NANO LETTERS
1D Nanofabrication with a Micrometer-Sized Laser Spot
2006 Vol. 6, No. 10 2358-2361
Daniel Dahlhaus, Steffen Franzka, Eckart Hasselbrink, and Nils Hartmann* Fachbereich Chemie, UniVersita¨t Duisburg-Essen, UniVersita¨tsstr. 5, 45141 Essen, Germany Received July 13, 2006; Revised Manuscript Received August 23, 2006
ABSTRACT A simple laser-assisted procedure for the fabrication of functional organic nanostructures is demonstrated. Native silicon samples are coated with alkylsiloxane monolayers and patterned with a focused beam of an Ar+ laser (λ ) 514 nm). After patterning, the coating is chemically functionalized following a robust preparation scheme. Despite a laser spot diameter of about 2.5 µm, this routine allows for the fabrication of well-confined organosiloxane stripes with widths below 100 nm. As shown, these structures provide a versatile means for building ordered surface architectures of nanoscopic components. In particular, gold nanoparticles (d ) 16 nm) self-assemble into one-dimensional arrangements, such as single chains.
Nanoparticles having diameters of d < 100 nm are widely recognized for their exceptional electronic, optoelectronic, and catalytic properties.1-3 Many advanced applications require an arrangement of these functional components into ordered structures.4,5 Single chains of nobel-metal nanoparticles, for example, have been shown to function as subwavelength waveguides.6,7 A general wet-chemical routine for the fabrication of such one-dimensional structures relies on the preparation of nanostructured templates, which allow for a selective adsorption of nanoparticles in predefined positions.7-13 This usually necessitates lithographic techniques with a lateral resolution in the sub-100 nm range, such as scanning probe techniques and e-beam lithography.7-10 In contrast, laser beam lithography, addressed here, typically yields structures with a lateral dimension on the order of the laser spot diameter and hence in most cases is limited to microfabrication.14 A simple means of extending the lateral resolution into the submicrometer range takes advantage of photothermal processes.15-21 In this case, the interplay between the laser-induced local temperature rise and the thermally activated process allows for the preparation of structures with lateral dimensions that are significantly smaller than the laser spot diameter.14 Photothermal routines that allow for patterning in the sub-100 nm regime, though, are rare.14,16 Recently, we developed some photothermal procedures for the preparation of submicrometer-structured alkylsiloxane monolayers.18-21 Despite a spot diameter of about 2.5 µm, well-confined lines with widths of 200 nm were prepared.20,21 Here we present a facile methodology for the fabrication of * Corresponding author. E-mail:
[email protected]. Phone: +49 201 183 3058. Fax: +49 201 1833228. 10.1021/nl061608u CCC: $33.50 Published on Web 09/12/2006
© 2006 American Chemical Society
organic structures with a lateral dimension below 100 nm. After chemical functionalization, these organic patterns are suitable for directing the adsorption and self-assembly of gold nanoparticles into micro- and nanostructured arrays. In particular, citrate-coated gold nanoparticles with an average diameter of 16 nm are shown to form single chains. For coating, samples cut from a commercial Si(100) wafer (p-type, 1-20 Ωcm) were first cleaned in piranha solution (caution: piranha solution should be handled with extreme care) and then immersed into a millimolar solution of octadecyltrichlorosilane (OTS) in toluene following common procedures.22,23 Long immersion times were chosen to ensure the formation of a complete octadecylsiloxane monolayer.24 Also, prior to patterning the coated samples were put aside for at least 1 day in order to ensure sufficient cross-linking and grafting of the self-assembled molecules.25 Patterning was carried out under ambient conditions using a laser direct writing setup described elsewhere.26 Briefly, the beam of an Ar+ laser operated at a wavelength of 514 nm is focused onto the sample and scanned across its surface. The laser spot exhibited a Gaussian intensity profile with an 1/e2 diameter of about 2.5 µm. Patterning with this setup is feasible at fast writing speeds up to 25 mm/s over large areas in the square millimeter to square centimeter range. For characterization of the patterns, atomic force microscopy (AFM, Autoprobe CP and Nanoscope IIIa from Veeco) has been used. All of the AFM images shown here display the topography as recorded in contact mode and tapping mode, respectively. As demonstrated previously, direct laser patterning of alkylsiloxane monolayers results in a decomposition of the coating along well-confined lines.20,21 Line widths are
Figure 1. Schematic drawing of the photothermal patterning strategy mentioned in the text. The dashed line depicts the intensity profile of the laser spot. The solid line shows the height profile after writing the first and the second line, as displayed at the top and at the bottom, respectively. Black arrows at the bottom indicate the narrow stripe of the monolayer, which is left between the laserwritten lines after patterning. Note that photothermal patterning generally yields lines with widths well below the laser spot diameter. Moreover, the lines exhibit extraordinarily sharp edges.
between 0.2 and 0.8 µm depending on the laser power and writing speed. Most notably, however, the width of the line edges is below 100 nm.20 As illustrated in Figure 1, this provides an opportunity to prepare narrow alkylsiloxane stripes upon drawing densely spaced lines. In particular, successive laser irradiation along adjacent lines results in a local decomposition of the coating in neighboring areas and leaves a narrow stripe behind. A typical line pattern that was created in this way is displayed in Figure 2a. Laser patterning in this case was carried out at constant parameters yielding lines with widths of about 400 nm. As shown in the height profile of Figure 2b, stripes with widths down to 80 nm are left behind. In some experiments, even stripes with widths of 60 nm are reached. Note that these values refer to the width at half-height. Stripes with widths below 120 nm exhibit a reduced height indicative for a partial decomposition of the monolayer structure. In Figure 2b, these stripes appear as hillocks. In fact, however, they represent flat structures. To illustrate this, a scaled profile across an 80 nm line is displayed in the inset of Figure 2b. For comparison, a black circle depicts the size of the gold nanoparticles that were deposited onto these structures as discussed further below. The results in Figure 2 clearly demonstrate the feasibility of creating organic nanostructures with widths of 80 nm. Note that the resolution of the stepper motor stages used for sample positioning is 100 nm. This currently limits controlled patterning below 100 nm. Another problem are mechanical vibrations. A regular pattern of stripes with widths of 100 nm is shown in Figure 3a. Nano Lett., Vol. 6, No. 10, 2006
Figure 2. (a) AFM image of a line pattern after photothermal patterning of an octadecylsiloxane monolayer. (b) Height profile at the position indicated in 2a. Note that in comparison to the length scale (x axis) the height scale (y axis) is enlarged by 3 orders of magnitude. For this reason, the narrow stripes appear as hillocks. The inset displays a 1:1 scale of the height profile across the 80 nm stripe, that is, the true topography of this structure. For comparison, a black circle depicts the average size of the gold nanoparticles used throughout this work.
Note that patterning is not restricted to line patterns as those displayed in Figure 3a. In Figure 3b, for example, a 2 × 2 array of dots is shown that was fabricated by intersecting lines. In principle, lines with bends and corners can also be created using positioning stages with continuous path control. Our current experimental setup, though, does not offer this feature. After patterning, the alkylsiloxane monolayer was chemically functionalized following a procedure described in the literature.27-29 Overall, this procedure involves three steps: (i) radical bromination in a solution of bromine upon irradiation with a tungsten lamp, (ii) azidation by substitution with NaN3, and (iii) reduction with LiAlH4 yielding amine groups. For characterization, coated samples were functionalized and investigated by photoelectron spectroscopy (XPS, Phi Quantum 2000 spectrometer) and water contact angle measurements (OCA 15 plus goniometer). The results are generally in agreement with those reported previously by Baker and Watling.28,29 In particular, an estimate considering the peak intensities in the XPS spectra suggests that, on average, each hydrocarbon chain carries between one and two functional groups. 2359
Figure 3. AFM images of (a) three parallel stripes and (b) a 2 × 2 array of dots.
Note that alternative procedures using ω-functionalized aminosilanes have been reported to form disordered coatings; that is, some terminal groups are buried within the monolayer.23,30 Hence, the effective density of amine groups at the surface in this case is expected to be comparatively low. In contrast, infrared spectroscopic measurements suggest that post functionalization as described above primarily takes place at the outer surface of the monolayer.29 Therefore, this routine appeared more promising to us. Most notably, though, the robust chemistry should ensure a functionalization of the alkylsiloxane stripes even if the monolayer was partially decomposed during patterning.31 In conjunction with the patterning procedure described above, this provides the basis for the preparation of functional organic nanostructures with widths of 100 nm and below as demonstrated here. As demonstrated in the following, the aminated patterns represent functional templates that allow us to direct the selfassembly of nanoscopic components into predefined domains. For this purpose, we immersed the samples in an aqueous solution of gold nanoparticles as prepared by the citrate method.32 The solution used here had a total gold concentration of 12.5 mg/L and was weakly acidic; that is, the pH was close to 6. Transmission electron microscopic measure2360
Figure 4. AFM images of aminated line patterns after selective deposition of gold nanoparticles. The immersion time in solution was 30 min. The nanoparticles appear as bright spots. (a) Stripes with widths between 400 and 700 nm. (b) Stripes with widths of about 100 nm (left) and 200 nm (right), respectively. The arrows depict the position of the height profile in Figure 2b. (c) Height profile across the lines shown in Figure 2b indicating the size of the gold nanoparticles.
ments (CM 200 FEG from Philips) revealed an average particle size of 16 nm. At weakly acidic conditions, the aminated coating is positively charged, whereas the nanoparticles are negatively charged.8 Hence, upon immersion of the substrates into the solution the particles primarily adsorb on top of the aminated alkylsiloxane stripes. In contrast, particle deposition in the surrounding areas is negligible. Apparently, patterning here results in a complete decomposition of the hydrocarbon chains. Typical AFM images are shown in Figure 4. Here, aminated stripes with varying widths are prepared in order to investigate the dependence of the colloidal self-assembly on the domain size. Most remarkably, single particle chains are formed on narrow stripes. This clearly emphasizes the Nano Lett., Vol. 6, No. 10, 2006
capabilities of the underlying patterning routine in nanofabrication. Alternative procedures for the preparation of such one-dimensional structures commonly involve nanolithographic techniques,7-13 for example, scanning probe techniques and e-beam lithography.7-10 Note that the width of the stripes on which the single chains form is close to 100 nm; that is, it is significantly larger than the size of the particles. Hence, a wider and lessordered assembly of the nanoparticles might be expected. The formation of comparatively ordered 1D structures such as those shown on the left side of Figure 4b could take place via a rearrangement of the deposited particles after the sample has been removed from solution. Such a two-step mechanism has been suggested by Aizenberg et al. for the colloidal assembly on micropatterned substrates.33 In solution, the particles at first attach to the patterns because of electrostatic interactions. Once the samples are removed from solution, the particles remain partially immersed in a liquid layer. Driven by capillary forces, the particles then start to rearrange. Single particles on circular-shaped domains, for example, have been shown to move into the center before drying is complete. In conclusion, photothermal patterning of alkylsiloxane monolayers in conjunction with post-functionalization schemes provides a facile means for nanofabrication. Upon irradiation with a micrometer-sized spot of an Ar+ laser (λ ) 514 nm) under ambient conditions, organic nanostructures with widths of 80 nm and below are prepared here. After amination, these structures represent functional templates that allow one to build ordered patterns of nanoscopic components. As an example, citrate-stabilized gold nanoparticles with an average diameter of 16 nm are shown to self-assemble into onedimensional arrangements. Most remarkably, single chains are formed on narrow organosiloxane stripes. As a sequential procedure, the overall routine offers a high degree of flexibility and hence is particularly useful in applications where the desired structures change frequently, for example, in research and development. Contrary to other sequential procedures, patterning is feasible over large areas at fast writing speeds. Acknowledgment. Financial support by the Deutsche Forschungsgemeinschaft (HA 1424/5-3, Graduiertenkolleg 689) and the BASF Coatings AG is gratefully acknowledged.
Nano Lett., Vol. 6, No. 10, 2006
We are indebted to Peter Thiessen for the XPS measurements and to Professor Matthias Epple and Ursula Giebel for supplying the nanoparticle solution. References (1) Schmid, G. Nanoparticles: From Theory to Application; WileyVCH: Weinheim, Germany, 2004. (2) Daniel, M.-C.; Astruc, D. Chem. ReV. 2004, 104, 293. (3) Schlo¨gl, R.; Abd Hamid, S. B. Angew. Chem. Int. Ed. 2004, 43, 1628. (4) Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem 2000, 1, 18. (5) Dziomkina, N. V.; Vancso, G. J. Soft Matter 2005, 1, 265. (6) Maier, S. A.; Brongersma, M. L.; Kik, P. G.; Meltzer, S.; Requicha, A. A. G.; Atwater, H. A. AdV. Mater. 2001, 13, 1501. (7) Maier, S. A.; Atwater, H. A. J. Appl. Phys. 2005, 98, 011101. (8) Zheng, J.; Zhu, Z.; Chen, H.; Liu, Z. Langmuir 2000, 16, 4409. (9) Demers, L. M.; Ginger, D. S.; Park, S.-J.; Li, Z.; Chung, S.-W.; Mirkin, C. A. Science 2002, 296, 1836. (10) Spatz, J. P.; Chan, V. Z.-H.; Mo¨βmer, S.; Kamm, F.-M.; Plettl, A.; Ziemann, P.; Mo¨ller, M. AdV. Mater. 2002, 14, 1827. (11) Xia, D.; Brueck, S. R. J. J. Vac. Sci. Technol., B 2004, 22, 3415. (12) Juillerat, F.; Solak, H. H.; Bowen, P.; Hofmann, H. Nanotechnology 2005, 16, 1311. (13) Maury, P.; Escalante, M.; Reinhoudt, D. N.; Huskens, J. AdV. Mater. 2005, 17, 2718. (14) Ba¨uerle, D. Laser Processing and Chemistry; Springer: Berlin, 2000. (15) Ehrlich, D. J.; Tsao, J. Y. Appl. Phys. Lett. 1984, 44, 267. (16) Mu¨llenborn, M.; Dirac, H.; Petersen, J. W. Appl. Phys. Lett. 1995, 66, 3001. (17) Kerner, G.; Asscher, M. Nano Lett. 2004, 4, 1433. (18) Balgar, T.; Franzka, S.; Hartmann, N.; Hasselbrink, E. Langmuir 2004, 20, 3525. (19) Balgar, T.; Franzka, S.; Hasselbrink, E.; Hartmann, N. Appl. Phys. A 2006, 82, 15. (20) Balgar, T.; Franzka, S.; Hartmann, N. Appl. Phys. A 2006, 82, 689. (21) Hartmann, N.; Balgar, T.; Bautista, R.; Franzka, S. Surf. Sci., in press, 2006, available online: http://dx.doi.org/10.1016/j.susc.2006.01.118. (22) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151. (23) Onclin, S.; Ravoo, B. J.; Reinhoudt, D. N. Angew. Chem., Int. Ed. 2005, 44, 6282. (24) Balgar, T.; Bautista, R.; Hartmann, N.; Hasselbrink, E. Surf. Sci. 2003, 532-535, 963. (25) Bautista, R.; Hartmann, N.; Hasselbrink, E. Langmuir 2003, 19, 6590. (26) Urch, H.; Franzka, S.; Dahlhaus, D.; Hartmann, N.; Hasselbrink, E.; Epple, M. J. Mater. Chem. 2006, 16, 1798. (27) Balachander, N.; Sukenik, C. N. Langmuir 1990, 6, 1621. (28) Baker, M. V.; Watling, J. D. Tetrahedron Lett. 1995, 36, 4623. (29) Baker, M. V.; Watling, J. D. Langmuir 1997, 13, 2027. (30) Bierbaum, K.; Kinzler, M.; Wo¨ll, C.; Grunze, M.; Ha¨hner, G.; Heid, S.; Effenberger, F. Langmuir 1995, 11, 512. (31) Carey, F. A.; Sundberg, R. J. AdVanced Organic Chemistry; Springer: Berlin, 2000. (32) Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday Soc. 1951, 11, 55. (33) Aizenberg, J.; Braun, P. V.; Wiltzius, P. Phys. ReV. Lett. 2000, 84, 2997.
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