Thin polymer films on metal : assessment of a conductivity mechanism via valence band spectra

Journal ofElectron Spectroscopyand RelatedPhenomena, 68 (1994) 541-546 0368-2048/94/$07.00 @ 1994 - Elsevier Science B.V. All rights reserved

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Thin polymer films on metal : assessmentof a conductivitymechauismvia

valencebandspecb

J.J. Pireauxa, Ch. GrCgoirea, J.A. Gardella Jrb, and P.A. Corneliob a Laboratoire Interdisciplinaire de Spectroscopic Electronique LISE FUNDP, 61 rue de Bruxelles, 5000-Namur, Belgium b

State University of New-York at Buffalo, IUCB 110 Parker Hall, Buffalo, NY 14214

When prepared as very thin layers on a conducting substrate, organic and polymer films are generally not prone to the charging effect when studied for example with XPS, HREELS or STM. For Langmuir-Blodgett layers of Arachic Acid, Behenic Acid, and Polyvinylstearate, it is shown that XPS valence band spectra exhibit an increased density of states just below the Fermi level. It is suggested that this effect is due to an electronic interaction with the substrate, allowing for electrical conductivity in the adsorbate layer. 1. INTRODUCTION Thick insulating materials (metal oxides, ceramics, polymers, . ..) inevitably charge up electrostatically when submitted to the bombardment of charged or ionizing particles. This well-known “charging effect”, can be compensated for, and even usefully studied, in many circumstances 111. The experimenters know that, most of the time, when a layer of an insulator is deposited - thin enough - on a conducting substrate, otherwise deleterious charging effect can be minimized if not completely avoided : this is true for thin metal oxide layers, thin organic or polymer films, etc. This property is often used for sample preparation in X-Ray Photoelectron Spectroscopy, or in High Resolution Electron Energy Loss Spectroscopy. Film thickness in the range of a few to tens of nanometers has proved good enough to avoid charging and thick enough to mask signals from the substrate. This property is at the origin of the success of Scanning Tunneling Microscopy, in imaging e.g. some details of proteins which are considered as insulating “molecules”. SSDI 0368-2048(94)02156-T

Different mechanisms have been proposed and studied for this “conduction in or through a thin insulating layer”. For XPS, we will disregard the contribution of photoelectrons coming from the substrate as this mechanism is not relevant for the other cited techniques. This questioning is particularly relevant for STM users and theoreticians. The most frequently cited mechanisms will be briefly recalled hereunder (section 2). The purpose of this contribution is to carefully analyse the XPS valence band spectra of three different LangmuirBlodgett layers of organic and polymer (otherwise insulating) materials. After a decription of the experimental protocol (section 31, we will show that these films repetitively show an increased density of states (DOS> just below the Fermi level (section 4), and conclude with a comparison with recent theoretical calculations of the electronic properties of a polymer-metal interface, and with other evidence from ultra-violet photoelectron spectroscopy (section 5).

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2. CONJMJCTIONMECHANISMS

3. EXPERIMENTAL

In thin insulating layers, even through on an individual molecules deposited otherwise conducting substrate, a tunneling current can be monitored, and a STM image recorded. Theoretical investigations and experiments were aimed to shed light on possible conduction mechanisms, and thus help a better understanding of the physics underlying the imaging process : this is absolutely mandatory in order to assess a relation between an observed image and the physical and/or chemical properties of the imaged target. Different electron transport processes from the surface of the adsorbate to the conducting substrate (or vice versa) - have been proposed to allow STM imaging from poor conductors and insulators. They include band-type conductivity, hopppingbulk conductivity, and surface conductivity; the object is supposed not to contain electronic states to take part in the as in resonant tunneling. The tunneling existence of surface states (intrinsic, extrinsic, defect induced) in the middle of the forbidden energy gap of the insulator is an alternative [Zl. For example, in Schottky barriers or heterojunctions, metal induced gap states (MIGS) have been evidenced 131. Let us mention also that for organic solids, intermolecular molecular interaction is not negligeable; even for these van der Waals solids, intra- and inter-molecular relaxation (electrostatic polarization within the molecule, or of surrounding molecules) accompanies the presence of any charge; hydrogen bonding and interaction with the substrate can modify the molecular packing and the density of occupied states (DOS) [41. All those processes should be considered in XPS, HREELS, STM, etc.

Langmuir-Blodgett films were deposited with a commercial instrument (KSV 2000) on silver foils (thickness : 0,25 mm; 99,999 ‘36 purity from Alfa). These substrates were washed in detergent, rinsed in distilled water, then plasma-oxidized in a glow discharge chamber in air. LB films were deposited immediately after this substrate preparation; one uncovered silver coupon was kept as reference. Amphiphilic molecules, Arachidic Acid (AA, CH3(CH2)13COOH), Behenic acid (BA, CH3(CH2)2OCOOH), and (PVS) from Sigma polyvinylstearate (Chromatographic Reference grade) were dissolved in benzene (1 mg/ml) and spread on a tri-distilled water subphase. After compression into a monomolecular layer (surface pressure : 25 x 10-a N/m), the films was transferred to the polycrystalline silver foil. In general, the transfer coefficient (deposition ratio) was 1.0 f 0.05. The XPS data have been recorded on a SSX-206 spectrometer, equipped with a monochromatic and focused (spot size : 300 Nominal resolution pm) AlKa source. setting was 0.9 eV. During the analysis, the operating pressure was 1 x low9 Torr. For the long accumulation (one to six hours) required for recording valence band spectra with good signal-to-noise ratio, the floodgun was turned on, but idled to zero volt energy. The spectrometer has been energy calibrated by positioning the Ag 3d5/2 line at 367.9 eV. A previous study confirmed first the correct stoichiomety of the prepared layers, their homogeneity on the substrate on the 300 pm scale, and some ageing in air or in vacuum or under X-ray exposure. The polymerized LB layers of polyvinylstearate are more homogeneous and stable systems r51. In this work, the very same types of films were prepared and studied : more precisely, these were AA (3 and 5 layers), BA (5 layers) and PVS samples (2 layers).

PROTOCOL

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XPS analyses were specifically aimed at recording valence band spectra. Core levels were also studied but only to assess the quality of the films and to ascertain the binding energy calibration of the spectra.

Further comparison of the spectra requires some simple data treatment. In order to selectively retrieve the AA valence signal from Fig lb, difference spectra are calculated, according to the following protocol :

4. LB LAYERS : VALENCE BAND SPECTRA

l- define the Fermi level and adjust the binding energy scale (if necessary) 2- adjust the intensity of the overall background al the right hand side of the Fermi level 3- scale the intensity of the “d band” region 4- subtract the reference silver spectrum from the LB sample one

Figure 1 reports the valence band (VB) spectra of the clean silver substrate (as reference) and of the sample of 5 LB layers of arachidic acid. The spectra - whose intensities have been normalized for the Indeed the - are very similar. presentation photoionization cross section of Ag electrons is quite high, by comparison with these factors for C2s and Qp electrons; therefore, the fingerprint of the AA sample is barely visible (Czs valence band) on the left hand side of the prominent silver signal. Ag valence band is well known, schematically composed of the intense spin-orbit doublet (4 d electrons), and of the low intense shoulder (5 s electrons) defining the Fermi level IS].

Different aids were used to ascertain the procedure, as shown on figure 2 : (a) background and Fermi edge do adjust perfectly, (b) second derivative spectra of the reference silver (line) and of the LB film on its substrate (dots) do adjust perfectly at the Fermi edge, and for the most intense (Ag 4 d character) peaks.

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1

Fermi edge

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1. XPS valence

band spectra (raw data but with normalized intensity, uncalibrated binding energy scale) of the as-prepared silver coupon (a), of the 5 LB layers of Arachidic Acid deposited onto the silver substrate (b).

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2. Illustration of two tests used to validate the data treatment on the silver (solid line) and LB film (dots) valence band spectra : superimposition of the Fermi edges (a); comparison of the second derivative spectra (b)

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The as-produced difference spectrum (Figure 3) s upposedly represents the fingerprinting valence band of the 5 layers of arachidic acid. One should immediately note weak resemblance of this VB with the one of bulk AA, or simply of polyethylene (PE) 171 : one can distinguish signal from the C2p electrons around 7 eV, and the band with C2s electrons emerging above 12 eV; this 5 LB system is almost only a hydrocarbon contamination layer on top of a silver substrate. However, Figure 3 clearly shows, besides the well defined zero intensity level (above the Fermi edge), regions of positive and negative intensities : electron densities have been brought by the hydrocarbon layer, but also transferred to and from the silver substrate. Positive/negative features are known to show up on computer subtracted spectra of adsorbed species; they are due to scattered electrons from the substrate causing a decrease at the onset of a substrate band and an increase at higher energy.

This effect may also explain part of features abserved at - 4 eV and above, where the substrate signal arises. But one particularly notes, very close to the Fermi level, a low intense positive electron density (arrow on fig 31, where the empty energy gap of this polymer-like system should extend : indeed, the gap of polyethylene is of about 8.8 eV [71. On the right hand side of the Fermi level where no genuine signal from the sample is expected, the noise is small, due to the spectrometer (essentially the detector and counting electronics); on the left hand side on the contrary, this “noise” is more intense, significantly “positive”, thus due to real photoelectrons. This “positive” increase density of occupied states, just at the edge of the Fermi level where no genuine signal from the adsorbate is expected, might account for the “conducting” behaviour of this thin organic film. In order to substantiate this rather small and noise-like effect,and to try to

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Figure 3. XPS valence band spectrum of the BLB-AA sample, after subtraction of the silver substrate contribution. Reported are a few typical information : the Fermi leel; the energy gap of polyethylene (8.8 eV ref 7); the (C2p-Hls) and the C2s bands.

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rule out an experimental artefact e.g. in the data treatment, similar spectra were recorded from other thin organic layers prepared according to the same protocol. Figure 4 reports these spectra for 3AA, 5AA, 5BA (2 samples), and 2PVS systems. As consistently a similar - low intense, noisy but “measurable” - increased density of states just below the Fermi level (dark shade on the spectra) is measured, the effect is thought to be significa_nt.

'2

6 a Binding Energy (eV)

Figure

4. XPS valence band spectra recorded for five different LB systems, after substraction of the silver substrate contribution : (a) 5AA, (b) 3AA, (c) and Cd) 5BA - two samples, (e) 2PVS layers.

5.DOSATTHEFERMILEYRL An increased DOS close to the Fermi level could originate from an interaction between the polymer-like layer and the conducting substrate : either formation of

chemical bonds results in a modifed DOS, or the interface is the source of defects, injecting surface (in this case interface) film, These states in the organic possibilities have already been mentioned in section 2. Recently, a theoretical study of the electronic structure of molecule-on-copper cluster ensembles, based on ab initio Hartree-Fock calculations, showed significant intermixing of the substrateadsorbate orbitals, and an intense DOS just at the Fermi level [$I. For self-assembled alkane-thiols adsorbed on a gold substrate, and particularly for carboxylic acid-terminated thiols, He-I UV photoemission spectra very clearly showed new DOS structures at the Fermi level related to chemical bond formation at the interface; in these cases, radiation wavelength via the photoionization cross section effect amplified the adsorbate bands with p symmetry, and lone-pair orbitals 191. Polymers and van der Waals organic solids are far from being perfect crystals; intrinsically they contain many defects (and most of the time impurities and/or additives, for polymers). However, this rarely reflects in valence band spectra. For polyethylene 171, but also for other polymers 1101 photoemission data are in agreement with the location of the Fermi level in the middle of the band gap. Therefore, impurities, defects, and surface states are probably not evidenced. To conclude, the increased DOS at the Fermi level of the thin organic layer substrate ensemble, that is probably responsible for the conducting behaviour of the thin film, is suggested to result from an electronic interaction between the adsorbate and the substrate. More precisely, the carboxylic groups molecular orbitals and the silver s (and d) electrons are probably involved: this interaction yields a polymer-metal adsorption complex with new electronic properties.

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ACKNOWLEDGEMENTS : This work has been supported by a NATO grant and a du Pont de Nemours donation (Ch. GrBgoire).

REFERENCES : Ill

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