Morphological Change of Molecular Assemblies through On-Surface Chemical Reaction

Abstract

Fabrication of molecular assemblies by arranging molecules through on-surface chemical reactions is an attractive way of functionalizing nanosheets chemically. Here, we study morphological changes of organometallic assemblies on a graphene oxide surface by controlling the formation of covalent bonds through on-surface chemical reactions with and without coupling agents. The difference in surface morphology originates from the different crystal growths of molecules diffusing on the surface, relating to the formation of covalent immobilization between the molecule and the surface. The molecules forming the assemblies have individual spins acquired by charge transfer between the molecule and the surface. The size and distribution of the assemblies on the surface produce morphology-related magnetism among the weakly interacting molecular spins. The present approach based on arranging simple molecules through on-surface chemical reactions provides new insights into molecular magnetism.

Weakly Interacting Molecular Spins in On‐Surface Synthesized Nanoclusters on a Graphene Oxide Nanosheet

“…Assuming g = 2, C gives a magnetic moment g𝜇 B S = 5.1 𝜇 B , confirming that the Fe ions are in the high-spin state (S = 5/2) rather than the low-spin state (S = 1/2) (Figure 1A). Note that this value was underestimated in our previous study [9] because it was determined in a magnetic field of 70 kOe and in an unsaturated M-H loop at T = 2 K. The constant Θ was −2.9 K, indicating that there were negative interactions, as predicted by mean-field theory. [18] Low-temperature zero-field 57 Fe Mössbauer spectroscopy [22,23] confirms that the Fe ion in the amino-ferrocene molecules changed from a non-magnetic S = 0 state to a S = 5/2 high-spin state when assembled by on-surface chemical reaction on a GO nanosheet.…”

“…The electronic states of the 3d electrons of the Fe ion in ferrocene or ferrocene-derivatives are dominated by the ligand field. [10] Inside the nanocluster, the Fe ion of a given amino-ferrocene molecule has unpaired electrons due to the through-bond electron transfer process between the Fe ion and the GO, [9] which behaves as a localized spin due to the geometry of the ferrocene unit (Figure 1A). Magnetic dipole interactions between localized spins in molecular assemblies are generally much weaker than intramolecular interactions and magnetic anisotropy in the molecule, because the dipole energy approximated as m 2 /r 3 is equivalent to only 0.0006 K assuming a magnetic moment m of 1 𝜇 B and a distance r between them of 1 nm.…”

“…The amino‐ferrocene molecules used in this study ( Figure A) can self‐assemble into nanoclusters on a graphene oxide nanosheet via on‐surface chemical reaction. [ 9 ] We have measured the magnetic properties of densely stacked weakly interacting molecular spins in a nanocluster with a diameter of ≈2 nm and discussed those properties by combining theoretical simulations and a systematic analysis of the experimental data.…”

“…[ 11,12 ] Here, the nearest dipole interactions dominate the formation of these properties. In our amino‐ferrocene‐based nanoclusters, the d m = 0.28 nm distance between neighboring localized spins [ 9 ] (Figure 1A) is much shorter than the d m between neighboring magnetic moments in Fe 12 cluster magnet crystals ( d m ≈ 1.4 nm) [ 7 ] and in nanoparticle assemblies ( d m = 2–10 nm), [ 11,12 ] leading to the completely different situation that all molecular spins within the nanocluster are weakly connected by magnetic dipole fields of the order of 10 2 –10 3 Oe. The interactions in the nanoclusters differ from those treated in a mean‐field‐based theory.…”

“…The interactions in the nanoclusters differ from those treated in a mean‐field‐based theory. [ 13 ] In addition, the controlled on‐surface self‐organization of our nanoclusters can produce a uniform nanocluster distribution with a relatively large inter‐cluster distance, [ 9 ] resulting in a surface structure close to an isolated magnetic nanocluster distribution on an insulating GO nanosheet. Here, we analyze the behavior of densely packed molecular spins resulting from such unique magnetic interactions in nearly 0D nanoclusters by measuring the magnetic susceptibility, magnetization, 57 Fe Mössbauer spectrum and by simulating the distribution of the spin orientations by Markov‐chain Monte Carlo methods.…”