J. Phys. Chem. C 2007, 111, 8543-8556
8543
Dynamics of Reactive Metal Adsorption on Organic Thin Films Gabriella Nagy and Amy V. Walker* Department of Chemistry and Center for Materials InnoVation, Washington UniVersity in St. Louis, Campus Box 1134, One Brookings DriVe, St. Louis, Missouri 63130 ReceiVed: NoVember 28, 2006; In Final Form: April 17, 2007
We have studied the interaction of vapor-deposited Mg and Ca on -CH3, -COOH, -OH, -OCH3, and -CO2CH3 terminated self-assembled monolayers (SAMs) on polycrystalline Au{111} using time-of-flight secondary ion mass spectrometry and density functional theory calculations. Magnesium has a very low initial sticking probability on all of the SAMs studied. It inserts into the C-O bonds of oxygen-containing terminal groups and penetrates through methyl-terminated SAMs. In contrast, vapor-deposited calcium vigorously reacts with all of the SAMs studied to form inorganic products. Ca has a high sticking probability on all of the SAMs studied except for methyl-terminated SAMs. The reaction of these metals with SAMs can be explained by a single, general scheme. In the first stages of deposition at low coverages, metal atoms are only adsorbed if they can form a weak complex at the SAM/vacuum interface; otherwise, they are scattered from the surface. The adsorbed metal atoms then either react with the terminal group or penetrate through the monolayer. For metals that do not form weak complexes, only impinging atoms with sufficient energy to react with the terminal group or the methylene chain will adsorb. For all metals, adsorbed atoms provide nucleation sites for the formation of metallic islands and/or overlayers. These findings contribute to a systematic understanding of the interactions of metals with organic surfaces.
1. Introduction Understanding and controlling the interaction of metals with organic thin films are critical to many technological applications, such as polymer light emitting diodes (PLEDs),1-4 photovoltaics (PVs),5 and organic/molecular electronics.6-14 PLEDs are a promising technology for the development of low-cost display devices,3,15 but they suffer from poor device lifetimes.3,15,16 Although the exact mechanism of device failure is unknown, it is likely due to oxidation and corrosion of the cathode material, which is typically Al, Mg, or Ca.3,16 A wide variety of molecular electronic devices have been prototyped, which include switches,17 rectifying diodes,9,18,19 transistors,10 and memory elements.11 However, these display device-to-device variations, which have been attributed to the quality of the metallic contacts, the structural integrity of the molecular layer, and contaminants.20,21 Therefore, it is crucial to understand the underlying metalorganic chemistries if the performance of such devices is to be improved. Because polymer surfaces are not easy to control systematically, recent studies22-30 have used self-assembled monolayers (SAMs), which have well-defined structures and controllable chemistries, as model organic thin films.31,32 Further, SAMs are employed in many molecular electronic devices, which are typically constructed by vapor-depositing metallic contacts on top of a SAM to form either metal/SAM/metal18 or semiconductor/SAM/metal devices.33 When a metal is vapor-deposited onto an organic thin film such as a SAM, a wide range of reaction pathways and behaviors is observed. When an unreactive metal, such as Cu, Ag, or Au, is deposited onto a SAM there is a competition between nucleation of islands at the SAM/vacuum interface, reaction with * Corresponding author. Phone: 314 935 8496; Fax: 314 935 4481; E-mail:
[email protected].
the SAM terminal groups, and penetration through the film to the substrate.22,27-29 Similar behaviors are observed when these metals are deposited onto polymer films: metallic nanoparticles form in the subsurface region of the polymer, which is then followed by deposition of a discontinuous overlayer at the polymer/vacuum interface.34 In contrast, reactive metals such as Ti have been shown to degrade SAM and polymer structures to form inorganic products such as metal oxides and carbides.23,29,30 One important parameter that characterizes the metal-organic interaction is the sticking probability (or condensation coefficient, S), which is defined as follows:
S)
Na N0
where Na and N0 are the number of metal atoms adsorbed and the number of incident metal atoms, respectively. Although the sticking probabilities of organic molecules on metals have been widely studied, for example, using the King and Wells method,35 little is known about the sticking probabilities of metals onto organic thin films. Zaporojtchenko, Faupel, and co-workers36-39 have demonstrated that Ag, Cu, and Au that have been vapordeposited onto polymer films display a wide range of sticking probabilities. For example, at room temperature the sticking probability of Ag is 0.95 on pyromellitic dianhydride-oxidianiline polyimide (PMDA-ODA) but is only 0.002 on Teflon AF. One would expect that the sticking probability would be high for very reactive metals, such as Ti and Mg. However, recent experiments have shown that vapor-deposited Ti and Mg can both have very low sticking probabilities. Tighe et al.40 observed that the initial sticking probability of Ti onto hexadecanethiolate SAMs is only ∼0.1 at room temperature. Nowak, Schlapbach, and Collaud41-44 observed that the sticking prob-
10.1021/jp0678960 CCC: $37.00 © 2007 American Chemical Society Published on Web 05/31/2007
8544 J. Phys. Chem. C, Vol. 111, No. 24, 2007 ability of Mg onto polypropylene ranged from zero to a maximum value of 0.3 and was dependent upon the surface pretreatment. Finally, Walker et al.45 measured the initial sticking probability of Mg onto a -OCH3 terminated SAM to be ∼0.005. In general, the strength of the metal-organic interaction does not determine the sticking probability, and the observed sticking probabilities are very dependent upon the chemistry of the organic surface. Ca, Mg, and Al are low work function metals that are used as cathodes in PLEDs. These metals all form stable carbides, oxides, and hydrides.46 However, their reactivities toward organic thin films are very different. Salaneck and coworkers47-54 have performed studies of the interactions of vapordeposited Ca, Mg, and Al on polymer surfaces, such as poly(pphenylene vinylenes) (PPVs), R-sexithiophene (6T), and R,ωdiphenyltetradecaheptaene (DP7). In general, they observed that Ca diffused into the surface to a depth of 2-3 nm and that it donated electrons to the polymer π system to form Ca2+ ions.48-51,54 For oxygen-containing PPV-type surfaces it was also observed that an interfacial layer of calcium oxide formed.50,51,54 In contrast, vapor-deposited Al first formed covalent bonds with both the polymer backbone and the oxygencontaining functional groups prior to forming a metallic overlayer.49-51 In agreement with these observations, further studies using Raman spectroscopy55 and X-ray photoelectron spectroscopy56 demonstrated that Al undergoes covalent addition into the vinylenes of PPV55 and that it reacted with the methoxy groups of poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) to form an oxycarbide species.56 Studies of the vapor-deposition of Mg on poly-1,4-(2,5-dialkoxy-pphenylene vinylene) (OC1C10-PPV)52 indicated that Mg did not diffuse into the polymer or react with the polymer backbone. However, in later experiments Li et al.57 observed that vapordeposited Mg did react with MEH-PPV films to form a carbidelike species as well as magnesium oxides, clusters, and overlayers. Recent studies have examined the interaction of vapordeposited Al, Mg, and Ca with functionalized SAMs.24-28,30,45 The most extensive studies have been carried out on the reaction of Al with -CH3,24 -OH,26 -COOH,25 -OCH3,26-28 and -CO2CH324 terminated SAMs. For -OH,26 -COOH,25 and -CO2CH324 terminated SAMs it was observed that Al inserts into the C-O bonds of the terminal group. In contrast, Al penetrates through -CH3 terminated SAMs at very low coverages, and at higher Al coverages a metallic overlayer was observed to form.24 Vapor-deposited Al weakly interacts with the -OCH3 terminal group and also penetrates through the SAM to the Au/S interface.26-28 Ca30 and Mg45 vapor-deposition on -OCH3 terminated SAMs have also been studied. The results are briefly summarized here. Unlike Al, Mg inserts into the C-O bond of the -OCH3 terminal group, does not penetrate through the SAM to the Au/S interface, and has a very low initial sticking probability.45 Vapor-deposited Ca exhibits behavior that is different from both Al and Mg; it reacts strongly with the -OCH3 terminated SAM to form inorganic products that include calcium carbide and oxide.30 It is not yet understood why Ca, Mg, and Al interact so differently with the -OCH3 SAM surface. In this study we investigate the interaction of vapor-deposited Ca and Mg with -CH3, -COOH, -OH, -OCH3, and -CO2CH3 terminated SAMs using time-of-flight secondary ion mass spectrometry (TOF SIMS) and density functional theory (DFT) calculations. We also compare the observed reaction pathways for Mg and Ca deposition with those observed for
Nagy and Walker Al. The results show that vapor-deposited Mg has a very low initial sticking probability on all of the SAMs studied. Mg inserts into the C-O bonds of oxygen-containing terminal groups, and it penetrates through the -CH3 terminated SAM to the Au/S interface. In contrast, vapor-deposited Ca reacts vigorously with all the SAMs studied to form inorganic products that include calcium carbide and oxides. Ca appears to react faster with oxygen-containing terminal groups than with -CH3 terminal groups. The interaction of vapor-deposited Mg and Ca, as well as Al, on functionalized SAMs can be rationalized in the following way. Vapor-deposited Mg is not stabilized at the SAM/vacuum interface by the metal-terminal group interaction; consequently, it has a short residence time on the surface. This lack of stabilization leads to the observed low initial sticking probability because only Mg atoms with sufficient energy to overcome the activation barrier for insertion into C-O bonds of -OH, -OCH3, -COOH, and -CO2CH3 terminal groups are adsorbed onto the surface. On -CH3 terminated SAMs, Mg penetrates to the Au/S interface if it is adsorbed at a defect site; otherwise, it is reflected from the surface. In contrast, vapor-deposited Al and Ca can be stabilized at the SAM/vacuum interface, which leads to longer residence times and to higher sticking probabilities for these metals, except for Ca deposition onto -CH3 terminated SAMs. In this case, vapor-deposited Ca is not stabilized at the SAM/vacuum interface. This leads to a short residence time on the surface, a lower sticking probability, and a slower reaction rate with the SAM. Once they are stabilized on the surface, Ca and Al atoms either react with a terminal group or penetrate through the SAM to the Au/S interface. In all cases (Mg, Ca, Al), as the deposition continues the initially reacted sites provide nucleation sites for either the formation of metallic islands and overlayers or, in the case of Ca, for the adsorption of atoms that react with the terminal groups or SAM methylene chain to form calcium carbide and oxide. The DFT analysis of interaction energies of these metals with -CH3, -OH, -OCH3, -COOH, and -CO2CH3 terminated SAMs correlates well with the observed behaviors. This suggests that calculations can provide useful guidelines for predicting the relative sticking probabilities of metals on organic thin films as well as information about possible reaction products. 2. Experimental Section 2.1. Materials and Sample Preparation. The materials for all metal depositions were obtained from Alfa Aesar Inc. and were of 99.995% purity. The preparation and characterization of the SAMs used in this study have been described in detail previously.24-26,58 Briefly, first Cr (∼10 nm) and then Au (∼100-200 nm) were sequentially thermally deposited onto clean Si native oxide covered wafers (, Addison Technology Inc.). Selfassembly of well-organized monolayers was achieved by immersing the Au substrates into a 1 mmol solution of the relevant hexadecanethiol (with -CH3, -COOH, -CO2CH3, -OCH3, or -OH terminal functional groups; obtained from Prof. D. L. Allara, Pennsylvania State University) in absolute ethanol (Aaper Alcohol and Chemical Co.) for 24 h at ambient temperature (21 ( 2 °C). To ensure that the SAMs were wellordered and that there was no significant chemical contamination for each batch of the monolayers produced, a sample (∼1 × 1 cm2) was taken and was characterized by both single-wavelength ellipsometry (Gaertner Inc.) and TOF SIMS prior to metal deposition. 2.2. TOF SIMS. TOF SIMS spectra were acquired by using an ION TOF IV spectrometer (ION TOF Inc.) equipped with a
Reactive Metals on Organic Thin Films
J. Phys. Chem. C, Vol. 111, No. 24, 2007 8545
TABLE 1: Characteristic Ions Observed in TOF SIMS Spectra upon Ca and Mg Deposition onto -CH3, -OCH3, -COOH, -OH, and -CO2CH3 Terminated SAMs metal deposited SAM terminal group -CH3 -OCH3 -COOH -OH -CO2CH3
Ca +,
+,
CaH+,
Mg
CaC+,
Cax CaSH2 CaC2+, AuxCaySzCax+, CaSH2+, CaH+, CaC+, CaC2+, CaO(, CaOH+, CaOCH3+, AuxCaySzCax+, CaSH2+, CaH+, CaC+, CaC2+, CaO(, CaOH+, Ca(OH)CH2+, AuxCaySzCax+, CaSH2+, CaH+, CaC+, CaC2+, CaO(, CaOH+, Ca(OH)CH2+, AuxCaySzCax+, CaSH2+, CaH+, CaC+, CaC2+, CaO(, CaOH+, CaOCH3+, AuxCaySz-
Au liquid metal ion gun (LMIG). Briefly, the instrument consists of a load lock, preparation, and analysis chambers that are each separated by a gate valve. The pressure of the preparation and analysis chambers were maintained at
