Mechanically-Controlled Reversible Spin Crossover of Single Fe

Mechanically-Controlled Reversible Spin Crossover of Single Fe-Porphyrin Molecules Guowen Kuang,† Qiushi Zhang,† Tao Lin,† Rui Pang,‡ Xingqiang Shi,‡ Hu Xu,‡ and Nian Lin*,† †

Department of Physics, The Hong Kong University of Science and Technology, Hong Kong, China Department of Physics, Southern University of Science and Technology of China, Nanshan District, Shenzhen, Guangdong 518055, China

‡

S Supporting Information *

ABSTRACT: Spin-crossover (SCO) molecules are thought to be ideal systems for molecular spintronics when SCO can be precisely controlled at the single-molecule level. This is demonstrated here in the single-molecule junctions of Feporphyrin formed in a scanning tunneling microscope. Experimentally, we find that the junctions feature a zerobias resonance in molecular conductance associated with the Fe spin center. When mechanically stretching or squeezing the junctions by adjusting the tip height, the line shape of the zero-bias resonance varies reversibly. First-principles calculations reveal that widening the junction gap by 2 Å transforms the macrocyclic core hosting the Fe center from a saddle to a planar conformation. This conformational change shortens the Fe−N bonds by 3%, which changes the Fe spin state from S = 2 to S = 1. KEYWORDS: density-functional theory, Fe-porphyrin, scanning tunneling microscopy, single-molecule junction, spin crossover the junction gap stretches the molecule and gradually flattens the macrocyclic core to a planar conformation, which results in an intermediate-spin (S = 1) Fe center. This process is fully reversible and can be repeated multiple times in every junction formed in our experiments.

T

he reversible manipulation of spin states of individual molecules is a critical step toward achieving singlemolecule spintronic devices.1,2 Spin crossover (SCO), a phenomenon by which molecules containing transition metal ions exhibit switchable magnetic bistability,3−5 represents an appealing mechanism for spin manipulation. The underlying force driving SCO is exerted by rearranging the coordination sphere of a metal ion. Such rearrangements can be induced by external stimuli, such as temperature, light, pressure, magnetic, or electric fields.4,5 All of these measures address ensembles of molecules but do not allow selectively controlling the SCO of individual molecules. Over the past decades, mechanically controllable break junctions (MCBJs) and scanning tunneling microscopes (STMs) have been used to control single-molecule SCO either mechanically6−8 or electrically.9−15 Here we demonstrate that the reversible SCO of single Fe-porphyrin (FeP, Figure 1a) molecules can be realized mechanically in a junction formed in STM. It has been reported that the spin state of transition metal functionalized porphyrin molecules can be changed through conformational modification and the attachment of small gaseous molecules at the metal center.16−22 In this study, we change the Fe spin state of single FeP molecules by varying the gap width of the single-molecule junctions. Experimentally, we observed that the zero-bias resonance associated with the Fe center varies its line shape against the gap width. The density-functional theory calculations reveal that when the junction gap is narrow, the macrocyclic core of the porphyrin adopts a saddle conformation that features a high-spin (S = 2) Fe center. Widening © 2017 American Chemical Society

RESULTS AND DISCUSSION Figure 1b shows a STM topograph of a single FeP molecule. The central protrusion highlights the Fe center. The tunneling spectrum (dI/dV) acquired with the tip positioned above the Fe center (the black cross in Figure 1b) features a zero-bias resonance with a Fano line shape (black curve in Figure 1d). Fitting the dI/dV spectrum with a Fano function results in a width (ΓFano‑Res) of 12 meV.23 To form a single-molecule junction, we placed the tip at a position marked with the red cross in Figure 1b, switched off the STM control feedback loop and moved the tip toward the molecule. A representative current trace against the tip displacement measured during this tip movement is presented in Figure 1c (red trace), showing that the current increased exponentially and jumped abruptly to saturation (50 nA) when the tip was displaced 0.2 nm from its set-point height. Furthermore, the tip was moved 0.2 nm toward the surface before being retracted. The blue trace in Figure 1c shows the current during tip retraction, which Received: April 13, 2017 Accepted: May 12, 2017 Published: May 12, 2017 6295

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configuration, the same as the tunneling spectrum acquired above the Fe center, while ΓFano‑Res = 25 meV in the junction configuration. We studied the evolution of the zero-bias resonance of the junction configuration against the tip displacement, i.e., the gap width of the junction. As illustrated in Figure 2a, we displaced the tip downward and upward (red and blue arrows) to vary the tip-to-substrate gap width and, in the meantime, acquired tunneling spectra at different tip heights. Figure 2b shows a series of dI/dV spectra acquired during the stepwise reduction in tip height from 3.4 Å (the top curve) to 1.2 Å (the bottom curve), while Figure 2c shows a series of dI/dV spectra acquired in the reversed tip displacement. In both cases, the zero-bias resonance becomes broader when the tip-to-substrate gap is narrowed, as is highlighted by the dotted curves. We also conducted similar measurements in the tunneling configuration. The tip was first positioned above the center of a molecule (cf. black cross in Figure 1b) and then displaced vertically. Figure 2d shows that the line shape of the zero-bias resonance is independent of tip height as the tip is displaced within the same range (∼2.2 Å) as in Figures 2b and 2c. In Figure 2e, ΓFano‑Res obtained from fitting the zero-bias resonance of the junction and tunneling configurations are plotted against the tip displacement. In the junction configuration, ΓFano‑Res remains almost constant at 25 meV as the tip is displaced from 1.2 to 2.2 Å and then starts to decrease linearly to 12 meV at 3.4 Å. Both the upward and downward tip displacements exhibit this trend. In contrast, in the tunneling configuration, ΓFano‑Res fluctuates between 10 and 12 meV against tip displacement. Figure 2f displays a series of dI/dV spectra in a colored presentation acquired at one junction while the tip was reversibly displaced by multiple downward and upward cycles. This figure demonstrates that the variation in the resonance line shape against tip displacement is not only reversible but also highly reproducible in the multiple tip displacement cycles. As control experiments, we conducted the same measurements on free-base porphyrin molecules and found that the zero-bias resonance was absent in the tunneling and the junction configuration, indicating that the zero-bias resonance is associated with the Fe center in the FeP molecule. A magnetic impurity screened by itinerary electrons gives rise to the Kondo effect, which exhibits zero-bias resonance below a characteristic temperature.24−28 Here we propose that the zero-

Figure 1. (a) Chemical structure of the FeP molecule. (b) STM topograph of the FeP molecule adsorbed on a Au(111) surface (scale bar: 1 nm). (c) Current as a function of tip height during tip approaching-retraction manipulation. (d) dI/dV spectra obtained in tunneling and junction configurations. The red and blue curves were acquired at triangle and square points respectively in (c).

remained saturated until the tip withdrew to ∼0.1 nm below the set-point height. When the tip was retracted to the set-point height, the current reached 30 nA, which is 600 times the initial set-point tunneling current. The two current traces evidence that the tip made contact with the molecule, forming a singlemolecule junction of tip-molecule-substrate. We acquired dI/dV spectra in the tunneling configuration (red curve in Figure 1d, measured when the tip was moved to the position marked with a triangle in Figure 1c) as well as in the junction configuration (blue curve in Figure 1d, measured when the tip was retracted to a position marked with a square in Figure 1c.). Both curves display a zero-bias resonance of a Fano line shape but with different widths: ΓFano‑Res = 12 meV in the tunneling

Figure 2. (a) Schematic illustration of varying the gap width of a molecular junction by varying tip height. (b) and (c) dI/dV spectra (normalized by averaged absolute value of current) acquired at varied tip heights in the junction configuration while lowering (b) and lifting (c) the tip. (d) dI/dV spectra at varied tip heights acquired at varied tip heights in the tunneling configuration. (e) ΓFano‑Res against tip displacement in the junction and the tunneling configurations. (f) dI/dV spectra acquired in a molecular junction undergoing multiple tip upward and downward displacement cycles. 6296

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Figure 3. (a) DFT-optimized structure of FeP sandwiched between two Au electrodes (a pyramid-shaped tip and a flat Au(111) substrate of 3 atomic Au layers) with a gap of 6, 7, 8, 10, and 12 Å, respectively. (b−f) SP-PDOS of Fe 3d orbitals of the five junctions.

polarized in the 6 and 7 Å junctions but becomes nonpolarized in the 8, 10, and 12 Å junctions as a spin-down channel shifts below the Fermi level. dx2 − y2 also becomes less polarized in the three wide junctions as a spin-up channel shifts above the Fermi level. Overall, the Fe spin state is S = 2 in the 6 and 7 Å junctions and changed to S = 1 in the 8, 10, and 12 Å junctions, indicating that SCO takes place when the gap is widened from 7 to 8 Å. As presented in Figure 4a, the magnetic moment of Fe is 3.8 and 3.5 μB in the narrow junctions of 6 and 7 Å, respectively, drops to 2.1 μB in the 8 Å junction, and remains at this level in the 10 and 12 Å junctions.

bias resonance is likely associated with the Kondo effect: in the tunneling configuration, the Au(111) surface provides the itinerary electrons and the molecule−substrate interaction regulates the exchange coupling between the substrate electrons and the Fe spin center.23 In the junction configuration, the Fe spin center is screened by the charge transported through the molecular junction.29 Generally speaking, the width of the Kondo resonance can be expressed as ΓFano‑Res ∼ e−1/ρJ (1), where J characterizes the screening strength and ρ represents the density of the itinerary electrons.30,31 Since the resonance of the tunneling configuration has a line shape that is independent of tip height (cf. Figure 2d,e), we argue that ρJ does not vary against vertical tip displacement in the tunneling configuration. Similar behavior was reported in two studies of Co adatoms on Cu(100).32,33 In the junction configuration, however, the dI/dV spectra presented in Figure 2 hint that ρJ varies with the gap width of the junction. Similar effects were reported by Park et al. that a Co complex exhibits changeable Kondo resonance in MCBJ devices when the junctions are stretched, which were ascribed to changed Co spin states.7 To investigate the underlying mechanisms of the evolution of the zero-bias resonance, we carried out spin-polarized density functional theory calculations to analyze a series of metalmolecule-metal junctions. Each junction consists of an FeP molecule sandwiched between a top electrode (with a pyramidshaped tip) and a bottom electrode of a flat Au(111) substrate of three-layer atoms (only one layer is shown in Figure 3a). To simulate the tip height displacement, we calculated five junctions with different top-bottom slab gaps of 6, 7, 8, 10, and 12 Å. The optimized structures of the five junctions are shown in Figure 3a. In the 6 Å junction, the molecule lies nearly parallel to the slab. When the gap is widened, the molecule gradually tilts up to about 50° in the 12 Å junction. As the molecule tilts, the conformation of the FeP undergoes relaxation: the macrocyclic core becomes flat (see detail discussion below). The spin-polarized projection density of states (SP-PDOS) of the Fe 3d orbitals of the five junctions are presented in Figures 3b−f. The definitions of the x-, y-, and z-axis are shown in the 12 Å junction in Figure 3a. Note that the XY plane is parallel to the macrocycle plane in all junctions. The three wide junctions with 8, 10, and 12 Å gap exhibit very similar SPPDOS, which are nevertheless different from those of the two narrow junctions (6 and 7 Å). In particular, the dxy orbital is

Figure 4. (a) Magnetic moment (from DFT calculations) and ρJ (calculated from the zero-bias resonance) as a function of tip− substrate gap. (b) Fe−N bond length as a function of tip−substrate gap. Inset: Heights of four nitrogen atoms of the macrocyclic core relative to the central Fe atom in the 6 and 12 Å junctions. 6297

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ACS Nano In comparison, we calculated five single-side contact junctions in which the FeP is flatly adsorbed on the bottom slab without contacting the tip electrode while the bottom-totop gaps are set at 6, 7, 8, 10, and 12 Å (Figure S1). These junctions simulate the experimental tunneling configuration with different tip heights. The calculations reveal that Fe is at a spin state of S = 2 in all five junctions. Figure 4a shows that the magnetic moment of the Fe centers in these junctions is in a range of 3.8 to 3.9 μB. As a result, widening the gap of the single-side contact junction does not change the Fe spin state. This result is consistent with the unchanged zero-bias resonance obtained in the tunneling configuration (Figure 2d,e). The magnitude of ρJ in eq 1 characterizes the spin density of the magnetic impurity.22 We calculated ρJ from the zero-bias resonance using ρJ =

result corroborates the constant magnetic moment shown in Figure 4a.

CONCLUSIONS In conclusion, we have precisely and reversibly tuned the gap width of the single FeP molecular junctions and simultaneously conducted transport measurements. The line shape of the zerobias resonance varies against the changed junction gap width, which is attributed to the different spin density of the Fe center. The first-principles calculations reveal that Fe center undergoes SCO from S = 2 to S = 1 when the junction gap is widened, which is associated with a conformational change in the FeP macrocyclic core from saddle to planar. The SCO is robust and can be reversibly controlled multiple times. Our results demonstrate that precise mechanical control can be harnessed to achieve spin manipulation in the ultimate limit of singlemolecule junctions.

1 A − ln(ΓFano − Res)

METHODS The experiments were performed in an ultrahigh-vacuum STM system (Omicron) with a base pressure of 2 × 10−10 mbar. TPP molecules were thermally evaporated using a molecular beam evaporator at 325 °C and deposited onto a clean Au(111) substrate which was held at room temperature. Subsequent deposition of Fe atoms and 80 °C annealing was performed to metalate porphyrin cores to form FeP molecules. STM and scanning tunneling spectroscopy measurements were conducted at 4.9 K. STS was performed using lock-in technique with a modulation frequency of 1.4 kHz and a modulation voltage of 4 mV. The first-principle calculations were performed using the Vienna ad initio Simulation Package (VASP) with the ion-electron interaction described by the projector-augmented wave potential (PAW).38,39 We employed an optimized version of van der Waals (vdW) density functional to describe the adsorption of porphyrin molecules on the surface.23,40,41 All the structural optimizations adopted gamma-pointonly K sampling. While for electronic-structure simulations, including projected density of states and magnetic moments, 4 × 4 × 1-K-point sampling and a Gaussian broadening of 0.02 eV were used. The planewave cutoff energy is 500 eV in all the calculations. The supercell consists of a FeP molecule between two Au(111)-(8 × 8) slabs with three atomic layers. The structures were found to be stable with all the atomic forces down to 10−2 eV/Å. A Hubbard U term with an effective value of U-J = 6.0 eV was used to describe the Coulomb repulsion of the Fe(II) 3d orbitals.42−47 The magnetic moment of Fe(II) were calculated on the base of Wigner Seitz radii.

where A is a dimensionless constant and ΓFano‑Res is obtained from fitting the zero-bias resonance. The calculated values are plotted in Figure 4a, which shows that the calculated ρJ decays monotonically as the tip height increases from 6 to 7.5 Å. The transition takes place while the gap width varies 1.5 Å, which is comparable with the changing trend of the calculated Fe magnetic moment. We propose that the evolution of the zerobias resonance line shape is associated with changing Fe magnetic moment. In a gas phase, the Fe center in FeP has a configuration of 3d6 with an intermediate spin state of S = 1.16,34,35 As adsorbed on Au(111), the spin state of Fe is switched to S = 2.23 The underlying mechanism was found to be associated with a conformational change in the porphyrin macrocyclic core: the core is planar in the gas phase but distorted to a saddle shape upon adsorption. Our calculations reveal that the macrocyclic core of FeP adopts a saddle conformation in the 6 Å junction. When the junction gap is widened, one side of the molecule is lifted from the bottom electrode. Consequently, the macrocyclic core is gradually flattened and becomes almost planar in the 12 Å junction. This structural transformation is explicitly depicted in the inset in Figure 4b, which plots the heights of four nitrogen atoms of the macrocyclic core relative to the central Fe atom. In the 6 Å junction, two N atoms are above and below the Fe by 0.26 and 0.28 Å, respectively, whereas these values are reduced to 0.08 and 0.06 Å in the 12 Å junction, featuring a nearly planar conformation. Figure 4b shows that accompanying this conformation change, the Fe−N bonds are shortened from 2.00 to 1.93 Å. It should be emphasized that the Fe is at a Fe(II) state and its 3d orbital has 6.20 and 6.29 electrons in the 6 and 12 Å junctions, respectively, which rules out that the change in the Fe(II) magnetic moment is caused by charge transfer. It is known that the spin state in FeP complexes is intimately coupled to the Fe−N bond distance.35,36 Bhandary et al. use first-principles calculations to reveal that an FeP molecule trapped at a divacancy site in graphene is switched from S = 2 to S = 1 when the Fe−N bond length is shortened from 2.03 to 1.96 Å.37 We argue that a similar mechanism is at work in our system. We also calculated the Fe−N bond length in the single-side contact junctions. As plotted in Figure 4b, the Fe−N bond length falls between 1.99 and 2.01 Å in the five junctions while the molecule preserves the saddle conformation (Figure S1). This

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b02567. Experimental and DFT methods and DFT results of the single-side contact junctions (PDF)

AUTHOR INFORMATION Corresponding Author

*[email protected]. ORCID

Hu Xu: 0000-0002-2254-5840 Nian Lin: 0000-0001-5693-4011 Author Contributions

G.K. and Q.Z. contributed equally to this work. Notes

The authors declare no competing financial interest. 6298

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