Supramolecular Motors on Graphite Surface Stabilized by Charge

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Supramolecular Motors on Graphite Surface Stabilized by Charge States and Hydrogen Bonds Kai Sun,†,⊥ Ji-Yong Luo,†,⊥ Xin Zhang,† Zhi-Jian Wu,‡ Ying Wang,*,‡ Hong-Kuan Yuan,† Zu-Hong Xiong,† Shao-Chun Li,§ Qi-Kun Xue,*,∥ and Jun-Zhong Wang*,† †

School of Physical Science and Technology and Key Laboratory of Luminescent and Real-Time Analytical Chemistry, Ministry of Education, Southwest University, Chongqing 400715, China ‡ State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China § School of Physics, Nanjing University and National Lab of Solid State Microstructure, Nanjing 210093, China ∥ Department of Physics, Tsinghua University, Beijing 100084, China S Supporting Information *

ABSTRACT: Molecular motors are nanoscale machines that convert external energies into controlled mechanical movements. In supramolecular motors, the rotator and stator are held together mechanically, and thus the rotation can be essentially barrier free when molecular conformation is negligible. However, nearly all the supramolecular motors appeared in solutions or host−guest complexes. Surface-mounted supramolecular motors have rarely been addressed, even though they are easily manipulated by external fields. Here we report a surface-mounted supramolecular motor assembled by charge states and hydrogen bonds. On a graphite surface, individual ethanol clusters can be charged with a scanning tunneling microscopy tip and then trap the ethanol chains with a permanent dipole moment. Serving as a rotator, the trapped ethanol chains rotate around a charged cluster driven by the inelastic tunneling electrons. Random rotation in clockwise or anticlockwise direction occurs in the chiral molecular chains through chiral flipping. Directional rotation with clockwise chirality can be realized by introducing a chiral branch to the near end of ethanol chains to suppress the chiral flipping with steric hindrance. KEYWORDS: ethanol, supramolecular motors, charge state, hydrogen bond, chiral flipping

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interfaces, and thus surface-mounted supramolecular motors have the most potential for nanoscience. Scanning tunneling microscope (STM) is a unique tool that can be used to visualize and manipulate single atoms or molecules on surfaces. For individual molecules on insulating surfaces such as NaCl22 or oxidized NiAl(110),23 a doublebarrier junction is formed by two barriers between a STM tip and molecules and between molecules and metal substrate. In such a junction, a molecule can be charged by the tunneling

evelopment of artificial molecular motors is a challenging project since it requires a molecular machine to convert external chemical,1−3 optical,4,5 or electrical6−12 energy into a controlled rotational motion.13−21 In covalent molecular rotors or motors, the stator and the rotor are connected to each other through covalent bonds. The cylindrical symmetry of chemical bonds does not guarantee that there is no rotational barrier because of electronic delocalization and steric interference between the molecular parts.21 However, in supramolecular motors, the rotator and stator are held together mechanically, and thus the rotation can be essentially barrier free when molecular conformation is negligible. In nature, most motors operate at © 2017 American Chemical Society

Received: July 9, 2017 Accepted: September 15, 2017 Published: September 19, 2017 10236

DOI: 10.1021/acsnano.7b04811 ACS Nano 2017, 11, 10236−10242

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Figure 1. STM images of ethanol monomer and dimer on graphite surface acquired at 78 K. (a) An ethanol monomer with trans conformation (1.5 V, 35 pA). (b) Conformational transition from trans to gauche induced by a voltage pulse (1.5 V, 35 pA). (c) Two ethanol dimers adsorbed on HOPG surface (3.0 V, 30 pA). (d) The same ethanol dimers as in (c) but now with the lower-right dimer charged by the dosing of tunneling electrons. (e, f) Side views of the optimized adsorption model of an ethanol monomer on graphite with trans (e) and gauche (f) conformations. C, H, and O atoms in ethanol are denoted by cyan, gray, and red spheres. (g) Cross section of the ethanol dimer before (dashed line) and after (solid line) charging. (h) I−V curve measured over an ethanol dimer (set point, 3.0 V, 100 pA). The forward and backward sweeps are indicated by the red and blue arrows, respectively.

In the case of ethanol clusters such as dimers, we find that a voltage pulse may charge the ethanol cluster by an injection tunneling electron from the tip, due to the presence of neighboring molecules and decoupling from the graphite surface. For example, the lower-right dimer in Figure 1c becomes much brighter in Figure 1d due to a voltage pulse. Although the nominal height increased 72% (Figure 1g), it is still located at the same position. Figure 1h shows a typical I−V curve acquired over a dimer, which exhibits a remarkable hysteresis. There is a sudden drop in tunneling current at +2.7 V during the forward sweep, indicating the dimer switched to a different state. In the backward sweep, another sudden drop occurs at −2.7 V, indicating the dimer switches back. Figure 2a shows a topographic STM image of an ethanol monomer dehydrogenated by a large voltage pulse of 5 V. Strong modification of the graphite lattice has been observed from the triangle pattern around the monomer due to √3 × √3 reconstruction, which corresponds to six bright spots inside the hexagon in Figure 2b. The spots outside the hexagon are second-order vectors. To find the detailed mechanism for dehydrogenation-induced surface reconstruction, we perform a DFTB calculation. As shown in Figure 2c, d, the hydrogen atom located at the ethyl group is the most easily removed. The carbon atom in ethyl group forms a C−C bond with the carbon atom in graphite. Meanwhile, the other carbon atoms appear to be “upward lifted”, but are instead a purely electronic not physical phenomenon forming a local √3 × √3 surface reconstruction. Figure 2e shows a charged ethanol cluster on a graphite surface. The intact graphite lattice nearby the cluster is clearly visible from the atomic-resolution image. Figure 2f is the fast Fourier transform (FFT) of Figure 2e. The dashed hexagon marks the intact 1 × 1 lattices of graphite surface. There is no additional spot caused by reconstruction. It should be pointed out that the brighter protrusion of the ethanol clusters is not caused by the dehydrogenation of ethanol molecule because the latter would lead to a √3 × √3 reconstruction of graphite surface due to the C−C bonding between ethanol and

electrons from tip, switching between neutral and negatively charged state. This controlled manipulation of the charge states resembles a single-molecule switch in logic devices. In addition to the insulating layers used to separate molecules from metallic substrate, other decoupling mechanisms were found in multilayer molecular film on Pb(111)24 or monolayer film composed of two different molecules on Au(111).25 Here, we demonstrate an interesting capability of charged states to trap individual molecular chains with a permanent dipole moment. On a graphite surface, individual ethanol clusters can be charged with a STM tip. Serving as a rotator, an ethanol chain may rotate randomly around the charged cluster in clockwise or anticlockwise direction. By introducing a chiral branch into the molecular chains, directional rotation with clockwise chirality can be realized by prohibiting the chiral flip, constituting a surface-mounted supramolecular motor.

RESULTS AND DISCUSSION Ethanol is a relatively simple alcohol molecule that consists of a polar hydroxyl group and an alkyl tail. It prefers to form hydrogen bonds that lead to molecular aggregation in the form of a zigzag chain or cyclic ring.26 When deposited on highly oriented pyrolytic graphite (HOPG) surface at 78 K, individual ethanol molecules are self-assembled into various clusters, which have been observed with a low-temperature STM. Figure 1a shows a typical STM image of an ethanol monomer adsorbed on HOPG surface. It exhibits an elliptical lobe shape, consistent with the stable adsorption configuration of a trans monomer calculated by spin-polarized self-consistent charge density-functional tight-binding theory (SCCDFTB) (Figure 1e). By applying a voltage pulse, the monomer protrusion becomes asymmetric with one end becoming higher (Figure 1b), consistent with the metastable adsorption configuration of a gauche monomer (Figure 1f). Thus, a voltage pulse is responsible for a conformational change of an ethanol monomer. 10237

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and negative ends of the dipole. Thus, an ethanol chain could be trapped by a charged cluster due to the electrostatic attraction. Furthermore, the trapped ethanol chains may rotate around the charged cluster, constituting a supramolecular rotor or motor. Figure 3a shows a supramolecular rotor composed of a pentamer chain that serves as a rotator and a charged pentamer ring that serves as a stator. The pentamer ring exhibits a very high protrusion with asymmetric shape due to the inhomogeneous charge distribution. The pentamer chain may rotate around the stator in clockwise or anticlockwise direction, due to the weak chiral feature of pentamer chain (Figure 3d). The rotator orbit reveals 12 protrusion maximums with six corresponding to the stable orientations and other six corresponding to the metastable orientations. The number of possible orientations is determined by the symmetry of graphite surface. If all of the six carbon atoms in the graphite honeycomb are taken into account, then the 12 possible orientations can be understood in terms of two separate types of three-fold symmetry. Moreover, we observed that there is a series of concentric ripples appearing on the orbit with a period of 2.4 ± 0.1 Å, nearly equal to the lattice constant of graphite surface. It means that the pentamer chain becomes transparent such that the HOPG lattices and pentamer chain are simultaneously imaged. In a separate experiment, the superposition of a static ethanol chain image with graphite lattices was also observed, which can be explained by the interference effects between tunneling channels.30,31 Figure 3b shows a larger supramolecular rotor composed of a heptamer chiral chain and a charged pentamer ring. The rotational orbit reveals six protrusion lobes corresponding to the six stable orientations, whereas the six metastable orientations are absent here because of the increased barrier brought by chirality. To our surprise, no rotational directionality has been observed in this chiral chain. Instead, it still performs a random rotation in clockwise or anticlockwise direction. Close inspection of the image of protrusion lobes, it is observed that each lobe has two mirrored branches with respect to the direction of graphite lattice, which can be attributed to the chiral flipping or chirality switching.32 It means that the stable orientations in clockwise rotation and anticlockwise rotation are not coincident, but have mirrored symmetry.18,19 The mechanism of chiral flipping can be attributed to the position where chiral branch is connected to the ethanol chain. As shown in Figure 3e, the chiral branch is an ethanol dimer, which appears at the far-end of a pentamer chain. This is a typical “branched” hydrogen bond chain though CH2−OH bonding.33 In this case, the barrier for chiral flipping is rather low due to the negligible steric hindrance. It is conceivable that, when the chiral branch moves the near-end of molecular chain, chiral flipping would be suppressed such that directional rotation with either clockwise or anticlockwise chirality can be realized. Indeed, we have observed such kind of directional rotation in clockwise chirality in the supramolecular motor shown in Figure 3c. This motor is composed of an octamer chiral chain and a charged trimer ring. The orbit reveals six protrusion lobes resembling a chiral pinwheel with clockwise rotation. Each lobe has no mirrored branches, indicating no chiral flip occurred. The chiral group is a dimer located at the near-end of the octamer chain, close to the central stator (Figure 3f). Thus, steric hindrance becomes strong such that chiral flipping is suppressed. Correspondingly, completely

Figure 2. Surface reconstruction induced by dehydrogenation of an ethanol monomer and the intact HOPG surface covered by a charged ethanol cluster acquired at 4.5K. (a) STM image of a dehydrogenated ethanol monomer by a large voltage pulse (0.3 V, 36 pA). (b) FFT of the image of (a). The six spots of √3 × √3 reconstruction are located inside the hexagon. (c, d) Side (c) and top (d) views of the optimized model of a dehydrogenated ethanol molecule on graphite surface. (e) STM image of a charged ethanol cluster on graphite surface with the substrate lattices clearly visible (20 mV, 36 pA). (f) FFT of the image of (e). The dashed hexagon represents 1 × 1 lattice of graphite.

substrate, while the charged ethanol cluster would not. However, we can only refer to the “charged” cluster as most the possible mechanism of the bonding situation of the rotator because the graphite substrate is not an insulator nor does it possess a gap in its surface density of states and a long lasting residual “charge” could exist on this substrate. Although we have ruled out covalent bonding, the ethanol cluster may bond to the graphite surface ionically, resembling an earlier study of fluorine bonding on graphite.27 It is also possible that the zapping of the ethanol molecules maybe fundamentally altering the ethanol into a chemical compound similar to the thioethers.28 In fact, we have observed four types of ethanol dimmers due to the different arrangement of hydrogen bonding and molecular conformation (Supplementary Figure 2). For ethanol trimers and pentamers, both the zigzag chain and cyclic ring have been observed on the graphite surface (Supplementary Figure 3 and Figure 4). DFTB calculations reveal that the zigzag chains have a much larger dipole moment than those in cyclic rings (Supplementary Table 1). According to the textbook of electrodynamics,29 in a non-uniform electrical field, there is a net force on a dipole in addition to a torque. This is caused by the imbalance of forces between the positive 10238

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Figure 3. Supramolecular rotors and motor composed of a stator and a rotator observed at 78 K. (a) Random rotation of a pentamer zigzag chain around a charged pentamer ring (1.5 V, 20 pA). (b) Random rotation of a heptamer chiral chain around a charged pentamer ring (2.5 V, 20 pA). (c) Directional rotation with clockwise chirality of an octamer chiral chain around a charged trimer ring (2.5 V, 25 pA). (d−f) Schematic models of the rotors and motor shown in (a−c), respectively. The structures of three rotators have been optimized with SCCDFTB method with dispersion correction. (g−i) Cross sections of the images of rotors and motor shown in (a−c), respectively, taken across the arrow positions. All scale bars: 2 nm.

random rotation similar to the supramolecular rotor in Figure 3b is absent. Instead, the motor exhibits a clockwise chiral feature, in which the chiral molecular chain makes more clockwise rotations than anticlockwise. It can be conceived that if the chiral molecular chain makes more anticlockwise rotations than clockwise, the supramolecular motor can exhibit an anticlockwise rotation. To validate the reliability of our predicted structural models, we have simulated the STM images for the molecular motors in Figure 3 and dimers in Figure S2. The simulated STM images are displayed in Figure S5g−i and Figure S2i−m, which are consistent with the experimental results. The rotational rate of a chiral chain was measured by monitoring the variation of tunneling current with time.6,28 By positioning the tip above the chiral chain at a fixed height (the cross in Figure 3c), the feedback loop was turned off, and the tunneling current (I) was monitored with respect to time (t). The trace of the tunneling current reflects residence time of the ethanol chain at a certain orientation. The I−t spectrum is displayed in Figure 4a, where the abrupt change in the tunneling current is associated with a rotation step under the tip. This rotation is based on electronic excitation by means of tunneling electron energy transfer to the ethanol chain. When the chiral chain passed under the tip, one may expect the tunneling current to fluctuate between three discrete values as reported by Sykes et al.,28 corresponding to the three energetically stable orientations of the molecular chain. However, only two values for the tunneling current have been observed, possibly because the size of the supramolecular rotor is too large to detect the tunneling events that occurred at the third nearest orientations. The number of stepwise rotational switching events decays exponentially when plotted against their time intervals, and the

Figure 4. Measurement of rotation rate acquired at 78K. (a) Current versus time spectra acquired at bias of 3.0 V with STM tip positioned at the cross in Figure 3c. (b) Exponential decay of high conductance state as a function of resident time. (c) Rotating rate in high conductance state as a function of tunneling current under the sample bias of 3.0 V. (d) A I−V curve acquired on a rotating ethanol chain (1.3 V, 100 pA). The arrow marks the energy position at 0.4 V, corresponding to the excitation of electronic states of the ethanol molecules by tunneling electrons.

rotation rate could be derived by plotting these events versus time (Figure 4b). Finally, the data collected over 700 rotational events at five different currents were used to determine the relationship R ∝ IN (Figure 4c), where N is the number of electrons involved in the excitation, and R is the rate. The slope of ln R versus ln I lead to a value of N = 1.04 ± 0.05, indicating a one-electron process is involved in the energy transfer to 10239

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Figure 5. Supramolecular rotor composed of a rotator and a bearing observed at 78 K. (a−c) Three sequential STM images showing an ethanol chain rotates around a spinning trimer (bearing). (d) Escape of the rotating chain with the spinning trimer left at the original position. The two dashed circles mark the traces of rotating chain. (e) Relaxation of the charged trimer. (f) Close-up view of the charged trimer. All the images were obtained at U = 3.4 V, I = 35 pA.

can be viewed as a superposition of a dc and ac field. It is very similar to the previous trapping mechanism for polar molecules using ac + dc field in a three-dimensional space proposed by Blümel,35 where a circular motion of a polar molecule around an electrical field was predicted. With the reduction in symmetry of charged clusters, the rotation around the charged cluster would be hindered or frustrated such that the 360° rotation would become 120° or 60° libration. Figure 6 presents a series of STM images showing

ethanol chain. In addition, we also perform the I−V spectrum measurement on ethanol chains (Figure 4d). There is a sudden increase in tunneling current at ∼0.4 V, followed by the excitation of electronic states of the ethanol molecules. We consider that the rotation can result from the excitation of the OH stretch mode in the ethanol chains driven by tunneling electrons. Another candidate responsible for the directional rotation is the electric field formed by the tip and substrate. However, the contribution of electric field can be neglected by comparing the two variation curves of electric field versus current34 and rotation rate versus current (see Supplementary Figure 6). As a stator, the charged cyclic pentamers in Figure 3a, b kept still during the ethanol chain rotating, evidenced by the small tails of the pentamer protrusions. When the stator was a smaller cluster, such as a cyclic trimer, the stator became a “bearing” rotating together with the rotator. Figure 5 displays a series of STM images of a supramolecular rotor composed of a rotating chain and a charged spinning trimer. As revealed by Figure 5a, the orbit of ethanol chain revealed six protrusion lobes corresponding to the six stable orientations. Each lobe had two mirrored branches with respect to the graphite lattice. It seems that a chiral flip occurred for the ethanol chain in analogy to the case of Figure 3b. However, there is no chiral feature from the image of ethanol chain. We speculate that the mirrored branches for each lobe are due to the coupling between two separate sets of random rotation: the rotating chain and spinning bearing. On the other hand, the charged trimer did not reveal a stable and repetitive orbit during continuous scanning from Figure 5a, b to c. It can be also attributed to the coupling or interference. From Figure 5d−f, we found that the charged trimer adsorbed tightly on the HOPG surface. It was still located at the original position, while the rotating chain was dragged away by the tip. It should be pointed that, due to the carried net charges, the spinning trimer

Figure 6. Libration of a long ethanol chain around a common charged trimer chain acquired at 78 K. (a) The long molecular chain liberating around the equilibrium position (3.0 V, 40 pA). (b) Shifting of the equilibrium position for the long chain (3.0 V, 40 pA). (c) Stopping libration when tunneling current is reduced (3.0 V, 35 pA). (d) Current versus time spectra acquired at a bias of 3.2 V with STM tip positioned at the cross position in (b). 10240

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ACS Nano the libration of a long molecular chain around a common linear cluster. Initially, the long molecular chain exhibits a 120° libration around the equilibrium position (Figure 6a). Interestingly, the equilibrium position of the long chain could be rotated by 120° (Figure 6b). When the tip was slightly withdrawn, the chain stopped librating and became linear rod (Figure 6c). A I−t spectrum was measured by positioning the tip over the ethanol chain (Figure 6d). It is obvious that the fluctuation in tunneling current reveals certain “noises” without distinct values corresponding to the stable orientations of the long chain. We also noticed that the libration frequency is much higher than that of the directional rotation shown in Figure 3c, beyond the response ability of STM controller.

Zu-Hong Xiong: 0000-0003-4729-300X Jun-Zhong Wang: 0000-0003-1311-3736 Author Contributions ⊥

These authors contributed equally to the work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is supported by the Natural Science Foundation of China (grant nos. 10974156, 21173170, 91121013, 21221061, 21273219, 21203174, 11604269), the Fundamental Research Funds for the Central Universities (XDJK2017C064, XDJK2016C065), Jilin Province Youth Fund (grant no.: 20130522141JH), and Jilin Province Computing Center (grant nos.: 20130101179 JC-08, 20130101179 JC-07). Z.J.W. and Y.W. also thank the financial support from Department of Science and Technology of Sichuan Province. We are grateful to Performance Computing Center of Jilin University and Computing Center of Jilin Province for essential support.

CONCLUSIONS In summary, we have fabricated several supramolecular rotors and motors of ethanol molecules mounted on a graphite surface by tip charging individual ethanol clusters. By varying the chirality of ethanol chains, random rotation and directional rotation with clockwise or anticlockwise chirality can be realized. Our findings show a direct route to fabricate supramolecular motors, which applies not only to ethanol molecules but also to other polar alcohol molecules.

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METHODS The experiments were conducted in an ultrahigh-vacuum lowtemperature STM system. The base pressure is better than 1.2 × 10−10 Torr. A HOPG flake attached to a thick Cu plate was used as the substrate. The ethanol molecules (Sigma-Aldrich, ≥ 99.8%) were dosed in situ onto a HOPG surface through a leak valve. The subsequent STM topographic images were acquired with a tungsten tip in constant-current mode with the substrate temperature kept at either 4.5 or 78 K. All voltages refer to the sample bias with respect to the tip. The differential conductance spectra were acquired using lock-in detection of the tunneling current modulated by a signal of 2505 Hz, 10 mV, added to the sample bias under open-loop condition. In the charge-state switching experiments, we positioned the STM tip on top of an ethanol molecule or a cluster and applied a voltage pulse of 2−3 V with the feedback loop turned off. In characterizing the rotation rates of ethanol chains, the tunneling current is monitored with respect to time with the feedback loop turned off.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b04811. Computational methodology of all the electronic structure calculations, sequential acquisition of a supramolecular rotor, STM images, structural models and simulated topographic images of ethanol clusters, calculated dipole moments of ethanol clusters, and variation of electric field versus tunneling current (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Kai Sun: 0000-0002-7179-7200 Ying Wang: 0000-0002-5437-8741 10241

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NOTE ADDED AFTER ASAP PUBLICATION Due to a production error, panel D of Figure 6 was truncated in the version published on September 18, 2017. The corrected version was reposted on September 22, 2017. 10242

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