Article pubs.acs.org/JPCC
Selective Cooperative Self-Assembly between an Organic Semiconductor and Native Adatoms on Cu(110) Bret Maughan,† Percy Zahl,‡ Peter Sutter,‡,∥ and Oliver L. A. Monti*,†,§ †
Department of Chemistry & Biochemistry, University of Arizona, 1306 East University Boulevard, Tucson, Arizona 85721, United States ‡ Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States § Department of Physics, University of Arizona, 1118 East Fourth Street, Tucson, Arizona 85721, United States S Supporting Information *
ABSTRACT: We investigate molecular adsorption, film growth, and selfassembly for titanyl phthalocyanine (TiOPc) on Cu(110) in ultrahigh vacuum using low-temperature scanning tunneling microscopy (LT-STM). Three unique molecular adsorption configurations are identified, two of which are referred to as “O-down” and one as “O-up”, each differing in the molecular registry with the surface. Even though disorder dominates film growth to coverages in excess of 1 monolayer in the native thin film, extended self-assembled 1D configuration-dependent nanoribbons form upon annealing of the film. The STM data reveal that the nanoribbons consist of “O-down” TiOPc and a Cu skeleton, anchoring cooperatively on the Cu(110) terraces. Agent-based simulations show that nanoribbons grow and elongate due to anisotropic adatom attachment rates along the two major surface directions. The study reveals the importance of molecule− adatom interactions for novel approaches toward nanostructuring organic semiconductor/metal interfaces.
1. INTRODUCTION Self-assembly of functional molecules such as organic semiconductors is emerging as a practical means for tailoring electronic properties at the nanoscale.1 This is particularly important for the development of rational design strategies toward novel and efficient organic optoelectronic devices such as photovoltaic cells, light-emitting diodes, or thermoelectrics. A detailed understanding of molecular self-assembly also opens new avenues to miniaturization with the ultimate goal of bottom-up design and synthesis of highly integrated singlemolecule devices.2 Further, collective effects in self-assembled monolayers lead to the emergence of electronic properties that may differ substantially from those of the isolated molecule.3−6 Self-assembly is inherently driven by the subtle interactions at the molecule−substrate interface and relies on the complex interplay of surface/molecule and intermolecular forces. The surface plays an important role, e.g., by templating film growth or by direct functionalization of the molecular adsorbates into supramolecular surface-confined organic nanoassemblies. Beyond creating novel supramolecular structures, electronic coupling at the interface may also alter the molecular and interfacial electronic structure.7−9 The electron-rich nature of coinage metal surfaces facilitates self-assembly of complex molecular nanostructures over a wide range of length scales,10,11 driven in part by the specific nature of the surface geometry and electronic structure. 12,13 Anisotropic, open surfaces such as the (110) surface of coinage © 2015 American Chemical Society
metals introduce the possibility for low-dimensional growth with tightly controlled molecular film structure.14−18 The combination of such anisotropic substrates with the tunable chemical nature of metal phthalocyanines (MPcs) with different coordinated metal centers affords the unique possibility to explore the factors that determine molecular self-assembly and the influence of molecular film growth on the interfacial electronic structure. The structural richness of this process necessitates an approach that reveals the surface−molecule interactions in atomic detail. Scanning tunneling microscopy (STM) is ideally suited to this purpose. In this work we investigate the prototypical system of TiOPc on the Cu(110) surface with low-temperature scanning tunneling microscopy (LT-STM) and agent-based modeling to report on surface−molecule interactions and uncover the relevant kinetic processes that drive surface order and selfassembly. We show that the presence of three unique adsorption configurations governs the thin film growth and is ultimately responsible for the cooperative formation of extended 1D configuration-dependent self-assembled nanostructures. We demonstrate that these nanostructures arise from selective interactions with native surface adatoms and Received: July 31, 2015 Revised: October 10, 2015 Published: November 9, 2015 27416
DOI: 10.1021/acs.jpcc.5b07436 J. Phys. Chem. C 2015, 119, 27416−27425
Article
The Journal of Physical Chemistry C
Figure 1. (a) 0.1 MLE TiOPc on Cu(110), shown across two separate terraces (sample bias VS = −0.2 V, tunneling current IT = 0.25 nA). (b) Adsorption configurations for TiOPc (VS = −0.4 V, IT = 0.6 nA). (c) Line profiles across ligand lobes, along the dashed lines in (b), for both molecular configurations.
3. RESULTS 3.1. Molecular Adsorption. We first consider the atomistic details of adsorption of individual TiOPc molecules on Cu(110) since the adsorption process underpins thin film growth and self-assembly into 1D nanostructures. Figure 1a shows an image of the Cu(110) surface with 0.1 MLE of TiOPc adsorbed, with a step edge running diagonally across the image. TiOPc molecules densely decorate the step edge, providing a first indication of considerable molecular diffusion during room temperature growth. On the terraces, TiOPc molecules form a lattice gas, as is common for MPcs on many coinage metal surfaces,23,24 and no island nucleation is observed. This reflects at least in part the symmetry mismatch between the C4v TiOPc molecules and the 2-fold symmetric (110) surface, preventing facile epitaxial growth. In addition, the presence of extended one-dimensional chains, as reported, e.g., for pentacene on Cu(110)14 and several phthalocyanine systems,25,26 is not observed either, suggesting that attractive intermolecular interactions play a limited role at this interface. Rather, the surface−molecule interaction appears to dominate, as born out, e.g., by the lack of strong bias-dependent spatial features in the molecule. This is indicative of hybridization of the molecule and surface wave functions.27 Further insight into why chain formation is not strongly favored is revealed from a close inspection of the adsorption geometry of isolated molecules. Figure 1b shows a representative zoomed-in image of a typical arrangement of TiOPc molecules. The TiOPc adsorbates exhibit two and only two distinct STM contrasts, which differ primarily by the apparent height of features at the molecular center. We assign molecules with a prominent circular feature at their center to TiOPc molecules oriented with the oxygen atom directed toward the surface and molecules without the circular feature to TiOPc molecules oriented with the oxygen atom directed away from the surface and toward vacuum. The inset in Figure 1b shows a cartoon of the two configurations, to which we refer in the remainder of the text as “O-up” and “O-down”, respectively. The presence of two configurations likely disfavors assembly into extended molecular chains for entropic reasons. The assignment of the two STM contrasts as “O-up” and “Odown” requires further justification: Assigning a molecular configuration while considering the height of a particular feature in the molecule is challenging since the tunneling matrix element contains also the tip density of states and height
develop a simple model to explain the growth of these nanoribbons.
2. EXPERIMENTAL SECTION TiOPc (Sigma-Aldrich) was obtained commercially and further purified by three cycles of gradient sublimation in a custombuilt furnace. The Cu(110) substrate was cleaned by Ar+ sputtering (1 keV, 5.5 μA/cm2) for 15 min, followed by annealing to 850 K and slow cooling to ensure straight step edges and large terraces. Imaging of the bare surface prior to thin film growth revealed a clean surface with sparsely scattered residual impurities and isolated Cu adatoms characteristic of the Cu(110) surface.18,19 Thin films of TiOPc were prepared by physical vapor deposition using a home-built water-cooled Knudsen cell in the sample preparation chamber (base pressure of 9 × 10−10 Torr) of a Createc low-temperature scanning tunneling microscope system. Prior to sublimation, the Knudsen cell was degassed for 24 h under ultrahigh vacuum conditions near the sublimation temperature (140 °C). The deposition rate and film thickness were monitored with a quartz crystal microbalance (QCM), and low-coverage films were grown typically at a rate of 0.1 monolayers (ML)/min on the Cu(110) substrate held at room temperature. Given the nonuniform packing displayed by this system, molecular coverages are reported as fractions of a hypothetical monolayer equivalent (MLE) corresponding to the percentage of substrate surface area covered (1 MLE ≈ 4.44 × 1013 molecules/cm2). Following deposition, the sample was held at RT for ∼2 min before rapid cooling and transfer to the imaging chamber housing the cryogenic STM (pressure
