Choongik Kim

Stretchable Microfluidic Radiofrequency Antennas

www.MaterialsViews.com C O M Stretchable Microfl uidic Radiofrequency Antennas M U N I By Masahiro Kubo , Xiaofeng Li , Choongik Kim , Michinao Hashimoto , Benjamin J. Wiley , Donhee Ham , and George M. Whitesides * C A IO N This paper describes a new method for fabricating stretchable radiofrequency antennas. The antennas consist of liquid metal (eutectic gallium indium alloy, EGaIn [ 1 , 2 ] ) enclosed in elastomeric microfl uidic channels. In particular, a microfl uidic structure made of two types of elastomers (polydimethylsiloxane (PDMS) and Ecofl ex (type 0030, Reynolds Advanced Materials)) with different stiffness has been developed to improve the stretchability and mechanical stability of the antennas. These antennas can be stretched up to a strain [defi ned as the percentage change in length or ( l – l 0 )/ l 0 ] of 120 %. This high stretchability allows the resonance frequencies of the antennas to be mechanically tuned over a wide range of frequencies. The antennas can also be repeatedly stretched, while retaining a high effi ciency (> 95 %) in radiation. “Stretchability” in electronics has the potential to open new opportunities, particularly for large-area devices and systems, and in systems that require the device to conform to a nonplanar surface, or to bend and stretch while in use. [ 3–5 ] Compared to “fl exible” electronics built on nonstretchable polymer or paper substrates, [ 6 , 7 ] stretchable electronics can cover almost arbitrarily curved surfaces and movable parts. Mechanical compliance may increase the comfort of the user for wearable electronics or implantable medical devices, and simplify the integration for a range of applications. [ 3–5 , 8 ] New approaches to stretchable electronics are now being developed. In a recent advance, Rogers et al. [ 4 , 5 ] described stretchable integrated circuits with elongation of up to 100 % using wavy, thin silicon ribbons on pre-stretched elastic substrates. Antennas offer new, attractive applications for stretchable electronics; these applications might include reconfi gurable antennas, [ 9 ] antennas for limited and nonplanar spaces, [ 10 ] and wearable sensors. Two methods are commonly used to build antennas for commercial applications. The most common method uses sheet-metal processing; in this method, a metal sheet is punched, bent, and welded into the desired structure. A second method uses chemical etching and plating to make small patterns of metal. This method can make fl exible antennas by patterning metal on a fl exible substrate. Neither

Printable cross-linked polymer blend dielectrics. Design strategies, synthesis, microstructures, and electrical properties, with organic field-effect transistors as testbeds.

We report here the synthesis and dielectric properties of optimized, cross-linked polymer blend (CPB) dielectrics for application in organic field-effect transistors (OFETs). Novel silane cross-linking reagents enable the synthesis of CPB films having excellent quality and tunable thickness (from 10 to approximately 500 nm), fabricated both by spin-coating and gravure-printing. Silane reagents of the formula X 3Si-R-SiX 3 (R = -C 6H 12- and X = Cl, OAc, NMe 2, OMe, or R = -C 2H 4-O-C 2H 4- and X = OAc) exhibit tunable reactivity with hydroxyl-containing substrates. Dielectric films fabricated by blending X 3Si-R-SiX 3 with poly(4-vinyl)phenol (PVP) require very low-curing temperatures ( approximately 110 degrees C) and adhere tenaciously to a variety of FET gate contact materials such as n (+)-Si, ITO, and Al. The CPB dielectrics exhibit excellent insulating properties (leakage current densities of 10 (-7) approximately 10 (-8) A cm (-2) at 2.0 MV/cm) and tunable capacitance values (from 5 to approximately 350 nF cm (-2)). CPB film quality is correlated with the PVP-cross-linking reagent reactivity. OFETs are fabricated with both p- and n-type organic semiconductors using the CPB dielectrics function at low operating voltages. The morphology and microstructure of representative semiconductor films grown on the CPB dielectrics is also investigated and is correlated with OFET device performance.

Replacing –CH2CH2- with -CONH- Does Not Significantly Change Rates of Charge Transport Through AgTS-SAM//Ga2O3/EGaIn Junctions

This paper describes physical-organic studies of charge transport by tunneling through self-assembled monolayers (SAMs), based on systematic variations of the structure of the molecules constituting the SAM. Replacing a -CH(2)CH(2)- group with a -CONH- group changes the dipole moment and polarizability of a portion of the molecule and has, in principle, the potential to change the rate of charge transport through the SAM. In practice, this substitution produces no significant change in the rate of charge transport across junctions of the structure Ag(TS)-S(CH(2))(m)X(CH(2))(n)H//Ga(2)O(3)/EGaIn (TS = template stripped, X = -CH(2)CH(2)- or -CONH-, and EGaIn = eutectic alloy of gallium and indium). Incorporation of the amide group does, however, increase the yields of working (non-shorting) junctions (when compared to n-alkanethiolates of the same length). These results suggest that synthetic schemes that combine a thiol group on one end of a molecule with a group, R, to be tested, on the other (e.g., HS~CONH~R) using an amide-based coupling provide practical routes to molecules useful in studies of molecular electronics.

Anthracenedicarboximide-based semiconductors for air-stable, n-channel organic thin-film transistors: materials design, synthesis, and structural characterization

A family of six n-channel organic semiconductors (1–6) based on the N,N′-dialkyl-2,3:6,7-anthracenedicarboximide (ADI) core was synthesized and characterized. These new semiconductors are functionalized with n-octyl (-n-C8H17), 1H,1H-perfluorobutyl (-n-CH2C3F7), cyano (–CN), and bromo (–Br) substituents, which results in wide HOMO and LUMO energy variations (∼1 eV) but negligible optical absorbance (λmax = 418–436 nm) in the visible region of the solar spectrum. Organic thin-film transistors (OTFTs) were fabricated via semiconductor vapor-deposition, and the resulting devices exhibit exclusively electron transport with good carrier mobilities (μe) of 10−3 to 0.06 cm2 V−1 s−1. Within this semiconductor family, cyano core-substitution plays a critical role in properly tuning the LUMO energy to enable good electron transport in ambient conditions while maintaining a low level of ambient doping (i.e., low Ioff). Core-cyanated ADIs 3 and 6 exhibit air-stable TFT device operation with electron mobilities up to 0.04 cm2 V−1 s−1 in air. Very high current on/off ratios of >107 are measured with positive threshold voltages (Vth = 5–15 V) and low off currents (Ioff = 10−9 to 10−12 A). Single-crystal structures of N,N′-1H,1H-perfluorobutyl ADIs 5 and 6 exhibit slipped-stack cofacial crystal packing with close π–π stacking distances of ∼3.2 A. Additionally, close intermolecular interactions between imide-carbonyl oxygen and anthracene core-hydrogen are identified, which lead to the assembly of highly planar lamellar layers. Analysis of the air-stability of 1–6 thin films suggests that air-stability is mainly controlled by the LUMO energetics, and an electrochemical threshold of Ered1 = −0.3 to −0.4 V is estimated to stabilize n-channel transport in this family of materials.

Asymmetric fused thiophenes for field-effect transistors: crystal structure–film microstructure–transistor performance correlations

New asymmetric phenyl and perfluorophenyl end-functionalized dithienothiophene (DTT)- and bisdithienothiophene (BDTT)-based fused-thiophene derivatives (FPP-DTT; 1 and FPP-BDTT; 3) were synthesized and characterized for organic thin-film transistor (OTFT) applications. For comparison, symmetric phenyl end-capped dithienothiophene and bisdithienothiophene derivatives DP-DTT (2) and DP-BDTT (4) were also explored in parallel. The crystal structures of all four molecules were determined via single-crystal X-ray diffraction. Asymmetric compounds 1 and 3 exhibit face-to-face π–π stacking, while symmetric 2 and 4 show herringbone stacking. Single-crystal and thin-film transistors based on these four materials were fabricated. For single-crystal transistors, asymmetric FPP-DTT and FPP-BDTT gave high p-channel mobilities of 0.74 and 0.73 cm2 V−1 s−1, respectively, as well as current on/off ratios of ∼105. Symmetric DP-DTT and DP-BDTT gave relatively lower p-channel mobilities of 0.36 and 0.41 cm2 V−1 s−1, respectively. For thin-film transistors, FPP-DTT and DP-DTT films deposited at 25 °C exhibited decent p-channel characteristics with a carrier mobility as high as 0.15 and 0.20 cm2 V−1 s−1, respectively for top-contact/bottom-gate OTFT devices. The device characteristics on various gate dielectrics have been correlated with the film morphologies and microstructures of the corresponding compounds.

Pentacene transistors fabricated on photocurable polymer gate dielectrics: tuning surface viscoelasticity and device response.

2010 WILEY-VCH Verlag Gm Organic thin-film transistors (OTFTs) have generated considerable interest for applications in low-cost electronics and as vehicles to understand the fundamental properties of organic semiconductors and dielectrics. To this end, most research efforts have targeted the development of new organic semiconductors, the OTFT components through which charge transport occurs. The other key OTFT material component is the gate dielectric, which enables the accumulation of charge carriers in the semiconductor upon application of the gate field. Among organic dielectrics, polymer dielectrics are ideal for OTFTs due to ease of processing from solution, printability, and mechanical flexibility. Since OTFT carrier transport is confined to a very thin channel ( 4–9 nm, depending on the semiconductor) in the organic semiconductor near the interface with the gate dielectric, it is well established that gate-dielectric surface properties, such as roughness, chemical functionalization, and surface energy, greatly affect the overlying semiconductor growth mode and microstructure, charge trapping density, and, hence, the OTFT performance. Our group recently reported that OTFT performance is also substantially affected by glassy polymer gate-dielectric viscoelasticity, i.e., the temperaturedependent segmental dynamics of the polymer chains. A traditional observable of this property is the glass-transition temperature of the bulk material, Tg(b), the temperature at which the material passes from the glassy to the rubbery state. With regard to polymer films, it is known that below a certain critical thickness, h, the film glass-transition temperature becomes thickness dependent (defined as Tg(h)), either elevated or depressed versus Tg(b), depending on the interplay between the free (top) and buried (bottom) interfaces of the film and the chain dynamics. This behavior is relevant to the fabrication of organic electronics, since devices are usually composed of stacked thin films with many interfaces. Previous results showed that glassy homopolymer dielectric chain motion at temperatures well below Tg(b) and Tg(h) perturbs the semiconductor film growth on the dielectric surface and modifies the OTFTperformance. We empirically defined a TFT-derived film glass-transition temperature, Tg(s), as the temperature at which the TFT carrier mobility falls by 50%. Besides providing technology-relevant strategies for device fabrication, this discovery shows that OTFT response can be used to probe and differentiate free-surface from bulk and thin-film viscoelastic properties. We showed that Tg(s) for these simple glassy polymers is always lower than both Tg(b) and Tg(h) by studying polymers where the Tg(b) was systematically varied via chemical structure, molecular weight, and film thickness. The above findings on simple glassy homopolymers raise intriguing questions concerning other polymer surface properties that might be interrogated via TFT response and whether these can be used to optimize the OTFT performance in new ways. Here, we report on the properties of pentacene transistors fabricated on photocrosslinkable polymer gate dielectrics. Introducing crosslinks in glassy polymers restricts chain segmental mobility, the degree of which is usually assessed via optical spectroscopy, changes in Tg(b) using differential scanning calorimetry (DSC), and/or dynamic mechanical analysis (DMA). Note that these techniques cannot readily distinguish between surface and bulk modifications and are often insensitive to crosslinking in photocurable polymers. For organic electronics, these techniques provide little information regarding film crosslinking, surface chain dynamics, overlying organic semiconductor film-growth mode, or the resulting TFT properties. We demonstrate here that pentacene growth characteristics and TFT response track the extent of photocrosslinking, a heretofore unexplored means to restrict gatedielectric chain dynamics. Furthemore, we show that fully photocrosslinked films are not required for maximum OTFT response, relevant to photon-efficient organic electronics production. Finally, device-performance trends can be clearly correlated with the evolution of pentacene film morphology and microstructure. Pentacene, a well-studied organic semiconductor, was employed, here, in combination with two UV-curable dielectric polymers, poly(vinyl cinnamate) (PVCi) and ActivInk D0150. Depending on the conditions, vacuum-deposited pentacene films grow in either of two polymorphs, the thin-film and the bulk phases, which are identified by d-spacings of 15.4 Å (2u1⁄4 5.748) and 14.5 Å (2u1⁄4 6.108), respectively (Fig. 1a). Furthermore, the thin-film phase is often characterized by very large grains (usually> 1mm) in contrast to the small bulk-phase crystallites, and the corresponding TFTs of the thin-film phase exhibit relatively large mobilities. The dominant pentacene film phase formed on SiO2 gate dielectrics is determined by several growth

Enhanced Performance of Benzothieno[3,2‐b]thiophene (BTT)‐Based Bottom‐Contact Thin‐Film Transistors

Three new benzothieno[3,2-b]thiophene (BTT; 1) derivatives, which were end-functionalized with phenyl (BTT-P; 2), benzothiophenyl (BTT-BT; 3), and benzothieno[3,2-b]thiophenyl groups (BBTT; 4; dimer of 1), were synthesized and characterized in organic thin-film transistors (OTFTs). A new and improved synthetic method for BTTs was developed, which enabled the efficient realization of new BTT-based semiconductors. The crystal structure of BBTT was determined by single-crystal X-ray diffraction. Within this family, BBTT, which had the largest conjugation of the BTT derivatives in this study, exhibited the highest p-channel characteristic, with a carrier mobility as high as 0.22 cm(2)  V(-1)  s(-1) and a current on/off ratio of 1×10(7) , as well as good ambient stability for bottom-contact/bottom-gate OTFT devices. The device characteristics were correlated with the film morphologies and microstructures of the corresponding compounds.

Solution-Processable BODIPY-Based Small Molecules for Semiconducting Microfibers in Organic Thin-Film Transistors

Electron-deficient π-conjugated small molecules can function as electron-transporting semiconductors in various optoelectronic applications. Despite their unique structural, optical, and electronic properties, the development of BODIPY-based organic semiconductors has lagged behind that of other π-deficient units. Here, we report the design and synthesis of two novel solution-proccessable BODIPY-based small molecules (BDY-3T-BDY and BDY-4T-BDY) for organic thin-film transistors (OTFTs). The new semiconductors were fully characterized by (1)H/(13)C NMR, mass spectrometry, cyclic voltammetry, UV-vis spectroscopy, photoluminescence, differential scanning calorimetry, and thermogravimetric analysis. The single-crystal X-ray diffraction (XRD) characterization of a key intermediate reveals crucial structural properties. Solution-sheared top-contact/bottom-gate OTFTs exhibited electron mobilities up to 0.01 cm(2)/V·s and current on/off ratios of >10(8). Film microstructural and morphological characterizations indicate the formation of relatively long (∼0.1 mm) and micrometer-sized (1-2 μm) crystalline fibers for BDY-4T-BDY-based films along the shearing direction. Fiber-alignment-induced charge-transport anisotropy (μ∥/μ⊥ ≈ 10) was observed, and higher mobilities were achieved when the microfibers were aligned along the conduction channel, which allows for efficient long-range charge-transport between source and drain electrodes. These OTFT performances are the highest reported to date for a BODIPY-based molecular semiconductor, and demonstrate that BODIPY is a promising building block for enabling solution-processed, electron-transporting semiconductor films.

Design, synthesis, and characterization of α,ω-disubstituted indeno[1,2-b]fluorene-6,12-dione-thiophene molecular semiconductors. Enhancement of ambipolar charge transport through synthetic tailoring of alkyl substituents

A series of indeno[1,2-b]fluorene-6,12-dione-thiophene derivatives with hydrocarbon substituents at α,ω-positions as side groups have been designed and synthesized. The new compounds were fully characterized by 1H/13C NMR, mass spectrometry, cyclic voltammetry, UV-vis absorption spectroscopy, differential scanning calorimetry, thermogravimetric analysis, and melting point measurements. The solid state structure of the indeno[1,2-b]fluorene-6,12-dione acceptor core has been identified based on single-crystal X-ray diffraction (XRD). The structural and electronic properties were also studied using density functional theory calculations, which were found to be in excellent agreement with the experimental findings and provided further insight. The detailed effects of alkyl chain size and orientation on the optoelectronic properties, intermolecular cohesive forces, thin-film microstructures, and charge transport performance of the new semiconductors were investigated. Two of the new solution-processable semiconductors, 2EH-TIFDKT and 2OD-TIFDKT, were deposited as thin-films via solution-shearing, drop-casting, and droplet-pinned crystallization methods, and their morphologies and microstructures were investigated by X-ray diffraction (XRD) and atomic force microscopy (AFM). The solution-processed thin-film transistors based on 2EH-TIFDKT and 2OD-TIFDKT showed ambipolar device operations with electron and hole mobilities as high as 0.12 cm2 V−1 s−1 and 0.02 cm2 V−1 s−1, respectively, with Ion/Ioff ratios of 105 to 106. Here, we demonstrate that rational repositioning of the β-substituents to molecular termini greatly benefits the π-core planarity while maintaining a good solubility, and results in favorable structural and optoelectronic characteristics for more efficient charge-transport in the solid-state. The ambipolar charge carrier mobilities were increased by two–three orders of magnitude in the new indeno[1,2-b]fluorene-6,12-dione-thiophene core on account of the rational side-chain engineering.

Gas-liquid mass transfer coefficient of methane in bubble column reactor

Biological conversion of methane gas has been attracting considerable recent interest. However, methanotropic bioreactor is limited by low solubility of methane gas in aqueous solution. Although a large mass transfer coefficient of methane in water could possibly overcome this limitation, no dissolved methane probe in aqueous environment is commercially available. We have developed a reactor enabling the measurement of aqueous phase methane concentration and mass transfer coefficient (kLa). The feasibility of the new reactor was demonstrated by measuring kLa values as a function of spinning rate of impeller and flow rate of methane gas. Especially, at spinning rate of 300 rpm and flow rate of 3.0 L/min, a large kLa value of 102.9 h−1 was obtained.