Using first-principles calculations within density functional theory, we investigate the electronic and chemical properties of a single-layer MoS(2) adsorbed on Ir(111), Pd(111), or Ru(0001), three representative transition metal substrates having varying work functions but each with minimal lattice mismatch with the MoS(2) overlayer. We find that, for each of the metal substrates, the contact nature is of Schottky-barrier type, and the dependence of the barrier height on the work function exhibits a partial Fermi-level pinning picture. Using hydrogen adsorption as a testing example, we further demonstrate that the introduction of a metal substrate can substantially alter the chemical reactivity of the adsorbed MoS(2) layer. The enhanced binding of hydrogen, by as much as ~0.4 eV, is attributed in part to a stronger H-S coupling enabled by the transferred charge from the substrate to the MoS(2) overlayer, and in part to a stronger MoS(2)-metal interface by the hydrogen adsorption. These findings may prove to be instrumental in future design of MoS(2)-based electronics, as well as in exploring novel catalysts for hydrogen production and related chemical processes.

Tuning the Electronic and Chemical Properties of Monolayer MoS$_2$ Adsorbed on Transition Metal Substrates

Two-dimensional crystals have emerged as a class of materials that may impact future electronic technologies. Experimentally identifying and characterizing new functional two-dimensional materials is challenging, but also potentially rewarding. Here, we fabricate field-effect transistors based on few-layer black phosphorus crystals with thickness down to a few nanometres. Reliable transistor performance is achieved at room temperature in samples thinner than 7.5 nm, with drain current modulation on the order of 10(5) and well-developed current saturation in the I-V characteristics. The charge-carrier mobility is found to be thickness-dependent, with the highest values up to ∼ 1,000 cm(2) V(-1) s(-1) obtained for a thickness of ∼ 10 nm. Our results demonstrate the potential of black phosphorus thin crystals as a new two-dimensional material for applications in nanoelectronic devices.

http://absimage.aps.org/image/MAR14/MWS_MAR14-2013-005031.pdf

Black Phosphorus Field-effect Transistors

Low-resistivity metal-semiconductor (M-S) contact is one of the urgent challenges in the research of 2D transition metal dichalcogenides (TMDs). Here, we report a chloride molecular doping technique which greatly reduces the contact resistance (Rc) in the few-layer WS2 and MoS2. After doping, the Rc of WS2 and MoS2 have been decreased to 0.7 kohm*um and 0.5 kohm*um, respectively. The significant reduction of the Rc is attributed to the achieved high electron doping density thus significant reduction of Schottky barrier width. As a proof-ofconcept, high-performance few-layer WS2 field-effect transistors (FETs) are demonstrated, exhibiting a high drain current of 380 uA/um, an on/off ratio of 4*106, and a peak field-effect mobility of 60 cm2/V*s. This doping technique provides a highly viable route to diminish the Rc in TMDs, paving the way for high-performance 2D nano-electronic devices.

Chloride Molecular Doping Technique on 2D Materials: WS2 and MoS2

A highly active and stable hybrid electrocatalyst 3D hierarchical CoS₂ nanosheets incorporated with WS₂ (CoS₂@WS₂) has been developed via a one-step sulfurization method for the first time, where the contents of WS₂ can be adjusted easily. We first prove that the addition of small amounts of WS₂ enhances the hydrogen evolution reaction (HER) performance of CoS₂, and vice versa. In other words, we validated the symmetric synergy for the HER between the Co- and W-based sulfide hybrid catalysts. In addition, we confirmed that the formation of nanointerfaces of Co–S–W between CoS₂ and WS₂ was responsible for the excellent HER activity (an overpotential of −97.2 mV at −10 mA cm⁻², a small Tafel slope of 66.0 mV dec⁻¹, and prominent electrochemical stability) of the hybrid electrocatalyst CoS₂@WS₂.

Symmetric synergy of hybrid CoS₂–WS₂ electrocatalysts for the hydrogen evolution reaction

Recently, two-dimensional transition metal dichalcogenides, particularly WS2, raised extensive interest due to its extraordinary physicochemical properties. With the merits of low costs and prominent properties such as high anisotropy and distinct crystal structure, WS2 is regarded as a competent substitute in the construction of next-generation environmentally benign energy storage and conversion devices. In this review, we begin with the fundamental studies of the structure, properties and preparation of WS2, followed by detailed discussions on the development of various WS2 and WS2-based composites for electrochemical energy storage and conversion applications. In the end, some prospective prospects and promising developments of WS2 in these fields are proposed.

Tungsten disulfide: synthesis and applications in electrochemical energy storage and conversion

By combining the electronic structures of graphene and monolayer AsSb via van der Waals force interaction, the intrinsic p-type Schottky contact can be obtained. Here, a series of theoretic calculations are performed to survey the effects of interlayer coupling and the band realignment of graphene-AsSb heterointerface. It reveals that intrinsic p-type Schottky barriers of 0.184 and 0.381 eV are formed for the two types of configurations. Besides, the intrinsic electronic properties of graphene and AsSb are roughly preserved. When the external electric field is applied, the Schottky barrier can be effectively tuned up by changing the external electric field intensity and further convert the p-type contact into the n-type contact. A variation of the Schottky barriers indicates a partial Fermi level pinning at the interfaces of AsSb. It results from the low density of interfacial states between graphene and AsSb. The barrier height of AsSb and the corresponding contact type can be flexibly tuned, which is of great importance in the design of novel transistors based two-dimensional materials and they provide meaningful guidelines.

Tuning the band alignment of p-type graphene-AsSb Schottky contact by electric field

Comparative Study of Blue Phosphorus-MoTe 2 /WTe 2 van der Waals Heterostructures on Electronic Structures and Band Realignment

Searching for novel two-dimensional (2D) materials based nanoscale electronic devices is a hot topic in the current research. A modulation of materials’ specific physical properties by altering external conditions has long been used. There are a variety of routes to improve the specific behavior of materials. In this paper, the structural, electronic, and the corresponding variational characteristics of the graphene(g)-MoTe2/WTe2 heterointerfaces are studied in detail based on ab initio calculations with nonlocal van der Waals (vdW) corrections. We performed research on the band alignments of g–MoTe2/WTe2 contacts and a concise routine to reduce the Schottky barrier and obtain Ohmic contact. The results predict a barrier height of 62 meV and 280 meV for g–MoTe2/WTe2, respectively, in a neutral state. In the applied electric field, the corresponding Schottky barriers can be effectively tuned by various electric fields. The height of the barrier further decreases to 0 under − 0.02/0.16 V/A and − 0.06/0.08 V/A for g–MoTe2/WTe2, respectively, and the numerical value of the barrier and the corresponding Schottky type can be regulated in a flexible way. Additionally, theoretical calculation results also demonstrate that g–MoTe2 has a smaller Fermi level pinning effect than g–WTe2, which plays a significant role in the fabrication of novel transistors based 2D materials and it should be a better choice for FETs application.

The band alignments modulation of g-MoTe 2 /WTe 2 van der Waals heterostructures

n-Type contact of Schottky barriers at two-dimensional (2D) materials/metal interfaces is a usual formalization in the modern FETs applications. It is common to modulate it from n- to p-type through some specific methods. In this work, we came up with two new intrinsic p-type contacts of graphene-GeC/GeS and further tune them from p-type to n-type by external electric fields. It proved that the electronic properties of graphene and GeC/GeS can be roughly preserved for the weak van der Waals (vdW) interaction. p-Type contacts with relatively small barriers are formed at g-GeC/GeS heterointerfaces. After external electric field applied, the Schottky barrier can be effectively tuned by different external electric and the p-type contact further turns into n-type. Variation of the Schottky barriers indicated a partial pinning for interfaces of g-GeC/GeS. This is because the interfacial states between graphene and GeC/GeS hardly exists. The barrier height of g-GeC/GeS and the corresponding contact type can be flexibly tuned, which is of great importance in the design of novel transistors-based 2D materials. Searching for novel nanoscale electronic equipment based on 2D materials is a hot topic in the current study. This work would provide meaningful guidelines for nanoscale devices.

Honglin Li

Papers

Tuning the band alignment of p-type graphene-AsSb Schottky contact by electric field

Comparative Study of Blue Phosphorus-MoTe 2 /WTe 2 van der Waals Heterostructures on Electronic Structures and Band Realignment

The band alignments modulation of g-MoTe 2 /WTe 2 van der Waals heterostructures

The modulation of Schottky contacts of p-type graphene-GeC/GeS heterointerface

The role of strain induced band modulation of WS 2 -GeC heterostructure for the hydrogen evolution