Annotation of the results obtained in 2018
- The effect of atmospheric air exposure on the properties of nanocluster films was studied, and the method for creating protective coatings was developed. To analyze changes in the electronic structure caused by atmospheric exposure (at room temperature), metal nanocluster films were investigated by XPS after exposure to the atmosphere. Analysis of the obtained spectra allowed us to conclude that the exposure of samples to the atmosphere leads to a strong oxidation of nanocluster films. For all samples, the formation of an oxide film with a thickness of about 2 nm was observed, under which a layer of metal nanoclusters was retained. This fact was confirmed by the ohmic type of conductivity of the obtained films measured after exposure to the atmosphere. A method was developed to create a protective coating from a thin layer of CaF2 using an electron beam evaporator in situ immediately after the deposition of nanocluster films. - The model of the battery was created on the basis of a radioisotope battery of micron size using the obtained nanostructured thermoelectric and beta-voltaic material. For this purpose, a structure of electrodes with a width of 100 µm and distances between the electrodes of 100 µm was developed, the substrate of which is monocrystalline silicon of p and n types of conductivity with a low concentration of charge carriers. The topology of structures was developed, a set of photomasks was made and the technological cycle of works on the production of structures on silicon substrates was performed. As a result, a set of samples was prepared for the subsequent deposition of an array of point sources of nuclear material using the developed technique of local electrochemical deposition, and deposition of nanocluster films was obtained. - Measurements of thermo-EMF in the created thermoelements, and optimization of the geometric structure of such elements were made. The developed measuring chip was used, which includes tracks of metal gold and nickel, and the track of a thin film consisting of nickel nanoclusters intersecting at one point formed on the surface of silicon oxide SiO2/Si. A pair of gold-nickel tracks is a simple thermocouple and allows one to control the heating temperature of the intersection point of the tracks. The silicon substrate has a thinning in the area of heating the plate performed by laser ablation, to provide a large temperature gradient. The simulation of the heating caused by the alpha decay of the nuclear material placed at this point was simulated by laser radiation from a single-mode laser with an energy of 300 mW and a wavelength of 532 nm. To control the power of laser radiation incident on the surface of the measuring chip, a set of gray filters was used. The analysis of the results showed that the characteristic maximum absolute values of the Seebeck coefficient obtained for Co nanoclusters with a diameter of about 5.0 nm were -230 µv/K. The values of the Seebeck coefficient of the Co nanocluster film are much higher than the one for bulk value. For Ni nanocluster film, the Seebeck coefficient changes sign and retains the absolute value compared to the bulk value. - Measurements of photo-EMF in the created nanocluster films were carried out. For this purpose, a thin film consisting of Ni nanoclusters with a gradient size distribution of nanoclusters was formed on the surface of the SiO2/Si substrate. Then, the laser radiation with a wavelength of 532 nm and a power of 300 mW was directed to the surface of the formed nanocluster film. In order to eliminate the effect of heating, silicon with a thin layer of SiO2 (100 nm) oxide on the surface was used as a substrate. Maximum values of photo-EMF for the same series of samples of nanocluster tracks Ni and Co were 7.0 mV and 1.0 mV, respectively. - The concept of using nanocluster thin film coatings as photosensitive elements and solar cells was developed. For this purpose, it is proposed to use a thin nanostructured film consisting of nanoscale metal clusters (2-15 nm in diameter) with spatial ordering of nanoclusters by size. This system is an ensemble of densely packed metal nanoclusters with a gradient size distribution of nanoparticles deposited on the surface of a broadband dielectric – silicon oxide. Due to the dimensional change of volume and surface contributions to the density of electronic states of nanoparticles, the Fermi energy of nanoparticles of nanometer sizes varies with the size, which leads to the spatial redistribution of the charge in the system as a whole, and the emergence of a chemical potential gradient. Since the nanoclusters are metallic, this makes it possible to detect photons of different wavelengths. Thus, the nanocluster film upon interaction with photons is the excitation of electrons and their transition into the conduction band with subsequent runoff through the contact between nanoclusters. – The electronic properties of two-dimensional clusters simulating real metal films of nanoclusters were investigated using the “density functional theory + cluster perturbation theory” method. The phase diagram of the existence of the superconducting, antiferromagnetic, and nematic phases was calculated. The method of variational cluster approximation was used, which accurately takes into account the dynamic local and nonlocal correlations and allows one to investigate several order parameters corresponding to the phases under study. Single-particle spectra of quasiparticle excitations, density of states, and order parameters of ordered phases were obtained. The areas of existence of various phase states at different degrees of doping were shown, the phase diagram was obtained for various doping of the system. It was shown that the simultaneous existence of the superconducting and antiferromagnetic phases is possible in a wide region of electron doping, which is a completely new result. The symmetry of the superconducting order parameter was investigated, the behavior of the Fermi surface, the density of electronic states at different points of the phase diagram were analyzed. Data have been obtained indicating the possibility of metal-superconductor-dielectric phase transitions (antiferromagnet) of the first kind.

 

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Annotation of the results obtained in 2016
The technique is developed of forming metal nanoclusters of Ta, Mo, Ag, Au by cluster deposition method in gaseous phase; the range of nanocluster sizes is 1.3-7.5 nm with a narrow size distribution on various substrates. Analysis of the chemical composition was carried out in situ and ex situ by XPS technique. It was found that the amount of adsorbed oxygen increases with the decreasing of the size of nanoclusters in nanocluster films. The samples of nanocluster films have metallic conductivity, which is confirmed by the analysis of XPS and EELS spectra. In the atmosphere, Mo and Au samples retain their conductivity. The size and the shape of large clusters were measured ex situ by analysis of images obtained in a scanning electron microscope. It was found that deposited nanoparticles in nanocluster films are rounded and quite tightly packed to form a continuous film. The vacuum annealing of Ta, Mo and Ag cluster films up to 600 C did not led to a change in the chemical composition of the upper layers and to the coagulation. Annealing at T = 300 C reduces the Schottky barrier at the interface between Ta(core) / Ta(shell) from 0.5 eV for initial clusters to 0.3 eV for clusters after annealing. Reducing the Schottky barrier increases the above-barrier current and reduces the overall resistance of nanocluster Ta films for clusters having a natural oxide on the surface. The developed technique with the use of scanning tunneling spectroscopy (STS) of individual nanoclusters and nanoclusters in nanocluster films allows to restore the value of the local thermal emf of a sample. According to the proposed technique, ensembles were analyzed of Au, Pd, and Pt nanoclusters deposited by pulsed laser deposition on a conducting substrate. It has been found that the value of the thermal emf of palladium nanoclusters increases with decreasing of their size, whereas the value of the thermal emf of gold nanoclusters formed by the same technique decreases with decreasing of the size of nanoclusters. It was concluded that metals with unfilled d-shell may be of interest since the value of thermal emf of nanoclusters is sensitive to the behavior of the electron density of states near the Fermi energy, and, in particular, to the presence of defined peaks (the value of the thermal emf is proportional to the logarithmic derivative of the electron density of states). Additional experiments have shown that the potential candidates for the role of nanostructured thermoelectric materials may also be Ni and Co. Thus, the following materials are determined for further studies of thermoelectric properties of nanocluster films: Ta, Mo, Au, Ni, Co. The layout of a radioisotope battery was modeled in ANSYS package, and a calculation of its thermal characteristics was carried out. The radioisotope battery is a thermostatic substrate of silicon, on which a heat-insulating layer of composite material based on fluoropolymer is deposited. This choice is due to extremely low values of thermal conductivity of such a composite material and high radiation resistance of fluoropolymer. Thin films of thermoelectric material based on gold and platinum nanoclusters were deposited on the surface of the heat insulator. On these films, the radioisotope source (228ThO2) with the shape of a disk was deposited. The temperature gradient generated by such a system was 60 C. The method is developed for the recovery of the full and spectral density of states of nanoclusters with the use of the combined method "gradient descent + Monte Carlo algorithm", which includes an iterative process and Metropolis algorithm. A special basis of hyper-site states is developed, which takes into account two orbitals in the tight-binding model. Quantum Monte Carlo algorithm is developed and adapted for studying of metal nanoclusters; the algorithm allows to calculate diagonal and off-diagonal system characteristics at finite temperatures. The spectral and the total density of states were restored in the entire Brillouin zone for two-dimensional two-orbital model for large clusters. Profiles of the Fermi surface were obtained for the entire Brillouin zone. The effect of the strength of the interaction, cluster size and temperature on the excitation spectrum was studied.

 

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Annotation of the results obtained in 2017
1) A series of Au, Pd, Pt samples formed on conductive substrates were formed. Deposition of films consisting of nanoclusters was performed using pulsed laser deposition at the room temperature in ultrahigh vacuum conditions. The size and tunnel current-voltage characteristics of the nanoclusters under study was measured using scanning tunneling microscope in the analysis chamber of surface analysis system Multiprobe MXPS RM VТ AFM-25. The scanning tunneling images and current-voltage characteristics of individual nanoclusters in films were obtained for the samples. The differential tunneling conductivity of individual clusters was measured for the studied materials. On the basis of the obtained experimental data the logarithmic derivative of the density of electron states at Fermi energy that is directly connected to the Seebeck coefficient was obtained. 2) The technique of in situ diagnostics on the basis of X-ray photoelectron spectroscopy (XPS) devoted to the assessment of Seebeck coefficient from the analysis of the XPS core lines shape. The idea of the technique to analyze the asymmetry of the XPS core lines spectra and restoration of the Anderson singularity index for the studied material. The correlation dependence of the Anderson singularity index on Seebeck coefficient for bulk materials was found. This dependence allows estimating Seebeck coefficient for thin Ta, Mo, Co and Ni nanocluster films. According to the estimations, Seebeck coefficient of Co and Ni nanocluster films consisting on nanoclusters of about 1.5 nm in diameter can reach 100 muV/K and more that is of an order of magnitude higher than the bulk values. The analysis showed that independently on typical nanocluster size all the studied nanocluster films has no bandgap and thus are conductive. 3) The experiments proved that the samples obtained has a metallic conductivity. It was also experimentally shown that the work function of the samples changes non monotonically with cluster’s size. 4) The influence of the deposition regimes on the properties of nanocluster films was studied. The analysis show that the cluster source modes (magnetron power, aggregation zone length) influence only on the typical nanocluster size. While the buffer gas flow (Ar, He) changes the oxidation degree of the obtained samples. It was shown that lower the nanocluster size is the higher is the oxidation degree. Moreover the oxidation mechanism is different for big (~5 nm) and small (<2 nm) clusters. In the first case the formation of oxide shell can be observed, while small surface layer of clusters is completely oxidized. The use of different substrate materials showed no influence on the nanocluster film electron properties. 5)The technique of the nanocluster tracks formation was developed. It allowed depositing the uniform films consisting of metal nanoclusters. The electrical resistance and geometrical parameters of the obtained films were measured. The conductivity of the obtained nanocluster tracks was calculated according to measured height profiles. The measurements showed that the samples have an ohmic-type conductivity that is close to the bulk value. 6) In order to form heat-insulating layer the technique of ultrathin heat-insulating PVdF membranes was developed. The technique allows one to obtain smooth membranes of 20*10^(-6) m in thickness. After the additional analysis the decision not to use the PVdF membranes due to a number of deficiencies like the complexity of the formation of thin enough membranes, deficient radiation resistance, deficient strength. An alternative heat insulating material was suggested – a thinned silicon substrate. The thinning of the heated area of substrate was made using laser ablation method. 7) As a result of the development of the technique and device for direct thermoelectric properties measurement of thin nanocluster films the measurement post was build. The post consists of the developed measurement chip onto which the nanocluster track is deposited, single mode green laser (300mW), that implements the local heating and voltmeters that measures thermal electromotive force. During the experiments we found the occurrence of photo electromotive force while irradiating the nanocluster tracks with laser. This effect was observed for the first time and will be the task for future studies as soon as nanocluster film can be an alternative for semiconductor photoelements. While measuring thermal electromotive force the coverage of nanocluster tracks from laser emission was used. Thus the developed measuring post allows direct measuring both thermal and photo electromotive force. 8) A model of a system of nanoclusters has been developed, which makes it possible to simulate two-dimensional and three-dimensional systems of nanoclusters and obtain data on their electronic properties. The model was used to simulate the transition from a system of isolated nanoclusters to a bulk sample by changing the intercluster distances. For a system of gold nanoclusters, at room temperature, the transition from the isolated state of a sparse system of individual nanoclusters to the metallic state of bulk system was observed. 9) For small nanoclusters, the method of exact diagonalization was realized, which makes it possible to obtain data on the spectrum, wave functions, and the Green's function. The method passed State registration procedure (Kashurnikov VA, Krasavin AV, Zhumagulov Ya.V., Podlivaev AI Exact calculation of the spectrum and wave functions of a system of particles with strong correlations with different quantum statistics. Certificate of State Registration No. 2017617589 from July 7, 2017). 10) The DFT + CPT (density functional theory + cluster perturbation theory) method was developed and implemented to calculate the electronic properties of ensembles of nanoclusters consisting of a large number of atoms. Densities of electronic states for various ensembles of gold nanoclusters were obtained. 11) A technique has been developed for reconstructing the density of electronic states from experimental XPS spectra using a stochastic procedure that accurately takes into account the instrumental broadening of the spectrometer. An essential feature of the technique is that to calculate the spectrometer broadening function it is not necessary to know the crystal structure of nanoclusters, it is necessary to have only XPS spectra of core levels. 12) The method for reconstructing the density of electronic states was used to calculate single-particle spectra of a two-dimensional system of tantalum nanoclusters of various sizes. The obtained results indicate the existence of a system of nanoclusters of a certain size, having a peak at the Fermi level and characterized by a large value of the Seebeck coefficient.

 

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