Photocatalytic water splitting is a promising renewable energy source as an alternative for limited fossil fuels. The effectiveness of the conversion from solar energy to hydrogen fuel relies primarily on the material. Previously, researchers studied different TMDs such as WS$_{2}$, and PdSe$_{2}$. These materials perform well in certain aspects such as strong adsorption stability and promising abilities for HER, however, their band gaps are still not ideal. In this paper, we studied a new TMD material WSe$_{2}$, which is currently used in heterostructure photocatalysts. To our knowledge, this is the first assessment of using transition metal doped WSe$_{2}$ as potential photocatalysts for photocatalytic water splitting. Using first principles calculations, we evaluated the band gaps and other photocatalytic abilities of pristine WSe2 as well as Cr, Mo, Ta, and Re doped WSe$_{2}$. Compared to previously studied TMD materials, three of our newly studied materials (pristine, Mo doped, and Ta doped WSe$_{2}$)demonstrated more desirable band gaps, which are closer to being ideal (1.23eV); The band edge positions of our materials are also closer to the ideal reduction potential of H$^{+}$/H$_{2}$ and the oxidation potential of O$_{2}$/H$_{2}$O. Furthermore, Mo and Ta doped WSe$_{2}$ monolayers undergo an exothermic process, indicating stable monolayers. Of the three selected materials, pristine WSe$_{2}$ exhibits the strongest water adsorption abilities. Our results substantiates pristine, Mo doped, and Cr doped WSe$_{2}$ as potential photocatalysts for water splitting.
First principles study of chalcogen vacancy effect on the optoelectronic and photocatalytic properties of transition metal dichalcogenides monolayers
Department of Control Engineering, Faculty of Electrical Engineering, Czech Technical University in Prague（布拉格捷克理工大学电气工程学院控制工程系）；Engineering Materials & nCATS, FEE, University of Southampton（南安普顿大学电气工程学院工程材料与nCATS）
Phase Switchable Photocatalytic Water Splitting via a Paraelectric-Ferroelectric Transition in Zr2Ge2S6 Monolayer: A Comprehensive Theoretical Insights
Low-dimensional transition metal dichalcogenides (TMDC) exhibit great photocatalytic performance and tunability. In this work, using first-principles simulations based on density functional theory (DFT), we demonstrate that external electric bias can be employed to further improve the photocatalytic hydrogen production efficiency of the six AB$_{2}$ (A=Mo, W and B=S, Se, Te) TMDC monolayers by exploiting their piezoelectric response. In particular, when subjected to a proper amount of electrically induced tensile biaxial strain, most TMDC monolayers turn into potentially ideal photocatalyst to…
We report on large exciton tuning in WSe$_2$ monolayers via substrate induced non-degenerate doping. We observe a redshift of $\sim$62 meV for the $A$ exciton together with a 1-2 orders of magnitude photoluminescence (PL) quenching when the monolayer WSe$_2$ is brought in contact with highly oriented pyrolytic graphite (HOPG) compared to the dielectric substrates such as hBN and SiO$_2$. As the evidence of doping from HOPG to WSe$_2$, a drastic increase of the trion emission intensity was observed. Using a systematic PL and Kelvin probe force microscopy (KPFM) investigation on WSe$_2$/HOPG, WS…
Photocatalytic water splitting is an effective way to obtain renewable clean energy. The challenge is to design tunable photocatalyst to meet the needs in different environments. At the same time, the oxygen and hydrogen evolution reactions (OER and HER) on the photocatalyst should be separated, which will be conducive to the separation of products. The electronic, optical and photocatalytic properties of Janus X2PAs (X = Si, Ge and Sn) monolayers are explored by first-principles calculation. All the strain-free X2PAs monolayers exhibit excellent photocatalytic properties with suitable band ed…
Semiconducting transition metal dichalcogenides (TMDCs) have attracted a lot of attention recently, because of their interesting electronic, optical, and mechanical properties. Among large numbers of TMDCs, monolayer of tungsten diselenides (WSe2) is of particular interest since it possesses a direct band gap and tunable charge transport behaviors, which make it suitable for a variety of electronic and optoelectronic applications. Direct synthesis of large domains of monolayer WSe2 and their growth mechanism studies are important steps toward applications of WSe2. Here, we report systematical …
We present the electronic and structural properties of monolayer WSe$_{2}$ grown by pulsed-laser deposition on monolayer graphene (MLG) on SiC. The spin splitting in the WSe$_{2}$ valence band at $\overline{\mathrm{K}}$ was $Δ_\mathrm{SO}=0.469\pm0.008$ eV by angle-resolved photoemission spectroscopy (ARPES). Synchrotron-based grazing-incidence in-plane X-ray diffraction (XRD) revealed the in-plane lattice constant of monolayer WSe$_{2}$ to be $a_\mathrm{WSe_2}=3.2757\pm0.0008 \mathrm{Å}$. This indicates a lattice compression of -0.19 % from bulk WSe$_{2}$. By using experimentally determined g…
We report a determination of the complex in-plane dielectric function of monolayers of four transition metal dichalcogenides: MoS2, MoSe2, WS2 and WSe2, for photon energies from 1.5 - 3 eV. The results were obtained from reflection spectra using a Kramers-Kronig constrained variational analysis. From the dielectric functions, we obtain the absolute absorbance of the monolayers. We also provide a comparison of the dielectric function for the monolayers with the corresponding bulk materials.
In this study, the interaction between the gas molecules, including H2O, N2, CO, NO, NO2 and N2O, with the WSe2 monolayer containing a Se vacancy (denoted as VSe) is theoretically studied. It is found that H2O and N2 molecules are highly prone to be physisorbed on the VSe surface. The presence of the Se vacancy should significantly enhance the sensing ability of the WSe2 monolayer toward the H2O and N2 molecules. In contrast, CO and NO molecules highly prefer to be molecularly chemisorbed on the VSe surface with the non-oxygen atom occupying the Se site. Further, the exposed O atoms of the mol…
The monolayer transition metal dichalcogenides have recently attracted much attention owing to their potential in valleytronics, flexible and low-power electronics and optoelectronic devices. Recent reports have demonstrated the growth of large-size 2-dimensional MoS2 layers by the sulfurization of molybdenum oxides. However, the growth of transition metal selenide monolayer has still been a challenge. Here we report that the introduction of hydrogen in the reaction chamber helps to activate the selenization of WO3, where large-size WSe2 monolayer flakes or thin films can be successfully grown…
$WSe_2$ Monolayers Grown by Molecular Beam Epitaxy on hBN
Institute of Experimental Physics, Faculty of Physics, University of Warsaw（华沙大学物理学院实验物理研究所）；Institute of Physics, Polish Academy of Sciences（波兰科学院物理研究所）；Research Center for Materials Nanoarchitectonics, National Institute for Materials Science（国家材料科学研究所材料纳米构造研究中心）；Research Center for Electronic and Optical Materials, National Institute for Materials Science（国家材料科学研究所电子与光学材料研究中心）
In the archetypal monolayer semiconductor WSe$_2$, the distinct ordering of spin-polarized valleys (low-energy pockets) in the conduction band allows for studies of not only simple neutral excitons and charged excitons (i.e., trions), but also more complex many-body states that are predicted at higher electron densities. We discuss magneto-optical measurements of electron-rich WSe$_2$ monolayers, and interpret the spectral lines that emerge at high electron doping as optical transitions of 6-body exciton states ("hexcitons") and 8-body exciton states ("oxcitons"). These many-body states emerge…
We investigate the effect on disorder potential on exciton valley polarization and valley coherence in monolayer WSe2. By analyzing polarization properties of photoluminescence, the valley coherence (VC) and valley polarization (VP) is quantified across the inhomogeneously broadened exciton resonance. We find that disorder plays a critical role in the exciton VC, while minimally affecting VP. For different monolayer samples with disorder characterized by their Stokes Shift (SS), VC decreases in samples with higher SS while VP again remains unchanged. These two methods consistently demonstrate …
The optical properties of two-dimensional transition metal dichalcogenide monolayers such as MoS$_2$ or WSe$_2$ are dominated by excitons, Coulomb bound electron-hole pairs. Screening effects due the presence of hexagonal-BN surrounding layers have been investigated by solving the Bethe Salpeter Equation on top of GW wave functions in density functional theory calculations. We have calculated the dependence of both the quasi-particle gap and the binding energy of the neutral exciton ground state E$_b$ as a function of the hBN layer thickness. This study demonstrates that the effects of screeni…
Doping a two-dimensional semiconductor with magnetic atoms is a possible route to induce magnetism in the material. We report on the atomic structure and electronic properties of monolayer WSe$_2$ intentionally doped with vanadium atoms by means of scanning transmission electron microscopy and scanning tunneling microscopy and spectroscopy. Most of the V atoms incorporate at W sites. These V$_W$ dopants are negatively charged, which induces a localized bound state located 140 meV above the valence band maximum. The overlap of the electronic potential of two charged V$_W$ dopants generates addi…
We explore the photocatalytic ability of honeycomb lattice of borocarbonitride, $BC_{6}N$, investigating its opto-electronic properties and band-edge alignment based on hybrid density functional theory. We observe that along with pronounced visible absorbance, $BC_{6}N$ exhibits a good reducing ability. It is capable of depositing heavy metal ions, hydrogen fuel generation and carbon dioxide sequestration through photoelectrocatalysis. To broaden its applicability we further tune its redox ability using biaxial strain. We observe even a slight expansion makes this material capable of performin…
Monolayers of transition metal dichalcogenide materials emerged as a new material class to study excitonic effects in solid state, since they benefit from enormous coulomb correlations between electrons and holes. Especially in WSe2, sharp emission features have been observed at cryogenic temperatures, which act as single photon sources . Tight exciton localization has been assumed to induce an anharmonic excitation spectrum, however, the evidence of the hypothesis, namely the demonstration of a localized biexciton, is elusive. Here, we unambiguously demonstrate the existence of a localized bi…
The search for high-quality transition metal dichalcogenides mono- and multi-layers grown on large areas is still a very active field of investigation nowadays. Here, we use molecular beam epitaxy to grow 15$\times$15 mm large WSe$_2$ on mica in the van der Waals regime. By screening one-step growth conditions, we find that very high temperature ($>$900$^{\circ}$C) and very low deposition rate ($<$0.015 nm/min) are necessary to obtain high quality WSe$_2$ films. The domain size can be larger than 1 $μ$m and the in-plane rotational misorientation less than $\pm$0.5$^{\circ}$. The WSe$_2$ monola…
Monolayer transition-metal dichalcogenides are direct gap semiconductors with great promise for optoelectronic devices. Although spatial correlation of electrons and holes plays a key role, there is little experimental information on such fundamental properties as exciton binding energies and band gaps. We report here an experimental determination of exciton excited states and binding energies for monolayer WS2 and WSe2. We observe peaks in the optical reflectivity/absorption spectra corresponding to the ground- and excited-state excitons (1s and 2s states). From these features, we determine l…
Diluted magnetic semiconductors including Mn-doped GaAs are attractive for gate-controlled spintronics but Curie transition at room temperature with long-range ferromagnetic order is still debatable to date. Here, we report the room-temperature ferromagnetic domains with long-range order in semiconducting V-doped WSe2 monolayer synthesized by chemical vapor deposition. Ferromagnetic order is manifested using magnetic force microscopy up to 360K, while retaining high on/off current ratio of ~105 at 0.1% V-doping concentration. The V-substitution to W sites keep a V-V separation distance of 5 nm…
Graphene like 2D materials have attracted tremendous attention in the field of photocatalytic water splitting. Indium Selenide (InSe) is one such potential material. Here, we report the effect of Tellurium on InSe monolayer for the photocatalytic water splitting by means of density functional calculations. The calculated bandgaps fall within the visible region of the solar spectrum indicating these materials could absorb a significant amount of solar light. Density of states calculations show that the covalent character is present between the atoms which is typical for the layered system. The …
Two-dimensional vertically polarized Hg3AsSe4I monolayer for efficient photocatalytic water-splitting: promoting carrier separation by intrinsic electric field and Rashba effect
Frontier Institute of Science and Technology, State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong University（西安交通大学前沿科学研究所、电气绝缘与电源设备国家重点实验室）；Key Lab of advanced optoelectronic quantum architecture and measurement (MOE), Beijing Key Laboratory of Quantum Matter State Control and Ultra-Precision Measurement Technology, and School of Physics, Beijing Institute of Technology（北京理工大学物理学院、光电量子架构与测量教育部重点实验室、北京市量子物质态控制与超精密测量技术重点实验室）
H$_2$ evolution reaction (HER) requires an electrocatalyst to reduce the reaction barriers for the efficient production of H$_2$. Platinum-group metal (PGM) elements such as Pt, Pd, etc. and their derivatives show excellent electrocatalytic activity for HER. The high cost and lack of availability of PGM elements bring constraints over their wide commercial applications, so discovering noble metal-free electrocatalysts with lower possible reaction barriers is paramount important. Two-Dimensional Transition Metal Dichalcogenides (2D TMDs) have emerged as a pinnacle group of materials for many po…
Near-field optical microscopy can be used as a viable route to understand the nanoscale material properties below the diffraction limit. On the other hand, atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs) are the materials of recent interest to study the spatial confinement of charge carriers, photon, and phonons. Heterostructures based on Mo or W based monolayer TMDs form type-II band alignment, and hence the optically excited carriers can be easily separated for applications pertaining to photonics and electronics. Mapping these spatially confined carriers or phot…
Atomically thin two-dimensional crystals have revolutionized materials science. In particular, monolayer transition metal dichalcogenides promise novel optoelectronic applications, due to their direct energy gaps in the optical range. Their electronic and optical properties, however, are complicated by exotic room-temperature excitons, whose fundamental structure and dynamics has been under intense investigation. While interband spectroscopy probes energies of excitons with vanishing centre-of-mass momenta, the majority of excitons has remained elusive, raising questions about their unusual in…
Many of the fundamental optical and electronic properties of atomically thin transition metal dichalcogenides are dominated by strong Coulomb interactions between electrons and holes, forming tightly bound atom-like excitons. Here, we directly trace the ultrafast formation of excitons by monitoring the absolute densities of bound and unbound electron-hole pairs in monolayers of WSe$_2$ following femtosecond non-resonant optical excitation. To this end, phase-locked mid-infrared probe pulses and field-sensitive electro-optic sampling are used to map out the full complex-valued optical conductiv…
Two-dimensional (2D) transition metal dichalcogenides MX2 (M = Mo, W, X = S, Se, Te) attracts enormous research interests in recent years. Its 2H phase possesses an indirect to direct bandgap transition in 2D limit, and thus shows great application potentials in optoelectronic devices [1]. The 1T' crystalline phase transition can drive the monolayer MX2 to be a 2D topological insulator. Here we realized the molecular beam epitaxial (MBE) growth of both the 1T' and 2H phase monolayer WSe2 on bilayer graphene (BLG) substrate. The crystalline structures of these two phases were characterized usin…
Monolayers (MLs) of MoS2 and WSe2 are 2D semiconductors with strong, direct optical transitions that are governed by tightly Coulomb bound eletron-hole pairs (excitons). The optoelectronic properties of these transition metal dichalcogenides are directly related to the inherent crystal inversion symmetry breaking. It allows for efficient second harmonic generation (SHG) and is at the origin of chiral optical selections rules, which enable efficient optical initialization of electrons in specific K-valleys in momentum space. Here we demonstrate how these unique non-linear and linear optical pro…
Mirror twin boundary (MTB) brings unique 1D physics and properties into two-dimensional transition metal dichalcogenides (TMDCs), but they were rarely observed in non-Mo-based TMDCs. Herein, by post-growth Nb doping, high density 4|4E-W and 4|4P-Se MTBs were introduced into molecular beam epitaxy (MBE) grown WSe2 monolayers. Of them, 4|4E-W MTB with a novel structure was discovered experimentally for the first time, while 4|4P-Se MTBs present a random permutations of W and Nb, forming a 1D alloy system. Comparison between the doped and non-doped WSe2 confirmed that Nb dopants are essential for…
We present a detailed first principles density functional theory study of intrinsic and extrinsic point defects in monolayer (ML) WSe2. Among the intrinsic point defects, Se vacancies (Sevac) have the lowest formation energy (disregarding Se adatoms that can be removed with annealing). The defects with the next smallest formation energies (at least 1 eV larger) are SeW (Se substituting W atoms in an antisite defect), Wvac (W vacancies) and 2Sevac (Se divacancies). All these intrinsic defects have gap states that are not spin-polarized. The presence of a graphite substrate does not change the f…
We report the fabrication and characterization of gate-defined hole quantum dots in monolayer and bilayer WSe$_2$. The devices were operated with gates above and below the WSe$_2$ layer to accumulate a hole gas, which for some devices was then selectively depleted to define the dot. Temperature dependence of conductance in the Coulomb blockade regime is consistent with transport through a single level, and excited state transport through the dots was observed at temperatures up to 10 K. For adjacent charge states of a bilayer WSe$_2$ dot, magnetic field dependence of excited state energies was…
2D semiconducting nanosheets of Transition Metal Dichalcogenides are attractive materials for solar energy conversion because of their unique absorption properties. Here, we propose Mo thio- and oxo-thio-complexes anchored on 2D p-WSe2 nanosheets for efficient water splitting under visible light irradiation with photocurrent density up to 2.0 mA cm-2 at -0.2 V/NHE. Besides developing high electro-catalytic activity, the Mo complexe films were shown to display ability to heal surface defects. We propose that the observed healing of surface defects arises from the strong adsorption on point defe…
Very recently, the two-dimensional (2D) form of MoSi2N4 has been successfully fabricated [Hong et al., Sci. 369, 670 (2020)]. Motivated by theses recent experimental results, herein we investigate the structural, mechanical, thermal, electronic, optical and photocatalytic properties using hybrid density functional theory (HSE06-DFT). Phonon band dispersion calculations reveal the dynamical stability of MoSi2N4 monolayer structure. Furthermore, the mechanical study confirms the stability of MoSi2N4 monolayer. As compared to the corresponding value of graphene, we find the Youngs modulus decreas…
Linear and nonlinear optical response of WSe$_{2}$ monolayer is investigated by two-dimensional Maxwell plus time-dependent density functional theory with spin-orbit interaction. By applying the chiral resonant pulses, the electron dynamics along with high harmonic generation are examined at weak and strong laser fields. WSe$_{2}$ monolayer shows the linear optical response at the intensity I = 10$^{10}$~W/cm$^{2}$ while a complex nonlinear behavior is observed at I = 10$^{12}$~W/cm$^{2}$. The nonlinear response of WSe$_{2}$ monolayer in terms of saturable absorption is observed at strong lase…
The optical properties of monolayer and bilayer transition metal dichalcogenide semiconductors are governed by excitons in different spin and valley configurations, providing versatile aspects for van der Waals heterostructures and devices. We present experimental and theoretical studies of exciton energy splittings in external magnetic field in neutral and charged WSe$_2$ monolayer and bilayer crystals embedded in a field effect device for active doping control. We develop theoretical methods to calculate the exciton $g$-factors from first principles and tight-binding for all possible spin-va…
Since hydrogen fuel involves the highest energy density among all fuels, production of this gas through the solar water splitting approach has been suggested as a green remedy for greenhouse environmental issues due to extensive consumption of fossil fuels. Low dimensional materials possessing a large surface-to-volume ratio can be a promising candidate to be used for the photocatalytic approach. Here, we used extensive first principles calculations to investigate the application of newly fabricated members of two dimensional carbon nitrides including tg-C3N4, hg-C3N4, C2N, and C3N for water s…
Recently, hydrogen generation by water-splitting photocatalysts is attracting attention as a sustainable and clean energy resource. Photocatalytic hydrogen-generation systems are much simpler, cheaper, and easier to scale up than the coupled systems of electrolysis and solar cells, wind-power generation, etc. However, photocatalytic hydrogen generation is currently inefficient. This paper proposes the monolayer of boron phosphide as a stable highly-efficient water-splitting photocatalyst by high-precision density-functional theory calculations using a HSE06 functional with a solvent effect. Th…
Monolayer WSe$_2$ hosts a series of exciton Rydberg states denoted by the principal quantum number n = 1, 2, 3, etc. While most research focuses on their absorption properties, their optical emission is also important but much less studied. Here we measure the photoluminescence from the 1s - 5s exciton Rydberg states in ultraclean monolayer WSe$_2$ encapsulated by boron nitride under magnetic fields from -31 T to 31 T. The exciton Rydberg states exhibit similar Zeeman shifts but distinct diamagnetic shifts from each other. From their luminescence spectra, Zeeman and diamagnetic shifts, we dedu…
The monolayer WSe2 is interesting and important for future application in nanoelectronics, spintronics and valleytronics devices, because it has the largest spin splitting and longest valley coherence time among all the known monolayer transition-metal dichalcogenides (TMDs). To obtain the large-area monolayer TMDs' crystal is the first step to manufacture scalable and high-performance electronic devices. In this letter, we have successfully fabricated millimeter-sized monolayer WSe2 single crystals with very high quality, based on our improved mechanical exfoliation method. With such superior…
In monolayer transition-metal dichalcogenide semiconductors, many-body correlations can manifest in optical spectra when photoexcited electron-hole pairs (excitons) are introduced into a 2D Fermi sea of mobile carriers. At low carrier densities, the formation of positively and negatively charged excitons ($X^\pm$) is well documented. However, in WSe$_2$ monolayers, an additional absorption resonance, often called $X^{-\prime}$, emerges at high electron density. Its origin is not understood. Here we investigate the $X^{-\prime}$ state via polarized absorption spectroscopy of electrostatically-g…
The variety of excitonic states in tungsten-based dichalcogenide monolayers stems from unique interplay between the spin and valley degrees of freedom. One of the exciton species is the indirect exciton (momentum or valley dark), which is responsible to a series of resonances when the monolayer is charge neutral. We investigate the short-range electron-hole exchange interaction of the indirect exciton, as well as its recombination mechanism mediated by impurities. The analysis provides thorough understanding of the energy and polarization of the zero-phonon indirect exciton resonance in the em…
Two-dimensional materials and their heterostructures have opened up new possibilities for magnetism at the nanoscale. In this study, we utilize first-principles simulations to investigate the structural, electronic, and magnetic properties of $\textrm{Fe}/\textrm{WSe}_2/\textrm{Pt}$ systems containing pristine, defective, or doped $\textrm{WSe}_2$ monolayers. The proximity effects of the ferromagnetic Fe layer are studied by considering defective and vanadium-doped $\textrm{WSe}_2$ monolayers. All heterostructures are found to be ferromagnetic, and the insertion of the transition-metal dichalc…
Janus transition-metal dichalcogenide monolayers are fully artificial materials, where one plane of chalcogen atoms is replaced by chalcogen atoms of a different type. Theory predicts an in-built out-of-plane electric field, giving rise to long-lived, dipolar excitons, while preserving direct-bandgap optical transitions in a uniform potential landscape. Previous Janus studies had broad photoluminescence (>15 meV) spectra obfuscating their excitonic origin. Here, we identify the neutral, and negatively charged inter- and intravalley exciton transitions in Janus WSeS monolayer with $\sim 6$ meV …
We report the observation and gate manipulation of intrinsic dark trions in monolayer WSe$_2$. By using ultraclean WSe$_2$ devices encapsulated by boron nitride, we directly resolve the weak photoluminescence of dark trions. The dark trions can be tuned continuously between negative and positive charged trions with electrostatic gating. We also reveal their spin triplet configuration and distinct valley optical emission by their characteristic Zeeman splitting under magnetic field. The dark trions exhibit large binding energy (14-16 meV). Their lifetime (~1.3 ns) is two orders of magnitude lon…
