First-Principles Prediction of the Charge Mobility in Black Phosphorus Semiconductor Nanoribbons

Renaissance of elemental phosphorus materials: properties, synthesis, and applications in sustainable energy and environment

The polymorphism of phosphorus-based materials has garnered much research interest, and the variable chemical bonding structures give rise to a variety of micro and nanostructures. Among the different types of materials containing phosphorus, elemental phosphorus materials (EPMs) constitute the foundation for the synthesis of related compounds. EPMs are experiencing a renaissance in the post-graphene era, thanks to recent advancements in the scaling-down of black phosphorus, amorphous red phosphorus, violet phosphorus, and fibrous phosphorus and consequently, diverse classes of low-dimensional sheets, ribbons, and dots of EPMs with intriguing properties have been produced. The nanostructured EPMs featuring tunable bandgaps, moderate carrier mobility, and excellent optical absorption have shown great potential in energy conversion, energy storage, and environmental remediation. It is thus important to have a good understanding of the differences and interrelationships among diverse EPMs, their intrinsic physical and chemical properties, the synthesis of specific structures, and the selection of suitable nanostructures of EPMs for particular applications. In this comprehensive review, we aim to provide an in-depth analysis and discussion of the fundamental physicochemical properties, synthesis, and applications of EPMs in the areas of energy conversion, energy storage, and environmental remediation. Our evaluations are based on recent literature on well-established phosphorus allotropes and theoretical predictions of new EPMs. The objective of this review is to enhance our comprehension of the characteristics of EPMs, keep abreast of recent advances, and provide guidance for future research of EPMs in the fields of chemistry and materials science.

Narrow Directed Black Phosphorus Nanoribbons Produced by A Reformative Mechanical Exfoliation Approach

Black phosphorus nanoribbons (PNRs) are ideal candidates for constructing electronic and optoelectronic devices owing to their unique structure and high bandgap tunability. However, the preparation of high-quality narrow PNRs aligned along the same direction is very challenging. Here, a reformative mechanical exfoliation approach combining tape and polydimethylsiloxane (PDMS) exfoliations to fabricate high-quality, narrow, and directed PNRs with smooth edges for the first time is developed. In this method, partially-exfoliated PNRs are first formed on thick black phosphorus (BP) flakes via the tape exfoliation and further peeled off to obtain separated PNRs via the PDMS exfoliation. The prepared PNRs have widths from a dozen to hundreds of nanometers (down to 15 nm) and a mean length of 18 µm. It is found that the PNRs can align along a same direction and the length directions of directed PNRs are along the zigzag direction. The formation of PNRs is attributed to that the BP prefers to be unzipped along the zigzag direction and has an appropriate magnitude of interaction force with the PDMS substrate. The fabricated PNR/MoS₂ heterojunction diode and PNR field-effect transistor exhibit good device performance. This work provides a new pathway to achieve high-quality, narrow, and directed PNRs for electronic and optoelectronic applications.

Prediction of highly stable two-dimensional materials of boron and phosphorus: structural and electronic properties

The discovery of two-dimensional (2D) semiconducting materials has attracted broad research interest, owing to their wide applications in spintronics and optoelectronics. Group III-V 2D materials such as hexagonal boron nitride (h-BN) have been demonstrated with remarkable electronic properties. However, the 2D materials consisting of boron and phosphorus have not been comprehensively explored. Using global structural search combined with first-principles calculations, we have hereby theoretically predicted several stable and metastable boron phosphorus (B_mP_n) monolayer 2D compounds that have lower formation enthalpies (ΔH) than black phosphorus and α-bulk boron and could be formed at stoichiometries of m/n ≤ 1. Two of these 2D B_mP_n compounds, i.e., P2₁/m B₁P₃ and Cm B₂P₄, are confirmed to be thermodynamically stable, with bandgaps less than 2 eV. In particular, Cm B₂P₄ features a narrow bandgap of ∼0.609 eV, near the short wavelength infrared ray (SWIR) region, and it possesses anisotropic mechanical properties. Moreover, we have demonstrated that these compounds can be converted into half-metallic spin-polarized states through charge doping, which promises their applications in spintronic devices.

Pressure-stabilized graphene-like P layer in superconducting LaP2

MgB₂-type superconductors are of great interest in chemistry and condensed matter physics due to their superconductivity dominated by the structural unit of graphene-like B. However, this kind of material is absent in phosphides resulting from the inherent lone pair electrons of phosphorus. Here, we report that a pressure-stabilized LaP₂, isostructural to MgB₂, shows superconductivity with a predicted T_c of 22.2 K, which is the highest among those of already known transition metal phosphides. Besides the electron-phonon coupling of graphene-like P, alike the role of the B layer in MgB₂, La 5d/4f electrons are also responsible for the superconducting transition. The distinct P atomic arrangement is attributed to its sp² hybridization and out-of-plane symmetric distribution of lone pair electrons. On the other hand, its dynamically stabilized pressure reaches as low as 7 GPa, a desirable feature of pressure-induced superconductors. Although P is isoelectronic to N and As, we hereby find the different stable stoichiometries, structures, and electronic properties of La phosphides compared with La nitrides/arsenides at high pressures.

Carrier mobility of one-dimensional vanadium selenide (V2Se9) monolayer and nanoribbon systems: DFT study

Vanadium selenide (V₂Se₉) is a true one-dimensional (1D) crystal composed of atomic nanochains bonded by van der Waals (vdW) interactions. Recent experiments revealed the mechanical exfoliation of newly synthesized V₂Se₉. In this study, we predicted the electronic and transport properties of V₂Se₉through computational analyses. We calculated the intrinsic carrier mobility of V₂Se₉monolayers (MLs) and nanoribbons (NRs) using density functional theory and deformation potential theory. We found that the electron mobility of the two-dimensional (2D) (010)-plane ML of V₂Se₉is highly anisotropic, reachingμ2D,ze=1327cm²V^-1s^-1across the chain direction. The electron mobility of 1D NR systems in a (010)-plane ML of V₂Se₉along the chain direction continuously increased as the thickness increased from 1-chain to 4-chain NR (width below 3 nm). Interestingly, the electron mobility of 1D 4-chain NR along the chain direction (μ1D,xe=775cm²V^-1s^-1) was higher than that of a 2D (010)-plane ML (μ2D,xe=567cm²V^-1s^-1). These results demonstrate the potential of vdW-1D crystal V₂Se₉as a new nanomaterial for ultranarrow (sub-3 nm width) optoelectronic devices with high electron mobility.

Theoretical Study of Anisotropic Carrier Mobility for Two-Dimensional Nb2Se9 Material

Finding new materials with satisfying all the desired criteria for nanodevices is an extremely difficult work. Here, we introduce a novel Nb2Se9 material as a promising candidate, capable of overcoming some physical limitations, such as a suitable band gap, high carrier mobility, and chemical stability. Unlike graphene, it has a noticeable band gap and no dangling bonds at surfaces that deteriorate transport properties, owing to its molecular chain structure. Using density functional theory (DFT) calculations with deformation potential (DP) theory, we find that the electron mobility of 2D Nb2Se9 across the axis direction reaches up to 2.56 × 103 cm2 V-1 s-1 and is approximately 2.5-6 times higher than the mobility of other 2D materials, such as MoS2, black phosphorous, and InSe, at room temperature. Moreover, the mobility of 2D Nb2Se9 is highly anisotropic (μ a /μ c ≈ 6.5). We demonstrate the potential of 2D Nb2Se9 for applications in nanoscale electronic devices and, possibly, mid-infrared photodetectors.

Tuning the electronic properties of highly anisotropic 2D dangling-bond-free sheets from 1D V2Se9 chain structures

True one-dimensional (1D) van der Waals materials can form two-dimensional (2D) dangling-bond-free anisotropic surfaces. Dangling bonds on surfaces act as defects for transporting charge carriers. In this study, we consider true 1D materials to be V2Se9 chains, and then the electronic structures of 2D sheets composed of true 1D V2Se9 chains are calculated. The (010) plane has indirect bandgap with 0.757 eV (1.768 eV), while the (111̅) plane shows a nearly direct bandgap of 1.047 eV (2.118 eV) for DFT-D3 (HSE06) correction, respectively. The (111̅) plane of V2Se9 is expected to be used in optoelectronic devices because it contains a nearly direct bandgap. Partial charge analysis indicates that the (010) plane exhibits interchain interaction is stronger than the (111̅) plane. To investigate the strain effect, we increased the interchain distance of planes until an indirect-to-direct bandgap transition occurred. The (010) plane then demonstrated a direct bandgap when interchain distance increased by 30%, while the (111̅) plane demonstrated a direct bandgap when the interchain distance increased by 10%. In mechanical sensors, this change in the bandgap was induced by the interchain distance.

Carbon phosphide nanosheets and nanoribbons: insights on modulating their electronic properties by first principles calculations

A carbon phosphide (CP) monolayer, a 2D structure derived from the same 3-fold coordination found both in graphene and phosphorene, has been successfully synthesized in an experiment recently. In this paper, we investigated the modulation of electronic structures and transport characteristics of 2D nanosheets and quasi-1D nanoribbons of CP nanomaterials in the α-phase by using first-principles density functional theory simulation. The calculated band structures show that the band gap of 2D CP nanosheets progressively increases as the uniform biaxial strain changes from compression to stretching. However, the biaxial strain cannot change the indirect band gap behavior of the original 2D CP nanosheet. In addition, the band structures of quasi-1D nanoribbons with different styles of H-passivated zigzag edges have also been studied. The results show that the H-passivated zigzag PC ribbons with two P edges are semiconductors with indirect band gaps, and the gaps decrease with increasing width of ribbons. However, the H-passivated CP nanoribbons with one P-atom terminated edge in combination with one P-atom edge, and H-passivated CC nanoribbons with two C-atom terminated edges display metallic behaviors. The semi-conductive or metallic behaviors of zigzag CP nanoribbons can be explained by presenting the wave function of their energy band around the Fermi level. Finally, the electronic transport properties of different CP nanoribbon based nanojunctions are studied in which arise the interesting negative differential resistance or rectification effects in their current-voltage characteristic curves.

Edge Defect-Free Anisotropic Two-Dimensional Sheets with Nearly Direct Band Gaps from a True One-Dimensional Van der Waals Nb2Se9 Material

Dangling-bond-free two-dimensional (2D) materials can be isolated from the bulk structures of one-dimensional (1D) van der Waals materials to produce edge-defect-free 2D materials. Conventional 2D materials have dangling bonds on their edges, which act as scattering centers that deteriorate the transport properties of carriers. Highly anisotropic 2D sheets, made of 1D van der Waals Nb2Se9 material, have three planar structures depending on the cutting direction of the bulk Nb2Se9 crystal. To investigate the applications of these 2D Nb2Se9 sheets, we calculated the band structures of the three planar sheets and observed that two sheets had nearly direct band gaps, which were only slightly greater (0.01 eV) than the indirect band gaps. These energy differences were smaller than the thermal energy at room temperature. The 2D Nb2Se9 plane with an indirect band gap had the shortest interchain distance for selenium ions among the three planes and exhibited significant interchain interactions on the conduction band. The interchain strain induced an indirect-to-direct band gap transition in the 2D Nb2Se9 sheets. These 2D sheets of Nb2Se9 with direct band gaps also had different band structures because of different interactions between chains, implying that they can have different charge mobilities. We expect these dangling-bond-free 2D Nb2Se9 sheets to be applied in optoelectronic devices because they allow for nearly direct band gaps. They can also be used in mechanical sensors because the band gaps can be controlled by varying the interchain strain.

Synthesis and electrical properties of single crystalline black phosphorus nanoribbons

Single crystal black phosphorus nanoribbons along the zigzag direction have been successfully grown by chemical vapor transport.

First-Principles Study of Gas Molecule Adsorption on C-doped Zigzag Phosphorene Nanoribbons

Phosphorene, due to its large surface-to-volume ratio and high chemical activity, shows potential application for gas sensing. In order to explore its sensing performance, we have performed the first-principles calculations based on density functional theory (DFT) to investigate the perfect and C-doped zigzag phosphorene nanoribbons (C-ZPNRs) with a series of small gas molecules (NH3, NO, NO2, H2, O2, CO, and CO2) adsorbed. The calculated results show that NH3, CO2, O2 gas molecules have relatively larger adsorption energies than other gas molecules, indicating that phosphorene is more sensitive to these gas molecules. For C-ZPNRs configuration, the adsorption energy of NO and NO2 increase and that of other gas molecules decrease. Interestingly, the adsorption energy of hydrogen is −0.229 eV, which may be suitable for hydrogen storage. It is hoped that ZPNRs may be a good sensor for (NH3, CO2 and O2) and C-ZPNRs may be useful for H2 storage.

Stability and carrier transport properties of phosphorene-based polymorphic nanoribbons

Few-layer black phosphorene has recently attracted significant interest in the scientific community. In this paper, we consider several polymorphs of phosphorene nanoribbons (PNRs) and employ deformation potential theory within the effective mass approximation, together with density functional theory, to investigate their structural, mechanical and electronic properties. The results show that the stability of a PNR strongly depends on the direction along which it can be cut from its 2D counterpart. PNRs also exhibit a wide range of line stiffnesses ranging from 6 × 10¹⁰ eV m^-1 to 18 × 10¹¹ eV m^-1, which has little dependence on the edge passivation. Likewise, the calculated electronic properties of PNRs show them to be either a narrow-gap semiconductor (E _g < 1 eV) or a wide-gap semiconductor (E _g > 1 eV). The carrier mobility of PNRs is found to be comparable to that of black phosphorene. Some of the PNRs show an n-type (p-type) semiconducting character owing to their higher electron (hole) mobility. Passivation of the edges leads to n-type ↔ p-type transition in many of the PNRs considered. The predicted novel characteristics of PNRs, with a wide range of mechanical and electronic properties, make them potentially suitable for use in nanoscale devices.

Direct observation of thermal disorder and decomposition of black phosphorus

Theoretical research has been devoted to reveal the properties of black phosphorus as a two-dimensional nanomaterial, but little attention has been paid for the experimental characterization. In this study, the thermal disorder and decomposition of black phosphorus were examined using in situ heating transmission electron microscopy experiments. We observed that the breaking of crystallographic symmetry begins at 380 °C under vacuum condition, followed by the phosphorus evaporates after long-term heating at 400 °C. This decomposition process can be initiated by the surficial vacancy and proceeds toward both interlayer ([010]) and intralayer ([001]) directions. The results on the thermal behavior of black phosphorus provide useful guidance for thin film deposition and fabrication processes with black phosphorus.

Electronic and thermoelectric properties of atomically thin C3Si3/C and C3Ge3/C superlattices

The nanostructuring of graphene into superlattices offers the possibility of tuning both the electronic and thermal properties of graphene. Using classical and quantum mechanical calculations, we have investigated the electronic and thermoelectric properties of the atomically thin superlattice of C₃Si₃/C (C₃Ge₃/C) formed by the incorporation of Si (Ge) atoms into graphene. The bandgap and phonon thermal conductivity of C₃Si₃/C (C₃Ge₃/C) are 0.54 (0.51) eV and 15.48 (12.64) W m^-1 K^-1, respectively, while the carrier mobility of C₃Si₃/C (C₃Ge₃/C) is 1.285 × 10⁵ (1.311 × 10⁵) cm² V^-1 s^-1 at 300 K. The thermoelectric figure of merit for C₃Si₃/C (C₃Ge₃/C) can be optimized via the tuning of carrier concentration to obtain the prominent ZT value of 1.95 (2.72).

Negative/zero thermal expansion in black phosphorus nanotubes

The thermal expansion of BPNTs has been given a comprehensive study. It is found that the abnormal thermal expansion can be tunable by changing diameter and bonding in the unique configurations of armchair and zigzag BPNTs.

Stability and carrier mobility of organic-inorganic hybrid perovskite CH3NH3PbI3 in two-dimensional limit

Recently, atomically thin organic-inorganic hybrid perovskites have been synthesized experimentally, which opens up new opportunities for exploring their novel properties in the 2D limit. Based on the comparative density functional theory calculation with and without spin-orbit coupling effects, the stability, electronic structure, and carrier mobility of the two-dimensional organic-inorganic hybrid perovskites MAPbI₃ (MA = CH₃NH₃) have been investigated systemically. Two single-unit-cell-thick 2D MAPbI₃ terminated by PbI₂ and CH₃NH₃I are constructed, and their thermodynamic stabilities are also evaluated using the first-principles constrained thermodynamics method. Our results indicate that both 2D MAPbI₃ with different terminations can be stable under certain conditions and have a suitable direct bandgap. Moreover, they are also found to have termination-dependent band edge and carrier mobility. The acoustic-phonon-limited carrier mobilities estimated using the deformation theory and effective mass approximation are on the order of thousands of square centimeters per volt per second and also highly anisotropic. These results indicate that 2D MAPbI₃ are competitive candidates for low-dimensional photovoltaic applications.

High-Mobility Transport Anisotropy in Few-Layer MoO3 and Its Origin

The novel two-dimensional semiconductors with high carrier mobility and excellent stability are essential to the next-generation high-speed and low-power nanoelectronic devices. Because of the natural abundance, intrinsic gap, and chemical stability, metal oxides were also recently suggested as potential candidates for electronic materials. However, their carrier mobilities are typically on the order of tens of square centimeters per volt per second, much lower than that for commonly used silicon. By using first-principles calculations and deformation potential theory, we have predicted few-layer MoO₃ as chemically stable wide-band-gap semiconductors with a considerably high acoustic-phonon-limited carrier mobility above 3000 cm² V^-1 s^-1, which makes them promising candidates for both electron- and hole-transport applications. Moreover, we also find a large in-plane anisotropy of the carrier mobility with a ratio of about 20-30 in this unusual system. Further analysis indicates that, because of the unique charge density distribution of whole valence electrons and the states near the band edge, both the elastic modulus and deformation potential are strongly directionally dependent. Also, the predicted high-mobility transport anisotropy of few-layer MoO₃ can be attributed to the synergistic effect of the anisotropy of the elastic modulus and deformation potential. Our results not only give an insightful understanding for the high carrier mobility observed in few-layer MoO₃ systems but also reveal the importance of the carrier-transport direction to the device performance.

ψ-Phosphorene: a new allotrope of phosphorene

Proposal of a new phosphorene allotrope with suitable geometrical and electronic properties for application in gas purification and solar cells.

Hydrogenations and electric field induced magnetic behaviors in armchair silicene nanoribbons

Using the first-principles calculations, we investigate the geometric, electronic and magnetic properties of armchair silicene nanoribbons with different edge hydrogenations. Our results show that the interesting magnetic behaviors such as the bipolar magnetic semiconductor can be found. Moreover, the addition of the transverse electric field can modulate the bipolar magnetic semiconductor to half-metal or spin-splitting metal. And the spin-up electrons are localized at one edge, the spin-down holes localized at the opposite edge under the external electric field. These results may present a new avenue for band engineering of silicene nanoribbons and benefit the design of silicon-based nano-spin-devices in nanoelectronics.