Achieving Low-Dose Rate X-Ray Imaging Based on 2D/3D-Mixed Perovskite Films.

X-ray detection and imaging are widely used in medical diagnosis, product inspection, security monitoring, etc. Large-scale polycrystalline perovskite thick films possess high potential for direct X-ray imaging. However, the notorious problems of baseline drift and high detection limit caused by ions migration are still remained. Here, ion migration is reduced by incorporating 2D perovskite into 3D perovskite, thereby increasing the ion activation energy. This approach hinders ion migration within the perovskite film, consequently suppressing baseline drift and reducing the lowest detection limit(LOD) of the device. As a result, the baseline drifting declines by 20 times and the LOD reduces to 21.1 nGy s-1, while the device maintains a satisfactory sensitivity of 5.6 × 103 µC Gy-1 cm-2. This work provides a new strategy to achieve low ion migration in large-scale X-ray detectors and may provide new thoughts for the application of mixed-dimension perovskite.

Terahertz Impedance Spectroscopy of Biological Nanoparticles by a Resonant Metamaterial Chip for Breathalyzer-Based COVID-19 Prompt Tests.

We propose a tested, sensitive, and prompt COVID-19 breath screening method that takes less than 1 min. The method is nonbiological and is based on the detection of a shift in the resonance frequency of a nanoengineered inductor-capacitor (LC) resonant metamaterial chip, caused by viruses and mainly related exhaled particles, when performing terahertz spectroscopy. The chip consists of thousands of microantennas arranged in an array and enclosed in a plastic breathalyzer-like disposable capsule kit. After an appreciable agreement between numerical simulations (COMSOL and CST) and experimental results was reached using our metamaterial design, low-scale clinical trials were conducted with asymptomatic and symptomatic coronavirus patients and healthy individuals. It is shown that coronavirus-positive individuals are effectively screened upon observation of a shift in the transmission resonance frequency of about 1.5-9 GHz, which is diagnostically different from the resonance shift of healthy individuals who display a 0-1.5 GHz shift. The initial results of screening coronavirus patients yielded 88% agreement with the real-time quantitative polymerase chain reaction (RT-qPCR) results (performed concurrently with the breath test) with an outcome of a positive predicted value of 87% and a negative predicted value of 88%.

The role of CdS doping in improving SWIR photovoltaic and photoconductive responses in solution grown CdS/PbS heterojunctions.

Low cost short wavelength infrared (SWIR) photovoltaic (PV) detectors and solar cells are of very great interest, yet the main production technology today is based on costly epitaxial growth of InGaAs layers. In this study, layers of p-type, quantum confined (QC) PbS nano-domains (NDs) structure that were engineered to absorb SWIR light at 1550 nm (Eg = 0.8 eV) were fabricated from solution using the chemical bath deposition (CBD) technique. The layers were grown on top of two different n-type CdS intermediate layers (Eg = 2.4 eV) using two different CBD protocols on fluoride tin oxide (FTO) substrates. Two types of CdS/PbS heterojunction were obtained to serve as SWIR PV detectors. The two resulting devices showed similar photoluminescence behavior, but a profoundly different electrical response to SWIR illumination. One type of CdS/PbS heterojunction exhibited a PV response to SWIR light, while the other demonstrated a photo-response to SWIR light only under an applied bias. To clarify this intriguing phenomenon, and since the only difference between the two heterojunctions could be the doping level of the CdS layer, we measured the doping level of this layer by means of the surface photo voltage (SPV). This yielded different polarizations for the two devices, indicating different doping levels of the CdS for the two different fabrication protocols, which was also confirmed by Hall Effect measurements. We performed current voltage measurements under super bandgap illumination, with respect to CdS, and got an electrical response indicating a barrier free for holes transfer from the CdS to the PbS. The results indicate that the different response does, indeed, originate from variations in the band structures at the interface of the CdS/PbS heterojunction due to the different doping levels of the CdS. We found that, unlike solar cells or visible light detectors having similar structure, in SWIR photodetectors, a type I heterojunction is formed having a barrier at the interface that limits the injection of the photo-exited electrons from the QC-PbS to the CdS side. Higher n-doped CdS generates a narrow depletion region on the CdS side, with a spike like barrier that is narrow enough to enable tunneling current, leading to a PV current. Our results show that an external quantum efficiency (EQE) of ∼2% and an internal quantum efficiency (IQE) of ∼20% can be obtained, at zero bias, for CBD grown SWIR sensitive CdS/PbS-NDs heterojunctions.

Highly sensitive liquid crystal optically addressed spatial light modulator for infrared-to-visible image up-conversion.

A liquid crystal optically addressed spatial light modulator based on an InGaAs photodiode array operating at low light levels is investigated in the short wavelength infrared (SWIR) spectral band to serve as a SWIR-to-visible imaging upconversion device. It consists of InGaAs/InP heterojunction photodetectors array sandwiched with a nematic LC layer. The photodiode array is composed of a 640×512 InGaAs/Inp heterojunctions, grown on InP substrate with a 15 µm pitch. Full up-converted visible images in stills and video modes were demonstrated with SWIR light intensities as low as 70 nW/cm2 or less than pW/pixel. The influence of operation frequency on the performance of the device was found theoretically and experimentally to be crucial for a proper operation of the device. The optimum sensitivity and contrast of the device was found at a frequency around 70 Hz. To the best of our knowledge, this is the first time that such a high performance upconversion device is presented and that actual visible images are obtained.

Controlling graphene work function by doping in a MOCVD reactor.

Here we demonstrate a new method for doping graphene using Metal Organic Chemical Vapor Deposition (MOCVD) reactor. The original undoped graphene was of a very high quality mounted on Si/SiO2 substrates, they were then doped in the MOCVD's reactor using tertiarybutylphosphine (TBP) and tertiarybutylarsene (TBA). Post process Raman spectroscopy confirmed the presence of a single layer of phosphor doped graphene (G/P) and Arsine doped graphene (G/As) when doped by TBP or by TBA, respectively. Blue shift of the 2D peak assured p-type doping. The work function determined by ultraviolet photoelectron spectroscopy varied from 4.5 eV for Pristine Graphene to 4.7, 4.8 eV for G/As, G/P, respectively. The increase of the work function is attributed to electron transfer from the graphene to the dopant. Our results suggest that doping graphene by MOCVD with TBA or TBP can easily and effectively alternate the work function by few tenths of eV and improve the electronic properties of graphene. The MOCVD technology of doping graphene opens a new route on which other semiconductors can be epitaxially grown on it in a continues process in the same MOCVD reactor.

A New Solid Solution Approach for the Study of Self-Irradiating Damage in non-Radioactive Materials.

A new method to produce a model system for the study of radiation damage in non-radioactive materials is presented. The method is based on homogenously dissolving minute amounts of 228Th ions in thin films in a controllable manner using a small volume chemical bath deposition technique. This approach is demonstrated for PbS films. The properties of the PbS (228Th) solid solution film activity were investigated by monitoring the accompanying radioactive processes. Electrical resistivity studies were performed and decay-event damage accumulation was measured, followed by isochronal annealing which presented two annealing stages and another two sub-stages. This is the first report on self-irradiating damage studies in IV-VI semiconductors and the resulting films present a novel method for the analysis of dilute defect systems in semiconductor thin films.