The spin-entangled Zhang-Rice (ZR) exciton observed in the van der Waals magnet NiPS3 has garnered significant interest due to its strong correlation with magnetic ordering and long coherence. Herein, we present a temperature- and polarization-dependent photoluminescence (PL) study of anion-substituted NiPS3-xSex (x = 0.008, 0.03, 0.06, and 0.09) to explore the nature and dynamics of the ZR exciton. Our results reveal that even a small percentage of anion substitution effectively destroys and modulates the ZR exciton, as evidenced by the emergence of a weaker, lower-energy PL peak in addition to the primary ZR peak. Both peaks share the same anisotropic polarization but differ in their peak energy shift and intensity evolution with Se substitution, suggesting varying charge transfers of p-orbitals. Notably, the ZR exciton undergoes thermal destabilization at much lower temperatures than two-magnon excitations, highlighting that p-orbital inhomogeneity beyond the magnetic ordering structure is a decisive factor in driving its thermal quenching.

Herein, direct evidence of intrinsic spin-layer coupling in bilayer MoS2 through polarization-resolved photoluminescence measurements on fully suspended samples is presented. By eliminating substrate-induced symmetry breaking, such as electrostatic potential gradients and unintentional strain, the intrinsic valley dynamics of bilayer MoS2 is isolated. The results reveal that the degree of circular polarization (DoCP) in bilayers remains significantly higher than that of monolayers across the entire temperature range. This observation cannot be explained by conventional thermal effects and instead indicates the presence of robust depolarization suppression mechanisms. In bilayer MoS2, spin-layer coupling not only inhibits interlayer scattering by locking spin and valley indices to individual layers but also suppression spin-flip-mediated intervalley scattering by constraining the spin dynamics. Together, these effects maintain a high DoCP even under conditions that would normally induce strong depolarization via rapid intervalley and interlayer scattering pathways. The observation of enhanced DoCP in bilayers, despite larger excess energy typically enhances depolarization through intervalley scattering, indicates that the polarization is intrinsically linked to spin-layer coupling. These findings establish a bilayer MoS2 as a compelling platform for exploring spin-valley-layer physics and advancing valleytronic applications.

Atomically thin two-dimensional (2D) transition-metal dichalcogenide (TMDC) films have emerged as promising semiconducting materials for use in thermoelectric (TE) applications. However, the utilization of such materials remains challenging owing to the relatively high intrinsic resistance as the size of the TMDC thin films increases to the centimeter scale. These 2D TMDC films can also form vertically stacked homo- or heterostructures at large interfaces with other 2D TMDC films, resulting in unique TE properties at room temperature. This article reports on the in-plane TE properties when the interfaces formed within a PtSe2/PtSe2 (3 nm/3 nm) homostructure are modulated as a function of O2 plasma treatment time. The results show enhanced Seebeck coefficients compared with that of the single-layer PtSe2 with the same thickness. The independent enhancement in the Seebeck coefficient while keeping the electrical conductivity leads to a substantial increase in the power factor. Such extra Seebeck voltage in 2D PtSe2/PtSe2 homostructures is mainly as a result of momentum exchange by charge carriers caused by the temperature gradient in the vertical direction, which occurs in-plane Seebeck coefficient measurements, at the interface between the PtSe2 layers in the in-plane temperature gradient along the samples. These results resemble the characteristics of the phonon drag effect at low temperatures, which can independently increase the Seebeck coefficient at room temperature.

The intricate interplay between spin and lattice degrees of freedom in two-dimensional magnetic materials plays a pivotal role in modifying their magnetic characteristics, engendering hybrid quasiparticles, and implementing functional devices. Herein, we present our comprehensive and in-depth investigations on magnetic and lattice excitations of MnPSe3-xSx 3 - x S x (x x = 0, 0.63, and 1.68) alloys, utilizing temperature- and polarization- dependent Raman scattering. Our experimental results reveal the occurrence of multiple phase transitions, evidenced by notable changes in phonon self-energy and the appearance or splitting of phonon modes. These emergent phases are tied to the development of long and short-range spin-spin correlations, as well as to spin reorientations or magnetic instabilities. Our analysis of two-magnon excitations as a function of temperature and composition showcases their hybridization with phonons whose degree weakens with increasing x . Moreover, the suppression of spin-dependent phonon intensity in chemically most-disordered MnPSe3-xSx 3 - x S x (x x = 1.68) . 68) suggests that chalcogen substitution offers a control knob of tuning spin and phonon dynamics by modulating concurrently superexchange pathways and a degree of trigonal distortions.

The intricate interplay between spin and lattice degrees of freedom in two-dimensional magnetic materials plays a pivotal role in modifying their magnetic characteristics, engendering hybrid quasiparticles, and implementing functional devices. Herein, we present our comprehensive and in-depth investigations on magnetic and lattice excitations of MnPSe3-xSx (x = 0, 0.5, and 1.5) alloys, utilizing temperature- and polarization-dependent Raman scattering. Our experimental results reveal the occurrence of multiple phase transitions, evidenced by notable changes in phonon self-energy and the appearance or splitting of phonon modes. These emergent phases are tied to the development of long and short-range spin-spin correlations, as well as to spin reorientations or magnetic instabilities. Our analysis of two-magnon excitations as a function of temperature and composition showcases their hybridization with phonons whose degree weakens with increasing x. Moreover, the suppression of spin-dependent phonon intensity in chemically most-disordered MnPSe3-xSx (x = 1.5) suggests that chalcogen substitution offers a control knob of tuning spin and phonon dynamics by modulating concurrently superexchange pathways and a degree of trigonal distortions.

Molybdenum disulfide (MoS2) has attracted great attention because of its unique physical properties and wide potential applications in electronic, optoelectronic devices. Chemical vapor deposition (CVD) turned out to be a powerful tool for producing MoS2 thin layers. Nonetheless, how the concentration of vapor-phase precursor influences the size and shape change of MoS2 is not clearly understood yet. In this work, we designed a gradient of sulfur (S) concentration during CVD growth of MoS2 to investigate how S concentration affects its size and shape change. The shape of MoS2 film changes from triangle to hexagon, then back to triangle as S concentration decreases. We also analyzed the uniformity and crystal quality of the CVD-grown MoS2 by using optical microscopy, Raman, photoluminescence (PL), circularly polarized PL, and atomic force microscopy.

We investigate the voltage control of magnetism in a van der Waals (vdW) heterostructure device consisting of two distinct vdW materials, the ferromagnetic Fe3-xGeTe2 and the ferroelectric In2Se3. It is observed that gate voltages applied to the Fe3-xGeTe2/In2Se3 heterostructure device modulate the magnetic properties of Fe3-xGeTe2 with significant decrease in coercive field for both positive and negative voltages. Raman spectroscopy on the heterostructure device shows voltage-dependent increase in the in-plane In2Se3 and Fe3-xGeTe2 lattice constants for both voltage polarities. Thus, the voltage-dependent decrease in the Fe3-xGeTe2 coercive field, regardless of the gate voltage polarity, can be attributed to the presence of in-plane tensile strain. This is supported by density functional theory calculations showing tensile-strain-induced reduction of the magnetocrystalline anisotropy, which in turn decreases the coercive field. Our results demonstrate an effective method to realize low-power voltage-controlled vdW spintronic devices utilizing the magnetoelectric effect in vdW ferromagnetic/ferroelectric heterostructures.

We investigate the voltage control of magnetism in a van der Waals (vdW) heterostructure device consisting of two distinct vdW materials, the ferromagnetic Fe 3- x GeTe 2 and the ferroelectric In 2 Se 3 . It is observed that gate voltages applied to the Fe 3- x GeTe 2 /In 2 Se 3 heterostructure device modulate the magnetic properties of Fe 3- x GeTe 2 with significant decrease in coercive field for both positive and negative voltages. Raman spectroscopy on the heterostructure device shows voltage-dependent increase in the in-plane In 2 Se 3 and Fe 3- x GeTe 2 lattice constants for both voltage polarities. Thus, the voltage-dependent decrease in the Fe 3- x GeTe 2 coercive field, regardless of the gate voltage polarity, can be attributed to the presence of in-plane tensile strain. This is supported by density functional theory calculations showing tensile-strain-induced reduction of the magnetocrystalline anisotropy, which in turn decreases the coercive field. Our results demonstrate an effective method to realize low-power voltage-controlled vdW spintronic devices utilizing the magnetoelectric effect in vdW ferromagnetic/ferroelectric heterostructures.

The Seebeck effect refers to the production of an electric voltage when different temperatures are applied on a conductor, and the corresponding voltage-production efficiency is represented by the Seebeck coefficient. We report a Seebeck effect: thermal generation of driving voltage from the heat flowing in a thin PtSe2/PtSe2 van der Waals homostructure at the interface. We refer to the effect as the interface-induced Seebeck effect. By exploiting this effect by directly attaching multilayered PtSe2 over high-resistance PtSe2 thin films as a hybridized single structure, we obtained the highly challenging in-plane Seebeck coefficient of the PtSe2 films that exhibit extremely high resistances. This direct attachment further enhanced the in-plane thermal Seebeck coefficients of the PtSe2/PtSe2 van der Waals homostructure on sapphire substrates. Consequently, we successfully enhanced the in-plane Seebeck coefficients for the PtSe2 (10 nm)/PtSe2 (2 nm) homostructure approximately 42% compared to that of a pure PtSe2 (10 nm) layer at 300 K. These findings represent a significant achievement in understanding the interface-induced Seebeck effect and provide an effective strategy for promising large-area thermoelectric energy harvesting devices using two-dimensional transition metal dichalcogenide materials, which are ideal thermoelectric platforms with high figures of merit.

The mechanical stability and failures of two-dimensional (2D) materials on the compliant polymer substrates under biaxial strain are investigated. As the polymer substrate swells, the fracture of the system is initiated with the crack-formation in 2D materials (graphene and MoS2 flakes), which propagates deep into the substrates. According to the fracture theory, the generation of deep cracks in such a system (thin hard material on a compliant substrate) is expected to be a universal behavior due to a large mismatch in the elastic moduli between the film and the substrate. The properties of crack formation in systems with varying crystallinity and size of 2D materials are also investigated. The present results provide important insights into what to consider for mechanical stability in designing flexible devices based on 2D materials.

•Extremely high quality In2S3 thin films with preferred growth direction of [103].•In2S3 thin film exhibited remarkably strong photoluminescence (PL) at room temp.•PL efficiency of In2S3 was comparable to that of quantum wells and quantum dots.•An important contribution to eco-friendly solar cell applications.

In2S3 is one of the fascinating materials for a range of optoelectronic applications due to its suitable bandgap and stability. Nonetheless, its photoluminescence (PL) originating from defect states within its bandgap were hardly reported. In this work, high quality In2S3 thin films on SiO2 substrates were synthesized by using physical vapor deposition of In2S3 powder in a hot-wall Quartz tube-furnace. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy studies of the grown In2S3 films showed that extremely high crystalline quality tetragonal beta-In2S3 films were synthesized with preferred growth direction of [103]. Absorption spectroscopy revealed the In2S3 films have a direct band of similar to 2.66 eV. Interestingly, our In2S3 films showed an extremely strong PL peak at room temperature at similar to 1.6 eV. The observed strong PL from our In2S3 films can be attributed to originating from the oxygen isoelectronic substitution at the sulfur site, and its efficiency was affected by oxygen concentrations in In2S3 films. Our In2S3 thin films exhibited remarkably high PL quantum yields (QY) of up to 14% which is comparable to that of GaAs quantum wells (QWs) and CdSe quantum dots (QDs).

Band-like transport behavior of H-doped transition metal dichalcogenide (TMD) channels in field effect transistors (FET) is studied by conducting low-temperature electrical measurements, where MoTe2 , WSe2 , and MoS2 are chosen for channels. Doped with H atoms through atomic layer deposition, those channels show strong n-type conduction and their mobility increases without losing on-state current as the measurement temperature decreases. In contrast, the mobility of unintentionally (naturally) doped TMD FETs always drops at low temperatures whether they are p- or n-type. Density functional theory calculations show that H-doped MoTe2 , WSe2 , and MoS2 have Fermi levels above conduction band edge. It is thus concluded that the charge transport behavior in H-doped TMD channels is metallic showing band-like transport rather than thermal hopping. These results indicate that H-doped TMD FETs are practically useful even at low-temperature ranges.

A facile synthesis method for the heterostructures of single‐walled carbon nanotubes (SWCNTs) and few‐layer MoS 2 is reported. The heterostructures are realized by in situ chemical vapor deposition of MoS 2 on individual SWCNTs. Field effect transistors based on the heterostructures display different transfer characteristics depending on the formation of MoS 2 conduction channels along SWCNTs. Under light illumination, negative photoresponse originating from charge transfer from MoS 2 to SWCNT is observed while positive photoresponse is observed in MoS 2 conduction channels, leading to ambipolar photoresponse in devices with both SWCNT and MoS 2 channels. The heterostructure phototransistor, for negative photoresponse, exhibits high responsivity (100–1000 AW −1 ) at low bias voltages (0.1 V) in the visible spectrum (500–700 nm) by combining high mobility conduction channel (SWCNT) with efficient light absorber (MoS 2 ).

We have investigated strong optical nonlinearity of monolayer MoS2(1-x)Se2x, across the exciton resonance, which is directly tunable by Se doping. The quality of monolayer alloys prepared by chemical vapor deposition is verified by atomic force microscopy, Raman spectroscopy, and photoluminescence analysis. The crystal symmetry of all of our alloys is essentially D3h, as confirmed by polarization-dependent second-harmonic generation (SHG). The spectral structure of the exciton resonance is sampled by wavelength dependent SHG (lambda = 1000-1800 nm), where the SHG resonance red shifts in accordance with the corresponding optical gap. Surprisingly, the effect of compositional variation turns out to be much more dramatic owing to the unexpected increase of B-exciton-induced SHG, which indeed dominates over the A-exciton resonance for x >= 0.3. The overall effect is therefore stronger and broader SHG resonance where the latter arises from different degrees of red-shift for the two exciton states. We report the corresponding absolute SHG dispersion of monolayer alloys, chi((2)), as a function of Se doping. We believe that our finding is a critical step toward engineering highly efficient nonlinear optical van der Waals materials working in a broader performance range.

The covalently bonded in-plane heterostructure (HS) of monolayer transition-metal dichalcogenides (TMDCs) possesses huge potential for high-speed electronic devices in terms of valleytronics. In this study, high-quality monolayer MoSe2WSe2 lateral HSs are grown by pulsed-laser-deposition-assisted selenization method. The sharp interface of the lateral HS is verified by morphological and optical characterizations. Intriguingly, photoluminescence spectra acquired from the interface show rather clear signatures of pristine MoSe2 and WSe2 with no intermediate energy peak related to intralayer excitonic matter or formation of MoxW(1-x)Se2 alloys, thereby confirming the sharp interface. Furthermore, the discrete nature of laterally attached TMDC monolayers, each with doubly degenerated but nonequivalent energy valleys marked by (K-M, K'(M)) for MoSe2, and (K-w, K'(w)) for WSe2 in k space, allows simultaneous control of the four valleys within the excitation area without any crosstalk effect over the interface. As an example, K-M and K-w valleys or K'(M) and K'(w) valleys are simultaneously polarized by controlling the helicity of circularly polarized optical pumping, where the maximum degree of polarization is achieved at their respective band edges. The current work provides the growth mechanism of laterally sharp HSs and highlights their potential use in valleytronics.

The interest in layered materials is largely based on the expectation that they will be beneficial for a variety of applications, from low-power-consuming, wearable electronics to energy harvesting. However, the properties of layered materials are highly dependent on thickness, and the difficulty of controlling thickness over a large area has been a bottleneck for commercial applications. Here, we report layer-by-layer growth of SnSe2, a layered semiconducting material, via van der Waals epitaxy. The films were fabricated on insulating mica substrates with substrate temperatures in the range of 210 °C–370 °C. The surface consists of a mixture of N and (N ± 1) layers, showing that the thickness of the film can be defined with monolayer accuracy (±0.6 nm). High-resolution transmission electron microscopy reveals a polycrystalline film with a grain size of ∼100 nm and clear Moiré patterns from overlapped grains with similar thickness. We also report field effect mobility values of 3.7 cm2 V−1 s−1 and 6.7 cm2 V−1 s−1 for 11 and 22 nm thick SnSe2, respectively. SnSe2 films with customizable thickness can provide valuable platforms for industry and academic researchers to fully exploit the potential of layered materials.

Control of living cells on biocompatible materials or on modified substrates is important for the development of bio-applications, including biosensors and implant biomaterials. The topography and hydrophobicity of substrates highly affect cell adhesion, growth, and cell growth kinetics, which is of great importance in bio-applications. Herein, we investigate the adhesion, growth, and morphology of cultured breast cancer cells on a silicon substrate, on which graphene oxides (GO) was partially formed. By minimizing the size and amount of the GO-containing solution and the further annealing process, GO-coated Si samples were prepared which partially covered the Si substrates. The coverage of GO on Si samples decreases upon annealing. The behaviors of cells cultured on two samples have been observed, i.e. partially GO-coated Si (P-GO) and annealed partially GO-coated Si (Annealed p-GO), with a different coverage of GO. Indeed, the spreading area covered by the cells and the number of cells for a given culture period in the incubator were highly dependent on the hydrophobicity and the presence of oxygenated groups on GO and Si substrates, suggesting hydrophobicity-driven cell growth. Thus, the presented method can be used to control the cell growth via an appropriate surface modification.

In this study, we utilized picosecond pulses from an Nd:YAG laser to investigate the nonlinear optical characteristics of monolayer MoSe 2 . Two‐step growth involving the selenization of pulsed‐laser‐deposited MoO 3 film was employed to yield the MoSe 2 monolayer on a SiO 2 /Si substrate. Raman scattering, photoluminescence (PL) spectroscopy, and atomic force microscopy verified the high optical quality of the monolayer. The second‐order susceptibility χ (2) was calculated to be ∼50 pm V −1 at the second harmonic wavelength ∼810 nm, which is near the optical gap of the monolayer. Interestingly, our wavelength‐dependent second harmonic scan can identify the bound excitonic states including negatively charged excitons much more efficiently, compared with the PL method at room temperature. Additionally, the MoSe 2 monolayer exhibits a strong laser‐induced damage threshold ∼16 GW cm −2 under picosecond‐pulse excitation . Our findings suggest that monolayer MoSe 2 can be considered as a promising candidate for high‐power, thin‐film‐based nonlinear optical devices and applications.

CdS nanowires were synthesized using a simple synthesis process using Au nanoparticles (average diameter: 13 nm) as catalyst. The growth conditions were optimized by varying the substrate temperature and the growth time of a thermal chemical-vapor-deposition system. CdS nanowires were successfully grown at temperatures above 480 degrees C. In fact, high-quality nanowires, with hexagonal wurtzite structures and an average diameter of 25 nm and length of 1.46 mu m, were obtained after a 30-min synthesis at a growth temperature of 520 degrees C. Electron microscope images revealed that our CdS nanowires, grown at relatively lower growth temperatures, have higher average-aspect-ratio and smaller average-diameter than those previously reported in the literature. Our synthesis method resulted in CdS nanowires only without producing nanorods and nanobelts, which make it unnecessary to filter and purify nanowires from the mixture of nanowires, nanorods, and nanobelts. (C) 2015 Elsevier B.V. All rights reserved.