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Carbon nanotube (CNT)-based transparent conducting films (TCFs) have been prepared by filtration of (i) surfactant-based aqueous dispersions and (ii) organic solutions obtained by reductive dissolution of an alkali metal salt of polyelectrolyte nanotubes. Starting from the same source of nanotubes, it is shown that films obtained by the reductive dissolution route present up to one order of magnitude better conductivity for the same transmittance. Light scattering experiments show that the average CNT length is much larger for the reductive dissolution-based organic solutions than for the sonication aided aqueous dispersions. Values of surface resistivity of 200 ohm per square have been obtained for 80% transmittance. Additionally, it is shown that the CNT-based TCFs are undistinguishable from indium tin oxide (ITO) as electrodes in regular environments, whereas they perform efficiently in acidic environments where ITO fails.
One of the challenges in developing a low thermal conductivity material addresses on searching lightweight ceramic without heavy or rare-earth (RE) elements. Mg2Al4Si5O18 interests us for its very low density and complex crystal structure. The first-principle calculations were performed to predict mechanical and lattice thermal conductivity of hexagonal and orthorhombic phases of Mg2Al4Si5O18. According to Debye approximation and the Slack model, the lattice thermal conductivity varies with temperature in 804.6/T and 719.7/T, yielding 2.95 and 2.64 W/(m·K) at room temperature, respectively. The high temperature limits of thermal conductivities are as low as 1.33 and 1.29 W/(m·K). The thermal conductivities of both polymorphs of Mg2Al4Si5O18 are lower than most of RE-containing silicates and zirconates. The present work suggests that Mg2Al4Si5O18 is a promising lightweight ceramic with extremely low thermal conductivity. We also highlight that enhancing complexity of the crystal structure rather than incorporating heavy RE elements may be an alternative wisdom to explore lightweight thermal insulators.
We report on the formation of highly flexible and transparent TiO2/Ag/ITO multilayer films deposited on polyethylene terephthalate substrates. The optical and electrical properties of the multilayer films were investigated as a function of oxide thickness. The transmission window gradually shifted toward lower energies with increasing oxide thickness. The TiO2 (40 nm)/Ag (18 nm)/ITO (40 nm) films gave the transmittance of 93.1% at 560 nm. The relationship between transmittance and oxide thickness was simulated using the scattering matrix method to understand high transmittance. As the oxide thickness increased from 20 to 50 nm, the carrier concentration gradually decreased from 1.08 × 1022 to 6.66 × 1021 cm−3, while the sheet resistance varied from 5.8 to 6.1 Ω/sq. Haacke's figure of merit reached a maximum at 40 nm and then decreased with increasing oxide thickness. The change in resistance for the 60 nm-thick ITO single film rapidly increased with increasing bending cycles, while that of the TiO2/Ag/ITO (40 nm/18 nm/40 nm) film remained virtually unchanged during the bending test.
The effects of an axial high magnetic field on the growth of the α-Al dendrites and the alignment of the iron-intermetallics (β-AlSiFe phases) in directionally solidified Al–7 wt% Si and Al–7 wt% Si–1 wt% Fe alloys were investigated experimentally. The results showed that the application of a high magnetic field changed the α-Al dendrite morphology significantly. Indeed, a high magnetic field caused the deformation of the α-Al dendrites and induced the occurrence of the columnar-to-equiaxed transition (CET). It was also found that a high magnetic field was capable of aligning the β-AlSiFe phases with the <001>-crystal direction along the solidification direction. Further, the Seebeck thermoelectric signal at the liquid/solid interface in the Al–7 wt% Si alloys was measured in situ and the results indicated that the value of the Seebeck signal was of the order of 10 µV. The modification of the α-Al dendrite morphology under the magnetic field should be attributed to the thermoelectric magnetic force acting on the α-Al dendrites. The magnetization force may be responsible for the alignment of the β-AlSiFe phases under the magnetic field.
Reducing the delay of backend interconnects is critical in delivering improved performance in next generation computer chips. One option is to implement interlayer dielectric (ILD) materials with increasingly lower dielectric constant (k) values. Despite industry need, there has been a recent decrease in study and production of these materials in academia and business communities. We have generated a backbone and porogen system that allows us to control porosity from 0 to 60% volume, achieve k-values from 3.4 to 1.6, maintain high chemical stability to various wet cleans, and deliver uniquely high mechanical strength at a given porosity. Finite element modeling and experimental results demonstrate that further improvements can be achieved through control of the pore volume into an ordered network. With hopes to spur more materials development, this paper discusses some molecular design and nanoscale hierarchical principles relevant to making next generation low-k ILD materials.
Characterization and Modeling of Radiation Damage on Materials: State of the Art, Challenges, and Protocols
Grain boundary (GB) segregation can markedly improve the stability of nanostructured alloys, where the fraction of GB sites is inherently large. Here, we explore the concept of entropically supported GB segregation in alloys with a tendency to phase-separate and its role in stabilizing nanostructures therein. These duplex nanocrystalline alloys are notably different, both in a structural and thermodynamic sense, from the previously studied “classical” nanocrystalline alloys, which are solid solutions with GB segregation of solute. Experiments are conducted on the W–Cr system, in which nanoduplex structures are expected. Upon heating ball-milled W–15 at.% Cr up to 950 °C, a nanoscale Cr-rich phase was found along the GBs. These precipitates mostly dissolved into the W-rich grains leaving behind Cr-enriched GBs upon further heating to 1400 °C. The presence of Cr-rich nanoprecipitates and GB segregation of Cr is in line with prediction from our Monte Carlo simulation when GB states are incorporated into the alloy thermodynamics.
To optimize the structure of the flexible piezoresistive sensor based on conductive polymer composite and widen the workable pressure range, a piezoresistive sensor with a multilayered structure based on carbon nanotubes/carbon black/silicone rubber conductive composite was designed and investigated. Different from the traditional monolayer structure, this novel multilayered sensor consisted of three microstructured piezoresistive composite films. The experimental data showed that the electrical resistance of the sensor varied regularly with a wide range of applied pressure (0–1.8 MPa at least). The high sensitivity, high flexibility, facile fabrication, and low cost were also the advantages of this pressure sensor. In addition, the piezoresistive mechanism was studied and shown to be the synergistic effects of the contact resistance mechanism and bulk resistance mechanism. Factors influencing the piezoresistive properties were also investigated. Moreover, the consecutive loading tests verified the feasibility and stability to use this sensor element for pressure measurement.
A nanoscale and pure, olivine-structured LiFePO4 was synthesized at ∼300 °C using an organic–inorganic steric entrapment method. Normally, when calcined and crystallized in air, this method leads to the synthesis of compounds where the cations are in their highest oxidation state. However, in this study, we found a way to produce compounds having lower oxidation states (e.g., compounds containing Fe2+), which may have wider applications in the synthesis of other compounds with complex chemistry that have variable oxidation states and, therefore, potential applications in electronic ceramics. The resulting LiFePO4 or (Li2O·2FeO·P2O5) was characterized by thermogravimetric analysis/differential thermal analysis, x-ray diffractometry, scanning electron microscopy, transmission electron microscopy, inductively coupled plasma emission spectroscopy, specific surface area by Brunauer–Emmett–Teller nitrogen absorption, and particle size analysis.
Responsive, biocompatible substrates are of interest for directing the maturation and function of cells in vitro during cell culture. This can potentially provide cells and tissues with desirable properties for regenerative therapies. Here, we demonstrate a straightforward and scalable approach to attach, align, and dynamically load cardiomyocytes on responsive liquid crystal elastomer (LCE) substrates. Monodomain LCEs exhibit reversible shape changes in response to cyclic heating, and when immersed in an aqueous medium on top of resistive heaters, shape changes are fast, reversible, and produce minimal temperature changes in the surroundings. We systematically characterized the strain response of LCEs in water and demonstrated the attachment and alignment of neonatal rat ventricular myocytes on LCE substrates. Cardiomyocytes attached to both static and stimulated LCE substrates, and under cyclic stimulation, cardiomyocytes aligned along the primary direction of strain. This work demonstrates the potential of LCEs as stimuli-responsive substrates for dynamic cell culture.
As electronic devices are indispensable in many aspects of our lives today, their integration with unconventional surfaces is increasingly essential. Electronic devices which maintain their electrical properties upon stretching are desirable for various wearable applications. Stretchable devices demonstrated are conventionally fabricated using semiconductor processing techniques. In this study, we demonstrate stretchable electrodes, which are basic components of electrical circuits, using screen printing, a large area printing method. It provides a low cost and scalable method to fabricate large area stretchable devices. Despite the larger width and thickness of the electrodes which increases the stiffness of the material, stretchability beyond 40% is demonstrated, which is suitable for certain wearable applications. The stretchable electrodes are integrated with light emitting diodes (LEDs) to demonstrate a stretchable LED matrix. The large area LED matrices exhibit variable stretchability, depending on the LED areal coverage. This technique is expected to be applicable in the fabrication of other stretchable, large area, and more complex electronic systems.
Single wall carbon nanotubes (SWCNTs) and liquid-phase exfoliated multilayer graphene (MLG) material thin films were assembled at a polarizable organic/water interface. A simple, spontaneous route to functionalize/decorate the interfacial assembly of MLG and SWCNTs with noble metal nanoparticles, at the interface between two immiscible electrolyte solutions (ITIES), is reported. The formation of MLG- or SWCNT-based metal nanocomposites was confirmed using various microscopic (scanning electron, transmission electron, and atomic force microscopy) and several spectroscopic (energy dispersive x-ray and Raman spectroscopy) techniques. Increasing the interfacial deposition time of the metal nanoparticles on the assembled low-dimensional carbon material increased the amount of the metal particles/structures, resulting in greater coverage of the MLG or SWCNTs with metal nanoparticles. This low-cost and convenient solution chemistry based impregnation method can serve as a means to prepare nanoscale carbonaceous material-based metal nanocomposites for their potential exploitation as electro-active materials, e.g., new generation catalysts or electrode materials.
Optical properties of Si nanowire (SiNW) arrays prepared on p-doped Si(111) and Si(100) substrates were studied. SiNWs were synthesized by self-assembly electroless metal deposition nanoelectrochemistry in an ionic silver HF solution through selective etching. Total reflectance (Rt) and total diffuse reflectance (Rdt) of SiNWs change drastically in comparison to polished Si. To understand these changes, diffuse reflectance (Rd) with polarized incident light was studied. For samples prepared on Si(111), the wave length integrated Rd (wIRd) shows maxima at certain angles of incidence θ, regardless of the incident light polarization. For samples prepared on Si(100), wIRd increases with θ and depends on incident light polarization. Also, Rd spectra show structures due to interference effects. Therefore, SiNWs prepared on Si(100) can be considered as thin films whose refractive index depends on light polarization. Moreover, Rdt of SiNWs prepared on Si(111) can be modeled as an ensemble of diffuse reflectors.
Two-dimensional (2D) materials, such as graphene, hexagonal boron nitride, and molybdenum sulfide (MoS2), have attracted considerable interest from the academia and industry because of their extraordinary properties. With the remarkable development of transmission electron microscope (TEM), nanolabs can be established inside the TEM to simulate a real environment by introducing external fields, such as electron irradiation, thermal excitation, electrical field, and mechanical force, into the system. In consequence, besides static structural characterization, in situ TEM can also realize dynamic observation of the evolution in structures and properties of 2D materials. This extension promises an enormous potential for manipulating and engineering 2D materials at the atomic scale with desired structures and properties for future applications. In this study, we review the recent progress of in situ electron microscopy studies of 2D materials, including atomic resolution characterization, in situ growth, nanofabrication, and property characterization.
We demonstrate a resonant Bragg structure formed by quasi-two-dimensional excitons in periodic systems of InGaN quantum wells (QWs) separated by GaN barriers. When the Bragg resonance and exciton–polariton resonance are tuned to each other, the medium exhibits an exciton-mediated resonantly enhanced optical Bragg reflection. The enhancement factor appeared to be largest for the system of 60 QWs. Owing to a high binding energy and oscillator strength of the excitons in InGaN QWs, the resonant enhancement was achieved at room temperature. The samples were grown by the metal–organic vapor-phase epitaxy (MOVPE) on GaN-on-sapphire templates. The most important technological problem of the developed structures is inhomogeneous broadening of the excitonic states due to nonuniform chemical composition of the QWs driven by InN–GaN phase separation trend. We addressed this problem by variation of the vapor pressure, growth rate, growth interactions, and admixing of hydrogen during the MOVPE. The lowest width of 74 meV at room temperature and 41 meV at 77 K was achieved for the excitonic emission line from a single InGaN QW.
Phase identification is an arduous task during high-throughput processing experiments, which can be exacerbated by the need to reconcile results from multiple measurement techniques to form a holistic understanding of phase dynamics. Here, we demonstrate AutoPhase, a machine learning algorithm, which can identify the presence of the different phases in spectral and diffraction data. The algorithm uses training data to determine the characteristic features of each phase present and then uses these features to evaluate new spectral and diffraction data. AutoPhase was used to identify oxide phase growth during a high-throughput oxidation study of NiAl bond coats that used x-ray diffraction, Raman, and fluorescence spectroscopic techniques. The algorithm had a minimum overall accuracy of 88.9% for unprocessed data and 98.4% for postprocessed data. Although the features selected by AutoPhase for phase attribution were distinct from those of topical experts, these results show that AutoPhase can substantially increase the throughput high-throughput data analysis.
This article features the importance of nanomaterial–protein interfaces, with a special interest on two-dimensional (2D) nanomaterials, for next generation sensors and electronics. Graphene, the first isolated and studied 2D nanomaterial, is taken as the material of most interest and then focused on its engineering by heteroatom doping. The success of graphene engineering for sensors widened the search for better and efficient biosensor platforms of other layered materials such as boron nitride and transition metal dichalcogenides. But functionalization of 2D backbones with biomolecules often ends up with the disruption of the biological activities due to various reasons. This has to be fundamentally studied and corrected for the clinical implementation of these materials based novel sensing platforms in point-of-care devices and micro-fluidic chips. At the end, importance of various 2D materials–biomolecule interfaces is discussed, and MoS2 based label-free biosensor is highlighted. A method for the modification of MoS2–biomolecule interaction via covalent functionalization of oxygen functionalities in MoS2 is also proposed.