Fe-phen molecules on Cu(100)

Figure 1c displays an STM image of isolated Fe-phen molecules on Cu(100). Each molecule shows two lobes. Because of the strong affinity of S to the Cu substrate, the NCS groups bind to the substrate and the two phen groups point upwards and are seen with the STM as the two lobes. There exist two molecular conformations for Fe-phen on Cu(100). Type I shows a larger inter-lobe distance than type II. To identify the spin state of the two types, differential conductance spectra of the tunnelling current (dI/dV) were measured (Fig. 1d). The spectrum measured atop a type II molecule′s centre exhibits only minor differential conductance variations that are likely related to the electronic structure of the molecule adsorbed on Cu(100). In contrast, the spectrum atop a type I molecule′s centre shows a strong resonance near the Fermi energy. The resonance exhibits a Fano line shape16, indicating a Kondo resonance. A Fano fit to the spectrum reveals a Kondo temperature T K of ≈317 K, a q-factor of 6.1 and a shift of the Kondo resonance due to Coulomb repulsion of 5.7 meV. This large T K underscores a strong electronic hybridization between the Fe spin and the Cu substrate similar to that found in Fe2+ ions of Fe-Phthalocyanine on Au(111) with Kondo temperatures between 357 and 598 K17. A dI/dV map of the Kondo feature (Fig. 1e,f) reveals that the Kondo cloud is centred around the expected position of the Fe-ion for a type I molecular conformation, while there is no resonance for a type II molecular conformation.

A Kondo resonance is a signature of unpaired spins and is caused by a many-body state including the substrate electrons. Thus, these spectroscopic experiments ascribe a type I molecular conformation as the HS and type II as the LS state. As further evidence, we have carried out X-ray absorption spectroscopy (XAS) measurements of ≈2 molecular layers (ML) of Fe-phen on Cu(100) at low temperatures.

As XAS is an element-specific spectroscopic method, this approach is very effective to elucidate the electronic structure of Fe ions, that is, to resolve the HS and LS states. During the measurements, no external illumination except for the X-rays was applied to avoid light-induced excited spin state trapping18. The surface structure of 2 ML thin films as verified by STM (Supplementary Fig. S1a,b) exhibits an ordered structure along the four-fold symmetry of the Cu(100) surface. A closer look at the film reveals the existence of two types of molecular conformations that are similar to the type I and type II conformations of individual molecules on Cu(100) shown in Fig. 1c. We first present XAS spectra of Fe-phen powder acquired above and below the SCO transition at T≈175 K14, as references to analyse thin film data. Top and middle of Fig. 2a display the XAS spectra of the HS state and the LS state measured at 200 K and 100 K, respectively. Clear differences exist between the spectra, such as a dominant peak located at ≈706 eV for the HS state and at ≈708 eV for the LS state of the Fe 2p L 3 absorption edge, in good agreement with previous studies19. The bottom of Fig. 2a displays the XAS spectrum of the molecular film on Cu(100) measured at 50 K. In agreement with the STM experiments, the XAS spectrum exhibits both peaks indicating the coexistence of the two spin states. To confirm this interpretation, we have reproduced the XAS 2 ML spectrum through a linear combination of the HS (46% weight) and LS state (54% weight) reference spectra at the L 3 and the L 2 edge (Fig. 2b,c). The validity of the mixed spin state in 2 ML Fe-phen thin film is further reinforced by X-ray magnetic circular dichroism (XMCD) measurements showing nearly identical spectra for the thin film sample (Fig. 2d) and the powder sample (Fig. 2e). Note that some of the LS state molecules are switched to the HS state owing to the soft X-ray-induced excited spin state trapping19, which can be seen in the XAS spectra. A spectrum change of XAS with respect to circular right- and left- polarized X-ray was observed only near the peak at ≈706 eV and no change was observed at ≈708 eV. This clearly proves the coexistence of the two spin states, as the polarization of the X-ray only affects the magnetic HS state spectrum mainly positioned at ≈706 eV and the non-magnetic LS state spectrum mainly positioned at ≈708 eV does not react to the polarization of the X-ray. To conclude this part, XAS and XMCD measurements obtained on thin films on Cu(100) show that the spectra are split into two contributions very similar to those obtained for the HS and the LS states in bulk samples. Thus, the two molecular states observed in STM can be assigned to the HS (S=2) and LS (S=0) states.

Figure 2: Coexistence of HS and LS states of Fe-phen molecules on Cu(100). (a) Fe 2p XAS spectra of Fe-phen powder reference sample obtained at 200 K (HS state: top) and 100 K (LS state: middle), and two monolayers of Fe-phen thin film on Cu(100) obtained at 50 K (bottom). Reproduction of Fe 2p XAS L 3 (b) and L 2 (c) edges of two monolayers Fe-phen using the reference spectra for the HS and LS states obtained on the powder sample. Green circles represent the experimental data of the thin film, black lines the reproduced spectra obtained from the linear combination of the powder spectra of the HS state (red dotted line) and LS state (blue dotted line). The black dotted line represents the background. XAS and XMCD spectra of (d) two monolayers Fe-phen thin film on Cu(100) measured at 1.5 K and 6.5 T, and (e) Fe-phen powder reference sample measured at 4 K and 6.5 T. To obtain dichroic signal, the helicity of the X-ray was reversed between circular right (CR) and circular left (CL). The black dotted line shows the zero line of the XMCD signal. Full size image

Despite applying high local electric fields (up to several GV m−1) in the junction of the STM, high tunnelling voltages (± 4 V) and high current densities (up to 1 μA per molecule), we were unable to electrically switch between the two types (Supplementary Fig. S2a–d). Switching between the states was only possible by moving molecules laterally with the STM tip. We infer that the strong S-Cu bond impairs spin switching with the tunnelling conditions given above.

Fe-phen molecules on CuN/Cu(100)

To reduce the chemical interaction between molecule and substrate, we inserted a monoatomic CuN layer, which significantly reduces the adsorption energy and ensuing hybridization20. Moreover, the bond of S to the oxidized Cu atoms in the CuN network is expected to be weaker. Figure 3a shows an STM image of isolated Fe-phen molecules on CuN/Cu(100). As with Cu(100), the STM images suggest that the molecules adsorb with the NCS groups onto the CuN surface, and two types of molecular conformations denoted as type α and type β are observed (for large-scale images of the CuN surface and the molecules on the CuN surface see Supplementary Fig. S3 a–d). The difference in the molecular shape as seen by STM is significantly smaller when compared with that on bare Cu, which might be due to the electronic decoupling of the molecules. The two types reveal slight differences in the height profiles (Fig. 3b): the central region of type α is higher than that of type β. dI/dV spectra near the Fermi energy recorded on the centre of both types show a clear Kondo resonance only on type α and a weak spectroscopic feature on type β (Fig. 3c). A similarly weak feature is also frequently found in the spectrum recorded on bare CuN. We associate it with inelastic vibronic excitations of the CuN layer. From the spectroscopic data, we therefore ascribe the type α (β) as the HS (LS) conformational state in analogy to the molecules on Cu(100). As further confirmation, spectral data recorded on the organic ligands reveal strong electronic differences between the two types (Supplementary Fig. S4), in agreement with theoretical predictions of electronic differences between the HS and LS molecules12. The weaker electronic hybridization expected on CuN compared with Cu results in a lowering of the fitted Kondo temperature to T K ≈105 K (q=4.9, Coulomb shift 5.5 meV). This reduced hybridization also results in molecular orbitals that are energetically better defined (Supplementary Fig. S4). Note, however, that the molecular states are still rather broad. From the observed widths of the molecular states, lifetimes of these excited states between 2 and 5 fs can be estimated. These short lifetimes indicate an efficient hybridization of the molecular states with the substrate and exclude the possibility that the observed peak of HS molecules at the Fermi level is caused by a charged state of the molecule.

Figure 3: SCO Fe-phen molecules on CuN/Cu(100). (a) STM image (3×5.5 nm2) of isolated Fe-phen molecules on the CuN/Cu(100) surface with two conformations denoted as α (HS) and β (LS) and (b) line scans across the long axis of the molecules showing the difference in their appearance. (c) dI/dV spectra taken on the centre of the two configurations of the Fe-phen molecules together with a Fano fit (dotted black line) to the Kondo resonance of type α. The black line is the spectrum recorded on bare CuN with the same tip. Full size image

Contrary to Fe-phen on bare Cu(100), Fe-phen on CuN/Cu(100) can be electrically switched between the HS and LS states reproducibly, leading to a single-molecule device whose state can be deterministically selected. When performing cyclic I(V) curves atop the molecule′s centre with the STM feedback loop opened, conformational switching can be seen as an abrupt jump in the I(V) curve2. Figure 4a displays a typical cyclic I(V) sweep with arrows indicating the sweep direction. An abrupt current increase (decrease) is seen on the forward (reverse) sweep at ≈+1.2 V (≈−0.8 V). The cyclic I(V) curves depict a hysteretic behaviour at intermediate voltages indicated by two branches of the I(V) curve (high conduction branch in red and low conduction branch in blue). Repeated forward and reverse I(V) sweeps reveal the switching between the two states to be fully reversible.

Figure 4: Spin and junction conductance switching of single molecules. (a) I(V) curves of isolated Fe-phen molecules on the CuN/Cu(100) surface recorded on the centre of the molecule showing a hysteretic switching behaviour (tunnelling conditions for tip stabilization: +1 V, 100 pA). The high (red) and low (blue) junction conductance states are visualized using STM images (b for HS state and c for LS state, each 3.7×3.7 nm2) and corresponding height profiles (d) along the molecular long axis acquired before and after the switching event. (e) dI/dV spectra recorded on the centre of the molecule before and after a switching event from the high-conductance to the low-conductance state and spectrum of the bare CuN surface recorded with the same tip. (b) to (e) indicate that the switching event is a transition from the HS to the LS state. Full size image

This switching behaviour falls into the general category of memristive effects21, which generically describe how it is electrically possible to reversibly alter a device′s conductance. In our case, the two branches of the I(V) curve are related to the HS and LS states of the Fe-phen molecule as seen by comparing STM images and dI/dV spectra of the molecule in each of the two branches. When ramping the sample bias up, the molecule in the HS-state (type α) is found to change its conformation after the abrupt drop in the tunnelling current to the LS-state (type β), as shown in Fig. 4b and c with corresponding height profiles in Fig. 4d. This observation is reinforced by the associated changes in the dI/dV spectra (Fig. 4e). Equally, switching from the LS to the HS state has been verified. This unambiguously ascribes the intrinsic SCO property of Fe-phen as the origin of this nanoscale memristive behaviour. We exclude the possible artifact of STM tip changes, as the switching event does not alter the STM images and dI/dV spectra of the surrounding molecules. The observed combined switching of the spin and conduction state of the molecule is in line with recent predictions for similar molecules22. Note that SCO is not caused by charge transfer or unpaired spins on the organic ligands as in the case of switchable Kondo effect23,24. This is clearly evidenced by the fact that the Kondo resonance is only observed on the position of the central Fe ion and not on the organic ligands. Thus, the spin of the molecule is associated with a magnetic moment on the metal ion3.

As the HS (LS) state of a single Fe-phen molecular junction can be prepared by applying a suitably large applied bias of negative (positive) polarity (Fig. 4), we can test the bistable deterministic nature of this memory device by applying a suitable train of bias pulses. As shown in Fig. 5a, alternating positive and negative bias pulses lead to molecular junctions with a current level measured at +0.1 V that is associated with each of the desired LS and HS states, respectively. We emphasize that it is not only the amplitude of the bias pulse but also its polarity that determines the state after the pulse. After the molecule has been switched from the HS to the LS state by a positive voltage, it is not switched back by a second positive pulse to the HS state. Only applying a negative pulse switches the molecule back to the HS state (Fig. 5b). The same holds for the opposite switching process (not shown). This proves that addressing the memory device is fully deterministic. Furthermore, the lifetime of each spin state is remarkably long at the observation temperature: no spontaneous switches were observed even after tens of hours of STM observation at low bias voltages (<±0.5 V).