J . Phys. Chem. 1987, 91, 5005-5008 further increased to 340 Torr the frequency increases. The frequency is highest when the sample is first exposed to the laser beam and gradually diminshes with exposure time. The spectra are shown in Figure 2. Data collection was initiated at the same time that the laser was first allowed to excite the sample; four consecutive 10-s accumulations were taken without moving the cell or changing the power. To obtain a spectrum of the orange emission, spectrum C (containing no transitory orange emission) was subtracted from spectrum A (containing many transitory orange emissions). The result, spectrum D, is a broad featureless band centered at about 16000 cm-I. The orange emission is accompanied by deposition of large amounts of black powder. The absorption spectrum of gas-phase Ni(C0)4 from 2000 to 5000 A consisted of a peak at 2050 8, with a long tail extending to about 3800 A. The peak may correspond to the peak at 2400 A seen by Lever et al. for Ni(CO)4 frozen in a CO matrix.6 They assigned this peak as a set of overlapping MLCT bands from the t2(d") and e(dr) orbitals on the Ni to R* orbitals on the carbonyls. The molar extinction coefficients (M-' cm-I) at the laser lines are t = 16 at 3400 8,;t = 5.2 at 3511 8,; and t = 1.5 at 3638 8,. Discussion The 13 700-cm-I band which is observed at low laser power (Figure 1) probably originates from a nickel carbonyl fragment early in the photofragmentation pathway. That it is not from excited N i ( C 0 ) 4 is strongly suggested by three facts. The first is the large separation between the end of the absorption spectrum of N i ( C 0 ) 4 (at about 26500 cm-I) and the E , band of the emission spectrum (about 18 500 cm-l). This 8000-cm-] gap suggests that the emission and absorption spectra belong to two different species. The second piece of evidence is provided by the vibronic structure. The 400-cm-l spacing is significantly larger than the 369-cm-I totally symmetric metal-ligand mode of Ni( 6 ) Lever, A. B. p.; Ozin, G. A,; Hanlan, A. J. L.; Power, W.J.; Gray, H. B. Inorg. Chem. 1979, 18, 2088.
5005
( c o ) 4 . 7 Both theory8 and experiment9 suggest that the energy of the totally symmetric metal carbon stretch of Ni(C0)3 should be greater than that of Ni(CO)+ Thirdly, the molecular orbital calculations by Rosch and co-workersI0 indicated that the luminescence from Ni(CO), should occur in the red region of the spectrum. These three facts, together with the observation that the spectrum in Figure 1 can be excited by unfocused laser powers as low as 2 mW, suggest that the emitting species is a nickel carbonyl early in the fragmentation pathway. Callear" has derived the following decomposition mechanism from his flash photolysis studies of Ni(CO),: Ni(C0)3 + CO Ni(C0)4 hv (step 1, reversible)
--
+ N i ( C 0 ) 3 + hv 2Ni(C0)2
Ni(C0)2 + CO
solid product
(step 2, reversible)
(step 3, spontaneous)
The red luminescence is most likely associated with a product or intermediate from step 1. The other luminescence bands, which are only observed at high powers, probably originate from the more highly dissociated fragments or from clusters from fragmentfragment and/or fragment-precursor reactions. Further characterization of the unusual dynamics and time dependences of the luminescence under C W excitation and of the spectra are in progress.
Acknowledgment. The support of the National Science Foundation (CHE85-09329) is gratefully acknowledged. D.M.P. acknowledges the receipt of a Dr. Ursula Mandell Fellowship. (7) Jones, L. H.; McDowell, R. S.; Goldblatt, M. J . Chem. Phys. 1968, 48, 2663.
(8) Cirsky, P.; Dedieu, A. Chem. Phys. 1986, 103, 265. (9) DeKock, R. L. Inorg. Chem. 1971,10, 1205. DeKock found that the C-0 stretch was 35 cm-' lower for the tricarbonyl than for the tetracarbonyl, which suggests there may be more backbonding in the tricarbonyl and an increase in the Ni-C stretching frequency. (10) Rosch, N.; Jorg, H.; Kotzian, M. J . Chem. Phys. 1987, 86, 4038. (11) Callear, A. B. Proc. R. SOC.,Ser. A . 1961, 265, 71.
Chemical Effects on the Optical Properties of Semiconductor Particles Y. Wang* and N. Herron* E . I. du Pont de Nemours and Company,? Central Research and Development Department, Experimental Station, Wilmingfon, Delaware I9898 (Received: April 16, I987; In Final Form: June 11, 1987)
The optical spectra of many recently synthesized colloidal semiconductor particles often show large deviation from the bulk semiconductor spectra. This is often attributed to quantum size effects. We provide experimental evidences to demonstrate that (1) optical properties of small semiconductorparticles can be modified by chemical interactions with surrounding molecules and (2) unexpected chemical species may be generated either as a result of the strong chemical interactions or as a by-product during semiconductor synthesis. One problem we have discovered so far is the facile oxidation reactions of I-, H,Se, H,As, and PH3 by molecular oxygen or by commonly used anions such as NO3- and C104-. The species generated, 13-, Se, As, and P, have strong UV-vis absorption and can interfere with the optical spectra of the semiconductor colloids under study.
Introduction Small semiconductor clusters and particles possess hybrid electronic properties that differ from both molecules and bulk. and Although the preparation of small semiconductor the observation of a quantum size effect were reported some time systematic studies began only recently."16 It appears now that many different semiconductor particles can be prepared in Contribution No. 4358.
0022-3654/87/2091-5005$01 .SO10
a variety of media including solutions,4-'2 gla~ses,'~ zeolite^,'^^^^ and polymers.I6 The availability of this new class of semiconductor (1) Berry, C. R. Phys. Rev. 1967, 161, 848. (2) Mumaw, C. T. Photogr. Sci. Eng. 1980, 24, 77. (3) (a) Chang, L. L.; Esaki, L.; Howard, W. E.; Ludeke, R. J . Vac. Sei. Technol. 1973, 10, 11. (b) Cho, A. Y.; Arthur, J. R. Prog. Solid State Chem. 1975, 10, 157. (c) Dingle, R.; Gossard, A. C.; Wiegmann, W. Phys. Reo. Lett. 1975, 34, 1327. (4) Brus, L. E. J . Phys. Chem. 1986, 90, 2555 and references therein. (5) Chestnoy, N.; Hull, R.; Brus, L. E. J . Chem. Phys. 1986, 85, 2237.
0 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 19, 1987
5006
Letters
composite materials will hopefully facilitate our quantitative understanding of the transition from molecular to bulk properties. Potential applications also exist for these composite materials in the area of optoelectronics.I6 It is commonly observed that as the semiconductor particle size decreases, the optical absorption edge shifts to the b l ~ e . ~ - ] ~ Sometimes it is also accompanied by the appearance of discrete absorption bands. This change in optical properties is usually attributed to the quantum size effect. Although detailed quantitative treatment is not yet available, qualitatively the quantum size effect can be understood on the simple Hiickel MO level. The simple “electron-hole in a box” model identifies the blue shift as mainly due to increasing kinetic energy of the electron-hole pair as a result of their spatial confinement, balanced by the screened Coulomb interaction. However, the optical properties of a semiconductor can also 0 z be modified by mechanisms other than the quantum size effect. If the structure of the particle changes slightly from the bulk, it can have a major effect. The magnitude of this effect can be 0.10+ \ estimated from the pressure coefficient of the gap and bulk compressibility. For example, a 1% volume change of the PbS 0.00 unit cell can induce a shift of the band gap by about 0.045 eV 230 330 430 530 630 730 800 eV/bar and the (taking the pressure coefficient to be 9.1 X WAVELENGTH, NM bulk compressibility to be 2 X 10” bar-] In colloidal solutions, Figure 1. Absorption spectra of pyridine-intercalated PbI, and Bil, effects of particle aggregation on the optical properties are still solids. These are calculated from Kubelka-Munk diffuse reflectance theory (not corrected for scattering). The samples are diluted with not clear. Finally, the semiconductor particles can interact with BaSOl and run against BaSO,. surrounding molecules and this interaction can also alter the optical properties. This effect can be large for small particles due to their large surface area. In this Letter we address this last issue, that is, chemical effects on the optical properties of semiconductor particles. We provide (a) Bi13/acetonitrile several examples to show that strong interactions exist between OD 0 - 1 5 semiconductors and several commonly used organic solvents and the nature of the semiconductor can be greatly modified due to the strong chemical interaction. Furthermore, significant chemical species may be generated either as a result of this interaction or as a by-product of the semiconductor synthesis procedures. These (b) Bilg/water OD. 0. 0.5 unexpected by-products may have strong UV-vis transitions which can interfere with the optical spectrum of the semiconductor particles under study, and misinterpretation may result. Results and Discussion Layered Semiconductors. Layered semiconductors are ideal for probing the effects of chemical interactions, since the weak (6) Weller, H.; Schmidt, H. M.; Koch, U.; Fojtik, A,; Baral, S.;Henglein, A,; Kunath, W.; Weiss, K.; Dieman, E. Chem. Phys. Lett. 1986, 124, 557 and references therein. (7) (a) Weller, H.; Fojtik, A.; Henglein, A. Chem. Phys. Lett. 1985, 117, 485. Note that cadmium phosphide can exist as Cd3P2,Cd2P3,Cd6P7,CdP2, and Cd7Plo. (b) Fojtik, A.; Weller, H.; Henglein, A. Chem. Phys. Lett. 1985, 120, 552. Note that cadmium arsenide can exist as Cd3As2,CdAs, or CdAs,. (8) (a) Nozik, A. J.; Williams, F.; Nenadovic, M. T.; Rajh, T.; Micic, 0. I . J. Phys. Chem. 1985, 89, 397. (b) Nedeljkovic, J. M.; Nenadovic, M. T.; Micic, 0. I.; Nozik, A. J. J. Phys. Chem. 1986, 90, 12. (9) Ramsden, J. J.; Webber, S. E.; Gratzel, M. J. Phys. Chem. 1985,89, 2740. ( I O ) Tricot, Y.-M.; Fendler, J. H. J. Phys. Chem. 1986, 90, 3369. ( 1 1 ) (a) Sandroff, C. J.; Hwang, D. M.; Chung, W. M. Phys. Rev. B.: Condens. Matter 1986, 33, 5953. (b) Sarid, D.;Rhee, B. K.; McGinnis, B. P.; Sandroff, C. J. Appl. Phys. Lett. 1986, 49, 1196. (c) Sandroff, C. J.; Farrow, L. A. Chem. Phys. Lett. 1986,130,458. (d) Sandroff, C. J.; Chung, W. M. J. Colloid Interface Sri. 1987, 115, 593. (e) Sandroff, C. J.; Kelty, S. P.; Hwang, D. M . J. Chem. Phys. 1986, 85, 5337. (12) Dannhauser, T.; O’Neil, M.; Johansson, K.; Whitten, D.; McLendon, G.J. Phys. Chem. 1986, 90, 6074. (13) (a) Ekimov, A. I.; Onushchenko, A. A. JETPLett. 1984, 40, 1136. (b) Ekimov, A. I.; Efros, AI. L.; Onushchenko, A. A. Solid State Commun. 1985. 56. 921. Wang, Y.; Herron, N. J. Phys. Chem. 1987, 91, 257. Parise, J. B.; Mac Dougall, J.; Herron, N.; Farlee, R.; Sleight, A. W.; Y . ;Bein, T.; Moller, K.; Moroney, L. M., submitted for publication. Wang, Y.; Mahler, W. Opt. Commun. 1987, 61, 233. (a) Landolt-Bornstein, New Series; Madelung, O., Ed.; SpringerNew York, 1983; Vol. 17, Subvol. f. (b) Landolt-Bornstein, New Madelung, O . , Ed.; Springer-Verlag: New York, 1983; Vol. 17, e.
Ghorayed, A. M.; Coleman, C. C.; Yoffe, A. D. J. Phys. C. 1984, 17,
( c ) Pb1,iwater OD: 0 - 5
(d) PbIpiacetonitrile OD: 0. 0.4
(e)
IVV ” 200
300
Kliacelonitrile, HN03
400
500
600
Wavelength (nm)
Figure 2. (a)-(d) Absorption spectra of dissolved PbI, and Bil, in water and acetonitrile. ( e ) Control experiment to demonstrate the formation of 13-, as explained in the text.
interactions between layers allow easy access and intimate contact with other molecules. We examined PbI, and BiI, (bulk powder form, puratronic grade from Alfa) in three organic solvents: water, acetonitrile, and pyridine. By simply immersing the semiconductor particles into various solvents, one can readily observe changes in the optical properties in either the solution or the semiconductor itself. These data are now presented below. The most dramatic change is observed for Pb12 and BiI, in pyridine. The yellow PbIz solid turned white, and the black BiI, turned orange (Figure 1 ) . This is attributed to the intercalation of pyridine into Pb12 and BiI, and the subsequent chemical in-
The Journal of Physical Chemistry, Vol. 91, No. 19, 1987 5007
Letters teractions.'* Since pyridine is known to be a very good ligand for ions such as Cd2+ and Pb2+, we attribute the underlying c h e m i ~ t r y 'to ~ ~the , ~ reaction Pb12
+ 2(pyridine) s P b ( p ~ r i d i n e ) ~ I ~
+ I2 @ I,-
(2)
I- has an absorption peak at 225 nm. Is- has known electronic transitions at about 225, 290, 360,440, and 565 nm, the exact locations depending on the environment and whether it is in a symmetric or asymmetric configuration.20 Equilibrium 2 exists because of the ease with which I- may be oxidized to 12. We verified this by simply adding one drop of HNO, to a 0.05 M KI/acetonitrile solution, in the absence of either Pb2+ or Bi3+. A yellow-brown color appears immediately. The spectrum is shown in Figure 2e. The absorption peaks of I- and I,- can be readily identified. In this example, NO3-, which was also used in the literature synthesis of Pb12 and Bi13,11 is the probable oxidizing agent. The following redox process is exothermic by about 0.41 V.21 21- s 12(s) + 2e-
EO,,
= -0.535
+ 3H+ + 2e- H N 0 2 + H 2 0 N o 3 - + 4H+ + 3e- s N O + 2 H 2 0
NO3-
Eorcd= 0.94
a
(1)
In other words, a different type of solid is formed due to the interaction between Pb12 (or BiI,) and pyridine. When Bi13 is immersed into water or acetonitrile, the solutions became yellow-brown (Figure 2a,b). The color of the undissolved BiI, remained black in acetonitrile but turned dark brown in water. Similarly, immersion of Pb12into water or acetonitrile gave yellow solutions (Figure 2c,d) while the color of solid Pb12 remained unchanged. These data indicate two things: (1) Water and acetonitrile have much less effect on the optical properties of PbI, and BiI, solids than pyridine, although the effect of water on BiI, is still noticeable. (2) Both Pb12and BiI, can be dissolved in water and acetonitrile. This dissolution process should be very important for small particles. In principle, the dissolved species can be either molecules, ions, and/or small clusters. The absorption spectra of solutions containing dissolved Pb12 and BiI, are shown in Figure 2a-d. Several prominent absorption bands can be observed at around 360,290,250, and 225 nm. We believe that they are predominantly due to I-, I
