A scanning tunneling microscopy study of the electronic and spin states of bis(phthalocyaninato)terbium(iii) (TbPc₂) molecules on Ag(111)

Abstract

In this article, we investigate a single molecule magnet bis(phthalocyaninato)terbium(iii) (TbPc) molecule film by using low temperature STM. In order to investigate the effect of molecule-substrate interaction on the electronic and spin properties of the adsorbed molecule, we tune the molecule-substrate coupling by switching the substrate between Au(111) and Ag(111), the latter of which provides stronger interaction with the molecule than the former. Despite the enhanced chemical reactivity of the Ag(111) surface compared with Au(111), a well-organized pseudo-square film is formed. In addition, a checker-board type contrast variation is identified, which is well explained by the existence of two types of molecules whose rotational angle between the top and bottom Pc is θ = 45° (bright molecule) and θ = 30° (dark molecule). The expected stronger molecule-substrate interaction, however, appears as an intriguing dI/dV mapping image which reveals the spatial distribution of the density of states (DOS). We identify the contrast reversal in the dI/dV mapping for the molecules of θ = 45° and θ = 30° at the sample voltages of V = 0.7 eV and 1.1 eV. Combined with the density functional theory (DFT) calculation, we attribute this change to the shift of an electronic state due to the rotation of the mutual angle between the top and bottom Pc. For the spin behavior, we previously observed a Kondo resonance for the TbPc molecule adsorbed on the Au(111) surface. On the Ag(111) surface, the Kondo resonance is hardly observed, which is due to the annihilation of the π radical spin by the charge transfer from the substrate to the molecule. Instead we observe a Kondo peak for the molecule on the second layer, for which the spin recovers due to the reduction of the coupling with the substrate. In addition, when a magnetic field of 2 T normal to the surface is applied, the second layer molecule shows a sharp dip at the Fermi level. We attribute this to the inelastic tunneling feature caused by the spin flipping. This feature is not observed for the TbPc/Au(111) system, suggesting that the decoupling between the TbPc molecule and Ag(111) by the presence of the first layer produces an inelastic feature in the tunneling spectra.

On-surface structural and electronic properties of spontaneously formed Tb2Pc3 single molecule magnets

“…1b), reveals two distinct molecular contrasts, dark and bright, which correspond to different apparent heights of molecules. Differences in height have been previously observed in STM measurements for TbPc 2 absorbed on Au(111) and Ag(111) surfaces 6,24 and additional species with higher apparent height were attributed to a second layer of TbPc 2 molecules. 24,27 However, apparent topography measured in STM on the same region may change substantially at different tip-sample biases.…”

“…The presence of the Kondo effect in TbPc 2 molecules deposited on different metal surface has already been reported by many groups. 6,9,11,15,19,[24][25][26][27]30,35 Consequently we attributed this zero bias anomaly to the Kondo resonance. Our spectroscopic measurements establish that the Kondo resonance appears only for the Tb 2 Pc 3 molecules having the HOMO orbital located closest to the Fermi level.…”

“…For certain bias voltages, the higher apparent height molecules are roughly double that of the lower apparent height molecules relative to the Ag(111) surface, which is likely why they have previously been identified as a second layer of TbPc 2 molecules. 24,27 z) spectroscopies taken on the three different regions indicated by the colored arrows on c). We use the minima from these curves to determine the heights of TbPc 2 and Tb 2 Pc 3 as 0.62 nm and 0.92 nm, respectively.…”

“…[5][6][7]11,[13][14][15][17][18][19][20]22,[24][25][26][28][29][30]35,36 The Kondo phenomenon is observed in all studies conducted on the monolayer TbPc 2 on Au(111) 19,27 but not on the Ag (111) substrate. 24 Nevertheless, in the case of the higher apparent height (i.e. brighter) molecules, on Au(111) the Kondo effect is quenched, but is present on Ag(111).…”

“…16,[32][33][34] One prevalent and widely studied phenomena on TbPc 2 molecules is the selective and controllable presence of a zero-bias Kondo resonance in the dI/dV spectra, localized to the Pc ligand lobes. 6,19,24,26,30,35 The location and strength of the Kondo resonance has been attributed to charge transfer between the substrate and the TbPc 2 molecules. [5][6][7]11,[13][14][15][17][18][19][20]22,[24][25][26][28][29][30]35,36 The Kondo phenomenon is observed in all studies conducted on the monolayer TbPc 2 on Au(111) 19,27 but not on the Ag (111) substrate.…”