Extrinsic polarization-controlled optical anisotropy in plasmon-black phosphorus coupled system

As the demand for data-driven technologies such as cloud computing, artificial intelligence, and the Internet of Things increases, the value of data centers rises. However, the cost of optical interconnects that convert optical signals to electrical signals and vice versa limits the bandwidth in these data centers. Historically the optoelectronic materials of group III–V semiconductor materials are grown using epitaxy and transferred to silicon microelectronics using robotic pick-and-place technology. This costly method restricts the large-scale production of on-chip optoelectronic components such as diodes, photodetectors, and lasers. In this dissertation, we investigated the potential of black phosphorus (BP) as a material for optoelectronic devices and aerosol jet printing (AJP) as a viable deposition method. AJP-compatible inks were synthesized by mixing liquid-phase exfoliated HEBM BP flakes with isopropanol and 2-butanol solvents. The resulting printed thin films of HEBM BP exhibited photoluminescence emission suitable for optically pumped devices. Similarly, exfoliated single-crystal BP flakes were mixed with isopropanol, water, and 2-butanol to produce an AJP-compatible ink. Thin films printed on SiO2 with chemical vapor transport grown BP (CVT-BP) inks exhibited photoluminescence emission and electrical properties suitable for optically and electrically pumped optoelectronic devices. We demonstrate that these inks can be printed on various surfaces, such as flexible polyimide, glass, conformal quartz tubes, pre-deposited graphene, and WS2. We examined the relationship between BP's microstructure and its electrical and optical properties using high-resolution transmission electron microscopy. Using a previously established analytical density gradient ultracentrifugation (DGU) model, we purified polydisperse converted-BP dispersions into monolayer dispersions to exploit the layer-dependent bandgap of BP. In addition, we demonstrated that cascade centrifugation and tangential flow filtration (TFF) could be used to generate larger quantities of purified samples. We show that combining cascade centrifugation and TFF BP is an effective solution processing method for BP inks by enabling purification, surfactant removal, solvent exchange, and dispersion concentration. Our results indicate that AJP-printed BP has the potential to function as an active material in optoelectronic devices.

The ability to achieve orientation selectivity in piezoelectric response and thereby identifying its active piezoelectric facets with superior response is imperative for the breakthrough in piezoelectricity; still, it remains a challenge due to the presence of the mixed facets in contemporary semicrystalline piezoelectric materials. Accurate determination of in-plane and out-of-plane piezoelectric coefficients in these materials is practically impossible due to the involvement of components of other piezoelectric tensors arising from the randomly oriented molecules. This report delves into the exploration of the piezoelectric and ferroelectric response at piezoelectric facets such as basal plane and edges of two-dimensional (2D) black phosphorus (BP) flake and thereby measures the in-plane and out-of-plane piezoelectric coefficients in their pure form. In this report, the localized piezoelectric facets (artificial edges) are meticulously sculpted using a low-power focused laser, resulting in precise piezoelectric response selectivity at facets of the BP flake. The experimental findings, supported by theoretical calculations, demonstrate that the in-plane piezoelectric response is prominent at the edges, while the out-of-plane response is significant at the basal plane of the flakes. Moreover, thinner flakes exhibit lower coercive voltage (VC) and voltage at the spontaneous polarization (VS), indicating a stronger ferroelectric response as compared to thicker flakes. The experimental results offer an innovative path for deciphering the active piezoelectric coefficient along different crystal facets of 2D materials, offering exciting prospects for further research beyond the confinement of contemporary piezoelectric materials.

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.

Two-dimensional (2D) black phosphorus (BP) has drawn tremendous attention in solar-light-driven catalytic processes for its intriguing chemical and physical properties. Benefiting from the highly anisotropic electronic structure induced by its puckered crystal geometry, 2D BP tends to have greater confinement with respect to traditional inorganic nanomaterials, thereby leading to robust many-body effects. Such Coulomb-interaction-mediated effects dominate the electronic and optical properties of 2D BP-based nanosystems, where exotic correlations between photoinduced species give rise to unique photoexcitation processes that are closely associated with the involved photocatalytic behavior. In this Perspective, we highlight the critical role of many-body effects in 2D BP-based photocatalysis and exemplify the relationships between the correlated photoinduced species-dominated photoexcitation processes and photocatalytic behavior involved therein. The relevant challenges and opportunities in pursuing efficient 2D BP-based solar energy utilization are also discussed.

Two-dimensional black phosphorus (BP) recently emerged as an outstanding material for optoelectronics and nanophotonics applications. In contrast to graphene, BP has a sufficiently large electronic bandgap and its high carrier mobility allows for efficient free-carrier absorption in the infrared and terahertz regimes. Here, we present a reflective structure to enhance the response of nanostructured monolayer BP at terahertz frequencies and investigate localized surface plasmon resonances in BP nanostrip arrays. Anisotropic absorption is observed in the proposed BP metamaterials due to the puckered crystal structure of the monolayer BP, and further investigations show that the plasmonic resonances are strongly depending on the geometric parameters of the nanostrips and the coupling between the adjacent nanostrips. We expect that the monolayer BP is an outstanding candidate of highly anisotropic plasmonic material for ultrascaled optoelectronic integration.

Phosphorus, one of the most abundant elements in the Earth (∼0.1%), has attracted much attention in the last five years since the rediscovery of two-dimensional (2D) black phosphorus (BP) in 2014. The successful scaling down of BP endows this 'old material' with new vitality, resulting from the intriguing semiconducting properties in the atomic scale limit, i.e. layer-dependent bandgap that covers from the visible light to mid-infrared light spectrum as well as hole-dominated ambipolar transport characteristics. Intensive research effort has been devoted to the fabrication, characterization, functionalization and application of BP and other phosphorus allotropes. In this review article, we summarize the fundamental properties and fabrication techniques of BP, with particular emphasis on the recent progress in molecular beam epitaxy growth of 2D phosphorus. Subsequently, we highlight recent progress in BP (opto)electronic device applications achieved via customized manipulation methods, such as interface, defect and bandgap engineering as well as forming Lego-like stacked heterostructures.

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.

As society continues to create new digital content, the telecommunications industry is seeking new technologies to enable higher bandwidth and lower costs to keep pace with the growing demand. Two-dimensional black phosphorus is proposed as a replacement for III-V compound semiconductors as the optically active material in next-generation silicon photonics as it can enable device scaling with lower power consumption. Therefore, the primary motivation of this dissertation was to investigate BP synthesis and chemical doping using an industrially scalable process, high energy ball milling. Initially, the work focused on understanding the ball mill conversion kinetics of red to black phosphorus, hitherto unknown, and is detailed in Chapter 2. The process follows a nucleation and growth dominated mechanism whose rate is controlled by the collision energy and milling intensity. Photoluminescence on mechanochemically synthesized BP showed visible and infrared emissions at the few-layer limit, indicating this process route provides optically viable BP suitable for silicon photonics. To address feasibility of doping, arsenic alloys with phosphorus were subsequently produced by ball milling in order to better understand how the crystal structure changes with substitutional doping; this work is described in Chapter 3. A similar conversion kinetics study was also performed showing a two-step mechanism. First, within a few minutes of milling, the trigonal PAs structure forms followed by a much slower phase transformation to the orthorhombic structure. This work provided a solid benchmark for how substitutional atoms affects the crystal structure, vibrational modes, binding energies, and photoluminescence. Candidate dopants for BP beyond arsenic included germanium, sulfur, selenium, and tellurium. Milling results for germanium phosphides are presented in Chapter 4 and results for phosphorus with sulfur, selenium, and tellurium are presented in Chapter 5. Germanium appears to dope BP (<1> at% Ge) as do sulfur (<10> at%) and selenium (<10> at%). Tellurium does not appear to form a stable dopant with black phosphorus via ball milling. Higher concentrations produced layered trigonal and monoclinic Ge-P crystals, while several crystalline and amorphous phosphorus sulfides and selenides are synthesized by this novel route. Together, Chapters 3-5 indicated that mechanochemical doping of BP with arsenic, germanium, sulfur and selenium is feasible with future work to explore electrical measurements. Finally, within the appendix, a discussion is presented challenges for ball mill doping of BP including milling material, red phosphorus purity, and candidate dopants; limited structural characterization of BP doped with germanium and selenium are included. Less comprehensive work on ball mill reactions of phosphorus with boron, tin, antimony, and bismuth are also reported in the appendix. These results confirmed inability to form phosphorus antimonides while several of the known tin phosphides were successfully synthesized. Independent of the black phosphorus work, a separate study on the synthesis of several intermetallic half-Heuslers for thermoelectric applications is also included in the Appendix. The half-Heusler work shows the versatility of ball milling to synthesize a wide range of intermetallic compounds and revealed nuances regarding challenges of milling together high temperature refractory metals, transition metals, and soft metalloids, in terms of particle size reduction, single phase synthesis, and milling media contamination.

Since its successful isolation in 2014, two-dimensional black phosphorus (BP) has triggered considerable interest ffrom physicists, chemists and material scientists. Benefitting from the unique structural and physicochemical properties, BP has been explored in various applications including photoelectric, biological and electrochemical fields. Besides, BP also shows great potential as a promising electrode material and electrocatalyst in energy storage and electrocatalytic applications, and tremendous progress has been made in these electrochemical fields in recent years. Here, this review highlights the recent experimental and theoretical progress of BP-based electrodes and electrocatalysts. The latest recent advances of BP-based functional materials in energy storage applications including lithium-, magnesium- and sodium-ion batteries, lithium–sulfur batteries and supercapacitors, are presented in detail. Further, the emerging electrocatalytic applications of BP for hydrogen evolution reaction, oxygen evolution reaction and nitrogen reduction reaction are systematically reviewed with achievements and challenges. Finally, we offer brief personal comments on the existing challenges and prospective outlook on the basis of current research progress.

Black phosphorus, which is a relatively rare allotrope of phosphorus, was first discovered by Bridgman in 1914. Since the advent of two-dimensional (2D) black phosphorus (which is known as phosphorene due to its resembling graphene sheets) in early 2014, research interest in the arena of black phosphorus was reignited in the scientific and technological communities. Henceforth, a myriad of research studies on this new member of the 2D world have been extensively emerged. Fascinatingly, 2D black phosphorus exhibits a distinctive wrinkled structure with the high hole mobility up to 1000 cm2 V−1 s−1, excellent mechanical properties, tunable band structures, anisotropic thermal, electrical and optical properties, thus leading to its marvelous prospects in device applications. This review firstly introduces the state-of-the-art development, structural properties and preparation routes of black phosphorus. In particular, anisotropy involved in mechanical properties, thermal conductivity, carrier transport as well as optical properties is comprehensively discussed. Apart from discussing the recent progress in black phosphorus which is applied to devices (i.e. field effect transistors and optoelectronic), the review also highlights the bottlenecks encountered by the society and finally casts an invigorating perspective and insightful outlook on the future direction of the next-generation 2D black phosphorus by harnessing its remarkable characteristics for energy production.

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.

Recently, black phosphorus (BP), an emerging layered two-dimensional (2D) material, has aroused much research interest. Distinguished from most of other 2D materials, BP is always a direct bandgap semiconductor regardless of the thickness, with the bandgap ranging from 0.3 eV (bulk) to 1.7 eV (monolayer), which is just fill the gap in the bandgap between graphene and transition metal dichalcogenides (TMDCs) in this frequency range. Besides, the BP exhibits many intriguing properties, such as high carrier mobility, highly tunable and anisotropic physical properties, which render the BP another star 2D material following graphene and TMDCs. In this review, we mainly focus on the advances in the optical properties of 2D BP, with the content covering the intrinsic optical properties and external perturbation effects on optical properties. Regarding the intrinsic optical properties, we introduce the anisotropic and layer-dependent optical absorption from interband transitions, the layer-dependent exciton binding energy and exciton absorption, visible-to-infrared photoluminescence, and stability of absorption and photoluminescence. As for external perturbation effects on optical properties, we introduce in-plane uniaxial and biaxial strain effects, gate-induced quantum confined Franz-Keldysh effect and Burstein-Moss effect. And finally we give a brief summary and outlook, pointing out some several interesting and important issues of BP, which need further studying urgently such as hyperbolic plasmons, intersubband transitions, optical properties in heterostructures and twist angle moiré superlattice and so on. This review gives an overview of the optical properties of BP and is expected to arouse the interest in further studying the BP.

Compared with other two-dimensional materials, two-dimensional black phosphorus (BP) has attracted extensive attention across various fields including transistors, photonics, optoelectronics, sensors, batteries and catalysis in recent years owing to its high carrier mobility, wide adjustable band gap, and in-plane anisotropy. However, the instability of black phosphorus in natural environmental conditions and the chanllenges in the fabrication of large-area, high-quality semiconducting black phosphorus nanostructures seriously have hampered its rapid development in the field of nanodevices, catalysis and biophotonics. In this work, we summarize the controlled fabrication and optical properties of highly stable BP nanostructures in recent years. The research advances in photoresponse applications, including photodetectors, nonlinear and linear photonics, photocatalysis and biophotonics, are highlighted. Additionally, the challenges and opportunities of BP nanostructures in future optoelectronic devices, photocatalysis and biophotonics are discussed.

Research discoveries in carbon nanostructures have encouraged scientists to look for the next generation of versatile nanomaterials with novel electronic and optical properties to generate new scientific knowledge and discover materials with unanticipated or enhanced functionality. One element that has become of great interest in the last few years is phosphorus. Although studies investigating phosphorus nanostructures and properties are still in their infancy, unique properties of various phosphorus nanostructures are expected. First, in a literature review, this work will summarize some optical and optoelectronic applications of both black and red phosphorus. The role of phosphorus as a photocatalyst is also discussed. Next, this work will explore liquid exfoliation techniques applied to 2D black phosphorus with several exotic and common laboratory solvents, both with and without the use of potentially damaging ultrasonication. Lastly, investigations into reactions of black phosphorus and boron trifluoride under ambient conditions will be undergone to study the formation of boron phosphate.

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.).