Well-defined Multilayer Structures Research Articles

Advanced Functionalized Materials Based on Layer-by-Layer Assembled Natural Cellulose Nanofiber for Electrodes: A Review.

The depletion of fossil fuel resources and its impact on the environment provide a compelling motivation for the development of sustainable energy sources to meet the increasing demand for energy. Accordingly, research and development of energy storage devices have emerged as a critical area of focus. The electrode materials are critical in the electrochemical performance of energy storage devices, such as energy storage capacity and cycle life. Cellulose nanofiber (CNF) represents an important substrate with potentials in the applications of green electrode materials due to their environmental sustainability and excellent compatibility. By utilizing the layer-by layer (LbL) process, well-defined nanoscale multilayer structure is prepared on a variety of substrates. In recent years, increasing attention has focused on electrode materials produced from LbL process on CNFs to yield electrodes with exceptional properties, such as high specific surface area, outstanding electrical conductivity, superior electrochemical activity, and exceptional mechanical stability. This review provides a comprehensive overview on the development of functional CNF via the LbL approach as electrode materials.

One-pot self-assembly fabrication of chitosan coated hollow sphere for pH/ glutathione dual responsive drug delivery

Chitosan-coated poly (methacrylic acid) (PMAA) hollow spheres with 64 ± 3% drug loading capacity and low drug leakage (7 ± 2%, 54 h) were prepared through a novel one-pot two-step self-assembly process. Site-specific doxorubicin (DOX) loading and chitosan coating were achieved by electrostatic interaction to fulfill efficient drug loading and well-controlled drug release behavior. In vitro drug release profile revealed the pH and glutathione (GSH) dual responsive fast triggered drug release behavior, reaching 62 ± 3% during the first 10 h. And completely drug release could be achieved in 54 h. The high drug content and sensitive tumor microenvironment responsibility lead to similar anti-cancer efficiency with free doxorubicin in in vitro MTT assay. This self-assembly guided one-pot two-step fabrication process was proved to be an effective and convenient way to prepare the well-defined multi-layer structure and might be further employed in fabricating high-performance drug delivery systems.

Membrane stiffness and myelin basic protein binding strength as molecular origin of multiple sclerosis

Myelin basic protein (MBP) and its interaction with lipids of the myelin sheath plays an important part in the pathology of multiple sclerosis (MS). Previous studies observed that changes in the myelin lipid composition lead to instabilities and enhanced local curvature of MBP-lipid multilayer structures. We investigated the molecular origin of the instability and found that the diseased lipid membrane has a 25% lower bending rigidity, thus destabilizing smooth >1,µm curvature radius structures such as in giant unilamellar vesicles. MBP-mediated assembling of lipid bilayers proceeds in two steps, with a slow second step occurring over many days where native lipid membranes assemble into well-defined multilayer structures, whereas diseased lipid membranes form folded assemblies with high local curvature. For both native and diseased lipid mixtures we find that MBP forms dense liquid phases on top of the lipid membranes mediating attractive membrane interactions. Furthermore, we observe MBP to insert into its bilayer leaflet side in case of the diseased lipid mixture, whereas there is no insertion for the native mixture. Insertion increases the local membrane curvature, and could be caused by a decrease of the sphingomyelin content of the diseased lipid mixture. These findings can help to open a pathway to remyelination strategies.

Control of surface functionalization of graphene-metal oxide polymer nanocomposites prepared by a hydrothermal method

The study of nanocomposites formed by doping of graphene oxide with a metal and polymer is presented in this paper. The graphene oxide contains oxygenated functional groups, like epoxy, hydroxyl and carboxyl, as shown by Fourier transform infrared spectroscopy (FTIR) studies. The existence of these functional groups on the surface of graphene oxide disperses the polymer chain and leads to a good interaction with the metal. The characterization of the prepared nanocomposite was done by X-ray diffraction, energy dispersive X-ray analysis, FTIR, Raman spectroscopy, scanning electron microscopy and high resolution transmission electron microscopy. The results showed that the obtained nanocomposite comprises nano-shaped thin stacked flakes with a well-defined multilayered edge structure. It was also seen that the metal particles were uniformly dispersed on the graphene surface with an average particle size of 1–25 nm. The synthesized nanocomposites can be used for adsorption and degradation of dyes.

ω-Thiolation of Phenolic Surfactants Enables Controlled Conversion between Extended, Bolaform, and Multilayer Conformations.

The self-assembly of ω-thiolated surfactants onto gold is a well-studied phenomenon; however, control over the final organization within the thin films is either limited or requires extensive pre- and post-deposition chemical modifications. On the other hand, Langmuir-Blodgett deposition from the air-water interfaces affords a high degree of control over lateral organization within the film, yet it is generally employed to create physisorbed, soft matter films. Despite this, relatively little is known about the impact of the ω-thiolation on either the air-water of deposited film organization. Here, we show that the introduction of a terminal hydrophilic thiol on a phenolic surfactant does not necessarily disrupt a highly organized film nor does it necessarily induce a bolaform conformation at the interface. We show that the relative proportions of different conformations can be controlled using pH, relaxation time, surface pressure, and combinations thereof. Moreover, at high pH, the system undergoes a monolayer-to-multilayer transition wherein well-defined multilayer structures and morphologies are generated. These multilayers appear to comprise a single bolaform conformation atop an extended-chain condensed phase. We demonstrate that these structures can be transferred using Langmuir-Blodgett deposition demonstrating that combining these two approaches has the potential to achieve greater control over the functional properties of robust, chemisorbed films.

Investigation of graphene-based nanocomposite for hydrogen storage

In this study, a simple, fast, and green one-pot method was developed for the preparation of Ni/Graphene nanocomposite for the storage of hydrogen. The precursor graphene oxide was prepared from expanded graphite flakes using a modified Hummer’s method. Graphene oxide was reduced effectively using natural reducing agents such as lemon juice as eco-friendly potential reducing agents instead of harmful hydrazine (N2H4). The characterization of as-synthesized graphene oxide and Ni/graphene nanocomposites were performed by powder X-ray diffraction (p-XRD), high-resolution electron microscopy (HRSEM) with energy dispersive spectroscopy (EDS) analysis and transmission electron microscopy (TEM) before investigating the performance for hydrogen storage. It was observed from the results obtained that the synthesized graphene contains thin stacked flakes of shapes having a well-defined multi-layered structure at the edge. And the round shaped Ni nanoparticles of 13 nanometres in size were dispersed around the graphene structure. The hydrogenation capacity of the Ni/graphene nanocomposites were tested using thermogravimetric analysis (TGA). The prepared Ni/graphene nanocomposites exhibited good hydrogen storage capacities.

Comparison of Optical and Electrical Properties of Different Hole-Transporting Materials for Solution-Processable Organic Light-Emitting Diodes.

Achieving well-defined multilayer structure is a key to improve device performance of organic light-emitting diodes (OLEDs), especially in solution-processed OLEDs. Use of cross-linkable hole-transporting materials (HTMs) is therefore gaining much attention to achieve such multilayer structures. One of representative solution-processable HTM is TFB, poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N-(4-sec-butylphenyl)diphenylamine)], but it does not have any cross-linkable units in its chemical structures. In this work, we performed basic physical characterization of a new cross-linkable HTM, HL-X026, and we also investigated its OLED characteristics. Actual solvent resistance by varying the thermal curing temperatures was measured. The thermally cross-linkable HL-X026 showed better device performances than simple TFB with higher luminance and efficiencies.

Multimodal investigation of a keloid scar by combining mechanical tests in vivo with diverse imaging techniques

Keloids are pathologic scars, defined as fibroproliferative diseases resulting from abnormal wound responses, which grow beyond the original wound margins. They develop on specific pro-keloid anatomic sites frequently characterized by high stress states. The initiation and growth mechanisms of keloid are not well-understood.This study relates multimodal investigation of a keloid by using mechanical tests in vivo and imaging techniques. A single case composed of a keloid, the healthy skin surrounding the keloid, and the contralateral healthy skin on the upper arms of a woman has been investigated in extension and suction by using non-invasive devices dedicated to in vivo skin measurement. The thickness and microstructure of these soft tissues have been observed by echography, tomography and confocal microscopy. Displacement fields have been obtained by using digital image correlation.Unlike healthy skin, keloid is not a well-defined multilayer structure: the frontier between epidermis and dermis disappears. The mechanical behavior of keloid is highly different from healthy skin one. The R-parameters have been deduced from suction curves. Physical parameters as tissue extensibility, initial and final tangent moduli have been identified from the stress-strain curves. The extensibility (respectively, initial rigidity) of keloid is highly lower (respectively, higher) than that of healthy skin. To compare the final rigidity of keloid versus healthy skin, further tests have to be performed with higher strain values.

Letter: Similarity model for corner roll in turbulent Rayleigh-Bénard convection

The corner roll (CR) in the Rayleigh-Bénard convection accounts for the behaviors of convection flow and heat transport in the corner. The three-dimensional direct numerical simulations for 108 < Ra < 5 × 109 show that CR presents a well-defined similarity and multi-layer structure. The CR can be described by a stream function derived from the homotopy and the structure ensemble dynamics. The scale of CR has a negative scaling r = 0.770Ra−0.085. A new scaling theory for CR suggests the scalings for Recr ∼ Ra0.248 and Nucr ∼ Ra1/3 very close to the simulated ones.

A co-fireable material system for ceramics and ferrites hetero-laminates in LTCC substrates

A method for solving the current challenges in manufacturing hetero-laminated material systems using Low Temperature Co-firing Ceramics (LTCC) technique is introduced in this paper. The system contains all three major materials that widely used in the LTCC industry, Zn2SiO4 ceramic, NiCuZn ferrite and silver. Previous reported methods either involve additional buffer layers which would increase the device thickness, or apply mechanical pressure during the co-firing which is not compatible to the standard LTCC process. Our method provides a facile route to manufacture hetero-laminated material systems that demands no buffer layer and follows the standard LTCC process. In this paper, four glass-free hetero-laminated material systems using LTCC technology, i.e. NiCuZn/Zn2SiO4/NiCuZn/Zn2SiO4/NiCuZn (termed “LTCC-5”), NiCuZn/Zn2SiO4-Ag/NiCuZn (termed “LTCC-3-NZAN”), NiCuZn/Ag/NiCuZn (termed “LTCC-3-NAN”) and NiCuZn/Zn2SiO4/NiCuZn (termed “LTCC-3-NZN”), have be were designed, manufactured and characterized. The shrinkage rates of these material systems show well-matched temperature spectrum. The sintering properties and interfacial diffusions of the material systems were characterized using optical microscopy, electron microscopy and Energy Dispersive Spectrometer (EDS) element mapping. Optical microscopy shows that all the material systems have well-defined multi-layered structure with controllable and tunable layer thickness. Electron microscopy images prove no microstructural cracks or defects at any of the hetero material interfaces. EDS analysis further proves that the inter-diffusions of the magnetic elements (Fe and Ni) at the NiCuZn/Zn2SiO4 interface is minimized. Such property is essential for achieving high performance devices for applications such as power inductors and DC/DC converters.

Enhanced resonant Raman scattering and optical emission of ZnO/ZnMgO multiple quantum wells

Multiple quantum wells (MQWs) with well-defined multilayer structure are at the foundation of fabricating optoelectronic devices with high quantum efficiency. In this study, we report the ZnO/ZnMgO MQWs fabricated under optimized conditions. At room temperature, the MQW samples, excited by a 325-nm He-Cd laser, exhibited intense ultraviolet photoluminescence (PL) tuned by the quantum confinement (QC) effect. At the same time, extremely enhanced multiple-phonon resonant Raman scattering (RRS) was observed in the first time in the PL spectra of ZnO-based MQW samples. The multiple-phonon RRS was ascribed to the ingoing and outgoing resonance with the barrier layers, and was found highly in accordance with that of ZnMgO epilayer deposited under the same conditions, thus strongly evidencing the MQW samples have well-defined multilayer structures. Using the PL spectra taken at ∼12 K, the PL shifts induced by QC effect were found in good agreement with the calculated results using Kronig-Penney model.

Facile hydrothermal preparation of niobium pentaoxide decorated reduced graphene oxide nanocomposites for supercapacitor applications

Facile synthesis of graphene–Nb2O5 composite has been reported. Graphene oxide was prepared by the modified Hummer's method. The metal oxide (Nb2O5) was introduced to the graphene to form the composite by the hydrothermal method. The prepared samples were characterized by X-ray diffraction, scanning electron microscopy, high resolution transmission electron microscopy, Fourier transform infrared (FTIR) and thermo gravimetric analysis (TGA). SEM and TEM results revealed that the metal oxide particles are uniformly dispersed on the surface of thin sheets of well-defined multilayered graphene structure. Thermal stability of the graphene metal oxide nanocomposites was also investigated. The CV measurements reveal a significant enhancement in the specific capacitance reaching 321Fg−1 at a scan rate of 10mVs−1. With promising electrochemical characteristics, Nb2O5 decorated graphene nanocomposite are explored as potential electrode material for supercapacitor applications.

Enhancement of the Si p-n diode NIR photoresponse by embedding β-FeSi2 nanocrystallites

By using solid phase epitaxy of thin Fe films and molecular beam epitaxy of Si, a p+-Si/p-Si/β-FeSi2 nanocrystallites/n-Si(111) diode structure was fabricated. Transmission electron microscopy data confirmed a well-defined multilayered structure with embedded nanocrystallites of two typical sizes: 3–4 and 15–20 nm, and almost coherent epitaxy of the nanocrystallites with the Si matrix. The diode at zero bias conditions exhibited a current responsivity of 1.7 mA/W, an external quantum efficiency of about 0.2%, and a specific detectivity of 1.2 × 109 cm × Hz1/2/W at a wavelength of 1300 nm at room temperature. In the avalanche mode, the responsivity reached up to 20 mA/W (2% in terms of efficiency) with a value of avalanche gain equal to 5. The data obtained indicate that embedding of β-FeSi2 nanocrystallites into the depletion region of the Si p-n junction results in expansion of the spectral sensitivity up to 1600 nm and an increase of the photoresponse by more than two orders of magnitude in comparison with a conventional Si p-n junction. Thereby, fabricated structure combines advantage of the silicon photodiode functionality and simplicity with near infrared light detection capability of β-FeSi2.

Structure evolution and properties of TiAlCN/VCN coatings deposited by reactive HIPIMS

2.5μm thick TiAlCN/VCN coatings were deposited by a reactive high power impulse magnetron sputtering (HIPIMS) process. Cross-sectional TEM showed gradual evolution of the structure of the coating with thickness. The initial structure is a nanoscale multilayer with sharp interlayer interfaces. This transforms to nanocomposite of TiAlCN and VCN nanocrystalline grains surrounded by a C-rich tissue phase and finally changes to an amorphous carbon rich Me-C phase. In contrast deposition in similar conditions using standard magnetron sputtering produces a well-defined nanoscale multilayer structure. Depth profiling by AES showed that the carbon content in the HIPIMS coating gradually increased from 25% at the coating substrate interface to 70% at the top thus supporting the TEM observations.Energy-resolved mass spectrometry revealed that HIPIMS plasma is a factor of 10 richer in C1+ ions, and therefore more reactive, as compared to the plasma generated by standard magnetron discharge at the same conditions. The peculiar structure evolution in HIPIMS is discussed in relation to target poisoning effect and carbon outward diffusion during coating growth.Highly abrasive AlSi9Cu1 alloy was dry machined using TiAlCN/VCN coated 25mm diameter end mills to investigate the coating-work piece material interaction. Green (532nm excitation) and ultraviolet (325nm excitation) Raman spectroscopy was employed to identify the phase composition of the built up material on the cutting edge and chip (swarf) surfaces produced during machining. These analyses revealed formation of lubricious Magnèli phases namely V2O5 and graphitic carbon as well as highly abrasive SiO2 and mixed (AlSi)O thus shedding light on the wear processes and coating tribological behavior during machining.

Synthesis and Characterization of Metal (Pt, Pd and Fe)-graphene Composites

In this study, we prepared graphene by using the modified Hummers-Offeman method and then introduced the metals (Pt, Pd and Fe) for dispersion on the surface of the graphene for synthesis of metal-graphene composites. The characterization of the prepared graphene and metal-graphene composites was performed by X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) analysis and transmission electron microscopy (TEM). According to the results, it can be observed that the prepared graphene consists of thin stacked flakes of shapes having a well-defined multilayered structure at the edge. And the metal particles are dispersed uniformly on the surface of the graphene with an average particle size of 20 ㎚.

Experimental Investigation of Charge Carrier Transport in Organic Thin-Film Transistors with “Buried Surface Layers”

We studied how the layers with different transport properties buried inside a semiconductor material affect the characteristics of organic thin film transistors (OTFT) using a well-defined multilayered structure fabricated by a contact film transfer method that we recently developed (ACS Appl. Mater. Interfaces 1, 2660 (2009)). A simple model with the charge distribution in the organic semiconductor film, the mobility dependence on the charge density, and the buried surface layers with a high mobility successfully reproduced the experimental mobility dependence on the thickness of the spin-coated films and the gate voltage. These results demonstrated that charge transport layers located far from the dielectric interface could contribute significantly to the total current in OTFTs.

The Impact of Multivalent Counterions, Al3+, on the Surface Adsorption and Self-Assembly of the Anionic Surfactant Alkyloxyethylene Sulfate and Anionic/Nonionic Surfactant Mixtures

The impact of multivalent counterions, Al(3+), on the surface adsorption and self-assembly of the anionic surfactant sodium dodecyl dioxyethylene sulfate, SLES, and the anionic/nonionic surfactant mixtures of SLES and monododecyl dodecaethylene glycol, C(12)E(12), has been investigated using neutron reflectivity, NR, and small angle neutron scattering, SANS. The addition of relatively low concentrations of Al(3+) counterions induces a transition from a monolayer to well-defined surface bilayer, trilayer, and multilayer structures in the adsorption of SLES at the air-water interface. The addition of the nonionic cosurfactant, C(12)E(12), partially inhibits the evolution in the surface structure from monolayer to multilayer interfacial structures. This surface phase behavior is strongly dependent upon the surfactant concentration, solution composition, and concentration of Al(3+) counterions. In solution, the addition of relatively low concentrations of Al(3+) ions promotes significant micellar growth in SLES and SLES/C(12)E(12) mixtures. At the higher counterion concentrations, there is a transition to lamellar structures and ultimately precipitation. The presence of the C(12)E(12) nonionic cosurfactant partially suppresses the aggregate growth. The surface and solution behaviors can be explained in terms of the strong binding of the Al(3+) ions to the SLES headgroup to form surfactant-ion complexes (trimers). These results provide direct evidence of the role of the nonionic cosurfactant in manipulating both the surface and solution behavior. The larger EO(12) headgroup of the C(12)E(12) provides a steric hindrance which disrupts and ultimately prevents the formation of the surfactant-ion complexes. The results provide an important insight into how multivalent counterions can be used to manipulate both solution self-assembly and surface properties.

Langmuir–Blodgett films of helical rigid-rod poly(quinoxaline-2,3-diyl)s

Optically pure, helical poly(quinoxaline-2,3-diyl)s (degree of polymerization, n=10–100), prepared by asymmetric living polymerization of 1,2-diisocyano-3,6-dimethyl-4,5-di(propoxymethyl)benzene with an optically pure organopalladium initiator, formed stable monolayers on a water surface. As the polymerization degree increased, more closely packed monolayers were formed, as indicated by surface pressure-area (π-A) isotherms. Up to 54 monolayers were deposited onto the solid support using the Langmuir–Blodgett (LB) technique. Polarized ultraviolet spectra of LB films showed that the helical axis was orientated parallel to the dipping direction. This optical property indicates that polymer molecules are organized anisotropically in LB films. Optically pure and helical poly(quinoxaline-2,3-diyl)s prepared by asymmetric living polymerization of 1,2-diisocyano-3,6-dimethyl-4,5-di(propoxymethyl)ben zene with an optically pure organopalladium initiator were found to take highly ordered and stable monolayer formation at the air/water interface, providing well-defined multilayer structures with 1.5 nm monolayer thickness.

Double spiral inductors using CoFeSiO/SiO2 multilayer granular films with very narrow ferromagnetic resonance bandwidth

In order to realize the integrated magnetic thin film devices, RF inductors have been fabricated using CoFeSiO/SiO2 multilayer granular film and copper spiral coil. Multilayer granular magnetic films which consist of alternate stacks of nano-sized granular and insulator layers exhibit very sharp ferromagnetic resonant (FMR) peak. Granular and insulator layers of 6 nm and 1 nm are suitable for obtaining a very narrow FMR linewidth. High-resolution transmission electron microscopy (HR-TEM) observation revealed a well-defined multilayer granular structure and a homogeneous CoFe grain size, which seem to be necessary for realizing a very narrow FMR linewidth. There is no thermal deterioration in frequency dependence of permeability of CoFeSiO/SiO2 films at the temperature range below 350°C. The fabricated inductor had a size of about 670 × 670 μm2, including a 3μm thick copper spiral coil, and a magnetic film with three different thicknesses of 100 nm, 200 nm and 500 nm. The slit patterned magnetic film and the division of conductor line are appeared to be useful for reducing a loss resistance of fabricated inductors.

CoFeSiO/SiO$_{2}$ Multilayer Granular Films With Very Narrow Ferromagnetic Resonant Linewidth

In order to fabricate GHz-band micro-magnetic devices, we have investigated the high-frequency permeability of CoFeSiO/SiO2 multilayer granular films comprising alternate stacks of granular and insulator layers, which can enable us to control the magnetic grain size and intergrain spacing along the film thickness direction independent of each other. A very narrow ferromagnetic resonance (FMR) linewidth was obtained under specific film conditions. A sharp FMR peak was obtained for granular layer and insulator layer thicknesses of 6 and 1 nm, respectively. The CoFe/SiO2 volume ratio of the granular layer has a strong influence on the FMR linewidth. A high CoFe/SiO2 volume ratio resulted in a very narrow FMR linewidth. HR-TEM observation of a film with a high CoFe/SiO2 volume ratio revealed a well-defined multilayer granular structure and a homogeneous CoFe grain size, which seem to be necessary for obtaining a very narrow FMR linewidth. The narrowest FMR linewidth observed in this study is 420 MHz, for an FMR frequency of 2.4 GHz, a permeability of 380, and a damping factor alpha of 0.007.