1680
Langmuir 1998, 14, 1680-1683
Cadmium Selenide Photoluminescence as a Probe for the Surface Adsorption of Dialkyl Chalcogenides Julie K. Lorenz,† T. F. Kuech,*,‡ and Arthur B. Ellis*,† Departments of Chemistry and Chemical Engineering, University of WisconsinsMadison, Madison, Wisconsin 53706 Received November 3, 1997. In Final Form: January 9, 1998 The band gap photoluminescence (PL) intensity of n-CdSe is reversibly enhanced by adsorption, from the gas phase, of two families of dialkyl chalcogenide compounds: (CH3)2E (E ) S, Se, Te) and R2S (R ) CH3, C2H5, n-C3H7, i-C3H7, and t-C4H9). A dead layer model was used to estimate adduct-induced reductions in depletion width thickness at saturation; values ranged from ∼100 to 500 Å. Binding constants of less than 10 atm-1 to as large as 500 atm-1 were estimated from fits to the Langmuir adsorption isotherm model. The magnitude of the PL responses and binding constants of the dialkyl sulfides increases with chain length for the straight-chain derivatives and correlates with trends in ionization potential and basicity. Branched-chain derivatives do not follow this correlation as strictly but still give substantial PL enhancements and binding constants. Varying the chalcogenide atom in the dimethyl chalcogenides has little effect on either the magnitude of the PL enhancements or the binding constants, which are relatively small. Steric and electronic factors contributing to these PL effects are discussed, as is the potential use of PL for on-line detection of these group VI precursor gases in the growth of materials by chemical vapor deposition processes.
Introduction Recent studies have shown that the photoluminescence (PL) intensity of single-crystal n-CdSe and other semiconductors can be modulated by molecules adsorbing onto the semiconductor’s surface from either the gas or solution phase.1-3 For n-CdSe, Lewis bases typically enhance the PL intensity while Lewis acids quench it, relative to a reference ambient; the PL changes are thought to arise from adduct-induced modulation of the semiconductor’s depletion region, which is modeled as being nonemissive. We have found that several group IV and group V chemical vapor deposition (CVD) precursor gases act as Lewis bases and reversibly enhance the PL of CdSe.4,5 A possible application of these effects is to the construction of online chemical sensors for monitoring precursor gases used in CVD processes. The rich coordination chemistry of sulfur-containing species suggests that they, too, could interact strongly with semiconductor surfaces, and several such studies have been reported. For example, Lunt et al. found that organic thiols passivate the surface of GaAs, increasing the PL intensity by orders of magnitude.6 Bruening et al. showed that cyclic disulfide groups changed the electron * To whom correspondence should be addressed. † Department of Chemistry. ‡ Department of Chemical Engineering. (1) (a) Meyer, G. J.; Lisensky, G. C.; Ellis, A. B. J. Am. Chem. Soc. 1988, 110, 4914. (b) Lisensky, G. C.; Penn, R. L.; Murphy, C. J.; Ellis, A. B. Science 1990, 248, 840. (c) Ellis, A. B.; Brainard, R. J.; Kepler, K. D.; Moore, D. E.; Winder, E. J.; Kuech, T. F.; Lisensky, G. C. J. Chem. Educ. 1997, 74, 680 and references therein. (2) Sweryda-Krawiec, B.; Chandler-Henderson, R. R.; Coffer, J. L.; Rho, Y. G.; Pinizzotto, R. F. J. Phys. Chem. 1996, 100, 13776 and references therein. (3) Harper, J.; Sailor, M. J. Anal. Chem. 1996, 68, 3713 and references therein. (4) Brainard, R. J.; Paulson, C. A.; Saulys, D.; Gaines, D. F.; Kuech, T. F.; Ellis, A. B. J. Phys. Chem. 1997, 101, 11180. (5) Winder, E. J.; Moore, D. E.; Neu, D. R.; Ellis, A. B.; Geisz, J. F.; Kuech, T. F. J. Cryst. Growth 1995, 148, 63. (6) Lunt, S. R.; Santangelo, P. G.; Lewis, N. S. J. Vac. Sci. Technol. B 1991, 9, 2333.
affinity of CuInSe2 crystals upon adsorption,7 and thiocarbamates and thiolate anions have been shown to photoelectrochemically oxidize to disulfides on CdS(e) surfaces by Wrighton and co-workers.8,9 Octadecanethiol has been adsorbed onto porous Si, InP, and GaAs surfaces, and self-assembled monolayer formation was reported on the InP and GaAs surfaces.10-12 In this paper we report that a variety of representative group VI precursor gases used in the growth of II-VI semiconductor materials cause substantial PL enhancements when adsorbed onto single-crystal n-CdSe substrates. The compounds studied in these experiments include two families of dialkyl chalcogenides: (CH3)2E (E ) S, Se, Te) and R2S (R ) CH3, C2H5, n-C3H7, i-C3H7 and t-C4H9) (Table 1). Collectively, they permit a systematic study of the effects of chalcogenide atom and of alkyl chain length and structure on binding to the CdSe surface. The reversible nature of the PL response may lend itself to on-line detection of these precursor molecules in the growth of II-VI semiconductors by CVD processes. Experimental Section Materials. Vapor-grown, c-plate, single crystals of CdSe with a resistivity of 2 Ω‚cm were obtained from Cleveland Crystals, Inc. The crystals were etched in Br2/MeOH (1:15, v/v) and rinsed and sonicated in methanol for 15 min prior to use. If the shiny Cd-rich surface was not revealed after this etch, the crystals were polished in a 0.3-µm alumina/water paste and re-etched. All experiments were performed on this Cd-rich face. Dimethyl sulfide (99+%), diethyl sulfide (98%), diisopropyl sulfide (99%), di-n-propyl sulfide (97%), and di-tert-butyl sulfide (98%) were (7) Bruening, M.; Cohen, R.; Guillemoles, J. F.; Moav, T.; Libman, J.; Shanzer, A.; Cahen, D. J. Am. Chem. Soc. 1997, 119, 5720. (8) Natan, M. J.; Thackeray, J. W.; Wrighton, M. S. J. Phys. Chem. 1986, 90, 4089. (9) Thackeray, J. W.; Natan, M. J.; Ng, P.; Wrighton, M. S. J. Am. Chem. Soc. 1986, 108, 3570. (10) Kim, D.-I.; Lee, C.-W. Bull. Korean Chem. Soc. 1995, 16, 1019. (11) Gu, Y.; Lin, Z.; Butera, R. A.; Smentkowski, V. S.; Waldeck, D. H. Langmuir 1995, 11, 1849. (12) Sheen, C. W.; Shi, J.-X.; Martensson, J.; Parikh, A. N.; Allara, D. L. J. Am. Chem. Soc. 1992, 114, 1514.
S0743-7463(97)01196-7 CCC: $15.00 © 1998 American Chemical Society Published on Web 02/20/1998
Surface Adsorption of Dialkyl Chalcogenides
Langmuir, Vol. 14, No. 7, 1998 1681
Table 1. Ionization Potentials, Proton Affinities, Gas-Phase Basicities, and Equilibrium Binding Constants for the Dialkyl Chalcogenides dialkyl chalcogenide S S S S
ionization potentiala (eV)
proton affinityb (kJ/mol)
gas-phase basicityb (kJ/mol)
Kc (atm-1)
8.67
831.0
801.3
20 ( 5
8.44
855.8
826.0
190 ( 20
8.34
861.9
833.0
260 ( 30
8.26
876.8
847.0
115 ( 20
8.07
894.2
864.4
500 ( 40
8.67
831.0
801.3
20 ( 5
8.4
d
d
10 ( 5
7.89
d
d
S
S Se Te
