Article pubs.acs.org/JPCC
Thermal Stability of N,N′‑Bis(1-ethylpropyl)perylene-3,4,9,10tetracarboxdiimide Films on Cu(100) Juan C. Moreno-López,† Oscar Grizzi,‡ and Esteban A. Sánchez*,‡ †
Instituto de Física del Litoral (CONICET and Universidad Nacional del Litoral), Güemes 3450, S3000GLN, Santa Fe, Argentina Centro Atómico Bariloche-Instituto Balseiro − CNEA − UNCuyo − CONICET, Avda. Ezequiel Bustillo 9500, 8400 S.C. de Bariloche, Río Negro, Argentina
‡
S Supporting Information *
ABSTRACT: The thermal stability of N,N′-bis(1-ethylpropyl)-perylene-3,4,9,10-tetracarboxdiimide (EP-PTCDI) thin films deposited on Cu(100) has been characterized by using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED). For one monolayer absorption, the molecules arrange 3 4 symmetry. After annealing the sample at 500 with a − 2 6 K, XPS measurements show that the molecules dissociate leaving the PTCDI core of the molecule on the surface. According to that, STM measurements show new images of molecules with the size of the PTCDI core, and without the characteristic ethylpropyl end groups. These remaining molecules cover most of the surface with ordered islands, presenting a different molecular arrangement. With the help of the experimental and simulated LEED patterns, the molecular 5 6 arrangement can be described by a − 3.5 3 superstructure and its four rotational equivalents. However, complementary measurements of STM images at different tip-to-sample bias voltages showed variations of the local density of states for 5 6 molecules absorbed at different absorption sites, reflecting a − 7 6 commensurated arrangement.
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fabrication to know the stability of the thin films under temperature variation. In a previous work, we presented experimental and theoretical results of the study of the initial growth stages of EP-PTCDI on Cu(100) surfaces. It was established that for the first monolayer the molecules cover the surface with 2D ordered islands, causing the reduction of the work function (∼0.7 eV) despite the fact that there was a net electron charge transfer toward the molecule.15 In the present work, we report the experimental characterization of the thermal stability of EPPTCDI thin films deposited at room temperature on a Cu(100) surface by using scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and X-ray photoelectron spectroscopy (XPS). STM and LEED experiments provided information about the ordering, while XPS gave information about the elemental composition of the film.
1. INTRODUCTION Organic molecules like PeryleneTetraCarboxylic DiImides (PTCDI) have been used in applications such as OFETs,1−3 OLEDs,4−7 and solar cells,8,9 among others. These molecules are promising technological candidates for organic optoelectronic applications due to their low cost and commercial availability together with their very good electronic, optical, and charge transport properties.10 To improve the functionality of the devices, it is common to tailor the PTCDI molecules by either introducing appropriate substituents in the imide position or by core substitution in the bay region. Recently, it was demonstrated that N,N′-bis(1-EthylPropyl)-Perylene3,4,9,10-TetraCarboxDiImide (EP-PTCDI) films have efficient performance for optoelectronic devices,11,12 but the detailed characteristics of the layer formation and the interaction with different noble metals still remain unknown. In particular, it is interesting to study how the EthylPropyl (EP-) end groups might affect order, taking into account that the mobility of the molecules on the surface and the lateral intermolecular interaction can be reduced by the presence of the end groups, as it was observed for DiMethyl-PTCDI (Me-PTCDI) molecules.13,14 In addition, it is also important for device © 2016 American Chemical Society
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Received: April 25, 2016 Revised: July 13, 2016 Published: August 10, 2016 19630
DOI: 10.1021/acs.jpcc.6b04157 J. Phys. Chem. C 2016, 120, 19630−19635
Article
The Journal of Physical Chemistry C
Figure 1. (a) Sketch of the EP-PTCDI molecule: H, O, N, and C atoms are drawn in light gray, red, blue, and gray, respectively. (b) STM image (100 × 100 nm2) of ∼0.7 ML of EP-PTCDI molecules deposited on Cu(100) at 300 K, acquired at IT = 60 pA and VS = +0.2 V. (c) 2D-FFT pattern 3 4 . performed in one of the islands seen in panel b that corresponds to the superstructure − 2 6
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2. EXPERIMENTAL SETUP The characterization of the EP-PTCDI film grown on Cu(100) and its behavior after annealing was performed in two ultrahigh vacuum (UHV) analysis chambers using scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and X-ray photoelectron spectroscopy (XPS) techniques. STM and LEED measurements were carried out in an UHV chamber (base pressure ∼10−10 mbar) equipped with a variable temperature STM (model VT AFM 25 DRH) and a LEED system, both from Omicron NanoTechnology.16 The STM images were acquired in a constant current mode using electrochemical etched tungsten tips, with the sample held at room temperature. The reported bias voltages VS are applied to the sample with respect to the STM tip; i.e., positive (negative) VS values correspond to unoccupied (occupied) state images. They were processed using the WS×M free software.17 XPS measurements were performed in a second UHV chamber (base pressure
