Glass-like Transport Dominates Ultralow Lattice Thermal Conductivity in Modular Crystalline BiOSeCl.

Crystalline BiOSeCl exhibits record-low 0.1 W/mK lattice thermal conductivity (κ), but the underlying transport mechanism is not yet understood. Using a theoretical framework which incorporates first-principles anharmonic lattice dynamics into a unified heat transport theory, we compute both the particle-like and glass-like components of κ in crystalline and pellet BiOSeCl forms.

Impact of Local Composition on the Emission Spectra of InGaN Quantum-Dot LEDs.

A possible solution for the realization of high-efficiency visible light-emitting diodes (LEDs) exploits InGaN-quantum-dot-based active regions. However, the role of local composition fluctuations inside the quantum dots and their effect of the device characteristics have not yet been examined in sufficient detail. Here, we present numerical simulations of a quantum-dot structure restored from an experimental high-resolution transmission electron microscopy image.

Predicting the Lattice Thermal Conductivity in Nitride Perovskite LaWN from ab initio Lattice Dynamics.

Using a density functional theory-based thermal transport model, which includes the effects of temperature (T)-dependent potential energy surface, lattice thermal expansion, force constant renormalization, and higher-order quartic phonon scattering processes, it is found that the recently synthesized nitride perovskite LaWN displays strong anharmonic lattice dynamics manifested into a low lattice thermal conductivity (κ ) and a non-standard κ ∝T dependence. At high T, the departure from the standard κ ∝T law originates in the dual particle-wave behavior of the heat carrying phonons, which includes vibrations tied to the N atoms. While the room temperature κ =2.

Four-phonon and electron-phonon scattering effects on thermal properties in two-dimensional 2H-TaS.

Thermal transport characteristics of monolayer trigonal prismatic tantalum disulfide (2H-TaS) are investigated using first-principles calculations combined with the Boltzmann transport equation. Due to a large acoustic-optical phonon gap of 1.85 THz, the four-phonon (4ph) scattering significantly reduces the room-temperature phononic thermal conductivity ().

Anisotropic Phononic and Electronic Thermal Transport in BeN.

Beryllium polynitride (BeN) has been recently synthesized under high-pressure conditions [Bykov . 2021, 126, 175501]. Its anisotropic lattice structure dependent on the applied pressure motivates exploration of its thermal transport properties with a theoretical framework that combines the Boltzmann transport equation with calculations.

Role of Phase Nanosegregation in the Photoluminescence Spectra of Halide Perovskites.

The study of MAPbI phase transitions based on temperature-dependent optical spectroscopy has recently gained a huge attention. Photoluminescence (PL) investigations of the tetragonal-orthorhombic transition suggest that tetragonal nanodomains are present below the transition temperature and signatures associated with tetragonal segregations are observed. We have studied the impact of phase nanosegregation across the orthorhombic-tetragonal phase transition of MAPbI on the system's properties employing a tight binding (TB) approach.

Phononic Thermal Transport along Graphene Grain Boundaries: A Hidden Vulnerability.

While graphene grain boundaries (GBs) are well characterized experimentally, their influence on transport properties is less understood. As revealed here, phononic thermal transport is vulnerable to GBs even when they are ultra-narrow and aligned along the temperature gradient direction. Non-equilibrium molecular dynamics simulations uncover large reductions in the phononic thermal conductivity (κ ) along linear GBs comprising periodically repeating pentagon-heptagon dislocations.

Quantum Phonon Transport in Nanomaterials: Combining Atomistic with Non-Equilibrium Green's Function Techniques.

A crucial goal for increasing thermal energy harvesting will be to progress towards atomistic design strategies for smart nanodevices and nanomaterials. This requires the combination of computationally efficient atomistic methodologies with quantum transport based approaches. Here, we review our recent work on this problem, by presenting selected applications of the PHONON tool to the description of phonon transport in nanostructured materials.

Spatial and orientational dependence of electron transfer parameters in aggregates of iridium-containing host materials for OLEDs: coupling constrained density functional theory with molecular dynamics.

The efficient transport of charge within the bulk of active molecular materials is one of the main factors affecting the efficiency and performance of organic electronic devices. In amorphous molecular aggregates, the observed effective mobility of charge carriers is usually considered as resulting from the convolution of the manifold of intermolecular configurations. In this picture, individual molecules are considered as spherically-symmetric scattering points for charge hopping.

Nanoscale morphology and electronic coupling at the interface between indium tin oxide and organic molecular materials.

The correlation between nanoscale morphology and charge injection rates at the interface between an organic semiconductor layer and a transparent metal oxide electrode was investigated by integrating molecular dynamics simulations with electronic structure calculations. The simulation approach proposed has been applied to the analysis of the hole injection mechanism at the interface between an amorphous layer of tris[(3-phenyl-1H-benzimidazol-1-yl-2(3H)-ylidene)-1,2-phenylene]Ir (DPBIC), a hole transport and emitter molecule, and the surface of indium tin oxide (ITO), a material commonly used as anode in OLEDs. The link between interface morphology and charge injection was investigated by implementing a two-step, top-down simulation approach.

Carrier transport and emission efficiency in InGaN quantum-dot based light-emitting diodes.

We present a study of blue III-nitride light-emitting diodes (LEDs) with multiple quantum well (MQW) and quantum dot (QD) active regions (ARs), comparing experimental and theoretical results. The LED samples were grown by metalorganic vapor phase epitaxy, utilizing growth interruption in the hydrogen/nitrogen atmosphere and variable reactor pressure to control the AR microstructure. Realistic configuration of the QD AR implied in simulations was directly extracted from HRTEM characterization of the grown QD-based structures.

Tuning quantum electron and phonon transport in two-dimensional materials by strain engineering: a Green's function based study.

Novel two-dimensional (2D) materials show unusual physical properties which combined with strain engineering open up the possibility of new potential device applications in nanoelectronics. In particular, transport properties have been found to be very sensitive to applied strain. In the present work, using a density-functional based tight-binding (DFTB) method in combination with Green's function (GF) approaches, we address the effect of strain engineering of the transport setup (contact-device(scattering)-contact regions) on the electron and phonon transport properties of two-dimensional materials, focusing on hexagonal boron-nitride (hBN), phosphorene, and MoS monolayers.

Influence of electromechanical coupling on optical properties of InGaN quantum-dot based light-emitting diodes.

The impact of electromechanical coupling on optical properties of light-emitting diodes (LEDs) with InGaN/GaN quantum-dot (QD) active regions is studied by numerical simulations. The structure, i.e.

Single-molecule electronics: Cooling individual vibrational modes by the tunneling current.

Electronic devices composed of single molecules constitute the ultimate limit in the continued downscaling of electronic components. A key challenge for single-molecule electronics is to control the temperature of these junctions. Controlling heating and cooling effects in individual vibrational modes can, in principle, be utilized to increase stability of single-molecule junctions under bias, to pump energy into particular vibrational modes to perform current-induced reactions, or to increase the resolution in inelastic electron tunneling spectroscopy by controlling the life-times of phonons in a molecule by suppressing absorption and external dissipation processes.

Efficiency Drop in Green InGaN/GaN Light Emitting Diodes: The Role of Random Alloy Fluctuations.

White light emitting diodes (LEDs) based on III-nitride InGaN/GaN quantum wells currently offer the highest overall efficiency for solid state lighting applications. Although current phosphor-converted white LEDs have high electricity-to-light conversion efficiencies, it has been recently pointed out that the full potential of solid state lighting could be exploited only by color mixing approaches without employing phosphor-based wavelength conversion. Such an approach requires direct emitting LEDs of different colors, including, in particular, the green-yellow range of the visible spectrum.

Role of Ferroelectric Nanodomains in the Transport Properties of Perovskite Solar Cells.

Metropolis Monte Carlo simulations are used to construct minimal energy configurations by electrostatic coupling of rotating dipoles associated with each unit cell of a perovskite CH3NH3PbI3 crystal. Short-range antiferroelectric order is found, whereas at scales of 8-10 nm, we observe the formation of nanodomains, strongly influencing the electrostatics of the device. The models are coupled to drift-diffusion simulations to study the actual role of nanodomains in the I-V characteristics, especially focusing on charge separation and recombination losses.

IETS and quantum interference: propensity rules in the presence of an interference feature.

Destructive quantum interference in single molecule electronics is an intriguing phenomenon; however, distinguishing quantum interference effects from generically low transmission is not trivial. In this paper, we discuss how quantum interference effects in the transmission lead to either low current or a particular line shape in current-voltage curves, depending on the position of the interference feature. Second, we consider how inelastic electron tunneling spectroscopy can be used to probe the presence of an interference feature by identifying vibrational modes that are selectively suppressed when quantum interference effects dominate.

Model of a realistic InP surface quantum dot extrapolated from atomic force microscopy results.

We report on numerical simulations of a zincblende InP surface quantum dot (QD) on In₀.₄₈Ga₀.₅₂ buffer.

Strong overtones modes in inelastic electron tunneling spectroscopy with cross-conjugated molecules: a prediction from theory.

Cross-conjugated molecules are known to exhibit destructive quantum interference, a property that has recently received considerable attention in single-molecule electronics. Destructive quantum interference can be understood as an antiresonance in the elastic transmission near the Fermi energy and leading to suppressed levels of elastic current. In most theoretical studies, only the elastic contributions to the current are taken into account.

The Green's function density functional tight-binding (gDFTB) method for molecular electronic conduction.

A review is presented of the nonequilibrium Green's function (NEGF) method "gDFTB" for evaluating elastic and inelastic conduction through single molecules employing the density functional tight-binding (DFTB) electronic structure method. This focuses on the possible advantages that DFTB implementations of NEGF have over conventional methods based on density functional theory, including not only the ability to treat large irregular metal-molecule junctions with high nonequilibrium thermal distributions but perhaps also the ability to treat dispersive forces, bond breakage, and open-shell systems and to avoid large band lineup errors. New results are presented indicating that DFTB provides a useful depiction of simple gold-thiol interactions.

The symmetry of single-molecule conduction.

We introduce the conductance point group which defines the symmetry of single-molecule conduction within the nonequilibrium Green's function formalism. It is shown, either rigorously or to within a very good approximation, to correspond to a molecular-conductance point group defined purely in terms of the properties of the conducting molecule. This enables single-molecule conductivity to be described in terms of key qualitative chemical descriptors that are independent of the nature of the molecule-conductor interfaces.

Molecular origins of conduction channels observed in shot-noise measurements.

Measurements of shot noise from single molecules have indicated the presence of various conduction channels. We present three descriptions of these channels in molecular terms showing that the number of conduction channels is limited by bottlenecks in the molecule and that the channels can be linked to transmission through different junction states. We introduce molecular-conductance orbitals, which allow the transmission to be separated into contributions from individual orbitals and contributions from interference between pairs of orbitals.

Effectiveness of laparoscopic sleeve gastrectomy (first stage of biliopancreatic diversion with duodenal switch) on co-morbidities in super-obese high-risk patients.

Background: We evaluated laparoscopic sleeve gastrectomy (LSG) on major co-morbidities (hypertension, type 2 diabetes / impaired glucose tolerance, obstructive sleep apnea syndrome (OSAS) and on American Society of Anesthesiologists (ASA) operative risk score in high-risk super-obese patients undergoing two-stage laparoscopic biliopancreatic diversion with duodenal switch (LBPD-DS).

Methods: 41 super-obese high-risk patients (mean BMI 57.3+/-6.

Laparoscopic splenectomy in the management of benign and malignant hematologic diseases.

Objectives: The use of laparoscopy to treat malignant hematological diseases is not completely accepted. Our aim was to analyze operative and postoperative results of laparoscopic splenectomy performed for benign versus malignant hematological disorders.

Methods: Between 1994 and 2003, 76 consecutive patients underwent laparoscopic splenectomy.

Understanding the inelastic electron-tunneling spectra of alkanedithiols on gold.

We present results for a simulated inelastic electron-tunneling spectra (IETS) from calculations using the "gDFTB" code. The geometric and electronic structure is obtained from calculations using a local-basis density-functional scheme, and a nonequilibrium Green's function formalism is employed to deal with the transport aspects of the problem. The calculated spectrum of octanedithiol on gold(111) shows good agreement with experimental results and suggests further details in the assignment of such spectra.