Addressing diffusion behavior and impact in an epoxy-amine cure system using molecular dynamics simulations.

To deepen understanding of diffusion-controlled crosslinking, molecular dynamics (MD) simulations are carried out by taking the diffusion image of 3,3'-diamino diphenyl sulfone (3,3'-DDS) and polyethersulfone (PES) with epoxy resin varying temperatures from 393.15 to 473.15 K over crosslinking conversion of 0-85%. The diffusion of PES and 3,3'-DDS into the bulk increased with increasing the temperature as a result of enhanced mobility of the molecules when the difference between the glass-transition temperature (Tg) and the curing temperature. Beyond the onset points of the converged crosslinking conversion ratio of 3,3'-DDS and PES, their diffusion properties are obviously restricted with crosslinking conversion ratio. At low crosslinking conversion ratios (> 10%), the diffusion coefficients of triglycidyl p-aminophenol (TGAP) were 1.1 times higher than those of diglycidyl ether of bisphenol F (DGEBF) because of the lower molecular weight of TGAP. On the other hand, the diffusion coefficients of TGAP decreased when the crosslinking ratio was up to ~ 60% because, compared with DGEBF, it had more functional groups available to react with the curing agent. At higher crosslinking ratios, the diffusion coefficients of both resins converged to zero as a result of their highly crosslinked structures.

High-Throughput Growth of Wafer-Scale Monolayer Transition Metal Dichalcogenide via Vertical Ostwald Ripening.

For practical device applications, monolayer transition metal dichalcogenide (TMD) films must meet key industry needs for batch processing, including the high-throughput, large-scale production of high-quality, spatially uniform materials, and reliable integration into devices. Here, high-throughput growth, completed in 12 min, of 6-inch wafer-scale monolayer MoS2 and WS2 is reported, which is directly compatible with scalable batch processing and device integration. Specifically, a pulsed metal-organic chemical vapor deposition process is developed, where periodic interruption of the precursor supply drives vertical Ostwald ripening, which prevents secondary nucleation despite high precursor concentrations. The as-grown TMD films show excellent spatial homogeneity and well-stitched grain boundaries, enabling facile transfer to various target substrates without degradation. Using these films, batch fabrication of high-performance field-effect transistor (FET) arrays in wafer-scale is demonstrated, and the FETs show remarkable uniformity. The high-throughput production and wafer-scale automatable transfer will facilitate the integration of TMDs into Si-complementary metal-oxide-semiconductor platforms.

Ultralow-dielectric-constant amorphous boron nitride.

Decrease in processing speed due to increased resistance and capacitance delay is a major obstacle for the down-scaling of electronics1-3. Minimizing the dimensions of interconnects (metal wires that connect different electronic components on a chip) is crucial for the miniaturization of devices. Interconnects are isolated from each other by non-conducting (dielectric) layers. So far, research has mostly focused on decreasing the resistance of scaled interconnects because integration of dielectrics using low-temperature deposition processes compatible with complementary metal-oxide-semiconductors is technically challenging. Interconnect isolation materials must have low relative dielectric constants (κ values), serve as diffusion barriers against the migration of metal into semiconductors, and be thermally, chemically and mechanically stable. Specifically, the International Roadmap for Devices and Systems recommends4 the development of dielectrics with κ values of less than 2 by 2028. Existing low-κ materials (such as silicon oxide derivatives, organic compounds and aerogels) have κ values greater than 2 and poor thermo-mechanical properties5. Here we report three-nanometre-thick amorphous boron nitride films with ultralow κ values of 1.78 and 1.16 (close to that of air, κ = 1) at operation frequencies of 100 kilohertz and 1 megahertz, respectively. The films are mechanically and electrically robust, with a breakdown strength of 7.3 megavolts per centimetre, which exceeds requirements. Cross-sectional imaging reveals that amorphous boron nitride prevents the diffusion of cobalt atoms into silicon under very harsh conditions, in contrast to reference barriers. Our results demonstrate that amorphous boron nitride has excellent low-κ dielectric characteristics for high-performance electronics.

Characteristics of ZrC Barrier Coating on SiC-Coated Carbon/Carbon Composite Developed by Thermal Spray Process.

A thick ZrC layer was successfully coated on top of a SiC buffer layer on carbon/carbon (C/C) composites by vacuum plasma spray (VPS) technology to improve the ablation resistance of the C/C composites. An optimal ZrC coating condition was determined by controlling the discharge current. The ZrC layers were more than 70 µm thick and were rapidly coated under all spraying conditions. The ablation resistance and the oxidation resistance of the coated layer were evaluated in supersonic flames at a temperature exceeding 2000 °C. The mass and linear ablation rate of the ZrC-coated C/C composites increased by 2.7% and 0.4%, respectively. During flame exposure, no recession was observed in the C/C composite. It was demonstrated that the ZrC coating layer can fully protect the C/C composites from oxidation and ablation.

High-performance and scalable metal-chalcogenide semiconductors and devices via chalco-gel routes.

We report a general strategy for obtaining high-quality, large-area metal-chalcogenide semiconductor films from precursors combining chelated metal salts with chalcoureas or chalcoamides. Using conventional organic solvents, such precursors enable the expeditious formation of chalco-gels, which are easily transformed into the corresponding high-performance metal-chalcogenide thin films with large, uniform areas. Diverse metal chalcogenides and their alloys (MQ x : M = Zn, Cd, In, Sb, Pb; Q = S, Se, Te) are successfully synthesized at relatively low processing temperatures (<400°C). The versatility of this scalable route is demonstrated by the fabrication of large-area thin-film transistors (TFTs), optoelectronic devices, and integrated circuits on a 4-inch Si wafer and 2.5-inch borosilicate glass substrates in ambient air using CdS, CdSe, and In2Se3 active layers. The CdSe TFTs exhibit a maximum field-effect mobility greater than 300 cm2 V-1 s-1 with an on/off current ratio of >107 and good operational stability (threshold voltage shift < 0.5 V at a positive gate bias stress of 10 ks). In addition, metal chalcogenide-based phototransistors with a photodetectivity of >1013 Jones and seven-stage ring oscillators operating at a speed of ~2.6 MHz (propagation delay of < 27 ns per stage) are demonstrated.

Degradation by water vapor of hydrogenated amorphous silicon oxynitride films grown at low temperature.

We report on the degradation process by water vapor of hydrogenated amorphous silicon oxynitride (SiON:H) films deposited by plasma-enhanced chemical vapor deposition at low temperature. The stability of the films was investigated as a function of the oxygen content and deposition temperature. Degradation by defects such as pinholes was not observed with transmission electron microscopy. However, we observed that SiON:H film degrades by reacting with water vapor through only interstitial paths and nano-defects. To monitor the degradation process, the atomic composition, mass density, and fully oxidized thickness were measured by using high-resolution Rutherford backscattering spectroscopy and X-ray reflectometry. The film rapidly degraded above an oxygen composition of ~27 at%, below a deposition temperature of ~150 °C, and below an mass density of ~2.15 g/cm3. This trend can be explained by the extents of porosity and percolation channel based on the ring model of the network structure. In the case of a high oxygen composition or low temperature, the SiON:H film becomes more porous because the film consists of network channels of rings with a low energy barrier.

Direct evidence of flat band voltage shift for TiN/LaO or ZrO/SiO2 stack structure via work function depth profiling.

We demonstrated that a flat band voltage (VFB) shift could be controlled in TiN/(LaO or ZrO)/SiO2 stack structures. The VFB shift described in term of metal diffusion into the TiN film and silicate formation in the inserted (LaO or ZrO)/SiO2 interface layer. The metal doping and silicate formation confirmed by using transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) line profiling, respectively. The direct work function measurement technique allowed us to make direct estimate of a variety of flat band voltages (VFB). As a function of composition ratio of La or Zr to Ti in the region of a TiN/(LaO or ZrO)/SiO2/Si stack, direct work function modulation driven by La and Zr doping was confirmed with the work functions obtained from the cutoff value of secondary electron emission by auger electron spectroscopy (AES). We also suggested an analytical method to determine the interface dipole via work function depth profiling.

Chemically Homogeneous and Thermally Robust Ni1-xPtxSi Film Formed Under a Non-Equilibrium Melting/Quenching Condition.

To synthesize a thermally robust Ni1-xPtxSi film suitable for ultrashallow junctions in advanced metal-oxide-semiconductor field-effect transistors, we used a continuous laser beam to carry out millisecond annealing (MSA) on a preformed Ni-rich silicide film at a local surface temperature above 1000 °C while heating the substrate to initiate a phase transition. The melting and quenching process by this unique high-temperature MSA process formed a Ni1-xPtxSi film with homogeneous Pt distribution across the entire film thickness. After additional substantial thermal treatment up to 800 °C, the noble Ni1-xPtxSi film maintained a low-resistive phase without agglomeration and even exhibited interface flattening with the underlying Si substrate.

Defect visualization of Cu(InGa)(SeS)2 thin films using DLTS measurement.

Defect depth profiles of Cu (In1-x,Gax)(Se1-ySy)2 (CIGSS) were measured as functions of pulse width and voltage via deep-level transient spectroscopy (DLTS). Four defects were observed, i.e., electron traps of ~0.2 eV at 140 K (E1 trap) and 0.47 eV at 300 K (E2 trap) and hole traps of ~0.1 eV at 100 K (H1 trap) and ~0.4 eV at 250 K (H2 trap). The open circuit voltage (VOC) deteriorated when the trap densities of E2 were increased. The energy band diagrams of CIGSS were also obtained using Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and DLTS data. These results showed that the valence band was lowered at higher S content. In addition, it was found that the E2 defect influenced the VOC and could be interpreted as an extended defect. Defect depth profile images provided clear insight into the identification of defect state and density as a function of depth around the space charge region.

Self-arrangement of nanoparticles toward crystalline metal oxides with high surface areas and tunable 3D mesopores.

We demonstrate a new design concept where the interaction between silica nanoparticles (about 1.5 nm in diameter) with titania nanoparticles (anatase, about 4 nm or 6 nm in diameter) guides a successful formation of mesoporous titania with crystalline walls and controllable porosity. At an appropriate solution pH (~1.5, depending on the deprotonation tendencies of two types of nanoparticles), the smaller silica nanoparticles, which attach to the surface of the larger titania nanoparticles and provide a portion of inactive surface and reactive surface of titania nanoparticles, dictate the direction and the degree of condensation of the titania nanoparticles, resulting in a porous 3D framework. Further crystallization by a hydrothermal treatment and subsequent removal of silica nanoparticles result in a mesoporous titania with highly crystalline walls and tunable mesopore sizes. A simple control of the Si/Ti ratio verified the versatility of the present method through the successful control of mean pore diameter in the range of 2-35 nm and specific surface area in the ranges of 180-250 m(2) g(-1). The present synthesis method is successfully extended to other metal oxides, their mixed oxides and analogues with different particle sizes, regarding as a general method for mesoporous metal (or mixed metal) oxides.

Advanced hybrid supercapacitor based on a mesoporous niobium pentoxide/carbon as high-performance anode.

Recently, hybrid supercapacitors (HSCs), which combine the use of battery and supercapacitor, have been extensively studied in order to satisfy increasing demands for large energy density and high power capability in energy-storage devices. For this purpose, the requirement for anode materials that provide enhanced charge storage sites (high capacity) and accommodate fast charge transport (high rate capability) has increased. Herein, therefore, a preparation of nanocomposite as anode material is presented and an advanced HSC using it is thoroughly analyzed. The HSC comprises a mesoporous Nb2O5/carbon (m-Nb2O5-C) nanocomposite anode synthesized by a simple one-pot method using a block copolymer assisted self-assembly and commercial activated carbon (MSP-20) cathode under organic electrolyte. The m-Nb2O5-C anode provides high specific capacity with outstanding rate performance and cyclability, mainly stemming from its enhanced pseudocapacitive behavior through introduction of a carbon-coated mesostructure within a voltage range from 3.0 to 1.1 V (vs Li/Li(+)). The HSC using the m-Nb2O5-C anode and MSP-20 cathode exhibits excellent energy and power densities (74 W h kg(-1) and 18,510 W kg(-1)), with advanced cycle life (capacity retention: ∼90% at 1000 mA g(-1) after 1000 cycles) within potential range from 1.0 to 3.5 V. In particular, we note that the highest power density (18,510 W kg(-1)) of HSC is achieved at 15 W h kg(-1), which is the highest level among similar HSC systems previously reported. With further study, the HSCs developed in this work could be a next-generation energy-storage device, bridging the performance gap between conventional batteries and supercapacitors.

Simple fabrication of flexible electrodes with high metal-oxide content: electrospun reduced tungsten oxide/carbon nanofibers for lithium ion battery applications.

A one-step and mass-production synthetic route for a flexible reduced tungsten oxide-carbon composite nanofiber (WO(x)-C-NF) film is demonstrated via an electrospinning technique. The WO(x)-C-NF film exhibits unprecedented high content of metal-oxides (∼ 80 wt%) and good flexibility (the tensile strength of the specimen was 6.13 MPa) without the use of flexible support materials like CNTs or graphene. The WO(x)-C-NF film is directly used as an anode in a lithium ion battery (LIB). Compared with previously reported tungsten oxide electrodes, the WO(x)-C-NF film exhibits high reversible capacity (481 mA h g(-1)total electrode), stable cycle, and improved rate performance, without the use of additive carbon, a polymeric binder and a current collector. Moreover, control electrodes fabricated by conventional processes support the positive effects of both the freestanding electrode and metal-oxide embedded carbon 1-D nanofiber structure.

Lead-free BaTiO3 nanowires-based flexible nanocomposite generator.

We have synthesized BaTiO3 nanowires (NWs) via a simple hydrothermal method at low temperature and developed a lead-free, flexible nanocomposite generator (NCG) device by a simple, low-cost, and scalable spin-coating method. The hydrothermally grown BaTiO3 NWs are mixed in a polymer matrix without a toxic dispersion enhancer to produce a piezoelectric nanocomposite (p-NC). During periodical and regular bending and unbending motions, the NCG device fabricated by utilizing a BaTiO3 NWs-polydimethylsiloxane (PDMS) composite successfully harvests the output voltage of ∼ 7.0 V and current signals of ∼ 360 nA, which are utilized to drive a liquid crystal display (LCD). We also characterized the instantaneous power (∼ 1.2 µW) of the NCG device by calculating the load voltage and current through the connected external resistance.

Effects of interfacial layer on characteristics of TiN/ZrO2 structures.

To minimize the formation of unwanted interfacial layers, thin interfacial layer (ZrCN layer) was deposited between TiN bottom electrode and ZrO2 dielectric in TiN/ZrO2/TiN capacitor. Carbon and nitrogen were also involved in the layer because ZrCN layer was thermally deposited using TEMAZ without any reactant. Electrical characteristics of TiN/ZrO2/TiN capacitor were improved by insertion of ZrCN layer. The oxidation of TiN bottom electrode was largely inhibited at TiN/ZrCN/ZrO2 structure compared to TiN/ZrO2 structure. While the sheet resistance of TiN/ZrCN/ZrO2 structure was constantly sustained with increasing ZrO2 thickness, the large increase of sheet resistance was observed in TiN/ZrO2 structure after 6 nm ZrO2 deposition. When ZrO2 films were deposited on ZrCN layer, the deposition rate of ZrO2 also increased. It is believed that ZrCN layer acted both as a protection layer of TiN oxidation and a seed layer of ZrO2 growth.

Catalytic conversion of 1,2-dichlorobenzene over mesoporous materials from zeolite.

In this study, 1,2-dichlorobenzene (DCB), an important precursor of PCDDs and PCDFs, was chosen as a suitable model compound for the catalyzed deep oxidation of dioxin. The recently developed mesoporous materials from zeolites (MMZ) were used for the first time as a support for an oxidation catalyst. The catalytic oxidation of 1,2-dichlorobenzene over Pt/MMZ was carried out, and the catalytic activity was compared with that of Pt/gamma-Al2O3, Pt/Al-MCM-41 and Pt/Beta catalysts. Pt/MMZ showed the highest catalytic activity among the catalysts tested. Interestingly, the catalytic activity of Pt/MMZ was maintained (> 40%) at low temperatures (250 degrees C) at which the other catalysts showed extremely low activity (< 5%). The high catalytic activity of Pt/MMZ was attributed to both the sufficient acidity and mesoporosity of the MMZ support.

Application of ordered nanoporous silica for removal of uranium ions from aqueous solutions.

Ordered nanoporous silica (MSU-H) with high surface area has been utilized as a solid substrate of a surface-modified hybrid sorbent for the application to the removal of U(VI). Carboxymethylated polyethyleneimine (CMPEI) with a strong complexing property has been introduced to the pore surface of MSU-H substrate. CMPEI-modified MSU-H (CMPEI/MSU-H) has been characterized by scanning electron microscopy and nitrogen sorption. In a kinetic experiment for 12.5 ppm U(VI) solution at pH 4.0, 99% U(VI) was removed from solution by the hybrid sorbent within less than 10 min, indicating that the sorption of U(VI) on the CMPEI/MSU-H proceeds very rapidly. It was evident that a U(VI) sorption capacity increased with pH in the range of 2.0 to 4.0. The CMPEI/MSU-H showed a high sorption capacity of 153 mg/g-sorbent at pH 4.0. In particular, the CMPEI/MSU-H showed a significantly high uranium loading stability. Only about 1% U(VI) was released out of CMPEI/MSU-H during 4 months, when the CMPEI/MSU-H was treated with polyacrylic acid.

Low-temperature pseudomorphic transformation of ordered hierarchical macro-mesoporous SiO2/C nanocomposite to SiC via magnesiothermic reduction.

A magnesiothermic reduction synthesis approach is reported for direct conversion of SiO(2)/C composite nanostructures to corresponding SiC materials without losing their nanostructure morphologies. Crystalline SiC materials can be obtained by this approach at a temperature as low as 600 degrees C, only approximately half of that applied in the generally used carbothermal reduction and preceramic polymer pyrolysis methods. An ordered hierarchical macro-mesoporous SiC material was synthesized for the first time as a demonstration. This pseudomorphic transformation can be regarded as a general synthesis method for different kinds of SiC nanostructures, and it can also be readily extended to other metal carbide materials as well as TiC.

Periodic mesoporous organosilica with a hexagonally pillared lamellar structure.

Ordered mesoporous materials (OMMs) with well-defined pore sizes (>2 nm) and pore geometries are important in various applications that require fast mass transfer or deal with large molecules. Extensive research has resulted in the discovery of OMMs with three types of mesostructures: (i) bi- or multicontinuous, (ii) columnar, and (iii) discontinuous (cagelike). However, another type, the pillared lamellar structure, which has long been sought and has been mathematically computed and known to exist in the research fields of surfactant and multiblock-copolymer mesophases, still remains a mesostructure that has not been observed in real OMMs for any specific symmetry. Herein, we report an unprecedented type of ordered mesoporous material with a hexagonally pillared lamellar (HPL) structure (P6(3)/mmc) that can be synthesized via a phase transformation from a lamellar mesophase by hydrothermal reaction in the presence of an organosilica precursor and a high concentration of a designed Gemini surfactant (Gem(16-3-16)) that has a large g value. The present GMO-HPL, which has an unique three-dimensional periodic structure with two-dimensionally connected pore channels running between the framework layers, provides a fascinating topological link between the lamellar and columnar (2D hexagonal) mesophases. It is unique in its application potential by making possible selective 2D diffusion in different directions.