Oxidation and Etching of CVD Diamond by Thermal and Hyperthermal

Oct 15, 2010 - E-mail: [email protected] and [email protected]. Phone: +972-8-9434412. Fax: +972-8-9434403., †. Soreq NRC. , ‡. Montana State ...
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Oxidation and Etching of CVD Diamond by Thermal and Hyperthermal Atomic Oxygen Zeev Shpilman,† Irina Gouzman,*,† Eitan Grossman,† Linhan Shen,‡ Timothy K. Minton,‡ Jeffrey T. Paci,§ George C. Schatz,| Rozalia Akhvlediani,⊥ and Alon Hoffman⊥ Space EnVironment Department, Soreq NRC, YaVne 81800, Israel, Department of Chemistry and Biochemistry, Montana State UniVersity, Bozeman, Montana 59717, United States, Department of Chemistry, UniVersity of Victoria, Victoria, British Columbia, V8W 3V6, Canada, Department of Chemistry, Northwestern UniVersity, 2145 Sheridan Road, EVanston, Illinois 60208, United States, and Schulich Faculty of Chemistry, Technion, Haifa 32000, Israel ReceiVed: July 25, 2010; ReVised Manuscript ReceiVed: September 21, 2010

The chemical bonding and morphology of chemical vapor deposited (CVD) diamond films exposed to thermal (∼0.04 eV) and hyperthermal (5 and 7.5 eV) atomic oxygen (AO) were studied by using high resolution electron energy loss spectroscopy (HREELS), atomic force microscopy, and theoretical simulations. Although exposure to thermal AO caused subtle changes to the surface morphology, hyperthermal AO resulted in selective etching of the diamond facets: (100) facets remained essentially unaffected, whereas (111)-oriented and other facets were severely etched. HREELS reveals that hydrogen is removed from the diamond surfaces during both thermal and hyperthermal AO exposures. By using isotopic labeling in the CVD growth procedure, it is observed that exposure to ambient conditions after the AO exposure leads to adsorption of adventitious hydrocarbons on the surface. The high background in the HREEL spectrum of samples exposed to hyperthermal AO suggests the presence of a graphitic layer. Simulations of the interaction between hyperthermal AO and (100) and (111) diamond surfaces were conducted by using direct dynamics based on density-functionalbased tight binding methods, in an attempt to elucidate relevant reaction mechanisms. They suggest mechanisms for the partial graphitization of the (111) surface and for etching of this surface by way of CO2 desorption. Such damaged graphitic layers have been previously shown to erode easily when exposed to a hyperthermal AO beam. The simulations also suggest that the (100) surface, fully covered with ketones, is inert to carbon removal upon exposure to hyperthermal oxygen atoms, which scatter inelastically from this surface without reaction. The simulations suggest that a nearly full ketone coverage is the steady-state configuration for a (100) diamond surface exposed to AO. 1. Introduction The reactivity of diamond in harsh environments and its surface functionalization have received considerable attention in recent years, for both fundamental and technological reasons. A harsh environment of particular interest to the work presented here is found in low Earth orbit (LEO). Atomic oxygen (AO) is the dominant species in the atmosphere at LEO altitudes.1 Orbital velocities of ∼8 km/s, coupled with the corotation of the Earth and its atmosphere, result in collisions between spacecraft and ambient AO that are equivalent to O atoms with ∼4.5 eV of kinetic energy striking the leading edges of spacecraft surfaces.2 Several studies of the interaction between diamond and AO suggest its high stability compared to other carbon-based materials.3-11 Bourdon et al. investigated the reactivity of hydrogenated amorphous carbon with a beam of 2.5 eV AO.3 They found erosion yields ranging from ∼3 × 10-26 to ∼1.8 × 10-24 cm3 atom-1, which strongly depended on the hydrogen content of the film. The AO erosion yield for graphite in space is ∼1.1 × 10-24 cm3 atom-1.4 Laboratory studies using 5 eV * To whom correspondence should be addressed. E-mail: [email protected] and [email protected]. Phone: +972-8-9434412. Fax: +972-8-9434403. † Soreq NRC. ‡ Montana State University. § University of Victoria. | Northwestern University. ⊥ Schulich Faculty of Chemistry.

AO beams have also been conducted.5-7 Nicholson et al. found temperature-dependent erosion yields that ranged from ∼4 × 10-25 to 1.1 × 10-24 cm3 atom-1 for graphite exposed at 25 and 220 °C, respectively.6 Studies of the interaction between diamond and hyperthermal AO are more limited.8,9 Joshi et al. studied the interaction of graphite and diamond with an oxygen plasma and mainly discussed the stability of sp3 bonded materials that were exposed to oxygen ions.10 The reaction of neutralized accelerated oxygen ions with chemical vapor deposited (CVD) diamond resulted in an erosion yield of 8 × 10-26 cm3 atom-1.11 Alternatively, we have found a lower erosion rate of ∼2 × 10-26 cm3 atom-1 for CVD diamond exposed to an oxygen plasma.12 The erosion yield for (111)-oriented single-crystal diamond exposed in space is ∼2 × 10-26 cm3 atom-1.8 The dependence of the crystallographic orientation on etching under these conditions has not been studied. The erosion yield of diamond is significantly lower than that of polymers that are commonly used in space, such as Kapton H polyimide, which has an erosion yield of 3.00 × 10-24 cm3 atom-1.2,8,13 Thus, diamond-based devices show promise for durable use in space applications, and the reactivity of AO with CVD diamond films, which are polycrystalline, is worth investigating further. Growth mechanisms of CVD diamond films have been studied over the past three decades. In particular, the behavior of different facets during growth has been investigated.14-16 It has been shown that the morphologies of the different facets are a function of growth conditions. Some facets, such as (100)

10.1021/jp1073208  2010 American Chemical Society Published on Web 10/15/2010

Oxidation and Etching of CVD Diamond by Thermal and Hyperthermal AO and (110), remain smooth whereas others become rough.14-16 The extent of roughening is related to the growth temperature and the details of the gas-phase chemistry. The exchange of deuterium for hydrogen in the feed gas is known to produce a slower and more ordered growth.17 This slower growth might be a consequence of slower diffusion of deuterium in the gas phase and in the diamond material relative to hydrogen. As a consequence, it has been observed that a relatively high number of square-faceted crystallites are formed,9 which are assigned as (100)-oriented.9,18,19 Silva et al.19 calculated that it is the (100) facet that is the most stable at slower growth rates. Previous studies reported that diamond can be selectively etched by using wet or dry chemical techniques at elevated temperatures. Zheng et al. found etch rates for different diamond facets in their studies by using KNO3 at 700 °C.20 The highest rates were for (111) surfaces. De Theije et al. studied thermochemical etching of (100)21 and (111)22 single-crystal diamond surfaces over a temperature range of 700-900 °C. In these studies, etching occurred as pits on the (100) crystal face. The (111) surface reacted differently, becoming totally rough. The authors suggested that the differences in the observed rates were the result of effective stabilization of the (100) surface by oxygen functionalization. However, the details of the chemistry involved were not fully elucidated. Some of the structures that might result from the oxidation of the diamond (100) surface have been investigated theoretically.23-27 In ref 26, the authors examined the stabilization caused by adding H atoms to the (2 × 1)-reconstructed surface, up to full surface coverage, and the energetic consequences of substituting O atoms in ether as well as ketone arrangements. The energetics of OH substitution and the addition of functional groups in various combinations were also described. This study suggests the types of functional groups expected, and their arrangement, when H, O, and OH are available in the gas phase in non-negligible concentrations near the surface. It indicates that full coverage with ketone-like oxygen atoms is the most probable because of its associated barriers to formation, even though ketone coverage is not quite as stable as full ether coverage. The work in ref 24 suggests the same, and it indicates that the ketones will be present on a (1 × 1), that is, undistorted surface because the (2 × 1) arrangement is unstable when functionalized with O atoms. The types of structures that might result from the oxidation of the (111) surface have been described by Larsson et al.25 This reference suggests that atop- and bridge-type oxygen structures are possible on surfaces that are otherwise hydrogenterminated, with atop oxygen structures being oxy radicals. The follow-up work in ref 27 suggests that it is the atop structure that is by far the most important. The reactivity of CVD diamond following exposure to AO was studied by some of us in recent years.9,12,28-30 It has been shown that following AO exposure and subsequent exposure to the ambient environment, the surface is oxidized and also contains hydrocarbons. Annealing led to desorption of most of the hydrocarbons. The nature of these surface species was not probed in great detail, for example, with the use of surface sensitive vibrational spectroscopy, such as high resolution electron energy loss spectroscopy (HREELS), which could reveal information about adsorbates as well as the carbon structure near the surface.12,28-30 In the present study, the growth of diamond films with both CH4/H2 and CD4/D2 and the use of HREELS have allowed us to determine that the source of the surface hydrocarbons is the ambient environment and that these

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adventitious hydrocarbons accumulate preferentially on surfaces that have been exposed to AO. Some of us recently reported the results of an atomic force microscopy (AFM) study of the morphological changes observed when polycrystalline diamond films were exposed to hyperthermal AO.9 The durability of (100) planes relative to other facets was shown, and a cursory model was suggested. In the present study, we have expanded the scope of the experiments to include thermal and hyperthermal AO, and we have conducted in-depth studies of the surface morphology (AFM) and chemistry (HREELS) of AO-exposed surfaces. In conjunction with the experiments, we have carried out theoretical direct dynamics simulations, which are well grounded in modern theory and provide a detailed explanation of the observed durability of the (100) surface relative to the (111) surface. 2. Experimental Methods Polycrystalline diamond films with submicrometer grain sizes were grown on 1.2 × 1.2 cm Si substrates in a standard hot filament (HF)-CVD system30 for 1-3 h. Before deposition, the Si substrate was treated in a mixed slurry of diamond and titanium particles, in order to increase the nucleation density. Deposition parameters were as follows: gas pressure of 50 Torr, gas flow of 100 sccm, substrate temperature of 800 °C, and gas mixtures of 1:99 for CH4: H2 or CD4:D2. The 1 h films were continuous with thicknesses of approximately 1 µm and 300 nm, respectively.17 A film grown from CD4/D2 for 3 h was ∼1 µm thick. Thermal AO exposure was carried out by using a modified RF-plasma system (LB-3000 by Advanced Energy) with a feed gas of 99.999% molecular oxygen.31 The system was operated at a pressure of 50 mTorr, a gas flow rate of 10 sccm, and a power of 2000 W. The samples were exposed in the afterglow of the plasma system, placed beyond two right-angle bends in the gas tube that emerged from the source. This location has been shown to contain mainly AO; the concentration of ions, electrons, excited species, and UV radiation at this location is negligible.31 The effective AO dose was determined by measuring the mass loss of a Kapton HN witness sample.13 By using an erosion yield of 2.8 × 10-24 cm3 O-atom-1 for Kapton HN exposed to LEO,32 the LEO-equivalent AO fluence of the diamond exposure was ∼6 × 1019 O atoms cm-2. Hyperthermal exposures were performed by using a laser detonation source,13 which was used to produce AO with energy distributions centered at either 5.0 or 7.5 eV, having full widths at half maxima of 1.6 and 3.0 eV, respectively. The samples were exposed to LEO-equivalent fluences of ∼6 × 1019 and 1.0 × 1020 O atoms cm-2, respectively, according to the recession of a Kapton H witness specimen, which has an agreedupon LEO erosion yield of 3.00 × 10-24 cm3 O-atom-1.13 The number of impacting O atoms was approximately the same for both exposures, and the apparent difference in O-atom fluence is the result of the fact that the etch rate of Kapton is higher with the higher-energy O atoms. The exposed samples were transferred ex situ to a combined X-ray photoelectron spectroscopy (XPS) and HREELS system. HREELS studies were carried out with a Delta 0.5 spectrometer (VSI-SPECS). HREELS data were smoothed by using a fivepoint second-order Savitzky-Golay method and analyzed by curve fitting by using the XPSpeak4.1 program. XPS spectra were obtained by Al KR (1486.6 eV) radiation (SPECS RQ 20/38 X-ray source) and a hemispherical analyzer. A more detailed description of the operating parameters can be found elsewhere.12 After each HREELS measurement, the sample was transferred in situ to an adjacent chamber for XPS analysis and

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Figure 1. AFM amplitude images of HF-CVD polycrystalline diamond film grown from CD4/D2 feed gases before (a) and after (b-d) exposure to a LEO-equivalent fluence of 6 × 1019 O atoms cm-2 of 5 eV hyperthermal AO. The image sizes are 2 × 2, 2 × 2, 1 × 1, and 0.5 × 0.5 µm, and the z scales are 500, 800, 600, and 200 mV, respectively. (100) and (111) facets are indicated in panel a.

annealing treatments. Annealing was conducted under vacuum lower than 1 × 10-7 Torr. The morphology of the samples was studied by AFM (Nanoscope IV MultiMode from Veeco) operated in tapping mode. AFM amplitude images are presented; this mode produced images with better contrast than height mode for these kinds of morphologies. 3. Theoretical Methods Theoretical investigations of 5 eV hyperthermal AO collisions with diamond (100) and (111) surfaces were conducted. Direct dynamics calculations were performed with forces and energies derived from self-consistent charge density-functional-based tightbinding (SCC-DFTB) calculations.33,34 The Γ point was used for Brillouin zone sampling. The (100) and (111) surface models each contained 72 carbon atoms, forming six atomic layers. Three-dimensional periodic boundary conditions were used. The two unit-cell vectors within the basal planes were optimized so that the pristine diamond was under zero strain. The lattice vector in the direction normal to the surfaces was set to 30 Å, a value large enough to prevent unrealistic interactions between neighboring unit cells in this direction. These unit-cell vectors were fixed at these values for all of the simulations. In the (100) case, the back sides (the surfaces opposite the sides with which the incoming AO collides) were (2 × 1) reconstructed, with the remaining dangling bonds terminated by using hydrogen atoms. In the (111) case, they were also hydrogen-terminated.

Intramolecular trajectories were run at 298 K for the atoms in the slabs, and these trajectories were sampled to determine initial velocities and positions for all but the incoming AO. Slabs were exposed to oxygen atoms traveling with 5 eV of translational energy. Normal incidence was considered, with impact locations chosen randomly. Trajectories were propagated for 1 ps by using a 10.0 au (0.24 fs) time step on the lowest triplet potential-energy surface. Initial AO-to-diamond-surface separations of ∼10 au (5.3 Å) were used. Different oxygen-, hydrogen-, and deuterium-containing surface coverages were systematically investigated. A subset of these simulations is described below. 4. Results and Discussion 4.1. Morphological Changes. AFM images of films grown from CD4/D2 before and after exposure to a LEO-equivalent fluence of ∼6 × 1019 O atoms cm-2 of 5 eV AO are shown in Figure 1. Figure 1a is an image of an area on the surface of a pristine film. (100)- and (111)-oriented facets in this film are indicated for clarity. It can be seen in Figure 1b that, following exposure to hyperthermal AO, the (100) facets, recognized by their square shape,9,19,35 were apparently not affected. Other facets, including triangular (111) facets, were severely etched, although the height differences of the facets did not change significantly as a result of the exposure. Higher-resolution images are shown in Figure 1c,d. The (100) facets appear to have

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Figure 2. AFM amplitude images of polycrystalline diamond film grown from CD4/D2. The film was exposed to a LEO-equivalent fluence of 1 × 1020 O atoms cm-2 of 7.5 eV hyperthermal AO. The image sizes 5 × 5 and 2 × 2 µm, and the z scales are 1 and 0.7 V for panels a and b, respectively.

been eroded by the AO only at their edges, which become rounded and which are where other crystal orientations dominate.19,35 Relatively small (100) crystallites became totally round (partially shown in Figure 1b), perhaps by etching at their sides. The (111) facets transformed into a rough granular phase. These facets transform into an array of steps and ledges having dimensions of tens of nanometers. Craters are sometimes observed; these craters are usually apparent on rectangular (110) facets, as will be discussed below (see Figure 3d). The morphological changes following exposure to a LEO-equivalent fluence of 1 × 1019 O atoms cm-2 of 5 eV AO were also measured (images not shown). The results showed the beginnings of the etching process and are consistent with the higher-fluence results. Figure 2 shows images of a film grown from CD4/D2 after exposure to a LEO-equivalent fluence of ∼1 × 1020 O atoms cm-2 of 7.5 eV AO. Figure 2a is a large-scale image with a few exposed (100) and (111) crystallites, and Figure 2b focuses on one exposed (100) crystallite and its vicinal facets. These figures indicate that the (100) facets endure, even when exposed to AO with high translational energies that can reach above 10 eV in the tail of the distribution. Figure 3 contains images of films grown from CH4/H2, before and after exposure to a LEO-equivalent fluence of ∼6 × 1019 O atoms cm-2 of 5 eV AO. Figure 3a is an image of the pristine film. The number of conspicuous (100) crystallites in this sample is relatively small. Among the different crystallite shapes, a few distinct rectangular facets were observed, which were assigned as (110) facets.19,35 Two representative (110) facets are indicated for clarity in Figure 3a. The film was severely etched by the AO exposure, as shown in Figure 3b-d. It can be observed that edges of large facets in this film became smooth, whereas smaller crystallites transformed into a nondescript granular phase. The (110) facets exhibit crater-like points on their surfaces and edges, with diameters of ∼50 nm (see the ellipse in Figure 3d). Images of a film grown from CH4/H2, following exposure to a LEO-equivalent fluence of 5.5 × 1019 O atoms cm-2 of thermal (∼0.04 eV) AO, have recently been published.28 Only subtle smoothing of the surface was observed following exposure. These changes are characterized by the smoothing of edges and terrace features on the diamond surface.28 A CVD diamond film was also exposed to 10 eV argon atoms in an attempt to distinguish between chemical and physical effects. It can be seen from Figure 4a,b (before and after exposure, respectively), that Ar exposure caused no appreciable

change to the surface morphology. This result suggests that the effects produced by hyperthermal AO are purely chemical and not the result of a sputtering process. 4.2. Chemical Modifications. HREELS measurements, which in the dipole scattering regime probe the top 1-3 layers of the films,36 taken before and after thermal and hyperthermal AO exposures are shown in Figures 5-7. The assignment of various vibrational peaks associated with diamond and different oxygen-, hydrogen-, and deuterium-bonded species have been published recently.12,37 The vibrational spectrum of a film grown from CH4/ H2 is shown in Figure 5a. Curve fitting plots are superimposed on the spectra for clarity. The spectrum in Figure 5a shows the vibrational spectrum of the pristine diamond surface consisting of (i) the C-C stretching- and C-H bend-mixed modes which produce a peak at 155 meV,12,37 (ii) the symmetric and antisymmetric C-H stretching modes which are responsible for the peak between 350 and 395 meV; this peak can be separated into sp2 (dCHx)1-2) and sp3 (-CHx)1-3) bonding vibrations,12,37-39 and (iii) the diamond first and second optical phonon (OP) overtones which produce peaks at 300 and 450 meV,12,37-39 respectively; these overtones are a fingerprint of diamond toplayer crystallinity. Exposing the film to thermal AO has a significant impact on the surface structure, as is evident from the HREEL spectra.12 The diamond OP overtones disappear (see Figure 5b); that is, the surface layers no longer contain carbon in the form of diamond. The peak that was centered at ∼155 meV becomes broadened, suggesting the presence of hydrocarbon species (sCHx)1-3 or dCH2 bend at 180 meV)12,40,41 and oxygencontaining functional groups [peroxide (C-O-O-C), ether (C-O-C), and carbonyl (CdO) at 125, 135, and 220 meV, respectively].12,39,42,43 Upon in situ annealing, the peaks attributed to the hydrocarbons and oxygen-bonded species almost completely disappear, and the diamond OP overtones are partially restored (see Figure 5c,d). The oxygen atomic percentage of the surface layers was determined by XPS and is shown in red in the HREELS figures (Figures 5-7). XPS can probe to a depth of ∼3-5 nm;44 therefore, the surface coverage of oxygen, which can only be present at the surface, must be very high after AO exposure. Note that experimental results indicate that the 3% oxygen present on the films before AO exposure is caused by water or other oxygen-containing molecules which can become physisorbed during exposure of the films to ambient conditions.

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Figure 3. AFM amplitude images of HF-CVD polycrystalline diamond film, grown from CH4/H2 feed gases before (a) and after (b-d) exposure to a LEO-equivalent fluence of 6 × 1019 O atoms cm-2 of 5 eV hyperthermal AO. The image sizes are 2 × 2, 2 × 2, 1 × 1, and 1 × 1 µm, and the z scales are 500, 1000, 300, and 300 mV, respectively. (110) facets are indicated in panel a.

Figure 4. AFM amplitude images of HF-CVD polycrystalline diamond film grown from CH4/H2 feed gases before (a) and after (b) exposure to 10 eV hyperthermal argon atoms. The image sizes are 2 × 2 µm, and the z scale is 500 mV.

Following annealing of the AO-exposed diamond films, the concentration of oxygen was significantly decreased. Annealing to 700 °C (see Figure 5d) results in a value of 4.3 at.% O, close to that of the pristine diamond film. The vibrational spectrum of a pristine film grown by using CD4/D2 feed gases is shown in Figure 6a. In addition to the peaks assigned to C-C and C-H vibrations shown in Figure

5a, analogous isotopically shifted peaks appear at 110 and 270 meV; these peaks can be assigned to the C-D bending and stretching modes, respectively. At the same time, the relative intensity of the C-H stretching mode decreases. The C-H peaks may be related to hydrocarbon contamination on the surface that desorbs from the surface only at ∼500 °C.12 Following exposure to thermal AO, the vibrational spectra of the

Oxidation and Etching of CVD Diamond by Thermal and Hyperthermal AO

Figure 5. HREELS vibrational spectra of HF-CVD polycrystalline diamond film grown from CH4/H2 feed gases before (a) and after exposure to a LEO-equivalent fluence of ∼6 × 1019 O atoms cm-2 of thermal AO and subsequent annealing to 300 °C (b), 500 °C (c), and 700 °C (d). The atomic percentages of oxygen, measured by XPS, in the surface layer are indicated next to each spectrum.

Figure 6. HREELS vibrational spectra of HF-CVD polycrystalline diamond film grown from CD4/D2 feed gases before (a) and after exposure to a LEO-equivalent fluence of ∼6 × 1019 O atoms cm-2 of thermal AO and subsequent annealing to 300 °C (b) and 500 °C (c). The atomic percentages of oxygen, measured by XPS, in the surface layer are indicated next to each spectrum.

CD4/D2 film (see Figure 6b,c) appears to be fundamentally the same as the similarly exposed CH4/H2 film (see Figure 5b,c). The isotopically shifted peaks are not present following AO exposure and do not reappear after annealing, whereas the diamond OP overtones are partially restored. The vibrational spectrum of a film grown from CD4/D2 following exposure to 5 eV AO is also suggestive of peaks resulting from hydrocarbons and oxygen-bonded species (see Figure 7b). In this case, these peaks are superimposed on a high background. This background may be related to collective electron excitations in a semimetallic material, which in this case would most likely be a graphitic layer. Π-symmetry energy

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Figure 7. HREELS vibrational spectra of HF-CVD polycrystalline diamond film grown from CD4/D2 feed gases before (a) and after exposure to a LEO-equivalent fluence of ∼6 × 1019 O atoms cm-2 of 5 eV hyperthermal AO and subsequent annealing to 300 °C (b), 500 °C (c), and 700 °C (d). The atomic percentages of oxygen, measured by XPS, in the surface layer are indicated next to each spectrum.

bands in graphite near the Fermi level enable such transitions.45 The spectra of the annealed films (see Figures 7b-d) are otherwise consistent with what was observed for those exposed to thermal AO. In previous work,12 it was not possible to determine the dominant source of the hydrocarbons that HREELS suggests are present after the exposure of CVD diamond to AO followed by exposure to the ambient environment. The question as to whether these hydrocarbons were intrinsic to the diamond sample or adventitious remained open. The present study provides insight into this issue; the C-D peaks in Figure 6 were present before exposure but were absent afterward. However, there is a large peak at 180 meV (see Figures 6 and 7) which is now specifically assigned to C-H hydrocarbons. Therefore, these spectra suggest that the hydrocarbons are adventitious. Furthermore, the absence of C-D peaks after exposure suggests that deuterium, and thus hydrogen, was removed during both thermal and hyperthermal AO exposures. 4.3. Direct Dynamic Simulations. Simulations indicate that the hydrogen or deuterium atoms which are expected to terminate the dangling bonds on both (111) and (2 × 1)reconstructed (100) surfaces at the end of CVD growth are readily abstracted by 5 eV AO. As the surfaces lose H/D atoms, the incoming O atoms tend to form ketones on the (100) surface and atop oxygen atoms (oxy radicals) on (111). The steadystate O-atom coverages on the various diamond surfaces were estimated to be those at which the probability of an incoming oxygen atom abstracting an O atom already on the surface was approximately equal to the probability of an incoming O atom adding itself to the surface. The (111) surface had ∼0.67 O atoms per surface carbon atom, whereas the (100) surface was almost completely covered with ketones at this steady state. These high coverages are consistent with the XPS results reported in Figure 7b. The (111) surface with this steady-state coverage is prone to partial graphitization. Five out of the 100 trajectories propagated with this coverage underwent this process. An example is

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Figure 8. Hyperthermal AO collisions can cause a partial graphitization of the (111) diamond surface. In this trajectory, the incoming O atom (left panel) becomes bound to the surface as an oxy radical (center panel), resulting in the collision-induced dissociation of a neighboring carbon-carbon bond (right panel) that previously vertically joined the diamond layers. The broken bond is indicated by the ellipse.

Figure 9. Hyperthermal AO collisions can cause the production of CO2 from the (111) diamond surface. In this trajectory, the incoming O atom (left panel) collides with the surface, causing a C-C bond to break (center panel). Two additional C-C bonds break, releasing a CO2 molecule (right panel). The surface is functionalized with less than the predicted steady-state oxygen coverage in this trajectory.

Figure 10. Hyperthermal AO collisions with the (100) surface almost always result in inelastic scattering. In this trajectory, the incoming AO (left panel) collides with a ketone-covered surface (center panel), transferring some of its kinetic energy. It then moves away from the surface (right panel).

illustrated in Figure 8, where snapshots of a trajectory are presented. CO2 evolution from the (111) diamond surface was also observed, and is illustrated in Figure 9. In this trajectory, the surface was covered with 0.5 O atoms per surface carbon atom, that is, somewhat less than the predicted steady-state coverage. Subsequent O-atom collisions with the surface shown in the right panel of Figure 9 are likely to produce defective graphite on the diamond surface. In a previous study, some of us have shown that graphite and especially damaged graphitic layers are quickly eroded by a 5 eV AO beam.46 Furthermore in this study, 43 of the one hundred (111)-surface trajectories produced O2 or peroxide radicals, whereas a different 43 resulted in the formation of new atop O atoms. These peroxides are expected to be unstable and to react quickly to form O2 and a carbon atom with a dangling bond. The (100) surface is close to fully covered with ketones at its steady state. It showed no mechanism for the removal of carbon atoms during any of the 100 trajectories that were propagated. A total of 74 trajectories showed inelastic scattering. One such trajectory is illustrated in Figure 10. One (100)-surface trajectory did show the formation of a partially detached

carbonyl group. However, given sufficient time, it may have lain back down, reforming a ketone and thus eliminating the two dangling bonds that it created when it was formed. Nevertheless, this trajectory suggests that it is possible that the (100) facets erode very slowly, layer by layer. A total of 23 of the trajectories resulted in O2 formation; therefore, the true steady-state coverage will probably include some carbon dangling bonds. However, subsequent O-atom collisions with these bonds are likely to result simply in the formation of new ketones. Therefore, nearly full ketone coverage seems to result in surface passivation, which suggests that the (100) surface can endure hyperthermal AO exposure. Note that graphitization of the diamond (111) surface at ambient pressure has been previously observed, and mechanisms for it have been proposed.47,48 Also, graphitization plays an important role in nanodiamonds.49 Therefore, the fact that it may occur as a result of the impact of a 5 eV AO is not entirely surprising. This latter work suggests that the propensity for graphitization may be a function of model size, with smaller models being more inclined than larger ones to undergo this

Oxidation and Etching of CVD Diamond by Thermal and Hyperthermal AO process. Additional simulations performed by using larger models might lead to useful insight in this regard. Nevertheless, these results offer a plausible explanation for the durability of the (100) surface and the etching of the (111) surface. They are consistent with the HREELS data, which suggests the presence of graphite, and with the AFM results, which show the endurance of the (100) and the etching of (111) surfaces. 4.4. Implications for the Use of CVD Diamond Films in LEO. On the basis of both experimental and simulation results, the erosion yield of the (100) surface in LEO is predicted to be vanishingly smallssignificantly lower than that of (111) and other crystal faces. (100) directionally grown polycrystalline diamond films can be manufactured either by applying a bias on the substrate during growth50 or by adding different gases to the growth ambient.51 Therefore, the observation of the resistance of the (100) surface to hyperthermal AO could be an important advance and provide a means to reduce deleterious LEO effects on carbon-based materials. (100) coatings could be applied to diamond-based devices. Examples of such devices include heat-sinks, radiation detectors, and solar cells. In addition, (100)-oriented films could be deposited over other oxidation-prone materials such as the reinforced carbon-carbon used on the space shuttle. 5. Conclusions Morphological changes of polycrystalline diamond films following exposure to thermal and hyperthermal AO were presented. Although exposure to thermal AO only results in mild smoothing, hyperthermal AO causes severe and preferential erosion. The (111)oriented facets become rough, whereas the (100) facets remain mostly smooth, showing evidence of etching only at their edges. HREELS of isotopically labeled samples suggests that exposure to AO results in a reactive surface, which upon exposure to ambient conditions adsorbs adventitious hydrocarbons on the surface. Furthermore, hydrogen is removed from the surface during exposure to both thermal and hyperthermal AO. An explanation for the observed selective erosion is offered on the basis of morphological, chemical bonding and simulation analyses. These analyses suggest that hyperthermal AO exposure of (111) diamond surfaces results in significant oxygen coverage and partial graphitization. Furthermore, damage to the (111) surface by CO2 desorption can occur. The combination of these two effects likely creates a damaged graphitic surface which can be easily eroded. Conversely, hyperthermal AO exposure of (100) diamond surfaces results in nearly full ketone coverage that does not seem to react further with the hyperthermal AO beam, providing a plausible explanation for its durability. Acknowledgment. This work was supported by the National Science Foundation (CHE-0943639) and by the Israeli Space Agency. G.C.S. was also supported by AFOSR Grant FA955010-1-0205. References and Notes (1) Tribble, A. C. The space enVironment: Implications for spacecraft design; Princeton University Press: Princeton, NJ, 1995. (2) Murad, E. J. Spacecr. Rockets 1996, 33, 131. (3) Bourdon, E. B. D.; Raveh, A.; Gujrathi, S. C.; Martinu, L. J. Vac. Sci. Technol. A 1993, 11, 2530. (4) Ngo, T.; Snyder, E. J.; Tong, W. M.; Williams, R. S. Surf. Sci. 1994, 314, L817. (5) Nicholson, K. T.; Minton, T. K.; Sibener, S. J. Prog. Org. Coatings 2003, 47, 443. (6) Nicholson, K. T.; Minton, T. K.; Sibener, S. J. J. Phys. Chem. B 2005, 109, 8476.

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