E. M. Balashov

Resonance tunneling spectroscopy of heteropoly compounds

The electron tunneling spectra of phosphomolybdic and phosphomolybdovanadic acids have been measured using a scanning tunneling microscope. A new mechanism of negative differential resistance (NDR) formation in tunneling nanocontacts is established, which is general for all systems featuring the Wannier-Stark localization effect. A two-center inelastic resonance tunneling model is constructed, which allows the values of both electron and vibrational energy parameters to be determined from the measured spectra.

Correlations between the catalytic activity of heteropoly acids and the special features of their tunneling spectra

The electronic-vibrational spectra of phosphomolybdic acid, a classic heteropoly acid with a Keggin anion, were measured in an ultrahigh vacuum with the use of scanning tunneling microscopy. The dependences of the resonance characteristics of the spectra, so-called negative differential resistances (NDRs), on the vacuum gap, the polarity of applied voltage, and the direction of tunnel current were determined. A new mechanism of the formation of NDRs was proposed based on the quantum effect of the localization of the electronic states of multicenter systems in strong electric fields. The character of correlations between NDRs and the catalytic properties of heteropoly acid catalysts was established.

Correlation between the catalytic activity of polyoxometallates and the special features of their tunnel and optical spectra

The tunnel spectra of phosphomolybdic acid, a classic heteropoly acid with a Keggin anion, were measured in ultrahigh-vacuum experiments with the use of scanning tunnel microscopy. The dependences of the resonance characteristics of the spectra, “negative differential resistances,” on the vacuum gap, material of contacts, and field polarity were studied. An earlier unknown mechanism of the formation of these characteristics was described. The mechanism included the action of strong electric fields in scanning tunnel microscope nanocontacts and a low degree of the delocalization of Keggin anion peripheral electrons. Strong electric fields (∼107 V/cm) characteristic of spectroscopic measurements with the use of scanning tunnel microscopes could break exchange bonds in heteropoly acids and their derivatives. This produced spectroscopic effects of interest for catalysis and nanoelectronics.

Negative differential resistance in polyoxometalates

Large-scale (with peak-to-valley ratios up to 100) controllable multiple negative differential resistances have been revealed in tunneling spectra of one of the polyoxometalates (ammonium decamolybdodicobaltate(III)) in experiments with a scanning tunneling microscope in air under normal conditions and standard humidity. Conditions necessary for the observation of such negative differential resistances have been determined. A mechanism of their formation common for all nanostructures with weak intramolecular bonds has been revealed. A strategy for the selection of materials for molecular electronics based on the optimization of the composition and architecture of metal oxides has been proposed.

Molecular diodes and negative differential resistances based on polyoxometalates

By measuring the tunnel conductivity of polyoxometalates (POM) and their organic derivatives in experiments with a scanning tunnel microscope (STM), effects have been found which are promising for use in nanoelectronics. Large-scale multiple negative differential resistances (with record peak-to-valley ratios up to 102) have been observed under conditions which do not require low temperatures and high vacuum. The diode properties of organo-polyoxometalate complexes with coefficients of rectification up to 35–40 are revealed. A mechanism of biresonance tunnel electron transport in strong electric fields, which explains the effects, has been developed. A strategy for selecting nanomaterials which can improve the functional properties of molecular electronic elements based on the optimization of the composition and architecture of polyoxometalate complexes has been proposed.

The influence of strong electric fields on the growth of platinum oxide nanoparticles and their interaction with molecular hydrogen

The influence of strong electric fields created by the tip of a scanning tunnel microscope on the growth, modification, and destruction of platinum oxide nanoparticles and their interaction with molecular hydrogen was studied in high-vacuum experiments by the “nanolaboratory” method. The dependences of the rate of platinum oxidation on the voltage applied to the contact, its polarity in particular, were measured and interpreted. The possibility of field-induced reversible changes in the atomic structure of oxide phases was demonstrated. For the first time, applied field polarity dependent autooscillations of chemical reactions accelerated by the field that occurred in the interaction of molecular hydrogen with PtOx were observed. The results obtained, which modeled the influence of the charges of small metal particles on their catalytic activity, introduced a new size parameter into the theory of deposited catalysts. This parameter determined maximum sizes of nanoparticles whose reactivity depended on their charge state.

Structure and electronic properties of imperfect oxides and nanooxides

Results of studies of the structure and physicochemical properties of perfect (defectless) and imperfect (containing point defects) nanoparticles of metal oxides by the methods of scanning tunnel microscopy (STM) and spectroscopy (STS) are summarized and reviewed. Using nanooxides of various metals (Al, Pt, W, and Ti), as examples, it was demonstrated that the modern STM and STS methods make it possible to measure the electronic spectra of perfect nanoparticles, detect individual and associated point defects, determine spatial distributions of defects, derive spatial and energetic distributions of electrons trapped on point defects (anionic vacancies, intercalation atoms, and adatoms), detect single electron spins (paramagnetic surface complexes), measure phonon and vibration spectra of individual nanoparticles and surface complexes, determine (from measured electronic and phonon spectra) the chemical composition and atomic structure of individual nanoparticles. It was emphasized that modern versions of the STM-STS methods can constitute the foundation of new analytical methods needed in nanometrology and nanodefectoscopy.

RYDMR spectra for reactions of triplet−triplet annihilation and photogeneration of triplet pairs in molecular crystals

The contribution to the RYDMR spectrum from an elementary event of collision of two triplet excitons during their mutual annihilation is calculated, and the reverse process of photogeneration of a pair of triplet excitons in a molecular crystal are considered. The interaction between the excitons in the cell is assumed weak in comparison with the Zeeman splitting and the fine-structure parameters for the triplet state of each of the molecules. For both the annihilation of excitons and the photogeneration of a pair, the RYDMR signal from a single-crystal sample disappears at certain orientations of the crystal relative to the external magnetic field. This effect was previously experimentally observed for the anthracene−tetracyanobenzene crystal and was explained in the case of stationary optical excitation of the molecule. The results obtained in this paper make it possible to consider an arbitrary, not only stationary, mode of irradiation sample. An interval of angles between the static magnetic field and the crystallographic axes of the crystal within which the lines in the spectrum can disappear is determined. The possibility of using the RYDMR method for studying nanosized objects, for which additional peaks in the RYDMR spectrum may arise, is briefly discussed.

Nature of tunneling electron spin resonance of an isolated surface spin

A phenomenological model has been proposed for tunneling electron spin resonance (ESR) of an isolated surface spin situated in a scanning tunneling microscope (STM), which explains the dependence of features (local maxima) of the tunneling current on the radio-frequency (RF) electric field and on the position of the tip with respect to the spin. A crossover of the line shape of the resonance signals, whose nature in weak and strong pumping fields corresponds to Lorentzian and Fano resonances, respectively, has been interpreted. New ESR–STM effects that are linear and nonlinear in the RF field and are promising for developing the methods of controlling spin qubits have been predicted.

The Special Features of the Electronic Structure of Imperfect and Interacting Low-Dimensional Nanoparticles

A new method for constructing analytic exactly solvable models of the electronic structure of imperfect and interacting low-dimensional nanoparticles is suggested. The method uses the approach based on multiple scattering theory. It allows characteristic equations to be written specifying the contributions of exchange interactions between separate atoms, nanoparticle fragments, or interacting nanoparticles. The possibilities of the method are demonstrated for the example of one-dimensional and quasi-one-dimensional finite regular structures with monovacancies and isolated attached atoms and diatomic molecules. We also consider regular structures composed of two interacting low-dimensional nanoparticles. The general characteristic of the electronic spectra of the systems under consideration is qualitative changes in the system of energy levels and electron density distributions as a result of the formation of defects or new bonds between subsystems. These changes can substantially influence all the physical and chemical properties of nanoparticles.