Synthetic Metals, 21 (1987) 57 - 61 57

UHV-COMPATIBLE PROCESSING OF CONDUCTING POLYMERS: POLYTHIOPHENE

W. R. SALANECK, C. R. WU and J. O. NILSSON

IFM, Link6ping University, S-581 83 Link6ping (Sweden)

J. L. BRI~DAS*

Laboratoire de Chimie, Thdorique Appliqude, Centre de Recherches sur les Matdriaux Avancds, Facultds Universitaires, Notre Dame de la Paix, Rue de Bruxelles, 61, B-5000 Namur (Belgium)

Abstract

The motivation for the study of the underlying physics and chemistry of the 'dry' processing of electrically-conducting organic polymers is discussed. Some details of the study of the radiation polymerization of thiophene under ultra-high vacuum (UHV) conditions are then presented.

Electronic circuit processing in the semiconductor industry today ex- hibits a trend toward 'dry' processing, i.e., toward non-wet chemical and often vacuum processing [1]. Electrically-conducting organic polymers (CPs) are also now prepared mainly by 'wet' techniques: catalytic chemistry, electro- chemistry, and chemical precipitation methods [2]. Although acknowledging that the 'best' applications of conducting polymers are yet to be discovered, we propose that it might be desirable to incorporate CP materials into semicon- ductor technology to form what might be termed 'integrated electronics'. In this sense, CP thin films might be considered, for example, as the active sensor components in an integrated (bio)gas sensor with built-in logic on a (V)LSI chip. One can also envisage other applications where the 'dry' processing of CPs might be desirable; perhaps in applications requiring the coating of irregularly-shaped objects for RFI shielding.

We propose that vacuum processing is a potentially attractive method for preparing thin films of CPs, especially for potential (V)LSI applications. There are (at least) three possible categories. First, appropriate monomer molecules can be deposited on a cooled substrate, and subsequently polymerized in s i tu

via radiation polymerization (RP) [3, 4]. Types of radiation that can be consid- ered include electrons, photons (i.r., u.v., soft X-rays, etc.), ions and others. Secondly, radiation treatment of otherwise insulating polymers can be used to modify their (surface) chemical structure to form 'conducting' polymers

*Chercheur Qualifi~ of the Belgian National Fund for Scientific Research (FRNS).

0379-6779/87/$3.50 ~(~: Elsevier Sequoia/Printed in The Netherlands

58

(which can then be doped) using all types of radiation (as above). In both cases, radiation treatment would be followed by exposure to gases of 'dopant' molecules. Thirdly, a (cooled) substrate in UHV can be exposed to individual streams of monomer, dopant and catalyst molecules simultaneously in order to accomplish directly a chemical vapour deposition (CVD) of a CP film [5]. Although other possibilities exist, these three areas are being studied in our laboratories, and some new results within the first category will be discussed further below.

Advantages of radiation treatment or CVD processes would be that they would be compatible with semiconductor vapour-phase processing technology, might allow at least modest spatial resolution (especially with the use of laser, focused electron or molecular beam sources) and might lend themselves to large area processing. One additional advantage of vapour-depositing a CP on a metal surface in UHV by either RP or CVD methods is control of the poly- mer-metal interface. This latter point could be of obvious importance in cer- tain future electronic applications.

In this report, we describe the radiation-polymerization of thiophene to prepare polythiophene (PT) under UHV conditions. This activity is an exten- sion of our previous work on radiation-polymerized polypyrrole [3, 4], and we present some comparison of the results below.

This study was performed in our own UHV system, which was especially designed and constructed for the preparation and characterization of organic molecular solid thin films and organic adsorbates on surfaces. A preparation chamber allows the vapour deposition of organic molecular solids from either a gas-handling system (gases and volatile liquids) or a low-temperature- shielded Knudsson cell (molecular solids) to take place in a base pressure of 2 x 10 -l° Torr. Vapour deposition is monitored by a mass spectrometer. Sam- ples are analysed in a separate analysis chamber in a vacuum of better than 1 x 10 10 Torr. Analysis is by X-ray photoelectron spectroscopy (XPS, also known as ESCA), ultraviolet photoelectron spectroscopy (UPS), Auger spec- troscopy and four-point probe electrical conductivity measurements.

In the present case, a gold substrate was cleaned in situ by Ne ÷ ion etching, and then a thin film of thiophene molecules was condensed on the gold substrate at T = -130 °C. This is the highest temperature at which the thio- phene will condense on a cooled substrate in our vacuum system, thus allow- ing the molecules the maximum mobility possible during the radiation polymerization process. The thickness of these thin films was estimated by observing the at tenuation of the XPS signal of the substrate. We used thick- nesses from 25 to 100/~. Both the UPS valence electron spectrum and the XPS core-level spectra of C(ls) and S(2p) electrons were recorded immediately after vapour deposition.

Representative core-level (XPS) and valence band (XPS and UPS) spec- tra are shown in Figs. 1, 2 and 3. During the recording of the C(ls) and S(2p) spectra, the 1254 eV soft X-ray photons begin polymerizing the thiophene molecules. The weak shoulders on the low binding-energy side of the C(ls) and the S(2p) spectra (Fig. 1, top) correspond to material that has already polymer-

59

C(Is)

/ /

S(2p)

"MONOMER" ( - ~ 3 o ° ~

~ X-RAYS (+6gC~ ~

CHEM. PT ° ~

CNDO/S3

\

U P S / P ~ ,"" ,,, [I ,,

,,/ UPS/M

CNDO/S3 ~

UPS

CI6S4 HIO ,,fir n

M

)

UPS/GAS /

i;8 2'o E F BINDING ENERGY (eV) BINDING ENERGY (eV)

Fig. 1. (Left) XPS core-level spectra of thiophene monomer (top), radiation polymerized polythio- phene (middle) and electrochemical polythiophene (bottom).

Fig. 2. (Right) The UPS spectra are compared with the results of CNDO/S3 calculations [7], as discussed in the text. The gas phase UPS spectrum is from ref. 12.

ized by the time the spectra were taken. Any work function changes that occur as a result of the polymerization have not been taken into account in con- structing Fig. 1, so that alignment of the spectra with those of the radiation- polymerized material (middle row) indicates the origin of the weak shoulders. The core-level spectra of the RP material were recorded at +60 °C after pro- longed exposure of the thiophene film to 1254 eV photons at -130 °C. At +60 '~C (or actually at any temperature much over -130 °C), the thiophene molecules would have evaporated from the substrate had the material not polymerized. In the bottom row of Fig. 1 is shown the corresponding core-level spectra for reduced (i.e., undoped) electrochemically-made PT for comparison. The XPS and UPS spectra of reduced and oxidized electrochemically-made PT are discussed by Wu and coworkers [6].

The valence band spectra in Fig. 2 are compared with the results of model CNDO/S3 calculations, since spectra of both (M) monomer and (P) 'polymer' (i.e., tetramer) have been published [7]. As in the case of RP polypyrrole [3, 4], polymerization results in the stabilization of the first major feature (peak) in the UPS spectrum of the monomer, as predicted in the CNDO/S3 results [7], near 4 eV binding energy. In PT, however, the deeper-lying broad peak near

60

X P S . ~ .~ , / IL j ; ~ , "~'.."

uPs_t / ' I " / "

)

70 ,'o

L EF

BINDING ENERGY (eV)

Fig. 3. Various valence band spectra are compared with the results of a VEH calculation [9], as discussed in the text.

8 eV in the monomer also stabilizes upon polymerization. Most importantly, the UPS spectrum of RP PT exhibits the ~-band development at lowest binding energies, as predicted [7]. Note that in RP polypyrrole, we used a much lower temperature for vapour deposition ( -175 °C). This presumably limited the molecular mobility, resulting in a more disordered RP film, which precluded the development of the 7t-band in the UPS spectrum [3, 4]. Also, in poly- metaphenylene, VEH (Valence Effective Hamiltonian) calculations show that electronic delocalization is significantly smaller than in polyparaphenylene and, as a result, the highest ~-band is much narrower [8]. By analogy, we expect that a PT material with a significant amount of/?-linkages would not show the development of the ~-band peak at very low binding energies. Thus presence of the ~-band in our case indicates a c o n t r a r i o that the linkages are predominantly ~-~'.

Figure 3 shows a comparison of various UPS and XPS spectra of thio- phene (electrochemical, reduced PT, i.e., PT °) and the results of a VEH calcula- tion of the valence spectra of PT [9]. The VEH results cover the entire valence band, while the CNDO/S3 results shown in Fig. 2 were reported for only the highest occupied orbitals [7]. The agreement between the VEH results and the XPS spectrum of reduced, electrochemical PT is excellent, and is discussed elsewhere in this volume [6]. The correspondence among the various UPS and XPS spectra can also be seen. The UPS spectra of RP and electrochemical PT are somewhat different, indicating that the two materials are still not identi- cal. The most important point, however, is that the u-band peak (labelled n in Fig. 3) appears in all of the polymer spectra shown. Only after many tries was

61

the ~-band finally observed in po lypyr ro le [10]. Final ly , the ~-band edge is found to be loca ted abou t 1 eV below the Fermi level, ind ica t ing t h a t the ma t e r i a l is a semiconduc tor . The co r respond ing va lue in reduced poly(3- me thy l th iophene ) has been r epor t ed to be 1.1 eV [11].

The RP PT ma te r i a l discussed here was p repa red as u l t r a - th in films on me ta l subs t r a t e s s t r ic t ly for spec t roscopic purposes . No a t t emp t s have been made to dope the th in films and s tudy the e lec t r ica l conduct iv i ty . Expe r imen t s on th i cke r films grown on quar tz subs t r a t e s us ing ha rd X-ray po lymer iza t ion are cu r r en t ly unde r way.

In summary , we propose t ha t v a c u u m process ing of conduc t ing po lymers is a des i rab le t echn ique for i nco rpo ra t i ng conduc t ing polymers , for example as specia l - funct ion ma te r i a l s in modern (V)LSI technology. We have p rev ious ly d e m o n s t r a t e d the chemica l v a p o u r deposi t ion of a conduc t ing po lymer [5], and the r ad ia t ion -po lymer iza t ion of po lypyr ro le [3, 4]. Here , we repor t t h a t thio- phene can also be po lymer ized by soft X-rays and have shown some resu l t s of a s tudy of the e lec t ron ic (and some chemical ) s t ruc tu re of the mate r ia l . By employ ing the h ighes t subs t r a t e t e m p e r a t u r e possible (sti l l only - 1 3 0 'C), PT tha t spec t roscop ica l ly looks very s imi lar to e l ec t rochemica l PT can be prepared .

Acknowledgements

This work was suppor ted by gran ts f rom the Swedish Board for Techn ica l Deve lopmen t (STU), the Swedish N a t u r a l Sciences Resea rch Counci l (NFR) and Imper ia l Chemica l Indust r ies , PLC (ICI), U.K.

References

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