Wechung Maria Wang

Direct Patterning of Gold Nanoparticles Using Dip-Pen Nanolithography

Various methods for the patterned assembly of metal nanoparticles have been developed in order to harness their unique electrical and optical properties for device applications. This paper discusses a method for direct writing of Au nanoparticles at nanoscale resolution using dip-pen nanolithography. First, a procedure was developed for increasing the loading of Au nanoparticles onto AFM tips to prolong patterning life. AFM tips were subsequently imaged by scanning electron microscopy to determine ink coverage and to gain insight into the deposition process. Next, surface interactions, relative humidity, and writing speed were controlled to determine an optimal range of conditions for deposition. Various ink-substrate combinations were studied to elucidate the dependence of deposition on interactions between Au nanoparticles and the substrate surface; inks consisted of positively and negatively charged particles, and substrates were SiO(2) surfaces modified as hydrophilic or hydrophobic and interacted electrostatically or covalently with Au nanoparticles. Results indicate that a highly hydrophilic surface is required for Au nanoparticle deposition, unless covalent binding can occur between the Au and substrate surface. The optimal range of relative humidity for patterning was found to be 40-60%, and Au nanoparticle deposition was not sensitive to writing speeds ranging from 0.01 to 2 microm/s.

Lyotropic Liquid-Crystalline Solutions of High-Concentration Dispersions of Single-Walled Carbon Nanotubes with Conjugated Polymers†

The 1D structure of single-walled carbon nanotubes (SWNTs) leads to unique physical properties, which have been investigated extensively. Numerous applications and device prototypes have been demonstrated; however, most have used SWNTs grown in situ by chemical vapor deposition. This limits throughput and choice of substrate owing to the high growth temperatures involved. Solution-based postsynthesis device fabrication, typically involving purification, solubilization, chemical functionalization, cutting, and/ or controlled assembly of SWNTs, is more desirable because of low cost, scalability to large areas, and compatibility with flexible plastic substrates. Unfortunately, SWNTs are not readily soluble, and chemical functionalization strategies for their solubilization usually alter their electronic properties. Furthermore, to take full advantage of the anisotropic charge-transport properties of SWNTs and to enhance their performance in high-strength composite materials, it is necessary to align them over a large area. Noncovalent functionalization of SWNTs is a particularly attractive avenue for dispersion because it enables modification of material properties without altering the chemical structure of the nanotubes. To date, most high-concentration dispersions (>1mg mL ) have been obtained in aqueous solutions by mixing SWNTs with surfactants, doubleor

Dip-Pen Nanolithography of Electrical Contacts to Single-Walled Carbon Nanotubes

This paper discusses a method for the direct patterning of Au electrodes at nanoscale resolution using dip-pen nanolithography, with proof-of-concept demonstrated by creating single-walled carbon nanotube devices. This technique enables insight into three key concepts at the nanoscale: using dip-pen nanolithography as an alternative to electron-beam lithography for writing contacts to carbon nanotubes, understanding the integrity of contacts and devices patterned with this technique, and on a more fundamental level, providing a facile method to compare and understand electrical and Raman spectroscopy data from the same isolated carbon nanotube. Electrical contacts to individual and small bundle single-walled carbon nanotubes were masked by an alkylthiol that was deposited via dip-pen nanolithography on a thin film of Au evaporated onto spin-cast, nonpercolating, and highly isolated single-walled carbon nanotubes. A wet Au etching step was used to form the individual devices. The electrical characteristics for three different single-walled carbon nanotube devices are reported: semimetallic, semiconducting, and metallic. Raman analysis on representative devices corroborates the results from AFM imaging and electrical testing. This work demonstrates a technique for making electrical contact to nanostructures of interest and provides a platform for directly corroborating electrical and optical measurements. The merits of using dip-pen nanolithography include flexible device configuration (such as varying the channel length and the number, size, and orientation of contacts), targeted patterning of individual devices with imaging and writing conducted in the same instrument under ambient conditions, and negligible damage to single-walled carbon nanotubes during the fabrication process.

Dip-pen nanolithography of electrical contacts to single graphene flakes.

This study evaluates an alternative to electron-beam lithography for fabricating nanoscale graphene devices. Dip-pen nanolithography is used for defining monolayer graphene flakes and for patterning of gold electrodes through writing of an alkylthiol on thin films of gold evaporated onto graphene flakes. A wet gold etching step was used to form the individual devices. The sheet resistances of these monolayer graphene devices are comparable to reported literature values. This alternative technique for making electrical contact to 2D nanostructures provides a platform for fundamental studies of nanomaterial properties. The merits of using dip-pen nanolithography include lack of electron-beam irradiation damage and targeted patterning of individual devices with imaging and writing conducted in the same instrument under ambient conditions.

Parallel Fabrication of Electrode Arrays on Single-Walled Carbon Nanotubes using Dip-Pen-Nanolithography-Patterned Etch Masks

This article presents a novel application of using dip-pen nanolithography (DPN) to fabricate Au electrodes concurrently in a high-throughput fashion through an etch resist. We have fabricated 26 pairs of electrodes, where cleanly etched electrode architectures, along with a high degree of feature-size controllability and tip-to-tip uniformity, were observed. Moreover, electrode gaps in the sub-100-nm regime have been successfully fabricated. Conductivity measurements of multiple electrodes in the array were all comparable to that of bulk Au, confirming the reliability and the low-resistance property of the electrodes. Finally, as a demonstration of electrode functionality, SWNT devices were fabricated and the electrical properties of an SWNT device were measured. Hence, our experimental results validate DPN as an effective tool in generating high-quality electrodes in a parallel manner with mild, simple processing steps at a relatively low cost.