Transition metal containing plasma polymersN. Morosoff, R. Haque, S. D. Clymer, and A. L. Crumbliss Citation: Journal of Vacuum Science & Technology A 3, 2098 (1985); doi: 10.1116/1.572931 View online: http://dx.doi.org/10.1116/1.572931 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/3/6?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Comparative Study of Catalysts containing Transition Metals in Production of Carbon Nanotubes AIP Conf. Proc. 633, 190 (2002); 10.1063/1.1514103 Dielectric breakdown of polymer films containing metal clusters J. Appl. Phys. 64, 336 (1988); 10.1063/1.341433 Organic films containing metal prepared by plasma polymerization J. Vac. Sci. Technol. A 5, 1828 (1987); 10.1116/1.574508 Summary Abstract: Properties of hydrocarbon polymer films containing metal clusters J. Vac. Sci. Technol. A 5, 1913 (1987); 10.1116/1.574489 A Metal Tube Plasma Container for Argon Ion Lasers Rev. Sci. Instrum. 43, 1216 (1972); 10.1063/1.1685887

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Transition metal containing plasma polymersa)

N. Morosoff and R. Haque Research Triangle Institute, Research Triangle Park, North Carolina 27709

S. D. Clymer and A. L. Crumbliss Chemistry Department, Duke University, Durham, North Carolina 27706

(Received 17 July 1985; accepted 21 July 1985)

The plasma polymerization of the C2 hydrocarbons in the presence of Fe(CO)5 vapor can lead to the formation of soluble cluster complexes (I) and/or high oxidation state iron both in the form of the oxide (II) and associated with the carboxylate or ,B-diketonate ion (III). Analysis of the electron spectroscopy for chemical analysis (ESCA) spectra obtained yields information regarding the nature of I on the one hand, and regarding the prevalence of II relative to III, on the other. The relative yield of I with respect to (II + III) can be controlled, at will, by variation of W / F. This permits control of the coating's morphology (continuity) as shown by the electrodeposition of Prussian blue on smooth surfaces coated with plasma polymer containing I + II + III.

I. INTRODUCTION

The incorporation of metals in plasma polymers has been studied by a number of authors, including some presenting papers at this session. Our emphasis 1-3 has been on the in­corporation of transition metals in plasma polymer matrices, and on the elucidation of the types of chemical structures that can be produced using a plasma process. In our studies we have introduced transition metal into the plasma in the form ofvolatile organometallic compounds. We have shown that depending on the power to monomer(s) flow rate (W / F) ratio employed, either transition metal cluster complexes (with a chemical structure similar to that of the monomer) or relatively simple inorganic compounds may be formed. In the latter case, obtained at the higher values of W / F, the metal is in a high oxidation state and may, in part, be ionical­ly bound to functional groups attached to the plasma poly­mer backbone. Also the transition metal containing mon­omer may be essentially considered as a source of bare metal atoms, in contrast to the low W / F case where portions of the monomer's chemical structure are preserved. In this paper we present additional facts relating to the control of the chemical structure of the plasma polymer formed from Fe(CO)s and acetylene, and an example of the utilization of this control for electrochemical experiments. It may be not­ed that infrared evidence suggests that the nature of the tran­sition metal product is not critically dependent on the C2 hydrocarbon (C2 H2 or C2 H6 ) employed. 3

II. RESULTS

A. Nature of plasma-produced product from ESCA spectra

The nature of the product produced in a plasma into which Fe(CO)s and C2 hydrocarbon are introduced has been extensively studied by a variety of techniques. 3 It has been shown that the high W / F (50 W /0.1 sccm) product contains iron oxide and iron bound to carboxylate or ,B-diketonate groups, while infrared spectra indicate thatthe low W / F (50 W /0.5 sccm) product contains iron complexed to carbon monoxide, as well. Additional information relating to the nature of this complex may be obtained from ESCA spectra.

2098 J. Vac. Sci. Technol. A 3 (6), Nov/Dec 1985

The C Is and 0 Is spectra obtained from high W / F and low W /Fproduct [Fe(CO)s + C2H6 monomer feed] are shown in Fig. 1. These are fitted with peaks at 284.6, 286, and at 288 eV for the C Is spectrum assigned to aliphatic carbon (284.6) and to carbons bound to oxygen. For the 0 Is spectrum, the corresponding peaks are at 530.2 and 532 eV, assigned to iron oxide and to oxygen bound to carbon, respectively. Consideration of the intensity ratios of these peaks has aided in the interpretation of the nature of the plasma product. For example, since the peaks at 286 and 288 eV can be assigned to carbon bound to oxygen and that at 532 to oxygen bound to carbon, the ratio of oxygen atoms obtained from the area of the 532 eV peak to carbon from the 286 and 288 eV peaks should be between 1 and 2. For the high W / Fsamples this is the case; in fact, the ratio is close to unity (0.8 to 1.2). One would expect the same result for the low W / F plasma pro­duct, particularly since one would tend to associate the somewhat more prominent peak at 288.5 eV with CO com­plexed to iron.

Surprisingly, the OS32 eV to C288 eV ratio is found to be 2.4. This quantitatively unreasonable result, combined with the absence ofa peak at 533.8 eV (as would be expected for CO complexed to a transition metal), suggests that CO species in the transition metal complexes are not the sole ligands in the iron clusters. For example, the spectrum at the bottom of Fig. 1 is that reported for carbon in the compound (PPNlz+ [Fe4 QCOh2]-2 with the high binding energy peak assigned to the carbido carbon (FeC).4 For the same compound the carbonyl C Is peak was observed at 284.3 eV. A similar as­signment is made for the compounds [Fes QCO)IS] and [Fe4 (CH)(CO)12 H], i.e., carbido carbon at 287.7 eV, the CO andCH C Is peak at 284.6 eVandO Is peak at 532 eV.4The ratio of the OS32 eV to C284.6 eV species in our low W / F pro­duct was observed to be 0.7. These observations suggest that for the soluble iron cluster complex the oxygen contributing to the 532 eV portion of the spectrum is attached to a carbon contributing to 284.6 eV portion of C Is, not to the 288 eV portion. This is consistent with the transition metal complex being an iron carbido carbonyl or an iron carbonyl cluster containing alkyl groups sigma bonded to the iron, as well. s

The use of atomic ratios, as indicated by portions of the

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2099 Moro8off et al. : Transition metal

HighW/F 5

LowW/F 4

(b)

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ESCA spectra, can also be used to indicate the nature of the counterion to the high oxidation state iron in the high WI F product as well. This can be demonstrated with the aid of Fig. 2. As stated above the O/C ratio shown in the figure remains close to 1. However, the OS3o.2/Fe ratio decreases with increasing distance from the monomer inlet, while the C288 eV /Fe ratio increases correspondingly. The infrared spectrum indicates that both iron oxide and iron bound to carboxylate or p-diketonate groups are present in the plas­ma polymer. The results shown in Fig. 2 indicate that iron in iron oxide becomes less prominent as the distance from the monomer inlet increases. The corresponding growth in pre­dominance of binding of iron to carboxylate or p-diketonate groups is associated with an excess of CO relative to iron (also indicated in the infrared spectrum by a peak at 1700 cm - \ for the plasma polymer deposited at 11 cm from the monomer inlet but not at other positions) and with a low concentration of iron in the plasma polymer.

The sensitivity of the nature of the plasma polymer to W IF suggests that the relative amount of cluster complex and of "high W IF" product can be controlled by W IF. This is indeed found to be the case, on the basis of infrared spec­troscopic studies. The ratio of the intensity of the peak at 1550 cm - \ (carboxylate or p-diketonate) to that at 2000 cm -\ (CO bonded to soluble iron cluster complex) is shown in Fig. 3 as a function of total monomer flow rate [C2 H2 + Fe(CO)s monomerina 1:1 Mratio, 50Wrfpower, 20--60 mTorr pressure]. As expected this decreases with in­creasing flow rate until at a flow rate of 0.6 sccm the infrared spectrum obtained shows no absorbance at 1550 cm - \ (for the material deposited directly underneath the monomer in-

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4

3

2

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FIG. I. ESCA spectra obtained for C Is and 0 Is for high W IFandlow W IFproducts. (Bot· tom) C Is spectrum for carbonyl, aryl, and carbido (FeC), carbon in (PPN)t [Fe. C(CO)12 1-2 (Ref. 4).

OC@288

J Fe

OO@530.2 Fe

O@532 <> C @ 286 + 288

0---- __ ,

"-" - "---.--..r... --~- ~

~- "­"-'8

o 7 11

Distance from Monomer Inlet (cm)

FIG. 2. Partial elemental ratios obtained from ESCA spectra for high WI F plasma products as a function of distance (cm) from the monomer inlet. Partial elemental ratios were obtained from areas of peaks curve fitted to ESCA spectra.

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2100 Morosoff et al : Transition metal

o 0.2 0.4 06 O.B

Flow Rale (seem)

FIG. 3. Ratio of intensities of IR peaks at 1550 cm - I (carboxylate or dike­tonate) and 2000 cm -1 (soluble CO complexed to iron) as a function of Row rate (sccm). Plasma conditions were: molar feed rate ratio for C2 H2 and Fe(CO)" 1:1, 2~ mTorr plasma pressure, 50 W rf power. (e = 0 cm from monomer inlet; • = 4 cm from monomer inlet).

let); it is dominated by the 2000 cm -I peak. Thus it is possi­ble to deposit a product consisting almost entirely of iron cluster complex.

B. Electrochemical studies of mixed valence hexacyanoferrate compounds bound to iron containing plasma polymers

We have demonstrated that Prussian blue can be bound to rough-surfaced electrodes, previously coated with iron con­taining plasma polymer.6-9 The presence of the plasma depo­sit is essential for the formation of an adherent Prussian blue coating; the result is a surface-modified electrode consisting of Pruss ian blue with unique properties. Recently, we have further demonstrated that this type of surface modification can be extended to the Prussian blue analog, ruthenium hex­acyanoruthenate, which also binds to a rough-surfaced car­bon electrode previously exposed to an iron-containing plas­ma. lo

The initial motivation for experiments relating to the elec­trochemistry ofhexacyanoferrate, using electrodes on which iron-containing plasma polymer had been coated, was to de­termine if the iron sites within the bulk of the plasma poly­mer were capable of binding to hexacyanoferrate ion. It was found that such binding was observed only if the iron-con­taining plasma polymer was deposited on a rough surfaced electrode; no cyclic voltammetric signal was observed if a smooth-surfaced platinum wire electrode, previously coated with iron-containing plasma polymer, was subjected to cy­clic voltammetric scans in a solution of hexacyanoferrate ions. For such a signal to be observed, in the latter case, three conditions need to be satisfied: (1) the hexacyanoferrate ion should penetrate the plasma polymer layer and bind to iron sites within the plasma polymer layer, (2) a mechanism must exist for transfer of charge from the electrode to the iron atoms in the hexacyanoferrate ions so that oxidation (or re­duction) may occur, (3) since the charge on the hexacyano­ferrate atom changes from the (II) to the (III) valence state, the plasma polymer must also allow the transport of

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counterions in and out, assuming that conditions 1 and 2 are satisfied, in order for the redox reaction to occur.

Satisfaction of conditions 1 and 3 is clearly related to the ability of the plasma polymer to swell in the solvent in which electrochemical measurements are carried out. Satisfaction of condition 2 is indirectly related to such a swelling capabili­ty, in as much as electron conduction in surface modified electrodes is often observed to occur via an electron hopping mechanism which obtains if oxidizable groups are present in the surface modifying layer directly adjacent to each other. The results described thus far therefore suggest that the iron containing plasma polymer layers we have made so far are insufficiently permeable or porous to allow redox reactions to occur by hexacyanofertate ion bound to iron within the bulk of the plasma polymer. On the other hand the results obtained on rough surfaced electrodes, where it is expected that only a discontinuous coat of iron containing plasma polymer is effected, suggest that binding ofhexacyanoferrate ion does occur at the surface of the plasma polymer, that such binding sites serve as nuclei for the growth of Prussian blue and that redox reactions become possible because of direct contact of such Prussian blue crystals with uncoated portions of the electrode.

The results shown in Fig. 3 suggest that the permeability or porosity of iron containing plasma polymers can be con­trolled. We have therefore deposited a plasma polymer using an [Fe(CO)s + acetylene] monomer mixture on an Sn02

electrode surface under low W / F conditions and subse­quently leached out the soluble cluster complex. A plasma polymer coating remains adherent to the electrode, which in hexacyanoferrate solution yields a cyclic voltammogram in­dicating penetration of the plasma polymer coating by hexa­cyanoferrate ion in water and subsequent growth of Pruss ian blue. Scanning electron microscopy has shown, however, that this is due, at least in part, to the presence of holes 5-20 Ii in diameter in the plasma polymer coating. Weare cur­rently studying the degree to which this morphology can be controlled and reproduced.

III. CONCLUSIONS

Two types of products can be formed in the plasma poly­merization of Fe(CO)s + C2 hydrocarbons. These are char­acterized by the oxidation state of the iron and are (a) a solu­ble cluster complex with iron in a low oxidation state and (b) an insoluble product containing iron oxide and iron bound to carboxylate and/or diketonate groups.

The nature of the cluster complex can be determined by analysis of ESCA spectra. These are compatible with the presence of iron carbido carbonyls or iron carbonyls in which alkyl groups are sigma bonded to the iron.

The relative amount of iron deposited in the form of iron oxide in the high W / Fproduct is found to vary with deposi­tion site in the reactor and/or the proportion of iron in the plasma polymer. The amount of iron bound to carboxylate and/or diketonate groups (relative to that present as iron oxide) increases with increasing distance of deposition site from the monomer inlet corresponding to increased dilution of iron by plasma polymer.

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2101 Morosoff et al. : Transition metal

The ratio of soluble cluster complex to insoluble product can be varied, at will, by variation of W / F.

This variation of the relative amount of soluble product has been used to vary the gross penetrability of such a coat­ing by hexacyanoferrate ion in water.

ACKNOWLEDGMENT

This work was made possible by a grant from the National Science Foundation.

a) This paper was presented at the 12th International Conference on Metal­lurgical Coatings, Los Angeles, 1985. Because the authors were unable to meet the Editor's acceptance deadline, it could not be included in the Conference Proceedings.

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