Graphene is not alone

Large-Area Epitaxial Growth of Transition Metal Dichalcogenides

Over the past decade, research on atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs) has expanded rapidly due to their unique properties such as high carrier mobility, significant excitonic effects, and strong spin-orbit couplings. Considerable attention from both scientific and industrial communities has fully fueled the exploration of TMDs toward practical applications. Proposed scenarios, such as ultrascaled transistors, on-chip photonics, flexible optoelectronics, and efficient electrocatalysis, critically depend on the scalable production of large-area TMD films. Correspondingly, substantial efforts have been devoted to refining the synthesizing methodology of 2D TMDs, which brought the field to a stage that necessitates a comprehensive summary. In this Review, we give a systematic overview of the basic designs and significant advancements in large-area epitaxial growth of TMDs. We first sketch out their fundamental structures and diverse properties. Subsequent discussion encompasses the state-of-the-art wafer-scale production designs, single-crystal epitaxial strategies, and techniques for structure modification and postprocessing. Additionally, we highlight the future directions for application-driven material fabrication and persistent challenges, aiming to inspire ongoing exploration along a revolution in the modern semiconductor industry.

Recent Advances in Non-Ti MXenes: Synthesis, Properties, and Novel Applications

One of the most fascinating 2D nanomaterials (NMs) ever found is various members of MXene family. Among them, the titanium-based MXenes, with more than 70% of publication-related investigations, are comparatively well studied, producing fundamental foundation for the 2D MXene family members with flexible properties, familiar with a variety of advanced novel technological applications. Nonetheless, there are still more candidates among transitional metals (TMs) that can function as MXene NMs in ways that go well beyond those that are now recognized. Systematized details of the preparations, characteristics, limitations, significant discoveries, and uses of the novel M-based MXenes (M-MXenes), where M stands for non-Ti TMs (M = Sc, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, and Lu), are given. The exceptional qualities of the 2D non-Ti MXene outperform standard Ti-MXene in several applications. There is many advancement in top-down as well as bottom-up production of MXenes family members, which allows for exact control of the M-characteristics MXene NMs to contain cutting-edge applications. This study offers a systematic evaluation of existing research, covering everything in producing complex M-MXenes from primary limitations to the characterization and selection of their applications in accordance with their novel features. The development of double metal combinations, extension of additional metal candidates beyond group-(III-VI)B family, and subsequent development of the 2D TM carbide/TMs nitride/TM carbonitrides to 2D metal boride family are also included in this overview. The possibilities and further recommendations for the way of non-Ti MXene NMs are in the synthesis of NMs will discuss in detail in this critical evaluation.

Recent Advances in Surface Modifications of Elemental Two-Dimensional Materials: Structures, Properties, and Applications

The advent of graphene opens up the research into two-dimensional (2D) materials, which are considered revolutionary materials. Due to its unique geometric structure, graphene exhibits a series of exotic physical and chemical properties. In addition, single-element-based 2D materials (Xenes) have garnered tremendous interest. At present, 16 kinds of Xenes (silicene, borophene, germanene, phosphorene, tellurene, etc.) have been explored, mainly distributed in the third, fourth, fifth, and sixth main groups. The current methods to prepare monolayers or few-layer 2D materials include epitaxy growth, mechanical exfoliation, and liquid phase exfoliation. Although two Xenes (aluminene and indiene) have not been synthesized due to the limitations of synthetic methods and the stability of Xenes, other Xenes have been successfully created via elaborate artificial design and synthesis. Focusing on elemental 2D materials, this review mainly summarizes the recently reported work about tuning the electronic, optical, mechanical, and chemical properties of Xenes via surface modifications, achieved using controllable approaches (doping, adsorption, strain, intercalation, phase transition, etc.) to broaden their applications in various fields, including spintronics, electronics, optoelectronics, superconducting, photovoltaics, sensors, catalysis, and biomedicines. These advances in the surface modification of Xenes have laid a theoretical and experimental foundation for the development of 2D materials and their practical applications in diverse fields.

The Role of Permanent and Induced Electrostatic Dipole Moments for Schottky Barriers in Janus MXY/Graphene Heterostructures: a First Principles Study

The Schottky barrier height (ESBH) is a crucial factor in determining the transport properties of semiconductor materials and it directly regulates the carrier mobility in opto-electronics devices. In principle, van...

A progressive metal-semiconductor transition in two-faced Janus monolayer transition-metal chalcogenides

Breaking the symmetry in the out-of-plane direction in two-dimensional materials to trigger distinctive electronic properties has long been predicted. Inspired by the recent progress in the experimental synthesis of a sandwiched S-Mo-Se structure (Janus SMoSe) at the monolayer limit [Zhang et al., ACS Nano, 2017, 11, 8192-8198], we investigate the transport and electronic structure of two-faced XMoY monolayers (X, Y = O, S, Se and Te) through first-principles calculations. It is found that all the monolayers are semiconductors except OMoTe, which is metallic. Interestingly, the "parents" of OMoTe (MoO2 and MoTe2) are both semiconductors. Further analysis shows that it is the out-of-plane asymmetry-induced strain that results in the metal-semiconductor transition between Janus OMoTe and its parents. By increasing the ratio of O atoms in one face of MoTe2, a progressive decreasing trend of the bandgap, as well as the transition to metallic, is found. In addition, a transition from the direct band gap semiconductor to the indirect one is also observed in the process. This could be used as an effective way to precisely control electronic structures, e.g., the bandgap. Different from other methods, this method uses the intrinsic features of the material, which can persist without the need of additional equipment. Moreover, such a modulating method is expected to be extended to many other transition-metal chalcogenides, showing great application potential.

Realizing Stable p-Type Transporting in Two-Dimensional WS2 Films

Two-dimensional (2D) semiconductors have become promising candidates for nanoelectronics applications due to their unique layered structure and rich physical properties. However, the significant lack of reproducible p-type doping methods that can avoid the instability induced by the widely used charge transfer doping method greatly limits the applications of these semiconductors in complementary metal-oxide-semiconductor (CMOS) integrated digital circuits. This work presents a new scheme to realize stable p-type doping for WS₂ with excellent layer controllability, wafer-level uniformity, and high reproducibility at the same time. The p-type WS₂ was produced by introducing substitutional doping of sulfur with nitrogen atoms during the sulfurization of WO_xN_y film. Nitrogen atoms acted as acceptors moving the Fermi level of WS₂ toward the valance band. Both experimental and theoretical investigations were designed to study the physical properties of the films fabricated. The WS₂ based field-effect transistors exhibited a well-defined p-type behavior with a large on/off current ratio of ∼10⁵ and a high hole mobility of ∼18.8 cm² V^-1 s^-1. This opens up a promising method to realize stable p-type doping of 2D materials, which is very attractive for future large-scale 2D CMOS device applications.

Thermal damage suppression of a black phosphorus saturable absorber for high-power operation of pulsed fiber lasers

Recent studies of black phosphorus (BP) have shown its future potential in the field of photonics. We determined the optical damage threshold of BP at 21.8 dBm in a fiber ring laser cavity, and demonstrated the high-power operation capacity of an evanescent field interaction-based BP saturable absorber. The long-term stability of a passively mode-locked fiber laser with a saturable absorber operating at the optical power of 23.3 dBm was verified for 168 h without any significant performance degradation. The center wavelength, spectral width, and pulse width of the laser output are 1558.8 nm, 14.2 nm, and 805 fs, respectively.

Thickness-Dependent Binding Energy Shift in Few-Layer MoS2 Grown by Chemical Vapor Deposition

The thickness-dependent surface states of MoS2 thin films grown by the chemical vapor deposition process on the SiO2-Si substrates are investigated by X-ray photoelectron spectroscopy. Raman and high-resolution transmission electron microscopy suggest the thicknesses of MoS2 films to be ranging from 3 to 10 layers. Both the core levels and valence band edges of MoS2 shift downward ∼0.2 eV as the film thickness increases, which can be ascribed to the Fermi level variations resulting from the surface states and bulk defects. Grainy features observed from the atomic force microscopy topographies, and sulfur-vacancy-induced defect states illustrated at the valence band spectra imply the generation of surface states that causes the downward band bending at the n-type MoS2 surface. Bulk defects in thick MoS2 may also influence the Fermi level oppositely compared to the surface states. When Au contacts with our MoS2 thin films, the Fermi level downshifts and the binding energy reduces due to the hole-doping characteristics of Au and easy charge transfer from the surface defect sites of MoS2. The shift of the onset potentials in hydrogen evolution reaction and the evolution of charge-transfer resistances extracted from the impedance measurement also indicate the Fermi level varies with MoS2 film thickness. The tunable Fermi level and the high chemical stability make our MoS2 a potential catalyst. The observed thickness-dependent properties can also be applied to other transition-metal dichalcogenides (TMDs), and facilitates the development in the low-dimensional electronic devices and catalysts.

Electronic transport properties of transition metal dichalcogenide field-effect devices: surface and interface effects

Recent explosion of interest in two-dimensional (2D) materials research has led to extensive exploration of physical and chemical phenomena unique to this new class of materials and their technological potential. Atomically thin layers of group 6 transition metal dichalcogenides (TMDs) such as MoS2 and WSe2 are remarkably stable semiconductors that allow highly efficient electrostatic control due to their 2D nature. Field effect transistors (FETs) based on 2D TMDs are basic building blocks for novel electronic and chemical sensing applications. Here, we review the state-of-the-art of TMD-based FETs and summarize the current understanding of interface and surface effects that play a major role in these systems. We discuss how controlled doping is key to tailoring the electrical response of these materials and realizing high performance devices. The first part of this review focuses on some fundamental features of gate-modulated charge transport in 2D TMDs. We critically evaluate the role of surfaces and interfaces based on the data reported in the literature and explain the observed discrepancies between the experimental and theoretical values of carrier mobility. The second part introduces various non-covalent strategies for achieving desired doping in these systems. Gas sensors based on charge transfer doping and electrostatic stabilization are introduced to highlight progress in this direction. We conclude the review with an outlook on the realization of tailored TMD-based field-effect devices through surface and interface chemistry.

Understanding the Intrinsic Water Wettability of Molybdenum Disulfide (MoS2)

2D semiconductors allow for unique and ultrasensitive devices to be fabricated for applications ranging from clinical diagnosis instruments to low-energy light-emitting diodes (LEDs). Graphene has championed research in this field since it was first fabricated; however, its zero bandgap creates many challenges. Transition metal dichalcogenides (TMDCs), e.g., MoS2, have a direct bandgap which alleviates the challenge of creating a bandgap in graphene-based devices. Water wettability of MoS2 is critical to device fabrication/performance and MoS2 has been believed to be hydrophobic. Herein, we report that water contact angle (WCA) of freshly exfoliated MoS2 shows temporal evolution with an intrinsic WCA of 69.0 ± 3.8° that increases to 89.0 ± 3.1° after 1 day exposure to ambient air. ATR-FTIR and ellipsometry show that the fresh, intrinsically mildly hydrophilic MoS2 surface adsorbs hydrocarbons from ambient air and thus becomes hydrophobic.

Two-Dimensional Pnictogen Honeycomb Lattice: Structure, On-Site Spin-Orbit Coupling and Spin Polarization

Because of its novel physical properties, two-dimensional materials have attracted great attention. From first-principle calculations and vibration frequencies analysis, we predict a new family of two-dimensional materials based on the idea of octet stability: honeycomb lattices of pnictogens (N, P, As, Sb, Bi). The buckled structures of materials come from the sp(3) hybridization. These materials have indirect band gap ranging from 0.43 eV to 3.7 eV. From the analysis of projected density of states, we argue that the s and p orbitals together are sufficient to describe the electronic structure under tight-binding model, and the tight-binding parameters are obtained by fitting the band structures to first-principle results. Surprisingly large on-site spin-orbit coupling is found for all the pnictogen lattices except nitrogen. Investigation on the electronic structures of both zigzag and armchair nanoribbons reveals the possible existence of spin-polarized ferromagnetic edge states in some cases, which are rare in one-dimensional systems. These edge states and magnetism may exist under the condition of high vacuum and low temperature. This new family of materials would have promising applications in electronics, optics, sensors, and solar cells.

Atomic healing of defects in transition metal dichalcogenides

As-grown transition metal dichalcogenides are usually chalcogen deficient and therefore contain a high density of chalcogen vacancies, deep electron traps which can act as charged scattering centers, reducing the electron mobility. However, we show that chalcogen vacancies can be effectively passivated by oxygen, healing the electronic structure of the material. We proposed that this can be achieved by means of surface laser modification and demonstrate the efficiency of this processing technique, which can enhance the conductivity of monolayer WSe2 by ∼400 times and its photoconductivity by ∼150 times.

Comparative Study of Potential Applications of Graphene, MoS2, and Other Two-Dimensional Materials in Energy Devices, Sensors, and Related Areas

Novel properties of graphene have been well documented, whereas the importance of nanosheets of MoS2 and other chalcogenides is increasingly being recognized over the last two to three years. Borocarbonitrides, BxCyNz, with insulating BN and conducting graphene on either side are new materials whose properties have been attracting attention. These two-dimensional (2D) materials contain certain common features. Thus, graphene, MoS2, and borocarbonitrides have all been used in supercapacitor applications, oxygen reduction reactions (ORRs), and lithium-ion batteries. It is instructive, therefore, to make a comparative study of some of the important properties of these layered materials. In this article, we discuss properties related to energy devices at length. We examine the hydrogen evolution reaction facilitated by graphene, MoS2, and related materials. We also discuss gas and radiation sensors based on graphene and MoS2 as well as gas storage properties of graphene and borocarbonitrides. The article should be useful in making a judicious choice of which 2D material to use for a particular application.

Fate and Transport of Molybdenum Disulfide Nanomaterials in Sand Columns

Research and development of two-dimensional transition metal dichalcogenides (TMDC) (e.g., molybdenum disulfide [MoS₂]) in electronic, optical, and catalytic applications has been growing rapidly. However, there is little known regarding the behavior of these particles once released into aquatic environments. Therefore, an in-depth study regarding the fate and transport of two popular types of MoS₂ nanomaterials, lithiated (MoS₂-Li) and Pluronic PF-87 dispersed (MoS₂-PL), was conducted in saturated porous media (quartz sand) to identify which form would be least mobile in aquatic environments. The electrokinetic properties and hydrodynamic diameters of MoS₂ as a function of ionic strength and pH were determined using a zeta potential analyzer and dynamic light scattering techniques. Results suggest that the stability is significantly decreased beginning at 10 and 31.6 mM KCl, for MoS₂-PL and MoS₂-Li, respectively. Transport study results from breakthrough curves, column dissections, and release experiments suggest that MoS₂-PL exhibits a greater affinity to be irreversibly bound to quartz surfaces as compared with the MoS₂-Li at a similar ionic strength. Derjaguin-Landau-Verwey-Overbeek theory was used to help explain the unique interactions between the MoS₂-PL and MoS₂-Li surfaces between particles and with the quartz collectors. Overall, the results suggest that the fate and transport of MoS₂ is dependent on the type of MoS₂ that enters the environment, where MoS₂-PL will be least mobile and more likely be deposited in porous media from pluronic-quartz interactions, whereas MoS₂-Li will travel greater distances and have a greater tendency to be remobilized in sand columns.

Few-layer black phosphorus field-effect transistors with reduced current fluctuation

We investigated the reduction of current fluctuations in few-layer black phosphorus (BP) field-effect transistors resulting from Al2O3 passivation. In order to verify the effect of Al2O3 passivation on device characteristics, measurements and analyses were conducted on thermally annealed devices before and after the passivation. More specifically, static and low-frequency noise analyses were used in monitoring the charge transport characteristics in the devices. The carrier number fluctuation (CNF) model, which is related to the charge trapping/detrapping process near the interface between the channel and gate dielectric, was employed to describe the current fluctuation phenomena. Noise reduction due to the Al2O3 passivation was expressed in terms of the reduced interface trap density values D(it) and N(it), extracted from the subthreshold slope (SS) and the CNF model, respectively. The deviations between the interface trap density values extracted using the SS value and CNF model are elucidated in terms of the role of the Schottky barrier between the few-layer BP and metal contact. Furthermore, the preservation of the Al2O3-passivated few-layer BP flakes in ambient air for two months was confirmed by identical Raman spectra.

Contrast and Raman spectroscopy study of single- and few-layered charge density wave material: 2H-TaSe₂

In this article, we report the first successful preparation of single- and few-layers of tantalum diselenide (2H-TaSe₂) by mechanical exfoliation technique. Number of layers is confirmed by white light contrast spectroscopy and atomic force microscopy (AFM). Vibrational properties of the atomically thin layers of 2H-TaSe₂ are characterized by micro-Raman spectroscopy. Room temperature Raman measurements demonstrate MoS₂-like spectral features, which are reliable for thickness determination. E₁g mode, usually forbidden in backscattering Raman configuration is observed in the supported TaSe₂ layers while disappears in the suspended layers, suggesting that this mode may be enabled because of the symmetry breaking induced by the interaction with the substrate. A systematic in-situ low temperature Raman study, for the first time, reveals the existence of incommensurate charge density wave phase transition in single and double-layered 2H-TaSe₂ as reflected by a sudden softening of the second-order broad Raman mode resulted from the strong electron-phonon coupling (Kohn anomaly).