Substrate Mediated Ordering in PmPV Thin Films
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ABSTRACT Monolayer films of poly(m-phenylenevinylene-co-2,5-dioctoxy-p-phenylenevinylene) (PmPV) supported on highly orientated pyrolytic graphite (HOPG) have been investigated using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). While there are areas of the surface covered with a featureless polymer layer, we also observe large regions with surprisingly long range order. Structural models of the polymer, as well as tunneling spectra, suggest that this ordering is derived from substrate-polymer epitaxy. INTRODUCTION Conjugated polymers, such as Poly(phenylene vinylenes), have been widely studied as the active emissive layer in organic light emitting devices [1]. Since light emission in these polymers arises from a polaronic exciton resulting from the injection of negative and positive charge at a contact, it is of central interest to such applications to understand the nature of the interaction of these polymers with solid surfaces. Specifically, morphologically induced changes in the polymer’s electronic structure due to interactions with a solid surface must be understood before ideal polymer-metal contacts can be engineered. In this study we observe crystalline ordering in monolayers of a soluble variation of PPV, poly(m-phenylenevinylene-co-2,5-dioctoxy-p-phenylenevinylene). Using STM/STS, long-range crystalline phases of PmPV are identified on HOPG substrates. We further suggest that such ordering is mediated by epitaxy with the underlying substrate. It is expected that such ordering will have a significant effect on polymer-substrate interface electronics. SAMPLE PREPARATION AND STRUCTURE The PmPV used in this study was prepared by a Horner reaction and is described elsewhere [2]. The incorporation of the meta-phenylene linkages in this PPV variation favors coiling of the polymer backbone for trans linkages, leading to a predominantly regular helical structure[3]. Due to the high proportion of trans linkages in PmPV made via this synthesis method (~70%), this polymer is highly ordered and crystalline. X-Ray Diffraction has shown the crystalline fraction is approximately 30% [4]. Table I gives a typical variation in the mass characteristics of the PmPV [2]. Figure 1 depicts the repeat unit structure of the PmPV.
Table I. Molecular weight characteristics of this PmPV
PmPV
Mw 6800336200
Mn 65009050
nav 14197
Where: Mw = weight average molecular weight (g/mol) Mn = number average molecular weight (g/mol) nav = average number repeat units
.4nm 2.2nm
1.1nm Figure 1. Repeat unit structure of poly(m-phenylenevinylene-co-2,5-dioctoxyp-phenylenevinylene) Figure 2 gives a schematic of the proposed helical structure of an all trans form of the polymer. Each ring in the helix is typically composed of 7 repeat units. From molecular modeling calculations [3], in free space, the helix pitch has been estimated to be 6 Å, and the calculated diameter of the helical structure is 20 Å. Naturally, this will be modified by substrate interactions. The PmPV chains are terminated at the ends, o
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