Direct Imaging of Precursor Adcomplex States during Cryogenic-Temperature On-Surface Metalation: Scanning Tunneling Microscopy Study on Porphyrin Array with Fe Adsorption at 78.5 K

Abstract

On-surface metalation of metal-free π-conjugated planar molecules, such as metal-free tetraphenylporphyrin (2HTPP), using 3d transition metals prepared in ultrahigh vacuum (UHV), attracted significant attention as a newly developed bottom-up method to create a building block for 1-nm-size nano devices, one-or two-dimensional nano networks, as a precursor of further on-surface synthesis or a magnetic atom array aligned on a well-ordered molecular template with an extreme low impurity concentration. Experimental and theoretical studies for this type of on-surface metalation via dehydrogenation (e.g., 2HTPP + Fe → H 2 + FeTPP) have been successfully demonstrated by depositing 3d metal atoms on metal-free π-conjugated planar molecular array at 300 K with subsequent annealing of 350−700 K in UHV. Significantly, photoemission spectroscopy study for the on-surface metalation process at cryogenic temperatures suggested a precursor adcomplex state different from the normal metalation state. In this study, we demonstrate that on-surface metalation at cryogenic temperature reduces thermal diffusion of both metal atoms and molecules, which allows gentle adsorption of the 3d metal atoms inside the molecule, providing precursor adcomplex states while preserving the original well-ordering molecular array structure. Using scanning tunneling microscopy (STM) we investigated Fe deposition on a well-ordered metal-free tetraphenylporphyrin (2HTPP) monolayer array islands with an Au(111) substrate temperature maintained at 78.5 K. Through direct STM topographic and spectroscopy imaging supported by density functional theory calculations, we identify three types of TPP appearance: (1) original 2HTPP, (2) 2HTPP with an Fe atom on top (precursor adcomplex "α" state), and (3) 2HTPP with an Fe atom inside (precursor adcomplex "γ" state, where a clear LUMO peak change was observed).

Growth of Transition-Metal Cobalt Nanoclusters on 2D Covalent Organic Frameworks

“…The utilization of organic compounds to immobilize magnetic atoms on a surface was initially demonstrated on the surface of π-conjugated molecular self-assembled monolayer (SAM) films, such as phthalocyanines or porphyrins. − However, a single molecule can only accommodate one atom at its core in this metalation process. Suppose the deposition of transition-metal atoms surpasses the number of organic molecules within the SAM film.…”

On-surface growth of transition-metal cobalt nanoclusters using a 2D crown-ether array

“…This is the clear difference compared to the previously studied pconjugated Pc/TPP molecules, where only one atom can be trapped at the core, but extra Co atoms diffused and disordered the SAM film. 16 In this sense, it is remarkable that the molecules surrounding the NC kept the original (7 Â 4) superstructure, as shown by the white box in Fig. 2(c), including 5 by 3 BrCR molecules without disordering or deformation.…”

“…In that case, the SAM orderliness is broken, and other atoms grow independently from the molecules, leading to a messy surface. 16 In this study, we demonstrated the adsorption of the ''guest'' 3d transition-metal cobalt (Co) on the ''host'' well-ordered ring molecular array at room temperature in ultra-high vacuum (UHV). Crown ether molecules in 4,4 0 ,5,5 0 -tetrabromodibenzo [18]crown-6 ether (BrCR) SAM films were used as a ''host,'' which have been studied since the 1960s as cyclic molecules with an intramolecular ring in bulk crystal and liquid solution.…”

“…In that case, the SAM orderliness is broken, and other atoms grow independently from the molecules, leading to a messy surface. 16…”

Metalation of 2HTCNPP on Ag(111) with Zn: Evidence for the Sitting atop Complex at Room Temperature

“…Yamada et al. were able to image these three different states of the metalation reaction of 2HTPP with low‐temperature STM for Fe on Ag(111) at 78.5 K. The original 2HTPP, a 2HTPP precursor adcomplex “ α ” state (Fe atom on top), and a 2HTPP precursor adcomplex “ γ ” state (Fe atom inside) were identified by DFT calculations, STM topography and scanning tunneling spectroscopy (STS) [25] . One interesting question in this context is, whether for a metal with a higher activation energy for metalation, such as Zn, the adcomplex can also be observed at room temperature, which is closer to operating conditions of real devices.…”