Method for fabricating nanostructures via nanotemplates using dip-pen nanolithography S. Sharma1, A. Salehi-Reyhani2, A. Bahrami1, E. Intisar1, H. Santhanam1, K. Michelakis3, A. Cass1 1
Department of Chemistry, Institute of Biomedical Engineering, Imperial College London, South Kensington Campus, Exhibition Road, London SW7 2AZ, United Kingdom 2 Department of Chemistry, Proxomics Project, Institute of Chemical Biology, Imperial College London, South Kensington Campus, Exhibition Road, London SW7 2AZ, United Kingdom 3 Advanced Technology Institute, University of Surrey, Guildford, Surrey, GU2 7XH, United Kingdom E-mail:
[email protected] Published in Micro & Nano Letters; Received on 10th September 2012
Dip-pen nanolithography (DPN) relies on the compatibility between the ink used and the substrate it is written on. This has lead to a significant reliance for DPN on thiol-gold chemistries for the fabrication of nanostructures. This reported work demonstrates a method for creating nanostructures through nanotemplates that allows a wider range of substrates and inks to be used. The substrate was spin-coated with a thin layer of polymer which is selectively removed using a scanning probe microscopy tip coated with a solvent. This produces nanotemplates that are subsequently used to create nanostructures by metallisation. Compared with directly written templates these nanotemplates facilitate longer interaction between inking material and the substrate. They also allow a controlled spatial resolution along the z-axis and eliminate the need for any blocking agents to prevent non-specific adsorption. This method allows DPN to be expanded to applications beyond thiol-gold chemistries. 1. Introduction: Nanostructures (,100 nm) offer an interesting platform for the study of physical and chemical properties at a single particle level. As the field of nanotechnology is expanding, new ways of fabricating nanostructures have been evolving. Traditionally, e-beam lithography involving the use of a highenergy electron beam has been used for nanofabrication; however, its expense prohibits its widespread adoption [1, 2]. Scanning probe lithography and nanoimprint lithography have become popular over the last decade [3]. Dip-pen nanolithography (DPN) offers a desktop route to high-throughput fabrication of nanostructures and for the nanopatterning of substrates [4 –6]; however, these techniques are limited by the choice of ink material and substrate surfaces [7, 8]. The substrates that can be used for DPN are mainly limited to gold, silicon and silicon dioxide. For DPN-fabricated nanostructures, their dimensions depend on dwell time, writing speed, solubility and viscosity of the ink. Nanotemplates by directly writing organic molecules such as mercaptoundecanoic acid and 1-octadecane-thiol have been reported. These templates allowed controlled deposition of single particles such as carbon-coated iron nanoparticles [9] and carbon nanotubes [10, 11]. Huang et al. have reported an approach which addresses the limitations of DPN direct writing techniques by using a poly(ethylene glycol) (PEG)-based ink and polymer lithography for patterning Au, Fe3O4 and C60 nanoparticles. PEG acts as a carrier of the nanoparticles and once deposited is removed by oxygen plasma and washing steps while the nanoparticles remain on the substrate [12]. However, to prevent non-specific adsorption to the substrate surface-blocking agents were required. We describe here an alternative technique to fabricate nanotemplates for nanostructures (Fig. 1). A polymer layer is spun on the substrate; the tip is dipped in solvent followed by contact with the substrate which then dissolves the polymer thereby creating a nanotemplate. Once the templates are obtained the nanostructures can be formed by metallisation (e-beam evaporation/electroplating), etching the exposed substrates or introducing samples using a transfer pipette. The advantage this method offers over nanotemplates formed by direct writing is that it eliminates the need for optimisation of the printing buffer and substrate chemistries. The selective removal of the protective polymer by the solvent provides a three-dimensional femtolitre volume reservoir. This enables longer incubation times 1038 & The Institution of Engineering and Technology 2012
for the ink to bind with the substrate thus offering a wider range of substrates to be used. Another feature of this technique is that it does not necessitate the need for intermediate blocking agents to prevent non-specific adsorption to the substrate surface [9– 11]. This allows the development of methods to obtain high-throughput nanopatterned structures for applications ranging from single cell assays to nanowires and nanosensors. The novel feature of our method lies in the use of a protective polymer spun on any substrate. The thickness of this polymer dictates the depth of the nanotemplates and the height of the resulting nanostructures. Therefore, in addition to the precise control over feature size and registration offered by DPN, our method extends this to nanostructure height. Control writing scans were performed using dry tips under similar conditions. Lines of 8 mm length were written at speeds of
Figure 1 Illustration of the method used for producing nanotemplates a Substrate is spin coated with a polymer layer b Using toluene as a solvent the polymer is removed to create nanotemplates c Nanotemplate is subjected to metallisation or direct transfer of material using transfer pipettes to allow binding with the substrate d Polymer layer is stripped off to produce a nanopatterned surface
Micro & Nano Letters, 2012, Vol. 7, Iss. 10, pp. 1038–1040 doi: 10.1049/mnl.2012.0698
0.05, 0.10, 0.20, 0.40 and 0.50 mm/s. The scans were repeated to ensure removal of the polymer. All image files were processed using Nanorule+ (version 2.5.05). Fig. 2a shows an atomic force microscopy (AFM) image after writing with a tip dipped in toluene. For the lines the width and depth of the lines decrease with p increased writing speed. As seen from Fig. 2b, there is a t dependence that supports the meniscus-based ink transport mechanism reported earlier [13 –15]. Arrays of dots were patterned onto substrates by selective removal of the polymer layer to create the Olympic rings (Fig. 3). The nanotemplates were tested by controlled e-beam evaporation of Ti/Pt (5 nm/30 nm). The nanostructures were imaged using AFM (NSCRIPTORTM ; NanoInk Inc., Skokie, IL, USA). After e-beam metallisation the polymer layer was stripped using acetone.
Figure 2 AFM image of nanotemplated lines created by removal of protective polymer layer using toluene, lines are 8 mm in length, and graph showing relationship between line width and the square root of dwell time t, calculated from the line writing speed a AFM image of nanotemplated lines b Graph showing relationship between line width and the square root of dwell time t
Often, DPN is described as a sequence of steps involving molecular deposition of the ink from the AFM tip to the substrate followed by lateral diffusion, substrate binding and final reorganisation of the ink. In our case the solvent diffuses laterally, thus swelling and dissolving the coated polymer layer; in the second writing step the tip removes the polymer producing the nanotemplates. The method offers more comprehensive control of the size of the nanostructures. We observed an relative standard deviation of ,10% which compares favourably to direct writing with high melting point solvents [15]. In summary, we have presented here an alternative method for the fabrication of nanostructures using nanotemplates. The use of other coating layers and removal solvents will enable the fabrication of nanostructures with a wider range of substrates. 2. Methods: For our experiments, a polymer commercially available as an image reversal photoresist (AZ5214E) (Clariant GmbH, Germany) was thinned with methoxypropyl acetate (Microchemicals GmbH) in the ratio (1:3) and spun for 1 min on a silicon substrate, followed by a soft bake at 908C. AZ5214E is commonly used in standard photolithography. It undergoes any changes in its properties only when exposed to ultraviolet light. The thickness of the spun polymer layers was measured using a Dek Tak profilometer to be 52 nm. Organic solvents such as toluene, decalin and squalene were used to remove the polymer layer because of their higher boiling points (.1108C) and their ability to dissolve the photoresist polymer. All DPN work was performed using the NSCRIPTORTM or the NLP2000 DPN System (NanoInk Inc). The DPN probes used were single pen, Type A or Type M. The solvent was loaded onto the tip using a double-dip coating method which involved 20 s dipping followed by a 10 s wait and a final 20 s dipping. Inkwell reservoirs were used with the NLP 2000 system. DPN writing was performed at 188C and 45% humidity. 3. Acknowledgments: E. Intisar acknowledges the Wellcome Trust Biomedical Vacation Scholarship Scheme, while A. Bahrami acknowledges the Imperial College’s UROP placement scheme. The authors also thank R. Marchmont and R. Stokes for the loan of the NLP2000 system and for useful discussions. 4
Figure 3 AFM images of solvent-assisted DPN on a polymer-coated surface a DPN tip coated with squalene is used selectively to dissolve the polymer and create a nanotemplate b AFM image of the Olympic rings nanostructure after metallisation by e-beam evaporation of platinum c Processed image showing 35 nm height of platinum metallised Olympic rings. Scale bar 5 mm
Micro & Nano Letters, 2012, Vol. 7, Iss. 10, pp. 1038–1040 doi: 10.1049/mnl.2012.0698
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Micro & Nano Letters, 2012, Vol. 7, Iss. 10, pp. 1038–1040 doi: 10.1049/mnl.2012.0698
