2H-tantalum disulfide (2H-TaS₂) is a layered metallic transition metal dichalcogenide (TMD) that has recently been studied from the perspective of new physics phenomena, including simultaneous lattice distortion and charge density modulation known as the charge density wave (CDW) phase. Here we explored the collapse of CDW states in few-layer 2H-TaS₂ induced by molecular interactions using Raman spectroscopy. Our results indicate that the CDW states disappear in few-layer 2H-TaS₂ with rhodamine 6G (R6G) adsorbed due to the charge transfer, which is reflected by the change of behaviors of lattice vibrational modes in 2H-TaS₂. We observed the 2-phonon mode that signifies the CDW formation in 2H-TaS₂, and becomes a phonon-hardened mode when R6G molecules are absorbed on its surface. R6G adsorption further induces the breakdown of the Raman polarization selection rule in 2H-TaS₂, which results in the alteration of the A_1g phonon mode polarization state of 2H-TaS₂. This study can shed light not only on the underlying mechanisms of CDW states but also on controlling the CDW states under a variety of environmental conditions.

Inserting hexagonal boron nitride (hBN) as barrier layers into bilayer transition metal dichalcogenides heterointerface has been proved an efficient method to improve two dimensional tunneling optoelectronic device performance. Nevertheless, the physical picture of interlayer coupling effect during incorporation of monolayer (1L-) hBN is not explicit yet. In this article, spectroscopic ellipsometry was used to experimentally obtain the broadband excitonic and critical point properties of WS₂/MoS₂ and WS₂/hBN/MoS₂ van der Waals heterostructures. We find that 1L-hBN can only slightly block the interlayer electron transfer from WS₂ layer to MoS₂ layer. Moreover, insertion of 1L-hBN weakens the interlayer coupling effect by releasing quantum confinement and reducing efficient dielectric screening. Consequently, the exciton binding energies in WS₂/hBN/MoS₂ heterostructures blueshift comparing to those in WS₂/MoS₂ heterostructures. In this exciton binding energies tuning process, the reducing dielectric screening effect plays a leading role. In the meantime, the quasi-particle (QP) bandgap remains unchanged before and after 1L-hBN insertion, which is attributed to released quantum confinement and decreased dielectric screening effects canceling each other. Unchanged QP bandgap as along with blueshift exciton binding energies lead to the redshift exciton transition energies in WS₂/hBN/MoS₂ heterostructures.

Substrates provide the necessary support for scientific explorations of numerous promising features and exciting potential applications in two-dimensional (2D) transition metal dichalcogenides (TMDs). To utilize substrate engineering to alter the properties of 2D TMDs and avoid introducing unwanted adverse effects, various experimental techniques, such as high-frequency Raman spectroscopy, have been used to understand the interactions between 2D TMDs and substrates. However, sample–substrate interaction in 2D TMDs is not yet fully understood due to the lack of systematic studies by techniques that are sensitive to 2D TMD–substrate interaction. This work systematically investigates the interaction between tungsten disulfide (WS₂) monolayers and substrates by low-frequency Raman spectroscopy, which is very sensitive to WS₂–substrate interaction. Strong coupling with substrates is clearly revealed in chemical vapor deposition (CVD)-grown monolayer WS₂ by its low-wavenumber interface mode. It is demonstrated that the enhanced sample–substrate interaction leads to tensile strain on monolayer WS₂, which is induced during the cooling process of CVD growth and could be released for monolayer WS₂ sample after transfer or fabricated by an annealing-free method such as mechanical exfoliation. These results not only suggest the effectiveness of low-frequency Raman spectroscopy for probing sample–substrate interactions in 2D TMDs, but also provide guidance for the design of high-performance devices with the desired sample–substrate coupling strength based on 2D TMDs.

Twisted van der Waals homo- and hetero-structures have aroused great attentions due to their unique physical properties, providing a new platform to explore the novel two-dimensional (2D) condensed matter physics. The robust dependence of phonon vibrations and electronic band structures on the twist angle has been intensively observed in transition metal dichalcogenide (TMD) homo-structures. However, the effects of twist angle on the lattice vibrational properties in the TMD heterostructures have not caused enough attention. Here, we report the distinct evolutions of Raman scattering and the underlying interlayer interactions in the twisted WS₂/MoS₂ heterostructures. The shifts and linewidths of E_2g(Γ) and A_1g(Γ) phonon modes are found to be twist angle dependent. In particular, analogous to that of the twisted TMD homostructures, the frequency separations between E_2g(Γ) and A_1g(Γ) modes of MoS₂ and WS₂ in the twisted heterostructures varying with twist angle correlate with the interlayer mechanical coupling, essentially originating from the spacing-related repulsion between sulfur atoms. Moreover, the opposite shift behaviors and broadening of A_1g(Γ) modes caused by charge transfer are also observed in the twisted heterostructures. The calculated interlayer distances and band alignment of twisted WS₂/MoS₂ through density functional theory further evidence our interpretations on the roles of the interlayer mechanical coupling and charge transfer in variations of Raman features. Such understanding and controlling of interlayer interaction through the stacking orientation are significant for future optoelectronic device design based on the newly emerged 2D heterostructures.

Chemical vapor deposition (CVD) is the most efficient method to grow large-area two dimensional (2D) transition metal dichiacogenides (TMDCs) in high quality. Monolayer molybdenum disulfide (MoS₂) and seed-assistant are the mostly selected 2D TMDC and growth strategy for such CVD processes, respectively. Though the advantages of seed catalysts in facilitating the nucleation, achieving higher yield and better repeatability, as well as their effects on the morphologies of as-grown MoS₂ have been studied, the influence of seeding promoters on both optical and electrical properties of as-grown monolayer MoS₂ is not known comprehensively, which is indeed critical for understanding fundamental physics and developing practical application of such emerging 2D semiconductors. In this report, we systematically investigated the effect of different seeding promoters on the properties of CVD-grown monolayer MoS₂. It is found that different seed molecules lead to different impacts on the optical and electrical properties of as-grown monolayer MoS₂. Among three different seed catalysts (perylene-3, 4, 9, 10-tetracarboxylic acid tetrapotassium salt (PTAS), copper phthalocyanine (CuPc), and crystal violet (CV)), PTAS performs better in obtaining large area monolayer MoS₂ with good optical quality and high electrical mobility than the other two. Our work gives a guide for modifying the properties of as-grown monolayer MoS₂ and other 2D transition metal dichalcogenides in seeding promoters-assisted synthesis process.

Possessing a valley degree of freedom and potential in information processing by manipulating valley features (such as valley splitting), group-VI monolayer transition metal dichalcogenides have attracted enormous interest. This valley splitting can be measured based on the difference between the peak energies of σ⁺ and σ^- polarized emissions for excitons or trions in direct band gap monolayer transition metal dichalcogenides under perpendicular magnetic fields. In this work, a well-prepared heterostructure is formed by transferring exfoliated WSe₂ onto a EuS substrate. Circular-polarization-resolved photoluminescence spectroscopy, one of the most facile and intuitive methods, is used to probe the difference of the gap energy in two valleys under an applied out-of-plane external magnetic field. Our results indicate that valley splitting can be enhanced when using a EuS substrate, as compared to a SiO₂/Si substrate. The enhanced valley splitting of the WSe₂/EuS heterostructure can be understood as a result of an interfacial magnetic exchange field originating from the magnetic proximity effect. The value of this magnetic exchange field, based on our estimation, is approximately 9 T. Our findings will stimulate further studies on the magnetic exchange field at the interface of similar heterostructures.

Two-dimensional (2D) semiconductors, represented by 2D transition metal dichalcogenides (TMDs), exhibit rich valley physics due to strong spin-orbit/spin-valley coupling. The most common way to probe such 2D systems is to utilize optical methods, which can monitor light emissions from various excitonic states and further help in understanding the physics behind such phenomena. Therefore, 2D TMDs with good optical quality are in great demand. Here, we report a method to directly grow epitaxial WS₂ and MoS₂ monolayers on hexagonal boron nitride (hBN) flakes with a high yield and high optical quality; these monolayers show better intrinsic light emission features than exfoliated monolayers from natural crystals. For the first time, the valley Zeeman splitting of WS₂ and MoS₂ monolayers on hBN has been visualized and systematically investigated. This study paves a new way to produce high optical quality WS₂ and MoS₂ monolayers and to exploit their intrinsic properties in a multitude of applications.

Two-dimensional transition metal dichalcogenides (2D TMDs) possess a tunable excitonic light emission that is sensitive to external conditions such as electric field, strain, and chemical doping. In this work, we reveal the interactions between DNA nucleobases, i.e., adenine (A), guanine (G), cytosine (C), and thymine (T) and monolayer WS₂ by investigating the changes in the photoluminescence (PL) emissions of the monolayer WS₂ after coating with nucleobase solutions. We found that adenine and guanine exert a clear effect on the PL profile of the monolayer WS₂ and cause different PL evolution trends. In contrast, cytosine and thymine have little effect on the PL behavior. To obtain information on the interactions between the DNA bases and WS₂, a series of measurements were conducted on adenine-coated WS₂ monolayers, as a demonstration. The p-type doping of the WS₂ monolayers on the introduction of adenine is clearly shown by both the evolution of the PL spectra and the electrical transport response. Our findings open the door for the development of label-free optical sensing approaches in which the detection signals arise from the tunable excitonic emission of the TMD itself rather than the fluorescence signals of label molecules. This dopant-selective optical response to the DNA nucleobases fills the gaps in previously reported optical biosensing methods and indicates a potential new strategy for DNA sequencing.