Electron Localization Effects Research Articles

Doping strategies for β-Ga2O3 based on high-throughput first-principles calculations

β-Ga2O3 is a promising material for advanced optoelectronic semiconductors due to its wide bandgap and high breakdown voltage. However, achieving efficient p-type doping remains a challenge. This investigation employs high-throughput (HT) first-principles calculations to explore doping effects in β-Ga2O3 systematically. The results of HT first-principles calculations indicate that changes in the Fermi levels (EF) can reflect the type of dopant elements. We systematically evaluated nine candidate dopants using GGA+U calculations. Through analysis of the (0/−1) transition levels, the ability to form shallow acceptors gradually decreases in the order of Zn, Mg, Cu, Hg, and Ag. Interestingly, due to the GGA+U optimization process considering electron correlation and localization effects, Mg-doped β-Ga2O3 exhibits an impurity level near the conduction band minimum (CBM), contributed by Mg2p and O2p orbitals. Mulliken population analysis reveals that Cu exhibits the highest charge density after doping compared to Hg, Zn, and Mg. Additionally, Ag-doped β-Ga2O3 shows potential for UV device applications within the 200 nm to 280 nm spectrum. The insights from this study delineate effective p-type doping strategies for β-Ga2O3, contributing to the advancement of optoelectronic device development involving semiconductors.

Breaking the scaling relationship via lattice expansion of Ag for CO2 electroreduction over a wide potential window

AbstractThe achievement of excellent catalytic activity for electricity‐driven CO2 reduction over a wide potential window is significant for the mature applications. However, the efficient potential range is always limited by the dilemma in CO2 activation and product desorption at different potentials, due to the scaling relationships of adsorption energy. Herein, we developed a nanostructured Ag‐CO3 electrocatalyst featuring metallic Ag with Ag(220) orientation on the surface and Ag2CO3 beneath. The interactions between Ag and Ag2CO3 lead to lattice expansion on the surface of Ag, resulting in electron localization and accordingly enhanced CO2 activation to *COOH on Ag. Furthermore, this lattice expansion induces a change in the *CO adsorption geometry from a bridge mode to a linear mode, which compensates the promotion effect of electron localization on *CO adsorption energy. As a result, the scaling relationship between *COOH and *CO adsorption energy was disrupted, leading to enhanced *COOH adsorption and inferior *CO adsorption. Consequently, the Ag‐CO3 catalyst exhibited a high FECO (>80%) over a robust potential window of −0.8 to −1.8 V (vs. RHE), with a maximum FECO of 95% at −1.2 V (vs. RHE). The mechanism described herein offers a universal design principle for the development of high‐efficiency CO2 electroreduction catalysts that operate effectively within a broad potential range.

Terahertz optoelectronic properties of synthetic single crystal diamond

A systematic investigation is undertaken for studying the optoelectronic properties of single crystal diamond (SCD) grown by microwave plasma chemical vapor deposition (MPCVD). It is indicated that, without intentional doping and surface treatment during the sample growth, the terahertz (THz) optical conduction in SCD is mainly affected by surface H-terminations, -OH-, O- and N-based functional groups. By using THz time-domain spectroscopy (TDS), we measure the transmittance, the complex dielectric constant and optical conductivity σ(ω) of SCD. We find that SCD does not show typical semiconductor characteristics in THz regime, where σ(ω) cannot be described rightly by the conventional Drude formula. Via fitting the real and imaginary parts of σ(ω) to the Drude-Smith formula, the ratio of the average carrier density to the effective electron mass γ = ne/m*, the electronic relaxation time τ and the electronic backscattering or localization factor can be determined optically. The temperature dependence of these parameters is examined. From the temperature dependence of γ, a metallic to semiconductor transition is observed at about T = 10 K. The temperature dependence of τ is mainly induced by electron coupling with acoustic-phonons and there is a significant effect of photon-induced electron backscattering or localization in SCD. This work demonstrates that THz TDS is a powerful technique in studying SCD which contains H-, N- and O-based bonds and has low electron density and high dc resistivity. The results obtained from this study can benefit us to gain an in-depth understanding of SCD and may provide new guidance for the application of SCD as electronic, optical and optoelectronic materials.

Deciphering the Effects of Molecular Dipole Moments on the Photovoltaic Performance of Organic Solar Cells.

The dielectronic constant of organic semiconductor materials is directly related to its molecule dipole moment, which can be used to guide the design of high-performance organic photovoltaic materials. Herein, two isomeric small molecule acceptors, ANDT-2F and CNDT-2F, are designed and synthesized by using the electron localization effect of alkoxy in different positions of naphthalene. It is found that the axisymmetric ANDT-2F exhibits a larger dipole moment, which can improve exciton dissociation and charge generation efficiencies due to the strong intramolecular charge transfer effect, resulting in the higher photovoltaic performance of devices. Moreover, PBDB-T:ANDT-2F blend film exhibits larger and more balanced hole and electron mobility as well as nanoscale phase separation due to the favorable miscibility. As a result, the optimized device based on axisymmetric ANDT-2F shows a JSC of 21.30mA cm-2 , an FF of 66.21%, and a power conversion energy of 12.13%, higher than that of centrosymmetric CNDT-2F-based device. This work provides important implications for designing and synthesizing efficient organic photovoltaic materials by tuning their dipole moment.

A generalized Drude–Smith model for ultrafast pump-and-probe experiments and its application for studying of electronic dynamic properties of polycrystalline diamond

In this work, we generalize the Drude–Smith model (DSM) proposed for optical conductivity in frequency-domain to the case of ultrafast pump-and-probe experiments, where the results are measured in time-domain. The generalized DSM (GDSM) considers the optically induced electronic backscattering or localization effect, and can be utilized for studying the electronic dynamics induced by pulsed radiation field in time-domain. By applying the optical-pump and terahertz-probe (OPTP) technique via normal reflection measurement, we study the electronic dynamic properties of polycrystalline diamond (PCD) on Si substrate and examine the validity of the proposed GDSM. We find that this model can lead to a better fitting between the experimental and theoretical results and can reasonably reflect the information about time resolution of the measurement system and the energy relaxation time for free carriers in PCD. For PCD subjected to 800 nm wavelength fs laser irradiation, the effect of carrier backscattering decreases and the electronic energy relaxation time increases with increasing pump radiation fluence. We hope that the GDSM from this work can be helpful for understanding the electronic dynamics in electronic and optoelectronic materials measured by, e.g., ultrafast pump-and-probe experiments.

Tunable nonradiative recombination dynamics and charge injection of graphene quantum dots for energy conversion applications by controllable functionalization

Understanding the dependence of optoelectronic properties and charge transfer processes on the specified functionalization pattern of the graphene quantum dot (GQD) surface is key to deciphering the photovoltaic and photocatalytic mechanisms. In the present work, the photophysical properties and energy conversion efficiency of OCH3-functionalized GQDs are investigated using first-principle calculations. Furthermore, the nonradiative electron–hole recombination dynamics is analyzed using Fermi’s golden rule. Our results show that the OCH3 group has different binding energies on the GQD surface depending on its binding configuration and forms different oxidation patterns of the GQD controlled by the reaction temperature. Both basal and edge oxidation reduce the bandgaps of GQDs due to the electron localization effect, resulting in differing chemical stability. In addition, basal oxidation provides more degrees of freedom with which to tune the wavelengths and oscillator strengths of the low absorption peaks. Although edge oxidation provides a stronger electron-injection driving force from the GQDs into the TiO2 and facilitates charge separation, it also leads to faster nonradiative recombination, which reduces charge separation. Overall, our work reveals a detailed mechanistic picture of energy conversion in oxidized GQDs.

Intrinsic instability to martensite phases in ferromagnetic shape memory alloy Ni2MnGa : Quasiparticle self-consistent GW investigation

We investigated the electronic structure of a ferromagnetic shape memory alloy ${\mathrm{Ni}}_{2}\mathrm{MnGa}$ utilizing an advanced approach, quasiparticle self-consistent $GW$, which takes account of electron localization effects without empirical parameters. The Ni ${e}_{g}$ orbitals in the cubic phase, which lead to martensite phase transition, were found to locate on the Fermi level, implying a clear definitive origin of band Jahn-Teller (JT) effect in comparison with the results obtained by the density functional approach of generalized gradient approximation. From the analysis of generalized susceptibility in the cubic phase, the instabilities responsible for the modulated structures of 10M, 14M, and 6M were found to be an intrinsic property in the electronic states. These states may stabilize the modulated one, accompanied by tetragonal local JT distortions. Their property of Fermi surface nesting sensitively depends on a subtle change in the magnetic moment, corresponding to the experimental fact that the modulated structure appears depending on temperature and the composition of the magnetic element. The secondary nesting vector along [110] direction was discussed in relation to a modulation alignment of the nanotwin boundary.

Two-dimensional charge localization at the perovskite oxide interface

The effects of atomic-scale disorder and charge (de)localization hold significant importance, and they provide essential insights to unravel the role that strong and weak correlations play in condensed matter systems. In the case of perovskite oxide heterostructures, while disorders introduced via various external stimuli have strong influences over the (de)localization of interfacial two-dimensional (2D) electrons, these factors alone could not fully account for the system's charge dynamics where interfacial hybridization holds very strong influence. Here, we determine that the displaced 2D free electrons have been localized in the specific hybridized states of the LaAlO3/SrTiO3 interface. This experimental study combines both transport measurements and temperature-dependent x-ray absorption spectroscopy and suggests that the localization of 2D electrons can be induced via temperature reduction or ionic liquid gating. Furthermore, this localization effect is found to be applicable to both amorphous and crystalline interfacial systems. In particular, we demonstrate that interfacial hybridization plays a pivotal role in regulating the 2D electron localization effects. Our study resolves the location where the 2D electrons are localized not only does it highlight the importance of interfacial hybridization but it also opens a new avenue for device fabrication in amorphous film systems where charge localization can be done at much great ease as compared to epitaxial crystalline heterostructures.

Effects of substrate on cavity plasmon polaritons in monolayer MoS2 embedded in an asymmetric cavity

The coupling between plasmons in two-dimensional materials and photons can be enhanced by a Fabry–Pérot (FP) cavity, leading to the formation of cavity plasmon polaritons (CPPs). In this work, we theoretically investigate the effects of substrate on CPPs in monolayer (ML) molybdenum disulfide ( M o S 2 ) embedded in an asymmetric FP cavity. The optical conductivity of ML M o S 2 used here is described by the Drude–Smith model, in which the substrate-induced electronic localization effect is considered. This effect has been experimentally demonstrated for chemical vapor deposition grown M o S 2 in our recent work. Herein, the investigation shows the existence of three types of CPPs in this asymmetric plasmonic system. Meanwhile, we also demonstrate that the substrate can affect the properties of these modes through different mechanisms. These results not only can give us insight into the effects of substrate on plasmon polaritons but also remind us that the differences between the realistic optical parameters of M o S 2 and the ideal ones cannot be neglected in many situations. Moreover, we hope this study can push forward the design and optimization of plasmonic devices based on M o S 2 , such as sensors, attenuators, and detectors.

Electronic transport properties and quantum localization effects monitored by selective functionalization in Bernal bilayer graphene

Monitoring electronic properties of 2D materials is an essential step to open a way for applications such as electronic devices and sensors. From this perspective, Bernal bilayer graphene (BLG) is a fairly simple system that offers great possibilities for tuning electronic gap and charge carriers' mobility by selective functionalization (adsorptions of atoms or molecules). Here, we present a detailed numerical study of BLG electronic properties when two types of adsorption site are present simultaneously. We focus on realistic cases that could be realized experimentally with adsorbate concentration c varying from 0.25% to 5%. For a given value of c, when the electronic doping is lower than c we show that quantum effects, which are ignored in usual semi-classical calculations, strongly affect the electronic structure and the transport properties. A wide range of behaviors is indeed found, such as gap opening, metallic behavior or abnormal conductivity, which depend on the adsorbate positions, the c value, the doping, and eventually the coupling between midgap states which can create a midgap band. These behaviors are understood by simple arguments based on the fact that BLG lattice is bipartite. We also analyze the conductivity at low temperature, where multiple scattering effects cannot be ignored. Moreover, when the Fermi energy lies in the band of midgap states, the average velocity of charge carriers cancels but conduction is still possible thanks to quantum fluctuations of the velocity.

Effect of electron localization in theoretical design of Ni-Mn-Ga based magnetic shape memory alloys

The precise determination of the stability of different martensitic phases is an essential task in the successful design of (magnetic) shape memory alloys. We evaluate the effect of electron delocalization correction on the predictive power of density functional theory for Ni-Mn-Ga, the prototype magnetic shape memory compound. Using the corrected Hubbard-model-based generalized gradient approximation (GGA+U), we varied the Coulomb repulsion parameter U from 0 eV to 3 eV to reveal the evolution of predicted material parameters. The increasing localization on Mn sites results in the increasing stabilization of 10M modulated structure in stoichiometric Ni2MnGa in agreement with experiment whereas uncorrected GGA and meta-GGA functional provide the lowest energy for 4O modulated structure and non-modulated structure, respectively. GGA+U calculations indicate that 10M structure is more stable than other martensitic structures for U > 1.2 eV.The key features of density of states (DOS) responsible for the stabilization or destabilization of particular martensitic phases calculated with GGA+U are found also in DOS calculated with advanced quasi-particle self-consistent GW (QSGW) method. It supports the physical background of Hubbard correction. Moreover, the calculations with U = 1.8 eV provide the best agreement with experimental data for lattice parameters of stoichiometric and off-stoichiometric alloys.

Microscopic Link between Electron Localization and Chemical Expansion in AMnO3 and ATiO3 Perovskites (A = Ca, Sr, Ba)

The microscopic origin of chemical expansion in perovskite oxides, due to formation of oxygen vacancies accompanied by formal reduction of a 3d transition metal, is studied by first-principles calculations. We compare the II–IV manganite and titanate series, having Ca, Sr, or Ba on the A site. In particular, the effect of electron localization is elucidated by systematically varying the Hubbard U, and we find that the localization behavior is significantly different in the manganites and titanates. The chemical expansion is explicitly calculated for all compounds, and we demonstrate that increasing on-site repulsion (Hubbard U) on the B site in the lattice yields increased chemical expansion in the manganites and reduced chemical expansion in the titanates. The opposite behavior of the manganites and titanates arises from different electrostatic screenings of oxygen vacancies. We show that this can be attributed to differences in electronic energy levels, specifically that Mn–O bonds are more covalent than Ti–O bonds. Fundamental understanding of electronic and crystal chemical origins of the important phenomenon of chemical expansion is required for rational design of oxide materials for energy technology, sensors, and actuators. We hope our analysis will inspire further fundamental studies of other oxides for solid state ionics applications.

Substrate-induced electronic localization in monolayer MoS2 measured via terahertz spectroscopy.

We study terahertz (THz) optoelectronic properties of monolayer (ML) MoS2 placed on different substrates such as SiO2/Si, sapphire, and quartz. Through the measurements of THz Fourier transform spectroscopy (2.5-6.5THz) and THz time-domain spectroscopy (TDS, 0.2-1.2THz), we find that the real part of optical conductivity increases for ML MoS2 on SiO2/Si and sapphire substrates and decreases for it on quartz with increasing radiation frequency. It is shown that the complex optical conductivity for ML MoS2, obtained from THz TDS measurements, can fit very well to the Drude-Smith formula. Thus, the dependence of optical conductivity of ML MoS2 on different substrates can be understood via a mechanism of electronic localization, and the electron density, relaxation time, and localization factor of the sample can be determined optically. Furthermore, we examine the influence of temperature on these key parameters in ML MoS2 on different substrates. The results obtained from this Letter indicate that THz spectroscopy is a very powerful tool in studying and characterizing ML MoS2-based electronic systems, especially in examining the electronic localization effect which cannot be directly measured in conventional electrical transport experiment. This Letter is relevant to an in-depth understanding of the optoelectronic properties of ML MoS2 and of the proximity effect induced by different substrates.

Locally defined quantum emission from epitaxial few-layer tungsten diselenide

Recently, single photons have been observed emanating from point defects in two-dimensional (2D) materials including WSe2, WS2, hexagonal-BN, and GaSe, with their energy residing in the direct electronic bandgap. Here, we report single photon emission from a nominal weakly emitting indirect bandgap 2D material through deterministic strain induced localization. A method is demonstrated to create highly spatially localized and spectrally well-separated defect emission sites in the 750–800 nm regime in a continuous epitaxial film of few-layer WSe2 synthesized by a multistep diffusion-mediated gas source chemical vapor deposition technique. To separate the effects of mechanical strain from the substrate or dielectric-environment induced changes in the electronic structure, we created arrays of large isotropically etched ultrasharp silicon dioxide tips with spatial dimensions on the order of 10 μm. We use bending based on the small radius of these tips—on the order of 4 nm—to impart electronic localization effects through morphology alone, as the WSe2 film experiences a uniform SiO2 dielectric environment in the device geometry chosen for this investigation. When the continuous WSe2 film was transferred onto an array of SiO2 tips, an ∼87% yield of localized emission sites on the tips was observed. The outcomes of this report provide fundamental guidelines for the integration of beyond-lab-scale quantum materials into photonic device architectures for all-optical quantum information applications.

The structure stability, diffusion behavior and elastic properties of stoichiometric ZrC bulk with interstitial hydrogen defect: A first-principles study

The structure stability, diffusion behavior of interstitial hydrogen, and its influence on the elastic properties of stoichiometric ZrC bulk are investigated by first-principles calculations. The interstitial hydrogen in stoichiometric ZrC bulk prefers to be trapped by the C atom with C-H distance of 1.15 Å, and its formation energy is 1.30 eV after ZPE correction. The interstitial hydrogen promotes the formation of Zr/C vacancy in ZrC bulk due to the electron localization effects and the effect on the C vacancy is more significant. A new energetically favorable diffusion path of hydrogen in ZrC bulk is predicted as from one center C-H site in the Zr/C cube to the other neighboring one by climbing the metastable Zr/C cubic center site with an energy barrier of 0.64 eV, and then followed by twice rotations around the nearest neighboring C atom with an energy barrier of 0.26 eV. A high hydrogen resistance can be expected on the ZrC ceramic due to the extremely low hydrogen diffusivity and the insignificant effects of hydrogen on the elastic properties.

Локализация электронов верхних долин в узкозонном канале --- возможный дополнительный механизм увеличения тока в DA-DpHEMT

Abstract: The theoretical estimation of the effect of electron localization in the upper valleys in the narrow-band channel of transistor heterostructures AlxGa1–xAs-GaAs with two-sided doping on the value оf drift velocity overshoot is carried out. It is shown that for transistor heterostructures with donor-acceptor doping, in which the proportion of electrons transferred from the narrow-band channel to the wide-band material is less than in conventional structures, in some cases, the drift velocity increase can reach 15 % due to the localization of electrons in the upper valleys in the narrow-band channel. The studied effect can be an additional mechanism for increasing the current in transistors based on heterostructures with donor-acceptor doping.

Dissipative non-equilibrium Green function methodology to treat short range Coulomb interaction: current through a 1D nanostructure

A methodology describing Coulomb blockade in the non-equilibrium Green function formalism is presented. We carried out ballistic and dissipative simulations through a 1D quantum dot using an Einstein phonon model. Inelastic phonons with different energies have been considered. The methodology incorporates the short-range Coulomb interaction between two electrons through the use of a two-particle Green function. Unlike previous work, the quantum dot has spatial resolution i.e. it is not just parameterized by the energy level and coupling constants of the dot. Our method intends to describe the effect of electron localization while maintaining an open boundary or extended wave function. The formalism conserves the current through the nanostructure. A simple 1D model is used to explain the increase of mobility in semi-crystalline polymers as a function of the electron concentration. The mechanism suggested is based on the lifting of energy levels into the transmission window as a result of the local electron–electron repulsion inside a crystalline domain. The results are aligned with recent experimental findings. Finally, as a proof of concept, we present a simulation of a low temperature resonant structure showing the stability diagram in the Coulomb blockade regime.

β”-(CNB-EDT-TTF)4BF4; Anion Disorder Effects in Bilayer Molecular Metals

The preparation and characterization of new salts based on the dissymmetrical TTF derivative CNB-EDT-TTF (cyanobenzene-ethylenedithio-tetrathiafulvalene) and BF4− anions, are reported. Depending on the electrocrystallization conditions salts with different stoichiometries, (CNB-EDT-TTF)BF4 and β”-(CNB-EDT-TTF)4BF4, can be obtained. The 1:1 salt is an electrical insulator isostructural to the ClO4 analogue previously described. The 4:1 salt is a new member of the family of 2D metals of this donor with different small anions X, (CNB-EDT-TTF)4X, characterized by a bilayer arrangement of the donors and it was obtained in a monoclinic polymorph with a β”-type donor packing pattern. The small anions in this compound are severely disordered between the donor bilayers, which present slightly larger lattice parameters than the isostructural ClO4 analogue. Both electrical conductivity and thermoelectric power measurements in single crystals denote metallic properties as predicted by electronic band structure calculations. As a consequence of the anion disorder the metallic regime of the electrical conductivity denotes electronic localization effects with a progressive increase of resistivity below ~25 K. Because of the larger lattice parameters the intermolecular interactions and electronic bandwidth are decreased compared to other (CNB-EDT-TTF)4X salts. The large and positive thermoelectric power S of this compound (~110 μV/K in the range 100–330 K) and its electrical conductivity σ = 20 S/cm at room temperature lead to a power factor S2σ = 24 μW/K2m, quite large among molecular conductors, placing these compounds as potential candidates for thermoelectric materials.

Description of light-element magnetic systems via density functional theory plus U with an example system of fluorinated boron nitride: An efficient alternative to hybrid functional approach

It is well known that for light-element magnetic materials density functional theory (DFT) in the local density approximation or generalized gradient approximation (LDA or GGA) underestimates the electron localization effects and tends to give misleading results. Hybrid functionals such as Heyd-Scuseria-Ernzerhof (HSE) perform much better while being computationally expensive, especially for extended systems. In order to go beyond semi-local DFT without needing to calculate the expensive Fock exchange, here we explore the performance of the more efficient GGA plus the Hubbard U correction (GGA+U) approach to light-element magnetic materials by considering fluorinated boron nitride (F-BN) sheets and nanotubes as model systems. By applying the Hubbard U correction to the N-2p orbitals with the value of U determined by fitting the HSE results in a particular F-BN sheet, it is found that the GGA+U approach shows a great improvement to GGA in describing the magnetic properties of F-BN systems with an accuracy close to that of the HSE hybrid functional approach. It indicates the possibility of using the ad hoc correction approach as an efficient alternative to study light-element magnetic materials, especially for large systems where calculations based on hybrid functionals become cost-demanding.

Direct evidence of traps controlling the carriers transport in SnO2 nanobelts

This work reports on direct evidence of localized states in undoped SnO2 nanobelts. Effects of disorder and electron localization were observed in Schottky barrier dependence on the temperature and in thermally stimulated currents. A transition from thermal activation to hopping transport mechanisms was also observed. The energy levels found by thermally stimulated current experiments were in close agreement with transport data confirming the role of localization in determining the properties of devices.