Electrochemical exfoliation of 2D materials beyond graphene

After the discovery of graphene in 2004, the field of atomically thin crystals has exploded with the discovery of thousands of 2-dimensional materials (2DMs) with unique electronic and optical properties, by making them very attractive for a broad range of applications, from electronics to energy storage and harvesting, and from sensing to biomedical applications. In order to integrate 2DMs into practical applications, it is crucial to develop mass scalable techniques providing crystals of high quality and in large yield. Electrochemical exfoliation is one of the most promising methods for producing 2DMs, as it enables quick and large-scale production of solution processable nanosheets with a thickness well below 10 layers and lateral size above 1 μm. Originally, this technique was developed for the production of graphene; however, in the last few years, this approach has been successfully extended to other 2DMs, such as transition metal dichalcogenides, black phosphorous, hexagonal boron nitride, MXenes and many other emerging 2D materials. This review first provides an introduction to the fundamentals of electrochemical exfoliation and then it discusses the production of each class of 2DMs, by introducing their properties and giving examples of applications. Finally, a summary and perspective are given to address some of the challenges in this research area.

Application of GaS nanotubes as efficient catalysts in photocatalytic hydrolysis: a first principles study

Photocatalytic hydrogen production is a promising and sustainable technology that converts solar energy into hydrogen energy with the assistance of semiconductor photocatalysts. Herein, we investigated the geometric structure and electronic and photocatalytic properties of single-walled GaS nanotubes under the framework of density functional theory with HSE06 as an exchange-correlation function. This paper presents the first study on the geometric structure, electron, and photocatalytic properties of single-walled GaS nanotubes. The results show that the strain energy and formation energy of GaS nanotubes decrease, while the structure is more stable, with increasing radius. Our study shows that after rolling from the slab, the nanotubes undergo a transition from an indirect band gap to a direct band gap and have appropriate band gaps for absorbing visible light. Moreover, it is speculated that the large disparity between the effective mass of electrons and holes can reduce charge carrier recombination. Among them, the nanotube with a diameter larger than (35, 0) showed promising band edge positions for photocatalytic hydrolysis redox potential with pH values between 0 and 7. Based on these properties, we believe that GaS nanotubes will be promising in photocatalytic hydrolysis.

Deciphering the ultra-high plasticity in metal monochalcogenides

The quest for electronic devices that offer flexibility, wearability, durability and high performance has spotlighted two-dimensional (2D) van der Waals materials as potential next-generation semiconductors. Especially noteworthy is indium selenide, which has demonstrated surprising ultra-high plasticity. To deepen our understanding of this unusual plasticity in 2D van der Waals materials and to explore inorganic plastic semiconductors, we have conducted in-depth experimental and theoretical investigations on metal monochalcogenides (MX) and transition metal dichalcogenides (MX2). We have discovered a general plastic deformation mode in MX, which is facilitated by the synergetic effect of phase transitions, interlayer gliding and micro-cracks. This is in contrast to crystals with strong atomic bonding, such as metals and ceramics, where plasticity is primarily driven by dislocations, twinning or grain boundaries. The enhancement of gliding barriers prevents macroscopic fractures through a pinning effect after changes in stacking order. The discovery of ultra-high plasticity and the phase transition mechanism in 2D MX materials holds significant potential for the design and development of high-performance inorganic plastic semiconductors.

Growth and Liquid-Phase Exfoliation of GaSe1-xSx Crystals

In recent years, interest in the liquid-phase exfoliation (LPE) of layered crystals has been growing due to the efficiency and scalability of the method, as well as the wide range of practical applications of the obtained dispersions based on two-dimensional flakes. In this paper, we present a comparative study of as-grown and liquid-phase exfoliated GaSe_1-xS_x flakes. Bulk GaSe_1-xS_x crystals with x ~ 0, 0.25, 0.5, 0.75, 1 were synthesized by melting stoichiometric amounts of gallium, selenium, and sulfur particles in evacuated ampoules. X-ray diffraction analysis showed that the crystal structure does not change considerably after LPE, while the analysis of the Raman spectra revealed that, after liquid-phase processing in IPA, an additional peak associated with amorphous selenium is observed in selenium-rich GaSeS compounds. Nevertheless, the direct and indirect transition energies determined from the Kubelka-Munk function for LPE crystals correlate with the band gap of the as-grown bulk GaSeS crystals. This finding is also confirmed by comparison with the data on the positions of the photoluminescence peak.

Nonlinear Optical Activities in Two-Dimensional Gallium Sulfide: A Comprehensive Study

The nonlinear optical (NLO) properties of two-dimensional (2D) materials are fascinating for fundamental physics and optoelectronic device development. However, relatively few investigations have been conducted to establish the combined NLO activities of a 2D material. Herein, a study of numerous NLO properties of 2D gallium sulfide (GaS), including second-harmonic generation (SHG), two-photon excited fluorescence (TPEF), and NLO absorption are presented. The layer-dependent SHG response of 2D GaS identifies the noncentrosymmetric nature of the odd layers, and the second-order susceptibility (χ²) value of 47.98 pm/V (three-layers of GaS) indicates the superior efficiency of the SHG signal. In addition, structural deformation induces the symmetry breaking and facilitates the SHG in the bulk samples, whereas a possible efficient symmetry breaking in the liquid-phase exfoliated samples results in an enhancement of the SHG signal, providing prospective fields of investigation for researchers. The generation of TPEF from 800 to 860 nm depicts the two-photon absorption characteristics of 2D GaS material. Moreover, the saturable absorption characteristics of 2D GaS are realized from the largest nonlinear absorption coefficient (β) of -9.3 × 10³, -91.0 × 10³, and -6.05 × 10³ cm/GW and giant modulation depths (T_s) of 24.4%, 35.3%, and 29.1% at three different wavelengths of 800, 1066, and 1560 nm, respectively. Hence, such NLO activities indicate that 2D GaS material can facilitate in the technical advancements of future nonlinear optoelectronic devices.

Two-Dimensional Gallium Sulfide as a Novel Saturable Absorber for Broadband Ultrafast Photonics Applications

Two-dimensional (2D) gallium sulfide (GaS) offers a plethora of exceptional electrical and optical properties, allowing it to be used in a wide range of applications, including photodetectors, hydrogen generation, and nonlinear optical devices. In this paper, ultrathin 2D GaS nanosheets are synthesized using the liquid-phase exfoliation method, and the structure, morphology, and chemical composition of the as-prepared nanosheets are extensively investigated. After depositing 2D GaS nanosheets on side polished fibers, successful saturable absorbers (SAs) are fabricated for the first time. The realized modulation depths are 10 and 5.3% at 1 and 1.5 μm, respectively, indicating the wideband saturable absorption performance of the prepared SAs. By integrating GaS-SAs into three different wavelength-based fiber laser cavities, stable mode-locked pulses are achieved, having pulse durations of 46.22 ps (1 μm), 614 fs (1.5 μm), and 1.02 ps (2 μm), respectively. Additionally, different orders of harmonic mode-locked pulses with the highest repetition rate of 0.55 GHz (45th order) and Q-switched pulses with the shortest pulse duration of 2.2 μs are obtained in the telecommunication waveband. These findings suggest that 2D GaS has a lot of potential for broadband ultrafast photonics in nonlinear photonics devices.

GaS:WS2 Heterojunctions for Ultrathin Two-Dimensional Photodetectors with Large Linear Dynamic Range across Broad Wavelengths

Two-dimensional (2D) photodetectors based on photovoltaic effect or photogating effect can hardly achieve both high photoresponsivity and large linear dynamic range at the same time, which greatly limits many practical applications such as imaging sensors. Here, the conductive-sensitizer strategy, a general design for improving photoresponsivity and linear dynamic range in 2D photodetectors is provided and experimentally demonstrated on vertically stacked bilayer WS₂/GaS_0.87 under a parallel circuit mode. Owing to successful band alignment engineering, the isotype type-II heterojunction enables efficient charge carrier transfer from WS₂, the high-mobility sensitizer, to GaS_0.87, the low-mobility channel, under illumination from a broad visible spectrum. The transferred electron charges introduce a reverse electric field which efficiently lowers the band offset between the two materials, facilitating a transition from low-mobility photocarrier transport to high-mobility photocarrier transport with increasing illumination power. We achieved a large linear dynamic range of 73 dB as well as a high and constant photoresponsivity of 13 A/W under green light. X-ray photoelectron spectroscopy, cathodoluminescence, and Kelvin probe force microscopy further identify the key role of defects in monolayer GaS_0.87 in engineering the band alignment with monolayer WS₂. This work proposes a design route based on band and interface modulation for improving performance of 2D photodetectors and provides deep insights into the important role of strong interlayer coupling in offering heterostructures with desired properties and functions.

Controlling Defects in Continuous 2D GaS Films for High-Performance Wavelength-Tunable UV-Discriminating Photodetectors

A chemical vapor deposition method is developed for thickness-controlled (one to four layers), uniform, and continuous films of both defective gallium(II) sulfide (GaS): GaS_0.87 and stoichiometric GaS. The unique degradation mechanism of GaS_0.87 with X-ray photoelectron spectroscopy and annular dark-field scanning transmission electron microscopy is studied, and it is found that the poor stability and weak optical signal from GaS are strongly related to photo-induced oxidation at defects. An enhanced stability of the stoichiometric GaS is demonstrated under laser and strong UV light, and by controlling defects in GaS, the photoresponse range can be changed from vis-to-UV to UV-discriminating. The stoichiometric GaS is suitable for large-scale, UV-sensitive, high-performance photodetector arrays for information encoding under large vis-light noise, with short response time (<66 ms), excellent UV photoresponsivity (4.7 A W^-1 for trilayer GaS), and 26-times increase of signal-to-noise ratio compared with small-bandgap 2D semiconductors. By comprehensive characterizations from atomic-scale structures to large-scale device performances in 2D semiconductors, the study provides insights into the role of defects, the importance of neglected material-quality control, and how to enhance device performance, and both layer-controlled defective GaS_0.87 and stoichiometric GaS prove to be promising platforms for study of novel phenomena and new applications.

2D GeSe2 amorphous monolayer

In this paper, GeSe2 thin film and glass ingot were prepared in a layered structure. Subsequently, the 2D amorphous monolayers were achieved from layered thin film and layered glass ingot. The thicknesses of monolayers from thin film range from 1.5 nm to 5 nm. And the thickness of monolayer from glass ingot is 7 μm. The fast cooling of material results in the formation of self-assembled monolayers. In the case of thin film, layers are connected with “bridge”. After doping of Ag, the precipitation of nano particles exfoliates the adjacent monolayers which can be further dispersed by etching of Ag particles. In the case of glass ingot, the composition changes at 1 % between adjacent monolayers, according to EDX (energy-dispersive X-ray spectroscopy) spectra. And the glass 2D monolayer can be mechanically peeled off from the glass ingot.

Layered transition metal dichalcogenide electrochemistry: journey across the periodic table

Studies on layered transition metal dichalcogenides (TMDs), in particular for Group VIB TMDs like MoS2 and WS2, have long reached a crescendo in the realms of electrochemical applications initiated by their remarkable catalytic and electronic properties. One area that garnered considerable attention is the fervent pursuit of layered TMDs as electrocatalysts for hydrogen evolution reaction (HER), driven by global efforts towards reducing carbon footprint and attaining hydrogen economy. This Tutorial Review captures the essence of electrochemistry of different classes of layered TMDs and metal chalcogenides across the period table and showcases their tuneable electrochemical and HER catalytic attributes that are governed by the elemental composition, structure and anisotropy. Of interest to the assiduously studied Group VIB TMDs, we describe the role of elemental constituents and material purity in aspects of surface composition and structure, on their electrochemistry. Across families of layered TMDs in the periodic table, we highlight the apparent trends in their electrochemical and electrocatalytic properties through diligent comparison. Inevitably, these trends vary according to the type of chalcogen or transition metal that constitutes the eventual TMD. Beyond layered TMDs, we discuss the electrochemistry and recent progress in HER electrocatalysis of other layered metal chalcogenides that are overshadowed by the success of Group VIB TMDs. At the pinnacle of the emergent applications of layered TMDs, it is prudent to demystify the intrinsic electrochemical behaviour that originates from the participation of the elemental constitution of transition metal or chalcogen. Moreover, knowledge of the catalytic and electronic properties of the various TMD families and emerging trends across the period or down the group is of paramount importance when introducing or refining their prospective uses. The annotations in this Tutorial Review are envisioned to promote discourse into the catalytic and electrochemical trends of TMDs that is currently absent.

Effect of illumination and Se vacancies on fast oxidation of ultrathin gallium selenide

Gallium selenide (GaSe) has recently emerged as a unique platform due to its exciting properties, namely, large and fast photo-response, high carrier mobility and non-linear optical properties. However, exposure for a few days causes the fast oxidation of ultrathin GaSe under ambient conditions and the oxidation mechanism remains unclear. By means of density functional theory calculations and ab initio molecular dynamics simulations, we comprehensively investigated the possible sources that cause oxidation of ultrathin GaSe. Our results show that illumination and Se vacancies induce the fast oxidation of GaSe. Under illumination, photo-excited electrons from the surface of GaSe are effectively transferred to oxygen molecules and thus, superoxide anions (O2-) are generated that react with GaSe. Moreover, Se vacancies directly react with O2. In both the cases, the Ga-Se bonds are continually replaced by Ga-O bonds, which eventually leads to complete degradation of GaSe, accompanied with the formation of the oxidation products Ga2O3 and elemental Se. The comprehensive degradation mechanism unveiled herein lays an important foundation for the development of suitable protecting strategies in GaSe-based devices.

Tuning Schottky barriers for monolayer GaSe FETs by exploiting a weak Fermi level pinning effect

Monolayer gallium selenide (GaSe), an emerging two-dimensional semiconductor, holds great promise for electronics and optoelectronics.