NANO LETTERS
Three-Dimensional Nanostructure Construction via Nanografting: Positive and Negative Pattern Transfer
2002 Vol. 2, No. 9 937-940
Jun-Fu Liu,† Sylvain Cruchon-Dupeyrat,†,| Jayne C. Garno,†,‡ Jane Frommer,§ and Gang-Yu Liu*,† Department of Chemistry, UniVersity of California, DaVis, California 95616, Department of Chemistry, Wayne State UniVersity, Detroit, Michigan 48202, IBM Almaden Research Center, 650 Harry Road, San Jose, California 95120 Received June 25, 2002; Revised Manuscript Received August 1, 2002
ABSTRACT Three-dimensional nanostructures can be constructed using scanning probe lithography in combination with selective surface reactions. This letter introduces a successful approach using AFM-based nanografting to produce two-dimensional nanopatterns within self-assembled monolayer resists. These nanopatterns serve as an anchor to construct nanostructures in the third dimension via surface reactions. In this way, the nanometer-scale 2D pattern is transferred to chemically distinct 3D nanostructures. This approach offers the advantages of high spatial precision and selectivity in pattern transfer.
Current and future developments in the microelectronics and biotechnology industries require successful nanofabrication technologies capable of extremely high spatial precision.1 Nanostructures need to be constructed in both two and three dimensions. Using conventional photolithography and lightdirected chemical reactivity, large arrays of molecules with designated chemical and biochemical functionalities have been produced.2,3 Microcontact printing provides a relatively simple means to create micropatterns of organic thin films such as SAMs and polymers.4-11 While microfabrication techniques are relatively established, nanometer-scale fabrication methods are still under development. Scanning probe lithography has been utilized to produce 2D nanostructures with molecular precision.12-15 Various approaches have been taken to control the probe-surface local interactions to achieve lithography, for example, nanografting,12,16 nanopen reading and writing (NPRW),17 dip-pen nanolithography,18-20 catalyst-induced surface modification,21 and electrochemical etching.22 In this letter, we introduce a method capable of building 3D nanostructures. First, nanostructures of functionalized thiol SAMs are produced using an AFM-based nanolithographic method, nanografting.16,23 Then, a new reactant is * Corresponding author. Tel: 530-754-9678; Fax: 530-752-8995; Email:
[email protected]. † University of California, Davis. ‡ Wayne State University. § IBM Almaden Research Center. | Current address: NanoInk, Inc., 1436 W. Randolph St. #402, Chicago, IL 60607. 10.1021/nl025670c CCC: $22.00 Published on Web 08/24/2002
© 2002 American Chemical Society
introduced to attach to the reactive termini on the nanoengineered areas, resulting in the selective attachment to the 2D nanostructures, i.e., 3D nanofabrication. The spatial precision of this 3D nanofabrication process is determined by the quality of the 2D nanopatterns and by the selectivity of the pattern transfer reaction. Freshly cleaved mica (clear ruby muscovite, S & J Trading Company, NY) was used as the substrate for the deposition of gold thin films (Alfa Aesar, 99.99%). Gold was evaporated in a Denton DV-502A high vacuum system at a base pressure of 2 × 10-6 Torr. The evaporation rate was 0.25-0.30 nm/s while the mica was maintained at 325 °C. After the deposition was completed, the gold films were annealed at 350 °C for 30 min. A hydrogen-flame treatment was employed afterward to improve the surface morphology and domain size. Monolayers of 1-octadecanethiol (CH3(CH2)17SH, 98%, ODT) and 11-mercapto-1-undecanol (HO(CH2)11S, 97%, MUD) from Aldrich Chemicals, were prepared by immersing these gold thin films into 1 mM ODT and 1 mM MUD solutions, respectively, for at least 24 h. Prior to use, the SAMs were rinsed with ethanol, blown dry with nitrogen, and cleaned with anhydrous decahydronaphthalene from Aldrich Chemicals (with 99+ % purity, water < 0.003%). The AFM used for this study employs a deflection type scanning head (Molecular Imaging, PicoSPM, Phoenix, AZ), controlled by SPM100 feedback electronics (RHK Technology Inc., Troy, MI). A new vector scan module enabled automated lithography, in combination with scripts written in-house.24 Sharpened Si3N4 microlevers (Thermomicro-
Figure 1. Schematic diagrams of two series of experiments for construction of 3D nanostructures. (a) Nanopatterns of MUD are grafted into a matrix of ODT/Au(111) SAMs. The hydroxyl termini within the nanostructure react with OTS, resulting in a positive pattern transfer. (b) Nanopatterns of ODT are grafted into a matrix of MUD/Au(111) SAMs. The hydroxyl termini in the matrix react with OTS, resulting in a negative pattern transfer.
scopes, CA) with a force constant of 0.50 N/m were used for both imaging and fabrication. Two series of nanofabrication experiments are shown schematically in Figure 1. In the first set of experiments (Figure 1a), 2D nanopatterns of MUD were produced by nanografting in a dilute thiol/decahydronaphthalene solution. Nanografting was described in detail in several of our previous publications.16,23 Briefly, an ODT SAM was first imaged by AFM under low imaging force. Fabrication locations were then selected based on the morphology viewed via AFM. The AFM tip was subsequently moved to the desired location, where high load was applied. ODT molecules were displaced during the scan, while MUD molecules adsorbed onto the newly exposed area of the gold surface following the fabrication trajectory of the AFM tip. After nanografting, the cell was rinsed three times with decahydronaphthalene to remove residual thiols, then octadecyltrichlorosilane (CH3(CH2)17SiCl3 or OTS, 90+ % from Aldrich) was introduced. The trichlorosilane headgroups reacted with the hydroxyl terminal groups, resulting in the formation of a Si-O network at the interface.25-28 Therefore, positive pattern transfer was accomplished for the MUD nanopatterns. The second series followed a procedure similar to the first, except that the composition of the matrix and nanopattern were reversed, as illustrated in Figure 1b. AFM results from the experiment shown in Figure 1a are summarized in Figure 2. The surface roughness of the ODT/ Au(111) matrix (Figure 2a) is less than 0.2 nm as determined from a histogram of the AFM topograph. In addition, a periodicity of 0.5 nm was observed, indicating a well-ordered and closely packed alkanethiol SAM. Three single atomic 938
Au(111) steps are clearly visible, which serve as landmarks for this in situ study. Two MUD/Au(111) nanopatterns, 100 × 100 nm2 and 50 × 50 nm2, were grafted into the terrace area of the matrix SAM (Figure 2b). The MUD/Au(111) nanopatterns were 1.2 ( 0.3 nm lower than the ODT monolayer matrix, which is consistent with the chain length difference between the two molecules in the known structure and packing of SAMs.29,30 Ten minutes after injection of 10 mM OTS, the two recessed nanopatterns became 1.1 ( 0.3 nm taller than the surrounding matrix, i.e., positive pattern transfer occurred, as shown in Figures 2c and 2d. The surface within the nanopatterns appeared less homogeneous. From image histogram analysis, the surface roughness of the nanopattern was 0.3 nm and domains ranged from 10 to 20 nm in lateral dimension. The inhomogeneity could be attributed to island growth.30,31 Island growth was also observed by an in situ AFM study of silanization of an oxidized silicon surface by OTS.26 The siloxane layer thickness measured 2.3 ( 0.4 nm (Figures 2c and 2d). Previous studies have shown that the tilt angle of siloxane layers is less than 15°,32-34 thus the expected layer thickness should be greater than 2.53 nm. The observed thickness is lower than the expected value. We attribute this observation to incomplete coverage of the surface, or to a relatively disordered monolayer, which is more compressible than the closely packed thiol matrix under AFM tip pressure. The results from the experiment described in Figure 1b are shown in Figure 3. Figure 3a shows the morphology of a MUD/Au(111) SAM. The surface roughness of the MUD/ Nano Lett., Vol. 2, No. 9, 2002
Figure 2. Nanografting of SAMs followed by a positive pattern transfer process (illustrated in Figure 1a). (a) A 600 × 600 nm2 AFM topograph of an ODT/Au(111) SAM. (b) Two MUD nanopatterns (50 × 50 nm2 and 100 × 100 nm2) are produced using nanografting. Fabrication and imaging are carried out in a decahydronaphthalene solution containing 0.4 mM MUD. The fabricating force is 75 nN at a scan speed of 100 nm/s. The grafted nanopatterns of MUD/Au(111) were 1.2 ( 0.3 nm shorter than the surrounding ODT/Au(111) matrix. (c) The same nanopatterns 10 min after reaction with 10 mM OTS in decahydronaphthalene. The height of the nanostructures increased by 2.3 ( 0.4 nm. (d) A combined cursor plot indicates the height evolution during the fabrication process. The gold surface is used as the origin.
Figure 3. Nanografting of an ODT/Au SAM followed by a negative pattern transfer process (illustrated in Figure 1b). (a) A 600 × 600 nm2 AFM topograph of an MUD/Au(111) matrix. (b) A square nanoframe (300 × 300 nm2) of ODT/Au(111) was grafted into the MUD/Au(111) matrix. The width of the frame is 100 nm. The nanoframe is 0.7 ( 0.2 nm taller than the matrix. (c) The same area after reaction with 10 mM OTS in decahydronaphthalene for 3 min. The height of the matrix areas increased by 1.7 ( 0.5 nm. As a result, the nanoframe exhibits negative contrast in the topographic image. (d) A combined cursor plot reveals the height evolution during the fabrication process.
Au(111) monolayer was less than 0.2 nm. Using automated lithography, a square nanoframe of ODT was produced, as shown in Figure 3b. From the combined cursor plot in Figure 3d, the MUD/Au(111) areas are 0.7 ( 0.2 nm lower than the grafted ODT/Au(111) nanoframe. This measured height difference is consistent with the known commensurate (x3 × x3)R30° structure of ODT and MUD SAMs with closely packed hydrocarbon chains and a tilt angle of 30°.35-37 Introducing OTS caused a reaction with the MUD/Au(111) matrix as well as within the nanosquare defined by the
hydrophobic frame. In contrast to Figure 2, where positive pattern transfer was achieved, the entire surface of the MUD/ Au(111) matrix was covered by siloxane except for the nanoframe. The measured thickness of the OTS layer is 1.7 ( 0.5 nm. This value is smaller than the anticipated length of an all-trans OTS molecule (2.62 nm) or the OTS layer thickness (2.53 nm). The reaction time in Figure 3c was 3 min, shorter than that in Figure 2c (10 min). The coverage in Figure 3c is, therefore, lower than that in Figure 2c. In addition, the formation of an Si-O network at the interface
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is much more random than that in thiol SAMs.38,39 All factors collectively make the OTS layer more susceptible to tip compression. The low coverage is consistent with the surface roughness measurement of 0.5 nm, larger than that of the ODT/Au(111) matrix in Figure 2a. In summary, 3D structures can be produced using AFMbased nanolithography followed by selective adsorption. The present work demonstrates two successful examples, where 2D nanopatterns are first created using nanografting. These 2D patterns are used as templates for a subsequent pattern transfer process, e.g., a trichlorosilane reaction with surface terminal hydroxyl groups. Both positive and negative pattern transfer processes exhibit high spatial precision and selectivity in pattern transfer. The procedures may be applied using other surface reaction protocols to construct multilayer nanostructures with desired spacer lengths and functionalities. Compared with monolayers of thiols, multilayers, especially those with siloxane groups, have the advantage of higher stability because of their greater mass, and the siloxane headgroups are not subject to further oxidation. More importantly, the desired interfacial properties, such as lubricity, protein adhesion or resistance, and electron transfer, may be regulated by incorporating various functional groups within organic molecules and at designated layers of the films. Acknowledgment. The authors gratefully acknowledge financial support from the University of California, Davis, and the National Science Foundation (Career Award 9733410, IGERT-970952, and the Stanford University-CPIMA program). We further acknowledge RHK Technology, Inc. and Molecular Imaging, Inc. for lending us the instrument used for this investigation, and for their technical support. Helpful discussions with Nabil Amro at the University of California, Davis, and Dr. Charles Wade at IBM are greatly appreciated. References (1) Jacobs, J. W.; Fodor, S. P. A. Trends Biotechnol. 1994, 12, 19-26. (2) Chen, Y.; Pepin, A. Electrophoresis 2001, 22, 187-207. (3) Fodor, S. P. A.; Read, J. L.; Pirrung, M. C.; Stryer, L.; Lu, A. T.; Solas, D. Science 1991, 251, 767-773. (4) Yan, L.; Huck, W. T. S.; Zhao, X.-M.; Whitesides, G. M. Langmuir 1999, 15, 1208-1214. (5) Harada, Y.; Li, X.; Bohn, P. W.; Nuzzo, R. G. J. Am. Chem. Soc. 2001, 123, 8709-8717. (6) Ghosh, P.; Amirpour, M. L.; Lackowski, W. M.; Pishko, M. V.; Crooks, R. M. Angew. Chem., Int. Ed. Engl. 1999, 38, 1592-1595.
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