Article pubs.acs.org/JPCC
Electric-Field-Induced Molecular Switch of Single Dipolar Phthalocyanine on Cu(111): A Scanning Tunneling Microscopy Study Tianchao Niu* State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, 865 Changning Road, Shanghai 200050, People’s Republic of China ABSTRACT: We demonstrate a molecular switch of a single Cl-down adsorbed choloraluminum phthalocyanine (ClAlPc) molecule on a Cu(111) substrate by scanning tunneling microscopy (STM) at liquid nitrogen temperature (77 K). The Cl-down adsorbed ClAlPc molecule exhibits an in-plane quadruple moment on the four-lobe which stems from the inequivalent charge distribution after anchoring on Cu(111). The quadruple moment under the nonuniform electric field between the STM tip and the sample surface is responsible for the rotation of these Cl-down adsorbed ClAlPc molecules within three metastable states. In response to the local nonuniform electric field, the switching of these Cl-down adsorbed ClAlPc molecules can be controlled by varying the applied biases.
1. INTRODUCTION
Phthalocyanines, a class of widely used functional molecular building blocks in technological applications26 and as the candidates for fundamental research27,28 in molecular electronics, represent an interesting model for molecular switches. Here, we show that the nonplanar dipolar chloroaluminum phthalocyanine (ClAlPc) molecule, which is anchored to a Cu(111) surface with the Cl-down configuration, can function as a molecular switch. The Cl-down adsorbed ClAlPc residing at the hollow site of Cu(111) exhibits an induced in-plane electrostatic quadruple moment due to the asymmetric charge distribution on the four-lobe which stems from the charge transfer from the Cu substrate to lift the degeneracy of the eg state of ClAlPc.29 As a result, the Cl-down adsorbed ClAlPc can reversibly switch between three metastable states under the nonuniform electric field between the STM tip and sample surface.
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A molecular switch is one of the basic components in electrical circuitry to build up molecular electronics, such as data storage and logical circuits.2−5 The inducing and reading of the switch can benefit from the bi-/multi-stable states of the molecules.6−9 To this end, conformational change,10,11 bond formation,12 chiral switching,13,14 charge state,15,16 tautomerization,17,18 and even the smallest single proton switch19,20 have been demonstrated in single molecular switches. Generally, an external stimuli used in these molecular switches is inelastic electron injection directly into the molecules through the tip of scanning probe microscopy (SPM).8,21 The coupling of tunneling electrons with vibrational modes of molecules can induce the conformational switch. For example, the rotation of a single acetylene molecule on Cu(001) includes three elementary processes, the excitation of the C−H stretch mode, electron transferring to the rotation mode, and the combination band process of inelastic tunneling and vibrational excitation.22 Instead, operating the molecular device without the accurate injecting current through the molecule would be useful to develop multifunctional molecular complexes without disturbing the molecule itself, such as the isomerization of azobenzene induced by the electric field between the STM tip and the sample surface.23 A novel concept differing from the previously reported switches is based on the Jahn−Teller effect by changing the local vicinity of the negatively charged CuPc/ NaCl(2 ML)/Cu(100), which can be applied to a molecular switch.24 Furthermore, it has shown that the local environment of a molecule plays an important role in controlling the intramolecular hydrogen transfer in a porphycene molecule on the Cu(110) surface, demonstrating the potential to realize controllable switching processes in a single molecule.25 © XXXX American Chemical Society
2. EXPERIMENTAL METHODS STM measurements were performed using a custom-built multichamber STM system with a pressure better than 1.0 × 10−10 mbar, housing an Omicron LT-STM interfaced to a Nanonis controller. The Cu(111) single crystal (MaTeck, Germany) was cleaned by standard Ar+ sputtering and annealing cycles. The surface structure and cleanliness of Cu(111) were characterized by LT-STM; after that, the ClAlPc molecules were evaporated in ultrahigh vacuum (UHV) from heated Knudsen cells (MBE Komponenten, Germany) onto the Cu(111) kept at room temperature. The molecular source was degassed for more than 24 h before the deposition. All the Received: May 5, 2015 Revised: June 30, 2015
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Figure 1. (A) Schematic model showing the nonplanar configuration of ClAlPc with the Cl atom protruding outside the molecular plane. (B) ClAlPc molecules adopt Cl-up and Cl-down configurations on the Cu(111) surface; three different orientations of the Cl-down adsorbed ClAlPc molecules are denoted by D-L, D-O, and D-R, respectively. Chemical structure of ClAlPc is shown in the inset. (C) Optimized most stable configuration of Cldown adsorbed ClAlPc at the hollow site on Cu(111); bottom shows the three orientations of Cl-down ClAlPc due to the 3-fold symmetry of Cu(111) substrate. The three states are denoted by “L: left”, “O: origin”, and “R: right”.
reduction, while the ClAlPc molecules with Cl-up configuration reserve the 4-fold symmetry.29 Figure 2 summarizes the switching processes of Cl-down adsorbed ClAlPc molecules under different coverages. Figure 2A demonstrates the switching processes under different tip biases after sequential scanning at the coverage of 0.3 ML. Six Cl-down adsorbed ClAlPc molecules underwent switching named a−f are highlighted (dashed circles in Figure 2A). The yellow dashed rectangles in Figure 2A represent the switched molecules under certain scanning conditions. The molecules a, b, and c experienced at least three orientations during the minimum four times scanning, whereas the molecules d, e, and f underwent only the “O” and “L” orientations. At first sight, the switching processes would be ascribed to (1) the interactions with the neighboring molecules and/or (2) the electric field between the STM tip and the sample. In order to elucidate the role of interactions between the switched molecules and their neighboring molecules, STM investigations on the switching processes at different coverage are conducted. Figure 2B representatively shows an STM image of ClAlPc molecules on Cu(111) at 0.1 ML, within which the intermolecular distance would be larger than that in Figure 2A (∼0.3 ML). We recorded the switched molecules after 28 sequential scans under different tip biases, but the same tunneling current (80 pA). Nearly ∼60% of the Cl-down adsorbed ClAlPc molecules switched. The dashed black circles represent the distance between the switched ClAlPc molecules and their neighbors. Similarly, the measurements at the coverage of ∼0.8 ML are shown in Figure 2C, and ∼31% of the Cl-down adsorbed ClAlPc molecules switched. However, the molecules are spatially confined after increasing the coverage to 1 ML (Figure 2D) with an intermolecular distance of ∼1.5 nm. The intermolecular distances between the switched molecules and their neighbors are ∼2.2, ∼3.1, ∼3.9, ∼4.6, and ∼5.4 nm (±0.05 nm), showing an arithmetic progression with a common difference of ∼0.8 nm. This is coincident with the long-range oscillatory of period λF/4 in the electronic density of Cu(111), because of the 2D nearly free electron gas features of the surface state electrons on Cu(111).36 However, the switch process is blocked when the Cl-down adsorbed ClAlPc molecules have a neighboring adsorbate away from ∼1.5 nm
STM images were measured at liquid nitrogen temperature (77 K) using the constant current mode and electrochemically etched tungsten (W) tip. I−V spectra were collected by using lock-in detection of the a.c. tunneling current driven by a 1013 Hz, 10 mV (r.m.s) signal added to the junction bias under open-loop conditions. The bias voltage was applied on the tip during the STM measurements.
3. RESULTS AND DISCUSSION ClAlPc (molecular structure is shown in Figure 1A and inset in Figure 1B) is a typical nonplanar phthalocyanine molecule with the central Cl atom protruding outside the molecular plane, which can adopt either Cl-up or Cl-down configurations after the adsorption on substrates.30,31 Previous studies revealed that ClAlPc tends to aggregate into unidirectionally aligned twodimensional molecular dipole dot arrays with predominant Clup configuration on HOPG,32 and graphene33 surface. A reversible single-molecular switch between the Cl-up and Cldown configurations can be realized within these ordered dipole dot arrays which are manipulated by applying tip pulses.11 The reversible switch has been demonstrated to be a general phenomenon in manipulating such kinds of dipole phthalocyanine molecules by STM, such as the vanadyl phthalocyanine (VOPc) on a graphite surface.34 However, when depositing on the Cu(111) surface, these nonplanar Pc molecules can adopt both the up and the down configurations.35 Figure 1B shows the coexistence of Cl-up and Cldown adsorbed ClAlPc on Cu; the four-lobe featured with a central bright dot represents a ClAlPc molecule with the Cl-up configuration, while the ones without are Cl-down adsorbed ClAlPc molecules. These ClAlPc molecules randomly dispersed even at ∼0.8 monolayer (Figure 2C) (ML, referring to a densely packed layer of flat-lying ClAlPc molecules fully covered the Cu(111) surface with their molecular π-plane parallel to the substrate). ClAlPc adopting a Cl-down configuration preferred to reside at the hollow site, and three equivalent molecular orientations of the Cl-down ClAlPc can be discerned due to the 3-fold rotation of the Cu(111) substrate (Figure 1B,C). It is noteworthy that the Cl-down adsorbed ClAlPc molecules are subjected to symmetry B
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Figure 2. Switching processes under different coverages. (A) 0.3 ML, sequential STM images demonstrating the switching of Cl-down adsorbed ClAlPc under different scanning conditions: (i) Vtip = 1.6 V, I = 1 nA; (ii) Vtip = 2 V, I = 1 nA; (iii) Vtip = 1.5 V, I = 1 nA; (iv) Vtip = 0.5 V, I = 1 nA. (B) 0.1 ML, the red circle represents one switched ClAlPc molecule which has the farthest neighbors away from ∼5.4 nm; the red triangle indicates that the ClAlPc molecules are surrounded by Cl-up adsorbed ClAlPc only; the red square indicates that the ClAlPc molecule is surrounded by Cldown adsorbed ClAlPc only. The dashed circles highlight the distance between the neighboring molecules and the switched molecules. (C) ∼0.8 ML, the switched Cl-down ClAlPc molecules are indicated by dotted circles; the average distance between the switched molecules and their neighbors is ∼2.1 nm. (D) 1 ML ClAlPc on Cu(111), nonswitching of the Cl-down adsorbed ClAlPc molecules has been observed due to the spatial confinement.
has a minimal effect on the switching. In particular, the highest ratio among all the studied cases happened under a tip bias of 0.02 V. This is in stark contrast to the inelastic tunnelinginduced vibration of molecules with a threshold voltage.37 It has been demonstrated that the interatomic forces, i.e., the electric field, chemical attraction, and repulsion between the tip and the single molecule can induce the movement of the molecules, which depends on the tip−sample distance.38−40 Electric field can induce the isomerization of azobenzene, the conformational change of the tetra-di-tert-butyl-phenyl porphyrin,41 and also can drive the unidirectional rolling motion of a nanocar on Au(111).40 One of the key factors involved in these electric-field-induced molecular movements is the presence of permanent or induced dipole in the molecules. Coupling between the dipole moment and the electric field can provide efficient energy to initiate the conformational trans-
(one pair of Cl-down adsorbed ClAlPc molecules, black circle in Figure 2A; and the 1 ML ClAlPc). This is why the ratio of switched molecules decreases after increasing the coverage. It can be stressed that the substrate-mediated intermolecular interactions play a dominant role in driving the molecular switches. Furthermore, to reveal the potential driving force to induce the switching of the Cl-down adsorbed ClAlPc, we continued to scan the same area under different tip biases and with different tunneling currents. Figure 3 summarizes the selective STM images among 40 sequential scans under different scanning parameters. The tip bias ranges from −2 to 2 V with the step of 0.05 V, and the set tunneling current is from 50 to 100 pA with the step of ∼5pA. It can be found that the switching ratio of the Cl-down adsorbed ClAlPc molecules depends strongly on the tip bias, while the tunneling current C
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Figure 3. STM images obtained under sequential scanning under different tip biases and tunneling currents show the tip bias dependent switching efficiency. The dashed circles indicate the switched molecules under certain tip bias and tunneling current. Scanning parameters are indicated on each STM image, the same size for all images.
Figure 4. (A) High-resolution STM image showing the symmetry reduction of the Cl-down adsorbed ClAlPc molecule. (B) The negatively charged ClAlPc after an intake of 1.3 e from Cu(111) is subjected to Jahn−Teller distortion to lift the degeneracy of the frontier eg state, forming an in-plane quadruple moment. (C) The I−V curve of the central (blue), bright and long lobe (black), and short lobe (red). (D) Schematic model showing the nonuniform electric field generated between the STM tip and the sample when applying a bias.
V measurements acquired over the center and the symmetryreduced lobes of a Cl-down adsorbed ClAlPc molecule; there is no steplike changes in the current during the tip bias ramping from −2 to 2 V, indicating no conductance variation. This is in contrast to the inelastic electron tunneling via a molecular electronic resonance at the threshold voltage.42 Furthermore, this molecule shows no switching after the I−V measurement. In addition, the collective rotation of the molecules during a single scan further proves that the molecular rotation of Cldown ClAlPc is induced by the electric field rather than the tunneling electrons. Although the ClAlPc has a permanent dipole moment which is perpendicular to the molecular π-
formation, while the coupling between the tunneling current and the molecule is negligible. In the present study, an in-plane quadruple moment was induced in the Cl-down adsorbed ClAlPc due to the asymmetric charge distribution on the four Pc lobes which stems from the Jahn−Teller effect of the negatively charged ClAlPc after the intake of 1.3 electrons from the Cu substrate (Figure 4B).29 The inequivalently charged Pc lobes with an inplane quadruple moment are sensitive to the local nonuniform electric field between the STM tip and the sample (Figure 4D), and hence, reversible switching of the Cl-down ClAlPc molecules can be efficiently induced. Figure 4C shows the I− D
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Sciences. T.N. would like to acknowledge his Ph.D. supervisor Professor Wei Chen and all group members in NUS for their contributions to the STM study. Helpful discussions with Prof. Ang Li and Prof. Zhi Liu from SIMIT are gratefully acknowledged. The author is supported by the National Natural Science Foundation of China (contract No. 11227902, contract No. 21403282) and the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (Grant No. XDB04040300).
plane, no rotation/diffusion of the Cl-up adsorbed ClAlPc has been observed during our measurements. This would be due to the lack of the anchoring center for rotation and the nonexistence of the in-plane quadruple moment in Cl-up adsorbed ClAlPc molecules. The electric field generated between the STM tip and the sample surface is asymmetric in the x−y plane and along the z direction. The energy source to drive the switching of these Cldown adsorbed ClAlPc molecules can be illustrated based on the forces of a quadruple under a nonuniform electric field (Figure 4D). Decreasing the tip bias reduces the tip−sample distance and further enhances the nonuniformity of the electric field. As such, more molecules can be driven to switch under lower tip bias (0.02 V). This is consistent with phenomena observed during the sequential STM scanning under different tip biases (Figure 3). As a proof-of-concept experiment, the features of planar manganese phthalocyanine (MnPc, nonpermanent dipole moment) and nonplanar, nondipole dichlorotin phthalocyanine (SnCl2Pc, central Cl atom protruding outside the molecular plane) on the Cu(111) were studied by STM under the same scanning parameters as that of the ClAlPc on Cu(111). Particularly, no in-plane quadruple moment or symmetry reduction has been observed in these molecules after the adsorption on Cu(111). Consequently, nonrotation of either phthalocyanines has been observed. We can form a conclusion that the molecular switching of these negatively charged Cldown adsorbed ClAlPc molecules depends crucially on the presence of the in-plane quadruple moments and the nonuniform electric field. The protruding Cl atom plays a the role of both the anchoring point on the substrate to stabilize the molecular switch and the charge transfer venue to the Pc ligand from Cu.
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4. CONCLUSIONS In conclusion, ClAlPc anchored on the Cu(111) surface with a Cl-down configuration can be reversibly switched between three metastable states. The inequivalent charge redistribution onto the four Pc lobes of the negatively charged ClAlPc induced an in-plane quadruple moment within the Cl-down adsorbed ClAlPc, and the relationship between the applied voltage and the switching ratio of the ClAlPc is determined by the nonuniform electric field between the STM tip and the sample surface. Comparing with other precisely controlled methods to switch the molecular conformation, the electricfield-controlled way described here has the advantages of the collective switching of many molecules during scanning and of using functional phthalocyanine molecules as basic components.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*Phone: +86-21-62511070. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The research described herein was supported by the Surface Science Lab, Department of Physics, National University of Singapore (NUS), and the State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology (SIMIT), Chinese Academy of E
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