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Robust and selective switching of a Fe(III) spin-crossover compound on Cu2N/Cu(100) with memristance behavior Torben Jasper-Toennies, Manuel Gruber, Sujoy Karan, Hanne Jacob, Felix Tuczek, and Richard Berndt Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b02481 • Publication Date (Web): 12 Oct 2017 Downloaded from http://pubs.acs.org on October 13, 2017

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Robust and selective switching of a FeIII spin-crossover compound on Cu2N/Cu(100) with memristance behavior Torben Jasper-Toennies,∗,† Manuel Gruber,∗,† Sujoy Karan,†,¶ Hanne Jacob,‡ Felix Tuczek,‡ and Richard Berndt† Institut für Experimentelle und Angewandte Physik, Christian-Albrechts-Universität, 24098 Kiel, Germany, and Institut für Anorganische Chemie, Christian-Albrechts-Universität, 24098 Kiel, Germany

Abstract

Molecular spintroncis aims at utilizing molecules for spin-based applications. 1 The main motivations to go beyond conventional spintronics are new properties emerging at the metal/molecule interfaces 2–16 and the possibility to incorporate functions (e.g., switching) within the molecules themselves. In this context, spincrossover (SCO) molecules are particularly interesting since they provide a spin-switching functionality. Consequently, SCO molecules on surfaces attracted considerable interest in recent years. 17–39 Two main difficulties were identified for SCO molecules on surfaces. First, the deposition of delicate SCO compounds on surfaces is challenging as it can lead to their dissociation. 12,24,32 Furthermore, strong molecule-substrate interaction often prevents switching of the adsorbed molecule. 21,33 In that respect, so far, all successful studies were limited to FeII SCO molecules, although compounds with different oxidation states or different central ions are available 40 and actually desirable as they exhibit different sets of spin states. Very recently, an FeIII SCO molecule, [Fe(pap)2 ]+ (pap = N-2-pyridylmethylidene-2-hydroxyphenylaminato), was successfully deposited on Au(111). 39 However, although switching events were observed, they occured with an extremely low yield, and thus the nature of the switching could not be determined. Here, we employ the same molecule, namely [Fe(pap)2 ]+ , and deposit it on a Cu2 N/Cu(100) substrate. Using scanning tunneling microscopy (STM), we show that the molecules are intact on the surface, although the charge state cannot be unambiguously identified. Furthermore, thanks to a reduced moleculesubstrate coupling, the molecule can be selectively and reversibly switched into three distinct states, characterized by different tunneling conductances, with a relatively high yield. With the help of gas-phase densityfunctional-theory (DFT) calculations, the spin states

The switching between two spin states makes spincrossover molecules on surfaces very attractive for potential applications in molecular spintronics. Using scanning tunneling microscopy, the successful deposition of [Fe(pap)2 ]+ (pap = N-2-pyridylmethylidene-2hydroxyphenylaminato) molecules on Cu2 N/Cu(100) surface is evidenced. The deposited FeIII spin-crossover compound is controllably switched between three different states, each of them exhibiting a characteristic tunneling conductance. The conductance is therefore employed to readily read the state of the molecules. A comparison of the experimental data with the results of density-functional-theory calculations reveals that all Fe(pap)2 molecules are initially in their high-spin state. The two other states are compatible with the low-spin state of the molecule but differ with respect to their coupling to the substrate. As a proof of concept, the reversible and selective nature of the switching is used to build a two-molecule memory.

Keywords spin crossover; switching; memristance; scanning tunneling microscopy ∗

To whom correspondence should be addressed Institut für Experimentelle und Angewandte Physik, Christian-Albrechts-Universität, 24098 Kiel, Germany ‡ Institut für Anorganische Chemie, ChristianAlbrechts-Universität, 24098 Kiel, Germany ¶ Institute of Experimental and Applied Physics, University of Regensburg, 93053 Regensburg, Germany †

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features. 39 Differences in the topographs are expected as the molecule should be less coupled to a Cu2 N substrate compared to a Au(111) substrate, hence leading to different hybridization of the molecular orbitals with that of the substrate. Indeed, the Cu2 N acts as an insulating layer to electronically decouple the investigated object from the substrate. 51 The topograph of Fe(pap)2 on Cu2 N (Fig. 1(d)) displays a four-lobe structure. Two lobes, placed diagonally around the center, are laterally more extended and thus define the long axis of the molecule. The two other lobes are located almost perpendicularly to the long axis. These lobes are closer to each other and appear higher. As discussed below, long and short axes are expected for [Fe(pap)2 ]+ from DFT calculations. Furthermore, the length along the long axis depends on the molecular spin state, and thus may be used as a readout channel. Yet, anticipating the discussion below, we overlay the calculated molecular structure of HS [Fe(pap)2 ]+ on the topograph (left molecule in Fig. 1(d)). The agreement is relatively good. The calculated length of 1.0 nm is shorter than the 1.9 nm apparent length in the topograph. Such a difference between model and topograph is expected as the orbitals of the molecule extend into vacuum and the topograph is related to the overlap of the molecular orbitals with those of the tip. Interestingly, the apparent height of Fe(pap)2 on Cu2 N (0.24 nm) is much larger than that of Fe(pap)2 on Au(111) (0.15 nm). 39 The different apparent height may reflect a reduced coupling to the substrate. In turn, a reduced molecule-substrate coupling facilitates switching of SCO molecules on surfaces. 21,26,32,33 In order to switch the molecule, the tip was positioned over the center of the molecule followed by an increase of the sample voltage above 1.8 V until a change in the tunneling current was detected (feedback off). Switching of Fe(pap)2 molecules is indeed possible with a much larger yield (discussed below) compared to that of Fe(pap)2 on Au(111) (Ref. 39). Interestingly, and in contrast to previous reports of switching of SCO compounds in direct contact to surfaces, 21,29,31 not only two but three different states are found. Figures 2(a–c) show the three conformations (A, B, and C) of the same Fe(pap)2 molecule upon switching. The first conformation (Fig. 2(a)), referred to as state A, has an elongated shape with an apparent length of 1.9 nm. All Fe(pap)2 molecules on Cu2 N are initially in state A, which presumably is the state of lowest energy. In conformations B and C, the Fe(pap)2 molecule appears shorter by 11 % (1.7 nm), leading to an approximately square shape of the molecule. The main difference between B and C is the orientation of the molecule relative to the Cu2 N lattice (see SI, section II). C is rotated by approximately 45◦ compared to A and B. Furthermore, the center of the molecule appears higher in state C. Actually, the apparent height of the molecular center is unique for each conformation (Fig. 2(a-c)). Consequently, the molecular conformation can directly be inferred from the apparent height (or the conductance)

of the molecule at its center. The three conformations of Fe(pap)2 exhibit very distinct topographic features. The question arises whether the different conformations of the molecules are related to different spin states. We performed gas-phase DFT calculations of [Fe(pap)2 ]+ in the HS (S = 5/2) and low spin (S = 1/2) states. The calculated molecular structures are presented in Fig. 2(d). In the HS (LS) state, the average Fe-O and Fe-N distances are 0.195 nm and 0.221 nm (0.189 nm and 0.194 nm), respectively. These calculated distances are very close (relative difference of 3 % or less) to 0.193 nm and 0.214 nm (0.188 nm and 0.195 nm) found by X-ray diffraction for the HS (LS) state. 41,42 The [Fe(pap)2 ]+ molecule in the HS state is longer (1.0 nm) than in the LS state (0.9 nm). This corresponds to a length reduction of 10 % upon switching from HS to LS. Considering that a similar relative length reduction of 11 % was observed for the transitions from conformation A to conformations B and C, we ascribe conformation A as the HS state. From topographic considerations, molecular conformations B and C are both compatible with the LS state. As the charge state of the adsorbed molecule is unknown, the topographs are also compared to the calculated geometries of the neutral Fe(pap)2 molecule (see SI, section III). This comparison leads to the same HSand LS-state assignments, i.e. conformation A (B-C) corresponds to the molecule in the HS (LS) state. To gain further insight into conformations A, B and C, differential-conductance spectra were measured over a Fe(pap)2 molecule that was successively switched into the different conformations. The normalized dI/dV spectrum taken atop the molecular center in state A shows a dominant feature around −1.24 V (blue curve in Fig. 2(e)). There is indication of a second peak at 0.71 V, whose amplitude, however, is close to the noise level. The corresponding spectra of the B and C states exhibit both an additional peak at −0.14 V (black and red curves in Fig. 2(e)). However, the amplitude of the peak is much more pronounced for state C. A second peak is observed at −1.25 V and −1.21 V in states B and C, respectively. These measurements reveal that A, B, and C have different but similar electronic structures. In particular, states B and C display additional states right below the Fermi level. While the HS state of other SCO molecules on surfaces could be inferred from the presence of a Kondo resonance, 20,21,33 in the present study no Kondo resonance was found for any of the states (like in Ref. 31). This is most probably due to a non-sufficient molecule-substrate coupling. Figure 2(f) shows the calculated projected density of states (PDOS) at the Fe center of HS and LS [Fe(pap)2 ]+ (see methods). It should be mentioned that the energy dependence of the PDOS is not expected to be quantitatively correct. 53 Henceforth, only a qualitative comparison can be made between the calculated PDOS and the experimental dI/dV spectra. The PDOS of LS and HS [Fe(pap)2 ]+ are rather similar with small differences. Furthermore, the LS state exhibits addi-


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