Topographically Flat, Chemically Patterned PDMS Stamps Made by Dip‐Pen Nanolithography

Abstract

Flexible nanolithographic and printing methods are essential tools in the field of nanoscience and nanotechnology. [1][2][3][4][5] Scanning probe techniques such as dip-pen nanolithography (DPN) [3,[6][7][8][9][10][11][12] and nanografting [13,14] are extremely useful since they allow one to make nanostructures from a wide variety of different chemical compositions. In the case of DPN, despite significant advances in parallelization, [2,[15][16][17] it is still challenged for certain applications with regard to throughput, especially when compared to lower resolution contact printing methods. Indeed, microcontact printing (μCP) has emerged as a "bench-top" technique for preparing patterns of various active structures in a massively parallel manner. [18][19][20] In conventional μCP, a soft elastomer stamp (typically polydimethylsiloxane, PDMS) with relief structures predefined by photo-or e-beam lithography is brought into contact with a substrate, where the ink is transferred from the stamp to the substrate surface at the area of contact.[21] PDMS has a Young's Modulus of 1 MPa, which is soft enough to make conformal contact with planar or nonplanar surfaces to facilitate ink transfer. However, since the relief structures are separated by air voids in the stamp, the mechanical instability (pairing, sagging, etc.) of the stamp deriving from the "soft" nature of PDMS, has been a major limitation for a variety of applications in μCP.[5,22,23] The volatility of the inks and their diffusion properties on a surface also can be limiting factors when printing small features (Scheme 1).[24] Although many solutions have been suggested to address both mechanical and ink-transport issues, such as using harder elastomers [25,26] and establishing control over printing time, [27] the lateral resolution of printed features and the design of such stamps are still limited to ~150 nm (in terms of alkanethiols) and a ~1/20 filling factor, respectively. [23,28] Herein, we report how one can use DPN to prepare a stamp for high resolution μCP in a way that keeps the PDMS surface topographically flat but chemically patterned. The mechanically stable flat stamp is prepared without a photomask (since the structure may be directly written onto the surface by DPN), and the pattern drawn by DPN combined with subsequent chemical modification of the exposed areas of the stamp, allows one to create a surface with well-defined chemical regions that can be used to confine ink transport to sub-100 nm dimensions and prevent the lateral diffusion of the ink (Scheme 1). Others have used contact inking, [29,30] metal contact masks, [31,32] and template-mediated selfassembly approaches[33] to fabricate (sub)micron-sized chemical patterns on PDMS, but the approach described herein is remarkably simple and straightforward to implement and does not require complex and expensive photo-or e-beam lithography. More importantly, the high registration and resolution of DPN allow one to create stamps using this approach that ** C.A.M. acknowledges the AFOSR, DA...

Polymer Nanostructures Made by Scanning Probe Lithography: Recent Progress in Material Applications

“…Reproduced with permission. [ 29 ] D) Optical image of an array of arbitrary photoresist patterns fabricated with PEG phase-shift lenses. The AFM image to the right shows that the letter "N" consists of well-shaped features.…”

“…[ 28 ] In another example, Zheng et al fabricated PEG nano structures with DPN on fl at PDMS as a resist for plasma treatment. [ 29 ] After backfi lling of various silane molecules on the unprotected areas and removal of the PEG resist, a topographically fl at PDMS stamp with well-defi ned chemical regions was formed, which could be used for microcontact printing of small molecules, proteins and polar inks at sub-100 nm. Furthermore, since DPN-deposited PEG dots on glass are hemispheric and transparent, they possess optical properties and can be employed as nanosized lenses for subwavelength photolithography: [ 30 ] parallel light passing through PEG was focused beneath the center of the PEG lens, with a lower intensity at the edges.…”

Molecular printing

“…If the feature aspect ratio on the stamp is not between 0.2 and 2, bending of the stamp can cause pattern defects, and, for features that are too widely separated (>0 h ), sagging of the stamp causes defects. Indeed, the fabrication of alkanethiol features smaller than 150 nm is a significant challenge using conventional methods 44 . By using DPN to introduce chemical modifications on a flat PDMS stamp and by passivating the surrounding areas with a perfluorinated monolayer, we have helped overcome this deficiency, and shown that features as small as 80 nm could be easily prepared with aspect ratios as low as 0.01.…”

Lithography-free production of stamps for microcontact printing of arrays

“…Microcontact printing of 1-hexadecanethiol on gold slides and subsequent chemical etching 26 Glass slides were soaked in 1 : 3 volume ratio of H 2 O 2 /H 2 SO 4 solution for at least an hour and thoroughly rinsed with 18 MU H 2 O, ethanol and dried under a stream of nitrogen. Cleaned glass slides were used for depositing chromium as the adhesion layer followed by gold deposition inside a thermal evaporator.…”