Sagalianov I. Yu.


Electrical conductivity of graphene with various distributions of substitutional defects and adsorbed atoms.
01.04.07 – solid state physics
Doctor of Philosophy degree (Candidate of science in Physics and Mathematics)

Thesis for a Doctor of Philosophy degree (Candidate of Science in Physics and Mathematics) by specialty 01.04.07—Solid State Physics. — Physics Department, Taras Shevchenko National University of Kyiv, MES of Ukraine, 2015.
The thesis is concerned with the electrical-transport properties of graphene with different types of point defects: substitutional atoms (nitrogen, N), vacancies or the adsorbed atoms (potassium, K).
During the research, a software package, which allows us to explore electronic structure and conductance of doped graphene in terms of the Kubo–Greenwood methodology for lattices consisting of millions of atoms, is developed. The influence of various types of point defects (substitutional defects or adatoms) on electronic transport characteristics of graphene is analysed. As shown, the modification of graphene lattice with substitutional defects in different spatial configurations (of pyridine-like and graphite-like types of substitution) substantially changes its electrical conductivity in both the quantitative aspect and the qualitative one. Charge-carrier-density dependence of the conductivity can be linear or nonlinear depending on relative concentrations of N dopants and vacancies, their configurations over the graphene-lattice sites, and type of carriers—electrons or holes.
As found, at the transition from the random spatial distribution of defects to the correlated (ordered) one, the electroconductivity is enhanced in several (tens) times, as a consequence of the dominance of the short-range scattering in the presence of long-range or short-range orders in the spatial distribution of point defects in graphene. Correlated and ordered N scatterers in graphene, modelled with the negative scattering potential, enhance the conductivity up to the several (3‒6) and tens (10‒30) times, respectively, as compared to their random distribution. Depending on electron density and type of adsorbing sites, the conductivity for correlated and ordered K adatoms is found to be enhanced in dozens (up to 7 and 15, respectively) of times as compared to their random arrangement. The correlation and ordering effects manifest themselves stronger for adatoms (acting as substitutional atoms) and weaker for those acting as interstitial atoms.
As shown, the ordered arrangement of substitutional defects or adatoms (adsorbed directly above the carbon atoms) leads to breaking of the symmetry of the graphene lattice and causes the opening gap with the width linearly dependent on the concentration of these defects. Correlation and especially ordering of adatoms (i.e. formation of complex substitutional defects) as well as increase of their adsorption height weaken and even can suppress the electron–hole asymmetry in the conductivity occurring for their random positions.
Demonstrated influences of the various point-defect-doping configurations, especially ordering effect and herewith band-gap opening, in the doped graphene suggest the possibility of tailoring graphene transport properties via the positioning neutral adatoms or charged impurities on the substrate. The theoretical results are in agreement with available experimental data.
Key words: graphene, electrical conductivity, point defects.

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