Improving Radio Frequency Transmission Properties of Graphene via Carrier Concentration Control toward High Frequency Transmission Line Applications

Graphene has been gradually studied as a high‐frequency transmission line material owing to high carrier mobility with frequency independence up to a few THz. However, the graphene‐based transmission lines have poor conductivity due to their low carrier concentration. Here, it is observed that the radio frequency (RF) transmission performance could be severely hampered by the defect‐induced scattering, even though the carrier concentration is increased. As a possible solution, the deposition of the amorphous carbon on the graphene is studied in the high‐frequency region up to 110 GHz. The DC resistance is reduced by as much as 60%, and the RF transmission property is also enhanced by 3 dB. Also, the amorphous carbon covered graphene shows stable performance under a harsh environment. These results prove that the carrier concentration control is an effective and a facile method to improve the transmission performance of graphene. It opens up the possibilities of using graphene as interconnects in the ultrahigh‐frequency region.     

Progressive NO2 Sensors with Rapid Alarm and Persistent Memory-Type Responses for Wide-Range Sensing Using Antimony Triselenide Nanoflakes

Antimony triselenide (Sb₂Se₃) nanoflake-based nitrogen dioxide (NO₂) sensors exhibit a progressive bifunctional gas-sensing performance, with a rapid alarm for hazardous highly concentrated gases, and an advanced memory-type function for low-concentration (<1 ppm) monitoring repeated under potentially fatal exposure. Rectangular and cuboid shaped Sb₂Se₃ nanoflakes, comprising van der Waals planes with large surface areas and covalent bond planes with small areas, can rapidly detect a wide range of NO₂ gas concentrations from 0.1 to 100 ppm. These Sb₂Se₃ nanoflakes are found to be suitable for physisorption-based gas sensing owing to their anisotropic quasi-2D crystal structure with extremely enlarged van der Waals planes, where they are humidity-insensitive and consequently exhibit an extremely stable baseline current. The Sb₂Se₃ nanoflake sensor exhibits a room-temperature/low-voltage operation, which is noticeable owing to its low energy consumption and rapid response even under a NO₂ gas flow of only 1 ppm. As a result, the Sb₂Se₃ nanoflake sensor is suitable for the development of a rapid alarm system. Furthermore, the persistent gas-sensing conductivity of the sensor with a slow decaying current can enable the development of a progressive memory-type sensor that retains the previous signal under irregular gas injection at low concentrations.     

Thermally Controlled,Patterned Graphene Transfer Printing for Transparent and Wearable Electronic/Optoelectronic System

Graphene has been highlighted as a platform material in transparent electronics and optoelectronics, including flexible and stretchable ones, due to its unique properties such as optical transparency, mechanical softness, ultrathin thickness, and high carrier mobility. Despite huge research efforts for graphene‐based electronic/optoelectronic devices, there are remaining challenges in terms of their seamless integration, such as the high‐quality contact formation, precise alignment of micrometer‐scale patterns, and control of interfacial‐adhesion/local‐resistance. Here, a thermally controlled transfer printing technique that allows multiple patterned‐graphene transfers at desired locations is presented. Using the thermal‐expansion mismatch between the viscoelastic sacrificial layer and the elastic stamp, a “heating and cooling” process precisely positions patterned graphene layers on various substrates, including graphene prepatterns, hydrophilic surfaces, and superhydrophobic surfaces, with high transfer yields. A detailed theoretical analysis of underlying physics/mechanics of this approach is also described. The proposed transfer printing successfully integrates graphene‐based stretchable sensors, actuators, light‐emitting diodes, and other electronics in one platform, paving the way toward transparent and wearable multifunctional electronic systems.     

A Temperature‐Controllable Microelectrode and Its Application to Protein Immobilization

This letter presents a smart integrated microfluidic device which can be applied to actively immobilize proteins on demand. The active component in the device is a temperature‐controllable microelectrode array with a smart polymer film, poly(N‐isopropylacrylamide) (PNIPAAm) which can be thermally switched between hydrophilic and hydrophobic states. It is integrated into a micro hot diaphragm having an integrated micro heater and temperature sensors on a 2‐micrometer‐thick silicon oxide/silicon nitride/silicon oxide (O/N/O) template. Only 36 mW is required to heat the large template area of 2 mm×16 mm to 40°C within 1 second. To relay the stimulus‐response activity to the microelectrode surface, the interface is modified with a smart polymer. For a model biomolecular affinity test, an anti‐6‐(2, 4‐dinitrophenyl) aminohexanoic acid (DNP) antibody protein immobilization on the microelectrodes is demonstrated by fluorescence patterns.     

Direct Transfer Printing with Metal Oxide Layers for Fabricating Flexible Nanowire Devices

A direct printing method for fabricating devices by using metal oxide transfer layers instead of conventional transfer media such as polydimethylsiloxane is presented. Metal oxides are not damaged by organic solvents; therefore, electrodes with gaps less than 2 μm can be defined on a metal oxide transfer layer through photolithography. In order to determine a suitable metal oxide for use as transfer layer, the surface energies of various metal oxides are measured, and Au layers deposited on these oxides are transferred onto polyvinylphenol (PVP). To verify the feasibility of our approach, Au source–drain electrodes on transfer layers and Si nanowires (NWs) addressed by the dielectrophoretic (DEP) alignment process are transferred onto rigid and flexible PVP‐coated substrates. Based on transfer test and DEP process, Al₂O₃ is determined to be the best transfer layer. Finally, Si NWs field effect transistors (FETs) are fabricated on a rigid Si substrate and a flexible polyimide film. As the channel length decreases from 3.442 to 1.767 μm, the mobility of FET on the Si substrate increases from 127.61 ± 37.64 to 181.60 ± 23.73 cm² V^?1 s^?1. Furthermore, the flexible Si NWs FETs fabricated through this process show enhanced electrical properties with an increasing number of bending cycles.     

Generalized eigen-combining algorithm for adaptive array systems in a co-channel interference environment

This paper presents a generalized eigen-combining algorithm for an adaptive array system that provides a diversity gain in angular spread, and a maximum signal-to-interference ratio (SIR) in the presence of strong interference. The proposed technique generates the maximum ratio of signal components distributed in the spatial channel subspace using channel bases that span the signal subspace and nullify the interference. Through extensive computer simulations, it is shown that the proposed algorithm represents a breakthrough in preventing performance saturation caused by strong co-channel interference.     

Lycopodium Spores: A Naturally Manufactured,Superrobust Biomaterial for Drug Delivery

Herein, the exploration of natural plant‐based “spores” for the encapsulation of macromolecules as a drug delivery platform is reported. Benefits of encapsulation with natural “spores” include highly uniform size distribution and materials encapsulation by relatively economical and simple versatile methods. The natural spores possess unique micromeritic properties and an inner cavity for significant macromolecule loading with retention of therapeutic spore constituents. In addition, these natural spores can be used as advanced materials to encapsulate a wide variety of pharmaceutical drugs, chemicals, cosmetics, and food supplements. Here, for the first time a strategy to utilize natural spores as advanced materials is developed to encapsulate macromolecules by three different microencapsulation techniques including passive, compression, and vacuum loading. The natural spore formulations developed by these techniques are extensively characterized with respect to size uniformity, shape, encapsulation efficiency, and localization of macromolecules in the spores. In vitro release profiles of developed spore formulations in simulated gastric and intestinal fluids have also been studied, and alginate coatings to tune the release profile using vacuum‐loaded spores have been explored. These results provide the basis for further exploration into the encapsulation of a wide range of therapeutic molecules in natural plant spores.     

Lipid production in Porphyridium cruentum grown under different culture conditions

Autotrophic growth of Porphyridium cruentum under 18:12 h and 12:12 h light:dark cycles showed the maximum cell concentration of 2.1 g-dry wt./L, whereas the specific growth rate, 0.042 (1/h), at 18:6 h is faster than that of 12:12 h, 0.031 (1/h), respectively. The highest lipid accumulation level, 19.3 (%, w/w), was achieved at 12:12 h cycle. Under dark cultivation condition with 10 g/L of glucose, the lipid accumulation in the cell was 10.9 (%, w/w), whereas the heterotrophic growth with glycerol as the carbon resource showed low level of cell concentration and lipid production, compared to that of glucose. The glucose was decided to be a suitable carbon resource for the heterotrophic growth of P. cruentum. The lipids from P. cruentum seemed be feasible for biodiesel production, because over 30% of the lipid was C₁₆–C_18:1. The cultivation time and temperature were important factors to increase the maximum cell concentration. Extending the cultivation time helps maintain the maximum cell concentration, and higher lipid accumulation was achieved at 25 °C, compared to 35 °C. The fed-batch cultures showed that, under the light condition, the specific production rate was slightly decreased to 0.4% lipid/g-dry wt./day at the later stage, whereas, under the dark condition, the specific production rate was maintained to be a maximum value of 1.1% lipid/g-dry wt./day, even in the later stage of cultivation. The results indicate that the heterotrophic or 12:12 h cyclic mixotrophic growth of P. cruentum could be used for the production of biodiesel in long-term fed-batch cultivation of P. cruentum.