Few-layer Black Phosphorus Research Articles

Electronic structures of air-exposed few-layer black phosphorus by optical spectroscopy

The electronic structure of few-layer(FL) black phosphorus(BP) sensitively depends on the sample thickness, strain and doping. In this paper, we show that it is also vulnerable to air-exposure. The oxidation of BP caused by air-exposure gives several optical signatures, including the broadening of resonance peaks and increased Stokes shift between infrared (IR) absorption and photoluminescence (PL) peaks. More importantly, air exposure causes blue shifts of all resonance peaks in IR absorption and PL spectra, with more prominent effects for thinner samples and higher order subband transitions. Our study alludes a convenient and exotic way for band-structure engineering of FL-BP through controllable air-exposure or defect creation.

Semiclassical Boltzmann transport theory of few-layer black phosphorus in various phases

Black phosphorus (BP), a two-dimensional (2D) van der Waals layered material composed of phosphorus atoms, has been one of the most actively studied 2D materials in recent years due to its tunable energy band gap (tunable even to a negative value) and its highly anisotropic electronic structure. Depending on the sign of the band gap tuning parameter, few-layer BP can be in a gapped insulator phase, gapless Dirac semimetal phase, or gapless semi-Dirac transition point between the two phases. Using the fully anisotropic multiband Boltzmann transport theory, we systematically study the dc conductivity of few-layer BP as a function of the carrier density and temperature by varying the band gap tuning parameter, and determine the characteristic density and temperature dependence corresponding to each phase.

Air-Stable Polyphosphazene-Functionalized Few-Layer Black Phosphorene for Flame Retardancy of Epoxy Resins.

Similar to graphene, few-layer black phosphorus (BP) features thermal stability, mechanical properties, and characteristic dimension effects, which has potential as a new member of nanofillers for fabricating polymer nanocomposites. Herein, a cross-linked polyphosphazene-functionalized BP (BP-PZN) is developed with abundant -NH2 groups via a one-pot polycondensation of 4,4'-diaminodiphenyl ether and hexachlorocyclotriphosphazene on the surface of BP nanosheets. Whereafter, the resulting BP-PZN is incorporated into epoxy resin (EP) to study the flame-retardant property and smoke suppression performance. Cone results show that the introduction of 2 wt% BP-PZN distinctly improves the flame-retardant property of EP, for instance, 59.4% decrease in peak heat release rate and 63.6% reduction in total heat release. The diffusion of pyrolysis products from EP during combustion is obviously suppressed after incorporating the BP-PZN nanosheets. Meanwhile, the EP/BP-PZN nanocomposites exhibit air stability after exposure to ambient conditions for four months. The air stability of the BP nanosheets in EP matrix is assigned to surface wrapping by PZN and embedded in the polymer matrix as dual protection. As a new member of the 2D nanomaterials, BP nanosheets have potential to be a new choice for fabricating high-performance nanocomposites.

Evaluating the Surface Chemistry of Black Phosphorus during Ambient Degradation.

Black phosphorus (BP) is emerging as a promising candidate for electronic, optical, and energy storage applications. However, its poor ambient stability remains a critical challenge. Evaluation of few-layer liquid-exfoliated BP during ambient exposure using X-ray photoelectron spectroscopy and attenuated total reflectance Fourier transform infrared spectroscopy allows its surface chemistry to be investigated. Oxidation of liquid-exfoliated few-layer BP initially occurs through nonbridging oxide species, which convert to bridging oxide species after ambient exposure. We demonstrate the instability of these bridging oxide species, which undergo hydrolysis to form volatile phosphorus oxides and evaporate from the BP surface. FTIR spectroscopy, scanning transmission electron microscopy, and atomic force microscopy were used to confirm the formation of liquid oxides through a continuous oxidation cycle that results in the decomposition of BP. Furthermore, we show that the instability of few-layer BP originates from the formation of bridging oxide species.

Ammonia Synthesis Under Ambient Conditions: Selective Electroreduction of Dinitrogen to Ammonia on Black Phosphorus Nanosheets.

Constructing efficient catalysts for the N2 reduction reaction (NRR) is a major challenge for artificial nitrogen fixation under ambient conditions. Herein, inspired by the principle of "like dissolves like", it is demonstrated that a member of the nitrogen family, well-exfoliated few-layer black phosphorus nanosheets (FL-BP NSs), can be used as an efficient nonmetallic catalyst for electrochemical nitrogen reduction. The catalyst can achieve a high ammonia yield of 31.37 μg h-1 mg-1 cat. under ambient conditions. Density functional theory calculations reveal that the active orbital and electrons of zigzag and diff-zigzag type edges of FL-BP NSs enable selective electrocatalysis of N2 to NH3 via an alternating hydrogenation pathway. This work proves the feasibility of using a nonmetallic simple substance as a nitrogen-fixing catalyst and thus opening a new avenue towards the development of more efficient metal-free catalysts.

Raman and electrical transport properties of few-layered arsenic-doped black phosphorus.

Black phosphorus (b-P) is an allotrope of phosphorus whose properties have attracted great attention. In contrast to other 2D compounds, or pristine b-P, the properties of b-P alloys have yet to be explored. In this report, we present a detailed study on the Raman spectra and on the temperature dependence of the electrical transport properties of As-doped black phosphorus (b-AsP) for an As fraction x = 0.25. The observed complex Raman spectra were interpreted with the support of Density Functional Theory (DFT) calculations since each original mode splits in three due to P-P, P-As, and As-As bonds. Field-effect transistors (FET) fabricated from few-layered b-AsP exfoliated onto Si/SiO2 substrates exhibit hole-doped like conduction with a room temperature ON/OFF current ratio of ∼103 and an intrinsic field-effect mobility approaching ∼300 cm2 V-1 s-1 at 300 K which increases up to 600 cm2 V-1 s-1 at 100 K when measured via a 4-terminal method. Remarkably, these values are comparable to, or higher, than those initially reported for pristine b-P, indicating that this level of As doping is not detrimental to its transport properties. The ON to OFF current ratio is observed to increase up to 105 at 4 K. At high gate voltages b-AsP displays metallic behavior with the resistivity decreasing with decreasing temperature and saturating below T ∼100 K, indicating a gate-induced insulator to metal transition. Similarly to pristine b-P, its transport properties reveal a high anisotropy between armchair (AC) and zig-zag (ZZ) directions. Electronic band structure computed through periodic dispersion-corrected hybrid Density Functional Theory (DFT) indicate close proximity between the Fermi level and the top of the valence band(s) thus explaining its hole doped character. Our study shows that b-AsP has potential for optoelectronics applications that benefit from its anisotropic character and the ability to tune its band gap as a function of the number of layers and As content.

Anisotropic buckling of few-layer black phosphorus

When a two-dimensional material, adhered onto a compliant substrate, is subjected to compression it can undergo a buckling instability yielding to a periodic rippling. Interestingly, when black phosphorus (bP) flakes are compressed along the zig-zag crystal direction, the flake buckles forming ripples with a 40% longer period than that obtained when the compression is applied along the armchair direction. This anisotropic buckling stems from the puckered honeycomb crystal structure of bP and a quantitative analysis of the ripple period allows the determination of the Youngs's modulus of few-layer bP along the armchair direction (EbP_AC = 35.1 ± 6.3 GPa) and the zig-zag direction (EbP_ZZ = 93.3 ± 21.8 GPa).

Directly anchoring 2D NiCo metal–organic frameworks on few-layer black phosphorus for advanced lithium-ion batteries

A 2D few-layer black phosphorus/NiCo MOF (BP/NiCo MOF) hybrid is rationally designed and directly utilized as a lithium-ion battery anode.

Large-yield exfoliation of few-layer black phosphorus nanosheets in liquid

A novel method for large-scale, few-layer phosphorus nanosheets and their potential antibacterial application.

Natural organic matter inhibits aggregation of few-layered black phosphorus in mono- and divalent electrolyte solutions

NOM coating enabled stabilization of BPs in CaCl2 solutions via steric repulsion and isolating Ca2+ ions from interaction with BPs.

Gate-tunable large spin polarization in a few-layer black phosphorus-based spintronic device.

We report a first principles study of the gate tunable spin transport properties in trilayer black phosphorus (BP) placed on a Ni(100) electrode. In the proposed spintronic device, the spin-dependent conductance and the corresponding spin polarization can be largely modulated by the gate voltage. Instead of resorting to the gate tunable spin orbital interaction (SOI) mechanism in the traditional spintronic devices, the control scheme here is achieved by the giant Stark effect induced semiconducting-metallic phase transition of the few-layer BP. It is found that the quantum tunneling effect is gradually reduced as the gate voltage increases. During this process, the conductances of spin up and spin down components increase differently and hence the spin polarization is largely varied from 7% to 77% in parallel configuration (PC) and from 1% to 54% in anti-parallel configuration (APC). More importantly, the maximum magnetoresistance (MR) value can reach as large as 80%. Finally, we find that the non-equilibrium spin-polarized current can also be controlled by the external bias voltage. For instance, a large turn on/off ratio can be realized by the gate voltage (4239 for spin down current and 1809 for spin up current) when the bias voltage is fixed at 0.01 V. Our theoretical findings pave a feasible way for the novel application of few-layer BP in spintronic nanodevices.

Complementary Black Phosphorus Tunneling Field-Effect Transistors.

Band-to-band tunneling field-effect transistors (TFETs) have emerged as promising candidates for low-power integration circuits beyond conventional metal-oxide-semiconductor field-effect transistors (MOSFETs) and have been demonstrated to overcome the thermionic limit, which results intrinsically in sub-threshold swings of at least 60 mV/dec at room temperature. Here, we demonstrate complementary TFETs based on few-layer black phosphorus, in which multiple top gates create electrostatic doping in the source and drain regions. By electrically tuning the doping types and levels in the source and drain regions, the device can be reconfigured to allow for TFET or MOSFET operation and can be tuned to be n-type or p-type. Owing to the proper choice of materials and careful engineering of device structures, record-high current densities have been achieved in 2D TFETs. Full-band atomistic quantum transport simulations of the fabricated devices agree quantitatively with the current-voltage measurements, which gives credibility to the promising simulation results of ultrascaled phosphorene TFETs. Using atomistic simulations, we project substantial improvements in the performance of the fabricated TFETs when channel thicknesses and oxide thicknesses are scaled down.

Performance and Reliability Improvement under High Current Densities in Black Phosphorus Transistors by Interface Engineering.

Few-layer black phosphorus (BP) has recently emerged as a promising two-dimensional (2D) material for electronic and optoelectronic devices because of its high mobility and tunable band gap. However, BP is known to quickly degrade and oxidize in ambient conditions by breaking of the P-P bonds. As a result, there is a growing need to encapsulate BP that avoids oxygen and water while retaining the high electric performance of the devices. Here, we demonstrate a hydrophobic polymer encapsulation technique with improved thermal conductivity for high current density, which preserves the electrical properties of BP back-gate transistors compared to the commonly used Al2O3 encapsulation with improved mobility and minimal traps. The on-off ratio increases by more than an order of magnitude at room temperature and more than 4 orders of magnitude at cryogenic temperatures. High field transport shows the first systematic study on unprecedented breakdown characteristics up to -5.5 V for the 0.16 μm transistors with a high current of 1.2 mA/μm at 20 K. These discoveries open up a new way to achieve high-performance 2D semiconductors with significantly improved breakdown voltage, on-off ratios, and stability under ambient conditions for practical applications in electronic and optoelectronic devices.

Anisotropic Quantum Well Electro-Optics in Few-Layer Black Phosphorus.

The incorporation of electrically tunable materials into photonic structures such as waveguides and metasurfaces enables dynamic, electrical control of light propagation at the nanoscale. Few-layer black phosphorus is a promising material for these applications due to its in-plane anisotropic, quantum well band structure, with a direct band gap that can be tuned from 0.3 to 2 eV with a number of layers and subbands that manifest as additional optical transitions across a wide range of energies. In this Letter, we report an experimental investigation of three different, anisotropic electro-optic mechanisms that allow electrical control of the complex refractive index in few-layer black phosphorus from the mid-infrared to the visible: Pauli-blocking of intersubband optical transitions (the Burstein-Moss effect); the quantum-confined Stark effect; and the modification of quantum well selection rules by a symmetry-breaking, applied electric field. These effects generate near-unity tuning of the BP oscillator strength for some material thicknesses and photon energies, along a single in-plane crystal axis, transforming absorption from highly anisotropic to nearly isotropic. Lastly, the anisotropy of these electro-optical phenomena results in dynamic control of linear dichroism and birefringence, a promising concept for active control of the complex polarization state of light, or propagation direction of surface waves.

Anisotropic Electron-Phonon Interactions in Angle-Resolved Raman Study of Strained Black Phosphorus.

Few-layer black phosphorus (BP) with an in-plane puckered crystalline structure has attracted intense interest for strain engineering due to both its significant anisotropy in mechanical and electrical properties and its high intrinsic strain limit. Here, we investigated the phonon response of few layer BP under uniaxial tensile strain (∼7%) with in situ polarized Raman spectroscopy. Together with the first-principles density functional theory (DFT) analysis, the anisotropic Poisson's ratio in few-layer BP was verified as one of the primary factors that caused the large discrepancy in the trend of reported Raman frequency shift for strained BP, armchair (AC) direction in particular. By carefully including and excluding the anisotropic Poisson's ratio in the DFT emulations, we rebuilt both trends reported for Raman mode shifts. Furthermore, the angle-resolved Raman spectroscopy was conducted in situ under tensile strain for systematic investigation of the in-plane anisotropy of BP phonon response. The experimentally observed thickness and crystallographic orientation dependence is elaborated using DFT theory as having a strong correlation between the strain-perturbated electronic-band structure and the phonon vibration modes. This study provides insight, both experimentally and theoretically, for the complex electron-phonon interaction behavior in strained BP, which enables diverse possibilities for the strain engineering of electrical and optical properties in BP and similar two-dimensional nanomaterials.

Performance Improvement in Hydrogenated Few-Layer Black Phosphorus Field-Effect Transistors☆☆Supported by the National Natural Science Foundation of China under Grant Nos 61474027 and 61774041.

A capping layer for black phosphorus (BP) field-effect transistors (FETs) can provide effective isolation from the ambient air; however, this also brings inconvenience to the post-treatment for optimizing devices. We perform low-temperature hydrogenation on Al2O3 capped BP FETs. The hydrogenated BP devices exhibit a pronounced improvement of mobility from 69.6 to 107.7 cm2v−1s−1, and a dramatic decrease of subthreshold swing from 8.4 to 2.6 V/dec. Furthermore, high/low frequency capacitance–voltage measurements suggest reduced interface defects in hydrogenated BP FETs. This could be due to the passivation of interface traps at both Al2O3/BP and BP/SiO2 interfaces with hydrogen revealed by secondary ion mass spectroscopy.

Black phosphorus inverter devices enabled by in-situ aluminum surface modification

Two-dimensional black phosphorus (BP) generally exhibits a hole-dominated transport characteristic when configured as field-effect transistor devices. The effective control of charge carrier type and concentration is very crucial for the application of BP in complementary electronics. Herein, we report a facile and effective electron doping methodology on BP, through in situ surface modification with aluminum (Al). The electron mobility of few-layer BP is found to be largely enhanced to ∼ 10.6 cm2·V–1·s–1 by over 6 times after aluminum modification. In situ photoelectron spectroscopy characterization reveals the formation of Al–P covalent bond at the interface, which can also serve as local gate to tune the transport properties in BP layers. Finally, a spatially-controlled aluminum doping technique is employed to establish a p–n homojunction on a single BP flake, and hence to realize the complementary inverter devices, where the highest gain value of ∼ 33 is obtained.

Black phosphorus: A novel nanoplatform with potential in the field of bio-photonic nanomedicine

Single- or few-layer black phosphorus (FLBP) has attracted great attentions in scientific community with its excellent properties, including biodegradability, unique puckered lattice configuration, attractive electrical properties and direct and tunable band gap. In recent years, FLBP has been widely studied in bio-photonic fields such as photothermal and photodynamic therapy, drug delivery, bioimaging and biosensor, showing attractive clinical potential. Because of the marked advantages of FLBP nanomaterials in bio-photonic fields, this review article reviews the latest advances of biomaterials based on FLBP in biomedical applications, ranging from biocompatibility, medical diagnosis to treatment.

Wideband wavelength-tunable ultrafast fiber laser based on black phosphorus saturable absorber

We report a wideband wavelength-tunable ultrafast fiber laser mode-locked with a few-layer black phosphorus (BP) saturable absorber. The proposed laser not only delivers picosecond stable conventional solitons but also exhibits flexible wavelength tunability from 1529 to 1592 nm. It is experimentally demonstrated that the broadband saturable absorption property of BP and birefringence filtering effects in the cavity contribute significantly to the aforementioned merits. In addition, the round-trip model is established to numerically demonstrate the spectral evolution, and numerical results are in good agreement with experimental observations. The proposed laser system is compact and wavelength-tunable, which provides an efficient solution for applications where a widely tunable ultrafast fiber laser is required.

Electronic, optical property and carrier mobility of graphene, black phosphorus, and molybdenum disulfide based on the first principles**Project supported by the National Key R&D Program of China (Grant No. 2017YFB0305800).

The band structure, density of states, optical properties, carrier mobility, and loss function of graphene, black phosphorus (BP), and molybdenum disulfide (MoS2) were investigated by the first-principles method with the generalized-gradient approximation. The graphene was a zero-band-gap semiconductor. The band gaps of BP and MoS2 were strongly dependent on the number of layers. The relationships between layers and band gap were built to predict the band gap of few-layer BP and MoS2. The absorption showed an explicit anisotropy for light polarized in (1 0 0) and (0 0 1) directions of graphene, BP, and MoS2. This behavior may be readily detected in spectroscopic measurements and exploited for optoelectronic applications. Moreover, graphene (, BP (, and MoS2 ( have high carrier mobility. These results show that graphene, BP, and MoS2 are promising candidates for future electronic applications.