Reducing the Schottky barrier height (SBH) of metal–MoS2 interface with no deteriorating the intrinsic properties of MoS2 channel layer is crucial to realize the high-performance MoS2 nanodevice. To realize this expectation, a promising approach is present in this study by doping the boron nitride (BN) buffer layer between metal electrode and MoS2 channel layer. Results demonstrate that no matter the types of concentrations and dopants the intrinsic electronic structure, low electron effective mass of MoS2 channel layer, and the weak Fermi level pinning effects of metal/BN–MoS2 interfaces are preserved and not deteriorated. More importantly, the n- and p-type SBHs of metal/BN–MoS2 interfaces are significantly reduced by the electron-poor and -rich dopants, respectively, when the doped BN buffer layer spreads all over the nanodevice, which is in contrast to the traditional doping rule. Moreover, both the n- and p-type SBHs are further decreased and even eliminated when the concentrations of dopants increase. The n-type SBH of doped Au/BxN–MoS2 interface and the p-type SBH of doped Pt/BNx–MoS2 interface can be reduced to −0.21 and −0.61 eV by doping with high concentrations of Li and O, respectively. This theoretical work provides an effective and promising method to realize high-performance MoS2 nanodevices with negligible SBHs.Keywords: BN buffer layer; density functional theory; doping; MoS2; Schottky barrier;

2D materials, represented by transition metal dichalcogenides (TMDs), have attracted tremendous research interests in photoelectronic and electronic devices. However, for their relatively small bandgap (<2 eV), the application of traditional TMDs into solar-blind ultraviolet (UV) photodetection is restricted. Here, for the first time, NiPS3 nanosheets are grown via chemical vapor deposition method. The nanosheets thinning to 3.2 nm with the lateral size of dozens of micrometers are acquired. Based on the various nanosheets, a linearity is found between the Raman intensity of specific Ag modes and the thickness, providing a convenient method to determine their layer numbers. Furthermore, a UV photodetector is fabricated using few-layered 2D NiPS3 nanosheets. It shows an ultrafast rise time shorter than 5 ms with an ultralow dark current less than 10 fA. Notably, this UV photodetector demonstrates a high detectivity of 1.22 × 1012 Jones, outperforming some traditional wide-bandgap UV detectors. The wavelength-dependent photoresponsivity measurement allows the direct observation of an admirable cut-off wavelength at 360 nm, which indicates a superior spectral selectivity. The promising photodetector performance, accompanied with the controllable fabrication and transfer process of nanosheet, lays the foundation of applying 2D semiconductors for ultrafast UV light detection.

Metal/insertion–MoS2 sandwich interfaces are designed to reduce the Schottky barriers at metal–MoS2 interfaces. The effects of geometric and electronic structures of two-dimensional (2D) insertion materials on the contact properties of metal/insertion–MoS2 interfaces are comparatively studied by first-principles calculations. Regardless of the geometric and electronic structures of 2D insertion materials, Fermi level pinning effects and charge scattering at the metal/insertion–MoS2 interface are weakened due to weak interactions between the insertion and MoS2 layers, no gap states and negligible structural deformations for MoS2 layers. The Schottky barriers at metal/insertion–MoS2 interfaces are induced by three interface dipoles and four potential steps that are determined by the charge transfers and structural deformations of 2D insertion materials. The lower the electron affinities of 2D insertion materials, the more are the electrons lost from the Sc surface, resulting in lower n-type Schottky barriers at Sc/insertion–MoS2 interfaces. The larger the ionization potentials and the thinner the thicknesses of 2D insertion materials, the fewer are the electrons that accumulate at the Pt surface, leading to lower p-type Schottky barriers at Pt/insertion–MoS2 interfaces. All Sc/insertion–MoS2 interfaces exhibited ohmic characters. The Pt/BN–MoS2 interface exhibits the lowest p-type Schottky barrier of 0.52 eV due to the largest ionization potential (∼6.88 eV) and the thinnest thickness (single atomic layer thickness) of BN. These results in this work are beneficial to understand and design high performance metal/insertion–MoS2 interfaces through 2D insertion materials.

Using first-principles calculations within density functional theory, vacancies in the BN buffer layer have been predicted to improve the Schottky barrier of the metal–MoS2 interface without deteriorating the intrinsic properties of the MoS2 layer. Here, the effects of concentrations, sizes and types of vacancies on the contact properties of metal/BN–MoS2 sandwich interfaces are comparatively studied. The results show that vacancies in the BN buffer layer not only don't deteriorate the charge scatterings and electronic properties of the MoS2 layer at the metal/BN–MoS2 interface, but also improve the charge density and contact resistance between the metal surface and the BN layer. Although these vacancies have a negligible influence on the Fermi level pinning effect of the metal/BN–MoS2 interface, both N-vacancies and B-vacancies significantly change the position of the Fermi level of the metal/BN–MoS2 interface and then tune the Schottky barriers. Moreover, the Schottky barriers of metal/BN–MoS2 interfaces can decrease at first with the increasing concentrations and sizes of vacancies. When the concentration of vacancies increases to 4%, the Schottky barriers of metal/BN–MoS2 interfaces can reduce to the minimum value. The lowest n-type and p-type Schottky barriers of Au/BN–MoS2 and Pt/BN–MoS2 interfaces can reduce to −0.16 and 0.28 eV, respectively. However, the Schottky barriers are deteriorated when the sizes and concentrations of vacancies continue to increase because vacancies with large sizes and concentrations obviously change the interfacial structures of metal/BN–MoS2 interfaces and disarrange the directions of interface dipoles. The predictions in this work provide a non-invasive method to achieve high performance metal–MoS2 interfaces with low Schottky barriers.

Novel MoS2/(MX2)n lateral and (MoS2)/(MX2)n–BN hybrid heterostructures have been designed on monolayer MoS2 to extend its applications. The electronic, interfacial and optical properties of the lateral and hybrid heterostructures have been investigated comparatively using first-principles calculations. It was found that the charge distributions, band gaps, band levels, electrostatic potentials, and optical absorption of the MoS2/(MX2)n lateral heterostructures depend greatly on the width n of MX2, irrespective of the size of the lateral heterostructures. The CBM states of the MoS2/(MX2)n lateral heterostructures dominated by the dz2 orbitals are localized around MoS2, whereas the VBM states of the MoS2/(MX2)n lateral heterostructures are dominated by the MX2 region. Through regulating the width n of the MX2 region in the MoS2/(MX2)n lateral heterostructures, the optical absorption of the lateral heterostructures under visible light can be increased, and the CBM and VBM states of the lateral heterostructures can be located above the hydrogen reduction potential and below the water oxidation potential, respectively. The similar characteristics were observed in the MoS2/(MX2)n–BN hybrid heterostructures, indicating that BN is a good substrate for the MoS2/(MX2)n lateral heterostructures. The analysis implies that forming the lateral and hybrid heterostructures is an effective way to extend the applications of monolayer MoS2 in photocatalytic water and photovoltaic devices.

The Schottky barrier has been detected in many field-effect transistors (FETs) based on transition metal dichalcogenide (TMD) semiconductors and has seriously affected the electronic properties of the devices. In order to decrease the Schottky barrier in WS2 FETs, novel Nb doping in WS2 monolayers has been performed and p-FETs based on Nb-doped WS2 (NbxW1−xS2) monolayers as the active channel have been fabricated for the first time. The monolayer Nb0.15W0.85S2 p-FET has a drain current of 330 μA μm−1, an impressive ION/IOFF of 107, and a high effective hole mobility of ∼146 cm2 V−1 s−1. The novel Nb doping in monolayer WS2 has eliminated the ambipolar behavior and reduced the Schottky barrier in WS2 FETs. The reduction of the Schottky barrier is ascribed to the hybridization between W 5d, Nb 4d and S 3p states near the EF and to the enhancement of the metallization of the contact between the Pd metal and monolayer NbxW1−xS2 after Nb doping.

Using first-principles calculations within density functional theory, we systematically studied the effect of BN–MoS2 heterostructure on the Schottky barriers of metal–MoS2 contacts. Two types of FETs are designed according to the area of the BN–MoS2 heterostructure. Results show that the vertical and lateral Schottky barriers in all the studied contacts, irrespective of the work function of the metal, are significantly reduced or even vanish when the BN–MoS2 heterostructure substitutes the monolayer MoS2. Only the n-type lateral Schottky barrier of Au/BN–MoS2 contact relates to the area of the BN–MoS2 heterostructure. Notably, the Pt–MoS2 contact with n-type character is transformed into a p-type contact upon substituting the monolayer MoS2 by a BN–MoS2 heterostructure. These changes of the contact natures are ascribed to the variation of Fermi level pinning, work function and charge distribution. Analysis demonstrates that the Fermi level pinning effects are significantly weakened for metal/BN–MoS2 contacts because no gap states dominated by MoS2 are formed, in contrast to those of metal–MoS2 contacts. Although additional BN layers reduce the interlayer interaction and the work function of the metal, the Schottky barriers of metal/BN–MoS2 contacts still do not obey the Schottky–Mott rule. Moreover, different from metal–MoS2 contacts, the charges transfer from electrodes to the monolayer MoS2, resulting in an increment of the work function of these metals in metal/BN–MoS2 contacts. These findings may prove to be instrumental in the future design of new MoS2-based FETs with ohmic contact or p-type character.

Composition, structure, optical and electrical properties of Al:WS2 (un-doped and Al-doped WS2) films prepared by atomic layer deposition (ALD) and CS2 vulcanization processing have been studied. Results show that Al:WS2 films grow with a preferential c⊥-orientation. The core-level binding energies (BEs) of W 4f, S 2p, O 1s and Al 2s decrease with increasing Al doping content, indicating that Al-doped WS2 films have a p-type conductivity. Optical property analysis shows that the absorption coefficient (∼107 m−1) is comparable to that of WS2 single crystals and that Al doping content can tune the optical band gap of the films. Hall measurements show that p-type conductive Al-doped WS2 films can be obtained by Al doping. Hall mobility values for the un-doped WS2 and 2.40% Al-doped WS2 films are 1.63 × 101 and 9.71 cm2 V−1 s−1, respectively. Comparing with un-doped WS2 films, the comparable Hall mobility of Al-doped WS2 films can be achieved by appropriate Al doping contents.

Electronic structure and optical properties of Cmcm orthorhombic SrHfO3 were computed, using the plane-wave ultrasoft pseudopotential technique based on the first-principles density functional theory (DFT). The equilibrium lattice parameters of orthorhombic SrHfO3 are in good agreement with experimental values. The band structure, the densities of states (DOS) and charge densities of Cmcm orthorhombic SrHfO3 have been obtained. The band structure shows that Cmcm orthorhombic SrHfO3 has direct band gap. The charge densities of Cmcm orthorhombic SrHfO3 indicate that bonding between Hf and O is mainly covalent whereas the bonding between Sr and O is mainly ionic. The complex dielectric function, refractive index and absorption coefficient of Cmcm orthorhombic SrHfO3 have been predicted. The imaginary and real parts of the calculated complex dielectric function are close to the results of experimental measurements.

The effect of vacancies in monolayer MoS2 on the electronic properties of a Ti–MoS2 top contact has been investigated using first-principles calculations. A Mo-vacancy is easier to form than a S-vacancy in a Ti–MoS2 top contact, especially under oxidation conditions. A Mo-vacancy eliminates the Schottky barrier of the Ti–MoS2 top contact, and a S-vacancy reduces the Schottky barrier from 0.28 to 0.15 eV. Mo-vacancies are beneficial for obtaining a high quality p-type Ti–MoS2 top contact, whereas S-vacancies are favorable to achieve a high quality n-type Ti–MoS2 top contact. Moreover, defective Ti–MoS2 top contacts have stronger dipole layers, a higher potential step and more transferred charges than a perfect ones. The electronic properties of Ti–MoS2 top contacts can be tuned by intrinsic vacancies in monolayer MoS2. Our findings provide important insights into the future design and fabrication of novel nanoelectronic devices with monolayer MoS2.

Revealing the influence of intrinsic defects in monolayer MoS2 on the electronic nature of metal–MoS2 contacts is particularly critical for their practical use as nanoelectronic devices. This work presents a systematic study toward electronic properties of Mo metal contacts to monolayer MoS2 with vacancies by using first-principles calculations based on density functional theory. Upon Mo- and S-vacancy formation in monolayer MoS2, both the height and the width of the tunnel barrier between Mo metal and monolayer MoS2 are decreased. Additionally, the Schottky barrier of 0.1 eV for the perfect Mo–MoS2 top contact is reduced to zero for defective ones. The partial density of states near the Fermi level of defective Mo–MoS2 top contacts is strengthened and electron densities at the interface of defective Mo–MoS2 top contacts are increased compared with those of the perfect one, suggesting that Mo- and S-vacancies in monolayer MoS2 have the possibility to improve the electron injection efficiency. Mo-vacancies in monolayer MoS2 are beneficial to get high quality p-type Mo–MoS2 contacts, whereas S-vacancies in monolayer MoS2 are favorable to achieve high quality n-type Mo–MoS2 contacts. Our findings provide important insights into future design and fabrication of nanoelectronic devices with monolayer MoS2.

•Under reducing conditions, S vacancies are more likely to form in monolayer MoS2.•VS2+VS2+ is the dominant defect in monolayer MoS2 under reducing conditions.•Under oxidizing conditions, Mo vacancies are easier to form in monolayer MoS2.•VMo4−VMo4− is the main source of defect in monolayer MoS2 under oxidizing conditions.•VMo4−VMo4− induces deep acceptor-like level, whereas VS2+VS2+ induces deep donor-like level.The formation energies of intrinsic vacancies in monolayer MoS2 as well as effect of the vacancies on electronic structures of monolayer MoS2 were investigated using the first-principles density functional theory. Results show that both Mo and S vacancies depend greatly on the Fermi level. With the increasing Fermi level, the formation energies of S vacancies increase whereas the formation energies of Mo vacancies decrease. Under reducing conditions, S vacancies are more likely to form and VS2+VS2+ is the dominant defect. In contrast, under oxidizing conditions, Mo vacancies are easier to form and VMo4−VMo4− is the main source of defect. After introducing the intrinsic vacancies, the valence and conduction bands of monolayer MoS2 were expanded toward lower energy and the band gaps of monolayer MoS2 were decreased. Moreover, VMo4−VMo4− brings about deep acceptor-like levels and p-type conductivities, whereas VS2+VS2+ induces deep donor-like levels and n-type conductivities.

•Elastic properties and Debye temperature of WS2 are firstly predicted in this work.•2H-WS2 is more stable than 3R-WS2 when pressure is less than 5.8 GPa.•Elastic anisotropies in compressibility and in shear were found for WS2 at 0 GPa.•The pressure dependence of elastic moduli, ΘDΘD and elastic anisotropy were obtained.•WS2 under pressure has higher hardness and better ductility than that at 0 GPa.The structure, mechanical stability and elastic properties of 2H- and 3R-WS2 under pressure have been investigated using first-principles calculations based on density functional theory (DFT). The equilibrium lattice parameters of 2H- and 3R-WS2 at 0 GPa are consistent with experimental and other theoretical values. 2H-WS2 is more stable than 3R-WS2 when pressure is less than 5.8 GPa whereas 3R-WS2 is more stable than 2H-WS2 when pressure is higher than 5.8 GPa. According to the mechanical stability criteria, both 2H- and 3R-WS2 exhibit mechanical stability under the pressure range from 0 to 20 GPa. With the increasing pressure, the elastic moduli (E, B, G), sound velocities (vs, vp, vm) and Debye temperatures of 2H- and 3R-WS2 increase monotonously whereas volume and specific heat decrease. Large elastic anisotropies in compressibility and in shear were demonstrated for 2H- and 3R-WS2 at 0 GPa. As the pressure increases, anisotropies in compressibility and in shear become weak for 2H- and 3R-WS2. Moreover, 2H- and 3R-WS2 under pressure have higher hardness and better ductility than those at 0 GPa.

The effect of vacancies in monolayer MoS2 on the electronic properties of a Ti–MoS2 top contact has been investigated using first-principles calculations. A Mo-vacancy is easier to form than a S-vacancy in a Ti–MoS2 top contact, especially under oxidation conditions. A Mo-vacancy eliminates the Schottky barrier of the Ti–MoS2 top contact, and a S-vacancy reduces the Schottky barrier from 0.28 to 0.15 eV. Mo-vacancies are beneficial for obtaining a high quality p-type Ti–MoS2 top contact, whereas S-vacancies are favorable to achieve a high quality n-type Ti–MoS2 top contact. Moreover, defective Ti–MoS2 top contacts have stronger dipole layers, a higher potential step and more transferred charges than a perfect ones. The electronic properties of Ti–MoS2 top contacts can be tuned by intrinsic vacancies in monolayer MoS2. Our findings provide important insights into the future design and fabrication of novel nanoelectronic devices with monolayer MoS2.

Thickness has been proved to have significant influence on the physical properties of two-dimensional (2D) materials and their corresponding devices. Understanding the effect of the thickness of 2D insertions on the contact properties of metal–monolayer MoS2 interfaces (viz. metal–mMoS2 interfaces) is vital to designing high performance mMoS2 devices. In this work, the electronic structures, Schottky barriers, contact resistance, and tunneling barriers of scandium–mMoS2 (Sc–mMoS2) interfaces with BN and graphene insertions have been comparatively studied by density functional theory. No Schottky barriers are found at Sc–mMoS2 interfaces with monolayer 2D insertions. Although the contact resistance and charge injection efficiency of Sc–mMoS2 interfaces with monolayer insertions deteriorate relatively to those of the Sc–mMoS2 interface, they are still sufficient to realize high-performance mMoS2-based devices. Note that, upon increasing the number of layers of 2D insertions, these contact properties are further deteriorated with the increasing number of layers of insertions. Moreover, additional significant Schottky barriers are introduced into Sc–mMoS2 interfaces with multilayer BN; the nature Dirac points of graphene insertions are opened, suggesting low performances of Sc–mMoS2 interfaces with multilayer BN and graphene insertions. These variations can be understood on the basis of the orbital hybridization and charge redistribution between the Sc slab and mMoS2 layer. In addition, these characteristics are expected to occur in other metal–mMoS2 interfaces with two-dimensional material insertions. Overall, monolayer rather than multilayer two-dimensional insertions can be used to improve the transport properties of mMoS2-based devices.

Using first-principles calculations within density functional theory, we systematically studied the effect of BN–MoS2 heterostructure on the Schottky barriers of metal–MoS2 contacts. Two types of FETs are designed according to the area of the BN–MoS2 heterostructure. Results show that the vertical and lateral Schottky barriers in all the studied contacts, irrespective of the work function of the metal, are significantly reduced or even vanish when the BN–MoS2 heterostructure substitutes the monolayer MoS2. Only the n-type lateral Schottky barrier of Au/BN–MoS2 contact relates to the area of the BN–MoS2 heterostructure. Notably, the Pt–MoS2 contact with n-type character is transformed into a p-type contact upon substituting the monolayer MoS2 by a BN–MoS2 heterostructure. These changes of the contact natures are ascribed to the variation of Fermi level pinning, work function and charge distribution. Analysis demonstrates that the Fermi level pinning effects are significantly weakened for metal/BN–MoS2 contacts because no gap states dominated by MoS2 are formed, in contrast to those of metal–MoS2 contacts. Although additional BN layers reduce the interlayer interaction and the work function of the metal, the Schottky barriers of metal/BN–MoS2 contacts still do not obey the Schottky–Mott rule. Moreover, different from metal–MoS2 contacts, the charges transfer from electrodes to the monolayer MoS2, resulting in an increment of the work function of these metals in metal/BN–MoS2 contacts. These findings may prove to be instrumental in the future design of new MoS2-based FETs with ohmic contact or p-type character.

Novel MoS2/(MX2)n lateral and (MoS2)/(MX2)n–BN hybrid heterostructures have been designed on monolayer MoS2 to extend its applications. The electronic, interfacial and optical properties of the lateral and hybrid heterostructures have been investigated comparatively using first-principles calculations. It was found that the charge distributions, band gaps, band levels, electrostatic potentials, and optical absorption of the MoS2/(MX2)n lateral heterostructures depend greatly on the width n of MX2, irrespective of the size of the lateral heterostructures. The CBM states of the MoS2/(MX2)n lateral heterostructures dominated by the dz2 orbitals are localized around MoS2, whereas the VBM states of the MoS2/(MX2)n lateral heterostructures are dominated by the MX2 region. Through regulating the width n of the MX2 region in the MoS2/(MX2)n lateral heterostructures, the optical absorption of the lateral heterostructures under visible light can be increased, and the CBM and VBM states of the lateral heterostructures can be located above the hydrogen reduction potential and below the water oxidation potential, respectively. The similar characteristics were observed in the MoS2/(MX2)n–BN hybrid heterostructures, indicating that BN is a good substrate for the MoS2/(MX2)n lateral heterostructures. The analysis implies that forming the lateral and hybrid heterostructures is an effective way to extend the applications of monolayer MoS2 in photocatalytic water and photovoltaic devices.