Photoluminescence studies of porous silicon microcavities

Narrow photoluminescence peaks with a full-width at half-maximum of 14–20 nm are obtained from porous silicon microcavities (PSM) fabricated by the electrochemical etching of a Si multilayer grown by molecular beam epitaxy. The microcavity structure contains an active porous silicon layer sandwiched between two distributed porous silicon Bragg reflectors; the latter were fabricated by etching a Si multilayer doped alternatively with high and low boron concentrations. The structural and optical properties of the PSMs are characterised by scanning electron microscopy and photoluminescence (PL). The wavelength of the narrow PL peaks could be tuned in the range of 700–810 nm by altering the optical constants.     

Fabrication and analysis of buried iron silicide microstructures using a focused low energy electron beam

We fabricated iron and iron silicide microstructures on an Si(1 0 0) clean surface via electron beam induced process of Fe(CO)₅ multilayer and subsequent annealing. The fabricated microstructures were in situ analyzed by Auger electron spectroscopy (AES) and scanning electron microscopy (SEM). We successfully analyzed the coverage and chemical states of the artificial deposited iron structure area-selectively by AES. The artificial iron structure was fabricated after heating to above 350 K to desorb residual Fe(CO)₅ species. The artificial structure was observed even after 1190 K annealing by SEM, but AES measurements showed it to be covered by Si atoms. We concluded that the buried iron silicide microstructure was formed by the present process.     

Stimuli-responsive self-assembled multilayer azo thin films: Photo-switchable absorbance and morphological characteristics

Light has been employed as a source of energy for surface modulation, tuning and control in photo-responsive azo-incorporating self-assembled polyelectrolyte multilayer thin films. A custom built setup for controlled exposure enabled successful photo-switching. Spectral changes were characterized through UV–vis spectroscopy, while scanning electron microscopy technique was employed to study structural properties and response of the films. The effects of chromophore aggregation and dispersion on thin film photo-responsiveness have been studied, with significant contributions made to a recently proposed model correlating structural and spectral properties of the azo thin films. Structural and optical photo-responses have been acquired for surface tuning and modulation, while photo-reversibility is also achieved to control surface properties cyclically.     

Laser melting of carbide tool surface: Model and experimental studies

Laser controlled melting is one of the methods to achieve structural integrity in the surface region of the carbide tools. In the present study, laser heating of carbide cutting tool and temperature distribution in the irradiated region are examined. The phase change process during the heating is modeled using the enthalpy–porosity method. The influence of laser pulse intensity distribution across the irradiated surface (β) on temperature distribution and melt formation is investigated. An experiment is carried out and the microstructural changes due to laser consecutive pulse heating is examined using the scanning electron microscope (SEM). It is found that melt depth predicted agrees with the experimental results. The maximum depth of the melt layer moves away from the symmetry axis with increasing β.     

Tuning vertically stacked InAs/GaAs quantum dot properties under spacer thickness effects for 1.3 μm emission

Coherent InAs islands separated by GaAs spacer (d) layers are shown to exhibit self-organized growth along the vertical direction. A vertically stacked layer structure is useful for controlling the size distribution of quantum dots. The thickness of the GaAs spacer has been varied to study its influence on the structural and optical properties. The structural and optical properties of multilayer InAs/GaAs quantum dots (QDs) have been investigated by atomic force microscopy (AFM), transmission electron microscopy (TEM), and photoluminescence (PL) measurements. The PL full width at half maximum (FWHM), reflecting the size distribution of the QDs, was found to reach a minimum for an inter-dots GaAs spacer layer thickness of 30 monolayers (ML). For the optimized structure, the TEM image shows that multilayer QDs align vertically in stacks with no observation of apparent structural defects. Furthermore, AFM images showed an improvement of the size uniformity of the QDs in the last layer of QDs with respect to the first one. The effect of growth interruption on the optical properties of the optimized sample (E30) was investigated by PL. The observed red shift is attributed to the evolution of the InAs islands during the growth interruption. We show the possibility of increasing the size of the QDs approaching the strategically important 1.3 m wavelength range (at room temperature) with growth interruption after InAs QD deposition.     

Probing the electronic structure and optical response of a graphene quantum disk supported on monolayer graphene

In this paper, we show that a graphene quantum disk (GQD) can be generated on monolayer graphene via structural modification using the electron beam. The electronic structure and local optical responses of the GQD, supported on monolayer graphene, were probed with electron energy-loss spectrum imaging on an aberration-corrected scanning transmission electron microscope. We observe that for small GQD, ~1.3 nm in diameter, the electronic structure and optical response are governed by the dominating edge states, and are distinctly different from either monolayer graphene or double-layer graphene. Highly localized plasmon modes are generated at the GQD due to the confinement from the edge of the GQD in all directions. The highly localized optical response from GQDs could find use in designing nanoscale optoelectronic and plasmonic devices based on monolayer graphene.     

Effects of the annealing duration of the ZnO buffer layer on structural and optical properties of ZnO rods grown by a hydrothermal process

In this study, the effects of the annealing duration of a zinc oxide (ZnO) buffer layer on structural and optical properties of ZnO rods grown by a hydrothermal process are discussed. A ZnO buffer layer was deposited on p-type Si (1 1 1) substrates by the metal organic chemical vapor deposition (MOCVD) method. After that, ZnO rods were grown on the ZnO-buffer/Si (1 1 1) substrate by a hydrothermal process. In order to determine the optimum annealing duration of the buffer layer for the growth of ZnO rods, durations ranging from 0.5 to 30 min were tried. The morphology and crystal structure of the ZnO/ZnO-buffer/Si (1 1 1) were measured by field emission scanning electron microscopy (FE-SEM) and x-ray diffraction (XRD). The optical properties were investigated by photoluminescence (PL) measurement.     

Excitonic absorption of spherical PbS nanoparticles in zeolite A

We report the optical and structural properties of PbS nanoparticles in zeolite A. The samples were obtained by sulfidation of the Pb²⁺ ion-exchanged zeolite in a thiourea solution at 50 °C. The optical properties of the samples were studied by diffuse reflectance spectroscopy. Their crystalline structure and morphology were studied by electron diffraction and by transmission and scanning electron microscopy. The results show that the nanoparticles are not inside the zeolite cages but outside, embedded in the zeolite matrix. Exciton absorption peaks at much higher energy than the fundamental absorption edge of bulk PbS indicate quantum confinement effects in the spherical shape nanoparticles as a consequence of their small size.     

High UV-shielding Performance of Zinc Oxide/High-Density Polyethylene Nanocomposites

Zinc oxide/high-density polyethylene nanocomposites with high-UV-shielding efficiency were reported. Zinc oxide nanoparticles were synthesized by the homogeneous precipitation method and calcination of the precursor at different temperatures. Zinc oxide/high-density polyethylene nanocomposites were subsequently prepared from high-density polyethylene and as-prepared zinc oxide nanoparticles via melt mixing process. The structural properties of the as-prepared zinc oxide nanoparticles and nanocomposites were studied in detail using X-ray diffractometer, Fourier transform infrared spectrometer, thermogravimetry, differential scanning calorimeter, ultrasonic pulse echo technique, scanning electron microscopy, and transmission electron microscope. The optical properties of the obtained nanocomposites were shown to depend on zinc oxide particle size and content. The nanocomposite containing zinc oxide nanoparticles with an average particle size of 25.22 nm after calcination at 350°C was found to have the most optimal optical properties, namely high-visible light transparency and high-UV light shielding efficiency, which are desirable for many important applications.     

温度对萘酞菁LB膜光学特性的影响

本文采用线性吸收谱研究了温度对两种不同结构的萘酞菁分子LB膜光学特性的影响。研究发现,加热可以使它们在LB膜中的聚集体离解,四叔丁基萘酞菁锌的LB膜结构较三叔丁基氰基萘酞菁的LB膜要稳定。     

Electronic structure and optical properties of the HoCoSi and ErNiSi compounds

The electronic structure and the optical properties of the HoCoSi and ErNiSi compounds are studied. Spin-polarized band calculations are performed in the local electron density approximation corrected for the strong electron–electron interactions in the 4f shell of a rare-earth ion (LSDA + U method [11]). The optical constants are measured by ellipsometry in a wide wavelength range, and the frequency dependences of a number of spectral parameters are determined. The calculated densities of states are used to interpret the structural features of the interband optical conductivities of the intermetallic compounds.     

Growth and properties of PbTe films on porous silicon

The structural and electrical characteristics of vacuum deposited PbTe films on Si substrate with buffer porous silicon (PS) layer were investigated. Auger electron spectroscopy, electron and optical microscopy data have shown the absence of cracks, pores, metal and chalcogen microinclusions. A mosaic structure with a grain size of 20–60 μm was detected by selective chemical etching and acoustic microscopy methods. The investigations of X-ray diffraction and X-ray pole figures showed that grains have [100] orientation along the growth direction. The cooling–heating (300–77–300 K) cycles of multilayer PbTe/sublayer/Si structures did not lead to the processes of peeling or appearance of cracks. It was found that thick amorphous layers on a PS surface change the nature of PbTe films growth.     

Characterization of ZnS nanoparticles synthesized by co-precipitation method

ZnS nanoparticles are prepared by homogeneous chemical co-precipitation method using EDTA as a stabilizer and capping agent. The structural, morphological, and optical properties of as-synthesized nanoparticles are investigated using x-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, ultraviolet-visible(UV-Vis)absorption, and photoluminescence spectroscopy. The x-ray diffraction pattern exhibits a zinc-blended crystal structure at room temperature. The average particle size of the nanoparticles from the scanning electron microscopy image is about50 nm. The ultraviolet absorption spectrum shows the blue shift in the band gap due to the quantum confinement effect.The photoluminescence spectrum of Zn S nanoparticles shows a blue visible spectrum.     

Supercapacitive cobalt oxide (Co3O4) thin films by spray pyrolysis

Uniform and adherent cobalt oxide thin films have been deposited on glass substrates from aqueous cobalt chloride solution, using the solution spray pyrolysis technique. Their structural, optical and electrical properties were investigated by means of X-ray diffraction (XRD), scanning electron micrograph (SEM), optical absorption and electrical resistivity measurements. Along with this, to propose Co₃O₄ for possible application in energy storage devices, its electrochemical supercapacitor properties have been studied in aqueous KOH electrolyte. The structural analysis from XRD pattern showed the oriented growth of Co₃O₄ of cubic structure. The surface morphological studies from scanning electron micrographs revealed the nanocrystalline grains alongwith some overgrown clusters of cobalt oxide. The optical studies showed direct and indirect band gaps of 2.10 and 1.60 eV, respectively. The electrical resistivity measurement of cobalt oxide films depicted a semiconducting behavior with the room temperature electrical resistivity of the order of 1.5 × 10³ Ω cm. The supercapacitor properties depicted that spray-deposited Co₃O₄ film is capable of exhibiting specific capacitance of 74 F/g.     

Direct comparison of the electrical,optical, and structural phase transitions of VO2 on ZnO nanostructures

We examined the temperature-dependent electrical, optical, and structural properties of VO₂ on ZnO nanorods with different lengths in the temperature range from 30 to 100 °C. ZnO nanorods with a uniform length were grown on Al₂O₃ substrates using a metal organic chemical vapor deposition, and subsequently, VO₂ was ex-situ deposited on ZnO nanorods/Al₂O₃ templates using a sputtering deposition. The optical properties of the VO₂/ZnO nanorods were measured simultaneously with direct current (DC) resistance using the reflectivity of an infrared (IR) laser beam with a wavelength of 790 nm. The local structural properties around V atoms of VO₂/ZnO nanorods were simultaneously measured with the DC resistance using x-ray absorption fine structure at the V K edge. Direct comparison of the temperature-dependent resistance, IR reflectivity, and local structure reveals that an optical phase transition first occurs, a structural phase transition follows, and an insulator-to-metal transition finally appears during heating.     

Characterization of Si nanocrystals into SiO2 matrix

Silicon nanocrystals (nc-Si) have gained great interest due to their excellent optical and electronic properties and their applications in optoelectronics. The aim of this work is the study of growth mechanism of nc-Si into a-SiO₂ matrix from SiO/SiO₂ multilayer annealing, using non-destructive and destructive techniques. The multilayer were grown by e-beam evaporation from SiO and SiO₂ materials and annealing at temperatures up to 1100 °C in N₂ atmosphere. X-rays reflectivity (XRR) and high resolution transmission electron microscopy (HRTEM) were used for the structural characterization and spectroscopic ellipsometry in IR (FTIRSE) energy region for the study of the bonding structure. The ellipsometric results gave a clear evidence of the formation of an a-SiO₂ matrix after the annealing process. The XRR data showed that the density is being increased in the range from 25 to 1100 °C. Finally, the HRTEM characterization proved the formation of nc-Si. Using the above results, we describe the growth mechanism of nc-Si into SiO₂ matrix under N₂ atmosphere.     

Nano-optical imaging and spectroscopy of order,phases, and domains in complex solids

The structure of our material world is characterized by a large hierarchy of length scales that determines material properties and functions. Increasing spatial resolution in optical imaging and spectroscopy has been a long standing desire, to provide access, in particular, to mesoscopic phenomena associated with phase separation, order, and intrinsic and extrinsic structural inhomogeneities. A general concept for the combination of optical spectroscopy with scanning probe microscopy emerged recently, extending the spatial resolution of optical imaging far beyond the diffraction limit. The optical antenna properties of a scanning probe tip and the local near-field coupling between its apex and a sample provide few-nanometer optical spatial resolution. With imaging mechanisms largely independent of wavelength, this concept is compatible with essentially any form of optical spectroscopy, including nonlinear and ultrafast techniques, over a wide frequency range from the terahertz to the extreme ultraviolet. The past 10 years have seen a rapid development of this nano-optical imaging technique, known as tip-enhanced or scattering-scanning near-field optical microscopy (s-SNOM). Its applicability has been demonstrated for the nano-scale investigation of a wide range of materials including biomolecular, polymer, plasmonic, semiconductor, and dielectric systems.

We provide a general review of the development, fundamental imaging mechanisms, and different implementations of s-SNOM, and discuss its potential for providing nanoscale spectroscopic including femtosecond spatio-temporal information. We discuss possible near-field spectroscopic implementations, with contrast based on the metallic infrared Drude response, nano-scale impedance, infrared and Raman vibrational spectroscopy, phonon Raman nano-crystallography, and nonlinear optics to identify nanoscale phase separation (PS), strain, and ferroic order. With regard to applications, we focus on correlated and low-dimensional materials as examples that benefit, in particular, from the unique applicability of s-SNOM under variable and cryogenic temperatures, nearly arbitrary atmospheric conditions, controlled sample strain, and large electric and magnetic fields and currents. For example, in transition metal oxides, topological insulators, and graphene, unusual electronic, optical, magnetic, or mechanical properties emerge, such as colossal magneto-resistance (CMR), metal–insulator transitions (MITs), high-T _C superconductivity, multiferroicity, and plasmon and phonon polaritons, with associated rich phase diagrams that are typically very sensitive to the above conditions. The interaction of charge, spin, orbital, and lattice degrees of freedom in correlated electron materials leads to frustration and degenerate ground states, with spatial PS over many orders of length scale. We discuss how the optical near-field response in s-SNOM allows for the systematic real space probing of multiple order parameters simultaneously under a wide range of internal and external stimuli (strain, magnetic field, photo-doping, etc.) by coupling directly to electronic, spin, phonon, optical, and polariton resonances in materials. In conclusion, we provide a perspective on the future extension of s-SNOM for multi-modal imaging with simultaneous nanometer spatial and femtosecond temporal resolution.     

Electron transport in modulation-doped GaAs v-groove quantum wires

We report the growth of modulation-doped GaAs/Al_xGa_1−xAs v-groove quantum wires and structural, electrical and optical investigations of their electronic states and transport properties. By using alternative group III precursors on partially SiO₂ masked pre-patterned GaAs substrates, samples have been fabricated which permit electrical measurements of single isolated wire structures without the need for additional electron-beam lithography. Magneto-transport was measured as a function of tilt angle of the incident magnetic field to identify the formation of low-dimensional electron gases in different parts of the structure. Photoluminescence investigations reveal 1D and 2D confined states which show different carrier heating when electric fields are applied along the wire structure.     

Effect of implantation regimes of silver ions on the structure and optical properties of zinc-oxide nanocrystalline films

Thin (about 270 nm) nanocrystalline films of zinc oxide (ZnO) are obtained on quartz substrates using ion sputtering and irradiated with Ag⁺ ions at an energy of 30 keV and relatively high fluences at ion current densities of 4, 8, and 12 µA/cm². The X-ray analysis, scanning electron microscopy, and optical spectroscopy are used to study the effect of irradiation dose and ion current density on the structural modification and optical properties of the ZnO films. Nontrivial dependences of the structural and optical parameters of the films on the ion irradiation regimes are due to radiation heating and film sputtering under the action of the ion beam, diffusion of impurity, formation of silver nanoparticles in the irradiated layer at high implantation fluences, and the diffusion of implanted impurity at relatively high ion current densities.