Rapid Screen for Antiviral T-Cell Immunity with Nanowire Electrochemical Biosensors.

The ability to rapidly assess and monitor patient immune responses is critical for clinical diagnostics, vaccine design, and fundamental investigations into the presence or generation of protective immunity against infectious diseases. Recently, findings on the limits of antibody-based protection provided by B-cells have highlighted the importance of engaging pathogen-specific T-cells for long-lasting and broad protection against viruses and their emergent variants such as in SARS-CoV-2. However, low-cost and point-of-care tools for detecting engagement of T-cell immunity in patients are conspicuously lacking in ongoing efforts to assess and control population-wide disease risk. Currently available tools for human T-cell analysis are time and resource-intensive. Using multichannel silicon-nanowire field-effect transistors compatible with complementary metal-oxide-semiconductor, a device designed for rapid and label-free detection of human T-cell immune responses is developed. The generalizability of this approach is demonstrated by measuring T-cell responses against melanoma antigen MART1, common and seasonal viruses CMV, EBV, flu, as well as emergent pandemic coronavirus, SARS-CoV-2. Further, this device provides a modular and translational platform for optimizing vaccine formulations and combinations, offering quick and quantitative readouts for acquisition and persistence of T-cell immunity against variant-driven pathogens such as flu and pandemic SARS-CoV-2.

Designing Sensitivity: A Comparative Analysis of Microelectrode Topologies for Electrochemical Oxygen Sensing in Biomedical Applications.

The monitoring of dissolved oxygen is a key parameter in many fields, namely the treatment and monitoring of various cerebral traumas. Leveraging existing manufacturing techniques, electrochemical sensors hold the potential for compact, simple, and scalable dissolved oxygen sensors. Past studies have focused on the general design of such sensors, but a comparative study on the impact of microelectrode geometries for cerebral applications has been forthcoming. We present here the results of a characterization study conducted across solid-state sensors with varying microelectrode geometries. The electrode structures were covered with a Nafion membrane and included variations of the classic interdigitated microelectrode array in addition to a circular microelectrode array variation. Voltage sweeps were conducted while monitoring the devices' sensing current responses across a 50.3 mmHg change in dissolved oxygen within a deionized aqueous solution. Half of the devices were identified as ultramicroelectrode designs that presented a greater dependence on electrode spacing and topology. The ultramicroelectrode-style (UME) interdigitated electrode (IDE) topology presented the greatest signal response at 25.24 nA/mmHg, an approximate eight-fold improvement in sensitivity from a non-UME variation with a sensitivity of 2.98 nA/mmHg. The design presented a linear response from 8.3 mmHg to 58.6 mmHg with r2 = 0.9743. The sensitivity improvement was attributed to the ultramicroelectrode structure's amplifying diffusive feedback, which was enabled by the IDE topology and short electrode spacings.

Etched-And-Regrown GaN P-N Diodes with Low-Defect Interfaces Prepared by In Situ TBCl Etching.

The ability to form pristine interfaces after etching and regrowth of GaN is a prerequisite for epitaxial selective area doping, which in turn is needed for the formation of lateral PN junctions and advanced device architectures. In this work, we report the electrical properties of etched-and-regrown GaN PN diodes using an in situ Cl-based precursor, tertiary butylchloride (TBCl). We demonstrated a regrowth diode with I-V characteristics approaching that from a continuously grown reference diode. The sources of unintentional contamination from the silicon (Si) impurity and the mediating effect of Si during the TBCl etching are also investigated in this study. This work points to the potential of in situ TBCl etching toward the realization of GaN lateral PN junctions.

Spectrally-resolved internal quantum efficiency and carrier dynamics of semipolar [Formula: see text] core-shell triangular nanostripe GaN/InGaN LEDs.

We investigate the spectrally resolved internal quantum efficiency (IQE) and carrier dynamics in semipolar [Formula: see text] core-shell triangular nanostripe light-emitting diodes (TLEDs) using temperature-dependent photoluminescence (TDPL) and time-resolved photoluminescence (TRPL) at various excitation energy densities. Using electroluminescence, photoluminescence, and cathodoluminescence measurements, we verify the origins of the broad emission spectra from the nanostructures and confirm that localized regions of high-indium-content InGaN exist along the apex of the nanostructures. Spectrally resolved IQE measurements are then performed, with the spectra integrated from 400-450 nm and 450-500 nm to obtain the IQE of the QWs mainly near the sidewalls and apex of the TLEDs, respectively. TDPL and TRPL are used to decouple the radiative and non-radiative carrier lifetimes for different regions of the emission spectra. We observe that the IQE is higher for the spectral region between 450 nm and 500 nm compared to the IQE between 400 and 450 nm. This result is in contrast to the typical observation that the IQE of planar GaN-based LEDs is lower for longer wavelengths (i.e., higher indium contents). We also observe a longer non-radiative recombination lifetime for the longer wavelength portion of the spectrum. Several explanations are proposed for the improved IQE and longer non-radiative lifetime observed near the apex of the nanostructures. The results show that nanostructures may be leveraged to design more efficient green LEDs, potentially addressing a long-standing challenge in GaN-based materials.

Scalable Top-Down Approach Tailored by Interferometric Lithography to Achieve Large-Area Single-Mode GaN Nanowire Laser Arrays on Sapphire Substrate.

GaN nanowires are promising for optical and optoelectronic applications because of their waveguiding properties and large optical band gap. However, developing a precise, scalable, and cost-effective fabrication method with a high degree of controllability to obtain high-aspect-ratio nanowires with high optical properties and minimum crystal defects remains a challenge. Here, we present a scalable two-step top-down approach using interferometric lithography, for which parameters can be controlled precisely to achieve highly ordered arrays of nanowires with excellent quality and desired aspect ratios. The wet-etch mechanism is investigated, and the etch rates of m-planes {11̅00} (sidewalls) were measured to be 2.5 to 70 nm/h depending on the Si doping concentration. Using this method, uniform nanowire arrays were achieved over a large area (>105 µm2) with an spect ratio as large as 50, a radius as small as 17 nm, and atomic-scale sidewall roughness (<1 nm). FDTD modeling demonstrated HE11 is the dominant transverse mode in the nanowires with a radius of sub-100 nm, and single-mode lasing from vertical cavity nanowire arrays with different doping concentrations on a sapphire substrate was interestingly observed in photoluminescence measurements. High Q-factors of ∼1139-2443 were obtained in nanowire array lasers with a radius and length of 65 nm and 2 µm, respectively, corresponding to a line width of 0.32-0.15 nm (minimum threshold of 3.31 MW/cm2). Our results show that fabrication of high-quality GaN nanowire arrays with adaptable aspect ratio and large-area uniformity is feasible through a top-down approach using interferometric lithography and is promising for fabrication of III-nitride-based nanophotonic devices (radial/axial) on the original substrate.

Carrier Dynamics and Electro-Optical Characterization of High-Performance GaN/InGaN Core-Shell Nanowire Light-Emitting Diodes.

In this work, we demonstrate high-performance electrically injected GaN/InGaN core-shell nanowire-based LEDs grown using selective-area epitaxy and characterize their electro-optical properties. To assess the quality of the quantum wells, we measure the internal quantum efficiency (IQE) using conventional low temperature/room temperature integrated photoluminescence. The quantum wells show a peak IQE of 62%, which is among the highest reported values for nanostructure-based LEDs. Time-resolved photoluminescence (TRPL) is also used to study the carrier dynamics and response times of the LEDs. TRPL measurements yield carrier lifetimes in the range of 1-2 ns at high excitation powers. To examine the electrical performance of the LEDs, current density-voltage (J-V) and light-current density-voltage (L-J-V) characteristics are measured. We also estimate the peak external quantum efficiency (EQE) to be 8.3% from a single side of the chip with no packaging. The LEDs have a turn-on voltage of 2.9 V and low series resistance. Based on FDTD simulations, the LEDs exhibit a relatively directional far-field emission pattern in the range of [Formula: see text]15°. This work demonstrates that it is feasible for electrically injected nanowire-based LEDs to achieve the performance levels needed for a variety of optical device applications.

Explanation of low efficiency droop in semipolar (202¯1¯) InGaN/GaN LEDs through evaluation of carrier recombination coefficients.

We report the carrier dynamics and recombination coefficients in single-quantum-well semipolar (202¯1¯) InGaN/GaN light-emitting diodes emitting at 440 nm with 93% peak internal quantum efficiency. The differential carrier lifetime is analyzed for various injection current densities from 5 A/cm2 to 10 kA/cm2, and the corresponding carrier densities are obtained. The coupling of internal quantum efficiency and differential carrier lifetime vs injected carrier density (n) enables the separation of the radiative and nonradiative recombination lifetimes and the extraction of the Shockley-Read-Hall (SRH) nonradiative (A), radiative (B), and Auger (C) recombination coefficients and their n-dependency considering the saturation of the SRH recombination rate and phase-space filling. The results indicate a three to four-fold higher A and a nearly two-fold higher B0 for this semipolar orientation compared to that of c-plane reported using a similar approach [A. David and M. J. Grundmann, Appl. Phys. Lett. 96, 103504 (2010)]. In addition, the carrier density in semipolar (202¯1¯) is found to be lower than the carrier density in c-plane for a given current density, which is important for suppressing efficiency droop. The semipolar LED also shows a two-fold lower C0 compared to c-plane, which is consistent with the lower relative efficiency droop for the semipolar LED (57% vs. 69%). The lower carrier density, higher B0 coefficient, and lower C0 (Auger) coefficient are directly responsible for the high efficiency and low efficiency droop reported in semipolar (202¯1¯) LEDs.

GaN nanowire tips for nanoscale atomic force microscopy.

Imaging of high-aspect-ratio nanostructures with sharp edges and straight walls in nanoscale metrology by atomic force microscopy (AFM) has been challenging due to the mechanical properties and conical geometry of the majority of available commercial tips. Here we report on the fabrication of GaN probes for nanoscale metrology of high-aspect-ratio structures to enhance the resolution of AFM imaging and improve the durability of AFM tips. GaN nanowires were fabricated using bottom-up and top-down techniques and bonded to Si cantilevers to scan vertical trenches on Si substrates. Over several scans, the GaN probes demonstrated excellent durability while scanning uneven structures and showed resolution enhancements in topography images, independent of scan direction, compared to commercial Si tips.

Internal quantum efficiency and carrier dynamics in semipolar (2021) InGaN/GaN light-emitting diodes.

The internal quantum efficiencies (IQE) and carrier lifetimes of semipolar (202¯1¯) InGaN/GaN LEDs with different active regions are measured using temperature-dependent, carrier-density-dependent, and time-resolved photoluminescence. Three active regions are investigated: one 12-nm-thick single quantum well (SQW), two 6-nm-thick QWs, and three 4-nm-thick QWs. The IQE is highest for the 12-nm-thick SQW and decreases as the well width decreases. The radiative lifetimes are similar for all structures, while the nonradiative lifetimes decrease as the well width decreases. The superior IQE and longer nonradiative lifetime of the SQW structure suggests using thick SQW active regions for high brightness semipolar (202¯1¯) LEDs.

Tailoring the morphology and luminescence of GaN/InGaN core-shell nanowires using bottom-up selective-area epitaxy.

Controlled bottom-up selective-area epitaxy (SAE) is used to tailor the morphology and photoluminescence properties of GaN/InGaN core-shell nanowire arrays. The nanowires are grown on c-plane sapphire substrates using pulsed-mode metal organic chemical vapor deposition. By varying the dielectric mask configuration and growth conditions, we achieve GaN nanowire cores with diameters ranging from 80 to 700 nm that exhibit various degrees of polar, semipolar, and nonpolar faceting. A single InGaN quantum well (QW) and GaN barrier shell is also grown on the GaN nanowire cores and micro-photoluminescence is obtained and analyzed for a variety of nanowire dimensions, array pitch spacings, and aperture diameters. By increasing the nanowire pitch spacing on the same growth wafer, the emission wavelength redshifts from 440 to 520 nm, while increasing the aperture diameter results in a ∼35 nm blueshift. The thickness of one QW/barrier period as a function of pitch and aperture diameter is inferred using scanning electron microscopy, with larger pitches showing significantly thicker QWs. Significant increases in indium composition were predicted for larger pitches and smaller aperture diameters. The results are interpreted in terms of local growth conditions and adatom capture radius around the nanowires. This work provides significant insight into the effects of mask configuration and growth conditions on the nanowire properties and is applicable to the engineering of monolithic multi-color nanowire LEDs on a single chip.

Optical properties of plasmonic light-emitting diodes based on flip-chip III-nitride core-shell nanowires.

In this work, we utilize the finite difference time domain (FDTD) method to investigate the Purcell factor, light extraction efficiency (EXE), and cavity quality parameter (Q), and to predict the modulation response of Ag-clad flip-chip GaN/InGaN core-shell nanowire light-emitting diodes (LEDs) with the potential for electrical injection. We consider the need for a pn-junction, the effects of the substrate, and the limitations of nanoscale fabrication techniques in the evaluation. The investigated core-shell nanowire consists of an n-GaN core, surrounded by nonpolar m-plane quantum wells, p-GaN, and silver cladding layers. The core-shell nanowire geometry exhibits a Purcell factor of 57, resulting in a predicted limit of 30 GHz for the 3dB modulation bandwidth.

Tunable microwave signal generator with an optically-injected 1310 nm QD-DFB laser.

Tunable microwave signal generation with frequencies ranging from below 1 GHz to values over 40 GHz is demonstrated experimentally with a 1310 nm Quantum Dot (QD) Distributed-Feedback (DFB) laser. Microwave signal generation is achieved using the period 1 dynamics induced in the QD DFB under optical injection. Continuous tuning in the positive detuning frequency range of the quantum dot's unique stability map is demonstrated. The simplicity of the experimental configuration offers promise for novel uses of these nanostructure lasers in Radio-over-Fiber (RoF) applications and future mobile networks.