Bilayer Nanofilms Research Articles

Rolling structure from bilayer nanofilm by mismatch

A continuum theoretical scheme for self-rolling nanotubes from bilayers by mismatch is obtained by considering surface elasticity, surface stress, and symmetry lowering effects. For an ultrathin nanofilm with only several nanometers in thickness, isotropic mismatch, and isotropic surface stress usually induce anisotropic rolling behavior. The isotropic Timoshenko formula should be modified anisotropically to explain the mechanical behavior of anisotropic rolling structure of nanotubes accurately. The nanofilm rolls up in tangential direction while remaining straight in cylindrical direction theoretically. Therefore, in this paper the anisotropic shape of nanotubes is taken into consideration. Along the cylindrical direction, although it maintains straight and its residual strain is uniform, the stress varies in the radial direction due to the Poisson’s effect of tangential strain. The results of the current theory applied to Si–Si nanotube, InAs–GaAs nanotube, and InGaAs–Cr nanotube systems show good agreement with the experimental data. Beside the surface elasticity effect and surface stress effect, the symmetry breaking and the anisotropic rolling structure are of great importance in theoretically describing the mechanical behavior of rolling-up of nanotubes.

Current–voltage characteristics of a Co/Ni bilayer nanofilm

Widespread interest in optical and electrical properties of nanostructured systems began more than 20 years ago. Here, optical and electrical properties of Co/Ni bilayer nanofilms are explored. These optical and electrical properties were described in terms of a common confined wavefunction spanning both Co and Ni layers. The effective relative electron mass parameter f estimated for this bilayer film was about 0.1489. Temperature dependence of the current–voltage characteristics of the Co/Ni bilayer nanofilms was described as that of a Schottky diode with the Schottky barrier height of US = 24 cm−1, very small compared to other semiconducting materials. Therefore, Co/Ni bilayer nanofilms apparently had semiconducting properties, which may approximately be described using Schottky diode theory. Such semiconducting properties of bilayer metal nanofilms could be used in special low-temperature low-consumption electronic devices.

Enhanced thermoelectric performance of the AlN/GaN bilayer

Improving the thermoelectric (TE) transformation efficiency has always been the focus in the TE fields. Inspired by the research progress of the predominant TE performance of graphitelike nanofilms, we constructed AlN/GaN bilayer nanofilms based on the hexagonal lattice planes along (001) surface of wurtzite (WZ) AlN and GaN bulk. By the first-principles calculation within the framework of density functional theory (DFT) combined with non-equilibrium Green's Function (NEFG) method and Landauer-Buttiker theory, the stability, the electronic structures, the TE parameters and the figure of merit ZT of AlN/GaN bilayer nanofilms are investigated and analyzed in detail. Relative to AlN and GaN monolayer, the maximum ZT of AlN/GaN bilayer is improved, which can reach 1.41 at 700 K for p-type doping. Our research provides an idea that bilayer heterojunction nanofilm has superior TE performance to that of the corresponding semiconductor monolayer.

Superhigh energy storage density on-chip capacitors with ferroelectric Hf0.5Zr0.5O2/antiferroelectric Hf0.25Zr0.75O2 bilayer nanofilms fabricated by plasma-enhanced atomic layer deposition.

Thanks to their excellent compatibility with the complementary metal-oxide-semiconductor (CMOS) process, antiferroelectric (AFE) HfO2/ZrO2-based thin films have emerged as potential candidates for high-performance on-chip energy storage capacitors of miniaturized energy-autonomous systems. However, increasing the energy storage density (ESD) of capacitors has been a great challenge. In this work, we propose the fabrication of ferroelectric (FE) Hf0.5Zr0.5O2/AFE Hf0.25Zr0.75O2 bilayer nanofilms by plasma-enhanced atomic layer deposition for high ESD capacitors with TiN electrodes. The effects of the FE/AFE thickness composition and annealing conditions are investigated, revealing that the Hf0.5Zr0.5O2 (1 nm)/Hf0.25Zr0.75O2 (9 nm) bilayer can generate the optimal ESD after optimized annealing at 450 °C for 30 min. This is mainly ascribed to the factor that the introduction of a 1 nm Hf0.5Zr0.5O2 layer enhances the formation of the tetragonal (T) phase with antiferroelectricity in the AFE Hf0.25Zr0.75O2 layer as well as the breakdown electric field of the bilayer while fixing the FE/AFE bilayer thickness at 10 nm. As a result, a ESD as high as 71.95 J cm-3 can be obtained together with an energy storage efficiency (ESE) of 57.8%. Meanwhile, with increasing the measurement temperature from 300 and 425 K, the capacitor also demonstrates excellent stabilities of ESD and ESE. In addition, superior electrical cycling endurance is also demonstrated. Further, by integrating the capacitor into deep silicon trenches, a superhigh ESD of 364.1 J cm-3 is achieved together with an ESE of 56.5%. This work provides an effective way for developing CMOS process-compatible, eco-friendly and superhigh ESD three-dimensional capacitors for on-chip energy storage applications.

Cascaded multilayer nano-kirigami for extensible 3D nanofabrication and visible light manipulation

Nano-kirigami enables direct and versatile shape transformations from two-dimensional predesigns to three-dimensional (3D) architectures in microscale/nanoscale. Here a new and extensible strategy for cascaded multilayer nano-kirigami is demonstrated in a gold/silicon nitride (Au/SiN) bilayer nanofilm for 3D nanofabrication and visible light manipulation. By employing a focused ion-beam-based Boolean irradiation, rich 3D shape transformation and nested multilayer nanostructures are precisely manufactured, which are well reproduced by developing a modified mechanical model. Based on the multilayer and deformable features of the nano-kirigami structures, potentials in manipulating the phase and intensity of visible light are explored. The developed new nano-kirigami strategies, as well as the novel exotic 3D nanostructures, could be helpful to build a novel platform for 3D nanofabrication and find potential applications in microelectromechanical/nanoelectromechanical systems, holographic display, plasmonics, nanophotonics, biophotonics, etc.

Integrated sensing layer of bacterial cellulose and polyethyleneimine to achieve high sensitivity of ST-cut quartz surface acoustic wave formaldehyde gas sensor

Surface acoustic wave (SAW)-based formaldehyde gas sensor using bi-layer nanofilms of bacterial cellulose (BC) and polyethyleneimine (PEI) was developed on an ST-cut quartz substrate using sol-gel and spin coating processes. BC nanofilms significantly improve the sensitivity of PEI films to formaldehyde gas, and reduces response and recovery times. The BC films have superfine filamentary and fibrous network structures, which provide a large number of attachment sites for the PEI particles. Measurement results obtained using in situ diffuse reflectance Fourier transform infrared spectroscopy showed that the primary amino groups of PEI strongly adsorb formaldehyde molecules through nucleophilic reactions, thus resulting in a negative frequency shift of the SAW sensor due to the mass loading effect. In addition, experimental results showed that the frequency shifts of the SAW devices are determined by thickness of PEI film, concentration of formaldehyde and relative humidity. The PEI/BC sensor coated with three layers of PEI as the sensing layer showed the optimal sensing performance, which had a frequency shift of 35.6 kHz for 10 ppm formaldehyde gas, measured at room temperature and 30 % RH. The sensor also showed good selectivity and stability, with a low limit of detection down to 100 ppb.

Synthesis and Characterization of Functional Nanofilm-Coated Live Immune Cells

Layer-by-layer (LbL) assembly techniques have been extensively studied in cell biology because of their simplicity of preparation and versatility. The applications of the LbL platform technology using polysaccharides, silicon, and graphene have been investigated. However, the applications of the above-mentioned technology using living cells remain to be fully understood. This study demonstrates a living cell-based LbL platform using various types of living cells. In addition, it confirms that the surplus charge on the outer surface of the coated cells can be used to bind the target protein. We develop a living cell-based LbL platform technology by stacking layers of hyaluronic acid (HA) and poly-l-lysine (PLL). The HA/PLL stacking results in three bilayers with a thickness of 4 ± 1 nm on the cell surface. Furthermore, the multilayer nanofilms on the cells are completely degraded after 3 days of the application of the LbL method. We also evaluate and visualize three bilayers of the nanofilm on adherent (AML-12 cells)-, nonadherent (trypsin-treated AML-12 cells)-, and circulation type [peripheral blood mononuclear cells (PBMCs)] cells by analyzing the zeta potential, cell viability, and imaging via scanning electron microscopy and confocal microscopy. Finally, we study the cytotoxicity of the nanofilm and characteristic functions of the immune cells after the nanofilm coating. The multilayer nanofilms are not acutely cytotoxic and did not inhibit the immune response of the PBMCs against stimulant. We conclude that a two bilayer nanofilm would be ideal for further study in any cell type. The living cell-based LbL platform is expected to be useful for a variety of applications in cell biology.

Interface-based two-way tuning of the in-plane thermal transport in nanofilms

Here, the two-way tuning of in-plane thermal transport is obtained in the bi-layer nanofilms with an interfacial effect by using the Boltzmann transport equation (BTE) and the phonon Monte Carlo (MC) technique. A thermal conductivity model was derived from the BTE and verified by the MC simulations. Both the model and the MC simulations indicate that the tuning of the thermal transport can be bidirectional (reduced or enhanced), depending on the interface conditions (i.e., roughness and adhesion energy) and the phonon property dissimilarity at the interface. For the identical-material interface, the emergence of thermal conductivity variation requires two conditions: (a) the interface is not completely specular and (b) the transmission specularity parameter differs from the reflection specularity parameter at the interface. When the transmission specularity parameter is larger than the reflection specularity parameter at the interface, the thermal conductivity improvement effect emerges, whereas the thermal conductivity reduction effect occurs. For the disparate-material interface, the phonon property perturbation near the interface causes the thermal conductivity variation, even when neither the above two conditions are satisfied. The mean free path ratio (γ) between the disparate materials was defined to characterize the phonon property dissimilarity. γ > 1 can lead to the thermal conductivity improvement effect, while γ < 1 corresponds to the thermal conductivity reduction effect. Our work provides a more in-depth understanding of the interfacial effect on the nanoscale thermal transport, with an applicable predictive model, which can be helpful for predicting and manipulating phonon transport in nanofilms.

Diffusion behavior of Ag in TiO2 nanofilms

Using magnetron sputtering, Ag/TiO2 bilayer nanofilms were deposited on glass substrates. Heat-treating the Ag/TiO2 nanofilms at 400°C led to the formation of Ag nanoparticles, which dispersed inside the TiO2 films as well as the free surface of the TiO2 films. After heat treatment the sample at 500°C, larger Ag nanoparticles could be observed on the surface, which means the particle size after heat treatment is temperature related. Anatase TiO2 phase was formed after the Ag/TiO2 films were heat-treated at 400°C. SEM and TEM were used to investigate the morphology and structure of the films and it was found that Ag was not oxidized during heat treatment. The diffusion process of Ag in TiO2 film is also related to the order of Ag layer and TiO2 layer on glass substrate. Moreover, the diffusion process is much easier in amorphous than that in anatase structure.

Au-TiO2 nanofilms for enhanced photocatalytic activity

Au/TiO2 nanofilms are prepared on indium-tin oxide glass using magnetron sputtering. The bilayer nanofilms are heat-treated at 500°C in air for 2h. The microstructure evolution of the prepared Au/TiO2 nanofilms is characterized, using scanning electron microscope, energy dispersive spectrometer, and X-ray diffraction. The heat treatment leads to the migration of Au atoms to form Au/TiO2 nanofilms with TiO2 being in anatase phase and the presence of Au films/islands on the surface of the Au/TiO2 nanofilms. The photocatalytic activity of the prepared Au/TiO2 nanofilms in an aqueous solution of methylene blue (MB) is evaluated under the irradiation of visible light. The experimental results show that the photocatalytic activity of the prepared Au/TiO2 nanofilms depends on the loading amount of Au. The rate constant for the photodegradation of the MB compound in the aqueous solutions with pure TiO2 nanofilms is less than those with the prepared Au/TiO2 nanofilms. Such behavior can be likely attributed to the activation of the photo-generated charge carriers and the increase of the electron–hole separation efficiency through localized surface plasmon resonance.

Edge effect of strained bilayer nanofilms for tunable multistability and actuation.

We employed both theoretical and computational models supported by experiments to study the multistable behavior of an edge-effect driven Si/Cr micro-claw. Our study showed that individual micro-claws demonstrate either monostability or bistability as the magnitude of the edge effect is varied.

A robust bilayer nanofilm fabricated on copper foam for oil–water separation with improved performances

In this work, a facile approach of covalent layer-by-layer assembly was applied to fabricate a bilayer nanofilm which renders roughened copper foam superhydrophobic with robust performances.

Photocurrent enhancement mechanisms in bilayer nanofilm-based ultraviolet photodetectors made from ZnO and ZnS spherical nanoshells.

Hollow-sphere bilayer nanofilm-based ultraviolet light photodetectors made from ZnO and ZnS spherical nanoshells show enhanced photocurrent, which are comparable to or even better than those of other semiconductor nanostructures with different shapes. In this work, the photocurrent enhancement mechanisms of these bilayer nanofilm-based ultraviolet light photodetectors are explained, which could be attributed to the strong light absorption based on the whispering gallery mode resonances, the separation of the photogenerated carriers through the internal electric field within the bilayer nanofilms, the hopping-like electrical transport, and the effective charge injection from Cr/Au contacts to the nanofilms.

Ammonia gas sensors based on ZnO/SiO2 bi-layer nanofilms on ST-cut quartz surface acoustic wave devices

Surface acoustic wave (SAW) ammonia gas sensors based on ZnO/SiO2 bi-layer nanofilms on ST-cut quartz surface acoustic wave devices were fabricated and characterized. The ZnO and SiO2 layers were coated onto SAW resonators by combining a sol–gel process and a spin-coating technique. The SEM and AFM results revealed the ZnO/SiO2 bi-layer films had porous structures. The gas sensing results showed that the sensitivity of sensors was dependent on the value of sheet conductivity of the sensing films. As a result, the bi-layer nanofilms were much more sensitive than the single layer films due to their appropriate sheet conductivity, and the absolute response value was dependent on the thickness of the top ZnO layer. The sensor based on the bi-layer nanofilm with 60nm top ZnO layer showed the best gas sensing property. It exhibited a frequency shift of 2000Hz in 30ppm ammonia gas with good repeatability and stability.

Dense Assembly of Gd2O3:0.05X3+ (X = Eu, Tb) Nanorods into Nanoscaled Thin-Films and Their Photoluminescence Properties

This paper presents a simple and effective oil-water interfacial self-assembly strategy to fabricate monolayer and bilayer nanofilms of densely packed Gd2O3:0.05X(3+) (X = Eu, Tb) nanorods with characteristic luminescence properties. In this process, Gd2O3:0.05X(3+) (X = Eu, Tb) nanotubes synthesized by a hydrothermal method are dispersed in deionized water; then, a certain amount of n-hexane is added to produce a hexane-water interface. With n-butanol added as initiator, the nanotubes are gradually trapped at the interface to form a densely packed nanofilm. A monolayer nanofilm of densely packed Gd2O3:0.05Eu(3+) nanorods is obtained after annealing. In addition, the bilayer nanofilm composed of Gd2O3:0.05X(3+) (X = Eu, Tb) nanorods still retains the luminescence properties of each monolayer nanofilm. Moreover, the adhesion of the film on the substrate is very strong, which is extremely beneficial for its future applications.

Patterned Free-Standing Conductive Nanofilms for Ultraconformable Circuits and Smart Interfaces

A process is presented for the fabrication of patterned ultrathin free-standing conductive nanofilms based on an all-polymer bilayer structure composed of poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonate) and poly(lactic acid) (PEDOT:PSS/PLA). Based on the strategy recently introduced by our group for producing large area free-standing nanofilms of conductive polymers with ultrahigh conformability, here an inkjet subtractive patterning technique was used, with localized overoxidation of PEDOT:PSS that caused the local irreversible loss of electrical conductivity. Different pattern geometries (e.g., interdigitated electrodes with various spacing, etc.) were tested for validating the proposed process. The fabrication of individually addressable microelectrodes and simple circuits on nanofilm having thickness ∼250 nm has been demonstrated. Using this strategy, mechanically robust, conformable ultrathin polymer films could be produced that can be released in water as free-standing nanofilms and/or collected on surfaces with arbitrary shapes, topography and compliance, including human skin. The patterned bilayer nanofilms were characterized as regards their morphology, thickness, topography, conductivity, and electrochemical behavior. In addition, the electrochemical switching of surface properties has been evaluated by means of contact angle measurements. These novel conductive materials can find use as ultrathin, conformable electronic devices and in many bioelectrical applications. Moreover, by exploiting the electrochemical properties of conducting polymers, they can act as responsive smart biointerfaces and in the field of conformable bioelectronics, for example, as electrodes on tissues or smart conductive substrates for cell culturing and stimulation.

Stacking‐Order‐Dependent Optoelectronic Properties of Bilayer Nanofilm Photodetectors Made From Hollow ZnS and ZnO Microspheres

Innovative bilayer nanofilms composed of semiconducting ZnS and ZnO hollow microspheres are successfully fabricated by an oil-water interfacial self-assembly strategy. The photocurrent of the bilayer film-based photodetectors is dependent on the stacking orders of the building blocks. The optimal optoelectronic properties of the ZnS(up)/ZnO(down) device are much better than those of the monolayer-film based device.

Spontaneous explosive crystallization and phase transformations in a selenium/copper bilayer nanofilm

Spontaneous explosive crystallization in a selenium/copper bilayer nanofilm has been experimentally studied. It has been shown that the formation of a microcrack network in the selenium film is a decisive factor for spontaneous explosive crystallization. The microcrack network is an efficient channel for the relaxation of the collected energy of elastic stresses in the selenium film. The relaxation of this energy accelerates crystallization and transfers it to the explosive regime. It has been found that crystalline phases appearing in the products of the reaction after spontaneous explosive crystallization depend on the ratio of the thicknesses of the copper and selenium films.

FEM Analysis on the Phenomenon of Bilayer Nanofilm Bending into Nanotube with Effects of Adsorption and Temperature

Nanotubes, fabricated by mechanical bending of thin films, have been widely used in numerous fields. For a bilayer nanofilm, it could bend itself driven by effects of adsorption and ambient temperature. Therefore, in this work, we simulate the phenomenon of bilayer AuSi nanofilm bending into nanotube with the effects of surface stress induced by vdW interaction between adsorbed O or Hg atoms and atoms on Au surface and ambient temperature change by finite element emulation. The curves of nanotube calculated by finite element software accord with theoretical prediction results very well, that demonstrate our theoretical prediction formula has a great feasibility and efficiency in practical application.

Size dependence in self-bending mechanism of Ge/Si bilayer nanofilms

The governing mechanism for a self-bending of Ge/Si bilayer epitaxial nanofilms that results in the formation of various nanostructures (nanotube, nanohelix, etc.) has been investigated using a molecular dynamics simulation. The effect of misfit stress between Ge and Si layers, surface stress due to the surface reconstruction and the anisotropy of elastic modulus on the bending behavior and the effect of film thickness on the relative strength of the individual factors are clarified. All experimentally and theoretically reported results that show sometimes a mutual disagreement could be reconciled, which enabled a more elaborate control of nanostructure formation.