On-Surface Synthesis of Organolanthanide Sandwich Complexes

Abstract

The synthesis of lanthanide-based organometallic sandwich compounds is very appealing regarding their potential for single-molecule magnetism. Here, it is exploited by on-surface synthesis to design unprecedented lanthanide-directed organometallic sandwich complexes on Au(111). The reported compounds consist of Dy or Er atoms sandwiched between partially deprotonated hexahydroxybenzene molecules, thus introducing a distinct family of homoleptic organometallic sandwiches based on six-membered ring ligands. Their structural, electronic, and magnetic properties are investigated by scanning tunneling microscopy and spectroscopy, X-ray absorption spectroscopy, X-ray linear and circular magnetic dichroism, and X-ray photoelectron spectroscopy, complemented by density functional theory-based calculations. Both lanthanide complexes self-assemble in close-packed islands featuring a hexagonal lattice. It is unveiled that, despite exhibiting analogous self-assembly, the erbium-based species is magnetically isotropic, whereas the dysprosium-based compound features an in-plane magnetization.

Keywords: STM/STS; XMCD; density functional theory; lanthanides; on‐surface synthesis; organometallic chemistry.

Scheme 1

Chemical route to prepare Er(p‐HOB)₂ or Dy(p‐HOB)₂ species on Au(111). Some ligands in panel (1) might already be partially deprotonated at room temperature.^{[ 22 ]}

Figure 1

Sequential addition of dysprosium atoms on H₆HOB/Au(111). a) STM image of only H₆HOB molecules deposited on a clean Au(111) substrate held at room temperature. From b) to d) STM images of sequential sublimation of Dy on a Au(111) substrate with close to a monolayer of H₆HOB molecules. Total Dy sublimation time: 1 min (b), 3 min (c), and 5 min (d). In all preparations, the substrate was held at 100 °C during Dy deposition, and the samples where post‐annealed at 100 °C for 10 min. The scale bar is 10 nm in (a) and 12 nm in (b–d). Scanning parameters: a) V_bias = 0.5 V, I_t = 20 pA, T = 4 K; b) V_bias = 0.5 V, I_t = 50 pA, T = 4 K; c) V_bias = 0.5 V, I_t = 10 pA, T = 4 K; d) V_bias = 1.0 V, I_t = 20 pA, T = 4 K.

Figure 2

Lanthanide‐based sandwich complexes on Au(111). a) STM image overview of as‐grown Er(p‐HOB)₂, with a close view of the island framed in a red square and displayed in b). The inset of (b) is a zoom‐in of the sandwich structure with the unit cell represented by a rhombus. The bar scale is 1 nm. c) STM image overview of partially grown Dy(p‐HOB)₂. d) A closer view of the purple frame is displayed in c) with a superimposed model (black circles) centered on a second layer molecule (white circle) showing that the second layer molecules are perfectly aligned with the first layer molecules. e) Height profile of the green line in d), in which the X = 0 point from the profile corresponds to the upper right end of the line. f–h) vdW‐DFT‐D3 calculation of the preferred adsorption geometry of the Dy(p‐HOB)₂. (f) top view showing the unit cell in dashed lines; g,h) the side views in (g) showing the partial deprotonation of the H₆HOB molecules. Yellow, cyan, red, grey, and white balls represent Au, Dy, O, C, and H atoms, respectively. Scale bars are: a) 14 nm, b) 4 nm, c) 8 nm, d) 2 nm. Scanning parameters: a) V_bias = 0.5 V, I_t = 10 pA, T = 4 K; b) V_bias = 0.5 V, I_t = 10 pA, T = 4 K, c) V_bias = 0.4 V, I_t = 10 pA, T = 4 K; d) V_bias = 0.4 V, I_t = 10 pA, T = 4 K.

Figure 3

Characterization of the electronic properties of Er(p‐HOB)₂ and Dy(p‐HOB)₂. a) From top to bottom, STS of Er(p‐HOB)₂ (blue plot), Dy(p‐HOB)₂ (red plot), and H₆HOB molecules (green plot) on Au(111). The Au(111) reference taken at a closer point is presented below every spectrum (grey plots). b–d) STM images of the location of the taken STS spectra. Scale bars are 2 nm in all STM images. Scanning parameters: b) V_bias = 500 mV, I_t = 30 pA, T = 4 K; c) V_bias = 500 mV, I_t = 50 pA, T = 4 K; d) V_bias = 500 mV, I_t = 40 pA, T = 4 K.

Figure 4

Magnetic characterization of Dy(p‐HOB)₂ and Er(p‐HOB)₂ species on Au(111). a–c) Dy(p‐HOB)₂; d–f) Er(p‐HOB)₂. a,d) XAS with positive (µ₊, green) and negative (µ_‐, orange) circularly polarized light and XMCD (µ_‐ − µ₊) taken at the Dy/Er M_4,5‐edges at normal (0°, red) and grazing (70°, blue) incidences (B = 6 T, T = 1.6 K). b,e) Magnetization curves constructed by measuring the XMCD intensity at the most intense peak of the Dy/Er M₅‐edge. (T = 1.6 K). c,f) XAS spectra acquired with vertical (µ_V, pink) and horizontal (µ_H, black) linearly polarized light and XLD (µ_V – µ_H, brown) taken at the Dy/Er M₅‐edge at grazing (70°) incidence (B = 0.05 T, T = 1.6 K).