Black Phosphorus Ink Formulation for Aerosol Jet Printing

Thin layers of black phosphorus present an ideal combination of a 2D material with a tunable direct bandgap and high carrier mobility. However the material suffers from degradation in ambient conditions due to an oxidation reaction which involves water, oxygen and light. We have measured the spatial profile of the conductivity on flakes of black phosphorus as a function of time using scanning microwave impedance microscopy. A microwave excitation (3 GHz) allows to image a conducting sample even when covered with a dielectric layer. We observe that on bare black phosphorus, the conductivity changes drastically over the whole surface within a day. We demonstrate that the degradation process is slowed down considerably by covering the material with a 10 nm layer of hafnium oxide. It is stable for more than a week, opening up a route towards stable black phosphorus devices in which the high dielectric constant of hafnium oxide can be exploited. Covering black phosphorus with a 15 nm boron nitride flake changes the degradation process qualitatively, it is dominated by the edges of the flake indicating a diffusive process and happens on the scale of days.

Unencapsulated, exfoliated black phosphorus (BP) flakes are found to chemically degrade upon exposure to ambient conditions. Atomic force microscopy, electrostatic force microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy are employed to characterize the structure and chemistry of the degradation process, suggesting that O2 saturated H2O irreversibly reacts with BP to form oxidized phosphorus species. This interpretation is further supported by the observation that BP degradation occurs more rapidly on hydrophobic octadecyltrichlorosilane self-assembled monolayers and on H-Si(111) versus hydrophilic SiO2. For unencapsulated BP field-effect transistors, the ambient degradation causes large increases in threshold voltage after 6 h in ambient, followed by a ∼ 10(3) decrease in FET current on/off ratio and mobility after 48 h. Atomic layer deposited AlOx overlayers effectively suppress ambient degradation, allowing encapsulated BP FETs to maintain high on/off ratios of ∼ 10(3) and mobilities of ∼ 100 cm(2) V(-1) s(-1) for over 2 weeks in ambient conditions. This work shows that the ambient degradation of BP can be managed effectively when the flakes are sufficiently passivated. In turn, our strategy for enhancing BP environmental stability will accelerate efforts to implement BP in electronic and optoelectronic applications.

This paper reviews the recent progress on electronic and optoelectronic devices based on 2D black phosphorus (BP). First, the crystal structure, band structure, and optical properties of BP, as well as some currently-known passivation methods used for making BP stable in ambient conditions are briefly summarized. Device architectures and operating principles of the state-of-the-art few-layer BP based electronic and optoelectronic devices will then be discussed in detail, with a focus on field-effect transistors, heterojunction diodes, and photodetectors. Next, solution-based exfoliation methods aimed for addressing the scalability challenge faced by BP are briefly discussed, followed by their potential applications in gas sensors and biomedicine. By reviewing recent process and discussing remaining challenges faced by BP, this paper aims to provide perspectives on opportunities and future research directions for utilizing BP and moving towards practical applications.

Two-dimensional black phosphorus (BP) has drawn extensive research interest due to its promising anisotropic photonic and electronic properties. Here, we study anisotropic optical absorption and photoresponse of exfoliated BP flakes at visible frequencies. We enhance this intrinsic optical anisotropy in BP flakes by coupling plasmonic rectangular nanopatch arrays that support localized surface plasmon resonances. In particular, by combining extrinsic anisotropic plasmonic nanostructures lithographically aligned with intrinsically anisotropic BP flakes, we demonstrate for the first time a combined anisotropic plasmonic-semiconductor coupling that provides significant control over the polarization-dependent optical properties of the plasmon-BP hybrid material system, enhancing polarization-sensitive responses to a larger degree. This hybrid material system not only unveils the plasmon-enhanced mechanisms in BP, but also provides novel controllable functionalities in optoelectronic device applications involving polarization-sensitive optical and electrical responses.

In the recent past, 2D black phosphorus (BP) has been intensively studied and examined due to its unique electronic, photonic, and mechanical properties. The tunable and moderate direct bandgap and the high carrier mobility of BP provide enormous potential in electronic and optoelectronic applications. In addition, the unique intrinsic anisotropic characteristics resulting from the puckered structure yield remarkable optical, electronic, transport, mechanical, and thermal characteristics that can be utilized for designing new devices. Significant efforts have been directed toward the synthesis, basic understanding, and applications of BP in the fields of nanoelectronics, ultrafast optics, nanophotonics, and optoelectronics. Here, the current development of electronic, photonic, optoelectronic, and optical devices based on BP is summarized, along with the recent advances in investigating its electronic, optical, and mechanical properties, which form the foundation for next‐generation chip‐scale integrated devices. In addition, a comprehensive discussion on the requirements for forthcoming studies to upgrade well‐systemized fabrication techniques toward large‐area, high‐yield, and perfectly shielded BP‐film production for the development of reliable devices in optoelectronic applications and other areas is provided. Finally, some existing challenges in implementing BP‐based optoelectronic and photonic devices are addressed and the prospects for future BP‐related research are discussed.

Black phosphorus is the most stable allotrope of phosphorus and exhibits a layered structure that can be exfoliated down to a single monolayer. This elemental 2D semiconductor offers several distinctive features unmatched by none of the other 2D materials. For instance, it offers high carrier mobilities, strongly anisotropic optical, electrical and mechanical properties along all three principal crystal axes, a thickness-tunable gap spanning the near-IR and the short wavelength IR spectral region, a direct gap that efficiently interacts with light irrespective of sample thickness, and, most usually, a gap that can be dynamically tuned by an external electric field to an extent rarely seen from other semiconductor materials. For these reasons, black phosphorus opens many interesting prospects for the design of optoelectronic devices with novel functionalities.I will introduce this 2D material, discuss its most intriguing characteristics and then present results from an elaborate Raman study. In addition to bulk modes, atomically-thin black phosphorus exhibits unexpected bulk-forbidden Raman-allowed modes. The most obvious ones result from the lack of translational symmetry in few-layer samples, where the combination of infrared-allowed modes in adjacent layers combine to generate new Raman-allowed modes. Normally very weak, this phenomenon is prominent in the bilayer due to resonant effects in the electronic band structure. Polarization-resolved measurements on first order modes reveal powerful insights on the complex Raman tensor elements, their relative strengths, and their evolution as a function of sample thickness. For instance, anisotropy dominates the monolayer response and interlayer interactions in multilayers reduce all observed anisotropies.In addition to first-order Raman modes, four new second-order modes are identified in the close vicinity of the well-established symmetrical Ag modes. Their evolutions as a function of sample thickness, excitation wavelength, and defect density indicate that they are defect-activated and involve high-momentum phonons in a doubly resonant Raman process. Ab-initio calculations of the monolayer band structure and Raman intensity simulations reveal that phonon scattering predominantly involves intravalley contributions either in the zigzag or armchair cristal directions. Second order modes have played an important role in the development of carbon-based materials, including graphene, and their existence in black phosphorus offers new ways to probe carrier mobility, defect density, doping levels as well as chemical reactivity and mechanical stress.Instrumental in the scientific and technological development of 2D materials, Raman spectroscopy appears to be an even more powerful technique for the study of orthorhombic materials: the Raman-allowed modes express changes in permittivity as a function of atomic motion, couple in a unique way with the anisotropic electronic band structure through very restrictive selection rules. These connections will be illustrated by experimental results discussed using arguments relating to the symmetry of black phosphorus.

Transistors were fabricated based on mechanical exfoliated few layer black phosphorus (BP) flakes, and performance change of these devices exposed to air was explored systematically. BP devices were found to suffer severe performance degradation in ambient conditions, and the field effect mobility drops to less than 1/10 of the original in no more than 120 hours after fabrication. However the current on/off ratio shows completely different time dependent behavior to the published result, i.e. increases with exposure time in air, since the minimum current decreases with exposure time to air, which is probably originated from the decrease of layer number in BP. A model is developed to estimate the bandgap change of BP according to the time dependent minimum current of the BP device.

Lithography-free fabrication of field effect transistor channels with randomly contact-printed black phosphorus flakes

Black phosphorus consists of stacked layers of phosphorene, a two-dimensional semiconductor with promising device characteristics. We report the realization of a widely tunable band gap in few-layer black phosphorus doped with potassium using an in situ surface doping technique. Through band structure measurements and calculations, we demonstrate that a vertical electric field from dopants modulates the band gap, owing to the giant Stark effect, and tunes the material from a moderate-gap semiconductor to a band-inverted semimetal. At the critical field of this band inversion, the material becomes a Dirac semimetal with anisotropic dispersion, linear in armchair and quadratic in zigzag directions. The tunable band structure of black phosphorus may allow great flexibility in design and optimization of electronic and optoelectronic devices.

Recent years have witnessed the rapidly growing interests in the rediscovered black phosphorus (BP), an elemental group‐V layered material with very high carrier mobility among all semiconducting layered materials. As a layered semiconductor, the bandgap of intrinsic BP varies from ≈0.3 to 2 eV depending on the thickness. This bandgap value can be tuned to below 50 meV by a moderate external electric field. Adsorption doping and external pressure can also effectively modify its bandgap. The largely tunable bandgap of BP makes it a promising material for infrared optics. Moreover, its unique puckered structure leads to the anisotropic in‐plane properties, making it ideal for the exploration of exotic physical phenomena and the realization of novel devices. Here, the fundamental optical properties are reviewed and latest developments on BP photonic and optoelectronic devices are discussed.

Black phosphorus (BP) has shown great potential as a semiconductor material beyond graphene and MoS2 because of its intrinsic band gap and high mobility. Moreover, the biocompatibility of the final biodegradation products of BP has led to extensive research on biomedical applications. Herein, physically transient field-effect transistors (FETs) based on black phosphorus have been demonstrated using peptide insulator as a gate dielectric layer. The fabricated devices show high hole mobility up to 468 cm2 V-1 s-1 and on-off current ratio over 103. The combined use of black phosphorus, peptide, and molybdenum provides rapid disappearance of the devices within 36 h. Dissolution kinetics and cytotoxicity of black phosphorus are assessed to clarify its availability to be applied in transient electronics. This work provides transient FETs with high degradability and high performance based on biocompatible black phosphorus.

Black phosphorus (BP) has recently been rediscovered as a new and interesting two-dimensional material due to its unique electronic and optical properties. Here, we study the linear and nonlinear optical properties of BP flakes. We observe that both the linear and nonlinear optical properties are anisotropic and can be tuned by the film thickness in BP, completely different from other typical two-dimensional layered materials (e.g., graphene and the most studied transition metal dichalcogenides). We then use the nonlinear optical properties of BP for ultrafast (pulse duration down to ~786 fs in mode-locking) and large-energy (pulse energy up to >18 nJ in Q-switching) pulse generation in fiber lasers at the near-infrared telecommunication band ~1.5 μm. We observe that the output of our BP based pulsed lasers is linearly polarized (with a degree-of-polarization ~98% in mode-locking, >99% in Q-switching, respectively) due to the anisotropic optical property of BP. Our results underscore the relatively large optical nonlinearity of BP with unique polarization and thickness dependence, and its potential for polarized optical pulse generation, paving the way to BP based nonlinear and ultrafast photonic applications (e.g., ultrafast all-optical polarization switches/modulators, frequency converters etc.).

Black phosphorus (BP) is a promising material for ultrafast and broadband photodetection because of its narrow bandgap from 0.35 eV (bulk) to 1.8 eV (monolayer) and high carrier mobility. Although photodetectors based on BP with different configurations have been reported, high photosensitivity was mostly observed in the visible range. A highly efficient BP-based infrared photodetector operated in the telecom spectral range, especially at 1550 nm, has not been demonstrated. Here, we report a Schottky-type photodetector based on thin BP flakes, operating in a broad spectral range from visible (635 nm) to infrared (1550 nm). A responsivity as high as 230 A·W−1 was achieved at 1550 nm with a source-drain bias of 1 V. The rise time is 4.8 ms, and the fall time is 6.8 ms. Under light illumination and external bias, the Schottky barrier between the BP and metal was reduced, leading to efficient photocurrent extraction. The unprecedented performance of the BP photodetector indicates intriguing potential for sensing, imaging, and optical communication.

Identification of the crystalline axis of anisotropic black phosphorus (BP) is important for investigating its physical properties, as well as for optical and electronic applications. Herein, it is showed that by applying in-plane uniaxial strain and measuring the changes of the Raman shifts, the crystalline axis of BP can be reliably determined. The strain effects on the Raman shifts are angle-dependent, and they can be expressed as a combination of the Raman responses under zigzag and armchair strain. Differing from previous polarized optical spectroscopic methods where the Raman intensity is analyzed, the proposed method uses the Raman frequency shift, which is less affected by laser polarization, excitation wavelength, the sample thickness, and the substrate. The effective strain applied on BP from the stretched substrate is estimated, and the results show that only 20 to 40% of the strain can be effectively transferred to BP flakes from a polyethylene terephthalate substrate. Our method provides not only an effective and robust approach to identify the crystalline orientation of layered BP, but it is also a model to extract additional information in strain-related studies. It can also be extended to other 2D anisotropic materials.

Black phosphorus (BP), especially in its two-dimensional (2D) form, is an intriguing material because it exhibits higher chemical sensing ability as compared to other thin-film and 2D materials. However, its implementation into hydrogen sensors has been limited due to its insensitivity toward hydrogen. We functionalized exfoliated BP flakes with Pt nanoparticles to improve their hydrogen sensing efficiency. Pt-functionalized BP sensors with back-gated field-effect transistor configuration exhibited a fast response/decay, excellent reproducibility, and high sensitivities (over 50%) at room temperature. Langmuir isotherm model was employed to analyze the Pt-catalyzed BP sensors. Furthermore, the activation energy of hydrogen adsorption on Pt-decorated BP was evaluated, which is equal to the change in work function resulting from hydrogen adsorption on the Pt(111) surface. These results demonstrate that Pt-catalyzed BP exhibits a great potential for next-generation hydrogen sensors.