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. 2020 Nov 10;12(7):2381-2388.
doi: 10.1039/d0sc04555a.

Room temperature conductance switching in a molecular iron(iii) spin crossover junction

Senthil Kumar Karuppannan  1 , Alejandro Martín-Rodríguez  2 , Eliseo Ruiz  2 , Phimphaka Harding  3 , David J Harding  3 , Xiaojiang Yu  4 , Anton Tadich  5 , Bruce Cowie  6 , Dongchen Qi  6 , Christian A Nijhuis  1   7
Affiliations

Room temperature conductance switching in a molecular iron(iii) spin crossover junction

Senthil Kumar Karuppannan et al. Chem Sci. .

Abstract

Herein, we report the first room temperature switchable Fe(iii) molecular spin crossover (SCO) tunnel junction. The junction is constructed from [FeIII(qsal-I)2]NTf2 (qsal-I = 4-iodo-2-[(8-quinolylimino)methyl]phenolate) molecules self-assembled on graphene surfaces with conductance switching of one order of magnitude associated with the high and low spin states of the SCO complex. Normalized conductance analysis of the current-voltage characteristics as a function of temperature reveals that charge transport across the SCO molecule is dominated by coherent tunnelling. Temperature-dependent X-ray absorption spectroscopy and density functional theory confirm the SCO complex retains its SCO functionality on the surface implying that van der Waals molecule-electrode interfaces provide a good trade-off between junction stability while retaining SCO switching capability. These results provide new insights and may aid in the design of other types of molecular devices based on SCO compounds.

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Conflict of interest statement

There are no conflicts to declare.

Figures

Fig. 1
Fig. 1. (A) Schematic illustration of the SCO molecule based device with the EGaIn top-electrode stabilized in a small through-hole in a rubber stamp (made of PDMS) and (B) the SCO based molecular junction of the form of Cu//SLG//[FeIII(qsal-I)2]NTf2//GaOx/EGaIn (EGaIn = eutectic alloy of Ga and In).
Fig. 2
Fig. 2. (A) Raman spectra of SLG before (red) and after adsorption (black) of [FeIII(qsal-I)2]NTf2. AFM amplitude images of (B) bare graphene and (C) after adsorption of [FeIII(qsal-I)2]NTf2 and the corresponding AFM height profiles (D). Angle dependent X-ray photoelectron spectra of (E) C 1s, and (F) N 1s (see section S1 for all other spectra).
Fig. 3
Fig. 3. (A) The X-ray absorption spectra (XAS) of the Fe L3 edge for a monolayer of [FeIII(qsal-I)2]NTf2 on graphene at different temperatures, along with simulated peaks for octahedral (Oh) Fe2+ and Fe3+ ions obtained with the CTM4XAS program. The XAS spectra of Fe L3 edges for two different external magnetic field directions at (B) 340 K, (C) 300 K, and (D) 260 K. The difference between the XAS signals observed at +1.5 T and −1.5 T defines the XMCD signal.
Fig. 4
Fig. 4. (A) Heatmap of log│J│ vs. (V) curves, the solid black line is the 〈log10|J|〉Gvs. V curve. (B) J(V) curves recorded as a function of temperature 340 → 240 → 340 K at intervals of 10 K for Cu//SLG//[FeIII(qsal-I)2]NTf2//GaOx/EGaIn junction. (C) J as a function of temperature at ±1.0 V applied bias (solid lines are visual guides), and (D) normalized differential conductance for the Cu//SLG//[FeIII(qsal-I)2]NTf2//GaOx/EGaIn junction at different temperature; solid lines are fits to the eqn (2).
Fig. 5
Fig. 5. Representation of the optimized unit cell for the high-spin system.
Fig. 6
Fig. 6. Projected density of states (PDOS) of [Fe(qsal-I)2]NTf2 for the high spin state (A) and low spin state (B). The projected transmission spectrum of [Fe(qsal-I)2]NTf2 for the high spin state (C) and low spin state (D). Red and blue colors stand for alpha and beta spin–orbitals. The filled curves are the PDOS of the FeIII metal.
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