Detection of Explosives by SERS Platform Using Metal Nanogap Substrates.

Detecting trace amounts of explosives to ensure personal safety is important, and this is possible by using laser-based spectroscopy techniques. We performed surface-enhanced Raman scattering (SERS) using plasmonic nanogap substrates for the solution phase detection of some nitro-based compounds, taking advantage of the hot spot at the nanogap. An excitation wavelength of 785 nm with an incident power of as low as ≈0.1 mW was used to excite the nanogap substrates. Since both RDX and PETN cannot be dissolved in water, acetone was used as a solvent. TNT was dissolved in water as well as in hexane. The main SERS peaks of TNT, RDX, and PETN were clearly observed down to the order of picomolar concentration. The variations in SERS spectra observed from different explosives can be useful in distinguishing and identifying different nitro-based compounds. This result indicates that our nanogap substrates offer an effective approach for explosives identification.

Photocurrent spectra from PbS photovoltaic infrared detectors using silver nanowires as plasmonic nano antenna electrodes.

We discussed structural and electrical properties of PbS films deposited by chemical bath deposition. The crystallite size of our films measured by transmission electron microscope was as large as 0.2 µm in a lateral direction and 1 µm in a vertical direction, and we obtained a high mobility value of 60 cm2 V-1 s-1 at room temperature. We also demonstrated PbS photovoltaic infrared detectors using silver nanowires as transparent electrodes, whose spectral response was measured by Fourier transform infrared spectrometer. The cut-off wavelength was ∼3 µm at room temperature and ∼4 µm at 10 K. At 100 K, a pronounced photocurrent peak was observed at λ = 3.7 µm. Using finite difference time domain simulations, we demonstrated that silver nanowires worked as nano antennas for generating surface plasmons, resulting in the enhancement of photocurrent. The pronounced photocurrent peak wavelength corresponds to the wavelength where the silver nanowires were located near the constructive interference.

Photoluminescence properties of lead selenide produced by selenization and a solvothermal method.

We studied temperature-dependent photoluminescence (PL) spectra of lead selenide (PbSe) dendrites and cubes grown by a solvothermal method. Their PL peaks were located at ∼8 µm at 10 K with a full width at half maximum (FWHM) of 10 meV. Using the temperature-dependent FWHM values, we obtained carrier-phonon coupling coefficients for PbSe. We also demonstrated mechanochemical synthesis of polycrystalline PbS nanoparticles and their successful conversion into a PbSe layer composed of nanocrystals by a selenization process with thermal treatment. The nanocrystals were found to be formed by the orientation alignment of small grains in the process. The PL peak energies of the PbSe layers as well as the PbSe dendrites and the cubes agreed well with their absorption edges in the transmission spectra, indicating that the photoluminescence originates from the band-edge emission. The band-edge emissions hold promise for the development of potential mid-infrared light sources using PbSe fabricated by these methods.

Optical properties and bridge photodetector integration of lead sulfide nanowires.

We studied optical properties and photocurrent characteristics of PbS nanowires grown by chemical vapor deposition. Distinct bandedge photoluminescence (PL) emission was observed in the mid-infrared spectral range and the quantum confinement effect estimated from the PL peak energy was within 40 meV, consistent with the average diameter of the nanowire (∼70 nm) being significantly larger than the exciton Bohr radius (∼18 nm). We also demonstrated interdigit photo detectors making use of these PbS nanowires suspended between two pre-patterned Ti electrodes, where Ti also acted as metal catalyst for the nanowire growth. The threshold wavelength of the photocurrent was found to be ∼3 µm at room temperature.

Temperature dependent photoluminescence from lead sulfide nanosheets and nanocubes.

We studied temperature dependent photoluminescence (PL) spectra in the mid-infrared range from lead sulfide (PbS) nanosheets with an average thickness of 25 nm and nanocubes grown by solvothermal and hydrothermal methods. Distinct bandedge PL emission was observed in the whole temperature range between 10 and 300 K, indicating the high optical quality of these nanostructures. The PL peak of the nanosheets was found at 0.326 eV at 10 K, about 40 meV higher than that of bulk PbS due to the quantum confinement effect, whereas no confinement effect was observed for the nanocubes. We also demonstrate that the absorption edges of the nanocubes and nanosheets in the transmission spectra agree very well with their fundamental bandgap.

Estimation of Environmental Effects and Response Time in Gas-Phase Explosives Detection Using Photoluminescence Quenching Method.

Detecting the presence of explosives is important to protect human lives during military conflicts and peacetime. Gas-phase detection of explosives can make use of the change of material properties, which can be sensitive to environmental conditions such as temperature and humidity. This paper describes a remote-controlled automatic shutter method for the environmental impact assessment of photoluminescence (PL) sensors under near-open conditions. Utilizing the remote-sensing method, we obtained environmental effects without being exposed to sensing vapor molecules and explained how PL intensity was influenced by the temperature, humidity, and exposure time. We also developed a theoretical model including the effect of exciton diffusion for PL quenching, which worked well under limited molecular diffusions. Incomplete recovery of PL intensity or the degradation effect was considered as an additional factor in the model.

Vapor phase detection of explosives by surface enhanced Raman scattering under ambient conditions with metal nanogap structures.

Non-invasive and passive detection of explosives in the vapor phase is advantageous for military, counter-terrorism, and homeland security applications. Detection of explosives using SERS has been an active research topic. However, the vapor pressures of most explosives are low at room temperature, and consequently, the vapor phase detection by SERS is highly challenging without intentionally heating explosive powder to increase the vapor pressure. In this work, we report the rapid and sensitive detection of 2,4,6-trinitrotoluene (TNT) and 2,4-dinitrotoluene (2,4-DNT) in the vapor phase, using a gold nanogap (AuNG) SERS substrate. The AuNG SERS substrate was fabricated with electron beam evaporation, rapid thermal annealing, and wet etching. SERS measurements were carried out with an incident power as low as 0.56 mW at 785 nm. To prevent the condensation effect, the TNT and 2,4-DNT powders inside the cuvette were taken out before inserting the nanogap substrate. Our SERS results demonstrate the feasibility of the non-invasive detection of vapor phase explosives under ambient conditions.

An Efficient p-n Heterojunction Copper Tin Sulfide/g-C3N4 Nanocomposite for Methyl Orange Photodegradation.

The discharge of toxic dye effluents from industry is a major concern for environmental pollution and toxicity. These toxic dyes can be efficiently removed from waste streams using a photocatalysis process involving visible light. Due to its simple synthesis procedure, inexpensive precursor, and robust stability, graphitic carbon nitride (g-C3N4, or CN) has been used as a visible light responsive catalyst for the degradation of dyes with mediocre performance because it is limited by its low visible light harvesting capability due to its wide bandgap and fast carrier recombination rate. To overcome these limitations and enhance the performance of g-C3N4, it was coupled with a narrow bandgap copper tin sulfide (CTS) semiconductor to form a p-n heterojunction. CTS and g-C3N4 were selected due to their good stability, low toxicity, ease of synthesis, layered sheet/plate-like morphology, and relatively abundant precursors. Accordingly, a series of copper tin sulfide/graphitic carbon nitride nanocomposites (CTS/g-C3N4) with varying CTS contents were successfully synthesized via a simple two-step process involving thermal pyrolysis and coprecipitation for visible-light-induced photocatalytic degradation of methyl orange (MO) dye. The photocatalytic activity results showed that the 50%(wt/wt) CTS/g-C3N4 composite displayed a remarkable degradation efficiency of 95.6% for MO dye under visible light illumination for 120 min, which is higher than that of either pristine CTS or g-C3N4. The improved performance is attributed to the extended light absorption range (due to the optimized bandgap), effective suppression of photoinduced electron-hole recombination, and improved charge transfer that arose from the formation of a p-n heterojunction, as evidenced by electrochemical impedance spectroscopy (EIS), photocurrent, and photoluminescence results. Moreover, the results of the reusability study showed that the composite has excellent stability, indicating its potential for the degradation of MO and other toxic organic dyes from waste streams.

Controlled ambipolar doping and gate voltage dependent carrier diffusion length in lead sulfide nanowires.

We report a simple, controlled doping method for achieving n-type, intrinsic, and p-type lead sulfide (PbS) nanowires (NWs) grown by chemical vapor deposition without introducing any impurities. A wide range of carrier concentrations is realized by adjusting the ratio between the Pb and S precursors. The field effect electron mobility of n-type PbS NWs is up to 660 cm(2)/(V s) at room temperature, in agreement with a long minority carrier diffusion length measured by scanning photocurrent microscopy (SPCM). Interestingly, we have observed a strong dependence of minority carrier diffusion length on gate voltage, which can be understood by considering a carrier concentration dependent recombination lifetime. The demonstrated ambipolar doping of high quality PbS NWs opens up exciting avenues for their applications in photodetectors and photovoltaics.

Direct synthesis of high-density lead sulfide nanowires on metal thin films towards efficient infrared light conversion.

We report chemical-vapor-deposition (CVD) synthesis of high-density lead sulfide (PbS) nanowire arrays and nano pine trees directly on Ti thin films, and the fabrication of photovoltaic devices based upon the PbS nanowires. The as-grown nanowire arrays are largely vertically aligned to the substrates and are uniformly distributed over a relatively large area. Field effect transistors incorporating single PbS nanowires show p-type conduction and high mobilities. These catalytic metal thin films also serve as photocarrier collection electrodes and greatly facilitate device integration. For the first time, we have fabricated Schottky junction photovoltaic devices incorporating PbS nanowires, which demonstrate the capability of converting near-infrared light to electricity. The PbS nanowire devices are stable in air and their external quantum efficiency shows no significant decrease over a period of 3 months in air. We have also compared the photocurrent direction and quantum efficiencies of photovoltaic devices made with different metal electrodes, and the results are explained by band bending at the Schottky junction. Our research shows that PbS nanowires are promising building blocks for collecting near-infrared solar energy.

Electric field dependent photocurrent decay length in single lead sulfide nanowire field effect transistors.

We determined the minority carrier diffusion length to be ∼1 µm in single PbS nanowire field effect transistors by scanning photocurrent microscopy. PbS nanowires grown by the vapor-liquid-solid method were p-type with hole mobilities up to 49 cm(2)/(V s). We measured a photoresponse time faster than 14 µs with near-unity charge separation efficiency at the contacts. For the first time, we also observed a field-dependent photocurrent decay length, indicating a drift dominant carrier transport at high bias.

Influence of Air Flow on Luminescence Quenching in Polymer Films towards Explosives Detection Using Drones.

Explosive detection has become an increased priority in recent years for homeland security and counter-terrorism applications. Although drones may not be able to pinpoint the exact location of the landmines and explosives, the identification of the explosive vapor present in the surrounding air provides significant information and comfort to the personnel and explosives removal equipment operators. Several optical methods, such as the luminescence quenching of fluorescent polymers, have been used for explosive detection. In order to utilize sensing technique via unmanned vehicles or drones, it is very important to study how the air flow affects the luminescence quenching. We investigated the effects of air flow on the quenching efficiency of Poly(2,5-di(2'-ethylhexyl)-1,4-ethynylene) (PEE) by TNT molecules. We treated the TNT molecules incorporated into the polymer film as non-radiative recombination centers, and found that the time derivative of the non-radiative recombination rates was greater with faster air flows. Our investigations show that relatively high air flow into an optical sensing part is crucial to achieving fast PL quenching. We also found that a "continuous light excitation" condition during the exposure of TNT vapor greatly influences the PL quenching.

Role of Estradiol Hormone in Human Life and Electrochemical Aptasensing of 17ß-Estradiol: A Review.

Estradiol is known as one of the most potent estrogenic endocrine-disrupting chemicals (EDCs) that may cause various health implications on human growth, metabolism regulation, the reproduction system, and possibly cancers. The detection of these EDCs in our surroundings, such as in foods and beverages, is important to prevent such harmful effects on humans. Aptamers are a promising class of bio-receptors for estradiol detection due to their chemical stability and high affinity. With the development of aptamer technology, electrochemical aptasensing became an important tool for estradiol detection. This review provides detailed information on various technological interventions in electrochemical estradiol detection in solutions and categorized the aptasensing mechanisms, aptamer immobilization strategies, and electrode materials. Moreover, we also discussed the role of estradiol in human physiology and signaling mechanisms. The level of estradiol in circulation is associated with normal and diseased conditions. The aptamer-based electrochemical sensing techniques are powerful and sensitive for estradiol detection.

Lateral PbS Photovoltaic Devices for High Performance Infrared and Terahertz Photodetectors.

We fabricated a lateral photovoltaic device for use as infrared to terahertz (THz) detectors by chemically depositing PbS films on titanium substrates. We discussed the material properties of PbS films grown on glass with varying deposition conditions. PbS was deposited on Ti substrates and by taking advantage of the Ti/PbS Schottky junction, we discussed the photocurrent transients as well as the room temperature spectrum response measured by Fourier transform infrared (FTIR) spectrometer. Our photovoltaic PbS device operates at room temperature for wavelength ranges up to 50 µm, which is in the terahertz region, making the device highly applicable in many fields.

Data for direct chemical deposition of PbS on chemical vapor deposition grown-graphene for high performance photovoltaic infrared photo-detectors.

Over the past decades, graphene has attracted much attention from the scientific community due to its broad applications in the optoelectronics industries [1]. Owing to graphene's high transmission and high electrical conductivity, diverse functional materials/graphene hybridized heterostructures and interfaces are under extensive investigation to satisfy the increasing interest in the need for bendable, flexible and high performance optoelectronic devices [2]. Due to the good atomic lattice structure of graphene, varying heterostructures have been formed by depositing different functional materials directly on graphene [3], [4], [5]. We fabricated a vertical photovoltaic type G/PbS/Ti device by making use of the Ti/PbS Schottky junction and discussed the photocurrent transient characteristics. Lead sulfide (PbS) was deposited directly on large area CVD (Chemical vapor deposition) graphene by CBD (Chemical bath deposition). Temperature dependent photocurrent spectra of our G/PbS/Ti photovoltaic devices were measured by a Fourier transformed infrared (FTIR) set-up. In this paper, we present the experimental procedures and the raw experimental data for the direct chemical deposition of PbS on CVD-graphene for high performance photovoltaic infrared photo-detectors. The manuscript is already available [6].