J. Phys. Chem. 1993,97, 8244-8249
8244
Carbon Films of Amorphous and Oriented Graphitic Structure from Fullerene Ion Beam Deposition H. Caber,+ H.-G.Busmann,'~+R. Hiss,+I. V. Hertel,? H. Romberg,$ J. Fink,* F. Bruder,f and R. B r e d Freiburger Materialforschungszentrum (FMF) and Fakultiit fZir Physik der Universitiit, Stefan-Meier-Strasse 31a, 0791 04 Freiburg, Germany, and Kernforschungszentrum Karlsruhe, Institut fir Nukleare Festkarperphysik, Posgach 3640, 076021 Karlsruhe. Germany Received: February 18, 1993; In Final Form: May 13, 1993
Energetic fullerene deposition has been performed with an intense, highly ionized beam in the kiloelectronvoltenergy range to form carbon thin films. The deposited material is amorphic and essentially free of hydrogen that is bonded to carbon. The films show graphitic nanocrystallites embedded in amorphous carbon which are preferentially oriented with the c-axis parallel to the substrate surface. With increasing substrate temperature, the size of the crystallites increases together with a decrease in the percentage of sp3-bonded carbon. There is evidence that the primary deposition product is mainly sp3-bonded carbon which then relaxes to form the oriented, graphitic nanocrystallites. The method of fullerene ion beam deposition has been used to form a dense amorphous carbon film on a single-crystalline fullerite film, where the crystalline quality of the fullerite film is only insignificantly affected by the deposition process.
1. Introduction
Diamond and graphite had long been regarded as the only solid phases of carbon.' However, remarkable new all-carbon materials have been found in recent years, where fulleritezJ and amorphous carbon4 are the new basic phases. Graphite is the most stable at room temperature, followed by diamond and fullerite. Amorphous carbon is probably the phase with the highest energy. There has been much fundamental scientific and economic interest in the question of whether, and by what route, pure carbon phases can be interconverted. In particular, squeezingthe fullerenes Cm and C70 into diamond by high pressure synthesis has been reported? and one might ask whether fullerenes might undergo phase-transitions in other ways. Recent experimental studiess-10 of Cm+-ionsimpacted onto surfaces in addition to dynamic modeling11 show that Cm is very stable at moderate initial kinetic energies, Ei,of some hundreds of electronvolts and does not penetrate into the surface. With increasing Ei,the deformation of the cluster, as well as that of the target surface, increases together with an increasinginternal energy of the outgoing cluster.12 For Ei in excess of about 500 eV, the reflected ion yield vanishes, because Cm or its collision products are deposited onto, or into, the ~urface.1~Thus, deposition of fullerene ions in the 1-keV energy range leads to conversion of fullerenes into other forms of carbon. For a rough estimate of the pressure during an Cwsurface encounter, the maximum deformation As is taken to be of the order of the radius Rc= 3.5AofC6oandthespringconstant kofthecageisestimated from Ef= kA$/2 15 eV, where this latter value is the final kinetic energy of C6o+-ions scattered from diamond (1 11) and graphite (OOOl).13 With an effective interaction area of A = .rrR,2, a maximum pressure P = kAslA = 2Ef/.rrRC3= 37 GPa is obtained, a value higher than the value of 20 f 5 GPa reported to squeeze C ~ into O diam~nd.~ In the present investigation, the pulsed fullerene ion beam of very low intensity used for the studies mentioned above has been replaced by a continuous, intensive beam ( c 6 0 ion current 10 PA) with 1 keV 5 Ei 5 10 keV, thus forming solid carbon films To whom the correspondence should be addressed.
t Freiburger Materialforschungszentrum and Fakultit fcr Physik der
Universitbt. t Institut fir Nukleare FestkBrperphysik.
0022-365419312097-8244$04.00/0
at a growth rate of 200 nm/h. Even when diamond is not formed in such an experiment, tetrahedral bond formation might occur, forming an overconstrained network. Existence of such material, in which the number of bonds per atom (constrains) is higher than the number of degrees of freedom of each atom (see ref 5 and references cited therein), has been reported to be formed by ablation of graphite with high power lased4Jsand mass selected C-ion beamdepositi~n.~~J~ Such materialshould have high energy due to the great variation of bond lengths and angle^.^*^ In amorphous hydrocarbon films, the number of constraints can always be adapted to form a fully constrained network via C-H bond formation. However, in amorphous carbon,free of hydrogen, formation of sp2-sites might lower the number of constraints. On the basis of simple energy considerations,6 groupingof the C e C bonds into aromatic, graphitic clusters is expected, which are embedded in an amorphous, partly sp3-bonded matrix.1*J9 This kind of material has been found, for example, by sputtering of graphite.20.z Amorphous carbon as presented in this paper has diamondlike chemical, mechanical, and/or optical properties, making it of high potential for applications where the unique properties of diamond are desirable, but where usage of diamond thin films is inhibited. Its deposition from an intense fullerene ion beam is reported here for the first time, and it is classified with regard to hydrogen content, graphitic cluster formation,and the fraction of sp3-bonding, thus allowing a classification according to the discussion in the foregoing paragraph. For the first application, a heteroepitaxialfullerene filmZZwas coated with an a-C film by fullerene ion beam deposition. The integrity of the crystalline films after coating may be understood by the ideas of ion cluster beam deposition23or energeticcluster i m p a ~ t , 2giving ~ . ~ ~fullerene ion beam deposition its own justification as a method for amorphous carbon film deposition. 2. Experimental Section
A schematic diagram of the experimental setup is shown in Figure 1. A mixture of about 90%Cm and 10% c70 is evaporated from an oven held at 500 OC, the background vacuum being in the 10-7 mbar range. The fullerenes are partially ionized using an efficient electron source (250 eV, 400 mA). The resulting fullerene ion current (=lo PA) is focused onto a heatable target at a distance of about 90 mm from the oven nozzle. These 0 1993 American Chemical Society
Carbon Films from Fullerene Ion Beam Deposition ion optics
$ ,
reflectron mass spectrometer
The Journal of Physical Chemistry, Vol. 97, No. 31, 1993 8245 a)
Energy (MeV) 200
0.2
0.4
0.6
I
1
I
1 .o
0.8
I
150
3 v)
2 100 Figure 1. Schematic diagram of the deposition apparatus with an illustration of the fullerene ion composition.
conditions lead to a measured ratio of about 1:9 for neutra1:ion flux. The composition of the ion beam was monitored with a reflection time-of-flight mass spectrometer. As illustrated in Figure 1, singly, doubly, and triply-charged Cm and C ~ions O are observed, together with some fragments. Films with a thickness ranging from 100 to 500 nm were grown at a rate of 200 nm/h. These films are typically very smooth and are difficult to remove from the substrate. No evidence for an influence of the ion accelerationvoltage in the range 1 keV IEi I10 keV on the film properties was found, in agreement with results on films formed by mass selected ion beam deposition (MSIB).16 This also shows that the presence of doubly- and triply-chargedions are of minor importance for the film properties. The results reported in the following were obtained from films deposited with 1-kV ion acceleration voltage onto a substrate held at room temperature, 730 OC, and 900 OC. Silicon (001) or NaCl substrates wereused depending on the requirements of the method used for subsequent characterization of the film. The concentration profile of hydrogen in the film was measured using elastic recoil detection (ERD). In this, a 2.8-MeV 4He+beam from a 7.5-MeV Van der Graaff accelerator strikes the film, from which the hydrogen is knocked out and monitored using an energydispersivedetector.26 Atoms ejected from deeper layers have lower energy due to stronger energy losses of the incident ions and the recoils. The energy spectra are thus a measure of the depth distribution of the hydrogen and can be converted into qualitative concentration profiles.z6 Light absorption of the films in the infrared spectral regime from 450 to 4000 cm-I was measured with an Fourier transform infrared (FTIR) spectrometer (IFS 88, Bruker). Although the measurements were performed in a flow of nitrogen, complete elimination of HzO-absorption lines was not possible. Silicon substrates polished on both sides were used in order to minimize scattering losses of the transmitting light Raman spectra were recorded in reflection geometry using the 5 14.5-nm line of an argon ion laser for excitation. Portions of the samples were removed from the substrate and mounted on electron microscopy grids. Electron diffraction measurements and measurements of the C 1s absorption edge were performed in transmission using a 170-keVelectron energyloss spectrometer.2' The spectra were taken with an energy resolution of 0.18 eV and with a momentum resolution of 0.04 and 0.2 A-1 for diffraction and C 1s absorption measurements, respectively. Due to performing electron energy-lossspectroscopy (EELS) in transmission, the sampling depth is the sample thickness, Le. 100-500 nm. The diameter of the electron beam is about 1 mm, and so an area of about 1 mm2 is probed. Thus, the results represent bulk properties and are not, as for example in the caseof STM results, surfacesensitive. Diffractionpatterns of the coated epitaxial fullerene films were obtained with a transmission electron microscope (Philip EM400).
3. Results and Discussion 3.1. Hydrogen Content. ERD and IR-absorption measurements were performed to confirm that the deposited carbon was essentially free of hydrogen. Figure 2 shows energy distributions of recoiled H-atoms of films deposited at substrate temperature T,= 730 O C and room temperature when exposed to 4He+(2.8
0
u
50
0 50
200
150
100
250
300
Channel Energy (MeV)
b) 1000
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I
I
$= RT
n
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v)
600
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c
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Figure 2. ERD spectra of films deposited at substrate temperatures of 730 "C (a) and room temperature (b).
I
I
I
I
4ooo35663ooo25002oO01soo1o0o
506
wavenumbet / cm" Figure 3. Infraredabsorptionspectrum of a film depositedat a substrate temperature of room temperature.
MeV). The increased intensity of protons of highest energy (maximum at about 0.77 MeV) is caused by an interface layer at the surface, which contains more hydrogen than the bulk. A qualitative analysis26shows that the hydrogen concentrationsin the bulk are 1-3% (T,= 730 "C) and 6-8% (T,= room temperature), where, e.g., 1% corresponds to 1 hydrogen/100 carbon atoms. Both films show a ---fold increased hydrogen concentration in an interface region of -15- and -50-nm thickness, respectively. Here the question arises whether the hydrogen is bonded to carbon or present in its molecular form. Figure 3 shows an IR-absorption spectrum of a film deposited at room temperature. It can be seen from this figure that the film is transparent in the range from 1800 to 4000 cm-I. Particularly no absorptions are found in the C-Hvibration regime ranging from 2850 to 3300 cm-1.2* Since hydrogen is definitely present in the film, it is concluded to be in its molecular form, and not saturating carbon bonding sites. Thus, the carbon can be considered to be essentially free of hydrogen that is bonded
Gaber et al.
0246 The Journal of Physical Chemistry, Vol. 97, No. 31, 1993
I T,=RT
_ 1
1
.
3 1 1100
1300
1500
1700
1100
1300
1500
1700
raman shift / cm-I Figure 4. Raman spectra of films deposited a t a substrate temperature of room temperature (a, top) and 900 OC (b, bottom).
to carbon. Although an all-carbon molecule is deposited, the growing film might act as a continuallyfresh surface for hydrogen absorption. This effect would be expected to decrease with increasing substrate temperature and also explains the observed hydrogen concentrations. For an assumed Hz-partial pressure of 3 X lo-' mbar in the deposition chamber and a H2-sticking coefficient of 1, one calculates a maximal concentration of hydrogen atoms in the films of 45% at T, = room temperature and 65% at T, = 730 O C . This values are higher than the values given above. Although these considerationsgive strong evidence for molecular rather than carbon bonded hydrogen, an unique determination of the hydrogen must await further studies. However, most important in this case is that the hydrogen concentrations are much lower than is required to form a fully constrained network of sp3-bonded carbon. 3.2. Graphitic Nanostructures. The main feature visible in Figure 3 is a broad band between 1050 and -1700 cm-I superimposed on the narrow IR-absorption lines of H2O in the range between 1450 and 1750 cm-I. It is possibly due to zone center phonons of graphitic material in the film at 1580 cm-l (El,-mode at the I'-point)29J0 broadened by disorder within the sheets or the presence of turbostratic graphite3O with a mutual orientation of the sheets around the (001 )-direction. However, this would imply a rather asymmetric broadening despite the overall symmetric shape of the band, since the maximum of the observed band is located at 1350cm-I. Raman emission of similar width and location is commonly found in spectra of amorphous carbon film~29-3~ and known as the D-band. It is attributed to phonons with k-vectors substantially different from zero in nanometer-sized graphitic crystals. This violation of the k = 0 selection rule is caused by an uncertainty of the wavevectors, Ak = 27r/&, larger than the vectors themselves. The same selection rule holds for IR absorption, and the broad IR band observed here might therefore be explained in a similar way as the Raman D-band, where the emission by phonons with k # 0 is caused by a small particle size as described above and the broadness of the band by disorder32or the presence of turbostratic graphite. Figure 4 shows Stokes Raman spectra of films deposited at T, = 900 O C and T,= room temperature. The spectra have been fitted by a single Gaussian peak centered around 1580 cm-1 (Gband), a single Lorentzian peak centered around 1360 cm-1 (Dband),j4 and a small band of low intensity at about 1550 cm-1 due to Raman emission from the sub~trate.3~ The G-band was found at 1580 and 1588 cm-I for the films with T, = room temperature and 900 O C , respectively, and the corresponding D-bands are located at 1370 and 1350 cm-1. The ratios of the
Figure 5. STM images of films deposited at high substrate temperature, Ts.The lateral extensions, height scales, and Tsare as follows: (a, top) 16 nm X 16 nm, 1 nm, 730 O C ; (b, bottom) 13 nm X 13 nm, 2.5 nm, 900 OC.
intensities of the maxima of the D-bands to that of the G-bands, ID/IG,are 3.6 and 1.5. From the relationship between this ratio and the sizeof the graphitic crystallites,L, as given by RobertsonP a decision as to which of the films has larger crystallites is not possible at this point. The islands in the films are 9 or 14 A in size for T, = room temperature and 4 or 27 A in size for T, = 900 OC. The Raman spectrum of the film of T, = room temperature is typical of films reported to be of relatively high, and the film of T, = 900 OC of less, sp3-content (see, e.g., ref 20). The surface structure of the films produced has been investigated by STM. Resolution on the atomic scale has not been achieved on films deposited at room temperature. However, on films deposited at 730 and 900 "C a variety of structures were visible. Figure 5 shows two STM images of films with Ts= 730 and 900 "C,which are typical for our measurements. Some chubby undulations with a diameter in the range from 20 to 50 A are visible in Figure 5a, superimposed with the structure on the atomic scale. Such chubby structure is not seen in Figure 5b; however, a dark-bright (high-low) undulation is visible on the same scale. The atomic structure here is more regular and extends over larger areas. The size of the undulations agrees with the size of the graphitic crystallites found by the analysis of the corresponding Raman spectrum when the higher value, L,,= 27 A, is taken. The prominent structure on the atomic scale visible in Figure 5 is shown on a larger scale in Figure 6a. It is taken from the lower left section of Figure 5b. A crystallinestructure is observed showing elongated spots with characteristic distances of 3.5 A in the direction of their largest extension and about 2.5 A
Carbon Films from Fullerene Ion Beam Deposition
The Journal of Physical Chemistry, VOI. 97, No.31, 1993 8247
A (101)
qdfilm (002) - 1
(004) I
Momentum transfer (As1) Figure 7. Electron diffraction patterns of the samples deposited at substrate temperaturesof 730 OC and room temperature. The data were taken with the EELS spectrometer with energy loss set equal to zero. The momentum transfer q was either parallel or at an angle of 45O to the film plane, which was achieved by rotating the sample with respect to the electron beam. The structures are labeled (hkl) for graphite (hkl) reflections.
Figure 6. STM images of films deposited at a substrate temperature Ts = 900 "C. The lateral extensions and height scales are as follows: (a, top) 4 nm X 4 nm, 1 nm; (b, middle) 1.4 nm X 1.4 nm, 2.5 nm; (c, bottom) 2.5 nm X 2.5 nm, 1.1 nm.
perpendicular to it. This structure agrees in detail with the structures recently reported by Cho et a1.,20who attributed it to turbostratic graphite cut along a (100)-plane. Figure 6a could be a view of a nanocrystalline graphitic layer of AAA ...stacking sequence and with the c-axis parallel to the surface,the hexagonal sheets being perpendicular to the film. The structure would correspond to a nonreconstructed surface possibly saturated by hydrogen. However, if such hydrogen would be present at the surface during growth but subsequently not in the bulk, it must desorb as growth proceeds. Although our images are dominated by this structure, others are also visible. The question of whether
the graphitic crystallites show a preferred orientation will be discussed below. Figure 6b shows a structure of roughly hexagonal symmetry and, in the inset, the pattern resulting from a two-dimensional Fourier transformation. From this, an average distance between the bright spots of 2.5 f 0.2 A is obtained, a value reported for highly oriented pyrolytic graphite (stacking sequence ABAB...). It is not possible to distinguish between turbostratic and wellstacked graphite with the aid of this image alone. The crystalline structures as shown so far are found only in some of the images made of the films deposited at 900 "C. Often, resolution on the atomic scale is not obtained or extremely poor, and sometimes noncrystalline structures such as those shown in Figure 6c are obtained. This example shows some undulations with a distance between the undulations of 2.5 f 0.3 A in the direction from the left, upper to the right, lower side. 3.3. Preferred Orientation of the Graphitic Nanocrystallites. Figure 7 shows electron diffraction results for samples with Ts = room temperature and Ts= 730 OC. For the sample of T,= 730 OC, strong peaks at 1.88,3.08, and 3.75 A-1 are seen, which can beassigned to the (002), (100) or (101), and (004) reflections of graphite, respectively. The data do not depend on changes of the direction of the momentum transfer, q, within the film plane. In contrast, turning q out of the film plane leads to a strong decrease of the (002) and (004) signals. This can be explained by strongly oriented graphiticstructuresverifying the explanation for the STM results given above. From the observed width of the (002) reflection (0.25-A-l full width, at half-maximum (FWHM)), a mean value of the extension of the crystalline graphite parallel to the c-axis of 25 A can be estimated, in agreement with the STM observation shown in Figure 5 and the crystal size obtained from the Raman spectrum in Figure 3. However, it should be noted that the unique identification of a preferred orientation was not possible with the former two methods. Amorphous or glassy carbon gives rise to the background for the sample of Ts= room temperature. However, a width of 10 for the graphitic clusterswas estimated from the width of the (002)-reflection. The graphitic structuresrepresent a significant volume fraction, which increases with increasing substrate temperature. 3.4. Chemical Bonding. In Figure 8, the C 1s core-level excitation spectra of the same samples, as shown in Figure 7, are shown in comparison to those of graphite, amorphous carbon
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Gaber et al.
8248 The Journal of Physical Chemistry, Vol. 97, No. 31, 1993 l
"
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i
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~
'
'
. . .
1
.
. .
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Figure 9. Electron diffraction pattern of a heteroepitaxial fullerite film grown on mica coated with an a-C film deposited by fullerene ion beam deposition. The pattern showsa face centered cubic film of single-crystal quality. The spotsare due to Bragg reflectionsat (220)-and (422)-planes with d = 5.0 and 2.9 A, respectively. T,= RT q(1film
Y I
fullerene film has been coated by a dense a-C film without being affected itself. 4. Conclusions
280
290
300
Energy (eV)
Figure 8. C Is absorption edges of samples deposited at substrate temperatures of 730 OC and room temperature in comparison with graphite, amorphous carbon, and diamonds2'
(a-C), and diamond.36 Neglecting core-hole effects, the energyloss spectra shown here reflect the structure of the C 2p derived unoccupied densityof states. Thestructures near 285 eV confirm the presence of carbon in the sp2-configuration. For the sample of T, = 730 OC, the observed higher intensity of around 285 eV when q is parallel rather than normal to the film plane, indicates that the *-orbitals are oriented predominantly parallel to the film plane, confirming the existence of graphitic sheets normal to the surface. The feature near 285 eV, compared to the rise near 290 eV, is a measure of the content of sp2-configuration. So diamond having pure sp3-configuration shows no intensity for energies E < 289 eV (Figure 8). By comparison with amorphous carbon (assuming 100% ~ p ~ - c o n f i g u r a t i oone n , ~may ~ ~ ~estimate ~ an sp2-percentage of more than 80% for the sample of T, = 730 "C. From the orientation dependence it may be estimated that more than 60% of the sp2-configurated carbon atoms are oriented according to the graphitic structures. The data cannot determine whether the non-oriented sp2amount (seen in Figure 8 near 285 eV for q perpendicular to the film) is due to slightly misoriented graphitic structures or the amorphous material surrounding these structures. However, the percentage of the amorphous carbon phaseis less than 40%. For the sample of T, = room temperature, about 65% sp2-configuration can be estimated from the C 1s absorption edges. The percentage of the amorphous carbon phase is estimated to be 80-90%. 3.5. Energetic Fullerene Impact To Form a Dense Carbon Coating on a Single-Crystalline Fullerite Film. As a first example of an a-C coating produced by fullerene ion beam deposition, a heteroepitaxial fullerene film on micaZ2was used as a substrate in our experiment. The layered structure (fullerene film/a-C) was removed from the substrate and mounted on grids of a transmission electron microscope for electron diffraction analysis. Figure 9 shows a typical diffraction pattern obtained at different locations of the film. The spots areclearly thoseof a face centered cubic fullerene ~rysta12~93~ and prove the integrity of the film after deposition. The film is still single-crystalline and has preserved its epitaxial relation to the substrate. By Raman measurements performed on films stored for 4 weeks in air, a content of oxygen in the film was found, which is much lower than that of uncoated films exposed to air.38 Thus the crystalline
Our novel, simple method of fullerene ion beam deposition forms hydrogen-free, very smooth, and strongly adhesive carbon films. The films show graphitic nanocrystallites embedded by amorphous carbon and preferentially oriented with the c-axis parallel to the substrate surface. With increasing substrate temperature, the size of the crystallites increases together with a decrease in the percentage of sp3-bonded carbon. The films deposited at high substrate temperature (730and 900 "C) show graphitic nanocrystallites with a size ranging from 20 to 50 A, as deduced from STM, and 25 A, as deduced from Raman spectroscopy and electron diffraction. The films deposited at room temperature show smaller crystallites, either 9 or 14 A, as deduced from the Raman spectroscopy,and an overall percentage of sp3-bondsof 35%, which might correspond to 50% sp3-bonding of the amorphous material. The graphitic crystallites in the film contain much more than 60 atoms in either case. The films show high similarity to films which are obtained by methods depositing neither fullerenes nor other large carbon molecules, and none of the film properties observed until now seem to be caused by a specific property of the fullerenessuch as its rigidity in surfacecollisionsor its graphitelike bond structure. Formation of the film is thus not attributed to any specific chemistry of the fullerenes. The fullerenes are completely disintegrated in the encounter, leaving many highly energetic atoms and radicals. The primary energy per atom of the projectiles ranges from 17 to 170 eV, which is the range where high density carbon films have been formed by e - i o n beam deposition.16 The observed film structure might result from formation of a variety of different nucleation sites, leading to different forms of carbon. The molecular mechanism of such nucleation processes should depend on the growth conditions and the deposition method, which is not observed. In all probability, the primary solid deposition product is mainly sp3-bonded, overconstrained amorphous carbon, which then relaxes to form graphitic nanocry~tallites.~.~ When it is assumed that the total energy of the film lowers with increasing phase separation, an increasingamount of graphitic material is expected with increasing substrate temperature. This has been observed in the present work and elsewhere.20 One difference of fullerene ion beam deposition to from a-C, as compared to the conventional methods, is the idea of energetic cluster impact. Less damage of the substrate is expected, because the primary kinetic energy is transformed into thermal and lateral kinetic energy of the fragments of the dissociated cluster. The a-C coating produced by fullerene ion beam deposition on the single-crystalline fullerene film clearly shows the high potential of fullerene ion beam deposition to form "diamond-like" amorphous carbon coatings with minor affects on the substrate. Acknowledgment. Part of this work was supported by the "Deutsche Forschungsgemeinschaft" (DFG), Project No. Bu 78 1/
Carbon Films from Fullerene Ion Beam Deposition
1-1, carried out under the auspices of the trinational “D-A-CH” cooperation of Germany, Austria, and Switzerland on the ‘Synthesis of Superhard Materials”. Moreover, this work was supported by the “Bundesministerium fiir Forschung und Technologie” (BMFT) under Contract 13 N 6073.
References and Notes (1) Several contributions to the relative phase stability of graphite and diamond can be found in: The Properties of Diamond, Field, J. E., Ed.; Academic Press: New York 1979. Davis, G. Diamond; Adam Hilger Ltd.: Bristol. U.K.. 1984. (2) Kroto, H. W.; Heath, J. R.; OBrien, S.C.; Curl, R. F.; Smalley, R. E. Nature 1985, 318, 162. (3) Krstschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R. Nature 1990, 347, 354. (4) Contributions to preparation methods and structure of this class of material can be found in articles written by Angus et al.5 and Robertson.6 ( 5 ) Angus, J. C.; Wang,Y. In ProceedingsoftheNATOAdvancedStudy Institute on Diamond and Diamond-like Films and Coatings; Clausing R., Horton, L., Angus, J. C., Koidl, P. Eds.; Plenum Press: New York, 1991; p 173. (6) Robertson, J. Proceedings of the NATO Advanced Study Institure
on Diamond and Diamond-like Films and Coatings; Clausing, R., Horton, L., Angus, J. C., Koidl, P. Eds.; Plenum Press: New York, 1991; p 37. (7) Regueiro, M. N.; Monceau, P.; Hodeau,J. L. Nuture 1992,355,237. (8) Beck, R. D.; St. John, P.; Alvarez, M. M.; Diederich, F.; Whetten, R. L. J . Phys. Chem. 1991, 95, 8402-8409. (9) Busmann, H.-G.;LiIl, Th.; Hertel, I. V. Chem. Phys. Lett. 1991,187, 459465. (10) Busmann, H.-G.;Lill, Th.; Reif, B.; Hertel, I. V. Surf.Sci. 1992,272, 146. (11) Mowray, R. C.; Brenner, D. W.; Dunlap, B. I.; Mintmire, J. W.; White, C. T. J. Phys. Chem. 1991, 95, 7138-7142. (12) Busmann, H.-G.; Lill, Th.; Reif, B.; Hertel, I. V.; Maguire, H. G. J. Chem. Phys. 1993, 98, 7574. (13) Lill, Th.;Busmann, H.-G.; Reif, B.; Hertel, I. V. Appl. Phys. 1992, ASS, 46 1. (14) Marquardt, C. L.; Williams, R. T.; Nagel, D. R. Mater. Res. SOC. Symp. Proc. 1985, 38, 325.
The Journal of Physical Chemistry, Vol. 97, No. 31, 1993 8249 (15) Collins, C. B.; Davanloo, D.; Jander, D. R.; Lee, T. J.; Park, H.; You, J . 11. J . Appl. Phys. 1991, 69, 7862. (16) Ishikawa, J.;Takeiri,Y.;Ogawa,K.;Takagi,T. J . Appl. Phys. 1987, 61, 2509. (17) Berger, S.D.; McKenzie, D. R.; Martin, P. J.; Philos. Mag. Lett. 1988, 57 (6), 285. (18) Bredas. J. L.; Street, G. B. J . Phys. Chem. 1985.18, L651. (19) Robertson, J. Adv. Phys. 1986, 35, 317. (20) Cho, N. H.; Veirs, D. K.; Ager. J. W..111: Rubin. M.D.; Hopper, C. B. J . Appl. Phys. 1992, 71, 2243. (21) Ullmann, J.; Schultze, S.;Erben, J.; Griinewald, W.; Heger, D.; Miihling, I. Thin Solid Films 1992, 219, 709. (22) Busmann, H.-G.; Hiss, R.; Gaber, H.; Hertel, I. V.; Surf.Sci. 1993, 289, 381. (23) Yamada, I.; Usui, H.; Takagi, T. Z . Phys. D 1986, 3, 137. (24) Haberland, H.; Karrais, M.; Mall, M.; Thurner, Y. J. Vac. Sci. Technol. 1992, AS, 3266. (25) Haberland, H.; Insepov, Z.; Moseler, M. Z . Phys. D, in press. (26) Bruder, F.; Brenn, R.; Stiihn, B.; Strobl, G. R. Macromolecules 1989, 22, 4434. (27) Fink, J. Ado. Electr. Electron Phys. 1989, 75, 121-232. (28) Dischler, B. In Proceedings of E-MRS (les editions de physique); Koidl, P., Oelhafen, P., Eds.;E-MRS: Les Ulis Cedex, 1987; Vol. XVII, p 189. (29) Nemanich, R. J.; S o h , S.A. Phys. Rev. B 1979, 20, 392. (30) AI-Jishi, R.; Dresselhaus, G. Phys. Rev. B 1982, 26, 4514. (31) Tuinstra, F.; Koenig, J . L. J . Chem. Phys. 1970, 53, 1126. (32) Knight. D. S.: White. W. B. J. Mater. Res. 1988. 4. 385. (33) Yo&ihikawa,’M.; Katagiri, G.; Ishida, H.; Ishitani, A. Appl. Phys. Lett. 1988, 52, 1639. (34) Bachmann, P. K.; Wiechert, D. U. Proc. NATO Advanced Studv
Institute on Diamond and Diamond-like Films and Coatings; Clausing, R:, Horton, L., Angus, J. C., Koidl, P., Eds.; Plenum Press: New York, 1991; p 677. (35) Kitabatake, M.; Wasa, K. J. Appl. Phys. 1985,58, 1693. (36) Fink, J.; Miiller-Heinzerling, Th.; Pfliiger, J.; Bubenzer, A,; Koidl, P.: Crecelius. G. Solid Stare Commun. 1983. 47. 687-691. ’(37) Krakow, W.; Rivera, N. M.; Roy, R.’A.;Ruoff, R. S.;Cuomo, J. J. J. Mater. Res. 1991, 7, 784. (38) Pichler, T.; Matus, M.; Kiirti. J.; Kuzmany, H. Phys. Rev. Lett. 1992, 845, 13841.
