This paper reports a novel low power, fast nonvolatile memory utilizing high frequency phonons, atomic force dual quantum wells, ferromagnetism, coupled magnetic dipoles and random accessed magnetic devices. Very high-speed memories, such as SRAM and DRAM, are mostly volatile (data are lost when power is off). Nonvolatile memories, including FLASH and MRAM, are typically not as fast has DRAM or SRAM, and the voltages for WRITE/ERASE operations are relatively high. This paper describes a silicon nonvolatile memory that is compatible with advanced sub-7nm CMOS process. It consists of only one transistor (MOSFET) – small size, and more cost effective, compared with a 6-Transistor SRAM. There is no need to refresh, as required by DRAM. The access time can be less than 1ns – close to the speed level of relaxation time - much faster than traditional FLASH memories and comparable to volatile DRAM. The operating voltages for all memory functions can be as low as high speed CMOS.

Silicon nanowires (SiNWs) show unique optoelectronic properties such as band gap, radiative and nonradiative relaxations. In this research, the optoelectronic properties of <211> SiNW are calculated by combining time-dependent density matrix methodology. Description of photo-excited dynamics processes is enabled by computing “on–the–fly” nonadiabatic couplings (NAC) between electronic and nuclear degrees of freedom using density functional theory (DFT). The dynamics of electronic degrees of freedom is propagated by the reduced density matrix with Redfield equation of motion. Oscillator strengths are used to compute radiative relaxation and to generate time resolved photoluminescence (PL) spectrum. Analysis of the simulated nonradiative decay shows that high-energy photoexcitation relaxes to the band gap edge on the order of 1 ps. We also simulate time-resolved emission spectra of the <211> SiNW that reveals optical emissions above the optical band gap. These emission features are attributed to the interband transitions. The results of this study can be useful for the material choice for optoelectronic applications.

The ground state structure, optical properties and charge carrier dynamics of silicon nanowire (SiNW) grown in <211> crystallographic direction is studied as a function of wavevector using density functional theory. This nanowire can be used as fundamental unit of nanoelectronic devices. The optical properties are computed under assumption of momentum conservation$\Delta \vec{k} = 0$. The on-the-fly non-adiabatic couplings for electronic degrees of freedom are obtained along the ab initio molecular dynamics nuclear trajectories, which are used as parameters for Redfield density matrix equation of motion. By investigating the photo-induced process on this nanowire, it is shown that high-energy photoexcitation relaxes to the band gap edge within 75 ps. The results of these calculations help to understand the mechanism of electron transfer process on the Si nanowire.

Using scanning tunneling microscopy (STM) we observed atomic scale interference patterns on quasi-freestanding WSe2 islands grown on top of graphene. The bias-independent double atomic size periodicity of these patterns and the sharp Brillouin zone edge revealed by 2D STM Fourier analysis indicate formation of optical phonon standing waves due to scattering on intercalating defects supporting these islands. Standing wave patterns of both synchronized and non-synchronized optical phonons, corresponding to resonant and non-resonant phonon scattering regimes, were experimentally observed. We also found the symmetry breaking effect for individual phonon wave packets, one of the unique features distinguishing phonon standing waves. We show that vibrational and electronic anharmonicities are responsible for STM detection of these patterns. A significant contribution to the interference contrast arises from quantum zero-point oscillations.

Germanene, the 2D graphene-like Ge nanosheet, has been recently the subject of many theoretical studies and experimental attempts to synthesize it on Ag(111), Au(111) and Pt(111) surfaces. The experimental and theoretical evidences of germanene show a 2D continuous honeycomb layer with a buckled conformation. Density functional theory (DFT) calculations have predicted a larger buckling for germanene than silicene whose origin is also associated with a pseudo Jahn–Teller (PJT) effect. In this work we show that despite the fact that both, silicene and germanene possess a buckled conformation with a PJT origin, their vibronic coupling have different origins. The analysis is based on the PJT puckering instability of the hexagermabenzene molecule, the single hexagonal unit of germanene. This is done through the linear vibronic coupling model between the ground and the lowest excited states, which leads to a puckering distortion of the more symmetric cluster. We study both, the multilevel superposition vibronic model and possible mixing of excited states of different irreducible representations, which have been used to show the origin of similar structural transitions in hexagonal silicon and gold ring systems respectively. We show that contrary to other cases with one six-member rings, for the hexagermabenzene molecule a mixture of both the multilevel PJT and a ground state coupling with two quasi-degenerate excited states is necessary for a satisfactory explanation of puckering. Our model allows a determination of the coupling constants and predicts simultaneously the Adiabatic Potential Energy Surface (APES) behavior for the ground and excited states around the maximum symmetry point. The analysis is based on a scalar relativistic DFT and time-dependent DFT (TD-DFT) calculations in the Zero Order Regular Approximation (ZORA) using the B3LYP hybrid functional.

We develop theoretical descriptions for charge transport in organic semiconductors and carbon nanomaterials. For the localized charges, we found the quantum nuclear tunneling effect is essential which could manifest isotope effect for mobility as well as exotic optical feature. Because the nuclear tunneling tends to favor electron transfer while heavier nuclei decrease the quantum effect, isotopic substitution should reduce carrier mobility. Moreover, the isotopic effect only occurs when the substituted nuclei contribute actively to vibrations with appreciable charge reorganization energy and coupling with carrier motion. For the band-like transport, we propose a Wannier extrapolation scheme for computing the electron-phonon interaction matrix for the Boltzmann equation. Our calculation indicates that the intrinsic electron-phonon scatterings in two-dimensional carbon materials are dominated by low-energy longitudinal-acoustic phonon scatterings over a wide range of temperatures, while by high-frequency optical phonons at high temperature. The electron mobilities of α- and γ-graphynes are predicted to be ca.104 cm2V-1s-1 at room temperature.

Resonant Raman scattering in molybdenum disulfide (MoS2) is studied as a function of the sample thickness. Optical emission from 1ML, 2ML, 3ML and bulk MoS2 is investigated both at room and at liquid helium temperature. The experimental results are analysed in terms of the recently proposed attribution of the Raman peaks to multiphonon replica involving transverse acoustic phonons from the vicinity of the high-symmetry M point of the MoS2 Brillouin zone. It is shown that the corresponding processes are quenched in a few monolayer samples much stronger than the modes involving longitudinal acoustic phonons. It is also shown that along with the disappearance of multiphonon replica, the Raman modes, which are in-active in bulk become active in a few-monolayer flakes.

In fission based nuclear reactors, uranium dioxide fuel is subject to an intense neutron environment that drives the fission chain reaction. In this process, fission fragments will be produced with an energy reaching 1 MeV/amu. These fragments will initially lose energy through inelastic interactions resulting in excitations of the electronic structure. The excitations subsequently transfer energy to the atomic lattice through electron-phonon (e-p) coupling resulting in a thermal spike which may enhance mobility of fuel atoms. Consequently, the enhanced mobility resulting from fission energy deposition is expected to promote annealing of lattice defects such as ion tracks. Classical molecular dynamics (MD) simulations of uranium dioxide were performed using the LAMMPS code to investigate the effects of fission enhanced mobility on ion tracks formed in the fuel. The MD model was composed of 10×60×60 unit cells, 432000 atoms, and used a Buckingham potential to describe interatomic interactions. A two-temperature model was used to capture the process of fission energy deposition in the electronic subsystem and its transfer to the atomic lattice through e-p coupling. Previous MD simulations demonstrated that fission-enhanced diffusion became more pronounced as the electronic system behavior was varied from metal-like to insulator-like, i.e., increasing the e-p coupling strength. In the present MD simulations, the annealing of an existing ion track (radius nearly 3.0 nm) due to the interaction with 18 keV/nm and 22 keV/nm fission fragments was observed. For a metal-like system (weak e-p coupling), it was found that the track persisted with a radius of nearly 3.0 nm. For an insulator-like system (strong e-p coupling), it was found that the track can be reduced significantly in size approaching a radius of 1.4 nm.

Dynamical effects of non-conservative forces in long, defect free atomic wires are investigated. Current flow through these wires is simulated and we find that during the initial transient, the kinetic energies of the ions are contained in a small number of phonon modes, closely clustered in frequency. These phonon modes correspond to the waterwheel modes determined from preliminary static calculations. The static calculations allow one to predict the appearance of non-conservative effects in advance of the more expensive real-time simulations. The ion kinetic energy redistributes across the band as non-conservative forces reach a steady state with electronic frictional forces. The typical ion kinetic energy is found to decrease with system length, increase with atomic mass, and its dependence on bias, mass and length is supported with a pen and paper model. This paper highlights the importance of non-conservative forces in current carrying devices and provides criteria for the design of stable atomic wires.

Phonons—quanta of crystal lattice vibrations—reveal themselves in electrical, thermal, optical, and mechanical phenomena in materials. Phonons carry heat, scatter electrons, and affect light–matter interactions. Nanostructures opened opportunities for tuning the phonon spectrum and related properties of materials for specific applications, thus realizing what was termed phonon engineering. Recent progress in graphene and two-dimensional van der Waals materials has led to a better understanding of phonon physics and created additional opportunities for controlling phonon interactions and phonon transport at room temperature. This article reviews the basics of phonon confinement effects in nanostructures, describes phonon thermal transport in graphene, discusses phonon properties of van der Waals materials, and outlines practical applications of low-dimensional materials that rely on phonon properties.

Diamond is a unique material in many respects. One of the most well-known extreme properties of diamond is its ultrahardness. This property of diamond actually turns out to have interesting consequences for charge transport, in particular at low temperatures. In fact, the strong covalent bonds that give rise to the ultrahardness results in a lack of short wavelength lattice vibrations which has a strong impact on both electron and hole scattering. In some sense diamond behaves more like a vacuum than other semiconductor materials. In this paper we describe some interesting charge transport properties of diamond and discuss possible novel electronic applications.

Surface plasmon polaritons (SPPs), which are coupled excitations of electrons bound to a metal-dielectric interface, show great potential for application in future nanoscale photonic systems due to the strong field confinement at the nanoscale, intensive local field enhancement, and interplay between strongly localized and propagating SPPs. The fabrication of sufficiently smooth metal surface with nanoscale feature size is crucial for SPPs to have practical applications. A template stripping (ST) method combined with PMMA as a template was successfully developed to create extraordinarily smooth metal nanostructures with a desirable feature size and morphology for plasmonics and metamaterials. The advantages of this method, including the high resolution, precipitous top-to bottom profile with a high aspect ratio, and three-dimensional characteristics, make it very suitable for the fabrication of plasmonic structures. By using this ST method, boxing ring-shaped nanocavities have been fabricated and the confined modes of surface plasmon polaritons in these nanocavities have been investigated and imaged by using cathodoluminescence (CL) spectroscopy, which has been turned out to be a powerful means to characterize the resonant SPPs modes confined in metal nanocavities [1∼5] . The mode of the out-of-plane field components of surface plasmon polaritons dominates the experimental mode patterns, indicating that the electron beam locally excites the out-of-plane field component of surface plasmon polaritons. Quality factors can be directly acquired from the spectra induced by the ultrasmooth surface of the cavity and the high reflectivity of the silver (Ag) reflectors. Because of its three-dimensional confined characteristics and the omnidirectional reflectors, the nanocavity exhibits a small modal volume, small total volume, rich resonant modes, and flexibility in mode control. Numerous applications, such as plasmonic filter, nanolaser, and efficient light-emitting devices, can be expected to arise from these developments.

Localized surface plasmon resonance (LSPR) is a label-free biosensing technique employing plasmonic nanostructures to detect local refractive index change induced by biomolecules in the vicinity of these nanostructures. In analogy to surface plasmon resonance (SPR) sensor in a cuvette, LSPR is resistant to bulk refractive index fluctuation yet remains comparably sensitive for biosensing purpose. LSPR has the advantage over SPR in that the overall system size is smaller, and not affected by normal temperature fluctuations during measurement. However, mass production of a cheap but effective LSPR substrate remains challenging. In this paper, a self-assembly gold nanoisland structure was synthesized on transparent glass substrate by a simple two-step deposition-growth process. The first step involved depositing an ultra thin film of gold with nominal thickness of 5 nm by thermal evaporation at 1× 10-7 torr. Then the gold coated substrate was placed into a high temperature oven and annealed at 450°C for 10 hours. By first observation, the annealed substrate turned from pale green to dark pink. Upon scanning with atomic force microscopy, it was revealed that nanoislands of about 100 nm to 150 nm wide with average height of 60 nm were formed. Optical extinction measurements showed that the absorption peak was about 560 nm with fullwidth-half-maximum of 100 nm, so dark pink color was observed. For the biosensing demonstration, Bovine serum albumin (BSA) and Anti-BSA bio-affinity interaction was measured using the self-assembly gold nanoisland LSPR sensor. Anti-BSA was functionalized onto the sensing site and BSA of known concentrations, i.e. 1 ug/ml was injected. The results showed LSPR spectral intensity change of 650 counts at the resonance slope of 634 nm. With standard deviation of spectral intensity fluctuation at 7 counts, the detection limit of BSA was estimated at about 0.5 nM which was comparable with that of LSPR systems with more elaborate nanostructures. The limit of detection of the present system can be further improved by implementing phase measurement and further nanostructure improvement.

In this paper we demonstrate that graphene is one of the best materials for new types of terahertz lasers as optical and/or injection pumping of graphene can exhibit negative-dynamic conductivity in the terahertz spectral range. We analyze the formation of nonequilibrium states in optically pumped graphene layers and in forward-biased graphene structures with lateral p-i-n junctions and consider the conditions of population inversion and lasing. The latter provides a significant advantage of the injection pumping in realization of graphene terahertz lasers. We benchmark graphene as a prospective material for injection-type terahertz lasers.

The modelling of of silicon gate-all-around nanowire transistors by non-equilibrium Green function methods requires the computation of self-energies for inelastic electron-phonon interactions. It is shown that many approximations designed to reduce numerical complexityto these self-energies in fact fail because they do not satisfy appropriate causality conditions. Four familiar approximations are discussed and their failures resolved. It is also shown that a condition for the spectral density sum rule to hold (and hence accurate density of states in energy) depends on a simple causality condition.

Dielectric materials with GDR (e.g. CaCu3Ti4O12 – CCTO and isostructural systems, co-doped NiO etc) attract major research interest due to their bright prospective in energy storage and memory devices. However, after years of intensive experimental and theoretical studies of GDR materials, physical nature of their extremely high complex dielectric permittivity (specifically, real part ∼ 104 - 106) is still not established convincingly. Another serious problem is excessively high imaginary part of the permittivity (which usually exceeds real one). Better understanding on physical mechanisms and limitations of GDR behavior in aforementioned dielectrics could be achieved based on polaronic phase transition criteria, proposed S. Fratini and P. Quémerais [Eur. Phys. Journ. B14, 99 (2000)]. In particular, ‘melting’ of Polaronic Wigner Crystal (PWC) either to ‘polaronic liquid’ or ‘electron liquid’ manifests two different scenarios of PWC phase transition at increment of concentrations of appropriate dopants. The former scenario is certainly preferable for ionic dielectrics with GDR behavior, while the latter one would yield in metal-like dielectric response with very high real permittivity, but unacceptable loss. Described approach provides physically transparent guidelines for selection of prospective host dielectrics with GDR behavior and quantitative estimations on critical dopant/polaron concentrations, corresponding to both aforementioned types of the phase transitions as well as temperature ranges suitable for GDR.

The aim of this article is to briefly review the progress made over the past few years in the theoretical description of the intrinsic charge-transport properties of organic molecular crystals. We first discuss the state-of-the-art methodologies used in the derivation of the electronic coupling and electron-phonon coupling constants. We illustrate the application of these techniques to two classes of semiconductors of interest for crystal-based organic electronics: crystals consisting of a single molecular building block, such as oligoacenes and their derivatives, and bimolecular crystals consisting of donor and acceptor compounds. After a brief overview of recent developments in the polaron modeling of the electronic and electrical properties of these systems, we examine the impact that the interplay between electronic interactions and various electron-phonon mechanisms has on the temperature dependence of the charge-carrier mobility.

Thermal scanning electron microscopy is a new temperature mapping technique based on thermal diffuse scattering in electron backscatter diffraction in a scanning electron microscope. It provides both nano-scale resolution and far-field non-contact temperature mapping capabilities no other methods can adequately combine. While a calculated spatial resolution of less than 100 nm has already been realized using 20 keV electrons, lower energy incident electrons should enable still higher spatial resolution (even down to 10 nm). In this paper, the feasibility of this approach is examined.

There is a growing body of evidence that a number of mixed-valent and heavy-fermion materials show renormalized hybridization gaps either at the Fermi-energy or close to the Fermi-energy. In the former case, a heavy-fermion semiconducting state occurs and in the later case, the system remains metallic at low temperatures. The magnitudes of the hy-bridization gaps are observed to decrease with increasing temperature. The existence of a low-energy electronic energy scale creates a possibility that the Born-Oppenheimer ap-proximation may fail and that there may be a resonant coupling between the phonons and the electronic excitations. Here we argue that such a mechanism may be the cause of the phonon anomalies observed in neutron scattering experiments on the high-temperature phase of alpha-uranium.

Recent advances in terahertz light amplification by stimulated emission of radiation in optically pumped graphene are reviewed. We present, within a picosecond time scale, fast relaxation and relatively slow recombination dynamics of photogenerated electrons and holes in an exfoliated graphene under infrared pulse excitation. We conduct time-domain spectroscopic studies using an optical pump and terahertz probe with an optical probe technique and show that graphene sheet amplifies an incoming terahertz field. The graphene emission spectral dependency on laser pumping intensity shows a threshold-like behavior, testifying to the occurrence of the negative conductivity and the population inversion. The emission spectra clearly narrow at a longer terahertz probe delay time, giving an evidence that the quasi-Fermi energy moves closer to the equilibrium at this longer terahertz probe delay time.