Single-layer Samples Research Articles

Interferometric characterization of a sub-wavelength near-infrared negative index metamaterial

Negative phase advance through a single layer of near-IR negative index metamaterial (NIM) is identified through interferometric measurements. The NIM unit cell, sub-wavelength in both the lateral and light propagation directions, is comprised of a pair of Au strips separated by two dielectric and one Au film. Numerical simulations show that the negative phase advance through the single-layer sample is consistent with the negative index exhibited by a bulk material comprised of multiple layers of the same structure. We also numerically demonstrate that the negative index band persists in the lossless limit.

Depth Profiling of Nanometer Coatings by Low Temperature Plasma Probe Combined with Inductively Coupled Plasma Mass Spectrometry

The development of material science increasingly calls for rapid characterization methods with low limits of detection and high spatial resolution. Here we report a depth profile analysis method for thin layer coatings by combining low temperature plasma (LTP) probe with inductively coupled plasma mass spectrometry (ICP-MS). The LTP probe with diameter of several tens of micrometers served as a tool for mass removal, which is generated by the discharge in a quartz capillary at ambient condition. The sample material is ablated by the LTP probe and converted into an aerosol, and transported by a carrier gas flow to the ICP-MS, where the decomposed and ionized aerosol particles are analyzed with high sensitivity. Scanning electron microscope (SEM) micrographs reveal that the trace after ablation by the LTP probe is a hole with a diameter less than 10 microm. A lateral resolution of approximately 200 microm has been achieved by analyzing an electron component with interval metal stripes. Depth profiling of a 100 nm single layer sample and a multiple layer sample (100 nm Al/250 nm SiO(2)/100 nm Au/50 nm Cr) on a silicon substrate have been successfully performed at ambient condition. The present method offers unique advantages in terms of high spatial resolution, fast analysis speed and ease of implementation. It might be considered a complementary technique to existing depth profiling methods such as GD-MS/OES, AES, and SIMS. In addition, the simple-to-fabricate LTP probe is easily coupled to various other elemental analysis tools for thin layer or direct solid sample analysis in micro area.

Observation and enhancement of visible photoluminescence of PECVD a‐SiOx:H (x < 2) thin films by microcavity effect

Abstracta‐SiOx:H (x < 2) thin film samples prepared in a PECVD system by using SiH4+CO2 gas mixture show strong room temperature visible photoluminescence (PL) at a photon energy of 2.1 eV. The PL spectrum of a single layer sample was observed to reshape due to the multiple reflection between two interfaces of the film. The orders of the resonance modes in the PL spectrum range of 1.5 eV‐2.5 eV, were calculated to be within a range of m=8 to m=13. The peak energy of the most intense mode (m=11) was observed at 2.1 eV and the half width and the quality factor of the microcavity were calculated as Δν=20 THz and Q=20, respectively. Two multi‐layered distributed Bragg reflectors (10 and 25 layer pairs) were fabricated with two different refractive indices (n1=3.2 and n2=2.4) of the layers with λ/4 (λ0=580nm) optical thickness. The PL intensity was detected increasing in intensity around the photon energy of 2 eV and the other modes were suppressed. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Indium tin oxide on copper phthalocyanine – an ellipsometry study

AbstractSpectroscopic ellipsometry was employed to evaluate the influence of ITO deposition on underlying copper phthalocyanine (CuPc) thin films grown on silicon with native oxide. At first the dielectric functions of ITO and copper phthalocyanine were derived from a multi‐sample analysis of single layer samples. While the CuPc films were isotropic in the substrate plane they showed a strong anisotropy in their optical response perpendicular to the film plane. Obvious higher values were observed below 1.9 eV for the real part, and over the whole spectral range (0.7 – 5 eV) for the imaginary part of the dielectric function perpendicular to the film plane. The dielectric functions were applied to model the double layer samples with ITO on top, resulting in a good match of modelled and measured ellipsometry spectra. This is a clear indication that the ITO deposition does not destroy or strongly modify the optically anisotropic CuPc layer. In a next step the CuPc dielectric function was varied to improve the fit resulting in a slight line shape change. This change might be a hint for possible phase transformation of a small part of the CuPc from α to β phase due to ITO deposition. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

PROGRESS OF WOOD DECAY AND VERIFICATION OF PREDICTION MODEL UNDER ISOTHERMAL CONDITIONS

This study focuses on clarification of wood decay process and verification of a prediction model under isothermal conditions. After moisture properties of decayed wood were investigated, wood decay tests considering moisture balance were implemented with the following conditions; one is stable test with multi-layer wood samples under constant relative humidity; the other is dynamic test with single-layer wood samples under variable relative humidity. Progress of the wood decay for the internal direction was demonstrated in the stable test, and impacts of grain on the moisture distribution of the multi-layer wood samples were observed. The results of the dynamic test indicated that wood decay process was temporarily suspended during hygroscopic conditions. The calculation results by the prediction model could simulate aforementioned phenomena related to both mass loss and moisture content of the wood samples. Additionally, accuracy of the prediction model was discussed in comparison to the results of the decay tests.

Homogeneous sound-absorbing structures for aircraft engine ducts

As applied to the ducts of aircraft engines, a new method is studied for extending the frequency range of sound absorption by using special homogeneous materials of a rigid structure. A through- or blind-hole perforation of such a homogeneous material is for the first time suggested with a view to substantially extend its capabilities. A theory is developed for sound-absorbing structures of perforated homogeneous material that allows for computing their wave parameters and impedance on the basis of those of the starting material. Based on this theory, one can calculate the impedance of any, no matter how complex a structure built up of several layers differing in thickness and perforation percentage and diameter. The results of calculations made for the impedance and sound absorption coefficient of single and multiple layer samples show good agreement with experimental data.

Structural and magnetic properties of GaGdN/GaN superlattice structures

GaGdN/GaN superlattices (SLs) having various GaGdN and GaN layer thicknesses were grown on Al 2O 3 (0001) substrates by radio frequency molecular beam epitaxy and their structural and magnetic properties were investigated. By changing the thickness of GaGdN and GaN layer in the GaGdN/GaN SL, we find that the lattice constant in the c-direction of GaGdN is larger than that of GaN. All SLs samples exhibited ferromagnetic characteristics at 10 K and 300 K. Comparing with the GaGdN single layer sample, the magnetization of SL samples is larger at the same unit volume of GaGdN. Magnetization data show that the SL samples having relatively thinner GaGdN layer and thicker GaN layer have larger magnetic moment per Gd atom. This phenomenon can be explained by considering that the carriers (electrons) flow from the GaN layers and are accumulated in the GaGdN layers; thinner GaGdN layer has higher carrier density, suggesting the carrier-enhanced magnetization in SL structure.

Structures, varistor properties, and electrical stability of ZnO thin films

In this letter, we report the structures, varistor properties, and electrical stability of ZnO thin films deposited by the gas discharge activated reaction evaporation (GDARE) technique. The X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements showed that the thin films thus prepared have polycrystalline structures with the preferred orientation along the (002) plane whose surface consists of ZnO aggregates with sizes of 50-200 nm. The ZnO thin films deposited by GDARE and annealed at 250 °C for 2 h have strong nonlinear varistor-type I- V characteristics. The nonlinear coefficient ( α) of a single-layered ZnO thin film sample was 33 and that of a triple-layered sample obtained by the many-time deposition was 62. The varistor voltages ( V 1mA) of the two samples are found rather close each other. Under a DC bias of 0.75 V 1mA and a temperature of 150 °C these thin films exhibit good electrical stability with a degradation rate coefficient K T of 0.05 mA/h 1/2.

단층 초전도케이블 샘플에서 교류손실의 수치해석에 대한 연구

AC loss is one of the important factors for commercialization of a high temperature superconducting (HTS) cable from an economic point of view. But AC loss characteristics of the HTS-cable are not elucidated completely because of its complex structure. As an earlier stage of analyzing the AC loss in the 22.9 kV/50 MVA, 100m HTS-cable system of Korea Electric Power Corporation (KEPCO) which is now in collaboration with us, a two-dimensional (2D) numerical model, which takes into account the nonlinear conductivity properties of a high temperature superconductor, has been developed. In order to examine our 2D model, we have prepared several single-layer cable samples whose AC losses are sufficiently reliable due to their simple structure. The AC losses of the samples were experimentally investigated and then compared with our 2D model. The results show that the numerically calculated AC losses are not in good agreement with the measured ones for the cylindrical cable and deca-cable samples with low critical current density. However, the numerically calculated and measured AC losses are relatively in good agreement for the deca-cable and hex-cable samples with high critical current density, although the difference between these two loss data in the deca-cable sample tends to increase in the low current region.

Porous silicon photonic devices using pulsed anodic etching of lightly doped silicon

The fabrication of porous silicon photonic structures using lightly doped, p-type, silicon wafers (resistivity: 14–22 Ω cm) by pulsed anodic etching is reported. The optical properties have been found to be strongly dependent on the duty cycle and frequency of the applied current. All the interfaces of the single layered samples were digitally analysed by calculating the mean interface roughness (Rm). The interface roughness was found to be maximum for the sample with direct current. The use of a duty cycle above 50%, in a certain range of frequencies, is found to reduce the interface roughness. The optical properties of some microcavities and rugate filters are investigated from the optimized parameters of the duty cycle and frequency, using the current densities of 10, 90 and 150 mA cm−2.

Magnetization dynamics in the presence of pure spin currents in magnetic single and double layers in spin ballistic and diffusive regimes

In this paper we study the spin transport by using the spin-pumping effect in epitaxial magnetic single and double layer film structures. For the magnetic single layer sample we show the spin-pumping-induced interface damping increases and saturates with the Au capping layer thickness. In addition magnetic double layer structures allowed us to investigate both the spin-pump and spin-sink effects. Coupling of pure spin currents to the magnetization via spin-sink effect is studied using time-resolved magneto-optical Kerr effect. These measurements were used to study the propagation of pure spin currents across a Au spacer layer between the two ferromagnets. The propagation of spin momentum density through the Au spacer layer was well described by spin-diffusion equation, which takes into account electron momentum and spin-flip scattering. The spin-diffusion theory was integrated into modified Landau-Lifshitz equations accounting in self-consistent manner for spin-pump/sink mechanism and spin momentum density propagation. Good agreement between theory and experimental data was found.

High-Density Stacked Ru Nanocrystals for Nonvolatile Memory Application

Stacked ruthenium (Ru) nanocrystals (NCs) are formed by rapid thermal annealing for the whole gate stacks and embedded in memory structure, which is compatible with conventional CMOS technology. Ru NCs with high density (3 × 1012 cm−2), small size (2–4 nm) and good uniformity both in aerial distribution and morphology are formed. Attributed to the higher surface trap density, a memory window of 5.2 V is obtained with stacked Ru NCs in comparison to that of 3.5 V with single-layer samples. The stacked Ru NCs device also exhibits much better retention performance because of Coulomb blockade and vertical uniformity between stacked Ru NCs.

High capacity double-layer surface modified Li[Li0.2Mn0.54Ni0.13Co0.13]O2 cathode with improved rate capability

Layered Li[Li0.2Mn0.54Ni0.13Co0.13]O2, which is a solid solution between layered Li[Li1/3Mn2/3]O2 and Li[Ni1/3Mn1/3Co1/3]O2, has been surface modified by single-layer coating with 2–5 wt.% AlPO4, CoPO4, and Al2O3 and double-layer coating with 2 wt.% AlPO4 or 2 wt.% CoPO4 inner layer and 2–3.5 wt.% Al2O3 outer layer. The samples have been characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS), and electrochemical measurements in lithium cells. The double-layer coated samples are found to exhibit lower irreversible capacity loss (Cirr) and higher discharge capacity values than both the pristine and the single-layer coated samples, which are due to the retention of a higher number of oxide ion vacancies in the layered lattice after the first charge. The double-layer coated composite cathodes exhibit Cirr values as low as 26 mAh g−1 with a high discharge capacity of ∼300 mAh g−1, which is two times higher than that achieved with layered LiCoO2. Moreover, the double-layer coated samples exhibit higher rate capabilities than the pristine and single-layer coated samples, which are attributed to the suppression of undesired SEI layers and the fast charge transfer reaction kinetics as indicated by the EIS data.

Microstructure and Characteristics of Thin-Film La0.8Sr0.2Co0.5Ni0.5O3/ZnO:Al Heterocontact CO Sensors Prepared by RF Magnetron Sputtering

Highly sensitive CO gas sensors based on heterocontacts of ZnO:Al on La0.8Sr0.2Co0.5Ni0.5O3 (LSCNO) were fabricated successfully. La0.8Sr0.2Co0.5Ni0.5O3 thin films were coated on (100) silicon wafers by a sol-gel method including the Pechini process followed by a spin-coating procedure. Then, ZnO:Al films prepared by radiofrequency (RF) magnetron sputtering at various oxygen partial pressures and deposited on as-deposited La0.8Sr0.2Co0.5Ni0.5O3 films were investigated. The results revealed that the CO sensing ability of the film prepared with the ratio of O2/Ar = 5/5 (ratio of volume flow rate) was the worst, owing to the highest (002) plane orientation in the ZnO:Al film. In contrast, the ZnO:Al film prepared with O2/Ar = 3/7 exhibited better CO sensitivity. Furthermore, all two-layer samples showed higher CO sensitivities than single-layer samples. The CO sensitivity of ZnO:Al/La0.8Sr0.2Co0.5Ni0.5O3 thin film was 45% for 500 ppm CO at a sensing temperature of 200°C.

High-throughput solution processing of large-scale graphene

The electronic properties of graphene, such as high charge carrier concentrations and mobilities, make it a promising candidate for next-generation nanoelectronic devices. In particular, electrons and holes can undergo ballistic transport on the sub-micrometre scale in graphene and do not suffer from the scale limitations of current MOSFET technologies. However, it is still difficult to produce single-layer samples of graphene and bulk processing has not yet been achieved, despite strenuous efforts to develop a scalable production method. Here, we report a versatile solution-based process for the large-scale production of single-layer chemically converted graphene over the entire area of a silicon/SiO(2) wafer. By dispersing graphite oxide paper in pure hydrazine we were able to remove oxygen functionalities and restore the planar geometry of the single sheets. The chemically converted graphene sheets that were produced have the largest area reported to date (up to 20 x 40 microm), making them far easier to process. Field-effect devices have been fabricated by conventional photolithography, displaying currents that are three orders of magnitude higher than previously reported for chemically produced graphene. The size of these sheets enables a wide range of characterization techniques, including optical microscopy, scanning electron microscopy and atomic force microscopy, to be performed on the same specimen.

Effects of Charge Impurities and Laser Energy on Raman Spectra of Graphene

The position and width of the Raman G-line was analyzed for unintentionally doped single-layered graphene samples. Results indicate a significant heating of the monolayer by the laser beam. Moreover, a weak additional component was resolved in the G-band. The position of the line is independent of the level of doping of the sample. We conclude that this new component is due to the phonons coupled to the intraband electronic transitions.

Measurement of linear-response Coulomb drag in insulating a- [formula omitted] bilayer systems

We report observations of the Coulomb drag effect between insulating a- Si 1− x Nb x films. We find that a linear-response transresistivity ( ρ d ) can only be realized over a limited range of experimental parameters. We observe an anomalous decrease in the single layer resistance and transresistance at low temperatures ( T < 3 K ) due to an additional non-linear response coupling, which is in need of further investigation. In this manuscript, we focus on the behaviour at not too low temperatures ( 3 K < T < 15 K ) , where the intralayer transport is unperturbed by the bilayer geometry, and the temperature dependence of ρ d is qualitatively consistent with the layers being Efros–Shklovskii-type (as opposed to Mott-type) Anderson insulators. This is in agreement with past studies on the low energy transport in insulating a- Si 1− x Nb x . We show that functionally the temperature dependence in this regime is consistent with the layers having a 3-d density of states. Comparison to single-layer samples shows that the effective layer thickness is indeed increased in the bilayer geometry.

Tuning the luminescence emission of {105}-faceted Ge QDs superlattice using proton implantation and thermal annealing

We report on the low temperature PL emission of self-assembled Ge/Si(001) hut cluster multilayers grown by molecular beam epitaxy at relatively low temperature. We found that PL spectrum of Ge/Si hut cluster superlattice is slightly blueshifted with respect to the emission of single layer. The dependence of PL spectra on rapid thermal annealing temperature was investigated. We found a hitherto unexpected result, namely a remarkable difference in PL emission thermal behavior between multilayers and single layer. In contrast to single layer case, annealing-enhanced PL intensity is found to be ineffective for multilayer structures. Moreover, an anomalous redshift is observed after annealing below 780 °C. The origin of this redshift is tentatively attributed to a possible enhancement of the tensile strain in Si interlayers during annealing in this range of temperature. Further increase in temperature induces a blueshift in PL emission. Post-annealing induced increase of PL intensity has been found to be less pronounced than in single layer samples. We also show that proton implantation can be used to tune the emission energy of hut cluster superlattice. PL spectra are found to shift upwards depending on ion dose by ~ 25 to 130 meV after implantation followed by a flash annealing at 750 to 900 °C. The detected blueshift is attributed to radiation defect mediated Si–Ge intermixing.

Raman fingerprint of charged impurities in graphene

We report strong variations in the Raman spectra for different single-layer graphene samples obtained by micromechanical cleavage. This reveals the presence of excess charges, even in the absence of intentional doping. Doping concentrations up to ∼1013cm−2 are estimated from the G peak shift and width and the variation of both position and relative intensity of the second order 2D peak. Asymmetric G peaks indicate charge inhomogeneity on a scale of less than 1μm.

Characterization of ion beam sputter deposited W and Si films and W/Si interfaces by grazing incidence X-ray reflectivity, atomic force microscopy and spectroscopic ellipsometry

An ion beam sputtering system, which uses a commercial ECR microwave based plasma ion source, has been designed and fabricated in-house for deposition of soft X-ray multilayer mirrors. To begin with, in the ion beam sputtering system W, Si thin films, W/Si bi-layer and W/Si/W tri-layer samples have been deposited on c-Si substrates as precursors to W/Si multilayer stack. The samples have been characterized by grazing incidence X-ray reflectivity (GIXR), atomic force microscopy (AFM) and spectroscopic ellipsometry (SE) techniques. By analyzing the results, density, thickness, surface roughness of the single layer samples and interface width of the bi-layer and tri-layer samples have been estimated.