7620
Langmuir 2007, 23, 7620-7625
In-Situ Infrared Spectroscopy of Buried Organic Monolayers: Influence of the Substrate on Titanium Reactivity with a Langmuir-Blodgett Film Carrie L. Donley,†,§ Jason J. Blackstock,† William F. Stickle,‡ Duncan R. Stewart,*,† and R. Stanley Williams† Quantum Science Research Group, Hewlett-Packard Labs, 1501 Page Mill Road, Palo Alto, California, and Analytical and DeVelopment Laboratories, Hewlett-Packard Company, CorVallis, Oregon ReceiVed October 8, 2006. In Final Form: April 24, 2007 The reactivity of metals vapor deposited onto organic monolayers has historically been correlated to the metal/ terminal organic group chemistry. Here we demonstrate that the chemical composition of the substrate unexpectedly plays a significant role as well. In particular, the reactivity of evaporated titanium toward a cadmium stearate LangmuirBlodgett (LB) film was found to depend on the substrate upon which the LB film was deposited. Infrared spectra taken in a modified ATR (Kretschmann) geometry with a thin Au substrate showed large changes in peak shape, peak position, and peak width in the C-H stretching region, indicating titanium penetration into the LB film and decomposition of the original well-packed monolayer structure. LB monolayers formed on a platinum oxide (PtOx) surface showed remarkably small changes after Ti deposition, indicating only a slight increase in disorder and no significant metal penetration into the film. Films on SiO2 substrates showed reactivity between that of Au and PtOx. These differences in reactivity can be correlated primarily with the amount of available oxygen associated with each substrate, including surface oxide layers and water incorporated within the LB film.
Introduction Many of the electrical contacts made in organic electronic devices are the result of metal deposition onto an organic thin film or monolayer, and the effect of this deposition on the organic layer depends on the metal deposited, the chemistry of the organic layer, and the deposition method employed.1-3 As the size of electronic devices scales down to the molecular level, these effects become even more critical to the electrical behavior observed. We are specifically interested in the buried metal/organic monolayer/metal active region contained within our molecular scale cross bar junctions fabricated for logic and memory applications.4-11 In these devices, the critical organic layer is typically only a few nanometers thick, and a detailed physical * To whom correspondence should be addressed. E-mail: duncan.
[email protected]. † Hewlett-Packard Labs. ‡ Hewlett-Packard Company. § Current address: University of North Carolina, Institute for Advanced Materials, 243 Chapman Hall, Chapel Hill, NC 27599. (1) Haynie, B. C.; Walker, A. V.; Tighe, T. B.; Allara, D. L.; Winograd, N. Appl. Surf. Sci. 2003, 203-204, 433. (2) Herdt, G. C.; Jung, D. R.; Czanderna, A. W. Prog. Surf. Sci. 1995, 50, 103. (3) Jung, D. R.; Czanderna, A. W.; Herdt, G. C. J. Vac. Sci. Technol. A 1996, 14, 1779. (4) Richter, C. A.; Stewart, D. R.; Ohlberg, D. A. A.; Williams, R. S. Appl. Phys. A. 2005, 80, 1355. (5) Stewart, D. R.; Ohlberg, D. A. A.; Beck, P. A.; Chen, Y.; Williams, R. S.; Jeppesen, J. O.; Nielsen, K. A.; Stoddart, J. F. Nano Lett. 2004, 4, 133. (6) Lau, C. N.; Stewart, D. R.; Williams, R. S.; Bockrath, M. Nano Lett. 2004, 4, 569. (7) Lau, C. N.; Stewart, D. R.; Bockrath, M.; Williams, R. S. Appl. Phys. A 2005, 80, 1373. (8) Chen, Y.; Jung, G.-Y.; Ohlberg, D. A. A.; Li, X.; Stewart, D. R.; Jeppesen, J. O.; Nielsen, K. A.; Stoddart, J. F.; Williams, R. S. Nanotechnology, 2003, 14, 462. (9) Wu, W.; Jung, G.-Y.; Olynick, D. L.; Straznicky, J.; Li, Z.; Li, X.; Ohlberg, D. A. A.; Chen, Y.; Wang, S.-Y.; Liddle, J. A.; Tong, W. M.; Williams, R. S. Appl. Phys. A. 2005, 80, 1173. (10) Jung, G. Y.; Ganapathiappan, S.; Li, X.; Ohlberg, D. A. A.; Olynick, D. L.; Chen, Y.; Tong, W. M.; Williams, R. S. Appl. Phys. A 2004, 78, 1169. (11) Chen, Y.; Ohlberg, D. A. A.; Li, X.; Stewart, D. R.; Jeppesen, J. O.; Nielsen, K. A.; Stoddart, J. F.; Olynick, D. L.; Anderson, E. Appl. Phys. Lett. 2003, 82, 1610.
and chemical characterization of this layer and adjacent interfaces is essential for understanding electronic behavior. This active region is buried within a complex materials stack formed in a series of processing steps, many of which induce physical and chemical modifications to the preceding materials layers. Reliable chemical information on the as-built devices can only be extracted from in situ analysis of the complete device stack. The effect of metal deposition onto organic monolayers has been previously studied by infrared (IR) spectroscopy,12-17,19-22 X-ray photoelectron spectroscopy (XPS),2-3,12-13,15,17-18,23 secondary ion mass spectroscopy (SIMS),1,12-17 atomic force microscopy (AFM),12,20 and Raman spectroscopy.24 Many combinations of metals (Al, Au, Ti, Ca, Cu, and Ag) and various end group functionalized self-assembled monolayers (-OCH3, -CH3, -COOH, -CO2CH3, -OH, -CN, -NO2, -phenyl) have been studied. This collection of work has illustrated the variety of behaviors that result, based on different metal/organic (12) Tighe, T. B.; Daniel, T. A.; Zhu, Z.; Uppili, S.; Winograd, N.; Allara, D. L. J. Phys. Chem. B 2005, 109, 21006. (13) 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. (14) Walker, A. V.; Tighe, T. B.; Stapleton, J.; Haynie, B. C.; Upilli, S.; Allara, D. L.; Winograd, N. Appl. Phys. Lett. 2004, 84, 4008. (15) Walker, A. V.; Tighe, T. B.; Haynie, B. C.; Uppili, S.; Winograd, N.; Allara, D. L. J. Phys. Chem. B 2005, 109, 11263. (16) Fisher, G. L.; Hooper, A. E.; Opila, R. L.; Allara, D. L.; Winograd, N. J. Phys. Chem. B 2000, 104, 3267. (17) 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. (18) Konstandinidis, K.; Zhang, P.; Opila, R. L.; Allara, D. L. Surf. Sci. 1995, 338, 300. (19) Jun, Y.; Zhu, X. Y. J. Am. Chem. Soc. 2004, 126, 13224. (20) deBoer, B.; Frank, M. M.; Chabal, Y. J.; Jiang, W.; Garfunkel, E.; Bao, Z. Langmuir 2004, 20, 1539. (21) Richter, C. A.; Hacker, C. A.; Richter, L. J. J. Phys. Chem. B 2005, 109, 21836. (22) Chang, S.-C.; Li, Z.; Lau, C. N.; Larade, B.; Williams, R. S. Appl. Phys. Lett. 2003, 83, 3198. (23) McGovern, W. R.; Anariba, F.; McCreery, R. L. J. Electrochem. Soc. 2005, 152, E176. (24) Nowak, A. M.; McCreery, R. L. J. Am. Chem. Soc. 2004, 126, 16621.
10.1021/la062960q CCC: $37.00 © 2007 American Chemical Society Published on Web 06/09/2007
Infrared Spectroscopy of Buried Organic Monolayers
Langmuir, Vol. 23, No. 14, 2007 7621
combinations: (i) metal over-layer formation with minimal penetration into the organic layer, (ii) reaction of the metal with the organic monolayer to form metal oxides, carbides, and hydrides, and (iii) complete penetration of the deposited metal to the substrate and displacement of the organic monolayer. This previous work has also shown that titanium is relatively reactive, often causing significant destruction of the organic monolayer upon which it is deposited.3,12,14,15,18-21 It should be noted, however, that most of the infrared spectroscopic work described above has been performed in a reflection absorbance infrared (RAIR) geometry in which data was obtained through an extremely thin top metal layer, and not on true buried device interfaces. A few IR studies have utilized an attenuated total reflectance (ATR) geometry19,20 where the sample stack was deposited directly on the trapezoidal ATR crystal, which allows for a thicker top metal layer. Our goal was to measure actual metal/molecule/metal device structures containing relatively thick top metal electrodes (g10 nm), and highly IR-absorbing Pt bottom electrodes, as these device structures have been the previous focus of extensive electrical characterization.4-11 Thick top metals required us to adopt an experimental geometry in which IR light could be coupled in through a thin bottom metal electrode. Highly absorbing Pt bottom electrodes required a geometry with only one or two reflections from the Pt metal. We describe here an ATR method based on a modified Kretschmann geometry25 for obtaining IR spectra of organic monolayers buried between highly absorbing metallic layers. Samples were built directly on partially ‘masked’ silicon trapezoidal ATR crystals, and the effect of titanium deposition on the IR spectra of Langmuir-Blodgett (LB) films formed on PtOx, Au, and SiO2 substrates was investigated. We demonstrate that the chemical composition of the substrate unexpectedly plays a significant role in the reactivity of the titanium toward the LB film. Experimental Methods The experimental geometry employed is illustrated in Figure 1a. Single-crystal silicon ATR trapezoids (50 × 10 × 3 mm3, 45° end facets) were purchased from Spectral Systems and were cleaned in a piranha solution (2:1 H2SO4/H2O2) before processing. The shorter back surface and all but an ∼11 × 10 mm2 window region of the longer front surface of the ATR trapezoid were coated or masked with Ti (2 nm) + thick Au (200 nm). This thick Au masking was critical to the experimental success, and the reasons for this are discussed in more detail below. The thin device structures were then fabricated on top of the exposed 11 × 10 mm2 window region of the Si surface. For the SiO2 sample, the LB monolayer was deposited directly onto this cleaned structure with the exposed 11 × 10 mm2 region of the native oxide covered silicon ATR crystal acting as the substrate (inset Figure 1c). For the PtOx electrode sample (inset Figure 1d), the masked ATR crystal first underwent a 5 min argon ion milling step to remove any adventitious species and to promote adhesion between the platinum and the bare silicon substrate. Seven nanometers of Pt were then sputter-deposited at a rate of 0.04 nm/s and at a base pressure of fwhmSiO2 > fwhmPtOx. Despite our hesitation to use peak heights or peak areas for quantitative purposes, it is interesting to note in Table 1 that the changes in peak height and peak area are largest for the Au substrate and they decrease on SiO2 and PtOx substrates, in agreement with the interpretation above suggesting increased LB film decomposition on Au substrates as compared to the other two substrates. If we were to naı¨vely assume that the modified post Ti deposition sample structures themselves did not affect the signal intensity, then approximately 5%, 16%, and 22% of the CH2 bonds on PtOx, SiO2, and Au substrates, respectively, were destroyed upon Ti deposition. Recent XPS experiments on samples similar to these have shown that the deposited titanium comprises primarily titanium dioxide (TiO2), titanium oxide (TiO), and titanium suboxide (TiOx) with a relatively small amount of titanium carbide (TiC). The amount of TiC depends on the substrate and increases as TiCPtOx < TiCSiO2 < TiCAu (see Supporting Information, Figure S1) in agreement with the metal reactivity inferred from the IR intensity data discussed above. That most of the titanium is oxidized indicates that our system contains a significant source of oxygen which acts to protect the LB film by reacting with titanium to produce oxide species (TiO2, TiO, and TiOx), and preventing significant formation of TiC. The reported standard enthalpies of formation for TiC and TiO2 differ quite dramatically
7624 Langmuir, Vol. 23, No. 14, 2007
with TiO2 formation highly favored over TiC formation (∆°Hf-TiC ) -60 to -90 kJ/mol,36 and ∆°Hf-TiO2 ) -944 kJ/mol37). Given a source of oxygen, a majority of oxidized titanium is therefore not surprising even if oxygen is not as prevalent at the sample surface as carbon. There are three possible sources of oxygen that could account for the titanium oxide formation including: (i) water trapped in the LB film during LB monolayer deposition or exposure to the ambient before deposition, (ii) water in the deposition chamber, and (iii) oxygen present in the substrates themselves. These possibilities are discussed below. Water within the LB film might be expected, since the samples are not annealed prior to titanium deposition. This water may originate from the LB subphase or exposure to ambient prior to titanium deposition. The amount of trapped water is likely to be substrate dependent, as interactions between the carboxylate head group of the stearic acid, the cadmium ions in the subphase, the water, and the substrate surface will determine the extent of water incorporation. Even alkane thiol SAM films on Au have recently been shown to be susceptible to water adsorption when exposed to ambient conditions, with the water interacting with the sulfur head group of the thiol molecules.38 A straightforward consideration of the molar volumes of water (∼18 cm3/mol) and TiO2 (∼20 cm3/mol) or TiO (∼13 cm3/mol) indicate that formation of even 5 nm of TiO would require ∼7 nm of H2O. Considering the 2.5 nm LB monolayer thickness, it seems infeasible that water trapped in the LB film could account for all of the oxygen incorporated in the titanium. A small amount of water trapped within the organic film may, however, account for some of the differences observed in the TiOx concentration on the three substrates. Given the pressure at which titanium was deposited (∼1 × 10-6 Torr), we would expect a significant amount of oxygen and water to be present in the deposition chamber. The presence of water in the deposition chamber, however, would not a priori account for the differences observed in the IR data as a function of the substrate. Titanium deposited under these same conditions onto a control sample with a clean platinum metal surface was shown to be mostly metallic by XPS but included a significant 25% atomic concentration of oxygen (Supporting Information Figure S1). This demonstrates that the high chamber pressure is responsible for some of the Ti oxide formation, but again cannot simply account for the differences between substrates. Finally, two of the three substrates under investigation contain oxygen explicitly, in the form of a surface oxide. The platinum oxide formed during oxygen plasma treatment has been shown to be relatively unstable; it readily decomposes under an argon plasma.26,27 In addition, XPS investigation has shown that this oxide is reduced to platinum metal when titanium is deposited on this sample stack,39 thereby releasing the surface-contained oxygen during the titanium deposition process. A similar reduction of SiO2 by titanium was reported through a ∼4 nm thick layer of either HfO2 or ZrO2,40 although in our experiments, the ∼1.5 nm native SiO2 seems relatively stable by XPS analysis. Finally, the Au substrate has no oxide. Thus, the amount of oxygen available in these three substrates varies as OPtOx > OSiO2 > OAu. (36) Frisk, K. Calphad 2003, 27, 367, and references therein. (37) CRC Handbook of Chemistry and Physics, 81st ed.; Lide, D.R., Ed.; CRC Press: Boca Raton, FL, 2000. (38) Long, D. P.; Lazorcik, J. L.; Mantooth, B. A.; Moore, M. H.; Ratner, M. A.; Troisi, A.; Yao, Y.; Ciszek, J. W.; Tour, J. M.; Shashidhar, R. Nat. Mater. 2006, 5, 901. (39) Blackstock, J. J.; Stickle, W. F.; Donley, C. L.; Stewart, D. R.; Williams, R. S. J. Phys. Chem. C 2007, 111, 16. (40) Kim, H.; McIntyre, P. C.; Chui, C. O.; Saraswat, K. C.; Stemmer, S. J. Appl. Phys. 2004, 96, 3467.
Donley et al.
In the PtOx case, the molar volume of PtO2 (∼19 cm3/mol) is comparable to the molar volumes of TiO2 and TiO, such that decomposition of the 2.5 nm PtO2 layer may be predicted to yield a relatively thick ∼2.5 nm of TiO2 or ∼3.4 nm of TiO. This excess oxygen available from PtOx substrates apparently acts to protect the LB film from the reactive titanium atoms and allows the LB film to retain a large portion of its original well-packed structure. In contrast, the LB film on Au is most likely to react with the incoming titanium atoms resulting in: (i) the largest amount of Ti penetration and disruption of the ordered LB film (the largest fwhm was observed in the IR data from both a shift toward higher wavenumbers due to more gauche defects and a softening due to metal interactions with intact C-H bonds) and (ii) the largest amount of monolayer decomposition and TiC formation (the largest IR C-H intensity loss and the largest TiC concentration in the XPS data were observed). Thus, while there is a significant amount of water in our deposition chamber which provides some of the oxygen ultimately incorporated into the titanium film, oxygen in the substrate itself and water trapped in the LB film are the likely causes of the differences in Ti reactivity with the LB film. Our results differ from other recent studies of titanium deposited onto alkane thiol SAM films formed on Au, where the organic monolayer shows a much larger degree of destruction and titanium carbide formation with minimal titanium oxide formation.12 The titanium depositions in the previous studies were performed at lower pressures (ranging from
