Detection of Methomyl,a Carbamate Insecticide,in Food Matrices Using Terahertz Time-Domain Spectroscopy

The aim of this study was to investigate the feasibility of detecting methomyl, a carbamate insecticide, in food matrices (wheat and rice flours) using terahertz time-domain spectroscopy (THz-TDS). In the frequency range 0.1–3 THz, the characteristic THz absorption peaks of methomyl at room temperature were detected at 1 (33.4 cm^?1), 1.64 (54.7 cm^?1), and 1.89 (63.0 cm^?1)?THz. For detailed spectral analysis, the vibrational frequency and intensity of methomyl were calculated using solid-state density functional theory to mimic molecular interactions in the solid state. Qualitatively, the simulated spectrum was in good agreement with the experimental spectrum. Analysis of the individual absorption modes revealed that all of the features in the THz spectrum of methomyl were mainly generated from intermolecular vibrations. The peak appearing at 1 THz (33.4 cm^?1) was then selected and tested for its suitability as a fingerprint for detecting methomyl in food matrices. Its absorbance was dose-dependently distinguishable from that of wheat and rice flours. The calibration curve of methomyl had a regression coefficient of >0.974 and a detection limit of <3.74 %. Accuracy and precision expressed as recovery and relative standard deviation in interday repeatability were in the ranges 78.0–96.5 and 2.83–4.98 %, respectively. Our results suggest that THz-TDS can be used for the rapid detection of methomyl in foods, but its sensitivity needs to be improved.     

Terahertz Frequency-Domain Spectroscopy of Low-Pressure Acetonitrile Gas by a Photomixing Terahertz Synthesizer Referenced to Dual Optical Frequency Combs

A terahertz (THz) frequency synthesizer based on photomixing of two near-infrared lasers with a sub-THz to THz frequency offset is a powerful tool for spectroscopy of polar gas molecules due to its broad spectral coverage; however, its frequency accuracy and resolution are relatively low. To tune the output frequency continuously and widely while maintaining its traceability to a frequency standard, we developed a photomixing THz synthesizer phase-locked to dual optical frequency combs (OFCs). While the phase-locking to dual OFCs ensured continuous tuning within a spectral range of 120 GHz, in addition to the traceability to the frequency standard, use of a broadband uni-traveling carrier photodiode for photomixing enabled the generation of CW-THz radiation within a frequency range from 0.2 to 1.5 THz. We demonstrated THz frequency-domain spectroscopy of gas-phase acetonitrile CH₃CN and its isotope CH₃ ¹³CN in the frequency range of 0.600–0.720 THz using this THz synthesizer. Their rotational transitions were assigned with a frequency accuracy of 8.42?×?10^?8 and a frequency resolution of 520 kHz. Furthermore, the concentration of the CH₃CN gas at 20 Pa was determined to be (5.41?±?0.05)?×?10¹⁴ molecules/cm³ by curve fitting analysis of the measured absorbance spectrum, and the mixture ratio of the mixed CH₃CN/CH₃ ¹³CN gas was determined to be 1:2.26 with a gas concentration of 10¹⁴–10¹⁵ molecules/cm³. The developed THz synthesizer is highly promising for high-precision THz-FDS of low-pressure molecular gases and will enable the qualitative and quantitative analyses of multiple gases.     

Terahertz Properties of Cellulose Nanocrystals and Films

Terahertz (THz) radiation properties of cellulose nanocrystal (CNC) films, a CNC powder, and a dissolving pulp film are examined using THz time-domain spectroscopy. The relative permittivity (real component) of the CNC samples are found to vary between 1.78 and 3.81, over the frequency range of 0.2–1.5 THz, despite the fact that they are made from the same linear chain of glucose monomers. The results show that the permittivity is strongly dependent on the source from which the CNC glucose monomers are extracted, as well as on the drying process used. The THz loss tangent (0.043?<?tan(δ)?<?0.145), absorption coefficient (3.5 cm^?1?<?α?<?63.7 cm^?1), and growth-varying permittivity, combined with other appealing thermal and mechanical characteristic of CNC, make such material attractive for use in both passive and potential THz bandwidth electronic components.     

Complementary Surface-Enhanced Raman Scattering (SERS) and IR Absorption Spectroscopy (SEIRAS) with Nanorods-on-a-Mirror

The surface-enhanced counterparts of Raman scattering (SERS) and infrared (IR) absorption (SEIRAS) are commonly used to probe and identify nanoscale matter and small populations of molecules. The contrasting selection rules offer complementary vibrational information of bulk solids or solutions. In this study, a complementary surface-enhanced vibrational spectroscopy approach is presented to probe the vibrational signature of metal-bound molecular monolayers. Nanocavities are designed and produced with sharp and tunable visible (VIS) and mid-IR gap resonances by placing nanorods on a mirror that is coated with a thin dielectric spacer. Their VIS resonances are tuned to match a 1.61 eV (770 nm) resonant excitation for SERS, while their mid-IR resonances span the 1500–2800 cm⁻¹ range (6.5–3.5 µm) in high resolution for SEIRAS, targeting CN bond vibrations at 2220 cm⁻¹. Both the VIS and mid-IR gap modes support spatially overlapping and highly enhanced near-fields ensuring strong SERS and SEIRAS signals from the same monolayer molecular population. The differences in the vibrational information obtained with the two surface-enhanced spectroscopies when probing coupled molecular vibrations are highlighted and the advantages of using such a platform for investigating cavity-modified chemical reactions are discussed.     

Reducing Lattice Thermal Conductivity of the Thermoelectric Compound AgSbTe2 (P4/mmm) by Lanthanum Substitution: Computational and Experimental Approaches

In this study we performed lattice dynamics first-principles calculations for the promising thermoelectric (TE) compound AgSbTe₂, and estimated the stability of its three polymorphs over a wide temperature range from 0 to 600 K. We calculated the vibrational density of states of the AgSbTe₂ (P4/mmm) phase. The results suggested that formation of substitutional defects at Ag-sublattice sites impedes lattice vibrations, thereby reducing lattice thermal conductivity. We focused on calculations based on the Debye approximation for the compound La_0.125Ag_0.875SbTe₂, and predicted reduction of the average sound velocity from 1684 to 1563 m s^?1 as a result of La doping. This is manifested as a ca. 14% reduction in thermal conductivity. To confirm the results from computation we produced two Ag–Sb–Te-based alloys, a ternary alloy without La addition and a quaternary alloy containing La. We measured the thermal conductivity of both alloys by use of the laser flash analysis method, and, as a result of La alloying, observed a reduction in thermal conductivity from 0.92 to 0.71 W m^?1 K^?1 at 573 K, as calculated from first principles.     

Vibrational and morphological properties of Sn-doped CdS films deposited by ultrasonic spray pyrolysis method

Cd_1?xSn_xS films (x=0.0, 0.1 and 0.2) were prepared by the ultrasonic spray pyrolysis (USP) method on the glass substrate at 300 °C. Effect of Sn doping on the vibrational and morphological properties of CdS films has been investigated by scanning electron microscopy (SEM), atomic force microscopy (AFM), FT-IR and Raman spectroscopy. The SEM and AFM measurements showed that the surface morphology of the films was affected by the tin incorporation. The Raman (200–700 cm^?1) and FT-IR (400–4000 cm^?1) spectra of Cd_1?xSn_xS were recorded. The Raman spectrum for Cd_1?xSn_xS films is dominated by an intense band at 300 cm^?1, assigned to the first-order longitudinal optic phonon and the second-order phonon peak 599 cm^?1. The absorption peaks in the FT-IR spectra of Cd_1?xSn_xS located at 540–700 and 1004–1045 cm^?1 can be assigned to the Cd–S and C–O stretching frequencies, respectively. Raman and FT-IR spectra shows decrease in the peak intensity with increasing Sn concentration.     

DLTS Analysis and Interface Engineering of Solution Route Fabricated Zirconia Based MIS Devices Using Plasma Treatment

In this work, we have fabricated low-temperature sol–gel spin-coated and oxygen (O₂) plasma treated ZrO₂ thin film-based metal–insulator–semiconductor devices. To understand the impact of plasma treatment on the Si/ZrO₂ interface, deep level transient spectroscopy measurements were performed. It is reported that the interface state density (D _it) comes down to 7.1 × 10¹⁰ eV^?1 cm^?2 from 4 × 10¹¹ eV^?1 cm^?2, after plasma treatment. The reduction in D _it is around five times and can be attributed to the passivation of oxygen vacancies near the Si/ZrO₂ interface, as they try to relocate near the interface. The energy level position (E _T) of interfacial traps is estimated to be 0.36 eV below the conduction band edge. The untreated ZrO₂ film displayed poor leakage behavior due to the presence of several traps within the film and at the interface; O₂ plasma treated films show improved leakage current density as they have been reduced from 5.4 × 10^?8 A/cm² to 1.98 × 10^?9 A/cm² for gate injection mode and 6.4 × 10^?8 A/cm² to 6.3 × 10^?10 A/cm² for substrate injection mode at 1 V. Hence, we suggest that plasma treatment might be useful in future device fabrication technology.     

Layered Sb2Te3 Nanoflakes as Chalcogenide Dielectrics

Dielectric nanoflakes of Sb₂Te₃ represent an important advance in science and technology due to their extraordinary properties. Polycrystalline layered Sb₂Te₃ nanoflakes have been successfully synthesized via a high-throughput chemical route at 60°C. The frequency and temperature dependence of the dielectric constant and dielectric loss of the layered Sb₂Te₃ nanoflakes have been measured in the frequency range from 30 Hz to 758,000 Hz and temperature range from 313 K to 373 K. As-synthesized Sb₂Te₃ nanoflakes are shown to be promising alternative dielectrics because of their high dielectric constant (ε′ ≈ 7.3 to 6022) and low dielectric loss (tan δ ≈ 0.2 to 9.2). These higher values of ε′ and lower values of tan δ of Sb₂Te₃ nanoflakes confirm that capacitors with capacity (C) of ～5.2 pF to 4336 pF may be fabricated for storing renewable energy. Raman spectroscopy confirms that the peak located at ～142 cm^?1 corresponds to one in-plane vibrational mode (E _g ² ) of layered Te–Sb–Te–Sb–Te lattice vibration.     

Sol–Gel Grown SnO<Subscript>2</Subscript> Nanoparticles: Evaluation of Structure,Morphology, and Raman Spectra

Tin oxide (SnO₂) nanoparticles (TONPs) were prepared using sol–gel method under different growth conditions. The influence of calcination temperature (450°C and 600°C) and molecular weight of polyethylene glycol (PEG 300 and PEG 4000) on the nanocrystallinity, surface morphology, and Raman spectra of as-prepared TONPs were evaluated. Variation of calcination temperature and dopant (sulfosuccinic acid, SA) was found to affect considerably the structure, surface morphology, and Raman activities of the TONPs. The size of TONPs estimated using Scherrer equation was discerned to be in the range of 15–32 nm. The observed intensity enhancement in the Raman vibrational modes at lower calcination temperature was attributed to the enlargement of TONPs size. The absorption of molecules at the TONPs surface led to a quenching in the A ₂ g and Eu Raman peaks. Raman peaks centered around 673 cm^?1, 799 cm^?1, 640 cm^?1 , and 432 cm^?1 corresponding to A1g, B2g, A1g, and Eg modes, respectively have manifested highest peaks intensity. Furthermore, the enhancement of the Eg mode due to the addition of SA dopant was ascribed to the Jahn–Teller distortion mechanism.     

The Synthesis and Characterization of Cerium Carbonate Hydroxide Nanorods as an Anode for Lithium-Ion Batteries

Nanorods cerium carbonate hydroxide, CeCO₃OH, was synthesized through a low-temperature reaction route. The data of x-ray diffraction and scanning electron microscopy revealed that the as-prepared samples were CeCO₃OH nanorods. The diameters of the nanorods were in the range of 50–100 nm, and the lengths were around 300–500 nm. As an anode of a lithium ion battery, the charge–discharge capacity, cyclability and lithium-ion diffusion kinetics of CeCO₃OH nanorods were investigated. The calculated lithium ion diffusion coefficient was 1.36 × 10^?19 cm² s^?1. The initial discharge capacity was about 621.6 mA h g^?1 at 0.2 mA cm^?2 in 0.05–2.5 V. After 100 cycles, the discharge capacity stabilized at about 362 mA h g^?1 and the Coulombic efficiency was nearly 98%, indicating the potential application in anodes of lithium-ion batteries.     

Features of the vibrational spectra of diamond-like and polymer-like <Emphasis Type="Italic">a</Emphasis>-C:H films

Comparative analysis of the IR spectra of multiple frustrated total internal reflection (MFTIR) in the range 4000–1000 cm^?1 is performed for diamond-like and polymer-like a-C:H films with the refractive indices n ≥ 2.0 and n ≤ 1.7, respectively. The films are obtained by chemical-vapor deposition from octane, cyclohexane, toluene, and acetylene under various conditions using a dc glow discharge plasma. Characteristic features are found in the vibrational spectra of a-C:H films with different refractive indices. A peak at 1250 cm^?1, which is independent of the initial hydrocarbon, is observed in the spectra of the diamond-like films. Additional peaks at 3400 and 1700 cm^?1, due to O-H and C=H vibrations, are present in the spectra of the polymer-like films. It is shown that the integrated intensity of the band of CH vibrations peaked at ～2900 cm^?1 decreases exponentially by an order of magnitude with an increase in n from 1.55 to 2.4.     

Generation of High-Power Sub-THz Waves in Magnetized Turbulent Electron Beam Plasmas

Sub-THz radiation can be generated by conversion of plasma waves into electromagnetic (EM) radiation in a plasma with strong Langmuir (LT) turbulence produced via a two-stream instability of a high current relativistic electron beam (REB). Nonlinear plasmon-plasmon merging results in the generation of photons nearby the 2^nd harmonic of the plasma frequency 2ω _p (“2ω _p -process”). For plasma densities of 10¹⁴???10¹⁵?cm^?3, these frequencies are in the range of sub-THz waves at 370–570 GHz. The specific power density of sub-THz-wave emission from plasmas in the multi-mirror magnetic trap GOL-3 (at BINP) during injection of a 10-μs-REB with a current density of about 1 kA/cm² at plasma densities n _e ?≈?5?10¹⁴?cm^?3, electron temperatures T _e ?≈?1.5 keV and magnetic induction B?≈?4 T was measured to be approx. 1 kW/cm³ in the frequency band around 300 GHz. In the case of a weakly relativistic 100-μs-electron beam (90 keV) with 250 A/cm² the corresponding results are 700 W/cm³ around 90 GHz with an efficiency of 1–2 % at n _e ?≈?3?10¹³?cm^?3 (total power?≈?30 kW). Theoretical investigations show that at a density of n _e ?≈?3?10¹⁵?cm^?3 and a turbulence level of 5 % the generated sub-THz power can reach?≈?1 MW/cm³.     

Properties of ZnO Thin Films Codoped with Lithium and Phosphorus

The properties of ZnO thin films codoped with lithium and phosphorus have been characterized. The films were deposited from high-purity ZnO and Li₃PO₄ solid targets onto c-plane sapphire substrates by radiofrequency (RF) magnetron sputtering. A substrate temperature of 900°C was determined as optimum for depositing undoped ZnO films with background electron concentration of 9.9 × 10¹⁵ cm^?3 as the buffer layer on the sapphire substrate. Postdeposition annealing was carried out using rapid thermal processing in O₂ at temperatures ranging from 500°C to 1000°C for 3 min. Analyses performed using low-temperature photoluminescence spectroscopy measurements revealed luminescence peaks at 3.356 eV, 3.307 eV, 3.248 eV, and 3.203 eV at 12 K for the codoped samples. X-ray diffraction 2θ-scans showed a single peak at about 34.4° with full-width at half-maximum of about 0.09°. Hall-effect measurements revealed initial p-type conductivities, but these were unstable and toggled between p-type and n-type over time with Hall concentrations that varied between 2.05 × 10¹³ cm^?3 and 2.89 × 10¹⁵ cm^?3. The fluctuation in the carrier type could be due to lateral inhomogeneity in the hole concentration caused by stacking faults in the films. An additional cause could be the small Hall voltages in the measurements, which could be significantly impacted by even small spikes in signal noise inherent in the measurements.     

Optical and Electrical Properties of Cu2ZnSnS4 Film Prepared by Sulfurization Method

Cu₂ZnSnS₄ (CZTS) films were prepared by sulfurization of sputtered Zn/Sn/Cu multilayer thin films. Raman peaks at 251 cm^?1, 289 cm^?1, 336 cm^?1, and 362 cm^?1 were detected, and the optical band gap energy of the CZTS was estimated to be about 1.53 eV. Energy-dispersive spectrometry and x-ray photoelectron spectroscopy reveal that the composition ratio of prepared CZTS film is close to stoichiometric. Photoresponse current measurements show persistent photoconductivity effect, with decay constants τ and β of 5.04 and 0.269, respectively.     

Terahertz luminescence of GaAs-based heterostructures with quantum wells under the optical excitation of donors

Spontaneous emission from selectively doped GaAs/InGaAs:Si and GaAs/InGaAsP:Si heterostructures is studied in the frequency range of ～3–3.5 THz for transitions between the states of the two-dimensional subband and donor center (Si) under the condition of excitation with a CO₂ laser at liquid-helium temperature. It is shown that the population inversion and amplification in an active layer of 100–300 cm^?1 in multilayered structures with quantum wells (50 periods) and a concentration of doping centers N _D ≈ 10¹¹ cm^?2 can be attained under the excitation-flux density 10²³ photons/(cm² s).     

Effects of Growth Temperature and Annealing on Properties of Zn3Sn2O7 Thin Films and Application in TFTs

The effects of growth temperature and annealing on the physical properties of Zn₃Sn₂O₇ thin films were investigated in this work. The Zn₃Sn₂O₇ thin films were deposited on glass substrates by radio frequency (rf) magnetron sputtering. It is found that the films are amorphous regardless of the growth temperature. The film grown at room temperature shows the highest mobility of 8.1 cm² V^?1 s^?1 and the lowest carrier concentration of 2.0 × 10¹⁵ cm^?3. The highest carrier concentration of 1.6 × 10¹⁹ cm^?3 is obtained at the growth temperature of 250°C. Annealing treatment of the Zn₃Sn₂O₇ thin films resulted in increases of carrier concentration and mobility. The average transmittance of the as-deposited and annealed films reaches 80%. By using a Zn₃Sn₂O₇ thin film as the channel and a Ta₂O₅ thin film as the insulating layer, we fabricated transparent Zn₃Sn₂O₇ thin-film transistors with field-effect mobility of 21.2 cm² V^?1 s^?1, on/off current ratio of 10⁵, threshold voltage of 0.8 V, and subthreshold swing of 0.8 V/decade.     

Terahertz Spectroscopic Analysis of Peptides and Proteins

Spectroscopic analysis using the Terahertz frequencies between 0.1-15 THz (3–500?cm^?1) has been underutilised by the biochemistry community but is starting to yield some scientifically interesting information. Analysis of structures from simple molecules like N-methylacetamide, to polyamides, peptides and relatively complex proteins provides different types of information dependant on the molecular size. The absorbance spectrum of small molecules is dominated by individual modes and specific hydrogen bonds, peptide spectra have peaks associated with secondary structure, while protein spectra are dominated by ensembles of hydrogen bonds and/or collective modes. Protein dynamics has been studied using Terahertz spectroscopy using proteins like bacteriorhodopsin, illustrating a potential application where this approach can provide complementary global dynamics information to the current nuclear magnetic resonance and fluorescence-based techniques. Analysis of higher-order protein structures like polyomavirus virus-like particles generate quite different spectra compared to their constituent parts. The presence of an extended hydration layer around proteins, first postulated to explain data generated using p-germanium spectroscopy may present a particularly interesting opportunity to better understand protein’s complex interaction with water and small solutes in an aqueous environment. The practical aspects of Terahertz spectroscopy including sample handling, the use of molecular dynamics simulation and orthogonal experiment design are also discussed.     

High resolution molecular spectroscopy in the submillimeter region

Fourier Transform laboratory measurements have been carried out, for the first time in the 8–85 cm^?1 spectral region, with an unapodized resolution of 3.3. 10^?3 cm^?1 and a frequency accuracy of 2. 10^?4 cm^?1. Samples from spectra of several molecules namely: CO, O₃, H₂O₂, NO, NO₂, HNO₃, SO₂, H₂S, HOCL, NOCL, HNCO, ND₃ and AsH₃ are presented to show both the quality of the measurements and the type of information supplied by high resolution spectroscopy in the submillimeter region.     

Suppression of Red Luminescence in Wire Explosion Derived Eu:ZnO

Europium oxide (Eu₂O₃) is coated on zinc (Zn) wire using the electrophoretic deposition process. The coated Zn wire is subjected to the wire explosion process (WEP) which is rapid (< 15 min), and chimie douce (soft chemical, low temperature), in nature; this results in the formation of Eu doped ZnO. The explosion chamber contains oxygen (99.9%) at atmospheric pressure. Electron micrographs indicate that the particle sizes are ～ 80 nm. Diffractogram-based analysis suggests that the crystallite size is ~ 18–20 nm in the as-prepared doped ZnO nanoparticles. Electron paramagnetic resonance shows the presence of Zn vacancies and the cryo-photoluminescence spectrum indicates that Eu exists in the + 3 state. A combined Williamson–Hall plot and Kisielowski’s model based analysis indicates that Eu is a substitutional dopant in WEP derived Eu:ZnO particles. It is estimated that this material has ～ 0.24 at.% doping. This analysis also shows that, unlike another popular material GaN, in the case of ZnO, Eu³⁺ strictly substitutes for Zn²⁺ (i.e., dopant replacing a cation–anion pair does not seem possible). It may be noted that Eu³⁺ in a suitable host is oftentimes reported to be an efficient luminophore. The IR spectra show a band shift from 486 cm^?1 to 493 cm^?1; with peak shifts from 436 cm^?1 to 430 cm^?1 in Raman spectra. These too indicate the presence of Eu in the samples. However, at room temperature, only green luminescence (centered at 534 nm) is observed from the sample indicating (1) high concentrations of O_Zn anti-site defects and Zn vacancies, and (2) concomitant quenching of the luminescence at room temperature. Our results suggest that WEP is viable for synthesizing rare earth doped ceramic materials. However, obtaining efficient phosphors using this approach will likely require, (1) reduction of defect densities, and (2) appropriate passivation using post-processing.     

Effect of Annealing Ambient on the Electrical and Optical Properties of Aluminum-Doped ZnO Films Produced via a Sol–Gel Process

In this study, aluminum-doped ZnO (AZO) thin films were prepared by a sol–gel with spin coating process. The AZO films were annealed by a two-step process. The films were first annealed in air or nitrogen at 500°C for 3 h, followed by annealing in three types of ambient, i.e., vacuum (10^?3 Torr or 10^?6 Torr) or forming gas (10% H₂/90% N₂), at 500°C for 4 h. The effect of the annealing ambient on the microstructure, electrical and optical properties of the AZO films was explored by x-ray diffraction, field-emission scanning electron microscopy, four-point probe sheet resistivity measurements, Hall voltage measurements, and ultraviolet–visible spectroscopy. The results showed that the size of AZO particulates in the films was determined mainly by the first annealing step. The films annealed in air in the first step were composed of larger AZO particulates than those annealed in nitrogen. The conductivities of the AZO films were significantly increased by the second annealing step. Second annealing in a high-vacuum system (10^?6 Torr) led to the highest AZO film conductivity among the three ambients. Regardless of the various annealing processes, the films remained transparent under visible light and exhibited a sharp absorption edge in the ultraviolet region. The highest conductivity, i.e., 168 S cm^?1, was obtained from films annealed first in air and then in vacuum of 10^?6 Torr.