Laser-assisted phosphorus doping is demonstrated on ultrathin transition-metal dichalcogenides (TMDCs) including n-type MoS2 and p-type WSe2 . Temporal and spatial control of the doping is achieved by varying the laser irradiation power and time, demonstrating wide tunability and high site selectivity with high stability. The laser-assisted doping method may enable a new avenue for functionalizing TMDCs for customized nanodevice applications.

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Recent studies have shown that vapor phase synthesis of structurally isotropic two-dimensional (2D) MoS2 and WS2 produces well-defined domains with clean grain boundaries (GBs). This is anticipated to be vastly different for 2D anisotropic materials like ReS2 mainly due to large anisotropy in interfacial energy imposed by its distorted 1T crystal structure and formation of signature Re-chains along [010] b-axis direction. Here, we provide first insight on domain architecture on chemical vapor deposited (CVD) ReS2 domains using high-resolution scanning transmission electron microscopy, angle-resolved nano-Raman spectroscopy, reflectivity, and atomic force microscopy measurements. Results provide ways to achieve crystalline anisotropy in CVD ReS2, establish domain architecture of high symmetry ReS2 flakes, and determine Re-chain orientation within subdomains. Results also provide a first atomic resolution look at ReS2 GBs, and surprisingly we find that cluster and vacancy defects, formed by collusion of Re-chains at the GBs, dramatically impact the crystal structure by changing the Re-chain direction and rotating Re-chains 180° along their b-axis. Overall results not only shed first light on domain architecture and structure of anisotropic 2D systems but also allow one to attain much desired crystalline anisotropy in CVD grown ReS2 for the first time for tangible applications in photonics and optoelectronics where direction-dependent dichroic and linearly polarized material properties are required.

a Zirconium trisulphide (ZrS 3 ), a member of the layered transition metal trichalcogenides (TMTCs) family, has been studied by angle-resolved photoluminescence spectroscopy (ARPLS). The synthesized ZrS 3 layers possess a pseudo one-dimensional nature where each layer consists of ZrS 3 chains extending along the b-lattice direction. Our results show that the optical properties of few-layered ZrS 3 are highly anisotropic as evidenced by large PL intensity variation with the polarization direction. Light is efficiently absorbed when the E-field is polarized along the chain (b-axis), but the field is greatly attenuated and absorption is reduced when it is polarized vertical to the 1D-like chains as the wavelength of the exciting light is much longer than the width of each 1D chain. The observed PL variation with polarization is similar to that of conventional 1D materials, i.e., nanowires, and nanotubes, except for the fact that here the 1D chains interact with each other giving rise to a unique linear dichroism response that falls between the 2D ( planar) and 1D (chain) limit. These results not only mark the very first demonstration of PL polarization anisotropy in 2D systems, but also provide novel insight into how the interaction between adjacent 1D-like chains and the 2D nature of each layer influences the overall optical anisotropy of pseudo-1D materials. Results are anticipated to have an impact on optical technologies such as polarized detectors, near-field imaging, communication systems, and bio-applications relying on the generation and detection of polarized light.

Transition metal trichalcogenides form a class of layered materials with strong in-plane anisotropy. For example, titanium trisulfide (TiS3) whiskers are made out of weakly interacting TiS3 layers, where each layer is made of weakly interacting quasi-one-dimensional chains extending along the b axis. Here we establish the unusual vibrational properties of TiS3 both experimentally and theoretically. Unlike other two-dimensional systems, the Raman active peaks of TiS3 have only out-of-plane vibrational modes, and interestingly some of these vibrations involve unique rigid-chain vibrations and S–S molecular oscillations. High-pressure Raman studies further reveal that the AgS–S S-S molecular mode has an unconventional negative pressure dependence, whereas other peaks stiffen as anticipated. Various vibrational modes are doubly degenerate at ambient pressure, but the degeneracy is lifted at high pressures. These results establish the unusual vibrational properties of TiS3 with strong in-plane anisotropy, and may have relevance to understanding of vibrational properties in other anisotropic two-dimensional material systems.

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 AB 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 in-situ 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.

Layered transition metal trichalcogenides (TMTCs) are a new class of anisotropic two-dimensional materials that exhibit quasi-1D behavior. This property stems from their unique highly anisotropic crystal structure where vastly different material properties can be attained from different crystal directions. Here, we employ density functional theory predictions, atomic force microscopy, and angle-resolved Raman spectroscopy to investigate their fundamental vibrational properties which differ significantly from other 2D systems and to establish a method in identifying anisotropy direction of different types of TMTCs. We find that the intensity of certain Raman peaks of TiS, ZrS, and HfS have strong polarization dependence in such a way that intensity is at its maximum when the polarization direction is parallel to the anisotropic b-axis. This allows us to readily identify the Raman peaks that are representative of the vibrations along the b-axis direction. Interestingly, similar angle resolved studies on the novel TiNbS TMTC alloy reveal that determination of anisotropy/crystalline direction is rather difficult possibly due to loss of anisotropy by randomization distribution of quasi-1D MX chains by the presence of defects which are commonly found in 2D alloys and also due to the complex Raman tensor of TMTC alloys. Overall, the experimental and theoretical results establish non-destructive methods used to identify the direction of anisotropy in TMTCs and reveal their vibrational characteristics which are necessary to gain insight into potential applications that utilize direction dependent thermal response, optical polarization, and linear dichroism.

and 2D isotropic systems, and are anticipated to have an impact in applications for electronic and optoelectronic devices systems. Examples of such quasi-1D materials include, but are not limited to, black phosphorus [6][7][8][9][10][11][12] (BPs), ReS 2 , [13][14][15][16] and ReSe 2 , [17,18] and group IV-VI compounds such as GaTe [19,20] and SnSe. [21,22] Despite the pioneering investigations on the above materials, group-IV transition metal trichalcogenides (TMTCs) with a chemical formula of MX 3 (M = Ti, Zr, or Hf, and X = S, Se, or Te) have recently emerged as new candidates for the next generation of electronics and optoelectronics owing to theoretically predicted high carrier mobility values. [13,[23][24][25] Such TMTCs consist of distorted MX 6 polyhedrons extending along the in-plane b-axis direction, [26] forming quasi-1D chains where the MX bond between adjacent chains (along a-axis) is estimated to be 80% less than that of within the chains (along b-axis), [27] thus they exhibit strong structural anisotropy that is directly evidenced by the whisker-like crystals and the elongated shape of exfoliated nanoflakes falling between nanowires and nanosheets. Our previous work focused on zirconium trisulfide (ZrS 3 ) has shown the angle-dependent vibrational nature [28] and the strong dichroic emission from nanoribbons. [13] However except for the PL spectral characterization, more efforts are needed in surveying its in-plane anisotropy in the polarization related devices, thus enabling efficient 2D anisotropic materials, such as black phosphorus, ReS 2 , and GaTe, have been shown to exhibit exciting direction-and polarization-sensitive material properties. Highly crystalline chemical-vapor-transport-grown ZrS 3 crystals exhibit large optical-absorption-coefficient anisotropy, which doubles under resonance conditions. The observed optical anisotropy manifests itself in angle-resolved photocurrent density polar plots with dichroic ratio (I pb /I pa ) of 1.73 excited by a laser source of λ = 450 nm and 1.14 by λ = 532 nm. The optical absorption and electronic dichroic response are fully explained through detailed band structure and polarization-sensitive optical-absorption-spectrum calculations. Not only is the family of 2D anisotropic semiconductors expanded into Zr-based trichalcogenides but fundamental insights on how crystalline anisotropy, optical absorption dichroism, and generated photocurrents are interrelated in van der Waals Zr-based trichalcogenides materials are also provided.