Strong light-matter interactions involved with photons and quasiparticles are fundamentally interesting to access the wealthy many-body physics in quantum mechanics. The emerging two-dimensional (2D) semiconductors with large exciton binding energies and strong quantum confinement allow to investigate exciton-photon coupling at elevated temperatures. Here we report room- temperature formation of Bragg polaritons in monolayer semiconductor on a dielectric mirror through the exciton-Bragg photon coupling. With the negative detuning energy of ~ 30 meV, angle-resolved reflection signals reveal anti-crossing behaviors of lower and upper polariton branches at ±18° together with the Rabi splitting of 10 meV. Meanwhile, the strengthened photoluminescence appears in the lower polariton branch right below the anti-crossing angles, indicating the presence of the characteristic bottleneck effect caused by the slowing exciton-polariton energy relaxation towards the band minimum. The extracted coupling strength is between the ones of weak and distinct strong coupling regimes, where the eigenenergy splitting induced by the moderate coupling is resolvable but not large enough to fully separate two polaritonic components. Our work develops a simplified strategy to generate exciton-polaritons in 2D semiconductors and can be further extended to probe the intriguing bosonic characteristics of these quasiparticles, such as Bose-Einstein condensation, polariton lasing and superfluidity, directly at the material surfaces.

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.

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.

A hybrid structure consisting of boron-doped porous carbon spheres and graphene (BPCS-G) has been designed and synthesized toward solving the polysulfide-shuttle problem, which is the most critical issue of current Li-S batteries. The proposed hybrid structure showing high surface area (870 m²·g^-1) and high B-dopant content (6.51 wt.%) simultaneously offers both physical confinement and chemical absorption of polysulfides. On one hand, the abundant-pore structure ensures high sulfur loading, facilitates fast charge transfer, and restrains polysulfide migration during cycling. On the other hand, our density functional theory calculations demonstrate that the B dopant is capable of chemically binding polysulfides. As a consequence of such dual polysulfide confinement, the BPCS-G/S cathode prepared with 70 wt.% sulfur loading presents a high reversible capacity of 1, 174 mAh·g^-1 at 0.02 C, a high rate capacity of 396 mAh·g^-1 at 5 C, and good cyclability over 500 cycles with only 0.05% capacity decay per cycle. The present work provides an efficient and cost-effective platform for the scalable synthesis of high-performance carbon-based materials for advanced Li-S batteries.

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.

Black phosphorus (BP) is a good candidate for studying strain effects on twodimensional (2D) materials beyond graphene and transition-metal dichalcogenides. This is because of its particular ability to sustain high strain and remarkably anisotropic mechanical properties resulting from its unique puckered structure. We here investigate the dependence of lattice vibrational frequencies on crystallographic orientations in uniaxially strained few-layer BP by in-situ strained Raman spectroscopy. The out-of-plane A_g¹ mode is sensitive to uniaxial strain along the near-armchair direction whereas the in-plane B_2g and A_g² modes are sensitive to strain in the near-zigzag direction. For uniaxial strains applied away from these directions, all three phonon modes are linearly redshifted. Our experimental observation is explained by the anisotropic influence of uniaxial tensile strain on structural properties of BP using density functional theory. This study demonstrates the possibility of selective tuning of in-plane and out-of-plane phonon modes in BP by uniaxial strain and makes strain engineering a promising avenue for extensively modulating the optical and mechanical properties of 2D materials.

In situ strain photoluminescence (PL) and Raman spectroscopy have been employed to exploit the evolutions of the electronic band structure and lattice vibrational responses of chemical vapor deposition (CVD)-grown monolayer tungsten disulphide (WS₂) under uniaxial tensile strain. Observable broadening and appearance of an extra small feature at the longer-wavelength side shoulder of the PL peak occur under 2.5% strain, which could indicate the direct-indirect bandgap transition and is further confirmed by our density-functional-theory calculations. As the strain increases further, the spectral weight of the indirect transition gradually increases. Over the entire strain range, with the increase of the strain, the light emissions corresponding to each optical transition, such as the direct bandgap transition (K-K) and indirect bandgap transition (Γ-K, ≥2.5%), exhibit a monotonous linear redshift. In addition, the binding energy of the indirect transition is found to be larger than that of the direct transition, and the slight lowering of the trion dissociation energy with increasing strain is observed. The strain was used to modulate not only the electronic band structure but also the lattice vibrations. The softening and splitting of the in-plane E' mode is observed under uniaxial tensile strain, and polarization-dependent Raman spectroscopy confirms the observed zigzag-oriented edge of WS₂ grown by CVD in previous studies. These findings enrich our understanding of the strained states of monolayer transition-metal dichalcogenide (TMD) materials and lay a foundation for developing applications exploiting their strain-dependent optical properties, including the strain detection and light-emission modulation of such emerging two-dimensional TMDs.

We report the thermal conductivities of monolayer (1L) and bilayer (2L) WS₂ grown by chemical vapor deposition (CVD), which are determined by use of temperature and excitation dependences of E_2g¹ and A_1g Raman modes. The first-order temperature coefficients of E_2g¹ and A_1g modes in both supported and suspended WS₂ layers were extracted. The frequency shift of the A_1g mode with temperature is larger than that of the E_2g¹ mode for 1L-WS₂, which is attributed to stronger electron-phonon coupling for the A_1g mode than that for the E_2g¹ mode. Moreover, by use of the shift of the phonon mode induced by laser heating, the thermal conductivities at room temperature were estimated to be 32 and 53 W/(m·K) for 1L- and 2L-WS₂, respectively. Our results provide fundamental information about the thermal properties of WS₂ layers, which is crucial for developing applications of atomically-thin WS₂ devices.