Langmuir 2009, 25, 2585-2587
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Identification and Passivation of Defects in Self-Assembled Monolayers Michael J. Preiner and Nicholas A. Melosh* Geballe Laboratory for AdVanced Materials, Stanford UniVersity, Stanford, California 94305 ReceiVed December 17, 2008. ReVised Manuscript ReceiVed January 27, 2009 We demonstrate imaging of nanoscale defects in self-assembled monolayers (SAMs). Atomic layer deposition of aluminum oxide (AlOx) onto hydrophobic SAMs is followed by imaging using scanning electron microscopy (SEM). The insulating AlOx selectively deposits onto the exposed substrate at defect sites and becomes charged during imaging, providing high contrast even for nanometer scale defects. The deposited AlOx also acts as a barrier for electron transfer, thereby simultaneously electrically passivating the defects in the SAM as it labels them.
Self-assembled monolayers have become a ubiquitous ingredient in many fields of scientific research. They provide templates for chemical and biological phenomena,1,2 tune both electrical and optical properties of optoelectronic devices,3,4 and form the active component of molecular electronic devices.2,5 The electrical properties of the SAM depend strongly on its microscopic structure, in particular, on the density and size of the defects in the SAM itself. The macroscopic properties of SAM defects have been studied with a host of techniques, such as ellipsometry, infrared spectroscopy, and electrochemistry.6-8 Nanoscale characterization of single defects with scanning tunneling microscopy (STM)9 or atomic force microscopy (AFM)10,11 provides more detailed information about the distribution, density, size, and local environment. However, scanning probes suffer from slow speeds and stringent sample preparation, making them unsuitable for routine analysis. There have also been reports of chemically amplifying single defects and then using SEM or AFM to characterize the film.12,13 This simplifies the imaging process but greatly adds to the complexity of sample preparation. In this paper we present a one step method that uses atomic layer deposition (ALD) of AlOx to highlight defects in hydrophobic SAMs that have been deposited on Au substrates. This enables imaging of defects in the SAM with SEM, as the deposited AlOx provides high contrast to the conducting substrate. Analysis of SEM images demonstrates that this process can reveal point defects, extended area defects, and even larger structural domains within the SAM. Additionally, the deposited aluminum oxide electrically passivates the defects that it identifies, thereby * To whom correspondence should be addressed. E-mail: nmelosh@ stanford.edu. (1) Falconnet, D.; Pasqui, D.; Park, S.; Eckert, R.; Schift, H.; Gobrecht, J.; Barbucci, R.; Textor, M. Nano Lett. 2004, 4, 1909–1914. (2) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103–1169. (3) Fendler, J. H. Chem. Mater. 2001, 13, 3196–3210. (4) Malinsky, M. D.; Kelly, K. L.; Schatz, G. C.; Duyne, R. P. V. J. Am. Chem. Soc. 2001, 123, 1471–1482. (5) Reed, M. A.; Zhou, C.; Muller, C. J.; Burgin, T. P.; Tour, J. M. Science 1997, 278, 252–254. (6) Porter, M.; Bright, T.; Allara, D.; Chidsey, C. J. Am. Chem. Soc. 1987, 109, 3559–3568. (7) Ulman, A. Chem. ReV. 1996, 96, 1533–1554. (8) Seitz, O.; Bocking, T.; Salomon, A.; Gooding, J. J.; Cahen, D. Langmuir 2006, 22, 6915–6922. (9) Poirier, G. E. Chem. ReV. 1997, 97, 1117–1127. (10) Uchihashi, T.; Ishida, T.; Komiyama, M.; Ashino, M.; Sugawara, Y.; Mizutani, W.; Yokoyama; Morita, S.; Tokumoto, H.; Ishikawa, M. Appl. Surf. Sci. 2000, 57, 244–250. (11) Yamada, H.; Ogata, M.; Koike, T. Langmuir 2006, 22, 7923–7927. (12) Zhao, X.-M.; Wilbur, J. L.; Whitesides, G. M. Langmuir 1996, 12, 3257– 3264. (13) O’Dwyer, C. Mater. Lett. 2007, 61, 3837–3841.
providing a convenient mechanism to repair defects that may compromise the electrical properties of the SAM. In principle, this process is compatible with any substrate conducive to both self-assembly and ALD. It has recently been shown that hydrophobic SAMs can act as effective resists for ALD processes that use H2O as a precursor.14,15 This is due to the inability of H2O to form a stable wetting layer on the hydrophobic SAMs, thus preventing nucleation and growth of the ALD film.16 In contrast, ALD films can be readily grown on the metallic substrates typically used for SAMs, such as Au, Ag, Cu, and Ni.15,17,18 Vacancy defects in hydrophobic SAMs thus serve as nucleation sites for ALD growth by allowing the precursors to reach the substrate. Subsequent imaging of this growth allows visualization of the initial defects in the SAM, shown schematically in Figure 1. To create SAMs, freshly evaporated Au films were cleaned with UV ozone for 5 min, and then placed in 1 mg/mL solutions of hexadecanethiol in tetrahydrofuran for =10 h. To induce varying numbers of defects in the SAMs, the samples were then heated in ethanol at 50 °C for times ranging from 5 to 60 min, a process known to create vacancy defects.19 Measurements of the static water contact angle (θc) verified that the samples had defect densities proportional to the heating time. The samples were then immediately placed in an ALD reactor (Cambridge Nanotech, Cambridge, MA), where they were alternately exposed to trimethyl aluminum (TMA) and water vapor. To avoid desorption of the SAM at high temperatures, the ALD reactor temperature was kept at 60 °C.15 The samples were exposed to a total of 20 ALD cycles, where each cycle conformally deposits =1.1 Å of AlOx onto the substrate. The number of ALD cycles used was a compromise between too few cycles (∼10), which made SEM imaging difficult, and too many (∼40), which often caused particle overlap for high defect density films. After deposition of AlOx, Auger spectroscopy was used to measure the amount of aluminum deposited onto the samples. Plots of the surface aluminum coverage versus cos (θc) show a linear dependence, and a linear fit to the data predicts a pinhole free film at a contact angle of 110.9° (Figure 2). This is in good agreement with previous studies that have compared the contact (14) Chen, R.; Bent, S. Chem. Mater. 2006, 18, 3733–3741. (15) Preiner, M.; Melosh, N. Appl. Phys. Lett. 2008, 92, 213301. (16) Chen, R.; Kim, H.; McIntyre, P. C.; Bent, S. F. Chem. Mater. 2005, 17, 536–544. (17) Groner, M.; Elam, J.; Fabreguette, F.; George, S. Thin Solid Films 2002, 413, 186–197. (18) Zhang, X.; Zhao, J.; Whitney, A. V.; Elam, J. W.; Duyne, R. P. V. J. Am. Chem. Soc. 2006, 128, 10304–10309. (19) Doudevski, I.; Schwartz, D. K. Langmuir 2000, 16, 9381–9384.
10.1021/la804162a CCC: $40.75 2009 American Chemical Society Published on Web 02/04/2009
2586 Langmuir, Vol. 25, No. 5, 2009
Letters
Figure 3. SEM images of AlOx passivated defects from samples with initial θc of 111.5°, 108.5°, 107°, and 105° (a-d, respectively). The insulating aluminum oxide charges and effectively scatters incident electrons and thus shows high contrast with the conducting substrate. There are no defects in (a), whereas three defects are highlighted in (b) for reference. Note the increasing number of defects with increasing water contact angle, and the different scale on (b).
angle and electronic quality of SAMs.8 Additionally, the fact that high quality SAMs (θc > 109.5) show very little aluminum coverage (60×, giving a fractional pinhole area (after passivation) of
