J. Phys. Chem. 1982, 86, 313-315
the excess platinum valence electrons provided by the fDed number of potassium adatoms. At low CO coverages, each CO may interact with a larger fraction of the excess electron density. This is also supported by the single broad CO thermal desorption peak for saturation CO coverages at a potassium coverage of OK = 0.3 (Figure 3). At any point during the CO desorption process, those CO molecules which remain on the surface become more tightly bound, and their stretching frequency decreases. This is reasonable as surface electron transfer processes take 10-14-10-'6 s, while the thermal desorption experiment occurs on a time scale of about 30 s. Contributions to this 325-cm-' decrease due to dipole-dipole interactions between the adsorbed CO molecules cannot be distinguished from back-bonding effects, but electron back-donation appears to be the dominant contribution. The change in relative occupancy of the surface sites is more difficult to rationalize. Recently, Nieu~enhuys'~ has shown a correlation between the work function of a metal surface and the degree of electron back-donation: the lower the work function, the higher the electron spillover into the back-bonding orbital. Also on many lower work function metal surfaces CO sits preferentially on the bridged site. This indicates that the bridged site permits more extensive dR to 2 ~ electron * ~overlap than the linear site. Another factor influencing site selectivity could be the energetically favorable decrease in the dipole separa-
313
tion of CO and its image achieved by moving from a linear top site to a bridged or threefold hollow site. The catalytic implications of the results shown above, especially the single-bond character of CO, are significant. An increase in the electron back-donation from a metal increases the probability of both the hydrogenation of the weakened CO molecule and the dissociative adsorption of CO. For the Fischer-Tropsch reaction (CO + H2 hydrocarbons), an increase in the rate of CO dissociationwill increase the carbon and oxygen surface coverage relative to that of hydrogen. Work in the past'J' on catalytic hydrocarbon reactions have shown a preference for longer chain hydrocarbons as well as oxygenated products when alkali oxides were added to catalysts. More exact reaction studies, combined with atomic level surface characterizations such as those presented here, will eventually lead to a more fundamental understanding of promoter effects in catalysis.
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Acknowledgment. This work has been supported by the Director, Office of Energy Research, Office of Basic Energy Science, Material Sciences Division of the U.S.Department of Energy, under contract W-7405-ENG-48,and the Dow Chemical Company Foundation. The authors thank G. Ertl and M. A. Van Hove for their thoughtful discussions. (17)T.P.Wilson, W. J. Bartley, and P. C. Ellgen, to be published.
Amorphous Organic Molecular Solids. Vapor-Deposited Tetracene R. EIermann,+ 0. M. Parkinson,# H. Bassler," and J. M. Thomas' Fachberelch Physlkalkche Chemle der Universltit Merburg, D 3550 Merburg/Lann, West Germany, and Department of Physical Chemistfy, Unhers@ of CambrMge, CambrMge CB2 1EP. England (Recelvd: October 12, 198 1; I n Final Form: December 4, 198 1)
Electron diffraction has been employed to study the structure of tetracene films prepared in situ by vapor condensation onto substrates held at temperatures Tfbetween 10 and 300 K. For Tf< 180 K diffuse diffraction rings are observed. They indicate that the sample is amorphous in the sence that there is short-range order derived from crystal structure but no long-range order. The degree of short-range order increases with increasing TC Deposition at Tf> 200 K, as well as annealing above 200 K, leads to crystallization.
Introduction Although amorphous forms of metallic, covalent, as well as chain and layered materials have been prepared, little or no attention has been paid to obtaining and characterizing amorphous analogues of well-known and extensively studied organic molecular crystals. There have been a few reports, based on spectral, rather than direct structural, assessment that aromatic hydrocarbons grown from solution onto specially prepared surfaces form metastable phases which are quite unlike their thermodynamically stable counterparts. There has, however, been no systematic structural investigation of amorphous organic solids even though many propertiest Fachbereich Physikalische Chemie der Universiat Marburg. *University of Cambridge. *Address corresponding to this author at the following address: Fachbereich Physikalieche Chemie der Universitiit Marburg, D. 3550 Marburg/Lahn, West Germany.
0022-3654/82/2086-0313$01.25/0
such as luminescent emission and conductivity-glow characteristics-have recently pointed to the occurrence of significant changes in behavior as between crystalline and quasi-crystalline or disordered counterpalts. Moreover, studying small molecular entities in the amorphous condensed state affords a flexibility in the variation of the conditions of preparation which is not available in the case of polymeric species. The intermolecular potential of planar aromatic molecules is strongly asymmetric. For the linear acenes, like anthracene and tetracene, the pair configuration of minimum potential energy is that in which the long molecular axes are parallel and the short axes form an angle close to 55O.l This is the basic unit of the crvstal structure. The question arises as to whether this str&ture will be established if the solid phase is formed under conditions far (1)A. M. Mathieson, J. M. Robertson, and V. C. Sinclair,Acta Crystallogr., 3,245 (1950).
0 1982 Amerlcan Chemical Society
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removed from thermodynamic equilibrium. Conceivably, there is a variety of metastable intermolecular conformations which are likely to be frozen-in in a solid formed, for instance, by vapor deposition onto a cold substrate provided that temperature and deposition rate are controlled in such a way as to ensure that the rate of deposition of material by far exceeds the rate of structural relaxation. Previous fluorescence studies performed on vapor-deposited anthraceneZand tetracene3 have established the presence of various molecular pair conformations on a percent level which give rise to excimer emission. However, absorption spectroscopy clearly indicates that the bulk material is subject to short-range order which derives from the crystal structure." The main feature of the disordered phase formed by vapor deposition seems to be a fluctuation of the intermolecular seDaration bv small amounts causine a fluctuation of the gas-to crystal-shift energy and giving rise to a Gaussian broadening of the absorption profile. The only reliable way to gain direct information on the structure of evaporated organic solids is by electron diffraction on in situ grown films. Sample fabrication inside the electron microscope is necessaw to exclude annealing effects when transfeiring the Sam-ple to the diffraction chamber. X-ray analysis, although applied previously? appears to be unsuitable to yield quantitative results, since samples of the required thickness cannot be produced in a homogeneous manner. hi^ letter presents the first on the of vapor-deposited tetracene films employing electron diffraction. They show that low-temperature-deposited material is amorphous in the sense that there is short-range order but no long-range order. The degree of short-range order, documented by the width of the diffraction rings, decreases with increasing deposition temperature. Annealing above 200 K finally leads to crystallization. Experimental Section With the development of a liquid-helium-cooled, doubel-tilt stage in adapted electron microscope, it is possible to conduct in situ studies of freshly prepared, vapor-deposited organic f h at temperatures from ca.10 K upward to room temperature, without breaking vacuum. At present electron diffraction has been the main chara&rizing technique, but electron-optical imaging and to a lesser extent luminescence can be employed. Full details of the adapted microscope are to be described elsewhere? It is to be noted, however, that (1) specimen temperatures (at evaporation) are measured by thermocouples positioned as close to the substrate as possible, (2) a built-in mass speetrometer readily deteda (on warming the sample) any H,O or CO impurities, which, at the low temperatures employed, ean be disturbing influences in that they yield quite sharp diffraction spots or rings, (3) specimen thicknesses in the range 500-2000 A and deposition rates of 0.5-4 A/s may routinely be generated, and (4) intense illumination, of visible or UV light, may be directed onto the fh (using a Of quartz fibre-optics and molybdenum mirrors). Commerical tetracene which had been resublimed was evaporated from a small Knudsen cell onto a temperature-controlled cold stage mounted inside the transmission electron microscope. Evaporated carbon films mounted (2)
J. Hoiiinann. K. P.h f e l d , W.Hoihrger, and H. Blisslar, Mol.
Phys. 51,973 (1979). (3) G. Peter and H. BBasler, Chem. Phys. 49.9 (1980). (4) R Heaes. W. Hoihrger. and H. Bhkee., Chem. Phw., 49, 201 (1980). (5) Y. byma and N.~ ~ a ~ achm. k i , Phyr. L ~ ~24.26 ~ . (1974). , (6) G. M.Parkinson and J. M.Thomas, in preparation.
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F@re 1. Electmn diffracton panems of a R h of teiracBne deposited cnto a subsbate cooled to 20 K (h same Sample as shown in Figue 2a and allowed to warm up slowly. Diffraction patterns recorded at temperatures of (a) 200, (b) 250. (c)275. and (d) 295 K and (e) after annealing at 3oo for days in ai,.
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2, Elecbcn diffra* ot a of Rms of tebacene produced wiih a constant rate of deposiilon and having the same average ihlckness. recorded at ihek fcfmation temperature wimOul annealing (a) 20. (b) 120. (c) 175, (d) 250, and (e) 300 K. ~~~~
on copper grids served as sample support. The evaporation rate was typically 0.4 pm/h; the film thickness varied between 0.05 and 0.3 rm. and ~ i ~ ~ A series of diffraction patterns of a tetracene film deposited at 20 K and recorded at various temperatures is presented in Figure 1. The diffraction pattern of a polycrystalline film deposited at 300 K (Figure 2) is to be
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The Journal of Physical Chemistry, Vol. 86, No. 3, 1982 315
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Flgure 3. Summary of the Interplanar spacings measured by electron diffraction as a function of formation temperature, compared with values calculated from the crystal structure of tetracene determined by Xqay diffractlon. Horizontal lines represent data fits indicating that within the experimental accuracy no systematic temperature dependence of the d spacing 1s detectable.
compared with Figure 1. Analysis of the latter indicates that the crystallites are oriented with their (0,0,1) planes approximately parallel to the substrate surface. In the diffraction patterns of samples formed below 180 K, only three diffuse rings are discernible. Their width slightly decreases upon warming from 80 to 180 K. Above 180 K significant sharpening occurs and additional features appear. A t 290 K further rings can be distinguished. Annealing for 18 h at 295 K causes recrystallizationevidenced by the appearance of a Bragg diffraction pattern. Both in the diffraction pattern and in the direct imaging of the texture of the film,the annealed sample closely resembles a polycrystalline film directly prepared by evaporation at room temperature. The fact that the diffraction patterns of annealed samples exhibit some features which are absent in those of as-grown polycrystalline samples can be explained on the basis of random orientation of the crystallites in the former whereas in the latter there is some preferential orientation during growth, the ab plane of the crystallites being approximately parallel to the substrate.
Interestingly enough, the diameter of the diffraction ring are both independent of recording temperature within the whole temperature range; i.e., those that appear in the unannealed samples also appear in the pattern of the polycrystalline form (Figure 2) and of formation temperature (Figure 3). The diffuse character of the diffraction pattern of unannealed samples indicates absence of long-range order, yet the fact that some spacings in the polycrystalline samples are retained provides evidence for the existence of short-range order which derives from the molecular packing within the crystal lattice. The low-temperature structures are amorphous in the sense that the intermolecular coordinates are subject to statistical fluctuation around average values which are identical with or at least close to, the crystal values. There is close analogy between the amorphous phases of a van der Waals solid and a covalently bonded solid like silicon with, however, an important difference in that with an extended structure like silicon there is bond rupture, whereas no such free valencies are present in amorphous phases of van der Waals solids. It is significant that photoinduced structural changes may be wrought in silicon whereas, in the films of tetracene examined by us, we were unable to detect any comparable transformations. Nevertheless, there are regions within these organic amorphous solids which, in embryo at least, are similar in the short-range order to their crystalline counterparts. The existence of short-range order is equivalent to the existence of molecular pair structures resembling the intermolecular conformations within the unit cell of the crystal. This deduction is made on the basis of the observed splitting of the S& transition in the absorption spectra of these films! Clearly the extent of this short-range order is governed by the deposition temperature and by the rate of deposition in that both of these factors determine the time available for a molecule to relax into a site of minimum energy. The dependence of the degree of disorder on the substrate temperature during deposition is demonstrated by comparing of Figures 2, showing the diffraction patterns of unannealed films grown near 10 K, at 120 K, and at 200 K. The diffraction pattern of a film deposited at 200 K closely resembles the pattern of a polycrystalline layer formed at 300 K as far as sharpness of the features is concerned. This observation directly c o n f i i s earlier measurements by Eiermann et al.7 monitoring the density of localized, intrinsic valence states, Le., hole trapping states, as a function of film formation temperature. Acknowledgment. We are greatly indebted to Dr. W. Hofberger for numerous discussions during the early stages of this work. Financial support by the Deutsche Forschungsgemeinschaft and the Science and Engineering Research Council of Great Britain is gratefully acknowledged. (7) R. Eiermann,W. Hofberger, and H. Bider, J.Non-Cryst. Solids, 28, 415 (1978).
