Few-layer Black Phosphorus Research Articles

Coulomb interactions and screening effects in few-layer black phosphorus: a tight-binding consideration beyond the long-wavelength limit

Coulomb interaction and its screening play an important role in many physical phenomena of materials ranging from optical properties to many-body effects including superconductivity. Here, we report on a systematic study of dielectric screening in few-layer black phosphorus (BP), a two-dimensional material with promising electronic and optical characteristics. We use a combination of a tight-binding model and rigorously determined bare Coulomb interactions, which allows us to consider relevant microscopic effects beyond the long-wavelength limit. We calculate the dielectric function of few-layer BP in the random phase approximation and show that it exhibits strongly anisotropic behavior even in the static limit. We also estimate the strength of effective local and non-local Coulomb interactions and determine their doping dependence. We find that the pz states responsible for low-energy excitations in BP provide a moderate contribution to the screening, weakening the on-site Coulomb interaction by less that a factor of two. Finally, we calculate the full plasmon spectrum of few-layer BP and discuss the effects beyond long-wavelengths.

Deriving phosphorus atomic chains from few-layer black phosphorus

Phosphorus atomic chains, the utmost-narrow nanostructures of black phosphorus (BP), are highly relevant to the in-depth development of BP into one-dimensional (1D) regime. In this contribution, we report a top-down route to prepare atomic chains of BP via electron beam sculpting inside a transmission electron microscope (TEM). The growth and dynamics (i.e. rupture and edge migration) of 1D phosphorus chains are experimentally captured for the first time. Furthermore, the dynamic behaviors and associated energetics of the as-formed phosphorus chains are further corroborated by density functional theory (DFT) calculations. The 1D counterpart of BP will serve as a novel platform and inspire further exploration of the versatile properties of BP.

Black phosphorus mid-infrared photodetectors

Few-layer black phosphorus (BP) has emerged as a promising 2D material for photodetection in the mid-infrared spectral range given its narrow bandgap. However, a comprehensive understanding of BP photodetector’s response in the mid-infrared is still lacking. In this paper, we study the photoresponse of few-layer BP photodetector in the mid-infrared range from 2.5 to 3.7 µm. We identify broadband photoresponse of BP photodetectors in the mid-infrared and observe saturation of the response with optical power. Through frequency and time domain measurements, we have also identified the two dominate mechanisms in our device to be due to the photovoltaic and photogating effects. Our results provide valuable information for optimization of BP mid-infrared photodetectors toward rivaling the current infrared photodetection technology based on compound semiconductors.

Electronic and transport properties of n -type monolayer black phosphorus at low temperatures

We present a detailed theoretical study of the electronic and transport properties of monolayer black phosphorus (BP). This study is motivated by recent experimental activities in investigating $n$-type few-layer BP systems. The electron density of states, the screening length, and the low-temperature electron mobility are calculated for monolayer BP (MLBP). In particular, the electron transport mobilities along the armchair and zigzag directions are examined on the basis of the momentum-balance equation derived from a semiclassical Boltzmann equation. The anisotropic electron mobilities in MLBP along different directions are demonstrated where the electron-impurity scattering is considered. Furthermore, we compare the results obtained from two electronic band structures of MLBP and find that the simplified model can describe quite rightly the electronic and transport properties of MLBP. This study is relevant to the application of few-layer BP based electronic systems as advanced electronic devices.

Multifunctional Homogeneous Lateral Black Phosphorus Junction Devices

We demonstrate a controllable doping technique of few-layer black phosphorus (BP) via surface charge transfer using an ionic liquid mixture of EMIM(C6H11N2+):TFSI(C2F6NO4S2–) [EMIM:TFSI, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide]. A wide range of hole carrier densities, from 1011 cm–2 (nondegenerate) to 1013 cm–2 (degenerate), can be obtained by controlling the weight percentage of the ionic liquid mixture. The doping method we proposed in this paper can be applied to make a multifunctional homogeneous lateral p–n junction device. By doping a fraction of the BP sample and by applying a gate voltage to the other fraction of the BP, we obtain homogeneous lateral p+–p, p+–n, p+–n+ junction diodes in a single BP channel. The homogeneous lateral BP p+–p and p+–n junctions display ideal rectifying behavior and a much stronger photoresponse due to the built-in potential. Furthermore, at high positive gate voltages, the interband tunneling enables the homogeneous lateral p+–n+ junction transistors to provide both a negative differential resistance (NDR) and a negative transconductance (NTC) in the current–voltage characteristics at room temperature. On the basis of our results, it is possible to build novel devices utilizing the large NDR and NTC in BP such as amplifiers, oscillators, and multivalued logic systems.

Interfacial thermal conductance in graphene/black phosphorus heterogeneous structures

Graphene, as a passivation layer, can be used to protect the black phosphorus (BP) from the chemical reaction with surrounding oxygen and water. However, BP and graphene heterostructures have low efficiency of heat dissipation due to its intrinsic high thermal resistance at the interfaces. The accumulated energy from Joule heat has to be removed efficiently to avoid the malfunction of the devices. Therefore, it is of significance to investigate the interfacial thermal dissipation properties and manipulate the properties by interfacial engineering on demand. In this work, the interfacial thermal conductance between few-layer BP and graphene is studied extensively using molecular dynamics simulations. Two important parameters, Pcr, the critical heat power density of maintaining thermal stability, and Pmax, the maximum heat power density with which the system can be loaded, are identified. Our results show that interfacial thermal conductance can be effectively tuned in a wide range by external strains and interfacial defects. The compressive strain can enhance the interfacial thermal conductance by one order of magnitude, while interface defects give a two-fold increase. These findings could provide guidelines in heat dissipation and interfacial engineering for thermal conductance manipulation of BP-graphene heterostructure-based devices.

Generation of Anisotropic Massless Dirac Fermions and Asymmetric Klein Tunneling in Few-Layer Black Phosphorus Superlattices.

Artificial lattices have been employed in a broad range of two-dimensional systems, including those with electrons, atoms, and photons, in the quest for massless Dirac fermions with high flexibility and controllability. Establishing triangular or hexagonal symmetry, from periodically patterned molecule assembly or electrostatic gating as well as from moiré pattern induced by substrate, has produced electronic states with linear dispersions from two-dimensional electron gas (2DEG) residing in semiconductors, metals, and graphene. Different from the commonly studied isotropic host systems, here we demonstrate that massless Dirac fermions with tunable anisotropic characteristics can, in general, be generated in highly anisotropic 2DEG under slowly varying external periodic potentials. In the case of patterned few-layer black phosphorus superlattices, the new chiral quasiparticles exist exclusively in certain isolated energy window and inherit the strong anisotropic properties of pristine black phosphorus. These states exhibit asymmetric Klein tunneling, in which the transmission probability of the wave packets with normal incidence is no longer unity and can be tuned and controlled. In general, the direction of wave packet incidence for perfect transmission and that of the normal incidence are different, and the difference can reach more than 50° under an appropriate barrier orientation in black phosphorus superlattices. Our findings provide insight into the understanding and possible utilization of these novel emergent chiral quasiparticles.

Electrically Tunable Energy Bandgap in Dual-Gated Ultra-Thin Black Phosphorus Field Effect Transistors**Supported by the National Basic Research Program of China under Grant Nos 2013CB921900 and 2014CB920900, the National Natural Science Foundation of China under Grant No 11374021) (S. Yan, Z. Xie, J.-H. Chen). K.W. and T.T. acknowledge the support from the Elemental Strategy Initiative conducted by the MEXT, Japan and

The energy bandgap is an intrinsic character of semiconductors, which largely determines their properties. The ability to continuously and reversibly tune the bandgap of a single device during real time operation is of great importance not only to device physics but also to technological applications. Here we demonstrate a widely tunable bandgap of few-layer black phosphorus (BP) by the application of vertical electric field in dual-gated BP field-effect transistors. A total bandgap reduction of 124 meV is observed when the electrical displacement field is increased from 0.10 V/nm to 0.83 V/nm. Our results suggest appealing potential for few-layer BP as a tunable bandgap material in infrared optoelectronics, thermoelectric power generation and thermal imaging.

Black phosphorus nanodevices at terahertz frequencies: Photodetectors and future challenges

The discovery of graphene triggered a rapid rise of unexplored two-dimensional materials and heterostructures having optoelectronic and photonics properties that can be tailored on the nanoscale. Among these materials, black phosphorus (BP) has attracted a remarkable interest, thanks to many favorable properties, such as the high carrier mobility, the in-plane anisotropy, the possibility to alter its transport via electrical gating, and the direct band-gap, which can be tuned by thickness from 0.3 eV (bulk crystalline) to 1.7 eV (single atomic layer). When integrated in a microscopic field effect transistor, a few-layer BP flake can detect Terahertz (THz) frequency radiation. Remarkably, the in-plane crystalline anisotropy can be exploited to tailor the mechanisms that dominate the photoresponse; a BP-based field effect transistor can be engineered to act as a plasma-wave rectifier, a thermoelectric sensor, or a thermal bolometer. Here we present a review on recent research on BP detectors operating from 0.26 THz to 3.4 THz with particular emphasis on the underlying physical mechanisms and the future challenges that are yet to be addressed for making BP the active core of stable and reliable optical and electronic technologies.

New Method to Determine the Schottky Barrier in Few-Layer Black Phosphorus Metal Contacts.

Schottky barrier height and carrier polarity are seminal concepts for a practical device application of the interface between semiconductor and metal electrode. Investigation of those concepts is usually made by a conventional method such as the Schottky-Mott rule, incorporating the metal work function and semiconductor electron affinity, or the Fermi level pinning effect, resulting from the metal-induced gap states. Both manners are, however, basically applied to the bulk semiconductor metal contacts. To explore few-layer black phosphorus metal contacts far from the realm of bulk, we propose a new method to determine the Schottky barrier by scrutinizing the layer-by-layer phosphorus electronic structure from the first-principles calculation combined with the state-of-the-art band unfolding technique. In this study, using the new method, we calculate the Schottky barrier height and determine the contact polarity of Ti, Sc, and Al metal contacts to few-layer (mono-, bi-, tri-, and quadlayer) black phosphorus. This gives a significant physical insight toward the utmost layer-by-layer manipulation of electronic properties of few-layer semiconductor metal contacts.

Gate-Tunable Giant Stark Effect in Few-Layer Black Phosphorus.

Two-dimensional black phosphorus (BP) has sparked enormous research interest due to its high carrier mobility, layer-dependent direct bandgap and outstanding in-plane anisotropic properties. BP is one of the few two-dimensional materials where it is possible to tune the bandgap over a wide energy range from the visible up to the infrared. In this article, we report the observation of a giant Stark effect in electrostatically gated few-layer BP. Using low-temperature scanning tunnelling microscopy, we observed that in few-layer BP, when electrons are injected, a monotonic reduction of the bandgap occurs. The injected electrons compensate the existing defect-induced holes and achieve up to 35.5% bandgap modulation in the light-doping regime. When probed by tunnelling spectroscopy, the local density of states in few-layer BP shows characteristic resonance features arising from layer-dependent sub-band structures due to quantum confinement effects. The demonstration of an electrical gate-controlled giant Stark effect in BP paves the way to designing electro-optic modulators and photodetector devices that can be operated in a wide electromagnetic spectral range.

Emerging Trends in Phosphorene Fabrication towards Next Generation Devices.

The challenge of science and technology is to design and make materials that will dominate the future of our society. In this context, black phosphorus has emerged as a new, intriguing two‐dimensional (2D) material, together with its monolayer, which is referred to as phosphorene. The exploration of this new 2D material demands various fabrication methods to achieve potential applications— this demand motivated this review. This article is aimed at supplementing the concrete understanding of existing phosphorene fabrication techniques, which forms the foundation for a variety of applications. Here, the major issue of the degradation encountered in realizing devices based on few‐layered black phosphorus and phosphorene is reviewed. The prospects of phosphorene in future research are also described by discussing its significance and explaining ways to advance state‐of‐art of phosphorene‐based devices. In addition, a detailed presentation on the demand for future studies to promote well‐systemized fabrication methods towards large‐area, high‐yield and perfectly protected phosphorene for the development of reliable devices in optoelectronic applications and other areas is offered.

Oxygen induced strong mobility modulation in few-layer black phosphorus

2D black phosphorus configured field-effect transistors generally show a hole-transport-dominated ambipolar characteristic, owing to the severely restricted electron mobility by air ambient. Here, we demonstrate the strongly modulated mobility of few-layer black phosphorus in contact with oxygen. Pure oxygen exposure can dramatically decrease the electron mobility of black phosphorus by over three orders of magnitudes without degrading the hole transport. In situ x-ray photoelectron spectroscopy characterization reveals the physisorption nature of oxygen on black phosphorus. Density functional theory calculations identify the unoccupied states of molecular oxygen physisorbed on few-layer black phosphorus, that serves as electron trap but not as hole trap, consistent with the aforementioned mobility modulation. In contrast, oxygen exposure upon light illumination exhibits a significant attenuation for both electron and hole transport, originating from the photoactivated oxidation of black phosphorus, as corroborated by in situ x-ray photoelectron spectroscopy measurements. Our findings clarify the predominant role of oxygen in modulating transport properties of black phosphorus, thereby providing deeper insight to the design of black phosphorus based complementary electronics.

Effects of Contact Placement and Intra/Interlayer Interaction in Current Distribution of Black Phosphorus Sub-10-nm FET

Black phosphorus (BP) has been proposed as the channel material in the next generation ultrascaled CMOS devices. In order to gain insight into the current characteristics in 2-D layered materials, the current distribution of a few-layer BP Schottky barrier FET is investigated via state-of-the-art quantum device simulations. Approximately 40% of the total current was found to be concentrated in the top layer when the device was switched on, with the remaining current distributed among the other layers. In comparison, ~80% of the current concentrated below the surface in a Si device with the same structure. These features are related to the strength of the intra/interlayer interaction in few-layer BP and are unique to 2-D layered materials. Moreover, the current distribution and the device performance were different for the top- and side-contacted devices, with the side-contacted devices yielding lower resistance compared with the top-contacted devices.

Atomic-scale imaging of few-layer black phosphorus and its reconstructed edge

Black phosphorus (BP) has recently emerged as an alternative 2D semiconductor owing to its fascinating electronic properties such as tunable bandgap and high charge carrier mobility. The structural investigation of few-layer BP, such as identification of layer thickness and atomic-scale edge structure, is of great importance to fully understand its electronic and optical properties. Here we report atomic-scale analysis of few-layered BP performed by aberration corrected transmission electron microscopy (TEM). We establish the layer-number-dependent atomic resolution imaging of few-layer BP via TEM imaging and image simulations. The structural modification induced by the electron beam leads to revelation of crystalline edge and formation of BP nanoribbons. Atomic resolution imaging of BP clearly shows the reconstructed zigzag (ZZ) edge structures, which is also corroborated by van der Waals first principles calculations on the edge stability. Our study on the precise identification of BP thickness and atomic-resolution imaging of edge structures will lay the groundwork for investigation of few-layer BP, especially BP in nanostructured forms.

Progress of fabrication and surface modification of 2D black phosphorus

Two-dimensional (2D) materials have attracted broad interest because of their low-dimensional effect. Black phosphorus (BP), a new 2D layered material as graphene, has attracted worldwide attention since its appearance in 2014. As the most thermodynamic stable allotrope of phosphorus, BP is a natural semiconductor with adjustable bandgap and predominant electrical properties, and is considered to be able to replace silicon as the core material of semiconductor industry. The band gap in bulk BP is 0.3 eV and can be expanded to 2.0 eV depending on the layer numbers, and its band gap is highly sensitive to the strain either in-plane or out-of-plane. The BP based field effect transistor (FET) exhibits high mobility and appreciably high on/off ratios, and the mobility is thickness dependent. The optical property of BP is also superior to other semiconductors. The range of BP band gap corresponds to an absorption spectrum from visible light to infrared. With a direct bandgap, which means the bottom of conduction band and the top of valence band are at the same position, BP can couple with photons directly and construct a new generation of photoelectric device, and its in-plane anisotropy makes it suitable for the detecting of polarized light. Moreover, BP has good biocompatibility because about 1% of human weight is made up by phosphorus. All these attractive properties make BP a promising material in electronic, optical and biomedical applications. However, as always, opportunities and challenges coexist in the study and utilization of new materials, and such is the case for BP. There are still fundamental obstacles hampering the application of BP, such as mass production and stability. The thickness of BP can be scaled down to the atomic layer scale known by mechanical exfoliation or liquid exfoliation, but it is still difficult to realize the massive production of high-quality BP with accurate control of size and height. Furthermore, although BP is the most stable allotrope of elemental phosphorus and bulk BP can be stable under ambient conditions for months, the monolayer or few-layer BP is found to be unstable in an atmospheric environment, being subject to severe degradation by moisture and oxygen in air. It has been demonstrated that BP is very reactive to oxygen and water under ambient conditions, resulting in compositional and physical changes and consequently considerable degradation in the electronic and optical properties. Long-term exposure of BP to humid air or water can even completely etch the materials away. This poses a severe limitation to the adoption of BP in flexible electronics and photoelectronics, and its instability in water further limits potential electrochemical and biomedical applications. In this perspective, we review recent progress in the research of BP, touching upon topics on properties, fabrication and surface modification. We highlight that the surface modification to gain BP with good stability again air and water is important to its application. Finally, the development trend and application perspective of BP are mentioned.

Sensitive Detection of Carcinoembryonic Antigen Using Stability-Limited Few-Layer Black Phosphorus as an Electron Donor and a Reservoir.

The instability of few-layer black phosphorus (FL-BP) hampers its further applications. Here, it can be demonstrated that the instability of FL-BP can also be the advantage for application in biosensor. First, gold nanoparticle/FL-BP (BP-Au) hybrid is facilely synthesized by mixing Au precursor with FL-BP. BP-Au shows outstanding catalytic activity (K = 1120 s-1 g-1 ) and low activation energy (17.53 kJ mol-1 ) for reducing 4-nitrophenol, which is attributed to the electron-reservoir and electron-donor properties of FL-BP, and synergistic interaction of Au nanoparticles and FL-BP. Oxidation of FL-BP after catalytic reaction is further confirmed by transmission electron microscope, X-ray photoelectron spectroscopy, and zeta potentials. Second, the catalytic activity of BP-Au can be reversibly switched from "inactive" to "active" upon treatment with antibody and antigen in solution, thus providing a versatile platform for label-free colorimetric detection of biomarkers. The sensor shows a wide detection range (1 pg mL-1 to -10 µg mL-1 ), high sensitivity (0.20 pg mL-1 ), and selectivity for detecting carcinoembryonic antigen (CEA). Finally, the biosensor has been used to detect CEA in colon and breast cancer clinical samples with satisfactory results. Therefore, the instability of BP can also be the advantage for application in detecting cancer biomarker in clinic.

Air-stable few-layer black phosphorus phototransistor for near-infrared detection

We have demonstrated a few-layer black phosphorus (BP) phototransistor of stable operation in ambient air environment and at near-infrared light (λ = 1550 nm). The air-stable electronic and optoelectronic properties of the few-layer BP phototransistor have been achieved by a proper Al2O3 passivation. The optical identification method and qualitative and quantitative electrical characterizations of the few-layer BP phototransistor in dark state confirmed that the device performance was robust in ambient air, to further chemical treatments, and storage of more than six months. In addition, the low-frequency noise characterizations had revealed that the noise spectral density related to the sensitivity of phototransistor was reduced. Owing to the suppression of interaction between few-layer BP and adsorbates arising from the Al2O3 passivation, a fast rise time of the few-layer BP phototransistor, less than 100 μs, had been observed, demonstrating the intrinsic photoresponse properties of few-layer BP. The low dark current of ∼4 nA at the operation bias and the reasonable responsivity of ∼6 mA W−1 were obtained under the condition lacking adsorbates interactions. Internally, the dark current and responsivity level was tunable by changing the operation bias. Our results are close to the intrinsic properties of the few-layer BP phototransistor, implying that it can be a building block of functioned few-layer BP photodetectors.

Few-layer black phosphorus: A bright future in electromagnetic absorption

Few-layer black phosphorus (FL-BP) was prepared by liquid phase exfoliation (LPE) and composites with different weight ratio of FL-BP reveal high electromagnetic absorption (EA) performance in multi-frequency bands. A composite loaded with 30wt.% of FL-BP showed the broadest effective EA at bandwidth of 6.20GHz under the thickness of 2.5mm. Furthermore, when the loading ratio increased to 50wt.%, FL-BP based absorber has effective EA bandwidth in S (2–4GHz) band.

Mode-locked ytterbium-doped all-fiber lasers based on few-layer black phosphorus saturable absorbers

In this paper, we demonstrated ytterbium-doped mode-locked fiber lasers based on saturable absorbers (SAs) made of few-layer black phosphorus (BP) with all normal dispersion. The few-layer BP was prepared with the liquid phase exfoliation method and was deposited onto fiber facets by an optically driven process. By incorporating the BP-SA into an ytterbium-doped fiber cavity, stable mode-locking laser operation in all-normal dispersion region was achieved with a repetition rate of 46.3MHz. The laser spectrum was centered at 1030.6nm with a 3dB bandwidth of 0.11nm. Maximum output power of 32.5mW was achieved and showing no signs of saturation.