Surface phonon dispersion on hydrogen-terminated Si(110)-(1 × 1) surfaces studied by first-principles calculations.

We studied the lattice constants, surface-phonon dispersion curves, spectral densities, and displacement vectors of the hydrogen-terminated Si(110)-(1 × 1) [H:Si(110)-(1 × 1)] surface using the first-principles calculations within the framework of density functional theory (DFT). The symmetry of the H:Si(110)-(1 × 1) surface belongs to the two-dimensional space group p2mg, which has two highly symmetric and orthogonal directions, ΓX¯ and ΓX(')¯, with the glide planes along the ΓX¯ direction. Because glide symmetry separates the even and odd surface phonon modes, we mapped the even surface modes in the first surface Brillouin zone (SBZ) and the odd surface modes in the second SBZ using the spectral densities and displacement vectors. The surface phonon modes were analyzed with respect to their physical origin, spatial localization properties, polarization, and the charge density of their electronic states. Our calculated surface phonon modes were in good agreement with recent high-resolution electron-energy-loss spectroscopy data in the first and second SBZs of the ΓX¯ direction. In the SBZ of the ΓX(')¯ direction, our calculated surface phonon modes agree well with the data in the energy region below 65 meV but are not satisfactorily compatible with those in the stretching and bending modes. In addition, we discuss the microscopic nature of the surface phonon dispersion of the H:Si(110)-(1 × 1) surface using the phonon eigen modes.

Anisotropic surface phonon dispersion of the hydrogen-terminated Si(110)-(1×1) surface: one-dimensional phonons propagating along the glide planes.

We have measured the surface phonon dispersion curves on the hydrogen-terminated Si(110)-(1×1) surface with the two-dimensional space group of p2mg along the two highly symmetric and rectangular directions of ΓX and ΓX' using high-resolution electron-energy-loss spectroscopy. All the essential energy-loss peaks on H:Si(110) were assigned to the vibrational phonon modes by using the selection rules of inelastic electron scattering including the glide-plane symmetry. Actually, the surface phonon modes of even-symmetry to the glide plane (along ΓX) were observed in the first Brillouin zone, and those of odd-symmetry to the glide plane were in the second Brillouin zone. The detailed assignment was made by referring to theoretical phonon dispersion curves of Gräschus et al. [Phys. Rev. B 56, 6482 (1997)]. We found that the H-Si stretching and bending modes, which exhibit highly anisotropic dispersion, propagate along ΓX direction as a one-dimensional phonon. Judging from the surface structure as well as our classical and quantum mechanical estimations, the H-Si stretching phonon propagates by a direct repulsive interaction between the nearest neighbor H atoms facing each other along ΓX, whereas the H-Si bending phonon propagates by indirect interaction through the substrate Si atomic linkage.

Periodic corner holes on the Si(111)-7×7 surface can trap silver atoms.

Advancement in nanotechnology to a large extent depends on the ability to manipulate materials at the atomistic level, including positioning single atoms on the active sites of the surfaces of interest, promoting strong chemical bonding. Here, we report a long-time confinement of a single Ag atom inside a corner hole (CH) of the technologically relevant Si(111)-7×7 surface, which has comparable size as a fullerene C60 molecule with a single dangling bond at the bottom center. Experiments reveal that a set of 17 Ag atoms stays entrapped in the CH for the entire duration of experiment, 4 days and 7 h. Warming up the surface to about 150 °C degrees forces the Ag atoms out of the CH within a few minutes. The processes of entrapment and diffusion are temperature dependent. Theoretical calculations based on density functional theory support the experimental results confirming the highest adsorption energy at the CH for the Ag atom, and suggest that other elements such as Li, Na, Cu, Au, F and I may display similar behavior. The capability of atomic manipulation at room temperature makes this effect particularly attractive for building single atom devices and possibly developing new engineering and nano-manufacturing methods.

HREELS, STM, and STS study of CH(3)-terminated Si(111)-(1x1) surface.

An ideally (1x1)-CH(3)(methyl)-terminated Si(111) surface was composed by Grignard reaction of photochlorinated Si(111) and the surface structure was for the first time confirmed by Auger electron spectroscopy, low energy electron diffraction, high-resolution electron energy loss spectroscopy (HREELS), scanning tunneling microscopy (STM), and scanning tunneling spectroscopy (STS). HREELS revealed the vibration modes associated to the CH(3)-group as well as the C-Si bond. STM discerned an adlattice with (1x1) periodicity on Si(111) composed of protrusions with internal features, covering all surface terraces. The surface structure was confirmed to be stable at temperatures below 600 K. STS showed that an occupied-state band exists at gap voltage of -1.57 eV, generated by the surface CH(3) adlattice. This CH(3):Si(111)-(1x1) adlayer with high stability and unique electronic property is prospective for applications such as nanoscale lithography and advanced electrochemistry.