Korea Electrotechnology Research Institute

Metal-organic frameworks (MOFs) are constructed from metal ions/clusters coordinated by organic linkers (or bridging-ligands). The hallmark of MOFs is their permanent porosity, which is frequently found in MOFs constructed from metal-clusters. These clusters are often formed in situ, whereas the linkers are generally pre-formed. The geometry and connectivity of a linker dictate the structure of the resulting MOF. Adjustments of linker geometry, length, ratio, and functional-group can tune the size, shape, and internal surface property of a MOF for a targeted application. In this critical review, we highlight advances in MOF synthesis focusing on linker design. Examples of building MOFs to reach unique properties, such as unprecedented surface area, pore aperture, molecular recognition, stability, and catalysis, through linker design are described. Further search for application-oriented MOFs through judicious selection of metal clusters and organic linkers is desirable. In this review, linkers are categorized as ditopic (Section 1), tritopic (Section 2), tetratopic (Section 3), hexatopic (Section 4), octatopic (Section 5), mixed (Section 6), desymmetrized (Section 7), metallo (Section 8), and N-heterocyclic linkers (Section 9).

While organic electronics is mostly dominated by light-emitting diodes, photovoltaic cells and transistors, optoelectronics properties peculiar to organic semiconductors make them interesting candidates for the development of innovative and disruptive applications also in the field of light signal detection. In fact, organic-based photoactive media combine effective light absorption in the region of the spectrum from ultraviolet to near-infrared with good photogeneration yield and low-temperature processability over large areas and on virtually every substrate, which might enable innovative optoelectronic systems to be targeted for instance in the field of imaging, optical communications or biomedical sensing. In this review, after a brief resume of photogeneration basics and of devices operation mechanisms, we offer a broad overview of recent progress in the field, focusing on photodiodes and phototransistors. As to the former device category, very interesting values for figures of merit such as photoconversion efficiency, speed and minimum detectable signal level have been attained, and even though the simultaneous optimization of all these relevant parameters is demonstrated in a limited number of papers, real applications are within reach for this technology, as it is testified by the increasing number of realizations going beyond the single-device level and tackling more complex optoelectronic systems. As to phototransistors, a more recent subject of study in the framework of organic electronics, despite a broad distribution in the reported performances, best photoresponsivities outperform amorphous silicon-based devices. This suggests that organic phototransistors have a large potential to be used in a variety of optoelectronic peculiar applications, such as a photo-sensor, opto-isolator, image sensor, optically controlled phase shifter, and opto-electronic switch and memory.

A transparent and stretchable all-graphene multifunctional electronic-skin sensor matrix is developed. Three different functional sensors are included in this matrix: humidity, thermal, and pressure sensors. These are judiciously integrated into a layer-by-layer geometry through a simple lamination process. Electronic skins (E-skins) are flexible circuitry matrices in which each sensor cell can transduce external stimuli on epidermis to electronic signals. E-skins are potentially useful in the fabrication of wearable human-machine interfacing devices, remote real-time health monitoring, implantable prosthetics, and multifunctional smart skins.1-6 For example, pressure-sensitive rubbers integrated with organic transistor matrices have been used to map pressure distributions.7-11 A variety of skin-like pressure or strain sensors fabricated by sandwiching elastomeric materials between soft conductors have also been reported.12-17 A piezoelectric material or interdigitated electrode design has been introduced into a graphene transistor matrix to prepare a stretchable tactile E-skin.18-20 Previous studies of E-skins have mainly focused on tactile sensors that transduce physical variables (pressure, shear, or strain) into electronic signals. Plausible mimics of multifunctional human skin will require multimodal detection, including temperature, humidity, and pressure, integrated into a single pixel. Practical E-skin matrices require two additional qualifications: (i) simultaneous multiple stimuli sensing and (ii) low-cost and facile fabrication processes that minimize materials. The specifications may be met by developing rational device architecture designs using simple sensing materials. Graphene, a 2D hybridized carbon layer with a hexagonal honeycomb lattice, has attracted attention for its utility in a variety of electronic device applications. Its unique charge transport allows a high carrier concentration (1013 cm–2) with a mobility exceeding 104 cm2 V–1 s–1 under ambient condition.21 Excellent mechanical properties extend the applicability of graphene to stretchable devices, and a good thermal conductivity enhances heat dissipation in highly integrated circuits.22, 23 High-quality large-area graphene may be used in transparent conducting electrodes fabricated through chemical vapor deposition (CVD) and subsequent roll-to-roll transfer processes. These graphene electrodes exhibit a sheet resistance as low as 125 Ω sq−1 and a 97% optical transmittance.24 Graphene oxide (GO) or its reduced form, reduced graphene oxide (rGO), two graphene derivatives, may also be mass-produced using a solution process called Hummers method by exfoliating the materials from graphite.25-27 The surface functional groups on GO or rGO, including hydroxyl, carboxyl, and epoxy groups, are very sensitive to environmental conditions, including humidity, chemicals, and temperature, and they have been widely used as sensing materials.28-33 The adoption of versatile graphene derivatives in E-skin applications paves the way to achieve transparent and multifunctional sensors with facile fabrication process. Here, we developed a transparent and stretchable all-graphene multifunctional E-skin sensor matrix. Three different functional sensors were included in this matrix: humidity, thermal, and pressure sensors, and were judiciously integrated into a layer-by-layer geometry through a simple lamination process. CVD-grown graphene was used to form the electrodes and interconnects for these three sensors, whereas GO and rGO were used as the active sensing materials for the humidity and temperature sensors, respectively. The top polydimethylsiloxane (PDMS) substrate, which bore the GO humidity sensor array, was laminated in a crisscross fashion onto the top of the bottom PDMS substrate, which bore the rGO temperature sensor array. The arrays were prepared to have the same geometry. The top PDMS substrate sandwiched between two CVD-graphene electrodes acted as an active layer for the capacitive pressure and strain sensors. Together, the sensors monitored a variety of daily life sensations (e.g., a hot wind blowing, breathing, and finger touching) with excellent sensitivity. Each sensor in the matrix exhibited simplex sensing performance: it was only sensitive to its specific stimulation and gave no response to other stimulations. The three sensors in the matrix detected external stimuli simultaneously and relayed independent electrical signals. 2D color mappings of the simultaneous multifunctional sensing were collected. The device architecture developed here for use as a multifunctional E-skin sensor matrix not only avoided the preparation of several materials separately; it enabled sensor integration using a simple lamination method. Figure 1a shows a schematic diagram of the method used to fabricate the multifunctional sensor matrix (a 6 × 6 sensors array in this work, demonstrated using a simple 2 × 2 array). The top GO-based humidity sensor and the bottom rGO-based thermal sensor were prepared using the same geometry: four CVD-graphene (Gr) subelectrodes on each substrate shared two CVD-Gr main electrodes. The sheet resistance of the CVD-graphene electrode was measured to be around 500 Ω sq−1. Prior to transferring the CVD-Gr electrodes onto the PDMS substrate, the substrate was treated with O2 plasma to form a hydrophilic surface. The GO or rGO dispersion (characterized by AFM, XPS, and FT-IR spectroscopy, as shown in Figure S1 in the Supporting Information) was spray-coated onto the region between the patterned CVD-Gr electrodes on the two PDMS substrates to form the sensing channel. The top PDMS substrate prepared with a GO-based humidity sensor array was then laminated onto the top of the bottom PDMS substrate prepared with the rGO thermal sensor array, after rotation through a 90° angle, followed by a subsequent degassing process in a vacuum chamber over 30 min to remove bubbles between the two layers. Prior to lamination, the backside of the top PDMS substrate was treated with O2 plasma to promote interfacial adhesion between the two PDMS layers via the dehydration reaction between -Si-OH groups present on the PDMS surface.34 The top PDMS substrate sandwiched between the top and bottom CVD-Gr electrodes formed capacitive pressure sensors. Note that only graphene derivatives such as CVD-Gr, GO, and rGO were utilized to fabricate the multimodal sensing matrix (Figure 1b), thereby avoiding the use of several distinct materials. The simple fabrication process, which included lamination and degassing steps, permitted the facile integration of a variety of sensors at high yield and low cost. An equivalent circuit diagram of the multifunctional sensors matrix is displayed in Figure 1c. The matrix consisted of three different sensors: a GO-based impedance humidity sensor (red) on the top layer, an rGO-based resistive thermal sensor (blue) on the bottom layer, and a PDMS-based capacitive pressure sensor (green) sandwiched between the top (black line) and bottom (gray line) CVD-Gr electrodes. The resistive sensors were tested using a DC input, whereas the impedance and capacitive sensors were tested using an AC input. In the multimodal E-skin sensor matrix, it was crucial to distinguish the specific output signals from each sensor simultaneously (discussed further below). The good matrix transparency and excellent mechanical properties of the graphene derivatives and PDMS-enabled conformal contact between the matrix and human skin, with a transmittance exceeding 90% over the range 400–1000 nm, as shown in Figure 1d. The sensing performance of each sensor in the matrix was characterized. Figure 2a plots the sensing properties of the GO-based humidity sensor. The variation in the GO capacitance was collected at a specific input frequency of 10 MHz. As the relative humidity (RH) was increased from 20% to 90%, the capacitance increased from 0.15 to 4.27 pF because the adsorbed water molecules increased the capacitance of GO. Two distinct regimes were present in the curves. Under a low RH environment (RH less than 60%), water molecules were adsorbed onto the GO surface through double hydrogen bonding. In this regime, the hopping of protons between adjacent hydroxyl groups increased the leak conductivity in the film, which enhanced the capacitance of the GO. As the RH was increased above 60%, a larger number of water molecules adsorbed onto the GO surface and penetrated the GO films. These water molecules facilitated the hydrolysis of various functional groups, including carboxyl, epoxy, and hydroxyl groups, on the GO. These ionic species dramatically enhanced the ionic conductivity and sharply increased the capacitance.35, 36 The impedance spectrum obtained from the GO-based humidity sensor could be modeled by an electrical equivalent circuit that included a single charge transfer resistance (Rct) value and two constant phase elements, CPE1 and CPE2, as shown in Figure S2a in the Supporting Information. The impedance of a constant phase element (ZCPE) is defined as ZCPE = Q–1(iω)–n (0 ≤ n ≤ 1), where Q is a real parameter, i is the imaginary unit, ω is the frequency, and n is a real parameter, the value of which can vary from 0 (pure resistor) to 1 (pure capacitor).29 The spectrum fit well to the proposed equivalent circuit across the entire frequency range (Figures S2b and S2c, Supporting Information), and the extracted fitting parameters Rct, n1, and n2 were consistent with those obtained from previous GO humidity sensors. Figure 2b plots the real-time humidity sensing properties of the device at different RHs of 30%, 40%, 50%, 60%, and 90%. The capacitance of GO increased gradually to higher values under each RH condition and then returned to the initial value after the RH was recovered to ambient humidity. The stable signal saturation behavior was monitored at three different RHs (40%, 60%, and 75%) as shown in Figure S3 in the Supporting Information, which is consistent with previous reports related with the GO humidity sensors.37, 38 In addition, the delamination issue of the GO layers from the channel (Figure S4, Supporting Information) should be addressed because the GO was exposed to the outside environment. Figure 2c plots the sensing properties of the rGO-based thermal sensor. A DC voltage of 1 V was applied between the two CVD-Gr electrodes. The resistance of rGO decreased linearly from 0.62 to 0.28 MΩ as the temperature increased from 0 to 100 °C. The temperature dependence of the rGO conductance (G) was examined, and a plot of G versus T–1/3 was fit numerically (Figure S5, Supporting Information). The experimental data fit well over the entire temperature range, suggesting that charge transport in the rGO multilayer films was governed by a 3D variable range hopping (VRH) model supplemented with parallel quantum tunneling.39, 40 In this model, the temperature-dependent conductance of the rGO film could be described as G = Gh·exp(–H/T1/3)+Gt, where H is a hopping parameter, Gh·exp(–H/T1/3) represents the hopping contribution, and Gt represents the quantum tunneling contribution. High temperatures facilitated thermally activated charge hopping among the localized states, which enhanced the electrical conductance of the film.39, 40 Figure 2d shows the real-time measurements of the resistance of the rGO sensor as the temperature was increased and then held at four different temperatures (30, 60, 80, and 100 °C) for several minutes. The output resistance of rGO decreased gradually and was maintained at each value. As the temperature decreased to 0 °C, the rGO resistance returned to its initial value. The sensing properties of the GO and rGO devices depended on the amount of GO or rGO deposited onto the channel (Figure S6, Supporting Information) and should be addressed through further systematic studies. These results suggested that both GO and rGO could be used as excellent sensing materials for humidity and thermal sensors, respectively. The sensing properties of the PDMS-based pressure and strain sensors prepared with the CVD-Gr/PDMS/CVD-Gr structure were characterized. A custom pressure gauge comprising a plastic pole terminated with a square-shaped glass unit (contact area = 1 mm2) was used to apply pressures ranging from 0 to 450 kPa onto the device. PDMS layers with three different thicknesses (3, 5, and 10 μm) were tested, as shown in Figure S7 in the Supporting Information. Although the 10 μm thick PDMS layer exhibited the highest sensitivity, it did not form conformal contact with human skin. The trade-off between the contact properties with human skin and the sensing performance was optimized, and a 5 μm thick PDMS layer was selected. Figure 2e plots the capacitance as a function of the pressures applied to the capacitive PDMS pressure sensor. The curve exhibited two distinct regimes. As the applied pressure dipped below 10 kPa, the capacitance increased from 7.32 to 7.52 pF in a steep slope because the initially high pressure easily deformed the PDMS layer. Under high pressures exceeding 10 kPa, however, the slope decreased due to the reduced deformation space available to the compressed PDMS layer. The sensitivity of the PDMS pressure sensor, defined as (ΔC/C0)/P, was 0.002 kPa−1 (Figure S8, Supporting Information). Figure 2f plots the real-time measured capacitance values as a function of pressure. Application of five different pressures: 2, 4, 20, 100, and 200 kPa, resulted in immediate capacitance responses over a short response time of <0.2 s. The minimum detectable pressure was actually lower than 0.5 kPa, but could not be displayed due to the detection limitation of our pressure gauge. Apart from its pressure sensing capabilities, the PDMS pressure sensor also detected bending strain, as shown in Figure S9 in the Supporting Information. The durability bending test applied over 2000 cycles confirmed that our lamination and degassing processes were useful for preparing matrix-bound capacitive PDMS pressure sensors (Figure S10, Supporting Information). A piezoelectric poly(vinylidene fluoride trifluoroethylene) [P(VDF-TrFE)] layer could be inserted between the two CVD-Gr electrodes in place of the PDMS layer for both pressure and strain sensing (Figure S11, Supporting Information).41, 42 Notably, all sensing materials in the matrix, including GO, rGO, and the sandwiched PDMS, were utilized as obtained. A higher sensitivity may potentially be achieved from the E-skin by further optimizing these materials. Stretchability test was also conducted on the E-skin matrix (Figure S12, Supporting Information). The sensing properties of each sensor were monitored during stretching cycles. The stretching strain of 3% was applied and released repeatedly to the matrix in the direction parallel with the sensing channel. Note that the sensing performances of each sensor were invariant even after 500 cycles. The good stretchability of the E-skin was attributed to the mechanical properties of the graphene derivatives used in the sensor matrix. The integration of all three sensors into a matrix required that each sensor provide output responses to a specific stimulus without displaying sensitivity to other stimuli. The simplex sensing performance of the GO-based humidity sensor was tested under three stimuli: temperature, humidity, and pressure, as shown in Figure 3a. The GO capacitance responded only to the humidity and not to the temperature and pressure. A slight increase in the GO capacitance above 80 °C was observed, but this change was much smaller than the response arising from a humidity change. The resistance of the rGO-based thermal sensor only changed with the temperature (Figure 3b). The rGO resistance decreased linearly with temperature, but no obvious variations were observed under varying pressures. The top PDMS substrate in the sensors matrix was laminated onto the bottom PDMS substrate bearing the rGO-based thermal sensor array; thus, the thermal sensors were passivated by the top 5 μm thick PDMS layer. This lamination process effectively protected the rGO that interacted with external water molecules (humidity), thereby reducing the humidity sensitivity of rGO.43 The pressure detection properties of the PDMS-based pressure sensor under different environmental conditions were tested at different temperatures and RHs (Figure 3c,d). The left panel in Figure 3c shows the real-time pressure sensing measurements conducted at different temperatures. As the temperature increased from 22 to 90 °C, the capacitance decreased from 7.3 to 5.7 pF because the thickness of the PDMS layer increased due to the thermal expansion of the PDMS;44 however, the corresponding pressure sensing properties at each temperature were not significantly affected. As shown in the right panel of Figure 3c, the absolute values of the capacitance (the difference between the capacitance values measured in the presence or absence of the applied pressures) were similar at different temperatures (indicated by the blue arrows). Similar pressure tests were conducted under different humidity conditions (Figure 3d). As RH was increased from 30% to 80%, the capacitance of the PDMS layer increased from 7.3 to 8.4 pF. This increase was attributed to the fact that higher numbers of water molecules were adsorbed onto the PDMS surface at higher RH to increase the capacitance of the PDMS layer.45, 46 The pressure sensing properties were less affected by the RH (blue arrows in the right panel of Figure 3d). Consequently, the pressures applied to the pressure sensor under different environments could be detected by measuring the signal variations. The ability of the sensors to mimic the multifunctionality of human skin was assessed by measuring the output signals of each sensor in our multifunctional sensor matrix induced by external stimuli. The single signal detection capabilities of our impedance analyzer permitted the simultaneous monitoring of two different output signals from the pressure and thermal sensors. The two output signals from the humidity and thermal sensors could also be monitored simultaneously under similar external stimuli. These two groups of measured output signals were analyzed in a single graph. Figure 4a–c plot the output signals collected from the three sensors under three different stimuli (hot wind blowing, finger and The hot wind on the matrix dramatically decreased the rGO resistance (blue curve in Figure but did not variations in the capacitance values of both the GO and The finger stimulus resulted in a change in the output signals of all three sensors (Figure Under the PDMS capacitance decreased curve in Figure This capacitance change the capacitance increase observed with the pressure applied using the plastic The was and to by the which decreased the The GO capacitance increased of from the finger curve in Figure the same the rGO resistance increased as the channel temperature increased to the human temperature (blue curve in Figure The temperature sensing response was to the other sensor responses due to the heat transfer from the the responses to human detected by our sensor matrix are in Figure The GO capacitance values were sensitive to the humidity induced by curve in Figure The temperature also increased as by the resistance of the rGO sensor (blue curve in Figure obvious in the pressure were These results that different output signals extracted from the corresponding sensors under external stimuli could be which is for multifunctional human skin. The E-skin sensor matrix × 6 fabricated using the in Figure 1a was tested for its ability to human The pressure, temperature, and humidity were monitored as a finger was at two of the matrix (Figure As shown in Figure the output sensing signals from different sensors were collected and to the corresponding 2D color The shown in Figure the of the three sensors under finger The capacitance and resistance sensitivity were defined as and respectively. The bottom color the of the corresponding temperature humidity and pressure (green) during the finger as from the in the In we demonstrated the fabrication of an all-graphene transparent multifunctional E-skin matrix. CVD-Gr was used as the electrodes in the matrix, and GO and rGO were as the sensing materials. A simple lamination process was used to the humidity, temperature, and pressure sensors into a single Each sensor was sensitive to its external stimulus but was not affected by the other two stimuli. all sensors simultaneously and on different The of the temperature, humidity, and pressure during finger were in 2D color This a facile fabrication process using a of graphene derivatives to a transparent E-skin device without using E-skin fabrication processes. sensors and may be integrated into this simple lamination process to in remote applications in the The use of several graphene derivatives as the main in the E-skin may also the use of Graphene oxide was prepared from using a Hummers 10 and were in a 30 were over 1 was for 2 in an water the been for at temperature, water were and the solution was for 10 min in an water of were then and the was for 2 at The was and to the oxide The oxide was then into GO in water by for 1 The GO was reduced in solution by the dispersion of the GO with to was then to the GO solution to a concentration of followed by at 100 °C for graphene electrodes were on a × 10 through The was into a and at °C under an at low pressures for 1 5 was introduced to graphene under a 30 the was and the was to temperature under the The graphene obtained on the was onto a PDMS substrate using and processes. The PDMS solution comprising a and a was onto the and then the was into a chamber to remove bubbles in the PDMS layer by the PDMS layer on the glass was at and then at °C for 1 The PDMS surface was treated with O2 plasma to form a hydrophilic surface. The CVD-grown graphene was onto the PDMS using a and then patterned using and subsequent The sensing were formed by rGO or GO onto the channel through a were then laminated with the of an The degassing process was conducted in the vacuum chamber over 30 and the fabricated matrix was to achieve conformal The properties of the thermal sensing devices were measured using and with a temperature impedance properties were measured using a with a humidity The pressure sensing properties were measured using an with a gauge The were deposited onto the of each CVD-graphene electrode for electrical with the the sensing signals were collected through the to the and to this This was by a from the for under the and and of the of by the of and As a to our and this by the materials are and may be for but are not or arising from than should be addressed to the The is not for the or of by the than should be to the corresponding for the

A bidirectional full-bridge CLLC resonant converter using a new symmetric LLC-type resonant network is proposed for a low-voltage direct current power distribution system. This converter can operate under high power conversion efficiency because the symmetric LLC resonant network has zero-voltage switching capability for primary power switches and soft commutation capability for output rectifiers. In addition, the proposed topology does not require any snubber circuits to reduce the voltage stress of the switching devices because the switch voltage of the primary and secondary power stage is confined by the input and output voltage, respectively. In addition, the power conversion efficiency of any directions is exactly same as each other. Using digital control schemes, a 5-kW prototype converter designed for a high-frequency galvanic isolation of 380-V dc buses was developed with a commercial digital signal processor. Intelligent digital control algorithms are also proposed to regulate output voltage and to control bidirectional power conversions. Using the prototype converter, experimental results were obtained to verify the performance of the proposed topology and control algorithms. The converter could softly change the power flow directions and its maximum power conversion efficiency was 97.8% during the bidirectional operation.

For at least the past ten years printed electronics has promised to revolutionize our daily life by making cost-effective electronic circuits and sensors available through mass production techniques, for their ubiquitous applications in wearable components, rollable and conformable devices, and point-of-care applications. While passive components, such as conductors, resistors and capacitors, had already been fabricated by printing techniques at industrial scale, printing processes have been struggling to meet the requirements for mass-produced electronics and optoelectronics applications despite their great potential. In the case of logic integrated circuits (ICs), which constitute the focus of this Progress Report, the main limitations have been represented by the need of suitable functional inks, mainly high-mobility printable semiconductors and low sintering temperature conducting inks, and evoluted printing tools capable of higher resolution, registration and uniformity than needed in the conventional graphic arts printing sector. Solution-processable polymeric semiconductors are the best candidates to fulfill the requirements for printed logic ICs on flexible substrates, due to their superior processability, ease of tuning of their rheology parameters, and mechanical properties. One of the strongest limitations has been mainly represented by the low charge carrier mobility (μ) achievable with polymeric, organic field-effect transistors (OFETs). However, recently unprecedented values of μ ∼ 10 cm(2) /Vs have been achieved with solution-processed polymer based OFETs, a value competing with mobilities reported in organic single-crystals and exceeding the performances enabled by amorphous silicon (a-Si). Interestingly these values were achieved thanks to the design and synthesis of donor-acceptor copolymers, showing limited degree of order when processed in thin films and therefore fostering further studies on the reason leading to such improved charge transport properties. Among this class of materials, various polymers can show well balanced electrons and holes mobility, therefore being indicated as ambipolar semiconductors, good environmental stability, and a small band-gap, which simplifies the tuning of charge injection. This opened up the possibility of taking advantage of the superior performances offered by complementary "CMOS-like" logic for the design of digital ICs, easing the scaling down of critical geometrical features, and achieving higher complexity from robust single gates (e.g., inverters) and test circuits (e.g., ring oscillators) to more complete circuits. Here, we review the recent progress in the development of printed ICs based on polymeric semiconductors suitable for large-volume micro- and nano-electronics applications. Particular attention is paid to the strategies proposed in the literature to design and synthesize high mobility polymers and to develop suitable printing tools and techniques to allow for improved patterning capability required for the down-scaling of devices in order to achieve the operation frequencies needed for applications, such as flexible radio-frequency identification (RFID) tags, near-field communication (NFC) devices, ambient electronics, and portable flexible displays.

We report the successful application of multiwall carbon nanotubes (CNTs) as electrocatalysts for triiodide reduction in a dye-sensitized solar cell (DSSC). Defect-rich edge planes of bamboolike-structure multiwall CNTs facilitate the electron-transfer kinetics at the counter electrode-electrolyte interface, resulting in low charge-transfer resistance and an improved fill factor. In combination with a dye-sensitized TiO2 photoanode and an organic liquid electrolyte, a multiwall CNT counter-electrode DSSC shows 7.7% energy conversion efficiency under 1 sun illumination (100 mW/cm(2), air mass 1.5 G). The short-term stability test at moderate conditions confirms the robustness of CNT counter-electrode DSSCs.

In this paper, the cooperative control strategy of microsources and the energy storage system (ESS) during islanded operation is presented and evaluated by a simulation and experiment. The ESS handles the frequency and the voltage as a primary control. And then, the secondary control in microgrid management system returns the current power output of the ESS into zero. The test results show that the proposed cooperative control strategy can regulate the frequency and the voltage, and the secondary control action can contribute to improve the control capability.

Abstract Recently SnSe, a layered chalcogenide material, has attracted a great deal of attention for its excellent p-type thermoelectric property showing a remarkable ZT value of 2.6 at 923 K. For thermoelectric device applications, it is necessary to have n-type materials with comparable ZT value. Here, we report that n-type SnSe single crystals were successfully synthesized by substituting Bi at Sn sites. In addition, it was found that the carrier concentration increases with Bi content, which has a great influence on the thermoelectric properties of n-type SnSe single crystals. Indeed, we achieved the maximum ZT value of 2.2 along b axis at 733 K in the most highly doped n-type SnSe with a carrier density of −2.1 × 10 19 cm −3 at 773 K.

This paper presents power-control strategies of a grid-connected hybrid generation system with versatile power transfer. The hybrid system is the combination of photovoltaic (PV) array, wind turbine, and battery storage via a common dc bus. Versatile power transfer was defined as multimodes of operation, including normal operation without use of battery, power dispatching, and power averaging, which enables grid- or user-friendly operation. A supervisory control regulates power generation of the individual components so as to enable the hybrid system to operate in the proposed modes of operation. The concept and principle of the hybrid system and its control were described. A simple technique using a low-pass filter was introduced for power averaging. A modified hysteresis-control strategy was applied in the battery converter. Modeling and simulations were based on an electromagnetic-transient-analysis program. A 30-kW hybrid inverter and its control system were developed. The simulation and experimental results were presented to evaluate the dynamic performance of the hybrid system under the proposed modes of operation.

Abstract Human skin imperfectly discriminates between pressure and temperature stimuli under mixed stimulation, and exhibits nonlinear sensitivity to each stimulus. Despite great advances in the field of electronic skin (E‐skin), the limitations of human skin have not previously been overcome. For the first time, the development of a stimulus‐discriminating and linearly sensitive bimodal E‐skin that can simultaneously detect and discriminate pressure and temperature stimuli in real time is reported. By introducing a novel device design and using a temperature‐independent material, near‐perfect stimulus discriminability is realized. In addition, the hierarchical contact behavior of the surface‐wrinkled microstructure and the optimally reduced graphene oxide in the E‐skin contribute to linear sensitivity to applied pressure/temperature stimuli over wide intensity range. The E‐skin exhibits a linear and high pressure sensitivity of 0.7 kPa −1 up to 25 kPa. Its operation is also robust and exhibits fast response to pressure stimulus within 50 ms. In the case of temperature stimulus, the E‐skin shows a linear and reproducible temperature coefficient of resistance of 0.83% K −1 in the temperature range 22–70 °C and fast response to temperature change within 100 ms. In addition, two types of stimuli are simultaneously detected and discriminated in real time by only impedance measurements.

Microorganisms such as fungi and bacteria cause many human diseases and therefore rapid and accurate identification of these substances is essential for effective treatment and prevention of further infections. In particular, contemporary microbial detection technique is limited by the low detection speed which usually extends over a couple of days. Here we demonstrate that metamaterials operating in the terahertz frequency range shows promising potential for use in fabricating the highly sensitive and selective microbial sensors that are capable of high-speed on-site detection of microorganisms in both ambient and aqueous environments. We were able to detect extremely small amounts of the microorganisms, because their sizes are on the same scale as the micro-gaps of the terahertz metamaterials. The resonant frequency shift of the metamaterials was investigated in terms of the number density and the dielectric constants of the microorganisms, which was successfully interpreted by the change in the effective dielectric constant of a gap area.

Bulk-type all-solid-state lithium-ion batteries (ASLBs) have the potential to be superior to conventional lithium-ion batteries (LIBs) in terms of safety and energy density. Sulfide SE materials are key to the development of bulk-type ASLBs because of their high ionic conductivity (max of ∼10–2 S cm–1) and deformability. However, the severe reactivity of sulfide materials toward common polar solvents and the particulate nature of these electrolytes pose serious complications for the wet-slurry process used to fabricate ASLB electrodes, such as the availability of solvent and polymeric binders and the formation of ionic contacts and networks. In this work, we report a new scalable fabrication protocol for ASLB electrodes using conventional composite LIB electrodes and homogeneous SE solutions (Li6PS5Cl (LPSCl) in ethanol or 0.4LiI–0.6Li4SnS4 in methanol). The liquefied LPSCl is infiltrated into the tortuous porous structures of LIB electrodes and solidified, providing intimate ionic contacts and favorable ionic percolation. The LPSCl-infiltrated LiCoO2 and graphite electrodes show high reversible capacities (141 and 364 mA h g–1) at 0.14 mA cm–2 (0.1 C) and 30 °C, which are not only superior to those for conventional dry-mixed and slurry-mixed ASLB electrodes but also comparable to those for liquid electrolyte cells. Good electrochemical performance of ASLBs employing the LPSCl-infiltrated LiCoO2 and graphite electrodes at 100 °C is also presented, highlighting the excellent thermal stability and safety of ASLBs.

Mesoporous α-Fe2O3 materials were prepared in large quantity by the soft template synthesis method using the triblock copolymer surfactant F127 as the template. Nitrogen adsorption−desorption isothermal measurements and transmission electron microscope observation revealed that the as-prepared mesoporous α-Fe2O3 nanostructures have large mesopores in a wide size range of 5−30 nm. It has been found that the Morin transition depends on thermal history of mesoporous α-Fe2O3, which is driven by surface anisotropy. Superparamagnetic behavior of mesoporous α-Fe2O3 is also associated with surface spins with blocking temperature around 50 K. When applied as gas sensors, mesoporous α-Fe2O3 nanostructures exhibited high gas sensitivity toward acetic acid and ethanol gas. As anodes in lithium ion cells, mesoporous α-Fe2O3 materials show a high specific capacity of 1360 mAh/g with excellent cycling stability and high rate capacity.

In this paper, a fast voltage control strategy of three-phase AC/DC pulsewidth modulation (PWM) converters applying a feedback linearization technique is proposed. First, incorporating the power balance of the input and output sides in system modeling, a nonlinear model of the PWM converter is derived with state variables such as AC input currents and DC output voltage. Then, by input-output feedback linearization, the system is linearized and a state feedback control law is obtained by pole placement. With this control scheme, output voltage responses become faster than those in a conventional cascade control structure. For robust control to parameter variations, integrators are added to the exact feedback control law. Since the fast voltage control is feasible for load changes, it is shown that the DC electrolytic capacitor size can be reduced. In addition, the capacitor current is analyzed for size reduction of the capacitor. As is usual with PWM converters, the input current is regulated to be sinusoidal and the source power factor can be controlled at unity. The experimental results are provided to verify the validity of the proposed control algorithm for a 1.5 kVA insulated gate bipolar transistor PWM converter system.

Strategies are proposed to reconfigure the feeder in distribution systems by using artificial neural networks (ANNs) with mapping ability. ANNs determine the appropriate system topology that reduces the power loss according to the variation of load pattern. The control strategy can be easily obtained on the basis of the system topology which is provided by ANNs. ANNs are designed in two groups. The first group estimates the proper load level from the load data of each zone. The second determines the appropriate system topology from the input load level. Several programs with the training set builder are developed for the design, the training, and the accuracy test of artificial neural networks. The performance of neural networks designed is evaluated on the test distribution system. Neural networks are implemented in FORTRAN language and trained on a 386 PC.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

In the resonance ratio control, which the authors proposed for vibration suppression and disturbance rejection in a torsional system, the estimation speed of the disturbance observer should have been much faster than the resonance frequency of the controlled system. However, too fast a disturbance observer sometimes causes an implementation problem. In this paper, the authors give the optimal speed of the disturbance observer and propose a novel technique named "slow resonance ratio control". It does not have any fast part in the controller. It also enables us to design the speed control and the vibration suppression control almost completely independently.

The 3 × 3 gas sensor array with different metal oxides and morphologies is fabricated to compare the sensitization effects of Au nanoparticles on various metal oxides and gases.The 3 × 3 gas sensor array with different metal oxides and morphologies is fabricated to compare the sensitization effects of Au nanoparticles on various metal oxides and gases.

A novel zero-voltage and zero-current-switching (ZVZCS) full-bridge (FB) pulse-width modulated (PWM) converter is proposed. The new converter overcomes the limitations of the zero-voltage-switching (ZVS)-FB-PWM converter, such as high circulating energy, loss of duty cycle, and limited ZVS load range for the lagging-leg switches. By using the DC blocking capacitor and adding a saturable inductor, the primary current during the freewheeling period is reduced to zero, allowing the lagging-leg switches to be operated with zero-current-switching (ZCS). Meanwhile, the leading-leg switches are still operated with ZVS. The new converter is attractive for high-voltage (400-800 V), high-power (2-10 kW) applications where IGBTs are predominantly used as the power switches. The principle of operation, features, and design considerations of the new converter are described and verified on a 2-kW, 100-kHz, IGBT-based experimental circuit.

We herein report on the large-scale synthesis of ultrathin Bi(2)Te(3) nanoplates and subsequent spark plasma sintering to fabricate n-type nanostructured bulk thermoelectric materials. Bi(2)Te(3) nanoplates were synthesized by the reaction between bismuth thiolate and tri-n-octylphosphine telluride in oleylamine. The thickness of the nanoplates was ~1 nm, which corresponds to a single layer in Bi(2)Te(3) crystals. Bi(2)Te(3) nanostructured bulk materials were prepared by sintering of surfactant-removed Bi(2)Te(3) nanoplates using spark plasma sintering. We found that the grain size and density were strongly dependent on the sintering temperature, and we investigated the effect of the sintering temperature on the thermoelectric properties of the Bi(2)Te(3) nanostructured bulk materials. The electrical conductivities increased with an increase in the sintering temperature, owing to the decreased interface density arising from the grain growth and densification. The Seebeck coefficients roughly decreased with an increase in the sintering temperature. Interestingly, the electron concentrations and mobilities strongly depended on the sintering temperature, suggesting the potential barrier scattering at interfaces and the doping effect of defects and organic residues. The thermal conductivities also increased with an increase in the sintering temperature because of grain growth and densification. The maximum thermoelectric figure-of-merit, ZT, is 0.62 at 400 K, which is one of the highest among the reported values of n-type nanostructured materials based on chemically synthesized nanoparticles. This increase in ZT shows the possibility of the preparation of highly efficient thermoelectric materials by chemical synthesis.

3D printing of reduced graphene oxide (rGO) nanowires is realized at room temperature by local growth of GO at the meniscus formed at a micropipette tip followed by reduction of GO by thermal or chemical treatment. 3D rGO nanowires with diverse and complicated forms are successfully printed, demonstrating their ability to grow in any direction and at the selected sites. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.