Decreasing Film Thickness Research Articles

Destruction of superconductivity through phase fluctuations in ultrathin a -MoGe films

We investigate the evolution of superconductivity with decreasing film thickness in ultrathin amorphous MoGe (a-MoGe) films using a combination of sub-Kelvin scanning tunneling spectroscopy, magnetic penetration depth measurements and magneto-transport measurements. We observe that superconductivity is strongly affected by quantum and classical phase fluctuations for thickness below 5 nm. The superfluid density is strongly suppressed by quantum phase fluctuations at low temperatures and evolves towards a linear-T dependence at higher temperatures. This is associated with a rapid decrease in the superconducting transition temperature, Tc, and the emergence of a pronounced pseudogap above Tc. These observations suggest that at strong disorder the destruction of superconductivity follows a Bosonic route where the global superconducting state is destroyed by phase fluctuations even though the pairing amplitude remains finite.

Simulation of primary breakup of molten slag in the gas quenching dry granulation process

Gas quenching dry slag granulation process is an energy-saving environmental protection method for waste utilization and waste heat recovery of molten slag, and the molten slag is broken up by supersonic airflow into droplets in this process. This paper performed the numerical investigations on the breakup behavior of slag by the coupled level-set and volume of fluid (CLSVOF) methodology and gain a deep insight into the breakup mechanism of the primary breakup and influence of gas Weber number (We) and liquid Reynolds number (Re) on the primary breakup. The results indicated that the primary breakup include flat film breakup and spherical cap film breakup, the both are typical perforation breakup; Aerodynamic force cause K-H (Kelvin Helmholtz) instability, K-H surface waves are the root cause of film breakup; The wavelength and the film thickness decrease with increasing We which is conducive to reducing the ligament diameter; The wavelength and the film thickness decrease first and then increase with increasing Re which is optimum in the range of 1068–1280; The empirical formula of the ligament diameter is obtained. The ligament diameter decrease with increasing airflow velocity and slag density and increase with increasing slag mass flow rate and slag viscosity.

Comparing refractive index and density changes with decreasing film thickness in thin supported films across different polymers.

Density changes in thin polymer films have long been considered as a possible explanation for shifts in the thickness-dependent glass transition temperature Tg(h) in such nanoconfined systems, given that the glass transition is fundamentally associated with packing frustration during material densification on cooling. We use ellipsometry to compare the temperature-dependent refractive index with decreasing thickness n(h) for supported films of poly(2-vinyl pyridine) (P2VP), poly(methyl methacrylate) (PMMA), and polystyrene (PS), as these polymers have different silica substrate interactions. We observe similar n(h) trends for all three polymers, with near equivalence of P2VP and PS, characterized by a large apparent increase in refractive index for h ≤ 40 nm-65 nm depending on the polymer. Possible sources of molecular dipole orientation within the film are tested by varying molecular weight, polydispersity, chain conformation, and substrate chemistry. Such film inhomogeneities associated with non-uniform polarizability would invalidate the use of homogeneous layer approximations inherent in most thin film analysis methods, which we believe likely explains recent reports of large unphysical increases in film density with decreasing thickness by a variety of different experimental techniques.

Measurement Method of Eyelid Pressure Distribution Using Thin Film Coating on Contact Lenses

Distribution measurement of eyelid pressure is an important technology for diagnosis and treatment on corneal disorders, dry eyes etc. The measurement methods are limited because of the eyeball surface curvature, as well as the eyelid pressure itself is fairly small ranging from 10 to 30 mmHg. So far, MEMS fabricated contact lenses and strip pressure sensors have been proposed, however, distribution measurement are impossible except an average or a local pressure. In this study, we propose a new method for the pressure and abrasive distribution measurement, in which, a color film coated contact lens works as a sensing device for the film thickness decrease due to blinking. In the first report, we experimentally evaluated thickness reduction characteristics of sucrose films, using a newly developed friction tester. Experimental results show its sufficient potential as a sensing material for eyelid pressure measurement.

Modeling confined ductile fracture – A void-growth and coalescence approach

In a composite material a soft, ductile matrix can be confined by a hard, brittle phase, altering its deformation and fracture behavior. Increasing confinement leads to embrittlement of the matrix and, in turn, also the composite. From a materials design perspective, it is usually desired to avoid brittle fracture without compromising the hardness of the material. Understanding confined ductile fracture is therefore critical for modeling the mechanical response of composite materials with fine microstructure. The present work is focused on confined ductile fracture of a thin ductile film, with elasto-plastic power-law hardening behavior, sandwiched between ideal linear elastic substrates. Fracture of the ductile layer is modeled by growth and coalescence of prescribed voids in 2D. Influences of material properties, initial void volume fraction, geometric constraints and elastic mismatch are investigated. The results show a loss of ductility with decreasing film thickness that is accompanied by a severe decrease in fracture initiation toughness as well as an increased stress at the interface. The influence of materials properties is significant in all cases while the effect of initial void volume fraction is comparatively less critical for highly confined materials than for bulk materials. Increasing confinement also results in increasing normal stress at the phase interface, promoting interface decohesion prior to ductile fracture of the film. The present approach and results are a step towards more detailed prediction of composite fracture toughness and crack-growth resistance.

Synthesis of Atomically Thin 1T‐TaSe2 with a Strongly Enhanced Charge‐Density‐Wave Order

AbstractBulk 1T‐TaSe2 as a charge‐density‐wave (CDW) conductor is of special interest for CDW‐based nanodevice applications because of its high CDW transition temperature. Reduced dimensionality of the strongly correlated material is expected to result in significantly different collective properties. However, the growth of atomically thin 1T‐TaSe2 crystals remains elusive, thus hampering studies of dimensionality effects on the CDW of the material. Herein, chemical vapor deposition (CVD) of atomically thin TaSe2 crystals is reported with controlled 1T phase. Scanning transmission electron microscopy suggests the high crystallinity and the formation of CDW superlattice in the ultrathin 1T‐TaSe2 crystals. The commensurate–incommensurate CDW transition temperature of the grown 1T‐TaSe2 increases with decreasing film thickness and reaches a value of 570 K in a 3 nm thick layer, which is 97 K higher than that of previously reported bulk 1T‐TaSe2. This work enables the exploration of collective phenomena of 1T‐TaSe2 in the 2D limit, as well as offers the possibility of utilizing the high‐temperature CDW films in ultrathin phase‐change devices.

A 3-D Magneto-Elastohydrodynamic Model of Liquid Metal Film at Rail–Armature Interface

To make a deep investigation into the lubrication characteristics of the liquid metal film (LMF), the existing 2-D hydrodynamic lubrication models cannot meet the requirement as it fails to describe the flow of the liquid film at the transverse direction. This article makes the first step toward a 3-D model of the LMF at the armature/rail (A/R) interface. A 2-D Reynolds equation is derived considering the self-acceleration of the film and the Lorentz force acting on it. By coupling the electromagnetic fields, stress, and fluid fields, a magneto-elastohydrodynamic model is established completely. Based on this, the distribution characteristics of the LMF are explored which includes the distribution of the film thickness, hydrodynamic pressure, and fluid velocity. Besides, the dynamic characteristics of the LMF in the launch process are analyzed. The results show that: 1) the LMF is trapped in an arching gap in the transverse direction. The liquid pressure is symmetrically distributed along the centerline. In addition, the positive transverse fluid velocity indicates the jetting of the molten metal which may do damage to the bore. 2) At the current down-slope, the film thickness and liquid pressure both decrease rapidly which may put the film at risk of break. This article can provide a reference to the modeling of LMF hydrodynamic lubrication and help to understand the flow behavior of the liquid film.

Explosive boiling of liquid argon films on flat and nanostructured surfaces

Because of the effects of the nanostructure, phase change behaviors on flat and nanostructured surfaces display distinct features. In this work, the molecular dynamics simulation method is employed to investigate the onset temperature of explosive boiling (Ts) with various film thicknesses, pillar heights, and wettability. The simulation results show that Ts decreases with the film thickness on both wettability flat surfaces. However, the decreasing rates have the significant distinctions, where the difference between two surfaces of Ts with the identical film thickness decreases. In addition, the simulation results demonstrate that all the values of Ts on nanostructured surfaces are lower than those on flat surfaces with the same film thickness. With the increase of the film thickness, Ts presents a decreasing trend on both wettability and nanostructured surfaces, especially with the liquid film with the thickness over 6 nm, where a completely opposite conclusion compared to the flat surface is represented.

Polymeric liquid layer densified by surface acoustic wave.

Creating densified and stable liquid is a straightforward strategy for the fabrication of strong and ultra-stable amorphous or glassy materials. The current study has discovered that a liquid polymeric thin film is densified under the application of a high frequency surface acoustic wave (SAW). The experimental evidence is the decrease in film thickness and the increase in refractive index, measured by ellipsometry, of polyisobutylene thin films deposited on the solid substrates, when a high frequency SAW (39.5 MHz) is applied to the system. Further investigations by polarization-resolved single molecule fluorescence microscopy have demonstrated that the rotational motion of fluorescent probes doped inside the liquid film is retarded and the dynamical heterogeneity is reduced. The results demonstrate that the application of SAW of high frequency makes the thin polymeric liquid film densified and more dynamically homogeneous.

The effect of film thickness on the modulated spin structure of terbium manganite

The Ginzburg–Landau method is used to conduct a phenomenological analysis of how the sample thickness impacts the incommensurate spin states in the TbMnO3 centrosymmetric ferroelectromagnet. The analysis is performed using the first-harmonic approximation, and provides a qualitative confirmation of the experimentally observed decrease in the film modulation vector at a decreasing temperature, as well as an increase in the vector as the film thickness decreases at a constant temperature. The effect that the film thickness has on the first- and second-order phase transition lines is analyzed, and the tendency of the incommensurate spin phase volume to decrease with a decrease in the film thickness is demonstrated.

Thickness-dependent band gap of α-In2Se3: from electron energy loss spectroscopy to density functional theory calculations

α-In2Se3 has attracted increasing attention in recent years due to its excellent electrical and optical properties. Especially, attention has been paid to its peculiar ferroelectric and piezoelectric properties which most other two-dimensional (2D) materials do not possess. This paper presents the first measurement of the thickness-dependent band gaps of few-layer α-In2Se3 by electron energy loss spectroscopy (EELS). The band gap increases with decreasing film thickness which varies from 1.44 eV in a 48 nm thick area to 1.64 eV in an 8 nm thick area of the samples. Further, by combining the improved exchange-correlation potential and proper screening of the internal electric field in an advanced 2D electronic structure technique, we have been able to obtain the structural dependence of the band gap within density functional theory up to hundreds of atoms. This is also the first calculation of a similar type for 2D ferroelectric materials. Both experiment and theory suggest that the variation of the band gap of α-In2Se3 fits well with the quantum confinement model for 2D materials.

Thickness-dependent thermal conductivity of mechanically exfoliated β -Ga2O3 thin films

Beta-phase gallium oxide (β-Ga2O3), the most thermally stable phase of Ga2O3, has stimulated great interest in power electronics due to its ultra-wide bandgap (∼4.9 eV) and high breakdown electric field. The relatively low thermal conductivity of β-Ga2O3, however, limits the device performance due to excessive temperature driven by self-heating. Recently, integrating β-Ga2O3 thin films on substrates with high thermal conductivities has been proposed to improve heat rejection and device reliability. In this work, we prepare high-quality single-crystal β-Ga2O3 thin films by mechanical exfoliation of bulk crystals and study their thermal transport properties. Both the anisotropic thermal conductivity of β-Ga2O3 bulk crystals and the thickness-dependent thermal conductivity of β-Ga2O3 thin films are measured using the time-domain thermoreflectance technique. The reduction in the thin-film thermal conductivity, compared to the bulk value, can be well explained by the size effect resulting from the enhanced phonon-boundary scattering when the film thickness decreases. This work not only provides fundamental insight into the thermal transport mechanisms for high-quality β-Ga2O3 thin films but also facilitates the design and optimization of β-Ga2O3-based electronic devices.

A theoretical investigation on the lubrication mechanism of moving particles in elliptical contact

Numerous friction and wear experiments have demonstrated that nanoparticles (NNPs) perform excellently in friction coefficient reduction and anti-wear. However, theoretical explanation of the facilitated lubrication mechanism of NNPs is of complexity. This paper proposed a revised Reynolds equation taking moving particles into consideration. Through mutually coupling the governing equations the causes of wear reduction contributed by NNPs are well understood. Considering thermal effect, time-varying effect and particles motion, effects of particles motion on the tribology performance of elliptical contact was investigated. Furthermore, the influences of two deterministic particles’ distributions compared with base oil were discussed. Results show that the motion of moving particles from inlet to outlet induces remarkable increases in film thickness and considerable decreases in coefficient of friction, while also leads to evident rise of temperature and significant backflows in inlet zone. Additionally, particles with appropriate concentration are beneficial to the enhancement of lubricating film performance.

Influences of transient impact and vibration on the lubrication performance of spur gears

In this paper, the mathematical model of gear elastohydrodynamic lubrication is presented. The transient impact operating condition and underdamped load condition are considered. Taking thermal effect and squeeze effect into account, the full numerical solution of gear pairs is obtained. In this numerical calculation, multigrid method is applied to compute the film pressure; multigrid integration technique is used to calculate the solid surface deformation; column by column scanning technique is employed to calculate temperature. The simulation results show that an entrapped film dimple forms under transient impact condition; transient impact causes remarkable increases in film pressure and film temperature. Compared with the normal case the minimum thickness of the impact case is smaller, which is not beneficial to teeth lubrication. Thermal effect induces some decreases in film thickness because of the viscosity–temperature relationship. Vibrational load with high damped frequency causes greater increases in film thickness and greater decreases in the coefficient of friction than that of low damped frequency. However, the film temperature of high damped frequency is higher than that of low damped frequency.

Abrupt enhancement of spin–orbit scattering time in ultrathin semimetallic SrIrO3 close to the metal–insulator transition

We report a magnetotransport study of spin relaxation in 1.4–21.2 nm epitaxial SrIrO3 thin films coherently strained on SrTiO3 substrates. Fully charge compensated semimetallic transport has been observed in SrIrO3 films thicker than 1.6 nm, where the charge mobility at 10 K increases from 45 cm2/V s to 150 cm2/V s with decreasing film thickness. In the two-dimensional regime, the charge dephasing and spin–orbit scattering lengths extracted from the weak localization/anti-localization effects show power-law dependence on temperature, pointing to the important role of electron–electron interaction. The spin–orbit scattering time τso exhibits an Elliott–Yafet mechanism dominated quasi-linear dependence on the momentum relaxation time τp. Ultrathin films approaching the critical thickness of metal–insulator transition show an abrupt enhancement in τso, with the corresponding τso/τp about 7.6 times of the value for thicker films. A likely origin for such unusual enhancement is the onset of strong electron correlation, which leads to charge gap formation and suppresses spin scattering.

Anisotropic expansion of PEDOT/PSS-PAM films in water

We have found for the first time that the PEDOT/PSS-PAM film expands in water when the film is consisted of water-soluble polyacrylamide (PAM). Furthermore, the expansion of the film is evidently anisotropic, i.e., the film area increases while the film thickness decreases. The expansion also has an induction period in the initial stage where the film thickness rapidly increases to attain maximum thickness, while the film area is kept in unchanged. The anisotropic expansion can be well-explained by our hydrophilic-lipophilic-competing (HLC) model proposed in terms of chemical potentials of PAM between the solid state and the solution, coupled with the character of water insoluble PEDOT segment. It is considerable that the larger the △μPAM was, the more the film expanded. The higher content of PAM in the film results in the larger μfPAM, then the larger △μPAM. The dilution of PAM in water also results in the lower μwPAM, then the larger △μPAM. Noting that the sign change of the fsur and fth where fsur and fth are forces biased on surface direction and thickness direction of the film, respectively, occurs at different immersing time, which is a main driving force of the anisotropic expansion of the film.

Effects of nitrogen-argon flow ratio on the microstructural and mechanical properties of AlCrN coatings prepared using high power impulse magnetron sputtering

In this study, AlCrN films were deposited from Al70Cr30 alloy targets by high-power impulse magnetron sputtering (HiPIMS). The structural characteristics of the AlCrN films were modulated by performing deposition under various N2/Ar flow ratios. The results indicated that increasing the nitrogen content during deposition leads to corresponding decreases in deposition rate and film thickness, as well as a structural transformation from amorphous to crystalline. A mixture of cubic and hexagonal AlCrN phases was found in the films. Three regions of hardness were observed, and hardness increased with increasing N2/Ar ratio and peaked at 33.6GPa. Good adhesion and wear resistance were observed in all samples.

Enhanced Dynamics of Confined Polymers near the Immiscible Polymer-Polymer Interface: Neutron Reflectivity Studies.

For polymer-blend films, local dynamics in confined polymer domains tend to differ from the bulk because of significant contributions from the polymer-polymer interface. Herein, we investigated the diffusion dynamics of entangled polymer thin films confined between different polymers in a direction perpendicular to the surface using neutron reflectivity. We found that a bilayer of poly(methyl methacrylate) (PMMA) and deuterated PMMA (dPMMA) sandwiched between polystyrene (PS) layers exhibited significant increase in mobility near the polymer-polymer interface with decreasing PMMA thickness. This indicates that the contribution of repulsive interactions at the immiscible polymer-polymer interface becomes more significant as the film thickness decreases. We also found that the interfacial roughness between PMMA and PS (28 Å at equilibrium) and soft confinement of PS layers did not significantly affect the change in the diffusion dynamics of the adjacent PMMA. This was evidenced by comparison with the diffusion results of multilayers with a flat interface (8 Å at equilibrium) between PMMA and hard PS by UV cross-linking.

Phonon confinement and transport in ultrathin films

Thermal transport by phonons in films with thicknesses of less than 10 nm is investigated in a soft system (Lennard-Jones argon) and a stiff system (Tersoff silicon) using two-dimensional lattice dynamics calculations and the Boltzmann transport equation. This approach uses a unit cell that spans the film thickness, which removes approximations related to the finite cross-plane dimension required in typical three-dimensional-based approaches. Molecular dynamics simulations are performed to obtain finite-temperature structures for the lattice dynamics calculations and to predict thermal conductivity benchmarks. Thermal conductivity decreases with decreasing film thickness until the thickness reaches four unit cells (2.1 nm) for argon and three unit cells (1.6 nm) for silicon. With a further decrease in film thickness, thermal conductivity plateaus in argon while it increases in silicon. This unexpected behavior, which we identify as a signature of phonon confinement, is a result of an increased contribution from low-frequency phonons, whose density of states increases as the film thickness decreases. Phonon mode-level analysis suggests that confinement effects emerge below thicknesses of ten unit cells (5.3 nm) for argon and six unit cells (3.2 nm) for silicon. Thermal conductivity predictions based on the bulk phonon properties combined with a boundary scattering model do not capture the low thickness behavior. To match the two-dimensional lattice dynamics and molecular dynamics predictions for larger thicknesses, the three-dimensional lattice dynamics calculations require a finite specularity parameter that in some cases approaches unity. These findings point to the challenges associated with interpreting experimental thermal conductivity measurements of ultrathin silicon films, where surface roughness and a native oxide layer impact phonon transport.

Temperature- and thickness-dependence of robust out-of-plane ferroelectricity in CVD grown ultrathin van der Waals α-In2Se3 layers

Two-dimensional (2D) ferroelectric materials with unique structure and extraordinary optoelectrical properties have attracted intensive research in the field of nanoelectronic and optoelectronic devices, such as optical sensors, transistors, photovoltaics and non-volatile memory devices. However, the transition temperature of the reported ferroelectrics in 2D limit is generally low or slightly above room temperature, hampering their applications in high-temperature electronic devices. Here, we report the robust high-temperature ferroelectricity in 2D α-In2Se3, grown by chemical vapor deposition (CVD), exhibiting an out-of-plane spontaneous polarization reaching above 200 °C. The polarization switching and ferroelectric domains are observed in In2Se3 nanoflakes in a wide temperature range. The coercive field of the CVD grown ferroelectric layers illustrates a room-temperature thickness dependency and increases drastically when the film thickness decreases; whereas there is no large variance in the coercive field at different temperature from the samples with identical thickness. The results show the stable ferroelectricity of In2Se3 nanoflakes maintained at high temperature and open up the opportunities of 2D materials for novel applications in high-temperature nanoelectronic devices.