Alexey T. Zayak

In the search for new nanoscale spectroscopic capabilities, we simulated Raman scattering at the interface between a physisorbed organic molecule tetracyanoquinodimethane (TCNQ) and graphene. The strength of the electronic coupling across the interface was modulated by applying external electric biases. It was expected that a stronger coupling would lead to a stronger Raman signal from TCNQ, as it was previously observed on metals and semiconductors. To a great surprise, the results of this work show an opposite effect: a stronger electronic coupling with graphene quenches the Raman activity of TCNQ. The quenching is attributed to the coexistence of two damping mechanisms. One is related to the orbital relaxation or the response of the environment to the electron perturbed by the external electric field (electron damping). Another mechanism is the damping of molecular vibrations caused by the dynamic interfacial charge transfer driven by the vibrations (phonon damping). While the charge transfer in electronic excitations is commonly considered as a mechanism for the chemical enhancement in surface-enhanced Raman spectroscopy (SERS), on the level of vibrations it acts as damping.

Diabetes mellitus is a chronic disease, and its management focuses on monitoring and lowering a patient’s glucose level to prevent further complications. By tracking the glucose-induced shift in the surface-enhanced Raman-scattering (SERS) emission of mercaptophenylboronic acid (MPBA), we have demonstrated fast and continuous glucose sensing in the physiologically relevant range from 0.1 to 30 mM and verified the underlying mechanism using numerical simulations. Bonding of glucose on MPBA suppresses the “breathing” mode of MPBA at 1071 cm^-1 and energizes the “constrained-bending” mode at 1084 cm^-1, causing the dominant peak to shift from 1071 cm^-1 toward 1084 cm^-1. The MPBA-glucose bonding is also reversible, allowing continuous tracking of ambient glucose concentrations, and the MPBA-coated substrates showed very stable performance over a 30-day period, making the approach promising for long-term continuous glucose monitoring. Using this Raman-mode constraining and miniaturized SERS implants, we also successfully demonstrated intraocular glucose measurements in six ex vivo rabbit eyes within ±0.5 mM of readings obtained using a commercial glucose sensor.

ZnO is a well-known wide band gap semiconductor with promising potential for applications in optoelectronics, transparent electronics, and spintronics. Computational simulations based on the density functional theory (DFT) play an important role in the research of ZnO, but the standard functionals, like Perdew–Burke–Erzenhof, result in largely underestimated values of the band gap and the binding energies of the Zn _{3 d} electrons. Methods like DFT + U and hybrid functionals are meant to remedy the weaknesses of plain DFT. However, both methods are not parameter-free. Direct comparison with experimental data is the best way to optimize the computational parameters. X-ray photoemission spectroscopy (XPS) is commonly considered as a benchmark for the computed electronic densities of states. In this work, both DFT + U and HSE methods were parametrized to fit almost exactly the binding energies of electrons in ZnO obtained by XPS. The optimized parameterizations of DFT + U and HSE lead to significantly worse results in reproducing the ion-clamped static dielectric tensor, compared to standard high-level calculations, including GW, which in turn yield a perfect match for the dielectric tensor. The failure of our XPS-based optimization reveals the fact that XPS does not report the ground state electronic structure for ZnO and should not be used for benchmarking ground state electronic structure calculations.

The magnetic and structural properties of a series of nonstoichiometric Ni–Mn–Ga Heusler alloys are theoretically investigated in terms of the density functional theory. Nonstoichiometry is formed in the coherent potential approximation. Concentration dependences of the equilibrium lattice parameter, the bulk modulus, and the total magnetic moment are obtained and projected onto the ternary phase diagram of the alloys. The stable crystalline structures and the magnetic configurations of the austenitic phase are determined.

In this work we present a systematic investigation of magnetic and structural properties of a broad range of Ni-Mn-Ga alloys by means of the density functional theory. Calculations are carried out for the cubic austenitic phase. The effect of chemical disorder is simulated by using the single-site coherent-potential approximation and the spin-polarized generalized gradient approximation. Equilibrium lattice parameters, bulk moduli, total magnetic moments, and formation energies of a wide range of Heusler alloys have been mapped on compositional ternary diagrams that give a bigger picture of the variety of physical properties of this family of alloys.