Optical parameters of Al-doped ZnO nanorod array thin films grown via the hydrothermal method.

ZnO seed layers were deposited onto a quartz substrate using the sol--gel method, and Al-doped ZnO (AZO) nanorod array thin films with different Al concentrations that ranged from 0 to 2.0 at. % were grown on the ZnO seed layers via the hydrothermal method. Optical parameters, including the optical band gap, the absorption coefficient, the Urbach energy, the refractive index, the dispersion parameter, and the optical conductivity, were studied to investigate the effects of Al doping on the optical properties of AZO nanorod array thin films. The optical band gaps of the ZnO and AZO nanorod array thin films were 3.206 at 0 at.%, 3.214 at 0.5 at.%, 3.226 at 1.5 at.%, and 3.268 at 2.0 at.%. The Urbach energy gradually decreased from 126 meV (0 at.%) to 70 meV (2.0 at.%) as the Al concentration was increased. The dispersion energy, the single-oscillator energy, the average oscillator wavelength, the average oscillator strength, the refractive index, and the optical conductivity of the AZO nanorod array thin films were all affected by Al doping.

Photoluminescence properties of defect emissions in Al-doped ZnO nanorod array thin films.

The power- and temperature-dependent photoluminescence properties of Al-doped ZnO nanorod array thin films grown by the hydrothermal method were investigated. The intensities of both the near-band-edge emission (NBE) and deep-level emission (DLE) as well as the overall spectral line shape were strongly affected by the excitation power. At low excitation power, the blue emission was found to show the highest intensity among the different emission lights. A low-temperature photoluminescence analysis revealed the bound-exciton-related luminescence peak at 3.362 eV. The dependence of peak energy with the excitation power indicates that these DLE processes are generated by DAP transitions. The overall intensity of DLE was found to decrease as the temperature increases. With regard to the blue emission (around 2.52 eV), it showed a well-pronounced shoulder at 200 K. The activation energy for this blue emission was 51.93 meV, which corresponds to the thermal dissociation energy required for the donor-acceptor pair transitions.

Magnesium content ratio effects of MgxZn1-xO seed layers on structural and optical properties of ZnO nanorods.

ZnO nanorods were grown on MgxZn1-xO seed layers with different content ratio ranging from 0 to 0.3 by hydrothermal method. Scanning electron microscopy (SEM), X-ray diffraction (XRD), and photoluminescence (PL) were carried out to investigate the effects of Mg content ratio for the MgxZn1-xO seed layers on the structural and optical properties of the ZnO nanorods. The surface morphology and structural properties of the MgxZn1-xO seed layers were changed by the Mg incorporation. However, the appearance, such as density, diameter, and shape, of the ZnO nanorods grown on the MgxZn1-xO seed layers was not changed significantly. The highest intensity ratio of the near-band-edge emission (NBE) to deep-level emission (DLE) and the narrowest full width at half maximum (FWHM) of the NBE peaks, indicating improvement in the crystallinity and luminescent properties of the ZnO crystals, were observed in the ZnO nanorods grown on the MgxZn1_xO seed layers with the content ratio of the 0.05.

Real-time ultra-sensitive detection of SARS-CoV-2 by quasi-freestanding epitaxial graphene-based biosensor.

We report the rapid detection of SARS-CoV-2 in infected patients (mid-turbinate swabs and exhaled breath aerosol samples) in concentrations as low as 60 copies/mL of the virus in seconds by electrical transduction of the SARS-CoV-2 S1 spike protein antigen via SARS-CoV-2 S1 spike protein antibodies immobilized on bilayer quasi-freestanding epitaxial graphene without gate or signal amplification. The sensor demonstrates the spike protein antigen detection in a concentration as low as 1 ag/mL. The heterostructure of the SARS-CoV-2 antibody/graphene-based sensor is developed through a simple and low-cost fabrication technique. Furthermore, sensors integrated into a portable testing unit distinguished B.1.1.7 variant positive samples from infected patients (mid-turbinate swabs and saliva samples, 4000-8000 copies/mL) with a response time of as fast as 0.6 s. The sensor is reusable, allowing for reimmobilization of the crosslinker and antibodies on the biosensor after desorption of biomarkers by NaCl solution or heat treatment above 40 °C.

Piezoresistive Graphene/P(VDF-TrFE) Heterostructure Based Highly Sensitive and Flexible Pressure Sensor.

We report on a novel graphene/P(VDF-TrFE) heterostructure based highly sensitive, flexible, and biocompatible pressure/strain sensor developed through a facile and low-cost fabrication technique. The high piezoelectric coefficient of P(VDF-TrFE) coupled with outstanding electrical properties of graphene makes the sensor device highly sensitive, with an average sensitivity of 0.76 kPa-1, a gauge factor of 445, and signal-to-noise ratio of 60.8 dB in the range of pressure up to 45 mmHg. A model was proposed to explain the sensor operation, based on carrier density and mobility changes induced by the piezoelectric charge generated in response to strain, which was supported by Hall measurements and Raman spectroscopy. Potential applications in wearable sensing for human activity monitoring were also demonstrated.

Facile synthesis and enhanced ultraviolet emission of ZnO nanorods prepared by vapor-confined face-to-face annealing.

In this study, we report a novel regrowth method of sol-gel-prepared ZnO films using a vapor-confined face-to-face annealing (VC-FTFA) technique in which mica was inserted between two films, followed by annealing with the FTFA method. The ZnO nanorods are regrown when zinc acetate dihydrate and zinc chloride (ZnCl2) are used as the solvent, because these generate ZnCl2 vapor. The near-band-edge emission intensity of the ZnO nanorods was enhanced through the VC-FTFA method, increasing significantly by a factor of 56 compared to that of ZnO films annealed in open air at 700 °C. Our method may provide a route toward the facile fabrication of ZnO nanorods.

Optical parameters of boron-doped ZnO nanorods grown by low-temperature hydrothermal reaction.

Sol-gel spin-coating was used to deposit ZnO seed layers onto quartz substrates, and ZnO nanorods doped with various concentrations of B (i.e., BZO nanorods) ranging from 0 to 2.0 at% were hydrothermally grown on the ZnO seed layers. The effects of B doping on the absorption coefficient, optical band gap, Urbach energy, refractive index, extinction coefficient, single-oscillator energy, dispersion energy, average oscillator strength, average oscillator wavelength, dielectric constant, and optical conductivity of the hydrothermally grown BZO nanorods were investigated. The optical band gaps were 3.255, 3.243, 3.254, 3.258, and 3.228 eV for the nanorods grwon at 0, 0.5, 1.0, 1.5 and 2.0 at% B, respectively. B doping increased the Urbach energy from 40.7 to 65.1 meV for the nanorods grown at 0 and 2.0 at% B, respectively, and significantly affected the dispersion energy, the single-oscillator energy, the average oscillator wavelength, the average oscillator strength, the refractive index, and the optical conductivity of the hydrothermally grown BZO nanorods.