Electroabsorption studies of undoped C60 thin films

Synthetic Metals, 49-50 (1992) 557-563

557

Electroabsorption studies of undoped C6o thin films S. Jeglinski and Z. V. V a r d e n y Department of Physics, University of Utah, Salt Lake City, UT 84112 (USA)

D. Moses, V. I. S r d a n o v and F. W u d l Institute of Polymers and Organic Solids and Physics Department, University of California, Santa Barbara, CA 93106 (USA)

Abstract Electroabsorption spectra have been measured for undoped C6o thin films at 80 and 300 K in the spectral range 1.76 to 5 eV. Absorption and thermal modulation spectra have also been obtained. The electronic energy levels of C6o are reviewed, and the electroabsorption spectra are discussed in terms of the Stark effect on energy levels of Ag, He and Tu symmetries.

Introduction T h e a b s o r p t i o n s p e c t r u m o f C6o has b e e n m e a s u r e d by a n u m b e r of r e s e a r c h g r o u p s [ 1 - 3 ] and displays s t r u c t u r e f r o m the ultraviolet t h r o u g h the n e a r - i n f r a r e d s p e c t r a l range. The e l e c t r o a b s o r p t i o n (EA) s p e c t r u m , o n the o t h e r hand, has n o t b e e n m e a s u r e d . T h e EA p r e s e n t e d h e r e is r i c h e r and m o r e i m p r e s s i v e t h a n t h a t f o u n d f o r m a n y materials, w h o s e generic EA f e a t u r e s consist o f a single large p r i m a r y oscillation in Aa at o r n e a r the a b s o r p t i o n e d g e and s e c o n d a r y oscillations that m a y or m a y n o t c r o s s zero and e v e n t u a l l y d e c a y to negligible v a l u e s at h i g h e r energies. In contrast, the C8o EA s p e c t r u m consists o f m a n y oscillations b e t w e e n 2 and 5 eV and p r o b a b l y e x t e n d s t o e v e n h i g h e r energies. H a d d o n et al. [4] have c a l c u l a t e d the e n e r g y levels o f C~0 b y m e a n s of a t h r e e - d i m e n s i o n a l Htickel m o l e c u l a r orbital (HMO) model. Numerical calculations o f the e n e r g y levels are r e q u i r e d b e c a u s e of the i m p o r t a n c e o f e l e c t r o n c o r r e l a t i o n effects. This results in an energy-level s c h e m e (Fig. 1) consisting o f a singlet g r o u n d state o f Ag s y m m e t r y , a five-fold d e g e n e r a t e first e x c i t e d state o f Hg s y m m e t r y , and a t h r e e - f o l d d e g e n e r a t e s e c o n d e x c i t e d state o f T1u s y m m e t r y . H a d d o n p r e d i c t s the s y m m e t r y - f o r b i d d e n l o w e r excitedstate transition Ag--, Hg, c o r r e s p o n d i n g to the e l e c t r o n i c transition hu--,tl~, to h a v e an e n e r g y o f 0.76/3, w h e r e fl is the HMO r e s o n a n c e integral, and the s y m m e t r y - a l l o w e d h i g h e r e x c i t e d - s t a t e transition Ag-* T~u, c o r r e s p o n d i n g to the e l e c t r o n i c t r a n s i t i o n h~--*t,g, t o have an e n e r g y o f ~ [4]. Saito a n d O s h i y a m a [5l h a v e calculated the e l e c t r o n i c e n e r g y levels o f the C6o m o l e c u l e a n d the resulting b a n d s t r u c t u r e for an fcc lattice of C6o

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558 other excited states

~

~

~

three-fold Tlu degenerate

~

m

~

five-fold Hg degenerate

hu-~tlg [hu--*tlu 2.5 - 2.9 eV 1 1 . 9 eV

/

__a.. Ag Fig. 1. Schematic diagram of C60 energy levels (not to scale).

molecules, yielding an hu-*tlu energy gap of 1.9 eV, implying that f l ~ 2 . 5 eV. Haddon's result would in turn imply that the hu -~ txg electronic transition energy is 2.5 eV [4], while Saito's result for the same hu-*tlg transition is 2.87 eV [5]. Kawabe e t al. [6] have suggested a theory of EA in ~r-conjugated polymers that utilizes the straightforward Stark-effect perturbation. Coo bears similarities to 7r-conjugated polymers. Our work attempts to explain EA results in Coo in the framework of the Haddon/Saito and Kawabe approaches [4-6].

Experimental set-up and procedure A thin film of purified C6o (thickness about 1000/~) containing less than 8% C7o was evaporated (at 450 °C and 5 × 10 -6 Torr [7]) onto an aluminum electrode that had been deposited on a sapphire substrate in an 'interlocking finger' geometry. A gap of 20 /zm between adjacent electrodes allowed the use of voltages less than 200 V ~ (root-mean-squared) to achieve the required field strengths of 104-105 V cm -1. A small sine-wave voltage was connected to a custom-built step-up transformer, the output of which was connected to the electrode. The electrode was housed inside a cryostat for experiments between 80 and 300 K. The electric-field modulation frequencies ranged from 250 Hz to 1 kHz. A light source (100 W tungsten lamp for the visible, 450 W Xe lamp for the ultraviolet) was focused on a computer-controlled 0.25 m f~4 monochromator, whose output was directed through a mechanical chopper, focused on the sample, and detected by a UV-enhanced silicon photodiode. This configuration was used to minimize any photodegradation of the C6o sample [8]. The photodiode electrical output was directed to a computer-controlled lock-in amplifier. For each spectrum, the transmission (T) was measured with the mechanical chopper in place and the electric field off. The differential transmission (AT) was subsequently measured without the chopper and with the electric field on, with the lock-in set to detect signals at twice the electric-field modulation frequency. The data were divided

559

to yield the - A T / T information, which is free of the spectral response function. Precautions were taken to pr e ve nt any thermal modulation (TM) from interfering with the EA data. Electric-field modulation frequencies of 250 Hz and above were employed, because the TM amplitude is inversely proportional to the modulation f r equency [9]. This, combined with the high electrical resistance of C6o, constrained TM to a negligible level. Approximate TM s p ectr a were g e ne r at ed by measuring - AT/T in 20 K increments at both 300 and 80 K. The m e a s u r e d TM spectra were not similar to the EA spectra, indicating that TM was not a factor. Results and discussion The optical absorption s p e c t r u m [a(eo)] of the thin-film C60 at 300 K is shown in Fig. 2. Prominent absorption lines occur at 3.6 and 4.7 eV. An absorption shoulder occurs at about 2.8 eV and there are very weak absorption features at and below 2 eV. A typical EA s pe ct rum for C60 at 300 K is also shown in Fig. 2. It shows a narrow modulation of a centered at about 2.37

C6o 1.0 OD

Q~

c~

0.5 0

2

3

I

4

5

EA

-1

-2 ~

2

3

4

5

Photon Energy (eV) Fig. 2. C6o thin-film absorption and electroabsorption spectra at 300 K.

560 eV and a broad modulation of a centered at about 3.49 eV. One can also identify a second broad modulation at about 4.37 eV and a narrower one at 4.70 eV. The EA amplitude shows a quadratic dependence (slope = 1.93) on the electric-field strength. As described by Haddon et al. [4], the ground- and excited-state wavefunctions (denoted by I ~ n > ) are calculated as three-dimensional Htickel molecular orbitals. Numerical calculations including electron-correlation effects indicate that the actual ordering of energy levels is that shown in Fig. 1. The important result is that an Hg state exists between the Au ground state and the Tlu excited state. The T1u state might be described as excitonic to distinguish it from higher, more closely spaced levels, which could form a continuum 'conduction' band. The symmetry-allowed Ag--* T~u 'exciton' might generally be taken as the onset of optical absorption. Haddon has stated that this transition could be 'exceedingly strong' [4], and so one might be inclined to assign this transition to the 3.6 eV absorption peak. However, the numerical results of Haddon and Saito [4, 5] imply that this transition is associated with the shoulder at 2.8 eV. The symmetry-forbidden Au--* Hg transition is not expected to produce features in the absorption spectrum. Saito predicts this transition energy to be 1.9 eV, coinciding with the weak absorption structure at 2 eV. If the Hg level is weakly split in the unperturbed C~0 by internal fields, the feature at 2 eV could be due to this transition. Alternatively, the presence of a phonon side band may split the transition. In principle, the ambiguity in assigning the numerical results to features in the absorption spectrum can be removed with the EA modulation spectroscopy technique, in which a symmetry-breaking electric field applied to a sample permits forbidden and allowed transitions to be seen in sharper detail. This advantage may be diminished in C~o because the electrical field will also likely split the degeneracy of the levels, possibly broadening some of the sharp features of the typical EA spectrum. With this caveat, we apply Kawabe's description of the effect of an external electric field F in terms of the Stark effect with the standard first-order non-degenerate perturbation result [6] 0

t

0

Iqtn> = [gto> + ~ < ~k[// [gtn>.[qtO> (non-degenerate) k*n E ~0 - E k 0

(1)

Equation (1) gives the perturbed wavefunction for the Ag ground state only. The degeneracy of the excited-state levels in C6o requires us to consider the Stark effect with the first-order degenerate perturbation result 0

p

0

[ q t > = [ ~ o > + ~ < ~k~r/ I ~ > I ~ o > (degenerate) k~.

E °-E

(2)

°

where Iq~° > are the zero-field wavefunctions, D represents the subspace of unperturbed degenerate wavefunctions, and H ' = ~ . F is the Hamiltonian for a dipole moment /z interacting with the external field. For a band gap on

561 the order of 1 eV, electric fields on the order of 104 or 105 V cm -x satisfy the requirements for the validity of perturbation theory. Equations (1) and (2) describe how the Ag, Hg, and Tlu states are mixed in the presence of the external field: all A~ and H ° states acquire some T~u character, and vice versa. In addition, the Hg and T~, states are split. In the presence of H ' , the transition probabilities are modified and contain electric-field-dependent contributions from all combinations of pairs of states with mixed parity, e.g., , where H " = e A . p is the electric-dipole interaction Hamiltonian for the incoming light beam described by vector potential A. The various dipole matrix elements are not generally known; they may be large or small, depending on the results of numerical calculations. The Au ~ Hg transition is allowed in the presence of the perturbing electric field. Its stxength and splitting are determined by the contributions of the above-mentioned dipole matrix elements. On the other hand, the Ag~T~u 'exciton' transition is probably still strong, but is split, shifted, and suppressed. The shift appears as a positive amplitude in the EA spectrum. Suppression and splitting of the transition appear as a broadening of the exciton line, thus imparting the characteristic oscillatory shape of the Aa spectrum. How much shift and how much suppression appear in the EA spectrum depends on the strength of the dipole matrix elements that couple the perturbed energy levels. The oscillations in the higher-energy region ( > 5 eV) of the EA spectrum eventually die out and may do so because of a cancellation of continuumband matrix elements. This possibility has been advanced for conjugated polymers [6]. The EA data shown in Fig. 2 indicate distinct transitions at energies of about 2.4, 3.5, 4.4, and 4.7 eV. Of these, the 3.5 and 4.7 eV transitions are probably associated with the absorption peaks at 3.6 and 4.7 eV. The 4.4 eV EA transition is not apparent in the absorption spectrum, and may be the result of a splitting of the 3.6 and 4.7 eV absorption spectrum features. The 2.4 eV EA transitions is peculiar because it lies between the weak 2 eV absorption and the 2.8 eV shoulder and is as strong as the 3.5 eV EA feature. The 2.4 eV EA transition may be the signature of the allowed and split A~--* Hg transition. The effect of differences in samples is important but difficult to take into account. The presence of C70 can have a non-negligible effect, because the differences in absorption between C60 and C70 are non-trivial [2]. Our sample contains no more than 8% C7o. C8o can also exist in solid crystalline form, in particular the fcc phase [5], forming a direct-gap semiconductor and causing changes in absorption and other optical properties. Finally, our C60 film may be described as an amorphous material, as has been verified by X-ray diffraction and low-resolution atomic force micrographs. Figure 3 shows an absorption spectrum for our thin film and also for a C~o solution dissolved in benzene. The broadening and red-shift in the thin-film spectrum with respect to the solution is a classic sign of the presence of disorder and reinforces the assertion that the film is amorphous. This substantially corn-

562 1.4

C6o 1.2 1.0 -5

~, o.8

so,x2o_,

/

j

s

-9 0.6

0

\

O.4 0,2

/"

///,/~-~ 2

3

4

Solution

]

5

I

Photon Energy (eV) Fig. 3. Comparison of the 300 K absorption spectra of C60 thin film and solution.

plicates the analysis of both a(w) and the EA spectrum. In future work, the EA o f C6o thin films cast from solution will be com pared to our present result.

Conclusions

Absorption and electroabsorption spectra for thin-film C60 have been presented. A quadratic dependence of EA on field strength was measured for the film. C60 is such a new and intriguing material that its optical properties are n o t yet fully understood. The symmetry-breaking electric field inherent in the EA technique has the ability to expose forbidden transitions that appear weakly (or not at all) in an absorption spectrum, but may also complicate the picture because of its ability to lift the degeneracy of the states. The complexity o f the EA spectra indicates that non-trivial physical processes form the basis of the relatively simple but extensive absorption spectrum. T h e o r y suggests that the highly degenerate Ag-Hg--Tlu energy scheme can be split into a rather large n u m b e r of different states that can interact with one another, providing a rich and perhaps surprising array of optical properties. Our future work will include EA investigations of C6o and C70, as well as studies of the differences in crystalline, amorphous, and soluble forms. These materials, although similar, will show important differences in their absorption and modulation spectra, thus providing useful clues about their molecular and solid-state band structure.

563

Acknowledgements The work at the University of Utah was supported in part by DOE grant DE-FG-02-89 ER 45409. The work at UCSB was supported by DMR 882 0 9 3 3 and by NSF grant #CHE 89-08323.

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