Black phosphorus (BP) has drawn great attention owing to its tunable band gap depending on thickness, high mobility, and large I/ I ratio, which makes BP attractive for using in future two-dimensional electronic and optoelectronic devices. However, its instability under ambient conditions poses challenge to the research and limits its practical applications. In this work, we present a feasible approach to suppress the degradation of BP by sulfur (S) doping. The fabricated S-doped BP few-layer field-effect transistors (FETs) show more stable transistor performance under ambient conditions. After exposing to air for 21 days, the charge-carrier mobility of a representative S-doped BP FETs device decreases from 607 to 470 cm V s (remained as high as 77.4%) under ambient conditions and a large I/ I ratio of ∼10 is still retained. The atomic force microscopy analysis, including surface morphology, thickness, and roughness, also indicates the lower degradation rate of S-doped BP compared to BP. First-principles calculations show that the dopant S atom energetically prefers to chemisorb on the BP surface in a dangling form and the enhanced stability of S-doped BP can be ascribed to the downshift of the conduction band minimum of BP below the redox potential of O/O. Our work suggests that S doping is an effective way to enhance the stability of black phosphorus.

Single-layer WS2 has shown excellent photoresponse properties, but its promising applications in high-sensitivity photodetection suffer from the atomic-thickness-limited adsorption and band-gap-limited spectral selectivity. Here we have carried out investigations on WS2 monolayer based phototransistors with and without decoration of SnSe nanocrystals (NCs) for comparison. Compared to the solely WS2 monolayer, SnSe NCs decoration leads to not only huge enhancement of photoresponse in visible spectrum but also extension to near-infrared. Under excitation of visible light in a vacuum, the responsivity at zero gate bias can be enhanced by more than 45 times to ∼99 mA/W, and the response time is retained in millisecond level. Particularly, with extension of photoresponse to near-infrared (1064 nm), a responsivity of 6.6 mA/W can be still achieved. The excellent photoresponse from visible to near-infrared is considered to benefit from synergism of p-type SnSe NCs and n-type WS2 monolayer, or in other words, the formed p-n heterojunctions between p-type SnSe NCs and n-type WS2 monolayer.

Graphene nanosheets (GNSs) were prepared by an efficient liquid-phase exfoliation method, and then the NiCo/GNS nanohybrids were fabricated using the single-mode microwave-assisted hydrothermal technique. The NiCo/GNS composites with different GNS proportions were investigated as microwave absorbers. Morphology investigation suggested that NiCo nanocrystals were uniformly anchored on the GNS without aggregation. The electromagnetic parameters of NiCo/GNS nanohybrids could be artificially adjusted by changing the GNS proportion, which led to an exceptional microwave-absorbing performance. A reflection loss (RL) exceeding -20 dB was obtained in the frequency range of 5.3-16.4 GHz for the absorber thicknesses of 1.2-3.2 mm, while an optimal RL of -30 dB was achieved at 11.7 GHz for a thickness of 1.6 mm. The enhanced microwave-absorbing performance indicated that the NiCo/10 wt % GNS composite has great potential for use as an excellent microwave absorber.

Promising BP fi eld-effect transistors and absorbers for solar cells have already been demonstrated [ 17,19 ] and proposed. [ 20 ] Apart from the promising applications as transistors or absorbers, the nonlinear response of this 2D semiconductor to applied fi elds is also amazing. The possible nonlinear response properties are determined by the crystalline structure based on the Neumann principle. [ 21 ] Structurally, bulk BP crystals belong to the space group C mca, which indicates that the multilayered BP crystal has a center of inversion and has no second-order nonlinearity, including piezoelectric, ferroelectric, Pockels, second harmonic generation, etc. effects. For the third-order nonlinearity, the saturable absorption of BP has been studied recently and the results pointed out that few-layered BP has a nonlinear response in the visible to mid-infrared range with a large saturable intensity, which is defi ned as the optical intensity at the point where the optical absorption coeffi cient is reduced to half of its original value. In addition to the direct bandgap, which determines the effi cient absorption and emission properties, the optical nonlinearity theoretically indicates that BP should be a potential broadband saturable absorber that can be applied for modulating visible to mid-infrared lasers with large pulse energy. However, up to now, only few BP applications have been demonstrated in 1.5-to 1.9-µm fi ber lasers [22][23][24][25] and there are no reports on their use in bulk lasers either. Here, we report for the fi rst time the experimental realization of a BP pulse modulator and its applications in crystal lasers in the wavelength range of the visible to the mid-infrared. The results widen the spectral range of BP applications and experimentally identify that BP is a promising broadband optical modulator in optics beyond electronics. Results and Discussion Characterization of BP Broadband Modulator and Saturable Absorption MeasurementsThe mechanically exfoliated BP fl akes were investigated by Raman spectroscopy. The Raman shifts after stimulation by a laser with a wavelength of 633 nm are presented in Figure 1 a. From this fi gure, we observe that the peaks appear at 369.2 cm −1 , 439.0 cm −1 , and 467.5 cm −1 , corresponding to one out-of-plane vibration mode A 1 g and two in-plane vibration modes B 2 g and A 2 g , respectively. [ 26,27 ] It is well known that Black phosphorus (BP), a two-dimensional (2D) material, has a direct bandgap that can be tuned by changing the layers and applied strain, which fi lls the lacuna left by graphene topological insulators and transition-metal dichalcogenides. Theoretically, the direct and tunable bandgap should enable broadband applications for optoelectronics with high effi ciencies in the spectral range from the visible to the mid-infrared. Here, a BP broadband optical modulator is experimentally constructed and passively modulated lasers at 639 nm (red), 1.06 µm (near-infrared), and 2.1 µm (mid-infrared) are realized by using a BP optical modulator as the...

Atomically thin two-dimensional (2D) van der Waals materials have exhibited many exotic layer-dependent physical properties including electronic structure, magnetic order, etc. Here, we report a striking even-odd layer dependent oscillation...