Altynbek Murat

Two-dimensional materials can be utilized to detect gas molecules in low concentration due to their high surface-to-volume ratios. In this respect, we investigate in the present work recently fabricated borophene, two-dimensional B, which has buckled and line-defective phases. Both are systematically studied for four gas molecules: NH3, NO, NO2, and CO. In each case, the adsorption energy is found to be high and borophene develops distinct wrinkles. Our results provide a thorough understanding of the interaction between borophene and the gas molecules. An excellent performance of borophene as gas sensor is demonstrated by simulating the material’s transport characteristics by means of the nonequilibrium Green’s function method.

The structural, electronic, and optical properties of 12 multicomponent oxides with layered structure $RAM$O${}_{4}$, where $R$${}^{3+}$ = In or Sc, $A$${}^{3+}$ = Al or Ga, and $M$${}^{2+}$ = Ca, Cd, Mg, or Zn, are investigated using first-principles density functional approach. The compositional complexity of $RAM$O${}_{4}$ leads to a wide range of band-gap values varying from 2.45 eV for InGaCdO${}_{4}$ to 6.29 eV for ScAlMgO${}_{4}$ as obtained from our self-consistent screened-exchange local density approximation calculations. Strikingly, despite the different band gaps in the oxide constituents, namely, 2--4 eV in CdO, In${}_{2}$O${}_{3}$, or ZnO, 5--6 eV for Ga${}_{2}$O${}_{3}$ or Sc${}_{2}$O${}_{3}$, and 7--9 eV in CaO, MgO, or Al${}_{2}$O${}_{3}$, the bottom of the conduction band in the multicomponent oxides is formed from the $s$ states of all cations and their neighboring oxygen $p$ states. We show that the hybrid nature of the conduction band in multicomponent oxides originates from the unusual fivefold atomic coordination of $A$${}^{3+}$ and $M$${}^{2+}$ cations, which enables the interaction between the spatially spread $s$ orbitals of adjacent cations via shared oxygen atoms. The effect of the local atomic coordination on the band gap, the electron effective mass, the orbital composition of the conduction band, and the expected (an)isotropic character of the electron transport in layered $RAM$O${}_{4}$ is thoroughly discussed.

To exploit the full potential of multicomponent wide-bandgap oxides, an in-depth understanding of the complex defect chemistry and of the role played by the constituent oxides is required. In this work, thorough theoretical and experimental investigations are combined in order to explain the carrier generation and transport in crystalline InGaZnO4. Using first-principles density functional approach, we calculate the formation energies and transition levels of possible acceptor and donor point defects as well as the implied defect complexes in InGaZnO4 and determine the equilibrium defect and electron densities as a function of growth temperature and oxygen partial pressure. An excellent agreement of the theoretical results with our Brouwer analysis of the bulk electrical measurements for InGaZnO4 establishes the Ga antisite defect, GaZn, as the major electron donor in InGaZnO4. Moreover, we show that the oxygen vacancies, long believed to be the carrier source in this oxide, are scarce. The proposed carrier generation mechanism also explains the observed intriguing behavior of the conductivity in In-rich vs Ga-rich InGaZnO4.

The microscopic origin of the p-type character of AuCl3 functionalized carbon nanotubes (CNTs) is investigated using first-principles self-interaction corrected density functional theory (DFT). Recent DFT calculations suggest that the p-type character of AuCl3 functionalized CNTs is due to the Cl atoms adsorbed on the CNTs. We test this hypothesis and show that adsorbed Cl atoms only lead to a p-type character for very specific concentrations and arrangements of the Cl atoms, which furthermore are not the lowest energy configurations. We therefore investigate alternative mechanisms and conclude that the p-type character is due to the adsorption of AuCl4 molecules. The unraveling of the exact nature of the p-doping adsorbates is a key step for further development of AuCl3 functionalized CNTs in water sensor applications.

The formation and distribution of oxygen vacancy in layered multicomponent In$AM$O${}_{4}$ oxides with ${A}^{3+}=\text{Al}$ or Ga and ${M}^{2+}=\text{Ca}$ or Zn and in the corresponding binary oxide constituents is investigated using first-principles density functional calculations. Comparing the calculated formation energies of the oxygen defect at six different site locations within the structurally and chemically distinct layers of In$AM$O${}_{4}$ oxides, we find that the vacancy distribution is significantly affected not only by the strength of the metal-oxygen bonding, but also by the cation's ability to adjust to anisotropic oxygen environment created by the vacancy. In particular, the tendency of Zn, Ga, and Al atoms to form stable structures with low-oxygen coordination results in nearly identical vacancy concentrations in the InO${}_{1.5}$ and GaZnO${}_{2.5}$ layers in InGaZnO${}_{4}$, and only an order of magnitude lower concentration in the AlZnO${}_{2.5}$ layer as compared to the one in the InO${}_{1.5}$ layer in InAlZnO${}_{4}$. The presence of two light-metal constituents in the InAlCaO${}_{4}$ along with Ca failure to form a stable fourfold coordination as revealed by its negligible relaxation near the defect, leads to a strong preference of the oxygen vacancy to be in the InO${}_{1.5}$ layer. Based on the results obtained, we derive general rules on the role of chemical composition, local coordination, and atomic relaxation in the defect formation and propose an alternative light-metal oxide as a promising constituent of multicomponent functional materials with tunable properties.