Laser generated Ag and Ag–Au composite nanoparticles for refractive index sensor

Localized surface plasmon resonance (LSPR) wavelength of metal nanoparticles (NPs) is highly sensitive to size, shape and the surrounding medium. Metal targets were laser ablated in liquid for preparation of spherical Ag and Ag@Au core–shell NP colloidal solution for refractive index sensing. The LSPR peak wavelength and broadening of the NPs were monitored in different refractive index liquid. Quasi-static Mie theory simulation results show that refractive index sensitivity of Ag, Ag–Au alloy and Ag@Au core–shell NPs increases nearly linearly with size and shell thickness. However, the increased broadening of the LSPR peak with size, alloy concentration and Au shell thickness restricts the sensing resolution of these NPs. Figure-of-merit (FOM) was calculated to optimize the size of Ag NPs, concentration of Ag–Au alloy NPs and Au shell thickness of Ag@Au core–shell NPs. The refractive index sensitivity (RIS) and FOM were optimum in the size range 20–40 nm for Ag NPs. Laser generated Ag@Au NPs of Au shell thickness in the range of 1–2 nm showed optimum FOM, where thin layer of Au coating can improve the stability of Ag NPs.     

On the morphology of Si/SiO2/Ni nanostructures with swift heavy ion tracks in silicon oxide

Si/SiO₂/Ni nanostructures are fabricated by the irradiation of an oxidized Si surface with swift heavy ions, nanopore etching in the SiO₂ layer, and the electrochemical deposition of nickel in the nanopores with their partial (～50%) or complete filling. Studies of the morphology of the metal in the nanopores shows that the nickel-cluster structure is rather homogeneous and formed by crystallites ～30–50 nm in size. The effects of deposition modes and structure morphology on current transport are analyzed with the use of test temperature dependences of the magnetoresistance. The reproducibility and stability of the magnetoresistance values for the case of homogeneous structure and pore filling with nickel make the Si/SiO₂/Ni system promising for application as the base element for high-sensitivity low-temperature magnetic field sensors.     

The phase behavior of gamma and laser irradiated polytetrafluorine

By using thermomechanical spectroscopy, an amorphous and three crystalline (high melting, intermediate, and low melting forms) blocks of the topological structures of polytetrafluoroetylene (PTFE) were characterized and their behavior under γ-radiation up to 2420 kGy was explored. The influence of γ-radiation on the powder and sheet of PTFE is essentially the same leading to formation of amorphous character. The glass transition and melting point temperatures of PTFE continuously decreased with increasing dose of irradiation. On exposure to continuous CO₂ laser radiation, PTFE degrades at a high rate and its clusters have a fibrous form. Initial γ-irradiation of PTFE enhances the laser ablation process.     

Enhanced effect of side-Ohmic contact processing on the 2DEG electron density and electron mobility of In0.17Al0.83N/AlN/GaN heterostructure field-effect transistors

From the capacitance–voltage curves and current–voltage characteristics of the In_0.17Al_0.83N/AlN/GaN heterostructure field-effect transistors (HFETs) with side-Ohmic contacts and normal-Ohmic contacts, two-dimensional electron gas (2DEG) electron mobility was calculated. It is found that the polarization Coulomb field scattering (PCF) is closely related to the normal-Ohmic contact processing, and the PCF was weakened by side-Ohmic contact processing in In_0.17Al_0.83N/AlN/GaN HFETs, similar to that in AlGaN/AlN/GaN HFET devices. Further, due to the stronger spontaneous polarization in the thinner In_0.17Al_0.83N barrier layer, the influence of the gate bias on the PCF in In_0.17Al_0.83N/AlN/GaN HFETs is greater than that in AlGaN/AlN/GaN HFETs. As a result, the PCF in In_0.17Al_0.83N/AlN/GaN HFETs with side-Ohmic contacts is stronger than that in AlGaN/AlN/GaN HFETs with side-Ohmic contacts. Moreover, the 2DEG electron density in the In_0.17Al_0.83N/AlN/GaN HFETs with side-Ohmic contacts is increased by more than twice compared with the 2DEG electron density in the In_0.17Al_0.83N/AlN/GaN HFETs with normal-Ohmic contacts.     

Looking at hydrogen motions in confinement

Why in a barren and hot desert, clays can contain a significant fraction of water? Why does concrete crack? How can we demonstrate that complexation of a drug does not alter its conformation in a way that affects its functionality? In this paper we present results on various studies using Quasi-Elastic Neutron Scattering aimed at clarifying these questions. To allow for a better understanding of neutron scattering, a brief introduction to the basics of its theory is presented. Following the theoretical part, experimental results dealing with the effects of confinement on the water dynamics caused by the interfaces in clays and the nano- and micro-pores of concrete are reviewed in detail. At the end, recent Quasi-Elastic Neutron Scattering investigations on the complexation of the local anesthetics Bupivacaine (BVC.HCl, C₁₈H₂₈N₂₀.HCl.H₂O) and Ropivacaine (RVC.HCl, C₁₇H₂₆N₂₀.HCl.H₂O) into the cyclic β-cyclodextrin oligosaccharide are presented. To conclude, the perspectives that the European Spallation Source brings to this subject are discussed.     

Label Free Fluorometric Characterization of DNA Interaction with Cholate Capped Gold Nanoparticles Using Ethidium Bromide as a Fluorescent Probe

We demonstrated label free ethidium bromide assisted characterization of DNA interaction with cholate capped AuNPs. Interactions between ss/ds DNA and AuNPs with two different lengths (0.5 and 0.85 kb) were analyzed through fluorescence spectrophotometer and agrose gel electrophoresis analysis. Further results were confirmed by UV–globally visible spectrophotometer, DLS and TEM. As 0.5 and 0.85 kb of ssDNA effectively interacted with AuNPs through the van der Waals interaction which consequently led to the prevention of salt induced aggregation, EtBr intercalations as well as fluorescence shift with less binding constant 0.098 and 0.108 μM, respectively. On the contrary, the same length of dsDNA (0.5 and 0.85 kb) not interacted with AuNPs which led to the NPs aggregation, EtBr intercalation as well as fluorescence shift with increased binding constant 0.166 and 0.599 μM, respectively. This approach helped to understand the mode of interactions of DNA with cholate capped AuNPs without any modifications in a simple method and the results could be readout through the naked eye under the UV transilluminator. Figure

Fluorometric characterization of interaction of different lengths of ss/dsDNA with cholate capped AuNPs using EtBr as fluorescence probe     

Effect of SiO2–metal–SiO2 plasmonic structures on InGaAs/GaAs quantum well intermixing

Dielectric–metal–dielectric sandwich structures have been fabricated on top of an InGaAs/GaAs single quantum well (QW) structure to enhance atomic interdiffusion across the QW interfaces at elevated temperature during rapid thermal annealing using a halogen lamp as the heating source. The QW intermixing enhancement is realized during rapid thermal annealing. By placing a properly designed SiO₂–Ag–SiO₂ structure on top of the QW sample, a blueshift in photoluminescence emission from 920 to 882 nm was observed, larger than that obtained in a SiO₂-capped QW annealed at the same condition. Finite-difference time-domain simulation and optical reflectance measurements showed that the enhanced QW intermixing is due to the plasmonic resonance-enhanced light absorption and suppressed light reflection from the SiO₂–Ag–SiO₂ structure.     

Stimulation of neurite growth under broadband pulsed THz radiation

The effect of low-intensity broadband pulsed THz radiation in the range from 0.05 to 1.2 THz, with power from 0.25 to 11.6 µW, on the growth of neurites of spinal ganglia of (10–12)-day chick embryos has been investigated. An analysis of the 3-min irradiation with powers in this range has demonstrated both stimulating and suppressive effects of THz radiation on the proliferative activity of neurites. This effect manifests itself with a decrease in the THz radiation power.     

Plasmonic enhancement of optical absorption of UV radiation by Au nanoparticles dispersed on ZnO thin film

Ultraviolet photoconductivity in sol gel-derived ZnO thin film loaded with gold (Au) nanoparticle (NPs) is investigated. Au-NPs loaded ZnO thin film (Au-NPs/ZnO) is found to exhibit high photoconducting gain (K) of about 4.12 × 10³ as compared to the bare ZnO thin film-based photodetector (4.9 × 10¹). The enhanced photoconductive gain of Au-NPs/ZnO detector has been achieved due to simultaneous lowering of dark current which occurs due to Schottky barrier effect originated after dispersing the Au NPs on the surface of ZnO thin film. The enhancement in photocurrent upon UV illumination is due to trapping of UV radiation via plasmonic effect caused by Au-NPs and subsequently coupling of absorbed light with the optical modes of underlying semiconducting ZnO surface.     

Characterization of hydrophobic,oxidized porous silicon layer formed by anodic etching of n+-type silicon surface in a HF:C2H5OH:HCl:H2O2:H2O electrolyte for bio-application

Porous silicon (PS) has been prepared in the dark by anodic etching of n⁺-type (111) silicon substrate in a HF:HCl:C₂H₅OH:H₂O₂:H₂O electrolyte. The processed PS layer is characterized by means of photoluminescence (PL) spectroscopy, scanning electron microscope (SEM), water contact angle (CA) measurements, Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and micro-Raman scattering. The CA of fresh PS layer is found to be ~142°. On aging at ambient conditions, the CA decreases gently to reach ~133° after 3 month, and then it is stabilized for a prolonged time of aging. The visible PL emission from the PS layer also exhibits a good stability against aging time. The FTIR and XPS measurements and analysis show that the stable aged PS layer has rather SiO₂-rich surface. The micro/nanostructure nature of the PS layer is revealed from SEM and micro-Raman results and correlated to CA results. Stable hydrophobic surface of oxidized PS layer is attractive for bio-applications. The efficiency of the produced PS layers as an entrapping template for specific immobilization of IgG2a antibody via physical absorption process is demonstrated.     

Direct laser writing of symmetry-broken nanocorrals and their applications in SERS spectroscopy

We propose a simple and fast approach to prepare surface-enhanced Raman scattering (SERS) substrates over a large area with high flexibility by using direct laser writing (DLW) technique. The proposal is demonstrated by the direct fabrication of an array and a complex of symmetry-broken nanocorrals with DLW followed by a metal deposition process. SERS measurements show significant SERS enhancement, which can be controlled through engineering the focused “hot spots” by changing the structural parameters. The experimental observations are further confirmed by our simulations with a finite-difference time-domain tool. The studies can be extended to versatile SERS substrates with arbitrary geometries.     

Interaction of One Anthraquinone Derivative with ctDNA Analyzed by Spectroscopic and Modeling Methods

The interaction of one anthraquinone derivative (AOMan) with calf thymus deoxyribonucleic acid (ctDNA) was systematically investigated at physiological pH 7.4 by fluorescence spectroscopy and molecular modeling. Binding constants of ctDNA with AOMan were calculated at different temperatures. Thermodynamic parameters, enthalpy and entropy changes were calculated according to Van’t Hoff equation, which indicated that the reaction was spontaneous and predominantly enthalpically driven. The increasing viscosity of ctDNA indicated that AOMan could intercalate into the base pairs of ctDNA. This conclusion was also demonstrated by the results obtained from KI quenching, denatured DNA studies and fluorescence polarization experiment. Furthermore, the molecular modeling results showed that anthraquinone ring tended to slide into the G–C rich region of ctDNA through the hydrogen bond, which are consistent with the results from experimental methods. Studying the binding interaction of target anthraquinones with DNA is one of the key steps in their DNA-changing action and the design of new drugs.     

THz monitoring of the dehydration of biological tissues affected by hyperosmotic agents

Different ways for monitoring the dehydration of tissues affected by hyperosmotic agents have been comparatively analyzed to increase the THz transparency of biological tissues. The data obtained with an original THz laser spectrometer, a Nicolet 6700 Fourier spectrometer, and a Callegari Soft Plus system for skin diagnostics are in good agreement. The corresponding responses of biological tissues (in the form of THz transmittance, reflectance, absorption coefficient, and hydration coefficient) to the effect of biologically compatible hyperosmotic agents have been studied.     

Development of a compact hardware/software package for noninvasive diagnostics of skin diseases in the THz frequency range

The key elements of a mobile hardware/software package for noninvasive diagnostics of skin diseases in the THz frequency range have been designed, produced, and approved in model experiments. These elements are a compact THz oscillator based on an all-fiber femtosecond laser system and a unit for recovering electrodynamic characteristics of layered objects from scattered THz radiation spectra. Generation of 250-fs optical pulses at a wavelength of 1.03 µm with energy of 0.3 µJ and a repetition frequency of 1MHz is demonstrated and the efficiency of optical-THz conversion is found to be 5×10^?6. The proposed algorithm is constructed based on an iterative procedure and can be used for dispersive and absorbing media. It has higher operating speed in comparison with the algorithms for solving inverse problems, which are based on functional minimization methods.     

Heat capacity of oxides in the Bi2O3-SiO2 system

The high-temperature heat capacity of Bi₄Si₃O₁₂, Bi₂SiO₅, and Bi₁₂SiO₂₀ has been investigated. It has been found that there is a correlation between the specific heat C _p ⁰ (298 K) and the composition of oxides in the Bi₂O₃-SiO₂ system.     

Effect of trapped electrons in ultrathin SiO2 on the two-state inversion capacitance at varied frequencies of metal-oxide-semiconductor capacitor

Two-state inversion capacitances of a metal-oxide-semiconductor capacitor (MOSCAP) at varied AC frequencies after negative/positive constant voltage stress (negative/positive CVS) treatments are investigated. When the device was biased into inversion, a low/high inversion-capacitance state (set state/reset state) was achieved after the negative/positive CVS treatments with/without a few trapped electrons in the ultrathin SiO₂ layer. The inversion capacitances of set states were frequency independent, whereas those of reset states increased with the decreasing frequencies. It is different from the general characteristics of an MOSCAP whose inversion capacitances disperse at low frequencies. For this observed finding of the two-state inversion capacitances at varied frequencies, a mechanism of trapped-electrons-induced screening effect on the inversion electrons is proposed. The number of the trapped electrons in the SiO₂ layer affects the number of the inversion electrons, and thus dominates the values of the inversion capacitances. Besides, simulation curves of the inversion capacitances of set states are demonstrated. They are fitted well with the experimental data utilizing the mechanism we proposed. This work investigates further into the influence of the trapped electrons in the ultrathin SiO₂ layer on the inversion capacitance response.     

Passivation of InAs/GaSb type II superlattice photodiodes

One of the major challenges faced by antimonide-based devices is a result of the large number of surface states that are generated. Surface passivation and subsequent capping of the surfaces are essential for any practical applicability of this material system, as evidenced by the comparison of unpassivated and passivated InAs/GaSb superlattice mid-infrared photodiodes herein. The surface passivation methods include silicon dioxide (SiO₂) coating after anodic sulfide, SiO₂ coating after anodic oxide, SiO₂ coating only, zinc sulfide (ZnS) coating after anodic sulfide, and ZnS coating after ammonium sulfide [(NH₄)₂S] chemical passivation. The leakage current as a function of bias voltage (I–V) for superlattice diodes obtained using different passivation methods has been examined at 77 K. The best performance was demonstrated by the SiO₂ after anodic sulfide passivation. The leakage current of the passivated diode is four orders of magnitudes less than that of the unpassivated diode.     

Application of time-domain THz spectroscopy for studying blood plasma of rats with experimental diabetes

The absorption spectra of rat blood plasma have been studied by time-domain THz spectroscopy in the frequency range from 0.1 to 2.0 THz. The absorption coefficient of blood plasma is found to be significantly reduced, depending on the diabetes severity.     

Surface plasmon resonance and surface-enhanced Raman scattering activity of SiO2–Au core-cap nanostructure arrays

SiO₂–Au core-cap nanostructure arrays were prepared by dip-coating technique combined with wet chemical reduction method. The surface morphologies, structures, and optical properties of the obtained samples were characterized by scanning electron microscopy, X-ray diffraction, and ultraviolet–visible spectrophotometer, respectively. The surface-enhanced Raman scattering (SERS) activity of SiO₂–Au core-cap nanostructure arrays substrates was investigated using leucine as probe molecule. And the relationship between the SERS effect and the surface plasmon resonance (SPR) peaks was discussed. High-quality, stable, and reproducible SERS spectra of leucine were successfully obtained. When the maximum SPR peak matched with the excitation wavelength, the substrate gave rise to the highest SERS enhancement. Furthermore, six different fluorescent dyes were also chosen as probe molecules. It was found that the substrate showed good Raman enhancement and highly efficient fluorescence quenching characteristic on these fluorescent dyes.     

THz photomixer with milled nanoelectrodes on LT-GaAs

A terahertz (THz) photomixer: (i) a meander type antenna with integrated nanoelectrodes on (ii) a low temperature grown GaAs has been fabricated and characterized. It was designed for spectral range of 0.3–0.4 THz where molecular fingerprinting and sensing are performed. By combination of electron beam lithography with post-processing using focused ion beam (FIB), milling the THz emitter was successfully fabricated. Nanogaps as small as 40 nm width in the active area of photomixer were milled by FIB. Nanocontacts enhance electric fields of the illuminated and THz radiation and contribute to a better collection of photo-electrons. THz emission was obtained and spectrally characterized.