J. Phys. Chem. B 2001, 105, 3803-3812
3803
Oxygen Adsorption on the (110)-Oriented Diamond Surface† Bob L. Mackey,‡,§ John N. Russell, Jr.,*,| John E. Crowell,*,‡ Pehr E. Pehrsson,| Brian D. Thoms,|,⊥ and James E. Butler| Chemistry DiVision, NaVal Research Laboratory, Washington, D.C. 20375-5342, and Department of Chemistry and Biochemistry, UniVersity of California, San Diego, La Jolla, California 92093-0314 ReceiVed: October 2, 2000; In Final Form: February 23, 2001
Multiple internal reflection infrared spectroscopy (MIRIRS) and temperature-programmed desorption (TPD) were used to investigate the interaction of oxygen with a diamond (110)-oriented surface. Exposure of the hydrogen-free diamond surface at 90 K to room-temperature O2 or thermally excited oxygen, O2* (produced with a heated iridium filament) resulted in a sharp infrared absorption at 657 cm-1, which disappeared on heating to 300 K. The 657 cm-1 absorption may indicate a surface peroxide. When the hydrogen-free diamond surface was dosed with O2 at room temperature, no oxygen adsorption was observed by Auger electron spectroscopy (AES) or TPD. In contrast, dosing the surface with O2* at 300 K led to oxygen chemisorption. The room-temperature diamond surface was saturated with oxygen after exposures of >2400 L O2*. When the oxidized surface was heated, only CO2 and CO desorption were observed, with peak maxima at 780 and 870 K, respectively. The peak desorption temperatures for CO2 and CO did not vary with O coverage, implying first-order desorption kinetics. MIRIR spectra of the oxygen-saturated (110)-oriented surface showed weak absorption modes at 790 and 980 cm-1. The exposure of the surface at 900 K to O2* led to (1) an increase in the coverage of oxygen species stable at high-temperature, (2) narrower, more intense, MIRIRS absorption modes (16O, 770, 934, and 980 cm-1; 18O, 747, 895, and 936 cm-1) and (3) a sharp, intense CO desorption peak at 1025 K. These observations imply that the low-temperature adsorption sites were etched away, thus favoring the additional adsorption of oxygen into the adsorption sites that are stable at high temperature.
1. Introduction While the chemical vapor deposition (CVD) of diamond films, using about 1% carbon in hydrogen,1 has made the exploitation of the unique and extreme materials properties of diamond a realistic goal, understanding the chemical interaction of oxygen with diamond surfaces remains a critical issue. Oxygen has considerable influence over the growth and properties of diamond films. It has been used in combination with hydrogen and hydrocarbons to improve the quality or alter the morphology of the grown film.2,3 In the growth process, it is believed to tie up carbon as unreactive CO as well as etch sp2 carbon at the surface. Oxygen absorption onto the diamond surface also alters key technological properties such as electrical conductivity,4 Schottky barrier height,4,5 and electron emission.6,7 Despite the importance of oxygen’s influence on these properties, little is known of the chemistry of specific sites and structures of adsorbed oxygen on diamond. Matsumoto et al.8 reported infrared absorptions on oxidized powder samples at 1800 and 1280 cm-1 that were attributed to carbonyl and ether groups, but the infrared spectra were not published. A detailed diffuse reflectance infrared study by Ando et al.9 on diamond powders used temperature-programmed †
Part of the special issue “John T. Yates, Jr. Festschrift”. * Corresponding authors. E-mail:
[email protected] and
[email protected]. ‡ University of California, San Diego. § Email:
[email protected]. | Naval Research Laboratory, Washington, D.C. 20375-5342 ⊥ Present address: Dept. of Physics, Georgia State University, Atlanta, GA.
desorption (TPD), temperature-programmed reaction (TPR), and thermogravimetric analysis (TGA) to distinguish infrared features that were attributed to ether, ketone, lactone, carboxylic acid, and carboxylic anhydride chemical structures. However, the broad mixture of surface orientations found on powdered samples precludes the assignment of particular vibrational frequencies or chemical structures to specific sites. Studies of polycrystalline diamond films grown on silicon substrates10,11 suffer from the same ambiguities. TPD studies of oxidized single-crystal C(100) observed both CO and CO2 desorption upon heating12 but were unable to distinguish between possible ether, carbonyl, and other surface moieties. Struck and D’Evelyn reported multiple internal reflection infrared spectra (MIRIRS) of C(100) after reaction with water;13,14 absorptions at 720 and 1200-1250 cm-1 were attributed to carbonyl bend and ether stretch modes, respectively, while features at 1080 and 1125 cm-1 were attributed to hydroxyls. The associated carbonyl stretch mode was not observable due to the bulk absorption of the diamond prism. X-ray photoelectron spectroscopy (XPS) studies of oxygen on single-crystal diamond surfaces have shown broadened O1s, O2s, and O2p features, indicating the oxygen is present in multiple moieties.15,16,17 One of these papers15 also showed by Rutherford backscattering (RBS) that thermal oxidation of C(110) at 700 K gave a saturation coverage of 0.3 ML, compared to 0.31 and 0.50 for C(111) and C(100) surfaces. Theoretical studies have considered the stable configurations of oxygen on C(100) and C(111) surfaces18-24 but have not investigated the C(110) surface. On the (100) surface, SLABMNDO molecular orbital methods21 and ab initio total energy
10.1021/jp003586k CCC: $20.00 © 2001 American Chemical Society Published on Web 04/03/2001
3804 J. Phys. Chem. B, Vol. 105, No. 18, 2001 calculations22 have predicted that ketone (on-top) and ether (bridging) oxygen structures have similar stabilities. Similar calculations using density functional methods24 also predict similar thermodynamic stabilities for ketone and ether functionalities. Despite the relative scarcity of prior results on C(110) surfaces, there is a wealth of published information on infrared spectra of other oxygen-containing carbon compounds. In addition to many detailed studies of diamond powders,8,9,25-28 the considerable background of organic chemistry literature includes a number of infrared studies of related compounds.29,30 Vibrational frequencies of many oxidized carbon structures are readily available in published compendia of spectra. Additional studies have used high-resolution electron energy loss spectroscopy (HREELS) to investigate the vibrational properties at diamond surfaces. Several have considered hydrogen on C(100) surfaces, but only recently have spectra been reported for oxygen on C(100).31-33 Recent work on the (100) diamond surface has begun to correlate reactions and vibrational features with particular surface oxygen species. HREELS was used by Hossain et al.33 to identify vibrational modes that were attributed to carbonyl stretching and bending modes (νCdO ) 1734 cm-1, δCdO ) 912 cm-1), and ether (νC-O-C ) 1210 cm-1) stretching frequencies. In this laboratory, HREELS studies by Pehrsson et al.31,32 have also attributed features to carbonyl (νCdO ) 1720-1820 cm-1, δCdO ) 870 cm-1) and ether modes (νC-O-C ) 970-980 cm-1), though at somewhat different frequencies. During CVD or high-pressure diamond growth, as in nature, the growth habit of diamond crystals commonly shows (100) and (111) faceting. This indicates that those faces are slower growing than other possible orientations. The remaining lowindex face, (110), can serve as a model of the more reactive sites during diamond growth. The C(110) surface also offers an opportunity specific to infrared studies. Unlike the (100) surface, each layer of atoms has the same orientation as that of adjacent layers; unlike the (111) surface, the (110) surface has C2 symmetry so that polarization may be used to distinguish the orientation of chemisorbed species. As described below, the present study of the diamond (110)-oriented surface finds results which are similar in some regards to prior observations of other diamond surfaces and reveals novel results which elucidate the reactivity of this particular surface. 2. Experimental Methods For practical reasons, the multiple internal reflection infrared spectroscopy (MIRIRS) and temperature-programmed desorption (TPD) experiments were performed in separate ultrahigh vacuum (UHV) systems (P ≈ 1 × 10-10 Torr) on different C(110)-oriented samples. Each apparatus was equipped with a cylindrical mirror analyzer for electron spectroscopies, tungsten filaments for atomic hydrogen dosing, and iridium filaments for thermally excited oxygen (O2*) dosing. The “IR chamber”34,35 had two KBr windows for transmission of infrared light. Infrared spectra were collected with a Fourier transform infrared (FTIR) spectrometer using a wideband HgxCd1-xTe (MCT-B) detector. The “TPD chamber”36 contained low-energy electron diffraction (LEED) optics for monitoring surface structure and a differentially pumped quadrupole mass spectrometer for temperature-programmed desorption measurements. For the infrared experiments, a type-IIA natural diamond (15.2 × 5.2 × 0.75 mm3) was prepared as a trapezoid-shaped internal reflection element34,37 (IRE). This prism was mounted on a molybdenum (Mo) block in such a way that both large
Mackey et al.
Figure 1. Single-beam s-polarized IR spectrum through the diamond (110) prism with a broadband MCT-B detector. The optical path length in the diamond crystal is 30 mm.
faces of the crystal were exposed. The Mo block was in thermal contact with a l-N2-cooled reservoir. The temperature of the prism was measured with a chromel-alumel thermocouple wedged into a laser-drilled through-hole in the prism. The diamond prism could be heated by electron bombardment from a tantalum filament passing inside the Mo block or cooled by addition of l-N2. The electrical potential of the Mo block could be independently controlled and was grounded when collecting Auger electron spectra. The temperature of the Mo block could also be measured with a type-C (tungsten-rhenium) thermocouple or by optical pyrometry. Prior to insertion in the chamber, the prism was cleaned in a series of acid baths (to remove any metals and amorphous carbon) and was subjected to about 30 min of hydrogen plasma cleaning at 1073 K on both reflective surfaces. This procedure has proven to be quite effective for cleaning and smoothing the (100)-oriented surface.38 Recently, it was found that the H-plasma treatment results in a fully hydrogenated (110) surface but that the surface becomes comprised of (111) microfacets that are enhanced by the H plasma treatment.39,40 Nevertheless, for simplicity, we refer to the crystal surfaces under study as the (110) or (110)-oriented surfaces. Infrared light from a Fourier transform infrared spectrometer was focused through a KBr window onto one beveled edge of the diamond IRE. After 33 internal reflections (incident angle ) 30°) from the two large-area (110)-oriented faces, the light exited the opposite end of the prism and passed through a slit before exiting the chamber through a second KBr window. The optical path length in the IRE was 30 mm. The light was then focused onto a l-N2-cooled wideband MCT-B detector. The single-crystal IRE was oriented such that the light propagated along the [001] direction, while the electric field of the p- and s-polarized light lay in the [110] and [110] directions, respectively (Figure 1, inset). Infrared spectra were collected at s- and p-polarization with 4 cm-1 resolution. The signal was averaged over 1000-5000 scans and each spectrum was ratioed against a background spectrum to give relative reflectance. Except where noted, all spectra were collected at 300 K. The single-beam infrared spectrum of the diamond (110)oriented IRE is shown in Figure 1. Due to the long optical path length through the crystal (∼30 mm), the prism is opaque in the 2-phonon (1700-2600 cm-1) and 3-phonon (3200-3800 cm-1) lattice absorption regions. Therefore, the most useful portion of the IR spectrum in the MIRIRS experiment was between 500 and 1600 cm-1. While the lattice absorption
(110)-Oriented Diamond Surface eliminated a considerable and interesting portion of the infrared spectrum, much can be learned from MIRIRS studies using diamond. For example, in other experiments, we have been able to observe the C-H stretch on the hydrogenated C(110)-oriented surface41 at 2880 cm-1 and the C-F stretching modes of chemisorbed fluoroalkyl groups35 between 1100 and 1400 cm-1. The MCT detector used in these experiments was not sufficiently sensitive to detect the weak C-H stretch. The valuable carbonyl stretching modes were unfortunately obscured by the diamond 2-phonon bulk absorption. A type-IA natural diamond, 3.88 × 4.75 × 1.75 mm3, with a surface oriented to within 1° of the (110) plane, was used in the TPD experiments. It was suspended between two tungsten wires laid in grooves cut in the top and bottom edges of the crystal. This arrangement is similar to the one described by Crowell et al.42 for mounting Al(111) crystals. The tungsten wires were in thermal contact and electrical isolation with a l-N2-cooled reservoir. The diamond temperature was measured with a type-K (chromel-alumel) thermocouple inserted into a laser-drilled hole in the side of the crystal. The sample could be cooled to 90 K and heated to 1373 K by resistively heating the tungsten wires. Using PID control, the sample was heated at 2 K/s. Prior to insertion into the UHV chamber, the sample was cleaned in a series of acid baths38 and then subjected to a hydrogen plasma treatment for 30 min. During the TPD experimental series, the crystal was heated to 1300 K to desorb the oxygen but was not removed from the chamber and retreated in a hydrogen plasma. In both chambers, a bare diamond (110)-oriented surface was prepared by annealing the crystal to 1373 K for about 1 min. HREELS studies on other crystals have shown that this method removes surface hydrogen.43 The H-free surfaces of the TPD and IRE (both sides) diamond crystals were oxidized by exposure to 10-6 Torr O2 in the presence of a hot iridium filament maintained at 1900 and 1673 K, respectively. Since it is not clear to what extent the oxygen is dissociated to atoms at these filament temperatures,44 we describe the dosed gas moiety as O2*, referring to the thermal excitation and possible molecular dissociation. The exposure is expressed in Langmuirs of O2 gas (1 L ) 1 × 10-6 Torr‚s) based on the O2 chamber pressure and the duration of the exposure to the hot filament. The H-free diamond surfaces were subjected to at least one cycle of oxidation and annealing to 1173 K before the reported experiments. Before and after the surface treatments of each crystal, the surface cleanliness was verified by AES. No elements other than carbon and oxygen were detected during these experiments (Figure 2). Severe charging on the fully oxidized surface during AES was minimized by the use of an electron-counting detection system, with beam currents of 50 nA or less. Illuminating the diamond with ultraviolet radiation, from a 300 W low-pressure mercury lamp, further reduced charging. 3. Results A range of diamond surface temperatures were selected for MIRIRS and TPD studies of oxygen adsorption, including simple adsorption reactions at 90 K, more complete adsorption reactions at 300-500 K, and combined adsorption and desorption reactions at 900 K. Initial MIRIRS experiments at 90 and 300 K indicate that O2 and O2* react with the annealed surface. TPD then shows that CO and CO2 desorb between 600 and 1100 K. Further MIRIRS and TPD experiments at 900 K are used to observe the vibrational modes of the surface species and to correlate those modes with desorption temperatures, providing clues toward the identification of the adsorbed species.
J. Phys. Chem. B, Vol. 105, No. 18, 2001 3805
Figure 2. Auger spectra of the 1173 K annealed (lower), 300 K O2* exposed (middle), and 900 K O2* exposed (upper) surfaces. Relative to the carbon kll derivative peak intensity, the oxygen kll peak intensities are 0.22, 0.10, and
