arXiv subjects

Qunfei Zhou

Publications and source records attributed to Qunfei Zhou.

6 recordsLinked to original sources

arXiv2026

Collective Electrostatics and Band Alignment in Janus MoSTe nanotubes

In this work, we investigate the collective electrostatic effects of one-dimensional (1D) Janus MoSTe nanotubes and their impacts on the band alignment of nanotube heterostructures. Using first-principles calculations based on Density Functional Theory, we find that the Janus nanotube generates a large and uniform electrostatic potential of over 1.3 V within the nanotube pores, which is accumulative for double wall nanotubes. We develop an analytical model to provide a quantitative understanding of the electrostatic potential and its dependence on the quadrupole moment and nanotube radius. For double wall MoSTe nanotube, we find a substantial band edge shift of about 1.0 eV for the inner tube originated from the electrostatic effects, leading to a type-II band alignment. These results demonstrate that the electrostatic effects of 1D nanotubes can be used to tune the electronic properties and band alignment of 1D nanotube heterostructures for optoelectronic and catalytic applications.

cond-mat.mtrl-sci↗

arXiv2022

Non-Abelian Stokes theorem and quantized Berry flux

Band topology of anomalous quantum Hall insulators can be precisely addressed by computing Chern numbers of constituent non-degenerate bands that describe quantized, Abelian Berry flux through two-dimensional Brillouin zone. Can Chern numbers be defined for $SU(2)$ Berry connection of two-fold degenerate bands of materials preserving space-inversion ($\mathcal{P}$) and time-reversal ($\mathcal{T}$) symmetries or combined $\mathcal{PT}$ symmetry, without detailed knowledge of underlying basis? We affirmatively answer this question by employing a non-Abelian generalization of Stokes' theorem and describe a manifestly gauge-invariant method for computing magnitudes of quantized $SU(2)$ Berry flux (spin-Chern number) from eigenvalues of Wilson loops. The power of this method is elucidated by performing $\mathbb{N}$-classification of \emph{ab initio} band structures of three-dimensional, Dirac materials. Our work outlines a unified framework for addressing first-order and higher-order topology of insulators and semimetals, without relying on detailed symmetry data.

cond-mat.mtrl-sci↗

arXiv2022

Analytical Theory of Near-Field Electrostatic Effects in Two-Dimensional Materials and van der Waals Heterojunctions

We derive and validate a quantitative analytical model of the near-field electrostatic effects in the vicinity (>=3Å) of two-dimensional (2D) materials. In solving the Poisson equation of a near-planar point charge ansatz for the electronic density of a 2D material, our formula quantitatively captures the out-of-plane decay and the in-plane modulation of density functional theory (DFT)-calculated potentials. We provide a method for quickly constructing the electronic density ansatz, and apply it to the case of hexagonal monolayers (BN, AlN, GaN) and monochalcogenides (GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbS, PbSe, PbTe) and their flexural and polar distortions. We demonstrate how our model can be straightforwardly applied to predict material-/angle-specific moiré potentials arising in twisted superlattices with periodicities beyond the reach of DFT calculations.

cond-mat.mtrl-sci↗

arXiv2022

A Review of Sc-containing "Scandate" Thermionic Cathodes

Although thermionic emission has been studied for more than 100 years, recent interest in novel electron devices for military and civilian use has led to a surge in demand for cathodes with enhanced emission properties (e.g. higher current density, more uniform emission, lower operating temperatures, or extended in-service longevity). Sc-containing "scandate" cathodes have been widely reported to exhibit superior emission properties compared to previous-generation thermionic cathodes, including oxide, B-, and M-type cathodes. Despite extensive study spanning several decades, the mechanism by which the addition of Sc enhances cathode emission remains ambiguous, and certain limitations -- non-uniform emission, low reproducibility, inconsistent longevity -- continue to prevent widespread commercial integration of scandate cathodes into electron devices. This review attempts to survey the literature to-date addressing the fabrication, structure, and properties of scandate cathodes, with particular attention to studies addressing the role of Sc in enhancing emission.

cond-mat.mtrl-sci↗

arXiv2021

Range-Separated Hybrid Functionals for Mixed-Dimensional Heterojunctions: Application to Phthalocyanines/MoS2

We analyze the electronic structure and level alignment of transition-metal phthalocyanine (MPc) molecules adsorbed on two-dimensional MoS$_2$ employing density functional theory (DFT) calculations. We develop a procedure for multi-objective optimal tuning of parameters of range-separated hybrid functionals in these mixed-dimensional systems. Using this procedure, which leads to the asymptotically-correct exchange-correlation potential between molecule and two-dimensional material, we obtain electronic structures consistent with experimental photoemission results for both energy level alignment and electronic bandgaps, representing a significant advance compared to standard DFT methods. We elucidate the MoS$_2$ valence resonance with the transition-metal phthalocyanine non-frontier 3$d$ orbitals and its dependence on the transition metal atomic number. Based on our calculations, we derive parameter-free, model self-energy corrections that quantitatively accounts for the effects of the heterogeneous dielectric environment on the electronic structure of these mixed-dimensional heterojunctions.

cond-mat.mtrl-sci↗

arXiv2021

Engineering the Electronic Structure of Two-Dimensional Materials with Near-Field Electrostatic Effects of Self-Assembled Organic Layers

We compute the electronic structure of two-dimensional (2D) materials decorated with self-assembled organic monolayers using density functional theory. We find that 2D materials are strongly impacted by near-field electrostatic effects resulting from high multipoles of the organic layer electronic density. We show that this effect can lead to significant (~0.5V) modulation of the in-plane potential experienced by electrons in 2D materials within ~4Åfrom the molecular layer, with a transition between near- and far-field depending on the lateral extent of the molecules. We develop a theory of this effect, showing that the electrostatic potential of the molecular layer can be approximated by a discretized planar charge density derived from the molecular structure and multipoles. Solving this model computationally and analytically, we propose implementations of this effect to generate novel electronic properties for electrons in 2D materials, such as band gap opening and anisotropic group velocity modulation for monolayer graphene from experimentally achievable molecular assemblies.

cond-mat.mtrl-sci↗