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Fabrication of Nanometer Scale Patterns within Self-Assembled Monolayers by Nanografting Song Xu,† Scott Miller,† Paul E. Laibinis,*,‡ and Gang-yu Liu*,† Department of Chemistry, Wayne State University, Detroit, Michigan 48202, and Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received May 28, 1999 A nanofabrication method, nanografting, has been developed to fabricate nanometer scale patterns on surfaces with specified size and geometry. The nanografting process combines the displacement of selected resist molecules by an atomic force microscopy tip and the adsorption of new adsorbate. The present work details the procedure for nanografting and discusses various kinds of patterns produced and the stability of the resulting patterns. Compared with other methods for microfabrication, nanografting allows a more precise control over the size and geometry of patterned features and their locations on surfaces. Nanopatterns comprising various thiol-based components can be produced, where we have demonstrated the fabrication of nanopatterns from thiols with either the same or different chain lengths and terminal groups from the matrix SAM. Furthermore, nanografting allows the fabricated patterns to be altered in situ without the need to change masks or repeat entire fabrication processes. The patterned SAMs produced by nanografting open new opportunities for systematic studies of such size-dependent properties as conductivity, nanotribology, and spatially-confined surface reactions.
Introduction Micropatterned self-assembled monolayers (SAMs) have attracted tremendous attention because of the utility of SAMs as resists for pattern transfer and as templates for directing the selective adsorption and growth of metals, polymer films, protein layers, and cells.1-3 Patterns with dimensions of 0.1 µm or larger have been fabricated within SAMs using photolithography,2 microcontact printing,3 microwriting,4,5 and micromachining.5 Argon ion and electron beam lithography has produced patterns within the films with dimensions of tens of nanometers under UHV conditions.6,7 A less controlled approach to produce patterns within SAMs on a nanometer scale has been the preparation of mixed SAMs from solutions that contain * To whom correspondence should be addressed. † Wayne State University. ‡ MIT. (1) Ulman, A. An Introduction to Ultrathin Organic Films-From Langmuir-Blodgett to Self-assembly; Academic Press: San Diego, CA, 1991. (2) (a) Wollman, E. W.; Kang, D.; Frisbie, C. D.; Lorkovic, I. M.; Wrighton, M. S. J. Am. Chem. Soc. 1994, 116, 4395. (b) Huang, J. Y.; Dahlgren, D. A.; Hemminger, J. C. Langmuir 1994, 10, 626. (c) Tarlov, M. J.; Burgess, D. R. F.; Gillen, G. J. Am. Chem. Soc. 1993, 115, 5305. (3) (a) Kumar, A.; Whitesides, G. M. Science 1994, 263, 60. (b) Xia, Y.; Whitesides, G. M. J. Am. Chem. Soc. 1995, 117, 3274. (c) Jeon, N. L.; Nuzzo, R. G.; Xia, Y.; Mrksich, M.; Whitesides, G. M. Langmuir 1995, 11, 3024. (d) Jeon, N. L.; Lin, W.; Erhardt, M. K.; Girolami, G. S.; Nuzzo, R. G. Langmuir 1997, 13, 3833. (e) Lo´pez, G. P.; Albers, M. W.; Schreiber, S. L.; Carroll, R.; Peralta, E.; Whitesides, G. M. J. Am. Chem. Soc. 1993, 115, 5877. (f) Lackowski, W. M.; Ghosh, P.; Crooks, R. M. J. Am. Chem. Soc. 1999, 121, 1419. (g) Husemann, M.; Mecerreyes, D.; Hawker, C. J.; Hedrick, J. L.; Shah, R.; Abbott, N. L. Angew. Chem., Int. Ed. Engl. 1999, 38, 647. (h) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1998, 37, 550. (4) (a) Kumar, A.; Abbott, N. L.; Kim, E.; Biebuyck, H. A.; Whitesides, G. M. Acc. Chem. Res. 1995, 28, 219. (b) Kumar, A.; Biebuyck, M. A.; Abbott, N. L.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 9188. (5) (a) Abbott, N. L.; Kumar, A.; Whitesides, G. M. Chem. Mater. 1994, 6, 596. (b) Abbott, N. L.; Folkers, J. P.; Whitesides, G. M. Science 1992, 257, 1380. (6) Berggren, K. K.; et al. Science 1995, 269, 1255. (7) (a) Sondag-Huethorst, J. A. M.; Van Helleputte, H. R. J.; Fokkink, L. G. J. Appl. Phys. Lett. 1994, 64, 285. (b) Allara, D. L.; Tiberio, R. C.; Craighead, H. G.; Lercel, M. Appl. Phys. Lett. 1993, 62, 476.
multiple adsorbates.8,9 The size and lateral distribution of these domains is determined by kinetic and thermodynamic factors of the self-assembly process rather than lithography.8,9 The challenging task remains to produce more microscopic, e.g. nanometer or molecular-scale, patterns of controlled lateral metrology. Attempts to produce patterns on this level have triggered the development of techniques based on scanning probe microscopy (SPM) and resulted in the emerging field of scanning probe lithography (SPL).10,11 The nanopatterned SAMs produced to date via SPL have primarily been negative patterns, where the surfaces of the patterned regions are lower in height than the surrounding matrix areas.11,12 Examples of such negative patterns include fabricated holes or trenches produced by the selective removal of surface atoms or adsorbate molecules within the adlayer.11-13 This process has been (8) (a) Laibinis, P. E.; Nuzzo, R. G.; Whitesides, G. M. J. Phys. Chem. 1992, 96, 5097. (b) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. Langmuir 1992, 8, 1330. (c) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M.; Deutch, J. J. Phys. Chem. 1994, 98, 563. (9) (a) Offord, D. A.; John, C. M.; Griffin, J. H. Langmuir 1994, 10, 761. (b) Offord, D. A.; John, C. M.; Linford, M. R.; Griffin, J. H. Langmuir 1994, 10, 883. (c) Tamada, K.; Hara, M.; Sasabe, H.; Knoll, W. Langmuir 1997, 13, 1558. (d) Bard, A. J.; Abruna, H. D.; Chidsey, C. E. D.; Faulkner, L. R.; Feldberg, S. W.; Itaya, K.; Majda, M.; Melroy, O.; Murray, R. W.; Porter, M. S.; Suriaga, M. P.; White, H. S. J. Phys. Chem. 1993, 97, 7148. (e) Chidsey, C. E. D.; Bartozzi, C. R.; Putvinski, T. M.; Majsie, T. M. J. Am. Chem. Soc. 1990, 112, 4301. (f) Hayes, W. A.; Kim, H.; Yue, X.; Perry, S. S.; Shannon, C. Langmuir 1997, 13, 2511. (g) Bilewicz, R.; Sawaguchi, T.; Chamberlain, R. V.; Majda, M. Langmuir 1995, 11, 2256. (10) (a) Ross, C. B.; Sun, L.; Crooks, R. M. Langmuir 1993, 9, 632. (b) Schoer, J. K.; Ross, C. B.; Crooks, R. M.; Corbitt, T. S.; HampdenSmith, M. J. Langmuir 1994, 10, 615. (c) Muller, W. T.; Klein, D. L.; Lee, T.; Clarke, J.; Mceven, P. L.; Schultz, P. G. Science 1995, 268, 272. (d) Piner, R. D.; Zhu, J.; Xu, F.; Hong, S. H.; Mirkin, C. A. Science 1999, 283, 661. (11) (a) Nyffenegger, R. M.; Penner, R. M. Chem. Rev. 1997, 97, 1195. (b) Liu, G.-Y.; Xu, S. In New Directions in Materials Synthesis; Ed. Winter, C. H., Hoffman, D., Eds.; American Chemical Society: Washington, DC, 1999; Vol. 727, p 199. (12) Xiao, X.-D.; Liu, G.-Y.; Charych, D. H.; Salmeron, M. Langmuir 1995, 11, 1600. (13) (a) Liu, G. Y.; Salmeron, M. B. Langmuir 1994, 10, 367. (b) Liu, G. Y.; Fenter, P.; Eisenberger, P.; Chidsey, C. E. D.; Ogletree, D. F.; Salmeron, M. B. J. Chem. Phys. 1994, 101, 4301.
10.1021/la9906727 CCC: $18.00 © 1999 American Chemical Society Published on Web 08/26/1999
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achieved mechanically by an atomic force microscopy (AFM) tip under a high pressure and shear force,13 electrochemically with an SPM tip serving as an electrode to direct redox reactions,8-17 or by displacing molecules under high local electrical fields using a scanning tunneling microscopy (STM) tip and bias voltage pulsing.10a-c Recently, we described a new SPM-based method, nanografting, that can produce both positive and negative patterns within SAMs.14 The nanopatterned SAMs provide an opportunity to study various size-dependent phenomena such as the nanoelasticity15 and spatially confined chemical reactivity16 in two-dimensional systems. In this paper, we present a systematic study of the nanografting technique, focusing on the procedure for nanografting, its ability to graft and modify various nanopatterns, and the stability of the fabricated nanopatterns. Experimental Method Preparation of SAMs. Octadecanethiol (HS(CH2)17CH3, abbreviated as C18SH), decanethiol (HS(CH2)9CH3 or C10SH), and 2-mercaptoethanol (HS(CH2)2OH or HOC2SH) were obtained from Aldrich and used as received. Docosanethiol (HS(CH2)21CH3 or C22SH) and 16-mercaptohexadecanoic acid (HS(CH2)15CO2H, or HO2CC15SH) were synthesized by reported procedures.17 Gold (Alfa Aesar, 99.99%) was deposited onto freshly cleaved mica substrates (Mica New York Corp., clear ruby muscovite) in a high vacuum evaporator (Denton Vacuum Inc., model DV502A) at ∼10-7 Torr. Before deposition, the mica was preheated to 325 °C by two quartz lamps mounted behind the mica to enhance the formation of terraced Au(111) domains.18 The typical evaporation rate was 3 Å/s, and the thickness of the gold films ranged from 150 to 200 nm. The mica temperature was maintained at 325 °C for 15 min after deposition for annealing. This method produced samples with flat Au(111) terraces as large as 300 × 300 nm2 according to our AFM measurements. These films were either used directly to prepare self-assembled monolayers or were fixed to a silicon or glass substrate with an epoxy (EPO-TEK 377, Epoxy Tech.).19 These latter samples were separated at the gold-mica interface by peeling immediately before immersion in a thiol solution. This procedure produced gold substrates with an exceptionally flat surface morphology due to the templating effect of the atomically flat mica surface.19 AFM images revealed that the surfaces of these gold films had a mean roughness of ∼5 Å over areas as large as several square micrometers. Self-assembled monolayers (SAMs) were formed by soaking gold thin films (immediately after vacuum deposition or peeling) in dilute (∼1 mM) thiol solutions. Each gold substrate remained in a thiol solution for 2-7 days at room temperature to ensure the formation of mature monolayers with high coverage and a low density of defects.20-22 Atomic Force Microscopy. The atomic force microscope utilized a home-constructed, deflection-type scanning head that (14) Xu, S.; Liu, G. Y. Langmuir 1997, 13, 127. (15) (a) Kiridena, W.; Jain, V.; Kuo, P. K.; Liu, G. Y. Surf. Interface Anal. 1997, 25, 383. (b) Jourdan, J. S.; Cruchon-Dupeyrat, S. J.; Huang, Y.; Kuo, P. K.; Liu, G. Y. Langmuir, in press. (16) Xu, S.; Laibinis, P. E.; Liu, G. Y. J. Am. Chem. Soc. 1998, 120, 9356. (17) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (18) (a) Chidsey, C. E. D.; Loaicano, D. N.; Sleator, T.; Nakahaza, S. Surf. Sci. 1988, 200, 45. (b) Lang, C. A.; Dovek, M. M.; Nogami, J.; Quate, C. F. Surf. Sci. Lett. 1989, 224, 1974. (c) Wo¨ll, Ch.; Chiang, S.; Wilson, R. J.; Lippel, P. H. Phys. Rev. B 1989, 39, 7988. (19) (a) Hegner, M.; Wagner, P.; Semenza, G. Surf. Sci. 1993, 291, 39. (b) Wagner, P.; Hegner, M.; Guntherodt, H.-J.; Semenza, G. Langmuir 1995, 11, 3867. (20) Xu, S.; Cruchon-Dupeyrat, S.; Garno, J.; Liu, G. Y.; Jennings, G. K.; Yong T.-H.; Laibinis, P. E. J. Chem. Phys. 1998, 108, 5002. (21) (a) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853. (b) Poirier, G. E.; Tarlov, M. J.; Rushmeierf, H. E. Langmuir 1994, 10, 3383. (c) Sondag-Huthorst, J. A. M.; Scho¨nenberger, C.; Fokkink, L. G. J. Phys. Chem. 1994, 98, 6826. (d) McDermott, C. A.; McDermott, M. T.; Green, J. B.; Porter, M. D. J. Phys. Chem. 1995, 99, 13257. (e) Bucher, J. P.; Santesson, L.; Kern, K. Langmuir 1994, 10, 979.
Figure 1. Schematic diagram of the nanografting process illustrating the procedures for fabrication and characterization of a nanopattern using AFM. (A) Matrix SAM imaged at a low imaging force in a 2-butanol solution of another thiol. (B) At a force greater than the displacement threshold, the AFM tip displaces the adsorbates at the desired areas, and the thiol molecules in the contacting solution self-assemble onto the exposed gold sites. (C) The nanopattern is imaged at a reduced imaging force. exhibits high mechanical stability.20 Samples were mounted inside a quartz cell to allow imaging in a liquid environment while injecting or withdrawing solutions. Solvent was added to the cell as needed to compensate for its evaporation during the experiment. The scanner was controlled by an STM1000 electronics controller (RHK Technology, Inc.). Sharpened Si3N4 microlevers (Park Scientific Instruments) with a force constant of 0.1 N/m were used for AFM imaging and fabrication. Images were acquired in 2-butanol, which is an effective solvent for thiols and a good imaging medium, to avoid capillary interactions.
Results and Discussion Basic Procedure of Nanografting. The process of nanografting includes three steps as illustrated in Figure 1. The first step is to characterize the matrix SAM and select areas for fabrication. During this process, the surface structure of the SAM (matrix) is imaged by AFM under a low load, normally below 1 nN. The second step is to fabricate desired patterns within the SAM. In this step, SAM molecules in selected regions of the surface are removed by scanning these areas with an AFM tip at a force greater than the threshold displacement force. As the matrix molecules are removed, new thiol molecules from the contacting solution immediately adsorb onto these areas following the scanning track of the AFM tip.14 The final step is to characterize the patterned SAMs using the same AFM tip at a reduced imaging force. The geometry of the resulting patterns is defined by the (22) (a) Camillone, N. C.; Chidsey, C. E. D.; Liu, G. Y.; Scoles, G. J. Chem. Phys. 1993, 98, 3503. (b) Camillone, N.; Chidsey, C. E. D.; Liu, G. Y.; Scoles, G. J. Chem. Phys. 1993, 98, 4234. (c) Fenter, P.; Eberhardt, A.; Liang, K. S.; Eisenberger, P. J. Chem. Phys. 1997, 106, 1600.
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Figure 2. (A) A 100 × 100 nm2 topographic scan of a C18S SAM at an imaging force of 0.1 nN. Four single atomic Au(111) steps are seen in the image. A portion of a Au(111) step within the fabricated area is highlighted. Etch pits are observed, and one example is highlighted by a triangle. The arrows point to four scars present in the matrix layer. (B) After nanografting in the central 50 × 45 nm2 area (the square defined by dotted lines) in a 0.2 mM C18SH solution. The displacement force was 2.0 nN, and the scan speed was 250 nm/s. The measured spatial precision for this fabrication was 3 nm. The scars within the fabricated area in (A) are absent in (B). (C) A higher resolution image of the region indicated in (B). (D, E) Corresponding cursor profiles of the same line labeled in images A and B, respectively. A comparison of the images and cursor profiles reveals the disappearance of scars and other changes in surface morphology.
scanning trajectory, and the spatial precision of the fabrication depends on the sharpness of the AFM tip and the thermal and mechanical stability of the AFM. The edge resolution of each pattern is measured from the highresolution images acquired at the boundary areas. The best spatial precision we have achieved is 1 nm.14 We investigated the influence of the following parameters on the nanografting process: the fabrication force, the scanning speed, and the concentration of the thiol solution. Since the geometry and chemical nature of AFM tips vary, the corresponding threshold force must be determined for each tip before initiation of a fabrication process. Typically, a small area of 5 × 5 nm2 was scanned under a gradually increasing load until the periodicity changed from that for a thiol monolayer to that for Au(111), i.e., from a hexagonal lattice (a ) 5.0 Å) to a smaller hexagonal lattice (a ) 2.9 Å).13 This change indicated the displacement of the matrix molecules from their adsorption sites. To ensure complete removal of the matrix molecules without causing plastic deformation of the underlying gold surface, the nanografting force was set 10-50% higher than the threshold force.14 The solution concentration of
thiol was not a critical parameter as we have successfully achieved nanografting with concentrations ranging from 2 µM to 2 mM. The results of nanografting were sensitive to scan rate as slow scans often produced pattern distortion due to thermal drifts while fast scans did not produce patterns with high coverage. We found that scan rates ranging from 100 to 2000 nm/s resulted in the rapid and reproducible formation of patterns with well-defined geometry and sharp edges. Thiol molecules within these patterns were well-ordered and densely packed. Homogeneous Nanografting: Fabrication of SAM Nanopatterns with the Same Component as the Matrix. Figure 2 shows an example in which C18SH was nanografted into a matrix of C18S/Au(111). In Figure 2A, two types of surface defects are clearly visible within the plateau areas between steps: scars and etch pits.21 On the basis of the topographic image, the scars are ∼0.9 nm in depth and have lateral dimensions of 5-20 nm; however, due to the geometry of the tip and the small opening of the scars, their actual depth may exceed 0.9 nm. Even after extended soaking in the thiol solution, the scars did not heal, possibly due to trapped contaminants at these
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regions of the gold surface.20 In Figure 2A, a high density of etch pits is also observed, which is consistent with previous reports.20,21 These pits have a depth of 0.24 nm and lateral dimensions of 2-5 nm. The depth of these pits correlates well with there being a missing layer of gold atoms beneath the thiol molecules. Figure 2B shows the same area as in Figure 2A imaged after nanografting. The fabricated 50 × 45 nm2 area can be readily identified due to the structural discontinuity at its boundary. The grafted area exhibits the same height as the surrounding SAM matrix, indicating that the newly grown C18SH pattern has the same packing structure as the mature SAM. The difference between the pattern and the matrix C18S layer is the lack of scars in the fabricated area. These four scars in the grafted region (marked 1-4 in Figure 2A) disappeared after nanografting as shown in Figure 2B. The monolayer within the nanografted pattern has the same thickness as the surrounding matrix SAM, confirming that the fabricated SAM and the matrix have the same packing density and tilt angle. Since nanografting can be used to repair scar defects within SAMs, patterned SAMs without scar defects may offer improved performance as masks for pattern transfer on the nanometer scale. One side product of nanografting is that the high shear force during the procedure may clean and sharpen the AFM tip. For example, the resolution in Figure 2B taken after nanografting is substantially higher than in Figure 2A, as evidenced by the more sharply resolved step edges and etch pits. Figure 2C displays a high-resolution image of a 40 × 40 nm2 area within the nanografted region. Domains consisting of multiple parallel stripes are resolved and have an interstripe distance of 0.83 nm. These domains either share the same orientation or orient 60° with respect to each other. Within each domain, the adsorbate molecules form a c(4×2) superlattice with respect to the basic (x3×x3)R30° periodicity for alkanethiols on Au(111). Previously reported high-resolution AFM images of SAMs show either perfectly ordered periodicity over small scanned areas or surface defects over larger scanning ranges23 but not the well-defined, laterally resolved molecular defects seen in STM images.21,24 The sharpening of the AFM tip and the use of imaging forces smaller than 0.05 nN made it possible to obtain the true molecular resolution in Figure 2B,C, in which the etch pits, vacancies, domain boundaries, and superlattices are clearly resolved. The displacement threshold for the experiment shown in Figure 2 was 1.5 nN. As the shaving force was 33% above this threshold, the AFM tip caused some lateral movement of gold atoms located at the step edges. The relatively smooth Au(111) step edge in Figure 2A before nanografting changed to a saw-tooth-shaped edge as shown in Figure 2B. In the upper-left corner of the nanografted area, an irregularly shaped pits0.25 nm deep and 10 nm in lateral dimensionsis observed after fabrication. Another effect of the fabrication process was the appearance of new nanoislands as shown in image 2B. These islands are covered by C18SH molecules. From the cursor profile shown in Figure 2E, these islands are 0.23 nm taller than the surrounding SAM and have lateral dimensions of 2-5 nm. Thus, the displaced gold atoms from the step edges were moved during the fabrication process to the plateau areas and formed new nanoislands on surface with heights of a single atomic Au(111) step. (23) (a) Alves, C. A.; Smith, E. L.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 1222. (b) Butt, H. J.; Seifert, K.; Bamberg, E. J. Phys. Chem. 1993, 97, 7316. (24) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145.
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Figure 3. Fabrication of a hydrophilic nanoisland within a hydrophobic SAM matrix. (A) A 700 × 700 nm2 topographic scan of a C18S SAM taken at an imaging force of 0.2 nN. The dark dots and lines are due to holes and grooves within the ultraflat gold thin film, and the white dots are likely due to losely attached adsorbates. (B) Corresponding frictional force image of (A). The image contrast is relatively homogeneous except for several bright spots corresponding to the white dots in (A). (C) Same area after nanografting a 70 × 300 nm2 rectangular area of HSC15CO2H in the center. The concentration of HSC15CO2H was 0.2 mM, and the fabrication force was 5 nN. The measured spatial precision for the pattern was 3 nm. No visible boundaries can be identified in the topographic image. (D) Corresponding frictional force image. The grafted HSC15CO2H pattern is easily identified due to the higher frictional force of the -CO2H groups than the surrounding -CH3 terminated areas.
Heterogeneous Nanografting: Fabrication of SAM Nanopatterns with Thiol Molecules Different from Those of the Matrix. Nanopatterns with various chemical functionalities can be fabricated using thiols with terminal groups that are different from those of the matrix. The resulting nanopatterns can be visualized from topographic and/or the corresponding frictional force images. In Figure 3, a hydrophilic nanoisland (70 × 300 nm2) of HSC15CO2H was produced within a hydrophobic SAM matrix by nanografting under a force of 5 nN. Figure 3A,C shows a 700 × 700 nm2 topographic image before and after the fabrication process. In Figure 3C, it is difficult to identify the fabricated area because of the similar thickness of the patterned and matrix SAMs (2.2 nm). However, the fabricated area could be identified from higher resolution images by comparing the local surface features (etch pit distributions, domain structures, etc.) before and after the fabrication. In contrast, it was easier to locate the nanopattern from frictional force images taken simultaneously with the topographic images. Figure 3B,D displays the frictional force images corresponding to images 3A,B, respectively. The CH3-terminated surface in Figure 3B shows relatively uniform friction with the exception of several bright spots (
