Zhengtao Zhu

Pattern Transfer Printing by Kinetic Control of Adhesion to an Elastomeric Stamp

An increasing number of technologies require large-scale integration of disparate classes of separately fabricated objects into spatially organized, functional systems1,2,3,4,5,6,7,8,9. Here we introduce an approach for heterogeneous integration based on kinetically controlled switching between adhesion and release of solid objects to and from an elastomeric stamp. We describe the physics of soft adhesion that govern this process and demonstrate the method by printing objects with a wide range of sizes and shapes, made of single-crystal silicon and GaN, mica, highly ordered pyrolytic graphite, silica and pollen, onto a variety of substrates without specially designed surface chemistries or separate adhesive layers. Printed p–n junctions and photodiodes fixed directly on highly curved surfaces illustrate some unique device-level capabilities of this approach.

Humidity sensors based on pentacene thin-film transistors

When a pentacene thin-film transistor is exposed to humidity, its saturation current decreases. This decrease was found to be reversible and can therefore be used to measure the amount of relative humidity in atmosphere. The sensitivity was found to depend on the thickness of the pentacene layer. The microscopic origin of the sensing mechanism is discussed.

Mechanically flexible thin-film transistors that use ultrathin ribbons of silicon derived from bulk wafers

This letter introduces a type of thin-film transistor that uses aligned arrays of thin (submicron) ribbons of single-crystal silicon created by lithographic patterning and anisotropic etching of bulk silicon (111) wafers. Devices that incorporate such ribbons printed onto thin plastic substrates show good electrical properties and mechanical flexibility. Effective device mobilities, as evaluated in the linear regime, were as high as 360cm2V−1s−1, and on/off ratios were >103. These results may represent important steps toward a low-cost approach to large-area, high-performance, mechanically flexible electronic systems for structural health monitors, sensors, displays, and other applications.

High-speed mechanically flexible single-crystal silicon thin-film transistors on plastic substrates

This letter describes the fabrication and properties of bendable single-crystal-silicon thin film transistors formed on plastic substrates. These devices use ultrathin single-crystal silicon ribbons for the semiconductor, with optimized device layouts and low-temperature gate dielectrics. The level of performance that can be achieved approaches that of traditional silicon transistors on rigid bulk wafers: effective mobilities>500cm/sup 2//V/spl middot/s, ON/OFF ratios >10/sup 5/, and response frequencies > 500 MHz at channel lengths of 2 /spl mu/m. This type of device might provide a promising route to flexible digital circuits for classes of applications whose performance requirements cannot be satisfied with organic semiconductors, amorphous silicon, or other related approaches.

Transparent flexible organic thin-film transistors that use printed single-walled carbon nanotube electrodes

Electrodes based on printed networks of single-walled carbon nanotubes (SWNTs) are integrated with ultrathin layers of the organic semiconductor pentacene to produce bendable, transparent thin-film transistors on plastic substrates. The physical and structural properties of the SWNTs lead to the remarkably good electrical contacts with the pentacene. Optical transmittances of ∼70%, device mobilities >0.5cm2V−1s−1, ON/OFF ratios >105 and tensile strains as large as 1.8% are achieved in devices of this type. These characteristics indicate promise for applications in power conserving flexible display systems and other devices.

Composite of TiO2 nanofibers and nanoparticles for dye-sensitized solar cells with significantly improved efficiency

A composite made of electrospun TiO2 nanofibers and conventional TiO2 nanoparticles is an innovative type of photoanode, which noticeably improves the harvesting of light without substantially sacrificing the attachment (uptake) of dye molecules for convenient fabrication of dye-sensitized solar cells with significantly improved efficiency.

Spin on Dopants for High-Performance Single-Crystal Silicon Transistors on Flexible Plastic Substrates

Free-standing micro/nanoelements of single-crystal silicon with integrated doped regions for contacts provide a type of material that can be printed onto low-temperature device substrates, such as plastic, for high-performance mechanically flexible thin-film transistors (TFTs). We present simple approaches for fabricating collections of these elements, which we refer to as microstructured silicon (μs-Si), and for using spin-on dopants to introduce doped regions in them. Electrical and mechanical measurements of TFTs formed on plastic substrates with this doped μs-Si indicate excellent performance. These and other characteristics make the material potentially useful for emerging large area, flexible ‘macroelectronic’ devices.Free-standing micro/nanoelements of single-crystal silicon with integrated doped regions for contacts provide a type of material that can be printed onto low-temperature device substrates, such as plastic, for high-performance mechanically flexible thin-film transistors (TFTs). We present simple approaches for fabricating collections of these elements, which we refer to as microstructured silicon (μs-Si), and for using spin-on dopants to introduce doped regions in them. Electrical and mechanical measurements of TFTs formed on plastic substrates with this doped μs-Si indicate excellent performance. These and other characteristics make the material potentially useful for emerging large area, flexible ‘macroelectronic’ devices.

Bendable integrated circuits on plastic substrates by use of printed ribbons of single-crystalline silicon

This letter presents studies of several simple integrated circuits—n-channel metal-oxide semiconductor inverters, five-stage ring oscillators, and differential amplifiers—formed on thin, bendable plastic substrates with printed ribbons of ultrathin single-crystalline silicon as the semiconductor. The inverters exhibit gains as high as 2.5, the ring oscillators operate with oscillation frequencies between 8 and 9MHz at low supply voltages (∼4V), and the differential amplifiers show good performance and voltage gains of 1.3 for 500mV input signals. The responses of these systems to bending-induced strains show that relatively moderate changes of individual transistors can be significant for the operation of circuits that incorporate many transistors.

A simple poly(3,4-ethylene dioxythiophene)/poly(styrene sulfonic acid) transistor for glucose sensing at neutral pH

We demonstrate a simple transistor based on the conducting polymer poly(3,4-ethylene dioxythiophene)/poly(styrene sulfonic acid), capable of sensing glucose in a neutral pH buffer solution by a mechanism involving sensing of hydrogen peroxide.

Aligned electrospun ZnO nanofibers for simple and sensitive ultraviolet nanosensors.

The current of uni-axially aligned electrospun ZnO nanofibers is modulated reversibly under UV irradiation, with the sensitivity of the UV nanosensors depending on the surface coating of the nanofibers, due to the effect on the photo-generated current.