Tunable inverted gap in monolayer quasi-metallic MoS2 induced by strong charge-lattice coupling

Dualistic insulator states in 1T-TaS2 crystals

While the monolayer sheet is well-established as a Mott-insulator with a finite energy gap, the insulating nature of bulk 1T-TaS2 crystals remains ambiguous due to their varying dimensionalities and alterable interlayer coupling. In this study, we present a unique approach to unlock the intertwined two-dimensional Mott-insulator and three-dimensional band-insulator states in bulk 1T-TaS2 crystals by structuring a laddering stack along the out-of-plane direction. Through modulating the interlayer coupling, the insulating nature can be switched between band-insulator and Mott-insulator mechanisms. Our findings demonstrate the duality of insulating nature in 1T-TaS2 crystals. By manipulating the translational degree of freedom in layered crystals, our discovery presents a promising strategy for exploring fascinating physics, independent of their dimensionality, thereby offering a "three-dimensional" control for the era of slidetronics.

1T'-transition metal dichalcogenide monolayers stabilized on 4H-Au nanowires for ultrasensitive SERS detection

Unconventional 1T'-phase transition metal dichalcogenides (TMDs) have aroused tremendous research interest due to their unique phase-dependent physicochemical properties and applications. However, due to the metastable nature of 1T'-TMDs, the controlled synthesis of 1T'-TMD monolayers (MLs) with high phase purity and stability still remains a challenge. Here we report that 4H-Au nanowires (NWs), when used as templates, can induce the quasi-epitaxial growth of high-phase-purity and stable 1T'-TMD MLs, including WS2, WSe2, MoS2 and MoSe2, via a facile and rapid wet-chemical method. The as-synthesized 4H-Au@1T'-TMD core-shell NWs can be used for ultrasensitive surface-enhanced Raman scattering (SERS) detection. For instance, the 4H-Au@1T'-WS2 NWs have achieved attomole-level SERS detections of Rhodamine 6G and a variety of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike proteins. This work provides insights into the preparation of high-phase-purity and stable 1T'-TMD MLs on metal substrates or templates, showing great potential in various promising applications.

Stacking engineering in layered homostructures: transitioning from 2D to 3D architectures

Artificial materials, characterized by their distinctive properties and customized functionalities, occupy a central role in a wide range of applications including electronics, spintronics, optoelectronics, catalysis, and energy storage. The emergence of atomically thin two-dimensional (2D) materials has driven the creation of artificial heterostructures, harnessing the potential of combining various 2D building blocks with complementary properties through the art of stacking engineering. The promising outcomes achieved for heterostructures have spurred an inquisitive exploration of homostructures, where identical 2D layers are precisely stacked. This perspective primarily focuses on the field of stacking engineering within layered homostructures, where precise control over translational or rotational degrees of freedom between vertically stacked planes or layers is paramount. In particular, we provide an overview of recent advancements in the stacking engineering applied to 2D homostructures. Additionally, we will shed light on research endeavors venturing into three-dimensional (3D) structures, which allow us to proactively address the limitations associated with artificial 2D homostructures. We anticipate that the breakthroughs in stacking engineering in 3D materials will provide valuable insights into the mechanisms governing stacking effects. Such advancements have the potential to unlock the full capability of artificial layered homostructures, propelling the future development of materials, physics, and device applications.

Gate tunable linear dichroism in monolayer 1T'-MoS2

The linear dichroism demonstrates promising applications in the fields of polarization-resolved photodetectors and polarization optical imaging. Herein, we study the optical properties of monolayer 1T'-MoS2 based on a four-band effective k · p Hamiltonian within the framework of linear response theory. Owing to the anisotropic band structure, the k-resolved optical transition matrix elements associated with armchair(x) and zigzag(y) direction polarized light exhibit a staggered pattern. The anisotropy of the optical absorption spectrum is shown to sensitively depend on the photon energy, the light polarization and the gate voltage. A gate voltage can continuously modulate the anisotropy of the optical absorption spectra, rendering it isotropic or even reversing the initial anisotropy. This modulation leads to linear dichroism conversions across multiple wavelengths. Our findings are useful to design polarized photodetectors and sensors based on monolayer 1T'-MoS2. Our results are also applicable to other monolayer transition metal dichalcogenides with 1T' structure.

Investigation of Interface Interactions Between Monolayer MoS2 and Metals: Implications on Strain and Surface Roughness

Achieving a low contact resistance has been an important issue in the design of two-dimensional (2D) semiconductor-metal interfaces. The metal contact resistance is dominated by interfacial interactions. Here, we systematically investigate 2D semiconductor-metal interfaces formed by transferring monolayer MoS2 onto prefabricated metal surfaces, such as Au and Pd, using X-ray photoelectron spectroscopy (XPS), atomic force microscopy, and Raman spectroscopy. In contrast to the MoS2/HOPG interface, the interfaces of MoS2/Au and MoS2/Pd feature the formation of weak covalent bonds. The XPS spectra reveal distinct peak positions for S-Au and S-Pd, indicating a higher doping concentration at the S-Au interface. This difference is a key factor in understanding the electronic interactions at the metal-MoS2 interfaces. Additionally, we observe that the metal surface roughness is a critical determinant of the adhesion behavior of transferred monolayer MoS2, resulting in different strains and doping concentrations. The strain on transferred MoS2 increases with an increase in substrate roughness. However, the strain is released when the roughness of metal surface surpasses a certain threshold. The dependence of the contact material and the influence of the substrate roughness on the contact interface provide critical information for improving 2D semiconductor-metal contacts and device performance.

Recent Progress on Phase Engineering of Nanomaterials

As a key structural parameter, phase depicts the arrangement of atoms in materials. Normally, a nanomaterial exists in its thermodynamically stable crystal phase. With the development of nanotechnology, nanomaterials with unconventional crystal phases, which rarely exist in their bulk counterparts, or amorphous phase have been prepared using carefully controlled reaction conditions. Together these methods are beginning to enable phase engineering of nanomaterials (PEN), i.e., the synthesis of nanomaterials with unconventional phases and the transformation between different phases, to obtain desired properties and functions. This Review summarizes the research progress in the field of PEN. First, we present representative strategies for the direct synthesis of unconventional phases and modulation of phase transformation in diverse kinds of nanomaterials. We cover the synthesis of nanomaterials ranging from metal nanostructures such as Au, Ag, Cu, Pd, and Ru, and their alloys; metal oxides, borides, and carbides; to transition metal dichalcogenides (TMDs) and 2D layered materials. We review synthesis and growth methods ranging from wet-chemical reduction and seed-mediated epitaxial growth to chemical vapor deposition (CVD), high pressure phase transformation, and electron and ion-beam irradiation. After that, we summarize the significant influence of phase on the various properties of unconventional-phase nanomaterials. We also discuss the potential applications of the developed unconventional-phase nanomaterials in different areas including catalysis, electrochemical energy storage (batteries and supercapacitors), solar cells, optoelectronics, and sensing. Finally, we discuss existing challenges and future research directions in PEN.

Evolution of local edge state braiding and spin topological transport characterization of Te-doped monolayer 1T'-MoS2

We conducted first-principles calculations to investigate the dynamic braiding of local edge states and the spin topological transport mechanism in a strong topological MoS1.75Te0.25 matrix. The presence of type-II Van Hove singularity in the middle of the X-S path indicates a strong cohesive interaction and a paring condensation mechanism within the matrix. The surface state data of MoS1.75Te0.25 clearly demonstrate the characteristic features of strong regular loop braiding in spin transport. The spin Hall conductivity of the matrix was determined from the anisotropic characteristics of the spin Berry curvature. The phase transition of the spin Hall conductivity was evidenced by the positive sign of local spin polarization strength, primarily contributed by the dz2 orbital of Mo atoms, and the negative sign of spin polarization strength, mainly contributed by the p-px orbitals of S atoms. Moreover, the inclusion of Te selectively tuned the spin transport efficiency of the dz2 and px orbitals. Comprehensive braiding and readout of edge states can be achieved using an artificially designed MoS1.75Te0.25 spintronic device. This 2D fractional braiding holds significant potential for applications in topological quantum computation.

Improving the Luminescence Performance of Monolayer MoS2 by Doping Multiple Metal Elements with CVT Method

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) draw much attention as critical semiconductor materials for 2D, optoelectronic, and spin electronic devices. Although controlled doping of 2D semiconductors can also be used to tune their bandgap and type of carrier and further change their electronic, optical, and catalytic properties, this remains an ongoing challenge. Here, we successfully doped a series of metal elements (including Hf, Zr, Gd, and Dy) into the monolayer MoS2 through a single-step chemical vapor transport (CVT), and the atomic embedded structure is confirmed by scanning transmission electron microscope (STEM) with a probe corrector measurement. In addition, the host crystal is well preserved, and no random atomic aggregation is observed. More importantly, adjusting the band structure of MoS2 enhanced the fluorescence and the carrier effect. This work provides a growth method for doping non-like elements into 2D MoS2 and potentially many other 2D materials to modify their properties.

Theoretical Studies on the Quantum Capacitance of Two-Dimensional Electrode Materials for Supercapacitors

In recent years, supercapacitors have been widely used in the fields of energy, transportation, and industry. Among them, electrical double-layer capacitors (EDLCs) have attracted attention because of their dramatically high power density. With the rapid development of computational methods, theoretical studies on the physical and chemical properties of electrode materials have provided important support for the preparation of EDLCs with higher performance. Besides the widely studied double-layer capacitance (C_D), quantum capacitance (C_Q), which has long been ignored, is another important factor to improve the total capacitance (C_T) of an electrode. In this paper, we survey the recent theoretical progress on the C_Q of two-dimensional (2D) electrode materials in EDLCs and classify the electrode materials mainly into graphene-like 2D main group elements and compounds, transition metal carbides/nitrides (MXenes), and transition metal dichalcogenides (TMDs). In addition, we summarize the influence of different modification routes (including doping, metal-adsorption, vacancy, and surface functionalization) on the C_Q characteristics in the voltage range of ±0.6 V. Finally, we discuss the current difficulties in the theoretical study of supercapacitor electrode materials and provide our outlook on the future development of EDLCs in the field of energy storage.

Emergent Kagome Electrides

In a two-dimensional (2D) Kagome lattice, the ideal Kagome bands including Dirac cones, van Hove singularities, and a flat band are highly expected, because they can provide a promising platform to investigate novel physical phenomena. However, in the reported Kagome materials, the complex 3D and multiorder electron hoppings result in the disappearance of the ideal Kagome bands in these systems. Here, we propose an alternative way to achieve the ideal Kagome bands in non-Kagome materials by confining excess electrons in the system to the crystal interstitial sites to form a 2D Kagome lattice, coined as a Kagome electride. Then, we predict two novel stable 2D Kagome electrides in hexagonal materials Li₅Si and Li₅Sn, whose band structures are similar to the ideal Kagome bands, including topological Dirac cones with beautiful Fermi arcs in their surface states, van Hove singularities, and a flat band. In addition, Li₅Si is revealed to be a low-temperature superconductor at ambient pressure, and its superconducting transition temperature T_c can be increased from 1.1 K at 0 GPa to 7.2 K at 100 GPa. The high T_c is unveiled to be the consequence of strong electron-phonon coupling originated from the sp-hybridized phonon-coupled bands and phonon softening caused by strong Fermi nesting. Due to the strong Fermi nesting, the charge density wave phase transition occurs at 110 GPa with the lattice reconstructed from hexagonal to orthorhombic, accompanied with the increase of T_c to 10.5 K. Our findings pave an alternative way to fabricate more real materials with Kagome bands in electrides.

Electronic Structure and Stacking Arrangement of Tungsten Disulfide at the Gold Contact

There is an intensive effort to control the nature of attractive interactions between ultrathin semiconductors and metals and to understand its impact on the electronic properties at the junction. Here, we present a photoelectron spectroscopy study on the interface between WS₂ films and gold, with a focus on the occupied electronic states near the Brillouin zone center (i.e., the Γ point). To delineate the spectra of WS₂ supported on crystalline Au from the suspended WS₂, we employ a microscopy approach and a tailored sample structure, in which the WS₂/Au junction forms a semi-epitaxial relationship and is adjacent to suspended WS₂ regions. The photoelectron spectra, as a function of WS₂ thickness, display the expected splitting of the highest occupied states at the Γ point. In multilayer WS₂, we discovered variations in the electronic states that spatially align with the crystalline grains of underlying Au. Corroborated by density functional theory calculations, we attribute the electronic structure variations to stacking variations within the WS₂ films. We propose that strong interactions exerted by Au grains cause slippage of the interfacing WS₂ layer with respect to the rest of the WS₂ film. Our findings illustrate that the electronic properties of transition metal dichalcogenides, and more generally 2D layered materials, are physically altered by the interactions with the interfacing materials, in addition to the electron screening and defects that have been widely considered.

Defect Engineering of Out-of-Plane Charge Transport in van der Waals Heterostructures for Bi-Direction Photoresponse

Defects are ubiquitous in two-dimensional (2D) transition-metal dichalcogenides (TMDs), generated by the initial growth- or the postprocessing. However, the defects may play negative roles in the photoelectronic properties of TMDs due to the reduction of in-plane transport of carriers. In this work, we demonstrate that the Se-vacancy defects in MoSe₂ side of the van der Waal heterostructure is able to switch direction of out-of-plane charge transport. Photoresponse spectra showed defect density enable modified surface potential of MoSe_2-x, leading to the barrier reverse between graphene and MoSe_2-x and switches of the photoresponse from the negative to the positive. This unexpected property stemmed from appearance of midgap states by defects at heterostructure, as demonstrated by the density functional theory calculation and scanning tunneling microscope results. MoSe_2-0.2/graphene heterostructure has a broadband response ranging from 450 to 1064 nm and exhibits comparable or higher positive responsivity (5.4 × 10³ A/W to -15.3 × 10³ A/W at 632.8 and 5.7 × 10³ A/W to -1.2 × 10³ A/W at 1064 nm) to the negative one of the pristine MoSe₂/graphene. Based on defect-engineered heterostructures, we construct optoelectronic OR and AND logic devices with a broadband operation. Our work elucidates an alternative avenue to tailor the out-of-plane charge transport in TMD-based heterostructure through defects, and potentially invokes applicable utilization for 2D photodetectors and optoelectronic logic gates.

Doping-Mediated Lattice Engineering of Monolayer ReS2 for Modulating In-Plane Anisotropy of Optical and Transport Properties

ReS₂ exhibits strong anisotropic optical and electrical responses originating from the asymmetric lattice. Here, we show that the anisotropy of monolayer (1L) ReS₂ in optical scattering and electrical transport can be practically erased by lattice engineering via lithium (Li) treatment. Scanning transmission electron microscopy revealed that significant strain is induced in the lattice of Li-treated 1L-ReS₂, due to high-density electron doping and the resultant formation of continuous tiling of nanodomains with randomly rotating orientations of 60°, which produced a nearly isotropic response of polarized Raman scattering and absorption of Li-treated 1L-ReS₂. With Li treatment, the in-plane conductance of 1L-ReS₂ increased by an order of magnitude, and its angle dependence became negligible. Our result that the asymmetric phase was converted into the isotropic phase by electron injection could significantly expand the optoelectronic applications of polymorphic two-dimensional transition metal dichalcogenides.

Recent developments in 2D transition metal dichalcogenides: phase transition and applications of the (quasi-)metallic phases

The advent of two-dimensional transition metal dichalcogenides (2D-TMDs) has led to an extensive amount of interest amongst scientists and engineers alike and an intensive amount of research has brought about major breakthroughs in the electronic and optical properties of 2D materials. This in turn has generated considerable interest in novel device applications. With the polymorphic structural features of 2D-TMDs, this class of materials can exhibit both semiconducting and metallic (quasi-metallic) properties in their respective phases. This polymorphic property further increases the interest in 2D-TMDs both in fundamental research and for their potential utilization in novel high-performance device applications. In this review, we highlight the unique structural properties of few-layer and monolayer TMDs in the metallic 1T- and quasi-metallic 1T'-phases, and how these phases dictate their electronic and optical properties. An overview of the semiconducting-to-(quasi)-metallic phase transition of 2D-TMD systems will be covered along with a discussion on the phase transition mechanisms. The current development in the applications of (quasi)-metallic 2D-TMDs will be presented ranging from high-performance electronic and optoelectronic devices to energy storage, catalysis, piezoelectric and thermoelectric devices, and topological insulator and neuromorphic computing applications. We conclude our review by highlighting the challenges confronting the utilization of TMD-based systems and projecting the future developmental trends with an outlook of the progress needed to propel this exciting field forward.

Theory-Guided Discovery of Novel Materials

The traditional approach for materials discovery has been the domain of experimentalists, where elemental composition and synthesis conditions are often based on a trial-and-error method. Such processes are time-consuming and expensive. To minimize cost and to develop new materials at a faster pace, an alternate approach is to use theory to predict new materials with tailored properties and have experiments validate such predictions. The phenomenal increase in computing power, development of new first-principles methodologies, and a myriad of advanced computer codes in recent years have enabled researchers to predict novel materials that can be verified by later experiments. In this Perspective, we present advances in density functional theory-based methods and computational procedures that have made possible the discoveries of materials with varying size, composition, and dimensionalities. The challenges and opportunities in theory-guided discovery of materials, going forward, are also discussed.

Strain, Doping, and Electronic Transport of Large Area Monolayer MoS2 Exfoliated on Gold and Transferred to an Insulating Substrate

Gold-assisted mechanical exfoliation currently represents a promising method to separate ultralarge (centimeter scale) transition metal dichalcogenide (TMD) monolayers (1L) with excellent electronic and optical properties from the parent van der Waals (vdW) crystals. The strong interaction between Au and chalcogen atoms is key to achieving this nearly perfect 1L exfoliation yield. On the other hand, it may significantly affect the doping and strain of 1L TMDs in contact with Au. In this paper, we systematically investigated the morphology, strain, doping, and electrical properties of large area 1L MoS₂ exfoliated on ultraflat Au films (0.16-0.21 nm roughness) and finally transferred to an insulating Al₂O₃ substrate. Raman mapping and correlative analysis of the E' and A₁' peak positions revealed a moderate tensile strain (ε ≈ 0.2%) and p-type doping (n ≈ -0.25 × 10¹³ cm^-2) of 1L MoS₂ in contact with Au. Nanoscale resolution current mapping and current-voltage (I-V) measurements by conductive atomic force microscopy (C-AFM) showed direct tunneling across the 1L MoS₂ on Au, with a broad distribution of tunneling barrier values (Φ_B from 0.7 to 1.7 eV) consistent with p-type doping of MoS₂. After the final transfer of 1L MoS₂ on Al₂O₃/Si, the strain was converted to compressive strain (ε ≈ -0.25%). Furthermore, an n-type doping (n ≈ 0.5 × 10¹³ cm^-2) was deduced by Raman mapping and confirmed by electrical measurements of an Al₂O₃/Si back-gated 1L MoS₂ transistor. These results provide a deeper understanding of the Au-assisted exfoliation mechanism and can contribute to its widespread application for the realization of novel devices and artificial vdW heterostructures.

Upconversion Photovoltaic Effect of WS2/2D Perovskite Heterostructures by Two-Photon Absorption

Photovoltaic devices work by converting sunlight energy into electric energy. The efficiency of current photovoltaic devices, however, is significantly limited by the transmission loss of photons with energies below the bandgap of channel semiconductors, which can be circumvented by photon energy upconversion. Energy upconversion has been widely employed to improve the efficiency of traditional solar cells. However, the employment of energy upconversion in two-dimensional (2D) heterostructure photovoltaic devices has not been investigated yet. Here, we report the upconversion photovoltaic effect of WS₂ monolayer/(C₆H₅C₂H₄NH₃)₂PbI₄ (PEPI) 2D perovskite heterostructures by below-bandgap two-photon absorption via a virtual intermediate state. An open circuit voltage of 0.37 V and short circuit current of 7.4 pA are obtained with a photoresponsivity of 771 pA/W and current on/off ratio of 130:1. This work demonstrates that upconversion by two-photon absorption may potentially be a strategy for boosting the efficiency of 2D material-based photovoltaic devices by virtue of the absorption of photons below the bandgap energy of channel semiconductors.

Photoluminescence upconversion of 2D materials and applications

Photoluminescence (PL) upconversion is a phenomenon involving light-matter interactions, where the energy of emitted photons is higher than that of the incident photons. PL upconversion is an intriguing process in two-dimensional materials and specifically designed 2D heterostructures, which have potential upconversion applications in optoelectronic devices, bioimaging, and semiconductor cooling. In this review, we focus on the recent advances in photoluminescence upconversion in two-dimensional materials and their heterostructures. We discuss the upconversion mechanisms, applications, and future outlook of upconversion in two-dimensional materials.

Immunoassay-Amplified Responses Using a Functionalized MoS2-Based SPR Biosensor to Detect PAPP-A2 in Maternal Serum Samples to Screen for Fetal Down's Syndrome

Background

Due to educational, social and economic reasons, more and more women are delaying childbirth. However, advanced maternal age is associated with several adverse pregnancy outcomes, and in particular a high risk of Down’s syndrome (DS). Hence, it is increasingly important to be able to detect fetal Down’s syndrome (FDS).

Methods

We developed an effective, highly sensitive, surface plasmon resonance (SPR) biosensor with biochemically amplified responses using carboxyl-molybdenum disulfide (MoS₂) film. The use of carboxylic acid as a surface modifier of MoS₂ promoted dispersion and formed specific three-dimensional coordination sites. The carboxylic acid immobilized unmodified antibodies in a way that enhanced the bioaffinity of MoS₂ and preserved biorecognition properties of the SPR sensor surface. Complete antigen pregnancy-associated plasma protein-A2 (PAPP-A2) conjugated with the carboxyl-MoS₂-modified gold chip to amplify the signal and improve detection sensitivity. This heterostructure interface had a high work function, and thus improved the efficiency of the electric field energy of the surface plasmon. These results provide evidence that the interface electric field improved performance of the SPR biosensor.

Results

The carboxyl-MoS₂-based SPR biosensor was used successfully to evaluate PAPP-A2 level for fetal Down’s syndrome screening in maternal serum samples. The detection limit was 0.05 pg/mL, and the linear working range was 0.1 to 1100 pg/mL. The women with an SPR angle >46.57 m° were more closely associated with fetal Down’s syndrome. Once optimized for serum Down’s syndrome screening, an average recovery of 95.2% and relative standard deviation of 8.5% were obtained. Our findings suggest that carboxyl-MoS₂-based SPR technology may have advantages over conventional ELISA in certain situations.

Conclusion

Carboxyl-MoS₂-based SPR biosensors can be used as a new diagnostic technology to respond to the increasing need for fetal Down’s syndrome screening in maternal serum samples. Our results demonstrated that the carboxyl-MoS₂-based SPR biosensor was capable of determining PAPP-A2 levels with acceptable accuracy and recovery. We hope that this technology will be investigated in diverse clinical trials and in real case applications for screening and early diagnosis in the future.

Hidden Vacancy Benefit in Monolayer 2D Semiconductors

Monolayer 2D semiconductors (e.g., MoS₂ ) are of considerable interest for atomically thin transistors but generally limited by insufficient carrier mobility or driving current. Minimizing the lattice defects in 2D semiconductors represents a common strategy to improve their electronic properties, but has met with limited success to date. Herein, a hidden benefit of the atomic vacancies in monolayer 2D semiconductors to push their performance limit is reported. By purposely tailoring the sulfur vacancies (SVs) to an optimum density of 4.7% in monolayer MoS₂ , an unusual mobility enhancement is obtained and a record-high carrier mobility (>115 cm² V^-1 s^-1 ) is achieved, realizing monolayer MoS₂ transistors with an exceptional current density (>0.60 mA µm^-1 ) and a record-high on/off ratio >10¹⁰ , and enabling a logic inverter with an ultrahigh voltage gain >100. The systematic transport studies reveal that the counterintuitive vacancy-enhanced transport originates from a nearest-neighbor hopping conduction model, in which an optimum SV density is essential for maximizing the charge hopping probability. Lastly, the vacancy benefit into other monolayer 2D semiconductors is further generalized; thus, a general strategy for tailoring the charge transport properties of monolayer materials is defined.

Phase Transition-Induced Temperature-Dependent Phonon Shifts in Molybdenum Disulfide Monolayers Interfaced with a Vanadium Dioxide Film

We report the optical phonon shifts induced by phase transition effects of vanadium dioxide (VO₂) in monolayer molybdenum disulfide (MoS₂) when interfacing with a VO₂ film showing a metal-insulator transition coupled with structural phase transition (SPT). To this end, the monolayer MoS₂ directly synthesized on a SiO₂/Si substrate by chemical vapor deposition was first transferred onto a VO₂/c-Al₂O₃ substrate in which the VO₂ film was prepared by a sputtering method. We compared the MoS₂ interfaced with the VO₂ film with the as-synthesized MoS₂ by using Raman spectroscopy. The temperature-dependent Raman scattering characteristics exhibited the distinct phonon behaviors of the E_2g¹ and A_1g modes in the monolayer MoS₂. Specifically, for the as-synthesized MoS₂, there were no Raman shifts for each mode, but the enhancement in the Raman intensities of E_2g¹ and A_1g modes was clearly observed with increasing temperature, which could be interpreted by the significant contribution of the interface optical interference effect. In contrast, the red-shifts of both the E_2g¹ and A_1g modes for the MoS₂ transferred onto VO₂ were clearly observed across the phase transition of VO₂, which could be explained in terms of the in-plane tensile strain effect induced by the SPT and the enhancement of electron-phonon interactions due to an increased electron density at the MoS₂/VO₂ interface through the electronic phase transition. This study provides further insights into the influence of interfacial hybridization for the heterogeneous integration of 2D transition-metal dichalcogenides and strongly correlated materials.

Nanogap-based all-electronic DNA sequencing devices using MoS2 monolayers

The realization of nanopores in atom-thick materials may pave the way towards electrical detection of single biomolecules in a stable and scalable manner. In this work, we theoretically study the potential of different phases of MoS2 nanogaps to act as all-electronic DNA sequencing devices. We carry out simulations based on density functional theory and the non-equilibrium Green's function formalism to investigate the electronic transport across the device. Our results suggest that the 1T'-MoS2 nanogap structure is energetically more favorable than its 2H counterpart. At zero bias, the changes in the conductance of the 1T'-MoS2 device can be well distinguished, making possible the selectivity of the DNA nucleobases. Although the conductance fluctuates around the resonances, the overall results suggest that it is possible to distinguish the four DNA bases for energies close to the Fermi level.

Reduction of the ambient effect in multilayer InSe transistors and a strategy toward stable 2D-based optoelectronic applications

Indium selenide (InSe) photodetection devices attract significant research interest. However, InSe is unstable and degrades rapidly in ambient conditions, thus it is still a challenge to fabricate stable optoelectronic devices. In this work, multilayer InSe FETs are fabricated, and their photoresponse properties are investigated. Both positive and negative photoconductivities are observed for the first time in the same InSe FET in a wide spectral range from 450 nm to 660 nm, which can be tuned through changing either the gate bias or the source-drain bias. A physical mechanism is proposed to explain the dual-photoresponse phenomenon in our devices. Based on the proposed physical mechanism, as a proof of concept, a facile and simple approach is used to eliminate the negative photoconductivity of the InSe FET. Our results will offer valuable strategies for stable multilayer InSe optoelectronic device design, and a practical scheme for improving the performance of other transition metal dichalcogenide devices as well.

Strain and Charge Doping Fingerprints of the Strong Interaction between Monolayer MoS2 and Gold

Gold-mediated exfoliation of MoS₂ has recently attracted considerable interest. The strong interaction between MoS₂ and Au facilitates preferential production of centimeter-sized monolayer MoS₂ with near-unity yield and provides a heterostructure system noteworthy from a fundamental standpoint. However, little is known about the detailed nature of the MoS₂-Au interaction and its evolution with the MoS₂ thickness. Here, we identify the specific vibrational and binding energy fingerprints of this interaction using Raman and X-ray photoelectron spectroscopy, which indicate substantial strain and charge doping in monolayer MoS₂. Tip-enhanced Raman spectroscopy reveals heterogeneity of the MoS₂-Au interaction at the nanoscale, reflecting the spatial nonconformity between the two materials. Micro-Raman spectroscopy shows that this interaction is strongly affected by the roughness and cleanliness of the underlying Au. Our results elucidate the nature of the MoS₂-Au interaction and guide strain and charge doping engineering of MoS₂.

Initial stage of MBE growth of MoSe2 monolayer

An atomically thin MoSe₂ layer has been synthesized on mica using molecular beam epitaxy (MBE). The polymorphous of the MoSe₂ layer depends on the coverage and the growth temperature. At low coverages and low growth temperature, 1T-MoSe₂ forms in addition to a comparable quantity of 2H-MoSe₂. The metastable 1T-MoSe₂ transfers gradually to the stable 2H-MoSe₂ before the completion of the first monolayer. The current result sheds some light on the complexity of the nucleation and growth of transition metal dichalcogenide (TMDC) monolayers and implies a possible route for a phase selective synthesis using MBE.

Ultrathin 1T-MoS2 Nanoplates Induced by Quaternary Ammonium-Type Ionic Liquids on Polypyrrole/Graphene Oxide Nanosheets and Its Irreversible Crystal Phase Transition During Electrocatalytic Nitrogen Reduction

Ultrathin nanoplates of metastable 1T-MoS₂ have been successfully stabilized and uniformly distributed on the surface of n-butyl triethyl ammonium bromide functionalized polypyrrole/graphene oxide (BTAB/PPy/GO) by a very simple hydrothermal method. BTAB as a typical kind of quaternary ammonium-type ionic liquids (ILs) played a crucial role in the formation of the obtained 1T-MoS₂/BTAB/PPy/GO. It was covalently linked with PPy/GO and arranged in a highly ordered order at the solid-liquid interface of PPy/GO and H₂O due to Coulombic interactions and other intermolecular interactions, which would induce and stabilize ultrathin 1T-MoS₂ nanoplates by morphosynthesis. The good electrocatalytic activity toward nitrogen reduction reaction (NRR) with strong durability and good stability can be achieved by 1T-MoS₂/BTAB/PPy/GO due to their excellent inorganic/organic hierarchical lamellar micro-/nanostructures. Especially, after the long-term electrocatalysis for NRR at a negative potential, metastable 1T-MoS₂ as the catalytic center undergoes two types of irreversible crystal phase transition, which was converted to 1T'-MoS₂ and Mo₂N, caused by the competitive hydrogen evolution reaction (HER) process and the electrochemical reaction between the electroactive 1T-MoS₂ and N₂, respectively. The new N-Mo bonding prevents Mo atoms from binding to other N atoms in N₂, resulting in the deactivation of the electrocatalysts to NRR after being used for 18 h. Even so, quaternary ammonium-type ILs would induce the crystal structures of transition-metal dichalcogenides (TMDCs), which might provide a new thought for the reasonable design of electrocatalysts based on TMDCs for electrocatalysis.

Thermal History-Dependent Current Relaxation in hBN/MoS2 van der Waals Dimers

Combining atomically thin layers of van der Waals (vdW) materials in a chosen vertical sequence is an emerging route to create devices with desired functionalities. While this method aims to exploit the individual properties of partnering layers, strong interlayer coupling can significantly alter their electronic and optical properties. Here we explored the impact of the vdW epitaxy on electrical transport in atomically thin molybdenum disulfide (MoS₂) when it forms a vdW dimer with crystalline films of hexagonal boron nitride (hBN). We observe a thermal history-dependent long-term (over ∼40 h) current relaxation in the overlap region of MoS₂/hBN heterostructures, which is absent in bare MoS₂ layers (or homoepitaxial MoS₂/MoS₂ dimers) on the same substrate. Concurrent relaxation in the low-frequency Raman modes in MoS₂ in the heterostructure region suggests a slow structural relaxation between trigonal and octahedral polymorphs of MoS₂ as a likely driving mechanism that also results in inhomogeneous charge distribution in the MoS₂ layer. Our experiment yields an aspect of vdW heteroepitaxy that can be generic to electrical devices with atomically thin transition-metal dichalcogenides.

Direct synthesis of metastable phases of 2D transition metal dichalcogenides

The different polymorphic phases of transition metal dichalcogenides (TMDs) have attracted enormous interest in the last decade. The metastable metallic and small band gap phases of group VI TMDs displayed leading performance for electrocatalytic hydrogen evolution, high volumetric capacitance and some of them exhibit large gap quantum spin Hall (QSH) insulating behaviour. Metastable 1T(1T') phases require higher formation energy, as compared to the thermodynamically stable 2H phase, thus in standard chemical vapour deposition and vapour transport processes the materials normally grow in the 2H phases. Only destabilization of their 2H phase via external means, such as charge transfer or high electric field, allows the conversion of the crystal structure into the 1T(1T') phase. Bottom-up synthesis of materials in the 1T(1T') phases in measurable quantities would broaden their prospective applications and practical utilization. There is an emerging evidence that some of these 1T(1T') phases can be directly synthesized via bottom-up vapour- and liquid-phase methods. This review will provide an overview of the synthesis strategies which have been designed to achieve the crystal phase control in TMDs, and the chemical mechanisms that can drive the synthesis of metastable phases. We will provide a critical comparison between growth pathways in vapour- and liquid-phase synthesis techniques. Morphological and chemical characteristics of synthesized materials will be described along with their ability to act as electrocatalysts for the hydrogen evolution reaction from water. Phase stability and reversibility will be discussed and new potential applications will be introduced. This review aims at providing insights into the fundamental understanding of the favourable synthetic conditions for the stabilization of metastable TMD crystals and at stimulating future advancements in the field of large-scale synthesis of materials with crystal phase control.

Anisotropic Collective Charge Excitations in Quasimetallic 2D Transition-Metal Dichalcogenides

The quasimetallic 1T' phase 2D transition-metal dichalcogenides (TMDs) consist of 1D zigzag metal chains stacked periodically along a single axis. This gives rise to its prominent physical properties which promises the onset of novel physical phenomena and applications. Here, the in-plane electronic correlations are explored, and new mid-infrared plasmon excitations in 1T' phase monolayer WSe2 and MoS2 are observed using optical spectroscopies. Based on an extensive first-principles study which analyzes the charge dynamics across multiple axes of the atomic-layered systems, the collective charge excitations are found to disperse only along the direction perpendicular to the chains. Further analysis reveals that the interchain long-range coupling is responsible for the coherent 1D charge dynamics and the spin-orbit coupling affects the plasmon frequency. Detailed investigation of these charge collective modes in 2D-chained systems offers opportunities for novel device applications and has implications for the underlying mechanism that governs superconductivity in 2D TMD systems.

Squeezed metallic droplet with tunable Kubo gap and charge injection in transition metal dichalcogenides

Shrinking the size of a bulk metal into nanoscale leads to the discreteness of electronic energy levels, the so-called Kubo gap δ. Renormalization of the electronic properties with a tunable and size-dependent δ renders fascinating photon emission and electron tunneling. In contrast with usual three-dimensional (3D) metal clusters, here we demonstrate that Kubo gap δ can be achieved with a two-dimensional (2D) metallic transition metal dichalcogenide (i.e., 1T'-phase MoTe2) nanocluster embedded in a semiconducting polymorph (i.e., 1H-phase MoTe2). Such a 1T'/1H MoTe2 nanodomain resembles a 3D metallic droplet squeezed in a 2D space which shows a strong polarization catastrophe while simultaneously maintaining its bond integrity, which is absent in traditional δ-gapped 3D clusters. The weak screening of the host 2D MoTe2 leads to photon emission of such pseudometallic systems and a ballistic injection of carriers in the 1T'/1H/1T' homojunctions which may find applications in sensors and 2D reconfigurable devices.

Recent progress of TMD nanomaterials: phase transitions and applications

Transition metal dichalcogenides (TMDs) show wide ranges of electronic properties ranging from semiconducting, semi-metallic to metallic due to their remarkable structural differences. To obtain 2D TMDs with specific properties, it is extremely important to develop particular strategies to obtain specific phase structures. Phase engineering is a traditional method to achieve transformation from one phase to another controllably. Control of such transformations enables the control of properties and access to a range of properties, otherwise inaccessible. Then extraordinary structural, electronic and optical properties lead to a broad range of potential applications. In this review, we introduce the various electronic properties of 2D TMDs and their polymorphs, and strategies and mechanisms for phase transitions, and phase transition kinetics. Moreover, the potential applications of 2D TMDs in energy storage and conversion, including electro/photocatalysts, batteries/supercapacitors and electronic devices, are also discussed. Finally, opportunities and challenges are highlighted. This review may further promote the development of TMD phase engineering and shed light on other two-dimensional materials of fundamental interest and with potential ranges of applications.

Probing Multiphased Transition in Bulk MoS2 by Direct Electron Injection

Structural phase transitions in layered two-dimensional (2D) materials are of significant interest owing to their ability to exist in multiple metastable states with distinctive properties. However, phase transition in bulk MoS₂ by nondestructive electron infusion has not yet been realized. In this study, we report the 2H to 1T' phase transition and in-between intermediates in bulk MoS₂ using MoS₂/[Ca₂N]⁺·e^- heterostructures, in which kinetic free electrons were directly injected into MoS₂. We observed various phases in MoS₂ ranging from heavily doped 2H to a distorted lattice state and then on to a complete 1T' state. Snapshots of the multiphase transition were captured by extraordinary Raman shift and bandgap reduction and were further elucidated by theoretical calculations. We also observed a weakening in interlayer coupling in the vicinity of the metallic regime, which led to an unusually strong photoluminescence emission, suggesting light-efficient bulk MoS₂. Our results thus suggest the optoelectronic applications that can fully utilize the multiphase transition of bulk 2D materials.

Three-Dimensional Resonant Exciton in Monolayer Tungsten Diselenide Actuated by Spin-Orbit Coupling

The intricate features of many-body interactions and spin-orbit coupling play a significant role in numerous physical phenomena. Particularly in two-dimensional transition metal dichalcogenides (2D-TMDs), excitonic dynamics are a key phenomenon that promises opportunities for diverse range of device applications. Here, we report the direct observation of a visible-range three-dimensional resonant exciton and its associated charged exciton in monolayer tungsten diselenide, as compared to monolayer molybdenum disulfide. A comprehensive experimental study that includes high-resolution TEM, Raman, high-resolution spectroscopic ellipsometry over a wide temperature range down to 4 K, high-energy temperature, and excitation power-dependent photoluminescence spectroscopy has been conducted. It is supported by first-principles calculations to unravel the influence of spin-orbit coupling in the formation of the resonant exciton and to identify its in-plane and out-of-plane features. Furthermore, we study the impact of temperature and thickness on the spin-orbit coupling strength in 2D-TMDs. This work is crucial in creating a platform in the fundamental understanding of high-energy resonant exciton in layered two-dimensional systems and that such high-energy optoelectronic features make them an increasingly attractive candidate for novel electronic and optoelectronic applications particularly in the aspects of solar cells and light-emitting diodes via the manipulation of excitonic states.

Local engineering of topological phase in monolayer MoS2

Monolayer transition metal dichalcogenides (TMDCs) with the 1T' structure are a new class of large-gap two-dimensional (2D) topological insulators, hosting topologically protected conduction channels on the edges. However, the 1T' phase is metastable compared to the 2H phase for most of 2D TMDCs, among which the 1T' phase is least favored in monolayer MoS₂. Here we report a clean and controllable technique to locally induce nanometer-sized 1T' phase in monolayer 2H-MoS₂ via a weak Argon-plasma treatment, resulting in topological phase boundaries of high density. We found that the stabilization of 1T' phase arises from the concerted effects of S vacancies and the tensile strain. Scanning tunneling spectroscopy (STS) clearly reveals a spin-orbit band gap (~60 meV) and topologically protected in-gap states residing at the 1T'-2H phase boundary, which are corroborated by density-functional theory (DFT) calculations. The strategy developed in this work can be generalized to a large variety of TMDCs materials, with potentials to realize scalable electronics and spintronics with low dissipation.

Understanding Phase Stability of Metallic 1T-MoS2 Anodes for Sodium-Ion Batteries

We discuss metallic 1T-MoS2 as an anode material for sodium-ion batteries (SIBs). In situ Raman is used to investigate the stability of metallic MoS2 during the charging and discharging processes. Parallel first-principles computations are used to gain insight into the experimental observations, including the measured conductivities and the high capacity of the anode.

Unraveling High-Yield Phase-Transition Dynamics in Transition Metal Dichalcogenides on Metallic Substrates

2D transition metal dichalcogenides (2D-TMDs) and their unique polymorphic features such as the semiconducting 1H and quasi-metallic 1T' phases exhibit intriguing optical and electronic properties, which can be used in novel electronic and photonic device applications. With the favorable quasi-metallic nature of 1T'-phase 2D-TMDs, the 1H-to-1T' phase engineering processes are an immensely vital discipline exploited for novel device applications. Here, a high-yield 1H-to-1T' phase transition of monolayer-MoS2 on Cu and monolayer-WSe2 on Au via an annealing-based process is reported. A comprehensive experimental and first-principles study is performed to unravel the underlying mechanism and derive the general trends for the high-yield phase transition process of 2D-TMDs on metallic substrates. While each 2D-TMD possesses different intrinsic 1H-1T' energy barriers, the option of metallic substrates with higher chemical reactivity plays a significantly pivotal role in enhancing the 1H-1T' phase transition yield. The yield increase is achieved via the enhancement of the interfacial hybridizations by the means of increased interfacial binding energy, larger charge transfer, shorter interfacial spacing, and weaker bond strength. Fundamentally, this study opens up the field of 2D-TMD/metal-like systems to further scientific investigation and research, thereby creating new possibilities for 2D-TMDs-based device applications.

Interface Engineering of Au(111) for the Growth of 1T'-MoSe2

Phase-controlled synthesis of two-dimensional transition-metal dichalcogenides (TMDCs) is of great interest due to the distinct properties of the different phases. However, it is challenging to prepare metallic phase of group-VI TMDCs due to their metastability. At the monolayer level, interface engineering can be used to stabilize the metastable phase. Here, we demonstrate the selective growth of either single-layer 1H- or 1T'-MoSe₂ on Au(111) by molecular-beam epitaxy; the two phases can be unambiguously distinguished using scanning tunnelling microscopy and spectroscopy. While the growth of 1H-MoSe₂ is favorable on pristine Au(111), the growth of 1T'-MoSe₂ is promoted by the predeposition of Se on Au(111). The selective growth of the 1T'-MoSe₂ on Se-pretreated Au(111) is attributed to the Mo intercalation induced stabilization of the 1T' phase, which is supported by density functional theory calculations. In addition, 1T' twin boundaries and 1H-1T' heterojunctions were observed and found to exhibit enhanced tunnelling conductivity. The substrate pretreatment approach for phase-controlled epitaxy could be applicable to other group-VI TMDCs grown on Au (111).

Janus monolayer of WSeTe, a new structural phase transition material driven by electrostatic gating

Phase transition materials are widely exploited in sensors, switches, and information storage devices. However, the dynamic control of structural phase transitions in low-dimensional materials is rarely reported, except for the recent demonstration of semiconductor-semimetal transition in monolayer MoTe2 modulated by electrostatic gating. Here, based on density functional theory calculations we screen in the Janus family of transition metal dichalcogenides, MXY where M = Mo or W, X/Y = S, Se, or Te, for new two-dimensional phase transition materials. We find that the Janus monolayer of WSeTe undergoes reversible phase transitions modulated by electrostatic gating, owing to the small energy difference between H and T' phases, ET' - EH = 48 meV. The gate voltage of 2.0 V (with high dielectric gating the injected charge is ∼1013 cm-2) is required to trigger the semiconductor-semimetal transition in WSeTe. The kinetic barrier for both forward and backward phase transitions is ∼0.66 eV, which is significantly lower than that in MoTe2, leading to three orders of magnitude increase in the transition rate and much more rapid response of devices.

Direct Observation of Semiconductor-Metal Phase Transition in Bilayer Tungsten Diselenide Induced by Potassium Surface Functionalization

Structures determine properties of materials, and controllable phase transitions are, therefore, highly desirable for exploring exotic physics and fabricating devices. We report a direct observation of a controllable semiconductor-metal phase transition in bilayer tungsten diselenide (WSe₂) with potassium (K) surface functionalization. Through the integration of in situ field-effect transistors, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy measurements, and first-principles calculations, we identify that the electron doping from K adatoms drives bilayer WSe₂ from a 2H phase semiconductor to a 1T' phase metal. The phase transition mechanism is satisfactorily explained by the electronic structures and energy alignment of the 2H and 1T' phases. This explanation can be generally applied to understand doping-induced phase transitions in two-dimensional (2D) structures. Finally, the associated dramatic changes of electronic structures and electrical conductance make this controllable semiconductor-metal phase transition of interest for 2D semiconductor-based electronic and optoelectronic devices.

Fabry-Perot Cavity-Enhanced Optical Absorption in Ultrasensitive Tunable Photodiodes Based on Hybrid 2D Materials

Monolayer two-dimensional (2D) transition metal dichalcogenides (TMDs) show interesting optical and electrical properties because of their direct bandgap. However, the low absorption of atomically thin TMDs limits their applications. Here, we report enhanced absorption and optoelectronic properties of monolayer molybdenum disulfide (MoS₂) by using an asymmetric Fabry-Perot cavity. The cavity is based on a hybrid structure of MoS₂/ hexagonal boron nitride (BN)/Au/SiO₂ realized through layer-by-layer vertical stacking. Photoluminescence (PL) intensity of monolayer MoS₂ is enhanced over 2 orders of magnitude. Theoretical calculations show that the strong absorption of MoS₂ comes from photonic localization on the top of the microcavity at optimal BN spacer thickness. The n/n⁺ MoS₂ homojunction photodiode incorporating this asymmetric Fabry-Perot cavity exhibits excellent current rectifying behavior with an ideality factor of 1 and an ultrasensitive and gate-tunable external photo gain and specific detectivity. Our work offers an effective method to achieve uniform enhanced light absorption by monolayer TMDs, which has promising applications for highly sensitive optoelectronic devices.