Nanocrystalline Films Research Articles

Electron transport across nanocrystalline diamond films: Field emission and conducting atomic force microscopic investigations

In this paper, we report synthesis of nano-crystalline diamond (n-C diamond) films using DC-plasma assisted hot filament chemical vapor deposition. The films are characterized by Raman spectroscopy, scanning electron microscopy, and atomic force microscopy. The films were found to be uniform and well adherent to crystalline ⟨100⟩ and ⟨111⟩ on silicon substrates. Comparative studies were carried out using field emission microscopy and conductive atomic force microcopy to investigate the mechanism of electron transport across the n-C diamond films in far field and near field geometries. The former is important in the context of field emission display devices, and the latter is important as a gate electrode for field effect transistors. The I–V characteristics in both the cases obeyed the Fowler–Nordheim equation. Various parameters, viz., turn-on voltage, threshold voltage, and field enhancement factors, were estimated. The power spectral density of noise in field electron emission current exhibited P(f) = A·I2/f2 behavior. The results are discussed in the light of the present understanding of the mechanism of field emission from n-C diamond films.

Dependence of the Crystallization Kinetics of Cr0.26Si0.74 Thin Films on Their Thickness

The thermoelectric properties and crystallization kinetics of Cr0.26Si0.74 thin films with the thicknesses 11, 14, 21, 31, 56, 74, and 115 nm are studied. The films are produced by magnetron-assisted sputtering onto a cold substrate and, in the initial state, are amorphous in structure. During thermal annealing, the amorphous mixture transforms into a two-phase nanocrystalline composite consisting of chromium disilicide and silicon. In-situ measurements of the thermoelectric properties of the films during annealing show that the temperature of the onset of crystallization decreases, as the film thickness is decreased, whereas the crystallization rate increases. The thermopower of the nanocrystalline films decreases, as the film thickness is increased, and the thermoelectric-power factor reaches its maximum in films with a thickness of 31 nm.

Solution-processed p-type nanocrystalline CoO films for inverted mixed perovskite solar cells

Inorganic p-type materials show great potential as the hole transport layer in perovskite solar cells with the merits of low costs and enhanced chemical stability. As a p-type material, cobalt oxide (CoO) has received so far not that level of attention despite its high hole mobility. Herein, solution-processed p-type CoO nanocrystalline films are developed for inverted mixed perovskite solar cells. The ultrafine CoO nanocrystals are synthesized via an oil phase method, which are subsequently treated by a ligand exchange process using pyridine solvent to remove the long alkyl chains covering the nanocrystals. From this homogeneous colloidal solution CoO films are obtained, which exhibit a smooth and pin-hole free surface morphology with high transparency and good conductivity. The ultraviolet photoelectron spectrum also indicates that the energy levels of the CoO film match well with the mixed perovskite Cs0.05(FA0.83MA0.17)0.95(I0.83Br0.17)3. Inverted solar cells based on crystalline CoO films with ligand exchange show a reasonable energy conversion efficiency, whereas devices based on CoO films without ligand exchange suffer from a strong S-shape JV-characteristic. Thus, the crystalline CoO films are foreseen to pave a new way of inorganic hole transport materials in the fields of perovskite solar cells.

Enhancement in the photoluminescence, linear and third order nonlinear optical properties of nanostructured Na-CdS thin films for optoelectronic applications

Cadmium sulphide (CdS) thin films are widely used for various devices applications. Here, we report on the development of nanostructured CdS and Na-CdS thin films with enhanced optical and emission properties using an inexpensive deposition method. High quality Na-CdS nanocrystalline films were fabricated using spin coating method on glass substrates with different doping concentration (1–4 wt%). X-ray diffraction studies of the prepared films reveal a cubic zinc blende structure. The crystallites sizes of the films were found to be about ~ 5–11 nm and significantly increase with increasing Na doping concentrations. Morphological imaging of the grown films shows spherical nano-size grains in the range of 9–27 nm. The nanocrystalline films show high transmittance (~ 85%) in the visible region and was found to increase with increasing Na doping content. Room temperature photoluminescence (RTPL) spectra of the films reveal a direct band to band transition with enormous amount of enhancement in near band emission (NBE) up to 400% for the films doped with 1 and 2 wt% Na concentrations which is about ten times higher as compared with pure CdS films. A good improvement was also observed with Na doping in the linear and third order nonlinear optical properties of the films. The reported results in the current study show the high potential of Na-doped CdS films as candidates for low cost high performance optoelectronic devices.

Semitransparent and conductive CaSi2 films for silicon device applications

The purpose of this work was to comparatively analyze the structural, optical and electrical properties of calcium silicides (CaSi, and CaSi2) on silicon in the form of epitaxial and nanocrystalline films grown on Si(001) and Si(111) substrates and to determine the field of use. An analysis of the structure of the grown films showed the presence of contributions from amorphous, nanocrystalline and crystalline phases (Ca2Si, CaSi and CaSi2) with an increase of the Si substrate temperature from 190 °C to 500 °C. It has been established that transparency in the photon energy range 0.2–1.3 eV in CaSi2 films is associated with a low density of states at the Fermi level, and high conductivity is determined by the concentration of free carriers up to 1021 cm–3 at T = 7–300 K. The latter provide high electrical conductivity up to 1200 (Ω cm)−1, low sheet resistance (26–40 Ω/square) and high reflection (80%–90%) at photon energies below 0.4 eV. The unique properties of CaSi2 films are promising for the creation of transparent conductive pads for silicon-based solar cells and the development of infrared photodiodes with a Schottky barrier on p-type silicon.

Thin films of nanocrystalline graphene/graphite: An overview of synthesis and applications

AbstractNanocrystalline graphite/graphene (NCG) has been studied well before graphene was successfully produced in 2004. Despite the fact that NCG thin films cannot reach the high performance of large‐area graphene in some of its properties, they are able to provide significant benefits for various applications, besides being easier to produce and integrate into devices with current processing technologies. We present here, a comprehensive account of the large variety of synthesis methods, as well as of the already large range of potential applications for NCG thin films.

Zero field cooled exchange bias effect in nano-crystalline Mg-ferrite thin film

The Zero Field Cooled (ZFC) Exchange Bias (EB) effect in a single phase nanocrystalline Mg-ferrite thin film, deposited on an amorphous quartz substrate using pulsed laser ablation technique, is reported. The film showed a high ZFC EB shift (HE ∼ 190 Oe) at 5 K. The ZFC EB shift decreased with increasing temperature and disappeared at higher temperatures (T > 70 K). This Mg-ferrite thin film also showed the Conventional Exchange Bias (CEB) effect, but unlike many CEB systems, the film showed a decrease in coercivity (HC) under the Field Cooled measurements. The film also showed the training effect in ZFC measurements, which followed the frozen spin relaxation behavior. The observed exchange bias could be attributed to the pinning effect of the surface spins of frozen glassy states at the interface of large ferrimagnetic grains.

Anomalous photoconductivity in chemical spray pyrolysis deposited nano-crystalline ZnO thin films

Nano-crystalline ZnO thin films were deposited by chemical spray pyrolysis technique by maintaining the substrate at a temperature of 493 K. Structural characterization using XRD analysis identified the preferential orientation of the deposited films to be along the (002) plane. Photoluminescence studies showed emissions at 381 nm, 395 nm and 469 nm from the samples. The samples under halogen illumination in ambient condition exhibited a photosensitivity of ∼ 106. Under the same illumination pulse width and intensity in vacuum condition of 5x 10−5 T, the photosensitivity was an order higher ∼ 107. The photo response of the films to UV illumination under vacuum condition showed persistent photoconductivity, whereas under air ambient showed an anomalous behaviour of decrease of photocurrent under continuous illumination. We model the anomalous behaviour based on the mechanism of oxygen absorption and desorption from the surface of the nano-crystalline ZnO thin film. The dual photoconduction exhibited by our ZnO thin films can help multi-functional applications in artificial intelligence.

Novel hydrothermal route for synthesis of photoactive Cu2ZnSn(S,Se)4 nanocrystalline thin film: efficient photovoltaic performance

This work reports single-step deposition of Cu2ZnSn(S,Se)4 (CZTSSe) nanocrystalline film via simple and non-toxic hydrothermal route. The hydrothermal growth of CZTSSe multinary nanocrystalline film formation based on Ostwald ripening law which results in highly oriented nanocrystalline nanoflakes. The UV–Vis spectroscopy illustrates that maximum absorption was observed in the range of 650–700 nm with band gap energy of 1.48 eV. Structural studies confirm the formation of pure-phase kesterite crystal structure. Further, Raman shift at 174 cm−1, 196 cm−1 and 236 cm−1 shows A1 mode of vibration corresponding to kesterite structure. Densely packed and compact nanoflake formation was observed in SEM studies. Compositional analysis confirms the stoichiometric film formation with expected valence states of CZTSSe stoichiometry: Cu+, Zn2+, Sn4+, S2− and Se2−. The photoelectrochemical cell performance of CZTSSe film photoelectrode was investigated using simple two-electrode system. In dark condition, J–V measurement demonstrates semiconductor behavior and under illumination shows generation of photocurrent of 3.64 mA/cm2 at a photovoltage of 400 mV. The CZTSSe films show photoconversion efficiency (η) of 3.41%, indicating that the hydrothermally grown CZTSSe photocathode is a promising material for film solar cell technology. Electron impedance spectroscopy illustrated generation of charge transport resistance of 465 Ω.

Preparation of Ni–Si–B nano-crystalline film and mechanism analysis using first-principle calculations

In this work, Ni–Si–B nano-crystalline film was prepared successfully based on supersaturated solid solution method using magnetron sputtering technology. The phase structure of Ni–Si–B nano-crystalline film is composed of Ni (111) phase completely, and the average diameter of Ni grain is 4.77 nm. The high-multiple morphology of the film shows the uniform paste-like structure, which is constituted by a little of pure nickel (Ni) and much of nickel-based solid solution (NiSS). The solid solution of elements Si and B in host Ni can enhance the nano-crystalline degree of the film effectively, and the solid solubility of Si is much larger than that of B, which are c (Si) = 14.6 at.% and c(B) = 0.00124 at.%, respectively. The solid solubility of substitutional solute element Si and interstitial solute element B are both influenced by the bonding contribution and structural contribution simultaneously. For substitutional solute element, the structural contribution and bonding contribution are negative for improving the solid solubility, which are harmful to the formation of nano-crystalline. While for interstitial solute element, although the influence of structural contribution is negative for improving the solid solubility, the bonding contribution plays the positive role, which is conductive to the formation of nano-crystalline.

Impact of calcination temperature on the structure, optical and photoluminescence properties of NanocrystallineCerium oxide thin films

Nanocrystalline cerium oxide films were effectively synthesized on glass substrates via hydrothermal and dip coating techniques at various calcination temperatures (CT) from 300 to 550 °C. Impact of CT on the structure, surface morphology, optical and photoluminescence (PL) properties of cerium oxide (CeOx) films has been investigated. The grazing incident in-plane X-ray diffraction (GIIXD) analysis revealed that thin films acquire cubic fluorite nanostructure of CeO2, with average crystallite size in the range of 3.77–7.63 nm for films of different CT. XPS results revealed the existence of amorphous Ce2O3 in nanostructure CeOx films with crystalline CeO2 phase that was identified by GIIXD results. AFM images manifestations of CeOx thin film at various temperatures with tinny nanoparticles which agglomerate as spherical grains appear randomly distributed with surface roughness ranging from 0.42 to 0.488 μm for films of different CT. Optical properties indicated a narrowing in direct and indirect band gaps of nanocrystalline films than bulk CeO2 to show some fluctuation with the influence of CT that affected with lowering in Ce3+/Ce4+ ratio. Other optical parameters were analysed with the influence of calcination temperatures CT. Photoluminescence (PL) studies indicate the presence of most intense PL emission peak appears at 419 nm revealed the fundamental transition of direct band gap of CeO2, in addition to some other PL emission peaks in the visible range attributed to structural and oxygen vacancies defects states among Ce4f and O 2p bands.

Scalable ultrafast epitaxy of large-grain and single-crystal II-VI semiconductors

A general problem for semiconductor applications is that very slow deposition on expensive single-crystal substrates yields high crystalline quality with excellent electro-optical properties, but at prohibitive costs and throughput for many applications. In contrast, rapid deposition on inexpensive substrates or nanocrystalline films yields low costs, but comparatively inferior crystallinity, carrier transport, and recombination. Here, we present methods to deposit single-crystal material at rates 2–3 orders of magnitude faster than state-of-the-art epitaxy with low-cost methods without compromising crystalline or electro-optical quality. For example, single-crystal CdTe and CdZnTe films that would take several days to grow by molecular-beam epitaxy are deposited in 8 minutes by close-spaced sublimation, yet retain the same crystalline quality measured by X-ray diffraction rocking curves. The fast deposition is coupled with effective n- and p-type in-situ doping by In, P, and As. The epitaxy can be extended to nanocrystalline substrates. For example, we recrystallize thin CdTe films on glass to deposit large grains with low defect density. The results provide new research paths for photovoltaics, detectors, infrared imaging, flexible electronics, and other applications.

Effect of anionic fluorine incorporation on structural, optical and electrical properties of ZnO nanocrystalline films

ZnO and ZnO:F (fluorine doped Zinc oxide) films have been deposited on glass substrates at 325 ± 5 °C through ultrasonic spray pyrolysis method (USP). X-ray diffractometer (XRD) has been utilized to determine the crystal structural and some structural parameters such as lattice constants, texture coefficients TC (hkl), crystallite size and dislocation densities of all the ZnO films. The XRD results have revealed that all the ZnO films have the hexagonal wurtzite structure and F incorporation shows no significant effect on the crystal structure. X-ray photoelectron spectroscopy (XPS) measurements have confirmed the presence of fluorine atoms in the ZnO lattice and also the existence of Zn–F bonding states. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) measurements have been carried out to investigate the effect of F doping on the surface morphology of the ZnO films. The optical properties of the films have been analyzed by utilizing transmittance, absorbance and reflectance measurements. Some optical parameters such as extinction coefficient (k), refractive index (n) and optical conductivity have been determined depending on the change in F incorporation. Increasing the F doping initially led to a decrease in the optical band gap, but later increased. Electrical resistivity and carrier concentration values of the deposited films have been determined with Hall effect measurement system and it has been detected that 6% F-doped ZnO film has the minimum resistivity and maximum carrier concentration.

The Chemresistive Properties of SiC Nanocrystalline Films with Different Conductivity Type

We have demonstrated the ability of thin nanocrystalline SiC films with various types of conductivity to detect oxidative (O2), reducing gases (CO, CH4) with the maximum allowable concentrations for human safety. It was shown that n-nc-SiC films with electronic conductivity had a higher gas sensitivity Sn than p-nc-SiC films with hole conductivity sensivity Sp to the action of gases in a wide concentration range. So, for the maximum permissible concentrations of O2 (3%), CO (0.1%), CH4 (10%) the sensitivity ratio of the films Sn/Sp was 2.9; 4.8 and 10, respectively. For research, we used a simple resistor geometry optimizing which it is possible to significantly increase the sensitivity of films to gases in order to detect extremely low gas concentrations. Thus, based on nc-SiC films, it is possible to develop high-temperature gas sensors to detect reactive gases in a wide range of concentrations, including threshold allowable values.

Structural, optical, and magnetic properties of Co-doped ZnO nanocrystalline thin films for spintronic devices

Different compositions of Zn1−xCoxO (0 ≤ x ≤ 0.15) nanocrystalline compounds were produced in terms of ball milling method, thin films of these compounds were prepared by electron beam evaporation method. XRD pattern was utilized to study the structural properties of these films. Hexagonal wurtzite-type structure was displayed for all of the films. Using XRD pattern, the crystallite and lattice strain were calculated. The crystallite size decreases but the lattice strain's value rises up with accumulating dopants of Co. Increasing in lattice strain may be related to the increase in the concentration of lattice imperfections. The optical constants, n and k of the Zn1−xCoxO nanocrystalline films were calculated in terms of spectroscopic ellipsometry measurements in a range of 300–1100 nm. With more doping of Co, the refractive index showed an increase, as well. According to Tauc relation, the optical energy gap of the Zn1−xCoxO films was calculated and proved to be direct transition. The energy gap found to be decreased with the increasing of the Co concentration. In addition, studying the magnetic properties of Zn1−xCoxO films reveal room temperature ferromagnetism.

Multiscale modeling of the ionic conductivity of acceptor doped ceria

The ionic conductivity of acceptor doped ceria is strongly influenced by grain boundaries and interfaces, with most experiments showing a conductivity decrease in these regions. Classical models explain this observation by the formation of space charge layers, that are depleted of mobile ionic charge carriers. However, some experiments demonstrate an increase in ionic conductivity and recent models show that the space charge layers can also be enriched in mobile ionic species. Because of these discrepancies, it is still not certain whether nanocrystalline or thin film ceria can offer superior ionic conductivity or not. Recently, we have demonstrated by means of Monte Carlo simulations that the ionic conductivity in space charge layers can indeed exceed the bulk value. In this work, we combine these Monte Carlo simulations with a continuum model to predict charge carrier concentration profiles. This multiscale approach allows for a realistic prediction of the grain boundary ionic conductivity.

Enabling remote quantum emission in 2D semiconductors via porous metallic networks

Here we report how two-dimensional crystal (2DC) overlayers influence the recrystallization of relatively thick metal films and the subsequent synergetic benefits this provides for coupling surface plasmon-polaritons (SPPs) to photon emission in 2D semiconductors. We show that annealing 2DC/Au films on SiO2 results in a reverse epitaxial process where initially nanocrystalline Au films gain texture, crystallographically orient with the 2D crystal overlayer, and form an oriented porous metallic network (OPEN) structure in which the 2DC can suspend above or coat the inside of the metal pores. Both laser excitation and exciton recombination in the 2DC semiconductor launch propagating SPPs in the OPEN film. Energy in-/out- coupling occurs at metal pore sites, alleviating the need for dielectric spacers between the metal and 2DC layer. At low temperatures, single-photon emitters (SPEs) are present across an OPEN-WSe2 film, and we demonstrate remote SPP-mediated excitation of SPEs at a distance of 17 μm.

Nonstoichiometric tin oxide films: study by X-ray diffraction, Raman scattering and electron paramagnetic resonance

Nonstoichiometric SnO/SnO2/SnO2−δ films were fabricated by DC magnetron sputtering and reactive DC magnetron sputtering of tin target with further 2-stage temperature annealing of synthesized materials. X-ray diffraction analysis, Raman spectroscopy and electron paramagnetic resonance (EPR) spectroscopy were employed to study the influence of oxygen content in the plasma during the sputtering process and the temperature of annealing on the stoichiometric and phase composition of synthesized films. It was found that the tin monoxide phase prevailed in the films fabricated by DC magnetron sputtering in the argon plasma followed by a 2-stage annealing process. Nanocrystalline films containing both tin monoxide and tin dioxide were synthesized when reactive magnetron sputtering with a small content of oxygen (of about 1 vol.%) was used. The increase of oxygen content in the plasma to the value of about 2 vol.% leads to the formation of amorphous films. The intensity of the Raman peaks inherent in SnO2 vibration modes was found to depend on the content of the tin monoxide phase in the films. This effect can be attributed to the dissipative transition of electronic excitation from tin monoxide to tin dioxide nanocrystallites.

Hydrogen as a probe for defects in materials: Isotherms and related microstructures of palladium-hydrogen thin films

Metal-hydrogen systems offer grand opportunities for studies on fundamental aspects of alloy thermodynamics. Palladium-hydrogen (Pd-H) thin films of nano crystalline, multi-oriented and epitaxial microstructures, electrolytically charged with hydrogen, serve as model systems. In these films thermodynamics of hydrogen absorption is modified by interface effects related to mechanical stress and to microstructural defects. Since in this respect hydrogen can be utilized to reveal the microstructural constituents of the films, we aim to investigate the distribution of sites (DOS) hydrogen occupies in the films’ solid solution regime. A σDOS model is proposed, taking the measured substrate-induced stress contribution to the chemical potential into account. This enables the determination of the different sites’ volume fractions and of pure site energy distributions by fitting measured isotherms. Interstitial sites, grain/domain boundary sites and deep traps are distinguished. Dislocations and vacancies are shown to have a minor impact on the films’ trapping of hydrogen atoms, while deep traps are related to the films’ surface. Enhanced binding energies in nano crystalline films can be ascribed to the tensile strain effect of grain boundaries acting on the grains. Measured surface trapping energies fit to the respective bulk values, while the trapping of hydrogen in grain/domain boundaries of the films is significantly increased. This can be interpreted with different grain/domain boundary structures. Different from octahedral interstitial site occupation, tetrahedral site occupation is suggested for grain/domain boundaries of the films.

Зависимость кинетики кристаллизации тонких пленок Cr-=SUB=-0.26-=/SUB=-Si-=SUB=-0.74-=/SUB=- от толщины

In this work we study thermoelectric properties of Cr0.26Si0.74 thin films with thickness of 11, 14, 21, 31, 56, 74, 115 nm. The films were produced by magnetron sputtering onto unheated substrate. The films had amorphous structure. Thermoelectric properties of samples were studied during thermal annealing in pure helium atmosphere. Changing of thermoelectric properties during annealing indicated changing in structure. It was found that kinetic of crystallization depends on the thickness of the films. The thermopower of nanocrystalline films decreases with increasing film thickness, and the power factor reaches its maximum value in films with a thickness of 31 nm.