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J. Phys. Chem. C 2009, 113, 4184–4187
Dip-Pen Nanolithography-Generated Patterns Used as Gold Etch Resists: A Comparison Study of 16-Mercaptohexadecanioc Acid and 1-Octadecanethiol Gang Lu,† Yanhong Chen,† Bing Li, Xiaozhu Zhou, Can Xue, Jan Ma, Freddy Y. C. Boey, and Hua Zhang* School of Materials Science and Engineering, Nanyang Technological UniVersity, 50 Nanyang AVenue, Singapore 639798, Singapore ReceiVed: December 6, 2008; ReVised Manuscript ReceiVed: January 1, 2009
The etch resist ability of dip-pen nanolithography (DPN)-generated dot patterns of different alkanethiols on Au was systematically studied. After 16-mercaptohexadecanioc acid (MHA) and 1-octadecanethiol (ODT) dots with different diameters were patterned on a Au substrate by DPN, the substrate was etched in a feri-/ ferrocyanide solution. Tapping mode AFM (TMAFM) was used to monitor the morphology change of the patterned dots during the wet chemical etching. The diameter and height of MHA and ODT dots at different etch time were measured by TMAFM. The resist ability of the patterned MHA and ODT SAMs on Au was compared. The result shows that the MHA patterns have better etch resist ability than do ODT patterns. Introduction
Experimental Section
Over the past decades, metallic nanostructures have attracted great interest due to their wide applications in the field of electronics, optics, biosensing, and catalysis.1-4 One of the important strategies for fabricating metal nanostructures on surfaces incorporates the patterning of self-assembled monolayers (SAMs) of alkanethiols on metal films by microcontact printing (µCP),1,5,6 dip-pen nanolithography (DPN),7 or recently developed polymer pen lithography,8 and wet chemical etching.1,9,10 DPN has been known as a powerful tool to produce patterns from micro- to nanometer scale on a surface using an “ink”-coated AFM tip that servers as a “pen”,11,12 which offers the capabilities of high registration and sub-50 nm resolution.7,10 It has been demonstrated that the alkanethiol patterns generated by DPN can be used as etch resist for fabrication of metal nanostructures.10 For example, by using 16-mercaptohexadecanoic acid (MHA) or 1-octadecanethiol (ODT) as etch resist, the 12-nm Au gap10b and sub-100 nm structures of Au,10b,e Ag,10c and Pd10c have been constructed on Si/SiOx substrates after wet chemical etching. It was found that the molecule weight of alkanethiols affects the transfer of alkanethiol molecules during the µCP process and subsequently the resist ability of the formed SAMs in wet chemical etching, which thereby affects the quality of the generated metal structures.13 However, no systematic study has been conducted on the resist ability of the nanometer scale alkanethiol patterns generated by DPN in wet chemical etching. To enhance the controllability and reproducibility of the size and morphology of etched metal nanostructures, it is critical to understand the interplay between the alkanethiol patterns and wet chemical etching. In this Article, we carry out a systematic study on the behavior of DPN-generated MHA and ODT patterns with different feature sizes in wet chemical etching, which reveals that the MHA and ODT patterns show different resist ability for the underlying Au layer.
Chemicals. 16-Mercaptohexadecanoic acid (MHA), 1-octadecanethiol (ODT), potassium ferricyanide, potassium ferrocyanide hydrate, and potassium hydroxide were purchased from Sigma-Aldrich Inc. (St. Louis, MO). Sodium thiosulfate anhydrous, sulfuric acid, and acetone were purchased from Mallinckrodt Baker Inc. (Phillipsburg, NJ). Hydrogen peroxide and acetonitrile were purchased from EMD Chemicals Inc. (Darmstadt, Germany). All of the chemicals were used without further purification. Milli-Q water (resistance > 18 MΩ cm) was used for all aqueous experiments. Gold Substrate Preparation. An oxidized silicon wafer (∼100 nm of oxide) was cut into 1 × 1 cm2 squares. The substrates were first sonicated in acetone for 10 min and rinsed with Milli-Q water, and then immersed into a freshly prepared piranha solution (V(H2SO4):V(H2O2) ) 7:3) at 100 °C for 30 min. After the cleaned substrates were rinsed with Milli-Q water and dried with nitrogen flow, they were coated with 2-3 nm Ti as an adhesion layer and subsequently with ∼22 nm Au using a magnetron sputtering system (base pressure
