Investigation of mechanical properties and structural integrity of graphene aerogels via molecular dynamics simulations.

Graphene aerogel (GA), a 3D carbon-based nanostructure built on 2D graphene sheets, is well known for being the lightest solid material ever synthesized. It also possesses many other exceptional properties, such as high specific surface area and large liquid absorption capacity, thanks to its ultra-high porosity. Computationally, the mechanical properties of GA have been studied by molecular dynamics (MD) simulations, which uncover nanoscale mechanisms beyond experimental observations. However, studies on how GA structures and properties evolve in response to simulation parameter changes, which provide valuable insights to experimentalists, have been lacking. In addition, the differences between the calculated properties via simulations and experimental measurements have rarely been discussed. To address the shortcomings mentioned above, in this study, we systematically study various mechanical properties and the structural integrity of GA as a function of a wide range of simulation parameters. Results show that during the in silico GA preparation, smaller and less spherical inclusions (mimicking the effect of water clusters in experiments) are conducive to strength and stiffness but may lead to brittleness. Additionally, it is revealed that a structurally valid GA in the MD simulation requires the number of bonds per atom to be at least 1.40, otherwise the GA building blocks are not fully interconnected. Finally, our calculation results are compared with experiments to showcase both the power and the limitations of the simulation technique. This work may shed light on the improvement of computational approaches for GA as well as other novel nanomaterials.

Cobalt Oxide-Decorated Silicon Carbide Nano-Tree Array Electrode for Micro-Supercapacitor Application.

A cobalt oxide (Co3O4)-decorated silicon carbide (SiC) nano-tree array (denoted as Co3O4/SiC NTA) electrode is synthesized, and it is investigated for use in micro-supercapacitor applications. Firstly, the well-standing SiC nanowires (NWs) are prepared by nickel (Ni)-catalyzed chemical vapor deposition (CVD) method, and then the thin layer of Co3O4 and the hierarchical Co3O4 nano-flower-clusters are, respectively, fabricated on the side-walls and the top side of the SiC NWs via electrodeposition. The deposition of Co3O4 on the SiC NWs benefits the charge transfer at the electrode/aqueous electrolyte interface due to its extremely hydrophilic surface characteristic after Co3O4 decoration. Furthermore, the Co3O4/SiC NTA electrode provides a directional charge transport route along the length of SiC nanowires owing to their well-standing architecture. By using the Co3O4/SiC NTA electrode for micro-supercapacitor application, the areal capacitance obtained from cyclic voltammetry measurement reaches 845 mF cm-2 at a 10 mV s-1 scan rate. Finally, the capacitance durability is also evaluated by the cycling test of cyclic voltammetry at a high scan rate of 150 mV s-1 for 2000 cycles, exhibiting excellent stability.

Mutanofactin promotes adhesion and biofilm formation of cariogenic Streptococcus mutans.

Cariogenic Streptococcus mutans is known as a predominant etiological agent of dental caries due to its exceptional capacity to form biofilms. From strains of S. mutans isolated from dental plaque, we discovered, in the present study, a polyketide/nonribosomal peptide biosynthetic gene cluster, muf, which directly correlates with a strong biofilm-forming capability. We then identified the muf-associated bioactive product, mutanofactin-697, which contains a new molecular scaffold, along with its biosynthetic logic. Further mode-of-action studies revealed that mutanofactin-697 binds to S. mutans cells and also extracellular DNA, increases bacterial hydrophobicity, and promotes bacterial adhesion and subsequent biofilm formation. Our findings provided an example of a microbial secondary metabolite promoting biofilm formation via a physicochemical approach, highlighting the importance of secondary metabolism in mediating critical processes related to the development of dental caries.

In situ formation of metal-organic framework derived CuO polyhedrons on carbon cloth for highly sensitive non-enzymatic glucose sensing.

Metal-organic frameworks (MOFs) are considered promising templates for the fabrication of nanostructured materials with high porosities and high surface areas, which are important parameters for enhanced performance in sensing applications. Here, a facile in situ synthetic strategy to construct MOF-derived porous CuO polyhedrons on carbon cloth (CC) is reported. Uniform Cu(OH)2 nanorods are first synthesized on carbon cloth, followed by the conversion of Cu(OH)2 nanorods into porous CuO polyhedrons via a copper-based MOF, Cu-BTC, as the intermediate species. When evaluated as a glucose sensing electrode, the as-fabricated CuO polyhedrons/CC composite exhibits a high sensitivity of 13 575 µA mM-1 cm-2 with a fast response time (t90) of 2.3 s and a low detection limit of 0.46 µM. This work exemplifies the rational fabrication of porous nanostructures on conductive substrates for enhanced performance in glucose detection.

Transistor-Based Work-Function Measurement of Metal-Organic Frameworks for Ultra-Low-Power, Rationally Designed Chemical Sensors.

A classic challenge in chemical sensing is selectivity. Metal-organic frameworks (MOFs) are an exciting class of materials because they can be tuned for selective chemical adsorption. Adsorption events trigger work-function shifts, which can be detected with a chemical-sensitive field-effect transistor (power ≈microwatts). In this work, several case studies were used towards generalizing the sensing mechanism, ultimately towards our metal-centric hypothesis. HKUST-1 was used as a proof-of-principle humidity sensor. The response is thickness independent, meaning the response is surface localized. ZIF-8 is demonstrated to be an NO2 -sensing material, and the response is dominated by adsorption at metal sites. Finally, MFM-300(In) shows how standard hard-soft acid-base theory can be used to qualitatively predict sensor responses. This paper sets the groundwork for using the tunability of metal-organic frameworks for chemical sensing with distributed, scalable devices.

Synthesis and Electrochemical Stability of Ultrahigh Aspect Ratio Nanoporous Gold after Calixarene-Phosphine Ligand Removal.

Leveraging our previous report on the synthesis of calixarene-capped ultrahigh aspect-ratio nanoporous gold, we now report a new class of nanoporous gold comprising removed calixarene-phosphine ligands using UV/ozone treatment. The removal of the calixarene ligands by this treatment is supported by XPS measurements. TEM further shows the extraordinary stability of the ∼1 nm building blocks comprising the nanoporous gold wall after UV/ozone treatment and subsequent strongly reducing electrochemical environments. Sensing of nitrobenzene is used as a method of characterization to show that the surface chemistry of the nanoporous gold assemblies has radically changed after the UV/ozone treatment.

Casting Nanoporous Platinum in Metal-Organic Frameworks.

Nanocasting based on porous templates is a powerful strategy in accessing materials and structures that are difficult to form by bottom-up syntheses in a controlled fashion. A facile synthetic strategy for casting ordered, nanoporous platinum (NP-Pt) networks with a high degree of control by using metal-organic frameworks (MOFs) as templates is reported here. The Pt precursor is first infiltrated into zirconium-based MOFs and subsequently transformed to 3D metallic networks via a chemical reduction process. It is demonstrated that the dimensions and topologies of the cast NP-Pt networks can be accurately controlled by using different MOFs as templates. The Brunauer-Emmett-Teller surface areas of the NP-Pt networks are estimated to be >100 m2 g-1 and they exhibit excellent catalytic activities in the methanol electrooxidation reaction (MEOR). This new methodology presents an attractive route to prepare well-defined nanoporous materials for diverse applications ranging from energy to sensing and biotechnology.

Direct Organization of Morphology-Controllable Mesoporous SnO2 Using Amphiphilic Graft Copolymer for Gas-Sensing Applications.

A simple and flexible strategy for controlled synthesis of mesoporous metal oxide films using an amphiphilic graft copolymer as sacrificial template is presented and the effectiveness of this approach for gas-sensing applications is reported. The amphiphilic graft copolymer poly(vinyl chloride)-g-poly(oxyethylene methacrylate) (PVC-g-POEM) is used as a sacrificial template for the direct synthesis of mesoporous SnO2. The graft copolymer self-assembly is shown to enable good control over the morphology of the resulting SnO2 layer. Using this approach, mesoporous SnO2 based sensors with varied porosity are fabricated in situ on a microheater platform. This method reduces the interfacial contact resistance between the chemically sensitive materials and the microheater, while a simple fabrication process is provided. The sensors show significantly different gas-sensing performances depending on the SnO2 porosity, with the highly mesoporous SnO2 sensor exhibiting high sensitivity, low detection limit, and fast response and recovery toward hydrogen gas. This printable solution-based method can be used reproducibly to fabricate a variety of mesoporous metal oxide layers with tunable morphologies on various substrates for high-performance applications.

Aluminum-induced dreierketten chain cross-links increase the mechanical properties of nanocrystalline calcium aluminosilicate hydrate.

The incorporation of Al and increased curing temperature promotes the crystallization and cross-linking of calcium (alumino)silicate hydrate (C-(A-)S-H), which is the primary binding phase in most contemporary concrete materials. However, the influence of Al-induced structural changes on the mechanical properties at atomistic scale is not well understood. Herein, synchrotron radiation-based high-pressure X-ray diffraction is used to quantify the influence of dreierketten chain cross-linking on the anisotropic mechanical behavior of C-(A-)S-H. We show that the ab-planar stiffness is independent of dreierketten chain defects, e.g. vacancies in bridging tetrahedra sites and Al for Si substitution. The c-axis of non-cross-linked C-(A-)S-H is more deformable due to the softer interlayer opening but stiffens with decreased spacing and/or increased zeolitic water and Ca2+ of the interlayer. Dreierketten chain cross-links act as 'columns' to resist compression, thus increasing the bulk modulus of C-(A-)S-H. We provide the first experimental evidence on the influence of the Al-induced atomistic configurational change on the mechanical properties of C-(A-)S-H. Our work advances the fundamental knowledge of C-(A-)S-H on the lowest level of its hierarchical structure, and thus can impact the way that innovative C-(A-)S-H-based cementitious materials are developed using a 'bottom-up' approach.

High Speed Epitaxial Perovskite Memory on Flexible Substrates.

Single-crystal perovskite ferroelectric material is integrated at room temperature on a flexible substrate by the layer transfer technique. Two terminal memory devices fabricated with these materials exhibit faster switching speed, lower operating voltage, and superior endurance than other existing flexible counterparts. The research provides an avenue toward combining the rich functionality of charge and spin states, offered by the general class of complex oxides, onto a flexible platform.

In Situ Localized Growth of Ordered Metal Oxide Hollow Sphere Array on Microheater Platform for Sensitive, Ultra-Fast Gas Sensing.

A simple and versatile strategy is presented for the localized on-chip synthesis of an ordered metal oxide hollow sphere array directly on a low power microheater platform to form a closely integrated miniaturized gas sensor. Selective microheater surface modification through fluorinated monolayer self-assembly and its subsequent microheater-induced thermal decomposition enables the position-controlled deposition of an ordered two-dimensional colloidal sphere array, which serves as a sacrificial template for metal oxide growth via homogeneous chemical precipitation; this strategy ensures control in both the morphology and placement of the sensing material on only the active heated area of the microheater platform, providing a major advantage over other methods of presynthesized nanomaterial integration via suspension coating or printing. A fabricated tin oxide hollow sphere-based sensor shows high sensitivity (6.5 ppb detection limit) and selectivity toward formaldehyde, and extremely fast response (1.8 s) and recovery (5.4 s) times. This flexible and scalable method can be used to fabricate high performance miniaturized gas sensors with a variety of hollow nanostructured metal oxides for a range of applications, including combining multiple metal oxides for superior sensitivity and tunable selectivity.

Nanowire-Assembled Hierarchical ZnCo2O4 Microstructure Integrated with a Low-Power Microheater for Highly Sensitive Formaldehyde Detection.

Nanowire-assembled 3D hierarchical ZnCo2O4 microstructure is synthesized by a facile hydrothermal route and a subsequent annealing process. In comparison to simple nanowires, the resulting dandelion-like structure yields more open spaces between nanowires, which allow for better gas diffusion and provide more active sites for gas adsorption while maintaining good electrical conductivity. The hierarchical ZnCo2O4 microstructure is integrated on a low-power microheater platform without using binders or conductive additives. The hierarchical structure of the ZnCo2O4 sensing material provides reliable electrical connection across the sensing electrodes. The resulting sensor exhibits an ultralow detection limit of 3 ppb toward formaldehyde with fast response and recovery as well as good selectivity to CO, H2, and hydrocarbons such as n-pentane, propane, and CH4. The sensor only consumes ∼5.7 mW for continuous operation at 300 °C with good long-term stability. The excellent sensing performance of this hierarchical structure based sensor suggests the advantages of combining such structures with microfabricated heaters for practical low-power sensing applications.

3D Stretchable Arch Ribbon Array Fabricated via Grayscale Lithography.

Microstructures with flexible and stretchable properties display tremendous potential applications including integrated systems, wearable devices and bio-sensor electronics. Hence, it is essential to develop an effective method for fabricating curvilinear and flexural microstructures. Despite significant advances in 2D stretchable inorganic structures, large scale fabrication of unique 3D microstructures at a low cost remains challenging. Here, we demonstrate that the 3D microstructures can be achieved by grayscale lithography to produce a curved photoresist (PR) template, where the PR acts as sacrificial layer to form wavelike arched structures. Using plasma-enhanced chemical vapor deposition (PECVD) process at low temperature, the curved PR topography can be transferred to the silicon dioxide layer. Subsequently, plasma etching can be used to fabricate the arched stripe arrays. The wavelike silicon dioxide arch microstructure exhibits Young modulus and fracture strength of 52 GPa and 300 MPa, respectively. The model of stress distribution inside the microstructure was also established, which compares well with the experimental results. This approach of fabricating a wavelike arch structure may become a promising route to produce a variety of stretchable sensors, actuators and circuits, thus providing unique opportunities for emerging classes of robust 3D integrated systems.

General Thermal Texturization Process of MoS2 for Efficient Electrocatalytic Hydrogen Evolution Reaction.

Molybdenum disulfide (MoS2) has been widely examined as a catalyst containing no precious metals for the hydrogen evolution reaction (HER); however, these examinations have utilized synthesized MoS2 because the pristine MoS2 mineral is known to be a poor catalyst. The fundamental challenge with pristine MoS2 is the inert HER activity of the predominant (0001) basal surface plane. In order to achieve high HER performance with pristine MoS2, it is essential to activate the basal plane. Here, we report a general thermal process in which the basal plane is texturized to increase the density of HER-active edge sites. This texturization is achieved through a simple thermal annealing procedure in a hydrogen environment, removing sulfur from the MoS2 surface to form edge sites. As a result, the process generates high HER catalytic performance in pristine MoS2 across various morphologies such as the bulk mineral, films composed of micron-scale flakes, and even films of a commercially available spray of nanoflake MoS2. The lowest overpotential (η) observed for these samples was η = 170 mV to obtain 10 mA/cm(2) of HER current density.

Air-Stable n-Doping of WSe2 by Anion Vacancy Formation with Mild Plasma Treatment.

Transition metal dichalcogenides (TMDCs) have been extensively explored for applications in electronic and optoelectronic devices due to their unique material properties. However, the presence of large contact resistances is still a fundamental challenge in the field. In this work, we study defect engineering by using a mild plasma treatment (He or H2) as an approach to reduce the contact resistance to WSe2. Material characterization by X-ray photoelectron spectroscopy, photoluminescence, and Kelvin probe force microscopy confirm defect-induced n-doping, up to degenerate level, which is attributed to the creation of anion (Se) vacancies. The plasma treatment is adopted in the fabrication process flow of WSe2 n-type metal-oxide-semiconductor field-effect transistors to selectively create anion vacancies at the metal contact regions. Due to lowering the metal contact resistance, improvements in the device performance metrics such as a 20× improvement in ON current and a nearly ideal subthreshold swing value of 66 mV/dec are observed. This work demonstrates that defect engineering at the contact regions can be utilized as a reliable scheme to realize high-performance electronic and optoelectronic TMDC devices.

High-Temperature All Solid-State Microsupercapacitors based on SiC Nanowire Electrode and YSZ Electrolyte.

We demonstrate a symmetric supercapacitor by using yttria-stabilized zirconia (YSZ) as the electrolyte and silicon carbide nanowires (SiC NWs) as the electrode. The stacked symmetric SiC NWs/YSZ/SiC NWs supercapacitors exhibit excellent thermal stability and high areal capacitance at temperatures above 300 °C. The supercapacitor functions well at a record high temperature of 450 °C, yielding an areal capacitance of 92 µF cm(-2) at a voltage scan rate of 100 mV s(-1). At this temperature, it is also capable of withstanding current densities up to 50 µA cm(-2), yielding a maximum areal power density of 100 µW cm(-2). Good cycling stability is demonstrated with a capacitance retention of over 60% after 10,000 cycles at the operation temperature of 450 °C and a scan rate of 200 mV s(-1).

Tuning the Friction Characteristics of Gecko-Inspired Polydimethylsiloxane Micropillar Arrays by Embedding Fe3O4 and SiO2 Particles.

In order to improve stiffness of polydimethylsiloxane (PDMS) pillars while maintaining high friction, the effects of embedding Fe3O4 and SiO2 particles on the friction behavior of PDMS micropillars are studied. Both types of added particles increase the stiffness of the PDMS composite, but affect the friction behavior differently. The frictional force of the fibrillar array fabricated with Fe3O4/PDMS composite decreases initially, then increases as the particle content increases. For silica/PDMS composite pillars, the frictional force is independent of the particle density. Characterization by scanning electron microscopy shows that Fe3O4 particles are distributed uniformly in the PDMS matrix at low concentration, but heterogeneous distribution is observed at high particle loading, with particles being hindered from penetrating into the pillars. For silica/PDMS composite pillars, the particles distribute homogeneously inside the pillars, which is attributed to the formation of hydrogen bonding between silica particles and PDMS. The difference in particle distribution behavior is used to explain the observed difference in the friction response of these two composite systems.