Thermo-responsive polymers undergo a reversible coil-to-globule transition in water after which the chains collapse and aggregate into bigger globules when passing to above its lower critical ...solution temperature (LCST). The hydrogen bonding with the amide groups in the side chains has to be contrasted with the hydration interaction of the hydrophobic main-chain hydrocarbons. In the present investigation we study molecular changes in the polymer poly(N-isopropyl acrylamide) (PNIPAM) and in its monomer N-isopropyl acrylamide (NIPAM) in solution across the LCST transition. Employing Fourier-transform infrared spectroscopy we probe changes in conformation and hydrogen bonding. We observe a nearly discontinuous shift of the peak frequencies and areas of vibrational bands across the LCST transition for PNIPAM whereas NIPAM exhibits a continuous linear change with temperature. This supports the crucial role of the polymer backbone with respect to hydration changes in the amide group in combination with cooperative interactions of bound water along the backbone chain.
ZnO nanorods doped with Ag and Sb have been synthesized by a facile hydrothermal technique. Crystal quality, morphology, chemical/electronic composition, local structure, and vibrational mode ...properties are investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and micro-Raman spectroscopy. Evidence of dopant incorporation is demonstrated in the XPS measurements of both Sb-doped and Ag-doped ZnO nanorods. From XRD data, it was found that the doped ZnO nanorods have a lower degree of crystallinity. The lattice constants of doped ZnO nanorods were slightly larger than that of the pure samples.
Poly(N-isopropylmethacrylamide) (PNIPMAM) is a thermoresponsive polymer, exhibiting lower critical solution temperature (LCST) behavior in an aqueous solution. We investigate the ...temperature-dependent phase behavior of PNIPMAM solutions in D2O using turbidimetry, differential scanning calorimetry (DSC), small-angle and very small angle neutron scattering (SANS and VSANS), and Raman spectroscopy, covering a large concentration range, and compare the results from PNIPMAM with the findings from its analogue poly(N-isopropylacrylamide) (PNIPAM). We find that the PNIPMAM chains only dehydrate 2–3 °C above the macroscopic cloud point temperature, T CP. Even in the one-phase state, loosely packed, large-scale inhomogeneities and physical cross-links are observed, and the chain conformation of PNIPMAM is more compact than the one of PNIPAM. This is attributed to the attractive intermolecular interactions between the hydrophobic moieties. The phase transition of PNIPMAM is broader than the one of PNIPAM. Upon heating to the two-phase state, the PNIPMAM chains collapse and form mesoglobules. These are larger and more hydrated than those for PNIPAM. This is attributed to the steric hindrance caused by the additional methyl groups, which weaken the intrapolymer interactions in the two-phase state. Thus, the methyl groups in the backbone of the PNIPMAM chains have a significant impact on the hydration and the structural behavior around the phase transition.
It has recently been demonstrated that electron beam injection into p-type β-gallium oxide leads to a significant linear increase in minority carrier diffusion length with injection duration, ...followed by its saturation. The effect was ascribed to trapping of non-equilibrium electrons (generated by a primary electron beam) at meta-stable native defect levels in the material, which in turn blocks recombination through these levels. In this work, in contrast to previous studies, the effect of electron injection in p-type Ga2O3 was investigated using cathodoluminescence technique in situ in scanning electron microscope, thus providing insight into minority carrier lifetime behavior under electron beam irradiation. The activation energy of ∼0.3 eV, obtained for the phenomenon of interest, is consistent with the involvement of Ga vacancy-related defects.
The influence of various energetic particles and electron injection on the transport of minority carriers and non-equilibrium carrier recombination in Ga2O3 is summarized in this review. In Ga2O3 ...semiconductors, if robust p-type material and bipolar structures become available, the diffusion lengths of minority carriers will be of critical significance. The diffusion length of minority carriers dictates the functionality of electronic devices such as diodes, transistors, and detectors. One of the problems in ultrawide-bandgap materials technology is the short carrier diffusion length caused by the scattering on extended defects. Electron injection in n- and p-type gallium oxide results in a significant increase in the diffusion length, even after its deterioration, due to exposure to alpha and proton irradiation. Furthermore, post electron injection, the diffusion length of an irradiated material exceeds that of Ga2O3 prior to irradiation and injection. The root cause of the electron injection-induced effect is attributed to the increase in the minority carrier lifetime in the material due to the trapping of non-equilibrium electrons on native point defects. It is therefore concluded that electron injection is capable of “healing” the adverse impact of radiation in Ga2O3 and can be used for the control of minority carrier transport and, therefore, device performance.
Ga-doped CdS thin films, with different Ga/Cd ratios, were grown using chemical bath deposition. The effect of Ga-doping on optical properties and bandgap of CdS films is investigated. Resistivity, ...carrier density, and mobility of doped films were acquired using Hall effect measurements. Crystal structure as well as crystal quality and phase transition were determined using X-ray diffraction (XRD) and Micro-Raman spectroscopy. Film morphology was studied using scanning electron microscopy, while film chemistry and binding states were studied using X-ray photoelectron spectroscopy (XPS). A minimum bandgap of 2.26
eV was obtained at Ga/Cd ratio of 1.7
×
10
−2. XRD studies showed Ga
3+ ions entering the lattice substitutionally at low concentration, and interstitially at high concentration. Phase transition, due to annealing, as well as induced lattice defects, due to doping, were detected by Micro-Raman spectroscopy. The highest carrier density and lowest resistivity were obtained at Ga/Cd ratio of 3.4
×
10
−2. XPS measurements detect an increase in sulfur deficiency in doped films.
Temperature dependent continuous and time-resolved cathodoluminescence measurements were employed to understand the luminescence from Si-doped β-Ga2O3 prior to irradiation and after 10 MeV proton and ...18 MeV alpha-particle irradiation. The shape and location of the luminescence components ultraviolet luminescence (UVL′) at 3.63 eV, UVL at 3.3 eV, and blue-luminescence at 2.96 eV obtained from Gaussian decomposition did not change in either width or peak location, indicating that new radiation-induced trap-levels were non-radiative in nature between the 4.5 and 310 K temperature range. Activation energies, associated with thermal quenching of UVL′ and UVL bands, show temperature dependence, suggesting ionization of shallow Si-donors and a thermally activated non-radiative process.
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