NANO LETTERS
Creating Nanoscale Patterns of Dendrimers on Silicon Surfaces with Dip-Pen Nanolithography
2002 Vol. 2, No. 7 713-716
Rachel McKendry,*,† Wilhelm T. S. Huck,† Brandon Weeks,‡ Maria Fiorini,† Chris Abell,† and Trevor Rayment† Department of Chemistry, UniVersity of Cambridge, Lensfield Road, Cambridge U.K., and MelVille Laboratory for Polymer Synthesis, Department of Chemistry, UniVersity of Cambridge, Pembroke Street, Cambridge, U.K. Received March 20, 2002; Revised Manuscript Received May 10, 2002
ABSTRACT Dip-pen nanolithography (DPN) is a nanowriting procedure that employs an AFM tip as a “nanopen” to deposit organic molecules onto a substrate surface. This paper describes the application of DPN to write with dendrimer “inks”. We have generated patterns with 100 nm features (∼20 dendrimer molecules) on a Si/SiOx surface and investigated how the resolution is affected by surface chemistry and molecular weight of the dendrimer ink.
As the size of microelectronic devices continues to shrink, control of the chemistry and structure of materials at the molecular level is critical. Conventional lithographic techniques which pattern polymeric thin films are beginning to reach their resolution limit and several alternative “bottomup” strategies have emerged that use the scanning probe microscope to manipulate matter at the atomic or molecular scale. Of these new techniques, dip-pen nanolithography (DPN)1-5 is particularly promising. The DPN methodology utilizes the tip of an atomic force microscope as a “nanopen” to transport an “ink” containing organic molecules onto a solid support via a water meniscus (see Figure 1). Using the same tip to write and subsequently read patterns, it is possible to create nanoscale patterns of alkanethiols with remarkable resolution ( 100 µm/s), acquiring simultaneously (a) topographic and (b) friction force images. The reading scan speed was the maximum possible for the AFM to reduce overwriting the diamonds. The pattern of closely spaced vertical bars is thus an artifact of the feedback loop and a function of the scan speed. The scale bar represents 500 nm.
* Corresponding Author. Address: Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW. tel: +44 1223 336 528. email:
[email protected] † Department of Chemistry. ‡ Melville Laboratory for Polymer Synthesis.
branching chains. The catalytic,9 binding,10 and optical properties11 of dendrimers have generated considerable interest, with potential applications in the field of drug delivery, chemical sensors, and photosensitive materials.12
10.1021/nl020247p CCC: $22.00 Published on Web 06/05/2002
© 2002 American Chemical Society
Figure 2. Blotting experiments as a function of tip-sample contact time for a stationary tip: (a) friction and (b) topographic images that result from contact times of 5, 10, and 15 s using a G5 DAB dendrimer; (c) typical line scan showing the cross section of a droplet of G5 DAB formed after 15 s contact; (d) plot of the blot area as a function of time for G1 and G4 PAMAM dendrimers; (e) table showing average dendrimer delivery rates, estimated from gradient of linear portion of the graph (d).
Here we report that DPN can be used to pattern dendrimers on silicon surfaces and have investigated the effect of surface chemistry and the molecular weight of different inks. We chose to investigate writing with dendrimer inks directly onto Si/SiO2 semiconductor surfaces which are immediately compatible with foreseeable device applications. In this work, two different classes of commercially available dendrimers were used: Starburst polyamidoamine dendrimers,8 PAMAM, and polypropylene imine dendrimers, DAB.12 The electrostatic interaction between basic amino terminated dendrimers with glass and silica has been studied by a number of techniques.13,14 Recently it has been shown that amino terminating dendrimers can be patterned directly onto Si/SiO2 substrates by microcontact printing, to form stable nanostructures.15 Therefore our initial experiments used amine-terminated generation 5 DAB dendrimers, (G5 DAB). In a prototypical experiment, tips coated with G5 DAB dendrimers16 were brought into contact with a silicon (100) surface, and a pattern of diamonds was traced slowly, delivering dendrimer to the surface. The patterns were “read” by imaging with the same tip over a very large area at high scan speed,17 acquiring simultaneous topographic and friction force images. It was necessary to read the patterns at high scan speed to avoid depositing more dendrimer over freshly written areas. The topographic image (Figure 1a) clearly resolves white diamonds of raised topography. The friction 714
image (Figure 1b) shows corresponding dark diamonds of lower friction compared with the underlying silicon surface due to lower surface energy of dendrimer-coated surface.18 We did not observe any change in the patterns when imaged again after 6 h, which indicates that the written patterns are stable and that the ink does not spread out over the surface.15 Control experiments using a bare silicon nitride tip produced no patterns. To understand the factors that affect the rate of dendrimer delivery to the surface, blotting measurements were taken as a function of tip-sample contact time for a stationary tip. Figure 2a shows friction images that result from contact times of 5, 10, and 15 s. Figure 2b shows the corresponding topographic three-dimensional image. Figure 2d shows a typical plot of the diameter as a function of contact time, over a 1 min time period. It can be seen that initially the diameter increases linearly, suggesting that dendrimers are being delivered at an approximately constant rate. The plateau region observed in Figure 2d can be explained by the fact that while a flat disk is formed initially, it thickens with time to form a mesoscopic droplet on the surface (see Figure 2c). The blotting behavior was investigated for a range of dendrimers of differing characteristics. The effect of molecular weight upon DPN was studied using four generations of amine terminated PAMAM dendrimers of increasing Nano Lett., Vol. 2, No. 7, 2002
molecular weight (G1-G4 PAMAM). The effect of surface chemistry was investigated using hydroxyl-terminated instead of amine-terminated PAMAM dendrimers (G4 OH). The results summarized in Figure 2e (table) reflect the average dendrimer delivery rate of 30 measurements with more than five different tips. Variations in humidity of the environment and tip shape make absolute quantitative measurements difficult. However, two trends are apparent: the rate of delivery of larger molecules is slower and the surface chemistry has a significant impact. As the PAMAM dendrimer series increases from G1 to G4, the rate of delivery decreases (Figure 2e), suggesting that delivery is affected by molecular weight or the number of surface functional groups. The comparison of G4 PAMAM (Mw 14 215) with G5 DAB (Mw 7179) is informative. Despite the difference in molecular mass, they have the same number (64) and nature (primary amines) of surface groups. The delivery rate is faster for G5 DAB and indeed is similar to that for G3 PAMAM (32 surface amines), which has approximately the same molecular weight. This implies that molecular weight is an important parameter, indeed when high molecular weight polyethylene imine PEI (Mw 750 000) is used, a very slow delivery rate is observed. Finally, two fourth-generation dendrimers, which differed only in surface functionalization, were compared: G4 OH and G4 amine. The hydroxylterminated dendrimer was delivered more rapidly, showing that the type of surface functionalization is an important parameter as well. The observed effect could be due to stronger electrostatic attraction between the positively charged amine and the negative silicon oxide surface or possibly the greater solubility of the hydroxyl dendrimer. More experiments are needed to investigate this further. Knowing the static delivery rate, we were able to reduce the feature sizes by simply scanning faster. At a scan rate of 20 µm/s, it was possible to write 300 nm wide lines with G1 PAMAM dendrimers, which have a delivery rate of 6.2 µm/s (Figure 3a and 3b). Tips modified with the larger G5 DAB, which spreads more slowly, produced line widths of 100 nm, corresponding to approximately 20 dendrimer molecules (Figure 3c and 3d). The major limitation of our approach is our write-read capabilities using the same AFM tip. The resolution of patterns could be greatly improved using one modified tip to write with polymeric inks and a clean tip to read the patterns at slow scan speeds. With this approach, Mirkin et al. have demonstrated that DPN with thiol inks can produce features with less than 15 nm resolution. Further fine-tuning of our experimental conditions, for example using sharper tips (perhaps carbon nanotubes), faster scan speeds, lower temperatures, and tailoring the solubility and size of our inks may also reduce feature sizes, approaching the regime where it may be possible to write down to the resolution of individual dendrimer molecules. Concluding, we have successfully demonstrated that dippen nanolithography can be used to write nanostructures with dendrimers and polymers directly onto silicon. The nature of the ink in DPN can be changed from thiols to polymers without losing control over the writing capabilities. An Nano Lett., Vol. 2, No. 7, 2002
Figure 3. (a) Friction and (b) topographic images of lines produced with G1 PAMAM dendrimers; (c) friction and (d) topographic images of lines produced with G5 DAB dendrimers.
increase in molecular weight leads to narrower lines, as was demonstrated by the use of heavier dendrimers with similar surface chemistry. Our smallest patterns to date were in the 100 nm range, but future optimizations of the ink should make it possible to write smaller patterns. At present the underlying physics of DPN is poorly understood. The dependence of the delivery rate upon the size (generation) of the dendrimer is consistent with work by Duijvenbode et al.13 who studied the adsorption kinetics of PAMAM dendrimers on glass. They found that the rate of adsorption was also dependent on the pH of the solution, suggesting an electrostatic interaction. Althought DPN kinetics will be quite different, the nature of the interaction with the surface will be the same. Further work is required to permit quantitative predictions of writing speeds based upon the physical and chemical properties of the ink. DPN has several important advantages over conventional electron beam or photolithographic techniques. It does not require special, expensive facilities (only a standard AFM) and patterns can be generated in seconds without the need for photoresists or masters. Using DPN it should be possible to couple different polymer structures, for example conducting or light emitting polymers,6 with functions that might find application in device fabrication. Acknowledgment. We thank Girton College and the Royal Society (R.M. Dorothy Hodgkin Fellowship), the Leverhulme Trust (M.F.), and the E.P.S.R.C. for financial support. References (1) Piner, R. D.; Zhu, J.; Xu, F.; Hong, S. H.; Mirkin, C. A. Science 1999, 283, 661-663. 715
(2) Hong, S. H.; Zhu, J.; Mirkin, C. A. Science 1999, 286, 523-525. (3) Hang, S. H.; Mirkin, C. A. Science 2000, 288, 1808-1811. (4) Wilson, D. L.; Martin, R.; Hong, S.; Cronin-Golomb, M.; Mirkin, C. A.; Kaplan, D. L. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 1366013664. (5) Ivanisevic, A.; Mirkin, C. A. J. Am. Chem. Soc. 2001, 123, 78877889. (6) Noy, A.; Miller, A. E.; Klare, J. E.; Weeks, B. L.; Woods, B. W.; BeYoreo, J. J. Nano Lett. 2002, 2, 109-112. (7) Maynor, B. W.; Filocamo, S. F.; Grinstaff, M. W.; Liu, J. J. Am. Chem. Soc. 2002, 124, 522-523. (8) Tomalia, D. A. AdV. Mater. 1994, 6, 529-539. (9) Bhyrappa, P.; Young, J. K.; Moore, J. S.; Suslick, K. S. J. Am. Chem. Soc. 1996, 118, 5708-5711. (10) Jansen, J. F. G. A.; De Brabander-van den Berg, E. M. M.; Meijer, E. W. Science 1994, 226, 1266-1299. (11) Sadamoto, R.; Tomioka, N.; Aida, T. J. Am. Chem. Soc. 1996, 118, 3978-3979.
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Weener, J. W.; Meijer, E. W. AdV. Mater. 2000, 12, 741. Duijvenbode, Rietveld, I. B. Langmuir 2000, 16, 7720-7725. Esumi, K.; Goino, M. Langmuir 1998, 14, 4466-4470. Li, H.; Kang, D.-J.; Blamire, M.; Huck, W. T. S. Nano Lett. 2002, 2, 347. (16) Loaded silicon nitride tips (Thermomicroscopes, triangular, spring constant 0.06 N/m) were prepared by immersion in a 10 µM ethanolic solution of dendrimer (Aldrich, U.K.) for 5 s and then left to air-dry for 5 min prior to use. Si(100) wafers were used as supplied from Compart Technology, U.K. (17) Experiments were conducted using an Eastcoast Scientific AFM, with a quadrant photodetector allowing simultaneous topographic and friction force measurements. Contact mode AFM was performed in air at 19.5 ( 0.5 °C and relative humidity of 52 ( 2%. (18) Zhang, X.; Wilhelm, M.; Klein, J.; Pfaadt, M.; Meijer, E. W. Langmuir 2000, 16, 3884-3892.
NL020247P
Nano Lett., Vol. 2, No. 7, 2002
