Vibrational spectra of hydrogen on diamond C(111)-(1×1)

Abstract

Infrared-visible sum-frequency generation was used to obtain vibrational spectra of hydrogen on C(l I I). From a fully relaxed (I && I) surface, a single sharp peak at -2830 cm ' was observed, which can be identified as the CH stretch mode from H on top sites. With the surface freshly transformed from (2X I) to (I && I), however, another peak at a higher frequency was detected. This is attributed to H adsorbed on a metastable (I X I) structure.Hydrogen on diamond C(l I I) is a subject of great current interest since it plays an important role in the growth of diamond films by vapor deposition. ' However, despite the rather extensive effort in the past, understanding of the system is still not complete. The low-energy electron-diff'raction (LEED) work showed that upon hydrogen adsorption, the reconstructed C(l I I)-(2X I) surface is transformed into a (I X I) bulk-terminated structure, but few details are known. The electron-energy-loss spectroscopy (EELS) results suggested that the adsorption forms CH3 groups truncating the dangling bonds of the (I x I ) surface, while the He scattering and diA'raction data and the angular distribution of protons desorbed by electron stimulation ' from H/C(111)-(I x I) favored a simple hydrogen truncation of the bulk (H on top sites).Recently, infrared-visible sum-frequency generation (SFG) has been proven to be an eA'ective tool for surface vibrational spectroscopic studies.It can be applied to any interface accessible by light and is ideally suited for investigation of hydrogen adsorption on diamond. Here, we report the first results on H/C(111) using this technique. We have found, for H on a fully relaxed (I x I) surface, a single CH stretch peak at -2830 cm ' in the vibrational spectrum with an intrinsic full width less than 10 cm '. This, together with the polarization dependence of the spectrum, assures the monohydride adsorption geometry of H sitting on top C(l I I) sites. We have also found, together with LEED and optical second-harmonic generation work, that adsorption of only a few percent of a full monolayer of H was sufficient to induce the (2X I ) (I x I ) phase transformation.The CH stretch mode observed on a freshly formed (I & I ) surface at a low H coverage without annealing, however, showed up as a peak at a higher frequency (2860 cm '), suggesting the freshly formed surface might not be fully relaxed. The full surface relaxation could be effected by either hightemperature annealing (without hydorgen desorption) or higher surface coverage of H.The experimental setup for the SFG measurements has been described elsewhere. Briefly, the sum-frequency output from a sample surface was generated by overlapping a 25-ps visible light pulse at 0.532 pm and a 25-ps tunable infrared pulse on the surface. It is known that the output is proportional to the absolute square of the surface nonlinear susceptibility g, -, which can be written as (2) (2) + (2) gs gNR gR gR g Q)tR Q)g+ffg where gN2R is the nonresonant contribution, co~R is the infrared laser frequency, and Ag, cog,...

Chemistry of hydrogen on diamond (100)

“…This assignment is based on the similarity of the frequency of the surface vibrational mode to CC-D bending modes of 901 and 918 cr -1 in adamantane-d16 [371 and (CD 3 ) 3 C-D [38], respectively. The peak has a full width at half maximum of = 20 cm-1 , comparable to that seen for the C-H stretching mode on diamond (111) [39] and the Si-H stretching mode on flat Si(100) [27(c)]. The linewidth for H on flat Si(100) was overwhelmingly due to inhomogeneous broadening [27(c)], which is almost certainly also the source of the linewidth seen here.…”

A few selected applications of surface nonlinear optical spectroscopy.

“…This result together with the polarization dependence of the spectrum indicates that H sits nearly vertically on top of C, in agreement with the theoretical prediction (69). As in the case of H/Si(111), other properties of the CH stretch mode of H/C(111), such as anharmonicity and vibrational lifetime, could be measured and compared with theories (58,59 (58,59,70) (see Fig. 10b).…”

“…Here we consider adsorptions of H and CH3 on diamond C(111) studied in our laboratory (58)(59)(60)(61).…”

“…Fig. 10a shows the SFG spectra of CH stretching and bending modes of H on a well-ordered C(111)-(1 x 1) surface (58,59). Similar to H/Si(111), a single sharp peak appears in both the CH stretch and bend regions.…”

Nonlinear optical spectroscopy of diamond surfaces

“…Sum fkequency generation (SFG) spectroscopy has obtained vibrational spectra of hydrogen adsorption on C( 11 1). 8 It was found that only a single peak in the CH stretch region was observed, indicating the presence of only a single monohydride species on the sdace. This has been confirmed by the work of Hamza et ul.…”