Magnetic Ordering in Gold Nanoclusters

Abstract

Several research groups have observed magnetism in monolayer-protected gold cluster samples, but the results were often contradictory, and thus, a clear understanding of this phenomenon is still missing. We used Au₂₅(SCH₂CH₂Ph)₁₈ ⁰, which is a paramagnetic cluster that can be prepared with atomic precision and whose structure is known precisely. Previous magnetometry studies only detected paramagnetism. We used samples representing a range of crystallographic orders and studied their magnetic behaviors using electron paramagnetic resonance (EPR). As a film, Au₂₅(SCH₂CH₂Ph)₁₈ ⁰ exhibits a paramagnetic behavior, but at low temperature, ferromagnetic interactions are detectable. One or few single crystals undergo physical reorientation with the applied field and exhibit ferromagnetism, as detected through hysteresis experiments. A large collection of microcrystals is magnetic even at room temperature and shows distinct paramagnetic, superparamagnetic, and ferromagnetic behaviors. Simulation of the EPR spectra shows that both spin-orbit (SO) coupling and crystal distortion are important to determine the observed magnetic behaviors. Density functional theory calculations carried out on single cluster and periodic models predict the values of SO coupling and crystal-splitting effects in agreement with the EPR-derived quantities. Magnetism in gold nanoclusters is thus demonstrated to be the outcome of a very delicate balance of factors. To obtain reproducible results, the samples must be (i) controlled for composition and thus be monodisperse with atomic precision, (ii) of known charge state, and (iii) well-defined in terms of crystallinity and experimental conditions.

Figure 1

Experimental (black) and calculated (red) cw-EPR spectra of an Au₂₅(SC2Ph)₁₈⁰ amorphous film at different temperatures (K), as indicated. In (a), the data were multiplied by a factor of 10 with respect to those in (b). In (b), the blue trace corresponds to the EPR cavity at 5 K.

Figure 2

Dependence of the double-integrated EPR intensity on the reciprocal of temperature. The solid line is the linear regression of the data (black square) at the higher temperatures.

Figure 3

Effect of temperature (K) on the cw-EPR spectra of one single crystal of Au₂₅(SC2Ph)₁₈⁰.

Figure 4

Orientation dependence of the cw-EPR spectra of one single crystal of Au₂₅(SC2Ph)₁₈⁰ uncovered (a) or covered (b) by frozen MeCN. Within each graph, the EPR tube was rotated by 0 (blue), 90 (red), and 180° (black) (T = 5 K).

Figure 5

Hysteresis cw-EPR experiment for a Au₂₅(SC2Ph)₁₈⁰ single crystal at 5 K. The direction and trace color of the three scans are indicated.

Figure 6

Effect of temperature (K) on the cw-EPR spectra of an Au₂₅(SC2Ph)₁₈⁰ collection of microcrystals.

Figure 7

Hysteresis cw-EPR experiments for a large collection of Au₂₅(SC2Ph)₁₈⁰ microcrystals. (a) Effect of decreasing the temperature from 60 to 9 K and (b) corresponding temperature increase. The black and the red traces indicate the low-to-high- and high-to-low-field directions, respectively.

Figure 8

Simulations of the cw-EPR spectrum obtained at 5 K for the Au₂₅(SC2Ph)₁₈⁰ film (black). The simulations include SO and distortion (red), only SO (blue), and only distortion (green).

Figure 9

Diagram of DFT/B3LYP HOMO orbital energies (eV) in Au₂₅(SCH₃)₁₈⁰ systems. From left to right: Au₂₅(SCH₃)₁₈⁰ at the scalar relativistic level in the Au₂₅(SCH₃)₁₈⁰-anion geometry, Au₂₅(SCH₃)₁₈⁰ at the scalar relativistic level in the Au₂₅(SCH₃)₁₈⁰-crystal geometry, which includes Jahn–Teller (J–T) effects, and Au₂₅(SCH₃)₁₈⁰ including SO coupling (SOC) in the Au₂₅(SCH₃)₁₈⁰-crystal geometry.

Figure 10

Schematic depiction of the direction and magnitude of atomic spins (green arrows) in the putative spin global minimum (the spins on the Au atoms are not shown as they would be out of scale). The image shows the unit cell as seen from the direction c; all clusters but the central one are thus incomplete. The color codes are Au = yellow, S = red, and C = gray. Au and S atoms and bonds are rendered as balls and sticks, whereas C is rendered as the stick style. H atoms have been removed for clarity.