Narrow Results

Exploring the morphogical and structural properties along with gas sensing applications both pure and Ti-doped SnO₂ ultra-thin films, were meticulously crafted on micromachined silicon substrate heater devices using a combination of classical soft chemical processes and hydrothermal techniques (SCPHTP). The fabrication process involved a two-step approach: initially, a 20nm layer of tin oxide was hydrothermally deposited onto the substrates, followed by annealing in wet air at 600^∘C for 5h using a standardized temperature variation protocol. Subsequently, secondary layers with thicknesses of 20, 40 and 60nm were sequentially deposited onto the tin dioxide devices and oxidized in wet air at 550^∘C and 600^∘C for 20h each, using the same temperature modulation scheme. Throughout this process, the hydrothermal deposition temperature remained constant at 180^∘C for both the initial and secondary layers of tin dioxide deposition. Additionally, Ti layers with thicknesses of 4 and 8nm were deposited onto the 20nm + 40nm system, subjected to annealing at 550^∘C for 20h, followed by 1-min annealing in dry O₂ at 700^∘C and 800^∘C, respectively, using a Rapid Thermal Annealing (RTA) system. Characterization of the crystalline and surface structures of the devices revealed a transformation of the soft chemical tin dioxide solution into the cassiterite structure of SnO₂, resulting in uniform large surface areas for the sensor devices. Moreover, Ti metal layers of 4 and 8nm thicknesses were fully converted into TiO₂ on the surface of the devices. Subsequent testing showcased higher current values in sandwich systems of 20nm + 60nm and 20nm + 40nm compared to the 20nm + 20nm configuration. Sensitivity and stability assessments for various volatile organic compounds (VOCs) and CO gases at a constant DC temperature of 400^∘C indicated excellent performance, with sensitivity to CO gas being contingent on relative humidity (RH). Notably, RTA-annealed and Ti-8 nm-doped sensor devices exhibited superior sensitivity and reproducibility, particularly when treated at 800^∘C in dry O₂ for 1min. This heightened performance can be attributed to the occupation of chloride ions in the oxygen sites of the as-synthesized SnO₂, resulting in enhanced sensing capabilities for VOC gases.

Nickel-Titanium (Ni-Ti) thin films have gained a lot of attention due to their unique features, such as the shape memory effect. Micro-actuators, micro-valve, micro-fluid pumps, bio-medical applications, and electronic applications have a lot of interest in these smart thin films. Sputter-deposited NiTi thin films have shown the potential to be very useful as a powerful actuator in micro-electro-mechanical systems (MEMS) because of their large recovery forces and high recoverable strains. Despite the advancement of improved deposition methods for the NiTi thin films, there are still certain unsolved challenges that impede accurate composition control throughout the deposition process. Many applications, spanning from the aerospace industries to a range of nanotechnologies, require knowledge of the sputtering characteristics of the materials that are subjected to bombardment, ejection, and deposition of ions. In recent decades, atomic scale modeling has been given a high emphasis in ion sputtering research, providing an adequate and precise description of collision cascades in solids using the Stopping and Range of Ions in Matter (SRIM) and Transport of Ions in Matter (TRIM). In this paper, SRIM is used to address how the heavy ions interact with the target materials. A variety of ion-solid interaction characteristics, including the sputter yield, have been determined by simulating collision cascades in the solids. On the other hand, TRIM was used to describe the range of ions that enter into the matter and cause damage to the target throughout the process. The simulation was carried out to compare the sputtering yield of Ni and Ti by varying the energy input (from 300V to 1300V). SRIM simulation was conducted by varying the thickness of the film, the angle of incidence of ions, and the energy involved in the sputtering process. The characterization of the films has been carried out using Field Emission Scanning Electron Microscopy (FESEM) to comprehend the surface and interface morphologies of the films and to validate the simulated results. With an increase in energy input (target voltage), the sputtering yield increased. The sputtering yield of the Ni target was higher than the Ti target indicating that Ni can be removed relatively easier than Ti.

With the advent of the semiconductor age and later the age of nanotechnology, the thin film and coating field have established their importance and reasons for doing in-depth studies. Different sophisticated physical techniques, like chemical vapor deposition, sputtering, evaporation, molecular beam epitaxy, etc., and the conventional spin coating or dip coating, have been employed to get thin films of specific materials/compounds. Of all these, physical techniques are particularly preferred for their ability to develop good quality thin film with high uniformity. In the field of experimental material science, there are tremendous efforts in thin film development and the study of their different properties. The properties include topological, electrical, electronic, optical, or other. At the same time, though less explored, there are developments of theoretical understanding regarding the basic mechanism of thin film growth by specific growth mechanism. In doing this, the basic mechanism of thin film growth has been categorized into different broad classes with specific features. The main features include the time dependence of interface width and values of different scaling exponents. Apart from these, studies also addressed different morphological, optical, or electrical properties of the as-grown thin films of specific material. This paper gathers the existing literature that reports the simulation-based theoretical studies related to thin film growth by different algorithms like random deposition, ballistic deposition, random deposition with surface relaxation, or their different combinations. Not only that, but the paper also summarizes different reports related to the simulation-based prediction of the material properties. As the topic is relatively new, the collection of reports added in the last 20 years has been considered.

The paper has different sections. Section 1 gives basic introductory ideas related to thin film development and its properties. Sections 2 and 3, respectively, deal with the basics of different existing models and the basic steps involved in the simulation. Section 4 gathers the related results reported by various researchers, followed by a short discussion and final concluding remarks. Undoubtedly, this paper is the first review work in this field and thus will serve as an invaluable source of information for future workers.

This study involved depositing thin films of copper oxide (CuO) and copper oxide films incorporated with neodymium oxide (Nd₂O₃) in various weight ratios using the pulsed laser deposition technique to fabricate an ammonia gas sensor. The characteristics of the developed films were characterized using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and photoluminescence spectroscopy (PL) techniques. The study investigated the impact of incorporating Nd₂O₃ into the CuO film on its sensing characteristics. The X-ray analysis revealed a polycrystalline structure of CuO with a crystal size of 19 nanometers. Furthermore, incorporating CuO and Nd₂O₃ formed a mixed phase comprised of both CuO and Nd₂O₃. The average crystallite size varied with the Nd₂O₃ incorporation rate from 19nm to 14.7nm. With increasing Nd₂O₃ incorporation ratios, the surface morphology changed from smooth to spherical-like nanostructures, and the average particle sizes varied from 28nm to 130nm. The PL spectrum analysis results confirmed that the incorporation of Nd₂O₃ resulted in a change in the peak intensity of the PL spectrum, with a significant increase in the peak associated with oxygen vacancies, contributing to the improvement of the sensor response. The incorporation of Nd₂O₃ greatly enhanced the sensitivity of the as-deposited CuO film. The sample with 5 wt.% Nd₂O₃ exhibited the maximum sensitivity, reaching 180% against 79 ppm NH₃ at a working temperature of 50${}^{\circ }$C.

Tetrapyridotetraazaporphyrinatozinc (TPyTAPZn) can be looked at as a substituted phthalocyanine. Thin films of TPyTAPZn were prepared on quartz glass by physical vapour deposition under high-vacuum conditions. During the deposition, island growth was observed by a characteristic change in the electrical conduction, indicating an increasing number of conduction pathways along the film. Deposition conditions could be optimized to yield an ordered rather than amorphous growth as detected by a characteristic absorption band in the visible range, strongly red-shifted from the absorption of the monomeric molecule in solution. A negative Seebeck coefficient confirmed n-type conduction for TPyTAPZn. In temperature-dependent measurements of the electrical conductivity and thermopower across the samples an activation energy of 0.31 eV was established for the conductivity and of 0.04 eV for charge carrier generation. From this difference it is concluded that a thermally activated charge carrier transport mechanism (hopping) rather than delocalized conduction (band model) is dominant in TPyTAPZn. Photoconduction turned out to be rather small in these samples, although light was absorbed quite efficiently. The time dependence of photoconduction indicated a significant trap density. Interaction with ammonia or triethylamine in the gas phase led to an increase in the conductivity; oxygen or water led to a decrease. The time dependence of these interactions indicated that triethylamine and water were only reacting with the surface region, whereas NH₃ and O₂ were also diffusing into the bulk of the films.

Electrochromic redox reactions of vacuum-evaporated thin films of tetrapyridotetraazaporphyrinatozinc(II) (TPyTAPZn) have been studied in aqueous KCl electrolytes by in situ spectrocyclic voltammetry, which combines cyclic voltammetry and in situ monitoring of the absorption spectrum of the film. Although the voltammetric response of TPyTAPZn thin film electrodes can hardly be understood by the conventional electrochemical approach, the spectral information linked to the voltammetric information elucidated the presence of two-step reductions and reoxidations by each electron per TPyTAPZn molecule, which, however, take place with significant overlap during the voltammetric scan. Reduction of the film turned the color of the film from blue to purple and it changed reversibly to regenerate the original spectrum upon reoxidation. These reversible electrochromic reactions could be repeated many times without causing any noticeable change in the voltammetric and spectral responses. While use of electrolytes with various cations did not affect the redox behavior of the film, the effect of electrolyte pH was clearly seen, changing the ratio of the two reduction and two reoxidation peak magnitudes in the voltammogram. XPS analysis of the reduced film indicated that charge compensation upon reduction of the film occurred by protonation of reduced TPyTAPZn molecules at their aza and/or pyrido nitrogen atoms. A reaction scheme is presented to describe the observed redox reactions of the TPyTAPZn thin film from thermodynamic and kinetic viewpoints.

Thin films of phthalocyaninatomanganese (PcMn) in the thickness range of 100 nm have been prepared by vapour deposition on quartz glass substrates. The films were characterized in situ during film growth and following film deposition by measurements of the electrical conductivity under DC applied electric fields parallel to the substrate surface. The dependence of the conductivity on the average film thickness was determined and the mechanism of film growth is discussed. Without breaking the vacuum, a temperature gradient was established again parallel to the substrate surface and a thermopower was detected. Its dependence on the size of the temperature gradient gave a Seebeck coefficient of –780 μV K^-1 at 460 K. From the negative sign it is evident that electrons are the majority carriers in PcMn in a freshly prepared thin film. The temperature dependence of the Seebeck coefficient gave the thermal activation of charge carrier generation as ΔE = 0.19 eV, which is discussed in terms of the position of the Fermi energy in the films. The temperature dependence of the electrical conductivity gave an activation energy E_A = 0.38 eV considerably higher, indicating a thermal activation of charge carrier transport and hence supporting a hopping mechanism rather than delocalized transport. Under exposure to oxygen the conductivity showed a fast small increase followed by a slow large decrease, which is discussed considering surface as well as bulk interactions. Hole conduction was measured for the completely oxidized films by a positive Seebeck coefficient. Optical spectroscopy performed ex situ was used to allow further discussion of redox interactions of the films. Evidence was found for the presence of Pc(-2)Mn(+3)O₂(-1) as well as Pc(-2)Mn(+3)O(-2)Mn(+3)Pc(-2) and Pc(-2)Mn(+2) in freshly prepared films. Films exposed to air for as long as several months were completely oxidized to Pc(-2)Mn(+3)O₂(-1).

A polycarbonate film of thin and uniform thickness was prepared. A casting solution for film-formation was made up by diluting a solution of poly (bisphenol A carbonate) in chloroform by a factor of two to three with benzene. A uniform film was created by dropping 0.2-0.3 cm³ of the casting solution slowly on a water surface within an aperture (20 mm diameter) of Mylar target frame floating on 50 wt% sucrose aqueous solution. Films of 0.14-0.27 mg/cm² thickness thus prepared offer a good combination of mechanical strength and low continuum backgrounds. To test PIXE analysis of anionic species in water, targets containing SO₄^2-, Cr₂O₇^2-, AsO₄^3-, and Ga³⁺-internal standard were prepared by depositing 100 μl of the test solutions onto the polycarbonate film, and irradiated in vacuum by 3 MeV proton beams. The difference between the nominal and the analyzed concentrations seldom exceeded ± 15 % in the range from 10 to 2000 ppb.

Applying the powerful thin film brick-wall model to the general Kerr–Newman black hole, we find out that the entropy calculation result can also satisfy the area theorem. Moreover, the area theorem is not only satisfied for the global black hole, but also for every area cell on its horizon, that means, every cell on the horizon contributes its own part of entropy if we choose a same temperature-related radial cutoff ε'. This new thin film brick-wall model can be used to calculate dynamic black hole which has different temperatures on the horizon. It tells us that the horizon is exactly the statistical origin of a black hole entropy, the total entropy of a black hole is just the sum of all the contributions from every area cell. For a Kerr–Newman black hole, there is also an important difference between the thin film brick-wall model and the original one, that is, we do not need any angular cutoff in the thin film model, and this makes the physical meaning clearer.

In this paper, the initial stage of films assembled by energetic C₃₆ fullerenes on diamond (001)–(2 × 1) surface at low-temperature was investigated by molecular dynamics simulation using the Brenner potential. The incident energy was first uniformly distributed within an energy interval 20–50 eV, which was known to be the optimum energy range for chemisorption of single C₃₆ on diamond (001) surface. More than one hundred C₃₆ cages were impacted one after the other onto the diamond surface by randomly selecting their orientation as well as the impact position relative to the surface. The growth of films was found to be in three-dimensional island mode, where the deposited C₃₆ acted as building blocks. The study of film morphology shows that it retains the structure of a free C₃₆ cage, which is consistent with Low Energy Cluster Beam Deposition (LECBD) experiments. The adlayer is composed of many C₃₆-monomers as well as the covalently bonded C₃₆ dimers and trimers which is quite different from that of C₂₀ fullerene-assembled film, where a big polymerlike chain was observed due to the stronger interaction between C₂₀ cages. In addition, the chemisorption probability of C₃₆ fullerenes is decreased with increasing coverage because the interaction between these clusters is weaker than that between the cluster and the surface. When the incident energy is increased to 40–65 eV, the chemisorption probability is found to increased and more dimers and trimers as well as polymerlike-C₃₆ were observed on the deposited films. Furthermore, C₃₆ film also showed high thermal stability even when the temperature was raised to 1500 K.

Optical properties of thermally evaporated nickel phthalocyanine thin films have been characterised using spectrophotometric measurements of transmittance and reflectance in spectral range 200–2100 nm. The refractive index n and the absorption index k were calculated. Some of the optical absorption parameters, namely optical absorption coefficient (α), molar extinction coefficient (ε_molar), oscillator strength (f), electric dipole strength (q²) and absorption half bandwidth (Δλ) of the principal optical transitions have been also evaluated. The analysis of the spectral behaviour of the absorption coefficient (α), in the absorption region revealed indirect transitions. The fundamental and the onset energy gaps were estimated as 2.77±0.02 eV and 1.58±0.01 eV, respectively. According to the analysis of dispersion curves, the dielectric constants and dispersion parameters were obtained. The absorption measurements recorded in the UV–VIS region show two well defined absorption bands of phthalocyanine molecule, namely the Soret band (B) and the Q band. The Q band shows its characteristic splitting (Davydov splitting), and ΔQ was obtained as 0.21 eV. Discussion of the obtained results and their comparison with the previous published data are also given.

The electron gas statistics in semiconducting thin films with arbitrary isotropic energy spectra was found in this theoretical work. General expressions for the density of states of the charge carriers and for the chemical potential were deduced according to the two-band Kane model approach. The ratio of the expression obtained for the density of states to that of the expression deduced according to the parabolic approach was shown to be a function of the energy. The Fermi energy of the electron gas was observed to be a function of the thin film thickness, the non-parabolic parameter, the concentration and the temperature. In the strong non-parabolic approach the dependency of the Fermi energy on the thin film thickness was shown to exhibit non-monotonic characteristics.

High N content of carbon nitride films are quickly deposited onto Si (100) substrates at room temperature by using DC hollow cathode plasma sputtering deposition technique. The deposition rate is up to 283 nm/min at the bias voltage of 400 V. The properties of the films are characterized by using XPS, Raman (scattering) and Fourier transformation infrared (FTIR) spectroscopy. Experiments results provide direct evidences that the structures of CN films can be controlled by regulating bias voltages. The maximum sp³C–N concentration up to 0.73 is obtained. Raman data is used to confirm XPS results. FTIR of the films clearly show C–N and C=N components (1000–1800 cm^-1) together with a tiny peak C≡N (2181 cm^-1). By reducing particle energy and substrate temperature, we have succeeded in suppressing the mechanisms of losing N-contain species during deposition, and achieved a large amount of sp³ bonds of CN films.

We have developed a novel method and device for measuring the mechanical properties of micro/nano structures. An atomic force microscope (AFM) was employed to sense applied force and displacement and a new AFM cantilever which overcame the critical problems associated with conventional AFM cantilever systems was fabricated using single crystal silicon (110). The symmetrically designed cantilever removed lateral motion of the probe during indentation and strip bending tests. Strip bending tests on fixed-fixed molybdenum (Mo) strips 1 μm in thickness using the assembled cantilever in AFM system showed that consistent load-displacement curves can be obtained. The effect of adhesive energy on mechanical tests in micro/nano-scale was revealed.

The grain growth kinetics of nanocrystalline copper thin film samples was investigated. The grain size of nanocrystalline copper samples was determined from the broadening of X-ray spectra. It was found that the grain size increased linearly with isothermal annealing time within the first 10 minutes, beyond which power-law growth kinetics is applied. The activation energy for grain growth was determined by constructing an Arrhenius plot, which shows an activation energy of about 21 – 30 kJ/mol. The low activation energy is attributed to the second phase particle drag and the porosity drag, which act as the pinning force for grain growth in nanocrystalline copper.

Local vibrational modes of Si-H is an important research area in recent years. Local vibrational modes of chemical bonds between Si atom and other impurity atoms such as H and O in amorphous silicon films produced by radio frequency sputtering were studied by means of Fourier transform infrared spectroscopy. The concentrations of Si-H, Si-O and Si-C in the sample were calculated. It was found that the concentration of Si-H bond varied significantly when the material was annealed at temperatures T_a>600 K and tended to zero for T_a>1000 K. The optical absorption edge was also found to depend strongly on the thermal history of the amorphous semiconductor. A strong correlation between the optical absorption coefficient and the concentration of Si-H was also observed.

In-situ observation of thermal stresses in thin films deposited on a silicon substrate was made by synchrotron radiation. Specimens prepared in this experiment were nano-size thin aluminum films with SiO₂ passivation. The thickness of the films was 10 nm, 20 nm and 50 nm. Synchrotron radiation revealed the diffraction intensities for these thin films and make possible to measure stresses in nano-size thin films. Residual stresses in the as-deposited state were tensile. Compressive stresses were developed in a heating cycle up to 300°C and tensile stresses were developed in a cooling cycle. The thermal stresses in the 50 nm film showed linear behavior in the first heating stage from room temperature to 250°C followed by no change in the stress at 300°C, however, linearly behaved in the second cycle. On the other hand, the thermal stresses in 20 nm and 10 nm films almost linearly behaved without any hysteresis in increasing and decreasing temperature cycles. The mechanism of thermal stress behavior in thin films can be explained by strengthening of the nano-size thin films due to inhibition of dislocation source and dislocation motion.

TFDC (Thomas-Fermi-Dirac-Cheng) electron theory is applied to analyzing the characteristics of the electrons inside the double layer of the nanometer composite thin films. This paper proposes the new mechanism about the high capacity in both theoretical analysis and experimental measurement.

A double buffer of EU₂CuO₃/YSZ has been used to grow highly epitaxial thin films of YBa₂Cu₃O_y (YBCO) on Si. These films showed enhanced superconductivity and improved crystallinity in comparing with that of films grown on Si directly. Well defined interfaces with no immediate layers were found. The YBCO film surface was more smooth and stable. The results obtained indicate that highly epitaxial YBCO thin films can be successfully grown on Si wafers, demonstrating advantages of such a double buffer structure.

Multi-wall carbon nanotubes (MWNTs) thin film was synthesized and its sensing for different flowing solutions was investigated in this paper. The voltage generated from MWNTs thin film was in sublinear fashion and a new discovery in this paper is that the charge-to-mass ratio (q/m) of the ions is an important factor. The results were inconsistent with the conventional idea that a streaming potential is the efficient factor. New mechanism has been discussed about their effects on the charge of MWNTs and the sublinear response based on a pulsating ratchet model. This result would be indicative in nanoscale sensors, detectors and power cells.