Effect of copper concentration and sulfur vacancies on electronic properties of MoS2 monolayer: a computational study

We investigated the geometrical and electronic properties of copper-doped MoS₂ by first principles calculations. The doping is done by Cu substitution with Mo (1 to 4 atoms) accompanied by study of S vacancies. Our outcomes show that the concentration of doping and vacancy of S leads to determine and finely tune the band gap in the range of 0.16 to 1.95 eV. This fine tuning of band gap results due to variation in concentration of impurity, changing dopant site, and production of S vacancies. The resulting arrangements show significant charge redistribution on replacement of local atoms with foreign atoms dictated by electronegativity determined from the Bader analysis. In addition, bonding mechanism occurring due to substitution of foreign elements is discussed. These results give pleasing data regarding fine desired value of the band gap of the MoS₂ which helps its utilization in semiconductor and other opto-electronic devices in addition to understanding the electrical conductivity.

Impact of S-Vacancies on the Charge Injection Barrier at the Electrical Contact with the MoS2 Monolayer

Making electrical contacts to semiconducting transition metal dichalcogenides (TMDCs) represents a major bottleneck for high device performance, often manifesting as strong Fermi level pinning and high contact resistance. Despite intense ongoing research, the mechanism by which lattice defects in TMDCs impact the transport properties across the contact-TMDC interface remains unsettled. Here we study the impact of S-vacancies on the electronic properties at a MoS₂ monolayer interfaced with graphite by photoemission spectroscopy, where the defect density is selectively controlled by Ar sputtering. A clear reduction of the MoS₂ core level and valence band binding energies is observed as the defect density increases. The experimental results are explained in terms of (i) gap states' energy distribution and (ii) S-vacancies' electrostatic disorder effect. Our model indicates that the Fermi level pinning at deep S-vacancy gap states is the origin of the commonly reported large electron injection barrier (∼0.5 eV) at the MoS₂ ML interface with low work function metals. At the contact with high work function electrodes, S-vacancies do not significantly affect the hole injection barrier, which is intrinsically favored by Fermi level pinning at shallow occupied gap states. Our results clarify the importance of S-vacancies and electrostatic disorder in TMDC-based electronic devices, which could lead to strategies for optimizing device performance and production.

Defect-Assisted Contact Property Enhancement in a Molybdenum Disulfide Monolayer

Contact engineering for two-dimensional (2D) transition metal dichalcogenides (TMDCs) is crucial for realizing high-performance 2D TMDC devices, and most studies on contact properties of 2D TMDCs have mainly focused on Fermi level unpinning. Here, we investigated electrical and photoelectrical properties of chemical vapor deposition (CVD)-grown molybdenum disulfide (MoS₂) monolayer devices depending on metal contacts, Ti/Pt, Ti/Au, Ti, and Ag, and particularly demonstrated the essential role of defects in MoS₂ in contact properties. Remarkably, MoS₂ devices with Ag contacts show a field-effect mobility of 12.2 cm² V^-1 s^-1, an on/off current ratio of 7 × 10⁷, and a photoresponsivity of 1020 A W^-1, which are outstanding compared to similar devices with other metal contacts. These improvements are attributed to a reduced Schottky barrier height, thanks to the small work function of Ag and Ag-MoS₂ orbital hybridization at the interface, which facilitates efficient charge transfer between MoS₂ and Ag. Interestingly, X-ray photoelectron spectroscopic analysis reveals that Ag₂S was formed in our defect-containing CVD-grown MoS₂ monolayer, but such orbital hybridization is not observed in a nearly defect-free exfoliated MoS₂. This distinction shows that defects existing in MoS₂ enable Ag to effectively couple to MoS₂ and correspondingly enhance multiple electrical and photoelectrical properties.

Thermal Stability of Titanium Contacts to MoS2

Thermal annealing of Ti contacts is commonly implemented in the fabrication of MoS₂ devices; however, its effects on interface chemistry have not been previously reported in the literature. In this work, the thermal stability of titanium contacts deposited on geological bulk single crystals of MoS₂ in ultrahigh vacuum (UHV) is investigated with X-ray photoelectron spectroscopy and scanning transmission electron microscopy (STEM). In the as-deposited condition, the reaction of Ti with MoS₂ is observed resulting in a diffuse interface between the two materials that comprises metallic molybdenum and titanium sulfide compounds. Annealing Ti/MoS₂ sequentially at 100, 300, and 600 °C for 30 min in UHV results in a gradual increase in the reaction products as measured by XPS. Accordingly, STEM reveals the formation of a new ordered phase and a Mo-rich layer at the interface following heating. Due to the high degree of reactivity, the Ti/MoS₂ interface is not thermally stable even at a transistor operating temperature of 100 °C, while post-deposition annealing further enhances the interfacial reactions. These findings have important consequences for electrical transport properties, highlighting the importance of interface chemistry in the metal contact design and fabrication.

Electronic properties of atomically thin MoS2 layers grown by physical vapour deposition: band structure and energy level alignment at layer/substrate interfaces

We present an analysis of the electronic properties of an MoS₂ monolayer (ML) and bilayer (BL) as-grown on a highly ordered pyrolytic graphite (HOPG) substrate by physical vapour deposition (PVD), using lab-based angle-resolved photoemission spectroscopy (ARPES) supported by scanning tunnelling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) for morphology and elemental assessments, respectively. Despite the presence of multiple domains (causing in-plane rotational disorder) and structural defects, electronic band dispersions were clearly observed, reflecting the high density of electronic states along the high symmetry directions of MoS₂ single crystal domains. In particular, the thickness dependent direct-to-indirect band gap transition previously reported only for MoS₂ layers obtained by exfoliation or via epitaxial growth processes, was found to be also accessible in our PVD grown MoS₂ samples. At the same time, electronic gap states were detected, and attributed mainly to structural defects in the 2D layers. Finally, we discuss and clarify the role of the electronic gap states and the interlayer coupling in controlling the energy level alignment at the MoS₂/substrate interface.

The band structure of defective, rotationally disordered 2D TMDC layers is reported.

Schottky barrier engineering via adsorbing gases at the sulfur vacancies in the metal-MoS2 interface

Sulfur vacancies (S-vacancies) are common in monolayer MoS₂ (mMoS₂). Finding an effective way to control rather than abolish the effect of S-vacancies on contact properties is vital for the application of mMoS₂. Here, we propose the adsorption of gases to passivate the S-vacancies in Pt-mMoS₂ interfaces. Results demonstrate that gases are stably and preferentially adsorbed at S-vacancies. The n-type Schottky barriers of Pt-mMoS₂ interfaces are reduced significantly upon the adsorption electron-donor gases, especially Cl₂. The n-type transport character of the Pt-mMoS₂ interface can be changed to p-type by the adsorption of electron-acceptor gases. As the adsorption concentration increases, both n- and p-type Schottky barriers are further reduced, and the lowest n- and p-type Schottky barriers are 0.36 and 0 eV, respectively. Note that the variations in Schottky barriers are independent of the oxidizing ability of gases but relative to the average number of valence electrons per gas atom. Analysis demonstrates that although gases at S-vacancies cannot cause gap states to vanish, and can even enhance Fermi level pinning, they modulate charge redistribution and the potential step at the interface region. Moreover, with increasing adsorption concentration, the valence band maximum of mMoS₂ shows the opposite variation tendency to that of the potential step. Our results suggest that adsorption of gases is an effective way to passivate S-vacancies to modulate the transport properties of Pt-mMoS₂ interfaces.

Theoretical analysis of the combined effects of sulfur vacancies and analyte adsorption on the electronic properties of single-layer MoS2

We report a first-principles theoretical investigation on the electronic structure and electron transport of defective single-layer (SL) MoS2, as well as of corresponding structures adsorbed with benzyl viologen (BV), which was shown to provide improved performance of a field effect transistor. O2 adsorption was included to gain an understanding of the response upon air-exposure. Following analysis of the structure and stability of sulfur single vacancy and line defects in SL MoS2, we investigated the local transport at the adsorbed sites via a transport model that mimics a scanning tunneling spectroscopy experiment. Distinct current-voltage characteristics were indicated for adsorbed oxygen species at a sulfur vacancy. The electronic structures of defective MoS2 indicated the emergence of impurity states in the bandgap due to sulfur defects and oxygen adsorption. Electron transport calculations for the MoS2 surface with an extended defect in a device setting demonstrated that physisorption of BV enhances the output current, while facile chemisorption by O2 upon air-exposure causes degradation of electron transport.

Designing high performance metal–mMoS2 interfaces by two-dimensional insertions with suitable thickness

The contact properties of metal–MoS₂ interfaces are improved by 2D insertions, but they deteriorate with the increasing thickness of insertions.

Single Layer Bismuth Iodide: Computational Exploration of Structural, Electrical, Mechanical and Optical Properties

Layered graphitic materials exhibit new intriguing electronic structure and the search for new types of two-dimensional (2D) monolayer is of importance for the fabrication of next generation miniature electronic and optoelectronic devices. By means of density functional theory (DFT) computations, we investigated in detail the structural, electronic, mechanical and optical properties of the single-layer bismuth iodide (BiI₃) nanosheet. Monolayer BiI₃ is dynamically stable as confirmed by the computed phonon spectrum. The cleavage energy (E_cl) and interlayer coupling strength of bulk BiI₃ are comparable to the experimental values of graphite, which indicates that the exfoliation of BiI₃ is highly feasible. The obtained stress-strain curve shows that the BiI₃ nanosheet is a brittle material with a breaking strain of 13%. The BiI₃ monolayer has an indirect band gap of 1.57 eV with spin orbit coupling (SOC), indicating its potential application for solar cells. Furthermore, the band gap of BiI₃ monolayer can be modulated by biaxial strain. Most interestingly, interfacing electrically active graphene with monolayer BiI₃ nanosheet leads to enhanced light absorption compared to that in pure monolayer BiI₃ nanosheet, highlighting its great potential applications in photonics and photovoltaic solar cells.

MoS2 armored polystyrene particles with a narrow size distribution via membrane-assisted Pickering emulsions for monolayer-shelled liquid marbles

Monolayer-shelled liquid marbles were successfully stabilized by MoS₂ armored polystyrene particles with a narrow size distribution via membrane-assisted Pickering emulsions.

Tuning the electronic properties of bondings in monolayer MoS2 through (Au, O) co-doping

This paper studies the electronic properties of un-, Au-, (Au, O) co-doped monolayer MoS₂ to analyze the effect of oxygen on the character of Au–S bonding, with the goal of improving the conductivity of Au–S bonding of MoS₂-device with electrode Au.