Linear Dichroism and Nondestructive Crystalline Identification of Anisotropic Semimetal Few-Layer MoTe2

In-plane anisotropic two-dimensional materials for twistronics

In-plane anisotropic two-dimensional (2D) materials exhibit in-plane orientation-dependent properties. The anisotropic unit cell causes these materials to show lower symmetry but more diverse physical properties than in-plane isotropic 2D materials. In addition, the artificial stacking of in-plane anisotropic 2D materials can generate new phenomena that cannot be achieved in in-plane isotropic 2D materials. In this perspective we provide an overview of representative in-plane anisotropic 2D materials and their properties, such as black phosphorus, group IV monochalcogenides, group VI transition metal dichalcogenides with 1T' and Tdphases, and rhenium dichalcogenides. In addition, we discuss recent theoretical and experimental investigations of twistronics using in-plane anisotropic 2D materials. Both in-plane anisotropic 2D materials and their twistronics hold considerable potential for advancing the field of 2D materials, particularly in the context of orientation-dependent optoelectronic devices.

Deep-Ultraviolet and Helicity-Dependent Raman Spectroscopy for Carbon Nanotubes and 2D Materials

Recent progress of Raman spectroscopy on carbon nanotubes and 2D materials is reviewed as a topical review. The Raman tensor with complex values is related to the chiral 1D/2D materials without mirror symmetry for the mirror in the propagating direction of light, such as chiral carbon nanotube and black phosphorus. The phenomenon of complex Raman tensor is observed by the asymmetric polar plot of helicity-dependent Raman spectroscopy using incident circularly-polarized lights. First-principles calculations of resonant Raman spectra directly give the complex Raman tensor that explains helicity-dependent Raman spectra and laser-energy-dependent relative intensities of Raman spectra. In deep-ultraviolet (DUV) Raman spectroscopy with 266 nm laser, since the energy of the photon is large compared with the energy gap, the first-order and double resonant Raman processes occur in general k points in the Brillouin zone. First-principles calculation is necessary to understand the DUV Raman spectra and the origin of double-resonance Raman spectra. Asymmetric line shapes appear for the G band of graphene for 266 nm laser and in-plane Raman mode of WS2 for 532 nm laser, while these spectra show symmetric line shapes for other laser excitation. The interference effect on the asymmetric line shape is discussed by fitting the spectra to the Breit-Wigner-Fano line shapes.

Ferroic Phases in Two-Dimensional Materials

Two-dimensional (2D) ferroics, namely ferroelectric, ferromagnetic, and ferroelastic materials, are attracting rising interest due to their fascinating physical properties and promising functional applications. A variety of 2D ferroic phases, as well as 2D multiferroics and the novel 2D ferrovalleytronics/ferrotoroidics, have been recently predicted by theory, even down to the single atomic layers. Meanwhile, some of them have already been experimentally verified. In addition to the intrinsic 2D ferroics, appropriate stacking, doping, and defects can also artificially regulate the ferroic phases of 2D materials. Correspondingly, ferroic ordering in 2D materials exhibits enormous potential for future high density memory devices, energy conversion devices, and sensing devices, among other applications. In this paper, the recent research progresses on 2D ferroic phases are comprehensively reviewed, with emphasis on chemistry and structural origin of the ferroic properties. In addition, the promising applications of the 2D ferroics for information storage, optoelectronics, and sensing are also briefly discussed. Finally, we envisioned a few possible pathways for the future 2D ferroics research and development. This comprehensive overview on the 2D ferroic phases can provide an atlas for this field and facilitate further exploration of the intriguing new materials and physical phenomena, which will generate tremendous impact on future functional materials and devices.

Universal visualization of crystalline orientation for black phosphorus by angle-resolved polarized photoacoustic microscopy

Anisotropic two-dimensional (2D) materials, such as black phosphorus (BP), normally possess unique directional in-plane electrical, optical, and thermal properties that are highly correlated with their crystalline orientations. Nondestructive visualization of their crystalline orientation is an indispensable premise for the 2D materials to harness their distinctive strengths in optoelectronic and thermoelectric applications. Here, by photoacoustically recording the anisotropic optical absorption variation under linearly polarized laser beams, an angle-resolved polarized photoacoustic microscopy (AnR-PPAM) is developed, capable of non-invasively determining and visualizing BP's crystalline orientation. We theoretically deduced the physical relationship between the crystalline orientation and polarized photoacoustic (PA) signals, and experimentally proved the ability of AnR-PPAM to universally visualize BP's crystalline orientation regardless of its thickness, substrate, and encapsulation layer. This method provides a new, to the best of our knowledge, strategy for crystalline orientation recognition of 2D materials with flexible measurement conditions, prefiguring important potential for the applications of anisotropic 2D materials.

Helicity-resolved Raman scattering of MoS2 bulk crystal

Helicity-resolved Raman spectroscopy (HRRS) can effectively distinguish the Raman modes of two-dimensional (2D) layered materials by phonon symmetry. In this paper, we systematically investigated the phonon helicity selection of basal and edge planes of MoS₂ bulk by HRRS. We find that the symmetry of the crystal structure changes the helicity selection of the E_1g, E¹ _2g, and A_1g modes in the edge plane. The theoretical calculation results confirm that the E¹ _2g and A_1g modes of the basal plane exhibit a perfect helicity exchange, and the helicity selections of the E¹ _2g and A_1g modes of the edge plane are eliminated or weakened. Our study provides references for phonon helicity selection of 2D layered materials represented by MoS₂.

Atmospheric-pressure CVD growth of two-dimensional 2H- and 1 T'-MoTe2films with high-performance SERS activity

Two-dimensional (2D) molybdenum ditelluride (MoTe₂) is a member of the transition-metal dichalcogenides family, which is an especially promising platform for surface-enhanced Raman scattering (SERS) applications, due to its excellent electronic properties. However, the synthesis of large-area highly crystalline 2D MoTe₂with controllable polymorphism is a huge challenge due to the small free energy difference (∼40 meV per unit cell) between semiconducting 2H-MoTe₂and semi-metallic 1 T'-MoTe₂. Herein, we report an optimized route for the synthesis of 2H- and 1 T'-MoTe₂films by atmospheric-pressure chemical vapor deposition. The SERS study of the as-grown MoTe₂films was carried out using methylene blue (MB) as a probe molecule. The Raman enhancement factor on 1 T'-MoTe₂was found to be three times higher than that on 2H-MoTe₂and the 1 T'-MoTe₂film is an efficient Raman-enhancing substrate that can be used to detect MB at nanomolar concentrations. Our study also imparts knowledge on the significance of a suitable combination of laser excitation wavelength and molecule-material platform for achieving ultrasensitive SERS-based chemical detection.

2D Re-Based Transition Metal Chalcogenides: Progress, Challenges, and Opportunities

The rise of 2D transition-metal dichalcogenides (TMDs) materials has enormous implications for the scientific community and beyond. Among TMDs, ReX2 (X = S, Se) has attracted significant interest regarding its unusual 1T' structure and extraordinary properties in various fields during the past 7 years. For instance, ReX2 possesses large bandgaps (ReSe2: 1.3 eV, ReS2: 1.6 eV), distinctive interlayer decoupling, and strong anisotropic properties, which endow more degree of freedom for constructing novel optoelectronic, logic circuit, and sensor devices. Moreover, facile ion intercalation, abundant active sites, together with stable 1T' structure enable them great perspective to fabricate high-performance catalysts and advanced energy storage devices. In this review, the structural features, fundamental physicochemical properties, as well as all existing applications of Re-based TMDs materials are comprehensively introduced. Especially, the emerging synthesis strategies are critically analyzed and pay particular attention is paid to its growth mechanism with probing the assembly process of domain architectures. Finally, current challenges and future opportunities regarding the controlled preparation methods, property, and application exploration of Re-based TMDs are discussed.

Nonlinear Optical Characterization of 2D Materials

Characterizing the physical and chemical properties of two-dimensional (2D) materials is of great significance for performance analysis and functional device applications. As a powerful characterization method, nonlinear optics (NLO) spectroscopy has been widely used in the characterization of 2D materials. Here, we summarize the research progress of NLO in 2D materials characterization. First, we introduce the principles of NLO and common detection methods. Second, we introduce the recent research progress on the NLO characterization of several important properties of 2D materials, including the number of layers, crystal orientation, crystal phase, defects, chemical specificity, strain, chemical dynamics, and ultrafast dynamics of excitons and phonons, aiming to provide a comprehensive review on laser-based characterization for exploring 2D material properties. Finally, the future development trends, challenges of advanced equipment construction, and issues of signal modulation are discussed. In particular, we also discuss the machine learning and stimulated Raman scattering (SRS) technologies which are expected to provide promising opportunities for 2D material characterization.

Controlled growth of large-scale uniform 1T' MoTe2 crystals with tunable thickness and their photodetector applications

The monoclinic-phase 1T' MoTe₂ crystal exhibits inversion symmetry as an anisotropic semi-metal, dictating its interesting quantum transport phenomenon and other novel physical properties. However, large-scale controllable growth of uniform MoTe₂ crystals still remains a great challenge, hindering its further fundamental research and applications for novel devices. Herein, we report a modified growth method for synthesizing few-layer 1T' MoTe₂ crystals with large-scale uniformity with the assistance of molecular sieves. The theoretical simulations demonstrated that due to the temperature-dependent formation energies of different edges, the edge of (010) orientation shows a higher thermodynamic stability than that of (100) orientation, and results in the anisotropic growth behavior of 1T' MoTe₂ crystals while the temperature changes. The photoresponse of tri-layer 1T' MoTe₂-based devices shows a broad-spectrum response from 532 nm to 1550 nm. The photo-response time of 1T' MoTe₂ crystals demonstrates that it supposes to be the synergistic mechanism of photo-conductive and photo-radiation effects. Our findings not only provide a method for the controllable growth of anisotropic two-dimensional materials at a wafer scale, but also explore a broad-spectrum photodetector with the MoTe₂-based device.

Symmetry-Reduction Enhanced Polarization-Sensitive Photodetection in Core-Shell SbI3 /Sb2 O3 van der Waals Heterostructure

Structural symmetry is a simple way to quantify the anisotropic properties of materials toward unique device applications including anisotropic transportation and polarization-sensitive photodetection. The enhancement of anisotropy can be achieved by artificial symmetry-reduction design. A core-shell SbI₃ /Sb₂ O₃ nanowire, a heterostructure bonded by van der Waals forces, is introduced as an example of enhancing the performance of polarization-sensitive photodetectors via symmetry reduction. The structural, vibrational, and optical anisotropies of such core-shell nanostructures are systematically investigated. It is found that the anisotropic absorbance of a core-shell nanowire is obviously higher than that of two single compounds from both theoretical and experimental investigations. Anisotropic photocurrents of the polarization-sensitive photodetectors based on these core-shell SbI₃ /Sb₂ O₃ van der Waals nanowires are measured ranging from ultraviolet (UV) to visible light (360-532 nm). Compared with other van der Waals 1D materials, low anisotropy ratio (I_max /I_min ) is measured based on SbI₃ but a device based on this core-shell nanowire possesses a relatively high anisotropy ratio of ≈3.14 under 450 nm polarized light. This work shows that the low-symmetrical core-shell van der Waals heterostructure has large potential to be applied in wide range polarization-sensitive photodetectors.

Polarization-Dependent Optical Properties and Optoelectronic Devices of 2D Materials

The development of optoelectronic devices requires breakthroughs in new material systems and novel device mechanisms, and the demand recently changes from the detection of signal intensity and responsivity to the exploration of sensitivity of polarized state information. Two-dimensional (2D) materials are a rich family exhibiting diverse physical and electronic properties for polarization device applications, including anisotropic materials, valleytronic materials, and other hybrid heterostructures. In this review, we first review the polarized-light-dependent physical mechanism in 2D materials, then present detailed descriptions in optical and optoelectronic properties, involving Raman shift, optical absorption, and light emission and functional optoelectronic devices. Finally, a comment is made on future developments and challenges. The plethora of 2D materials and their heterostructures offers the promise of polarization-dependent scientific discovery and optoelectronic device application.