Dip-Pen Nanolithography & Cantilever Free Nanolithography

Research in the Dip-Pen Nanolithography (DPN) subgroup of the Mirkin Group focuses on the development of novel scanning probe-based lithography techniques and the application of these techniques to address questions in a variety of fields, including surface assembly, nanoelectronics, cell-surface interactions, and catalysis.

Recently, our subgroup has developed a number of massively parallel, cantilever-free deposition techniques (outlined below) that can be used for large-area, low-cost, arbitrary surface patterning.  These tools are the first of their kind and enable one to study the chemical consequences of materials miniaturization, opening avenues for nanocombinatorial studies in fields such as cell biology and heterogeneous catalysis, not possible with other methodologies.

DPN was invented in 1999 by the Mirkin Group, and it can be used to deposit molecules and materials on surfaces with sub-50 nm resolution.  This method employs an atomic force microscope (AFM) probe “pen” coated with a molecule- or materials-based “ink” that, upon contact with a surface, deposits the ink by diffusion through a water meniscus that forms under ambient conditions between the tip and substrate (Figure 1). In the first demonstrations of this technique, small organic molecules, such as alkanethiols (octadecanethiol (ODT) and mercaptohexadecanoic acid (MHA)), were written onto gold substrates; these molecules were chosen for their ability to form well-ordered self-assembled monolayers (SAMs). Since the initial demonstrations involving alkanethiols on gold, DPN has been used to write or template many different types of molecules and materials on a variety of surfaces (including metals, semiconductors and insulators) by controlling various experimental parameters such as ambient humidity, writing speed, and dwell time. These materials include polymers, colloidal nanoparticles (e.g. magnetic nanocrystals, carbon nanotubes), sol-gel precursors, small organic molecules, biomolecules (proteins and oligonucleotides) and even single virus particles and bacteria.

The first incarnations of DPN were serial processes, and were thus lower in throughput than stamping or optical lithography-based techniques. In order to compete with these techniques, it was necessary to increase the throughput of DPN by parallelizing the deposition process. To address this, we developed parallel DPN approaches that relied on 2D cantilever arrays,8 thus enabling large-area high-throughput patterning. While these 2D arrays (containing up to 55,000 pens/cm2) allowed facile patterning of large areas, they are also mechanically fragile and expensive. Thus, we developed several new methods for lower cost, high-throughput printing, which are discussed below.

Figure 1

(A) Schematic representation of molecular deposition by DPN through a water meniscus formed between the scanning probe tip and the substrate surface. (B) AN AFM image of a pattern with nanometer resolution created by DPN.

Polymer Pen Lithography (PPL)

The Mirkin group recently developed Polymer-Pen Lithography (PPL), which combines the advantages of DPN with microcontact printing while eliminating many of the limitations of the two techniques. A typical PPL array (Figure 2) is made from an elastomer, which is cured in a Si mold fabricated by photolithography and then mounted on a flat, transparent substrate (glass or quartz).  The array is then used in a AFM as one would use a cantilever in a DPN experiment. We have fabricated PPL arrays with as many as ~11,000,000 pens, thus allowing high-throughput, large-scale patterning at a low cost. In addition, in PPL, tip-substrate contact force can be used to control feature size, thus allowing rapid generation of micro and nanoscale features by varying the compression of the tips. By using the Si master has an inkwell, multiplexed patterns can be generated by individually addressing each tip with different inks. This methodology is the first example of scanning probe lithography performed in a cantilever-free mode – i.e. the tip was used as both the spring and printing tool. Our group is also using protein patterns made via PPL to address fundamental questions pertaining to cell-surface interactions and stem cell growth and differentiation.

Figure 2

(A) Photograph of a 4-inch the PPL array with 11 million pens. (B) A scanning electron micrograph n image of the nanometer-scale elastomeric tips within the array. (C) A schematic representation of multiplexed protein printing by PPL after inking of the individual tips from inkwells filled through inkjet printer deposition. (D) A fluorescence optical micrograph image of the resultant multiplexed PPL patterns of fluorophore-labeled proteins patterns.

Scanning Probe Block Copolymer Lithography (SPBCL)

Due to their size-dependent optoelectronic and chemical properties, there is an increasing interest in synthesizing ordered arrays of single nanoparticles for applications ranging from biomedical sensors to single-electron transistors. Indeed, the precise positioning and synthesis of sub-10 nm particles over a large area are exceedingly difficult tasks when using current lithographic techniques, particularly those readily available to those in an academic research lab. Toward this end, we recently developed a new high resolution scanning probe-based technique called Scanning Probe Block Copolymer Lithography (SPBCL). In a typical SPBCL experiment, a block copolymer (typically, PEO-b-P2VP)-based metal-salt solution (Figure 3) is used as an ink for conventional DPN and PPL.  The PEO chains provide water solubility and the P2VP chains serve to concentrate the metal ions.

By tuning the precursor ink composition, large arrays of metal nanoparticles (NPs) can be produced. We have also studied the mechanism for particle formation using this technique, finding that metal precursors diffuse in the polymer matrix to form a single nanoparticle through three pathways. In addition to monometallic nanoparticles, SPBCL has been used to make patterns of CdS nanoparticles, bimetallic nanoparticles, and trimetallic nanoparticles. Current research in the group is focused on using SPBCL as a novel tool for studying the fundamental science and potential applications of nanoparticles in areas such as heterogeneous catalysis, magnetics, and plasmonics.

Figure 3

(A) Schematic of the SPBCL process used to make individual nanoparticles.15 The process consists of four steps: i. Multiple metal ion precursors are coordinated onto a block copolymer, poly(ethylene oxide)-block-poly(2-vinylpyridine) (PEO-b-P2VP). ii. The polymer is cast onto AFM tips or polymer pen arrays and then deposited at desired locations on a substrate. iii. The substrate is annealed at 150 °C under Ar, allowing the metal ions to aggregate in the polymer nanoreactors. iv. The substrate is thermally annealed at 500 °C to reduce the metal ions and decompose the polymer, forming single nanoparticles in each reactor. (B) Generalization of nanoparticle synthesis by SPBCL. HRTEM images show the crystallinity of the nanoparticles. (Scale bars: 2 nm).

Beam Pen Lithography (BPL)

Our group recently invented and developed a technique termed Beam-Pen Lithography (BPL), which combines the advantages of PPL and near-field scanning optical microscopy. A typical BPL array is fabricated by coating a PPL array with an opaque metal (i.e. gold) and then opening apertures at the tips of the coated pens (Figure 4). This allows for the generation of arbitrary patterns with features smaller than the wavelength of incident light. Additionally, individual pens can be addressed by using a digital micromirror device (DMD). This approach uses macroscale features (the base of each pyramid) to control tip addressability, which is notoriously difficult due to the nanoscale dimensions of the tips. Additionally, tip movement allows for arbitrary pattern generation. Importantly, much like PPL, this technique is scalable with as many as hundreds of thousands of tips being individually addressed at once. Furthermore, this approach provides a unique platform for performing high-throughput nano- to macroscale photochemistry in combination with material transport, which is important in chemistry, biology and medicine.

Figure 4

(A) Schemes for individual tip during BPL, demonstrating exposure of select light-sensitive photoresist-coated regions. (B) Optical microscopy image of developed photoresist patterns. Arbitrary patterning capabilities are demonstrated by patterning an image of the Chicago skyline. (C) Schematic of the principle of operation of actuated BPL. A digital micromirror device (DMD) is illuminated with ultraviolet light, which is selectively directed onto the back of a near-field aperture array. (D) Magnified photograph of an arbitrary pattern of 9×9 mm2 size produced with actuated BPL. Insert scanning electron microscopy (SEM) showing a portion of a molecule in the ‘ocean’ region.

Hard Tip-Soft Spring Lithography (HSL)

PPL was a significant advance in that it eliminated most of the drawbacks of DPN and µ-contact printing and combined almost all of their attributes; however, it does not offer the sub-50 nm resolution afforded by high resolution SPL or alternative electron-beam technologies. In order to address this, our group recently developed two techniques: Hard-tip, Soft-spring Lithography (HSL) and hard transparent arrays for PPL. HSL that utilizes sharp Si tips on an elastomeric PDMS backing (Figure 5). By coating PPL pen arrays composed of polydimethylsiloxane (PDMS) with silica using plasma-enhanced chemical vapor deposition (PECVD), hard transparent arrays capable of patterning small molecules and polymers at high resolution without dependence on tip-sample contact force were formed (Figure 6).

Both techniques are cantilever-free and retain all of the advantages of PPL, such as low fabrication cost and high-throughput patterning, while retaining the resolution of DPN. HSL represents a significant advance for our group, as it is the first time that large arrays of sharp Si tips have been fabricated in a low-cost, large-area manner. Hard transparent arrays take it a step further by reducing the fabrication steps, keeping the optical transparency of PPL, and increasing the tip array robustness compared to HSL and PPL. In doing so, HSL and hard transparent arrays retain the benefits of PPL – low cost, large scale arrays – while maintaining the sub-50 nm resolution of DPN.  Current efforts in the group focus on utilizing these tip arrays to pattern high density patterns of functional materials.

Figure 5

Fabricated Si pen arrays on SiO2/PDMS/glass A) Si wafer (2×2cm) on cured PDMS surface on glass slide before etching B) Actual pen array after etching in KOH C) SEM image of Si pen array on SiO2/PDMS/glass with 160μm pitch. Pens have bottom width:30±0.6μm corresponding to ~47±0.9μm in pen height. Inset shows array in large area D) {311} planes introduced during wet etching.

Figure 6

a) Bright-field image of silica deposited onto a slab of PDMS, 200 µm scale bar. (b) Dark-field image of hard transparent array with a silica thickness of 175 nm, 50 µm scale bar. (c) AFM of a dot array with a 175 nm pitch in a hexagonal pattern array written using hard transparent array, 200 nm scale bar. (d) Single polymer feature with a circle of small droplets from original meniscus, 20 nm scale bar. (e) Dark-field optical microscopy of array with 500 nm pitch written using hard transparent array with PPL with 14,641 features written per pen, 100 µm scale bar, and (f) zoomed in image pattern made form a single pyramidal pen, 25 µm scale bar.

DPN Subgroup Members

Standing (L-R): Bo Shen, Liliang Huang, Issac Larkin, Brian Meckes, Ed Kluender, Xinpeng Zhang, Liban Jibril, Rustin Golnabi, Jingshan Du (subgroup leader)

Sitting (L-R): Pengcheng Chen, Donghoon Shin, Millicent Lin, Abha Gosavi, EunBi Oh, Namrata Ramani

Not pictured: Jordan Swisher (subgroup leader), Carolin Wahl, David Xu, Peter Smith, Kent Miao