Fermi Charge Research Articles

(Invited) Electronic Doping in Two-Dimensional Semiconductors

Quantum-confined semiconductors provide highly tunable optical and electrical properties for a wide variety of emerging applications. Two-dimensional semiconductors possess tunable bandgaps, high charge carrier mobilities, and strong light-matter interactions that can enable applications in photovoltaics, quantum information processing, and spintronics. Electronic doping, the intentional introduction of controlled charge carrier density and type (electrons or holes) is a fundamental enabler for many emerging applications of two-dimensional semiconductors. While electrostatic doping in transistor geometries is useful for certain applications, it is important to develop additional methods based on e.g. molecular redox doping and substitutional atomic doping that can provide tunable Fermi level control in the absence of an applied bias. In this presentation, I will discuss our efforts to develop such doping strategies for a variety of two-dimensional semiconductors. I will discuss how electronic doping modulates opto-electronic properties, including ground-state absorbance, charge transport, and excited-state dynamics in these 2D semiconductors. These studies provide fundamental insights into the degree to which molecular and substitutional doping strategies can modulate Fermi level, charge transport, and excitonic optical transitions in 2D semiconductors.

In-situ construction of Co/CoSe Schottky heterojunction with interfacial electron redistribution to facilitate oxygen electrocatalysis bifunctionality for zinc-air batteries

Design of low-cost and high-performance bifunctional electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is a key topic in the development of zinc-air batteries. In this work, a facile in-situ partial reduction strategy is proposed to fabricate Co/CoSe Schottky junctions encapsulated in carbon (Co/CoSe@NC). The heterojunction in the Co/CoSe@NC can tune Fermi level and charge redistribution to accelerate the charge transfer, while the metal–organic framework derived skeleton provides well-defined structure and abundant mesopores to facilitate mass transport. The accelerated charge transfer through the Mott-Schottky heterostructure can improve the intrinsic electrocatalytic activities for both electron-gaining ORR and electron-losing OER, which are confirmed by systematic characterizations and density functional theory calculations. Furthermore, the Zn-air battery with the as-prepared Co/CoSe@NC heterojunction shows better performance than the Pt/RuO2-based counterpart, demonstrating good feasibility for practical applications.

Observing separate spin and charge Fermi seas in a strongly correlated one-dimensional conductor.

An electron is usually considered to have only one form of kinetic energy, but could it have more, for its spin and charge, by exciting other electrons? In one dimension (1D), the physics of interacting electrons is captured well at low energies by the Tomonaga-Luttinger model, yet little has been observed experimentally beyond this linear regime. Here, we report on measurements of many-body modes in 1D gated wires using tunneling spectroscopy. We observe two parabolic dispersions, indicative of separate Fermi seas at high energies, associated with spin and charge excitations, together with the emergence of two additional 1D “replica” modes that strengthen with decreasing wire length. The interaction strength is varied by changing the amount of 1D intersubband screening by more than 45%. Our findings not only demonstrate the existence of spin-charge separation in the whole energy band outside the low-energy limit of the Tomonaga-Luttinger model but also set a constraint on the validity of the newer nonlinear Tomonaga-Luttinger theory.

Modeling Hydrodynamic Charge Transport in Graphene

Graphene has exceptional electronic properties, such as zero band gap, massless carriers, and high mobility. These exotic carrier properties enable the design and development of unique graphene devices. However, traditional semiconductor solvers based on drift-diffusion equations are not capable of modeling and simulating the charge distribution and transport in graphene, accurately, to its full extent. The effects of charge inertia, viscosity, collective charge movement, contact doping, etc., cannot be accounted for by the conventional Poisson-drift-diffusion models, due to the underlying assumptions and simplifications. Therefore, this article proposes two mathematical models to analyze and simulate graphene-based devices. The first model is based on a modified nonlinear Poisson’s equation, which solves for the Fermi level and charge distribution electrostatically on graphene, by considering gating and contact doping. The second proposed solver focuses on the transport of the carriers by solving a hydrodynamic model. Furthermore, this model is applied to a Tesla-valve structure, where the viscosity and collective motion of the carriers play an important role, giving rise to rectification. These two models allow us to model unique electronic properties of graphene that could be paramount for the design of future graphene devices.

Systematic application of the M3Y NN forces for describing the capture process in heavy-ion collisions involving deformed target nuclei

We present results of a systematic study of the capture process through the barrier penetration model. The nucleus-nucleus interaction potential is calculated using the double-folding model (DFM) with the M3Y Paris $\mathit{NN}$ forces. The nucleon densities entering the model are generated from the experimental three-parameter Fermi charge densities. The DFM has been extended to the case of deformed target nuclei. It is shown that the density-dependent M3Y $\mathit{NN}$ forces with the finite range exchange part can be mimicked successfully by the zero-range density-independent forces. The latter option significantly reduces the required computer time. For the nucleon densities and target nuclei deformations, we employ the values from the commonly used data bases. Thus, we do not vary any parameters to reach a better agreement with the data. The resulting cross sections are compared with data for 20 reactions with the product of the charge numbers ${Z}_{P}{Z}_{T}$ ranging from 216 up to 2576. We discuss the opinion met in the literature that the M3Y $\mathit{NN}$ forces provide a poorer description of the capture cross sections in heavy-ion collisions in comparison to the $\mathit{NN}$ forces coming from the relativistic mean-field approach. Our calculations show that the M3Y $\mathit{NN}$ forces give an agreement with the data which is not perfect yet is not worse than the one resulting from the relativistic mean-field approach $\mathit{NN}$ forces.

A chemical kinetics perspective on thermoelectric transport

There is a growing community of chemists and materials scientists exploring thermoelectric properties of organic, polymeric, hybrid, and composite materials. Many of these materials are nondegenerate semiconductors, meaning that the Fermi and transport charge transport energy levels are significantly offset. Others are ionic conductors. While the meaning of the essential parameters, namely, electrical conductivity (σ) and Seebeck coefficient (S, the voltage difference per degree of temperature difference) are accessible, the origins of S are not readily apparent to one trained mainly in chemistry. The purpose of this manuscript is to illustrate S as being the result of a steady state difference between concentrations of two designations of charge carriers, those on hotter and those on colder sides of a material sample, analogous to the equilibrium among interconverting chemical species that differ in enthalpy and entropy. The ion-based Seebeck coefficient, also known as the Soret effect, can be explained using principles similar to those applicable to electrons and holes. We hope that this analysis leads to wider understanding of the origins of S through an explanation using the language of chemical kinetics and thermodynamics and appreciation of ways that thermoelectric efficiency can be enhanced in emerging materials.

Restricted Channel Migration in 2D Multilayer ReS2.

When thickness-dependent carrier mobility is coupled with Thomas-Fermi screening and interlayer resistance effects in two-dimensional (2D) multilayer materials, a conducting channel migrates from the bottom surface to the top surface under electrostatic bias conditions. However, various factors including (i) insufficient carrier density, (ii) atomically thin material thickness, and (iii) numerous oxide traps/defects considerably limit our deep understanding of the carrier transport mechanism in 2D multilayer materials. Herein, we report the restricted conducting channel migration in 2D multilayer ReS2 after a constant voltage stress of gate dielectrics is applied. At a given gate bias condition, a gradual increase in the drain bias enables a sensitive change in the interlayer resistance of ReS2, leading to a modification of the shape of the transconductance curves, and consequently, demonstrates the conducting channel migration along the thickness of ReS2 before the stress. Meanwhile, this distinct conduction feature disappears after stress, indicating the formation of additional oxide trap sites inside the gate dielectrics that degrade the carrier mobility and eventually restrict the channel migration. Our theoretical and experimental study based on the resistor network model and Thomas-Fermi charge screening theory provides further insights into the origins of channel migration and restriction in 2D multilayer devices.

Exotic Topological Bands and Quantum States in Metal-Organic and Covalent-Organic Frameworks.

ConspectusMetal-organic and covalent-organic frameworks (MOFs/COFs) have been extensively studied for fundamental interests and their promising applications, taking advantage of their unique structural properties, i.e., high porosity and large surface-to-volume ratio. However, their electronic and magnetic properties have been somewhat overlooked because of their relatively poor performance as conductive and/or magnetic materials. Recent experimental breakthroughs in synthesizing two-dimensional (2D) π-conjugated MOFs/COFs with high conductivity and robust magnetism through doping have generated renewed and increasing interest in their electronic properties. Meanwhile, comprehensive theoretical studies of the underlying physical principles have led to discovery of many exotic quantum states, such as topological insulating states, which were only observed in inorganic systems. Especially, the diversity and high tunability of MOFs/COFs have provided a playground to explore novel quantum physics and quantum chemistry as well as promising applications.The band theory has empowered us to understand the most fundamental electronic properties of inorganic crystalline materials, which can also be used to better understand MOFs/COFs. The first obvious difference between the two is that instead of atomic orbitals residing at lattice sites of inorganic crystals, molecular orbitals of organic ligands are predominant in MOFs/COFs. The second key difference is that usually all atomic orbitals in an inorganic crystal are subject to one common group of lattice symmetry, while atomic orbitals of metal ion and molecular orbitals of different organic ligands in MOFs/COFs belong to different subgroups of lattice symmetries. Both these differences will impact the band structure of MOFs/COFs, in particular making it more complex. Consequently, which subset of bands are of most importance depends strongly on the location of Fermi level, i.e., electron counting and charge doping. Furthermore, there are usually two types of characteristic electrons coupled in MOFs, i.e., strongly correlated localized d and f electrons and diffusive s and p electrons, which interplay with lattice, orbital, and spin degrees of freedom, leading to more exotic topological and magnetic band structures.In this Account, we present an up-to-date review of recent theoretical developments to better understand the exotic band structures of MOFs/COFs. Starting from three fundamental 2D lattice models, i.e., honeycomb, Kagome, and Lieb lattices, exotic Dirac and flat bands as well as the intriguing topological quantum states they host, e.g., quantum spin Hall and quantum anomalous Hall states, are outlined. In addition to the single-lattice models, we further elaborate on combined lattice model Hamiltonians, which give rise to overlapping bands hosting novel quantum states, such as nodal-line Dirac semimetal and unconventional superconducting states. Also, first-principles predictions of candidate MOFs/COFs that host these exotic bands and hence quantum phases are reviewed, which greatly extends the pool of materials beyond inorganic crystals for hosting exotic band structures.

Elastic, electronic, vibrational and optical properties of filled skutterudite compound SrRu4As12: Insights from DFT-based computer simulation

Structural, elastic, electronic, and optical properties of filled skutterudite compound SrRu4As12 have been studied by using density functional theory with CASTEP code for the first time. The optimized structural parameters show good agreement with available experimental and theoretical values. The calculated single elastic constants are positive and agreed with the well-known Born stability criteria that indicate the mechanical stability of this compound. The phonon dispersion curve exhibit dynamical stability. The studied compound is found to be semi-metallic, anisotropic and brittle in nature. The highest bond population exists in As–As, indicating the highly covalent nature of SrRu4As12. Peierls stress value indicates that the dislocation moves very easily for filled skutterudite compounds. The calculated Debye temperature, minimum thermal conductivity, and melting temperature reveal the possibility to be used as a TBC material. High reflectivity also suggests that SrRu4As12 could be a promising protective coating material against solar heating. The Fermi surface, charge density, atomic, and bond population have also been calculated to analyze the bonding nature.

Structural, Vibrational, Electronic and Thermodynamical properties of B2–HgMg amalgam: A DFT study

The First principle study of structural, elastic, electronic and thermal properties for B2–HgMg amalgam has been carried out using Local Density Approximation (LDA) and Generalized Gradient Approximation (GGA) exchange correlation functionals. The presently ground state properties; lattice constant, bulk modulus, elastic constants, ductility and enthalpy show good agreement with available experimental results. The ductility of B2–HgMg is proven from Pugh's relation; the obtained B/GH values are 4.17 and 3.61 for LDA and GGA, respectively. The dynamical stability of B2–HgMg at 0 K and 900 K is confirmed from the positive frequency in the phonon spectra. The electronic band structure and density of states reveal the metallic nature of B2–HgMg and major contribution of Hg-d electrons for core configuration. The electron like Fermi surfaces and charge density along [111] and [100] planes illustrate strong bonding between Hg–Hg atoms compare to Hg–Mg atoms. The Grunesian parameter and Bulk modulus at 900 K decreases to less than half of its value at 0 K indicates compressible nature of B2– HgMg.

Modulation of the transport properties of metal/MoS2 interfaces using BN-graphene lateral tunneling layers

Modulating the n- and p-type interfacial charge transport properties of the metal–semiconductor interface is vital to realizing high performance two-dimensional material nanodevices and is still a significant challenge. Here, a boron nitride (BN)-graphene lateral heterostructure (LH) was used as the interfacial tunneling layer to control the Schottky barrier, Fermi level pinning and charge injection efficiency of the metal–MoS2 interface. The BN-graphene LH with graphene-N junction structure decreased the n-type vertical Schottky barrier and enhanced the interfacial tunneling probability, while the graphene-B junction structure decreased the p-type vertical Schottky barrier. Consequently, the n-type Au/LH–MoS2 interface with Ohmic character and high tunneling probability (∼0.242) and the p-type vertical Schottky barrier of about 0.20 eV for the Pt/LH–MoS2 interface were achieved. Compared to other reported BN or graphene tunneling layers, such a BN-graphene LH tunneling layer not only suppressed the charge scattering from the metal electrode to the MoS2 layer and the Fermi level pinning effect, but also reduced the contact resistance between metal electrode and tunneling layer. The underlying mechanisms were revealed to be due to the charge transfer, orbitals and interfacial dipole. This work improves the current understanding of the metal–MoS2 interface and proposes a new way to overcome the current severe contact issues for future nanoelectronic and optoelectronic applications.

Effect of interlayer tunneling barrier on carrier transport and fluctuation in multilayer ReS2

Multilayer rhenium disulfide (ReS2) has recently attracted significant attention because of the decoupled van der Waals interaction between its adjacent layers that leads to a much higher interlayer resistivity than that in other layered materials. Although the carrier transport in multilayer materials is well described by the interlayer resistance and Thomas–Fermi charge screening length (λ) in theoretical resistor network models, the understanding of the effect of electric field-dependent interlayer tunneling barrier (Eint) on current fluctuation in two-dimensional (2D) multilayer materials is limited. Herein, we report the effects of Eint on carrier transport and charge fluctuation in multilayer ReS2. The electrostatic back-gate (VBG)- and drain bias (VD)-dependent Eint causes channel migration along the c-axis in 2D multilayer systems and consequently results in two plateaus in the transconductance curve, thereby allowing us to determine the top and bottom carrier mobilities of multilayer ReS2 separately. Furthermore, the strong correlation between Eint and the Coulomb scattering parameter in multilayer ReS2 is elucidated via low-frequency noise spectroscopy. The results of our study provide a clear insight into the origins of carrier transport and current fluctuation in 2D multilayer devices.

Structural stability, electronic and thermoelectric properties of ruthenium silicide

The structural phase transition, electronic and transport properties of RuSi intermetallic compound has been studied using density functional theory (DFT). The pressure induced phase transition from B20 to B2 phase is observed at 6.4 GPa and compared with other theoretical results. The calculated ground state, electronic and transport properties are in good agreement with the previously reported results. The stability of RuSi at 0 and high pressure is also confirmed by phonon dispersion curve [see supplementary information].The band structure and density of states show that RuSi is metallic in B2 phase while semiconductor in B20 phase with indirect band gap of 0.34 eV. We have also calculated the transport properties of RuSi in terms of electrical conductivity (σ/τ), electronic thermal conductivity (κe/τ), Seebeck coefficient (S) and power factor (S2σ/τ) in B20 phase. Our results show that RuSi is p-type semiconductor and is in good agreement with the experimental data. Bonding nature is discussed using Fermi surface, band structure and charge density difference plots.

Erratum to: On the vacuum-polarization Uehling potential for a Fermi charge distribution

Some typographical errors remain after the publication of my paper “On the vacuum-polarization Uehling potential for a Fermi charge distribution”, Eur. Phys. J. D 72, 61 (2018). In order to improve its clarity and accuracy, the corrections are itemized below:

Discoloration Effect and One-Step Synthesis of Hydrogen Tungsten and Molybdenum Bronze (H x MO3) using Liquid Metal at Room Temperature.

This paper presents a new route to one-step fabrication and in situ application of hydrogen tungsten and molybdenum bronze (HxMO3) at room temperature and triggers the interdisciplinary research of multifunctional materials between liquid metal and transition-metal oxides (TMOs). Gallium-based liquid metal (GBLM) enables the discoloration effect on TMOs in acid electrolytes at ambient temperature. The underlying mechanism behind this phenomenon can be ascribed to the redox effect at the interface of liquid metal and TMOs in acid electrolytes. Both the theoretical calculations and the experimental results demonstrate that the increasing intercalation of H+ ions into the lattice of WO3 raises the electron density at the Fermi level and charge carriers. H+ ion content in the obtained HxMO3 can be controlled in our approach to meet different requirements. Taking advantage of the one-step fabrication and room-temperature liquid phase nature of the liquid metal, HxMO3 is synthesized under ambient conditions in a very short time, which is inaccessible with conventional solution-processed mechanical alloying, or other methods. The HxMO3 obtained in this one-step approach enables convenient and simple applications for biomimetic camouflage, cost-effective energy storage, H+ ion sensor, and electronic switch.

Effect of ferroelectric domains on electric properties of single layer graphene

Ferroelectric materials with tailored domain structures are promising “smart” substrates providing additional degrees of functionality for the graphene-based devices. We demonstrate the effect of periodical domain structure in lithium niobate substrate on the electrical properties of graphene studied by Raman spectroscopy. The spatial variations of the charge carriers and Fermi levels in graphene corresponding to oppositely polarized domains have been elucidated. The observed effects are supposed to be driven by the lateral component of electric field at the surface related to the gradient of spontaneous polarization. The five orders of magnitude decay of charge density in graphene is due to screening of depolarization field.

Computational predictions of electronic properties of graphene with defects, adsorbed transition metal-oxides and water using density functional theory

The interfacial interactions between transition metal oxides (vanadium oxide VO2, vanadium pentoxide V2O5, cobalt oxide CoO and Co3O4, manganese oxide MnO2) and water adsorbates on graphene supports as solvated interfaces and influence of defects in graphene are studied using periodic density functional theory (DFT) calculations in view of their significance for applied electrochemistry. DFT complemented and synergized our experimental work. The optimized metal oxide adatom-graphene geometries identified the preferred adatom sites, whereas metal oxide-graphene strengths are correlated with the adatom distance from the graphene plane, the Metal-C overlap populations, and the adsorption energies. The presence of finite electronic density of states (DOS) near Fermi level and charge transfers between the adatom top layer and graphene supports reflect primarily covalent bonding nature. The presence of small orbital overlap integral of bonds between the s and p (and d) orbitals of the nearest carbon (graphene), carbon oxide (graphene oxide) and metal oxide atoms reveal localized orbital re-hybridization resulting in changes in DOS yielding high electrochemical activity. Moreover, for increased adatom coverage the extent of charge transfer reverses resulting in limited electroactivity. In fact, DFT calculations are corroborated with experimental findings, where graphene-based supports decorated with optimal mass loaded nanostructured Co3O4 and MnO2 (as well as V2O5) were capable of delivering maximum specific energy storage capacity (Cs) > 550 F·g−1 (Gupta et al. J. Mater. Res. 32, 301 (2017)) in contrast to higher or lower loading. The presence of defects in graphene materials results in new electronic states to endow unique functionalities that is not otherwise possible in the bulk and with adsorbed water molecules besides optimum C/O ratio in graphene oxide nanosheets that show redshift thus a decreasing bandgap and finite charge transfer from graphene to water molecules. The case examples studied in this work represent a first glimpse of what may become routine and integral step in materials design and discovery for alternative energy and sustainable environmental technologies.

Synthesis, Structure, and Electrical Properties of the Single Crystal Ba8Cu16As30

Single crystals of clathrate-I Ba8Cu16As30 have been synthesized and their structure and electronic properties determined using synchrotron-based X-ray diffraction and first-principles calculations. The structure is confirmed to be Pm3̅ n (No. 223), with lattice parameter a = 10.4563(3) Å, and defined by a tetrahedrally bonded network of As and Cu that forms two distinct coordination polyhedra, with Ba residing inside these polyhedra. All crystallographic positions are fully occupied with no vacancies or superstructure with the Cu atoms, while occupying all framework sites in the network, exhibiting a preference for the 6c site. Agreement between the experimental and theoretically predicted structures was achieved after accounting for spin-orbit coupling. Our calculated Fermi surface, electron localization, and charge transfer, as well as a comparison with the results for elemental As46, provide insight into the fundamental properties of this clathrate-I material.

On the vacuum-polarization Uehling potential for a Fermi charge distribution

We present analytical formulas for the vacuum-polarization Uehling potential in the case where the finite size of the nucleus is modeled by a Fermi charge distribution. Using a Sommerfeld-type development, the potential is expressed in terms of multiple derivatives of a particular integral. The latter and its derivatives can be evaluated exactly in terms of Bickley-Naylor functions, which connection to the Uehling potential was already pointed out in the pure Coulomb case, and of usual Bessel functions of the second kind. The cusp and asymptotic expressions for the Uehling potential with a Fermi charge distribution are also provided. Analytical results for the higher-order-contribution K\"all\`en-Sabry potential are given.

Ab initio study of electronic structure, elasticity, bonding features and mechanical behaviour of zinc intermetallics

The structural stability, electronic structures, elastic, mechanical and thermal properties of XZn (X = Sc, Y, Ti and Zr) intermetallic compounds have been reported using ab-initio density functional theory with the generalized gradient approximation for exchange and correlation potentials. The various structural parameters, such as lattice constant (a0), bulk modulus (B), and its pressure derivative (B′) are analysed and compared. The investigation of elastic constant reveals that all studied XZn compounds are elastically stable in CsCl (B2 phase) structure. The electronic structure and metallic nature have been analysed quantitatively from band structure and DOS. To better illustrate the nature of bonding and charge transfer, we have studied the Fermi surfaces and charge density plots. The three well known criterion of ductility namely Pugh's rule, Cauchy's pressure and Frantsevich rule elucidate the ductile nature of these compounds.