2D Violet phosphorene with highly anisotropic mobility and its vdW heterojunction design for device applications

High Ambipolar Mobility and Long-Range Carrier Transport in Violet Phosphorus Nanosheet

Carrier transport capacity with high mobility and long-range diffusion length holds particular significance for the advancement of modern optoelectronic devices. Herein, we have unveiled the carrier dynamics and transport properties of a pristine violet phosphorus (VP) nanosheet by a transient absorption microscopy. Under the excitation (2.41 eV) above the exciton band, two photoinduced absorption peaks with the energy difference of approximately 520 meV emerge within a broadband transient absorption background which originates from the prompt generation of free carriers and the concomitant formation of excitons (lifetime of 467.21 ps). This observation is consistent with the established band-edge model of VP. Intriguingly, we have determined the ambipolar diffusion coefficient and mobility of VP to be approximately 47.32 cm2·s-1 and 1798 cm2·V-1·s-1, respectively, which further indicate a long-range carrier transport of approximately 2.10 μm. This work unveils the significant carrier transport capacity of VP, highlighting its potential for future optoelectronic and excitonic applications.

Enhanced electron transport and optical properties of experimentally synthesized monolayer Si9C15: a comprehensive DFT study for nanoelectronics and photocatalytic applications

Two-dimensional silicon-carbide (SixCy) materials stand out for their compatibility with current silicon-based technologies, offering unique advantages in nanoelectronics and photocatalysis. In this study, we employ density functional theory and nonequilibrium Green's function methods to investigate the electronic properties, electron transport characteristics, and optoelectronic qualities of experimentally synthesized monolayer Si9C15. Utilizing the modified deformation potential theory formula, we unveil Si9C15's significant directional anisotropy in electron mobility (706.42 cm2 V-1 s-1) compared to holes (432.84 cm2 V-1 s-1) in the a direction. The electrical transport calculations reveal that configurations with a 3 nm channel length demonstrate an ON state when biased, reaching a peak current of 150 nA. Moreover, this maximum current value escalates to 200 nA under tensile strain, marking an increase of approximately 100 times compared to the 5 nm channel, which remains in an OFF state. Si9C15 exhibits high light absorption coefficients (∼105 cm-1) and suitable band edge positions for water splitting at pH 0-7. Applying 1-5% tensile strain can tune the conduction band minimum and valence band maximum closer to the standard redox potentials, enhancing photocatalytic water splitting efficiency. Remarkably, under illumination at pH 0 and 7, Si9C15 can spontaneously catalyze water splitting, demonstrating its potential as a highly efficient photocatalyst. Our findings emphasize the importance of strain control and device length optimization for performance enhancement in nanoelectronics and renewable energy applications, positioning Si9C15 as a promising material for high-performance field-effect transistors and photocatalytic water splitting.

Modifying electronic and optical properties of violet phosphorus through variable fluorine coverage

We present a comprehensive investigation of the electronic properties of fluorinated monolayer violet phosphorus using first-principles calculations. Our results reveal a strong dependence of the electronic properties on the different fluorine coverages of fluorination. As the fluorine coverage increases, monolayer violet phosphorus undergoes a significant transition from a wide direct bandgap semiconductor to a narrow indirect bandgap semiconductor. Moreover, both semi-fluorinated and fully fluorinated monolayer violet phosphorus exhibit advantageous semiconducting characteristics, with a tunable bandgap of 0.50 ~ 1.04 eV under biaxial strain ranging from -6% to 6%. Notably, the fully fluorinated monolayer violet phosphorus demonstrates a higher coefficient of light absorption within the visible range. Therefore, our findings highlight the tunability of monolayer violet phosphorus properties through the absorption of various fluorine coverages, providing valuable insights for the design and development of novel semiconductor devices based on this material.

Nonlinear Optical Saturable Absorption Properties of 2D VP Nanosheets and Application as SA in a Passively Q-Switched Nd:YVO4 Laser

Two-dimensional (2D) violet phosphorus (VP) plays a significant role in the applications of photonic and optoelectronic devices due to its unique optical and electrical properties. The ultrafast carrier dynamics and nonlinear optical absorption properties were systematically investigated here. The intra- and inter-band ultrafast relaxation times of 2D VP nanosheets were measured to be ~6.83 ps and ~62.91 ps using the pump-probe method with a probe laser operating at 1.03 μm. The nonlinear absorption coefficient βeff, the saturation intensity Is, the modulation depth ΔR, and the nonsaturable loss were determined to be -2.18 × 104 cm/MW, 329 kW/cm2, 6.3%, and 9.8%, respectively, by using the Z-scan and I-scan methods, indicating the tremendous saturable absorption property of 2D VP nanosheets. Furthermore, the passively Q-switched Nd:YVO4 laser was realized with the 2D VP nanosheet-based SA, in which the average output power of 700 mW and the pulse duration of 478 ns were obtained. These results effectively reveal the nonlinear optical absorption characteristics of VP nanosheets, demonstrating their outstanding light-manipulating capabilities and providing a basis for the applications of ultrafast optical devices. Our results verify the excellent saturable absorption properties of 2D VP, paving the way for its applications in pulsed laser generation.

Probing the Strain Direction-Dependent Nonmonotonic Optical Bandgap Modulation of Layered Violet Phosphorus

Recent theoretical investigations have well-predicted strain-induced nonmonotonic optical band gap variations in low-dimensional materials. However, few two-dimensional (2D) materials are experimentally confirmed to exhibit nonmonotonic optical band gap variation under varying strain. Here, a strain-induced nonmonotonic optical bandgap variation in violet phosphorus (VP) nanosheets is observed, as evidenced by photoluminescence spectroscopy, which is reported in a few other 2D materials in knowledge. The optical bandgap variations are characterized to show the modulation rates of 41 and -24 meV/% with compression and tensile strains, respectively. Remarkably, first-principle calculations predict and clarify the nonmonotonic modulation accurately, highlighting its relationship with the strain direction-dependent asymmetric distribution of conduction band minimum wavefunctions, demonstrating that this unique nonmonotonic optical bandgap modulation is determined by the distinctive crystal structure of VP. This work provides a deep insight into the design of 2D materials toward optoelectronic and photoelectrochemical applications via strain engineering.

Ultrafast nonlinear optical response of layered violet phosphorus for femtosecond noise-like pulse generation

As a new member of two-dimensional (2D) phosphorene, 2D layered violet phosphorus (VP) has unique optoelectronic properties and good environmental stability, showing its huge advantages in optoelectronic applications. In this paper, the ultrafast nonlinear optical (NLO) properties of layered VP nanosheets at 1 µm band were explored, which exhibit an obvious saturable absorption response with a modulation depth of ∼1.97%. Meanwhile, the fast and slow carrier lifetimes of VP nanosheets at 1µm band were also determined as 295.9 fs and 2.36 ps, respectively, which are much shorter than that of most reported 2D materials. The excellent saturable absorption response combined with ultrashort carrier lifetimes indicate the prospect of layered VP nanosheets as a fast saturable absorber (SA) for ultrafast laser modulation. Then we demonstrated a Yb-doped fiber laser based on the VP-deposited taper-shaped fiber (TSF) SA, which delivers stable Q-switched mode-locked (QSML) pulses, dual-wavelength mode-locked pulses and 404-fs noise-like pulses. This work fully demonstrates the great potential of 2D VP materials for 1 µm ultrashort laser pulse generation.

Strong In-Plane Optoelectronic Anisotropy and Polarization Sensitivity in Low-Symmetry 2D Violet Phosphorus

Anisotropic optoelectronics based on low-symmetry two-dimensional (2D) materials hold immense potential for enabling multidimensional visual perception with improved miniaturization and integration capabilities, which has attracted extensive interest in optical communication, high-gain photoswitching circuits, and polarization imaging fields. However, the reported in-plane anisotropic photocurrent and polarized dichroic ratios are limited, hindering the achievement of high-performance anisotropic optoelectronics. In this study, we introduce novel low-symmetry violet phosphorus (VP) with a unique tubular cross-linked structure into this realm, and the corresponding anisotropic optical and optoelectronic properties are investigated both experimentally and theoretically for the first time. Remarkably, our prepared VP-based van der Waals phototransistor exhibits significant optoelectronic anisotropies with a giant in-plane anisotropic photocurrent ratio exceeding 10 and a comparable polarized dichroic ratio of 2.16, which is superior to those of most reported 2D counterparts. Our findings establish VP as an exceptional candidate for anisotropic optoelectronics, paving the way for future multifunctional applications.

Type-II 2D AgBr/SiH van der Waals heterostructures with tunable band edge positions and enhanced optical absorption coefficients for photocatalytic water splitting

Utilizing two-dimensional (2D) heterostructures in photocatalysis can enhance optical ab-sorption and charge separation, thereby increasing solar energy conversion efficiency and tackling environmental issues. Density functional theory (DFT) was employed in this study to investigate the structural and optoelectronic properties of the AgBr/SiH van der Waals (vdW) heterostructures. All three configurations (A1, A2, and A3) were stable, with direct bandgaps of 1.83 eV, 0.99 eV, and 1.36 eV, respectively. The type-II band alignment in these structures enables electrons to be transferred from the SiH layer to the AgBr layer, and holes to move in the opposite direction. In the ultraviolet region, the optical absorption coefficients of the A1, A2, and A3 configurations are approximately 4.0 × 105 cm-1, significantly higher than that of an isolated AgBr monolayer (2.4 × 104 cm-1). In the visible light region, the A1 configuration has an absorption coefficient of 4 × 104 cm-1, higher than that of an isolated AgBr (2.2 × 104 cm-1). The band edges of the A1 configuration satisfy the redox potential for photocatalytic water splitting at pH 0-7. When the biaxial tensile strain is 5% for the A2 configuration and 2% for the A3 configuration, it can allow photocatalytic water splitting from half-reactions without strain to photocatalytic overall water splitting at pH 0-7. With a 5% biaxial tensile strain in the visible light region, the A1 and A3 configurations experience a rise in the maximum absorption coefficient of 5.7 × 104 cm-1 and 4.6 × 104 cm-1, respectively. The findings indicate that the AgBr/SiH vdW heterostructure configurations could be utilized in photocatalytic water-splitting processes with great potential.

Air-Stable Violet Phosphorus/MoS2 van der Waals Heterostructure for High-Responsivity and Gate-Tunable Photodetection

Violet phosphorus (VP), a newly emerging elemental 2D semiconductor, with attractive properties such as tunable bandgap, high carrier mobility, and unusual structural anisotropy, offers significant opportunities for designing high-performance electronic and optoelectronic devices. However, the study on fundamental property and device application of 2D VP is seriously hindered by its inherent instability in ambient air. Here, a VP/MoS2 van der Waals heterostructure is constructed by vertically staking few-layer VP and MoS2 , aiming to utilize the synergistic effect of the two materials to achieve a high-performance 2D photodetector. The strong optical absorption of VP combining with the type-II band alignment of VP/MoS2 heterostructure make VP play a prominent photogating effect. As a result, the VP/MoS2 heterostructure photodetector achieves an excellent photoresponse performances with ultrahigh responsivity of 3.82 × 105  A W-1 , high specific detectivity of 9.17 × 1013 Jones, large external quantum efficiency of 8.91 × 107 %, and gate tunability, which are much superior to that of individual MoS2 device or VP device. Moreover, the VP/MoS2 heterostructure photodetector indicates superior air stability due to the effective protection of VP by MoS2 encapsulation. This work sheds light on the future study of the fundamental property and optoelectronic device application of VP.

Degenerate and non-degenerate all-optical switches using violet phosphorus nanosheets

As an allotrope of phosphorus, layered violet phosphorus (VP) has a wide range of applications in electronics, photonics, and optoelectronics. However, its nonlinear optical properties remain to be explored. In this work, we prepare and characterize VP nanosheets (VP Ns), investigate their spatial self-phase modulation (SSPM) effects, and develop them in all-optical switching applications. The ring forming time of SSPM and the third-order nonlinear susceptibility of monolayer VP Ns were found to be about 0.4 s and 10^-9 esu, respectively. The mechanism of SSPM formed by coherent light-VP Ns interaction is analyzed. Using the superior coherence electronic nonlinearity of VP Ns, we realize degenerate and non-degenerate all-optical switches based on the SSPM effect. It is demonstrated that the performance of all-optical switching can be controlled by adjusting the intensity of the control beam and/or the wavelength of the signal beam. The results will help us to better design and realize non-degenerate nonlinear photonic devices based on two-dimensional nanomaterials.

Polymorphic Phosphorus Applied to Alkali-Ion Battery Electrodes

The polymorphic phosphorus materials such as amorphous red and black ones have been used as the anodes for alkali-ion batteries. As the research field of 2D materials is pioneered, the fibrous red and violet phosphorus begin to be investigated and predicted for various devices. Meanwhile, they are not only applied to the active materials of electrodes but also the formation of protective layers for battery application. This article briefly introduces the primary allotropes of phosphorus, their research progress, and their potential for the application of alkali-ion batteries. Next, the recent studies concerning their applications of electrodes and protective layers for alkali-ion batteries are discussed in detail. Finally, the merits and drawbacks of preparation approaches, the strategies for improvement of battery performance, and the urgent challenges as well as possible solutions for future development of alkali-ion batteries using the electrodes or protective layers made from phosphorus materials, are summarized.