LETTER pubs.acs.org/NanoLett
A Molecular Switch Based on Current-Driven Rotation of an Encapsulated Cluster within a Fullerene Cage Tian Huang,† Jin Zhao,†,§,^ Min Feng,† Alexey A. Popov,‡ Shangfeng Yang,‡,§,|| Lothar Dunsch,‡ and Hrvoje Petek*,† †
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Department of Physics and Astronomy and Petersen Institute of NanoScience and Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States § Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China (USTC), Hefei 230026, China ^ Department of Physics, University of Science and Technology of China (USTC), Hefei 230026, China ‡ Department of Electrochemistry and Conducting Polymers, Leibniz-Institute for Solid State and Materials Research (IFW), Dresden, Germany Department of Materials Science and Engineering, University of Science and Technology of China (USTC), Hefei 230026, China
bS Supporting Information ABSTRACT: By scanning tunneling microscopy imaging and electronic structure theory, we investigate a single-molecule switch based on tunneling electron-driven rotation of a triangular Sc3N cluster within an icosahedral C80 fullerene cage among three pairs of enantiomorphic configurations. Bias-dependent action spectra and modeling implicate the antisymmetric stretch vibration of Sc3N cluster as the gateway for energy transfer from the tunneling electrons into the cluster rotation. Hierarchical switching of conductivity among multiple stationary states while maintaining a constant molecular shape, offers an advantage for the integration of endohedral fullerene-based single-molecule switches into multiple logic state molecular devices. KEYWORDS: Endohedral fullerene, enantiomerization, conductance switching, vibrational and electronic excitation, molecular machine, STM
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olecular materials present rich possibilities for construction of molecular switching devices based on structural isomerization stimulated by electrons or light.15 The primary switching events in vision and bacterial photosynthesis rely on a conformational change through cistrans isomerization within a protein framework to trigger quantal transduction.5 Photobiology has inspired studies of single-molecule switching in azobenzene6 and its derivatives,7,8 and more distantly in molecules such as rotaxane,9 catenane,10 phenylene ethynylene oligomers,11 and porphyrin derivatives,12 all of which undergo significant structural changes. Switches based on tautomerization13,14 of NH bonds minimize structural changes, but are vulnerable to chemical perturbations to their functional groups. By contrast to bare molecules, the inner space of fullerenes provides a more protected environment akin to a Faraday cage. Indeed, monatomic endohedral fullerenes have been imaged by low-temperature scanning tunneling microscopy (STM) and the internal structure of ionic bonding of metal atoms to the fullerene cage has been described.15,16 For some molecules, the internal atom/cluster motion is known to occur even below room r 2011 American Chemical Society
temperature for moieties with a close match between the ionic bond length and the cage radius.17,18 The potentially facile motion of the encapsulated moiety through electronic or optical stimulation, such as has recently been described in the case of charge-transfer excitation of the Li@C60 molecule, presents an attractive target for realization as a single molecule switch.19 We describe the conductance switching of single Sc3N@C80 molecules in a junction formed by an atomically ordered Cu(110)-(2 1)-O substrate and atomically sharp LT-STM tip. The switching involves hierarchical, STM bias voltage-dependent, multiple-axis rotation of the encapsulated planar equilateraltriangle Sc3N cluster within the icosahedral C80 cage. We demonstrate that the dominant low-bias switching mechanism occurs through excitation of the ScN stretching vibrations by the inelastic scattering of tunneling electrons, which consequently Received: August 16, 2011 Revised: November 4, 2011 Published: November 14, 2011 5327
dx.doi.org/10.1021/nl2028409 | Nano Lett. 2011, 11, 5327–5332
Nano Letters
Figure 1. Tunneling electron stimulated isomerization of Sc3N@C80. (a) Topographic STM image of a single Sc3N@C80 molecule on Cu(110)-(2 1)-O substrate. (b) Image of the same molecule after isomerization by applying a voltage pulse of 100 mV. The tunneling conditions are Vbias = 50 mV and Iset point = 0.1 nA for both panels a and b. (c) The difference pattern obtained by subtracting panel a from panel b and taking the absolute value. (d) Image of the same molecule as in panels a and b recorded at Vbias = 100 mV and Iset point = 0.4 nA that is symmetric because of fast isomerization during the imaging process. (e) Three-dimensional pseudo color plot of panel d. The superposed dashed curves delineate regions characterized by stable (S) and fluctuating (F) tunneling current. (f) Switching pattern obtained by high-pass filtering of panel d. The filtered image highlights the areas of fluctuating current, which correspond to the bright regions of the difference image panel c. (g) It trace taken with Vbias = 100 mV by fixing the tip at the asterisk in panels d and e and disabling the feedback loop. Arrows in panels c and d indicate the σ001 mirror plane. The dotted line meshes in panels d and f indicate the substrate lattice and the scale bars correspond to 1 nm in all subframes.
drives simple rotations of the Sc3N cluster between three pairs of chiral conformations. Whereas simple fullerenes like Li@C60 are difficult to isolate in a purified form,20 stable metal nitride cluster fullerenes can be prepared and isolated by well-known methods and have been characterized by a broad range of chemical, physical, and theoretical techniques.18 Sc3N@C80 is the most abundant endohedral fullerene product of the trimetallic nitride cluster synthesis,17,18 although neither the Sc3N cluster nor the C80 cage are stable separately. This fullerene is produced as an isomeric mixture of the dominant Ih and less stable D5h isomers, which can be separated by chromatographic or chemical techniques.21 Its structural, optical, and electronic properties have been characterized by X-ray diffraction, optical, IR, Raman, NMR, and ESR spectroscopy, and electrochemical methods.18 NMR and ESR measurements show that at 300 K, the planar Sc3N cluster exists in an isotropic environment consistent with thermally activated rotation within the C80 cage.17,22 Although one can ascribe formal transfer of six electrons from the cluster to the cage, the cluster-cage interaction has a covalent character of coordination bonds such as found in transition metal complexes. Density functional theory (DFT) calculations for an isolated, neutral
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Sc3N@C80 molecule give barriers of 74 and 100 meV for the internal in-plane and out-of-plane rotations of the Sc3N cluster.23 STM measurements were performed on single Sc3N@C80 molecules evaporated on a Cu(110)-(2 1)-O surface [Cu(110)-O]. The surface consists of highly anisotropic and atomically perfect CuO chains running in the Æ001æ crystallographic direction of the substrate.24 Figure 1a shows an STM topographic image of a single Sc3N@C80 molecule recorded at a 50 mV bias voltage at 4.7 K. Under these conditions, the molecule is stable, whereas applying a voltage pulse of ∼100 mV can alter its image (Figure 1b). Although Sc3N@C80 molecules are nearly spherical, the images before and after pulsing are mirror images, that is, they are enantiomorphic. The apparent distortion of the molecular shape arises from how the internal conformation of the Sc3N cluster affects molecular conductivity. A mirror symmetry plane σ001 parallel to the Æ001æ crystallographic direction of the substrate appearing in Figure 1c, which shows the absolute value of the difference between Figure 1a and Figure 1b, establishes the enantiomorphism. By contrast to the chirality found at 50 mV, when being imaged at 100 mV the same molecule appears to be nearly spherical (Figure 1d). Representing the same image in a threedimensional plot (Figure 1e) reveals rough patches corresponding to regions of noisy tunneling signal within the molecule. The noise can be characterized through recording a current versus time (It) trend at a fixed bias (100 mV) by locating the STM tip above a noisy area and turning off the feedback loop. The measurement reveals the current switching behavior between two stationary states (telegraph noise; Figure 1g), which, together with the symmetrized topography, is attributed to the interconversion of enantiomorphs. The image in Figure 1d is made symmetric by interconversion of the enantiomorphs. and the noise amplitude maps the position-dependent conductance differences. The tunneling current switches the molecule stochastically between two isomers during the imaging process. We attribute this behavior to the tunneling current-induced changes of the orientation and alignment of the Sc3N cluster within the C80 cage, because the cage itself and the adsorption site are expected to be stable, whereas the cluster is known to undergo thermally induced internal motion.17,22 We can represent the spatial distribution of the switching amplitude by applying a high-pass filter to the original image in Figure 1d to remove the topographic contribution. The resulting switching pattern (SP; Figure 1f) maps the same contrast as the difference pattern in Figure 1c and is characteristic of the alignment and orientation of both the cage with respect to the substrate and the cluster with respect to the cage. Therefore, we will use such SPs to characterize chemisorption structure and various switching behaviors of cluster among multiple stationary states. By varying the bias between 0 and 1200 mV, we found six stable configurations of the Sc3N cluster within single Sc3N@C80 molecules for a specific cage orientation. The 50 mV bias STM images of these isomers and the SPs for their interconversion at progressively increasing bias voltages are displayed in Figure 2. We classify the configurations into three enantiomorphic pairs, (I, I0 ), (II, II0 ), and (III, III0 ), based on the switching patterns, SPI, SPII, and SPIII. Each pair is related by reflection in the σ001 plane bisecting the symmetric SPs. The same SPs are observed in multiple molecules with consistent properties. The evolution of the SPs with respect to bias voltage traces a reproducible hierarchical switching behavior starting with nearly equal 5328
dx.doi.org/10.1021/nl2028409 |Nano Lett. 2011, 11, 5327–5332
Nano Letters
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Figure 2. Hierarchical switching processes in Sc3N@C80. STM images of six stable configurations at 50 mV and SPs at different bias voltages recorded on a single Sc3N@C80 molecule. The curved arrows at specific voltages indicate onsets of switching between different configurations. The vertical arrows indicate the increasing rate of specific switching processes at higher voltages. The bias voltage dependent SPs reveal the evolution of the switching from intra- to interchiral pairs with the increasing energy. Note that the switching between (I,I0 ) and (II,II0 ,III,III0 ) with a threshold above 1000 mV is so slow that it appears only once in the SP at 1200 mV at the instant indicated by the arrow.
thresholds below 100 mV. The shape of SPI remains constant, but the click rate increases as the bias is increased up to 1000 mV; the same is true for SPII and SPIII. Above 500 mV, however, these two merge into another SP, indicating an onset of a new switching process with a higher threshold and lower switching rate than the primary SPs. This new process interconverts II, II0 , III, and III0 , however, it can be viewed as bistate switching between II* and III*, where II* and III* stand for the averaged configurations of rapidly interconverting pairs IIII0 and IIIIII0 . At an even higher threshold of ∼1200 mV, switching occurs among all six configurations. On the basis of these observations, we conclude that the enantiomorphs correspond to rotational isomers of the encapsulated Sc3N cluster. The junction conductivity of an Sc3N@C80 molecule changes deterministically among the isomers reproducibly in response to setting of the bias voltage to induce specific rotations of the Sc3N cluster relative to the C80 cage. To explain the observed switching behavior, we examine the symmetry properties of the Sc3N@C80 molecule in Figure 3a. A C80 cage has 10 equivalent C3 rotational symmetry axes passing through opposing pairs of carbon atoms at the vertex of three hexagons, which we designate as C666. Pairs of nearest-neighbor C3 axes on opposite sides of hexagons define reflection symmetry planes. Within the C80 cage, one C3 axis passes through the N atom of the Sc3N cluster normal to the cluster plane, and a reflection plane containing the same C3 axis cuts the cluster at a (18.5 angle relative to an ScN bond.17,23 For each C3 axis, the Sc3N cluster can occupy four azimuths (Figure 3b), therefore an isolated Sc3N@C80 molecule has 40 energetically equivalent configurations. The elementary transformations of the Sc3N among these equivalent structures can be classified into two types: in-plane rotation by 37 around C3 axis, and axis-switching rotation by 41 between two C3 axes on the opposite sides of a carbon hexagon. As already noted, for an isolated, neutral molecule these
processes have respective calculated barriers of 74 and 100 meV.23 We postulate that the switching processes between the chiral pairs with
