Modulation of the Electronic Properties of Ultrathin Black Phosphorus by Strain and Electrical Field

The structural and electronic properties of the bulk and ultrathin black phosphorus and the effects of in-plane strain and out-of-plane electrical field on the electronic structure of phosphorene are investigated using first-principles methods. The computed results show that the bulk and few-layer black phosphorus from monolayer to six-layer demonstrates inherent direct bandgap features ranging from 0.5 to 1.6 eV. Interestingly, the band structures of the bulk and few-layer black phosphorus from X point via A point to Y point present degenerate distribution, which shows totally different partial charge dispersions. Moreover, strong anisotropy in regard to carrier effective mass has been observed along different directions. The response of phosphorene to in-plane strain is diverse. The bandgap monotonically decreases with increasing compressive strain, and semiconductor-to-metal transition occurs for phosphorene when the biaxial compressive reaches −9%. Tensile strain first enlarges the gap until the strain reaches around 4%, after which the bandgap exhibits a descending relationship with tensile strain. The bandgaps of the pristine and deformed phosphorene can also be continuously modulated by the electrical field and finally close up at about 15 V/nm. Besides, the electron and hole effective mass along different directions exhibits different responses to the combined impact of strain and electrical field

Vapor Phase Growth and Imaging Stacking Order of Bilayer Molybdenum Disulfide

Various stacking patterns have been predicted in few-layer MoS₂, strongly influencing its electronic properties. Bilayer MoS₂ nanosheets have been synthesized by vapor phase growth. It is found that both A-B and A-A′ stacking configurations are present in bilayer MoS₂ nanosheets through optical images, and the different stacking patterns exhibit distinctive line shapes in the Raman spectra. By theory calculation, it is also concluded that the A-B and A-A′ stacking are the most stable and lowest-energy stacking in the five predicted stacking patterns of bilayer MoS₂ nanosheets, which proves the experimental observations

Polarization-Resolved Near-Infrared PdSe₂ p‑i‑n Homojunction Photodetector

Constructing high-quality homojunctions plays a pivotal role for the advancement of two-dimensional transition metal sulfide (TMDC) based optoelectronic devices. Here, a lateral PdSe2 p-i-n homojunction is constructed by electrostatic doping. Electrical measurements reveal that the homojunction diode exhibits a strong rectifying characteristic with a rectification ratio exceeding 104 and an ideality factor approaching 1. When functioning in photovoltaic mode, the device achieves a high responsivity of 1.1 A/W under 1064 nm illumination, with a specific detectivity of 1.3 × 1011 Jones and a high linearity of 45 dB. Benefiting from the lateral p-i-n structure, the junction capacitance is significantly reduced, and an ultrafast response (3/6 μs) is obtained. Additionally, the photodiode has the capability of polarization distinction due to the unique in-plane anisotropic structure of PdSe2, exhibiting a dichroic ratio of 1.6 at a 1064 nm wavelength. This high-performance polarization-sensitive near-infrared photodetector exhibits great potential in the next-generation optoelectronic applications

Enhanced Electrical and Optoelectronic Characteristics of Few-Layer Type-II SnSe/MoS₂ van der Waals Heterojunctions

van der Waals heterojunctions formed by stacking various two-dimensional (2D) materials have a series of attractive physical properties, thus offering an ideal platform for versatile electronic and optoelectronic applications. Here, we report few-layer SnSe/MoS₂ van der Waals heterojunctions and study their electrical and optoelectronic characteristics. The new heterojunctions present excellent electrical transport characteristics with a distinct rectification effect and a high current on/off ratio (∼1 × 10⁵). Such type-II heterostructures also generate a self-powered photocurrent with a fast response time (<10 ms) and exhibit high photoresponsivity of 100 A W^–1, together with high external quantum efficiency of 23.3 × 10³% under illumination by 532 nm light. Photoswitching characteristics of the heterojunctions can be modulated by bias voltage, light wavelength, and power density. The designed novel type-II van der Waals heterojunctions are formed from a combination of a transition-metal dichalcogenide and a group IV–VI layered 2D material, thereby expanding the library of ultrathin flexible 2D semiconducting devices

Novel Surface Molecular Functionalization Route To Enhance Environmental Stability of Tellurium-Containing 2D Layers

Recent studies have shown that tellurium-based two-dimensional (2D) crystals undergo dramatic structural, physical, and chemical changes under ambient conditions, which adversely impact their much desired properties. Here, we introduce a diazonium molecule functionalization-based surface engineering route that greatly enhances their environmental stability without sacrificing their much desired properties. Spectroscopy and microscopy results show that diazonium groups significantly slow down the surface reactions, and consequently, gallium telluride (GaTe), zirconium telluride (ZrTe₃), and molybdenum ditelluride (MoTe₂) gain strong resistance to surface transformation in air or when immersed under water. Density functional theory calculations show that functionalizing molecules reduce surface reactivity of Te-containing 2D surfaces by chemical binding followed by an electron withdrawal process. While pristine surfaces structurally decompose because of strong reactivity of Te surface atoms, passivated functionalized surfaces retain their structural anisotropy, optical band gap, and emission characteristics as evidenced by our conductive atomic force microscopy, photoluminescence, and absorption spectroscopy measurements. Overall, our findings offer an effective method to increase the stability of these environmentally sensitive materials without impacting much of their physical properties

Unusual Pressure Response of Vibrational Modes in Anisotropic TaS₃

We report on the unique vibrational properties of 2D anisotropic orthorhombic tantalum trisulfide (<i>o</i>-TaS₃) measured through angle-resolved Raman spectroscopy and high-pressure diamond anvil cell studies. Our broad-spectrum Raman measurements identify optical and low-frequency shear modes in pseudo-1D o-TaS₃ for the first time, and introduce their polarization resolved Raman responses to understand atomic vibrations for these modes. Results show that, unlike other anisotropic systems, only the S_∥ mode at 54 cm^–1 can be utilized to identify the crystalline orientation of TaS₃. More notably, high-pressure Raman measurements reveal previously unknown four distinct types of responses to applied pressure, including positive, negative, and nonmonotonic dω/d<i>P</i> behaviors which are found to be closely linked to atomic vibrations for involving these modes. Our results also reveal that the material approaches an isotropic limit under applied pressure, evidenced by a significant reduction in the degree of anisotropy. Overall, these findings significantly advance not only our understanding of their fundamental properties of pseudo-1D materials but also our interpretations of the vibrational characteristics that offer valuable insights about thermal, electrical, and optical properties of pseudo-1D material systems

Synthesis and Transport Properties of Large-Scale Alloy Co_0.16Mo_0.84S₂ Bilayer Nanosheets

Synthesis of large-scale highly crystalline two-dimensional alloys is significant for revealing properties. Here, we have investigated the vapor growth process of high-quality bilayer Co_<i>x</i>Mo_1–<i>x</i>S₂ (<i>x</i> = 0.16) hexagonal nanosheets systematically. As the initial loading of the sulfur increases, the morphology of the Co_<i>x</i>Mo_1–<i>x</i>S₂ (0 < <i>x</i> ≤ 1) nanosheets becomes hexagons from David stars step by step at 680 °C. We find that Co atoms mainly distribute at the edge of nanosheets. When the temperature increases from 680 to 750 °C, high-quality cubic pyrite-type crystal structure CoS₂ grows on the surface of Co_<i>x</i>Mo_1–<i>x</i>S₂ nanosheet gradually and forms hexagonal film induced by the nanosheet. Electrical transport measurements reveal that the Co_<i>x</i>Mo_1–<i>x</i>S₂ nanosheets and CoS₂ films exhibit n-type semiconducting transport behavior and half-metallic behavior, respectively. Theoretical calculations of their band structures agree well with the experimental results

Strain and Interference Synergistically Modulated Optical and Electrical Properties in ReS₂/Graphene Heterojunction Bubbles

Two-dimensional (2D) material bubbles, as a straightforward method to induce strain, represent a potentially powerful platform for the modulation of different properties of 2D materials and the exploration of their strain-related applications. Here, we prepare ReS2/graphene heterojunction bubbles (ReS2/gr heterobubbles) and investigate their strain and interference synergistically modulated optical and electrical properties. We perform Raman and photoluminescence (PL) spectra to verify the continuously varying strain and the microcavity induced optical interference in ReS2/gr heterobubbles. Kelvin probe force microscopy (KPFM) is carried out to explore the photogenerated carrier transfer behavior in both strained ReS2/gr heterobubbles and ReS2/gr interfaces, as well as the oscillation of surface potential caused by optical interference under illumination conditions. Moreover, the switching of in-plane crystal orientation and the modulation of optical anisotropy of ReS2/gr heterobubbles are observed by azimuth-dependent reflectance difference microscopy (ADRDM), which can be attributed to the action of both strain effect and interference. Our study proves that the optical and electrical properties can be effectively modulated by the synergistical effect of strain and interference in a 2D material bubble

In-Plane Optical Anisotropy and Linear Dichroism in Low-Symmetry Layered TlSe

In-plane anisotropy of layered materials adds another dimension to their applications, opening up avenues in diverse angle-resolved devices. However, to fulfill a strong inherent in-plane anisotropy in layered materials still poses a significant challenge, as it often requires a low-symmetry nature of layered materials. Here, we report the fabrication of a member of layered semiconducting A^IIIB^VI compounds, TlSe, that possesses a low-symmetry tetragonal structure and investigate its anisotropic light–matter interactions. We first identify the in-plane Raman intensity anisotropy of thin-layer TlSe, offering unambiguous evidence that the anisotropy is sensitive to crystalline orientation. Further <i>in-situ</i> azimuth-dependent reflectance difference microscopy enables the direct evaluation of in-plane optical anisotropy of layered TlSe, and we demonstrate that the TlSe shows a linear dichroism under polarized absorption spectra arising from an in-plane anisotropic optical property. As a direct result of the linear dichroism, we successfully fabricate TlSe devices for polarization-sensitive photodetection. The discovery of layered TlSe with a strong in-plane anisotropy not only facilitates its applications in linear dichroic photodetection but opens up more possibilities for other functional device applications

Tuning the Optical, Magnetic, and Electrical Properties of ReSe₂ by Nanoscale Strain Engineering

Creating materials with ultimate control over their physical properties is vital for a wide range of applications. From a traditional materials design perspective, this task often requires precise control over the atomic composition and structure. However, owing to their mechanical properties, low-dimensional layered materials can actually withstand a significant amount of strain and thus sustain elastic deformations before fracture. This, in return, presents a unique technique for tuning their physical properties by “strain engineering”. Here, we find that local strain induced on ReSe₂, a new member of the transition metal dichalcogenides family, greatly changes its magnetic, optical, and electrical properties. Local strain induced by generation of wrinkle (1) modulates the optical gap as evidenced by red-shifted photoluminescence peak, (2) enhances light emission, (3) induces magnetism, and (4) modulates the electrical properties. The results not only allow us to create materials with vastly different properties at the nanoscale, but also enable a wide range of applications based on 2D materials, including strain sensors, stretchable electrodes, flexible field-effect transistors, artificial-muscle actuators, solar cells, and other spintronic, electromechanical, piezoelectric, photonic devices