Low Density Of States Research Articles

Relating band edge DOS occupancy statistics associated excited state electron entropy generation to free energy loss and intrinsic Voc deficit of solar cells.

Ever since the invention of solar cells, thermodynamics has been used to assess their performance limits and investigate advances in materials science and photovoltaic technology to reduce the gap between practical efficiencies and thermodynamic limits to photovoltaic energy conversion. By systematically addressing thermodynamic efficiency losses in current photovoltaics, ultrahigh efficiency photovoltaic can be expected. Currently, the non-radiative recombination of some ultrahigh efficient solar cells is almost completely suppressed, and the radiative recombination loss is the key to restricting the further improvement of device performance. This work relates the energy band edge electronic density of states (DOS) of semiconductor absorber and transport layer, excited/transfer state electronic entropy to thermodynamically inevitable energy loss during photoelectric conversion in solar cells. Through comprehensive theoretical analysis and device simulation, it is revealed why solar cells based on semiconductor material with a low DOS have higher Voc. On account of the basic limitations of thermodynamic laws on the energy conversion process, this work reveals a hidden variable that affects the photovoltaic performance and puts forward the band edge DOS engineering as a new dimension in performance optimization of solar cells apart from the traditional material and defect passivation engineering, etc. This work highlights the great importance of DOS engineering for further improving the performance of solar cell devices.

Tuning the lattice thermal conductivity of Sb2Te3 by Cr doping: a deep potential molecular dynamics study.

Element doping is a prominent method for reducing the lattice thermal conductivity and optimizing the thermoelectric performance of materials in the thermoelectric field. However, determination of the thermal conductivity of element-doped systems is a challenging task, especially when the elements are randomly doped. In this work, a first-principles based deep neural network potential (NNP) is developed to investigate the lattice thermal transport properties of Cr-doped Sb2Te3 using molecular dynamics simulations. Compared with pure Sb2Te3, the thermal conductivity of orderly Cr-doped Sb2Te3 with Cr atoms locating at specific atomic layer positions decreases slightly in the in-plane direction, but sharply in the out-of-plane direction. The decrease of the low frequency phonon density of states and the enhancement of phonon scattering near 2.5 THz are the primary reasons for the decrease in the thermal conductivity of Cr-doped Sb2Te3, while the decrease of phonon velocity due to band flattening is the reason for the sharp decrease of thermal conductivity in the out-of-plane direction. Moreover, the thermal conductivities of randomly Cr-doped Sb2Te3 with different Cr concentrations are also investigated using the NNP. It is found that the thermal conductivities in both the in-plane and out-of-plane directions are reduced by 76% and 80%, respectively, for Sb36Cr36Te108. Furthermore, the influence of different Cr dopant arrays on the thermal conductivity of Sb2Te3 is also predicted using the NNP. Our work provides a good example for predicting the thermal conductivity of element-doped systems using the NNP combined with molecular dynamics simulations.

Magnetic field enabled in situ control over the structure and dynamics of colloids interacting via SALR potentials.

Colloidal suspensions are an ideal model for studying crystallization, nucleation, and glass transition mechanisms, due to the precise control of interparticle interactions by changing the shape, charge, or volume fraction of particles. However, these tuning parameters offer insufficient active control over interparticle interactions and reconfigurability of assembled structures. Dynamic control over the interparticle interactions can be obtained through the application of external magnetic fields that are contactless and chemically inert. In this work, we demonstrate the dual nature of magnetic nanoparticle dispersions to program interactions between suspended nonmagnetic microspheres using an external magnetic field. The nanoparticle dispersion simultaneously behaves as a continuous magnetic medium at the microscale and a discrete medium composed of individual particles at the nanoscale. This enables control over a depletion attractive potential and the introduction of a magnetic repulsive potential, allowing a reversible transition of colloidal structures within a rich phase diagram by applying an external magnetic field. Active control over competing interactions allows us to create a model system encompassing a range of states, from large fractal clusters to low-density Wigner glass states. Monitoring the dynamics of colloidal particles reveals dynamic heterogeneity and a marked slowdown associated with approaching the Wigner glass state.

Allee effect considerations to support the spatial management of a sedentary marine species: the queen conch (

Context Density-dependent marine sedentary species exhibit heterogeneous distribution in response to biological needs and exploitation. Sustainable management requires consideration of factors influencing this distribution, including Allee effects and source–sink dynamics. Aim This study investigates the implications of the Allee effect and source–sink dynamics in the management of a sedentary species, queen conch (Aliger gigas) on the Pedro Bank Jamaica. Method We used spatial analysis of spatio-temporal survey data obtained over an 11-year period to determine spatial distribution and preferences. A depensation model along with knowledge of the connectivity of the population was used to model the Allee effect and define source and sink areas. Key results We found that mate-finding Allee effects and exploitation are major drivers of source–sink dynamics in this population. Sources (the effective spawning stock) consisted of less than 30% of total adult population and were being driven to a low-density stable state because of intensification of the Allee effect from high exploitation. Conclusions Management must explicitly consider Allee effects and source–sink dynamics to avoid overestimations of stock productivity and spatial mismatches of biological and management units. Stocks should be managed well above determined critical-density thresholds because stocks are unlikely to recover once they fall below them.

Correlation-driven electronic nematicity in the Dirac semimetal BaNiS2

In BaNiS2, a Dirac nodal line band structure exists within a two-dimensional Ni square lattice system, in which significant electronic correlation effects are anticipated. Using scanning tunneling microscopy (STM), we discover signs of correlated-electron behavior, namely electronic nematicity appearing as a pair of C2-symmetry striped patterns in the local density-of-states at ∼60 meV above the Fermi energy. In observations of quasiparticle interference, as well as identifying scattering between Dirac cones, we find that the striped patterns in real space stem from a lifting of degeneracy among electron pockets at the Brillouin zone boundary. We infer a momentum-dependent energy shift with d-form factor, which we model numerically within a density wave (DW) equation framework that considers spin-fluctuation-driven nematicity. This suggests an unusual mechanism driving the nematic instability, stemming from only a small perturbation to the Fermi surface, in a system with very low density of states at the Fermi energy. The Dirac points lie at nodes of the d-form factor and are almost unaffected by it. These results highlight BaNiS2 as a unique material in which Dirac electrons and symmetry-breaking electronic correlations coexist.

Using infodemiology metrics to assess patient demand for oculoplastic surgeons in the United States: insights from Google Search Trends

ABSTRACT Purpose The aim of this study was to compare state-by-state concentrations of oculoplastic surgeons against patient demand using Google Search Trends data, in order to identify potential areas of unmet need. Methods Google Trends data from 2004 to 2019 was collected to determine relative search volumes for the keyword “blepharoplasty” in each US state and the District of Columbia. Oculoplastic surgeon density was calculated by dividing the number of active American Society of Plastic and Reconstructive Surgeons members in 2019 by the State Census Bureau population estimates. Relative search volume values were divided by the local concentration of surgeons, and results were normalized between 0 and 100 to obtain a relative demand index for each state. Results Oculoplastic surgeon density varied widely across the country. The greatest concentrations of surgeons per 100,000 people were in D.C. (0.708) and Rhode Island (0.378), while the lowest were in Montana, New Mexico, North Dakota, South Dakota, and Wyoming (all 0). Relative search volumes were tightly distributed, ranging between 100 (Hawaii) and 45 (Vermont). The highest relative demand was found in low surgeon density states, such as Hawaii, Montana, New Mexico, North Dakota, South Dakota, and Wyoming. The lowest relative demand was found in DC (5), Rhode Island (12), and Utah (12). Conclusions Our results revealed vast disparities in surgical concentrations across the US and highlighted a number of areas with a relative undersupply of oculoplastic surgeons. Further investigation is necessary to examine the underlying factors impacting the supply and distribution of oculoplastic surgeons.

Photon density of states effect on Lamb shift in plasmas

A possible effect of the low photon density of states in plasma on the Lamb shift is analysed. It is found that because of a significant contribution of high-energy virtual photons to the Lamb shift, its modification in plasma does not exceed 1 % with respect to vacuum even at electron concentrations as high as 1022 cm–3. This behavior results from an asymptotic tendency of plasma properties to vacuum ones at an unlimited frequency growth.

The performance analysis of near-field thermophotovoltaics considering temperature dependence of indium tin oxide emitter

Near-field thermophotovoltaics (NFTPV) is of great interest because it can achieve both high power density and high efficiency at medium to high temperature. One key to efficient photovoltaic conversion is an emitter that can selectively emit spectrum. Indium tin oxide (ITO) is considered as the emitter for NFTPV because its plasma frequency is tunable by simple technical means. However, as a typical degenerate semiconductor, the temperature dependence of its dielectric constant is often neglected in studying the performance of NFTPV. Therefore, we propose a NFTPV model considering the temperature-dependent property of ITO. By changing the applied voltage V, the vacuum gap d, and the emitter temperature T1, the performance of NFTPV with and without temperature dependence is compared and shows a significant difference. We find that the heat flux is always underestimated as well as the optimal efficiency is overestimated when the temperature dependence is not considered. At d = 10 nm and T1 = 1000 K, the optimal efficiency is even overestimated by about 270%. Without considering the temperature dependence, the output power is overestimated at d < 20 nm, while the opposite is true at d ≥ 20 nm. We clarify the main reasons for these misjudgments at a small gap originate from the underestimation of the intensity of surface phonon polaritons (SPhPs) below the bandgap and the overestimation of the intensity of surface plasmon polaritons (SPPs) above the bandgap. The main factor affecting the two kinds of surface modes in near-field regime is that high temperatures lead to low electromagnetic local density of states (LDOS) for ITO emitter near the SPPs resonant frequency, but high LDOS in the ultralow frequencies. In a larger gap, it is attributed to the underestimation of the waveguide modes above the bandgap and the nearly-full-band propagation modes. Our work illustrates the necessity of considering temperature dependence, enriches the understanding of NFTPV, and facilitates the development of related applications.

Energetic stability and spatial inhomogeneity in the local electronic structure of relaxed twisted trilayer graphene

We study the energetic stability and the local electronic structure of the general twisted trilayer graphene (TTG) with the top and bottom layers rotated with respect to the middle layer respectively by $\theta$ and $\theta'$. Approximate supercells of the moir\'{e}-of-moir\'{e} superlattices with $\theta$ and $\theta^{\prime}$ within $1^{\circ}\sim 2^{\circ}$ are established to describe the structural and electronic properties of relaxed TTG with the periodic boundary condition. Full relaxation demonstrates that the commensurate TTG with $\theta=\theta^{\prime}$ has the local minimum total energy ($E_{tol}$) at a fixed $\theta$, while $E_{tol}$ first reaches a local maximum and begins to drop with decreasing $\theta^{\prime}$ for $\theta^{\prime} < \theta$. Some regions exhibit enhanced in-plane relaxation in the top and bottom layers but suppressed relaxation in the middle layer and form a hexagonal network with the moir\'{e}-of-moir\'{e} length scale. The stacking configurations with the atoms in the three layers vertically aligned at the origin of the relaxed TTG supercells at $\theta$ around $1.6^{\circ}$ and $\theta^{\prime}$ around $1.4^{\circ}$ have a high density of states (DOS) near the Fermi level ($E_F$), which can reach that of the mirror symmetric TTG with equal twist angles of about $1.7^{\circ}$. In contrast, some other stackings can have rather low DOS around $E_F$. The significant stacking dependence of DOS for some TTG supercells demonstrates that the local electronic structure of TTG can exhibit strong spatial inhomogeneity when the twist angles are slightly away from those of the small supercells with large variations of DOS among different stackings. Moreover, the structural relaxation of TTG plays a crucial role in the high DOS and its strong stacking dependence.

Electron transport mechanism in colloidal SnO2 nanoparticle films and its implications for quantum-dot light-emitting diodes

Charge transport behavior in SnO2 nanoparticle (NP) films is rather crucial to the optoelectronic devices. Temperature-dependent electrical results show that the electron transport in SnO2 NP films is dominated by the Mott variable-range hopping processes, i.e. the electrons are transported between different NPs through surface states rather than the conduction band of the nanocrystals, which is identical to the commonly used ZnO NP solids. Compared with ZnO, SnO2 films exhibit similar electron mobility but lower density of states (DOS). Therefore, we deduce that the low DOS in the SnO2 NP films should be the key factor limiting the device performance in compared with the ZnO as reported in most of the quantum-dot light-emitting diodes (QLEDs). Our work sheds light on optimizing SnO2 NP films for QLEDs. Moreover, we believe that the SnO2 remains a desirable candidate as the electron transport material for the QLEDs due to its excellent physicochemical stability.

Amplified emission and lasing in photonic time crystals.

Photonic time crystals (PTCs), materials with a dielectric permittivity that is modulated periodically in time, offer new concepts in light manipulation. We study theoretically the emission of light from a radiation source placed inside a PTC and find that radiation corresponding to the momentum bandgap is exponentially amplified, whether initiated by a macroscopic source, an atom, or vacuum fluctuations, drawing the amplification energy from the modulation. The radiation linewidth becomes narrower with time, eventually becoming monochromatic in the middle of the bandgap, which enables us to propose the concept of nonresonant tunable PTC laser. Finally, we find that the spontaneous decay rate of an atom embedded in a PTC vanishes at the band edge because of the low density of photonic states.

Spectral characteristics and electronic structure of semimetallic ScSb and YSb

A detailed study of the optical properties of semi-metallic compounds ScSb and YSb has been performed by means of ellipsometric method and first-principle TB-LMTO-ASA calculations. The calculated optical conductivity spectra are compared with experimental ones showing qualitative agreement. The low level of intra- and interband absorption in the infrared range of the spectrum is consistent with band calculations, which indicate a low density of electron states near the Fermi energy.

On the long decay time of the 7F5 level of Tb3+

The Tb3+7F5 level has been shown to play an important role in the energy transfer involving this ion. This is due to the unusually long lifetime of this level which is commonly disregarded when treating energy transfer involving this ion. In this work, we give a rationalized explanation for this phenomenon. It is discussed that the 7F5 level can be fed by an energy transfer process and the population is trapped due to a low decay rate, which is related to the energy gap to the 7F6 ground level in the far infrared and the correspondingly low photon density of states. This is in agreement with experimental data from direct measurements of the total decay lifetimes of the 7F5 level and the emission quantum yields when this level is considered as an energy acceptor in Tb3+ compounds.

Entropy Scaling for Viscosity of Pure Lennard-Jones Fluids and Their Binary Mixtures

In this work, entropy scaling approaches for viscosity of pure Lennard-Jones (LJ) fluids and their mixtures have been investigated. To do so, we have employed reliable viscosity database available in literature for the pure LJ fluids, and performed the molecular dynamics simulation to generate viscosity database over a wide range of thermodynamic condition for the LJ mixture fluids. It has shown that for the pure LJ fluid, the entropy scaling approaches using the macroscopic properties for the reduction of viscosity yield noticeably better collapse of data than the one using the zero-density viscosity in the dense fluid region. In addition, we have developed viscosity correlations based on these approaches. It has been obtained that the correlations of macroscopic properties approaches predict the pure LJ fluid viscosity with average absolute deviation of around 4% more coming from the low-density states, whereas it is of around 8.50% more coming from the dense states for the other one. Finally, the viscosity correlations have been applied to the LJ mixtures. Interestingly, the correlations of macroscopic properties approaches are able to provide good estimations for all mixtures studied. However, it deteriorates for the viscosity of dense mixtures when the other employed.

Yttrium‐doped Sn3O4 two‐dimensional electron transport material for perovskite solar cells with efficiency over 23%

AbstractThe two‐dimensional mixed‐valence tin oxide Sn3O4 with a low defect density of states and suitable energy band structure has shown great potential as the electron transport layer (ETL) for perovskite solar cells (PSCs). However, the electron density of Sn3O4 is low and should be modulated to demonstrate its application for more efficient PSCs. Here, Sn3O4 is doped with yttrium (Y) to optimize its electronic properties and improve the performance of PSCs based on Sn3O4 ETL. The conductivity of Sn3O4 is improved from 6.6 × 10−4 to 2.05 × 10−3 S m−1 with increased electron density by Y doping. Meanwhile, the Fermi level of Sn3O4 is also upshifted after doping. The increased electron density and upshifted Fermi level enhance the electron extraction capability of Sn3O4 and built‐in potential in PSCs. By doping the Sn3O4 ETL with Y, the power conversion efficiency of PSCs is successfully boosted to 23.05%.image

Decompression dynamics of high density amorphous ice above and below the liquid-liquid critical point

We investigate via numerical simulations of the TIP4P/Ice model the isothermal decompression of high density amorphous ice (HDA) mirroring the experimental protocol followed by recent experiments [H. K. Kim, et al., Science 370 , 978, 2020]. Taking advantage of the recent determination of the TIP4P/Ice liquid-liquid critical point, we decompress our samples at temperatures both below and above the critical temperature. We follow the crossover between high and low density, complementing the time evolution of the structure factor with the time evolution of microscopic descriptors. Our results support the interpretation of the experimental pathway as a structural change from high to low density states through a first-order transition line. In the simulation study we do not observe nucleation events, but rather a spinodal-like non-equilibrium transformation, which offers an explanation on why the decompression process even at sub-critical temperatures may result in a continuous time evolution of the density and of the microscopic descriptors.

Molecular-scale insights on structure-efficiency relationship of silane-based waterproofing agents

While silane-based materials are widely used to improve the impermeability of concrete, the nano-scale interaction mechanismson the interfaces are still unclear. In this study, molecular simulations were employed to explore the inhibiting behaviors of three commonly-used alkoxysilanes(Aminopropyltriethoxysilane (APTES), ethyltriethoxysilane (ETES), and γ-(methylpropyleneoxy)propyltrimethoxysilane (MPS)) on the transport of aggressive ions and water throughout a nano-pore of calcium silicate hydrates (C-S-H). The results show that during the penetration process of silane-doped systems, both water and ions exhibit a low density state and a longitudinal stratification owing to the barrier effect of silanes, where the ranking was APTES < ETES < MPS. The order can be explained by differences in the molecular structure. MPS demonstrates the best inhibiting effect because it possesses maximum hydrophobic groups. For APTES and ETES with similar molecular weights, the drag effect of amino groups, results in water molecules being more likely to penetrate the interface area and escape, causing a weaker inhibiting effect in APTES. The mechanisms interpreted in this work may shed new light on the molecular design of silane-based waterproofing agents.

Oxygen NMR of high-density and low-density amorphous ice.

Using oxygen-17 as a nuclear probe, spin relaxometry was applied to study the high-density and low-density states of amorphous ice, covering temperatures below and somewhat above their glass transitions. These findings are put in perspective with results from deuteron nuclear magnetic resonance and with calculations based on dielectrically detected correlation times. This comparison reveals the presence of a wide distribution of correlation times. Furthermore, oxygen-17 central-transition echo spectra were recorded for wide ranges of temperature and pulse spacing. The spectra cannot be described by a single set of quadrupolar parameters, suggesting a distribution of H-O-H opening angles that is broader for high-density than for low-density amorphous ice. Simulations of the pulse separation dependent spin-echo spectra for various scenarios demonstrate that a small-step frequency diffusion process, assigned to the presence of homonuclear oxygen-oxygen interactions, determines the shape evolution of the pulse-separation-dependent spectra.

Symmetrical Acceptor–Donor–Acceptor Molecule as a Versatile Defect Passivation Agent toward Efficient FA0.85MA0.15PbI3 Perovskite Solar Cells

AbstractDespite the swift development in perovskite solar cells (PSCs), suppressing the ion defects in the perovskite bulk and further extending the long‐lasting stability of the cells remain the concerned issues that are yet to be solved. Here, a symmetrical organic acceptor−donor−acceptor (A−D−A) molecule with the core architecture of indaceno[1,2‐b:5,6‐b']dithiophene (IDT) and bilateral arms of oxindole, named IDT‐OD, as a versatile defect passivation agent, is adopted to inactivate the nonradiative recombination sites in the perovskite absorber. The S element in the IDT unit and carbonyl group CO in the bilateral acceptor unit as the Lewis‐base contributes to the passivation sites that are the under‐coordinated Pb2+ cation defects and the N−H group in oxindole unit interacts with halide dangling bonds. The molecular structure with its symmetrical double arms assists the formation of a superior perovskite layer with enlarged grain size, smooth surface topography, hydrophobic property, and low density of defect state. Consequently, the corresponding PSCs with the proper IDT‐OD additive yield a remarkable increase in efficiency from 22.77% to 24.04%, along with excellent long‐term environmental and thermal stabilities. This study offers a propitious approach for ionic defect passivation engineering toward high‐performance PSCs.

Vortex-pair annihilation in arrays of photon cavities

We investigate theoretically the evolution of the vortex number in an array of photon condensates that is brought from an incoherent low-density state to a coherent high-density state by a sudden change in the pumping laser intensity. We analyze how the recombination of vortices and antivortices depends on the system parameters such as the coefficients for emission and absorption of photons by the dye molecules, the rate of tunneling between the cavities, the photon loss rate and the number of photons in the condensate.