Layer-by-layer
thin films with up to 11 layers of N,N′-bis(2-phosphonoethyl)-3,4,9,10-perylenediimide
(PPDI) were deposited on indium tin oxide (ITO) substrates by the
zirconium phosphonate method. Film growth was studied by UV–visible
absorption spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray
reflectivity (XRR), and atomic force microscopy (AFM). It was found
that the films grow in a linear fashion, with the same amount of dye
incorporated in each deposition cycle, and the dye molecules were
π-stacked within the films. For a 10-layer film, film thickness
was estimated as 20 nm, giving 2 nm per layer. Electrochemical characterization
of the films was performed using cyclic voltammetry (CV) and electrochemical
impedance spectroscopy (EIS). CV measurements showed that peak currents
were proportional to the number of layers, for both the first and
second reductions of PPDI. The effect of applied bias on the EIS response
was studied in both the impedance (Bode plots) and capacitance (Cole–Cole
plots) complex planes. The width of the semicircles in the Cole–Cole
plots was proportional to the number of layers when the electrode
was biased at −0.6 V, which is within the redox window for
PPDI reduction. The pseudocapacitance of the films was obtained from
the width of the semicircles, allowing the calculation of electroactive
surface coverages and electron-transfer rates. Surface coverages of
ca. 1 × 10–10 mol/cm2 per layer
were obtained, whereas electron transfer rates decreased with film
thickness, from 41 s–1 in a monolayer to 1.3 s–1 in a 10-layer film. The present study shows that
compact PPDI films were formed, with efficient electronic coupling
between the PPDI units, rendering the films attractive as electron
transport layers for organic electronics.
Hybrid thin films containing N,N′-bis(2-phosphonoethyl)-1,4,5,8-naphthalenediimide
(PNDI)
and zinc cations (PNDI/Zn films) were built on silicon and indium
tin oxide (ITO) substrates by a simple layer-by-layer deposition process.
Silicon substrates primed with a layer of phosphonate groups were
immersed alternately into zinc nitrate and PNDI aqueous solutions,
yielding PNDI/Zn films containing up to 40 layers. ITO substrates,
on the other hand, were used without priming, and the deposition sequence
began with a PNDI layer. All film growth steps were conducted at room
temperature, using aqueous solutions, thus assuring an environmentally
clean process. The PNDI/Zn films were studied by X-ray reflectivity
and grazing angle X-ray diffraction, using synchrotron radiation source.
The films were constituted by crystallites, containing zinc phosphonate
layers oriented nearly parallel to the substrate. PNDI/Zn films on
ITO were reduced to stable free radicals, which were observed by UV–visible
spectroscopy. Moreover, PNDI/Zn bulk materials with structural analogy
with the films were produced.
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