On-surface molecular self-assembly is one of the key paradigms for understanding intermolecular interactions and molecule−substrate interactions at the atomic scale. Phthalocyanines are planar π-conjugated systems capable of selfassembly and can act as versatile, robust, and tunable templates for surface functionalization. One of the ways to tailor the properties of phthalocyanines is by pendant group substitution. How such a scheme brings about changes in the properties of the phthalocyanines at the nanoscale has not been greatly explored.Here we present an atomic-scale picture of the self-assembly of copper phthalocyanine, CuPc, and compare it with its cyano analogue, CuPc(CN) 8, on Au(111) using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) in ultrahigh vacuum (UHV) at 77 K. STM imaging reveals a tetramer unit cell to be the hallmark of each assembly. The periodicity of herringbone reconstruction of Au( 111) is unchanged upon CuPc(CN) 8 adsorption, whereas for CuPc adsorption this periodicity changes. STS measurements show an increment in the highest occupied−lowest unoccupied molecular orbital (HOMO−LUMO) gap from CuPc to CuPc(CN) 8 . Extensive ab initio calculations within density functional theory (DFT) match well with the experimental observations. STM imaging shows adsorption-induced organizational chirality for both assemblies. For CuPc(CN) 8 at LUMO energy, the individual molecule exhibits an orbital-energy-dependent chirality on top of the existing organizational chirality. It remains achiral at HOMO energy and within the HOMO−LUMO gap. No such peculiarity is seen in the CuPc assembly. This energy-selective chiral picture of CuPc(CN) 8 is ascribed to the cyano groups that participate in antiparallel dipolar coupling, thereby enhancing intermolecular interaction in the CuPc(CN) 8 assembly. Thus, our atomically resolved topographic and spectroscopic studies, supplemented by DFT calculations, demonstrate that pendant group substitution is an effective strategy for tweaking intermolecular interactions and for surface functionalization.
(a) Self-assembly (SA) of CuPc on the step edges of the Bi2Se3 surface, b) CuPc 1D chains in the SA, and c) HOMO–LUMO gap variation of CuPc.
We unravel the bidirectional influence of the adsorbate−substrate hybrid interface in the case of CuPc doping of the topological insulator (TI) surface Bi 2 Se 3 . Using ultrahigh vacuum scanning tunneling microscopy at low temperature (77 K), we observe that for a dilute concentration, single CuPc molecules are dispersed on terraces of Bi 2 Se 3 as individual entities or as clusters. The site-dependent submolecular resolution images of CuPc on Bi 2 Se 3 reveal three different sites for CuPc adsorption. Scanning tunneling spectroscopy (STS) measurements show a rigid shift of the Dirac point toward negative voltage by 336 meV upon CuPc deposition. This is a clear signature that the topological surface state experiences a charge transfer because of CuPc. The highest occupied molecular orbital (HOMO)−lowest unoccupied molecular orbital (LUMO) energy gap of CuPc on Bi 2 Se 3 is also measured using STS. It is found to be larger than that on the Au(111) substrate because of the reduced screening offered by the TI Bi 2 Se 3 surface state as compared to the Au noble metal surface state. We also find that a higher concentration of CuPc results in an unconventional standing-up stacking of CuPc molecules at the step edge. This is suggestive of a magnetic coupling between the CuPc layers analogous to the observations in magnetic phthalocyanine thin films. Thus, the CuPc/Bi 2 Se 3 system is a potent combination where CuPc induces a charge transfer, a change in the HOMO−LUMO gap on the TI surface of Bi 2 Se 3 , and the step edges of Bi 2 Se 3 act as an anchor to guide the unique vertical alignment of CuPc molecules proposing a new route to harness the TI nature and the magnetic behavior of the system.
Step edges of single-crystal surfaces play an important role in tuning the electronic properties of the surfaces and in guiding the application of surfaces as catalytic reaction centers. Modification of step edges by molecular adsorption can be an effective strategy for bottom-up nanofabrication of surfaces. A detailed submolecular level understanding of step-edge adsorption is mandatory to exploit the properties of step edges for a variety of applications. Though a variety of phthalocyanine (Pc) molecules have been investigated on surfaces, there is a huge void in the literature about step-edge behavior of Pcs on surfaces. With this perspective, the adsorption characteristics of copper Pc (CuPc) and copperoctacyano Pc (CuPc(CN) 8 ) have been investigated on Au(111) monoatomic (MA) step edges using low-temperature scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. At very low coverage, the adsorption of CuPc and CuPc(CN) 8 leads to the formation of onedimensional chains along the step edge. At higher coverage, both CuPc and CuPc(CN) 8 guided by tetramer unit cell formation self-assemble on flat terraces and cross over the step edge of Au(111). CuPc adsorption along MA step edge shows only one geometric configuration, whereas two different geometric configurations occur for CuPc(CN) 8 . The spectroscopic signature of these two configurations, probed using scanning tunneling spectroscopy (STS), manifests in a shift of the peak position of the highest occupied molecular orbital for the CuPc(CN) 8 molecule at the MA step edge with respect to the molecule over the flat terrace of Au(111). The STM images simulated on the basis of DFT calculations for specific configurations agree with the experimental results. These findings also advance our understanding of the role played by the pendant groups of the Pc molecules in step-edge adsorption.
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