Hou Tee Ng

Bottom-up approach for carbon nanotube interconnects

We report a bottom-up approach to integrate multiwalled carbon nanotubes (MWNTs) into multilevel interconnects in silicon integrated-circuit manufacturing. MWNTs are grown vertically from patterned catalyst spots using plasma-enhanced chemical vapor deposition. We demonstrate the capability to grow aligned structures ranging from a single tube to forest-like arrays at desired locations. SiO2 is deposited to encapsulate each nanotube and the substrate, followed by a mechanical polishing process for planarization. MWNTs retain their integrity and demonstrate electrical properties consistent with their original structure.

Optical properties of single-crystalline ZnO nanowires on m-sapphire

ZnO nanowires have been synthesized using a catalyst-assisted heteroepitaxial carbothermal reduction approach on a m-sapphire substrate. Intricate and uniform arrays have been obtained with each nanowire forming an angle ∼30° with the substrate normal. Photoluminescence studies at room temperature for wavelengths between 335 and 620 nm reveal a strong single exciton peak at ∼380 nm (3.26 eV) with accompanying deep-level blueshifted emission peaks at ∼486, 490, and 510 nm. UV resonant Raman spectroscopy has been used to characterize the nanowires at room temperature with multiphonon scattering exhibiting phonon quantum confinement.

The fabrication and electrochemical characterization of carbon nanotube nanoelectrode arrays

We report a novel approach for the fabrication of nanoelectrode arrays using vertically aligned multi-walled carbon nanotubes (MWCNTs) embedded within a SiO2 matrix. Cyclic voltammetry and pulse voltammetry are employed to characterize the electrochemical properties of the MWCNT array. The unique graphitic structure of the novel MWCNT nanoelectrodes is compared with model systems such as highly oriented pyrolytic graphite and glassy carbon electrodes. Low-density MWCNT nanoelectrode arrays display independent nanoelectrode behavior showing diffusion-limited steady-state currents in cyclic voltammetry over a wide range of scan rates. Electroactive species can be detected at concentrations as low as a few nM. In addition, ultrasensitive DNA/RNA sensors are demonstrated using the low-density MWCNT arrays with selectively functionalized oligonucleotide probes. This platform can be widely used in analytical applications as well as fundamental electrochemical studies.

Ultrasensitive label-free DNA analysis using an electronic chip based on carbon nanotube nanoelectrode arrays

We report the detection of DNA PCR amplicons using an ultrasensitive label-free electronic technique based on multiwalled carbon nanotube (MWNT) nanoelectrode arrays embedded in an SiO(2) matrix. Specific PCR amplicons are reliably detected using electrochemical (EC) methods through allele-specific oligonucleotide hybridization. The inherent guanine bases in the DNA amplicon target of [Formula: see text] bases serve as signal moieties with the aid of Ru(bpy)(3)(2+) mediators, providing an amplified anodic current associated with the oxidation of guanine groups at the nanoelectrode surface. The reduced size and density of the nanoelectrode array provided by MWNTs dramatically improves the sensitivity of EC detection. In addition, the abundant guanine bases in target DNA produce a large signal. Less than [Formula: see text] target amplicons can be detected on a microspot, approaching the sensitivity limit of conventional laser-based fluorescence techniques. This method also eliminates the labelling requirement and makes the measurements much simpler. This platform can be employed for developing highly automated electronic chips with multiplex nanoelectrode arrays for quick DNA analysis.

Electronic properties of multiwalled carbon nanotubes in an embedded vertical array

We demonstrate integration of carbon nanotubes into large scale vertically aligned electrode arrays, by filling the as-grown samples with conformal SiO2 using chemical vapor deposition. Subsequent mechanical polishing yields a flat surface with only the very ends of the nanotube array exposed. The electronic properties of individual carbon nanotubes in the array are measured using current-sensing atomic force microscopy. These vertical nanotube arrays are suitable for fabricating various electronic devices and sensors.

A high-performance glucose biosensor based on monomolecular layer of glucose oxidase covalently immobilised on indium-tin oxide surface.

In this paper, a mediatorless amperometric glucose biosensor based on direct covalent immobilisation of monomolecular layer of glucose oxidase (GOx) on a semiconducting indium-tin oxide (ITO) is demonstrated. The abundance of surface hydroxyl functional group of ITO allows it to be used as a suitable platform for direct covalent immobilisation of the enzyme for sensor architecture. The anodic current corresponding to electrochemical oxidation of the enzymatic product, hydrogen peroxide, at a sputtered Pt electrode at 0.500 V (vs. SCE) was obtained as the sensor signal. It was found that the biosensor based on the direct immobilisation scheme shows a fast biosensor response, minimum interference from other common metabolic species and ease of biosensor miniaturisation. A linear range of 0-10 mM of glucose was demonstrated, which exhibits a high sensitivity as far as performance per immobilised GOx molecule is concerned. A detection limit as low as 0.05 mM and long-term stability were observed. Even more important, the biosensor design allows fabrication through a dry process. These characteristics make it possible to achieve mass production of biosensor compatible with the current electronic integrated circuit manufacturing technologies.

Combinatorial chips for optimizing the growth and integration of carbon nanofibre based devices

The growth of carbon nanofibres (CNF) by plasma enhanced chemical vapour deposition (PECVD) and the integration of CNFs into devices are studied using high throughput methodology. A growth compatibility chip containing candidate metal contact underlayers and transition metal catalyst layers are used to explore growth activity from various pairings of these two layers. In addition, catalyst density microarray chips (CDMCs) where each chip consists of catalyst patterns with various feature sizes and densities were used to investigate CNF growth and integration. The CDMCs were used to efficiently explore CNF growth conditions as well as subsequent downstream integration and testing for applications in ultrasensitive biosensing and field emission. The methodology described here is widely applicable as a general approach for optimizing the bottom-up growth and integration of nanostructures.

Carbon nanotube networks by chemical vapor deposition

We have demonstrated assembly of two- and three-dimensional networks of single-walled carbon nanotubes (SWNTs) using a microsphere assembly approach. The catalyst microcapsules are made from the solution based impregnation of uniform diameter, porous polystyrene microspheres. Chemical vapor deposition on the microcapsule arrays produces highly interconnected SWNT networks. Varying the microsphere diameter and catalyst solution composition allows varying the pattern spacing, catalyst yield, and network interconnectivity.

Carbon nanotube interconnects: a process solution

The susceptibility of common interconnect metals to electromigration at current densities of 10/sup 6/ A/cm/sup 2/ or greater has been a concern. The ITRS Roadmap emphasizes interconnect technology as a critical element and calls for innovative material and process solutions. This talk will present the potential of carbon nanotubes (CNTs) as interconnects and a processing scheme to integrate them in device fabrication.

Vertically aligned carbon nanotube heterojunctions

The bottom-up fabrication and electrical properties of end-to-end contacted multiwalled carbon nanotube (MWCNT) heterojunctions are reported. The vertically aligned MWCNT heterojunction arrays are formed via successive plasma-enhanced chemical vapor deposition processing to achieve the layered junction architecture. Electron microscopy and current-sensing atomic force microscopy are used to reveal the physical nature of the junctions. Symmetric, nonlinear I–V curves of the as-fabricated junctions indicate that a tunnel barrier is formed between the end-to-end contacted MWCNTs. Repeated high bias I–V scans of many devices connected in parallel fuses the heterojunctions, as manifested by a shift to linear I–V characteristics.The bottom-up fabrication and electrical properties of end-to-end contacted multiwalled carbon nanotube (MWCNT) heterojunctions are reported. The vertically aligned MWCNT heterojunction arrays are formed via successive plasma-enhanced chemical vapor deposition processing to achieve the layered junction architecture. Electron microscopy and current-sensing atomic force microscopy are used to reveal the physical nature of the junctions. Symmetric, nonlinear I–V curves of the as-fabricated junctions indicate that a tunnel barrier is formed between the end-to-end contacted MWCNTs. Repeated high bias I–V scans of many devices connected in parallel fuses the heterojunctions, as manifested by a shift to linear I–V characteristics.