J . Phys. Chem. 1990, 94, 1214-1216
1214
from the tetrahedral bonding more than the network structure of hyperquenched glassy water. We conclude that the crystal to amorphous transition leads to an H-bonded network structure thermodynamically distinct from the one obtained by configurationally freezing-in during hyperquenching of liquid water and, further, that their states at T > T,, but before crystallization to cubic ice, maintain this distinction for a significant length of time and do not transform from one to the other. This suggests that the two "fluid states" at T > Tgare both thermodynamically and kinetically different, a feature reminiscent of mesomorphic state of liquids where steric hindrance effects allow a liquid to exist in different structures. The increase in heat capacity, ACp, at T estimated from the thermogram of Figure l e is 1.8 J K-'mol-' (scaled to the amount of cubic ice transformed under pressure) and is comparable with that of hyperquenched glassy waterZobut is higher than that of hyperquenched LiC1-H20 solution of comparable Tg.21 Despite the estimated 14-20% cubic ice that remained untransformed under the uniaxial load, the temperature range of the stability against crystallization of the fluid water is remarkably greater than that observed on heating hyperquenched glassy water.
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This is evident from the plateau region in Figure le above the glass liquid transition. For hyperquenched glassy water (Figure lg), the endothermic step was followed almost immediately by the crystallization exotherm. This implies that the rate of nucleation and/or crystal growth is much slower in the amorphized cubic ice than in hyperquenched glassy water. The melting point of cubic ice is not known, but thermodynamics requires that because of a positive AV(cubic ice is bulkier than water at T < 273 K) its (aT,,,/BP) should be negative. Nevertheless, it is also conceivable that the suggested "pressure melting" of hexagonal ice at 77 K (refs 6 and 10) is equivalent to the Occurrence of an elastic instability at the grain boundaries of small crystals in a polycrystalline mass, when a large uniaxial stress becomes concentrated at these sites. This has been observed for metal al10ys~~~ and and suggested in refs 8, 9, and 11. The recovery of the initial crystal phase on heating its pressure-amorphized form is an important first observation for an amorphous state of a material. Acknowledgment. We are grateful for financial support by the "Forschungsforderungsfonds" of Austria.
Modulation of the Time-Resolved Photoluminescence of Cadmium Selenide by Adduct Formation with Gasgous Amines Larry K. hung: Gerald J. Meyer: George C. Lisensky,f and Arthur B. Ellis**+ Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, and Department of Chemistry, Beloit College, Beloit. Wisconsin 5351 1 (Received: October 24, 1989)
The time-resolved, band edge photoluminescence (PL) of cleaved samples of single-crystal n-CdSe has been measured in the presence of N2, NH3, and CH3NH2(30% mixtures of the amines in N2). Nonexponential decay profiles are modeled j3 reflects and varies inversely with the distribution width of a set by using the Kohlrausch equation, Z ( t ) = Io exp[-(t/~)@]; of exponential decay times, and the lifetime T represents the peak of the distribution. At a low incident average power of -0.1 mW, both T and j3 reversibly increase relative to a N2 ambient upon exposure of CdSe to NH3 and CH3NH2,which engage in adduct formation with the semiconductor surface; the magnitude of the enhancement in both decay parameters is larger for the more basic CH3NH2than for NH3. Increases in incident power cause the absolute values of T and j3 to increase while reducing the dependence of these parameters on gaseous ambient. Interfacial properties that contribute to these effects are discussed and comparisons with steady-state PL properties are presented.
Introduction We have recently shown that steady-state photoluminescence (PL) measurements afford a contactless, in situ technique for characterizing the semiconductor-gas interface.' In particular, we have demonstrated that band edge PL from single-crystal n-CdS and CdSe is affected by surface adduct formation: gaseous Lewis acids like SOZquench PL intensity relative to a N2ambient, and series of Lewis bases, viz., amines and butenes, enhance the PL intensity in accord with relative basicity.'b*c For etched samples, these changes in PL intensity fit a dead-layer model, allowing determination of the gas-induced change in depletion width, and to a Langmuir adsorption isotherm model, permitting acquisition of adduct formation constants. The amine-induced PL changes for cleaved samples of n-CdSe (cleaved along the c axis), while qualitatively similar to changes observed with etched samples, did not fit the dead-layer model. This indicates that the PL changes arise, at least in part, from changes in surface recombination velocity. We inferred from this result that amine adsorption strongly perturbs the semiconductor's carrier recombination kinetics, an effect that should be evident
* Author to whom correspondence should be addressed. University of Wisconsin-Madison.
* Beloit College.
0022-3654/90/2094-1214$02.50/0
in temporal PL measurements. Adducts of CdS and CdSe with solution species have, in fact, been characterized by time-resolved PL in the picosecond domain, through reductions in PL efficiency and lifetime.2 In this Letter, we demonstrate that time-resolved PL measurements can reveal adduct formation at the semiconductor-gas interface. Specifically, we show that the PL decay curves of single-crystal n-CdSe are nonexponential and well fit using the two-parameter Kohlrausch equation, Z(t) = Io exp[-(t/~)@],in N2 and amine ambients; the parameter j3 reflects and varies inversely with the distribution width of a set of exponential decay times, and the lifetime T represents the peak of the di~tribution.~ With cleaved samples, using low incident power, both T and j3 reversibly increase in amine ambients relative to their values in (1) (a) Ellis, A. B. In Chemistry and Structure at Interfaces: New Laser and Optical Techniques; Hall, R. B., Ellis, A. B., Us.; VCH: Deerfield
Beach, FL, 1986; Chapter 6. (b) Meyer, G. J.; Lisensky, G. C.; Ellis, A. B. J. Am. Chem. Soc. 1988,110,4914. (c) Meyer, G. J.; Leung, L. K.; Yu,J. C.; Lisensky, G. C.; Ellis, A. B. J . Am. Chem. Soc. 1989, I l l , 5146. (2) (a) Evenor, M.; Gottesfeld, S.;Harzion, Z.; Huppert, D.; Feldberg, S . W . J . Phys. Chem. 1984,88,6213. (b) Gottcsfeld,S.Ber. Bunsen-Ges. Phys. Chem. 1987, 91, 362. (c) Benjamin, D.; Huppert, D. J. Phys. Chem. 1988, 92, 4616. (3) Palmer, R. G.; Stein, D. L.;Abraham, E.; Anderson, P. W. Pbys. Rev. Lert. 1984, 53, 958.
0 1990 American Chemical Society
Letters
The Journal of Physical Chemistry, Vol. 94, No. 4, 1990 1215
a N 2 ambient, with the effect being larger for the more basic CH3NH2than for NH3. Increases in incident power cause the absolute values of 7 and /3 to increase while reducing the dependence of these parameters on gaseous ambient. Experimental Section Single-crystal, c plates of n-CdSe, having a resistivity of - 2 Q-cm, were obtained from Cleveland Crystals, Inc. Samples were cut to -4 X 4 X 1 mm3 and were either etched with Br2/MeOH (1 :30 v/v; 5 s) or cleaved parallel to the c axis with a razor blade. They were then ultrasonicated in MeOH for 15 min before use. The sample holder was a 1 X 1 X 5 cm3 aluminum block whose bottom half was partially cut away to produce a wedge-shaped slab to which the sample was affixed with Duco cement. When this block was inserted into a 1 X 1 X 3 cm3 quartz cuvette and sealed with Dow Corning high-vacuum grease, the sample was reproducibly positioned to be at 30' relative to the incident laser excitation beam. Two holes were drilled through the top half of the block, serving as inlet and outlet ports for gas flow. The gases employed were used as received: nitrogen (Badger Welding; 99.95%), ammonia (Matheson; 99.99%), methylamine (Aldrich; 98+%), and dimethylamine (Aldrich; 99+%). An all-glass apparatus for creating 30% mixtures of the amines with N 2 was used,Ib and total flow rates of 100 mL/min were employed. Excitation was provided by a Coherent Antares Nd:YAG laser system. The mode-locked Nd:YAG laser was used to synchronously pump a Coherent Model 700 dye laser that was cavity dumped. Dyes were obtained from Exciton Chemical Co., Inc. Using Styrol 8 and IR140 (the saturable absorber), the dye laser produced an average power of 40 mW at a repetition rate of 3.8 MHz with a pulse duration of -2 ps. The 770-nm dye laser output was doubled with a LiI03 crystal, yielding approximately 0.1 mW of average power at 385 nm. For 587-nm excitation, rhodamine 6G was used, producing an average power of 100 mW. The beam was focused to a 1-mm-diameter point on the crystal, yielding a maximum output of or lo1*photons/ cm3/pulse for 385- and 587-nm excitation, respectively. Neutral density filters were used to reduce the laser intensity. Timecorrelated single photon counting was employed to measure the PL decay profiles4 Scattered incident light was eliminated by placing a Corning 2-58 filter at the entrance slit to the Thermo Jarrell Ash emission monochromator. The instrument response function is about 90 ps. Fits of data were performed to the Kohlrausch form, using a modification of the program CURFIT by BevingtomS The luminescence decays were iteratively reconvoluted from the instrument response function.
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Results and Discussion Steady-state PL data for the red band edge emission of single-crystal n-CdSe samples revealed that, relative to a N2 ambient, exposure to amines enhanced the PL intensity in the order NH3 < CH,NH2 < (CH3)*NH.IbThis order corresponds to increasing gas-phase basicity, as measured by proton The PL changes were readily reversible with NH3, slightly less so with CH3NH2,and difficult to reverse with (CH3)2NH. Samples that were etched in Br2/MeOH were far less emissive than those that were simply cleaved along the c axis. As noted above, amineinduced PL changes in etched materials appear to be driven predominantly by changes in electric field (depletion width), whereas those in cleaved samples do not. In conducting time-resolved experiments, we examined PL decay profiles, initially using 385-nm ultrabandgap excitation, from etched and cleaved n-CdSe under four different gaseous ambients: N,, NH3, CH3NH2,and (CH3)2NH. We employed 30% mixtures (4) O'Connor, D. Z.; Phillips, D. In Time-Correlated Single Photon Counrins Academic Press: New York, 1985. (5) Bevington, P. R. In Data Reduction And Error Analysis f o r Physical Sciences; McGraw-Hill: New York, 1969. (6) Aue, D. H.; Bowers, M. T. In Gas Phase Chemistry; Bowers, M. T., Ed.; Academic: New York, 1979; Chapter 9, Vol. 2. (7) Huheey, J. E. Inorganic Chemistry, 3rd ed.; Harper & Row: New York, 1983; pp 299-302.
* E-c Y
wb z
H
5.0
0.0
10.0
TIME (ns) Figure 1. Luminescence decay from cleaved CdSe under a N 2 ambient using 385-nm excitation at an estimated average power of -0.1 mW. The dashed curve is the fit to the Kohlrausch equation. The fit parameters are @ = 0.49 and r = 1470 ps.
TABLE I: Kohlrausch Parameters for CdSeO sampleb etched cleaved
power: mW
c1ea v ed
0.08
cleaved
0.1 0.1
0.04
P
gasd
f N2 NH3 CH3NH2 N2 NH3 CH3NH2 N2 NH3 CH3NH2
0.25 0.50 0.56 0.59 0.42 0.51
f 0.02 f 0.01 f 0.01
f 0.01 f 0.01
f 0.01 0.53 f 0.01 0.34 f 0.01 0.46 f 0.01 0.50 f 0.01
7 : PS 50 f 20
1480 f 60 2130 f 90 2540 i 80 690 f 40 1460 f 70 1720 f 80 230 i 40 1020 f 90 1510 i 90
a Analysis of amine-induced changes in band edge P L decay profiles of CdSe samples. For the experiments in this table involving a cleaved sample, one sample was used and exposed to the indicated gases in the sequence listed using 385-nm excitation; this procedure was then repeated at the different powers listed, in order of decreasing power. Data in N 2 were repeated after each amine exposure to demonstrate reversibility. *The sample was etched or cleaved, as described in the Experimental Section. CThepower of the excitation was a rough estimate, made by assuming that the doubling crystal has -3% efficiency. "The amines listed were 30% mixtures in N 2 (total pressure of 1 atm; flow rate of 100 mL/min). CParameters @ and T , obtained from the Kohlrausch equation by averaging data from three trials. /Values for @ and r were experimentally indistinguishable in N2, NH3, and CHIN H 2 ambients.
of the amines in N2,since both amine-induced steady-stateIb and time-resolved (vide infra) PL changes had saturated by this concentration. Decay curves were invariably nonexponential and, as has been noted for other semiconductors,*-I0 could be well fit to the two-parameter Kohlrausch equation, I ( t ) = Io exp[-(t/~)fi], as illustrated in Figure 1. The parameter p (0 < /3 < 1) has been related to a distribution of exponential decay times that are serially linked ( p increases as the distribution of decay times becomes more homogeneous), and the parameter 7, the lifetime, corresponds to the maximum in this d i ~ t r i b u t i o n .For ~ ~ ~the ~ system at hand, a distribution of linked relaxation times might be expected in the likely scenario that a variety of recombination sites having different decay times is present. Table I summarizes the temporal PL data obtained. For weakly emitting etched samples, values of 7 and p are 50 f 20 ps and 0.25 f 0.02, respectively, for both the N2 and amine ambients investigated.'l These data are consistent with our inference from (8) Albery, W. J.; Bartlett, P. N.; Wilde, C. P.; Darwent, J. R. J . A m . Chem. SOC.1985, 107, 1854. (9) Queisser, H. J. Phys. Rev. Lett. 1985, 54, 234. (10) Alivisatos, A. P.; Arndt, M. F.; Efrima, S.; Waldeck, D. H.; Harris, C. B. J . Chem. Phys. 1987, 86,6540.
Letters
1216 The Journal of Physical Chemistry, Vol. 94, No. 4, 1990
c
0.0
2.0
4.0
6.0
8.0
TIME (ns) Figure 2. Comparison of CdSe luminescence decay in different gaseous ambients with 385-nm excitation a t an estimated average power of -0.08 mW. Curves a, b, and c correspond to N2,NH,, and CH3NH2ambients; they have all been normalized to a common maximum intensity. Curve d is the instrument response function. All the amines were present as 30% mixtures in N,.
steady-state results that the surface recombination velocity is either very large in both N 2 and amine ambients and/or independent of ambient.Ib In sharp contrast, Figure 2 demonstrates that the decay profiles for the more emissive cleaved samples are strongly affected by amine adsorption. In the presence of amines, r exceeds 1 ns and /3 is -0.5-0.6. Absolute PL intensities, not shown in Figure 2, increase with ambient in the order N2 < NH, < CH3NH2 < (CH3)2NH, but the PL changes caused by (CH,),NH were sufficiently difficult to reverse that quantitative measurements were not made in this ambient; these observations of relative intensity and reversibility mirror the steady-state results.1b Kohlrausch parameters for the cleaved samples (Table I) show that T and /3 depend on the base and incident power: Values of r increase by 40-340% in passing from N2 to the NH, ambient, and by even larger percentages, 70-560%, in passing from N,to the more basic CH3NH2ambient. Correspondingly, /3 increases by 12-35% from N2 to NH3 and by still larger percentages, 18-47'35, in passing from N,to CH3NH2. Table I reveals that increasing the incident power results in larger absolute values of T and /3 for a given ambient and smaller variations among the three ambients. For these systems, adduct formation is most obvious at low incident powers. The increases in r and /3 with amine adsorption at low excitation intensity demonstrate that molecular-surface interactions have (11) Time-resolved PL studies on chemically etched CdSe in air have previously been fit to a biexponential expression, yielding a short component of 400 f 100 ps and a long one of 8 2 ns: Harzion Z.; Huppert D.; Gottesfeld, S.; Croitoru, N. J . Elecrroanal. Chem. 1983, 150, 5 7 1 .
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significantly perturbed carrier recombination processes in the solid: qualitatively, the trend in T indicates that amine adsorption reduces the number and/or influence of sites of fast nonradiative decay, and the trend in /3 reveals that adsorption reduces the dispersion in site decay times. Assuming comparable surface coverage, the greater effect on r and /3 observed with CH3NH2relative to NH, presumably reflects the stronger surface interaction expected for the stronger base. The mechanism by which coordination of amine molecules to the surface (to Cd2+sites, presumably) perturbs PL is, however, difficult to evaluate, since changes in both recombination kinetics and depletion width can accompany adsorption under our experimental conditions.12 We initially chose 385 nm as our excitation wavelength to accentuate surface contributions to PL; we estimate the penetration depth ( a - l ) at this wavelength as -500 A.13 However, use of 587-nm excitation, which penetrates roughly 3 times as deeply,I3 also yields nonexponential PL decay curves that are well fit by using the Kohlrausch equation. In fact, the same trends are observed with the two excitation wavelengths, viz., increasing intensity causes T and /3 to increase while reducing their dependence on gaseous ambient. At the highest intensities available to us with 587-nm light,I2 we saw saturation in T and /3 values ( - 2 ns and 0.5, respectively) and no difference among gaseous ambients within experimental error. The intensity dependence of T and /3 suggests that we might be increasingly sampling bulk properties as the incident intensity increases: with increasing intensity, the resulting contraction of the depletion width would reduce the efficiency of hole migration to the surface, and the limited number of recombination sites at the surface may be overwhelmed, allowing carriers to return to the bulk; photodesorption processes, if they occur, would also be more important at higher intensity. Such an accentuation of bulk recombination and transport processes would, of course, blur the distinctions among PL decay curves resulting from different gaseous ambients, as observed. In summary, at low incident power, temporal PL experiments are shown to complement steady-state PL data in demonstrating that recombination kinetics of CdSe are substantially altered by amine adsorption. Experiments on related semiconductor-molecular interactions are in progress. Acknowledgment. We are grateful to Professors M. Ediger and C. B. Harris, Drs. D. Waldow, S. Zuhowski, S. Gottesfeld, and P. Gourley, and Ms. Jennifer Hane for assistance with our measurements and for helpful discussions. The Office of Naval Research and the 3M Company are thanked for generous financial support.
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(12) Surface recombination velocities have been extracted from time-resolved PL measurements under conditions of strong illumination, 1019-10m photons/cm3/pulse, wherein the depletion width can be eliminated, ref 2. We have employed weaker illumination, up to lok5photons/cm3/pulse at 385 nm and lo'* photons/cm)/pulse at 587 nm. (13) Parsons, R.B.; Wardzynski, W.; Yoffe, A. D. Proc. R. SOC.London, A 1961, 262, 120.
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