Proceedings of The National Conference

Dip Pen Nanolithography Using a Polyacrylonitrile Based Ink O. V. Manzano, G. J. P. Medina, R. Furlan, A.N. R. da Silva and L. G. Rosa ECS Trans. 2012, Volume 49, Issue 1, Pages 241-245. doi: 10.1149/04901.0241ecst Email alerting service

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© 2012 ECS - The Electrochemical Society

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ECS Transactions, 49 (1) 241-245 (2012) 10.1149/04901.0241ecst ©The Electrochemical Society

Dip Pen Nanolithography Using a Polyacrylonitrile Based Ink O. V. Manzanoa, G. J. P. Medinaa, R. Furlana, A. N. R. da Silvab,c and L. G. Rosaa a

Department of Physics and Electronics, University of Puerto Rico at Humacao, Humacao 00792, USA b Department of Engineering of Electronic Systems, University of Sao Paulo, Sao Paulo 05508, Brazil c Materials, Process, and Electronic Components Course, College of Technology of Sao Paulo, Sao Paulo 01124, Brazil Dip-pen nanolithography (DPN) is an outstanding nanofabrication technique with a combination of resolution and versatility. In this work we explored the use of a novel ink composed of polyacrylonitrile (PAN) and N, N dimethylformamide (DMF), as solvent, to obtain nanostructures. PAN is a largely used polymer that has been explored as a precursor to obtain carbon nanofibers. Our results show the feasibility of defining PAN lines on gold substrates using DPN. Humidity and contamination issues, like PAN oxidation play a major role in this nanolithography process.

Introduction Dip-pen nanolithography (DPN), the direct transfer of a molecular ink from a tip to a substrate, has been considered one of the most popular Atomic Force Microscopy (AFM) nanolithography techniques (1). The transport of material from tip to sample is influenced by (2, 3): chemical nature of the involved parts (tip, substrate, ink, solvent), humidity, temperature, instrumental parameters (scan speed, dwell time, contact force, etc.), and tip coating procedures, shape and contact area. DPN can be used with a wide range of biological, organic, and inorganic materials (4). In particular, the capability to direct-write polymeric materials using DPN opens interesting possibilities of application in nanofabrication. For example, poly(ethylene glycol) (PEG) based ink, requiring no chemical reaction with substrate has been developed, and DPN templates made with this material have been used as either a protective or sacrificial layer to generate raised or recessed structures on surfaces (5, 6). Nanowires of luminescent conductive polymers defined with DPN have been investigated envisioning the fabrication of nano-LED prototypes. Also, nanowires of these polymers represent an ideal system for studying photoluminescence and electron transport in organic one-dimensional materials (7). Other possibilities of application of nanowires made of conductive polymers include the fabrication of interconnections, part of electronic devices and mechanical actuators (3, 8). In this work we investigate the use of a novel ink containing polyacrylonitrile (PAN) and N, N dimethylformamide (DMF), as solvent, to obtain nanostructures with DPN. PAN is a largely used polymer that has been explored as a precursor to obtain carbon nanofibers (9, 10).

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ECS Transactions, 49 (1) 241-245 (2012)

Experimental Dip-pen nanolithography (DPN) was performed on freshly deposited gold films with a thickness of 500 nm. Au (111) was thermally evaporated on oxidized silicon wafers. Gold was used as an initial option but other types of surface (silicon, silicon dioxide, etc.) will be explored further. The solutions for DPN were prepared using polyacrylonitrile (PAN) and N,N, Dimethylformamide (DMF), as solvent. All solutions were prepared by dissolving 25 mg of PAN in 5 ml of DMF. Nanostructuring and imaging were both carried out using a NanoInk Inc. DPN 5000 system. The process of DPN has been described extensively elsewhere (11 – 15). Basically, an AFM tip is used, at low force, to image the surface morphology and select a region for nanolithography. DPN was performed under ambient conditions in an acoustic isolation box at controlled ambient humidity ranging from 40% to 60%. In different experimental runs, 500 nm x 500 nm squares and 3 µm long lines with different widths (200 nm to 2 µm) were defined. The space between lines was keep constant and equal to 1 µm. Consequently, AFM imaging was performed in contact mode in which the tip is placed in contact with the surface. During the contact mode imaging the load force of the tip on the surface was controlled to be less than 1 nN. Finally, cross-sectional height and amplitude analyses were carried out on the acquired topographic image. The software used for the data processing was Scanning Probe Image Processor (SPIP 5.0.5). Results and Discussion Before AFM nanofabrication, images were obtained with a normal force applied by the AFM probe controlled to remain below 1 nN, at ambient conditions. The grain size diameter distribution, obtained from the Au (111) surface, is presented in Figure 1. The surface is atomically flat, with only a few atomic steps and average grain size and roughness of 35 nm and 49 pm, respectively.

Figure 1 – Grain size diameter distribution obtained from the Au (111) surface. Figure 2 demonstrates that the formation of polyacrylonitrile (PAN) lines is feasible. In this case six lines were fabricated using ambient humidity of 40% (Figure 2a) and 60% (Figure 2b), respectively. The cross-sectional analysis, included in Figure 2, which



makes direct height measurements of the fabricated line by producing line profiles, indicates the thickness of the line. The measurement of the line height has been a useful tool to determine the conformation of the molecules in it (11 – 15). As can be seen, the line heights are of the order of 1 nm or less.

(a)

(b) Fig. 2 - Calibration patterns containing six different lines fabricated with relative humidity of: (a) 40% and (b) 60%. The tip to surface diffusion of the solution at the surface during the lithography process is presented in Figure 3 as a function of humidity. This parameter was determined by measuring the width of the line versus the inverse of tip velocity (3, 4). It is evident that diffusion varies upon the concentration of water present in the ambient. This is an expected result due to the hydrophilic properties of PAN/DMF solution (16).



At higher humidity (60%), as clearly observed in Figure 2b, thin patterns of PAN are overlapped with a second substructure (wider and less visible line). It can be speculated that the thinner lines formed during this process are a consequence of the deposition of a second unknown material presente in the solution. This effect points to the occurrence of oxidation of PAN in the solutions, what has to be further investigated. Also, the lithography of PAN on gold substrate is reproducible to some extent and the results are strongly correlated to the environmental conditions.

Fig. 3 - Diffusion coefficient for PAN solutions on Au (111) substrate after transfer by Dip-Pen lithography. Conclusion Dip-pen nanolithography not only has been a powerful tool in the laboratory for scientific discovery, but also has great potential in industrial applications with the development of arrays of AFM probes that may realize massively parallel patterning. Exploration into the mechanism of the lithography process can help understand material transfer, mechanical deformation and chemical reactions at the nanoscale. In this work we demonstrated that dip-pen lithography can be used to fabricate nanoscale patterns of polyacrylonitrile (PAN) on gold substrates. Humidity and contamination issues, like PAN oxidation have to be further investigated. Such controlled PAN lines fabrication will result on predetermined patterned template for carbon nanostructures growth. Acknowledgments The support of NSF – DMR 0923021, NSF – EPS 0701525, NSF – EPS 1002410, DoDARO-59012-RT-REP, Puerto Rico Industrial Development Company (PRIDCO), and Program MARC (NIH) is greatly acknowledged.



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