Molecular-scale dynamics of light-induced spin cross-over in a two-dimensional layer

Abstract

Spin cross-over molecules show the unique ability to switch between two spin states when submitted to external stimuli such as temperature, light or voltage. If controlled at the molecular scale, such switches would be of great interest for the development of genuine molecular devices in spintronics, sensing and for nanomechanics. Unfortunately, up to now, little is known on the behaviour of spin cross-over molecules organized in two dimensions and their ability to show cooperative transformation. Here we demonstrate that a combination of scanning tunnelling microscopy measurements and ab initio calculations allows discriminating unambiguously between both states by local vibrational spectroscopy. We also show that a single layer of spin cross-over molecules in contact with a metallic surface displays light-induced collective processes between two ordered mixed spin-state phases with two distinct timescale dynamics. These results open a way to molecular scale control of two-dimensional spin cross-over layers.

Figure 1. Ordered phase of HS and LS molecules.

(a) Schematic of molecule 1 with the z axis along the B–Fe–B direction (grey: C atoms, blue: N atoms, pink: B atoms, red: Fe atom and white: H atoms). (b) STM image acquired at 0.3 V in constant height mode showing the mixed spin-state superstructure S_1/3 (<I>=50 pA). Inset, topographic STM image acquired at V=−1.5 V showing the full molecular crystal (I=20 pA). The scale bar corresponds to 2 nm and is common for both images. The lattice vectors of the molecular network (A and B, black) and the S_1/3 superstructure (a and b, white) are indicated. The full (empty) dots indicate the position of the dark (bright) molecules at 0.3 V. (c) Projected density of states (PDOS) on the ligand atoms for the LS state. (d) PDOS on the d-orbitals of the central Fe^II atom for 1 in LS (S=0). (e) PDOS on the d-orbitals of the central Fe^II atom for 1 in HS (S=2) states. For c–e, the energy of HOMOs is set as zero.

Figure 2. Molecular vibrational modes for both spin states.

(a) Low-energy dI/dV curves for both types of molecules in the S_1/3 superstructure. 1 and 2 mark the position of the pronounced inelastic steps observed for the molecules in the HS state. (b) DFT coupling constants of the vibrational modes associated with the -derived molecular orbitals calculated for the free molecule in both spin states. (c) The displacement vectors (red arrows) for the two active modes indicating their longitudinal character (atoms move mostly along the B–Fe–B axis).

Figure 3. Light-induced SCO and thermal relaxation at the molecular scale.

(a–c) STM images (4.6 K) of the same area (a) in its initial state presenting the S_1/3 superstructure, (b) under blue light illumination after 9 h and 45 min of exposure and (c) in its relaxed state 9 h and 45 min after stopping the blue light illumination (V=0.3 V, I=20 pA). Scale bars, 10 nm. (d–f) Fourier-transformed images of a–c, respectively. (g) Model of the molecular network and both superstructures. For the full crystal representation, the Fe^II centres are enlightened and the lattice vectors A and B are indicated in black (indirect A and B angle), the S_1/3 superstructure is represented by the red circles and lattice vectors a and b the S_1/2 one by the blue circles and lattice vectors c and d. (h) Reciprocal space of both S_1/3 and S_1/2 superstructures (same colour code). (i) Time evolution of the normalized peak intensities x. The data (squares) are fitted using a least squares method by mono-exponential increase and decrease under blue light and for the thermal relaxation, respectively (solid lines). The arrows indicate the position of the STM images (a–c).

Figure 4. Internal dynamics of the photoexcited phase.

(a) STM image taken under blue illumination after 11 h and 15 min of exposure (V=0.3 V, I=20 pA). Scale bar, 10 nm. (b) Zoom on a S_1/3 area, marked by a orange square in a. (c) Zoom on a S_1/2 area, marked by a yellow square in a. (d) Apparent heights of molecules A and B, displayed in b versus time. Their apparent heights are followed with the illumination off and on. (e) Apparent height of molecule C, displayed in c, which switches between LS and HS states under continuous illumination. (f,g) Distribution of t_LS and t_HS, respectively. t_LS (t_HS) is the duration for a molecule in a LS (HS) state before switching in a HS (LS) state. The size of the boxes corresponds to the acquisition time of a STM image (5 min 51 s).