8216
J. Phys. Chem. 1995,99, 8216-8221
Scanning Tunneling Spectroscopy Investigation of Charge Transfer in Model Intercalation Compounds Til+,& C. WangJ L. Dotson: M. McKelvy,***and W. Glaunsingert7t Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287-1604 and Center for Solid State Science, Arizona State University, Tempe, Arizona 85287-1704 Received: April 13, 1994; In Final Form: January 18, 1995@
An ultrahigh vacuum (UHV) scanning tunneling microscopy/spectroscopy (STWSTS) system was used to study the density of states of Til+& (0.090 L x I0.002) near the Fermi level (EF).The effect of excess Ti on the local band structure and the position of EF was investigated. The Til+& single crystals studied were prepared by sulfur transport and cleaved under UHV conditions prior to in situ UHV STM/STS investigations. No new features or major changes were observed in the normalized conductance, (dl/dV)/(NV), as a function of x . The band-gap energy of 0.5 f 0.1 eV was essentially composition independent. These results can be explained within the rigid-band model, with electrons from the intercalated Ti being donated to the host conduction band. It has been demonstrated that STWSTS is a very sensitive atomic-level surface probe of intercalation charge transfer, especially for x I 0.05. Direct measurements of the EF shift can provide a semiquantitative measure of charge transfer to the external basal planes. These unique capabilities of STS can be used to probe the roles of adsorbate and guest-host charge transfer during intercalation at an unprecedented level of resolution.
Introduction The novel low-dimensional nature, properties, and synthetic flexibility of lamellar intercalation compounds have generated broad interest in these materials over the last two decades. In particular, transition-metal dichalcogenide intercalation compounds (TMDICs) have been the subject of widespread attention.'-I0 These compounds exhibit a wide variety of unusual phenomena, including superconductivity, charge density waves, redox reactions, highly anisotropic electrical and ionic conductivity, and staging (phase) transition^.'-^ They also have current and potential applications in catalysis, high-energydensity batteries, and lubrication.I-I0 One of the most thoroughly investigated TMD intercalation hosts is Ti&. The lamellar Tis2 structure consists of strongly bound S-Ti-S layers, with Ti occupying the octahedral sites between two hexagonally-close-packed layers of sulfur atoms. I I These S-Ti-S layers are held together by relatively weak van der Waals (vdW) forces, which allows the intercalation of a variety of guest species into the vdW gaps of the h o ~ t . ' ~ , ' ~ Many theoretical studies have been undertaken to explore the energy band structure of TiS2. Various calculations indicate that the conduction band is derived from Ti 3d orbitals and the valence bands from S 3s3p orbitals, with most of these studies predicting a band gap between the conduction and valence bands.I4-l8 Experimental measurements have lead to the wide acceptance of Tis2 as a small-band-gap semiconductor. These include Hall coefficient measurements as a function of pressure up to 20 kbar, compositional analysis of highly stoichiometric Tis2 (Ti1.m2*0,mlS2), carrier concentrations derived from electrical transport and reflectivity measurements, and angle-resolved photoemission spectroscopy (ARPS) s t ~ d i e s . ' ~ -Further *~ support comes from a recent scanning tunneling microscopy/ spectroscopy (STWSTS) study.26 ARPS studies found an energy gap of 0.3 f 0.2 eV,23which is consistent with a variety Department of Chemistry and Biochemistry,
* Center for Solid State Science.
@Abstractpublished in Advance ACS Abstracts, April 15, 1995
0022-365419512099-8216$09.00/0
of previous studies that favor a gap of -0.5 eV.27 More recently, STM/STS measurements taken at 4.2 K gave a similar gap of -0.4 eV for nonstoichiometric Tis2 (Le., However, what effect excess Ti in the vdW gap may have on the band gap is unknown. Recent STM studies have provided insight into the structure of lamellar intercalation compounds at very high resolution. For example, STM has been used to successfully image superstructure formation in alkali-metal graphite intercalationcompounds2* and to reveal a novel triangular charge pattern on nominally stoichiometric titanium disulfide (Til .m*0,01S2).29 STM is not only a powerful structural tool, it can also be used in the spectroscopy mode as a sensitive probe of the local density of states (LDOS) in the vicinity of the Fermi level, as demonstrated by Tersoff and ham an^^.^^ Such studies can be undertaken with atomic-level positioning of the STM tip. Zero voltage bias between the tip and sample in an STS experiment represents the position of the Fermi level (EF) of the sample. The normalized conductance, (dI/dV)/(I/V), is, to a first approximation, proportional to the LDOS of the sample near EF. Thus, STS measurements as a function of the bias potential can be used to identify semiconductor band gaps as reduced conductance regions around EF, with sharp increases in conductance at the gap edges. However, STS measurements can also be affected by various factors, such as the tunneling barrier, temperature effects, etc. Detailed discussions conceming these factors and the various aspects of STS measurements have been published el~ewhere.~' STM/STS has been successfully applied to measure the energy gaps in high-T, superconductors and in materials containing charge density wave^.^^^^^ Qualitative Fermi-level shifts have also been observed for doped Si33and, more recently, for the intercalation of CoC12 into graphite.34 Thus, STS can be used as a qualitative surface probe of charge transfer associated with the electronic doping of materials including the intercalation of lamellar materials. However, the extent to which STS can be used to probe the degree of charge transfer is unknown. In this regard, Til+*S2 offers an excellent model
0 1995 American Chemical Society
Investigation of Charge Transfer in Ti I +xS2 intercalation system to test the ability of STS to follow intercalation charge transfer and the effects of intercalation on the host band structure near EF because (i) has a pronounced energy gap structure readily identifiable by STS;26 and (ii) at least for low x ( x 5 0.05), excess ( x ) Ti transfers four electrons per titanium atom to the host conduction band,20.22.24.3s which allows the direct study of intercalates with known levels of charge transfer. Since the intercalation behavior of Tis2 is representative of TMD hosts and also possesses many chemical and structural similarities to graphite intercalation: the study of intercalation charge transfer for Tis2 should also provide more general insight into both TMD and graphite intercalation processes. Herein we report the results of ultrahigh vacuum (UHV) STM/STS investigations of the host band structure near EF as a function of x and guest-host charge transfer for crystalline Til+.& intercalation compounds, in which 0.090 2 x 2 0.002. The primary objective of this investigation is to ascertain to what extent STS can be used to directly probe the charge transfer associated with the intercalation of the model TMD host Tis2 by determining the effect of guest species (Ti) on the position of EF and the band structure of the host near EF.
J. Phys. Chem., Vol. 99, No. 20, 1995 8217 TABLE 1: Stoichiometry of Til+& Crystals Determined by Thermogravimetric Oxidation to Ti02 on Heating to 900 "C wt w P 71.44 7 I .90 72.48 73.55 74.12 74.89
0.002 0.0 I3 0.028 0.055 0.070 0.090
Weight percent = (wt of TiOz/wt of Til+.rS2) x 100. Overall, the results were reproducible within 0.06%. The actual values for x shown are 0.001 lower than those calculated from the weight percent directly due to the presence of small amounts of oxygen (500 ppm) in the titanium wire from which the sulfide was synthesized and trace residual sulfur (5800ppm) in the Ti02 resulting from oxidation."'
'
CONTACT PAD
CLEAVING PAD TORR SEAL
Experimental Section The samples used in this investigation were Til+,& crystals of known composition. Both the crystals and their polycrystalline powder precursors were prepared and handled under rigorous inert conditions.20 Highly stoichiometric TiS2, Ti I .~2S2, was used for crystal growth. It was prepared from the elements and characterized, as reported previously.20 The crystals were grown by chemical vapor transport from 700 to 600 "C in sealed quartz ampules. An excess of 10 mg/cm3 of sulfur was used as the transport agent. The crystal growth time ranged from 6 to 8 weeks. The resulting crystals grew as thin platelets, ranging up to 5-6 mm on a side and 0.05 mm thick. The crystals were combined with 40 mg/cm3 of excess sulfur in a sealed quartz ampule and were annealed at 640 "C for a week to give an elemental composition of Ti I . ( ~ ) 2 S 2 .Nonstoichiometric Ti I + . 3 2 crystals were prepared from the Ti 1 .o(~S2crystals by partial removal of their sulfur by vapor transport in sealed quartz ampules evacuated to Torr. Crystals with an initial composition of Ti1.~2S2were heat-treated at 640 f l "C at one end of an ampule, while the temperature at the opposite end was 0-80 "C cooler, depending on the final desired composition. The crystals were heat-treated for several days to attain homogeneous, equilibrium compositions.' The synthesis of nonstoichiometric Ti I + . 3 2 crystals was carried out at these relatively low temperatures to avoid exciting additional Ti atoms into the vdW gap via titanium vacancy-interstitial pair formation, which occurs during high-temperature (e.g. 1000 "C) .~ynthesis.*~.~~ Crystal compositions were analyzed by oxidative thermogravimetric analysis of representative crystals using a Perkin-Elmer TGS-2 system (ultimate 0.1 pg sensitivity and 0.01 % weight resolution) and a 40 cm3/min flow of 99.995% oxygen.") Prior to oxidation, the crystals were held at 110 "C for 10 min, during which there was no weight change. The crystals were then heated at 2 "C/min to 900 "C, held at 900 "C for 3 h, where the weight was stable to 3~0.02%over the last 2.5 h, and then cooled back to 110 "C to determine the final weight change. The resulting weight percents and the corresponding crystal compositions for the crystals studied are given in Table 1.*" The STM used for these studies is a modified ultrahigh vacuum (UHV) STM/STS system from Park Scientific Instruments, Inc. The UHV system used for this study incorporates
Figure 1. A diagram showing the mounting of single-crystal Til+.& samples for subsequent UHV cleaving in the load-lock chamber. The sample is cleaved using a wobble stick to remove the cleaving pad and expose a virgin basal plane for STM/STS studies.
a UHV STM chamber, a load-lock, and a main chamber connecting them. The UHV STM and main chambers can be evacuated to Torr after bakeout, while the load-lock can be evacuated into the high Torr region. Fresh sample surfaces for STMISTS study were prepared by in situ UHV cleaving. The crystals were mounted on the STM sample holder and cleaved in the load-lock chamber. The STM sample was then transferred rapidly to the STM sample stage via a magnetic linear drive, and the STM chamber was immediately isolated. Prior to cleaving, the crystals were mounted on the STM sample holder using Torr Seal, with a slight amount of Torr Seal being applied between the exposed external basal plane of the crystal and the cleaving pad, as illustrated in Figure 1. Torr Seal was selected as the adhesive in this study because of its extremely low vapor pressure, which is essential for UHV operations. A pad clamped to the sample surface provided electrical contact between the surface of the sample and the sample holder. The crystals were cleaved by lifting the cleaving pad with a wobble stick. It was possible to routinely cleave the Til +,& crystals parallel to the Tis2 layers to expose a virgin basal plane for UHV STM/ STS experiments. Cleavage was easiest for the most stoichiometric samples and became more difficult with increasing x due to the pinning of the Tis2 layers by excess Ti in the vdW gap.27-37This cleavage procedure should result statistically in approximately half of the excess Ti originally in the vdW gap that was cleaved being present on the exposed basal plane used for the UHV STM/STS experiments. As soon as the sample holder was transferred to the STM stage and the STM chamber was isolated, a mechanically-formed Pt(80%)Ir(20%) tip was brought into the tunneling position by a stepper motor, after which the STM was in full operation. The complete process from sample cleaving to the initiation of
Wang et al.
8218 J. Phys. Chem., Vol. 99, No. 20, I995
I Voltage
W
i
P
w
+Bias
0 0 0 0 0 0Sample T
0
0
0
0
lPre-amp
0
vdW Gap
000000 0 0 0 . 0 000000
I
I
-t
I
I
I
I
I
Figure 2. An illustration showing the operation of a scanning tunneling microscope on a lamellar transition-metal disulfide (TS2) surface. The bias voltage induces a current to flow between the tip and sample, and the STM response is then amplified and detected. In Til+S2, the excess Ti resides in the van der Waals gap.
the STM scans in the isolated STM chamber takes only a few minutes. During the STM/STS studies, surface features, such as atomic lattices and local defects, were observable for periods ranging from hours to days, thus demonstrating the long-term stability of the freshly-cleaved surfaces. The surfaces were frequently reexamined by STM during the STS studies to verify the stability of the sample surface. No more than eight STS measurements were taken before reexamining the sample surface by STM. STS measurements were not continued in a particular region if there were any changes in surface topography, as determined by STM. The STM was usually operated in the constant-current scanning mode, in which the tunneling current is maintained at a constant value while the tip scans the sample surface, as illustrated in Figure 2. The tunneling bias voltage was selected to be large enough to generate appreciable current at a desired distance from the sample as well as to cover the entire voltage range for spectroscopy measurements. The initial bias voltage was normally set at 300-500 mV, and the tunneling current was set at about 1 nA. Spectroscopy measurements were performed by adding a small dithering voltage (typically a 1 kHz, 20 mV peak-to-peak sinusoidal wave) to the tunneling bias voltage and then ramping the bias voltage through a preselected range, while processing the experimental dZ/dV vs V data with a lock-in amplifier. During the spectroscopy measurements the feedback loop of the STM was turned off, so that the tip-sample spacing remained unchanged. The Park STM system could not record dUdV vs V and I vs V data simultaneously. Consequently, the I vs V data was calculated numerically from the dZldV vs V data and the known set point (I,V) for a specific measurement [ W ) = Isetpoint - SYpoint (dZldV),e,”,dAV, assuming both Isetpoint and Vsetpoint are positive]. The normalized conductance, (dZ/dV)/(I/V), was then calculated from the dZ/dV vs V and I vs V data and is the quantity reported in this paper. Results and Discussion I. Topographical Imaging. Figure 3a shows a typical STM image of a freshly-UHV-cleaved Ti1.~2S2 surface in the constant-current mode showing atomic-level resolution of the long-range-ordered hexagonal surface. The concentration of
;08 Figure 3. (a) A typical three-dimensional STM image of a Ti1.002S2 basal-plane surface under UHV. The tunneling set point is 300 mV, and the tunneling current is 2 nA. The total z-deflection is about 1.5 A. (b) A three-dimensional STM image of Ti1.0~S2.Note the irregular z-deflection and occasional short-range order, including areas suggestive of local 2a, -6.8 A, repeat distances, such as in the lower right comer of the image.
excess Ti is so low in these crystals that it was possible to routinely observe large, well-ordered regions of the basal planes with atomic-level resolution. The observed in-plane hexagonal lattice constant of 3.4 A, which is equal to the nearest-neighbor S-S or Ti-Ti spacing in the basal plane, is in good agreement with that observed by X-ray powder diffraction [a = 3.4073(2) A].37 Since none of the occupied intralayer Ti orbitals are directed between the S layers bounding the Tis2 basal layer, they should not contribute to the observed STM images. Therefore, the STM images of highly stoichiometric Tis*, as shown in Figure 3a, should represent the positions of the S atoms. It was also observed that the STM images taken at a variety of bias voltages and reverse polarities are all essentially the same. This follows from the presence of a small amount of covalent orbital mixing between the Ti d states and S 3p states that results in the conduction band possessing -10% S 3p ~haracter:~which allows electrons to tunnel both out of and into terminating sulfur layers at bias voltages near EF. In contrast to the routine atomic-level resolution of longrange-ordered surfaces observed for highly stoichiometric Ti1.~2S2,as depicted in Figure 3a, nonstoichiometric sample surfaces exhibited only local hexagonal ordering. The absence of long-range order for nonstoichiometric Til +,& is probably due to both the exposure of interlayer Ti during cleaving and the disruption of the surface associated with the breaking of strong interlayer Ti-S bonds that can be very effective at pinning the host layers together.37 As expected, this loss of order was particularly acute for Ti1.090S2. Areas suggestive of local 2a repeat distances were also occasionally observed for
Investigation of Charge Transfer in Til+&
1.6
J. Phys. Chem., Vol. 99, No. 20, 1995 8219 TABLE 2: Energy-Gap Edge Positions for the Top of the Valence Band (Em) and Bottom of the Conduction Band (&e) Relative to EF for Til+&
i
X
0.002 0.013 0.028 0.055 0.070 0.090
I
0.8
2.5
0.5
I
i
i
I
j
0.8I
j
800
400
0
-400
-800
VOLTAG E(mV) Figure 4. Normalized conductance curves versus bias voltage for Til+&, where x = 0.002, 0.028, 0.090. For Til.WZS2, the positions of the conduction-band and valence-band edges are indicated. Note the negative shift of the center of the energy gap relative to EF with increasing x .
Ti1.woS2 crystals, as seen in Figure 3b. Since the 2a x 2a superstructure would correspond to 25% occupancy of the Ti surface sites, these local repeat distances suggest the possibility of a small amount of clustering of the excess Ti on the basal plane surfaces exposed by cleaving. In this study, the triangular surface pattems previously found for Til,m*o.o,S2, which have been correlated to intralayer Ti vacancy-interstitial pair format i ~ nwere , ~ ~not observed. The absence of such pattems may be related to higher crystal growth temperatures in the former study, since higher temperatures are known to induce analogous interlayer Ti vacancy-interstitial pair formation.36
EVB(eV) f 0.04 i 0.07
-0.30 -0.39 -0.41 -0.51 -0.67 -0.62
f 0.03 f 0.04 +c
0.06
& 0.03
ECB(ev) 0.26 i.0.07 0.17 i 0.05 0.09 f 0.03 -0.06 f 0.02 -0.09 f 0.04 -0.13 i.0.07
energy gap (eV) 0.56 f 0.08 0.56 f 0.09 0.50 f 0.03 0.45 f 0.04 0.58 f 0.07 0.49 f 0.08
11. Spectroscopy. UHV STS measurements were carried out simultaneously with the UHV STM observations. A typical curve of normalized conductance versus bias voltage for Til . ~ 2 S 2 is shown in Figure 4. The slope of the curve increases rapidly both above and below EFin the regions identified by the arrows. This indicates the presence of a band gap with a sharply reduced LDOS, in direct support of the prevalent view that stoichiometric Tis2 is a small-band-gap semiconductor. These rapid increases in the slope of the conductance curve indicate the positions of the bottom of the conduction (negative tip bias) and top of the valence (positive tip bias) bands,38whose energies are ECBand EVB,respectively. The positions of these rapid slope changes were highly reproducible. They were observed repeatedly for many different freshly-UHV-cleaved crystals and were independent of the surface location being probed. The band gap (Egap)for Ti1.&2 was found to be 0.56 f 0.08 eV, which is in good agreement with the value of -0.5 eV from previous investigation^.^^^^^^^^ A substantial contribution to the error comes from the presence of thermally excited electrons at ambient temperature which cause smearing of the band features.26 Although tip-induced band bending can substantially distort STS spectra and affect the measurement of Egap, ECB and EVB for semiconductors with carrier concentrations lower than -3 x 10's/cm3,38 it should not significantly affect the observed values for Til+&, due to the relatively high carrier concentration in these materials, which starts at 1.4 x 102'/cm3 for Til .&32 and increases with increasing x.20,22,24,35 Similar well-defined band-gap structures are also observed by STS for Til+,S2 (x = 0.013,0.028,0.055,0.070, and 0.090). Representative spectra are shown in Figure 4. Although the exact shape of the conductance curves varied somewhat, the positions of rapid slope changes, which indicate the locations of the band-gap edges,38 were highly reproducible for each sample composition. The values for ECBand EVB as a function of x are given along with the band-gap energies in Table 2 . ECB and EVB were again independent of the surface location where the spectroscopy measurements were taken. It is evident in Figure 4 that the intercalation of excess Ti does not result in any new features in the LDOS near EF. The only significant change in the spectra as a function of titanium excess is a shift of EF away from the middle of the reduced conductance region with increasing x. Although EF continuously moves further away from the middle of the band gap into the conduction band with increasing x (and carrier concentration), the band gap itself remains essentially unchanged, which confirms that band bending does not significantly affect the measurement of Egap, ECB,or EVB (e.g., even though the carrier concentrations for Til.wzS2 and Ti1.013S2are substantially different, 1.4 x 1020/ cm3 and 9.1 x 1020/cm3,respectively, the same band gap, 0.56 eV, is o b ~ e r v e d ) . ~ ~ , ~ ~ These observations can be interpreted in terms of the rigidband model for intercalation. In this model, the host band structure is, to a first approximation, unperturbed by the intercalationprocess. The electrons donated by the guest species are simply donated to the host conduction band resulting in the observed rise in EFrelative to the host band structure. The low,
Wang et al.
8220 J. Phys. Chem., Vol. 99, No. 20, 1995
-0.80I
,
I
0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 C IO
x (Excess Ti)
Least-squares refinement of the data gave the functions Figure 5. Positions of ECB(4) and EVB(A)relative to EF as a function of x for ECB- EF = -0.67 - 0.24 ln(x + 0.02) and Eve = ECB- 0.52, which are shown together with the experimental observations. but nonzero, tunneling current observed in the band-gap region suggests that the LDOS in the band gap is also low, but not zero. This results in the continuous increase of EF with increasing x shown in Figure 5. The shift of EFrelative to ECB can be fitted very well to a logarithmic function of the excess Ti concentration. A least-squares refinement of the data yields the empirical relation:
EcB- EF= -0.67 - 0.24 ln(x
+ 0.02)
(1)
which is shown together with the experimental values in Figure 5. EVBcan be well described as a linear function of ECB:
EVB= ECB- 0.52 indicating that the band-gap energy for Til+& (0.090 2 x 2 0.002) is largely independent of x. The somewhat better fit for the ECBdata relative to the EVBdata may be associated with the type of orbitals that comprise the bands. Whereas the conduction band primarily consists of Ti d orbitals into which delocalized guest electrons are donated, the highest occupied valence band states mainly contain S 3p orbitals, whose energy can be more easily perturbed by local guest-host bonding interactions due to their proximity to the vdW gap. Figure 5 can also be interpreted in terms of guest-host charge transfer, since each excess Ti donates four electrons to the conduction band up to at least x = 0.05. This region of quantitative charge transfer for x 5 0.05 follows directly from compositional analysis and transport measurements of Til +,&, including Hall coefficient measurements of the electron carrier concentrations for Til +& and thermoelectric power measurementS.20.22.24.35 However, in comparing x to charge transfer, one must consider the distribution of the excess Ti that was previously located in a vdW gap and is exposed on the basal plane by cleaving. The excess Ti associated with the cleaved layer is expected to be approximately evenly distributed between the two host-layer surfaces exposed by cleaving. This expectation, as well as the homogeneity of the crystals, is confirmed by the highly reproducibile EF shifts observed for many different freshly-cleaved sample surfaces for the same composition. Furthermore, any differences in the amount of Ti exposed on the basal plane in repeated experiments should fluctuate
randomly above and below 50% of the excess Ti originally in the vdW gap, thereby increasing the standard deviation of the observed EF shift, but having little affect on the magnitude of the shift itself. Since these equally-distributed surface Ti atoms should transfer four electrons each, as interlayer excess Ti does, to the host layer(s), the net charge transfer to the basal plane should be the same as that in the bulk. Support for this line of reasoning has been derived from recent measurements of the EF shift of -0.06 f 0.03 eV from ECBfor ammonia-intercalated til,^&,^^ which is known to have a bulk charge transfer of 0.23 & 0.02 electrons per Tis2 unit.6 The observed shift corresponds to a charge transfer of 0.24 electrons per Tis2 unit based on eq 1 and the value of four electrons per excess Ti atom.
Conclusions The STS measurements of the density of states of Til+,& near the Fermi level indicate that any changes in the host band structure associated with Ti intercalationare at most minor, since no new features are observed as a function of x. The semiconducting band-gap energy of 0.5 ir 0.1 eV observed for Ti1+$2 was found to be largely independent of composition in the range 0.090 1 x 1 0.002. Therefore, the rigid band model is a reasonable approximation within the accuracy of these studies for the self-intercalation of Ti into Tis2 to form TilfxS2 (0.090 1 x 2 0.002), with electrons from the intercalated Ti being donated to an essentially unperturbed host conduction band. It has also been demonstrated that STS offers a very sensitive atomic-level surface probe of intercalation charge transfer for the model compound This work indicates that direct measurements of the shift of EF relative to the band-gap-edge energies can be used to evaluate the degree of charge transfer. From Figure 5, it is evident that this technique will be most sensitive at x values below about 0.05, which corresponds to a charge transfer of 0.20 electrons per Tis2 unit. The unique capability of STS to probe surface charge transfer should allow the elucidation of the degree of charge transfer associated with the early events in intercalation, including guest-species adsorption and intercalant-host charge transfer, at an unprecedented level of resolution. Such investigations will be the subject of future publications.
Investigation of Charge Transfer in Acknowledgment. We wish to acknowledge the National Science Foundation for support through grant DMR 9 1-06792. We also wish to thank the Center for Solid State Science for use of its Materials Preparation Facility, S. M. Lindsay for helpful discussions, and Young Sir Chung and Tim Karcher for important technical assistance. References and Notes (1) Levy, F., Ed. Intercalated Layered Materials; D. Reidel: Dordrecht, Holland, 1979. (2) Whittingham, M. S., Jacobson, A. J., Eds. Intercalation Chemistly; Academic Press: New York, 1982. (3) Atwood, J. L., Davies, J. E. D., MacNicol, D. D., Eds. Inclusion Compounds; Academic Press: London, 1984. (4) Dresselhaus, M. S., Ed. Intercalation in Layered Materials; Plenum Press: New York, 1986. (5) Legrand, A. P., Flandrois, S., Eds. Chemical Physics of Intercalation; Plenum Press: New York, 1987. (6) McKelvy, M. J.; Glaunsinger, W. S. Annu. Rev. Phys. Chem. 1990, 41, 497. (7) Divigalpitiya, W. M. R.; Frindt, R. F.; Morrison, S. R. Science 1989, 246, 369. (8) Grange, P.; Delmon, B. J. Less-Common Met. 1974, 36, 353. (9) Stupian, G. W.; Cosse, P. J. Vac. Sci. Technol. 1976, 13, 684. (10) Whittingham, M. S. In Materials Science and Energy Technology; Libowitz, G. G., Whittingham, M. S., Eds.; Academic Press: New York, 1978; p 455. (1 1) Chianelli, R. R.; Scanlon, J. C.; Thompson, A. H. Mater. Res. Bull. 1975. 10, 1379. (12) Subba Rao, G. V.; Shafer, M. W. In ref 1; p 99. (13) Rouxel, J. In ref 5; p 127. (14) Murrav. R. B.: Yoffe. A. D. J. Phvs. C 1972, 5, 3038. (15) Myron, H. W.; Freeman, A. J. Phjs. Rev. B 1975, 11, 2087. (16) Zunger, A.; Freeman, A. J. Phys. Rev. B 1977, 16, 906. (17) Bullett, D. W. J. Phys. C 1978, 11, 4501.
J. Phys. Chem., Vol. 99, No. 20, 1995 8221 (18) Isomaki, H. M.; von Boehm, J.; Krusius, P. J. Phys. C 1979, 12, 3239. (19) Friend. R. H.: Jerome. D.; Liane, W. Y.; Mikkelsen, J. C.; Yoffe, A. D. J. Phys. C 1977, 10, L705. (20) McKelvv. M. J.; Glaunsinger, - W. S. J. Solid State Chem. 1987, 66,'181. (21) Logothetis, E. M.; Kaiser, W. J.; Kukkonen, C. A,; Faile, S. P.; Colella, R.; Gambold, J. J. Phys. C 1979, 12, L521. (22) Kukkonen, C. A,; Kaiser, W. J.; Logothetis, E. M.; Blumenstock, B. J.; Schroeder, P. A.; Faile, S. P.; Colella, R.; Gambold, J. Phys. Rev. B 1981, 24, 1691. (23) Chen, C. H.; Fabian, W.; Brown, F. C.; Woo, K. C.; Davies, B.; DeLong, B.; Thompson, A. H. Phys. Rev. B 1980, 21, 615. (24) Barry, J. J.; Hughes, H. P.; Klipstein, P. C.; Friend, R. H. J. Phys. C 1983, 16, 393. (25) Klipstein, P. C.; Friend, R. H. J. Phys. C 1984, 17, 2713. (26) Wang, C.; Slough, C. G.; Coleman, R. V. J. Vac. Sci. Technol. B 1991, 9, 1048. (27) Wilson, J. A. Phys. Status Solidi B 1978, 86, 11. (28) Kelty, S. P.; Lieber, C. M. Phys. Rev. B 1989, 40, 5856. (29) van Bakel, G. P. E. M.; De Hosson, J. Th. M. Appl. Phys. Lett. 1990, 56, 2402. (30) Tersoff, J.; Hamann, D. R. Phys. Rev. B 1985, 31, 805. (31) Kirtley, J. R. Int. J. Mod. Phys. B 1990, 4, 201. (32) Wane. C.: Giambattista. B.: Slough, C. G.; Coleman, R. V. Phys. Rev. B 1990,"42, 8890. (33) Feenstra. R. M.; Stroscio, J. A,; Fein, A. P. Surf Scz. 1987, 181, 295. ' (34) Tanaka, M.; Mizutani, W.; Nakashizu, T.; Morita, N.; Yamazalu, S.; Bando, H.; Ono,M.; Kajimura, K. J. Microsc. (Oxford) 1988,152, 183. (35) Friend, R. H.; Yoffe, A. D. Adv. Phys. 1987, 36, 1. (36) Takeuchi, S.; Katsuta, H. J. Nippon Kinzoku Gukkaishi 1970, 34, 758. (37) Thompson, A. H.; Gamble, F. R.; Symon, C. R. Mater. Res. Bull. 1975, 10, 915. (38) Feenstra, R. M. Phys. Rev. B 1994, 50, 4561. (39) Wang, C.; McKelvy, M.; Glaunsinger, W. In preparation.
-
I
JP9409247
