Tunable Direct Bandgap Research Articles

Efficient charge carrier separation and excellent visible light photoresponse in Cu2O nanowires

Last decade has witnessed a surge of research pertaining to Cu2O nanowires for fields of photocatalysts, sensors, solar cells and rechargeable battery systems owing to their unique physicochemical properties. Their atomic properties, especially the size dependent electronic structures, however, remain unknown. Herein, by combining systematic first-principles calculations with material synthesis, characterization, device fabrication and measurement, we investigated physical properties of Cu2O nanowires and their applications in visible light photodiodes. We explored Cu2O nanowires with triangular and hexagonal cross sections of a series of diameter size, and found that they have favorable formation energies, indicative of facile synthesis. These nanowires have tunable direct band gap ranging from 2.2 to 5.4 eV, and a simple model is derived to effectively predict gap size. Remarkably, obvious spatially separated charge distribution of conduction and valance band edges was observed, ensuring long lifetime of excited electron-hole pairs that may greatly benefit performance of optoelectronic devices. Experimentally, we synthesized well-crystallized Cu2O nanowires and fabricated photodiodes, which exhibit a fast rise time of 9.86 ms, decay time of 27.37 ms, and high responsivity of 10 A/W. We expect these results to shed new light on Cu2O nanostructures for scalable, versatile and high-performance optoelectronic devices.

Black phosphorus quantum dot-based field-effect transistors with ambipolar characteristics

Semiconductor quantum dots have intriguing electronic and optical properties distinguished from bulk owing to quantum confinement effects. Among the two-dimensional materials, black phosphorus (BP) has generated enormous excitement due to its tunable direct band gap and high p-type semiconducting properties. We prepared BP quantum dots (BPQDs) by simple liquid exfoliation using distilled water and ethanol solution. Our structural data show the uniform distribution of circular BPQDs with the average lateral size of 4.08 ± 0.66 nm and the height of 1.13 ± 0.32 nm. We fabricated BPQD field-effect transistors (FETs) to investigate the electrical characteristics of BPQD-based devices and found that both hole and electron transport can be probed in the BPQD FETs. The BPQD FETs exhibited unprecedentedly ambipolar behavior with the mobility of 0.11 cm2 V−1 s−1 for p type and 0.09 cm2 V−1 s−1 for n type at 300 K. Our results provide the simple preparation methods to fabricate ambipolar BPQD FETs with the comparable hole and electron transport for large-area applications in solar cells and optoelectronic devices.

Anisotropic visible photoluminescence from thermally annealed few-layer black phosphorus

Black phosphorus, a two-dimensional material, with high carrier mobility, tunable direct bandgap and anisotropic electronic properties has attracted enormous research interest towards potential application in electronic, optoelectronic and optomechanical devices. The bandgap of BP is thickness dependent, ranging from 0.3 eV for bulk to 1.3 eV for monolayer, while lacking in the visible region, a widely used optical regime for practical optoelectronic applications. In this work, photoluminescence (PL) centered at 605 nm is observed from the thermally annealed BP with thickness ≤20 nm. This higher energy PL is most likely the consequence of the formation of higher bandgap phosphorene oxides and suboxides on the surface BP layers as a result of the enhanced rate of oxidation. Moreover, the polarization-resolved PL measurements show that the emitted light is anisotropic when the excitation polarization is along the armchair direction. However, if excited along zigzag direction, the PL is nearly isotropic. Our findings suggest that the thermal annealing of BP can be used as a convenient route to fill the visible gap of the BP-based optoelectronic and optomechanical devices.

(Invited) Photodetection from Mid-IR to UV Using Semiconductor Heterostructures

I will discuss the use of semiconductor heterostructures with proper bandgaps and architectures to detect photons with different wavelengths. First, for the most pursued visible light detection, hybrid perovskites have aroused lots of interest because of their exceptional properties, such as low-cost solution processing, tunable direct bandgap, high light absorption and long carrier diffusion length. Our experiments demonstrate that combining 3D perovskites with high-mobility 1D carbon nanotubes or 2D two-dimensional metal dichalcogenides significantly enhances charge transport and device performance. Second, we demonstrate a mid-infrared hybrid graphene photodetector enabled via coupling graphene with a narrow bandgap semiconductor Ti2O3. This hybrid photodetector achieves high responsivity of ~200 A/W in a broadband wavelength range up to 10 μm. Finally, using epitaxial ferroelectric/semiconductor junctions with a current-perpendicular-to-plane geometry, we achieve a colossal X-ray-induced photoconductivity with a magnitude of six orders. This X-ray-induced persistent photoconductivity persists for days with negligible decay. This new device scheme open a new route towards X-ray sensing and imaging applications.

Recent Advances in Black‐Phosphorus‐Based Photonics and Optoelectronics Devices

AbstractIn 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 as a new lubricant

In recent years, a new 2D-layered material—black phosphorus (BP)—has been a rising star after the era of graphene owing to its high charge carrier mobility, tunable direct bandgap and unique in-plane anisotropic structure. With the development of the synthesis and modification methods of BP, its extensive applications, e.g., transistors, batteries and optoelectronics have emerged. In order to explore its full potential, research into the tribological properties of BP 2D-layered materials such as lubrication additives and fillers in self-lubricating composite materials would be not only of high scientific value but also of practical significance. In this work, recent advances on the friction and lubrication properties of BP nanosheets made by our group, including the micro-friction properties, the lubrication properties of BP nanosheets as water-based and oil-based lubrication additives, and the friction and wear of BP/PVDF composites will be presented. Finally, the future challenges and opportunities in the use of BP materials as lubricants will be discussed.

Solution Deposited Cu2BaSnS4–xSex from a Thiol–Amine Solvent Mixture

The discovery that cation antisite defects in Cu2ZnSn(S,Se)4 (CZTS) thin film photovoltaic absorbers are a major contributor to lower than expected device efficiencies has spurred recent interest in material design to develop similar materials that maintain the desirable optoelectronic properties of CZTS, while mitigating this source of loss. Recent reports have shown that this can be accomplished by replacing zinc with a larger cation of the same valency, such as barium, to give Cu2BaSn(S,Se)4. Until this point, Cu2BaSn(S,Se)4 has only been synthesized by solid-state and physical deposition methods. Herein, we use a thiol-amine solvent mixture to dissolve bulk Cu2S, BaS, and SnO to prepare a precursor ink that upon low temperature annealing yields phase-pure trigonal Cu2BaSnS4-xSex (nominal x = 0, 1, 2, 3) alloys with tunable direct band gaps from 1.56 – 1.86 eV.

Tunable direct band gap photoluminescent organic semiconducting nanoparticles from lignite

Fluorescent organic semiconducting dots (OSDs) with tunable particle size and surface functionality are synthesized from lignite by chemical oxidation method followed by ultra-sonication techniques and dialysis. The defects and oxygen functionalities play a vital role in the photoluminescent property of the synthesized nanoparticles along with quantum confinement effect. These nanomaterials are suitable for imaging and chemical sensing applications as there is no photobleaching and quenching even after a continuous UV exposure of 24 hours and storage of 2 years. The excellent excitation dependent luminescence of the synthesized carbon dots can be utilized for making a low-cost carbon-based sensor for Cu2+ metal ions sensing. The OSDs show good ratiometric fluorescent sensing and can be used as a reliable probe for the detection of Cu2+ ions. They exhibit excellent detection limit of copper ion in acidic solution to a very low concentration of 0.0089 nM. The fluorescent nanodots synthesized from such an abundant and cost-effective precursor exhibiting high copper ion sensitivity is being reported for the first time.

Geometric structure and photovoltaic properties of mixed halide germanium perovskites from theoretical view

Abstract Inspired by the high efficiency of mixed halide perovskite CH3NH3PbI3-xClx due to the long diffusion and superior optical property, we pay our attention to the mixed halide germanium perovskite. In this paper, we present a detail theoretical investigation of geometric and electronic structure of mixed halide perovskite CH3NH3GeI3-xClx to predict its charge transport and optical properties. The calculated results show that this system exhibits tunable direct bandgap from 1.9 eV for CH3NH3GeI3 to 2.8 eV for CH3NH3GeCl3 compounds. With the increase of iodine atom doping ratio, the transport and optical properties significantly improve. In addition, we systematically discuss how the electronic structure depends on the doping position of iodine atoms by taking mixed halide CH3NH3GeI1Cl2 compound as examples. The results prove the mixed halide CH3NH3GeI1Cl2 demonstrates more superior transport and optical properties by doping iodine atoms at polar plane. This work will help to deepen the understanding of halogen doping mechanism by changing the doping proportion and doping position and these findings may lay foundation for experimental design to obtain higher photovoltaic properties in future.

Fluorinated Phosphorene: Electrochemical Synthesis, Atomistic Fluorination, and Enhanced Stability.

Phosphorene has attracted great interest due to its unique electronic and optoelectronic properties owing to its tunable direct and moderate band-gap in association with high carrier mobility. However, its intrinsic instability in air seriously hinders its practical applications, and problems of technical complexity and in-process degradation exist in currently proposed stabilization strategies. A facile pathway in obtaining and stabilizing phosphorene through a one-step, ionic liquid-assisted electrochemical exfoliation and synchronous fluorination process is reported in this study. This strategy enables fluorinated phosphorene (FP) to be discovered and large-scale, highly selective few-layer FP (3-6 atomic layers) to be obtained. The synthesized FP is found to exhibit unique morphological and optical characteristics. Possible atomistic fluorination configurations of FP are revealed by core-level binding energy shift calculations in combination with spectroscopic measurements, and the results indicate that electrolyte concentration significantly modulates the fluorination configurations. Furthermore, FP is found to exhibit enhanced air stability thanks to the antioxidation and antihydration effects of the introduced fluorine adatoms, and demonstrate excellent photothermal stability during a week of air exposure. These findings pave the way toward real applications of phosphorene-based nanophotonics.

Prediction of Green Phosphorus with Tunable Direct Band Gap and High Mobility.

Black phosphorus is an emerging material in nanoelectronics and nanophotonics due to its high carrier mobility and anisotropic in-plane properties. In addition, the polymorphism of phosphorus leads to numerous searches for new allotropes that are more attractive than black phosphorus in a variety of applications. On the basis of ab initio evolutionary crystal structure search computation, we report the prediction of a phosphorus allotrope called green phosphorus (λ-P), which exhibits direct band gaps ranging from 0.7 to 2.4 eV and strong anisotropy in optical and transport properties. Free-energy calculations show that a single-layer form, termed green phosphorene, is energetically more stable than blue phosphorene, and a phase transition from black to green phosphorene can occur at temperatures above 87 K. We suggest that green phosphorene can be synthesized on corrugated metal surfaces rather than clean surfaces due to its buckled structure, providing guidance to achieving epitaxial growth.

Field‐Induced n‐Doping of Black Phosphorus for CMOS Compatible 2D Logic Electronics with High Electron Mobility

Black phosphorus (BP) has been considered as a promising two‐dimensional (2D) semiconductor beyond graphene owning to its tunable direct bandgap and high carrier mobility. However, the hole‐transport‐dominated characteristic limits the application of BP in versatile electronics. Here, we report a stable and complementary metal oxide semiconductor (COMS) compatible electron doping method for BP, which is realized with the strong field‐induced effect from the K+ center of the silicon nitride (SixNy). An obvious change from pristine p‐type BP to n type is observed after the deposit of the SixNy on the BP surface. This electron doping can be kept stable for over 1 month and capable of improving the electron mobility of BP towards as high as ~176 cm2 V–1 s–1. Moreover, high‐performance in‐plane BP p‐n diode and further logic inverter were realized by utilizing the n‐doping approach. The BP p‐n diode exhibits a high rectifying ratio of ~104. And, a successful transfer of the output voltage from “High” to “Low” with very few voltage loss at various working frequencies were also demonstrated with the constructed BP inverter. Our findings paves the way for the success of COMS compatible technique for BP‐based nanoelectronics.

Three-layer phosphorene-metal interfaces

Phosphorene has attracted much attention recently as an alternative channel material in nanoscale electronic and optoelectronic devices due to its high carrier mobility and tunable direct bandgap. Compared with monolayer (ML) phosphorene, few-layer (FL) phosphorene is easier to prepare, is more stable in experiments, and is expected to form a smaller Schottky barrier height (SBH) at the phosphorene-metal interface. Using ab initio electronic structure calculations and quantum transport simulations, we perform a systematic study of the interfacial properties of three-layer (3L) phosphorene field effect transistors (FETs) contacted with several common metals (Al, Ag, Au, Cu, Ti, Cr, Ni, and Pd) for the first time. The SBHs obtained in the vertical direction from projecting the band structures of the 3L phosphorene-metal systems to the left bilayer (2L) phosphorenes are comparable with those obtained in the lateral direction from the quantum transport simulations for 2L phosphorene FETs. The quantum transport simulations for the 3L phosphorene FETs show that 3L phosphorene forms n-type Schottky contacts with electron SBHs of 0.16 and 0.28 eV in the lateral direction, when Ag and Cu are used as electrodes, respectively, and p-type Schottky contacts with hole SBHs of 0.05, 0.11, 0.20, 0.30, 0.30, and 0.31 eV in the lateral direction when Cr, Pd, Ni, Ti, Al, and Au are used as electrodes, respectively. The calculated polarity and SBHs of the 3L phosphorene FETs are generally in agreement with the available experiments.

GaAsP/Si tandem solar cells: Realistic prediction of efficiency gain by applying strain-balanced multiple quantum wells

The combination of tunable direct bandgap III-V absorbers with active Si substrates promises high-efficiency tandem solar cells. To yield the ideal III-V bandgaps between 1.6 and 1.8eV which is considered to achieve current matching with the Si bottom-cell, however, either a big lattice-mismatch or dilute nitrides had to be considered so far. Here, we propose a new structure for a two-terminal III-V-on-Si solar cell which bases on strain-balanced III-V multi-quantum wells (MQWs) embedded in a metamorphic GaAsP top-cell matrix. Strain-balanced MQWs extend the absorption edge to a longer wavelength and enable a reduction of the As content in the GaAsP metamorphic top-cell matrix. And accumulation of carriers in MQWs favors radiative recombination, which is beneficial for high efficiency while deep quantum wells lower the charge carrier collection efficiency. Here, we predict solar energy conversion efficiencies over 42.6% with an entire MQW as thin as 500nm. The applied model takes into account the drawbacks of MQWs, such as limited light absorption and the bottleneck of charge carrier collection from the confinement.

Testbeds for Transition Metal Dichalcogenide Photonics: Efficacy of Light Emission Enhancement in Monomer vs Dimer Nanoscale Antennae

Monolayer transition metal dichalcogenides (TMDs) are materials with unique potential for photonic and optoelectronic applications. They offer well-defined tunable direct band gaps in a broad electromagnetic spectral range. The small optical path across them naturally limits the light–matter interactions of these two-dimensional (2-D) materials, due to their atomic thinness. Nanoscale plasmonic antennae offer a substantial increase of field strength over very short distances, comparable to the native thickness of the TMD. For instance, it has been demonstrated that plasmonic dimer antennae generate hot-spot field enhancements by orders of magnitude when an emitter is positioned exactly over the middle of their gap. However, 2-D materials cannot be grown or easily transferred, to reside midgap of the metallic dimer cavity. Hence, it is not plausible to simply take the peak intensity as the emission enhancement factor. Here we show that the emission enhancement generated in a 2-D TMD film by a monomer anten...

Exotic Physics and Chemistry of Two-Dimensional Phosphorus: Phosphorene.

Phosphorene, the monolayer form of black phosphorus, is the most recent addition to graphene-like van der Waals two-dimensional (2D) systems. Due to its several interesting properties, namely its tunable direct band gap, high carrier mobility, and unique in-plane anisotropy, it has emerged as a promising candidate for electronic and optoelectronic devices. Phosphorene (Pn) reveals a much richer phase diagram than graphene, and it comprises the two forms namely the stapler-clip like (black Pn, α form) and chairlike (blue Pn, β form) structures. Regardless of its favorable properties, black Pn suffers from instability in oxygen and water, which limits its successful applications in electronic devices. In this Perspective, the cause of structural diversity of Pn, which leads to different properties of both black and blue Pn, is discussed. We provide possible solutions for protecting phosphorene from chemical degradation and its applications in the field of energy storage namely for Li and Na ion batteries.

Black Phosphorus Rediscovered: From Bulk Material to Monolayers.

Phosphorus is a nonmetal with several allotropes, from the highly reactive white phosphorus to the thermodynamically stable black phosphorus (BP) with a puckered orthorhombic layered structure. The bulk form of BP was first synthesized in 1914, but received little attention until it was rediscovered in 2014 as a member of the new wave of 2D layered nanomaterials. BP can be exfoliated to a single sheet that acts as a semiconductor with a tunable direct band gap, a high carrier mobility at room temperature, and an in-plane anisotropy. The development of BP applications is hampered by surface degradation, thus efforts to achieve effective BP passivation are ongoing, such as its integration in van der Waals heterostructures. BP has been tested as a novel nanomaterial in batteries, transistors, sensors, and photonics. This Review begins with the origin of the BP story, following the path from a bulk material to modern few/single layers. The physical and chemical properties are summarized, and the state-of-the-art of BP applications highlighted.

Antimonene Oxides: Emerging Tunable Direct Bandgap Semiconductor and Novel Topological Insulator.

Highly stable antimonene, as the cousin of phosphorene from group-VA, has opened up exciting realms in the two-dimensional (2D) materials family. However, pristine antimonene is an indirect band gap semiconductor, which greatly restricts its applications for optoelectronics devices. Identifying suitable materials, both responsive to incident photons and efficient for carrier transfer, is urgently needed for ultrathin devices. Herein, by means of first-principles computations we found that it is rather feasible to realize a new class of 2D materials with a direct bandgap and high carrier mobility, namely antimonene oxides with different content of oxygen. Moreover, these tunable direct bandgaps cover a wide range from 0 to 2.28 eV, which are crucial for solar cell and photodetector applications. Especially, the antimonene oxide (18Sb-18O) is a 2D topological insulator with a sizable global bandgap of 177 meV, which has a nontrivial Z2 topological invariant in the bulk and the topological states on the edge. Our findings not only introduce new vitality into 2D group-VA materials family and enrich available candidate materials in this field but also highlight the potential of these 2D semiconductors as appealing ultrathin materials for future flexible electronics and optoelectronics devices.

PV Absorber Layered Materials Formation Using Potential Pulse Atomic Layer Deposition (PP-ALD),

Thin film photovoltaic solar cells are a promising avenue for solar technology. However, they currently are not as cost effective as fossil fuels. Several absorber layer materials have been formed using various methods in the past. CuIn(1-x)GaxSe2(CIGS) is a promising material due to its tunable direct bandgap, allowing thin films to be used in photovoltaic devices. Previous studies of CIGS done by the author’s group investigated thin film deposition using electrochemical atomic layer deposition (E-ALD). Potential Pulse Atomic Layer Deposition (PP-ALD) is an electrochemical method of atomic layer deposition that relies on alternating cathodic and anodic potential pulses. Cathodic potential pulses long enough to form fractions of a monolayer are used. This is followed by an anodic potential which makes use of the underpotential deposition (UPD) phenomenon to strip away excess bulk, allowing stoichiometric deposits. Because short pulses are used, there is not enough time for excess elemental layers to be formed. PP-ALD is also a one solution co-electrodeposition technique. This differs from the previous E-ALD, which required multiple solution switching, allowing for PP-ALD to have shorter deposition cycles, saving solution and time. Initial results for the formation of In2Se3 and other components of the CIGS compound will be discussed.

Optical and Electrical Changes in 2-Dimentional Black Phosphorus Under Electron-Beam Irradiation

Black phosphorus (BP) has a puckered honeycomb structure with a tunable bandgap (direct bandgap from 0.3 eV (bulk) to 2.0 eV (monolayer)), where each layer is held together by a weak van der Waals interaction. With its high surface to volume ratio, tunable direct bandgap, excellent electrical and optical properties, BP is expected to greatly contribute to the improvement in nanotechnology. In previous studies, the properties of BP were studied using different characterization methods including transmission electron microscopy and scanning electron microscopy (SEM) and various nanoscale devices were fabricated by electron beam lithography (EBL) process. Thus widely used technologies that utilize electron beam (e-beam) in semiconductor fields imply that discovering the influence of electron-beam irradiation on BP would attribute to better understanding and make use of full potential that BP contains. In our experiment we have employed EBL equipment in order to irradiate e-beam onto BP flakes with precisely controlled beam intensity and area dose. As BP tends to degrade rapidly with the presence of oxygen and water, the vulnerability of BP under e-beam irradiation was also investigated with increasing exposure time under air condition. The mechanically exfoliated BP flakes were irradiated with e-beam after storing each sample in air ambient for different periods of time. It was found that the degradation of BP under e-beam irradiation is highly dependent on the degree of oxidation (air exposure time). The surface morphology was observed in situ by SEM while e-beam was irradiated inside EBL chamber while optical and electrical properties of the BP samples were analyzed using Raman spectroscopy and electrostatic force microscopy, respectively. Realizing the role of e-beam, accurate results can be obtained from further researches on BP. Further results and discussion will be presented in detail.