Tip enhanced Raman spectroscopy evidence for amorphous carbon contamination on gold surfaces

We show that the origin of the spectral fluctuations frequently observed in tip enhanced Raman spectroscopy (TERS) experiments can be mainly related to the presence of surface-contaminating amorphous carbon-based species. We have monitored the spectral fluctuations originating from the sharp metallic tips used as apertureless near field probes, as well as from commonly used noble metal substrates. A correlation between the tip surface roughness and the carbon-based spectral fluctuations has been revealed. An Au-(1 1 1) bare substrate has been mapped with sub-wavelength resolution by TERS, evidencing the localization of the carbon contaminants on the surface steps and grain boundaries.     

Surface-adsorbate-induced fluorescence-type Raman background of Pb(Zr0.4Ti0.6)O3 nanotubes

We report the effects of adsorbates on the Raman spectra of Pb(Zr_0.4Ti_0.6)O₃ nanotubes (PZT-NTs). PZT-NTs with adsorbates were prepared by exposing PZT-NTs to air for 1 day. We found that the presence of surface adsorbates can cause an intense and broad Raman background that hinders the observation of the Raman vibrational modes of the PZT-NTs. This background can be suppressed by adjusting the Raman excitation wavelength from 514.5 nm to 414.6 nm. It can also be removed by post-annealing the PZT-NTs at 500 °C, but it returns readily after re-exposure to air. X-ray photoelectron and Fourier transform infrared spectroscopies show that hydroxyl and carbonate groups are adsorbed onto the surfaces of the PZT-NTs during exposure to air. Moreover, dispersive Raman spectra reveal that the background is a tail of a very broad band from 1000 to 4000 cm^?1, which corresponds to hydroxyl groups. These results suggest that the main contribution to the Raman background of the PZT-NTs is that of surface hydroxyl groups rather than carbonate groups.     

Spectroscopic studies of the structural properties of Ni substituted spinel LiMn2O4

Microstructure and local structure of spinel LiNi_xMn_2 − xO₄ (x = 0, 0.1 and 0.2) were studied using X-ray diffraction (XRD) and a combination of X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge spectroscopy (XANES) and Raman scattering with the aim of getting a clear picture of the local structure of the materials responsible for the structural stability of LiNi_xMn_2 − xO₄. XRD study showed that Ni substitution caused the changes of the materials’ microstructure from the view of the lattice parameter, mean crystallite size, and microstrain. XPS and XANES studies showed the Ni oxidation state in LiNi_xMn_2 − xO₄ was larger than + 2, and the Mn oxidation state increased with Ni substitution. The decrease of the intensity of the 1s → 4p_z shakedown transition on the XANES spectra indicated that Ni substitution suppressed the tetragonal distortion of the [MnO₆] octahedron. The Mn(Ni)–O bond in LiNi_xMn_2 − xO₄, which is stronger than the Mn–O bond in LiMn₂O₄ was responsible for the blue shift of the A_1g Raman mode and could enhance the structural stability of the [Mn(Ni)O₆] octahedron.     

Hole superconductivity in Nd2 − zCezCuO4

The reclassification of Nd_2 − zCe_zCuO₄from an electron superconductor to a hole superconductor is suggested by simple valence and electrostatics arguments: In Nd_2 − zCe_zCuO₄, isolated Ce substituting for Nd is not an electron donor; the proposed actual dopant is a defect-complex of Ce and interstitial oxygen which provides holes, making Nd_2 − zCe_zCuO₄a hole superconductor.     

In situ Raman spectroscopy of cubic boron nitride to 90 GPa and 800 K

The temperature and pressure dependences of the Raman spectrum of the transverse-optical mode of cubic boron nitride were calibrated for applications to a Raman spectroscopy pressure sensor in optical cells to about 800 K and 90 GPa. A significant deviation from linearity of the pressure dependence is confirmed at pressures above 20 GPa. At ambient temperature, dv/dP slopes are 3.41(7) and 2.04(7) cm⁻¹/GPa at 0 and 90 GPa, respectively. A polynomial expression is used to fit the pressure–temperature dependence of the Raman line. The pressure dependence does not significantly change with temperature, as determined from experiments conducted up to 800 K. At 0 GPa, the dv/dP slope is 3.46(7) cm⁻¹/GPa at 800 K. At pressures above 90 GPa, the Raman spectrum of the transverse-optical mode cannot be observed because of an overlap of the signals of cubic boron nitride and diamond used as the anvils in the high-pressure cell.     

Efficient,long-life and Lambertian source of top-emitting white OLEDs using low-reflectivity molybdenum anode and co-doping technology

A top-emitting white organic electroluminescent device (TWOLED) established on low-reflectivity molybdenum (Mo) anode (R ∼ 58% in visible range) for alleviating the undesirable microcavity effect and achieving broad as well as similar emission spectra at different viewing angles has been demonstrated based on a dual-layer emitter composed of doped blue and yellow dopant in the same blue host. The electroluminescent (EL) efficiency and operational lifetime of TWOLED can be further enhanced by a factor of 1.2 and 3.4, respectively, when a co-doping technology of adding another blue dopant into the yellow emitter was incorporated. The enhanced efficiency is attributed to the improved energy transfer from blue host to yellow dopant by cascade process.     

Modelling the directional and energy dependence of 5–10 keV Ar+ ion-induced secondary electron yields from Cu crystals

The primary ion directional effects observed in secondary electron yields induced by ion bombardment [5 keV Ar⁺ → Cu(100)] are simulated using a semi-empirical molecular dynamics model. The directional effects are presumed to arise from inelastic energy transfers that take place in close binary atomic encounters. The latter are estimated using the Oen-Robinson model, in combination with a critical apsidal distance (R_c). The connection between the measured kinetic electron emission (KEE) yields (γ_KEE) and the predicted inelastic energy loss in a binary atomic collision (ΔE_i) is established through a semi-empirical fitting procedure involving R_c and other parameters in the following model: γ_e = γ₀ + γ_KEE = γ₀ + 〈ΔE_i(z)exp(? z/λ)〉, where z is the collision depth. The directional effects are best reproduced by fitting the model to Ar–Cu inelastic collisions for two azimuthal incident directions: R_c is estimated to be 0.47 ± 0.03 Å; the parameter, λ (an effective electron attenuation length), is estimated to be 18 ± 2 Å. The same model also describes the γ_KEE energy dependence for 5–10 keV Ar⁺ normally incident on low-index Cu crystal targets [Phys. Rev. 129 (1963) 2409]. The spatial and temporal distributions of the hard collisions that initiate KEE are discussed on the basis of the model.     

Diffraction patterns and nonlinear optical properties of gold nanoparticles

Stable gold nanoparticles have been prepared by using soluble starch as both the reducing and stabilizing agents; this reaction was carried out at 40 °C for 5 h. The obtained gold nanoparticles were characterized by UV–Vis absorption spectroscopy, transmission electron microscopy (TEM) and z-scan technique. The size of these nanoparticles was found to be in the range of 12–22 nm as analyzed using transmission electron micrographs. The optical properties of gold nanoparticles have been measured showing the surface plasmon resonance. The second-order nonlinear optical (NLO) properties were investigated by using a continuous-wave (CW) He–Ne laser beam with a wavelength of 632.8 nm at three different incident intensities by means of single beam techniques. The nonlinear refractive indices of gold nanoparticles were obtained from close aperture z-scan in order of 10^?7 cm²/W. Then, they were compared with diffraction patterns observed in far-field. The nonlinear absorption of these nanoparticles was obtained from open aperture z-scan technique. The values of nonlinear absorption coefficient are obtained in order of 10^?1 cm/W.     

Single-molecule surface- and tip-enhanced raman spectroscopy

A review is given on single-molecule surface- and tip-enhanced Raman spectroscopy (SERS and TERS). It sketches the historical development along different routes toward huge near-field enhancements, the basis of single-molecule enhanced Raman spectroscopy; from SNOM to apertureless SNOM to tip-enhanced Raman spectroscopy (TERS) and microscopy; from SERS to single-molecule SERS to single-molecule TERS. The claim of extremely high enhancement factors of 10¹⁴ in single-molecule SERS is critically discussed, in particular in the view of recent experimental and theoretical results that limits the electromagnetic enhancement to ? 10¹¹. In the field of TERS only very few reports on single-molecule TERS exist: single-molecule TERS on dyes and on a protein (cytochrome c). In the latter case, TERS ‘sees’ even subunits of this protein, either amino-acids or the heme, depending on the orientation of the protein relative to the tip. The former case concerns the dye brilliant cresyl blue adsorbed either on a Au surface under ambient conditions or on a Au(111) surface in ultra high vacuum. These results indicate that significant progress is to be expected for TERS in general and for single-molecule TERS in particular.     

Mechanical characterization of sintered piezo-electric ceramic material using scanning acoustic microscope

Lead Zirconate Titanate (PZT) is a piezo-electric ceramic material that needs to be characterized for its potential use in microelectronics. Energy dispersive X-ray analysis (EDX) is conducted to determine the chemical composition of the PZT ceramics. The scanning electron microscope (SEM) is performed to study the surface morphology, grain structure and grain boundaries. The SEM image helps us to understand the surface wave propagation and scattering phenomena by the PZT and the reason for its anisotropy and inhomogeneity due to the grain structure. In this paper scanning acoustic microscopy at 100 MHz excitation frequency is conducted for determining mechanical properties of PZT. Earlier works reported only the longitudinal wave speed in PZT while in this paper longitudinal, shear and surface acoustic wave speeds of sintered PZT are measured from its acoustic material signature (AMS) curves, also known as V(z) curves. AMS or V(z) curve is the variation of the output voltage as a function of the distance between the acoustic lens focal point and the reflecting surface. The average velocities of longitudinal, shear and surface acoustic waves in a PZT specimen are determined from its V(z) curve generated at 100 MHz excitation frequency and found to be over 5000 m/s, over 3000 m/s and between 2500 and 3000 m/s, respectively. From these velocities all elastic constants of the specimen are obtained.     

High performance GdBa2Cu3O7−z film preparation by non-fluorine chemical solution deposition approach

Biaxially textured GdBa₂Cu₃O_7?z (GdBCO) films with T_c above 93 K have been prepared on (0 0 l) LaAlO₃ substrate by self-developed non-fluorine polymer-assisted chemical solution deposition (PA-CSD) approach. The GdBCO films show smooth and crack-free morphology. Many nanoscale particles with homogeneous distribution are observed in the GdBCO films, which have not been observed yet in the YBa₂Cu₃O_7?z (YBCO) films prepared by the same processing technique. Besides a high J_c (77 K, 0 T) of 2.28 MA/cm², the optimized GdBCO films show a better J_c–B behavior and an improved high-field J_c, compared to the YBCO films.     

LFN and LSCFN perovskites — structure and transport properties

Structural, electronic and transport properties of LFN (LaFe_1−zNi_zO₃) and LSCFN (La_1−xSr_xCo_1−y −zFe_yNi_zO₃) perovskites synthesized by a modified citric acid method were studied. Structure of the samples was characterized by X-ray studies with Rietveld method analysis. Magnetic properties and valence states of iron ions were characterized by ⁵⁷Fe Moessbauer spectroscopy performed at RT, which were found to be greatly dependent on the chemical composition of the samples. Electrical conductivity was measured in the 20–800 °C temperature range and for some compositions relatively high values (exceeding 100 S cm^− 1) were observed in the 600–800 °C range. Chemical stability studies in relation to Ce_0.8Gd_0.2O_1.9 electrolyte, performed for selected perovskite samples, revealed decreasing stability with increasing Ni concentration and formation of solid solutions in CGO/perovskite composites. The coefficient of thermal expansion (CTE) of LFN perovskites was found to match that of CGO electrolyte (CTE in the 10–13 · 10^− 6 K^− 1 range).     

Continuous-wave 560 nm light generated by intracavity SrWO4 Raman and KTP sum-frequency mixing

An LD end-pumped Nd:YAG/SrWO₄ continuous-wave 560 nm laser is presented based on intracavity sum-frequency mixing of the fundamental and first-Strokes light. The maximum output power of 330 mW at 559.6 nm was obtained for the diode pump power of 13.7 W and the conversion efficiency was about 2.5%. The intense blue emission was also observed in the SrWO₄ crystal when the Raman laser was operating above threshold. This blue emission is centered at 473 nm, which also happened to YVO₄.     

Surface strain-field determination of tympanic membrane using 3D-digital holographic interferometry

In order to increase the understanding of soft tissues mechanical properties, 3D Digital Holographic Interferometry (3D-DHI) was used to quantify the strain-field on a cat tympanic membrane (TM) surface. The experiments were carried out applying a constant sound-stimuli pressure of 90 dB SPL (0.632 Pa) on the TM at 1.2 kHz. The technique allows the accurate acquisition of the micro-displacement data along the x, y and z directions, which is a must for a full characterization of the tissue mechanical behavior under load, and for the calculation of the strain-field in situ. The displacements repeatability in z direction shows a standard deviation of 0.062 µm at 95% confidence level. In order to realize the full 3D characterization correctly the contour of the TM surface was measured employing the optically non-contact two-illumination positions contouring method. The x, y and z displacements combined with the TM contour data allow the evaluation its strain-field by spatially differentiating the u(m,n), v(m,n), and w(m,n) deformation components. The accurate and correct determination of the TM strain-field leads to describing its elasticity, which is an important parameter needed to improve ear biomechanics studies, audition processes and TM mobility in both experimental measurements and theoretical analysis of ear functionality and its modeling.     

The evolution of electromagnetic response at low frequencies with the development of superconducting condensation in Tl2Ba2CaCu2O8+δ

We report the in-plane temperature dependent ac impedance spectrum of Tl₂Ba₂CaCu₂O_8+δ (Tl-2212) single crystal film with thickness 3000 Å and T_c = 96 K. In process of the superconducting condensation, the impedance spectrum z(ω,T₀) changed considerably and was very similar with the evolution of the spectrum under various dc bias voltages in the shallow superconducting state. The spectrum weight transfers from high frequency regions to lower ones and eventually diverges towards the dc limit. From the analysis, it seems that the superconducting ‘islands’ form first and then grow larger, till the formation of a ‘continent’. The impedance spectrum z(ω,B_I) at various dc bias B_I have also been studied.     

Mixed oxygen ion and electron conducting hollow fiber membranes for oxygen separation

Phase inversion spinning technique was employed to prepare dense perovskite hollow fiber membranes made from composition BaCo_xFe_yZr_zO_3−δ (BCFZ, x + y + z = 1.0). Scanning electron microscope (SEM) shows that such hollow fibers have an asymmetric structure, which is favored to the oxygen permeation. An oxygen permeation flux of 7.6 cm³/min cm² at 900 °C under an oxygen gradient of 0.209 × 10⁵ Pa/0.065 × 10⁵ Pa was achieved. From the Wagner Theory, the oxygen permeation through the hollow fiber membrane is controlled by both bulk diffusion and surface exchange. The elements composition of fresh fiber and the fiber after long-term experiments were analyzed by energy-dispersive X-ray spectra (EDXS). Compared to the fresh fiber, sulphur was found on the tested hollow fiber membrane surface exposed to the air side and in the bulk, and Ba segregations occur on the tested hollow fiber membrane surface exposed to the air side. A decrease of the oxygen permeation flux was observed, which was probably due to the sulphur poisoning.     

Mixed conductivity and electrocatalytic performance of SrFeO3-δ–SrAl2O4 composite membranes

Oxygen-ionic and electronic transport in dense (SrFe)_1−x(SrAl₂)_xO_z composites, consisting of strontium-deficient Sr(Fe,Al)O_3-δ and SrAl₂O₄ phases, is determined by the properties of perovskite-like solid solution. Increasing the content of SrAl₂O₄, with a total conductivity as low as 5 × 10^− 7 −  10^− 5 S × cm^− 1 at 973–1273 K in air, results in the gradual decrease of the partial conductivities, but also enables the suppression of thermal expansion. Compared to single-phase SrFe_1−xAl_xO_3-δ, (SrFe)_1−x(SrAl₂)_xO_z composites exhibit enhanced thermomechanical properties, while the oxygen permeability of these materials has similar values. The composite membranes exhibit stable performance under air/(H₂–H₂O–N₂) and air/(CH₄–He) gradients at 973–1173 K. The oxidation of dry methane by oxygen permeating through (SrFe)_0.7(SrAl₂)_0.3O_z results in dominant total oxidation, suggesting the necessity to incorporate a reforming catalyst into the ceramic reactors for natural gas conversion.     

Structural and vibrational properties of PVT grown Bi2Te3 microcrystals

High-quality Bi₂Te₃ microcrystals have been grown by physical vapor transport (PVT) method without using a foreign transport agent. The microcrystals grown under optimal temperature gradient are well facetted and they have dimensions up to ～100 μm. The phase composition of grown crystals has been identified by X-ray single crystal structure analysis in space group R3?m, a=4.3896(2) Å, b=30.5019(10) Å, Z=3 (R=0.0271). Raman microspectrometry has been used to describe the vibration parameters of Bi₂Te₃ microcrystals. The FWHM parameters obtained for representative Raman lines at 61 cm^?1 and 101 cm^?1 are as low as 3.5 cm^?1 and 4.5 cm^?1, respectively.     

Simulation of flux dynamics in a superconducting bulk magnetized by multi-pulse technique

We have simulated the time and spatial dependence of local field B_z(t, r) and temperature T(t, r) on the superconducting bulk during pulsed field magnetization (PFM) using the finite element method (FEM). A modified multi-pulse technique with step-wise cooling (MMPSC) was performed to the cryo-cooled bulk, which was experimentally confirmed to the effective PFM technique to enhance the trapped field B_z higher than 5 T. In the simulation, the B_z value at the center of the bulk surface was enhanced at the 2nd stage of the MMPSC method, in which the results of the simulation reproduced the experimental ones. The enhancement of B_z results from the reduction in the temperature because of the already trapped flux in the bulk at the 1st stage of the MMPSC method.