Article

Mapping the first electronic resonances of a Cu phthalocyanine STM tunnel junction

IOP Publishing
Journal of Physics: Condensed Matter
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Abstract

Using a low temperature, ultrahigh vacuum scanning tunneling microscope (STM), dI/dV differential conductance maps were recorded at the tunneling resonance energies for a single Cu phthalocyanine molecule adsorbed on an Au(111) surface. We demonstrated that, contrary to the common assumption, such maps are not representative of the molecular orbital spatial expansion, but rather result from their complex superposition captured by the STM tip apex with a superposition weight which generally does not correspond to the native weight used in the standard Slater determinant basis set. Changes in the molecule conformation on the Au(111) surface further obscure the identification between dI/dV conductance maps and the native molecular orbital electronic probability distribution in space.

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... Since STM images are a convolution of electronic and geometric structure, charge transfer in the imaged system can affect the appearance of the STM images, but the relative importance of the individual effects, electronic or geometric, is often difficult to determine. In this context, an intriguing observation is that in STM images surface-adsorbed MPcs might appear to have a lower symmetry than their gas-phase equivalents [6,[12][13][14][15][16][17][18][19][19][20][21][22][23][24]. While the planar gas phase MPcs adhere to the D 4h point group and thus could be expected to have C 4v symmetry upon adsorption, in STM symmetry breaking ...
... Since STM images are a convolution of electronic and geometric structure, charge transfer in the imaged system can affect the appearance of the STM images, but the relative importance of the individual effects, electronic or geometric, is often difficult to determine. In this context, an intriguing observation is that in STM images surface-adsorbed MPcs might appear to have a lower symmetry than their gas-phase equivalents [6,[12][13][14][15][16][17][18][19][19][20][21][22][23][24]. While the planar gas phase MPcs adhere to the D 4h point group and thus could be expected to have C 4v symmetry upon adsorption, in STM symmetry breaking ...
... Since STM images are a convolution of electronic and geometric structure, charge transfer in the imaged system can affect the appearance of the STM images, but the relative importance of the individual effects, electronic or geometric, is often difficult to determine. In this context, an intriguing observation is that in STM images surface-adsorbed MPcs might appear to have a lower symmetry than their gas-phase equivalents [6,[12][13][14][15][16][17][18][19][19][20][21][22][23][24]. While the planar gas phase MPcs adhere to the D 4h point group and thus could be expected to have C 4v symmetry upon adsorption, in STM symmetry breaking towards C 4 , C 2 , or C 1 has been observed frequently, for adsorption on both transition metal surfaces [6,[12][13][14][15][16][17][18][19][20][21] and insulator surfaces [22,23]. ...
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Subtle changes in the geometric and electronic properties of supported molecules, with a potential impact on the functioning of molecular devices, can typically be imaged by scanning probe microscopy, but their exact origin and nature often remain unclear. Here we show explicitly that the symmetry reduction of iron phthalocyanine upon adsorption on Cu(111) can be observed not only in scanning tunneling microscopy, but also in core-level spectroscopy, and that it is related to nonisotropic charge transfer into the two principal molecular axes, but in combination with topographic influences.
... One interesting issue is that the LUMO state of phase II locates almost at the center of the LUMO and LUMO + 1 in phase I (Fig. 6a). We expect adsorption-induced splitting of LUMO in phase I, which usually takes place in the system of molecule adsorbed on metal surfaces [39]. The free picene molecule has a quasi-degenerate LUMO state as well [2]. ...
... The free picene molecule has a quasi-degenerate LUMO state as well [2]. After adsorbed on metal surfaces, a strong interaction with the substrate can break the intrinsic symmetry and cause the splitting of electronic state [39]. ...
... As can be seen from Fig. 1.1, the gas phase molecule has a perfect fourfold (D 4h ) symmetry, and thus metal phthalocyanines are expected to have a C 4v symmetry upon adsorption. However, this is not always the case as has been observed for M Pc adsorbed on transition metal [31][32][33][34][35][36][37][38][39][40][41] and some isolator surfaces [52,53], where the adosrbates undergo symmetry reduction towards C 4 , C 2 , or C 1 symmetries. It has been shown that whether symmetry breaking occurs or not is a function both of the metal center in the compound and of the support. ...
... However, these conclusions have been somewhat controversial. While some STM groups claim that the symmetry breaking of MPcs is purely geometric in origin (topographic) [37,40], other groups argue that it is either purely electronic [32] or that it is a geometry-driven electronic effect [31,34,35,41]. Currently, there exists no consensus as to the origin of the observed symmetry breaking in the surface science community, and one of the aims of this thesis is directed towards resolving this issue by combined imaging and spectroscopy approaches. ...
Thesis
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The work presented in this thesis addresses the investigation of the electronic, magnetic, and structural properties of late transition metal complexes supported on various surfaces. The research is aimed at studying the interaction between the molecules and the support, together with the intermolecular interaction. This knowledge is essential e.g. for the development of organic molecule-based devices and the creation of active and stable catalysts. In this work, the modification of the electronic states of the iron phthalocyanine (FePc) complex induced by a Cu(111) surface was extensively investigated. These studies were motivated by the role that phthalocyanines play in charge injection devices and molecular electronics. The analysis revealed a non-isotropic charge transfer from the surface, which arises from the rehybridisation of molecular and metal electronic states and results in the breaking of the perfect fourfold symmetry of the molecule. In addition, I demonstrate a surface-driven thermal modification of the electronic and structural properties of the phthalocyanine molecules when deposited on Cu(111) support. This knowledge is essential because a thermal evaporation of adsorbates is the most common preparation technique for the creation of molecular monolayers. This technique is widely used for commercial purposes such as the creation of molecular switches and data storage devices. The FePc molecule is also quite unique due to the similarity of its structural and magnetic properties to that of the reactive site in haemoproteine, a molecule known to perform the activation, storage, and transport of molecular oxygen. Therefore, I also investigated FePc as a synthetic model of the iron porphyrine in the haem reactive site. This investigation revealed that despite its structural similarity to haem, the molecule interacts with molecular oxygen only as a result of the stronger electronic coupling of the FePc molecules to the surface. These studies can help to obtain a better understanding of mechanisms of the haem-oxygen interaction. Motivated by the success in the development and mass production of green and affordable surface-supported transition metal complex catalysts, this thesis incorporates the full characterisation of two new N-heterocyclic carbene palladium complexes anchored to a silica surface. The study is aimed at providing a comprehensive knowledge about stability, surface orientation, and catalytic activity of late transition metal complexes at surfaces for heterogeneous catalysis, as opposed to the more commonly used homogeneous catalysis in inorganic chemistry. In summary, the strength of this thesis lies in the provision of a comprehensive overview of the interaction and surface-driven modifications of supported transition metal complexes on various surfaces and new insight into the magnetic and electronic properties of single molecules, monolayer, and multilayers.
... The observed negative shift is ascribed to a push-back effect due to weak CuPc/Au(111) interaction (physisorption). [42][43][44] Thus, the larger VL shift of − 0.89 eV for the AN ClAlPc ML on Au(111) suggests stronger moleculesubstrate interactions. No HOMO broadening and larger VL shift at LT are found on spectrum (7) in Fig. 3(b), further confirming the weak CuPc/Au(111) interaction, which is in contrast to the ClAlPc/Au(111) case. ...
... In the case of weak physisorption, the contribution of the VL shift from the electric dipole moments of the ClAlPc molecules is about + 0.43 eV, e.g., the AN ML on graphite, where all ClAlPc molecules are also uniformly aligned in the Cl-up configurations and the dipole effect is considered the main contribution. When the push-back effect (weak electronic coupling [42][43][44] ) exists because of the Au(111)-Pc plane interaction, the typical Pc ML (e.g., CuPc) physisorbed on Au(111) induces a − 0.69-eV shift of the WF. Hence, a VL shift of − 0.23 eV (VL shift = 0.46 − 0.69 eV) is expected if we assume the molecular dipole layer is physisorbed on Au(111). ...
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The molecular orientation–dependent electronic properties of monolayer dipolar molecule chloroaluminum phthalocyanine (ClAlPc) on Au(111) are investigated by ultraviolet photoemission spectroscopy and scanning tunnelingmicroscopy. The relation between geometrical and electronic structures has been revealed in the binding energies of the highest occupied molecular orbital states and vacuum level (VL) shifts. Two molecular orientations, Cl-up- and Cl-down-oriented molecules, coexist in the as-grown monolayer ClAlPc films on Au(111) without the formation of staggered molecular pairs to cancel the dipoles and phase separation, as is the case on graphite. After annealing, only the Cl-up-oriented molecules remain on Au(111), as on graphite. Interestingly, an extraordinarily large VL shift of −0.89 eV is observed in the annealed monolayer ClAlPc film on Au(111), which is opposite to that of +0.46 eV on graphite even though the molecular dipoles are oriented similarly.
... We speculate that splitting of LUMO states of tilted molecules arises from the breaking of intrinsic symmetry of picene molecules driven by the molecule-substrate interaction. 16,31 The dI/dV spectrum for the upright-standing molecules in the crystalline (001) monolayer is shown in Fig. 6(b). It is found that the HOMO and LUMO peaks appear at −2.3 eV and +2.5 eV, respectively. ...
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... 49 In particular, their structural properties were of the first to be investigated by many different techniques, including x-ray crystallography, STM and AFM. 61 Investigations found that phthalocyanines exhibit excellent photoconductive properties with very intense optical absorption in the visual spectrum and are very robust against thermal and chemical decomposition. The properties of these molecules can be modified by changing the core metal atom, as well as with the addition of functional groups or incorporation into metal organic frameworks. ...
Thesis
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... This difference comes probably from a slight deformation of the molecular skeleton at the hexagonal carbon ring. 37 To theoretically simulate the -1.8 V and +2.45 V dI/dV maps, it was necessary to superpose several ESQC calculated DIP molecular orbital maps. This is an indication that those two resonances cannot be described solely by a single molecular orbital contribution of the corresponding many-body Slater determinants normally describing those resonances. ...
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... Considering the influence of the strong electric field from STM tip, in the order of 1 V/nm (10 9 V/m), deformation of the molecule under an STM tip is unavoidable. Soe et al. [24] demonstrated with their STM study that the D 4h flat structure of copper phthalocyanine is distorted into C 4v structure due to the adsorption on Au(111) surface [25]. Girard et al. [26] calculated the local electric field in the tipsample gap, and field induced diffusion processes of a physisorbed atom were discussed. ...
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