Nanocrystalline Films Research Articles

Study of variable range hopping conduction mechanism in nanocrystalline carbon thin films deposited by modified anodic jet carbon arc technique: application to light-dependent resistors

In this study, we report the deposition of nanocrystalline carbon thin films by modified anodic jet carbon arc technique assisted with inert-helium and reactive-nitrogen gaseous environments. The modified anodic jet technique facilitates to generate a very high localized pressure near the arc spot for the growth of nanocrystallites of carbon at high arc temperature and high pressure. The deposited films were analyzed for the growth of nanocrystallites in amorphous carbon structure under high-resolution transmission electron microscopy (HRTEM). The HRTEM studies reveal the distribution of nanocrystallites in the amorphous carbon matrix. The temperature-dependent conduction behavior of the deposited films has also been analyzed under Mott’s variable range hopping conduction mechanism. Both sets of the film are found to follow three-dimensional variable range hopping conduction mechanisms for the transport of charge carriers. The deposited films are also analyzed for their applications to energy-saving light-dependent resistors under an illumination intensity of ~ 100 mW/cm2 of white light. The reasonably high value of detectivity values ~ 1.26 × 106 and 3.12 × 106 Jones and relatively lower values of trap depth 0.6826 and 0.6834 eV for the nanocrystalline films deposited in helium and nitrogen environments suggest trapped state-assisted significantly high power conversion efficiency. This supports the suitability of the deposited films for light-dependent resistor applications.

Characterization of chemically deposited nanocrystalline CdSe thin films from transmittance spectra

Optical properties of nanocrystalline films of CdSe prepared by chemical bath deposition technique are studied as a function of molar concentration. Analyses of the transmittance spectra over 350–900 nm photon wavelengths of CdSe films indicates the existence of direct optical transition. A blue shift of absorption edge was observed which indicates a decrease in crystallite size. The optical band gap (Eg) was found to be in the range of 1.93–2.10 eV for as-prepared films. After annealing the band gap was found to decrease. The crystallite size was found in the range of 3.34–4.99 nm using the effective mass approximation model. Different optical parameters such as absorption coefficient, refractive index, extinction coefficient, optical conductivity, dielectric constants were determined from the transmittance spectra.

Mechanical behavior of nanocrystalline and ultrafine-grained NiTi thin films

Nanoscale NiTi thin films are highly suitable for high frequency thermal actuation in microelectromechanical devices because of their small thermal mass and large surface-to-volume ratio. However, there have been few studies on the mechanical behavior of NiTi films with thicknesses below 200 nm. Here, we synthesized nanocrystalline (NC) and ultrafine-grained (UFG) NiTi films, 150 and 120 nm thick, respectively, by crystallizing amorphous NiTi films embedded with Cr seed crystals, and investigated their mechanical behavior via uniaxial tensile experiments. The experiments reveal that the stress–strain response of NC and UFG NiTi film samples is considerably different from their bulk counterparts. NC NiTi film samples show a combination of pseudoelasticity and shape memory effect even when the deformation temperature is above the austenite finish temperature, while UFG samples show significant increase in stress during phase transformation from austenite to martensite. Furthermore, small changes in the microstructure cause marked differences in the stress–strain response of the NiTi thin films, emphasizing the need to carefully control their chemical composition and thermal history to achieve reproducible properties.

Exploring the microstructural, optoelectronic properties of deposition time dependent Cu2Sn(S,Se)3 thin film synthesized by non-vacuum arrested precipitation technique

A novel chemical approach is utilized to synthesize Cu2Sn(S,Se)3 (CTSSe) thin films by non-vacuum arrested precipitation technique (APT) via controlled release of ions from complexed solution. Effect of deposition time on the microstructural and optoelectronic properties of CTSSe thin films was studied comprehensively. The optical absorption studies demonstrate a decrease in band gap energy from 1.84 to 1.62 eV as the deposition time increases. Nanocrystalline thin films with improved grain size were illustrated from XRD studies. The FESEM micrographs depict progress in surface microstructure from a bunch of nanospheres to aggregated and densely packed nanospheres. EDS spectrum confirms the stoichiometric formation of Cu2Sn(S,Se)3 thin film. While, the XPS spectrum validates Cu1+, Sn4+, S2-, Se2-valence states of the elements in CTSSe thin film. Furthermore, the photoelectrochemical cell performance of 0.3 % for bare CTSSe thin film was elucidated with cell configuration glass-FTO/CTSSe/(I-/I3-)/graphite without any post deposition treatments. Also, the lowering of charge transfer resistance from EIS measurements corroborates for improved photoconversion efficiency. Finally, synthesis of CTSSe thin films by a simple APT finds a promising approach to fabricate efficient CTSSe photocathode for photovoltaic applications.

Plasma enhanced atomic layer deposition of titanium nitride-molybdenum nitride solid solutions

As part of improving the tribological properties of TiN-based coatings, researchers have introduced additional elements to the binary TiN system. Addition of a self-lubricating and oxide-forming substitutional element such as Mo to the rock salt TiN system deposited by sputtering has been widely studied. But, the TiN-MoN solid solution system grown by atomic layer deposition (ALD) is yet to be reported. Our current work is motivated by the need to understand and probe the structure of TixMo1−xN solid solutions with respect to the ALD growth process. In this work, thin films of TixMo1−xN (0 ≤ x ≤ 1) were deposited by plasma enhanced atomic layer deposition (PEALD) at 250 °C. Tetrakis(dimethylamido) titanium, bis(tert-butylimido)bis(dimethylamido) molybdenum, and N2 plasma were used as sources for Ti, Mo, and N, respectively. X-ray diffraction revealed nanocrystalline films with a rock salt crystal structure for all compositions of TixMo1−xN except for MoNx, which consisted of multiple phases with cubic MoN being the dominant phase. The elemental composition determined by x-ray photoelectron spectroscopy deviated from the pulse ratio of TiN:MoN. This study revealed that nearly the whole solid solution of the TiN-MoN system can be accessed by PEALD.

High-sensitivity photoelectrochemical visible-blind ultraviolet detector using SrTiO3 nanocrystalline for weak irradiation

We have demonstrated a photoelectrochemical solar-blind ultraviolet (UV) detector based on SrTiO3 (STO) nanocrystalline film in this work. The assembled UV detector presents a high on/off ratio of about 6433 under periodic UV irradiation, a high photocurrent density of 220.6 μA cm−2, and a fast response time of 9 ms. By introducing a block layer (BL) on fluorine-doped tin oxide substrate, the response and decay times are shortened to 6 ms and 8 ms, respectively. The BL plays a crucial part in hindering the recombination of electrons from electrolyte carriers and avoiding short circuits. Due to its excellent light capture capability, the detector based on SrTiO3 nanocrystalline exhibits high sensitivity to weak UV light (5 μW cm−2). Moreover, the detector also exhibits visible-blind characteristics and a good linear response.

FUNCTIONAL PROTECTIVE ZrN COATINGS ON IMPLANTS FOR TRAUMA SURGERY

The nano-crystalline films of zirconium nitride have been synthesized on implants for trauma surgery made of AISI 316 L stainles steel by using vacuum-arc deposition under RF-biasing mode in “Bulat” type device. Structure examinations – X-ray diffraction analysis (XRD), X-ray fluoriscent analysis (XRF), scanning electron microscopy (SEM) with microanalysis (EDX), nanoidentation method – were performed to study phase and chemical composition, surface morphology, microstructure and nanohardness of ZrN coatings. The corrosion resistance of coatings has been tested in 0.9 % quasiphysiological NaCl solution.

Effect of Transition Metals on the Mechanical Properties of ZnO Thin Films, Prepared by Sol-Gel Method

Undoped and transition metals (TM = Cr, Ni, Mn and Cd) doped zinc oxide (ZnO) thin films were prepared by sol-gel dip-coating method on glass substrates at 300 °C. In this study, the effect of dopant material on the structural, morphological, optical, electrical and mechanical properties of ZnO thin films is investigated by using XRD, AFM, UV-Vis, Hall effect and nanoindentation techniques, respectively. Nanocrystalline films with a ZnO hexagonal wurtzite structure and two preferred orientations (002) and (103) were obtained. UV-Vis transmittance spectra showed that all the films are highly transparent in the visible region (> 80 %). Moreover, the optical band gap of the films decreased to 3.13 eV with an increasing orbital occupation number of 3d electrons. AFM-topography shows that the films are dense, smooth and uniform, except for the high roughness RMS =26.3 nm obtained for Cd-doped ZnO. Finally, the dopant material is found to have a significant effect on the mechanical behavior of ZnO as compared to the undoped material. For Ni and Cd dopants, analysis of load and unload data yields an increase in the hardness (8.96 ± 0.22 GPa) and Young’s modulus (122 ± 7.46 GPa) of ZnO as compared to Cr and Mn dopants. Therefore, Ni and Cd are the appropriate dopants for the design and application of ZnO-based nanoelectromechanical systems.

Resistive switching control in forming-free nanocrystalline zinc oxide films

The influence of control parameters on the resistive switching effect in forming-free nanocrystalline zinc oxide films was studied. It was shown, resistive switching from HRS to LRS was observed at (+3.3±0.4) V, and from LRS to HRS at (-2.8±0.6) V. Changing the triangular sweep voltage signal phase by 90 degrees leads to a decrease R HRS from (52.7±5.2) kΩ to (38.3±20.2) kΩ, to an increase R LRS from (3.3±2.2) kΩ to (4.5±3.1) kΩ, and to a decrease R HRS /R LRS ratio from 17 to 9. Experimental results obtained showed, that an increase in amplitude of the sweep voltage U A from 2V to 6V and sweep time t A from 1s to 5s leads to a decrease in the R HRS /R LRS ratio from (25.1±2.4) to (9.3±0.6) and from (23.2±1.8) to (10.4±0.8), respectively. The results can be useful for the development of technological fundamentals of new-generation micro- and nanoelectronics elements manufacturing, including ReRAM elements for neuromorphic structures based on forming-free ZnO nanocrystalline films.

Effect of substrate temperature on morphology and properties of ZnO:In nanocrystalline films grown by PLD

The purpose of this work is to study the formation regularities of ZnO:In nanocrystalline films grown by pulsed laser deposition (PLD) for application as contact layers of photosensitive elements. It was found that increasing in the substrate temperature from 300 °C to 400 °C the films have a continuous granular structure with a grain size of 9±2 nm and 13.5±4.5 nm, respectively. The sample obtained at the substrate temperature of 150 °C is characterized by the presence of whisker-like structures with 200±47 nm long and 12±8 nm wide on the film’s surface. With increasing substrate temperature from 150 °C to 400 °C, the value of resistivity decreases from 9.3·10−1 Ω·cm to 2.9·10−2 Ω·cm, the concentration of current carriers increases from 2.12·1018 cm−3 to 3.5·1019 cm−3, and the mobility of current carriers also increases from 2.51 cm2/(V·s) to 6.17 cm2/(V·s). The results obtained can be used in the development and manufacture of highly efficient photoelectric converters based on hybrid nanostructures.

A novel approach for the development of bio-sensitized solar cell using cell lysate of a haloarchaeon Halostagnicola larsenii RG2.14 (MCC 2809) containing bacteriorhodopsin

Bacteriorhodopsin (BR) present in the cell membrane of haloarchaea and capable of converting light energy into electrical energy is being explored as biosensitizer in bio-sensitized solar cells (BSSC). The haloarchaeon, Halostagnicola larsenii RG2.14 (MCC 2809) containing bacteriorhodopsin (BR), was used to prepare BSSC. The cell lysate of the culture containing BR was immobilized on TiO2 nanocrystalline films. The BSSC was characterized for photoelectric performance, UV–visible spectroscopy, Scanning electron microscopy, and chronopotentiometry-chronoamperometry. Studies on the parameters like the use of ethanol for the cell lysate preparation resulted in the maximum open-circuit voltage of 0.52 V, short circuit photocurrent density of 120 μA/cm2 fill factor of 0.54 and overall efficiency of 0.11%. The present investigation indicates that the BR containing haloarchaeal culture cell lysate could be used in the preparation of BSSC.

Investigation of the processes of the formation of a nonequilibrium phase-structural state in FeTiB films obtained by magnetron sputtering

The main trends of modern developing magnetic microelectronics are miniaturization and speed, while ensuring efficient operation in the MHz and GHz frequency ranges of magnetic fields. Developing new magnetic materials featured by properties that ensure the implementation of these trends is the key fundamental and applied problem of materials science. In this regard, Fe-Me-X nanocrystalline soft magnetic alloys (Me is one of the metals from Group IVb of the Periodic Table, X is one of the N, C, O, B light elements) obtained in the form of films are of interest. As shown earlier by the authors of this article on Fe-Zr-N films, such films featuring by the Fe/MeX two-phase structure can provide a combination of high saturation induction (Bs), low coercive force (Hc), high hardness, and thermal stability of the structure. The films were produced by magnetron sputtering. The data obtained and published by the authors on the Fe–Ti–B films earlier indicate great prospects for their application in modern microelectronics. There are no any other published results of FeTiB film studies in the context of microelectronics applications. In this paper, we continue the studies of FeTiB films started earlier to identify the chemical and phase composition providing the level of properties required for film application in microelectronics. Nanocrystalline films containing 0 to 14.3 at.% Ti and 0 to 28.9 at.% B were obtained by DC magnetron sputtering. The phase-structural state of the films was studied by X-ray diffraction and transmission electron microscopy. All films are divided into 3 groups according to phase composition: single-phase (supersaturated solid solution of Ti in α-Fe), two-phase (α-Fe(Ti)/α-Ti, α-Fe(Ti)/TiB2, α-Fe (Ti)/FeTi, α-Fe(Ti)/Fe2B) and XRD amorphous. It is shown that XRD amorphous films feature by a mixed structure represented by a solid solution of α-Fe(Ti) with a grain size between 0.7 and 2 nm and an amorphous phase. A reasonable assumption is made on the amorphous phase enrichment by boron. A quantitative assessment of the α-Fe(Ti) phase grain size and its dependence on the chemical and phase composition of the films is given. The mechanisms of solid solution and dispersion hardening determine the grain size of this phase.

Spin crossover and cooperativity in nanocrystalline [Fe(pyrazine)Pt(CN)4] thin films deposited by matrix-assisted laser evaporation

Thin film structures with controlled functionalities are critical to exploit the exceptional application capacity of spin crossover complexes. Here, we report spin crossover (SCO) in thin films of [Fe(pyrazine)Pt(CN)4] nanocrystalline particles (NPs) deposited on monocrystalline Si by laser-mediated evaporation of a cryogenically frozen suspension of the title compound. Both X-ray diffraction and microscopic imaging of the films (thickness ~ 120 nm) confirm resembled structure of pristine nanocrystals. However, their spin crossover behavior is dramatically altered going from the cooperative and sharp transition within temperature hysteresis (16 K) around the room temperature region for reference NPs to a gradual transition type shifted downwards to 170 K for the thin films. The decrease in cooperativity within the film is due to side effect of laser-induced desorption that results in rearrangement of the pyrazine-pillared 3D framework into stacked 2D Fe[Pt(CN)4] sheets in the laser affected NP regions. The observed effects have significant implications for future study on the cooperative spin transition in laser-engineered nanocrystalline films.

The impact of processing conditions and post-deposition oxidation on the opto-electrical properties of hydrogenated amorphous and nano-crystalline Germanium films

Low-cost multijunction photovoltaic devices are the next step in the solar energy revolution. Adding a bottom junction with a low bandgap energy material through plasma enhanced chemical vapor deposition (PECVD) processing could potentially provide a low-cost boost in conversion efficiency. A logical candidate for this low bandgap material is germanium. In this work we investigate the growth of PECVD processed hydrogenated amorphous/nano-crystalline germanium (a/nc-Ge:H), by characterizing over 100 samples, processed with a wide range of deposition pressures, powers, temperatures and GeH4 dilution in hydrogen, using elemental analysis, vibrational analysis and analysis of the opto-electrical properties. We have identified a small processing window in which nc-Ge:H films are processed reproducibly. We also report on the strong correlation between the refractive index of the films and the presence- and extent of post-deposition oxidation. Notably, the oxidation generally increased the photoresponse of the films, as it results in a decrease of room temperature σd by 1-3 orders of magnitude. However, oxidation results in an increase of the bandgap energy and therefore impedes the development of a low bandgap material. The lowest E04 we report is about 1.1eV, with an ETauc of 0.9eV and an σph/σd of 3.4.

ZnO-Based Nanocrystalline Films Obtained in a Single Vacuum Cycle

ZnO-Based Nanocrystalline Films Obtained in a Single Vacuum Cycle

Optical and Structural Properties of Titanium Dioxide Papered by DC Magneto-Sputtering as a NO2 Gas Sensor

In this work, a reactive DC magnetron sputtering technique was used to prepare TiO2 thin films. The variation in argon and oxygen gases mixing ratios (4:1, 2:1, 1:1, 1:2, 1:4) was used to achieve optimal properties for gas sensing. In addition, an analysis of the optical XRD properties of TiO2 thin films is presented. High-quality and uniform nanocrystalline films were obtained at a working gas pressure of 0.25 mbar and 1:4 (Ar/O2) gas mixture. The optical properties showed a transparent thin film with uniform adherence to the substrate. The average transmission of the TiO2 films deposited on the glass substrates was higher than 95% over the range of 400 to 800 nm. The optical band gap varied from 3.84 eV to 3.93 eV as a function of oxygen/argon ratios. The XRD pattern showed that the films have an amorphous structure, which is shifted to polycrystalline with increasing oxygen to argon ratio. The sensitivity, response time, and recovery time were measured for TiO2 thin films using NO2 oxidizing gas.

Bonding and stoichiometry in low-energy radio frequency magnetron sputtered ZnO thin films on flexible substrate

Spectroscopic and electrical studies of ZnO thin films grown by radio frequency magnetron sputtering at low power without substrate heating revealed improved stoichiometry and reduced free carrier concentration with increasingly energetic growth conditions, which is opposite to what is typically observed. Hall measurements showed a decrease in carrier concentration from 6.9 × 1019 cm−3 to 1.4 × 1019 cm−3 as power increased from 40 W to 120 W, and a decrease in carrier concentration from 8.6 × 1019 cm−3 to 2.6 × 1019 cm−3 as the deposition pressure decreased from 1.2 to 0.4 Pa. These findings correlated well with XRD, x-ray photoelectron spectroscopy and photoluminescence results, which showed that more energetic conditions increased the fraction of oxygen that was stoichiometrically bonded. The change in the absorption edge, likely due to a Burstein-Moss shift, was also in qualitative agreement. In the range of conditions used, energy transfer from the plasma to the growing film governs bonding, stoichiometry and hexagonal nanocrystalline film formation and growth.

Ion irradiation induced phase transformation in gold nanocrystalline films

Gold is a noble metal typically stable as a solid in a face-centered cubic (FCC) structure under ambient conditions; however, under particular circumstances aberrant allotropes have been synthesized. In this work, we document the phase transformation of 25 nm thick nanocrystalline (NC) free-standing gold thin-film via in situ ion irradiation studied using atomic-resolution transmission electron microscopy (TEM). Utilizing precession electron diffraction (PED) techniques, crystallographic orientation and the radiation-induced relative strains were measured and furthermore used to determine that a combination of surface and radiation-induced strains lead to an FCC to hexagonal close packed (HCP) crystallographic phase transformation upon a 10 dpa radiation dose of Au4+ ions. Contrary to previous studies, HCP phase in nanostructures of gold was stabilized and did not transform back to FCC due to a combination of size effects and defects imparted by damage cascades.

Electronic structure spectroscopy of organic semiconductors by energy resolved-electrochemical impedance spectroscopy (ER-EIS)

Organic electronic applications are envisioned to address broad markets, which includes flexible displays, electronic papers, sensors, disposable and wearable electronics, and medical and biophysical applications, leading to a tremendous amount of interest from both academia and industry in the study of devices. These fields of science and technology constitute interdisciplinary fields that cover physics, chemistry, biology, and materials science, leading, as a wanted output, to the elucidation of physical and chemical properties, as well as structures, fabrication, and performance evaluation of devices and the creation of new knowledge underlying the operation of organic devices using new synthesized organic materials—organic semiconductors. We testify the situation when the available organic electronic applications sometimes lack a theoretical background. The cause may be the complicated properties of disordered, weak bounded, molecular materials with properties different from their inorganic counterparts. One of the basic information-rich resources is the electronic structure of organic semiconductors, elucidated by the methods, hardly possible to be transferred from the branch of inorganic semiconductors. Electrochemical spectroscopic methods, in general, and electrochemical impedance spectroscopy, in particular, tend and seem to fill this gap. In this Perspective article, the energy resolved-electrochemical impedance spectroscopic method for electronic structure studies of surface and bulk of organic semiconductors is presented, and its theoretical and implementation background is highlighted. To show the method’s properties and strength, both as to the wide energy and excessive dynamic range, the basic measurements on polymeric materials and D–A blends are introduced, and to highlight its broad applicability, the results on polysilanes degradability, gap engineering of non-fullerene D–A blends, and electron structure spectroscopy of an inorganic nanocrystalline film are highlighted. In the outlook and perspective, the electrolyte/polymer interface will be studied in general and specifically devoted to the morphological, transport, and recombination properties of organic semiconductors and biophysical materials.

Properties of nanocrystalline silicon probed by optomechanics

Abstract Nanocrystalline materials exhibit properties that can differ substantially from those of their single crystal counterparts. As such, they provide ways to enhance and optimize their functionality for devices and applications. Here, we report on the optical, mechanical and thermal properties of nanocrystalline silicon probed by means of optomechanical nanobeams to extract information of the dynamics of optical absorption, mechanical losses, heat generation and dissipation. The optomechanical nanobeams are fabricated using nanocrystalline films prepared by annealing amorphous silicon layers at different temperatures. The resulting crystallite sizes and the stress in the films can be controlled by the annealing temperature and time and, consequently, the properties of the films can be tuned relatively freely, as demonstrated here by means of electron microscopy and Raman scattering. We show that the nanocrystallite size and the volume fraction of the grain boundaries play a key role in the dissipation rates through nonlinear optical and thermal processes. Promising optical (13,000) and mechanical (1700) quality factors were found in the optomechanical cavity realized in the nanocrystalline Si resulting from annealing at 950°C. The enhanced absorption and recombination rates via the intragap states and the reduced thermal conductivity boost the potential to exploit these nonlinear effects in applications including Nanoelectromechanical systems (NEMS), phonon lasing and chaos-based devices.