pubs.acs.org/Langmuir © 2009 American Chemical Society
Building Robust and Reliable Molecular Constructs: Patterning, Metallic Contacts, and Layer-by-Layer Assembly Amy V. Walker* Department of Materials Science and Engineering, University of Texas at Dallas, 800 West Campbell Road, RL 10, Richardson, Texas 75080-3021 Received October 16, 2009. Revised Manuscript Received November 30, 2009 We describe recent progress in our laboratories to build stable complex two- and three-dimensional molecular constructs. We have introduced a simple and robust method for constructing complex molecular devices using top-down and bottom-up techniques based on self-assembled monolayers (SAMs), lithography, and site-selective reactions. It has significant advantages over other methods; it is easily scaled up, affords precise nanoscale placement, and is extensible to many different materials. Several recent developments are discussed including the UV photopatterning and electron beam lithography of SAMs adsorbed on semiconductors, the site-selective deposition of metals using electroless deposition and low-temperature chemical vapor deposition, and layer-by-layer assembly using covalent coupling. Optimization and further development of these techniques requires a detailed understanding of the reaction pathways involved in the lithography of SAMs and of the interaction of SAMs with metals, organometallic compounds, ions, and other compounds.
Introduction Molecular assemblies that function as electronic, optical, magnetic and biological devices are an exciting new area of science with many applications, including in molecular/organic electronics,1-6 polymer/organic light-emitting diodes (PLEDs/ OLEDs),7,8 photovoltaics,9,10 and sensing.11-14 While many molecular devices have been demonstrated, there are a number of challenges to be overcome before they can be used in everyday products. For instance, a wide variety of molecular electronic devices has been prototyped including memory elements, switches, and rectifying diodes.1-6 However, device-to-device performance variations are often observed that stem from the structural integrity of the monolayer and the quality of the metallic contacts. Chen et al.1 reported that for a molecular switch based on [2]rotaxane with Ti/Pt nanowire electrodes only ∼75% of 36 devices displayed reversible switching properties while the other 25% failed by shorting or displayed open circuit behavior. Some of these failures were due to defects within the device structure but several “defect free” devices were also *To whom correspondence should be addressed. Ph: 972 883 5780. Fax: 972 883 5725. E-mail:
[email protected]. (1) Chen, Y.; Ohlberg, D. A. A.; Li, X.; Stewart, D. R.; Williams, R. S.; Jeppesen, J. O.; Nielsen, K. A.; Stoddart, J. F.; Olynick, D. L.; Anderson, E. Appl. Phys. Lett. 2003, 82, 1610–1612. (2) Reed, M. A. Proc. IEEE 1999, 87, 652–658. (3) Carroll, R. L.; Gorman, C. B. Angew. Chem., Int. Ed. 2002, 41, 4378–4400. (4) Salomon, A.; Cahen, D.; Lindsay, S.; Tomfohr, J.; Engelkes, V. B.; Frisbie, C. D. Adv. Mater. 2003, 15, 1881–1890. (5) Tour, J. M. Acc. Chem. Res. 2000, 33, 791–804. (6) Metzger, R. M. Chem. Rev. 2003, 103, 3803–3834. (7) Halls, J. J. M.; Walsh, C. A.; Greenham, N. C.; Marseglia, E. A.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Nature 1995, 376, 498–500. (8) Kugler, T.; L€ogdlund, M.; Salaneck, W. R. IEEE J. Sel. Top. Quant. 1998, 4, 14–23. (9) Gledhill, S. E.; Scott, B.; Gregg, B. A. J. Mater. Res. 2005, 20, 3167–3179. (10) Sofos, M.; Goldberger, J.; Stone, D. A.; Allen, J. E.; Ma, Q.; Herman, D. J.; Tsai, W.-W.; Lauhon, L. J.; Stupp, S. I. Nat. Mater. 2008, 8, 68–75. (11) Zhao, W.; Xu, J.-J.; Chen, H.-Y. Electroanalysis 2006, 18, 1737–1748. (12) Lutkenhaus, J. L.; Hammond, P. T. Soft Matter 2007, 3, 804–816. (13) Carrara, S.; Riley, D. J.; Bavastrello, V.; Stura, E.; Nicolini, C. Sens. Actuators, B 2005, 105, 542–548. (14) Davis, F.; Higson, S. P. J. Biosens. Bioelectron. 2005, 21, 1–20.
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observed to fail. Similarly, a wide range of molecular conductances have been reported for devices with nominally the same chemical structure,4 and contact structure is also critical in sensing13 and photovoltaic devices.9,10 Other factors that affect the use of molecular devices in practical applications include stability under varying conditions such as pH and temperature.11,15 There are two approaches for building molecular devices, topdown and bottom-up.16 The bottom-up approach employs either the natural propensity of molecules to self-assemble, or positional assembly techniques such as dip pen nanolithography. In contrast, top-down approaches employ conventional microfabrication methods such as photolithography and selective etching to create nanoscale structures from bulk materials such as silicon wafers. Both approaches have advantages and disadvantages. Top-down approaches can produce many devices simultaneously but for the smallest devices becomes slow and prohibitively expensive. Further, the lithography of organic layers is not as well developed as for semiconductors.17,18 A second issue is the reliable formation of interconnects small enough to reproducibly contact molecules.2 Bottom-up approaches are inexpensive but require precise control of the chemistry and the self-assembly process, which at present has not been fully achieved for complex structures.16,19-21 These approaches can also be slow especially using AFM or STM tips to assemble nanostructures.22 (15) Quinn, J. F.; Johnston, A. P. R.; Such, G. K.; Zelikin, N.; Caruso, F. Chem. Soc. Rev. 2007, 36, 707–718. (16) Abr~una, H. D.; Ratner, M. A.; van Zee, R. D. Building Electronic Function into Nanoscale Molecular Architectures; Report of a National Science Foundation Workshop, Washington, DC, 2007. http://www.wtec.org/MolecularElectronics/ MolecularElectronics-enc.pdf. (17) G€olzh€auser, A.; Eck, W.; Geyer, W.; Stadler, V.; Weimann, T.; Hinze, P.; Grunze, M. Adv. Mater. 2001, 13, 806–809. (18) Sun, S.; Chong, K. S. L.; Leggett, G. J. J. Am. Chem. Soc. 2002, 124, 2414– 2415. (19) Ulman, A. In Characterization of Organic Thin Films; Ulman, A., Fitzpatrick, L. E., Eds.; Butterworth-Heinemann Manning: Woburn, MA, 1995; pp 21-31. (20) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151–256. (21) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103–1169. (22) Smith, R. K.; Lewis, P. A.; Weiss, P. S. Prog. Surf. Sci. 2004, 75, 1–68.
Published on Web 12/11/2009
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Invited Feature Article
Figure 1. A schematic illustration of the top-down, bottom-up method to construct complex 2D and 3D structures. Physical vapor deposition and room temperature chemical vapor deposition can be employed to selectively deposit Mg28 and Al,29 respectively, on patterned -COOH/-CH3-terminated SAMs. Compound semiconductors, such as ZnS,30 can also be selectively deposited using chemical bath deposition on -COOH-terminated SAMs, leading to the formation of nanoflowers.30 Finally, streptavidin binds selectively to biotin-functionalized SAMs, prepared by selective biotinylation of -COOH-terminated SAMs.31
We have introduced a simple, robust method for the construction of complex two- and three-dimension structures using a combination of top-down and bottom-up methods. This method employs self-assembled monolayers (SAMs), lithography and site-selective reactions. It has significant advantages over other methods. It is extensible to many different types of materials, easily scaled up, and affords precise nanoscale placement. The basic process has three steps (Figure 1). First an image in one SAM (SAM#1) is created by using either UV light shone through a mask (UV photopatterning)18,23,24 or an energetic electron beam (electron beam lithography).25-27 Second, a SAM of different composition (SAM#2) is adsorbed in the area where the first SAM has been removed. By using SAMs with different terminal groups we can control how and where metals, semiconductors, organic compounds, and biomolecules deposit on the construct. Figure 1 displays four example constructs prepared using physical vapor deposition (PVD), chemical vapor deposition (CVD), chemical bath deposition (CBD), and wet chemistry. Optimization and application of these techniques requires detailed understanding of the lithography of SAMs and the interaction of SAMs with metals, organometallic compounds, ions, and other compounds. In this paper we will review our recent progress in lithography using SAM resists, metal deposition on organic surfaces, and the construction of stable three-dimensional (3D) molecular devices.
Analytical Methods To fully characterize molecular devices requires determination simultaneously of the chemistry and morphology with nanometer resolution in both lateral and depth directions. At present, no (23) Zhou, C.; Walker, A. V. Langmuir 2006, 22, 11420–11425. (24) Zhou, C.; Walker, A. V. Langmuir 2007, 23, 8876–8881. (25) Eck, W.; Grunze, M. Bunsen-Magazin 2009, 11, 3–13. (26) Zharnikov, M.; Grunze, M. J. Vac. Sci. Technol., B 2002, 20, 1793–1807. (27) Zhou, C.; Jones, J. C.; Trionfi, A.; Hsu, J. W. P.; Walker, A. V. Langmuir, 2009, in press.
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single analytical technique can accomplish this, so we employ multiple techniques, including time-of-flight secondary ion mass spectrometry (TOF SIMS), infrared spectroscopy (IRS), X-ray photoelectron spectroscopy (XPS), ellipsometry, and scanning electron microscopy (SEM). These not only provide detailed information about the constructs but also allow us to determine the reaction pathways involved in their formation. Our principal analytical technique is SIMS because it provides surface-specific molecular chemical information with ∼1 nm depth and ∼200 nm lateral resolution.32,33 It also requires that the surface does not need be specially prepared, and so can be used to obtain mass spectrometric images from a wide variety of materials including tissues, cells, organic thin films, artifacts, and geochemical samples.32,33 We employ SIMS to determine whether deposited species react with the terminal of the organic film, if there is penetration of metals and other deposited materials through the layer, the chemical identity of deposited overlayers, and to follow the extent of reactions. IRS and XPS complement SIMS by determining the bonds formed/broken during reactions (IRS)34,35 and the oxidation states of the elements deposited (XPS).36,37 We note that electron spectroscopies, including XPS and ultraviolet photoelectron spectroscopy, can be employed to investigate the electronic properties of organic devices.38 However, the focus of our work has been to investigate the reaction pathways involved in forming stable and robust organic devices and not in the investigation of the electronic structure. Single wavelength ellipsometry (SWE) is employed to measure the thickness of thin films and therefore to follow the formation of 3D constructs using layer-by-layer assembly. Finally, SEM is employed to characterize the morphology of the deposited films with nanometer lateral resolution.
Lithography One of the simplest ways to produce a surface with multiple functionalities is to first pattern a SAM (SAM#1) and then displace the damaged/photooxidized areas with a second SAM (SAM#2).23,24 By correct choice of the terminal groups of the two SAMs, areas that are selectively reactive or inert toward deposited biomolecules, semiconductors, and other compounds can be created. A wide variety of lithographic techniques have been developed or adapted to pattern SAMs, including UV photopatterning,18,23,24 electron beam lithography,25-27 dip pen nanolithography,22,39 nanoimprinting22 and microcontact printing.21,22 Of these methods, electron beam lithography and UV (28) Zhou, C.; Nagy, G.; Walker, A. V. J. Am. Chem. Soc. 2005, 127, 12160– 12161. (29) Lu, P.; Demirkan, K.; Opila, R. L.; Walker, A. V. J. Phys. Chem. C 2007, 112, 2091–2098. (30) Lu, P.; Walker, A. V. ACS Nano 2009, 3, 370–378. (31) Zhou, C.; Qi, K.; Wooley, K. L.; Walker, A. V. Colloids Surf., B 2008, 65, 85–91. (32) Walker, A. V. Anal. Chem. 2008, 80, 8865–8870. (33) ToF-SIMS: Surface Analysis by Mass Spectrometry; Vickerman, J. C., Briggs, D., Eds.; IM Publications and Surface Spectra Limited: Chichester, UK, 2001. (34) Allara, D. L. In Characterization of Organic Thin Films; Ulman, A., Fitzpatrick, L. E., Eds.; Butterworth-Heinemann Manning: Woborn, MA, 1995; pp 57-86. (35) Walker, A. V.; Fisher, G. L.; Hooper, A. E.; Tighe, T.; Opila, R. L.; Winograd, N.; Allara, D. L. In Metallization of Polymers 2; Sacher, E., Ed.; Kluwer Academic/Plenum: New York, 2002; pp 117-126. (36) Hooper, A.; Fisher, G. L.; Konstadinidis, K.; Jung, D.; Nguyen, H.; Opila, R.; Collins, R. W.; Winograd, N.; Allara, D. L. J. Am. Chem. Soc. 1999, 121, 8052– 8064. (37) Fisher, G. L.; Hooper, A. E.; Opila, R. L.; Allara, D. L.; Winograd, N. J. Phys. Chem. B 2000, 104, 3267–3273. (38) Hirose, Y.; Kahn, A.; Aristov, V.; Soukiassian, P.; Bulovic, V.; Forrest, S. R. Phys. Rev. B 1996, 54, 13748–13758. (39) Piner, R. D.; Zhu, J.; Xu, F.; Hong, S.; Mirkin, C. A. Science 1999, 283, 661–663.
DOI: 10.1021/la903937u
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Figure 2. UV photopatterned ECT/ODT SAM adsorbed on GaAs (001). Negative ion TOF SIMS images centered at m/z 313 (S(CH2)19CH3-, molecular ion of ECT) and 285 (S(CH2)17CH3molecular ion of ODT). The ECT was UV photopatterned for times between 10 and 60 min. Also shown is the extent of photo3 oxidation of ECT, χ. χ ¼ ½MSO½MSO - where [MSO3 ] and [MS ] 3 þ ½MS are the ion intensities of MSO3 and MS , respectively, and M = (CH2)19CH3. The images are shown using a heat scale, and the scale bar displays the maximum and minimum number of counts per pixel. Area of analysis: 500 500 μm2, 128 128 pixels2.
photopatterning are particularly attractive because they produce chemically well-defined, contamination-free patterns on the nanometer scale.18,23-26,40 An additional advantage of UV photopatterning is that it can be used over large sample areas.23,24 Electron beam lithography also has particular advantages. After electron beam exposure, the damaged areas may be either soluble41,42 or insoluble25,26,41 depending on their chemistry, which enables further manipulation of the surface. A major issue for lithography of SAMs adsorbed on using electrons or UV light is that for SAMs adsorbed on metals very high electron doses (.1 mC cm-2)25,26 or long exposure times (g1 h using Hg arc lamps)23,43 are required to produce welldefined patterns surfaces. This suggests that using SAMs as their own resists is not practical. However, we have shown that at least on semiconductors it is not necessary to completely degrade the SAM to produce well-defined multifunctional pattern surfaces.27 For alkanethiolate SAMs adsorbed on GaAs (001) only ∼20% of the SAM must be degraded to form a well-defined patterned SAM, corresponding to an electron dose of 750 μC cm-2 for -CH3-terminated SAMs and 450 μC cm-2 for -COOH-terminated SAMs at a electron beam energy of 40 kV. Using UV photopatterning, we observe that only ∼50% of the monolayer must be photooxidized to form sharply patterned SAM surfaces. This corresponds to a 3-fold decrease in the time required for photopatterning. As an example, Figure 2 displays a UV photopatterned eicosanethiol (ECT) SAM that was backfilled with octadecanethiol (ODT). A well-defined ECT/ODT pattern is observed after only 20 min UV exposure, corresponding to an extent of photooxidation of 0.49. The electron dose required to produced well-defined patterned SAM surfaces is similar to the critical electron dose for poly(methyl methacrylate) (PMMA), 350 μC cm-2 at 50 kV,44 a positive resist commonly used in electron lithography. Similarly, the UV exposures required for SAMs adsorbed on GaAs (40) Zhou, C.; Jones, J. C.; Trionfi, A.; Hsu, J. W. P.; Walker, A. V. Langmuir, in press. (41) Lercel, M. J.; Rooks, M.; Tiberio, R. C.; Craighead, H. G.; Sheen, C. W.; Parikh, A. N.; Allara, D. L. J. Vac. Sci. Technol., B 1995, 13, 1139–1143. (42) Lercel, M. J.; Redinbo, G. F.; Pardo, F. D.; Rooks, M.; Tiberio, R. C.; Simpson, P.; Craighead, H. G.; Sheen, C. W.; Parikh, A. N.; Allara, D. L. J. Vac. Sci Technol., B 1994, 12, 3663–3667. (43) Cooper, E.; Leggett, G. J. Langmuir 1999, 15, 1024–1032. (44) McCord, M. A.; Rooks, M. J. In SPIE Handbook of Microlithography, Micromachining and Microfabrication; Rai-Choudhury, P., Ed.; SPIE Publications: Bellingham, WA, 1997; Vol. 1: Microlithography, pp 139-250.
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(octadecanethiol - 15 min) are only slightly longer than those required for conventional photoresists (including prebake, exposure, and postbake).45 UV photopatterning and electron beam lithography of SAMs on GaAs and other semiconductors are therefore compatible with current device processing and equipment. In contrast, SAMs adsorbed on metals must be almost fully degraded to produce well-defined patterned surfaces.27 For UV photopatterning of SAMs on Au substrates we have shown that at least 85% of SAM#1 must be photooxidized (exposure times >90 min) to form sharp patterns. Very high electron doses are also required; for -CH3-terminated SAMs on Au well-defined patterns do not form even after an electron dose of 4000 μC cm-2. UV photopatterning and electron beam lithography are therefore impractical methods for producing devices on metal surfaces. The most likely reason for the differences observed in the patterning processes on Au and GaAs is the monolayer structure.19,20,46 SAMs with more than nine methylene units in the alkanethiolate backbone adsorbed on Au are well-ordered19,20 with large domain sizes (∼250 nm2 47). In contrast, SAMs adsorbed on GaAs (001) are well-ordered only if there are at least 15 methylene units in the backbone and have only small domain sizes (ODT: ∼49 nm2).46 This suggests that these SAMs contain many domain boundaries that facilitate alkanethiol diffusion, adsorption, and the formation of SAM#2 in the damaged/photooxidized areas.
New Methods for the Metallization of Organic Surfaces To date most molecular/organic electronic devices have been produced using physical vapor deposition (PVD) to form the top metal contact. Many outcomes have been observed, from metal atom penetration through the organic layer (e.g., Al,36,48 Cu49 on -CH3-terminated SAMs) to destruction of the monolayer via carbide formation (e.g., Ti,50 Ca48 on -OCH3-terminated SAMs) to formation of strongly bound complexes and stable contact formation (e.g., Al,37,48 Mg48 on -COOH-terminated SAMs). We have demonstrated that the interaction of vapor-deposited metals with functionalized SAMs is governed by a single general scheme. In the first stages of metal deposition, at low coverages atoms of metals such as Al,36,37,48,51,52 Cu,49,52 and Ca48 are adsorbed because they form weak complexes at the SAM/vacuum (air) interface. These metals have high condensation (sticking) coefficients on the organic surface. Metals, such as Mg,48,53 which do not form such complexes, are scattered from the surface and so have low condensation coefficients. Once weakly adsorbed, metal atoms either then react with the SAM terminal groups or penetrate through the monolayer. In some cases, such as Cu deposition on -COOH-terminated SAMs,49 most adsorbed metal atoms eventually penetrate through the SAM, while only (45) Levinson, H. J.; Arnold, W. H. In SPIE Handbook of Microlithography, Micromachining and Microfabrication; Rai-Choudhury, P., Ed.; SPIE Publications: Bellingham, WA, 1997; Vol. 1: Microlithography, pp 13-138. (46) McGuiness, C. L.; Blasini, D.; Masejewski, J. P.; Uppili, S.; Cabarcos, O. M.; Smilgies, D.; Allara, D. L. ACS Nano 2007, 1, 30–49. (47) Yamada, R.; Sakai, H.; Uosaki, K. Chem. Lett. 1999, 667–668. (48) Nagy, G.; Walker, A. V. J. Phys. Chem. C 2007, 111, 8543–8556. (49) Nagy, G.; Walker, A. V. J. Phys. Chem. B 2006, 110, 12543–12554. (50) Walker, A. V.; Tighe, T. B.; Stapleton, J. J.; Haynie, B. C.; Allara, D. L.; Winograd, N. Appl. Phys. Lett. 2004, 84, 4008–4010. (51) Fisher, G. L.; Walker, A. V.; Hooper, A. E.; Tighe, T. B.; Bahnck, K. B.; Skriba, H. T.; Reinard, M. D.; Haynie, B. C.; Opila, R. L.; Winograd, N.; Allara, D. L. J. Am. Chem. Soc. 2002, 124, 5528–5541. (52) Walker, A. V.; Tighe, T. B.; Cabarcos, O. M.; Reinard, M. D.; Haynie, B. C.; Uppili, S.; Winograd, N.; Allara, D. L. J. Am. Chem. Soc. 2004, 126, 3954– 3963. (53) Walker, A. V.; Tighe, T. B.; Cabarcos, O.; Haynie, B. C.; Allara, D. L.; Winograd, N. J. Phys. Chem. C 2007, 111, 765–772.
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a small proportion react with the terminal group. For metals that do not form weak complexes, only atoms with energy sufficient to react with the terminal group or methylene chain can adsorb. In all cases, adsorbed metal atoms provide sites for nucleation of metallic islands and/or overlayers. Although the interaction of vapor-deposited metals with SAMs is governed by a single reaction pathway, it is not easy to predict the outcome of a particular interaction because it is not possible to calculate the energy barriers involved.48 We have recently demonstrated that the conductivity of a Au/ODT/GaAs device is strongly dependent on its structure,54 which in turn is related to the metal deposition conditions used. We vapor-deposited 150 A˚ of Au on an ODT SAM adsorbed on GaAs under three different deposition conditions, (a) direct deposition on a 300 K sample; (b) direct deposition on a 77 K sample; and (c) using a gentle deposition technique, Ar backfilling, on a 300 K sample. For all samples, Au was observed to penetrate through the SAM to the S/GaAs interface. However ∼4 and ∼2 more gold diffused through the SAM after direct deposition at 300 and 77 K, respectively, than under Ar backfill conditions. Substantially different conductances and rectification behaviors were observed for the three devices. The Ar-backfilled and 77 K samples gave ∼200 and ∼70 larger conductances, respectively, than the 300 K sample. Technologies besides PVD to deposit metals and other materials in Si-based microelectronics include CVD,55 atomic layer deposition (ALD),56 and electrodeposition.57 CVD is employed in a broad range of applications to grow oxide, metal, semiconductor, glass, and compound thin films.55 We have developed several low-temperature (T < 40 °C) CVD methods to selectively deposit metals, including Al, on functionalized SAMs.29 CVD is not normally used on organic thin films because high thermal activation temperatures are often required (Tsubstrate g 200 °C),55 which are incompatible with most organic films, including SAMs.20 However, CVD is an otherwise attractive technique for the deposition of metals on such films because the CVD precursor, typically an organometallic compound, can selectively react with functional groups present on the surface allowing the directed deposition of metals. We have demonstrated that aluminum (and alumina) can be selectively deposited on SAMs at room temperature using the ALD precursor, trimethyl aluminum (TMA).29 TMA reacts with -OH- and -COOH-terminated SAMs to form a surface-bound dimethyl aluminum complex, but it does not react with terminal groups that do not contain oxygen, for example, -CH3-terminated SAMs. If the deposition is carried out under very high vacuum (
