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.

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.

Artificial Optoelectronic Synapse Based on Violet Phosphorus Microfiber Arrays.

Memristor-based artificial synapses are regarded as the most promising candidate to develop brain-like neuromorphic network computers and overcome the bottleneck of Von-Neumann architecture. Violet phosphorus (VP) as a new allotrope of available phosphorus with outstanding electro-optical properties and stability has attracted more and more attention in the past several years. In this study, large-scale, high-yield VP microfiber vertical arrays have been successfully developed on a Sn-coated graphite paper and are used as the memristor functional layers to build reliable, low-power artificial synaptic devices. The VP devices can well mimic the major synaptic functions such as short-term memory (STM), long-term memory (LTM), paired-pulse facilitation (PPF), spike timing-dependent plasticity (STDP), and spike rate-dependent plasticity (SRDP) under both electrical and light stimulation conditions, even the dendritic synapse functions and simple logical operations. By virtue of the excellent performance, the VP artificial synapse devices can be conductive to building high-performance optic-neural synaptic devices simulating the human-like optic nerve system. On this basis, Pavlov's associative memory can be successfully implemented optically. This study provides a promising approach for the design and manufacture of VP-based artificial synaptic devices and outlines a direction with multifunctional neural devices.

Pressure-Driven Polymorphic Transition, Emergent Insulator-To-Metal Transition, and Photoconductivity Switching in Violet Phosphorus.

Polymorphic phase transition is an essential phenomenon in condensed matter that the physical properties of materials may undergo significant changes due to the structural transformation. Phase transition has thus become an important means and dimension for regulating material properties. Herein, this study demonstrates the pressure-induced multi-transition of both structure and physical properties in violet phosphorus, a novel phosphorus allotrope. Under compression, violet phosphorus undergoes sequential polymorphic phase transitions. Concomitant with the first phase transition, violet phosphorus exhibits emergent insulator-metal transition, superconductivity, and dramatic switching from positive to negative photoconductivity. Remarkably, the resistance of violet phosphorus shows a sudden drop of around 107 along with the phase transition. In addition, piezochromism from translucent red to opaque black and suppression of photoluminescence are observed upon compression. Of particular interest is that the sample irreversibly transforms into black phosphorus with a pronounced discrepancy in physical properties from the pristine violet phosphorus after decompression. The abundant polymorphic transitions and property changes in violet phosphorus have significant implications for designing novel pressure-responsive electronic/optoelectronic devices and exploring concealed polymorphic transition materials.

Fabrication of Single-Crystal Violet Phosphorus Flakes For Ultrasensitive Photodetection.

Violet phosphorus (VP) has attracted a lot of attention for its unique physicochemical properties and emerging potential in photoelectronic applications. Although VP has a van der Waals (vdW) structure similar to that of other 2D semiconductors, direct synthesis of VP on a substrate is still challenging. Moreover, optoelectronic devices composed of transfer-free VP flakes have not been demonstrated. Herein, a bismuth-assisted vapor phase transport technique is designed to grow uniform single-crystal VP flakes on the SiO2/Si substrate directly. The size of the crystalline VP flakes is an order of magnitude larger than that of previous liquid-exfoliated samples. The photodetector fabricated with the VP flakes shows a high responsivity of 12.5 A W-1 and response/recovery time of 3.82/3.03 ms upon exposure to 532 nm light. Furthermore, the photodetector shows a small dark current (<1 pA) that is beneficial to high-sensitivity photodetection. As a result, the detectivity is 1.38 × 1013 Jones that is comparable with that of the vdW p-n heterojunction detector. The results reveal the great potential of VP in optoelectronic devices as well as the CVT technique for the growth of single-crystal semiconductor thin films.

P-N Bonds-Mediated Atomic-Level Charge-Transfer Channel Fabricated between Violet Phosphorus and Carbon Nitride Favors Charge Separation and Water Splitting.

Heterostructures are widely employed in photocatalysis to promote charge separation and photocatalytic activity. However, their benefits are limited by the linkages and contact environment at the interface. Herein, violet phosphorus quantum dots (VPQDs) and graphitic carbon nitride (g-C3N4) are employed as model materials to form VPQDs/g-C3N4 heterostructures by a simple ultrasonic pulse excitation method. The heterostructure contains strong interfacial P-N bonds that mitigate interfacial charge-separation issues. P-P bond breakage occurs in the distinctive cage-like [P9] VPQD units during longitudinal disruption, thereby exposing numerous active P sites that bond with N atoms in g-C3N4 under ultrasonic pulse excitation. The atomic-level interfacial P-N bonds of the Z-scheme VPQDs/g-C3N4 heterostructure serve as photogenerated charge-transfer channels for improved electron-hole separation efficiency. This results in excellent photocatalytic performance with a hydrogen evolution rate of 7.70 mmol g-1 h-1 (over 9.2 and 8.5 times greater than those of pure g-C3N4 and VPQDs, respectively) and apparent quantum yield of 11.68% at 400 nm. Using atomic-level chemical bonds to promote interfacial charge separation in phosphorene heterostructures is a feasible and effective design strategy for photocatalytic water-splitting materials.

Partially Oxidized Violet Phosphorus as an Excellent Lubricant Additive for Tribological Applications.

As a novel two-dimensional material, violet phosphorus (VP) has attracted a considerable amount of attention due to its high carrier mobility, anisotropy, wide band gap, stability, and easy stripping properties. In this work, the microtribological properties of partially oxidized VP (oVP) and the mechanism of reducing friction and wear as additives in oleic acid (OA) oil were studied systematically. When adding oVP to OA, the coefficient of friction (COF) decreased from 0.084 to 0.014 with the steel-to-steel pair, and the ultralow shearing strength tribofilm consisting of amorphous carbon and phosphorus oxides that formed resulted in the reductions of COF and wear rate individually by 83.3% and 53.9%, respectively, compared with those of pure OA. The results extended the application scenarios for VP in the design of lubricant additives.

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.

Correspondence to "Structure of Violet Phosphorus and Its Phosphorene Exfoliation".

The structure described in the publication "Structure of Violet Phosphorus and Its Phosphorene Exfoliation" (Angew. Chem. Int. Ed. 2020, 59, 1074-1080) is identical to the structure by Thurn and Krebs determined already in 1969 and therefore by no means a new modification.

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.

Violet phosphorus transmission and photoconductance spectroscopy.

Violet phosphorus is a semiconducting allotrope of phosphorus with a layered crystalline structure consisting of orthogonally oriented layers of phosphorus chains composed of P2[P8]P2[P9] repeating units. Here, we report optical transmission spectroscopy and photoconductivity measurements of exfoliated flakes of violet phosphorus in the thin-film bulk limit. The violet phosphorus was synthesized by chemical vapour transport, and subsequently protected from oxidation with an inert gas environment. A peak photoconductive responsivity ofR= 7 mA W-1at photon energy 2.8 eV was observed. The spectral dependence of optical transmission and photoconductivity of violet phosphorus leads us to identify optical transitions at van Hove singularities corresponding to energiesE1= 1.80 ± 0.05 eV andE2= 1.95 ± 0.05 eV. Density functional theory was applied to the calculation of violet phosphorus (vP) bandstructure, and a dipole transition analysis shows that optical transitions at theZandA0points of the Brillouin zone are in agreement with experimental observations. Exposure to ambient environmental conditions for several minutes is sufficient to significantly reduce vP photoconductivity, while longer exposure leads to blistering due to oxidation. Thus, a locally inert chemical environment is essential to accessing vP intrinsic optoelectronic properties.

Structure and Properties of Violet Phosphorus and Its Phosphorene Exfoliation.

Black phosphorene has attracted much attention as a semiconducting two-dimensional material. Violet phosphorus is another layered semiconducting phosphorus allotrope with unique electronic and optoelectronic properties. However, no confirmed violet crystals or reliable lattice structure of violet phosphorus had been obtained. Now, violet phosphorus single crystals were produced and the lattice structure has been obtained by single-crystal x-ray diffraction to be monoclinic with space group of P2/n (13) (a=9.210, b=9.128, c=21.893 Å, ß=97.776°). The lattice structure obtained was confirmed to be reliable and stable. The optical band gap of violet phosphorus is around 1.7 eV, which is slightly larger than the calculated value. The thermal decomposition temperature was 52 °C higher than its black phosphorus counterpart, which was assumed to be the most stable form. Violet phosphorene was easily obtained by both mechanical and solution exfoliation under ambient conditions.

Lateral Heterostructures Fabricated via Artificial Pressure Gradient.

Hydrostatic conditions are generally pursued in high-pressure research, maintained to prevent the intrinsic pressure gradient on the culets of diamond anvil cells (DACs) from introducing heterogeneity to the structure and physical properties of the regulated materials. Here, a pioneering route to fabricate lateral heterostructures is proposed via artificial pressure gradients intentionally designed in DACs. Under the tailored pressure gradients, different structural phases emerge in distinct parts of the material, resulting in the formation of heterostructures. Harnessing the polymorphic transition nature of violet phosphorus under high pressure, violet/blue and violet/black phosphorus lateral heterostructures with different electrical properties have been successfully prepared by the pressure gradient method. This achievement highlights the potential of artificial pressure gradients as a portable and universal strategy for the fabrication of lateral heterostructures, shedding new light on the preparation and regulation of lateral heterostructures across a wider range of materials.

Photodegradation and van der Waals Passivation of Violet Phosphorus.

Violet phosphorus (VP), a novel two-dimensional (2D) nanomaterial, boasts structural anisotropy, a tunable optical bandgap, and superior thermal stability compared with its allotropes. Its multifunctionality has sparked widespread interest in the community. Yet, the VP's air susceptibility impedes both probing its intrinsic features and device integration, thus making it of urgent significance to unveil the degradation mechanism. Herein, we conduct a comprehensive study of photoactivated degradation effects on VP. A nitrogen annealing method is presented for the effective elimination of surface adsorbates from VP, as evidenced by a giant surface-roughness improvement from 65.639 nm to 7.09 nm, enabling direct observation of the intrinsic morphology changes induced by photodegradation. Laser illumination demonstrates a significant thickness-thinning effect on VP, manifested in the remarkable morphological changes and the 73% quenching of PL intensity within 160 s, implying its great potential for the efficient selected-area etching of VP at high resolution. Furthermore, van der Waals passivation of VP using 2D hexagonal boron nitride (hBN) was achieved. The hBN-passivated channel exhibited improved surface roughness (0.512 nm), reduced photocurrent hysteresis, and lower responsivity (0.11 A/W @ 450 nm; 2 µW), effectively excluding adsorbate-induced electrical and optoelectrical effects while disabling photodegradation. Based on our experimental results, we conclude that three possible factors contribute to the photodegradation of VP: illumination with photon energy higher than the bandgap, adsorbed H2O, and adsorbed O2.

Synergistic photoinduced charge transfer resonance from porous ZIF-67 decorated violet phosphorus array for SERS immunoassay of SARS-CoV-2 spike protein.

As a rapid, highly sensitive, and user-friendly technique, surface-enhanced Raman scattering (SERS) has an extraordinary appeal to home self-test of COVID-19 during the post pandemic era. However, most of the existing SERS substrates have been still criticized in stability, repeatability, and sample enrichment. To address these obstacles, a novel non-metallic SERS substrate with porous surfaces and array geometry was developed by in-situ growing ZIF-67 particles on two-dimensional violet phosphorus (VP) matrix. Chemical enhancement was prominently promoted by the synergistic photoinduced charge transfer resonance in the hybrid band structure of the ZIF-67@VP substrate, facilitating a noble metal-similar enhancement factor of 6.11 × 107. The biocompatible ZIF-67@VP porous array with attractive enhancement capability and high anchoring efficiency was further utilized to monitoring SARS-CoV-2 spike protein in practical saliva samples based on a sandwich immunostructure, achieving a limit of detection of 1.7 ng/mL assisted by black phosphorus nanosheets. This nonmetallic immunoassay strategy with exceptional sensitivity and specificity is predicted to extend the utilization of SERS obstacle in daily infectious disease screening.

Constructing Interfacial Charge Transfer Channels and Electric Field in Violet Phosphorus-Based van der Waals Heterojunction for Phototherapy of Periodontitis.

Periodontitis, a chronic oral disease instigated by bacteria, severely compromises human oral health. The prevailing clinical treatment for periodontitis involves mechanical scraping in conjunction with antibiotics. Phototherapy is employed to rapidly remove the bacteria and achieve periodontitis treatment, effectively circumventing the adverse effects associated with traditional therapies. Constructing 2D/2D van der Waals (VDW) heterojunctions is a key strategy for obtaining excellent photocatalytic activity. Herein, a 2D/2D violet phosphorus (VP)/Ti3C2 VDW heterojunction is designed using an interfacial engineering strategy. By constructing an electron transport "bridge" (P-Ti bond) at the heterogeneous interface as an effective transfer channel for photogenerated carriers, a compact monolithic structure between the VP and Ti3C2 phases is formed, and the spatial barrier for electron transfer at the interface is eliminated. Meanwhile, the strong directional built-in electric field induced by the intensive electron-coupling effect at the heterogeneous interface served as an internal driving force, which greatly accelerates the exciton dissociation and charge transfer in the photocatalytic process. These excited photogenerated electrons and holes are trapped by O2 and H2O on the surfaces of Ti3C2 and VP, respectively, and are subsequently catalytically converted to antibacterial reactive oxygen species (ROS). The VP/Ti3C2 VDW heterojunction eradicated 97.5% and 98.48% of Staphylococcus aureus and Escherichia coli, respectively, by photocatalytic and photothermal effects under visible light for 10 min. The VP/Ti3C2 nanoperiodontal dressing ointment effectively attenuated inflammatory response, reduced alveolar bone resorption, and promoted periodontal soft and hard tissue repair. Its periodontitis therapeutic effect outperforms the clinically used Periocline.

Optoelectronic Synapses Based on MXene/Violet Phosphorus van der Waals Heterojunctions for Visual-Olfactory Crossmodal Perception.

The crossmodal interaction of different senses, which is an important basis for learning and memory in the human brain, is highly desired to be mimicked at the device level for developing neuromorphic crossmodal perception, but related researches are scarce. Here, we demonstrate an optoelectronic synapse for vision-olfactory crossmodal perception based on MXene/violet phosphorus (VP) van der Waals heterojunctions. Benefiting from the efficient separation and transport of photogenerated carriers facilitated by conductive MXene, the photoelectric responsivity of VP is dramatically enhanced by 7 orders of magnitude, reaching up to 7.7 A W-1. Excited by ultraviolet light, multiple synaptic functions, including excitatory postsynaptic currents, paired-pulse facilitation, short/long-term plasticity and "learning-experience" behavior, were demonstrated with a low power consumption. Furthermore, the proposed optoelectronic synapse exhibits distinct synaptic behaviors in different gas environments, enabling it to simulate the interaction of visual and olfactory information for crossmodal perception. This work demonstrates the great potential of VP in optoelectronics and provides a promising platform for applications such as virtual reality and neurorobotics.

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.

Customization of 2D Atomic-Molecular Heterojunction with Manipulatable Charge-Transfer and Band Structure.

Manipulating the properties of 2D materials through meticulously engineered artificial heterojunctions holds great promise for novel device applications. However, existing research on the crucial charge-transfer interactions and energy profile regulation is predominantly focused on 2D van der Waals structures formed via weak van der Waals forces, limiting regulatory efficiency at high costs. Herein, a refined atomic-molecular heterojunction strategy featuring strong covalent bonds between organic molecule and 2D violet phosphorus (VP) atomic crystal is developed, which enables enhanced charge-transfer dynamics and customizable band structure regulation at the molecular level. Both experimentally and theoretically, it is demonstrated that grafting efficiency, charge redistribution, and energy gap regulation critically depend on organic electronegativity, providing a low-cost yet high-efficiency regulatory effect on a large scale. As a proof of concept, the novel VP-molecular heterojunctions exhibit optimized performance in diverse application domains, presenting a general platform for future high-performance device applications.

Strain-Prompted Giant Flexo-Photovoltaic Effect in Two-Dimensional Violet Phosphorene Nanosheets.

As a second-order nonlinear optical phenomenon, the bulk photovoltaic (BPV) effect is expected to break through the Shockley-Queisser limit of thermodynamic photoelectron conversion and improve the energy conversion efficiency of photovoltaic cells. Here, we have successfully induced a strong flexo-photovoltaic (FPV) effect, a form of BPV effect, in strained violet phosphorene nanosheets (VPNS) by utilizing strain engineering at the h-BN nanoedge, which was first observed in nontransition metal dichalcogenide (TMD) systems. This BPV effect was found to originate from the disruption of inversion symmetry induced by uniaxial strain applied to VPNS at the h-BN nanoedge. We have revealed the intricate relationship between the bulk photovoltaic effect and strain gradients in VPNS through thickness-dependent photovoltaic response experiments. A bulk photovoltaic coefficient of up to 1.3 × 10-3 V-1 and a polarization extinction ratio of 21.6 have been achieved by systematically optimizing the height of the h-BN nanoedge and the thickness of VPNS, surpassing those of reported TMD materials (typically less than 3). Our results have revealed the fundamental relationship between the FPV effect and the strain gradients in low-dimensional materials and inspired further exploration of optoelectronic phenomena in strain-gradient engineered materials.