Vanya Darakchieva

Bio: Vanya Darakchieva is an academic researcher from Linköping University. The author has contributed to research in topics: Epitaxy & Phonon. The author has an hindex of 30, co-authored 163 publications receiving 3554 citations. Previous affiliations of Vanya Darakchieva include Sofia University & University of Lisbon.

Infrared dielectric functions and phonon modes of high-quality ZnO films

Abstract: Infrared dielectric function spectra and phonon modes of high-quality, single crystalline, and highly resistive wurtzite ZnO films were obtained from infrared (300–1200 cm−1) spectroscopic ellipsometry and Raman scattering studies. The ZnO films were deposited by pulsed-laser deposition on c-plane sapphire substrates and investigated by high-resolution x-ray diffraction, high-resolution transmission electron microscopy, and Rutherford backscattering experiments. The crystal structure, phonon modes, and dielectric functions are compared to those obtained from a single-crystal ZnO bulk sample. The film ZnO phonon mode frequencies are highly consistent with those of the bulk material. A small redshift of the longitudinal optical phonon mode frequencies of the ZnO films with respect to the bulk material is observed. This is tentatively assigned to the existence of vacancy point defects within the films. Accurate long-wavelength dielectric constant limits of ZnO are obtained from the infrared ellipsometry anal...

On the organization and thermal behavior of functional groups on Ti3C2 MXene surfaces in vacuum

Abstract: The two-dimensional (2D) MXene Ti3C2T x is functionalized by surface groups (T x ) that determine its surface properties for, e.g. electrochemical applications. The coordination and thermal properties of these surface groups has, to date, not been investigated at the atomic level, despite strong variations in the MXene properties that are predicted from different coordinations and from the identity of the functional groups. To alleviate this deficiency, and to characterize the functionalized surfaces of single MXene sheets, the present investigation combines atomically resolved in situ heating in a scanning transmission electron microscope (STEM) and STEM simulations with temperature-programmed x-ray photoelectron spectroscopy (TP-XPS) in the room temperature to 750 °C range. Using these techniques, we follow the surface group coordination at the atomic level. It is concluded that the F and O atoms compete for the DFT-predicted thermodynamically preferred site and that at room temperature that site is mostly occupied by F. At higher temperatures, F desorbs and is replaced by O. Depending on the O/F ratio, the surface bare MXene is exposed as F desorbs, which enables a route for tailored surface functionalization.

Two-dimensional electron gas density in Al1-xInxN/AlN/GaN heterostructures (0.03 <= x <= 0.23)

Abstract: Compared to the AlGaN alloy, which can only be grown under tensile strain on GaN, the AlInN alloy is predicted by Vegard's law to be lattice-matched (LM) on fully relaxed GaN templates for an indium content of ~17.5%, i.e., it can be grown either tensely or compressively on GaN. The effect of strain on the polarization induced sheet charge density at the Al1-x Inx N/AlN/GaN heterointerfaces is carefully investigated for 6 and 14 nm thick AlInN barriers including a 1 nm thick AlN interlayer. The barrier indium content ranges at 0.03=x=0.23 for 6 nm thick barriers and 0.07=x=0.21 for 14 nm thick barriers. It is found that the two-dimensional electron gas (2DEG) density varies between (3.5±0.1) × 1013 cm-2 and (2.2±0.1) × 1013 cm-2 for 14 nm thick barriers. Finally, a 2DEG density up to (1.7±0.1) × 1013 cm-2 is obtained for a nearly LM AlInN barrier with ~14.5% indium on GaN as thin as 6 nm. © 2008 American Institute of Physics.

2D Transition Metal Carbides (MXenes) for Carbon Capture

Abstract: Global warming caused by burning of fossil fuels is indisputably one of mankinds greatest challenges in the 21st century. To reduce the ever-increasing CO2 emissions released into the atmosphere, d ...

Tailoring Structure, Composition, and Energy Storage Properties of MXenes from Selective Etching of In-Plane, Chemically Ordered MAX Phases.

Abstract: The exploration of 2D solids is one of our time's generators of materials discoveries. A recent addition to the 2D world is MXenes that possses a rich chemistry due to the large parent family of MAX phases. Recently, a new type of atomic laminated phases (coined i-MAX) is reported, in which two different transition metal atoms are ordered in the basal planes. Herein, these i-MAX phases are used in a new route for tailoriong the MXene structure and composition. By employing different etching protocols to the parent i-MAX phase (Mo2/3 Y1/3 )2 AlC, the resulting MXene can be either: i) (Mo2/3 Y1/3 )2 C with in-plane elemental order through selective removal of Al atoms or ii) Mo1.33 C with ordered vacancies through selective removal of both Al and Y atoms. When (Mo2/3 Y1/3 )2 C (ideal stoichiometry) is used as an electrode in a supercapacitor-with KOH electrolyte-a volumetric capacitance exceeding 1500 F cm-3 is obtained, which is 40% higher than that of its Mo1.33 C counterpart. With H2 SO4 , the trend is reversed, with the latter exhibiting the higher capacitance (≈1200 F cm-3 ). This additional ability for structural tailoring will indubitably prove to be a powerful tool in property-tailoring of 2D materials, as exemplified here for supercapacitors.

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

Abstract: 抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

A comprehensive review of zno materials and devices

Abstract: The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by Bunn [Proc. Phys. Soc. London 47, 836 (1935)], studies of its vibrational properties with Raman scattering in 1966 by Damen et al. [Phys. Rev. 142, 570 (1966)], detailed optical studies in 1954 by Mollwo [Z. Angew. Phys. 6, 257 (1954)], and its growth by chemical-vapor transport in 1970 by Galli and Coker [Appl. Phys. ...

Light-emitting diodes

Abstract: Light-Emitting Diodes. (Monographs in Electrical and Electronic Engineering.) By A. A. Bergh and P. J. Dean. Pp. viii+591. (Clarendon: Oxford; Oxford University: London, 1976.) £22.

A review of Ga2O3 materials, processing, and devices

Abstract: Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics, solar blind UV photodetectors, solar cells, and sensors with capabilities beyond existing technologies due to its large bandgap. It is usually reported that there are five different polymorphs of Ga2O3, namely, the monoclinic (β-Ga2O3), rhombohedral (α), defective spinel (γ), cubic (δ), or orthorhombic (e) structures. Of these, the β-polymorph is the stable form under normal conditions and has been the most widely studied and utilized. Since melt growth techniques can be used to grow bulk crystals of β-GaO3, the cost of producing larger area, uniform substrates is potentially lower compared to the vapor growth techniques used to manufacture bulk crystals of GaN and SiC. The performance of technologically important high voltage rectifiers and enhancement-mode Metal-Oxide Field Effect Transistors benefit from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. However, the absence of clear demonstrations of p-type doping in Ga2O3, which may be a fundamental issue resulting from the band structure, makes it very difficult to simultaneously achieve low turn-on voltages and ultra-high breakdown. The purpose of this review is to summarize recent advances in the growth, processing, and device performance of the most widely studied polymorph, β-Ga2O3. The role of defects and impurities on the transport and optical properties of bulk, epitaxial, and nanostructures material, the difficulty in p-type doping, and the development of processing techniques like etching, contact formation, dielectrics for gate formation, and passivation are discussed. Areas where continued development is needed to fully exploit the properties of Ga2O3 are identified.