Showing papers by "Jian-Hao Chen published in 2007"

Atomic Structure of Graphene on SiO2

Abstract: We employ scanning probe microscopy to reveal atomic structures and nanoscale morphology of graphene-based electronic devices (i.e., a graphene sheet supported by an insulating silicon dioxide substrate) for the first time. Atomic resolution scanning tunneling microscopy images reveal the presence of a strong spatially dependent perturbation, which breaks the hexagonal lattice symmetry of the graphitic lattice. Structural corrugations of the graphene sheet partially conform to the underlying silicon oxide substrate. These effects are obscured or modified on graphene devices processed with normal lithographic methods, as they are covered with a layer of photoresist residue. We enable our experiments by a novel cleaning process to produce atomically clean graphene sheets.

Printed Graphene Circuits

Abstract: we have fabricated transparent electronic devices based on graphene materials with thickness down to one single atomic layer by the transfer printing method. The resulting printed graphene devices retain high field effect mobility and have low contact resistance. The results show that the transfer printing method is capable of high-quality transfer of graphene materials from silicon dioxide substrates, and the method thus will have wide applications in manipulating and delivering graphene materials to desired substrate and device geometries. Since the method is purely additive, it exposes graphene (or other functional materials) to no chemical preparation or lithographic steps, providing greater experimental control over device environment for reproducibility and for studies of fundamental transport mechanisms. Finally, the transport properties of the graphene devices on the PET substrate demonstrate the non-universality of minimum conductivity and the incompleteness of the current transport theory.

Transmission line impedance of carbon nanotube thin films for chemical sensing applications.

Abstract: The authors measure the resistance and frequency-dependent gate capacitance of carbon nanotube (CNT) thin films in ambient, vacuum, and under low pressure (10−6Torr) analyte environments. They model the CNT film as a RC transmission line and show that changes in the measured capacitance as a function of gate bias and analyte pressure are consistent with changes in the transmission line impedance due to changes in the CNT film resistivity alone; the electrostatic gate capacitance of the CNT film does not depend on gate voltage or chemical analyte adsorption. However, the CNT film resistance is enormously sensitive to low pressure analyte exposure.

Transfer Printing as a Method for Fabricating Hybrid Devices on Flexible Substrates

Abstract: Printing methods are becoming important in the fabrication of flexible electronics. A transfer printing method has been developed for the fabrication of organic thin-film transistors (OTFT), capacitors, resistors and inductors onto plastic substrates. The method relies primarily on differential adhesion for the transfer of a printable layer from a transfer substrate to a device substrate. A range of materials applications is illustrated, including metals, organic semiconductors, organic dielectrics, nanotube and nanowire mats, a patterned inorganic semiconductor and graphene. Transfer printing can be used to create complex structures including many disparate materials sequentially printed onto the flexible substrate, with no mixed processing steps performed on the device substrate. Specifically, the fabrication and performance of model OTFT devices consisting of a polyethylene terephthalate (PET) substrate, gold (Au) gate and source/drain electrodes, a poly(methyl methacrylate) (PMMA) dielectric layer and either a pentacene (Pn) or a poly(3hexylthiophene) (P3HT) organic semiconductor layer will be presented. These transfer printed OTFTs on plastic outperform non-printed devices on a Si substrate with a SiO2 dielectric layer (SiO2/Si). Transfer printed Pn OTFTs on a plastic substrate have exhibited mobilities of 0.237 cm 2

Organic and carbon-based electronics printed onto flexible substrates

Abstract: In this paper, examples of organic and carbon-based thin-film transistors (TFT) that have been fabricated onto plastic substrates. The fabrication of these devices was achieved using the transfer printing method. With this method, each of the components (Au electrodes, polymer dielectric layer and semiconductor layer) were printed onto a plastic substrate using only pressure and temperature. This constitutes a very simple fabrication process that relies primarily on differential adhesion and eliminates all chemical processing on the device substrate.