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Electron-Induced Reactions of MeCpPtMe3 Investigated by HREELS M. N. Hedhili,† J. H. Bredeho¨ft, and P. Swiderek* UniVersita¨t Bremen, Institute for Applied and Physical Chemistry, Fachbereich 2 (Chemie/Biologie), Leobener Strasse/NW 2, Postfach 330440, 28334 Bremen, Germany ReceiVed: December 9, 2008; ReVised Manuscript ReceiVed: May 27, 2009
Multilayer condensed films of trimethyl(methylcyclopentadienyl)-platinum(IV) (MeCpPtMe3) have been exposed to low-energy electrons at incident electron energies of 15, 30, and 500 eV. The reactions occurring under exposure have been investigated using high-resolution electron energy loss (HREEL) spectroscopy. The observed changes in the HREEL spectra upon exposure are similar for the three different energies. Contrary to previous results obtained for electron irradiation at 500 eV using reflection absorption infrared spectroscopy (RAIRS) and pointing toward a complete loss of hydrogen a comparable electron exposure in the present experiment yields a product that still contains a significant amount of C-H bonds. In accord with results from thermal desorption spectrometry also obtained after exposure to electrons at lower electron energy, a material is obtained that does not evaporate at room temperature. A crude estimate suggests that the overall reaction rate is consistent with previous results. 1. Introduction Electron-beam-induced deposition (EBID) is a versatile technique for the controlled growth of nanostructures of arbitrary shape at surfaces.1 It relies on the decomposition of volatile organometallic precursors under the effect of a highly focused high-energy electron beam. Low-energy electrons backscattered from the surface are recognized as playing an important role in the decomposition reactions. It is thus of interest to study the chemical reactions of the precursors induced by direct exposure to low-energy electrons. Trimethyl(methylcyclopentadienyl)-platinum(IV) (MeCpPtMe3)2,3 is an important precursor for the growth of platinum nanostructures using EBID. In the ideal case, the EBID process would completely remove the organic material, leaving behind pure platinum. Often, though, this is not the case. A recent study of electron-induced chemistry in thin molecular films of MeCpPtMe3 has been performed under UHV conditions using a combination of temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), mass spectrometry (MS), and reflection absorption infrared spectroscopy (RAIRS).4 The results suggested that decomposition at an incident electron energy of 500 eV is far from complete with as many as eight carbon atoms remaining per Pt. RAIRS, on the other hand, hinted toward a complete loss of hydrogen at 500 eV. The present results obtained by high-resolution electron energy loss (HREEL) spectroscopy show that exposure at low electron energies (15 and 30 eV) but also at 500 eV leads to a material that still contains a significant amount of C-H bonds. 2. Experimental Section The experiments were performed using a µ-metal-shielded UHV chamber equipped with a HREEL spectrometer consisting of a rotating cylindrical double pass monochromator and a single-pass electron-energy analyzer.5 The base pressure of the * To whom correspondence should be addresssed. E-mail: swiderek@ uni-bremen.de. † Permanent address: Unite´ de recherche´ Mate´riaux Avance´s et Optronique, Faculte´ des Sciences de Tunis, 1060 Tunis, Tunisia.
system reaches 10-11 mbar through the combined action of an ion pump and a titanium sublimation pump. The HREEL chamber is connected to a sample preparation chamber with a base pressure of about 2 × 10-9 mbar. MeCpPtMe3 was purchased from Acros Organics at a stated purity of 99% and degassed by repeated freeze-pump-thaw cycles. The experiments were performed on thin molecular films deposited on a polycrystalline Pt substrate (surface area ∼3.8 cm2) cooled by means of a closed-cycle Helium refrigerator (Leybold Vacuum). A small heater unit was used to vary the temperature of the substrate between ∼20 and 30 K and room temperature. Prior to each deposition, the substrate was cleaned by direct resistive heating to an orange glow. To produce thin films of MeCpPtMe3, the vapor present in the reservoir containing the solid compound at room temperature was expanded into a small calibrated volume where the absolute pressure is measured with a capacitance manometer. For most experiments a calibrated amount of the vapor corresponding to a pressure drop of 3 µbar was leaked via a stainless steel capillary whose end is located just in front of the metal substrate. By comparison with previous monolayer calibrations for benzene5 and cyclopropane,6 it was deduced that this amount of vapor produces films with an average thickness of at least 10 monolayers. The samples were exposed to electrons at incident energies (E0) of 15, 30, or 500 eV using a commercial flood gun (estimated resolution of 0.5-1 eV) located in the sample preparation chamber and delivering at these E0 currents of the order of a few tens of µA/cm2 as measured at the sample. After deposition and after each exposure the cryostat carrying the sample was transferred to the HREEL chamber for data acquisition. All HREEL spectra were acquired with an incident electron energy of 5.5 eV. The spectrometer was operated with a resolution of 11-13 meV, measured as the fwhm of the elastic peak, for currents transmitted through the samples of the order of 0.2-0.3 nA. The typical acquisition time of a full spectrum was 30 min. Both the incidence and scattering angle were set at 60° from the surface normal. Spectra of the samples annealed to 300 K were actually recorded at this temperature while all
10.1021/jp810834r CCC: $40.75 2009 American Chemical Society Published on Web 07/02/2009
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TABLE 1: Assigment of MeCpPtMe3 Vibrational Bands in the HREEL Spectraa MeCpPtMe3 ∆E, HREELS 384 368 179 153 127
CpPtMe3 b
∆E, IR 386 384 368 177 156 151 130 124
c
∆E, Ramanc 386 383 369 367 176 156 151
assignment to CH3 modesd
ν(C-H), a1 ν(C-H), e1 ν(C-H), e ν(C-H), e δCH3, e δCH3, a1 δCH3, e δCH(|), e1
100 103 73
assignment to Cp modesd
98 73 69
33 31
73 69 33 30
δCH(⊥), a1 δCH(⊥), e1 ν(Pt-C), e ν(Pt-C), a1 ν(Pt-Cp), a1 δPtC3, a1?
a Excitation energies given in meV. b This work. c Solid CpPtMe3, from ref 7. Only bands that correlate with signals in the HREEL spectra are listed. d Assignments as given in ref 7 and including the modes concerning movement of the ligand with respect to the Pt.
Figure 1. HREEL spectra of a multilayer film of MeCpPtMe3 before and after increasing electron exposure at E0 ) 15 eV. The arrows in the upper graphs indicate the maxima of the two distinct CH stretching bands before exposure.
other spectra were aquired at 20 K. In addition, a reference transmission infrared (IR) spectrum of MeCpPtMe3 in a KBr pellet was acquired at room temperature using a Nicolet Avatar 370 spectrometer. 3. Results and Discussion 3.1. Assignment of the HREEL Spectrum. The vibrational bands observed in the HREEL spectra of MeCpPtMe3 are listed in Table 1 and were assigned by comparison with a previous study on the vibrational spectra of the related compound trimethyl(cyclopentadienyl)-platinum(IV) (CpPtMe3).7 Figure 1 also includes these assignments. From a comparison with other methylcyclopentadienyl complexes,8 the vibrations of the additional CH3 group attached to the cyclopentadienyl (Cp) ring in MeCpPtMe3 are expected to have similar frequencies as the CH3 groups attached to Pt. A distinction between the two different CH3 groups is thus not attempted here. Several bands in the HREEL spectrum can be assigned to specific subunits of MeCpPtMe3. In the CH stretching region,
vibrations of the CH3 groups (368 meV) can be clearly distinguished from those of the Cp ring (384 meV). Equally, the strong band at 103 meV relates to the out-of-plane CH deformation of the Cp ring and the signal at 73 meV to the Pt-C stretching vibrations of the CH3 groups. This will be important for the discussion of the chemical modifications occurring under electron exposure. Most importantly, the CH stretching bands of the Cp ring carry an important intensity in the HREEL spectrum. This is in striking contrast to the findings from the previous RAIRS study which could not detect these bands,4 most likely due to the very low IR activity of the Cp CH stretching vibrations as observed in the reference IR spectrum and reported in previous literature.7 3.2. Changes within the HREEL Spectrum upon Electron Exposure. Upon exposure to electrons at 15, 30, and 500 eV, the HREEL spectra of MeCpPtMe3 films are modified (Figures 1-3). It must be noted here and for the experiments described in the following section that the initial electron exposure interval often leads to a decrease or increase of the overall HREEL spectral intensity. This is most likely due to a rapid evolution of a certain charging equilibrium at the surface during the early stages of exposure that lead to changes in focusing properties of the HREEL spectrometer. The absolute HREEL spectral intensities are thus not a reliable measure of the quantity of a material under the present conditions while relative intensities of the observed signals may well serve for an analysis of the modifications occurring under electron exposure.9 The changes in the HREEL spectra of MeCpPtMe 3 films are very similar for the three investigated values of E0 at which exposure was performed. The two most obvious effects of electron exposure are the decrease of the Pt-C stretching vibration involving the methyl groups at 73 meV and the loss of intensity of the out-of-plane CH deformations of the Cp ring at 103 meV. Within the CH stretching region, the Cp CH stretching band appears to slightly shift and therefore merge with the methyl CH stretching signal, which, on the other hand, loses intensity relative to the Cp CH stretch but remains at a constant position. The loss of methyl Pt-C stretching intensity is in accord with the conclusion from the previous UHV study on electron-induced reactions in MeCpPtMe3 at E0 ) 500 eV. XPS results pointed toward a loss of one methyl group under electron exposure leaving behind a material with eight C atoms per Pt atom.4 On the other hand, the changes in the CH stretching region of the present HREEL spectra suggest that
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Figure 2. HREEL spectra of a multilayer film of MeCpPtMe3 before and after increasing electron exposure at E0 ) 30 eV. The uppermost spectrum was obtained after the final exposure and an additional annealing cycle to 100 K to evaporate possible low-mass reaction products. The arrows in the upper graphs indicate the maxima of the two distinct CH stretching bands before exposure.
Figure 3. HREEL spectra of a multilayer film of MeCpPtMe3 before and after increasing electron exposure at E0 ) 500 eV. The arrows in the upper graphs indicate the maxima of the two distinct CH stretching bands before exposure, as well as a shoulder at lower energy loss appearing after the final exposure interval.
the Cp ring is also affected by electron exposure because the Cp CH stretching band gradually fades into a shoulder on the high-energy side of the methyl CH stretching band. The previous UHV study4 has also monitored desorption of methane from adsorbed MeCpPtMe3 under 500 eV electron exposure at 195 K. This infers that methyl radicals produced by Pt-C bond cleavage attack adjacent material and abstract an H atom. If this abstraction takes place in the Cp ring, the Cp CH vibrations would also be affected as observed in the present HREEL spectra. A shoulder appearing at 359 meV after an exposure of 96 000 µC at E0 ) 500 eV may also hint toward formation of -CH2- groups10 produced by abstraction of an H atom from
Hedhili et al. one of the methyl groups and recombination of the resulting radical with another radical. The assumption that electron exposure preferentially removes a CH3 ligand from the complex is reasonable because this ligand shares only one electron pair with the central metal atom in the form of a σ-bond. Removal of one electron from this bond by electron impact ionization or attachment of an electron into the corresponding antibonding σ* orbital thus weakens the Pt-C bond sufficiently to favor dissociation. The η5-bonded Cp ring, on the other hand shares up to six electrons with the central metal atom. Here, removal of one electron from a bonding orbital or addition of one electron to an antibonding orbital will only weaken the bonding Pt-Cp interaction to a much lower extent leading to a smaller dissociation probability. As the present exposure experiments were performed at a temperature at which methane can be condensed as multilayer film,11 methane, or alternatively ethane resulting from recombination of methyl radicals formed under electron exposure, could be retained in the film. In order to verify if such trapped products contribute to the observed changes within the HREEL spectra, the substrate was annealed to 100 K after an electron exposure of 16 000 µC at 30 eV (Figure 2, uppermost spectrum). As this does not lead to clearly noticeable changes, we conclude that the amount of small products trapped within the MeCpPtMe3 film upon electron exposure is not significant. The previous RAIRS experiments used to monitor the modification of a MeCpPtMe3 film upon electron exposure at 500 eV and 195 K revealed a complete loss of the methyl CH stretching bands.4 As will be discussed in more detail in Section 3.4, the electron exposure per surface area in the previous study was roughly comparable to the present experiments. It is therefore striking that a strong overall decrease of the CH stretching intensity is not observed in the HREEL spectra. Part of this effect might be attributed to the low sensitivity of RAIRS toward the Cp CH stretching vibrations while the methyl CH stretching bands were clearly visible there.4 On the other hand, the difference in processing temperature in the two experiments may also be responsible for these conflicting results. This is addressed in the following section. 3.3. Production of Nonvolatile Species. Additional annealing experiments were performed to verify if electron exposure of MeCpPtMe3 films in fact produces a material that does not evaporate at room temperature. To exclude a dominant role of catalytic effects in the production of this material, the first experiment aimed at monitoring the desorption of nonexposed MeCpPtMe3 upon increase of the sample temperature. Figure 4 shows that the Pt substrate prior to deposition of a MeCpPtMe3 film was essentially free of organic material. Annealing of a freshly deposited film to 220 K only leads to a slight reduction of the original intensity, while warming the substrate to room temperature removes virtually all of the adsorbed material. An electron exposure of 32 000 µC at 30 eV and subsequent annealing to room temperature, however, clearly produces a product that is not volatile at room temperature (Figure 5). The top of Figure 5 shows that even after removal of the potentially remaining MeCpPtMe3 by annealing to 300 K, the resulting residue contains a noticeable amount of CH bonds as seen from the CH stretching vibrations while again the Cp CH stretching bands are not clearly resolved from the methyl CH stretches anymore. Furthermore, apart from a difference in absolute intensity, the HREEL spectrum of this residue is very similar to the spectrum of MeCpPtMe3 after an electron exposure of 32 000 µC at 30 eV (Figure 5, middle). The intensities may differ because the spectrum of the exposed film was recorded
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Figure 6. HREEL spectra obtained after dosing 2 mbar of MeCpPtMe3 at room temperature and after dosing 1 mbar of MeCpPtMe3 at room temperature under electron exposure at E0 ) 30 eV (∼11 000 µC).
Figure 4. HREEL spectra of a multilayer film of MeCpPtMe3 deposited at 20 K and annealed to 220 and 300 K. The lowest spectrum was obtained before film deposition.
Figure 5. HREEL spectra of a multilayer film of MeCpPtMe3 deposited at 20 K and subsequently exposed to electrons at E0 ) 30 eV (16 000 µC) and annealed to 300 K.
at 20-30 K while the spectrum of the sample after annealing was acquired at 300 K. The otherwise close similarity between the band positions in the spectra of the product and the original MeCpPtMe3 film suggest that a chemically related material is formed. If radical sites are formed on MeCpPtMe3 molecules by attack of a CH3 fragment and consequent H abstraction as suggested by the previously observed formation of CH44 the ligands of two or more complexes carrying radical sites may recombine to yield a heavier product. The observed nonvolatile material may thus be a losely cross-linked network of organic ligands still containing the Pt atoms. This interpretation is supported by previous TDS results also obtained at energies between 40 and 180 eV showing that a new desorption peak in the range between 273 and 373 K and with a mass of 289 amu appears after electron exposure.12 This mass is consistent with a fragment containing, besides Pt and carbon, also H atoms. As a final test that electron exposure and not a catalytic effect is responsible for the observed chemical modifications in the MeCpPtMe3 films, it was also attempted to deposit the compound at room temperature (Figure 6). In the first experi-
ment, 2 µbar of MeCpPtMe3 were leaked onto the substrate at room temperature but with a positive bias so that electrons from the flood gun could not reach the surface. The HREEL spectrum recorded immediately afterward with the sample still at room temperature shows only negligible amounts of organic material in line with the annealing experiment shown in Figure 4. Dosing 1 µbar of the vapor, while at the same time exposing the surface to electrons at E0 ) 30 eV to reach an overall accumulated charge of ∼11 000 µC, leads to a small amount of residue that again clearly shows the signature of CH stretching vibrations. Electron exposure is thus clearly responsible for the effects observed in this study. The finding that CH stretching bands are still present when the electron exposure proceeds at room temperature, on the other hand, suggests that the temperature does not significantly influence the outcome of the reaction. 3.4. Comparison of Electron Exposures with Previous Results. While the fluctuations of the absolute HREEL spectral intensity upon electron exposure observed here do not allow for directly deducing a cross section for the decomposition of MeCpPtMe3 in a similar way as done previosuly,4,13 a qualitative comparison with the previous XPS and RAIRS results4 shows that the electron exposure applied to the MeCpPtMe3 film in the present HREEL study is of similar order of magnitude (for E0 ) 15 eV) or even larger (for E0 ) 30 and 500 eV). For example, in RAIRS the CH stretching band has completely vanished between electron exposures of 12 000-30 000 µC onto a 5.7 cm2 sample area while in the XPS experiment complete conversion of the Pt to a reduced form was obtained at roughly 10000 µC onto a 1.8 cm2 surface.4 This translates into 2100-5250 and 5500 µC/cm2, respectively, for completion of the reaction at E0 ) 500 eV. Similarly, an exposure of ∼20 000 µC (11 000 µC/cm2) in the same setup as used for XPS was required until electron-stimulated desorption of CH4 as observed by MS had ceased.4 Comparison of Figures 2 and 5 shows that the most rapid changes to the HREEL spectrum at E0 ) 30 eV occur at exposures up to 16 000 µC. Taking into account the surface area of 3.8 cm2, this translates to 4200 µC/cm2 which is comparable to the exposures applied previously. The fast change observed here and leading to the formation of a nonvolatile cross-linked material (see Section 3.3) thus correlates with the reduction of Pt and concurrent removal of one carbon atom as well as the completion of CH4 desorption.4 Still, while the same exposure at E0 ) 500 eV also produces a similar change to the HREEL spectrum, Figure 3 shows that slower modifications occur over a wider exposure range. This must relate to further
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chemical modifications of the organic component of the initially formed cross-linked material the exact nature of which can not be deduced with certainty here. In contrast to the conclusion drawn previously,4 the present results show that the processes related with transformation of MeCpPtMe3 films under electron exposure are complex and proceed on different time scales. The combination of as many methods as possible is required to obtain comprehensive information on the reactions. It must be noted that in the present experiment the exact exposure at E0 ) 500 eV is more difficult to quantify than at lower E0 because the electron beam produced by the flood gun is less divergent at higher E0. Measurements of the current density as function of position of a small probe with respect to the center of the beam have been performed in a separate test setup. From these measurements we conclude that the relative current density delivered to the substrate used in the HREEL experiments and thus the exposures per square centimeter drops from 100% at the center to roughly 50% at the edges. In contrast to the experiments at lower E0, the values of the exposure reported in Figure 3 thus yield only a current density that is averaged over the sample area. An uncertainty in the actual exposure per square centimeter represented by the HREEL spectra results from the fact that the HREEL beam position on the sample is not exactly known although from visual inspection of the setup it should be approximately centered. The actual exposure per unit surface area leading to the observed changes at E0 ) 500 eV is thus probably even higher than deduced from the total current on the sample. In any case, the electron exposures applied here at E0 ) 500 eV clearly exceed those applied in the previous study.4 In contrast to the conclusion drawn there, CH bonds remain even under such drastic conditions. While a more elaborate experimental procedure involving an internal standard in the HREEL spectra is required to directly deduce cross sections from the decay of the parent compound, the identification of the exposure required to reach a certain progress of the conversion could in future studies be used to obtain at least a rough estimate of the relative cross sections as function of E0. 4. Conclusions The present results obtained by use of HREEL spectroscopy show that electron exposure of multilayer films of MeCpPtMe3
Hedhili et al. at E0 ) 15, 30, and 500 eV at 20 K, as well as at room temperature, leads to a material that does not evaporate at room temperature but that still contains a significant amount of C-H bonds. This is in contrast to previous results obtained at E0 ) 500 eV and 195 K which suggested a complete removal of hydrogen from the film.4 The present study provides evidence that neither the electron energy nor the difference in temperature is responsible for this discrepancy. As the electron exposure applied to the condensed MeCpPtMe3 film in the present experiments is of comparable order of magnitude as was observed previously by XPS and RAIRS,4 very likely, the higher mode selectivity of RAIRS as compared to HREEL spectroscopy explains the different result. Acknowledgment. Support provided by the Cost Action CM0601 “Electron Controlled Chemical Lithography” (ECCL) and the ESF program “Electron Induced Processes At the Molecular Level” (EIPAM) is gratefully acknowledged. The authors also thank C. W. Hagen for valuable discussions. References and Notes (1) Utke, I.; Hoffmann, P.; Melngailis, J. J. Vac. Sci. Technol. B 2008, 26, 1197. (2) Xue, Z.; Strouse, M. J.; Shuh, D. K.; Knobler, C. B.; Kaesz, H. D.; Hicks, R. F.; Williams, R. S. J. Am. Chem. Soc. 1989, 111, 8779. (3) Xue, Z.; Tridandam, H.; Kaesz, H. D.; Hicks, R. F. Chem. Mater. 1992, 4, 162. (4) Wnuk, J. D.; Gorham, J. M.; Rosenberg, S.; van Dorp, W. F.; Madey, T. E.; Hagen, C. W.; Fairbrother, D. H. J. Phys. Chem. C 2009, 113, 2487. (5) Swiderek, P.; Winterling, H. Chem. Phys. 1998, 229, 295. (6) Winterling, H.; Haberkern, H.; Swiderek, P. Phys. Chem. Chem. Phys. 2001, 3, 4592. (7) Hall, J. R.; Smith, B. E. Aust. J. Chem. 1971, 24, 911. (8) Parker, D. J.; Stiddard, M. H. B. J. Chem. Soc. A 1970, 1040. (9) Swiderek, P.; Deschamps, M. C.; Michaud, M.; Sanche, L. J. Phys. Chem. B 2004, 108, 11850. (10) Swiderek, P.; Burean, E. J. Chem. Phys. 2007, 127, 214506. (11) Conley, R. T. Infrared Spectroscopy; Allyn and Bacon, Inc.: Boston, 1972. (12) van Dorp, W. F., Zalkind, S., Yakshinsky, B., Madey, T. E., Hagen, C. W., to be published. (Presented as Poster 07 at the Second international workshop FEBIP, 2008, Thun, Switzerland.) (13) Botman, A.; de Winter, D. A. M.; Mulders, J. J. M. J. Vac. Sci. Technol. B 2008, 26, 2460.
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