Inducing Magnetism in Pure Organic Molecules by Single Magnetic Atom Doping

We report on in situ chemical reactions between an organic trimesic acid (TMA) ligand and a Co atom center. By varying the substrate temperature, we are able to explore the Co–TMA interactions and create novel magnetic complexes that preserve the chemical structure of the ligands. Using scanning tunneling microscopy and spectroscopy combined with density functional theory calculations, we elucidate the structure and the properties of the newly synthesized complex at atomic or molecular size level. Hybridization between the atomic orbitals of the Co and the $\pi$ orbitals of the ligand results in a delocalized spin distribution onto the TMA. The here demonstrated possibility to conveniently magnetize such versatile molecules opens up new potential applications for TMAs in molecular spintronics.

Figure 1

(a), (b), and (c) Voltage-dependent STM images ($5.8\times 5.5{\mathrm{nm}}^2$, 90 pA) of the TMA superstructure containing the two types of molecules, TMA I and TMA II, highlighted by solid and dotted triangles, respectively. (d) $dI/dV(V)$ spectra of the TMA I, TMA II, and the Au(111) surface exposed in the pores measured at 4.5 K (set point: 0.3 nA, $-0.8\mathrm{V}$). (e) $dz/dV(V)$ spectra measured at 4.5 K (set point: 0.1 nA, 1.4 V). (f) Apparent height analysis of TMA I and TMA II at 0.5 V.

Figure 2

(a) and (b) Simulated STM images of the CoTMA porous structure at 0.7 and 1.6 V, respectively (tip height of 3.5 Å). The chemical structure of the TMA molecule and the unit cell of the simulation are added. (c) The relaxed structure of the CoTMA dimer on Au(111). (d) Apparent height analysis of TMA and CoTMA at 0.7 V.

Figure 3

(a) $dI/dV(V)$ spectra measured at the locations marked in the inset at 4.5 K. (b) ZBP measured from 4.5 K to 35 K (set points: 0.2 V, 0.4 nA). Solid lines in (a) and (b) represent Fano-line shape fits. (c) Temperature dependence of the ZBP width; error bars represent the statistical deviations of the measured data. The solid line is a fit to Eq. (1) with $\alpha =4.56(\pm 0.2)$. (d) STM image ($7.4\times 5.3{\mathrm{nm}}^2$) of the molecular network containing TMA I and TMA II ($-0.2\mathrm{V}$ and 0.16 nA). The solid (dotted) triangle outlines the TMA I (TMA II) molecule. (e) $dI/dV$ map recorded at 5 mV for the area shown in (d).

Figure 4

(a) Energy levels of the gas phase CoTMA complex. The percentages indicate the orbital contribution of the Co fragment. (b) SP PDOS of the gas phase CoTMA complex together with the PDOS contribution of the two fragments. Theory level: B3LYP with the $6\text{−}31{\mathrm{G}}^{**}/6\text{−}31\mathrm{G}$ basis. (c) SP PDOS of the CoTMA complex after adsorption on the gold surface calculated using the LSDA. Fermi level is at 0 V.