2D Ionic Liquid-Like State of Charged Rare-Earth Clusters on a Metal Surface

Abstract

Rare-earth complexes are vital for separation chemistry and useful in many advanced applications including emission and energy upconversion. Here, 2D rare-earth clusters having net charges are formed on a metal surface, enabling investigations of their structural and electronic properties on a one-cluster-at-a-time basis using scanning tunneling microscopy. While these ionic complexes are highly mobile on the surface at ≈100 K, their mobility is greatly reduced at 5 K and reveals stable and self-limiting clusters. In each cluster, a pair of charged rare-earth complexes formed by electrostatic and dispersive interactions act as a basic unit, and the clusters are chiral. Unlike other non-ionic molecular clusters formed on the surfaces, these rare-earth clusters show mechanical stability. Moreover, their high mobility on the surface suggests that they are in a 2D liquid-like state.

Keywords: Au(111) surface; ionic liquid; rare‐earth metals; scanning tunneling microscopy; triflate anions.

Figure 1

Structures of lanthanum complexes. a) A model of [La(pcam)₃]³⁺. b) A model of complex [La(pcam)₃X]²⁺. c) STM image of [La(pcam)₃]³⁺ (A) and [La(pcam)₃X]²⁺ (B) on a Cu(111) surface. The blue arrow indicates a location where the [La(pcam)₃]³⁺ complex is moved during scanning while the white arrow indicates the shorter side of the [La(pcam)₃X]²⁺ complex. (Tunneling parameters: V _t = 1 V, I _t = 2.6 × 10⁻¹¹ A). d) An angled view of [La(pcam)₃X]²⁺ where the triflate counterion is indicated with a dashed oval. e) DFT calculated adsorption geometry of [La(pcam)₃]³⁺ on Au(111) surface in top view, and f) in side view. g) DFT calculated adsorption geometry of [La(pcam)₃X]²⁺ on Au(111) surface in top view, and h) in a side view.

Figure 2

Rare‐earth clusters. a) An STM image acquired at 120 K substrate temperature on Au(111) shows the ionic liquid‐like condition of deposited salt. The arrows indicate highly fluid ionic species. (Tunneling parameters: V _t = 1.8 V, I _t = 6 × 10⁻¹¹ A). b) An STM image acquired at 5 K substrate temperature on Au(111) shows the clusters of rare‐earth complexes. The arrows indicate the mobility of the clusters while the ovals mark the complexes having two triflate ions. (Tunneling parameters: V _t = 1 V, I _t = 3.8 × 10⁻¹¹ A). c–e) STM images of two, four, and six‐unit clusters and corresponding models, respectively. [La(pcam)₃]³⁺ and [La(pcam)₃X]²⁺ complexes are marked with light blue and light green triangles, respectively. (Tunneling parameters: V _t = 0.5 V, I _t = 3.3 × 10⁻¹¹ A for (c); V _t = −2 V, I _t = 3.8 × 10⁻¹¹ A for (d); and V _t = 1 V, I _t = 1.2 × 10⁻¹¹ A for (e)).

Figure 3

Calculated structure of a complex pair on Au(111). a) A DFT calculated charge density plot of a complex pair adsorbed on Au(111) slab showing optimal geometry. b) A side view of the complex pair where the two La atoms (light blue balls) are separated by ≈11 Å. The position of the triflate counterion is shown by a dashed oval. c) Calculated potential energy as a function of distance between the two La atoms of complex pair. d) Calculated charge density plot showing local electrostatic interactions (indicated with an oval) between the two complexes. Blue: charge gain, and red: charge lost.

Figure 4

Cluster mobility and electronic structures. a) An STM image shows an abrupt lateral movement (indicated with an arrow) of a four‐unit cluster during scanning. b) When the same area is imaged again, the cluster is laterally displaced to the left but remains intact. (Tunneling parameters for “a” and “b”: V _t = 1.0 V, I _t = 3.8 × 10⁻¹¹ A). c) STM image of a six‐unit cluster (Tunneling parameters: V _t = 1.0 V, I _t = 1.2 × 10⁻¹¹ A) and d) a simultaneously acquired dI/dV map at +1 V reveals bright protrusions (yellow regions) on [La(pcam)₃]³⁺ complexes in the cluster while the signal is absent on the [La(pcam)₃X]²⁺ complexes. e) dI/dV tunneling spectroscopy of [La(pcam)3]³⁺ and [La(pcam)₃X]²⁺ complexes acquired at the locations shown with dots in (c). The Au(111) surface state, SS, is also recorded in both spectra at −0.45 V. Notice that the LUMO is absent in the [La(pcam)₃X]²⁺ complex. The upper spectrum is vertically shifted for clarity.

Figure 5

Cluster chirality. a,b) STM images of two‐unit cluster enantiomers and corresponding models. (Tunneling parameters: V _t = 0.5 V, I _t = 3.3 × 10⁻¹¹ A for (a), V _t = 0.25 V, I _t = 2.0 × 10⁻¹¹ A for (b)). c) An STM image showing a pair of four‐unit cluster enantiomers and corresponding models. (Tunneling parameters: V _t = 1.0 V, I _t = 9.5 × 10⁻¹² A). “M” and “P” labeling is arbitrary, as the chirality of each unit in the cluster cannot be assessed by STM.