Nanocrystalline Films Research Articles

Comparisons of surface and optical properties of the heavily carbon-doped GaN nanocrystalline films deposited by thermionic vacuum arc method

In this paper, heavily C-doped GaN samples were deposited on glass and PET substrates by the thermionic vacuum arc (TVA) method, for the first time. Microstructure, surface and optical properties of the carbon-doped GaN samples were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy, atomic force microscopy, and UV–Vis spectrophotometer. According to XRD pattern, fullerene (C60) structures were able to deposit in doped samples, for the first time. The fullerene structures were detected at heavily carbon-doped GaN. The obtained band gap changes of heavily carbon-doped GaN samples are determined. The band gap of CGaN on PET substrate decreased to a lower value, about 100 meV. The band gaps were shifted to the lowest wavelengths towards the edge of the UV region by heavily doped carbon. (002) and (004) peaks for GaN were also detected.

A study of microstructural properties and quantum size effect in SILAR deposited nano-crystalline CdS thin films

The paper reports deposition and characterization of CdS thin films by micro-controlled SILAR technique using CdCl2 and Na2S as precursors. The thickness of the films was found to increase with number of immersion cycles, and also with the concentration of precursor solutions. But this increase in thickness was observed only up to a certain limit beyond which films showed peeling effect. The films were found to be polycrystalline with cubic structure. The lattice constant, crystallite size and average strain on the films were found from detailed XRD analysis. Optical characterization revealed that the films exhibit quantum size effect due to their nano crystalline nature.

Pinning in high performance MgB2 thin films and bulks: Role of Mg-B-O nano-scale inhomogeneities

The comparison of nano-crystalline MgB2 oxygen-containing thin film (140nm) and highly dense bulk materials showed that the critical current density, Jc, depends on the distribution of Mg-B-O nano-scale inhomogeneities. It has been shown that MgB2 bulks with high Jc in low (∼106 A/cm2 in 0-1 T at 10K) and medium magnetic fields contain MgB0.6-0.8O0.8-0.9 nano-inclusions, where δTc or a combined δTc (dominant) / δl pinning mechanism prevails, while in bulk MgB2 with high Jc in high magnetic fields (Birr(18.5K)=15 T, Bc2(0K)=42.1T) MgB1.2-2.7O1.8-2.5 nano-layers are present and δl pinning prevails. The structure of oxygen-containing films with high Jc in low and high magnetic fields (Jc (0 Т)=1.8×107 А/сm2 and Jc (5 Т)=2×106 А/сm2 at 10 К) contains very fine oxygen-enriched Mg-B-O inhomogeneities and δl pinning is realized. The results of DOS calculations in MgB2-xOx cells for x=0, 0.125, 0.25, 0.5, 1 demonstrate that all compounds are conductors with metal-like behaviour. In the case of ordered oxygen substitution for boron the binding energy, Eb, does not increase sufficiently as compared with that for MgB2, while when oxygen atoms form zigzag chains the calculated Eb is even lower (Eb=−1.15712 Ry).

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.

AFM investigation and optical band gap study of chemically deposited PbS thin films

The interest into deposition of nanocrystalline PbS thin films, the potential of designing and tailoring both the topographical features and the band gap energy (Eg) by controlling growth parameters, has significant technological importance. Nanocrystalline thin films of lead sulfide were grown onto glass substrates by chemical bath deposition (CBD) method. The experiments were carried out by varying deposition temperature. We report on the modification of structural and optical properties as a function of deposition temperature. The morphological changes of the films were analyzed by using SEM and AFM. AFM was also used to calculate average roughness of the films. XRD spectra indicated preferred growth of cubic phase of PbS films in (200) direction with increasing deposition time. Optical properties have been studied by UV-Spectrophotometer. From the diffused reflectance spectra we have calculated the optical Eg shift from 0.649-0.636 eV with increasing deposition time.

High Coverage Formation of CdS Quantum Dots on TiO2 by the Photocatalytic Growth of Preformed Seeds

Photoelectrochemical experiments and density functional theory calculations indicated that visible-light irradiation of the CdS quantum dots (QDs)–TiO2 direct coupling system (CdS/TiO2) causes the electron injection from the valence band (VB) of CdS into the conduction band (CB) of TiO2 (path 2) in addition to the inter-CB electron transfer from CdS to TiO2 (path 1). Path 2 can be induced by the sub-bandgap excitation of CdS QDs to extend the spectral response of the CdS/TiO2 system. For path 2 as well as path 1 to effectively work, CdS QDs should be directly deposited on the TiO2 surface with high coverage. According to the guideline, a photocatalytic growth of the preformed seed (PCGS) technique has been developed. Transmission electron microscopy observation and X-ray photoelectron spectroscopy measurements of the CdS/TiO2 prepared by the PCGS technique indicated that the TiO2 surface is highly covered by CdS QDs. The technique was applied to mesoporous TiO2 nanocrystalline films (mp-TiO2) to yield CdS...

Structural transition, subgap states, and carrier transport in anion-engineered zinc oxynitride nanocrystalline films

In this work, anion alloying is engineered in ZnON nanocrystalline films, and the resultant evolution of the structural transition, subgap states, and carrier transport is investigated. A broad distribution of sub-gap states above the valence band maximum is introduced by nitrogen due to the hybridization of N 2p and O 2p orbitals. The phase transition from partially amorphous states to full crystallinity occurs above a characteristic growth temperature of 100 °C, and the localized states are suppressed greatly due to the reduction of nitrogen composition. The electronic properties are dominated by grain boundary scattering and electron transport across boundary barriers through thermal activation at band edge states at high temperatures. The conductivity below 130 K exhibits a weak temperature dependence, which is a signature of variable-range hopping conduction between localized states introduced by nitrogen incorporation.

Enhancement of second harmonic generation in nanocrystalline SiC films based natural microcavities

We explore second harmonic generation (SHG) features of nanocrystalline SiC films based natural microcavities (MC) with resonance modes in the vicinity of the fundamental radiation at 1064 nm wavelength for nanocrystalline films of 24R, 27R polytypes characterized by high radiation doubling frequency coefficients. The natural walls of the SiC NC serve a role of cavities. To learn the nonlinear resonance features of the MC near the fundamental wavelength 1064 nm three series of the nc-SiC films indicated as 7495, 7517 and 7522 with thicknesses varying within 250–600 nm were fabricated. The role of the coherent and non-coherent contributions with taking into an account of cavity resonance modes is discussed. The giant increase of the SHG was achieved which may be used for further fabrication of the frequency transducers and nonlinear optical triggers.

Size-Dependence of the Activity of Gold Nanoparticle-Loaded Titanium(IV) Oxide Plasmonic Photocatalyst for Water Oxidation.

Mesoporous TiO2 nanocrystalline film was formed on fluorine-doped tin oxide electrode (TiO2 /FTO) and gold nanoparticles (NPs) of different sizes were loaded onto the surface with the loading amount kept constant (Au/TiO2 /FTO). Visible-light irradiation (λ>430 nm) of the Au/TiO2 /FTO photoanode in a photoelectrochemical cell with the structure of photoanode|0.1 m NaClO4 aqueous solution|Ag/AgCl (reference electrode)|glassy carbon (cathode) leads to the oxidation of water to oxygen (O2 ). We show that the visible-light activity of the Au/TiO2 /FTO anode increases with a decrease in Au particle size (d) at 2.9≤d≤11.9 nm due to the enhancement of the charge separation and increasing photoelectrocatalytic activity.

Low-temperature Process for Fabrication of Porous Nanocrystalline Tin(IV) Oxide Film for Electrochromic Devices

We report a low-temperature annealing process for the fabrication of an electrochromic device based on a SnO2 nanocrystalline film. The device exhibits high transmittance and fast response times in the bleached state, and the difference in transmittance between the colored state and the bleached state is higher than that of a TiO2 film fabricated by low-temperature annealing.

Cavitation Erosion Resistance of Sputter-Deposited Cr3Si Film on Stainless Steel

In this technical brief, a Cr3Si nanocrystalline film was deposited on 304 stainless steel (SS) substrate using a double glow discharge plasma technique. The film was characterized by X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy, nanohardness tester, and scratch tester. The as-deposited film with a thickness of 5 μm consisted of A15 structured Cr3Si phase with an average grain size of 8 nm. The hardness values of the film were determined to be 26 GPa, which was ten times greater than 304 SS. A self-designed ultrasonic vibration cavitation erosion apparatus was employed to evaluate the cavitation erosion resistance of the Cr3Si film. The results showed that after cavitation tests of 30 hrs, the erosion mass loss of the film was only 60% of that for 304 SS substrate. SEM observation of the erosion surfaces indicated that the surface damage degree of the Cr3Si film is significantly less than that of 304 SS.

Low thermal conductivity in nanocrystalline Zn3P2

Semiconductors with low lattice thermal conductivity are important in the search for more efficient thermoelectric materials. The thermal conductivity of nanocrystalline (<7 nm) Zn3P2, fabricated in thin film form by pulsed laser deposition, was measured from 80 K to 294 K. The thermal conductivity of the film showed weak temperature dependence in this temperature range and at 294 K had its highest value of 0.49 W/m K. Although Zn3P2 and its family of isomorphic compounds are known to have intrinsically low thermal conductivity, at room temperature the thermal conductivity of this nanocrystalline film is 25% smaller than the calculated minimum thermal conductivity for Zn3P2. Analyzing the thermal conductivity data with the Callaway model revealed that the data could be well fit by considering only boundary scattering and point defect scattering. The boundary scattering length was in good agreement with the film’s average crystallite size of 4.1 nm and the magnitude of the point defect scattering required the formation of VZn-Zni pairs from approximately 23% of the Zn sites. It is believed that a large number of point defects are responsible for the intrinsically low thermal conductivity of bulk Zn3P2 and therefore the exceptionally low thermal conductivity found in the present study results from the nanometer dimensions of the crystallites. As previous studies have reported high Seebeck coefficients and electronic properties that are insensitive to grain boundaries in Zn3P2, the low thermal conductivity observed in the present study suggests that nanocrystalline Zn3P2 should be further explored for use in thermoelectric applications.

Achieving highly-enhanced UV photoluminescence and its origin in ZnO nanocrystalline films

ZnO is an efficient luminescent material in the UV-range ∼3.4 eV with a wide range of applications in optical technologies. Sputtering is a cost-effective and relatively straightforward growth technique for ZnO films; however, most as-grown films are observed to contain intrinsic defects which can significantly diminish the desirable UV-emission. In this research the defect dynamics and optical properties of ZnO sputtered films were studied via post-growth annealing in Ar or O2 ambient, with X-ray diffraction (XRD), imaging, transmission and Urbach analysis, Raman scattering, and photoluminescence (PL). The imaging, XRD, Raman and Urbach analyses indicate significant improvement in crystal morphology and band-edge characteristics upon annealing, which is nearly independent of the annealing environment. The native defects specific to the as-grown films, which were analyzed via PL, are assigned to Zni related centers that luminesce at 2.8 eV. Their presence is attributed to the nature of the sputtering growth technique, which supports Zn-rich growth conditions. After annealing, in either environment the 2.8 eV center diminished accompanied by morphology improvement, and the desirable UV-PL significantly increased. The O2 ambient was found to introduce nominal Oi centers while the Ar ambient was found to be the ideal environment for the enhancement of the UV-light emission: an enhancement of ∼40 times was achieved. The increase in the UV-PL is attributed to the reduction of Zni-related defects, the presence of which in ZnO provides a competing route to the UV emission. Also, the effect of the annealing was to decrease the compressive stress in the films. Finally, the dominant UV-PL at the cold temperature regime is attributed to luminescent centers not associated with the usual excitons of ZnO, but rather to structural defects.

Near infrared to ultraviolet optical properties of bulk single crystal and nanocrystal thin film iron pyrite

We report optical properties of iron pyrite (FeS2) determined from ex situ spectroscopic ellipsometry measurements made on both a commercially available bulk single crystal and nanocrystalline thin film over a spectral range of 0.735–5.887 eV. The complex dielectric function, ε (E) = ε1 (E) + iε2 (E), spectra have been determined by fitting a layered parametric model to the ellipsometric measurements. Spectra in ε are modeled using a Kramers–Kronig consistent critical point parabolic band model involving seven critical points for the bulk single crystal and four critical points for the nanocrystalline film. Absorption coefficient spectra for both types of samples are also determined from ε. Critical point features in the nanocrystalline films are broader, have lower amplitude and lower energy critical points detected having a small blue shift when compared to the single crystal sample.

Grain size and lattice parameter's influence on band gap of SnS thin nano-crystalline films

Tin sulphide nano-crystalline thin films were fabricated on glass and Indium Tin Oxide (ITO) substrates by thermal evaporation method. The crystal structure orientation of the films was found to be dependent on the substrate. Residual stress existed in the films due to these orientations. This stress led to variation in lattice parameter. The nano-crystalline grain size was also found to vary with film thickness. A plot of band-gap with grain size or with lattice parameter showed the existence of a family of curves. This implied that band-gap of SnS films in the preview of the present study depends on two parameters, lattice parameter and grain size. The band-gap relation with grain size is well known in the nano regime. Experimental data fitted well with this relation for the given lattice constants. The manuscript uses theoretical structure calculations for different lattice constants and shows that the experimental data follows the trend. Thus, confirming that the band gap has a two variable dependency.

Influence of microstructure on the corrosion resistance of Fe–44Ni thin films

An Fe–44Ni nanocrystalline (NC) alloy thin film was prepared through electrodeposition. The relation between the microstructure and corrosion behavior of the NC film was investigated using electrochemical methods and chemical analysis approaches. The results show that the NC film is composed of a face-centered cubic phase (γ-(Fe,Ni)) and a body-centered cubic phase (α-(Fe,Ni)) when it is annealed at temperatures less than 400°C. The corrosion resistance increases with the increase in grain size, and the corresponding corrosion process is controlled by oxygen reduction. The NC films annealed at 500°C and 600°C do not exhibit the same pattern, although their grain sizes are considerably large. This result is attributed to the existence of an anodic phase, Fe0.947Ni0.054, in these films. Under this condition, the related corrosion process is synthetically controlled by anodic dissolution and depolarization.

Contribution of selective scattering to increase in the thermoelectric power of nanocrystalline films Cr1–x Si x

The results of the experimental investigation of the thermoelectric power and electrical conductivity of amorphous and nanocrystalline films in the Cr1–xSix (0.65 < x < 0.89) system at temperatures ranging from 300 to 800 K have been presented. It has been shown that the amorphous films rapidly crystallize at temperatures above 550 K. During the crystallization, the amorphous films transform into the nanocrystalline state. The rate of crystallization rapidly decreases with a decrease in the temperature. The in situ measurements of the thermoelectric power and electrical resistivity of the CrSi2 film have been performed during isothermal annealing at a temperature of 496 K. It has been demonstrated that the crystallization leads to an additional contribution to the thermoelectric power due to selective scattering of charge carriers at the boundaries of the nanocrystals.

Ultra-low NO2 detection by gamma WO3 synthesized by Reactive Spray Deposition Technology

A porous tungsten oxide (WO3) NO2 sensor was developed by a one-step flame based process called Reactive Spray Deposition Technology (RSDT). This nano-crystalline WO3 film was deposited directly on gold interdigitated electrodes. The sensing characteristics of this NO2 sensor was measured at the parts per million (ppm) level, (0.17–5ppm in air) at 300°C. The sensors showed a relatively fast response time (∼7s) and recovery time (∼5min), respectively. The stability of the sensor was evaluated for 300h in 0.5ppm NO2 at 300°C in (2000 response-recovery cycles). The sensor was stable up to 6days (∼150h) of continuous operation and degraded between 150–300h. The morphology and surface properties of the WO3 film were investigated with XRD, Raman spectroscopy, BET, SEM, TEM, and HRTEM.

Highly selective H2S gas sensor based on Cu-doped ZnO nanocrystalline films deposited by RF magnetron sputtering of powder target

Cu – doped ZnO (CZO) films were synthesized by RF magnetron sputtering of composite target prepared with powders of ZnO and copper. Crystalline nature, morphology and chemical/electronic composition of the films were investigated by X-ray diffraction (XRD), Atomic Force Microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). XRD showed hexagonal wurtzite structure with strong c-axis orientation for both doped and undoped films. AFM study revealed uniform deposition of nanocrystallites and XPS confirmed the Cu incorporation into ZnO lattice. Further the H2S sensing properties of the CZO films were investigated at different operating temperatures and gas concentrations. While all the doped films were highly selective to H2S, 2% CZO film showed highest sensitivity at an operating temperature of 250 °C.

Fabrication of Nb/Pb structures through ultrashort pulsed laser deposition

This work reports the fabrication of Nb/Pb structures with an application as photocathode devices. The use of relatively low energy densities for the ablation of Nb with ultrashort pulses favors the reduction of droplets during the growth of the film. However, the use of laser fluences in this ablation regime results in a consequent reduction in the average deposition rate. On the other hand, despite the low deposition rate, the films present a superior adherence to the substrate and an excellent coverage of the irregular substrate surface, avoiding the appearance of voids or discontinuities on the film surface. Moreover, the low energy densities used for the ablation favor the growth of nanocrystalline films with a similar crystalline structure to the bulk material. Therefore, the use of low ablation energy densities with ultrashort pulses for the deposition of the Nb thin films allows the growth of very adherent and nanocrystalline films with adequate properties for the fabrication of Nb/Pb structures to be included in superconducting radiofrequency cavities.