Topological Proximity-Induced Dirac Fermion in Two-Dimensional Antimonene.

Antimonene is a promising two-dimensional (2D) material that is calculated to have a significant fundamental bandgap usable for advanced applications such as field-effect transistors, photoelectric devices, and the quantum-spin Hall (QSH) state. Herein, we demonstrate a phenomenon termed topological proximity effect, which occurs between a 2D material and a three-dimensional (3D) topological insulator (TI). We provide strong evidence derived from hydrogen etching on Sb2Te3 that large-area and well-ordered antimonene presents a 2D topological state. Delicate analysis with a scanning tunneling microscope of the evolutionary intermediates reveals that hydrogen etching on Sb2Te3 resulted in the formation of a large area of antimonene with a buckled structure. A topological state formed in the antimonene/Sb2Te3 heterostructure was confirmed with angle-resolved photoemission spectra and density-functional theory calculations; in particular, the Dirac point was located almost at the Fermi level. The results reveal that Dirac fermions are indeed realized at the interface of a 2D normal insulator (NI) and a 3D TI as a result of strong hybridization between antimonene and Sb2Te3. Our work demonstrates that the position of the Dirac point and the shape of the Dirac surface state can be tuned by varying the energy position of the NI valence band, which modifies the direction of the spin texture of Sb-BL/Sb2Te3 via varying the Fermi level. This topological phase in 2D-material engineering has generated a paradigm in that the topological proximity effect at the NI/TI interface has been realized, which demonstrates a way to create QSH systems in 2D-material TI heterostructures.

Portable Automatic Microring Resonator System Using a Subwavelength Grating Metamaterial Waveguide for High-Sensitivity Real-Time Optical-Biosensing Applications.

The slow light sensor techniques have been applied to bio-related detection in the past decades. However, similar testing-systems are too large to carry to a remote area for diagnosis or point-of-care testing. This study demonstrated a fully automatic portable biosensing system based on the microring resonator. An optical-fiber array mounted on a controller based micro-positioning system, which can be interfaced with MATLAB to locate a tentative position for light source and waveguide coupling alignment. Chip adapter and microfluidic channel could be packaged as a product such that it is cheap to be manufactured and can be disposed of after every test conducted. Thus, the platform can be more easily operated via an ordinary user without expertise in photonics. It is designed based on conventional optical communication wavelength range. The C-band superluminescent-light-emitting-diode light source couples in/out the microring sensor to obtain quasi-TE mode by grating coupler techniques. For keeping a stable chemical binding reaction, the cost-effective microfluidic pump was developed to offer a specific flow rate of 20 µL/min by using a servo-motor, an Arduino board, and a motor driver. The subwavelength grating metamaterial ring resonator shows highly sensitive sensing performance via surface index changes due to biomarker adhered on the sensor. The real-time peak-shift monitoring shows 10 µg/mL streptavidin detection of limit based on the biotin-streptavidin binding reaction. Through the different specific receptors immobilized on the sensor surface, the system can be utilized on the open applications such as heavy metal detection, gas sensing, virus examination, and cancer marker diagnosis.

Role of carrier-transfer in the optical nonlinearity of graphene/Bi2Te3 heterojunctions.

Two-dimensional (2D) topological insulators (TIs) have attracted a lot of attention owing to their striking optical nonlinearity. However, the ultra-low saturable intensity (SI) of TIs resulting from the bulk conduction band limits their applications, such as in mode-locking solid-state lasers. In this work, through fabricating a graphene/Bi2Te3 heterojunction which combines monolayer graphene and a Bi2Te3 nanoplate, the optical nonlinearities are analyzed. Moreover, the thickness-dependent characteristics are also investigated by varying the thickness of the Bi2Te3 when synthesizing the heterojunctions. Furthermore, with the aid of the estimated junction electron escape time, a model of the photo-excited carrier-transfer mechanism is proposed and used to describe the phenomena of depression of ultra-low saturable absorption (SA) from the Bi2Te3 bulk band. The increased modulation depth of the graphene/Bi2Te3 heterojunction can accordingly be realized in more detail. In addition, a Q-switched solid-state laser operating at 1064 nm with heterojunction saturable absorbers is built up and characterized for validating the proposed model. The laser performance with varied Bi2Te3 thickness, such as pulse duration and repetition rate, agrees quite well with our proposed model. Our work demonstrates the functionality of optical nonlinear engineering by tuning the thickness of the graphene/Bi2Te3 heterojunction and demonstrates its potential for applications.

Light Trapping Induced High Short-Circuit Current Density in III-Nitride Nanorods/Si (111) Heterojunction Solar Cells.

An effective-area photovoltaic efficiency of 1.27% in power conversion, excluding the grid metal contact area and under 1 sun, AM 1.5G conditions, has been obtained for the p-GaN/i-InGaN/n-GaN diode arrays epitaxially grown on (111)-Si. The short-circuit current density is 14.96 mA/cm2 and the open-circuit voltage is 0.28 V. Enhanced light trapping acquired via multiple reflections within the strain and defect free III-nitride nanorod array structures and the short-wavelength responses boosted by the wide bandgap III-nitride constituents are believed to contribute to the observed enhancements in device performance.

Pedestal subwavelength grating metamaterial waveguide ring resonator for ultra-sensitive label-free biosensing.

Mode volume overlap factor is one of the parameters determining the sensitivity of a sensor. In past decades, many approaches have been proposed to increase the mode volume overlap. As the increased mode volume overlap factor results in reduced mode confinement, the maximum value is ultimately determined by the micro- and nano-structure of the refractive index distribution of the sensing devices. Due to the asymmetric index profile along the vertical direction on silicon-on-insulator platform, further increasing the sensitivity of subwavelength grating metamaterial (SGM) waveguide based sensors is challenging. In this paper, we propose and demonstrate pedestaled SGM which reduces the asymmetricity and thus allows further increasing the interaction between optical field and analytes. The pedestal structure can be readily formed by a controlled undercut etching. Both theoretical analysis and experimental demonstration show a significant improvement of sensitivity. The bulk sensitivity and surface sensitivity are improved by 28.8% and 1000 times, respectively. The detection of streptavidin at a low concentration of 0.1 ng/mL (∼1.67 pM) is also demonstrated through real-time monitoring of the resonance shift. A ∼400 fM streptavidin limit of detection is expected with a 0.01nm resolution spectrum analyzer based on the real-time measurement of streptavidin detection results from two-site binding model fitting.

Growth and characterization of textured well-faceted ZnO on planar Si(100), planar Si(111), and textured Si(100) substrates for solar cell applications.

In this work, textured, well-faceted ZnO materials grown on planar Si(100), planar Si(111), and textured Si(100) substrates by low-pressure chemical vapor deposition (LPCVD) were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and cathode luminescence (CL) measurements. The results show that ZnO grown on planar Si(100), planar Si(111), and textured Si(100) substrates favor the growth of ZnO(110) ridge-like, ZnO(002) pyramid-like, and ZnO(101) pyramidal-tip structures, respectively. This could be attributed to the constraints of the lattice mismatch between the ZnO and Si unit cells. The average grain size of ZnO on the planar Si(100) substrate is slightly larger than that on the planar Si(111) substrate, while both of them are much larger than that on the textured Si(100) substrate. The average grain sizes (about 10-50 nm) of the ZnO grown on the different silicon substrates decreases with the increase of their strains. These results are shown to strongly correlate with the results from the SEM, AFM, and CL as well. The reflectance spectra of these three samples show that the antireflection function provided by theses samples mostly results from the nanometer-scaled texture of the ZnO films, while the micrometer-scaled texture of the Si substrate has a limited contribution. The results of this work provide important information for optimized growth of textured and well-faceted ZnO grown on wafer-based silicon solar cells and can be utilized for efficiency enhancement and optimization of device materials and structures, such as heterojunction with intrinsic thin layer (HIT) solar cells.

Effect of Nascent Peptide Steric Bulk on Elongation Kinetics in the Ribosome Exit Tunnel.

All proteins are synthesized by the ribosome, a macromolecular complex that accomplishes the life-sustaining tasks of faithfully decoding mRNA and catalyzing peptide bond formation at the peptidyl transferase center (PTC). The ribosome has evolved an exit tunnel to host the elongating new peptide, protect it from proteolytic digestion, and guide its emergence. It is here that the nascent chain begins to fold. This folding process depends on the rate of translation at the PTC. We report here that besides PTC events, translation kinetics depend on steric constraints on nascent peptide side chains and that confined movements of cramped side chains within and through the tunnel fine-tune elongation rates.

Determinants of Helix Formation for a Kv1.3 Transmembrane Segment inside the Ribosome Exit Tunnel.

Proteins begin to fold in the ribosome, and misfolding has pathological consequences. Among the earliest folding events in biogenesis is the formation of a helix, an elementary structure that is ubiquitously present and required for correct protein folding in all proteomes. The determinants underlying helix formation in the confined space of the ribosome exit tunnel are relatively unknown. We chose the second transmembrane segment, S2, of a voltage-gated potassium channel, Kv1.3, as a model to probe this issue. Since the N terminus of S2 is initially in an extended conformation in the folding vestibule of the ribosome yet ultimately emerges at the exit port as a helix, S2 is ideally suited for delineating sequential events and folding determinants of helix formation inside the ribosome. We show that S2's extended N terminus inside the tunnel is converted into a helix by a single, distant mutation in the nascent peptide. This transition depends on nascent peptide sequence at specific tunnel locations. Co-translational secondary folding of nascent chains inside the ribosome has profound physiological consequences that bear on correct membrane insertion, tertiary folding, oligomerization, and biochemical modification of the newborn protein during biogenesis.

Enhanced Ferromagnetic Interaction in Modulation-Doped GaMnN Nanorods.

In this report, ferromagnetic interactions in modulation-doped GaMnN nanorods grown on Si (111) substrate by plasma-assisted molecular beam epitaxy are investigated with the prospect of achieving a room temperature ferromagnetic semiconductor. Our results indicate the thickness of GaN layer in each GaN/MnN pair, as well as Mn-doping levels, are essential for suppressing secondary phases as well as enhancing the magnetic moment. For these optimized samples, structural analysis by high-resolution X-ray diffractometry and Raman spectroscopy verifies single-crystalline modulation-doped GaMnN nanorods with Ga sites substituted by Mn atoms. Energy dispersive X-ray spectrometry shows that the average Mn concentration can be raised from 0.4 to 1.8% by increasing Mn fluxes without formation of secondary phases resulted in a notable enhancement of the saturation magnetization as well as coercive force in these nanorods.

Robustness of a Topologically Protected Surface State in a Sb2Te2Se Single Crystal.

A topological insulator (TI) is a quantum material in a new class with attractive properties for physical and technological applications. Here we derive the electronic structure of highly crystalline Sb2Te2Se single crystals studied with angle-resolved photoemission spectra. The result of band mapping reveals that the Sb2Te2Se compound behaves as a p-type semiconductor and has an isolated Dirac cone of a topological surface state, which is highly favored for spintronic and thermoelectric devices because of the dissipation-less surface state and the decreased scattering from bulk bands. More importantly, the topological surface state and doping level in Sb2Te2Se are difficult to alter for a cleaved surface exposed to air; the robustness of the topological surface state defined in our data indicates that this Sb2Te2Se compound has a great potential for future atmospheric applications.

Approaching Defect-free Amorphous Silicon Nitride by Plasma-assisted Atomic Beam Deposition for High Performance Gate Dielectric.

In the past few decades, gate insulators with a high dielectric constant (high-k dielectric) enabling a physically thick but dielectrically thin insulating layer, have been used to replace traditional SiOx insulator and to ensure continuous downscaling of Si-based transistor technology. However, due to the non-silicon derivative natures of the high-k metal oxides, transport properties in these dielectrics are still limited by various structural defects on the hetero-interfaces and inside the dielectrics. Here, we show that another insulating silicon compound, amorphous silicon nitride (a-Si3N4), is a promising candidate of effective electrical insulator for use as a high-k dielectric. We have examined a-Si3N4 deposited using the plasma-assisted atomic beam deposition (PA-ABD) technique in an ultra-high vacuum (UHV) environment and demonstrated the absence of defect-related luminescence; it was also found that the electronic structure across the a-Si3N4/Si heterojunction approaches the intrinsic limit, which exhibits large band gap energy and valence band offset. We demonstrate that charge transport properties in the metal/a-Si3N4/Si (MNS) structures approach defect-free limits with a large breakdown field and a low leakage current. Using PA-ABD, our results suggest a general strategy to markedly improve the performance of gate dielectric using a nearly defect-free insulator.

Numerical simulations of the current-matching effect and operation mechanisms on the performance of InGaN/Si tandem cells.

Numerical simulations are conducted to study the current-matching effect and operation mechanisms in and to design the optimized device structure of InGaN/Si tandem cells. The characteristics of short circuit current density (J sc), open circuit voltage (V oc), fill factor (FF), and conversion efficiency (η) of InGaN/Si tandem cells are determined by the current-matching effect. The similar trend of η to that of J sc shows that J sc is a dominant factor in determining the performance of InGaN/Si tandem cells. In addition, the combined effects of the J sc, V oc, and FF lead to an optimized η in the medium-indium, [Formula: see text], InGaN/Si tandem cell. At [Formula: see text], the J sc of the InGaN subcell is equal to that of the Si subcell such that an InGaN/Si tandem cell reaches the current matching condition to operate at the maximum power point. Similar to the J sc and FF, the η for low- [Formula: see text] and high-In [Formula: see text] InGaN/Si tandem cells are InGaN- and Si subcell-limited, respectively. Furthermore, the p- and n-layer thicknesses, indium content, and position of depletion region of InGaN subcell should be adjusted to reapportion the light between the two subcells and to achieve the maximum conversion efficiency. With appropriate thicknesses of p- and n-InGaN, In0.5-0.6Ga0.5-0.4 N/Si tandem cells can exhibit as high as approximately 34% to 36.5% conversion efficiency, demonstrating that a medium-indium InGaN/Si tandem cell results in a high-efficiency solar cell. Simulation results determine that the current-matching effect and operation mechanisms of InGaN/Si tandem cells can be utilized for efficiency enhancement through the optimized device structures.

Fabrication and characterization of back-side illuminated InGaN/GaN solar cells with periodic via-holes etching and Bragg mirror processes.

In this study, the design and fabrication schemes of back-side illuminated InGaN/GaN solar cells with periodic via-holes etching and Bragg mirror processes are presented. Compared to typical front-side illuminated solar cells, the improvements of open-circuit voltage (V(oc)) from 1.88 to 1.94 V and short-circuit current density (J(sc)) from 0.84 to 1.02 mA/cm(2) are observed. Most significantly, the back-side illuminated InGaN/GaN solar cells exhibit an extremely high fill factor up to 85.5%, leading to a conversion efficiency of 1.69% from 0.66% of typical front-side illuminated solar cells under air mass 1.5 global illuminations. Moreover, the effects of bottom Bragg mirrors on the photovoltaic characteristics of back-side illuminated solar cells are studied by an advanced simulation program. The results show that the J(sc) could further be improved with a factor of 10% from the original back-side illuminated solar cell by the structure optimization of bottom Bragg mirrors.

Effects of free carriers on piezoelectric nanogenerators and piezotronic devices made of GaN nanowire arrays.

This study investigates the role of carrier concentration in semiconducting piezoelectric single-nanowire nanogenerators (SNWNGs) and piezotronic devices. Unintentionally doped and Si-doped GaN nanowire arrays with various carrier concentrations, ranging from 10(17) (unintentionally doped) to 10(19) cm(-3) (heavily doped), are synthesized. For SNWNGs, the output current of individual nanowires starts from a negligible level and rises to the maximum of ≈50 nA at a doping concentration of 5.63 × 10(18) cm(-3) and then falls off with further increase in carrier concentration, due to the competition between the reduction of inner resistance and the screening effect on piezoelectric potential. For piezotronic applications, the force sensitivity based on the change of the Schottky barrier height works best for unintentionally doped nanowires, reaching 26.20 ± 1.82 meV nN(-1) and then decreasing with carrier concentration. Although both types of devices share the same Schottky diode, they involve different characteristics in that the slope of the current-voltage characteristics governs SNWNG devices, while the turn-on voltage determines piezotronic devices. It is demonstrated that free carriers in piezotronic materials can influence the slope and turn-on voltage of the diode characteristics concurrently when subjected to strain. This work offers a design guideline for the optimum doping concentration in semiconductors for obtaining the best performance in piezotronic devices and SNWNGs.

Type II GaSb quantum ring solar cells under concentrated sunlight.

A type II GaSb quantum ring solar cell is fabricated and measured under the concentrated sunlight. The external quantum efficiency confirms the extended absorption from the quantum rings at long wavelength coinciding with the photoluminescence results. The short-circuit current of the quantum ring devices is 5.1% to 9.9% more than the GaAs reference's under various concentrations. While the quantum ring solar cell does not exceed its GaAs counterpart in efficiency under one-sun, the recovery of the open-circuit voltages at higher concentration helps to reverse the situation. A slightly higher efficiency (10.31% vs. 10.29%) is reported for the quantum ring device against the GaAs one.

Type II GaSb quantum ring solar cells under concentrated sunlight.

A type II GaSb quantum ring solar cell is fabricated and measured under the concentrated sunlight. The external quantum efficiency confirms the extended absorption from the quantum rings at long wavelength coinciding with the photoluminescence results. The short-circuit current of the quantum ring devices is 5.1% to 9.9% more than the GaAs reference's under various concentrations. While the quantum ring solar cell does not exceed its GaAs counterpart in efficiency under one-sun, the recovery of the open-circuit voltages at higher concentration helps to reverse the situation. A slightly higher efficiency (10.31% vs. 10.29%) is reported for the quantum ring device against the GaAs one.

Determinants of pore folding in potassium channel biogenesis.

Many ion channels, both selective and nonselective, have reentrant pore loops that contribute to the architecture of the permeation pathway. It is a fundamental feature of these diverse channels, regardless of whether they are gated by changes of membrane potential or by neurotransmitters, and is critical to function of the channel. Misfolding of the pore loop leads to loss of trafficking and expression of these channels on the cell surface. Mature tetrameric potassium channels contain an α-helix within the pore loop. We systematically mutated the "pore helix" residues of the channel Kv1.3 and assessed the ability of the monomer to fold into a tertiary reentrant loop. Our results show that pore loop residues form a canonical α-helix in the monomer early in biogenesis and that disruption of tertiary folding is caused by hydrophilic substitutions only along one face of this α-helix. These results provide insight into the determinants of the reentrant pore conformation, which is essential for ion channel function.

Transmembrane segments form tertiary hairpins in the folding vestibule of the ribosome.

Folding of membrane proteins begins in the ribosome as the peptide is elongated. During this process, the nascent peptide navigates along 100Å of tunnel from the peptidyltransferase center to the exit port. Proximal to the exit port is a "folding vestibule" that permits the nascent peptide to compact and explore conformational space for potential tertiary folding partners. The latter occurs for cytosolic subdomains but has not yet been shown for transmembrane segments. We now demonstrate, using an accessibility assay and an improved intramolecular crosslinking assay, that the helical transmembrane S3b-S4 hairpin ("paddle") of a voltage-gated potassium (Kv) channel, a critical region of the Kv voltage sensor, forms in the vestibule. S3-S4 hairpin interactions are detected at an early stage of Kv biogenesis. Moreover, this vestibule hairpin is consistent with a closed-state conformation of the Kv channel in the plasma membrane.

Effects of crystallinity and point defects on optoelectronic applications of ß-Ga2O3 epilayers.

This study evaluates the effect of crystallinity and point defects on time-dependent photoresponsivity and the cathodoluminescence (CL) properties of ß-Ga2O3 epilayers. A synchrotron high-resolution X-ray technique was used to understand the crystalline structure of samples. Rutherford backscattering spectroscopy was used to determine the net chemical composition of the samples to examine the type and ratio of their possible point defects. The results show that in functional time-dependent photoresponsivity of photodetectors based on ß-Ga2O3 epilayers, point defects contribution overcomes the contribution of crystallinity. However, the crystalline structure affects the intensities and emission regions of CL spectra more than point defects.

Gigahertz coherent guided acoustic phonons in AlN/GaN nanowire superlattices.

The generation of guided acoustic phonons in the GHz range in GaN/AlN superlattices grown atop a GaN nanowire is presented. Combined with a femtosecond laser, ultrafast pump-probe spectroscopy allows the generation and detection of guided acoustic phonons at different frequencies in the nanowire superlattices. The capability of the nanowire superlattices to be excellent detectors of acoustic phonons at specific frequencies is then used to observe the strong dispersion, as a result of nanoconfinement, of guided acoustic phonons after their propagation in the nanowire. The generation of high frequency coherent guided acoustic phonons could be useful not only to realize an acoustic transducer with a nanolateral size but also as a source to understand the thermal behavior of nanowires.