Doped Monolayer Research Articles

Tuning Electrical Conductance in Bilayer MoS2 through Defect-Mediated Interlayer Chemical Bonding.

Interlayer interaction could substantially affect the electrical transport in transition metal dichalcogenides, serving as an effective way to control the device performance. However, it is still challenging to utilize interlayer interaction in weakly interlayer-coupled materials such as pristine MoS2 to realize layer-dependent tunable transport behavior. Here, we demonstrate that, by substitutional doping of vanadium atoms in the Mo sites of the MoS2 lattice, the vanadium-doped monolayer MoS2 device exhibits an ambipolar field effect characteristic, while its bilayer device demonstrates a heavy p-type field effect feature, in sharp contrast to the pristine monolayer and bilayer MoS2 devices, both of which show similar n-type electrical transport behaviors. Moreover, the electrical conductance of the doped bilayer MoS2 device is drastically enhanced with respect to that of the doped monolayer MoS2 device. Employing first-principle calculations, we reveal that such striking behaviors arise from the presence of electrical transport networks associated with the enhanced interlayer hybridization of S-3pz orbitals between adjacent layers activated by vanadium dopants in the bilayer MoS2, which is nevertheless absent in its monolayer counterpart. Our work highlights that the effect of dopant not only is confined in the in-plane electrical transport behavior but also could be used to activate out-of-plane interaction between adjacent layers in tailoring the electrical transport of the bilayer transitional metal dichalcogenides, which may bring different applications in electronic and optoelectronic devices.

Dark trions govern the temperature-dependent optical absorption and emission of doped atomically thin semiconductors

We perform absorption and photoluminescence spectroscopy of trions in hBN-encapsulated WSe$_2$, WS$_2$, MoSe$_2$, and MoS$_2$ monolayers, depending on temperature. The different trends for W- and Mo-based materials are excellently reproduced considering a Fermi-Dirac distribution of bright and dark trions. We find a dark trion, $\rm{X_D^-}$ 19 meV $\textit{below}$ the lowest bright trion, $\rm{X}_1^-$ in WSe$_2$ and WS$_2$. In MoSe$_2$, $\rm{X_D^-}$ lies 6 meV $\textit{above}$ $\rm{X}_1^-$, while $\rm{X_D^-}$ and $\rm{X}_1^-$ almost coincide in MoS$_2$. Our results agree with GW-BSE $\textit{ab-initio}$ calculations and quantitatively explain the optical response of doped monolayers with temperature.

Strain robust spin gapless semiconductors/half-metals in transition metal embedded MoSe2 monolayer

The realization of spin gapless semiconductor (SGS) and half-metal (HM) behavior in two-dimensional (2D) transition metal (TM) dichalcogenides is highly desirable for their applications in spintronic devices. Here, using density functional theory calculations, we demonstrate that Fe, Co, Ni substitutional impurities can not only induce magnetism in MoSe2 monolayer, but also convert the semiconducting MoSe2 to SGS/HM system. We also study the effects of mechanical strain on the electronic and magnetic properties of the doped monolayer. We show that for all TM impurities we considered, the system exhibits the robust SGS/HM behavior regardless of biaxial strain values. Moreover, it is found that the magnetic properties of TM–MoSe2 can effectively be tuned under biaxial strain by controlling the spin polarization of the 3d orbitals of Fe, Co, Ni atoms. Our findings offer a new route to designing the SGS/HM properties and modulating magnetic characteristics of the TM–MoSe2 system and may also facilitate the implementation of SGS/HM behavior and realization of spintronic devices based on other 2D materials.

Phonon-mediated superconductivity in doped monolayer materials

Insight into why superconductivity in pristine and doped monolayer graphene seems strongly suppressed has been central for the recent years' various creative approaches to realize superconductivity in graphene and graphene-like systems. We provide further insight by studying electron-phonon coupling and superconductivity in doped monolayer graphene and hexagonal boron nitride based on intrinsic phonon modes. Solving the graphene gap equation using a detailed model for the effective attraction based on electron tight binding and phonon force constant models, the various system parameters can be tuned at will. Consistent with results in the literature, we find slight gap modulations along the Fermi surface, and the high energy phonon modes are shown to be the most significant for the superconductivity instability. The Coulomb interaction plays a major role in suppressing superconductivity at realistic dopings. Motivated by the direct onset of a large density of states at the Fermi surface for small charge dopings in hexagonal boron nitride, we also calculate the dimensionless electron-phonon coupling strength there, but the comparatively large density of states cannot immediately be capitalized on, and the charge doping necessary to obtain significant electron-phonon coupling is similar to the value in graphene.

Edge plasmon-polaritons on isotropic semi-infinite conducting sheets

From a three-dimensional boundary value problem for the time harmonic classical Maxwell equations, we derive the dispersion relation for a surface wave, the edge plasmon-polariton (EP), which is localized near and propagates along the straight edge of a planar, semi-infinite sheet with a spatially homogeneous, scalar conductivity. The sheet lies in a uniform and isotropic medium and serves as a model for some two-dimensional (2D) conducting materials such as the doped monolayer graphene. We formulate a homogeneous system of integral equations for the electric field tangential to the plane of the sheet. By the Wiener–Hopf method, we convert this system to coupled functional equations on the real line for the Fourier transforms of the fields in the surface coordinate normal to the edge and solve these equations exactly. The derived EP dispersion relation smoothly connects two regimes: a low-frequency regime, where the EP wave number, q, can be comparable to the propagation constant, k0, of the ambient medium, and the nonretarded frequency regime in which |q| ≫ |k0|. Our analysis indicates two types of 2D surface plasmon-polaritons on the sheet away from the edge. We extend the formalism to the geometry of two coplanar sheets.

Spin-polarized current in wide bandgap hexagonal boron nitrides containing 4|8 line defects

Line defects (LDs) which are commonly present in two-dimensional materials play an important role in spintronic applications for generating and transporting spin current. We theoretically predict of the electronic and magnetic properties of n-type doped monolayers of hexagonal boron nitride (h-BN) with 4|8 LDs. The LDs can create two deep narrow bands in the band gap of the h-BN sheet with an energy separation about 2.75 eV at Γ point. Interestingly, at a certain concentration of electrons introduced by n-type doping, a tantalizing spontaneous one-dimensional ferromagnetic ordering emerges due to the partially-occupied narrow conduction band. Moreover, it is found that this electron doping induced spin polarization (of charge current) is up to 90% in the bias voltage range from 0.1 V to 0.4 V. Our findings indicate that the h-BN monolayer can functionalize a semiconductor for spintronic devices applications, which is beyond a common sense of h-BN tunneling barrier.

Defective and doped aluminum nitride monolayers for NO adsorption: Physical insight

An atomistic insight into the pristine, defective and doped AlN monolayers for NO adsorption is conducted using Density Functional Theory. The adsorption energy of NO on N-vacancy monolayer (−7.374 eV) is much larger than that of pure AlN (−0.397 eV). The Al-vacancy or B-doped monolayer are likely to be utilized for NO sensing due to the favorable band gap changes (0.969 eV and 0.836 eV respectively) compared with the pure monolayer. This work is to give physical insight into the NO adsorption mechanism of AlN monolayer and help experimentalists to design better 2-D materials for molecular adsorption and/or sensing applications.

Rotons in optical excitation spectra of monolayer semiconductors

Optically generated excitons dictate the absorption and emission spectrum of doped semiconductor transition metal dichalcogenide monolayers. We show that upon increasing the electron density, the elementary optical excitations develop a roton-like dispersion, evidenced by a shift of the lowest energy state to a finite momentum on the order of the Fermi momentum. This effect emerges due to Pauli exclusion between excitons and the electron Fermi sea, but the robustness of the roton minimum in these systems is a direct consequence of the long-range nature of the Coulomb interaction and the nonlocal dielectric screening characteristic of monolayers. Finally, we show that the emergence of rotons could be related to hitherto unexplained aspects of photoluminescence spectra in doped transition metal dichalcogenide monolayers.

Low-loss two-dimensional plasmon modes in antimonene

The effects of spin-orbit (SOC) and electron-phonon coupling on the collective excitation of doped monolayer Sb$_2$ are investigated using density functional and many-body perturbation theories. The spin-orbit coupling is exclusively important for the monolayer Sb$_2$ and it leads to the reconstruction of the electronic band structure. In particular, plasmon modes of monolayer Sb$_2$ are quite sensitive to the SOC and are characterized by very low damping rates owing to small electron-phonon scatterings. Our results show plasmons in antimonene are significantly less damped compared to monolayer graphene when plasmon energies are $\hbar \omega> 0.2$ eV due to smaller plasmon-phonon coupling in the former material.

Conductivity models for electron energy loss spectroscopy of graphene in a scanning transmission electron microscope with high energy resolution

Recent advancements in the energy resolution and probing capabilities of monochromated electron-beam spectroscopy instruments have made this experimental technique increasingly useful for investigating and understanding the plasmonic, photonic, and electronic properties of graphene-enhanced systems. We develop herein an empirical model for the in-plane conductivity of doped monolayer graphene, comparing with ab initio data from the terahertz (THz) to the upper range of frequencies accessible with the valence electron energy loss spectroscopy (VEELS). Along with our ab initio data, this model is employed to calculate the energy loss spectra using a relativistic formulation, allowing us to analyze the effects that different electron beam parameters have on the response of graphene in a monochromated scanning transmission electron microscope setup. In particular, we explore the effects of reducing the collection angle of scattered electrons, thereby deducing a computational procedure for extracting the real and imaginary parts of the optical conductivity of graphene layers from VEELS measurements. Our modeling ultimately provides insight into how the optoelectronic properties of graphene are expected to manifest in the VEELS obtained via monochromated beams, with the effects of graphene doping, the excitation of its plasmon-polaritons, and relativistic contributions included comprehensively.

Magnetic and optical properties of δ-SnSe doped by transition-metal atoms: A first-principle calculation

By employing the first-principles calculations, the effect of transition-metallic doped δ-SnSe monolayer have been investigated. All considered transition-metal (TM, TM = Co, Cu, Mn, Fe, and Ni) dopants induce magnetic moment except for Cu. Based on the binding energies, the Mn- and Fe-doped systems are more structurally stable. We further study the magnetic coupling of these two systems. For the Mn-doped system, its antiferromagnetic (AFM) state is its ground state, and it changes to nonmagnetic (NM) sate as the Mn–Mn distance increases. Owing to the strong spin-orbit coupling between Fe-3d and Se-4p, different phenomena show in the Fe-doped system. Both AFM and ferromagnetic (FM) coupling is observed, which is mainly determined by the Fe–Fe distance and the crystal direction. Moreover, due to the introduction of Cu and Mn atoms, the absorption strength in visible light might be enhanced. These results show that the δ-SnSe monolayer doped with TM atoms might give some potential applications in spintronic devices and optical fields.

Exploring single atom catalysts of transition-metal doped phosphorus carbide monolayer for HER: A first-principles study

• The single Ir atom doped α-PC has much smaller Gibbs free adsorption energy of H atom comparing to the reported Pt(111). • The Gibbs free adsorption energy of H atom is highly related to the d-band center and the period of the doped TM atom. • The Volmer–Heyrovsky step has smaller energy barrier than the Volmer-Tafel step on Ir-αPC. • The Ir-PC is physically and thermally stable according to phonon calculation and AIMD result. Hydrogen has been identified as one of the most promising sustainable and clean energy. Developing hydrogen evolution reaction (HER) catalyst with high activity is essential for satisfying the future requirements. Considering novel advantages of two-dimensional materials and high catalytic activity of atomic transition metal, in this study, using density functional theory calculation, the HER on single transition-metal (23 different TM atoms) doped phosphorus carbide monolayer (α-PC) has been investigated. The Volmer–Tafel and Volmer–Heyrovsky reaction mechanisms, and the stability of the most promising HER catalyst are also included. The results show that Ir-αPC with high physical and thermal stability has the most optimal value of Gibbs free adsorption energy for H atom. The relationship of d band center and the HER activity shows a volcano-like curve. The calculation of reaction energy barrier indicates that the Volmer-Heyrovsky step is more favorable than the Volmer-Tafel step.

Plasmon-Phonon-Polaritons in Encapsulated Phosphorene

We consider a system consists of a doped monolayer phosphorene embedded between two hexagonal Boron Nitride (hBN) slabs along the heterostructure direction. The wavevector azimuthal angle dependence of the plasmon-polariton and plasmon-phonon-polariton modes of the hybrid system are calculated based on the random phase approximation at finite temperature. The collective modes illustrate strong anisotropy and strong coupling with phonon modes of the polar media and furthermore, the Landau damping occurs due to the intraband processes when plasmon enters intraband electron-hole continuum. Our numerical results show that the plasmon mode is highly confined to the surface along the zigzag direction. Owing to the strong electron-phonon interaction, the phonon dispersions in the Reststrahlen bands are also angle-dependent. These results are also in agreement with those of the semiclassical model obtained in our calculations.

Adsorption of small gas molecules on strained monolayer WSe2 doped with Pd, Ag, Au, and Pt: A computational investigation

Herein, we report on the doping of monolayer WSe2 with Pd, Ag, Au, and Pt as a promising approach to improve the gas sensing performance of the 2D material. The adsorption of CO2, NO2 and SO2 gas molecules on Pd, Ag, Au, and Pt-doped monolayer WSe2 are studied in terms of adsorption energy, charge transfer and band structure. Results suggest that NO2 adsorption on Ag-WSe2 monolayer is energetically favorable giving rise to the most stable configuration. NO2 is chemisorb on the doped monolayer surface due to a strong interaction. We study the influence of applying strain on the gas molecule/TM-WSe2 adsorption systems. Increasing the stain, the adsorption properties decrease for NO2. Our results indicate that Ag-WSe2 monolayer is a great candidate for the development of efficient NO2 gas sensors.

Extended 1D defect induced magnetism in 2D MoS2 crystal

Exploring two dimensional (2D) magnetic materials is important for both fundamental research and practical applications in nanoscale spintronics. Although dispersive doping of atoms in 2D nonmagnetic transition-metal dichalcogenides (TMD) has been broadly studied in recent years, the regular linear substitution inside 2D nonmagnetic TMD is rarely explored. Herein, based on first-principles calculations, we report a series of hybrid magnetic structures formed by linear atomic doping in MoS2 monolayer. We demonstrate that F and Fe atoms linear-doped MoS2 are ferromagnetic semi-metals while Mn and Co atoms linear-doped MoS2 are ferromagnetic semiconductors in their ground states. Except for F dopant, the magnetic ground states of Mn, Fe, or Co atom linear-doped MoS2 are independent of the width of linear defect. The thermal and lattice dynamical stabilities of linear-doped MoS2 monolayer are confirmed with the molecular dynamics simulations and phonon spectra. A ferromagnetic semi-metal or semiconductor to half-metallic ferromagnet transition in doped MoS2 monolayer is revealed with applying strain. Further, atomically thin magnetic zones with different shapes can also be achieved by arranging the dopants. The induced magnetic properties render linear-doped MoS2 a promising material for spintronics in the nanoscale.

Orbital localization induced magnetization in nonmetal-doped phosphorene

Density functional theory calculations are performed to explore the nature of magnetization in group-VI (O, S, Se, Te) and group-VII (F, Cl, Br, I) elements doped phosphorene monolayers. All these dopants, except for F, can introduce localized spin-polarized states in the gap of phosphorene, resulting in the magnetization of these systems. Orbital coupling analysis suggests that the distinct spin-polarization behaviors between group-VI-substituted and group-VII-substituted phosphorene result from the different orbital coupling between the dopants and neighboring P atoms. It is also demonstrated that the stability of magnetization, characterized by polarization energy, increases with the localization of spin-polarized bands. Our predictive results may inspire further experimental and theoretical exploration on the potential applications of doped 2D materials in spintronics.

Doping induced charge density wave in monolayer TiS2 and phonon-mediated superconductivity

Using the first-principles calculations, we have investigated the effects of the charge doping and biaxial strain on the charge density wave (CDW) of monolayer octahedral titanium disulfide (1T-TiS2). Our results show that proper electron doping can suppress the (2×2) CDW instability and the larger electron doping promotes a (7×3) CDW phase involving a spontaneous breaking of the hexagonal crystal symmetry. Strain can affect the stability of doped monolayer 1T-TiS2, but neither compressive strain nor tensile strain alone can stabilize the pristine one. When TiS2 layers are decorated by metal atoms, such as Li, Na, K, Ca, and Al, the transferred charge can also suppress the (2×2) CDW instability. The stable doped 1T-TiS2 is predicted to exhibit good phonon-mediated superconductivity, which can be further enhanced by strain. Our results reveal a unique CDW phenomenon in the electron-doped monolayer 1T-TiS2 and that doping combined strain is efficient to modulate phonon softening, electron-phonon coupling, and superconductivity.

Improved photocatalytic property of monolayer MoS2 by B and F co-doping: First principles study

Visible light photocatalysts have drawn wide attention due to their great prospect in solving environmental problems, such as air pollution and water pollution. In this work, using first principles study, we report the doping site, formation energy, electronic and optical properties of B, F single doped and co-doped monolayer MoS2. It is found that B and F co-doping can greatly improve the photocatalytic activity of MoS2 in visible light range. This is attributed to the lower recombination rate of photo-excited carriers, stronger oxidation (or reduction) capacity, higher absorption efficiency of visible light and more catalytic active sites. This work helps to understand the intrinsic photocatalytic mechanisms of MoS2, and provides an important guidance to design more efficient MoS2-based visible light photocatalysts.

Controlling the electronic and optical properties of HfS2 mono-layers via lanthanide substitutional doping: a DFT+U study.

Two dimensional HfS2 is a material with potential applications in the field of photo-catalysis and advanced solid state devices. Density functional theory with the Hubbard U parameter (DFT+U) calculations were carried out to investigate the structural, electronic and optical properties of lanthanide dopant atoms (LN = La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) in the HfS2 mono-layer. The calculated electronic band gap for a pristine HfS2 mono-layer is 1.30 eV with a non-magnetic ground state. The dopant substitutional energies under both Hf-rich and S-rich conditions were evaluated, with the S-rich condition for the dopant atoms being negative. This implies that the incorporation of these LN dopant atoms in the HfS2 is feasible and experimental realization possible. The introduction of LN dopant atoms in the HfS2 mono-layer resulted in a significant change of the material properties. We found that the presence of LN dopant atoms in the HfS2 mono-layer significantly alters its electronic ground states by introducing defect states as well as changes in the overall density of states profile resulting in a metallic ground state for the doped mono-layers. The doped mono-layers are all magnetic with the exception of La and Lu dopant atoms. We found that LN dopant atoms in the HfS2 mono-layer influence the absorption and reflectivity spectra with the introduction of states in the lower frequency range (<1.30 eV). Furthermore, we showed that the applicability of doped HfS2 mono-layers as photo-catalysts is very different compared with the pristine HfS2 mono-layer.

Modulating the electro-optical properties of doped C3N monolayers and graphene bilayers via mechanical strain and pressure

The effect of atomic doping on the electronic properties of C3N monolayer and graphene bilayer is investigated. We found that doped C3N monolayer and doped graphene bilayer are a direct semiconductor. Our result show that the electronic properties of the studied structures can be modulated by electric field and mechanical strain.