Teng-Fang Kuo

DNA-directed synthesis of zinc oxide nanowires on carbon nanotube tips

This paper describes a class of three component hybrid nanowires templated by DNA directed self-assembly. Through the modification of carbon nanotube (CNT) termini with synthetic DNA oligonucleotides, gold nanoparticles are delivered, via DNA hybridization, to CNT tips that then serve as growth sites for zinc oxide (ZnO) nanowires. The structures we have generated using DNA templating represent an advance toward building higher order sequenced one dimensional nanostructures with rational control.

Electron transport characteristics of the carbon nanotubes/Si heterodimensional heterostructure

The properties of nanosize heterojunctions are of increasing interest as the trend of scaling down the size of electronic devices continues. We present here the direct growth of carbon nanotubes on a silicon substrate to form a heterodimensional heterojunction. Current-voltage measurements reveal the characteristics of a Schottky diode. However, a close examination of the data suggests that the device is limited in the forward bias direction by space charge limited current. In the reverse direction, it is functionally altered by the heterodimensionality of the junction and its associated enhancement of field emission.

Photocurrent response of the carbon nanotube-silicon heterojunction array

A highly ordered array of parallel, identical carbon nanotubes is grown non-lithographically in a bottom-up fabrication approach to form a heterojunction with a silicon substrate. Evidence of a space-charge separated region at the nanotube–silicon interface is present in the form of diode rectification and a closed-circuit zero-bias photocurrent in response to infrared light. Because carbon nanotubes are narrow bandgap semiconductors, their heterojunction with silicon was analysed spectrally via Fourier transform infrared photocurrent spectroscopy with the aim of investigating the suitability of this structure for infrared (IR) detector applications. IR photoresponse shows signs of temperature-dependent activation that is complex but consistent with estimates of the heterojunction barrier height. Considering the many interesting benefits and properties of carbon nanotubes, these results despite their earliness suggest that nanotube–silicon heterojunction systems could form the foundation for a new kind of infrared detection device.

Electron transport in carbon nanotube-silicon heterodimensional heterojunction array: An experimental investigation

Current-voltage experiments on a highly ordered array of carbon nanotubes interfaced with silicon reveal interesting features arising from the regular array of heterojunctions between one-dimensional (1D) and three-dimensional materials. At high temperature, the vertically aligned and ordered nanotubes behave as an array of point junction contacts to the silicon below, which merge into a planar junction as temperature decreases. This model is further supported by the observation of signature space charge limited conduction, whose origin is attributed to deep levels in the silicon substrate and to the strong field enhancement due to the quasi-1D nanotubes.

Optoelectrical characteristics of individual zinc oxide nanorods grown by DNA directed assembly on vertically aligned carbon nanotube tips

The authors describe the properties of electronically active nanowires that can be assembled via DNA directed growth on a nanostructured array. DNA-modified nanoparticles are used to site-specifically address the tips of vertically aligned carbon nanotubes (CNTs) that serve as catalysts for the growth of zinc oxide (ZnO) nanorods. Using conductive probe atomic force microscopy, they measured the conductance characteristics of single ZnO-CNT structures under various force and illumination conditions and at different sites in a large array, thereby establishing that DNA directed formation of multimaterial, optically active nanostructures can yield devices that are electronically functional at the nanometer scale. The inherent ability of DNA to carry and convey encoded information provides the basis for targeted synthesis of nanostructured devices.

Growth and application of highly ordered array of vertical nanoposts

In this article, we discuss a few recent advances in fabrication and application of highly ordered nanopost arrays. These arrays are vertically aligned and are uniform in diameter, height, and spacing. They can be made from a large variety of materials ranging from metals, semiconductors, and carbon nanotubes. The keys to achieving a high degree of uniformity and ordering are in the formation and the use of a highly ordered nanopore array as a growth template or mask. The uniformity and vertical orientation greatly facilitate the use of these arrays in biomolecular interfacing, field (optical and acoustic) sensing and modulation, as well as field emitters.

The carbon nanotube-silicon heterojunction as infrared detector

Carbon nanotubes are a unique material that can be either metallic or semiconducting, usually with a small bandgap inversely proportional to tube diameter and with interesting optical properties. However, their general randomness in length, diameter, and chirality, and the challenges in aggregating sufficiently large quantities of precisely uniform nanotubes, render its applications in optical detection so far unattainable beyond simple absorptive coating. The highly-ordered carbon nanotube array, as grown by the non-lithographic methods described here, surmounts many of these obstacles while presenting a geometry that is useful for focal plane array applications. A nanoporous alumina template assists the nanotube growth, which proceeds by carbon vapor deposition in a technique that is compatible with integration on silicon. We report on the experimental treatment of one possible platform for applying carbon nanotubes in infrared detection: a heterojunction photodiode with silicon. The nanotube-silicon heterojunction has rectifying characteristics that are consistent with silicon doping type, nanotube work function, and silicon-nanotube bandgaps. We investigate this hybrid nanostructure with spectral photocurrent measurements in the near and mid-infrared regime in both cooled and uncooled modes of detection. Transient photocurrent analysis suggests that both pyroelectric and direct optoelectronic effects are sources of photoresponse. First-principle theoretical treatments of nanotube-silicon heterojunction detection imply that performance parameters such as D* could be greatly optimized in future generations of samples. We explore the suitability of this detector prototype for spaceborne applications where many known properties of carbon, such as chemical and mechanical durability as well as strong covalent bonding and therefore radiation hardness, merit its consideration.

Hybrid nanostructures for mid-infrared to near-infrared detection

We will present our advance in the utilization of a non-lithographic approach for formation of periodic nanosized arrays and formation of hybrid structures suitable for light detection. We explore a self-organization process for formation of periodical nanopores in anodized aluminum oxide, the transfer of this pattern, and the subsequent growth within the pores. This approach was successfully demonstrated for a system having carbon nanotubes as kernel. The carbon nanotubes by themselves are very attractive for detector applications. It is theoretically predicted and experimentally proven that their band gap is adjustable in broad spectral range, their charge carrier mobility is high, and their thermal and mechanical properties are unmatched by other materials. The nanotemplate we use for growth of the nanotubes allows their controlled placement in a regular array, without restriction of the curvature of the surface to be covered. There are no principal limitations for scaling of the process. The third element of our approach is the integration with silicon which provides the compatibility with the well elaborated silicon technology. We will demonstrate the suitability of these structures for light detection.

Periodic nanometric superstructures for photonic applications

In this paper, we will introduce a non-lithographic technique that utilizes self-organized, highly ordered anodized aluminum oxide (AAO) porous membrane as a template to form a variety of periodic nanometric superstructures. Materials as different as metals, semiconductors, and carbon nanotubes (CNT) can be adapted into this platform with equal ease. The flexibility with materials, the accurate control over the fabrication process, and the command over the alumina template attributes offers us a new degree of freedom of engineering various physical properties by templating the nanostructures with the desired shape, size, composition, and doping. The versatile nanometric superstructure platform realized by this unique approach can enable a broad range of applications including infrared detector and quantum dot lasers and so on.