Gen-Wen Hsieh

Inkjet-printed graphene electronics.

We demonstrate inkjet printing as a viable method for large-area fabrication of graphene devices. We produce a graphene-based ink by liquid phase exfoliation of graphite in N-methylpyrrolidone. We use it to print thin-film transistors, with mobilities up to ∼95 cm(2) V(-1) s(-1), as well as transparent and conductive patterns, with ∼80% transmittance and ∼30 kΩ/□ sheet resistance. This paves the way to all-printed, flexible, and transparent graphene devices on arbitrary substrates.

High performance nanocomposite thin film transistors with bilayer carbon nanotube-polythiophene active channel by ink-jet printing

Nanocomposite thin film transistors (TFTs) based on nonpercolating networks of single-walled carbon nanotubes (CNTs) and polythiophene semiconductor [poly[5,5′-bis(3-dodecyl-2-thienyl)-2,2′-bithiophene] (PQT-12)] thin film hosts are demonstrated by ink-jet printing. A systematic study on the effect of CNT loading on the transistor performance and channel morphology is conducted. With an appropriate loading of CNTs into the active channel, ink-jet printed composite transistors show an effective hole mobility of 0.23 cm2 V−1 s−1, which is an enhancement of more than a factor of 7 over ink-jet printed pristine PQT-12 TFTs. In addition, these devices display reasonable on/off current ratio of 105–106, low off currents of the order of 10 pA, and a sharp subthreshold slope (<0.8 V dec−1). The work presented here furthers our understanding of the interaction between polythiophene polymers and nonpercolating CNTs, where the CNT density in the bilayer structure substantially influences the morphology and transisto...

Zinc Oxide Nanostructures and High Electron Mobility Nanocomposite Thin Film Transistors

This paper reports on the synthesis of zinc oxide (ZnO) nanostructures and examines the performance of nanocomposite thin-film transistors (TFTs) fabricated using ZnO dispersed in both n- and p-type polymer host matrices. The ZnO nanostructures considered here comprise nanowires and tetrapods and were synthesized using vapor phase deposition techniques involving the carbothermal reduction of solid-phase zinc-containing compounds. Measurement results of nanocomposite TFTs based on dispersion of ZnO nanorods in an n-type organic semiconductor ([6, 6]-phenyl-C61-butyric acid methyl ester) show electron field-effect mobilities in the range 0.3-0.6 cm2 V-1s-1, representing an approximate enhancement by as much as a factor of 40 from the pristine state. The on/off current ratio of the nanocomposite TFTs approach 106 at saturation with off-currents on the order of 10 pA. The results presented here, although preliminary, show a highly promising enhancement for realization of high-performance solution-processable n-type organic TFTs.

Zinc Oxide Nanowire-Poly(Methyl Methacrylate) Dielectric Layers for Polymer Capacitive Pressure Sensors

Polymer capacitive pressure sensors based on a dielectric composite layer of zinc oxide nanowire and poly(methyl methacrylate) show pressure sensitivity in the range of 2.63 × 10(-3) to 9.95 × 10(-3) cm(2) gf(-1). This represents an increase of capacitance change by as much as a factor of 23 over pristine polymer devices. An ultralight load of only 10 mg (corresponding to an applied pressure of ∼0.01 gf cm(-2)) can be clearly recognized, demonstrating remarkable characteristics of these nanowire-polymer capacitive pressure sensors. In addition, optical transmittance of the dielectric composite layer is approximately 90% in the visible wavelength region. Their low processing temperature, transparency, and flexible dielectric film makes them a highly promising means for flexible touching and pressure-sensing applications.

Bond-and-transfer scanning probe array for high-density data storage

This paper reports a study of a wafer-level bond-and-transfer technique for scanning probe arrays and its future application on probe-based data storage. The bonding performance between sodium ion-rich glass and silicon-nitride-deposited silicon substrate has been characterized. The effects of tool pressure, bonding time, surface properties, and cleanliness were thoroughly discussed. Furthermore, the silicon-nitride-based scanning probe array with pyramidal tip and 1.5-/spl mu/m-thick cantilevers were successful bonded and transferred to Pyrex 7740 substrate by the optimized condition of wafer-scale electrostatic force bonding and transferring processes. The nano-patterning capabilities of scanning probe array for high-density data storage were also discussed.

Selective carbon nanotube growth on silicon tips with the soft electrostatic force bonding and catalyst transfer concepts

To integrate a single carbon nanotube (CNT) onto a silicon tip, the area of deposited catalyst on the apex of the silicon tip, that influences the quantity of grown CNTs, is critical. Therefore, a method of selective CNT growth with soft electrostatic force bonding and catalyst transfer concepts has been proposed in this paper. In this work, silicon tips were fabricated by bulk micromachining. The tiny catalyst dot on the tip apex was transferred from a nickel-deposited sodium-ion-rich glass wafer by wafer bonding. Finally, CNTs were synthesized on the tip apexes by methane thermal chemical vapour deposition. From the experimental results, the oriented CNTs were observed on roughly 10% of the silicon tips. The yield could be further increased by optimizing the silicon tip structure, contact pressure of soft bonding, and catalyst deposition.

Anodic bonding of glass and silicon wafers with an intermediate silicon nitride film and its application to batch fabrication of SPM tip arrays

This paper reports a detailed study of wafer-level anodic bonding with a dielectric intermediate layer and its application to the fabrication of scanning probe microscope (SPM) probe arrays. First, the bonding performance between sodium-ion rich glass and silicon nitride coated silicon substrate is characterized. The effects of voltage, tool pressure, bonding time, surface properties, and cleanliness are thoroughly studied. Then, the silicon nitride based SPM probe arrays consisted of pyramidal tip and 1.5 μm-thickness cantilever are successful bonded and transferred to Pyrex 7740 glass substrate by use of our optimized wafer-scale electrostatic force bonding condition. The nano-imaging capability of the scanning probe array is also demonstrated.

Microstructuring characteristics of a chemically amplified photoresist synthesized for ultra-thick UV-LIGA applications

The thick-film photoresists are essential to fabricate high-aspect-ratio microstructures by the UV-LIGA process. However, current thick-film photoresists have some weaknesses including a thickness of only up to 100 µm, a poor line-width resolution and difficulty in being stripped. Consequently, a new type of thick-film photoresist is required. This work presents a novel positive-tone MMA/TBMA photoresist, formed by combining copolymerization and chemically amplification (CA) for use in the ultra-thick UV-LIGA process. An MMA/TBMA photoresist film with a thickness of 500 µm is easily achieved. For MMA/TBMA photoresist layers with thicknesses from 100 µm to 500 µm, an exposure dose from 80 to 100 mJ cm−2 per micron is required to remove all of the exposed photoresist, revealing that the selectivity between radiated and non-radiated zones during a long development process is sufficiently high; the sidewall verticality and aspect ratio of the microstructure are excellent; stress-induced cracks are not observed in the non-radiated zones after development. MMA/TBMA photoresist is demonstrated to fabricate open microstructures with aspect ratios of at least 10 and close microstructures with aspect ratios of not more than 10, such values of aspect ratio are still sufficient for most ultra-thick mold applications. Moreover, MMA/TBMA photoresist can undergo erosion by acidic electrolyte and easily be stripped using usual organic solvents. These findings demonstrate that MMA/TBMA photoresist has the potential to replace SU-8 resist in the ultra-thick UV-LIGA process.

Thin-film transistors based on poly(3,3‴-dialkyl-quarterthiophene) and zinc oxide nanowires with improved ambient stability

The ambient stability of thin-film transistors (TFTs) based on zinc oxide (ZnO) nanowires embedded in poly(3,3‴-dialkyl-quarterthiophene) was monitored through time dependence of electrical characteristics over a period of 16 months. The hybrid-based TFT showed an initial hole mobility in the linear regime of 4.2×10−4 cm2/V s. After 16 months storage in ambient conditions (exposed to air, moisture, and light) the mobility decreased to 2.3×10−5 cm2/V s. Comparatively the organic-based TFT lost total carrier mobility after one month storage making the hybrid-based TFTs more suitable for transistor applications when improved stability combined with structural flexibility are required.

Fabrication of individual aligned carbon nanotube for scanning probe microscope

This paper demonstrates the fabrication of a micro-cantilever equipped with a high aspect ratio carbon nanotube (CNT) tip to perform surface characterization with high spatial resolution. A single vertical CNT, synthesized on a freestanding tip-lessmicro cantilever, is intended to be used for scanning probe microscopy (SPM). E-beam lithography and lift-off technique was adopted to define a nano-sized catalyst particle. An individual CNT was then synthesized by using an inductively coupled plasma chemical vapor deposition (ICP-CVD) system. The freestanding individual aligned CNT probe was then used to characterize a silicon nano pillar structure. The surface morphology was scanned in a contact mode and 0.1 nN set-point force. The ultra fine resolution can be characterized without wearing the probe and sample. The CNT probe shows a great potential as a high sensitive sensor operating in an optimally adjusted atomic force microscopy (AFM) system.