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).1-5 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.2,4,6,7
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.
Polymer Pen Lithography (PPL)
The Mirkin group recently developed Polymer-Pen Lithography (PPL),9 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 generated10by 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.
Scanning Probe Block Co-polymer 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 (SPBCPL).13 In a typical SPBCPL 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 concentration, large arrays of sub-5 nm single crystalline Au nanoparticles (NPs) can be produced. We have also studied the mechanism for particle formation using this technique, finding that both NP coalescence (Smoluchowksi ripening) and Ostwald ripening occur, with coalescence being the dominant ripening mode for NPs < 5 nm apart and Ostwald ripening seen for NPs that were further apart.
In addition to the ripening work described above, SPBCL has been used to make patterns of CdS nanoparticles15 and individual proteins.16 Current research in the group is focused on extending SPBCL to make arrays of other types of metallic, metal oxide, and semiconducting nanoparticles, which can be used to enable studies in catalysis and plasmonics.
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.11 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 is accomplished by either placing the pens in contact with an adhesive surface or using focused-ion beam (FIB) lithography. 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 photomask. 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 used at once.
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 a technique – termed Hard-tip, Soft-spring Lithography (HSL)12 – that utilizes sharp Si tips on an elastomeric PDMS backing (Figure 5). HSL is a cantilever-free technique that retains 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. In doing so, HSL retains 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 increasing the robustness of these tips and creating conductive tips.
 Piner, R. D.; Zhu, J.; Xu, F.; Hong, S.; Mirkin, C. A. “”Dip-Pen” Nanolithography,” Science 1999, 283, 661-663. DOI: 10.1126/science.283.5402.661
 Ginger, D. S.; Zhang, H.; Mirkin, C. A. “The Evolution of Dip-Pen Nanolithography,” Angew. Chem. Int. Ed. 2004, 43, 30-45. DOI: 10.1002/anie.200300608
 Rozhok, S.; Sun, P.; Piner, R.; Lieberman, M.; Mirkin, C. A. “AFM Study of Water Meniscus Formation between an AFM Tip and NaCl Substrate,” J. Phys. Chem. B 2004, 108, 7814-7819. DOI: 10.1021/jp0401269
 Salaita, K.; Wang, Y.; Mirkin, C. A. “Applications of Dip-Pen Nanolithography,” Nat. Nanotechnol. 2007, 2, 145-155. DOI: 10.1038/nnano.2007.39
 Braunschweig, A. B.; Huo, F.; Mirkin, C. A. “Molecular Printing,” Nat. Chem. 2009, 1, 353-358. DOI: 10.1038/nchem.258
 Lenhert, S.; Sun, P.; Wang, Y.; Fuchs, H.; Mirkin, C. A. “Massively Parallel Dip-Pen Nanolithography of Heterogeneous Supported Phospholipid Multilayer Patterns,” Small 2007, 3, 71-75. DOI: 10.1002/smll.200600431
 Vega, R. A.; Shen, C. K. F.; Maspoch, D.; Robach, J. G.; Lamb, R. A.; Mirkin, C. A. “Monitoring Single-Cell Infectivity from Virus-Particle Nanoarrays Fabricated by Parallel Dip-Pen Nanolithography,” Small 2007, 3, 1482-1485. DOI: 10.1002/smll.200700244
 Salaita, K.; Wang, Y.; Fragala, J.; Vega, R. A.; Liu, C.; Mirkin, C. A. “Massively Parallel Dip-Pen Nanolithography with 55 000-Pen Two-Dimensional Arrays,” Angew. Chem. Int. Ed. 2006, 45, 7220-7223. DOI: 10.1002/anie.200603142
 Huo, F.; Zheng, Z.; Zheng, G.; Giam, L. R.; Zhang, H.; Mirkin, C. A. “Polymer Pen Lithography,” Science 2008, 321, 1658-1660. DOI: 10.1126/science.1162193
 Zheng, Z.; Daniel, W. L.; Giam, L. R.; Huo, F.; J., S. A.; Zheng, G.; Mirkin, C. A. “Massively Multiplexed Protein Arrays Enabled by Polymer Pen Lithography: Addressing the Challenge of Multiplexed Inking,” Angew. Chem Int. Ed. 2009, 48, 7626-7629. DOI: 10.1002/anie.200902649
 Huo, F.; Zheng, G.; Liao, X.; Giam, L. R.; Chai, J.; Chen, X.; Shim, W.; Mirkin, C. A. “Beam Pen Lithography,” Nat. Nanotechnol. 2010, 5, 637-640. DOI: 10.1038/nnano.2010.161
 Shim, W.; Braunschweig, A. B.; Liao, X.; Chai, J.; Lim, J. K.; Zheng, G.; Mirkin, C. A. “Hard-tip Soft-spring Lithography,” Nature 2011, 469, 516-520. DOI: 10.1038/nature09697
 Chai, J.; Huo, F.; Zheng, Z.; Giam, L. R.; Shim, W.; Mirkin, C. A. “Scanning Probe Block Copolymer Lithography,” Proc. Nat. Acad. Sci. 2010, 107, 20202-20206. DOI: 10.1073/pnas.1014892107.
 Chai, J.; Liao, X.; Giam, L. R.; Mirkin, C. A. “Nanoreactors for Studying Single-Particle Coarsening,” J. Am. Chem. Soc. 2012, 134, 158-161. DOI: 10.1021/ja2097964
 Giam, L. R.; He, S.; Horwitz, N. E.; Eichelsdoerfer, D. J.; Chai, J.; Zheng, Z.; Kim, D.; Shim, W.; Mirkin, C. A. “Positionally Defined, Binary Semiconductor Nanoparticles Synthesized by Scanning Probe Block Copolymer Lithography,” Nano Lett. 2012, 12, 1022-1025. DOI: 10.1021/nl204233r
 Chai, J.; Wong, L. S.; Giam, L. R.; Mirkin, C. A. “Single-molecule Protein Arrays Enabled by Scanning Probe Block Copolymer Lithography,” Proc. Nat. Acad. Sci. 2011, 108, 19521-19525. DOI: 10.1073/pnas.1116099108