J. Phys. Chem. 1987, 91, 6455-6458 better agreement with the empirically determined microscopic solvation times. This latter observation seems to support the notion the et rates of small barrier reactions are controlled by solvent relaxation. Apparently,, the failure of (1) is a consequence of the failure of the dielectric continuum model for solvent dynamics of any type, including et and simple solvation, in these solvents. The validity of (1) has already been questioned for a limited number of polar aprotics by Maroncelli and Fleming14* and Castner et Perhaps continuum models fail to correctly predict solvation and electron-transfer times because, while they include long-range correlations of the solvent motion, they ignore the molecular interactions and structure in the inner solvation shells. In fact, theoretical treatments12J3 that go beyond the continuum approximation and actually consider molecular interactions correctly predict relaxation components that are considerably longer than q. Indeed, two other predictions of contemporary theories for transient solvation are experimentally observed in our measurements and other data, namely nonexponential solvation functionsI4J6 and a measurable probe dependence of the T~ values;20 see Table 11. One potentia] problem in interpreting the et times of BA is that torsional motion about the bond that connects the two aromatic
6455
rings might play a role in the et mechanism.lg This seems unlikely, however, since we have observed that a highly sterically hindered derivative of BA that has restricted torsional motion exhibits similar et times to unsubstituted BA.Zo It is also interesting to note that the solvent dependence of et for BA is qualitatively different to that for the excited-state et process of the molecule (dimethy1amino)benzonitrile (DMABN)' in polar aprotics. For this molecule T~~is a function of the solvent polarity rather than the solvent dynamics. Perhaps the molecules differ simply because DMABN has a larger activation barrier as a result of vibrational reorganization energy associated with the reaction. Nevertheless, the striking difference between these molecules is worthy of further investigation. Future work from our group on et reactions and solvation will deal with the extremely early time (less than 300 fs) behavior of the various molecules mentioned in this paper. This is a critical time scale for solvent motion effects on reactions in solution because the actual barrier-crossing process of thermal reactions occurs on this time scale.
Acknowledgment. Acknowledgment is made to the National Science Foundation (Grant CHE-825 1158), the National Institutes of Health (shared Instrument Grant RR01439), Rohm and Haas, and Unisys for supporting this research. M.A.K. was generously supported by a fellowship from Dow and the Graduate School at the University of Minnesota. P.F.B. thanks Prof. John Simon for helpful discussions.
(20) Kang, T.-J.; Kahlow, M. A.; Gieser, D.; Swallen, S.;Nagarajan, V.; Jarzeba, W.; Barbara, P. F. J . Phys. Chem., submitted for publication.
Quantum Effects In Anisotropic Semiconductor Clusters: Colloidal Suspensions of BI,S, and Sb,S3 B. .F. Variano, D. M. Hwang, C. J. Sandroff,* Bell Communications Research, Red Bank, New Jersey 07701
P. Wiltzius, AT& T Bell Laboratories, Murray Hill, New Jersey 07974
T. W. Jing, and N. P. Ong Department of Physics, Princeton University, Princeton, New Jersey 08540 (Received: June 30, 1987)
Transmission electron microscopy (TEM), optical absorption, dynamic light scattering, and scanning tunneling microscopy (STM) have been used to characterize colloidal clusters of BizSl and Sb2S3. These layered semiconductor clusters were found by TEM to grow to sizes ranging from 16 to 90 A. Consistent with quantum confinement of charge carriers in small microcrystalline volumes, the optical absorption spectra were blue-shifted =0.7 eV from the bulk band gaps of the materials. These small clusters escaped detection by in situ dynamic light scattering, and only particles roughly 5 times larger could be readily detected. Preliminary results with a scanning tunneling microscope seem to support the anisotropic disklike geometry of the clusters, and for Bi2S3we estimate cluster thicknesses of 20-30 A. With the TEM and STM measurements of the cluster geometry and a simple particle-in-a-box model, we calculate band edge shifts which are in reasonable accord with the optical experiments.
There has been an intense focus on the properties of clusters in recent years. Though the major experimental emphasis has been placed on clusters generated in the gas phase,'s2 increasing attention has been devoted to the properties of clusters synthesized as colloidal particles in ~olution.~-'~ While most of the gas-phase (1) Heath, J. R.; Liu, Y.;OBrien, S.C.; Zhang, Q.-L.; Curl, R. F.; Tittle, F. K.; Smalley, R. E. J . Chem. Phys. 1985, 83, 5520. (2) Whetten, R. L.; Cox,D. M.; Trevor, D. J.; Kaldor, A. J. Phys. Chem. 1985, 89, 566. (3) Rossetti, R.; Hull, R.; Gibson, J. M.; Brus, L. E. J. Chem. Phys. 1985, 82, 552. (4) Brus, L. IEEE J. Quantum Electron. 1986, QE-22, 1909. ( 5 ) Brus, L. E. J . Phys. Chem. 1986, 90, 2555. (6) Fojtik, A.; Weller, H.; Koch, U.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1984,88,969.
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work has concentrated on clusters composed of a single element, the colloidal clusters are generally binary compounds made by (7) Fojtik A.; Weller, H.; Henglein, A. Chem. Phys. Lett. 1985, 120, 552. (8) Nozik, A. J.; Williams, F.; Nenadovic, M. T.; Rajh, T.;Micic, 0. I. J . Phys. Chem. 1985,89, 391. (9) Nedeljkovic, J. M.; Nenadovic, M. T.; Micic, 0. I.; Nozik, A. J. J . Phys. Chem. 1986,90, 12. (10) Sandroff, C. J.; Hwang, D. M.; Chung, W. M. Phys. Reu. E: Condens. Matter 1986, 33, 5953. (1 1) Sandroff, C. J.; Kelty, S.P.; Hwang, D. M. J . Chem. Phys. 1986,85, 5337. (12) Sarid, D.; Rhee, B. K.; McGinnis, B. P. Appl. Phys. Lett. 1986, 49, 1196. ~~~.
(13) Micic, 0. J.; Nenadovic, M. T.; Peterson, M. W.; Nozik, A. J. J . Phys. Chem. 1987, 91, 1295.
0 1987 American Chemical Society
6456 The Journal of Physical Chemistry, Vol. 91, No. 26, 1987 300
500
I
I
Wavelength (nm) 700 -900
Letters
11OQ
I
ibzS3 Bulk)
4A I
5.0
I
3.0
I
2.0 1.5 Energy (eV)
1
I
1.3
1.1
Figure 1. Optical absorption spectra of bulk and colloidal semiconductors of Sb2S3and Biz&. In the colloids of both systems there is a blue shift of ~ 0 . 7 eV from the bulk band gaps due to the confinement of charge carriers in the small microcrystalline volumes.
mixing the appropriate salts in a polar solvent. When the compounds are semiconductors and the particle size becomes comparable to the Bohr radius for electron-hole pairs, quantization effects can be observed due to the quantum confinement of charge carriers in the finite volume of the microcrystalline solid. Generally, the clusters grow isotropically in three dimensions so the radius of the cluster is the only parameter which scales the quantum size effect. In layered materials however, where highly anisotropic structural forces are at play, the situation can be somewhat more complicated. In these systems which appear to grow with the geometry of disks, it has recently been suggested that quantum size effects are determined by two microcrystalline length scales: the thickness of the cluster and its lateral dimen~ i o n . ' ~In' ~this paper we further explore the structural and optical properties of these quantum disks. Like the layered ionic semiconductors, Pb12,10 BiI3I1and Hg12,13Bi2S3and Sb2S3clusters, whose syntheses and characterization are presented in this paper, are expected to show novel optical properties resulting from their structural anisotropy. But unlike the halides, the sulfides are highly insoluble so that the formation of molecular complexes is predicted to be rather unlikely. Thus, by studying Bi2S3and Sb2S3, it should be easier to understand the optical properties of quantum disks, since competing optical absorption from other species in solution should be virtually a b ~ e n t . ~ Colloidal suspensions of Bi2S3were prepared in water at room temperature with solutions of Bi(N03)3 and N a 2 S 9 H 2 0as the starting materials. Typically, 2 cm3 of a 0.01 M solution of Bi(N03)3containing 12% concentrated nitric acid by volume was added to 100 cm3 of water. To this solution we added 4 cm3 of 0.01 M sodium hexametaphospate as a stabilizing agent and then added 2 cm3 of 0.01 M Na2S=9H20dropwise while stirring. This procedure resulted in a clear, dark brown suspension which remained stable for several days. Colloids of Sb2S3were prepared in a similar way, except that Sb2O3 rather than the appropriate nitrate salt was used as starting material and concentrated HCl was used instead of nitric acid. Sb2S3colloids prepared in this way had a clear yellow appearance and were somewhat less stable than their bismuth analogues. In figure 1 we display the optical absorption spectra of the colloidal materials. Readily observable in both Bi2S3and Sb2S3 (14) The histogram represents 407 particles. Agglomerates, the largest of which measured =140 A in diameter, were excluded from the count.
Figure 2. TEM micrograph of Bi2S3 microcrystallites. The roughly constant contrast in the images suggests that the clusters are of constant thickness consistent with the anisotropic structure of the material.
0
Lateral Dimension (A)
Figure 3. Histogram of the Bi2S3clusters in Figure 2. The size distribution is broad and featureless with an average size of =32 A.
is a = 0.7-eV blue shift in the absorption edge relative to the band gaps in the bulk materials. Such large blue shifts are consistent with the confinement of charge carriers in microcrystallites with at least one dimension comparable to the Bohr radius of electron-hole pairs in the bulk solid. In what follows, we shall focus on the size and geometry of Bi2S3clusters, as inferred from several techniques in an attempt to relate cluster structure to the optical properties of the colloidal suspensions. With transmission electron microscopy (TEM) particle distributions were analyzed by depositing several microliters of a colloidal suspension onto amorphous carbon film substrates and allowing the solvent to evaporate. To make the carbon grids more hydrophilic, we generally exposed them to UV radiation from a xenon lamp for several hours. The particles sizes were measured directly from micrographs obtained with a JEOL 4000 FX electron microscope capable of 2.6-A point-to-point resolution. In Figure
Letters
Figure 4. STM images of two Bi2S3microcrystallites adsorbed on the graphite surface. The images indicate disk-shaped clusters with a step height between 20 and 30 A.
2 we display a typical TEM micrograph of Bi2S3colloids prepared as described above. The particles generally deposited on the c a r h n grid as isolated clusters, but occasionally the highly irregular shape of the particle provided clear evidence for agglomeration. The particle size distribution is summarized in a histogram in Figure 3 which reveals an extremely broad distribution of microcrystallite sizes ranging from 16 to 90 A whose mean diameter is roughly 32 A.I3 Unlike previous work on PbIz and Bi13 clusters, we find no evidence for peaks in the size distributions of Bi2S3colloids, an observation consistent with the featureless absorption edge measured in our optical experiments. The range of particle sizes seen by TEM are similar to those observed in other preparations of semiconductor microcrystallites and can qualitatively account for the blue-shifted band edge of the Bi& and Sb2S3colloids. To model these shifts more quantiatively, we shall employ the anisotropic particle-in-a-box model which has been discussed previously in conjunction with the band gap shifts, AE,, of layered halideslO
where L , is the lateral dimension of the anisotropic microcrystallite, L, is its thickness, and the p’s are the reduced effective masses of electron-hole pairs in the appropriate directions. To apply eq 1 to Bi2S3,we first identify Lv with the average particle size in Figure 2. Hence, we assume that Bi2S3clusters grow as disks mirroring the structural anisotropy found in the bulk solid and that they lie flat on the carbon grid surface. Some supporting evidence for the disklike geometry of the Bi2S3clusters comes from the TEM micrographs. Even though clusters can vary in size by a factor of 5, the contrast seen in the cluster images remains essentially constant. This observation is not consistent with isotropic clusters spherical in shape. Such clusters would show a factor of =S variation in contrast between the smallest and largest microcrystallites in the TEM micrographs. Instead, the nearly constant contrast suggests that transmission of electrons is occurring through microcrystallites of nearly equal thickness, a situation compatible with clusters evincing the symmetry of disks. The thickness of the cluster is the other geometrical parameter needed to calculate the band edge shifts for small Bi2S3colloids. In previous work on the layered halides the thickness was determined by a fit of eq l to optical absorption data, allowing L, to take on values corresponding to integral multiples of layer sandwich thicknesses. For the case of Bi2S3,we attempted a direct measurement of L, using a scanning tunneling microscope (STM). Briefly, we deposited a drop of the colloidal suspension on a single crystal of graphite from whose surface atomic scale images had been previously obtained. After evaporation of the solvent, the tungsten tip was brought to the surface and scanned across the substrate. A STM image obtained by using this procedure is depicted in Figure 4. On two occasions we were able to identify stable topographic features which we ascribe to Bi2S3clusters adsorbed on the graphite surface. During these two encounters distinct step heights between 20 and 30 A could be measured. Once on top of the step, the STM tip could be scanned across the top of the elevated structure and brought back down to the gra-
The Journal of Physical Chemistry, Vol. 91, No. 26, 1987 6457 phite substrate, supporting the picture of these clusters as flat, anisotropic disks. The values for the reduced effective masses are the final parameters needed to calculate the band edge shifts in small anisotropic particles. Unfortunately, we have been unable to find any experimental determination of these quantities in Bi2S3or the isostructural Sb2S3. In other layered semiconductors containing heavy metals, e.g. Pb12 and Hg12, pxv is about 0.22 and pz varies from 0.26 to 0.41 in units of electron mass.13*15However, these materials have layered structures composed of alternating sheets of metal and anion while the layer structure in Bi2S3is formed from threadlike molecules which crystallize in a two-dimensional array.I6 Given the different crystal structure in the sulfides, we cannot be confident that their effective masses fall within the range quoted above. But using these values as a qualitative guide puts the band edge shifts in the range from 0.43 to 0.71 eV.” The experimental shift of 0.7 eV falls at the extreme end of this range, but better values for the effective masses and narrower size distributions need to be obtained before the validity of eq 1 can be fairly tested. Some recent work on layered semiconductor colloids made from Hg12 has drawn attention to the complicated nature of these and related s ~ s p e n s i o n s .Though ~~ small particles do exist when the components making up the compound are mixed under the right conditions, other species-such as molecular and oligomeric complexes-can be present in the solution as well. Thus, in the ionic halides the relationship between the optical absorption spectra and the particle size distribution is not a straightforward one. In Bi2S3, however, we believe the fundamental optical properties derive almost exclusively from microcrystallites in suspension and that the possibility of forming complexes or other conflicting absorbing species is rather remote. Firstly, the solubility of Bi2S3 in water is extremely small so that only minute concentrations of molecular or oligomeric species are expected to form. Indeed, a standard procedure for separating arsenic and antimony from bismuth entails adding an excess of sulfide ion to acidified solutions of the metals. When sulfur is in large excess, Bi2S3 readily precipitates while soluble complexes form between sulfur and the other metals. Moreover, since our syntheses are carried out in excess metal ion, we do not expect any conflicting absorption from sulfides or sulfur complexes, and metal ion absorption is strong only in the hard ultraviolet. Thus, rather than molecular species, we think it is reasonable to associate the optical features in Figure 2 with the presence of anisotropic Bi2S3microcrystallites in the quantum size regime. The broad, blue-shifted absorbance observed in Bi2S3contrasts sharply to the well-defined features seen in the layered halide^.'^'^ As suggested by several workers, such sharp features could originate from molecular species in solution, obscuring the less pronounced absorbance from the microcrystalline semicondu~tors.~~J~ Another important issue raised when trying to relate TEMdetermined cluster size distributions to optical properties of colloidal suspensions is the possibility that the measured sizes do not accurately reflect the in situ size distributions whose optical properties are being measured. We addressed this question by measuring particle sizes in situ using dynamic light scattering. The experimental setup consisted of a fvted-angle optical detection system” using the 5145-A argon laser line with a typical incident power of 500 mW. The light scattering samples were filtered through 0.22-pm Millipore filters. Temperature was stabilized at 25 f 1 OC. The autocorrelation functions of the light scattered a t an angle of typically 78’ were analyzed by using cumulant (15) Bloch, P. D.; Hodby, J. W.; Jenkins, T. E.; Stacey, D. W.; Lang, G.; Levy, F.;, Schwab, C. J. Phys. C . 1978, 1 1 , 4997. Bloch, P.D.; Hodby, J. W.; Schwab, C.; Stacey, D. W. J . Phys. C 1978, 11, 2579. (16) Krebs, H. Fundamenrals of Inorganic Crystal Chemistry; McGrawHill: London, 1968. (17) This range is calculated by using two sets of parameters: L , = 32 A, pxy = 0.22, Lx = 30 A, wr = 0.41 and L, = 32 A, pxy = 0.22, L, = 20 A, wz = 0.26. (18) Wang, Y.; Herron, N. J . Phys. Chem., to be published. (19) Haller, H. R.; Destor, C.; Cannel], D. S. Rev. Sei. Instrum. 1975,54,
973.
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J. Phys. Chem. 1987, 91, 6458-6461
analysis.20 The distribution of scatterers was in all cases fairly monodisperse, and from the thus determined diffusion coefficient we calculated via the Einstein-Stokes relationship an equivalent hydrodynamic radius RH. RH was found to be very sensitive to preparation and handling techniques and varied from one sample to another in the range 150 A < RH C 250 A. One of the possible causes might be impurities in the water as evidenced by the observation that preparation with distilled but not deionized water ~
~~
(20) Berne, B. J.; P a r a , R. Dynamic Light Scattering Wiley: New York, 1976.
An Electronlc Absorption Spectrum of C2+:
leads to the formation of large (several thousand angstroms) aggregates. We have not been able to detect any significant amount of scatterers with RH C 100 A. We are thus led to postulate that in addition to the distribution of small colloids seen in transmission electron micrographs there are additional particles in the 200-A range. Since the amount of light scattered in the Rayleigh regime varies like the square of the volume, one diskshaped particle with RH = 200 A scatters as much light as 10000 particles with RH = 20 A, which might explain why a small number of larger particles can pass unnoticed in the TEM histogram analysis and yet dominate the light scattering signal.
i42;
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z42g
Daniel Forney, Hartwig Altbaus, and John P. Maier" Institut fur Physikalische Chemie der Universitat Basel, CH-4056Basel, Switzerland (Received: July 24, 1987; In Final Form: September 14, 1987)
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The B4Zu- g42; electronic transition of C2+in a 5 K neon matrix has been identified. The 0-0, 1-0, and 2-0 bands are observed in absorption when lzC2+and 13C2+are produced in the matrix by photolysis followed by photoionization of various precursors, the most favorable one being chloroacetylene. The carrier and symmetry of the transition are assigned on the basis of spectroscopic and chemical evidence and by comparison to recent calculations. The 1-1 band is also observed in the wavelength-dispersed fluorescence spectrum of Cz+,subsequent to laser excitation of the 1-0 absorption transition. The derived spectroscopic constants (in neon matrix) are voo = 19765 (4) cm-', w,I = 1507 (4) cm-I, w,x,' = 8 (4) cm-', and ~"(1-0) = 1359 (6) cm-I. Introduction modulation and a photodiode or photomultiplier in conjunction with phase-sensitive detection and an on-line LSI 11/03 computer The open-shell ion C2+is one of the few simple, fundamental system.' Use of the waveguide technique enhanced the absorption species whose spectroscopy remains virtually unknown or unby a factor of -200 by passing the light through =2 cm of a certain. This is in spite of the assumed importance of C2+in matrix, which is 130 Mm thick.* comets, in chemical schemes for the formation of hydrocarbons The molecular species were embedded in the neon matrix with in interstellar media, and in plasma reactions.' Of the two a dilution in the range 1:3000--1:8OOO. In the present experiments spectroscopic studies dealing with Cz+, one assignment2 has been vacuum-UV radiation ( H Ly a, 121.6 nm; Xe I, 147.0 nm) was questioned in view of the two theoretical calculations carried first used to photolyze the molecular precursor to produce fragwhereas the other one was a low-resolution a p p r o a ~ h . ~ ments, which were subsequently photoionized by Ne I (73.6 nm) An absorption spectrum (at 249 nm) observed in a flash disphotons. In some of the experiments, H 2 0 was codeposited for 211utransition of charge of acetylene was assigned to a 2Z; the purpose of acting as an electron scavenger. However, it was C2+.* However, as the ground state of Cz+has the symmetry 4Z9, found that this was usually not necessary for the observation of the transition would have to be between excited electronic states of C2+. Unfortunately, in neither of the two theoretical w ~ r k s ~ . ~ the C2+ signals. The various precursors used either were obtained commercially is there a satisfactory match with the experimental observation. (HCCH, H13C13CH,DCCD, NCCN, C2HF3)or were synthesized A recent translational energy loss spectrum of C2+showed according to documented procedures9 (ClCCH, CICCD, ClCCCl, several broad bands,5 which were attributed to electronic tranBrCCH) and were purified by vacuum distillations. sitions within the quartet and doublet manifolds by comparison In the laser excitation experiments a pulsed dye laser (0.02-nm to the calculated state energiese3 Thus, the most detailed inforbandwidth) was oriented -45' to the matrix. The fluorescence mation at present still stems from the two computational was viewed along the thin side, in the same direction as in the In this contribution we present the spectrum of the B4Z; absorption, with a monochromator and registered with gated R4Z; electronic absorption of C2+embedded in a 5 K neon matrix electronics. and the evidence for the assignment. The waveguide absorption technique, which has proved successful in the characterization Results and Discussion of parent open-shell molecular ions,6v7 was employed. In Figures 1 and 2 are shown portions of the absorption spectra Experimental Section following vacuum-W irradiation of 5 K neon matrices containing Absorption measurements were carried out with a waveacetylene and chloroacetylene in dilute concentrations. No ablength-selecting monochromator (0.1-nm band-pass), using light sorption bands are apparent prior to irradiation. The top traces are observed after several minutes of photolysis with either H Ly (1) Winnewisser, G. Top. Curr. Chem. 1981, 99, 39. Liist, R. Ibid. 1981, a (Figure l a ) or Xe I (Figure 2a), the purpose of which is to 99, 13. produce Cz, evidenced by bands of two well-known systems of C2, (2) Meinel, H. Can. J . Phys. 1972, 50, 158. X'Z,+ and the A'II, X'Z,+ transitions.I0 With the D'Z,' (3) Petrongolo, C.; Bruna, P. J.; Peyerimhoff, S. D.; Buenker, R. J. J . acetylene the C2 bands are intense enough after H Ly cy but barely Chem. Phys. 1981, 74,4594. (4) Rosmus, P.; Werner, H.-J.; Reinsch E.-A,; Larsson, M. J . Electron perceptible after Xe I irradiation. Both sources lead to intense
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effort^.^*^
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Spectrosc. Relat. Phenom. 1986, 41, 289. (5) O'Keefe, A.; Derai, R.; Bowers, M. T. Chem. Phys. 1984, 91, 161. (6) Bondybey, V. E.; Miller, T. A. In Molecular Ions: Spectroscopy, Structure and Chemistry; Miller, T . A,, Bondybey, V. E., Eds.; North-Holland: New York, 1983; p 125. (7) Fulara, J.; Leutwyler, S.; Maier, J. P.; Spittel, U. J. Phys. Chem. 1985, 89, 3 190.
0022-3654/87/2091-6458$01.50/0
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(8) Rossetti, R.; Brus, L. E. Rev. Sci. Instrum. 1980, 51, 467. (9) Brandsma, L. Preparative Acetylene Chemistry; Elsevier: Amsterdam, 1971. (IO) Milligan, D. E.; Jacox, M. E.; Abouaf-Marguin, L. J . Chem. Phys. 1967,46, 4562.
0 1987 American Chemical Society
