Stacking Fault Enriching the Electronic and Transport Properties of Few-Layer Phosphorenes and Black Phosphorus

S-Scheme Boron Phosphide/MoS2 Heterostructure with Excellent Light Conversion Ability for Solar Cells and Water Splitting Photocatalysts

Monolayer molybdenum disulfide (MoS2) with a suitable direct band gap and strong optical absorption is very attractive for utilization in solar cells and photocatalytic water splitting. Nevertheless, the broader utilization of MoS2 is impeded by its low carrier mobility and limited responsiveness to infrared light. To overcome these challenges, we constructed a variety of stackings for the boron phosphide (BP)/MoS2 van der Waals heterostructure (vdWH), all of which display S-scheme band alignments except for the AC' stacking. The constituent BP monolayer has superior carrier mobility and strong infrared and visible light response, which makes up for the shortcomings of MoS2. The study revealed that the AB stacking exhibits a remarkable power conversion efficiency of 22.27%, indicating its significant application prospect in solar cells. Additionally, the AB stacking also exhibits a promising application prospect in photocatalytic water splitting due to its suitable band structure, S-scheme band alignment, strong optical adsorption characteristic, high solar-to-hydrogen efficiency, and robust built-in electric field. Meanwhile, applying uniaxial tensile strains along the x-axis direction is more beneficial for photocatalytic water splitting. Hence, the AB-stacked BP/MoS2 vdWH shows significant potential for use in both solar cells and photocatalytic water splitting. This work paves the way for exploring the application of S-scheme heterostructures in solar energy conversion systems.

Neodymium (3+)-Coordinated Black Phosphorus Quantum Dots with Retrievable NIR/X-Ray Optoelectronic Switching Effect for Anti-Glioblastoma

Heteroatom interaction of atomically thin nanomaterials enables the improvement of electronic transfer, band structure, and optical properties. Black phosphorus quantum dots (BP QDs) are considered to be candidate diagnostic and/or therapeutic agents due to their innate biocompatibility and exceptional photochemical effects. However, BP QDs are not competitive regarding second near-infrared (NIR-II) window medical diagnosis and X-ray induced phototherapy. Here, an Nd³⁺ ion coordinated BP QD (BPNd) is synthesized with the aim to sufficiently improve its performances in NIR-II fluorescence imaging and X-ray induced photodynamic therapy, benefitting from the retrievable NIR/X-ray optoelectronic switching effects between BP QD and Nd³⁺ ion. Given its ultrasmall size and efficient cargo loading capacity, BPNd can easily cross the blood-brain barrier to precisely monitor the growth of glioblastoma through intracranial NIR-II fluorescence imaging and impede its progression by specific X-ray induced, synergistic photodynamic chemotherapy.

Pd Speciation on Black Phosphorene in CO and C2H4 Atmosphere: A First-principles Investigation

Deposited transition metal clusters and nanoparticles are widely used as catalysts and have long been thought stable in reaction conditions. We investigated the electronic structure and stability of freestanding and...

Large-scale growth of few-layer two-dimensional black phosphorus

Two-dimensional materials provide opportunities for developing semiconductor applications at atomistic thickness to break the limits of silicon technology. Black phosphorus (BP), as a layered semiconductor with controllable bandgap and high carrier mobility, is one of the most promising candidates for transistor devices at atomistic thickness^1-4. However, the lack of large-scale growth greatly hinders its development in devices. Here, we report the growth of ultrathin BP on the centimetre scale through pulsed laser deposition. The unique plasma-activated region induced by laser ablation provides highly desirable conditions for BP cluster formation and transportation^5,6, facilitating growth. Furthermore, we fabricated large-scale field-effect transistor arrays on BP films, yielding appealing hole mobility of up to 213 and 617 cm² V^-1 s^-1 at 295 and 250 K, respectively. Our results pave the way for further developing BP-based wafer-scale devices with potential applications in the information industry.

Robust charge spatial separation and linearly tunable band gap of low-energy tube-edge phosphorene nanoribbon

Versatile applications have been proposed for phosphorene nanoribbons (PNRs), whose properties depend strongly on the edge structures. Recently, a unique tube-reconstruction at the zigzag edge (ZZ[Tube]) of PNRs was discovered to be the lowest configuration. Therefore, studies on PNRs should be reconsidered. In this paper, we systemically explore the width and strain effects on zigzag PNRs with different edge structures, including ZZ[Tube], ZZ and ZZ[ad] edges. ZZ PNRs always have small band gaps which are nearly independent of both width and strain. A remarkable band gap exists in ZZ[ad] PNRs which increases with a decrease in the ribbon width but is not sensitive to strain. In contrast, the band gaps of ZZ[Tube] PNRs change from 1.08 to 0.70 eV as the width increases from 12 to 65 Å. In addition, the band gaps of ZZ[Tube] PNRs show a linear response under a certain range of strain. In addition, the carrier effective masses (0.50 m ₀ for electrons and 0.94 m ₀ for holes) of ZZ[Tube] PNRs are much lower than for ZZ[ad], and the VBM and CBM charges are robustly spatially separated even under strains ranging from -5% to 5%. Their ease of formation, lowest energy, light effective mass, linear band gap response to strain and robust charge spatial separation provide ZZ[Tube] PNRs with potentially excellent performance in microelectronic and opto-electric applications.

Hexagonal layered group IV-VI semiconductors and derivatives: fresh blood of the 2D family

New phases of group IV-VI semiconductors in 2D hexagonal structures are predicted and their unusual physical properties are revealed. The structures of monolayer group IV-VI semiconductors are similar to those of blue phosphorene and each unit has the same ten valence electrons. The band gap of 2D hexagonal group IV-VI semiconductors depends on both the thickness and stacking order. Atomic functionalization can induce ferromagnetism, and the Curie temperature can be tuned. Gapped Dirac fermions with zero mass are developed and this makes it exceed that of graphene. The Fermi velocity can be compared to or even above that of graphene.

Rectifying behavior in twisted bilayer black phosphorus nanojunctions mediated through intrinsic anisotropy

We explore the possibility of using van der Waals bonded heterostructures of stacked together 2D bilayer black phosphorus (BP) for nanoscale device applications. The electronic properties of BP in AA stacking and 90° twisted are studied with density functional theory. Furthermore, we study the homogeneous nanojunction architecture of BP to use its anisotropic properties. Using the first principles simulations along with the NEGF approach, we calculate the quantum transport properties of the nanojunction setup. The interlayer direction dependent current characteristics are explained in different setups. Our result revealed that the 90° twisted nanojunction device would be a potential rectifier despite having no p-n junction characteristics only due to the intrinsic anisotropy of the material, making tunneling between armchair- and zigzag-directional BP sheets asymmetric.

Physical Fingerprints of the 2O-tαP Phase in Phosphorene Stacking

The 2O-tαP phase is a bilayer phosphorene stacking twisted by ∼70.5° standing out from all the potential candidates predicted by our previous work. Here, by linear response theory, we directly verified that the 2O-tαP phase preserves the intrinsic features of phonon spectrum of the existing AB phase, reflecting a stable thermodynamic behavior. Then we provided three distinct fingerprints to help finding this new phase: upon comparison to the existing shifting bilayer phosphorene, the in-plane elastic constants showed a much weaker anisotropic response, providing a characteristic mechanical criterion; the calculated Raman spectrum revealed for the low frequency rang the layer-breathing mode and the out-of-plane twisted mode, L-A¹ and L-A², both of which together stabilize the twisted structure; in particular, the simulated scanning tunneling microscope image presented recognizable cross stripes, which should withstand an examination of exfoliated bilayer and few-layer black phosphorus.

The Electronic and Magnetic Properties of Multi-Atom Doped Black Phosphorene

Recently, substitutional doping is proved to be an effective route to induce magnetism to black phosphorene for its application in spintronics. Herein, we investigate the thermodynamic stability, electronic and magnetic properties of doped black phosphorene with multi Al or Cl atoms using first-principles calculations. We find these doped phosphorenes are thermodynamically stable at 0 K and the stability first improves and then deteriorates with the number of dopant atom increasing. Corresponding to the variety of stability, the amount of electrons transferred between impurity and neighboring phosphorus atoms also first increase and then reduce. However, the band gap of Al-doped phosphorene reduces monotonically from 0.44 eV to 0.13 eV while that of Cl-doped phosphorene first decreases from 0.10 eV to 0 and then becomes flat, which is a result of the impurity levels emerging and splitting. Besides, in doped phosphorenes with an even number of impurity atoms, the antiferromagnetic order is favored by energy. Through computing the magnetic moment and spin distribution, we further confirm the antiferromagnetic order existing only in the doped phosphorenes with two and four Cl atoms. These results may provide some help for future applications of black phosphorene in spintronics.

Photo-oxidative Degradation and Protection Mechanism of Black Phosphorus: Insights from Ultrafast Dynamics

The environmental instability and protection of black phosphorus (BP) is one of the most attractive hotspots in two-dimensional materials. The generation of superoxide is believed to be the key culprit, while the photogenerated electron dynamics is yet to be known. In this work, we carry out time domain ab initio nonadiabatic molecular dynamics to understand the photogenerated electron dynamics at the molecule/BP interface. It is found that oxygen can trap the photogenerated electrons of BP rapidly owing to strong electron-phonon (e-p) coupling and becomes an active superoxide under light illumination. A good protection layer, such as perylene diimide (PDI), has comparable capabilities of trapping photogenerated electrons to oxygen, which can efficiently prevent the formation of superoxide and thus suppress the degradation of BP. Moreover, PDI can enhance the separation of photogenerated electron-hole pairs of BP by prolonging the time of holding photogenerated electrons of BP. In contrast, 7,7,8,8-tetracyano- p-quinodimethane (TCNQ) has weak e-p coupling and a large band edge offset between trapping and donor states and thereby poor trapping ability for photogenerated electrons, leading to poor protection efficiency. This study provides the first in-depth understanding of the BP degradation and protection mechanism from excited-state dynamics and can be applicable to other 2D photo-oxidative degradation.

Quantum oscillation in carrier transport in two-dimensional junctions

Two-dimensional (2D) junction devices have recently attracted considerable attention. Here, we show that most 2D junction structures, whether vertical or lateral, act as a lateral monolayer-bilayer-monolayer junction in their operation. In particular, a vertical structure cannot function as a vertical junction as having been widely believed in the literature. Due to a larger electrostatic screening, the bilayer region in the junction always has a smaller bandgap than its monolayer counterpart. As a result, a potential well, aside from the usual potential barrier, will form universally in the bilayer region to affect the hole or electron quantum transport in the form of transmission or reflection. Taking black phosphorus as an example, our calculations using a non-equilibrium Green function combined with density functional theory show a distinct oscillation in the transmission coefficient in a two-electrode prototypical device, and the results can be qualitatively understood using a simple quantum well model.

CO Adsorption on Metal-Decorated Phosphorene

Using first principle calculations, we have investigated the adsorption of CO gas on various metal-decorated phosphorene. Almost all of the metals were considered to decorate phosphorene. By comparing binding energy (E _b) and cohesive energy (E _c), only 10 metals (Li, Na, K, Rb, Cs, Ca, Sr, Ba, Pd, and La) can stably decorate phosphorene and avoid clustering. CO adsorptions on these metal-decorated systems were calculated, and the mechanism of interaction between CO and metal atoms was analyzed in detail. E _a shows a significant improvement after metal decoration, excerpt for Rb and Cs. The results imply that Li-, Na-, K-, Ca-, Sr-, Ba-, and La-decorated phosphorene could be used as CO elimination or reversible CO storage.

Stacking sequences of black phosphorous allotropes and the corresponding few-layer phosphorenes

Possible bulk black phosphorus (BP) allotropes are constructed based on single-layer BP with various stacking sequences. Our stacking algorithm shows that there are eight possible allotropes with two stacking layers in their unit cells possessing relatively high symmetries, and six of them are retained after structural relaxation using a van der Waals correction of optB88-vdW. The AF, AG, and AH bulk structures are presented for the first time. The structural relationship of these configurations has been explained via an interlayer slipping process. The total energy of the AF allotrope is closest to the most stable bulk BP structure (AB stacking) among all explored 2-layer stacked bulk structures. The calculated band structure of the AF allotrope using HSE06 shows a direct band gap of 0.48 eV with anisotropic electronic structures. We also presented six possible BP allotropes with three stacking layers in their unit cells. The newly reported AAF and ABC stacked structures show semiconducting and metallic features, respectively. After the bulk structures were explored, we further built the corresponding few-layer phosphorene structures and investigated their electronic properties. The results show that all the few-layer phosphorenes show semiconducting features. The AE, AAE, and AEA phosphorenes have indirect band gaps while the other explored phosphorenes possess direct band gaps located at the Γ point.

Covalent Functionalization of Black Phosphorus from First-Principles

The chemical functionalization is proven to be an effective and controllable approach to modify the properties of black phosphorus (BP), and improve the air-stability of BP and its nanoelectronic applications [Nat. Chem., 2016, 8, 597]. However, covalent functionalization of BP and related properties are poorly understood. Here we present a theoretical investigation on the electronic structure and transport property of chemically modified BP. Our calculations reveal that the molecule modification generates a rather flat energy band within the bandgap, which leads to a reduced hole mobility of BP. Alternatively, we propose to use polymers bonded to BP surface, aiming at a balance between functionality and carrier mobility. The polymer-BP composites preserve both electron and hole mobility of pristine BP. Meanwhile, the stability of polymer-BP composites in ambient condition is enhanced as well.

Impact of edge states on device performance of phosphorene heterojunction tunneling field effect transistors

Black phosphorus (BP) tunneling field effect transistors (TFETs) using heterojunctions (Hes) are investigated by atomistic quantum transport simulations. It is observed that edge states have a great impact on the transport characteristics of BP He-TFETs, which results in the potential pinning effect and deterioration of gate control. However, the on-state current can be effectively enhanced by using hydrogen to saturate the edge dangling bonds in BP He-TFETs, by which means edge states are quenched. By extending layered BP with a smaller band gap to the channel region and modulating the BP thickness, the device performance of BP He-TFETs can be further optimized and can fulfil the requirements of the international technology road-map for semiconductors (ITRS) 2013 for low power applications. In 15 nm 3L-1L and 4L-1L BP He-TFETs along the armchair direction the on-state currents are over two times larger than the current required by ITRS 2013 and can reach above 10³ μA μm^-1 with the fixed off-state current of 10 pA μm^-1. It is also found that the ambipolar effect can be effectively suppressed in BP He-TFETs.

In situ TEM visualization of superior nanomechanical flexibility of shear-exfoliated phosphorene

Recently discovered atomically thin black phosphorus (called phosphorene) holds great promise for applications in flexible nanoelectronic devices. Experimentally identifying and characterizing nanomechanical properties of phosphorene are challenging, but also potentially rewarding. This work combines for the first time in situ transmission electron microscopy (TEM) imaging and an in situ micro-manipulation system to directly visualize the nanomechanical behaviour of individual phosphorene nanoflakes. We demonstrate that the phosphorene nanoflakes can be easily bent, scrolled, and stretched, showing remarkable mechanical flexibility rather than fracturing. An out-of-plane plate-like bending mechanism and in-plane tensile strain of up to 34% were observed. Moreover, a facile liquid-phase shear exfoliation route has been developed to produce such mono-layer and few-layer phosphorene nanoflakes in organic solvents using only a household kitchen blender. The effects of surface tensions of the applied solvents on the ratio of average length and thickness (L/T) of the nanoflakes were studied systematically. The results reported here will pave the way for potential industrial-scale applications of flexible phosphorene nanoelectronic devices.