397
J. Phys. Chem. 1985,89, 397-399
Size Quantization in Small Semiconductor Partlcied A. J. Nozik,* Ferd Williams,$ Solar Energy Research Institute, Golden, Colorado 80401
M. T. Nenadovic, T. Rajh, and 0. I. MiCiE* Boris Kidric Institute of Nuclear Sciences, Belgrade, Yugoslavia 11001 (Received: September 1 1 , 1984; In Final Form: November 12, 1984)
Optical effects due to size quantization in three dimensions have been observed for CdS and PbS colloids with particle diameters less than 50 8, and 20 to 200 A, respectively. The optical absorption edge is blue shifted -0.9 eV for the CdS colloid; for PbS colloids the shift is -2 eV for particle diameters 20-30 A, and 1 eV for particle diameters 50-200 A. The results are consistent with perturbation of the semiconductor band structure due to carrier confinement resulting in an increase in the effective band gap,
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Quantization effects that arise from the confinement of charge carriers in semiconductors with potential wells of small dimensions have been extensively studied in recent years. The vast bulk of the work to date has been done in thin-film, laminar These structures are one-dimensional quantum wells; when multiple wells are electronically coupled the resulting layered structures are termed superlattices.' Quantum confinement in two dimensions results in quantum well wires, and such systems have recently been r e p ~ r t e d . ~Quantum ,~ confinement in three dimensions should occur with small semiconductor particles. We report here initial experiments with small colloidal particles of CdS ( 200 A is compared with that for D, < 50 8, in Figure 1; the CdS concentration is 1.75 X lo4 M for both colloids. Also shown in Figure 1 is the expected absorbance for the CdS colloid calculated from optical absorption data for single crystal C d S 9 The calculated absorbance is in reasonably good agreement with the results for the CdS colloids with D, > 200 A. For the CdS colloid with D, < 50 A the absorption edge is blue shifted by about 0.9 eV (140 nm). Curves a and b are blank spectra of 0.1% polyethylene glycol in acetonitrile and 1.75 X lo4 M Na2S containing 0.1 8% H 2 0in acetonitrile, respectively. The spectra of CdS colloids in water with equivalent particle sizes is similar to Figure 1. Photoluminescence emission spectra of CdS colloids with D, < 50 A show an emission peak at -360 nm with excitation from 278 to 333 nm. A small amount of emission also occurs at about 725 nm. The absorption spectra of PbS colloids with D, 20 A and D, 50-200 A are shown in Figure 2, b and a, respectively. The band gap of PbS is 0.4 eV, and colloids with large particle sizes (D, > 200 A) are black and opaque. Emission spectra of the small particle samples show peaks at 436 and 600 nm; the larger particle samples have only one peak at 600 nm. Excitation for these emission peaks were at 355 or 410 nm. We believe the absorption spectra presented in Figures 1 and 2 and the emission results reflect effects of size quantization. The
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(1) K. Ploog and G. H. Dohler, Adu. Phys., 32, 285 (1983). (2) R. Dingle, Adu. Solid Store Phys., 15, 21 (1975). (3) P. M. Petroff, A. C. Gbssard, R. A. Logan, and W. Wiegmann, Appl. ~ h y sLett., . 41, 633 (1982). (4) M. Laviron, P. Averbuch, H. Godfrin, and R. E. Rapp, J. Phys., Lett. (Orsoy, Fr.), 44, L-1021 (1983). (5) R. Rossetti, S. Nakahara, and L. E. Brus, J . Chem. Phys., 79, 1086 (1983). (6) L. E. Brus, J . Chem. Phys., 79, 5566 (1983). (7) R. Rossetti, J. L. Ellison, J. M. Bigson, and L. E. Brus,J. Chem. Phys.,
--.(8) L . E. Brus, J . Chem. Phys., 80, 4403 (1984).
80. 4464 11984). ,--I
'Dedicated to the memory of Ferd Williams. *DeceasedJune, 1984; work performed while on sabbatical leave from the Department of Physics, University of Delaware, Newark, DE 19711.
(9) D. Dutton, Phys. Reu., 112,785 (1958); E. Khawaja and S . G. Tomlin, J . Phys., D,8, 581 (1975).
0022-3654/85/2089-0397$01.50/00 1985 American Chemical Society
398 The Journal of Physical Chemistry, Vol. 89, No. 3, 1985
, , 4;O
3;5
Energy (eV) 3;O
2;5
,
r
1
CdS colloid spectra in acetonitrile
0.5
Letters
Solutlon conduction band
0.4 m C
!! 0.3 n 2
a
0.2
t
E,
0.1 0.0
275 300 325 350 375 400 425 450 475 500 525
550
Wavelength (nm)
lo4 M) in acetonitrile. Blank spectra of polyethylene glycol and NazS solutions are given in curves a and b, respectively (see text).
Figure 1. Optical absorption spectra of CdS colloids (1.75 X
o,8
.,
4.03.5 .,. 3.0
Energy (eV) 2;5
2;O
1;5
,
PbS colloid spectra in acetonitrite
0.6
Wavelength (nm)
Figure 2. Optical absorption spectra of PbS colloids (1.0 X lo4 M) in acetonitrile: (a) particle diameter -50-200 A; (b) particle diameter -20-30 A (see text).
charge camers in the semiconductor particles are in potential wells defined by the conduction and valence bands of the solid and liquid phase (see Figure 3). This carrier confinement leads to the creation of discrete levels in the conduction and valence bands if Dp becomes small. Such quantum sizes effects on the structure of semiconductor band states have been discussed for laminar structures;2 we believe this effect will also be present in small semiconductor particles.l0 Furthermore, carrier confinement may produce correlation between the confined electron and hole that results in large excitonic effects? We believe that the predominant effect will depend upon the fate of the photoexcited carriers. If they are rapidly removed from the particle through photoemission or fast photoinduced charge transfer, then the major effect of small particle size is quantization of band states. If the photogenerated carriers are confined to the particle for longer times, then large increases in the energy of excitons created during photoexcitation become evident.* The spectra in Figures 1 and 2 do not show significant excitonic effects but rather show large shifts in the fundamental absorption edge reflecting an increase in the effective band gap. For CdS colloids with Dp 30-40 A, the band edge shift is -0.9 eV (bulk gand gap = 2.4 eV); for PbS particles the bulk band gap is 0.4 eV and the observed shifts in the band edges are -2.0 eV for D, 20-30 A, and 1 eV for Dpbetween 50 and 200 A. The larger shift for PbS is consistent with it having a lower electron effective mass (me* = 0.1) compared to CdS (me*= 0.2)."
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(10) F. Williams and A. J. Nozik, Nature (London), 311, 21 (1984).
$ !]:
(Large Particles)
Solution valence band
Figure 3. Energy diagram showing quantization effects in small semiconductor particles in a liquid matrix.
The photoluminescence emission spectra of CdS and PbS colloids at 360 and 436-600 nm, respectively, are also consistent with an increased band gap. The small amount of emission at 725 nm for CdS is attributed to the presence of a small number of large particles with bulk properties which normally emit in the red. These emission spectra also confirm that the absorption shifts in Figures 1 and 2 are not simply due to mass concentration effects. For quantized particles that have a very uniform size, one would expect to see welldefined structure in the absorption edge because of the discrete levels. However, the present colloids do not have a sufficiently narrow size distribution to exhibit such structure in the optical spectra. The absence of significant excitonic peaks suggests that perturbation of the band structure is more important in our samples than electron-hole correlation. This implies that there is significant leakage of either the electron or hole out of the particle before full excitonic effects can be developed. Figure 1 also shows a shoulder followed by a steep rise in absorption at -300 nm for D, < 50 A. This may be a quantum confinement effect caused by variations in the perturbation of the band structure depending on the effective masses of the r-,X-, and L-band minima. We expect the I?-band minimum to be more sensitive to quantization effects and perturbed to higher energies compared to the L- and X-band minima because of the lower effective mass of the I' point. This could lead to degeneracy of the r-,X-, and L-band minima and result in enhanced absorption. However, according to the band structure of cubic CdS,'* it would be expected that this degeneracy should occur at -230 nm (5.4 eV) rather than at 300 nm. Further work is required to establish whether this effect is indeed present in small semiconductor colloids. The absorption edge for PbS sample b in Figure 2 shows a long tail that arises from the broad distribution of particle sizes; sample a has a narrower size distribution and shows much less of a tail. The photoluminescence emission of PbS is consistent with the absorption spectra. The smaller particle size samples show emission at 436 and 600 nm, while the larger particle size samples only show emission at 600 nm. The shift of -2 eV in the effective gand gap for 20-30 A PbS is quite dramatic. The existence of size quantization effects in semiconductor colloids may have important implications for the photoelectrochemistry of such systems. In addition to the very large effects on optical properties, size quantization could lead to major changes in the effective redox potential of the photogenerated carrier^.^.'^ In particular, creation and utilization of type I1 hot carriers13may (1 1) J. I. Pankove, "Optical Processes in Semiconductors", Prentice Hall, Englewood Cliffs, NJ, 1981. (12) A. Zunger and A. J. Freeman, Phys. Rev. B, 17, 4850 (1978).
399
J. Phys. Chem. 1985, 89, 399-401 be possible, and this would lead to higher conversion efficiencies. Further work is underway to explore the photochemistry, as well as the photophysics, of small, quantized semiconductor particles. Note: It was pointed out by a referee that, after this paper was submitted, another publication appeared (ref 14) which deals with (13) G. Cooper, J. A. Turner, B. A. Parkhon, and A. J. Nozik, J . A P d . Phys., 54, 6463 (1983). (14) H. Weller, V. Koch, M. Gutierrez, and A. Henglein, Eer. Eusenges. Phys. Chem., 88, 649 (1984).
the unique optical properties of very small ZnS particles. Acknowledgment. We thank Kim Jones for obtaining TEM pictures of the colloids, Ken Marsh and John Connolly for obtaining emission spectra, and Brad Thacker for technical assistance in several experiments. A.J.N. was supported by the U S . Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences. O.I.M, M.T.N, and T.R.were supported by the US-Yugoslavia Joint Research Fund. Registry No. CdS, 1306-23-6; PbS, 1314-87-0.
Proton Transfer Tautomerism in the Excited State of Indazoie in Acetic Acid: Tautomerization via Double Proton Switching Masayo Noda, Tokai Women's College, Kagamigahara, Gifu 504, Japan
Noboru Hirota,* Department of Chemistry, Faculty of Science, Kyoto University, Kyoto 606, Japan
Minoru Sumitani, and Keitaro Yoshihara Institute for Molecular Science, Myodaiji, Okazaki 444, Japan (Received: October 3, 1984)
It is shown that the SI state of 1H-indazole in acetic acid undergoes double proton transfer along the hydrogen bonds in the indazole-acetic acid complex. The proton transfer converts 1H-indazole into 2H-indazole. The rate of proton transfer is estimated to be 2.7 X lo9 s-I from transient fluorescence measurements.
Introduction Excited-state tautomerism associated with proton transfer is a topic of considerable current interest and kinetic details of the transfer processes have been investigated actively by picosecond spectroscopy.l-I0 In the course of our investigation of the lowest excited triplet (T,) state of indazole in a benzoic acid host we found that the TI state of indazole exists in two tautomeric forms, 1Hand 2H-indazole (abbreviated as 1H and 2H, respectively)."
I
H
2H-indarole
1H-indazole
(1) Hertherington, W. M.; Micheels, R. H.; Eisenthal, K. B.Chem. Phys. Lett. 1979, 66, 230. (2) Wang, Y . ;Eisenthal, K. B. J . Chem. Phys. 1982, 77, 6076. (3) Itoh, M.; Tokumura, K.; Tanimoto, Y.; Okada, Y.; Takeuchi, H.; Obi, K.; Tanaka, I. J . Am. Chem. SOC.1982, 104,4146. (4) Strandjord, A. J. G.; Courtney, S. H.; Friedrich, D. M.; Barbara, P. F.J . Phys. Chem. 1983,87, 1125. ( 5 ) Flom, S. R.; Barbara, P. F. Chem. Phys. Lett. 1983, 94, 488. (6) Strandjord, A. J. G.; Barbara, P. F. Chem. Phys. Lett. 1983,98, 21. (7) Barbara, P. F.; Rentzepis, P. M.; Brus, L. E. J. Am. Chem. SOC.1980, 102, 2186. (8) Campillo, A. J.; Clark, J. H.; Shapiro, S . L.; Winn, K. R.; Woodbridge, P. K. Chem. Phys. Lert. 1979.67, 218. (9) Smith, K. K.; Kaufmann, K. J.; Huppert, D.; Gutman, M. Chem. Phys. Lett. 1979, 64, 522. (10) Thistlethwaite, P. J.; Corkill, P. J. Chem. Phys. Lett. 1982,85, 317. (11) Noda, M.; Hirota, N. J . Am. Chem. SOC.1983, 105, 6790.
0022-3654/85/2089-0399$01.50/0
Since indazole in the ground state is considered to exist only in the 1H form, it was thought that proton transfer in the excited singlet (S,) state is responsible for the formation of 2H in the T1 state. 1H in the benzoic acid host is hydrogen bonded with benzoic acid and double proton switching along hydrogen bonds can convert 1H into 2H. In order to examine the possibility of such proton transfer tautomerism in indazole, we have studied the fluorescence properties of indazole in various protic solvents. Here we report observations in acetic acid which clearly demonstrated the occurrence of such tautomerism in the S, state of indazole. Experimental Section Indazole (Aldrich) was purified by repeated recrystallization from water and vacuum sublimation. 1-Methylindazole and 2-methylindazole were synthesized according to the procedures given by AuwersI2 and Shad,13respectively. Acetic acid, ethanol, and cyclohexane (Wako Spectrosole) were used without further purification. Concentrations of the solutions were 2 X 10" M. Absorption spectra were recorded with a Shimadzu U-125MU spectrophotometer. Fluorescence emission and excitation spectra were taken with a Hitachi MPF-2A spectrofluorimeter. The fluorescence rise and decay curves were recorded with a picosecond spectroscopic system consisting of a mode-locked Nd:YAG laser, optoelectronics to produce 10-ps, 266-nm excitation pulses, a Hamamatsu C979 streak camera, and a microcomputer (Hamamatsu ClOOO) to process transient ~igna1s.I~
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(12) Auwers, V.;Duisberg, M. Eerichte 1920, 53, 1179. (13) Shad, P. Eerichte 1893, 26, 218. (14) Takagi, Y.; Sumitani, M.; Yoshihara, K. Rev. Sci. Instrum. 1981, 52, 1003.
0 1985 American Chemical Society
