Few-layer Black Phosphorus Research Articles

Polymer Ionic Liquid Stabilized Black Phosphorus for Environmental Robust Flexible Optoelectronics

AbstractBlack phosphorus (BP) has attracted great attentions in the last few years, but their applications in optoelectronics have been strongly hindered by the poor environmental stability, especially under light irradiation. Herein, the fabrication of surface passivated few‐layer BP assisted by polymer ionic liquids (PILs) is reported. The PIL‐modified 2D BP exhibits excellent stability under ambient conditions, with negligible deterioration in 100 d. Furthermore, the PILs attached on the BP surface provide reliable and flexible electrical contact between the few‐layer BP and other device components. As a proof of concept, PIL‐modified BP nanosheets are successfully applied in flexible photodetectors, which show high flexibility, good detectivity with no obvious performance deterioration in 120 h. This work demonstrates that the PIL modification can endow BP nanosheets with excellent environmental stability and good conductivity, so that may significantly expand the applications of BP in flexible optoelectronics.

Stabilizing black phosphorus nanosheets via edge-selective bonding of sacrificial C60 molecules

Few-layer black phosphorus (BP) with an anisotropic two-dimensional (2D)-layered structure shows potential applications in photoelectric conversion and photocatalysis, but is easily oxidized under ambient condition preferentially at its edge sites. Improving the ambient stability of BP nanosheets has been fulfilled by chemical functionalization, however this functionalization is typically non-selective. Here we show that edge-selective functionalization of BP nanosheets by covalently bonding stable C60 molecules leads to its significant stability improvement. Owing to the high stability of the hydrophobic C60 molecule, C60 functions as a sacrificial shield and effectively protects BP nanosheets from oxidation under ambient condition. C60 bonding leads to a rapid photoinduced electron transfer from BP to C60, affording enhanced photoelectrochemical and photocatalytic activities. The selective passivation of the reactive edge sites of BP nanosheets by sacrificial C60 molecules paves the way toward ambient processing and applications of BP.

Quantitative Tracking of the Oxidation of Black Phosphorus in the Few-Layer Regime.

Previous theoretical reports have described the oxidation of few-layer black phosphorus and its effects on the electronic properties. Theoretically, native oxide layers bring opportunities for band gap engineering, but the detection of the different types of oxides is still a challenge at the experimental level. In this work, we uncover a correlation between thermal processes and Raman shift for the Ag1, B2g, and Ag2 vibrational modes. The thermal expansion coefficients (temperature range, 290–485 K) for the Ag1, B2g, and Ag2 were −0.015, −0.027, and −0.028 cm–1 K–1, respectively. Differential scanning calorimetry analysis shows an endothermic process centered at 528 K, and it was related with a mass increase according to thermogravimetric analysis. Raman shift temperature dependence was correlated to theoretical lattice thermal expansion, and a significant deviation was detected in the stacking direction at 500 K.

Electrically tunable optical properties of few-layer black arsenic phosphorus

Black arsenic phosphorus (b-AsP) is a promising two-dimensional material for various optoelectronic applications, bridging the wavelength gap between two-dimensional molybdenum disulfide and graphene. In particular, it has intriguing potential in photodetectors and great advantages in the mid-infrared field. However, its optoelectronic modulation has yet to be elucidated, which requires a fundamental understanding of its field-effect optical modulation. Here, we report the measurements of the lower-energy infrared anisotropic optical response of thin b-AsP under different electrical gating. We reveal that in addition to band edge absorption, amplitude modulation of sub-band absorption up to ten percent is also obtained in reflection extinction. These in-gap absorptions are attributed to spin–orbital coupling and free carrier absorption. Our results suggest the important potential for use of b-AsP in mid-infrared optoelectronic modulator applications.

Tunable bright interlayer excitons in few-layer black phosphorus based van der Waals heterostructures

Few-layer black phosphorus (BP) is a direct band gap material with large exciton binding energies, and shows great promise in optoelectronic applications. Here, we study the excitons in BP-based heterostructures with encapsulation and spacer 2D layers, using first principles GW and Bethe–Salpeter equation (BSE) methods. The 2D layers chosen are germanium sulfide (GeS) and hexagonal boron nitride (hBN), representing respectively strong and weak hybridization with BP. Except for hBN-encapsulated BP, all systems host bright interlayer (or indirect) excitons. In contrast to 2D indirect gap heterostructures, the interlayer excitons here are much brighter. Strong hybridization between GeS and BP increases the effective mass and room temperature exciton lifetimes. In contrast, the hBN spacer layer decouples the BP monolayers in BP/hBN/BP, resulting in the lowest energy exciton being dark. Surprisingly, however, BP/hBN/BP hosts interlayer BP excitons that are even brighter than those in bilayer BP. This lowest energy bright exciton lies very close in energy to the dark state, resulting in an increased effective lifetime. Our work uncovers the interplay between interlayer interactions and the physics of interlayer excitons, and paves the way for the use of bottom-up materials design to optimize the dipole oscillator strengths and lifetimes of interlayer excitons for excitonic device applications.

Silicon-Phosphorene Nanocavity-Enhanced Optical Emission at Telecommunications Wavelengths.

Generating and amplifying light in silicon (Si) continues to attract significant attention due to the possibility of integrating optical and electronic components in a single material platform. Unfortunately, silicon is an indirect band gap material and therefore an inefficient emitter of light. With the rise of integrated photonics, the search for silicon-based light sources has evolved from a scientific quest to a major technological bottleneck for scalable, CMOS-compatible, light sources. Recently, emerging two-dimensional materials have opened the prospect of tailoring material properties based on atomic layers. Few-layer phosphorene, which is isolated through exfoliation from black phosphorus (BP), is a great candidate to partner with silicon due to its layer-tunable direct band gap in the near-infrared where silicon is transparent. Here we demonstrate a hybrid silicon optical emitter composed of few-layer phosphorene nanomaterial flakes coupled to silicon photonic crystal resonators. We show single-mode emission in the telecommunications band of 1.55 μm ( Eg = 0.8 eV) under continuous wave optical excitation at room temperature. The solution-processed few-layer BP flakes enable tunable emission across a broad range of wavelengths and the simultaneous creation of multiple devices. Our work highlights the versatility of the Si-BP material platform for creating optically active devices in integrated silicon chips.

Gate-tunable weak antilocalization in a few-layer InSe

Indium selenide (InSe) has attracted tremendous research interest due to its high mobility and potential applications in next-generation electronics. However, the underlying transport mechanism of carriers in thin InSe at low temperatures remains unknown. Here we report the gate voltage and temperature-dependent magnetotransport properties of $\ensuremath{\gamma}$-InSe transistor devices with Hall mobility up to $2455\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ at the temperature of 1.7 K. We observe a gate-tunable weak antilocalization behavior at lower magnetic field $B$, which shows a transition to weak localization at higher $B$ region. We find that the magnetotransport data agree well with the Hikami-Larkin-Nagaoka theory. The conductivity and temperature dependence of phase-coherence length reveal that the electron-electron ($e\text{\ensuremath{-}}e$) interactions are dominated dephasing mechanism for electronic transport in $\ensuremath{\gamma}$-InSe at low temperatures. The maximum phase-coherence length is found to be 320 nm at 1.7 K, larger than that of monolayer $\mathrm{Mo}{\mathrm{S}}_{2}$ and few-layer black phosphorus. These results enrich the fundamental understanding of electronic transport properties of InSe.

Performance Enhancement of SPR Biosensor Based on Phosphorene and Transition Metal Dichalcogenides for Sensing DNA Hybridization

Two-dimensional (2-D) materials have attracted a lot of attention for using in surface plasmon resonance (SPR) biosensors. High sensitivity and figure of merit are two desirable parameters in sensor performance analysis. Using heterostructures of phosphorene, graphene, and transition metal dichalcogenides is proposed to enhance the sensitivity of biosensor. Although the proposed biosensor can be used for detection of different analytes with a wide refractive index range, DNA hybridization is the sensing target in this paper. Sensing parameters of different biosensor configurations based on a few-layer black phosphorus (BP) and other 2-D materials are compared with the conventional Au-based SPR sensor and the highest sensitivity of 187°/RIU for the structure consists of 10-layer BP and monolayer WS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> is obtained. The suggested sensor structure provides the sensitivity more than two times of the conventional biosensor. This high sensor performance may have potential applications in medical diagnosis and biochemical detection.

Passively Q-Switched Nd:GdVO4 1.3 μm Laser with Few-Layered Black Phosphorus Saturable Absorber

By using a few-layer black phosphorus (BP) as the saturable absorber (SA), a laser-diode end-pumped stable Q-switched Nd:GdVO4 laser operating at 1.3 μm was realized. A maximum average output power of 452 mW was obtained at 2.22 W absorbed pump power, corresponding to a 34.5% slope efficiency. The shortest pulse width, highest pulse repetition rate, and largest peak power were determined to be 72 ns, 625 kHz, and 10.04 W, respectively. To the best of our knowledge, this pulse duration was the shortest ever reported for passive Q-switched bulk lasers based on two-dimensional materials at 1.3 μm. Results suggested that BP can be used to achieve short laser pulses with high repetition rate at 1.3 μm spectral region.

Highly-sensitive gas sensor based on two-dimensional material field effect transistor

Atomically thin two-dimensional (2D) materials are ideal gas sensing materials for achieving an ultra-low detection limit, due to the high surface-to-volume ratio, low electronic noise and sensitively tunable Fermi level. However, the sensitivity of 2D materials to their surrounding environment may also severely degrade the long-term stability of sensing devices, since most of them use the same 2D material flake as both the sensing and conduction material. In this work, we report a gas sensor based on a 2D material field effect transistor (FET) which uses few-layer black phosphorus (BP), boron nitride (BN) and molybdenum disulfide (MoS2) as the top-gate, dielectric layer and conduction channel, respectively. In this device configuration, the top-gate of BP with a superior gas adsorption capability serves as the sensing material, while the conduction channel of MoS2 is isolated from ambient environment by the coverage of the BN dielectric layer. The separation of the sensing and conduction materials not only improves the long-term stability of the device, but also enables us to use different materials for gas adsorption and conduction purposes to achieve optimum sensing performances. In addition, the adsorption kinetics of the gas molecules on the sensing channel can be sensitively detected by the current/resistance variation of the conduction channel, since the adsorbed gas molecules can effectively tune the Fermi level of sensing and conduction materials (BP and MoS2, respectively) through band alignment. We experimentally demonstrated that the proposed 2D material FET not only achieved a detection limit of 3.3 ppb to NO2, but was also capable to differentiate oxidizing and reducing gases.

Valley-Selective Linear Dichroism in Layered Tin Sulfide

Valley-related physics, optics, and electronics have been extensively investigated in transition metal dichalcogenides with a honeycomb lattice such as molybdenum disulfide. Their photoemission properties exhibit a circular dichroism due to energy degenerate but inequivalent valleys located at K and K′ in momentum space. Recently linear dichroism was observed in few-layer black phosphorus with highly anisotropic in-plane optical properties. Here, we report a novel valley-selective linear dichroism of the photoluminescence in layered thin-film tin sulfide (SnS). We identified two PL emission peaks, exhibiting an orthogonal linear dichroism, arising from two inequivalent valleys in the energy band. The photon emission from the Γ-X valley is completely x-polarized, while in contrast, the photon emission from the Γ-Y valley is purely y-polarized. This unique valley-selective linear dichroism in SnS is further confirmed by the band-edge absorption measurements along the Γ-X and Γ-Y directions, respectively. Ou...

Facile Synthesis of Highly Dispersed Co3O4 Nanoparticles on Expanded, Thin Black Phosphorus for a ppb-Level NO x Gas Sensor.

Expanded few-layer black phosphorus nanosheets (FL-BP NSs) were functionalized by branched polyethylenimine (PEI) using a simple noncovalent assembly to form air-stable overlayers (BP-PEI), and a Co3O4@BP-PEI composite was designed and synthesized using a hydrothermal method. The size of the highly dispersed Co3O4 nanoparticles (NPs) on the FL-BP NSs can be controlled. The BP-C5 (190 °C for 5 h) sensor, with 4-6 nm Co3O4 NPs on the FL-BP NSs, exhibited an ultrahigh sensitivity of 8.38 and a fast response of 0.67 s to 100 ppm of NO x at room temperature in air, which is 4 times faster than the response of the FL-BP NS sensor, and the lower detection limit reached 10 ppb. This study points to a promising method for tuning properties of BP-based composites by forming air-stable overlayers and highly dispersed metal oxide NPs for use in high-performance gas sensors.

Encapsulation-Free Stabilization of Few-Layer Black Phosphorus.

Under ambient conditions and in H2O and O2 environments, reactive oxygen species (ROS) cause immediate degradation of the mobility of few-layer black phosphorus (FLBP). Here, we show that FLBP degradation can be prevented by maintaining the temperature in the range ∼125-300 °C during ROS exposure. FLBP devices maintained at elevated temperature show no deterioration of electrical conductance, in contrast to the immediate degradation of pristine FLBP held at room temperature. Our results constitute the first demonstration of stable FLBP in the presence of ROS without requiring encapsulation or a protective coating. The stabilization method will enable applications based on the surface properties of intrinsic FLBP.

Acetone improves the topographical homogeneity of liquid phase exfoliated few-layer black phosphorus flakes

Liquid phase exfoliation of 2D materials has issues related to the sorption of the solvent, the oxidation of the sample during storage, and the topographical inhomogeneity of the exfoliated material. N-methyl-2-pyrrolidone (NMP), a common solvent for black phosphorus (BP) exfoliation, has additional drawbacks like the formation of by-products during sonication and poor solvent volatility. Here we demonstrate an improvement in the topographical homogeneity (i.e. thickness and lateral dimensions) of NMP-exfoliated BP flakes after resuspension in acetone. The typical size of monolayers and bilayers stabilised in acetone was 99.8 ± 27.4 nm and 159.1 ± 57 nm, respectively. These standard deviations represent a threefold improvement over those of the NMP-exfoliated originals. Phosphorene can also be exfoliated directly in acetone by very long ultrasonication. The product suspension enjoys the same dimensional homogeneity benefits, which confirms that this effect is an intrinsic property of the acetone–BP system. The quality and stability of the exfoliated flakes was checked by XRD, TEM, electron diffraction and Raman spectroscopy. Thermal expansion coefficients of the B2g and Raman modes were calculated for drop-casted samples as −0.018 28 cm−1 K–1, −0.030 56 cm−1 K–1 and −0.032 19 cm−1 K–1, respectively. The flakes withstand 20 min in O2 flow at 373 K without lattice distortion.

Covalent Functionalization of Few-Layer Black Phosphorus Using Iodonium Salts and Comparison to Diazonium Modified Black Phosphorus

Since the isolation of two-dimensional (2D) phosphorene, black phosphorus (BP) has gained popularity due to its high carrier mobility and tunable bandgap. Poor ambient stability of BP remains a key issue and impedes its use in electronic applications. Here we report a new stabilization strategy based on covalent functionalization of liquid exfoliated few-layer BP using aryl iodonium salts. Arylation of BP using iodonium salts enables covalent modification without inducing oxidation and alters the degradation chemistry of BP by inhibiting bridged oxygen formation through attachment to surface oxygen sites. In comparison, functionalization using aryl diazonium salts results in oxidation and aryl multilayer formation and does not adequately disrupt noncovalent solvent passivation. Aryl functionalization of BP using iodonium salts displays superior ambient stability compared to arylation using diazonium salts associated with greater covalent functionalization as characterized using X-ray photoelectron spectro...

Many-Body Effect and Device Performance Limit of Monolayer InSe.

Due to a higher environmental stability than few-layer black phosphorus and a higher carrier mobility than few-layer dichalcogenides, two-dimensional (2D) semiconductor InSe has become quite a promising channel material for the next-generation field-effect transistors (FETs). Here, we provide the investigation of the many-body effect and transistor performance scaling of monolayer (ML) InSe based on ab initio GW-Bethe-Salpeter equation approaches and quantum transport simulations, respectively. The fundamental band gap of ML InSe is indirect and 2.60 eV. The optical band gap of ML InSe is 2.50 eV for the in-plane polarized light, with the corresponding exciton binding energy of 0.58 eV. The ML InSe metal oxide semiconductor FETs (MOSFETs) show excellent performances with reduced short-channel effects. The on-current, delay time, and dynamic power indicator of the optimized n- and p-type ML InSe MOSFETs can satisfy the high-performance and low-power requirements of the International Technology Roadmap for Semiconductors 2013 both down to 3-5 nm gate length in the ballistic limit. Therefore, a new avenue is opened to continue Moore's law down to 3 nm by utilizing 2D InSe.

Effects of plasma-treatment on the electrical and optoelectronic properties of layered black phosphorus

Exfoliated few-layer black-phosphorus (BP) has been explored for a variety of electrical and optoelectronic applications. Plasma-assisted thinning of BP has emerged as an exciting pathway to achieve BP crystals of desired thickness. However, to fully realise the true potential of plasma-assisted thinning of BP and other emerging 2D materials, it is critical to understand the effects of different plasma environments on the electrical and optoelectronic properties of the resultant material. Here, we investigate the influence of Ar and O2 plasma on the electrical and optoelectronic properties of plasma-treated BP flakes. It is revealed that by manipulating the environment under which BP is exposed to the plasma, it is possible to engineer defects that lead to new photoluminescence (PL) emission peaks without compromising the switching ratios or carrier mobilities of BP-based field effect transistors (FETs). Overall, our study finds the use of O2 plasma as a more suitable approach to retain and enrich the intrinsic (opto)electronic properties of BP. Additionally, our study, for the first time, experimentally reveals the ability of BP to respond to UV excitation.

Photoluminescence Quenching in Self-Assembled CsPbBr3 Quantum Dots on Few-Layer Black Phosphorus Sheets.

An ordered self-assembly of CsPbBr3 quantum dots (QDs) was generated on the surface of few-layer black phosphorus (FLBP). Strong quenching of the QD fluorescence was observed, and analyzed by time-resolved photoluminescence (TR-PL) studies, DFT calculations, and photoconductivity measurements. Charge transfer by type I band alignment is suggested to be the cause of the observed effects.

Photoluminescence Quenching in Self‐Assembled CsPbBr3 Quantum Dots on Few‐Layer Black Phosphorus Sheets

AbstractAn ordered self‐assembly of CsPbBr3 quantum dots (QDs) was generated on the surface of few‐layer black phosphorus (FLBP). Strong quenching of the QD fluorescence was observed, and analyzed by time‐resolved photoluminescence (TR‐PL) studies, DFT calculations, and photoconductivity measurements. Charge transfer by type I band alignment is suggested to be the cause of the observed effects.

Realizing Long-Term Stability and Thickness Control of Black Phosphorus by Ambient Thermal Treatment.

Few-layer black phosphorus (BP) has shown great potential for next-generation electronics with tunable band gap and high carrier mobility. For the electronic applications, the thickness modulation of a BP flake is essential due to its thickness-dependent electronic properties. However, controlling the precise thickness of few-layer BP is a challenge for the high-performance device applications. In this study, we demonstrate that thermal treatment under ambient condition precisely controls the thickness of BP flake. The thermal etching method utilizes the chemical reactivity of BP surface with oxygen and water molecules by the repeated formation and evaporation of phosphoric acid during thermal annealing. Field-effect transistor of the thickness-modulated BP sheet by thermal etching method shows a high hole mobility of ∼576 cm2 V-1 s-1 and a high on-off ratio of ∼105. The stability of the BP devices remained for 1 month under ambient condition without an additional protecting layer, resulting from the preservation of active BP layers below native surface phosphorus oxide.