Taisong Pan

Field effect transistors with layered two-dimensional SnS2−xSex conduction channels: Effects of selenium substitution

A thorough characterization of field effect transistors with conduction channels made of SnS2−xSex nanocrystals having different selenium content is presented. The main effect of increasing the selenium content is a suppression of the drain-source current modulation by the gate voltage. The temperature dependence of SnS2−xSex conductivity for all compositions is characterized by an activation energy that gradually decreases with x. A simple donor model, with parameters of SnS2 and SnSe2 deduced from density functional theory, suggests that the change in the activation energy is mostly due to enhanced dielectric constants that accompany the band gap reduction in SnS2−xSex.

Epitaxial growth and metal-insulator transition of vanadium oxide thin films with controllable phases

Vanadium oxide thin films with well controlled phases such as rhombohedra V2O3 and monoclinic VO2 were synthesized on Al2O3 (0001) substrates by optimizing the processing parameters of a polymer assisted deposition technique. X-ray diffraction and high-resolution transmission electron microscopy studies revealed that both V2O3 and VO2 films can be well controlled with good epitaxial quality. The temperature dependency of electrical resistivity demonstrated sharp metal-insulator transitions (MITs) for V2O3 and VO2 films. The crystallinity and the strains in the films are believed to play critical roles in determining the MIT properties.

Enhanced thermal conductivity of polycrystalline aluminum nitride thin films by optimizing the interface structure

The growth-temperature dependency and interface structure effects on the thermal conductivity of the highly textured AlN thin films on (001) Si substrates were systematically studied by characterizing the crystal structures, surface morphologies, interface structures, chemical compositions, and thermal conductivity using x-ray diffraction analysis, atomic force microscopy, high resolution transmission electron microscopy, x-ray photoelectron spectroscopy, and 3-omega method, respectively. By optimizing the interface microstructure and the growth temperature, thermal conductivity of polycrystalline AlN thin films can be greatly enhanced from 9.9 to 26.7 W/mK, when the growth temperature increases from 330 to 560 °C. This achievement is considered to be associated with the diminishment of the amorphous and disordered layer at the AlN/Si interface.

Highly stretchable and shape-controllable three-dimensional antenna fabricated by “Cut-Transfer-Release” method

Recent progresses on the Kirigami-inspired method provide a new idea to assemble three-dimensional (3D) functional structures with conventional materials by releasing the prestrained elastomeric substrates. In this paper, highly stretchable serpentine-like antenna is fabricated by a simple and quick “Cut-Transfer-Release” method for assembling stretchable 3D functional structures on an elastomeric substrate with a controlled shape. The mechanical reliability of the serpentine-like 3D stretchable antenna is evaluated by the finite element method and experiments. The antenna shows consistent radio frequency performance with center frequency at 5.6 GHz during stretching up to 200%. The 3D structure is also able to eliminate the hand effect observed commonly in the conventional antenna. This work is expected to spur the applications of novel 3D structures in the stretchable electronics.

Thermal Release Transfer Printing for Stretchable Conformal Bioelectronics

Abstract Soft neural electrode arrays that are mechanically matched between neural tissues and electrodes offer valuable opportunities for the development of disease diagnose and brain computer interface systems. Here, a thermal release transfer printing method for fabrication of stretchable bioelectronics, such as soft neural electrode arrays, is presented. Due to the large, switchable and irreversible change in adhesion strength of thermal release tape, a low‐cost, easy‐to‐operate, and temperature‐controlled transfer printing process can be achieved. The mechanism of this method is analyzed by experiments and fracture‐mechanics models. Using the thermal release transfer printing method, a stretchable neural electrode array is fabricated by a sacrificial‐layer‐free process. The ability of the as‐fabricated electrode array to conform different curvilinear surfaces is confirmed by experimental and theoretical studies. High‐quality electrocorticography signals of anesthetized rat are collected with the as‐fabricated electrode array, which proves good conformal interface between the electrodes and dura mater. The application of the as‐fabricated electrode array on detecting the steady‐state visual evoked potentials research is also demonstrated by in vivo experiments and the results are compared with those detected by stainless‐steel screw electrodes.

Carbon nanotube-graphene junctions studied by impedance spectra

Two kinds of carbon nanotube (CNT)-graphene structures, vertical CNT-graphene and paralleled CNT–graphene, were fabricated to investigate the geometrical effect on the transport properties of the CNT–graphene junctions by using AC impedance spectra. The results demonstrated that the geometrical structure showed obvious impact on the resistance rather than the capacity of the junction. It is proposed that the difference caused by the geometrical structure may be associated with the dangling bonds terminated by –OH or –COOH of the open-ended CNTs. The unsymmetrical chemical bonds will increase the dipole moment in CNTs, which enhance the interaction between vertical CNTs and graphene and reduce the contact resistance.

Ultrafast response flexible breath sensor based on vanadium dioxide

Real-time monitoring of breath can provide clinically relevant information about apnea syndrome and other important aspects of human physiology. Here, we introduce a flexible skin-like breath sensor developed by transfer-printing vanadium dioxide (VO2) thin films on PDMS substrates. This flexible breath sensor can conformably laminate on the skin under the nose with different curvatures and operate at different environment temperatures through day and night. Attributed to the high temperature coefficient of resistance of VO2, the enhanced breath sensing performance was demonstrated and the response time and recovery time can be as fast as 0.5 s. The excellent sensing performance and fast response time indicate that the VO2-based breath sensor is feasible in monitoring breath for prevention of apnea syndrome.

Microstructure dependence of heat sink constructed by carbon nanotubes for chip cooling

Carbon nanotube (CNT) arrays with aligned growth orientation and sponge CNTs which consist of flocculent carbon fibers with tiny CNTs on their surface were synthesized by chemical vapor deposition. Heat sinks based on CNT arrays or sponge CNTs were made to investigate their heat dissipation performance. It is found that their microstructures have strong impacts on the thermal performance by changing the coefficient of air convection. The long CNT arrays have good heat dissipation performance even under natural convection for its aligned structure and large contacting area with the air, while the sponge CNTs show larger improvement in heat dissipation ability under airflow due to their porous structure. The results give a good reference for developing low-cost, light-weight, and high-performance CNT-based heat sinks for chip cooling under different working conditions.

Three-dimensional integrated stretchable electronics

Stretchable electronics is an emerging technology that creates devices with the ability to conform to nonplanar and dynamic surfaces such as the human body. Current stretchable configurations are constrained to single-layer designs due to limited material processing capabilities in soft electronic systems. Here we report a framework for engineering three-dimensional integrated stretchable electronics by combining strategies in material design and advanced microfabrication. Our three-dimensional devices are built layer by layer through transfer printing pre-designed stretchable circuits on elastomers and creating vertical interconnect accesses using laser ablation and controlled soldering. Our approach enables a higher integration density on stretchable substrates than single-layer approaches and allows new functionalities that would be difficult to implement with conventional single-layer designs. Using this engineering framework, we create a stretchable human–machine interface testbed that is based on a four-layer design and offers eight-channel sensing and Bluetooth data communication capabilities.By combining strategies in material design and advanced microfabrication, three-dimensional integrated stretchable electronic devices can be created, including an eight-channel sensing system with Bluetooth communication capabilities that can be used to extract an array of signals from the human body.

ZnO particles enhanced graphene-based hybrid light sensors

Graphene with excellent mechanical properties shows potential applications in flexible devices. By decorating graphene with ZnO nanoparticles, the response of graphene-based flexible light sensor to light was enhanced. The light sensor showed different response rates with light intensities under different wavelength which indicated that the as-designed sensor can be used to detect the wavelength and intensity of light simultaneously. The microstructures and photonic properties of the hybrid device revealed that the interface played an important role on the performance of device. This result confirmed the method to prepare graphene/ZnO hybrid light sensor are convenient and economical, which may be applied in wearable devices.