Glass-like Transport Dominates Ultralow Lattice Thermal Conductivity in Modular Crystalline Bi4O4SeCl2.

Crystalline Bi4O4SeCl2 exhibits record-low 0.1 W/mK lattice thermal conductivity (κL), 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 κL in crystalline and pellet Bi4O4SeCl2 forms. The model includes intrinsic three- and four-phonon scattering processes and extrinsic defect and extended defect scattering contributing to the phonon lifetime, as well as temperature-dependent interatomic force constants linked to phonon frequency shifts and anharmonicity. Bi4O4SeCl2 displays strongly anisotropic complex crystal behavior with dominant glass-like transport along the cross-plane direction. The uncovered origin of κL underscores an intrinsic approach for designing extremely low κL materials.

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. The PHONON tool is a module developed as part of the Density-Functional Tight-Binding (DFTB) software platform. We discuss the anisotropic phonon band structure of selected puckered two-dimensional materials, helical and horizontal doping effects in the phonon thermal conductivity of boron nitride-carbon heteronanotubes, phonon filtering in molecular junctions, and a novel computational methodology to investigate time-dependent phonon transport at the atomistic level. These examples illustrate the versatility of our implementation of phonon transport in combination with density functional-based methods to address specific nanoscale functionalities, thus potentially allowing for designing novel thermal devices.

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. Yet, the details of the molecular structure and the topology of the electronic states involved in the charge transport mechanism affect dramatically the intermolecular electronic coupling even in amorphous materials. In this work, we link the morphology of aggregates, in terms of intermolecular configurations, as obtained from atomistic molecular dynamics, to the distribution of diabatic electronic couplings and charge transfer energies, computed by constrained density functional theory simulations. In particular, we focus on aggregates of an organometallic system with multidentate ligands, the iridium complex fac-tris(1,3-diphenyl-benzimidazolin-2-ylidene-C,C2')iridium(iii) (DPBIC), commonly used in OLEDs as host transporter and emitter. Despite the quasi-spherical symmetry of the molecule, our simulations suggest a strong correlation between intermolecular orientation and electronic coupling, indicating a strong impact of the mutual orientation of molecules on charge transport in bulk molecular materials.

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. Multi-scale 2D simulations of the carrier transport inside the multiple QD AR have revealed a non-trivial pathway for carrier injection into the dots. Electrons and holes are found to penetrate deep into the multi-layer AR through the gaps between individual QDs and get into the dots via their side edges rather than via top and bottom interfaces. This enables a more homogeneous carrier distribution among the dots situated in different layers than among the laterally uniform quantum well (QWs) in the MQW AR. As a result, a lower turn-on voltage is predicted for QD-based LEDs, as compared to MQW ones. Simulations did not show any remarkable difference in the efficiencies of the MQW and QD-based LEDs, if the same recombination coefficients are utilized, i.e. a similar crystal quality of both types of LED structures is assumed. Measurements of the current-voltage characteristics of LEDs with both kinds of the AR have shown their close similarity, in contrast to theoretical predictions. This implies the conventional assumption of laterally uniform QWs not to be likely an adequate approximation for the carrier transport in MQW LED structures. Optical characterization of MQW and QD-based LEDs has demonstrated that the later ones exhibit a higher efficiency, which could be attributed to better crystal quality of the grown QD-based structures. The difference in the crystal quality explains the recently observed correlation between the growth pressure of LED structures and their efficiency and should be taken into account while further comparing performances of MQW and QD-based LEDs. In contrast to experimental results, our simulations did not reveal any advantages of using QD-based ARs over the MQW ones, if the same recombination constants are assumed for both cases. This fact demonstrates importance of accounting for growth-dependent factors, like crystal quality, which may limit the device performance. Nevertheless, a more uniform carrier injection into multi-layer QD ARs predicted by modeling may serve as the basis for further improvement of LED efficiency by lowering carrier density in individual QDs and, hence, suppressing the Auger recombination losses.

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. the shape and the average In content of the QDs, has been directly derived from experimental data on out-of-plane strain distribution obtained from the geometric-phase analysis of a high-resolution transmission electron microscopy image of an LED structure grown by metalorganic vapor-phase epitaxy. Using continuum [Formula: see text] calculations, we have studied first the lateral and full electromechanical coupling between the QDs in the active region and its impact on the emission spectrum of a single QD located in the center of the region. Our simulations demonstrate the spectrum to be weakly affected by the coupling despite the strong common strain field induced in the QD active region. Then we analyzed the effect of vertical coupling between vertically stacked QDs as a function of the interdot distance. We have found that QCSE gives rise to a blue-shift of the overall emission spectrum when the interdot distance becomes small enough. Finally, we compared the theoretical spectrum obtained from simulation of the entire active region with an experimental electroluminescence (EL) spectrum. While the theoretical peak emission wavelength of the selected central QD corresponded well to that of the EL spectrum, the width of the latter one was determined by the scatter in the structures of various QDs located in the active region. Good agreement between the simulations and experiment achieved as a whole validates our model based on realistic structure of the QD active region and demonstrates advantages of the applied approach.

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 MoS2 monolayers. Considering unstretched contact regions, we show that the electronic bandgap displays an anomalous behavior and the thermal conductance continuously decreases after increasing the strain level in the scattering region. However, when the whole system (contact and device regions) is homogeneously strained, the bandgap for hBN and MoS2 monolayers decreases, while for phosphorene it first increases and then tends to zero with larger strain levels. Additionally, the thermal conductance shows specific strain dependence for each of the studied 2D materials. These effects can be tuned by modifying the strain level in the stretched contact regions.

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. We demonstrate that holes and electrons separate into different nanodomains following different current pathways. From our analysis we can conclude that even antiferroelectric ordering can ultimately lead to an increase of photoconversion efficiencies thanks to a decrease of trap-assisted recombination losses and the formation of good current percolation patterns along domain edges.

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. This range, however, suffers from a systematic drop in efficiency, known as the "green gap," whose physical origin has not been understood completely so far. In this work, we show by atomistic simulations that a consistent part of the green gap in c-plane InGaN/GaN-based light emitting diodes may be attributed to a decrease in the radiative recombination coefficient with increasing indium content due to random fluctuations of the indium concentration naturally present in any InGaN alloy.

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. Under bias the current and the molecule exchange energy, which typically results in heating of the molecule. However, the opposite process is also possible, where energy is extracted from the molecule by the tunneling current. Designing a molecular "heat sink" where a particular vibrational mode funnels heat out of the molecule and into the leads would be very desirable. It is even possible to imagine how the vibrational energy of the other vibrational modes could be funneled into the "cooling mode," given the right molecular design. Previous efforts to understand heating and cooling mechanisms in single molecule junctions have primarily been concerned with small models, where it is unclear which molecular systems they correspond to. In this paper, our focus is on suppressing heating and obtaining current-induced cooling in certain vibrational modes. Strategies for cooling vibrational modes in single-molecule junctions are presented, together with atomistic calculations based on those strategies. Cooling and reduced heating are observed for two different cooling schemes in calculations of atomistic single-molecule junctions.

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. Our model is strictly based on experimental structures, since we extrapolated a three-dimensional dot directly by atomic force microscopy results. Continuum electromechanical, [Formula: see text] bandstructure and optical calculations are presented for this realistic structure, together with benchmark calculations for a lens-shape QD with the same radius and height of the extrapolated dot. Interesting similarities and differences are shown by comparing the results obtained with the two different structures, leading to the conclusion that the use of a more realistic structure can provide significant improvements in the modeling of QDs fact, the remarkable splitting for the electron p-like levels of the extrapolated dot seems to prove that a realistic experimental structure can reproduce the right symmetry and a correct splitting usually given by atomistic calculations even within the multiband [Formula: see text] approach. Moreover, the energy levels and the symmetry of the holes are strongly dependent on the shape of the dot. In particular, as far as we know, their wave function symmetries do not seem to resemble to any results previously obtained with simulations of zincblende ideal structures, such as lenses or truncated pyramids. The magnitude of the oscillator strengths is also strongly dependent on the shape of the dot, showing a lower intensity for the extrapolated dot, especially for the transition between the electrons and holes ground state, as a result of a relevant reduction of the wave functions overlap. We also compare an experimental photoluminescence spectrum measured on an homogeneous sample containing about 60 dots with a numerical ensemble average derived from single dot calculations. The broader energy range of the numerical spectrum motivated us to perform further verifications, which have clarified some aspects of the experimental results and helped us to develop a suitable model for the spectrum, by assuming a not equiprobable weight from each dot, a model which is extremely consistent with the experimental data.

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. That is, we expand the understanding of propensity rules in inelastic electron tunneling spectroscopy to molecules with destructive quantum interference.

Predicting the Lattice Thermal Conductivity in Nitride Perovskite LaWN3 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 LaWN3 displays strong anharmonic lattice dynamics manifested into a low lattice thermal conductivity (κL ) and a non-standard κL ∝T-0.491 dependence. At high T, the departure from the standard κL ∝T-1 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 κL =2.98 W mK-1 arises mainly from the conventional particle-like propagation of phonons, there is also a significant atypical wave-like phonon tunneling effect, leading to a 20% glass-like heat transport contribution. The phonon broadening effect lowers the particle-like contribution but increases the glass-like one. Upon T increase, the glass-like contribution increases and dominates above T = 850 K. Overall, the low κL with a weak T-dependence points to a new utility for LaWN3 in energy technology applications, and motivates synthesis and exploration of nitride perovskites.

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. A single InGaN island with the size of ten nanometers and nonuniform indium content distribution is analyzed. A number of two- and three-dimensional models of the quantum dot are derived from the experimental image by a special numerical algorithm, which enables electromechanical, continuum k→·p→, and empirical tight-binding calculations, including emission spectra prediction. Effectiveness of continuous and atomistic approaches are compared, and the impact of InGaN composition fluctuations on the ground-state electron and hole wave functions and quantum dot emission spectrum is analyzed in detail. Finally, comparison of the predicted spectrum with the experimental one is performed to assess the applicability of various simulation approaches.

Anisotropic Phononic and Electronic Thermal Transport in BeN4.

Beryllium polynitride (BeN4) has been recently synthesized under high-pressure conditions [Bykov et al. Phys. Rev. Lett. 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 ab initio calculations. The bonding anisotropy (impacting the phonon and electron group velocities) and bonding anharmonicity (captured through three- and four-phonon scatterings) are reflected in the strong anisotropy of both phononic and electronic components of the thermal conductivity. Moreover, the pressure-driven evolution of the interlayer Be-N bonding, from partially covalent (under high-pressure synthesis conditions) to van der Waals (under ambient pressure), drives a largely interlayer thermal conductivity. These findings highlight an alternative strategy for achieving directional control of the thermal transport in synthetic materials.

Four-phonon and electron-phonon scattering effects on thermal properties in two-dimensional 2H-TaS2.

Thermal transport characteristics of monolayer trigonal prismatic tantalum disulfide (2H-TaS2) 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 (κph). With the further inclusion of phonon-electron scattering, κph reduces to 1.78 W mK-1. Nevertheless, the total thermal conductivity (κtotal) of 7.82 W mK-1 is dominated by the electronic thermal conductivity (κe) of 6.04 W mK-1. Due to the electron-phonon coupling, κe differs from the typical estimation based on the Wiedemann-Franz law with a constant Sommerfeld value. This work provides new insights into the physical mechanisms for thermal transport in metallic 2D systems with strong anharmonic and electron-phonon coupling effects. The phonon scattering beyond three-phonon (3ph) scattering and even κe are typically overlooked in computations, and the constant Sommerfeld value is widely used for separating κe and κph from the experimental thermal conductivity. These conclusions have implications for both the computational and experimental measurements of the thermal properties of transition metal dichalcogenides.

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 (κp ) along linear GBs comprising periodically repeating pentagon-heptagon dislocations. Green's function calculations and spectral energy density analysis indicate that the origin of the κp reduction is hidden in the periodic GB strain field, which behaves as a reflective diffraction grating with either diffuse or specular phonon reflections, and represents a source of anharmonic phonon-phonon scattering. The non-monotonic dependence with dislocation density of κp uncovered here is unaccounted for by the classical Klemens theory. It can help identify GB structures that can best preserve the integrity of the phononic transport.

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

The study of MAPbI3 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 MAPbI3 on the system's properties employing a tight binding (TB) approach. The particle swarm optimization has been used to obtain a consistent set of TB parameters, where the target properties of the system have been derived by first-principles calculations. The theoretical results have been compared with the measured PL spectra for a temperature range going from 10 to 100 K. Our model effectively captures the carriers' localization phenomenon induced by the presence of residual tetragonal nanodomains and demonstrates that the assumption of phase nanosegregation can explain the low-energy features in the PL spectra of MAPbI3.

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. Namely, nanoscale molecular aggregation phenomena at the organic/electrode interface were first assessed by molecular dynamics simulations, mimicking different processing conditions, and followed by density functional theory calculations of the electronic coupling between molecular levels and the manifold of electrode states involved in the charge injection process. The correlation between structural parameters and electronic coupling suggests a significant role of specific molecule/electrode configurations on charge transport processes at the interface, resulting in a broad distribution of charge injection rates, and highlights the link between molecular structure, nanoscale aggregation and processing in the realization of heterointerfaces for efficient charge injection in organic electronic devices.

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+/-6.5 kg/m(2), age 44.6+/-9.7 years) were entered into a prospective study (BMI > or = 60, or BMI > or = 50 with at least two severe co-morbidities, no Prader-Willi syndrome, no conversion, minimum follow-up 12 months). 9 patients had BMI > or = 60. 17 patients (41.4%) had OSAS on C-PAP therapy. In 10 patients, at least one intragastric balloon had been positioned and 4 had undergone laparoscopic adjustable gastric banding, all with unsatisfactory results. At surgery, 41.5% were classified ASA 4 and 58.5% as ASA 3 (mean ASA score 3.4+/-0.5). Patients underwent evaluation every 3 months postoperatively and were restaged at 12 months and/or before the second step. RESULTS: 60% of major co-morbidities were cured and 24% improved. Average BMI after 6 and 12 months was 44.5+/-8.1 and 40.8+/-8.5 respectively (mean follow-up 22.2+/-7.1 months). After 12 months, 57.8% of the patients were co-morbidity-free and 31.5% had only one major co-morbid condition. At restaging, 20% of patients were still classified as ASA score 4 (OSAS on C-PAP therapy). 3 patients showed BMI <30 and were co-morbidity-free 12 months after LSG. CONCLUSIONS: LSG represents a safe and effective procedure to achieve marked weight loss as well as significant reduction of major obesity-related co-morbidities. The procedure reduced the operative risk (ASA score) in super-obese patients undergoing two-stage LBPD-DS.

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. The first 38 cases were performed by using an anterior approach, whereas in the remaining 38 cases a semilateral position was used. RESULTS: Baseline characteristics showed that patients with malignant diseases were significantly older (56.9 vs 32.6 years, P < 0.001). Seventy-two (94.7%) procedures were completed laparoscopically. Conversion was required in 4 cases (5.2%). Mean operative time was 138.5 minutes for benign and 151.0 minutes for malignant diseases, (P > 0.05, ns). The hand-assisted technique was used in 3 patients with massive splenomegaly. Pathologic features showed that spleen volume was higher in patients with malignant diseases (mean interpole diameter 18.1 cm vs 13.7 cm, P < 0.001). Massive splenomegaly (interpole diameter over 20 cm, weight over 1000 g) was present in 13 patients (17.1%); 9 had malignant diseases. Overall perioperative mortality was 1.3% and major postoperative complications occurred in 6 patients (7.8%). Postoperative splenoportal partial thrombosis was identified in 9.7% of patients. CONCLUSIONS: Laparoscopic splenectomy is a well-accepted, less-invasive procedure for hematological disorders. Neoplastic diseases or splenomegaly, or both, do not seem to limit the indications for a minimally invasive approach after the learning curve.