Thin Island Films Research Articles

Can the Sublimation Enthalpy Be Obtained Using Atomic Force Microscopy with Heating? A PETN Nanofilm Case.

Vaporization is an important aspect of the performance and detection of energetic materials. While the traditional techniques concentrate on bulk property changes during sublimation, atomic force microscopy (AFM) offers the possibility to track particle volume changes under heating. Ideally, this will enable the investigation of chemicals that are challenging to study using conventional vaporization analysis methods, i.e., those having low thermal stability and/or low volatility. However, prior studies have demonstrated that novel structural effects at the nanoscale may interfere with sublimation mass loss. The present work aims to provide a comprehensive investigation of the sublimation of pentaerythritol tetranitrate (PETN) thin films with respect to the measurement parameters, the heating technique, the sample composition, and the type of the substrate. We observed the low-temperature recrystallization of thin-film islands during heating together with the sublimation process; this was demonstrated by the unexpected local increase in volume with temperature. Overall, AFM allows us to set up a precise nanoscale vaporization experiment and, in some instances, to obtain a reliable estimate of the sublimation enthalpy. However, it is crucial to consider the sample's morphology as well as any concurrent structural transformations in order to ensure the validity of the results.

Terahertz Photoconductive Antenna Based on a Topological Insulator Nanofilm

In this study, the efficient generation of terahertz radiation by a dipole photoconductive antenna, based on a thin island film of a topological insulator, was experimentally demonstrated. The performance of the Bi1.9Sb0.1Te2Se antenna was shown to be no worse than those of a semiconductor photoconductive antenna, which is an order of magnitude thicker. The current–voltage characteristics were studied for the photo and dark currents in Bi1.9Sb0.1Te2Se. The possible mechanisms for generating terahertz waves were analyzed by comparing the characteristics of terahertz radiation of an electrically biased and unbiased topological insulator.

Calculation and design of a technological module for formation of island thin films and nanostructures in vacuum

The application areas of island thin films are considered, and the relevance of their application is presented. The choice of the method for formation island thin films and nanostructures is justified. A design of the thermal evaporation module is proposed and its documentation is developed. The use of it will allow to form continuous and island coatings. Variants for placing the module in the laboratory equipment - MVTU-11-1MS are considered. The analysis of the evaporator configurations was carried out and the type of evaporator – «tape» evaporator, was selected. The necessary calculations are presented: evaporator resistance (2,31 mOm) and thermal calculation (1200 K, 20), – this contributed the design of the module and the materials selection of its components.

Evolution of plasmonic nanostructures under ultra-low-energy ion bombardment

Metal nanostructures supported on a dielectric substrate possess unique plasmonic properties that are highly dependent on their morphologies. We show that such nanostructures can be modified upon exposure to an ultra-low-energy ion-beam bombardment (< 100 eV) near the sputtering threshold of the metal. This plasma treatment can be easily implemented in a vacuum deposition chamber and allows the achievement of morphologies, and therefore optical properties, that are difficult to achieve otherwise by physical vapor deposition. We present the effects obtained (i) on self-organized Ag nanoparticles with unidirectional orientation and bimodal distribution, and (ii) on Ag layers in the form of percolated, quasi-continuous or island thin films. Our results show that the plasmonic dichroism of self-organized assemblies can be tuned thanks to the preferential sputtering of the smaller particles associated with gradual erosion of the larger ones. Ion-induced modifications of percolated or quasi-continuous films lead to the formation of isolated tiny particles with areal densities smaller than 500 μm−2. In both cases, the duration of the ion bombardment makes it possible to finely control the desired structural and plasmonic modifications.

The observation of tin islands in Nb3Sn thin films deposited by magnetron sputtering

Superconducting Nb3Sn thin films have been produced on copper substrate by dc magnetron co-sputtering technique successfully. XRD and magnetic moment measurement demonstrate the exist of Nb3Sn crystal. By changing the ratio between niobium target sputtering current to tin target, the tin concentration in different films is varied. However, the experiment results indicate critical temperature TC of different Nb3Sn thin film are nearly invariable while the composition of Nb-Sn of those are changed. Highest TC of those films annealing at 650°C are around 12K, while 750°C annealing increase it to 15K. The SEM and element analysis results manifest inhomogenous distribution of tin atoms, where some tin islands appear over the film. The exist of tin-rich areas like tin islands is responsible for such particular performance of Nb3Sn films.

Fabrication of large scale uniform copper-island thin film for ultrasensitive surface enhanced Raman scattering

Nanostructured metals with designable and controllable structures have received increasing attention in surface enhanced Raman scattering (SERS) due to the single molecular detection limit. Great challenges still remain in creating large scale substrates with high-density ‘hotspots’ to provide a uniform and stable enhancement of Raman signals. Here, we fabricated a copper island thin film over an 80 cm2 scale substrate with tunable particle sizes by combining sputtering with dealloying processes. The island size can be tailored from 150 nm to 370 nm by controlling parameters and etching conditions and possesses an optimized surface morphology structure. The detection limit of crystal violet (CV) molecules reached 0.1 pM. Meanwhile, the copper island thin film presents good homogeneity and stability. Our method is promising to repeatedly fabricate novel metal SERS substrates on a large scale with standard properties for sensing applications.

Dynamics of instability in plasmonic response of nanostructured gold thin films on ambient ageing

Instability in the plasmonic response of nanostructured gold (Au) thin films on ambient ageing has been investigated. Different types of Au nanostructures (island, percolated, and continuous thin film) are obtained at the early stage of thin film growth by variation in film thickness using sputtering technique. Absorbance spectra of as-deposited island Au film shows a systematic blue-shift in the localized surface plasmon resonance (LSPR) peak position on ageing. The rate of blue-shift in the LSPR peak position is fitted with single-exponential decay function to analyse the dynamics of the plasmonic response. The as-deposited percolated Au film displays a transformation of broadened plasmonic response into a wavelength independent absorbance profile on ageing. The immersion of a new plasmonic band in the absorbance spectra is noted for continuous Au thin film on ageing. The change in plasmonic responses of nanostructured Au thin films on ambient ageing is found to be directly correlated with the re-organization of the surface morphology. Finally, ageing-induced solid-state dewetting and crystallization are found to be the main responsible processes of the underlying mechanism for morphological re-organizations, leading to change in the plasmonic response of the nanostructured Au thin films.

MOCVD deposition of zinc and bismuth chalcogenides films on the surface of silica optical fibres

Metal organic chemical vapour deposition (MOCVD) technology is adapted for the deposition of thin zinc and bismuth chalcogenides films on the surface of silica optical fibres with short tapered sections. Growth runs were carried out in a special tubular quartz reactor at atmospheric pressure of hydrogen at 425°C temperature using ZnEt2, BiMe3, Et2Te and i-Pro2Se as organometallic precursors. During the deposition of chalcogenides, the transmittance spectra of the fibre were recorded in regular short time intervals. In the transmittance spectra of the fibre with a tapered section coated by ZnSe and ZnTe, lossy mode resonances (LMR) were observed at a diameter of the tapered waist below 30 μm. After the deposition of very thin Bi2Te3 and Bi2Se3 island films on the tapered waist with a diameter about 10 μm optical fibres were built into erbium fibre ring lasers. A pulsed generation mode was achieved in some of lasers due to resonator Q-factor modulation. These results can be applied for the design of LMR fibre sensors and passively Q-switch pulsed fibre lasers.

Radial non-uniform piezoelectric response of perovskite islands in thin PZT films

A comparative study of the morphology and piezoelectric response of island and continuous perovskite thin PZT films deposited on a platinized silicon substrate Si/SiO2/TiO2/Pt was carried out. It was shown that the self-polarization value of micron-size island films was about 1.4 times higher than that of continuous films. It is assumed that the difference is due to the relaxation of tensile mechanical stresses caused by silicon substrate at the island periphery.

Mathematical modeling of geometrical and electrical characteristics of island films growth initial stages

The metal island nanostructures application fields in science and technology are presented. Experimental studies were carried out using the UVN-1M vacuum unit to test the metal thin films formation modes. Presents the control principle at the initial stages of growth of the tunneling current change between the island thin films. A series of copper and aluminum ITF samples were obtained. The derived samples thicknesses were measured using an atomic-force microscope, as a result, the dependence of the geometric and electrical characteristics of thin films and island nanostructures on the process parameters were simulated.

Concentration dependent properties lead to plastic ratcheting in thin island electrodes on substrate under cyclic charging and discharging

It is known that the mechanical properties of electrodes in lithium-ion batteries, such as modulus, yield stress, and interfacial strength, can depend strongly on lithium concentration. Here we show that a thin film island electrode with properties dependent on lithium concentration naturally undergoes plastic ratcheting with accumulative deformation and failure during cyclic charging and discharging. Some key predictions from numerical simulations are validated by galvanostatic tests. Strategies to avoid ratcheting include limiting the electrode size and/or selecting a balanced combination of concentration dependent materials properties.

Influence of the Geometric Dimensions of a Nanosystem on the Main Characteristics of Charge Exchange with a Negative Ion

The problem of charge exchange between a negative hydrogen ion and nanosystems (thin metal films and island films) is considered. Quantum-mechanical calculations show that the effectiveness of charge exchange is determined by the ion-surface distance and the structure of discrete electron energy levels inside the film. In turn, the structure of the discrete levels depends on the film’s geometric dimensions. The characteristics of charge exchange (the effectiveness of the electron transfer, the probability of charge exchange, the propagation velocity, and the average coordinate of the electron’s position inside the film) are calculated via solution of the nonstationary Schrodinger equation. It is shown that these quantities can also be approximated by a simple linear model of electron propagation.

A computational study of cyclic thermal response of shape memory alloy thin-film island structures

With a view toward contributing to the state of the art in the research on shape memory alloy memory storage devices, the thermal response of a shape memory alloy (SMA) thin film during a complete cycle of heating and cooling is computationally addressed in this communication. The ultimate objective is to provide a better understanding of shape memory alloys within the context of the characteristics of thermally activated phase-transforming materials as candidate materials for memory devices. In this study, the effect of the current density, thickness of the SMA thin film, convection and boundary conditions on the transformation of the SMA thin film are investigated. In particular, we study the effect of pulse heating as that is important for memory devices. A comparison is made between the cyclic response time of a an SMA unit cell and an Ovonic unit memory (OUM) cell. The total cycle time for the SMA cell and the OUM cell is 0.7 and 60 ns, respectively. The total energy consumption per cycle for the SMA cell and the OUM cell is 0.441 and 37.8 pJ, respectively. While the response of the SMA cell is almost two orders of magnitude better than the OUM cell, we want to strike a cautionary note that the results for the SMA cell are computational and those for the OUM cell are experimental in nature. Regardless, we hope that the outcomes for the SMA cell will provide some motivation for research in SMA-based memory devices.

White Light Generation and Anisotropic Damage in Gold Films near Percolation Threshold

Strongly enhanced and confined electromagnetic fields generated in metal nanostructures upon illumination are exploited in many emerging technologies by either fabricating sophisticated nanostructures or synthesizing colloid nanoparticles. Here we study effects driven by field enhancement in vanishingly small gaps between gold islands in thin films near the electrically determined percolation threshold. Optical explorations using two-photon luminescence (TPL) and near-field microscopies reveals super-cubic TPL power dependencies with white-light spectra, establishing unequivocally that the strongest TPL signals are generated with close to the percolation threshold films, and occurrence of extremely confined (~ 30 nm)and strongly enhanced (~ 100 times) fields at the illumination wavelength. For linearly polarized and sufficiently powerful light, we observe pronounced optical damage with TPL images being sensitive to both wavelength and polarization of illuminating light. We relate these effects to thermally induced morphological changes observed with scanning electron microscopy images. Fascinating physics involved in light interaction with near-percolation metal films along with their straightforward and scalable one-step fabrication procedure promises a wide range of fascinating developments and technological applications within diverse areas of modern nanotechnology, from bio-molecule optical sensing to ultra-dense optical data storage.

Super-resolved thickness maps of thin film phantoms and in vivo visualization of tear film lipid layer using OCT.

In optical coherence tomography (OCT), the axial resolution is directly linked to the coherence length of the employed light source. It is currently unclear if OCT allows measuring thicknesses below its axial resolution value. To investigate spectral-domain OCT imaging in the super-resolution regime, we derived a signal model and compared it with the experiment. Several island thin film samples of known refractive indices and thicknesses in the range 46 - 163 nm were fabricated and imaged. Reference thickness measurements were performed using a commercial atomic force microscope. In vivo measurements of the tear film were performed in 4 healthy subjects. Our results show that quantitative super-resolved thickness measurement can be performed using OCT. In addition, we report repeatable tear film lipid layer visualization. Our results provide a novel interpretation of the OCT axial resolution limit and open a perspective to deeper extraction of the information hidden in the coherence volume.

Interpreting plasmonic response of epitaxial Ag/Si(100) island ensembles

Associating features in the experimentally measured optical response of epitaxial Ag islands grown on Si(100) with the localized surface plasmon resonances (LSPRs) hosted by the Ag islands is challenging due to the variation of the Si dielectric function over the energy range under consideration. However, it is possible to conclusively identify features in the experimental spectra with LSPR modes oscillating both parallel and perpendicular to the epitaxial interface by simulating the optical response. The Abeles matrix method is used to describe the composite layered system and the Ag islands are modeled using the thin island film model developed by Bedeaux and Vlieger. By incorporating island morphology parameters determined by quantitative analysis of electron micrographs, the simulation faithfully reproduces the main features of the experimental spectra. Individually zeroing the dipoles associated with the LSPR modes enables conclusive identification of their contribution to the optical response of the composite system.

Sharp interface model for solid-state dewetting problems with weakly anisotropic surface energies

We propose a sharp interface model for simulating solid-state dewetting where the surface energy is (weakly) anisotropic. The morphology evolution of thin films is governed by surface diffusion and contact line migration. The mathematical model is based on an energy variational approach. Anisotropic surface energies lead to multiple solutions of the contact angle equation at contact points. Introduction of a finite contact point mobility is both physically based and leads to robust, unambiguous determination of the contact angles. We implement the mathematical model in an explicit finite difference scheme with cubic spline interpolation for evolving marker points. Following validation of the mathematical and numerical approaches, we simulate the evolution of thin film islands, semi-infinite films, and films with holes as a function of film dimensions, Young's angle $\theta_i$, anisotropy strength and crystal symmetry, and film crystal orientation relative to the substrate normal. We find that the contact point retraction rate can be well described by a power-law, $l \sim t^n$. Our results demonstrate that the exponent $n$ is not universal -- it is sensitive to the Young's angle $\theta_i$ (and insensitive to anisotropy). In addition to classical wetting (where holes in a film heal) and dewetting (where holes in a film grow), we observe cases where a hole through the film heals but leave a finite size hole/bubble between the continuous film and substrate or where the hole heals leaving a continuous film that is not bonded to the substrate. Surface energy anisotropy (i) increases the instability that leads to island break-up into multiple islands, (ii) enhances hole healing, and (iii) leads to finite island size even under some conditions where the Young's angle $\theta_i$ suggests that the film wets the substrate.

Removal of Ge Islands in al-induced layer-exchanged Ge thin films on glass substrates by selective etching technique

Removal of Ge Islands in al-induced layer-exchanged Ge thin films on glass substrates by selective etching technique

High-performance parallel solver for 3D time-dependent Schrodinger equation for large-scale nanosystems

A parallelized three-dimensional (3D) time-dependent Schrodinger equation (TDSE) solver for one-electron systems is presented in this paper. The TDSE Solver is based on the finite-difference method (FDM) in Cartesian coordinates and uses a simple and explicit leap-frog numerical scheme. The simplicity of the numerical method provides very efficient parallelization and high performance of calculations using Graphics Processing Units (GPUs). For example, calculation of 106 time-steps on the 1000⋅1000⋅1000 numerical grid (109 points) takes only 16 hours on 16 Tesla M2090 GPUs. The TDSE Solver demonstrates scalability (parallel efficiency) close to 100% with some limitations on the problem size. The TDSE Solver is validated by calculation of energy eigenstates of the hydrogen atom (13.55 eV) and affinity level of H- ion (0.75 eV). The comparison with other TDSE solvers shows that a GPU-based TDSE Solver is 3 times faster for the problems of the same size and with the same cost of computational resources. The usage of a non-regular Cartesian grid or problem-specific non-Cartesian coordinates increases this benefit up to 10 times. The TDSE Solver was applied to the calculation of the resonant charge transfer (RCT) in nanosystems, including several related physical problems, such as electron capture during H+–H0 collision and electron tunneling between H- ion and thin metallic island film.

Nanosilver surface plasmon response enhanced the photovoltaic performance of organic solar cells

Nanosilver island thin films with different thickness were synthesized by vacuum vapor deposition between ITO and PEDOT:PSS for organic solar cells, forming the structure of ITO/AgNPs layer/PEDOT:PSS/P3HT:PCBM/LiF/Al. Surface morphology and UV–vis absorption spectrum were investigated by AFM and UV–vis scanning spectrophotometer. It was found that after adding the nanosilver film, the optical properties of the device were enhanced with increasing the thickness of nanosilver island films. When the thickness of nanosilver thin films is 3.0nm, the most significant surface plasmon response and red-shift of the resonance absorption peak appeared. Meanwhile, short circuit current density of the device increased from 9.93mA/cm2 to 12.98mA/cm2, the fill factor increased from 49.35% to 52.79% and the power conversion efficiency increased from 3.05% to 4.01%. These results provided a theoretical guidance to optimize the design and increase the performance of solar cells.