Electronic structure of HgTe nanocrystals: An observation of p-d weakening

Photoemission studies to identify the electronic structure of the HgTe nanocrystals revealed a new phenomenon of p-d weakening, as a consequence of size quantization effect associated with the mean crystalline size, 5.35 ± 0.83 nm. The weakening of the p-d hybridization by a factor of 0.33, to that of the bulk HgTe suggests the valence band maxima and core level shifts toward higher binding energy. The widening of the band gap due to size quantization is confirmed from optical absorption and photoluminescence measurements. The upward and downward shift of the conduction band minima and the valence band maxima with respect to the bulk value of HgTe are found to be 1.6 eV and 0.54 eV respectively.     

Kinetics of luminescence of interface defects and resonant Er ions in nanostructured SnO2:SiO2

Silica glass with SnO₂ nanocrystals and Er³⁺ ions are prepared by the sol-gel route and treatment above 1000 °C. Transmission electron microscopy evidences a homogeneous dispersion of nanoclusters 4-6 nm in size in the amorphous silica matrix. Photoluminescence spectra excited at 3.5 eV, outside erbium transitions, show an inhomogeneous spectral distribution of light emission from interface defects, in the range 1.9-2.4 eV, resonant with transitions of erbium ions. The analysis of kinetics and temperature dependence of luminescence allows to quantify the efficiency of the energy transfer channel between nanoclusters and erbium ions.     

Spectroscopic ellipsometry of layer by layer deposited colloidal HgTe nanocrystals exhibiting quantum confinement

We have explored the optical properties of bilayers of Mercury telluride (HgTe) nanocrystals (NCs) embedded in polymer which were prepared from a colloidal solution. These NCs show strong luminescence in the near infrared at room temperature, which makes them an interesting material for the telecommunication area. The emission wavelength can efficiently be tuned by controlling the size of the NCs. We report spectroscopic ellipsometry measurements, which clearly show an energy shift of the critical points (CPs) in the dielectric function to higher energies compared to the HgTe bulk properties. This is caused by quantum confinement in the crystals. The exact peak energies of the transitions are fitted with line-shape models for CPs. Surprisingly, concepts coming from semiconductor bulk optics, as CPs, can be applied to NCs with a diameter of less than 5 unit cells.     

Refractive index, oscillator parameters and temperature-tuned energy band gap of Tl4In3GaS8-layered single crystals

Transmission and reflection measurements in the wavelength region 450-1100 nm were carried out on Tl₄In₃GaS₈-layered single crystals. The analysis of the room temperature absorption data revealed the presence of both optical indirect and direct transitions with band gap energies of 2.32 and 2.52 eV, respectively. The rate of change of the indirect band gap with temperature dE_gi/dT=-6.0×10⁻⁴ eV/K was determined from transmission measurements in the temperature range of 10-300 K. The absolute zero value of the band gap energy was obtained as E_gi(0)=2.44 eV. The dispersion of the refractive index is discussed in terms of the Wemple-DiDomenico single-effective-oscillator model. The refractive index dispersion parameters: oscillator energy, dispersion energy, oscillator strength and zero-frequency refractive index were found to be 4.87 eV, 26.77 eV, 8.48×10¹³ m⁻² and 2.55, respectively.     

Calculated plasmonic enhancement of spontaneous emission from silicon nanocrystals with metallic gratings

The spontaneous emission rate (SER) enhancement due to the surface plasmon polariton (SPP) band gap effect of metallic gratings was evaluated by calculating the dispersion relation and electromagnetic field distribution simultaneously. Within the luminescence range of silicon nanocrystals (Si-NC) (?ω = 1.9 eV-1.6 eV), the calculated peak Purcell factors for Au-, Ag-, and Al-Si₃N₄ gratings are 266.9-30.1, 80.2-22.6, and 78.7-20.3, respectively. These results indicate that the common metals such as Au, Ag and Al, whose SPP resonance frequencies/energies are rather higher, can be adopted to enhance the SER of Si-NCs with the help of metallic gratings.     

Use of the direct negative Cu ion beam deposition for the control of the properties of Cu thin film

Direct metal ion beam deposition (DMIBD) technique for Cu thin film metallization is characterized. With suitable operating conditions, secondary Cu⁻ ion yield, ion/atom arrival rate ratio, ion beam energy spreads were optimized at 15%, 0.3, and 10%, respectively.After optimization of DMIBD system, the effect of Cu ion beam energy on the resistivity, adhesion strength, and surface morphology of Cu thin film was investigated. TEM micrograph shows that the film prepared at 75 eV was polycrystalline, while the film prepared at 0 eV was vertical columnar structure.As ion beam energy is increased from 25 to 75 eV, the resistivity is decreased from 6.21 to 2.09 μΩ cm, while the critical load to cause adhesion failure was increased to about 13 N at 200 eV, which is four-times higher that that of 25 eV.     

Preparation of CuInGeSe4 thin films by selenization method using the Cu-In-Ge evaporated layer precursors

CuInGeSe₄ quaternary compounds are known to have a chalcopyrite-like structure and have band gaps of about 1.3 eV, suitable for optimum conversion efficiency for solar cells. We have prepared the CuInGeSe₄ thin films by the selenization method using the Cu-In-Ge evaporated layer precursors. The analyses of X-ray diffraction show that the single phase of CuInGeSe₄ is obtained by the selenization of precursors at 450-500 °C. The SEM observation of film surface shows that the grain sizes are in the order of 1-2 μm. The band gaps of selenized films close to 1.6 eV are wider than that of bulk crystals (about 1.3 eV). These films have p-type conduction and higher electrical resistivities than more 10⁵ Ω cm at room temperature.     

Synthesis of nanocrystalline and mesoporous zinc sulphide from a single precursor Zn(SOCCH3)2Lut2 complex

We report the formation of mesoporous zinc sulphide, composed by the fine network of nanoparticles, which was formed via a single precursor Zn(SOCCH₃)₂Lut₂ complex. The complex was chemically synthesized using zinc carbonate basic, 3,5-lutidine and thioacetic acid, in air. The metal precursor complex was characterized using different conventional techniques. Thermogravimetric analysis (TGA) result indicates that the decomposition of the complex starts at 100 °C and continues up to 450 °C, finally yielding ZnS. ZnS nanocrystals were characterized by powder X-ray diffraction (XRD) technique, field emission scanning electron microscopy (FESEM), N₂-sorption isotherm, UV-vis spectroscopy and photoluminescence (PL) spectroscopy. The grain diameter of nanocrystals was found to be 4-5 nm. The material followed Type-IV N₂-sorption isotherm, which is the characteristic of mesoporous materials. The band gap energy, as obtained from optical measurements was around 3.8 eV.     

Modeling the influence of the porosity of laser-ablated silicon films on their photoluminescence properties

Nanostructured porous Si-based films produced by pulsed laser ablation (PLA) from a silicon target in residual helium gas can exhibit both size-dependent (1.6-3.2 eV) and fixed photoluminescent (PL) bands (1.6 and 2.2 eV) with their relative contributions depending on the film porosity. We study the influence of prolonged oxidation in ambient air on properties of the fixed PL bands, associated with oxidation phenomena, and their correlation with structural properties of the films. In addition, we propose a model describing the appearance of surface radiation states for oxidized films of various porosities. Our experiments and numerical simulations led to a conclusion that the 1.6 eV PL is due to a mechanism involving a recombination through the interfacial layer between Si core and an upper oxide of nanocrystals. This mechanism gives the optimal porosity of 73% for the most efficient production of 1.6 eV PL centers that is in excellent agreement with our experimental results.     

Temperature induced size selective synthesis of hybrid Cds/pepsin nanocrystals and their photoluminescence investigation

There is growing interest in materials chemistry for taking advantage of the physical and chemical properties of biomolecules in the development of next generation nanoscale materials for opto-electronic applications. A biomimetic approach to materials synthesis offers the possibility of controlling size, shape, crystal structure, orientation, and organization. The great progress has been made in the control that can be exerted over optical materials synthesis using biomolecules (protein, nucleic acid)/mineral interfaces as templates for directed synthesis. We have synthesized the CdS nanocrystals using pepsin by biomimetic technique at four different set temperatures. X-ray diffraction (XRD) and small angle X-ray scattering (SAXS) results showed that we are able to tune the size and distribution profile just by tuning the reaction (Rx) temperature and goes towards excitonic Bhor radius (2.5 nm) at low temperature (4 °C). The narrow absorption peak at 260 nm from Cd²⁺-pepsin complex dominates and indicates the size dispersion of the modified CdS nanoparticles are fairly monodisperse. Effective mass approximation (EMA) shows large blue-shift (~1 eV) in the band gap for the cubic phase from bulk hexagonal CdS. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra are dominated by a strong and narrow band-edge emission tunable in the blue region indicating a narrow size distribution. The reduction in PL efficiency is observed when the Rx temperature increases however no change in PLE spectra and temporal profiles of the band-edge PL is observed. At 4 °C, high emission efficiency with shift of PL spectrum in the violet region is observed for 1.7 nm size CdS quantum dots (QDs). Presence of pepsin has slowed the PL decay which is of the order of 100 μs.     

Energy distribution of plasma-assisted electron and ion emission from TGS single crystals

Electron and ion emission accompanying non-thermal plasma processes, produced at the surface of TGS single crystals under driving ac electric field exceeding 10³ V/cm, have been carried out. These plasma-assisted emission of electrons and ions were examined by means of time and energy distribution measurements. The intensity of registered charges (electrons and ions) displayed on the 2 ms time scale are represented by two distinct peaks. Time dependent energy spectrum of charges, detected under our experimental conditions, involves electrons and ions with maximum energy up to 30-40 eV for first peaks and up to 70-80 eV for second one. Additionally, the energy of electrons is focused at about 10-15 eV for first and second peaks and about 60-70 eV for second ones; the ion energy spectrum for both peaks exhibits only distinct low energy maximum focused at about 5-15 eV.     

Transport and optical properties of amorphous carbon and hydrogenated amorphous carbon films

In this paper we report on the electrical and optical properties of amorphous carbon (a-C) and hydrogenated amorphous carbon (a-C:H) films. Resistivity of both types of films decreases with increase in temperature. At lower temperatures (60-250 K) the electron transport is due to variable range hopping for the a-C films. At higher temperatures (300-430 K) it is thermally activated for both types of films. Analysis of the heterojunction between diamond-like carbon (DLC) and bulk silicon (Si) leads to the conclusion that our a-C films are of n-type and our a-C:H films are of p-type. The optical measurements with DLC revealed a Tauc bandgap of 0.6 eV for the a-C films and 1-1.2 eV for the a-C:H films. An Urbach energy around 170 meV could be determined for the a-C:H films. Strain versus resistance plots were measured resulting in piezoresistive gauge factors around 50 for the a-C films and in between 100 and 1200 for the a-C:H films.     

Synthesis and characterization of zinc oxide nanoparticles by laser ablation of zinc in liquid

We report formation of colloidal suspension of zinc oxide nanoparticles by pulsed laser ablation of a zinc metal target at room temperature in different liquid environment. We have used photoluminescence, atomic force microscopy and X-ray diffraction to characterize the nanoparticles. The sample ablated in deionized water showed the photoluminescence peak at 384 nm (3.23 eV), whereas peaks at 370 nm (3.35 eV) were observed for sample prepared in isopropanol. The use of water and isopropanol as a solvent yielded spherical nanoparticles of 14-20 nm while in acetone we found two types of particles, one spherical nanoparticles with sizes around 100 nm and another platelet-like structure of 1 μm in diameter and 40 nm in width. The absorption peak of samples prepared in deionized water and isopropanol are seen to be substantially blue shifted relative to that of the bulk zinc oxide due to the strong confinement effect. The technique offers an alternative for preparing the nanoparticles of active metal.     

CdTe aggregates in KBr crystalline matrix

In this work, we report the experimental results on the fabrication and optical characterization of Czochralski (Cz) grown KBr single crystals doped with CdTe crystallites. The results of the optical absorption have shown two bands, the first one located at 250 nm demonstrates the incorporation of cadmium atoms in the KBr host followed by a partial chemical decomposition of CdTe, the second band located at 585 nm shows an optical response of CdTe aggregates. Photoluminescence spectra at room temperature before annealing showed a band located at 520 nm (2.38 eV), with a blue shift from the bulk gap of 0.82 eV (E_g (CdTe)=1.56 eV). While the photoluminescence spectra after annealing at 600 °C showed a band situated at 640 nm (1.93 eV), these bands are due to band-to-band transitions of CdTe nanocrystals with a blue shift from the bulk gap at 0.38 eV. Blue shift in optical absorption and photoluminescence spectra confirm nanometric size of dopant. X-ray diffraction (XRD) spectra have shown the incorporation of CdTe aggregates in KBr.     

Investigation of intrinsic plasmon energy losses associated with the Fe 1s core level in metallic iron

A recently developed Cu Kα₁ (hν = 8047.8 eV) X-ray source/ESCA300 electron spectrometer combination has been used to investigate the intrinsic plasmon energy losses associated with the Fe 1s core level (binding energy = 7111 eV) in metallic iron. The surface and bulk intrinsic plasmon energy losses were separated and it was found that using the theoretically calculated extrinsic energy loss cross-section to represent the bulk intrinsic energy loss cross-section gave an overall intrinsic loss probability which is approximately the same as if a Lorentzian type cross-section is used. However, this approach does not separate the surface and bulk intrinsic losses properly and is not a good approximation for peak shape analysis in the near peak region. A more realistic approximation is provided by using a Lorentzian type energy loss cross-section to represent the bulk intrinsic energy losses. It has also been shown that for the Fe 1s core level of metallic iron the probability that a photoelectron will suffer an intrinsic energy loss is higher at the surface than in the bulk. Also for this core level the excitation probability for the intrinsic plasmons is greater than that of the extrinsic plasmons. Hence ignoring the intrinsic plasmons would cause considerable error in peak shape analysis in the near peak region.     

Molecular dynamics simulation of homogeneous nucleation of KBr cluster

We perform molecular dynamics simulations to study the homogeneous nucleation in the freezing of molten potassium bromide clusters. The nucleation rates tend to decrease with increasing cluster size and temperature. The solid-liquid interfacial free energy σ_sl of 42.4-52.3 mJ/m² is close to the values predicted by Turnbull's relation and comparable to the experimental observation by Buckle and Ubbelohde. It is interesting to find that there is no cluster size effect on the critical nucleus size. Critical nucleus sizes inferred from classical nucleation theory are of 6.5-20.7 K⁺Br⁻ ionic pairs in the temperature range of 400-600 K. The critical nucleus size at bulk MD freezing temperature obtained by extrapolation is about 45 K⁺Br⁻ ionic pairs, which is comparable to the experimental value of NaCl.     

Dependence of film thickness on the structural and optical properties of ZnO thin films

ZnO thin films are prepared on glass substrates by pulsed filtered cathodic vacuum arc deposition (PFCVAD) at room temperature. Optical parameters such as optical transmittance, reflectance, band tail, dielectric coefficient, refractive index, energy band gap have been studied, discussed and correlated to the changes with film thickness. Kramers-Kronig and dispersion relations were employed to determine the complex refractive index and dielectric constants using reflection data in the ultraviolet-visible-near infrared regions. Films with optical transmittance above 90% in the visible range were prepared at pressure of 6.5 × 10⁻⁴ Torr. XRD analysis revealed that all films had a strong ZnO (0 0 2) peak, indicating c-axis orientation. The crystal grain size increased from 14.97 nm to 22.53 nm as the film thickness increased from 139 nm to 427 nm, however no significant change was observed in interplanar distance and crystal lattice constant. Optical energy gap decreased from 3.21 eV to 3.19 eV with increasing the thickness. The transmission in UV region decreased with the increase of film thickness. The refractive index, Urbach tail and real part of complex dielectric constant decreased as the film thickness increased. Oscillator energy of as-deposited films increased from 3.49 eV to 4.78 eV as the thickness increased.     

Probability of ionization of sputtered particles as a function of their energy: Part I: Negative Si ions

In this study we have investigated how the probability of ionization of sputtered Si atoms to form negative ions depends on the energy of the atoms. We have determined the ionization probability from experimental SIMS energy distributions using a special experimental technique, which included de-convolution of the energy distribution with an instrumental transmission function, found by separate measurements.We found that the ionization probability increases as a power law ∼E^0.677 for particles sputtered with energies of 0-10 eV, then becomes a constant value (within the limits of experimental error) for particles sputtered with energies of 30-100 eV. The energy distributions of Si⁻ ions, measured under argon and cesium ion sputtering, confirmed this radical difference between the yields from low and high-energy ions.To explain these results we have considered ionization mechanisms that are different for the low energy atoms (<10 eV) and for the atoms emitted with higher energy (>30 eV).     

Dissociation of water molecule at three-fold oxygen coordinated V site on the InVO4 (0 0 1) surface

Water molecule adsorption properties at the surface of InVO₄ have been investigated using an ab initio molecular dynamics approach. It was found that the water molecules were adsorbed dissociatively to the three-fold oxygen coordinated V sites on the (0 0 1) surface. The dissociative adsorption energy was estimated to be 0.8-0.9 eV per molecule. The equilibrium distance between V and O of the hydroxyl -OH was almost the same as the V-O distance of tetrahedra VO₄ in the InVO₄ bulk crystal (1.7-1.8 Å).     

Synthesis and photoluminescence properties of Bi2S3 nanowires via surfactant micelle-template inducing reaction

Single-crystalline Bi₂S₃ nanowires, with diameters in the range of 80-200 nm and lengths up to tens of micrometers, have been successfully synthesized through surfactant micelle-template inducing reaction at ambient-pressure and low-temperature. The synthetic route is simple, effective and can provide great opportunities for both fundamental and technological applications. The optical properties of the Bi₂S₃ nanowires with different diameters were firstly examined by means of photoluminescence spectroscopy at room temperature. The representative photoluminescence spectrum exhibits a great blue-shift from the band gap of 1.30 eV of bulk Bi₂S₃ to high energy of 1.44 eV, which indicated that these nanostructures showed quantum confinement effects.