NANO LETTERS
Nanometer Scale Patterning of Langmuir−Blodgett Films of Gold Nanoparticles by Electron Beam Lithography
2002 Vol. 2, No. 1 43-47
Martinus H. V. Werts,† Mathieu Lambert,† Jean-Philippe Bourgoin,*,† and Mathias Brust‡ SerVice de Chimie Mole´ culaire, CEA/Saclay, F-91191 Gif-sur-YVette Cedex, France, and Department of Chemistry, UniVersity of LiVerpool, LiVerpool L697ZD, U.K. Received September 19, 2001; Revised Manuscript Received November 5, 2001
ABSTRACT Electron beam lithography on Langmuir−Blodgett films of alkanethiol-capped gold nanoparticles is shown to be a viable strategy to define nanometer scale structures of such particles. Sub-50 nm wide “nanowires”, the thickness of which is controlled at the single particle level, are created with e-beam doses in the mC/cm2 range. It is shown that the patterns are formed by radiation-induced cross-linking of the alkyl chains and that they can be contacted and studied electrically.
Metallic and semiconducting nanoparticles capped with protecting ligand shells have recently been demonstrated to be promising building blocks for molecular electronics and optoelectronics.1-10 They also find medical and biological applications, such as in the labeling of cells and in highthroughput screening technologies.11-13 The scientific interest in these particles is stimulated by the fact that their nanometer size gives rise to clearly observable quantum effects such as Coulomb blockade or a size dependence of optical properties. For molecular electronics, as well as for other applications, the ability to arrange these nanoparticles into wires, films, or three-dimensional assemblies is a crucial point. In most cases, films of nanoparticles are formed either by spin coating, sedimentation, or by simply dipping a substrate into a solution of particles.6,14 With those techniques the thickness of the films is difficult to control. Conversely, the LangmuirBlodgett technique provides a suitable way to control the thickness of the film, which can be tuned layer by layer.8,15,16 Patterning of nanoparticle assemblies can be achieved in various ways.6,17-21 For example, optical or electron beam lithography can be used to generate a mask through which a surface is functionalized with molecules to which the nanoparticles bind selectively. Such particle-binding patterns may also be put onto a surface by microcontact printing. DNA templates to which nanoparticles, capped with strands * Corresponding author. E-mail:
[email protected]. Fax: ++33 1 69 08 66 40. † CEA/Saclay. ‡ University of Liverpool. 10.1021/nl015629u CCC: $22.00 Published on Web 12/11/2001
© 2002 American Chemical Society
that are complementary to those of the template, are attached can also be considered a promising way to spatially control assemblies of nanoparticles.22-24 At present, however, none of the aforementioned methods has demonstrated the capability to make wire-like assemblies of sub-100 nm width. Very recently, several groups25-28 have demonstrated that patterns can be written in films of gold nanoclusters by means of electron-beam lithography (EBL). The exposed areas become insoluble, and the lithograms can be developed by dissolving the unexposed particles. We have been investigating EBL for submicron patterning of arrays of gold nanoclusters. In our case, films of alkanethiol-stabilized gold nanoparticles are deposited on solid substrates by the Langmuir-Blodgett technique, allowing precise control over the film thickness and enabling us to cover large areas with uniform films of particles. Subsequently, patterns are written in the films using EBL. We will first demonstrate that lines in the sub-50 nm range can be prepared, the thickness of which is controlled at the single particle level. Then it will be shown that the doses needed for lithography are lower than those reported recently. We propose a different mechanism for particle immobilization that involves cross-linking of the ligand shells rather than their complete removal upon e-beam irradiation. The development process and the influence of the chain length of the capping agent have been studied in more detail. Finally, we demonstrate that these lines can withstand a lithographic process aimed at electrically contacting them with electrodes.
We worked with alkanethiol-capped gold nanoparticles that were obtained using the well-known two-phase synthesis originally developed by Brust et al.29 The ligand shell might be tailored either by using different thiols during the particle synthesis, by exchange of ligands, or by direct chemical modification of the side chains.5 In this work, 3 nm diameter particles were used with either an n-hexanethiol or ndodecanethiol capping. They will be referred to as AuC6 and AuC12, respectively. The Langmuir-Blodgett (LB) films were prepared as follows. Typically, a 1 µM solution of gold nanoparticles in 1,1,1-trichloroethane was slowly spread (approximately 1 mL in 5 min) on the water surface of a custom-made LB trough. After spreading, the clusters self-assemble into small monolayer rafts floating on the water surface. Isothermal compression of the surface moves these rafts closer together until they merge into a brittle two-dimensional array of gold particles. Upon further compression, the surface pressure starts rising dramatically.8 At a surface pressure of 12 mN/m typically, the layer was transferred to a solid substrate either by horizontal or vertical transfer. Obviously, the latter way is preferrable since it allows for the controlled deposition of multiple layers. However, horizontal transfer yields better results with substrates to which the Langmuir film does not adhere strongly. It was found that the transfer was most successful with substrates that are sufficiently hydrophobic. Freshly cleaned SiO2-coated Si wafers (which are completely hydrophilic) gave poor transfer ratios, although regions suitable for lithography could be identified with the aid of an optical microscope. [Cleaning included 20 min in freshly prepared piranha solution (30% H2O2/concentrated H2SO4, 1:3 by volume, CAUTION!), followed by extensive rinsing with deionized water, and drying in a nitrogen jet. Just before use, the wafers were cleaned once more by 20 min of ultraviolet/O3.] Silanization was then carried out as follows. The substrates were introduced in a vacuum chamber. (Aminopropyl)triethoxysilane (APTS), hexamethyldisilazane (HMDS), or methyltrichlorosilane (MTS) vapor was introduced, and the substrates were allowed to react for 1 min. After removal from the vacuum chamber, the substrates were heated for 10 min at 150 °C on a hot plate. Silanization with MTS, HMDS, or APTS made the substrates more hydrophobic and greatly improved adhesion of the nanocluster monolayer to the substrate. Once a proper monolayer is formed on the substrate, transfer of subsequent layers proceeds smoothly. Test patterns consisting of lines of varying doses were written using a Philips SFEG 30 field-emission scanning electron microscope controlled by a Raith ELPHY lithography system. After irradiation, the patterns were developed during 1 min in toluene, rinsed with the same solvent, 2-propanol, and finally with deionized water (MilliQ). Any remaining water droplets were removed by a jet of dustfree, dry nitrogen gas. The developed patterns were studied by tapping-mode AFM (Digital Instruments Nanoscope IIIa and Dimension 3100, 225 µm long Nanoprobe cantilevers), SEM (Philips SFEG 30), and using optical microscopy (Nachet Vision). 44
Figure 1. AFM image of a test pattern written at 25 keV on a three layer thick AuC6 LB film. The e-beam dose is increased from 0.8 mC/cm2 (left) to 6 mC/cm2 (right). The lines are 40 nm wide, with a center-to-center distance of 100 nm. The right-most pair of lines is broadened as a result of saturation.
The developed test patterns show contrast in AFM (Figure 1), SEM and, depending on the size of the lithographic features, optical microscopy (see Supporting Information). The nanoparticle film behaves as a negative e-beam resist: the area exposed to the electron beam becomes insoluble in toluene. The height of fully exposed structures observed in tapping-mode AFM corresponds well to the initial film thickness. In the case of underexposure, heights that are nonintegral multiples of a monolayer are observed in largescale (e.g., 80 µm × 80 µm) images. However, zooming in (e.g., 1 µm × 1 µm) reveals that such structures in fact consist of small islands of mono- or multilayer thickness. Overexposure leads to broadening of the lines, because the substrate scatters electrons into areas not intended to be exposed. Figure 2 gathers plots of the heights and the line widths of test patterns as a function of e-beam dose for mono- and multilayers of AuC6 and AuC12 particles. For that set of experiments, lines of 100 nm were irradiated at 25 keV at various doses and developed using toluene. The height and line width after development was measured using AFM. From the plots, it is possible to deduce the sensitivity of the resist, which is defined as the dose at which half the thickness of the resist is preserved.30 We found a sensitivity of 1.6 mC/cm2 and 0.5 mC/cm2 for AuC6 and AuC12 clusters, respectively. Thus, the sensitivity depends on the size of the organic capping agent. The optimal dose might be defined as the threshold dose above which the thickness of the developed pattern becomes equal to the thickness of the original layer. This optimal dose Nano Lett., Vol. 2, No. 1, 2002
Figure 3. 14.3 µm × 14.3 µm TEM image of a two layer thick AuC12 LB film after irradiation at 25 keV and subsequent development.
Figure 2. E-beam dose-response curves for writing 100 nm lines. Top: AuC6, monolayer (open squares) and trilayer (filled squares). Bottom: AuC12, 1 (open triangles) and 4 layers (filled triangles). Height and width refer to the actual height and line width measured by AFM.
was found to be practically independent of the number of layers for a given alkanethiol chain length. In other words, to produce four layer thick features on a four layer sample, one needs the same dose as for producing a one layer thick feature on a one layer sample. Except for the length of the capping alkanethiol, the only other parameter that critically influences the effectiveness of the e-beam writing process is the acceleration voltage. Writing with 5 keV electrons generally requires a dose 5 times less than writing with 25 keV electrons. To compare our results with those recently published by other groups, we consider the threshold dose, i.e., the dose above which a full pattern remains after development.25-28 In our case, the minimum dose needed to produce a full lithogram in a film of AuC12 particles (1 mC/cm2) is considerably lower than the 2.9 and 5.4 mC/cm2 reported by Bedson et al. or the 7.7 mC/cm2 found by Lin et al. However, we confirm the observation made by Bedson et al., who stressed that the highest resolution is obtained close to the threshold dose. It is indeed clear from the plots of Figure 2 that the line width continues to increase with dose after the height has reached stability. This allows us to define the optimal dose more precisely. It corresponds to the dose at which the height of the produced feature is that of the original film and at which the width is minimal. This optimal dose corresponds typically to twice the sensitivity of the resist: namely 3 mC/cm2 and 1 mC/cm2 for AuC6 and AuC12 clusters, respectively. Using these optimal doses, it was possible to reproducibly write continuous lines of 40 nm wide separated by 60 nm (edge-to-edge) as shown in Nano Lett., Vol. 2, No. 1, 2002
Figure 1. Some preliminary attempts proved successful down to 25 nm line width for isolated lines. An important issue is the mechanism by which the particles are immobilized. First, we observed by transmission electron microscopy of lithograms (see Supporting Information) on carbon grids that the particle cores remain intact and do not sinter during e-beam irradiation, a result also found by Lin et al.28 Moreover, the large scale TEM image in Figure 3 contains a clarifying artifact. The initially straight lithographed lines did not adhere strongly to the carbon grid they were written on and during TEM sample preparation folded into the bent shape shown. Apparently, e-beam irradiation has connected the particles into strings by linking the organic ligand shells. It is known that e-beam irradiation of saturated hydrocarbons such as linear alkanes and polyethylene leads to crosslinking of the individual molecules and the formation of double bonds, accompanied by the evolution of hydrogen gas.31 Recent work has shown that these processes also take place when irradiating self-assembled alkane monolayers, with the additional formation of oxygenated species if oxygen is present.32 Apparently, secondary electrons generated in the substrate interact with the alkanethiol coating of the nanoparticles to give rise to radical species that undergo cross-linking and related radical reactions. The cross-linked nanoparticles then become insoluble in toluene. This explanation constrasts with the one proposed by Lin et al. that the ligand shells are fully stripped off by the e-beam28 and that this would cause the immobilization of the particles. Several observations rule out this stripping mechanism as the origin of the patterning mechanism, at least for the low irradiation doses used in the present work. First, AFM characterization of the patterns before development showed no reduction of the film thickness in the areas that were irradiated. Second, the infared spectrum of a four-layered AuC12 film (see Supporting Information) still shows the presence of the alkanethiol related C-H stretch vibrations (2850-2950 cm-1) after e-beam irradiation and subsequent development. In addition, the greater e-beam sensitivity of AuC12 films as compared to AuC6 films is also consistent 45
Figure 5. Four probe electrodes fabricated on top of a one monolayer thick, 100 nm wide AuC12 line.
Figure 4. Exposure of 100 nm AuC6 lines (3 layers thick) to 1 vol % of n-hexanethiol in toluene. Top graph shows the height vs dose curves for different exposure times. Zero minutes corresponds to only developing in toluene. In the bottom graph, the line width is plotted as a function of dose for patterns developed in pure toluene (squares), and 40 min in 1% hexanethiol (triangles).
with the cross-linking mechanism proposed here. AuC12 has more cross-linkable organic matter available per unit volume than AuC6 does and is thus expected to have a bigger crosssection for e-beam induced formation of radicals. On the basis of a stripping mechanism, one would expect AuC6 particles to be more sensitive, since fewer electrons would be needed to fully deprive the gold cores of their ligand shells than are needed for AuC12. We have also considered the effect of using solutions of alkanethiols instead of neat toluene for developing the patterns, as originally proposed by Lin et al. Prolonged exposure of AuC6 lines to a solution of n-hexanethiol (C6H13SH) in toluene (1 vol %) gradually reduces both the height and the width of the lines (Figure 4). The rate at which this happens is inversely related to the initial dose that the lines have been written with. A probable mechanism is replacement of the cross-linked ligand shell by a “fresh” capping agent, thereby slowly making the particles soluble again. Such a mechanism is in line with the observed dose dependence of the rate of this “etching”. Moreover, it implies that chemical modification of the nanoparticles by ligand exchange is possible even after lithography. Interestingly, etching of AuC6 lines using a solution of n-dodecanethiol (C12H25SH) instead of n-hexanethiol proceeds much more slowly, whereas it is quite rapid for AuC12 patterns. Nanostructures such as those investigated may be interesting for electronics applications if they can (i) be adressed electrically and (ii) be tailored a posteriori by chemical modifications. Here, we briefly address the first point. Figure 5 shows a patterned nanoparticle nanowire contacted by gold electrodes. After e-beam patterning of a AuC12 monolayer, an e-beam resist bilayer (MMA/MAA copolymer, followed 46
by PMMA) was spun onto the sample and annealed (165 °C, 10 min for each layer). Four electrodes were subsequently made using EBL and subsequent evaporation of 2 nm Ti and 30 nm Au through the patterned resist. Finally, the resist layer was lifted off in nearly boiling acetone. The patterned nanoclusters can withstand a standard lithographic process, as becomes clear in Figure 5. It should be noted that we also found that it is possible to pattern a film on top of predefined electrodes. Preliminary electrical measurements made on test patterns contacted using the process detailed above have shown that the patterned dodecanethiol nanoparticle films have a conductivity that is decreased by about 1 order of magnitude compared to pristine LangmuirBlodgett films.8 We have shown that it is possible to prepare and electrically address sub-50 nm structures based on nanoparticles through direct e-beam irradiation of LB films. This adds an important tool to the molecular electronics toolbox, since the cores of the nanoparticles remain intact and probably accessible to chemical modifications. We are currently investigating the insertion of bridging molecules into the patterned array to improve their electrical characteristics. Acknowledgment. We are grateful to Dr. Elisabeth Lefe`vre for the help with the TEM, and Ste´phane Auvray for the preparation of APTS functionalized Si substrates. This work was supported by the EU NANOMOL IST-1999-12603 project. Supporting Information Available: Three figures further describing the effect of e-beam irradiation on AuC12 monolayers. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Klein, D. L.; Roth, R.; Lim, A. K. L.; Alivisatos, A. P.; McEuen, P. L. Nature 1997, 389, 699-701. (2) Park, H.; Lim, A. K. L.; Alivisatos, A. P.; Park, J.; McEuen, P. L. Appl. Phys. Lett. 1999, 75, 301-303. (3) Persson, S. H. M.; Olofsson, L.; Gunnarsson, L. Appl. Phys. Lett. 1999, 74, 2546-2548. Nano Lett., Vol. 2, No. 1, 2002
(4) Andres, R. P.; Bielefeld, J. D.; Henderson, J. I.; Janes, D. B.; Kolagunta, V. R.; Kubiak, C. P.; Mahoney, W. J.; Osifchin, R. G. Science 1996, 273, 1690-1693. (5) Templeton, A. C.; Wuelfing, M. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27-36. (6) Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem 2000, 1, 1852. (7) Klimov, V. I.; Mikhailovsky, A. A.; Xu, S.; Malko, A.; Hollingsworth, J. A.; Leatherdale, C. A.; Eisler, H. J.; Bawendi, M. G. Science 2000, 290, 314-317. (8) Bourgoin, J.-P.; Kergueris, C.; Lefe`vre, E.; Palacin, S. Thin Solid Films 1998, 327-329, 515-519. (9) Chi, L. F.; Hartig, M.; Drechsler, T.; Schwaack, T.; Seidel, C.; Fuchs, H.; Schmid, G. Appl. Phys. Lett. 1998, 66, S187-190. (10) Schmid, G.; Liu, Y. P.; Schumann, M.; Raschke, T.; Radehaus, C. Nano Lett. 2001, 1, 405-407. (11) Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013-2016. (12) Chan, W. C. W.; Nie, S. M. Science 1998, 281, 2016-2018. (13) Mayer, C.; Stich, N.; Palkovits, R.; Bauer, G.; Pittner, F.; Schalkhammer, T. J. Pharm. Biomed. Anal. 2001, 24, 773-783. (14) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. ReV. Mater. Sci. 2000, 30, 545. (15) Heath, J. R.; Knobler, C. M.; Leff, D. V. J. Phys. Chem. B 1997, 101, 189-197. (16) Brust, M.; Stuhr-Hansen, N.; Norgaard, K.; Christensen, J. B.; Nielsen, L. K.; Bjornholm, T. Nano Lett. 2001, 1, 189-191. (17) Vossmeyer, T.; Jia, S.; Delonno, E.; Diehl, M. R.; Kim, S. H.; Peng, X.; Alivisatos, A. P.; Heath, J. R. J. Appl. Phys. 1998, 84, 36643670. (18) Vossmeyer, T.; De Ionno, E.; Heath, J. R. Angew. Chem., Int. Ed. Engl. 1997, 36, 1080-1083.
Nano Lett., Vol. 2, No. 1, 2002
(19) Palacin, S.; Hidber, P. C.; Bourgoin, J. P.; Miramond, C.; Fermon, C.; Whitesides, G. M. Chem. Mater. 1996, 8, 1316-1325. (20) Choi, K. H.; Bourgoin, J. P.; Auvray, S.; Esteve, D.; Duesberg, G. S.; Roth, S.; Burghard, M. Surf. Sci. 2000, 462, 195-202. (21) Parker, A. J.; Childs, P. A.; Palmer, R. E.; Brust, M. Appl. Phys. Lett. 1999, 74, 2833-2835. (22) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607-609. (23) Mirkin, C. A. Inorg. Chem. 2000, 39, 2258-2272. (24) Alivisatos, A. P.; Johnsson, K. P.; Peng, X. G.; Wilson, T. E.; Loweth, C. J.; Bruchez, M. P.; Schultz, P. G. Nature 1996, 382, 609-611. (25) Bedson, T. R.; Palmer, R. E.; Wilcoxon, J. P. Appl. Phys. Lett. 2001, 78, 2061-2063. (26) Bedson, T. R.; Palmer, R. E.; Jenkins, T. E.; Hayton, D. J.; Wilcoxon, J. P. Appl. Phys. Lett. 2001, 78, 1921-1923. (27) Clarke, L.; Wybourne, M. N.; Yan, M. D.; Cai, S. X.; Keana, J. F. W. Appl. Phys. Lett. 1997, 71, 617-619. (28) Lin, X. M.; Parthasarathy, R.; Jaeger, H. M. Appl. Phys. Lett. 2001, 78, 1915-1917. (29) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801-802. (30) Introduction to microlithography; Thompson, L. F., Willson, C. G., Bowden, M. J., Eds.; ACS Symposium Series 219; American Chemical Society: Washington DC, 1983; p 106. (31) Miller, A. A.; Lawton, E. J.; Balwit, J. S. J. Phys. Chem. 1956, 60, 599-604. (32) Seshadri, K.; Froyd, K.; Parikh, A. N.; Allara, D. L.; Lercel, M. J.; Craighead, H. G. J. Phys. Chem. 1996, 100, 15900-15909.
NL015629U
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