Scanning Probe Microscopy

Scanning probe microscopy (SPM) scans a fine tip close to a surface and measures the tunneling current (STM) or force (SFM), based on many possible tip-surface interactions. STM provides atomic resolution imaging, or the local electronic structure (spectroscopy) as a function of bias voltage, and is also used to manipulate adsorbed atoms on a clean surface. STM operates in two modes— constant current or height—and requires a conducting specimen. SFM uses a cantilever (force sensor) to measure short range (< 1 nm) chemical, and a variety of long-range (< 100 nm) forces, depending on the tip and the specimen; a conducting specimen is not required. In static mode, the tip height is controlled to maintain a constant force, and measure surface topography. In dynamic mode, changes in the vibrational properties of the cantilever are measured using frequency, amplitude, or phase modulation as feedback to control the tip-surface distance and form the image. Dynamic imaging includes contact and non-contact modes, but intermittent contact or tapping mode is common. SPMs measure properties (optical, acoustic, conductance, electrochemical, capacitance, thermal, magnetic, etc.) using appropriate tips, and find applications in the physical and life sciences. They are also used for nanoscale lithography.

Diffraction of Electrons and Neutrons

Electron scattering, significantly stronger than that for X-rays, is sensitive to surfaces and small volumes of materials. Low-energy electron diffraction (LEED) provides information on surface reconstruction and the arrangement of adsorbed atoms. Reflection high energy electron diffraction (RHEED) probes surface crystallography, and monitors, in situ, mechanisms of thin film growth. Transmission electron diffraction reveals a planar cross-section of the reciprocal lattice, where intensities are products of the structure and lattice amplitude factors determined by the overall shape of the specimen. The amplitude of any diffracted beam at the exit surface oscillates with thickness (fringes) and the excitation error (bend contours). Selected area diffraction produce spot or ring patterns, where low-index zone-axis orientations reflect the symmetry of the crystal, and double-diffraction shows positive intensities even for reflections forbidden by extinction rules. Kikuchi lines appear as pairs of dark and bright lines, and help in tilting the specimen. A focused probe produces convergent beam electron diffraction (CBED), useful for symmetry analysis at nanoscale resolution. Neutrons interact with the nucleus and the magnetic moment of the atom via the spin of the neutron; the latter finds particular use in studies of magnetic order. The atomic scattering factor for neutrons shows negligible angular dependence.

The influence of adsorbed atoms concentration on the temperature coefficient of resonant frequency of the quasi-Rayleigh wave

Within the model of self-consistent connection of quasi-Rayleigh wave with adsorbed atoms, a method of constructing a new class of radiometric sensors of the temperature and concentration of adsorbed atoms on the surface acoustic waves is proposed. Based on the developed theory of the dispersion and acoustic mode width of a quasi-Rayleigh wave on the adsorbed surface of monocrystals with a Zinc blende structure, the temperature coefficient of the resonant frequency of the surface acoustic wave is calculated depending on the temperature and on the concentration of adsorbed atoms.

Adsorption of Silicon-Containing Dendrimers: Effects of Chemical Composition, Structure, and Generation Number

We studied the conformational behavior of silicon-containing dendrimers during their adsorption onto a flat impenetrable surface by molecular dynamics (MD) simulations. Four homologous series of dendrimers from the 4th up to the 7th generations were modeled, namely, two types of carbosilane dendrimers differing by the functionality of the core Si atom and two types of siloxane dendrimers with different lengths of the spacers. Comparative analysis of the fractions of adsorbed atoms belonging to various structural layers within dendrimers as well as density profiles allowed us to elucidate not only some general trends but also the effects determined by dendrimer specificity. In particular, it was found that in contrast to the carbosilane dendrimers interacting with the adsorbing surface mainly by their peripheral layers, the siloxane dendrimers with the longer –O–Si(CH3)2–O spacers expose atoms from their interior to the surface spreading out on it. These findings are important for the design of functional materials on the basis of silicon-containing dendrimers.

Квантово-механический подход к определению энергии активации поверхностной диффузии

A quantum-mechanical approach is proposed for determining the activation energy of surface diffusion for adsorbed atoms of copper, nickel, zinc and iron on a copper substrate during electrocrystallization for various substrate overpotential. The calculation of the activation energy of surface diffusion is performed through the total energy of the crystal. An increase in the activation energy of surface diffusion with an increase in the surface potential is associated with an increase in the binding energy of the ad-atom with the substrate

Atomic-scale thermal manipulation with adsorbed atoms on a solid surface at a liquid-solid interface

Modulating thermal transport through interfaces is one of the central issues in nanoscience and nanotechnology. This study examined thermal transport between atoms adsorbed on a solid surface and a liquid phase based on non-equilibrium molecular dynamics. The heat flux was detected at sub-atomic spatial resolution, yielding a two-dimensional map of local heat flux in the vicinity of the adsorbed atoms on the surface. Based on the detected heat flux, the possibility of atomic-scale thermal manipulation with the adsorbed atoms was examined by varying the interaction strengths between the liquid molecules and atoms adsorbed on the surface. The results of the local heat flux at the single-atom scale clearly showed effects of the adsorbed atoms on the thermal transport through the liquid-solid interface; they can significantly enhance the heat flux at the single-atom scale using degrees of freedom normal to the macroscopic temperature gradient. The effect was especially evident for a low wettability surface, which provides key information on local enhancement at the single-atom scale of the thermal transport through a liquid-solid interface.