| Summary: | 博士 === 國立清華大學 === 物理學系 === 96 === Dislocation theory is a powerful tool in the investigation of crystal defects. In the past dislocation theory was adopted to explain the results of scanning tunneling microscopy (STM) experiments on face-centered crystal surfaces. Here, we used the STM observations and dislocation theory to investigate the surface defects on body-centered cubic transition metal Mo(110) surface, and discussed the reasonable formation mechanism of them.
STM can be used to resolve the surface structures of conductive samples. On the other hand, the surface reactivity of transition metal can be modified by forming carbide structures. Combining STM with low energy electron diffraction (LEED), we successfully determined three carbide structures on the Mo(110) surface. By different carburization conditions, the coverages of carbon of structures (50-26), (12×4)–2C and (4×4) formed on the Mo(110) surface are 1/30, 1/24 and 1/16 ML, respectively. Besides, increasing the annealing time will change a high-coverage structure to a low-coverage one. This is caused by the inward diffusion of carbon atoms at high temperatures.
STM can also be used to observe the reactivity of the surface toward different adsorption molecules. When a methanol or an ammonia molecule interacts with the Si(111)-7×7 surface, two reacted sites will be produced: a Si adatom and its nearest neighboring rest atom. Comparing the STM images, prior to and after the exposure of gas molecule, and counting the reacted sites, we obtained the reacted ratio of different adsorption sites. Thus, the reactivities toward gaseous molecule between different Si atom sites can be studied. Furthermore, the dissociation of adsorbed molecules can be stimulated by a scanning STM tip. At sample voltages lower than �{1.5 V, the absorbed ammonia molecules on the Si(111)-7×7 surface can be dissociated by the inelastic tunneling electrons. By counting the dissociation yield or rate of NH3 molecules by inelastic tunneling electrons, the dissociation mechanism of NH3 on the surface can be investigated, and this investigation is useful in interpretation of the growth mechanism of silicon nitride thin film on Si(111)-7×7 surface.
|
|---|
