10758
2007, 111, 10758-10760 Published on Web 06/29/2007
Field-Assisted Nanopatterning Jun-Fu Liu and Glen P. Miller* Department of Chemistry and Materials Science Program, UniVersity of New Hampshire, Durham, New Hampshire 03824 ReceiVed: May 29, 2007
A new atomic force microscopy (AFM)-based lithography method called field-assisted nanopatterning (FAN) has been demonstrated. Through the use of a conventional atomic force microscope with no alterations, FAN controllably patterns solid or liquid organic and inorganic molecules in the air under ambient conditions. In this manner, patterns can be produced with feature sizes that range from tens of microns to sub-20 nm. Examples include the high-resolution FAN of [60]fullerene, N-methylpyrrole, naphthalene, poly-3-octylthiophene, polyaniline, meso-tetraphenylporphyrin, and gold. These molecules have been patterned onto highly ordered pyrolytic graphite, indium-tin oxide, Au, and passivated Au. The molecules are first coated on a standard AFM tip and then are deposited onto the substrate when a threshold tip bias is achieved. The deposition process is a field-assisted transfer of the molecules from the tip to the substrate. Patterning is turned on or off by controlling tip bias, and the same tip is used for both patterning and imaging. Pattern dimensions are controlled by varying tip bias and fabrication (tip) speed.
Because the probe tip-surface interaction is confined to the nanoscale, scanning probe microscope-based techniques continue to be important for the fabrication of inorganic and organic nanostructures as well as the atomic-scale manipulation of atoms. Examples of atomic force microscopy (AFM) and scanning tunneling microscopy nanofabrication include field evaporation of metal atoms from the tip,1 tip-induced local surface reactions,2 electrophoretic differentiation and patterning of DNA molecules,3 and dip-pen nanolithography (DPN).4 We now report an AFM-based lithography method called fieldassisted nanopatterning (FAN) that can be used to pattern a large range of either solid or liquid organic and inorganic molecules in air under ambient conditions. FAN is achieved by applying a voltage between an AFM tip and a conducting or semiconducting substrate. In this manner, [60]fullerene, N-methylpyrrole, naphthalene, poly-3-octylthiophene, polyaniline, meso-tetraphenylporphyrin, and gold have all been successfully patterned onto either highly ordered pyrolytic graphite (HOPG), indium-tin oxide (ITO), Au, or passivated Au surfaces with feature sizes that range from many microns to sub-20 nm. Of these substrates, HOPG provides the most uniform, continuous nanoscale features followed by ITO, passivated Au, and finally Au. Nanopatterns fabricated on Au are less uniform, possibly because of rapid contamination of the Au surface in air. FAN is turned ON when a negative tip bias is applied, OFF when the tip bias is removed, back ON when the tip bias is re-established, etc. Thus, the nanopattern need not be a continuous one. FAN is performed in the contact mode using standard silicon probes5 for both patterning and imaging. Organic and inorganic molecules are first dip-coated onto standard Si tips (with native SiO2 film) using solutions/ * To whom correspondence should be addressed. E-mail: glen.miller@ unh.edu.
10.1021/jp074144c CCC: $37.00
suspensions in organic solvent. The tips are dried before use. FAN is achieved by executing standard lithography software one time. Feature sizes, gaps between features, fabrication (tip) speeds, and the tip bias are all controlled by the lithography software and can be modified as appropriate. During FAN, the tip force is maintained at 2 nN. All experiments are performed in air at ambient temperature. FAN of [60]fullerene onto octadecanethiol-coated gold at different negative tip bias values is illustrated in Figure 1. The [60]fullerene nanolines are 40-100 nm in the lateral dimension and 2-12 nm higher than the passivated gold surface. The height value of the nanolines indicates multilayers of [60]fullerene molecules bound to each other and to the substrate surface via van der Waals interactions, which are individually weak but collectively strong. Nanodots composed of [60]fullerene have also been prepared with dimensions (50-80 nm wide and 0.91 nm high) that suggest monolayer islands composed of only a few thousand molecules. An array of parallel polyaniline (PANI) nanolines fabricated at tip speeds of 100 and 500 nm/s is illustrated in Figure 2. When writing at a tip speed of 100 nm/s, the PANI lines are 70-90 nm in the lateral dimension, 0.8-1.3 nm higher than the HOPG surface, and less regular than the PANI lines fabricated at a tip speed of 500 nm/s. At the higher tip speed, the PANI lines are only 20 nm wide, 0.3-0.4 nm high, and uniform. This latter height suggests a monolayer of PANI in which the aromatic rings lie flat on the HOPG surface to maximize van der Waals interactions. Greater fabrication speeds are equivalent to shorter molecular transfer times that equate to smaller, more shallow (and in this case more uniform) feature sizes. Complex nanostructures can also be fabricated using the FAN method. Figure 3 shows a molecular grid composed of a crisscrossing network of [60]fullerene and naphthalene nanolines. © 2007 American Chemical Society
Letters
J. Phys. Chem. C, Vol. 111, No. 29, 2007 10759
Figure 1. Field-assisted nanopatterning of [60]fullerene nanolines on octadecanethiol-coated Au(111) at different tip bias values. The lines were fabricated using tip bias values of -6, -7, -8, -9, and -10 V (from left to right in the left image) and a fabrication speed of 100 nm/s.
Figure 2. Height images of field-assisted nanopatterned lines of polyaniline on HOPG. The lines were fabricated at a tip bias of -8 V and with fabrication speeds of 500 nm/s (left) and 100 nm/s (right).
Figure 3. Field-assisted nanopatterning of criss-crossing naphthalene and [60]fullerene lines on HOPG. The naphthalene lines were fabricated using a tip bias of -9 V and a fabrication speed of 100 nm/s. The [60]fullerene lines were patterned using a tip bias of -10 V and a fabrication speed of 100 nm/s.
This nanopattern was created by first depositing parallel naphthalene nanolines onto HOPG followed by deposition of a separate set of [60]fullerene lines that are collectively orthogonal to the naphthalene lines. High-resolution images of several crossing points reveal them to be higher than either individual line. Thus, FAN may be capable of creating multilayer nanostructures that are analogous to those produced by thermal DPN methods6 but with much smaller feature sizes and with options for heterogeneous assembly.
There are numerous compelling indications that the FAN process does indeed directionally deposit the tip-coating materials. These include each of the following: (1) there is no deposition in the absence of a tip bias; (2) there is no deposition when using an uncoated tip; (3) the dimensions of nanopatterned features are a function of either tip speed (nanolines) or tip holdtime (nanodots) with feature heights ranging from less than one nm (e.g., high tip speeds, short tip hold times) to greater than 20 nm (e.g., slow tip speeds, long tip hold times); (4) the semivolatile liquid N-methylpyrrole (bp 111-113 °C) was deposited onto HOPG using FAN and the resulting nanopatterns were observed to slowly disappear (i.e., evaporate) over a period of a few hours; (5) nanopatterns created using nonvolatile organic and inorganic materials do not disappear; (6) nanopatterned [60]fullerene is observed to slowly shrink and in some cases completely disappear upon addition of either toluene or diethylenetriamine, both of which dissolve [60]fullerene; (7) [60]fullerene monolayers created using FAN have measured heights of 0.9-1 nm, consistent with the diameter of [60]fullerene; (8) monolayers of meso-tetraphenylporphyrin deposited on HOPG have measured heights of 3-4 Å, consistent with porphyrin molecules lying flat on the HOPG surface to achieve maximum π-π-stacking interactions; (9) monolayers of PANI deposited on HOPG also have measured heights of 3-4 Å, consistent with PANI molecules lying flat on the HOPG surface to achieve maximum π-π-stacking interactions. We propose a mechanism for FAN involving field evaporation of molecular species, a process of electric field-induced ionization and ejection of molecules from the Si tip1
10760 J. Phys. Chem. C, Vol. 111, No. 29, 2007
Figure 4. A schematic representation of field-assisted nanopatterning using an AFM tip.
(Figure 4). According to this field evaporation mechanism, a threshold ionization and ejection potential exists for each molecule. The value of the threshold ejection potential depends upon several factors including tip geometry, the nature of the molecules to be deposited, the nature of the substrate, the gap between the tip and the substrate (if any), and the location of the molecule on the tip surface. At sufficiently negative tip bias values, electrostatic repulsions exist between molecules and also between the tip and the molecules. Ultimately, an ejection potential is achieved causing the molecules on the tip to be directionally deposited onto the electrostatically attractive substrate surface. We believe that the molecules on the sidewall of the tip are mobile under the high electric field as has recently been demonstrated for DNA molecules under similar conditions.3 This mobility model is consistent with our observation that FAN line resolution does not progressively worsen upon using the same coated tip for multiple experiments. Instead, the probe tips simply stop writing when all of the molecular coating has been consumed. Occasionally, a temporary loss of writing function is observed, resuming again after fresh molecules travel closer to the tip nadir. The field evaporation-mobility mechanism suggests that features as small as 1-2 nm could be achieved using an ultrasharp AFM tip. We are currently investigating this possibility. FAN is not directly related to dip-pen nanolithography (DPN) or electrochemical dip-pen nanolithography7 (E-DPN). While FAN, DPN, and E-DPN all utilize coated tips, FAN does not utilize a water meniscus for transfer. This fact was demonstrated unambiguously by placing the entire AFM apparatus inside a large glove bag that was either purged with dry nitrogen to achieve dry conditions or with water vapor to achieve high humidity conditions. In this way, we performed FAN experiments at relative humidities (RH) that ranged from 0 to 80%. In all cases (including 0% RH), [60]fullerene was successfully nanopatterned on HOPG. Thus, FAN does not require a water meniscus as does DPN and E-DPN. As noted above, FAN preferentially utilizes a negative tip bias. The use of a positive tip bias does allow directional deposition of some metals, but it does not work well for most organic and inorganic compounds, including pyrrole and polythiophene, both of which have relatively low oxidation potentials. This fact further supports our contention that FAN is unrelated to E-DPN and is instead a field-assisted process.
Letters Whereas DPN is largely confined to the transfer of liquid thiols and E-DPN to the transfer of water soluble metallic salts7a and polar monomers,7b FAN is capable of transferring a wide range of liquid and solid (organic and inorganic) molecules irrespective of their functionality and polarity. Because the FAN technique utilizes an electric field for deposition, there is no requirement that the molecules be mobile on the tip surface at a tip bias of 0 V. Furthermore, FAN deposits these molecules onto a wide range of conducting and semiconducting substrates and utilizes the same tip for both patterning and imaging. In summary, a new AFM-based lithography method called field-assisted nanopatterning (FAN) has been demonstrated. Through the use of a conventional atomic force microscope with no alterations, FAN controllably patterns solid or liquid organic and inorganic molecules in the air under ambient conditions with feature sizes that range from tens of microns to sub20 nm. The deposition process is a field-assisted transfer of the molecules from the tip to the substrate. Patterning is turned on or off by controlling tip bias, and the same tip is used for both patterning and imaging. Acknowledgment. The authors acknowledge support of this work by the Nanoscale Science and Engineering Center for High-Rate Nanomanufacturing (NSF Award No. 0425826). They also acknowledge the suggestion of a reviewer to perform FAN experiments at several different percent relative humidities. Supporting Information Available: Multiple AFM images of field-assisted nanopatterned species. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Mamin, H. J.; Guethner, P. H.; Rugar, D. Phys. ReV. Lett. 1990, 65, 2418. (b) Hosaka, S.; Koyanagi, H.; Kikukawa, A.; Maruyama, Y.; Imura, R. J. Vac. Sci. Technol., B 1994, 12, 1872. (c) Bessho, K.; Hashimoto, S. Appl. Phys. Lett. 1994, 65, 2142. (d) Houel, A.; Tonneau, D.; Bonnail, N.; Dallaporta, H.; Safarov, V. I. J. Vac. Sci. Technol., B 2002, 20, 2337. (e) Fujita, D.; Kumakura, T. Appl. Phys. Lett. 2003, 82, 2329. (f) Song, J. Q.; Li, C. Z.; He, H. X.; Chen, Y.; Wang, L.; Liu, Z. F. Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 1997, 51, 294. (2) (a) Dagata, J. A.; Schneir, J.; Harary, H. H.; Evans, C. J.; Postek, M. T.; Bennett, J. Appl. Phys. Lett. 1990, 56, 2001. (b) Dagata, J. A. Science 1995, 270, 1625. (c) Dai, H. J.; Franklin, N.; Han, J. Appl. Phys. Lett. 1998, 73, 1508. (d) Maoz, R.; Frydman, E.; Cohen, S. R.; Sagiv, J. AdV. Mater. 2000, 11, 55. (e) Ahn, S. J.; Jang, Y. K.; Lee, H.; Lee, H. Appl. Phys. Lett. 2002, 80, 2592. (f) Stievenard, D.; Fontaine, P. A.; Dubois, E. Appl. Phys. Lett. 1997, 70, 3272. (g) Legrand, B.; Stievenard, D. Appl. Phys. Lett. 1999, 74, 4049. (3) Unal, K.; Frommer, J.; Wickramasinghe, H. K. App. Phys. Lett. 2006, 88, 183105. (4) (a) Piner, R. D.; Zhu, J.; Xu, F.; Hong, S.; Mirkin, C. A. Science 1999, 283, 661. (b) Hong, S.; Mirkin, C. A. Science 2000, 288, 1808. (c) Ginger, D. S.; Zhang, H.; Mirkin, C. A. Angew. Chem., Int. Ed. 2004, 43, 30. (5) The tip radius is less than 10 nm. The tip height is 20-25 µm. The tip cone angle is less than 30°. The cantilever width is 35 µm, and the cantilever length is 130 µm. Most of the Si/SiO2 tips utilized have a force constant of 0.6 N/m. We have also performed FAN studies using commercial Au-coated Si tips, Cr-coated Si tips, and Au/Cr-coated Si tips (MikroMasch Co.) but only in the context of directionally depositing the metals from the tip. (6) Yang, M.; Sheehan, P. E.; King, W. P.; Whitman, L. J. J. Am. Chem. Soc. 2006, 128, 6774. (7) (a) Li, Y.; Maynor, B. W.; Liu, J. J. Am. Chem. Soc. 2001, 123, 2105. (b) Maynor, B. W.; Filocamo, S. F.; Grinstaff, M. W.; Liu, J. J. Am. Chem. Soc. 2002, 124, 522.
