdS powder electrodes in the same electrolyte [8]. The EEL spectrum obtained from a sintered CdS powder electrode is included for comparison. However, the films were found to discolour rapidly under cathodic biasses. This could be reduced by pre-heating the films, but this in turn resulted in particle growth and red-shifted absorption spectra, indicating particle aggregation. After
4. Discussion Weller and coworkers have shown that careful activation of hexametaphosphate coated CdS particles enables almost complete removal of the red surface state emission and simultaneous enhancement of the exciton emission from CdS crystallites to be achieved [9,10]. For CdS, blue EL is only feasible from excitonic emission and not from
Fig. 5. Electroluminescence spectra from CdS QDEs: (a) peak wavelength =478 nm, CdS preparation pH 9.8; (b) peak wavelength =486 nm, CdS preparation pH 9.1; (c) peak wavelength =516 nm, CdS preparation pH 8.1; (d) CdS powder electrode, peak wavelength =525 nm (green) and 667 nm (red). EEL spectra measured at an applied voltage of −2.5 V versus Ag AgCl for CdS QDEs (a); (b); and (c), and at − 1.2 V versus Ag AgCl for the CdS powder electrode (d). Solutions contained 1 M Na2S2O8, pH 10.5. Intensity scale is arbitrary.
K. Tanno et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 146 (1999) 293–298
trap states. The mechanism for EEL is well established, and may be described by the following reaction sequence: − + CdS ecb + hvb
(1)
− + S2O28 − SO4− +SO24 − ecb
(2)
SO h + SO
(3)
− + ecb + hvb exciton blue light
(4)
h + e red light
(5)
+ vb
− 4
+ vb
− ss
2− 4
In the dry EL effect, hot electrons from the injecting cathode are postulated to cause electronhole generation within the active medium via impact ionization, or through an Auger type energy transfer to the lattice. By comparison, an electrolytic cell set-up does not require hot electrons. The energy required for hole formation within the n-type semiconducting CdS is ‘stored’ within the persulfate radical ion, which can directly inject holes into the valence band. This radical can be generated directly via reaction with conduction electrons arriving at the film surface. As a result, the QDE needs to sustain conduction electron transport to the solution interface, but does not rely on hot electron transport for hole formation. In a bulk semiconductor film, the application of a cathodic bias results in band bending, which serves to accelerate electron flow to the surface. Individual QDs cannot support band bending [16], but the overall film will do so [17]. Under accumulation, electrons arriving at the CdS–H2O interface reduce persulfate ion to persulfate radical anions, which in turn inject holes into the valence band (Eq. (3)). These are immediately annihilated by reaction with conduction electrons, resulting in blue emission (Eq. (4)). The fact that hole injection into the particle results in EL, not anodic corrosion of the particles, clearly indicates that the particles are under accumulation during emission. The efficiency of light emission is strongly dependent on film preparation and drying conditions. Lower DC film resistance generally results in improved EL yield. The weak EL tail at longer wavelengths is attributed to the + recombination of hvb with trapped electrons that have filled the surface states when the CdS film is under accumulation (Eq. (5)). This tail is not
297
evident in the PL spectra which are collected under ‘depletion’ conditions where the surface states are vacant. To determine how critical the stabilizer thickness is, we constructed Q–CdS@SiO2 particle films, which yielded good PL spectra [18]. The QDEs had high ohmic resistance and displayed very low EEL. The silica shell in these films was only 4 nm thick, but this was sufficient to drastically reduce electron transport. An advantage of EEL over solid state EL is that simultaneous electron and hole transport through the QDE is not really required. In the thin film devices conceived by Alivisatos and coworkers, holes enter one side of the film and percolate through to meet electrons injected by the cathode. A hole-conducting polymer is essential for efficient operation. Conversely, in EEL only those particles which can sustain cathodic currents can drive persulfate reduction. Only ‘active’ particles can generate the hole injecting anion, and consequently hole injection is directed precisely to those particles capable of EL. Emission is confined to the surface particles, and good hole transport is not essential. This simplifies film structure. However, the fundamental difficulty in such devices remains. Electron transport between particles implies electronic contact whereas quantized emission requires retention of localized band structures and electronic isolation. For quantized EL emission, individual particle integrity is necessary, but a completely impervious coating such as silica that minimizes coalescence adversely affects electron transport and lowers quantum efficiency.
5. Conclusions We have demonstrated that cathodic potentials can induce electron transfer from ITO to CdS quantum dots, an effect reported previously by Bard using STM [15] and Hodes [19] by photoelectrochemical measurements. Electron transfer from the particles to persulfate results in hole injection and subsequent electroluminescence via exciton formation. The colour can be controlled by varying the particle size. Blue emission from
298
K. Tanno et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 146 (1999) 293–298
Q–CdS particles was observed for the first time. The EEL spectrum is not quite the same as the PL spectrum, but shows a weaker tail at longer wavelengths. This is due to hole annihilation by electrons in filled surface states on the particle surfaces. A serious, but hopefully tractable problem, is that hydrogen evolution causes the films to blister within a few minutes of operation.
Acknowledgements We thank James Chon for assistance with the AFM measurements, Geoff Stevens for valuable discussions and the Advanced Mineral Products Research Centre for financial support.
References [1] V. Colvin, M. Schlamp, A. P. Alivisatos, Nature 370 (1994) 354. [2] B.O. Dabbousi, M.G. Bawendi, O. Onitsaka, M.F. Rubner, Appl. Phys. Lett. 66 (1995) 1316. [3] Y. Yang, S. Xue, S. Liu, J. Huang, J. Shen, Appl. Phys. Lett. 69 (1996) 377.
.
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