A ReaxFF potential for Al - ZnO systems

A reactive field force (ReaxFF) potential has been created in order to model the structural effects of low percentage dopant aluminium in a zinc oxide system. The potential’s parameters were fitted to configurations computed with Density Functional Theory (DFT): cohesive energies, binding energies and forces were all considered for bulk crystals, surface structures and ZnAl alloys. As a first application of the model, the energetic deposition (0.1 - 40 eV) of an aluminium atom onto the polar surface of a ZnO (000 ̄1) is considered. For low energies the Al atom attaches to two preferred sites on the surface but as the energy increases above ≈ 15 eV subplantation is preferred at near normal incidence, with high diffusion barriers between stable sites. At these energies, reflection of the Al atom occurs at incident angles above ≈ 55◦.

Comparative Study of Bulk and Nanoengineered Doped ZnSe

Chromium- and cobalt-doped zinc selenide nanoparticles were synthesized using a low-temperature reactive solution growth method. The morphological and optical characteristics were compared to those of doped zinc selenide (ZnSe) bulk crystals grown by the physical vapor transport (PVT) method. We observed agglomeration of particles; however, the thioglycerol capping agent has been shown to limit particle grain growth and agglomeration. This process enables doping by addition of chromium and cobalt salts in the solution. A slightly longer refluxing time was required to achieve cobalt doping as compared with chromium doping due to lower refluxing temperature. The nanoparticle growth process showed an average particle size of approximately 300 nm for both Cr- and Co-doped zinc selenide. The optical characterization of Co:ZnSe is ongoing; however, preliminary results showed a very high bandgap compared to that of pure ZnSe bulk crystal. Additionally, Co:ZnSe has an order of magnitude higher fluorescence intensity compared to bulk Cr:ZnSe samples.

Doping dependent electronic and magnetic ordering in mixed-valent La1−xSrxMnO3 thin films

We have investigated the collective electronic and magnetic orderings of a series of La1−xSrxMnO3 thin films grown epitaxially strained to (001) oriented strontium titanate substrates as a function of doping, x, for 0 ≤ x ≤ 0.4. We find that the ground states of these crystalline thin films are, in general, consistent with that observed in bulk crystals and thin film samples synthesized under a multitude of techniques. Our systematic study, however, reveal subtle features in the temperature dependent electronic transport and magnetization measurements, which presumably arise due to Jahn-Teller type distortions in the lattice for particular doping levels. For the parent compound LaMnO3 (x = 0), we report evidence of a strain-induced ferromagnetic ordering in contrast to the antiferromagnetic ground state found in bulk crystals.

Deep level transient spectroscopy and photoluminescence studies of hole and electron traps in ZnSnP2 bulk crystals

ZnSnP2, an emerging inorganic material for solar cells, was characterized by deep level transient spectroscopy (DLTS) and photoluminescence (PL). Acceptor- and donor-like traps with shallow energy levels were detected by DLTS analysis. The previous study based on first-principle calculation also suggested such traps were due to antisite defects of Zn and Sn. PL measurements also revealed sub-gap transitions related to these trap levels. Additionally, DLTS found a trap with deep level in ZnSnP2. A short lifetime of minority carrier in previous work might be due to such trap, coming from phosphorus vacancies and/or zinc interstitials suggested by first-principle study.

Effect of Sr Doping on Structural and Transport Properties of Bi2Te3

Search for doped superconducting topological insulators is of prime importance for new quantum technologies. We report on fabrication of Sr-doped Bi2Te3 single crystals. We found that Bridgman grown samples have p-type conductivity in the low 1019 cm−3, high mobility of 4000 cm2V−1s−1, crystal structure independent on nominal dopant content, and no signs of superconductivity. We also studied molecular beam epitaxy grown SrxBi2−xTe3 films on lattice matched (1 1 1) BaF2 polar surface. Contrary to the bulk crystals thin films have n-type conductivity. Carrier concentration, mobility and c-lattice constant demonstrate pronounced dependence on Sr concentration x. Variation of the parameters did not lead to superconductivity. We revealed, that transport and structural parameters are governed by Sr dopants incorporation in randomly inserted Bi bilayers into the parent matrix. Thus, our data shed light on the structural position of dopant in Bi2Te3 and should be helpful for further design of topological insulator-based superconductors.

Binding Energy of Cubic Carbon Atoms

Energy spectrum of gas particles in plasma is broad, ranging from fractions to 10s of electron volts. Proportion of particles with required energetic parameters, participating in cubic carbon c-C synthesis, is small. External energy deposition can transfer an inert carbon atom to active state and change its electronic configuration. Binding energy of c-C atom depends on energy sources interaction. In this work, the calculations found the binding energy value that was compared with value of energy of the bond between the carbon atoms in ethane. The advisability of external source, activated carbon atoms generator, is marked. It has been established that by adding accelerated carbon atoms with energy of 9,687 eV into reactor it is possible to increase productivity of films, coatings and bulk crystals growth.

Elasticity-based-exfoliability measure for high-throughput computational exfoliation of two-dimensional materials

Two-dimensional (2D) materials are promising candidates for uses in next-generation electronic and optoelectronic devices. However, only a few high-quality 2D materials have been mechanically exfoliated to date. One of the critical issues is that the exfoliability of 2D materials from their bulk precursors is unknown. To assess the exfoliability of potential 2D materials from their bulk counterparts, we derived an elasticity-based-exfoliability measure based on an exfoliation mechanics model. The proposed measure has a clear physical meaning and is universally applicable to all material systems. We used this measure to calculate the exfoliability of 10,812 crystals having a first-principles calculated elastic tensor. By setting the threshold values for easy and potential exfoliation based on already-exfoliated materials, we predicted 58 easily exfoliable bulk crystals and 90 potentially exfoliable bulk crystals for 2D materials. As evidence, a topology-based algorithm indicates that there is no interlayer bonding topology for 93% predicted exfoliable bulk crystals, and the analysis on packing ratios shows that 99% predicted exfoliable bulk crystals exhibit a relatively low packing ratio value. Moreover, literature survey shows that 34 predicted exfoliable bulk crystals have been experimentally exfoliated into 2D materials. In addition, the characteristics of these predicted 2D materials were discussed for practical use of such materials.