Observation of electronic states on Si(111)-(7 x 7) through short-range attractive force with noncontact atomic force spectroscopy

We experimentally reveal that the short-range attractive force between a Si tip and a Si(111)-(7 x 7) surface is enhanced at specified bias voltages; we conduct force spectroscopy based on noncontact atomic force microscopy with changing bias voltage at a fixed separation. The spectra exhibit prominent peaks and a broad peak, which are attributed to quantum mechanical resonance as the energy levels of sample surface states are tuned to those of the tip states by shifting the Fermi level through changing bias voltage, and to the resonating states over a lowered tunneling barrier, respectively.     

Low temperature scanning force microscopy of the Si(111)-(7x7) surface

A low temperature scanning force microscope (SFM) operating in a dynamic mode in ultrahigh vacuum was used to study the Si(111)- (7x7) surface at 7.2 K. Not only the twelve adatoms but also the six rest atoms of the unit cell are clearly resolved for the first time with SFM. In addition, the first measurements of the short range chemical bonding forces above specific atomic sites are presented. The data are in good agreement with first principles computations and indicate that the nearest atoms in the tip and sample relax significantly when the tip is within a few A of the surface.     

Electron-phonon interaction at the Si(111)-7 x 7 surface

It is shown that electron-phonon interaction provides a natural explanation for the unusual band dispersion of the metallic surface states at the Si(111)-(7 x 7) surface. Angle-resolved photoemission reveals a discontinuity of the adatom band at a binding energy close to the dominant surface phonon mode at h(omega0) = 70 meV. This mode has been assigned to adatom vibrations by molecular dynamics calculations. A calculation of the spectral function for electron-phonon interaction with this well-defined Einstein mode matches the data. Two independent determinations of the electron-phonon coupling parameter from the band dispersion and from the temperature-dependent phonon broadening yield similar values of lambda = 1.09 and lambda = 1.06.     

Na adsorption on the Si111-(7 x 7) surface: from two-dimensional gas to nanocluster array

We have systematically investigated Na adsorption on the Si(111)-(7 x 7) surface at room temperature using scanning tunneling microscopy (STM). Below the critical coverage of 0.08 monolayer, we find intriguing contrast modulation instead of localized Na adsorbates, coupled with streaky noise in the STM images, which is accompanied by monotonic work function drop. Above the critical coverage, Na clusters emerge and form a self-assembled array. Combined with first-principles theoretical simulations, we conclude that the Na atoms on the (7 x 7) surface are, while strongly bound ( approximately 2.2 eV) to the surface, highly mobile in "basins" around the Si rest atoms, forming a two-dimensional gas phase at the initial coverage, and that the cluster at the higher coverage consists of six Na atoms together with three Si adatoms.     

High resolution imaging of the dolomite (104) cleavage surface by atomic force microscopy

In this paper we present high resolution atomic force microscopy (AFM) images of dolomite (104) cleavage surfaces immersed in pure water. These images show a rectangular lattice with surface unit cell dimensions in general agreement with those derived from the dolomite bulk structure. Furthermore, the two-dimensional fast Fourier transform (2D-FFT) plots of the high resolution images exhibit a pattern of periodicities consistent with both the alternate orientation of the carbonate groups and the positions for calcium and magnesium atoms on the dolomite (104) surface. However, the Mg²⁺ and Ca²⁺ sublattices could not be resolved. Finally, the images in both the real and the Fourier space do not reveal any clear evidence of reconstruction of the dolomite (104) surfaces.     

Electron dynamics of silicon surface states: second-harmonic hole burning on Si(111)-(7 x 7)

We report the first all-optical study of homogeneous linewidths of surface excitations by the spectral-hole-burning technique with surface-specific second-harmonic generation as a probe. Measurement of transient spectral holes induced by a 100 fs pump pulse in excitations of the surface dangling-bond states of Si(111)-(7 x 7) led to a pump-fluence-dependent homogeneous linewidth as broad as approximately 100 meV or a dephasing time as short as 15 fs. The hole-burning spectra also revealed a strong coupling between the localized dangling-bond states and the associated surface phonon mode at 570 cm(-1). Carrier-carrier scattering was responsible for the linear dependence of the dephasing rate on pump fluence, and the carrier screening effect appeared to be weak.     

Disentangling surface, bulk, and space-charge-layer conductivity in Si(111)-(7 x 7)

A novel approach for extracting genuine surface conductivities is presented and illustrated using the unresolved example of Si(111)-(7 x 7). Its temperature-dependent conductivity was measured with a microscopic four point probe between room temperature and 100 K. At room temperature the measured conductance corresponds to that expected from the bulk doping level. However, as the temperatures is lowered below approximately 200 K, the conductance decreases by several orders of magnitude in a small temperature range and it saturates at a low temperature value of approximately 4 x 10(-8) Omega(-1), irrespective of bulk doping. This abrupt transition is interpreted as the switching from bulk to surface conduction, an interpretation which is supported by a numerical model for the measured four point probe conductance. The value of the surface conductance is considerably lower than that of a good metal.     

Scanning force microscopy on the Si(111)7×7 surface reconstruction

Scanning force microscopy images of the Si(111)7×7 surface reconstruction are presented which are taken in the contact mode in ultrahigh vacuum. Topographic and lateral force data are acquired simultaneously. A special treatment of the sensing tip with PTFE helps to overcome the strong adhesion and wear effects that normally occur on this particular surface.     

Electron spin resonance observation of the Si(111)-(7x7) surface and its oxidation process

Electron spin resonance (ESR) observation of dangling-bond states on the Si(111)-(7x7) surface is demonstrated for the first time. The ESR spectra clearly show that a reaction of molecular oxygen with the Si(111)-(7x7) surface is associated with the appearance of a new dangling-bond center at unreacted Si adatoms. Most of the oxidized surface sites do not show ESR signals, but in a minor part of the surface another new type of surface defect is detected. The well known P(b) center at the SiO2/Si interface is found to evolve at an oxide thickness as thin as 0.3 nm.