Modification of Molecular Spin Crossover in Ultrathin Films - Nano

Letter pubs.acs.org/NanoLett

Modification of Molecular Spin Crossover in Ultrathin Films Alex Pronschinske,† Yifeng Chen,† Geoffrey F. Lewis,‡ David A. Shultz,‡ Arrigo Calzolari,§,∥ Marco Buongiorno Nardelli,∥,⊥ and Daniel B. Dougherty*,† †

Department of Physics, North Carolina State University, Raleigh, North Carolina 27695, United States Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695, United States § CNR-NANO, Instituto Nanoscienze, Centro S3 I-41125, Modena, Italy ∥ Department of Physics and Department of Chemistry, University of North Texas, Denton, Texas 76203, United States ⊥ CSMD, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ‡

S Supporting Information *

ABSTRACT: Scanning tunneling microscopy and local conductance mapping show spin-state coexistence in bilayer films of Fe[(H2Bpz2)2bpy] on Au(111) that is independent of temperature between 131 and 300 K. This modification of bulk behavior is attributed in part to the unique packing constraints of the bilayer film that promote deviations from bulk behavior. The local density of states measured for different spin states shows that high-spin molecules have a smaller transport gap than low-spin molecules and are in agreement with density functional theory calculations.

KEYWORDS: Molecular spintronics, spin crossover, scanning tunneling microscopy, density functional theory

M

In addition to temperature, SCO can occur in response to mechanical pressure, magnetic field, and photoexcitation.8 The latter has been shown to result in light-induced excited spinstate trapping (LIESST)10 when a low-spin sample is photoexcited at sufficiently low temperature to prevent rapid activated relaxation.11 This has been of particular interest for optical memory and display elements12 and is a clear example of the practical value of the tunable properties of SCO-based materials. SCO materials have recently been predicted to show large changes in charge transport properties in response to a change in their molecular spin state,13,14 which are connected in a straightforward way with the change in their frontier orbitals. A related class of compounds called “valence tautomers” that undergo a simultaneous SCO and intramolecular charge transfer have been shown to have similar spin-state-dependent charge transport in both molecular and polymeric forms.15 Furthermore, density functional theory calculations suggest that the spin transition in valence tautomers is remarkably tunable by the application of an electrostatic field.16 On the experimental side, films of the SCO compound Fe[HBpz3]2 have been reported to show a large decrease in hopping conductivity when cooling from the high-spin to the low-spin state, and the shape of the conductivity temperature dependence was observed to approximately mimic the temperature-dependent changes in high-spin composition of

olecular and organic materials are of interest for spinbased electronics applications due to the extraordinary degree to which molecular spin-state can be controlled by synthetic chemistry.1 It has already been demonstrated that unexpected spintronic effects such as giant magnetoresistance2 and giant tunneling magnetoresistance3 can be obtained in diamagnetic organic semiconductor films. Spin injection4,5 and spin-dependent interfacial coupling6,7 across organic semiconductor−ferromagnet interfaces have also been demonstrated. Extending prototype spintronic device investigations to highly tunable paramagnetic molecular materials is an important goal that is likely to lead to new molecular functionality associated with the intrinsic molecular spin state, and its intimate connection to the electronic structure. An intriguing class of materials for molecular spintronics exhibits spin crossover (SCO) as a function of external stimulus.8 In the most common example, octahedral coordination compounds of Fe2+ with intermediate field ligands (often involving Fe−N bonding) show a change from high-spin (S = 2) at high temperatures to low-spin (S = 0) at low temperatures. The transition to a high-spin state at high temperature has a strong entropic origin; this is due to an increase in spin multiplicity, as well as a simultaneous increase in vibrational entropy, due to softening of Fe−N bond stretching force constants.9 It is well-known that the electronic structure of a SCO compound also changes dramatically across the spin transition, as evidenced by the strong visible light absorption of the metal-to-ligand charge transfer band, present only in the low-spin state.8 © 2013 American Chemical Society

Received: November 21, 2012 Revised: February 5, 2013 Published: March 21, 2013 1429

dx.doi.org/10.1021/nl304304e | Nano Lett. 2013, 13, 1429−1434

Nano Letters

Letter

the bulk SCO material.17 Nanoscale electrical transport measurements have shown hysteresis effects in current−voltage characteristics18 and ligand spin control.19 In a similar vein, two recent reports have shown evidence for scanning tunneling microscope (STM)-induced spin-state switching,20,21 suggesting new possibilities for electrically based control of these important materials. The characterization of films of SCO materials with thicknesses in the range of several hundreds of nanometers has shown that the spin transition persists essentially unchanged compared to the spin transition in a bulk powder.22 In nanostructured environments such as ultrathin films or single molecules in small junctions, it is not obvious that this similarity will persist. In particular, a bilayer film of Fe[(phen)2(NCS)2 ] on Cu(100) was shown by STM and X-ray absorption spectroscopy to have significant coexistence of high-spin and low-spin forms, even at temperatures far below the sharp spin transition in the bulk powder.21 Understanding the origin of such deviations from bulk behavior is important to the development of nanoscale spintronic devices based on the motif of spin-crossover. In this Letter we describe STM observations of unique spinstate coexistence in bilayer films of Fe[(H2Bpz2)2bpy] on Au(111). We observe temperature-independent spin composition in stark contrast to bulk behavior. High-spin molecules can be locally distinguished by their different apparent height, submolecular structure, and by STS measurements of their local density of states. The identification of different spin-states by imaging and spectroscopy is supported by direct comparison with density functional theory (DFT) calculations and simulated STM images. The Fe[(H2Bpz2)2bpy] (“Fe-bpy”, Figure 1a) compound was prepared as previously described by Real et al.23 The purple product material was confirmed to exhibit the known spin transition at ∼160 K22,23 by magnetometry in a Quantum Design MPMS 7XL SQUID magnetometer. STM experiments were carried out in an ultrahigh vacuum system (base pressure ∼5 × 10−11 Torr) connected by a gate valve to a molecular film growth chamber (base pressure 140 K), which we consider unlikely given the possible variability of intermolecular interactions in different packing structures. Instead, bilayer growth at room temperature, where the bulk Fe-bpy material is expected to be completely high-spin, may be unfavorable due to a mismatch between the preferred substratecontrolled film packing and the high-spin molecular geometry with long Fe−N bonds. As a result, condensation of the bilayer in only high-spin form is less favorable since it cannot fit in the preferred π-stacked bilayer structure. This interpretation of a strong geometric constraint on high spin fraction is consistent with the results of Gopakumar et al.20 who report that electroninduced low-spin to high-spin conversion at 5 K in a related molecule results in a dramatic increase in apparent size when carried out in a low-spin domain. In our experiments, during growth at room temperature, a significant conversion to lowspin configurations promotes assembly of the bilayer structure. High-spin molecules exist in this ordered molecular assembly as essentially point lattice “defects”. Once this coexisting like-spin domain structure is established at room temperature, it is likely to be frozen upon cooling. It is crucial to note that the strong spatial correlations observed in the location of high-spin isomers are clearly visible in the conductance maps in Figure 4. The high spin pairs (low conductance) have a strong tendency to aggregate in meandering domains. Monte Carlo simulations of SCO dynamics show similar spatial correlations in like-spin and elastic domain patterns during the course of a spin transition

dependent transmission function background that is challenging to accurately deconvolve.28 The conductance map shown in Figure 3b shows a particularly strong contrast due to the differences in the occupied DOS around −1 V. We have observed notable bias dependence to the STM imaging and conductance mapping (see sequence in Supporting Information, Figure S2) that provides further evidence for the assignment of dark features as high-spin molecules. As the bias changes, the apparent height in the depressions tends to increase relative to its surroundings, consistent with the relative DOS increase in the HS molecules compared to LS molecules (Figures 2b and 3a). In our experiments, STM imaging in the unoccupied density of states was often unstable (as evidenced in Figure S2) which we attribute to the prevelance of electron-induced excitations similar to those reported by Gopakumar et al.20 Our experiments, at relatively high temperatures, likely make these excitations especially significant even at relatively low bias. The systematic temperature dependence of the spin composition in the Fe-bpy bilayer structure was monitored using STM imaging and local conductance maps of the Fe-bpy bilayer at around 1 V below the Fermi level. At this energy, the local conductance of the high-spin species is lower than the local conductance of the low-spin species (Figure 3a), and it is easier to identify the different spin species. These conductance maps reveal striking spatial correlations in the location of highspin molecules. They tend to aggregate in low conductance “domains” with a meandering periphery as shown in Figure 3b. Figure 4a shows a topographic STM image measured at 131 K. A conductance map (Figure 4c) measured simultaneously

Figure 4. (a) STM image at 131K (36 nm × 36 nm, V = −1 V, I = 1 nA) and (b) STM image at 300 K (60 nm × 60 nm, V = −1, I = 1 nA) with corresponding conductance maps at (c) 131 K and (d) 300 K. (e) Depression count versus sample temperature, error bars indicate standard Poisson counting error.

with the topography shows more prominently the depressions due to high-spin pairs in the local conductance at −1 V. Remarkably, even at room temperature (∼ 300 K), both lowspin and high-spin features can be identified (Figure 4b and d). A significant population of low-spin molecules at this temperature is entirely unexpected based on bulk magneto1432

dx.doi.org/10.1021/nl304304e | Nano Lett. 2013, 13, 1429−1434

Nano Letters that result from elastic interactions between near-neighbor molecules.31,32 This 2D pattern formation supports the importance of intermolecular interactions in establishing spinstate coexistence in the bilayer. Intermolecular interactions can establish long-range spatial correlations in the spin state of the film which are frozen in during growth and subsequent lower temperature STM and STS experiments. As a result, our observations provide important direct confirmation of the predicted nanoscale patterns formed during spin-state separation. It is of interest to consider whether thermal evolution of the spatial distribution of spin-state domains can be measured in this or other related systems using STM imaging. Furthermore, the dramatic modification of bulk behavior in the ultrathin films described here points to the value of using control of molecular and supramolecular assembly on surfaces to control spin state. It is obviously crucial to find methods for controlling the electronic inhomogeneity associated with spin coexistence as molecular spintronic devices continue to be explored. This is because efficient spin injection is a crucial and sometimes limiting step in spintronic device operation.3,5,33 The impact of interfacial modifications on molecular spin transitions needs to be considered in optimizing band alignment at interfaces in devices. Our observations show that careful structural design of molecular assemblies on surfaces can, in principle, be used to dramatically alter the thermodynamics of SCO behavior, analogous to well-defined bulk and solution cooperative effects. In summary, we have observed spin-state coexistence in STM images, local tunneling spectra, and local conductance maps of bilayer films of Fe[(H2Bpz2)2bpy] on Au(111). The different spin states have dramatically different electronic propertiesin agreement with DFT calculationsand can be identified by different STM imaging and local conductance. A weak temperature dependence of spin composition in the bilayer, including a substantial low-spin population at room temperature, suggests a very different character of the spin transition compared to the bulk solid or even thicker films. The origin of this difference is attributed to the constraints on elastic interactions determined by molecule−substrate interactions that promote significant low-spin composition even at room temperature. Domain patterns exhibit spatial correlations consistent with Monte Carlo simulations of the SCO process.31 The results described in this Letter indicate significant opportunities for controlling molecular spin-state through the design of molecular assemblies. Given the intimate connection between spin state and charge transport shown here and in several very recent works,13,14,17,20,21 the motif of SCO is an extraordinarily promising approach to creating highly tunable materials for molecular spintronics.

■

ACKNOWLEDGMENTS

■

REFERENCES

We are grateful to Xin Zhang and Peter Dowben for pointing out the possibility creating films by vacuum evaporation of Fe[(H2Bpz2)2bpy]. This work was funded by the National Science Foundation through the phase I Center for Chemical Innovation: Center for Molecular Spintronics (CHE-0943975).

(1) Sanvito, S. Chem. Soc. Rev. 2011, 40 (6), 3336−3355. (2) Xiong, Z. H.; Wu, D.; Vardeny, Z. V.; Shi, J. Nature 2004, 427 (6977), 821−824. (3) Barraud, C.; Seneor, P.; Mattana, R.; Fusil, S.; Bouzehouane, K.; Deranlot, C.; Graziosi, P.; Hueso, L.; Bergenti, I.; Dediu, V.; Petroff, F.; Fert, A. Nat. Phys. 2010, 6 (8), 615−620. (4) Drew, A. J.; Hoppler, J.; Schulz, L.; Pratt, F. L.; Desai, P.; Shakya, P.; Kreouzis, T.; Gillin, W. P.; Suter, A.; Morley, N. A.; Malik, V. K.; Dubroka, A.; Kim, K. W.; Bouyanfif, H.; Bourqui, F.; Bernhard, C.; Scheuermann, R.; Nieuwenhuys, G. J.; Prokscha, T.; Morenzoni, E. Nat. Mater. 2009, 8 (2), 109−114. (5) Cinchetti, M.; Heimer, K.; Wustenberg, J. P.; Andreyev, O.; Bauer, M.; Lach, S.; Ziegler, C.; Gao, Y. L.; Aeschlimann, M. Nat. Mater. 2009, 8 (2), 115−119. (6) Iacovita, C.; Rastei, M. V.; Heinrich, B. W.; Brumme, T.; Kortus, J.; Limot, L.; Bucher, J. P. Phys. Rev. Lett. 2008, 101 (11), 116602− 116604. (7) Brede, J.; Atodiresei, N.; Kuck, S.; Lazic, P.; Caciuc, V.; Morikawa, Y.; Hoffmann, G.; Blugel, S.; Wiesendanger, R. Phys. Rev. Lett. 2010, 105 (4), 047204. (8) Gutlich, P.; Garcia, Y.; Goodwin, H. A. Chem. Soc. Rev. 2000, 29 (6), 419−427. (9) Gütlich, P.; Goodwin, H. A.; Tuchagues, J.-P.; Bousseksou, A.; Molnár, G.; McGarvey, J.; Varret, F. Spin Crossover in Transition Metal Compounds; Springer: Berlin, 2004; Vol. 235, pp 23−38. (10) Lawthers, I.; McGarvey, J. J. J. Am. Chem. Soc. 1984, 106 (15), 4280−4282. (11) Decurtins, S.; Gutlich, P.; Kahler, C. P.; Spiering, H.; Hauser, A. Chem. Phys. Lett. 1984, 105 (1), 1−4. (12) Kahn, O.; Martinez, C. J. Science 1998, 279 (5347), 44−48. (13) Aravena, D.; Ruiz, E. J. Am. Chem. Soc. 2012, 134 (2), 777−779. (14) Baadji, N.; Sanvito, S. Phys. Rev. Lett. 2012, 108 (21), 217201. (15) Calzolari, A.; Chen, Y.; Lewis, G. F.; Dougherty, D. B.; Shultz, D.; Buongiorno Nardelli, M. J. Phys. Chem. B 2012, 116 (43), 13141. (16) Droghetti, A.; Sanvito, S. Phys. Rev. Lett. 2011, 107 (4), 047201. (17) Mahfoud, T.; Molnar, G.; Cobo, S.; Salmon, L.; Thibault, C.; Vieu, C.; Demont, P.; Bousseksou, A. Appl. Phys. Lett. 2011, 99 (5), 053307. (18) Prins, F.; Monrabal-Capilla, M.; Osorio, E. A.; Coronado, E.; van der Zant, H. S. J. Adv. Mater. 2011, 23 (13), 1545−1549. (19) Meded, V.; Bagrets, A.; Fink, K.; Chandrasekar, R.; Ruben, M.; Evers, F.; Bernand-Mantel, A.; Seldenthuis, J. S.; Beukman, A.; van der Zant, H. S. J. Phys. Rev. B 2011, 83 (24), 245415. (20) Gopakumar, T. G.; Matino, F.; Naggert, H.; Bannwarth, A.; Tuczek, F.; Berndt, R. Angew. Chem., Int. Ed. 2012, 51 (25), 6262− 6266. (21) Miyamachi, T.; Gruber, M.; Davesne, V.; Bowen, M.; Boukari, S.; Joly, L. c.; Scheurer, F.; Rogez, G.; Yamada, T. K.; Ohresser, P.; Beaurepaire, E.; Wulfhekel, W. Nat. Commun. 2012, 3, 938. (22) Naggert, H.; Bannwarth, A.; Chemnitz, S.; von Hofe, T.; Quandt, E.; Tuczek, F. Dalton Trans. 2011, 40 (24), 6364−6366. (23) Real, J. A.; Muñoz, M. C.; Faus, J.; Solans, X. Inorg. Chem. 1997, 36 (14), 3008−3013. (24) Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; Dabo, I.; Dal Corso, A.; de Gironcoli, S.; Fabris, S.; Fratesi, G.; Gebauer, R.; Gerstmann, U.; Gougoussis, C.; Kokalj, A.; Lazzeri, M.; Martin-Samos, L.; Marzari, N.; Mauri, F.; Mazzarello, R.; Paolini, S.; Pasquarello, A.; Paulatto, L.; Sbraccia, C.; Scandolo, S.; Sclauzero, G.; Seitsonen, A. P.;

ASSOCIATED CONTENT

S Supporting Information *

Clarifying STM images and conductance maps. This material is available free of charge via the Internet at http://pubs.acs.org.

■

■

Letter

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 1433

dx.doi.org/10.1021/nl304304e | Nano Lett. 2013, 13, 1429−1434

Nano Letters

Letter

Smogunov, A.; Umari, P.; Wentzcovitch, R. M. J. Phys.: Condens. Matter 2009, 21 (39), 395502. (25) Droghetti, A.; Alfe, D.; Sanvito, S. J. Chem. Phys. 2012, 137 (12), 124303. (26) Zein, S.; Borshch, S. A.; Fleurat-Lessard, P.; Casida, M. E.; Chermette, H. J. Chem. Phys. 2007, 126 (1), 014105. (27) Sato, D.; Shiota, Y.; Juhász, G.; Yoshizawa, K. J. Phys. Chem. A 2010, 114 (49), 12928. (28) Koslowski, B.; Dietrich, C.; Tschetschetkin, A.; Ziemann, P. Phys. Rev. B 2007, 75 (3), 035421. (29) Murray, K. S.; Kepert, C. J. Spin Crossover in Transition Metal Compounds I; Springer: Berlin, 2004; Vol. 233, pp 195−228. (30) Bin-Salamon, S.; Brewer, S.; Franzen, S.; Feldheim, D. L.; Lappi, S.; Shultz, D. A. J. Am. Chem. Soc. 2005, 127 (15), 5328−5329. (31) Nicolazzi, W.; Pillet, S. Phys. Rev. B 2012, 85 (9), 094101. (32) Nicolazzi, W.; Pillet, S.; Lecomte, C. Phys. Rev. B 2008, 78 (17), 174401. (33) Jiang, J. S.; Pearson, J. E.; Bader, S. D. Phys. Rev. B 2008, 77 (3), 035303−035307.

1434

dx.doi.org/10.1021/nl304304e | Nano Lett. 2013, 13, 1429−1434