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Mechanical strain induced changes in the electronic properties of two-dimensional (2D) materials is of great interest for both fundamental studies and practical applications. The anisotropic 2D materials may further exhibit different electronic changes when the strain is applied along different crystalline axes. The resulting anisotropic piezoresistive phenomenon not only reveals distinct lattice–electron interaction along different principle axes in low-dimensional materials but also can accurately sense/recognize multidimensional strain signals for the development of strain sensors, electronic skin, human–machine interfaces, etc. In this work, we systematically studied the piezoresistive effect of an anisotropic 2D material of rhenium disulfide (ReS2), which has large anisotropic ratio. The measurement of ReS2 piezoresistance was experimentally performed on the devices fabricated on a flexible substrate with electrical channels made along the two principle axes, which were identified noninvasively by the reflectance difference microscopy developed in our lab. The result indicated that ReS2 had completely opposite (positive and negative) piezoresistance along two principle axes, which differed from any previously reported anisotropic piezoresistive effect in other 2D materials. We attributed the opposite anisotropic piezoresistive effect of ReS2 to the strain-induced broadening and narrowing of the bandgap along two principle axes, respectively, which was demonstrated by both reflectance difference spectroscopy and theoretical calculations.

Optical anisotropy is one of the most fundamental physical characteristics of emerging low-symmetry two-dimensional (2D) materials. It provides abundant structural information and is crucial for creating diverse nanoscale devices. Here, we have proposed an azimuth-resolved microscopic approach to directly resolve the normalized optical difference along two orthogonal directions at normal incidence. The differential principle ensures that the approach is only sensitive to anisotropic samples and immune to isotropic materials. We studied the optical anisotropy of bare and encapsulated black phosphorus (BP) and unveiled the interference effect on optical anisotropy, which is critical for practical applications in optical and optoelectronic devices. A multi-phase model based on the scattering matrix method was developed to account for the interference effect and then the crystallographic directions were unambiguously determined. Our result also suggests that the optical anisotropy is a probe to measure the thickness with monolayer resolution. Furthermore, the optical anisotropy of rhenium disulfide (ReS2), another class of anisotropic 2D materials, with a 1T distorted crystal structure, was investigated, which demonstrates that our approach is suitable for other anisotropic 2D materials. This technique is ideal for optical anisotropy characterization and will inspire future efforts in BP and related anisotropic 2D nanomaterials for engineering new conceptual nanodevices.

Two-step rapid thermal thinning obtains monolayer black phosphorus with a high surface quality.

Tunable photo-response is highly desirable by photodiodes for future optoelectronic applications. As compared to bulk semiconducting materials, the atomically thin two-dimensional (2D) materials may be one of the potential candidates to fabricate such adaptive photodiodes, since they possess not only excellent but also widely tunable optoelectronic properties. The most extensively applied device structure for the 2D materials based photodiodes is the vertically aligned van der Waals heterostructure. However, fabricating the vertical 2D material heterostructures is usually complicated, involving manually stacking multiple 2D material flakes together, which is undesirable for industry applications. In this work, we developed a vertical MoO 3 /MoS 2 heterojunction for photodetection and photovoltaic applications. The device used MoS 2 and its oxidation layer of MoO 3 as the n-and p-type regions, respectively, which can greatly simplify the fabrication process of 2D vertical heterojunctions. Moreover, the device exhibited prominent photo-response with photoresponsivity of 670 mA W −1 , detectivity of 4.77 × 10 10 Jones and power conversion efficiency (PCE) of 3.5% under 0 V bias. The device also presented efficient gate tunability on photocurrent with on/ off ratio of 10 3 . This research provides an alternative way to fabricate 2D materials based vertical heterojunctions for optoelectronic applications with tunable photo-responses.

To improve the safety of HMX without sacrificing energy properties, the composites of TNT and an energetic material (HP-1) were used to coat HMX particles by a method of integrating solvent -nonsolvent with aqueous suspension-melting. SEM (scanning electron microscopy) and XPS (X-ray photoelectron spectrometry) were employed to characterize the samples. The effect of the processing parameters, such as mass ratio of HP-1 to TNT (MRHT), stirring speed, and cooling rate, on the quality of coated samples were investigated and discussed. The mechanical sensitivity, thermal sensitivity, thermal decomposition characteristic, and heat of detonation of raw and coated HMX samples were also measured and contrasted. Results show that when MRHT, stirring speed in the second stage and cooling rate are 1 : 5, 1000 r · min À1 and 5 8C · min À1 respectively, the optimal coating effect is achieved. Compared with that of raw HMX, both impact and friction sensitivity of HMX coated with 2.5 wt.-% TNT and 0.5 wt.-% HP-1 decrease obviously, whereas there is a slight change in their thermal sensitivity and thermal decomposition characteristics. Meanwhile, such surface coating does not result in the decrease of its energy properties.

The selective and sensitive detection of chemical agents is demanded by a wide range of practical applications. In particular, sensing of volatile organic compounds (VOCs) at parts-per-billion level is critical for environmental monitoring, process control, and early diagnosis of human diseases. In this report, we demonstrate a specific and highly sensitive detection of ketone compounds using two-dimensional (2D) molybdenum ditelluride (MoTe2). We investigated the effects of UV activation on the sensing performance to a variety of VOCs. It is found that the MoTe2 field-effect transistor (FET) exhibits an opposite sensing response to ketone compounds before and after UV light activation, whereas the responses to other types of VOCs remain in the same direction regardless of the illumination. This unique behavior enables the discriminative detection of ketone molecules including acetone and pentanone from other VOCs in a gas mixture. The activation of UV light also results in a very high sensitivity and low detection limit toward acetone (∼0.2 ppm). Moreover, the MoTe2 FET shows a stable sensing performance in a high humidity environment. The results demonstrate the potential of MoTe2 as a promising candidate for high-performance acetone sensors in important applications such as human breath analysis. The scheme of light-tunable sensing can be applied to a broad range of sensing platforms based on 2D materials.