arXiv subjects

D. Holec

Publications and source records attributed to D. Holec.

6 recordsLinked to original sources

arXiv2021

Graphene on silicon: effects of the silicon surface orientation on the work function and carrier density of graphene

Density functional theory has been employed to study graphene on the (111), (100) and (110) surfaces of silicon (Si) substrates. There are several interesting findings. First, carbon atoms in graphene form covalent bonds with Si atoms, when placed close enough on Si (111) and (100) surfaces, but not on the (110) surface. The presence of a Si (111) surface shifts the Fermi level of graphene into its conduction band, resulting in an increase of the work function by 0.29 eV and of the electron density by three orders of magnitude. The carrier density of graphene can also be increased by eighty times on a Si (100) substrate without doping, due to the modification of the density of states near the Dirac point. No interfacial covalent bond can be formed on Si (110). These striking effects that different orientations of a silicon substrate can have on the properties of graphene are related to the surface density of the silicon surface. Applying the results to a real device of a specific orientation requires further consideration of surface reconstructions, lattice mismatch, temperature, and environmental effects.

cond-mat.mtrl-sci↗

arXiv2018

Size-dependent surface energies of Au nanoparticles

Motivated by often contradictory literature reports on size dependence of surface energy of gold nanoparticles, we performed an atomistic study combining molecular dynamics and \textit{ab initio} calculations. We show that in the case of Au nanocubes, their surface energy converges to a value for $(0\,0\,1)$ facets of bulk crystals. A fast convergence to a single valued surface energy is predicted also for nanosheres. In this case, however, the value of the surface energy is larger than that of any low-index surface facet of bulk Au crystal. This fact can be explained by the complex structure of the surface with an extensive number of broken bonds due to edge and corner atoms. A similar trend was obtained also for the case of cuboctahedron-shaped nanoobjects. As the exact surface area of the nanoparticles is an ill-defined quantity, we introduced the surface-induced excess energy and discussed this quantity as a function of (i) number of atoms forming the nanoobject or (ii) the nanoobject characteristic size. In case (i), a universal power-law behaviour was obtained independent of the nanoparticle shape. Importantly, we show that the size-dependence of the surface is hugely reduced is the surface area correction due to the extend of electronic cloud is considered, a phenomenon specifically important for small nanoparticles.

cond-mat.mtrl-sci↗

arXiv2017

Graphite Under Compression: Shift of Layer Breathing and Shear Modes Frequencies with Interlayer Spacing

Layered materials have huge potential in various applications due to their extraordinary properties. To determine the interlayer interaction (or equivalently the layer spacing under different perturbations) is of critical importance. In this letter, we focus on one of the most prominent layered materials, graphite, and theoretically quantify the relationship between its interlayer spacing and the vibrational frequencies of its layer breathing and shear modes, which are measures of the interlayer interaction. The method used here to determine the interlayer interaction can be further applied to other layered materials.

cond-mat.mtrl-sci↗

arXiv2015

Graphite under uniaxial compression along c-axis: a new parameter relates out-of-plane strain to in-plane phonon frequency

Stacking graphene sheets forms graphite. Two in-plane vibrational modes of graphite, E1u and E2g(2), are derived from graphene E2g mode, the shifts of which under compression are all considered as results of in-plane bond shortening. Values of Gruneisen parameter have been reported to quantify such relation. However, the reason why the shift rates of these three modes with pressure differ is unclear. In this work, we introduce a new parameter to quantify the contribution of out-of-plane strain to the in-plane vibrational frequencies, suggesting that the compression of π-electrons plays a non-negligible part in both graphite and graphene under high pressure.

cond-mat.mes-hall↗

arXiv2014

Complementary ab initio and X-ray nano-diffraction study of Ta2O5

Numerous different crystal structures of Ta2O5 are reported in literature. Although experimentally and computationally obtained lattice parameters and mechanical properties are in excellent agreement there is a pronounced deviation when it comes to electronic structures of Ta2O5. Based on ab initio studies and nano-beam X-ray diffraction of sputtered Ta2O5 thin films, we introduce an orthorhombic basic structure with a = 0.6425, b = 0.3769, and c = 0.7706 nm, which is stabilized by flipping of an oxygen atom in neighboring c-planes. The calculated energy of formation is with -3.209 eV/atom almost as low as -3.259 eV/atom for the well-known Stephenson superstructure. We propose the new structure based on the fact that it allows for a good description of orthorhombic Ta2O5 even with a small and simple unit cell, which is especially advantageous for computational studies.

cond-mat.mtrl-sci↗

arXiv2012

Magnetic field strength influence on the reactive magnetron sputter deposition of Ta2O5

Reactive magnetron sputtering enables the deposition of various thin films to be used for protective as well as optical and electronic applications. However, progressing target erosion during sputtering results in increased magnetic field strengths at the target surface. Consequently, the glow discharge, the target poisoning, and hence the morphology, crystal structure and stoichiometry of the prepared thin films are influenced. Therefore, these effects were investigated by varying the cathode current Im between 0.50 and 1.00 A, the magnetic field strength B between 45 and 90 mT, and the O2/(Ar+O2) flow rate ratio between 0 and 100%. With increasing oxygen flow ratio a sub-stoichiometric TaOx oxide forms at the metallic Ta target surface which further transfers to a non-conductive tantalum pentoxide Ta2O5, impeding a stable DC glow discharge. These two transition zones (from Ta to TaOx and from TaOx to Ta2O5) shift to higher oxygen flow rates for increasing target currents. Contrary, increasing the magnetic field strength (e.g., due to sputter erosion) mainly shifts the TaOx to Ta2O5 transition to lower oxygen flow rates while marginally influencing the Ta to TaOx transition. To allow for a stable DC glow discharge (and to suppress the formation of non-conductive Ta2O5 at the target) even at a flow rate ratio of 100% either a high target current (Im >= 1 A) or a low magnetic field strength (B <= 60 mT) is necessary. These conditions are required to prepare stoichiometric and fully crystalline Ta2O5 films. Our investigations clearly demonstrate the importance of the magnetic field strength, which changes during sputter erosion, on the target poisoning and the resulting film quality.

cond-mat.mtrl-sci↗