Article pubs.acs.org/Langmuir
In Situ Scanning Tunneling Microscopy Investigation of Subphthalocyanine and Subnaphthalocyanine Adlayers on a Au(111) Electrode Jing-Ying Gu,†,‡ Bo Cui,†,‡ Ting Chen,† Hui-Juan Yan,† Dong Wang,*,† and Li-Jun Wan*,† †
CAS Key Laboratory of Molecular Nanostructure and Nanotechnology, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China S Supporting Information *
ABSTRACT: The adsorption behaviors of subphthalocyanine (SubPc) and subnaphthalocyanine (SubNc) on the Au(111) surface were investigated by electrochemical scanning tunneling microscopy (ECSTM). Two types of ordered adlayer structures of SubPc were observed at 550 mV versus the reversible hydrogen electrode (RHE). All of the SubPc molecules take the Cl-down adsorption configuration on Au(111) in both structures. The ordered adlayers exist in the potential range between 350 and 650 mV. The SubNc molecules adsorb on Au(111) in a less-ordered pattern than the SubPc molecules. The present work provides direct evidence for understanding the potential-controlled adsorption behaviors of SubPc and SubNc on the Au(111) surface.
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INTRODUCTION
orientational switching of ClAlPc by controlling the polarity of the voltage pulse.16 The subphthalocyanine (SubPc) (Figure 1), which has only three isoindolyl groups, is a special member of the phthalocyanine family. Although the central boron atom is sp3-coordinated, which leads to a nonplanar cone-shaped structure, SubPc has a 14 π-electron aromatic conjugated system. SubPc's have a wide range of applications in solar cells,25−27 nonlinear optics,28,29 and organic light-emitting diodes (OLEDs)30,31 because of their unique photophysical and electronic properties. The adsorption behavior of SubPc's on crystalline surfaces has been studied previously.32−41 Berner et al. reported that a solid ordered phase of SubPc coexists with a 2D lattice gas on Ag(111) at room temperature, medium coverage (0.2−0.5 monolayer).32,41 In contrast, a highly ordered structure of SubPc was observed only for coverage at or above one monolayer, owing to the high mobility of SubPc on Au(111) at room temperature.38 At cryogenic temperatures where the diffusion of SubPc molecules is significantly suppressed, more complicated polymorphism of SubPc on metal surfaces has been observed. For example, SubPc can form coverage-dependent ordered chiral phases on Au(111) at 77 K.36 Trelka et al. found that SubPc can grow into bilayer and trilayer nanocrystallites on Cu(111).33
Porphyrins and phthalocyanines are promising components in molecular electronic devices because of their extraordinary optical and electronic properties and have attracted a great amount of attention over the past two decades.1−6 It is well demonstrated that the interfacial structures, especially the very first few layers between active optoelectronic functional molecules and electrodes, have a great effect on their performance.7,8 The porphyrin and phthalocyanine adlayers on surfaces have been investigated by a variety of advanced surface characterization techniques, such as X-ray photoelectron spectroscopy (XPS),9−11 atomic force microscopy (AFM),12−14 and scanning tunneling microscopy (STM).15−17 Among these techniques, STM is a powerful tool for providing visual structural information about porphyrins and phthalocyanines on single-crystal surfaces with submolecular resolution under different conditions, including electrochemical, ambient, and ultrahigh vacuum (UHV) environments.7,18−23 For instance, a “chessboard” structure was formed by the assembly of porphyrin and phthalocyanine molecules under electrochemical conditions, and the “chessboard” could serve as a template to trap C60 selectively.19 Hulsken et al. clarified the catalytic mechanism of metal porphyrin by monitoring the contrast of manganese porphyrin in real-time at a solid/liquid interface.24 Recently, Chen et al. reported the close-packed monolayer of chloroaluminum phthalocyanine (ClAlPc) under UHV conditions, and they demonstrated the reversible © 2012 American Chemical Society
Received: October 28, 2012 Revised: December 10, 2012 Published: December 10, 2012 264
dx.doi.org/10.1021/la3042742 | Langmuir 2013, 29, 264−270
Langmuir
Article
were used as an electrochemical cell. All electrode potentials were reported with respect to the reversible hydrogen electrode (RHE) in 0.1 M HClO4. The STM tips were prepared by electrochemically etching W wire (0.25 mm diameter) in 0.6 M KOH at 12−15 V ac. The tips were coated with transparent nail polish to minimize the residual faradaic current. All of the STM images were recorded in constant-current mode with a high-resolution scanner.
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RESULTS AND DISCUSSION SubPc Self-Assembled Adlayers. Adlayer (I). SubPc can form two different adlayer structures on the Au(111) surface at the potential between 350 and 650 mV. Figure 2a shows a
Figure 1. Chemical structures and optimized models of (a, b) SubPc and (c, d) SubNc.
In the present article, we report the detailed investigation of adlayer structures of SubPc and its homologue subnaphthalocyanine (SubNc) on the Au(111) electrode under potential control. The chemical structures of SubPc and SubNc are shown in Figure 1. Unlike the UHV environment, the adsorbate−substrate interaction can be modulated by electrode potential under electrochemical conditions.42−45 In situ STM results indicate that SubPc can form two ordered adlayer structures at the electrochemical solid/liquid interface by regulating the substrate potential. The substrate potential plays an important role in the adsorption behavior of SubPc on Au(111). The ordered SubPc adlayer exist in the potential range from 350 to 650 mV. SubNc can also form two ordered structures, but the adlayer of SubNc is less ordered than that of SubPc.
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Figure 2. (a) Large-scale and (b) high-resolution STM images of SubPc adlayer (I) on the Au(111) surface. The white arrows indicate the lattice directions of the Au(111) substrate. (c) Proposed structural model for the SubPc adlayer (I) with a top view (left) and side view (right). Image conditions: (a) E = 550 mV, Ebias = −451.6 mV, and It = 1.068 nA. (b) E = 550 mV, Ebias = −602.6 mV, It = 1.187 nA.
typical large-scale STM image of SubPc adlayer (I) obtained at 550 mV, indicating that adsorbed SubPc molecules can selforganize into a long-range-ordered hexagonal honeycomb pattern on Au(111). Different from the adsorption behavior in UHV condition,38 SubPc can form ordered structures on Au(111) at room temperature under potential control, even when the coverage is less than one monolayer (Figure S1 in Supporting Information). This implies that the substrate potential plays an important role in the adsorption of the SubPc molecules on the surface. The reconstruction lines of the Au(111) substrate, which can serve as a guide to deduce the adsorption direction of the adlayer related to the substrate, are still visible in the presence of adsorbed molecules, suggesting that the adsorbate−substrate interaction is not strong enough to lift the Au(111) reconstruction. Further details about the internal structure and orientation of adlayer (I) are revealed in the high-resolution STM image (Figure 2b). Each molecule appears as a triangle with dimension of ca. 1.2 nm, consistent with the optimized molecular size (Figure 1b). The adjacent SubPc molecules adopt a edge-to-edge configuration and form a honeycomb structure. The adlattice of the honeycomb adlayer is along the ⟨121⟩ direction of the Au(111) substrate. Each molecule can be recognized as a set of three bright lobes that are referred to as
EXPERIMENTAL SECTION
Subphthalocyanine (SubPc), subnaphthalocyanine (SubNc), and N,Ndimethylformamide (DMF) were purchased from Sigma-Aldrich and used without further purification. SubPc and SubNc were dissolved in DMF to obtain a saturated solution because of their poor solubility in aqueous solution. The adlayer of SubPc was prepared by immersing the gold bead into a SubPc/DMF saturated solution for 1 to 2 s and then mounting it in an electrochemical cell. The adlayer of SubNc was prepared by the same method as mentioned above using a 10-fold diluted SubNc/DMF saturated solution. Electrolyte solution (0.1 M HClO4) was prepared by diluting ultrapure HClO4 (Kanto Chemical Co.) with Milli-Q water (18.2 MΩ·cm, total organic carbon
