STRUCTURAL INSTABILITY OF THE DIAMOND C(111) SURFACE-INDUCED BY HYDROGEN CHEMISORPTION

The low energy electron diffraction technique was used to study the hy drogen chemisorption induced structural instability on the diamond C(1 11) surface. From the quantitative analysis of diffraction spots inten sity on the as-dosed, partially desorbed, and annealed hydrogenated C( 111) surfaces, the correlation between the (1x1)<->(2x1) phase transfo rmation, hydrogen coverage, and surface temperature is shown. Thermal treatment with partial hydrogen desorption on the fully hydrogenated C (111) surface induces a (1x1)-(2x1) reconstruction with the observable half-order spots intensity (I-1/2) emerging only after heating the su bstrate to 1270 K. Conversely, thermal annealing of the partially hydr ogenated C(111) surface without desorbing H causes the size shrinking of the (2x1) domains as well as the relaxation of the hydrogenated dom ains. The temperature effect of I-1/2 summarized from both thermal stu dies reveals that the (2x1) domain instability originated from the rel axation of the hydrogenated domains at elevated temperatures. In addit ion, the H chemisorption behavior on C(111) at different surface tempe ratures suggests that the terrace edges could be the preferential site s for the initial H adsorption and the growth of the hydrogenated doma ins might predominantly start from the terrace boundaries at a surface temperature as low as 125 K. The present study also allows us to tent atively propose that there might exist a low-temperature chemisorption state in addition to the hydrogenated metastable state as suggested b y the sum-frequency generation spectroscopy and theoretical studies. A possible mechanism for the hydrogen chemisorption induced structural transformation is also discussed. (C) 1998 American Institute of Physi cs. [S0021 -9606(98)70445-3].