Papers by Keyword: Surface States

Abstract: Red-emitting carbon dots (r-CDs) are among the most challenging categories of carbon-based luminescent materials to synthesize with high emission color purity, photoluminescent quantum yield, and photostability. While r-CDs are used as red phosphors in white LEDs to compensate for spectral deficiencies caused by strong blue and green emissions, their emission mechanisms warrant further investigation. Herein, we synthesized r-CDs via a facile, one-step, environmentally benign solvothermal method. We analyzed the structure of our r-CDs using Transmission electron microscopy (TEM), X-ray diffraction (XRD), and Fourier infrared spectrometry (FTIR). Subsequently, the optical properties were investigated using a UV-Vis spectrophotometer and spectrofluorometer. The TEM image showed well-dispersed quasi-spherical dots with an average diameter of 3.47 nm. The photophysical investigation revealed that the r-CDs exhibit excitation-dependent emissions from 400 to 750 nm in water, whereas in DMSO, the emission spectra are excitation-independent, with a peak centered at 680 nm. The excitation-independent spectra observed were attributed to DMSO-induced deprotonation of surface functional groups on r-CDs, resulting in solvent-induced red emission and a high photoluminescent quantum yield (PLQY) of 31.3%.

Abstract: In metal cutting, a severe thermo-mechanical load collective determines the friction and wear behavior at the tool-chip interface. The inaccessibility of this interface complicates studies and thus the understanding of tribological effects in metal cutting. During a tool’s lifetime, local friction conditions change drastically as coatings and tool geometry wear down. This paper shall provide a comprehensive overview of current methods to understand and describe friction conditions in metal cutting and how cutting induced surface layer states may influence the friction and wear behavior of the finished workpiece.

Abstract: I-V, C-V characteristics and current change kinetics of the Ni-TiO_x-p/Si-Ni heterojunction were studied under different speeds of voltage sweep, in darkness and under illumination of various spectral regions. It was found that Ni-TiO_x-p/Si-Ni heterojunction shows pronounced hysteretic behavior and can act as memristor cell. Results of studies of photosensitivity and current kinetics under abrupt changes of applied voltage and illumination reveal considerable role of surface states recharging in TiO_x oxide layer or at TiO_x-p/Si interface in the switching effects.The studied Ni-TiO_x-p/Si-Ni heterostructure is prospective as a basis for low-cost, CMOS- and SOI-compatible microelectronic devices with non-volatile memory.

Abstract: Thin 30 nm SiO₂ films on silicon substrate implanted with Ge⁺ ions and flash-annealed at 900°C are investigated by means of optically stimulated electron emission (OSEE). The materials under study may find a number of technological applications in optical devices and ultraviolet sensors. Samples attestation was performed by electron microscopy and x-ray photoelectron spectroscopy (XPS). XPS data revealed strong dependence between the germanium atoms state and annealing time. The application of Urbach rule and power Kane dependence allowed to fit OSEE spectra at different excitation energy ranges and to retrieve the important structure and energy parameters. Observed correlations between parameter values of Urbach-and Kane-related models suggest the implantation-induced changeover of both the vibronic subsystem and energy band structure.

Abstract: Nanometer-sized ZnO crystals with the diameter from 20 nm to 110 nm were prepared by homogenous precipitation method (HPM). The photoluminescence (PL) spectra of as-prepared nanoparticles under excitation at the wavelength of 320 nm were detected. The PL spectra were fitted with Gaussian curves, in which a good fitting consisting of six Gaussian peaks was obtained. We observed that the multi-peak centers do not change much, while the relative amplitude of Gaussian combination to the band-to-band emission decreases rapidly with the increased grain size. It shows that the broadband emission at the lower energy is associated with the surface states.

Abstract: We study the effect of magnetic doping on appearance of ferromagnetic (FM) order on the surface of a three-dimensional topological insulator. Assuming the exchange coupling of the Dirac fermions with the dilute magnetic ions, we find that the system under consideration is unstable with respect to a spontaneous uniform magnetization along the out-of-plane direction. The appearance of the magnetization is accompanied by opening of a gap in the spectrum of surface states. In the framework of a mean-field approach, we analyze the possibility of the FM order on the magnetically doped surface of topological insulator at different temperatures and positions of the chemical potential.

Abstract: This paper is a summary of the experimental study of deep levels in a SiC crystal lattice caused by diffusion welding (DW). Investigations were carried out by DLTS and Kelvin Probe methods. Investigations revealed that DLTS method is not applicable for identification of surface states. Research conducted by the Kelvin Probe method has shown an increase in the density of surface states after the diffusion welding from 2x10¹⁵ cm^-2 to 3.5x10¹⁶ cm^-2.

Abstract: A simple and versatile on-chip tensile testing method is proposed for the statistical evaluation of size effects on the mechanical strength of silicon thin films along with the simultaneous study of (from low to ultra) strain effects on the carrier transport. Mechanical results are presented on the fracture strength of micro-nano scale silicon beams, followed with a discussion on interface states and problems facing reliable nano-electronic and nano-electromechanical characterizations.

Abstract: An analytical direct matching procedure within the Kronig-Penney model applied to analyze the dispersion behavior of the localized surface states supported in the surface of a semiinfinite one-dimensional photonic crystals truncated with air. The photonic crystal containing alternating layers of positive and negative media. In the case of the conventional indices, as the incident light changes from normal to oblique incidence, the surface modes shifts into higher frequencies. For this reason, the phenomenon of photonic band gap has been used only under a narrow range of frequencies of light incident at a particular angle or in a particular angular range. We have showed that by choosing some proper value for the PC parameters, zero and negative dispersion of surface modes emerge in a large range of k|| , and, due to the different nature of the band structure the surface modes are more localized compared to those appeared in the conventional PC.

Abstract: It is generally accepted that the Schottky barrier height (SBH) is affected by the initial band bending at the bare nGaN surface as well as by an additional contribution following metal deposition. In this work the effect of processing used for device fabrication on the surface band bending of bare c-plane nGaN was studied by surface potential electric force microscopy (SP-EFM). An increase of the initial upward band bending from 1.0 ± 0.1eV for the as-grown GaN to 1.9 ± 0.1eV after RTA treatment in N2 ambient was observed. No significant dependence of band bending on N2 or Ar as ambient gas during the RTA treatment was observed. The increase of the initial upward band bending was also confirmed by photoluminescence (PL) measurements. We suggest that the RTA treatment causes a high density of surface states, possibly as a result of high temperature reaction of ambient gas and remnant contamination.