Nanoscale Thin Films Research Articles

A novel technique for Interfacial Thermal Resistance measurement for nanoscale thin films

Thermal resistance of interfaces is one of the most challenging measurands in experimental heat transfer, particularly for micro and nanoscale thin films. It is also the dominating quantity in thermal management in microelectronic and energy conversion devices. We present a novel technique that exploits the size dependence of thermal resistance in thin films to magnify the discontinuous temperature drop at the interface, which is then measured with infrared thermometry. We show that orders of magnitude amplification in interfacial temperature drop is possible by making the specimen freestanding, which facilitates the measurement. We present experimental results on metal–dielectric interfaces involving aluminum, copper, silicon di-oxide and hafnium oxide for validation of the novel technique.

The Elastic Mechanical Response of Nanoscale Thin Films of Miscible Polymer/Polymer Blends

Nanoindentation measurements of the elastic moduli Er of thin polymer films supported by stiff substrates with moduli Es ≫ Er show an increase of Er with decreasing h, for h less than a threshold thickness ht. In the thickness range h < ht , the value of the modulus manifests the influence of various interactions associated with the “stiff” substrate. We show that ht is a function of composition for the miscible blend of polystyrene (PS) and tetramethyl bisphenol-A polycarbonate (TMPC). The modulus Er,TMPC of TMPC films supported by SiOx increases for h < ht(TMPC) ∼ 300 nm while ht(PS) ∼ 450 nm for the modulus Er,PS of PS films, supported by the same substrate. The threshold thicknesses of the blends and the moduli of the blends appear to be reasonably described by an effective medium approximation. This behavior is rationalized in terms of the vibrational force constants f, determined using incoherent neutron scattering experiments, of the materials.

Ionic properties of ultrathin yttria-stabilized zirconia thin films fabricated by atomic layer deposition with water, oxygen, and ozone

We compared the ionic properties of yttria-stabilized zirconia (YSZ) thin films prepared by atomic layer deposition (ALD) using various oxidants including water, oxygen, and ozone. Cross-plane conductivity measurements were performed at low temperature (50°C) and high temperature (450°C) using AC impedance spectroscopy. As a result, we have confirmed that the conductivity of ALD YSZ films below 300°C is greater by several orders of magnitude compared to the nano-scale YSZ thin films synthesized by other conventional techniques. Among the ALD YSZ samples, ALD YSZ fabricated using water showed the highest conductivity while ALD YSZ fabricated using ozone showed the lowest. We have analyzed this result in relation with grain morphology characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM), and the chemical binding states measured by X-ray photoelectron spectroscopy (XPS).

The origin of ferroelectricity in Hf1−xZrxO2: A computational investigation and a surface energy model

The structural, thermal, and dielectric properties of the ferroelectric phase of HfO2, ZrO2, and Hf0.5Zr0.5O2 (HZO) are investigated with carefully validated density functional computations. We find that the free bulk energy of the ferroelectric orthorhombic Pca21 phase is unfavorable compared to the monoclinic P21/c and the orthorhombic Pbca phase for all investigated stoichiometries in the Hf1−xZrxO2 system. To explain the existence of the ferroelectric phase in nanoscale thin films, we explore the Gibbs/Helmholtz free energies as a function of stress and film strain and find them unlikely to become minimal in HZO films for technological relevant conditions. To assess the contribution of surface energy to the phase stability, we parameterize a model, interpolating between existing data, and find the Helmholtz free energy of ferroelectric grains minimal for a range of size and stoichiometry. From the model, we predict undoped HfO2 to be ferroelectric for a grain size of about 4 nm and epitaxial HZO below 5 nm. Furthermore, we calculate the strength of an applied electric field necessary to cause the antiferroelectric phase transformation in ZrO2 from the P42/nmc phase as 1 MV/cm in agreement with experimental data, explaining the mechanism of field induced phase transformation.

Nano-scale liquid film sheared between strong wetting surfaces: effects of interface region on the flow

In this paper, we use molecular dynamics (MD) simulations to investigate changes in fluid flow at a solid/liquid interface. The flow is driven by shearing FCC structured solid molecular walls under isothermal conditions using previously developed interactive thermal wall models. For the nano-scale thin liquid film flows, a fluid molecular layer attached to the wall molecules behaves as an extended wall layer, which induces increased shearing in the middle of the fluid by reducing the width of the flow region. Small variations in molecular diameter length at the interface significantly affect flow characteristics. Shear locking on strong wetting surfaces caused by the dynamic structuring of fluid molecules (i.e., the fluid molecules layering on the solid surface due to the wall force field) increases the density and viscosity and decreases the shear rate and the heat dissipation ratio on the interface, which are important in nano-scale fluid flow analysis.

Some properties of solutions for a parabolic equation modeling epitaxial thin film growth

In this paper, we study a fourth-order parabolic equation with Dirichlet boundary which arises in epitaxial growth of nanoscale thin films. We proved the equation with condition possesses a global attractor in , which attracts any bounded subset of in the -norm. In addition to this, we consider the equation under the conditions is blow-up.

Functionalized Polyelectrolytes Assembling on Nano‐BioFETs for Biosensing Applications

A new surface functionalization scheme for nano‐Bio field effect transistors (FETs) using biocompatible polyelectrolyte thin films (PET) is developed. PET assemblies on Si nanowires (Si‐NWs) are driven by electrostatic interactions between the positively charged polymer backbone and negatively charged Si/SiO2 surface. Such assemblies can be directly coated from PET aqueous solutions and result in a uniform nanoscale thin film, which is more stable compared to the conventional amine silanization. Short oligo‐ethylene glycol chains are grafted on the PETs to prevent nonspecific protein binding. Moreover, the reactive groups of the polymer chains can be further functionalized to other chemical groups in specific stoichiometry for biomolecules detection. Therefore, it opens a new strategy to precisely control the functional group densities on various biosensor surfaces at the molecular level. In addition, such assemblies of the polymers together with the bound analytes can be removed with the pH stimulation resulting in regeneration of a bare sensor surface without compromising the integrity and performance of the Si‐NWs. Thus, it is believed that the developed PET coating and sensing systems on Si‐NW FETs represent a versatile, promising approach for regenerative biosensors which can be applied to other biosensors and will benefit real device applications, enhancing sensor lifetime, reliability, and repeatability.

Vibration Damping Materials and Their Applications in Nano/Micro-Electro-Mechanical Systems: A Review.

The present review explores an overall view of the vibration damping materials ranging from traditionally used viscoelastic materials for macroscale damping to hybrid thin film heterostructures for micro-electro-mechanical systems (MEMS). Vibration damping materials like rubbers, polymers, metals, metal-matrix composites and smart materials are reviewed in terms of damping capacity, stiffness, mechanical strength and figure of merit. Nanoscale shape memory alloys, piezoelectric materials, carbon nanotubes, their composites and thin films are promising materials for future nanoscale damping devices. The main focus of this article is on our development of new vibration damping approach for MEMS structures comprising of ferroelastic/ferroelastic thin film heterostructures. For the first time, nanoindentation has been explored as an alternative tool to evaluate the damping capability of actual components (e.g., thin films for MEMS) where production of dynamic mechanical analyzer (DMA) test samples is not feasible. A comprehensive insight on the existing vibration damping materials and our new approach would definitely trigger some important applications in nano- and micro-electro-mechanical systems.

Nanoscale thin films of silicon can serve as lenses

Nanoscale thin films of silicon can serve as lenses

Characterization of molecular transport in ultrathin hydrogel coatings for cellular immunoprotection.

PEG hydrogels are routinely used in immunoprotection applications to hide foreign cells from a host immune system. Size-dependent transport is typically exploited in these systems to prevent access by macromolecular elements of the immune system while allowing the transport of low molecular weight nutrients. This work studies a nanoscale hydrogel coating for improved transport of beneficial low molecular weight materials across thicker hydrogel coatings while completely blocking transport of undesired larger molecular weight materials. Coatings composed of PEG diacrylate of molecular weight 575 and 3500 Da were studied by tracking the transport of fluorescently labeled dextrans across the coatings. The molecular weight of dextran at which the transport is blocked by these coatings are consistent with cutoff values in analogous bulk PEG materials. Additionally, the diffusion constants of 4 kDa dextrans across PEG 575 coatings (9.5 × 10(-10)-2.0 × 10(-9) cm(2)/s) was lower than across PEG 3500 coatings (5.9-9.8 × 10(-9) cm(2)/s), and these trends and magnitudes agree with bulk scale models. Overall, these nanoscale thin PEG diacrylate films offer the same size selective transport behavior of bulk PEG diacrylate materials, while the lower thickness translates directly to increased flux of beneficial low molecular weight materials.

Analysis of Reflectance and Transmittance Characteristics of Optical Thin Film for Various Film Materials, Thicknesses and Substrates

Optical thin film can control the reflection, refraction and transmission of specific wavelength of electromagnetic radiation and have numerous applications such as reflector, anti-reflector, attenuator, photovoltaic cell and refractor in the electromagnetic radiation spectrum. To perform the particular function for which they are designed, precise control of various film parameters such as thickness, composition, roughness, uniformity is necessary. Silicon dioxide, Zinc oxide, Zinc telluride are the most promising materials in optoelectronic and optical applications in the visible region as well as in the infrared and UV region. In this research, the optical properties such as spectral reflectance and transmittance of nano-scale optical thin films have been studied and analyzed for those materials on different substrates to obtain the optimal configuration for the best performance. To obtain this, the equations of reflectance and transmittance for thin film have been derived, then simulated and visualized by Matlab code. By changing film materials, substrates and film thickness, transmittance and reflectance characteristics have been observed and analyzed. The result showed that the reflectance and transmittance characteristics of optical thin film are strongly dependent on the wavelength of electromagnetic spectra. The film and substrate materials are very effective on the reflectance and transmittance characteristics of the optical thin films. It is also found that the reflectance and transmittance characteristics are greatly affected by film thickness. This research work will benefit and enhance the value of nano coating technology to determine the best thin film/substrate configuration in the development of micro- and nano optoelectronic devices.

Thermoresistive properties of p-type 3C–SiC nanoscale thin films for high-temperature MEMS thermal-based sensors

We report for the first time the thermoresistive property of p-type single crystalline 3C–SiC (p-3C–SiC), which was epitaxially grown on a silicon (Si) wafer, and then transferred to a glass substrate using a Focused Ion Beam (FIB) technique.

Unique phase diagrams in ultra-thin Ising films with dilutions

The phase diagrams of two nanoscaled thin films with site (or bond) dilution, described by the spin-1 (or spin-1/2) Ising model, are investigated by the use of the effective field theory with correlations. They are consisted of two layers. Some unique features have been found in the phase diagrams, while they show many similar behaviors for both the spin-1/2 films and the spin-1 films with a positive single-ion anisotropy. It is clarified that the unique behavior of critical concentration in the spin-1 system with site dilution and a positive single-ion anisotropy at which the transition temperature reduces to zero is completely equivalent to that in the spin-1/2 system with site dilution.

Assessing Collagen Nanoscale Thin Films Heterogeneity by AFM Multimode Imaging and Nanoindetation for NanoBioMedical Applications

Collagen due to its filamentous shape and unique properties is a very promising molecule for the development of nanostructures, scaffolds, cell culture platforms and nanobiomaterials. Furthermore, collagen thin films are of great interest as they can cover non-biological surfaces in medical devices and sensors so as to offer them biocompatibility. Since surface nanotopography and mechanical properties can influence cell-biomaterial interactions it is of crucial im- portance to nano-characterize collagen surface and heterogeneity. Nano-characterization can be performed with Atomic Force Microscopy (AFM), which operates in a variety of modes/techniques and can offer a wide range of information, from topography to mechanical properties. In this paper it was sought to gain insights of structural and mechanical hetero- geneity of collagen fibers in thin films by combining AFM multimode-imaging, including phase imaging, with quantita- tive measurements through nanoindentation. The results demonstrated that the overlap-gap regions on collagen fibers (D-periodicity) yield a significant phase contrast, due to different mechanical properties. In addition, phase contrast was also demonstrated in collagen 'kinks', which provides evidence that collagen fiber shell and core possess different properties. The mechanical heterogeneity of the collagen fibers in the kink areas was confirmed by AFM-nanoindentation and the obtained quantitative measurements. In addition, AFM multimode imaging performance was demonstrated on fibroblasts cultured on collagen thin films. The correlation between the heterogeneous structure and the mechanical properties of collagen fibers in thin films will enable the design and development of biomaterials and tissue scaffolds with improved properties.

Clues to the electrical switching mechanism of carbazole-containing polyimide thin films.

The mechanism behind electrical memory behavior of carbazole-containing polyimides (PIs) in nanoscale thin films was investigated. For this investigation, a series of poly(3,3'-dihydroxy-4,4'-biphenylene-co-3,3'-bis(N-ethylenyloxycarbazole)-4,4'-biphenylene hexafluoro-isopropylidenedi-phthalimide)s (6F-HAB-HABCZn PIs) with various compositions was synthesized as a model carbazole-containing polymer system. The thermal properties, band gaps, and molecular orbital levels of the PIs were determined. Furthermore, the chemical compositions, as well as the nanoscale thin film morphologies and electron densities, were analyzed, providing detailed information on the population and positional distribution of carbazole moieties in thin films of the PIs. PI Devices were fabricated with aluminum electrodes and tested electrically. The PI thin film layers in the devices exhibited electrically permanent memory behavior, which was driven by trap-limited space-charge limited conduction and ohmic conduction. The permanent memory characteristics were found to be attributed to the incorporated carbazole moieties rather than from the other chemical components. Furthermore, the memory characteristics depended significantly on the population and positional distribution of carbazole moieties in the PI layer, as well as the film thickness. Considering that the backbone is not conjugated, the present results collectively indicate that the electrical switching behavior of the PI films is driven by the carbazole moieties acting as charge traps and a hopping process using the carbazole charge-trap sites as stepping-stones.

ChemInform Abstract: On the Variability of Reported Ionic Conductivity in Nanoscale YSZ Thin Films

AbstractReview: 58 refs.

The Temperature-sensitivity of a Critical Electric Field Induced Magnetic Easy-axis Reorientation Ferromagnetic/ferroelectric Layered Heterostructures

We have investigated the effects of temperature on the magnetic easy axis reorientation (EARs) in polycrystalline ferromagnetic (FM) nano-scale thin films, i.e. (001)-oriented cubic films such as Fe, Ni, CFO and (0001)-oriented hexagonal films such as Co, grown on ferroelectric (FE) substrates, for example PZT, BTO substrates. The model of FM/FE bilayered heterostructures has been applied to study the total free energy of the FM films. By minimizing this energy, we have shown that temperature has a significant effect on a critical electric field induced magnetic reorientation.

Size-dependent mechanical behavior of free-standing glassy polymer thin films

Abstract

Self-Assembly and Digital Memory Characteristics of an Oxadiazole-Containing Brush Polymer in Nanoscale Thin Films

Self-Assembly and Digital Memory Characteristics of an Oxadiazole-Containing Brush Polymer in Nanoscale Thin Films

Hierarchical Structures in Thin Films of Miktoarm Star Polymers: Poly(n-hexyl isocyanate)(12K)–Poly(ε-caprolactone)1–3(5K)

A series of miktoarm star polymers, [poly(n-hexyl isocyanate)(12K)]–[poly(ε-caprolactone)1–3(5K)] (PHIC–PCL1–3) (composed of a rigid self-assembling PHIC arm and one to three flexible crystallizable PCL arms), were investigated to examine the polymers’ thermal properties and nanoscale thin film morphologies. The miktoarm polymers were stable up to 180 °C. The PHIC and PCL arm components underwent phase separation during the solution casting, drying, and post toluene-annealing processes, forming interesting but very complex thin film morphologies. The resulting thin film morphologies were examined in detail for the first time using synchrotron grazing incidence X-ray scattering (GIXS) measurements and quantitative data analysis. All of the miktoarm star polymer films formed vertically well-oriented lamellar structures, regardless of the number and length of PCL arms. These structures were quite different from the cylindrical structures commonly observed in conventional flexible diblock copolymer films having comparable volume fractions. The individual PHIC and PCL lamellar domains self-assembled to form their own respective morphological structures. The PHIC lamellae consisted of a mixture of horizontal and vertical multibilayer structure domains, as observed in the PHIC homopolymer film. The PCL lamellae formed fringed micelle-like crystals and/or highly imperfect folded crystals that differed significantly from the structures found in a PCL homopolymer film composed of typical folded lamellar crystals. These PCL crystals were formed with a mixture of vertical and horizontal orthorhombic lattices. Overall, the GIXS analysis revealed that the parameters that characterized the hierarchical structures in the thin films depended significantly on the number and length of the PCL arm and its crystallization characteristics as well as the chain rigidity and multibilayer structure formation characteristics of the PHIC arm.