J. Phys. Chem. 1992, 96, 1156-1160
1156
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Wavenumber Figure 7. Comparison of the relative intensities for the 126 and 108 cm-' bands (labeled by *) in phascs I1 and 111: (A) phase 11; (B)phase 111. (Before the curve was fit, the intensities of the Raman spectra were corrected for temperature and frequency factors according to the procedures described in refs 12 and 13.) low (less than 8 kcal m ~ l - ' ) . ~Therefore, .~ when the system is quenched down to the low temperature to form phase 11, it is very likely that other higher energy boat-chair conformers are frozen in this metastable phase along with the boat-chair-3 form. The line shapes of these modes are Lorentzian in phase I11 but Gaussian in phase 11. The two distinct carbonyl bending modes observed at 601 and 504 cm-'assigned as carbonyl out-of-plane and in-plane bending vibrations, respectively, both split into
doublets in the Raman spectrum, and the higher frequency component was also split into two components in the IR spectrum. These results imply that the vibrations associated with the carbonyl group are very sensitive to pseudorotation. It is interesting to note that the band position of the mode, appearing at 154 cm-' in phase I, was totally independent of temperature in both the stable phases I and 111, but in phase 11, it shifted linearly toward higher energies with increasing temperature (Figure 5 ) . This behavior indicates the role played by this particular vibration in the I1 -* I11 transition process. Careful examination of the peak positions in the IR and Raman spectra reveals that most of the coincident vibrational frequencies of the IR- and Raman-active bands in phases I and I11 tend to move apart from each other in phase I1 (Table I), especially the carbonyl stretching modes which are coincident in phases I and I11 at 1697 and 1687 cm-l in the IR and Raman spectra, respectively. This is indicative of that the rule of mutual exclusion applies and the unit cell symmetry of phase I1 is centrosymmetric. If the observed splittings in the ring deformation region of the Raman spectrum of phase I1 are only due to the existence of multiconformers and not to factor group interactions, then no correlation splitting occurred. Since all the conformations,which are energetically close to the boat-chaii-3 form, have no symmetry, the molecules can only occupy the C1sites. Therefore, the space group symmetry of phase I1 is probably triclinic with C, unit cell symmetry.
Conclusions The present vibrational studies of solid cyclooctanone confiied the existence of three solid phases. The vibrational spectra confirm that phase I is disordered with the molecules involved in rotational and conformationalmotions. The transition at 230 K occu~sfrom a rigid structure in which molecular motion is restricted, in agreement with the NMR ~tudies.2~ The nature of phase I1 was identified to be metastable with multiconformerscoexisting in this phase. The unit cell symmetry of phase I11 is most probably non-centrosymmetricwith a monoclinic space group of either C, or C, symmetry. Phase 11, however, apparently belongs to a centrosymmetric triclinic system with Ci symmetry. The spectral region associated with the ring deformations is highly sensitive to pseudorotation and conformational changes. Acknowledgment. This research was supported by operating and equipment grants to I.S.B. and D.F.R.G. from the following agencies: NSERC (Canada), EMR (Canada), and FCAR (Quebec). Y.H.thanks McGill University for the award of a fellowship.
Optical Properties of GaAs Nanocrystals Hiroyuki Uchida,**tCalvin J. Curtis,* hashant V. Kamat,t Kim M. Jones, and Arthur J. Nods* Solar Energy Research Institute, Golden, Colorado 80401 -3393 (Received: August 19, 1991) A modification of a previous synthesis for the formation of GaAs nanccrystals is reported. Unlike the previous synthesis, the new procedure produces quantized GaAs colloids which do not also contain molecular species which interfere with the optical properties of the GaAs particles. The GaAs colloids have been characterized by transmission electron microscopy, electron diffraction, ultrafiltration,absorption and photoluminescence spectroscopy, and transient pumpprobe spectroscopy; these experiments indicate that the particles have a size ranging from 20 to 80 A.
Introduction There is much current interest in the s y n t h d of sexniconductor nanostructures, such as GaAs, which can exhibit quantum size 'Current address: Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565, Japan. *University of Notre Dame, Radiation Laboratory, Notre Dame, IN 46556.
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effects in threedimensions,both as ordered arrays (quantum boxes or dots)' and as colloidal particles.24 The study of three-di(1) (a) Bryant, G. W .Phys. Rev.1988,837,8763. (b) Kash, K . J. Lumin. 1990,46, 69. ( 2 ) Brus, L.IEEE J . Quutum Electronics 1986, QE-22, 1909. Brus, L. Nouv. J . Chim. 1%7, 11, 123. (3) Steigerwald, M.L.; Brus, L. E. Ann. Rev.Muter. Sci. 1%9,19,471. 1992
American Chemical Society
Optical Properties of GaAs Nanocrystals mensionally quantized GaAs particles is a natural extension of the very large body of research on GaAs films quantized in one dimension,' as well as GaAs quantized in two dimensions (viz. quantum wires). b*8 Recently, a synthesis of GaAs particles was reported that was based on the reaction of GaC1, with ((CH,),Si),As in tolueneg and quinoline1° at reflux temperatures (111 and 237 "C, respectively). The former system produced large crystals that were not in the quantum size regime, while the latter system produced quantized colloidal nanocrystals (3&50 A). However, the optical properties of the colloidal nanocrystals produced in quinoline were found" to be masked by the presence of interfering molecular species which were believed to be either quinoline oligomers or gallium-quinoline complexes. Here, we report a new synthesis of colloidal GaAs nanocrystals that does not also produce accompanying molecular species that interfere with the optical properties of the nanocrystals. The synthesis involves the reaction of gallium(II1) acetylacetonate (Ga(acac),) and ((CH,),Si),As at reflux (216 "C) in triethylene glycol dimethyl ether (triglyme). We have characterized the resulting colloid by electron diffraction, transmission electron microscopy (TEM), ultrafiltration, absorption and photoluminescence spectroscopy, and transient picosecond pumpprobe spectroscopy to confirm the formation of quantized GaAs nanocrystals.
Experimeatal Seetion Gallium(II1) acetylacetonate (Ga(acac),) was purchased from Strem Chemicals and used without further purification. Tris(trimethylsily1)arsine (A~(siMe,)~) was prepared according to the literature'* and purified by vacuum distillation. These reagents were stored in a drybox. Triethylene glycol dimethylether (triglyme) was purchased from Aldrich and distilled over CaH2under reduced pressure (- 10 Torr). All compounds were manipulated in a standard vacuum line or in a purified nitrogen atmosphere. A mixture containing 0.5 mmol of Ga(acac), and 0.5 mmol of As(SiMe,), in 50 mL of triglyme was heated at reflux (216 "C) for 70 h. A blank sample with a concentration of 0.5 "01 of Ga(acac), in 50 mL of triglyme was heated under the same conditions. A series of ultrafiltrations of the GaAs colloids were performed under nitrogen atmosphere using Spectra/Por type F filters with molecular weight cutoffs (MWCO) of 5000 (average pore size (6)= 15 A), 1 0 0 (d~ = 100 A) and 1OOOOOO (d = 700 A). A modified stirred cell (S25-10, Spectrum)was employed to avoid the formation of a gel on the filtering medium which could reduce the effective pore size (concentration polarization). The optical absorption spectra of the filtrates were measured with an HP Model 8450 UV/vis photodiode array spectrophotometer. The optical path length was 2 mm in all cases. TEM (Philips Kevex Model CM30, accelerating voltage 300 kV) was used to obtain photomicrographs and electron diffraction patterns of the GaAs (4)Henglein, A. Topics in Current Chemistry; Springer-Verlag: Berlin, Heidelberg, Vol. 143, 1988;p 115. Henglein, A. Chem. Rev. 1989, 89, 1861-1873. (5) Sandroff, C. J.; Harbison, J. P.; Ramcah, R.; Andrejco, M. J.; Hedge, M. S.; Hwang, D. M.; Vogel, E. M. Science 1989,245, 391. (6) Williams, F.;Nozik, A. J. Norure 1984,311, 21. Nozik, A. J.; Williams, F.; Nenadovic, M. T.; Rajh, T.; Micic, 0. J . Phys. Chem. 1985,89, 397. Nedeljkovic, J. M.; Nenodovic, M. T.; Micic, 0.I.; Nozik, A. J. J. Phys. Chem. 1986,90,12. (7) Dingle, R., Ed. Semiconductors and Semimetals; Academic Press: New York, 1987;Vol. 24. Jaros, M. Physics ond Applicorions of Semiconducror Microsrnrcrures; Oxford Pres: Oxford, U.K., 1989. (8) Weiner, J. S.; Danan, G.; Pinczuk, A.; Valladares, J.; Pfeiffer, L. N.; West, K. Phys. R w . Lcrrs. 1989,63, 1641. Hirayama, Y.;Tarucha, S.; Suzuki, Y.; Okamoto, H. Phys. R w . 1988,837,2774. (9)Wells, R. L.;Pitt, C. G.; McOhail, A. T.; Purdy, A. P.; Shafieezad, S.; Hallock, R. B. Chem. Morer. 1989,I, 4. (10)Olshavsky, M. A.; Goldstein, A. N.; Alivisatos, A. P. J . Am. Chem. Soc. 1990,112,9438. (1 1) Uchida, H.; Curtis, C. J.; Nodk, A. J. J. Phys. Chem. 1991,95,5383. (12)Becker, G.;Gutehnst, G.; Wessely, H. J. Z . Anorg. AIIg. Chem. 1980,462,113.
The Journal of Physical Chemistry, Vol. 96, No. 3, I992 1157 particles which were deposited onto amorphous carbon overlayers on a Cu grid. Picosecond laser pumpprobe experiments" were conducted using a mode-locked Quantel YG-301DP Nd:YAG laser generating 532- or 355-nm pulses with a width of 18 ps. The repetition frequency of the pulse was chosen to be 10 Hz. The white continuum picosecond probe pulse was generated by passing the residual fundamental output (1064 nm) through a D 2 0 / H 2 0 solution. The output was fed to a spectrograph (HR-320, Instruments, Inc.) with fiber optic cables and was analyzed with a dual diode array detector (PrincetonInstruments, Inc.) interfaced with an IBM AT computer. The transient absorption spectra were corrected for laser scattering by taking the difference between the spectra with and without the probe pulse. Although this method greatly minimizes the contribution of scattered light to the absorption spectra, it docs not eliminate it completely. The spectra presented here may have some residual scatter, but its contribution is believed to be small. The delay in the probe pulse was introduced by using a computerantrolled optical delay line. The time zero in these experiments corresponded to the end of excitation pulse. The optical path length for the sample was 2 mm. ReSdtS (a) TEM Studies of GaAs Colloids. Orange-brown turbid slurries were obtained after the reaction of As(SiMe3), with Ga(acac), in triglyme at the reflux temperature. Colloids were formed from the slurries by filtering them through a 700 A filter; colloids stored under nitrogen in the dark did not aggregate for at least 6 months. We confirmed the formation of GaAs nanocrystals in these colloids by TEM and electron diffraction data. Figure l a shows GaAs particles in a colloid prepared from Ga(acac),; the individual particle diameter ranges from 20 to 80 A, and electron diffraction gives clear hkl zinc blende patterns of (1 1l), (220), (31 l), and weak (422) for GaAs. In the high-resolution picture (Figure lb), lattice planes can be seen and their spacing of 3.2 A corresponds to d(ll1) of GaAs. For comparison, we show in Figure IC the high-resolution TEM photos for a GaAs colloid we prepared in quinoliie using the prior procedurelothat was modified to include washing and redispersing isolated GaAs nanocrystds in quinoline." (b) Spectd Studies. A series of ultrafiltrations was performed to separate particles with different sizes. Figure 2a shows the optical absorption spectra of the GaAs colloid after a series of ultrafiltrations through 700, 100, and 15 A pore size filters. The filtrates through the 700 and 100 A pore size filters exhibit similar absorption spectra; the onset of absorption occurs at about 600 nm with a shallow rise with decreasing wavelength that steepens at about 470 nm and peaks at about 440 nm. The absorbance of the 100-A filtrate was slightly less than that of the 700 A filtrate; their difference spectra are shown in Figure 2b. The spectrum for the 15-A filtrate displayed in Figure 2a is quite different from those of the 700 and 100 A filtrates and is rather similar to that of the blank sample consisting of Ga(acac), refluxed in triglyme. The absorbance begins at about 525 nm and rises very weakly without any peaks; difference spectra are shown in Figure 2b. As discussed above, TEM results for the colloid showed that the particle size ranged from 20 to 80 A. This is consistent with the o tical absorption data in that the spectra for the 700 and 100 filtrates were quite similar, while that for the 15 A filtrate was similar to a blank Ga(acac), solution containing no particles. (c) F%unp-Fhbe Experiments. Transient photobleaching (Le., hole burning) of the GaAs colloid was ohserved during picosecond laser pumpprobe experiments. Figures 3 and 4 show the changes in the absorbance observed for the various filtrates of the colloid, along with results for a blank sample consisting of Ga(acac), in triglyme. In Figure 3, it is seen that with excitation at 532 nm the 700 and 100 A filtrates show appreciable hole burning be-
R
(13) Kamat, P. V.; Dimitrijevic. N. M.; Nozik,A. J. J. Phys. Chem. 1989. 93,2873. Kamat, P. V.; Gopidas, K. R. Proc. SPZE 1990,1209,115.
Uchida et al.
1158 The Journal of Physical Chemistry, Vol. 96, No. 3, 1992
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Figure 2. (a, top) Optical absorption spectra of the GaAs colloid after being passed through 700, 100, and 15 A pore size filters. A blank spectrum of Ga(acac), in triglyme after being passed through a 700 A filter is also shown. (b, bottom) difference spectra for the results described in (a): curve 1 = 700 - IS A; curve 2 = 700 - IS A; curve 3 = 700 - 100A.
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Figure 3. Pump-probe experiment for 700 (curve a) and 100 A (curve b) filtrates of the GaAs colloid showing photobleaching (Le., hole buming). Excitation was at 532 nm; the spectrum at 100 ps is displayed. A blank sample of Ga(acac), in triglyme shows no photobleaching(curve c).
Figure 1. (a, top) TEM bright-field image and the correspondingelectron diffraction pattern of GaAs nanocrystals made from the reaction of Ga(acac), with ((CH3)3Si)3As in triglyme. Arrows point to nanocrystals ranging from about 20 to 80 A in size. The dhklspacings in angstroms determined from the diffraction pattern and compared to the Joint Committee on Powder Diffraction Standards values (in brackets in angstroms)are as follows: ( 1 1 I), 3.16 [3.26]; (220), 1.95 [1.99];(31 I), 1.71 [1.70]. (b, middle) High-resolution TEM image of GaAs nanocrystal from (a) showing ( 1 1 1) lattice fringes with 3.2-A spacing. (c, bottom) High-resolution TEM of GaAs nanocrystals made from the reaction of GaCI, with ((CH3)3Si)3Asin quinoline. The particles are remarkably uniform disks showing excellent lattice fringe patterns.
ginning near 600nm and peaking near the pump wavelength; the photobleaching onset is slightly blue shifted for the 100 A filtrate compared to that of the 700 A filtrate. The blank sample of Ga(aca& in triglyme shows no significant hole burning. The data in Figure 3 were taken 100 ps after the pump pulse. The results of pum probe experiments for the 15 A pore size filtrate and the 700 pore size filtrate are shown in Figure 4; in Figure 4a the probe spectra are shown 0 ps after the pulse, while in Figure 4b the spectra are shown 500 ps after the pulse. For these data the wavelengths for the probe beam extend from 400 to 650 nm. It is seen from Figure 4 that the 15 filtrate does not show hole burning, while the 700 A sample again shows a large hole-burning effect. Moreover, Figure 4 shows that, in addition to photobleaching around the pump wavelength, the GaAs colloid also shows photobleaching that onsets at higher energy (475 nm). Spectra taken at various times after the laser pulse show that
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The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 1159
Optical Properties of GaAs Nanocrystals
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Figure 6. Photoluminesccnccspectra of 700, 100, and 15 A filtrate of GaAs colloids and of a blank sample of Ga(acac), in triglyme; excitation was at 530 nm.
Wavelength (nm)
Figure 4. Pumpprobe experiment for 700 (solid line, curve b) and 15 A (dot-dashed line, curve a) filtrates of the GaAs colloids showing a wider wavelength range for the probe spectrum: (a, top) 0 p after the pump pulse; (b, bottom) 500 ps after the pump pulse. Excitation was at 532 nm. Hole burning for the 700 A filtrate is observed centered at the pump wavelength and also beginning at 475 nm. No hole burning is observed for the 15 A filtrate.
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Discpssion The present experiments demonstrate that the reaction of Ga(acac), with ((CH,),Si),As at reflux temperatures produces quantized particles of GaAs with a size distribution ranging from 20 to 80 A. Also, unlike the previously reported synthesis of GaAs nanocrystals,loJl the present synthesis does not produce accompanying molecular species that seriously interfere with the optical properties of the GaAs nanocrystals over the wavelength range 600-470 nm; the situation below 470 nm is not as definite. As seen in Figure 2, the absorption from 600 to 470 nm has a shallow rise which can be attributed to GaAs nanocrystals. Using the simple model of Brus,Z the onset at 600 nm would correspond to a maximum particle size of about 65 A. This is consistent with the TEM results (Figure 1) showing particles from 20 to 80 A and the ultrafiltration results (Figures 2-6) showing that the 700 and 100 A filtrates had similar optical absorptions, PLs, and photobleaching behaviors, while the 15 A filtrates showed much diminished responses that were similar to those of a blank solution of Ga(acac),. Below 470 nm the absorption increases more sharply and peaks at 440 nm (Figure 2). In F w e 2, the 15 A filtrate does not show this peak. The difference spectra between the 700 and 100 A filtrates (Figure 2b) also show a small peak at 440 nm. The GaAs article size corresponding to absorption at 440 nm is about 40 ;the small decrease in peak intensity between the 700 and 100 A filtrates could be caused by concentration polarization effects during filtration which reduce the effective pore size. However, from the results to date, we cannot conclude unequivocally whether the 440-nm peak arises from GaAs particles or from molecular species produced during the synthesis. Further work is required to establish the origin of the absorption at 440 nm with certainty; however, the absorption between 600 and 470 nm can be definitely attributed to GaAs nanocrystals with a distribution of sizes from about 65 to 40 A. The TEM photos in Figure 1 show rather remarkable latticeimaged disklike particles for GaAs prepared in quinoline with GaCl, compared to the more irregular-shaped particles produced by the present synthesis. The more regular nature of the former system may be related to the stronger interaction of quinoline with Ga which, in addition to producing the galliumquinoline and/or polyquinoline species that dominate the optical properties throughout the visible region, could p i b l y also produce a better capping layer leading to better particle morphology. The pumpprobe experiments provide further proof that GaAs particles are present in the 700 and 100 A filtrates but are removed from the 15 A filtrates. With pump excitation at 532 nm, the latter filtrate shows no photobleaching while the former filtrates show pronounced photobleaching in two regions: one centered at the pump energy and another at higher energies (below 475 nm). The photobleaching at high energy is quite unusual and may be caused by two-photon absorption processes in the GaAs particles. The hole burning centered around the pump energy is the
650
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Figure 5. Pumpprobe spectra as a function of pump light intensity for the 100 A filtrate.
photobleaching of the GaAs colloid, both at the pump energy and at the higher energy seen in Figure 4, increasea slowly with time to about 100-200 ps and then decreases slowly. Figure 4 only shows the small decrease in photobleaching intensity that occurs between 0 and 500 ps; spectra between 0 and 200 ps (not shown) show a slight increase in photobleaching intensity. Figure 5 shows the photobleaching spectra after 100 ps as a function of pump intensity. As expected, the intensity and line width of the burned hole increeses with increasing pump intensity. (a) Lmninerance Studies. Photoluminescence (PL) spectra arc shown in F w 6 for the various GaAs colloidal filtrates and a blank of Ga(acac), in triglyme; the excitation wavelength was 530 nm. The spectral shapes and intensities of the PL spectra for the 700 and 100 A GaAs colloidal filtrates are similar. The emission onsets near 720 m and rises steadily until 550 nm. On the other hand, the emission for the 15 A filtrate is much reduced and is similar in shape and intensity to that of the Ga(aca& blank; the onset is near 650 nm, and there is only a small increase which plateaus at 580 m.These data again support the conclusion that GaAs nanocrystals that have a size distribution between 20 and 80 A are formed in the initial colloid.
1160
J. Phys. Chem. 1992, 96, 1160-1 164
GaAs colloidal particles in the quantum size regime have been produced by reacting G a ( a ~ a c and ) ~ ((CH3)3Si)3A~ in the tri-
ethylene glycol dimethyl ether at reflux temperature (216 OC). The resulting colloid contains GaAs particles that have a size distribution ranging from 20 to 80 A. This has been confirmed through experiments involving TEM, electron diffraction, ultrafiltration, optical and photoluminescence spectroscopy, and transient hole-burning spectroscopy. TEM and electron diffraction results indicate that the GaAs particles range from 20 to 80 A. Absorption spectra show an absorption onset at 600 nm which corresponds to a particle size of about 65 A. Absorption, photoluminesxnce, and holeburning spectroscopy experiments of filtrates passed through 700, 100, and 15 A pore size ult<crs show that the colloids do not contain GaAs particles s d e r than about 15 A or larger than about 100 A. Unlike the previously reported synthesis,lO*llthe present preparation does not produce molecular species that obscure the optical properties of the GaAs particles over the wavelength range 600-470 nm. Below 470 A, the absorption rises to a peak at 440 A; the present work cannot establish whether this peak is due to GaAs particles (40 A in diameter) or to molecular species. Hole-burning experiments show higher energy photobleaching of the GaAs nanocrystals beginning at 475 nm, in addition to the usual hole-burning peak centered at the pump wavelength (532 nm); the high-energy photobleaching may be caused by twephoton absorption. The rise time and decay of photobleaching in the GaAs nanocrystals is slow (hundreds of p) and is not understood. Further work is in progress to understand the optical properties of colloidal GaAs nanocrystals.
(14) Peyghambarian, N.; Fluegel, B.;Huylin, D.; Migus, A,; Joffre, M.; Antonetti, A,; Koch,S. W.; Lindberg, M. IEEE J. Quant. Electronics 1989, 25,2517. Roussingnol, P.; &card, D.; Flytzanis,C.; Neumth, N . Phys. Rev. Letts. 1989, 62, 312. Alivisatos, A. P.; Hams, A. L.; Levinos, N . J.; Steigerwald, M. L.; Bms, L. E. J . Chem. Phys. 1988,89, 4001.
Acknowledgment. This work was funded by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences. H.U. was supported by the U.S.-Japan Cooperative Program on Photoconversion and Photosynthesis.
usual photobleaching process that is well established for small semiconductor parti~1es.l~The time dependence of the photobleaching for the GaAs nanocrystals is unexpectedly long. The photobleaching protxss takes from 100 to 200 p to develop fully and then decays slowly (seeFigure 4). Further work is in progress to understand the details of the hole-burning phenomena in our GaAs colloids. Finally, the emission spectrum in Figure 6 also supports the presence of GaAs nanocrystals in the 700 and 100 A filtrates but not in the 15 A filtrate. Under excitation at 530 nm, the former shows appreciable PL that begins at 720 nm while the latter shows much weaker emission that resembles the blank solution of Ga( a c a ~ ) ~The . onset of emission for the GaAs nanocrystals is red shifted from the absorption edge at 600 nm. This is expected if traps and defects are present in the GaAs. However, the fact that PL emission is observed at all in the untreated GaAs colloid is of interest since surface states usually quench PL in GaAs nanocrystals,5 where the ratio of surface atoms to bulk atoms is very high. Our results therefore indicate that the nonradiative surface states are passivated to some degree by the use of triglyme and G a ( a c a ~for ) ~ the synthesis. The details of thii surface chemistry and passivation are not known. Future work will also address this issue.
Inelastic Electron Tunneling In ACAI,OS-Au Junctions Exposed to Thlocyanate Ion: Sensitive Thiocyanate Detection K.W.Hipps* and Ursula Manv Department of Chemistry, Washington State University, Pullman, Washington 99164-4630 (Received: August 22, 1991)
Inelastic electron tunneling spectra are reported for A1-Al2O3-Au junctions externally doped with KSCN from methanol solution. Spectra of the thiocyanate ion can be observed over a wide range of concentration, 10-'-lW2 M. Because the tunnel diodes are completely fabricated prior to the doping process, and because the doping process is simple and only requires a few minutes, these devices may serve as solid-state chemical sensors. The spectra observed for externally doped junctions are essentially the same as those for junctions doped with KSCN during device fabrication. While most of the bands are easily assigned, some questions remain about the assignment of the band@)near 350 cm-'. Scanning tunneling micrcxscopy images of A1-Al2O3-Au junctions (prepared on mica) before and after infusion are also presented.
Introduction Inelastic electron tunneling spectroscopy (IETS) is now two decades old.' It is an all electronic (photon-free) technique that provides vibrati~nal*-~ and electronic2.@ spectra of microscopic (1) Lambe, J.; Jaklevic, R. C. Phys. Rev.1968,165,821. ( 2 ) Hansma, P. K. Tunneling Spectroscopy; Plenum Press: New York, 1982. (3) Hansma, P. K. Phys. Rep. C 1977, 30, 145. (4) Weinberg, W. H. Vib. Spectra Struct. 1982, 11, 1. (5) Y a w , J. T., Madey, T. E., Eds. Mbrational Spectroscopy of Molecules on Surfaces; Plenum Press: New York, 1987. (6) Hipps, K.W.; Mazur, U. J. Am. Chem. Soc. 1987, 109, 3861.
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quantities of material (S1014molecules). The object of study, a metal-insulator-metal sandwich, can be made small enough to be integrated with other electronic circuitry on a single chip. In fact, tunneling spectroscopy is the only broad-band spectroscopic technique wherein the sample and the entire spectrometer might be placed in a single integrated circuit. Thus, it would seem natural that IETSbe applied to sensor development. There have been, however, several barriers to its application in analysis. The first and most obvious problem relates to how these devices arc (7) Hipps, K. W.; Mazur, U. Sur/. Sci. 1989, 207, 385. (8) Hipps, K. W.; Mazur, U. J. Phys. Chem. 1987, 91, 5218. (9) Hipps, K. W. J . Phys. Chem. 1989,93, 5958.
Q 1992 American Chemical Society
