-
na* Transitions in TMTCBD X
The Journal of Physical Chemistry, Vol. 85,No. 25, 198 1 3777
-
A2 A2
4
N
"CSb2 "Ob2
TMTCBD X'0,Y.S
region, however, it is clear that, apart from a dramatic (-800 cm-l) isotope shift of the 21 325-cm-' (-h12)band, there is no significant change in the spectrum upon deuteration. The band near 17050 cm-l is assigned to the 3A2(na*Cs) 'Al transition because of its weakness, its separation ( 1300 cm-l) from the corresponding SS transition, and the fact that it is exclusively c(y and/or 2 ) polarized. Several vibronic origins of different polarizations would have been expected for a SS transition. This assignment is consistent with that for the triplet-singlet (TS) transition in TMCBDTFfiand the arguments supporting it are outlined more fully in ref 5. The TS absorption is probably z (rather than y ) polarized, made allowed by interaction with a 'Al state through the A2(R,) component of the spin-orbit operator. A similar mechanism occurs in TMCBD,'t2, TMCBDT,5*6,H,CO,l3 and H2CS.12 The stronger absorption between 18000 and 23000 cm-l must record the corresponding 'Az(nT*cs) 'Al transition. The large isotope shift of the c-polarized 21 325-cm-' (-h12) band leads to its assignment as a vibronic origin involving an a2 and/or bl CH/CD stretching mode as found in TMCBD' and TMCBDT.5 It follows that the forbidden electronic origin must lie near 18350 cm-', close to the onset of SS absorption. The c-polarized band at 18625 cm-' and the b-polarized shoulder at 18725 cm-l must also record vibronic origins involving lower-frequency modes of a2 and/or bl, and bz symmetry, respectively, as may other bands in this region. It is interesting that the CH/CD stretching modes are active in the c(y and/or 2)-polarized spectrum, but are absent or weak along b ( x ) , while in TMCBD,l where n-, y-, and z-polarized spectra could be separately observed, they appeared in all three polarizations. One or two vibronic channels of this type operate in TMTCBD rather than three as in the dione. In TMCBDT5 crystals these modes are active along c(y) and in the ab plane ( x and/or z), which could be consistent with either of the cases above. The large widths of the visible bands are surprising since one might have expected a sharper spectrum than in TMCBDT where closely spaced singlet na* states perturb each other. This diffuseness seems to be characteristic of the singlet and triplet na*cs states themselves, perhaps indicating very short lifetimes, rather than being due to the crystalline environment, since the weak bands near 27 500 cm-l are much sharper under the same conditions. We tentatively assign these sharp, weak, exclusively c(y and/or 2)-polarized bands near 27 500 cm-l to a z-polarized 3A2(na*co) 'Al transition, by analogy with the 3A, 'A, system of TMCBD's2 and using the same arguments presented above for the n a * c ~TS system. The approximate 300-cm-' spacing between the sharp bands is reminiscent of the progression in u3, a totally symmetric ring distortion mode, found in TMCBD' and TMCBDT5 but the first two bands, which may be perturbed, do not fit into this pattern. The corresponding SS transition is probably hidden under the stronger absorption which begins near 29000 cm-l. This strong, structureless absorption, which between 29 000 and 33 000 cm-l is more intense along c than b, may also include nu* and ua* transitions. Much of the c-polarized intensity may be borrowed from the intense 43 500-cm-' system, presumably aa*cs and z polarized.
TMCBD: X=Y=O TMCBDT X=Y =S D2h
C2"
Figure 1. Molecular axes and molecular orbitals for TMCBD and its sulfur derivatives.
-
33
31
29
27
23
21
19
17
wcml lo3 Flgure 2. Densitometer traces of the TMTCBD crystal absorption spectrum: (solid curves) TMTCBD-hl2; (broken curve) TMTCBD-d,,; (top) c(y+z)polarized; (bottom) b(x)polarized; (i)I-mm thick crystal, long exposure; (ii) 1-mm, short exposure; (iii) 0.3 mm; (iv) 0.5 mm.
cia1 2-propanol-d8as described in ref 1,was used to make TMTCBD-dlz. Single crystals of TMTCBD, volatile red plates 0.2-1.0 mm thick, were grown with difficulty by vacuum sublimation. Their orientation was determined by X-ray diffraction using a precession camera with the volatile crystal sealed in a Lindemann glass capillary. Spectra were recorded photographically as described elsewherell at liquid-nitrogen and liquid-helium temperatures.
Results and Discussion TMTCBD owes its red color to a weak absorption (A, = 520 nm, -E = 10 L mol-I cm-', f = 1.3 X in hexane) which is readily assigned to the na*@ transition by analogy with the spectra of TMCBDT5 and other thiocarbonyl compounds such as H2CS.12 Significantly, its intensity is just half that in the corresponding dithione. Several stronger absorptions are found beyond 29 000 cm-'. Densitometer traces of the crystal absorption spectra are shown in Figure 2, and measured peaks are listed in Table 11. The visible bands are disappointingly broad (fwhm 1000 cm-l) compared to those in TMCBDT and sharpen only marginally between 77 K and liquid-helium temperature. However, a few much sharper (fwhm = 150 cm-'), very weak, c-polarized bands are observed near 27 500 cm-l in thick crystals at liquid-helium temperature. Because of limited material, we were not able to grow a good-quality, thick single crystal of TMTCBD-d,,; hence, its spectrum (broken line in Figure 2) is a mixture of b and c polarizations, and no features corresponding to the sharp, weak ultraviolet bands could be detected. In the visible (11) R. D. Gordon and R. F. Yang,J . Mol. Spectrosc., 34,266 (1970). (12) R. H. Judge, D. C. Moule, and G. W. King, J. Mol. Spectrosc., 81, 37 (1980); R. H. Judge and G. W.King, ibid., 74, 175 (1979).
-
+-
Conclusions Transitions from the ground state of TMTCBD to a 3 A z ( n ~ * a~ ~'Az(na*cS), ), and, tentatively, a 3A2(na*co) (13) D. C. Moule and
A. D. Walsh, Chem. Rev., 75, 67 (1975).
3778
J. mys. Chem. 1981, 85,3778-3787
T A B L E 111: nm* Transitions in TMCBD and Derivatives (crn-’ )
Its Sulfur
TMCBD‘
16829
lowest nn*cs singlet lowest nn*co triplet
TMTCBD
17050
lAU,‘ B , g , -18000
3Au, 25 720
lowest nn*co singlet
All
-18350 3-42,
27 430 ? l-42,
9
27 130
‘Reference 1.
TMCBDTb
SA,,
lowest nn*cS triplet
transitions. CH/CD stretching modes of the peripheral methyl groups are vibronically active in the SS transition, suggesting that, as in TMCBD and TMCBDT, the effects of excitation are widely delocalized. However, since the visible bands are very broad and little structure is resolved, nothing can be said on Franck-Condon grounds about the geometry of the excited states. The diffuseness of the visible spectrum suggests that the triplet and singlet nx*cs states may be very short-lived.
> 2 9 000 ?
Reference 5 .
state have been identified. Observed transition energies are compared in Table I11 to those in TMCBD and TMCBDT. The polarization of the TS transitions is ,Al consistent with their deriving intensity from ‘A,
-
Acknowledgment. We thank Dr. R. D. Heyding for the use of his X-ray equipment, Dr. R. A. Whitney for advice in synthetic chemistry, and the Queen’s University Advisory Research Committee and the Natural Sciences and Engineering Research Council of Canada for financial support.
Deliberate Modification of the Behavior of n-Type Cadmium Telluride/Electrolyte Interfaces by Surface Etching. Removal of Fermi Level Pinning Shinlchi Tanaka,‘ James A. Bruce, and Mark S. Wrighton* Department of Chemistry, Massachusetts Instltute of Technology, Cambridge, Massachusetts 02 139 (Received: July 13, 198 1; In Final Form: August 25, 1981)
Single-crystal,n-type CdTe (Eg= 1.4 eV) has been studied with respect to barrier height, E B , when contacting a liquid electrolyte solution containing a fast, one-electron, outer-sphere redox reagent. We approximate EB as equal to the photovoltage measured by cyclic voltammetry of various redox couples at illuminated n-CdTe vs. a reversible electrode. n-CdTe surfaces pretreated with an oxidizing etch give an E B of -0.5 V f 0.1 V in HzO/O.l M NaC104or CH3CN/0.1M [n-Bu4N]C104that is independent of the Ellzof the added redox couple. A reducing etch pretreatment gives an E B in either of the electrolyte solutions that depends on Ellzof the redox couple in a manner consistent with a nearly ideal semiconductor. The reduced CdTe exhibits an E B of up to 0.9 V for a redox couple having Ellz near 0.0 V vs. SCE, whereas couples having Ellznegative of --1.0 V vs. SCE give zero photovoltage. Auger spectroscopy and X-ray photoelectron spectroscopy (XPS) of the reduced and oxidized surfaces are qualitatively different. The reduced surface exhibits signals for Cd and Te in relative intensitives that are consistent with a close to stoichiometric (1/1)surface. The oxidized surface exhibits little or no detectable Cd signal, and the Te signal is consistent with a thick overlayer of elemental Te. The data are consistent with the conclusion that the CdTe/Te interface is Fermi level pinned ( E B independent of contacting medium);the semimetallic Te overlayer behaves as a metal contacting CdTe, and the CdTe/Te interface energetics are therefore not influenced by changes in the contacting medium.
Results from this laboratory were recently reported2 showing that n-type CdTe photoanodes give an open-circuit photovoltage, Ev, of -0.5 V independent of the electrochemical potential, Eredox,of the contacting electrolyte solution. Certain aspects of the results were independently r e p ~ r t e d . ~A nearly constant barrier height, EB, has been found4 for n-CdTe contacted by metals having different work functions, 4. The value of Ev is generally close to E B at high illumination intensity. The finding of a constant E B for n-CdTe, independent of Erdox or 4, leads to the conclusion that n-CdTe is Fermi level ~inned.~?~ (1)Participant in programs of the Center for Advanced Engineering Studies at M.I.T. while on leave from Toyobo Co., Ltd., Osaka, Japan, 1980-1981. (2) Aruchamy, A.; Wrighton, M. S. J. Phys. Chem. 1980,84, 2848. (3)(a) Nadjo, L.J.ElectroanaL Chem. 1980,108,29.(b) Sculfort, J. L.; Baticle, A. M. Rev. Phys. Appl. 1980,15,1209. (4)(a) Ponpon, J. P.; Siffert, P. Rev. Phys. Appl. 1977,12,427. (b) McGill, T. C. J. Vac. Sci. Technol. 1974,11,935.(c) Mead, C. A.; Spitzer, W. G. Phys. Rev. A 1964,134,713.
0022-3654/81/2085-3778$01.25/0
We take the term “Fermi level pinned” to refer to a semiconductor that is measured to have a constant EB, independent of the contacting medium for a wide range of Edoxor 4. The origin of a constant E B can be attributed to surface states situated between the top of the valence band, EVB, and the bottom of the conduction band, ED. The density and distribution of surface states can control the value of E B as a function of Eredox or +.6*6 When the region between E C B and E V B is free of surface states, EB of an n-type semiconductor is expected to vary with Edox according to eq 1’ for Erdoxsituated between ED and ECB (1) E, zz E B = lEredox - EFBl where EFB is the electrochemical potentia1 of the semiconductor, Ef,when there is no band bending. When Ef (5) Bard, A. J.; Bocarsly, A. B.; Fan,F.-R. F.; Walton, E. G.; Wrighton, M. S. J. Am. Chem. SOC.1980,102, 3671. (6) Lin, M. S.;Hung, N.; Wrighton, M. S., to be submitted for publication. (7) Gerischer, H. J. ElectroanaL Chem. 1975,58,263.
0 1981 American Chemical Society
