Defect states in magnetically "seasoned" WSSe - adsorption and doping effects of magnetic transition metals X = V, Cr, Mn, Fe, Co - a comprehensive first-principles study

Besides the symmetry breaking of Janus transition metal dichalcogenides (TMDs), Janus-based Diluted Magnetic Semiconductors (DMS) are attractive to study considering the local symmetry of transition metal (TM) dopant/adatom. This study conducts a first-principles calculation of magnetic properties in TM (V, Cr, Mn, Fe, and Co) -- doped and adsorbed Janus WSSe. Our results reveal that TM's atomic/ionic size impacts d-p-d orbital overlap, affecting bond length/angle and defect state positions. The result shows that V-doped WSSe exhibits long-range ferromagnetic order sustained by itinerant carriers and W atom spin-orbital coupling (SOC). The mechanism of d-p-d orbital hybridization is highlighted with an overlapping density of states and spin-density plots. Moreover, an enhanced magnetic anisotropy energy (MAE) is observed in Fe/Co-doped and Mn/Fe-adsorbed systems, with the easy axis aligning to the c-axis. The orbital contribution of MAE provided explains the relationship between states near the Fermi and MAE. On the other hand, the adsorption of V and Cr aligns the easy axis to the a-b plane, for which we have systematically explained the contribution of the spin-flip term in MAE. This research provides insights and a guideline for further exploration of 2D DMS for spintronics and spin-related phenomena.

Improving the Luminescence Performance of Monolayer MoS2 by Doping Multiple Metal Elements with CVT Method

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) draw much attention as critical semiconductor materials for 2D, optoelectronic, and spin electronic devices. Although controlled doping of 2D semiconductors can also be used to tune their bandgap and type of carrier and further change their electronic, optical, and catalytic properties, this remains an ongoing challenge. Here, we successfully doped a series of metal elements (including Hf, Zr, Gd, and Dy) into the monolayer MoS2 through a single-step chemical vapor transport (CVT), and the atomic embedded structure is confirmed by scanning transmission electron microscope (STEM) with a probe corrector measurement. In addition, the host crystal is well preserved, and no random atomic aggregation is observed. More importantly, adjusting the band structure of MoS2 enhanced the fluorescence and the carrier effect. This work provides a growth method for doping non-like elements into 2D MoS2 and potentially many other 2D materials to modify their properties.

Advances in Two-Dimensional Magnetic Semiconductors via Substitutional Doping of Transition Metal Dichalcogenides

Transition metal dichalcogenides (TMDs) are two-dimensional (2D) materials with remarkable electrical, optical, and chemical properties. One promising strategy to tailor the properties of TMDs is to create alloys through a dopant-induced modification. Dopants can introduce additional states within the bandgap of TMDs, leading to changes in their optical, electronic, and magnetic properties. This paper overviews chemical vapor deposition (CVD) methods to introduce dopants into TMD monolayers, and discusses the advantages, limitations, and their impacts on the structural, electrical, optical, and magnetic properties of substitutionally doped TMDs. The dopants in TMDs modify the density and type of carriers in the material, thereby influencing the optical properties of the materials. The magnetic moment and circular dichroism in magnetic TMDs are also strongly affected by doping, which enhances the magnetic signal in the material. Finally, we highlight the different doping-induced magnetic properties of TMDs, including superexchange-induced ferromagnetism and valley Zeeman shift. Overall, this review paper provides a comprehensive summary of magnetic TMDs synthesized via CVD, which can guide future research on doped TMDs for various applications, such as spintronics, optoelectronics, and magnetic memory devices.

Influence of Ce, Nd, Eu and Tm Dopants on the Properties of InSe Monolayer: A First-Principles Study

Doping of foreign atoms may substantially alter the properties of the host materials, in particular low-dimension materials, leading to many potential functional applications. Here, we perform density functional theory calculations of two-dimensional InSe materials with substitutional doping of lanthanide atoms (Ce, Nd, Eu, Tm) and investigate systematically their structural, magnetic, electronic and optical properties. The calculated formation energy shows that the substitutional doping of these lanthanide atoms is feasible in the InSe monolayer, and such doping is more favorable under Se-rich than In-rich conditions. As for the structure, doping of lanthanide atoms induces visible outward movement of the lanthanide atom and its surrounding Se atoms. The calculated total magnetic moments are 0.973, 2.948, 7.528 and 1.945 μB for the Ce-, Nd-, Eu-, and Tm-doped systems, respectively, which are mainly derived from lanthanide atoms. Further band structure calculations reveal that the Ce-doped InSe monolayer has n-type conductivity, while the Nd-doped InSe monolayer has p-type conductivity. The Eu- and Tm-doped systems are found to be diluted magnetic semiconductors. The calculated optical response of absorption in the four doping cases shows redshift to lower energy within the infrared range compared with the host InSe monolayer. These findings suggest that doping of lanthanide atoms may open up a new way of manipulating functionalities of InSe materials for low-dimension optoelectronics and spintronics applications.

Estimation ofsp-dexchange constants revisited

We present a new computational method for estimating thesp-dexchange constant,Jeffsp-d, applicable to transition metal doped diluted magnetic semiconductors, transition metal oxides, and 2D- and 3D- dichalcogenides. The proposed method is based on results describing the variation of the magnetic features of a doped system with the variation of its magnetization density (M). The results forJeffsp-d(M)obtained with the proposed method are compared with the corresponding results,Jeffsp-d(ΔEVBM), obtained from estimations of the spin electron orbital splitting, ΔE^VBM, at the valence band maximum (VBM). The latter is estimated in two ways; either directly from plots of the band structure calculations or by calculating the energy difference between the band-centers of the spin-up and spin-down electron density of states of the doped systems. Despite the inherent drawbacks in these two estimation methods for ΔE^VBM, they lead to equivalent results and the correspondingJeffsp-d(ΔEVBM)are in good agreement with theJeffsp-d(M)ones.Ab initioresults obtained for the 2D-MoS₂doped with 3d-series transition metals are presented to demonstrate the validity and applicability of the proposed computational schemes for obtainingJeffsp-d. The proposed methods can be utilized as useful tools in the search of new materials for spintronics and valleytronics applications.

Magnetism manipulation of Co n -adsorbed monolayer WS2 through charge injection

The design and manipulation of magnetism in low-dimensional systems are desirable for the development of spin electronic devices. Here, we design two kinds of Co-adsorbed monolayer WS₂ frameworks, i.e. Co₁/WS₂ and Co₂/WS₂, and comprehensively explore the dependences of their magnetic properties on injected charge by using first-principles calculations. The value of magnetic moment can be tuned almost linearly through injecting charge due to the modulated interaction and charge transferring between Co atom and monolayer WS₂. A transition from ferromagnetism to non-ferromagnetism occurs in Co₁/WS₂ system when 1 e/unit cell charge is injected. Furthermore, the magnetic anisotropy can be manipulated by injecting charge as well. The magnetic anisotropy energy (MAE) in Co₁/WS₂ system sharply increases from -4.16 to 2.47 (0.99) meV when injected charge vary from 0.0 to 0.2 (-0.2) e/unit cell, meaning a transition of the magnetic easy axis from in-plane to out-of-plane direction. Similarly, in Co₂/WS₂ system, the magnetic easy axis also can be modified to out-of-plane direction through injecting 0.1 e/unit cell charge. It is found that the changes of Co-3d states are responsible for the tunable magnetic anisotropy. This work provides a theoretical understanding on effective manipulation of magnetism in low-dimensional system.

Dual-Additive Assisted Chemical Vapor Deposition for the Growth of Mn-Doped 2D MoS2 with Tunable Electronic Properties

Doping of bulk silicon and III-V materials has paved the foundation of the current semiconductor industry. Controlled doping of 2D semiconductors, which can also be used to tune their bandgap and type of carrier thus changing their electronic, optical, and catalytic properties, remains challenging. Here the substitutional doping of nonlike element dopant (Mn) at the Mo sites of 2D MoS₂ is reported to tune its electronic and catalytic properties. The key for the successful incorporation of Mn into the MoS₂ lattice stems from the development of a new growth technology called dual-additive chemical vapor deposition. First, the addition of a MnO₂ additive to the MoS₂ growth process reshapes the morphology and increases lateral size of Mn-doped MoS₂ . Second, a NaCl additive helps in promoting the substitutional doping and increases the concentration of Mn dopant to 1.7 at%. Because Mn has more valance electrons than Mo, its doping into MoS₂ shifts the Fermi level toward the conduction band, resulting in improved electrical contact in field effect transistors. Mn doping also increases the hydrogen evolution activity of MoS₂ electrocatalysts. This work provides a growth method for doping nonlike elements into 2D MoS₂ and potentially many other 2D materials to modify their properties.

Photogalvanic Effect in Nitrogen-Doped Monolayer MoS2 from First Principles

We investigate the photogalvanic effect in nitrogen-doped monolayer molybdenum disulfide (MoS₂) under the perpendicular irradiation, using first-principles calculations combined with non-equilibrium Green function formalism. We provide a detailed analysis on the behavior of photoresponse based on the band structure and in particular the joint density of states. We thereby identify different mechanisms leading to the existence of zero points, where the photocurrent vanishes. In particular, while the zero point in the linear photovoltaic effect is due to forbidden transition, their appearance in the circular photovoltaic effect results from the identical intensity splitting of the valance band and the conduction band in the presence of Rashba and Dresslhaus spin-orbit coupling. Furthermore, our results reveal a strong circular photogalvanic effect of nitrogen-doped monolayer MoS₂, which is two orders of magnitude larger than that induced by the linearly polarized light.

Dependence of the magnetic interactions in MoS2 monolayer on Mn-doping configurations

Understanding the magnetic properties of the various Mn doping configurations that can be encountered in 2H-MoS₂ monolayer could be beneficial for its use in spintronics. Using density functional theory plus Hubbard term (DFT  +  U) approach, we study how a single isolated, double- and triple-substitution configurations of Mn atoms within a MoS₂ monolayer could contribute to its total magnetization. We find that the doping-configuration plays a critical role in stabilizing a ferromagnetic state in a Mn-doped MoS₂ monolayer. Indeed, the Mn-Mn magnetic interaction is found to be ferromagnetic and strong for Mn in equidistant substitution positions where the separation average range of 6-11 [Formula: see text]. The strongest ferromagnetic interaction is found when substitutions are in second nearest neighbor Mo-sites of the armchair chain. Clustering is energetically favorable but it strongly reduces the ferromagnetic exchange energies. Furthermore, in term of electronic properties, we show that the Mn-doped MoS₂ monolayer can change its electronic behavior from semiconductor to half-metallic depending on the doping configuration. Our results suggest that ordering the Mn dopants on MoS₂ monolayer is needed to increase its potential ferromagnetism.

Strain engineering on the electronic states of two-dimensional GaN/graphene heterostructure

Combining two different layered structures to form a van der Waals (vdW) heterostructure has recently emerged as an intriguing way of designing electronic and optoelectronic devices. Effects of the strain on the electronic properties of GaN/graphene heterostructure are investigated by using first-principles calculation. In the GaN/graphene heterostructure, the strain can control not only the Schottky barrier, but also contact types at the interface. Moreover, when the uniaxial strain is above -1% or the biaxial strain is above 0%, the contact type transforms to ohmic contact. These results provide a detailed understanding of the interfacial properties of GaN/graphene and help to predict the performance of the GaN/graphene heterostructure on nanoelectronics and nanocomposites.

Effect of interfacial defects on the electronic properties of graphene/g-GaN heterostructures

To gain deep insights into their interactions, the effects of interfacial defects on the structural and electronic properties of graphene/g-GaN heterostructures were investigated by using first-principles calculations. The graphene/g-GaN-VGa heterostructure maintains a p-type Schottky contact in the spin-up channel and the Schottky barrier height (SBH) is decreased to 0.332 eV, but there is not a metal/semiconductor contact in the spin-down channel. However, the n-type SBH is negative for the graphene/g-GaN-VN heterostructure, indicating an ohmic contact. Furthermore, the SBH in the graphene/g-GaN heterostructure can be effectively modulated by the interlayer distance. The research could provide a strategy for the development and fabrication of efficient novel nanoelectronic devices.

Enhanced magnetic properties and tunable Dirac point of graphene/Mn-doped monolayer MoS2 heterostructures

Graphene is one of the most promising spintronic materials due to its high carrier mobility. However, the absence of a band gap and ferromagnetic order in graphene seriously limit its applications in spintronics. How to utilize its high carrier mobility as well as mediate its electronic structure remains a challenge. Herein, we design a novel composite, which is composed of graphene and Mn-doped monolayer MoS₂. The magnetic properties and electronic structures of graphene/Mn-doped monolayer MoS₂ heterostructures were studied by using density functional theory (DFT) with the van der Waals (vdW) correlations (DFT-D). We found that the heterostructures show increased magnetic moments and more stable ferromagnetic (FM) states compared with that of isolated Mn-doped MoS₂ monolayer. Our further studies show that many electrons are transferred to Mn-doped MoS₂ monolayer from graphene, which causes the Fermi level to shift down below the Dirac cone about 0.59 eV. The transfered electrons also enhance the FM coupling between Mn ions. Graphene is partially spin polarized because of the magnetic proximity effect, which leads to the spin-dependent gaps for spin-up (16.1 meV) and spin-down (5 meV) at Dirac point, respectively. The introduction of sulfur (S) vacancy to the interface results in a much more stable FM structure and a higher total magnetic moment of the FM state; furthermore, it raises the spin polarization of graphene π orbitals and opens up a small band gap of about 7 meV. These findings propose a new route to facilitate the design of spintronic devices which both need stable ferromagnetism and finite band gap.