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Scanning Tunneling Microscope Tip-Induced Formation of a Supramolecular Network of Terarylene Molecules on Cu(111) Jan Patrick Dela Cruz Calupitan, Olivier Galangau, Olivier Guillermet, Roland Coratger, Takuya Nakashima, Gwenael Rapenne, and Tsuyoshi Kawai J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09370 • Publication Date (Web): 23 Oct 2017 Downloaded from http://pubs.acs.org on October 24, 2017

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The Journal of Physical Chemistry

Scanning Tunneling Microscope Tip-induced Formation of a Supramolecular Network of Terarylene Molecules on Cu(111) Jan Patrick Dela Cruz Calupitan,†,‡,§ Olivier Galangau, †,‡ Olivier Guillermet, § Roland Coratger, § Takuya Nakashima,‡ Gwénaël Rapenne,*,†,§ and Tsuyoshi Kawai*,†,‡ †

NAIST-CEMES International Collaborative Laboratory, CEMES-CNRS, BP 94347, 31055

Toulouse, France ‡

Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5

Takayama, Ikoma, Nara, Japan §

CEMES, Université de Toulouse, CNRS, Toulouse, France

ACS Paragon Plus Environment

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ABSTRACT.

Bottom-up approaches allow for molecular-level control of engineering new nanostructures. For the next generation molecular circuits, machines, and phase-change materials, supramolecular structure formation and interactions must be investigated on a two-dimensional solid substrate. Scanning tunneling microscopy (STM) provides not only a method to address such supramolecular structures at the molecular level on a solid surface but also to locally control and manipulate such structures. In this article, we show that a terarylene molecule, a sub-family of photoswitching diarylethenes that promise a lot of applications from molecular switches to photoelectronics, assembles upon application of a bias voltage pulse from an STM tip. We show that molecules organize themselves in order to align their dipole moment to follow those of the electric field induced in the tunneling junction. The 2D assembly are stabilized by π - π stacking interactions and van der Waals interactions. This expands the repertoire of currently available bottom-up techniques for fabrication of supramolecular nanostructures.


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INTRODUCTION Bottom-up approaches relying on supramolecular interactions are foundations for new molecular-scale machines, switches, and other energy transfer devices at the nanoscale.1 For such applications as phase-change materials or molecular circuits, it is necessary to address candidate molecules or supramolecular structures on a solid surface so that the past decade has seen enormous research attention on supramolecular interactions and functions in 2D assemblies.2-3 Although these studies deal with one less dimension than that of solution-based supramolecular chemistry, the presence of the solid substrate complicates and at the same time affords new opportunities for designing and making new functions by giving an orientation to the studied system.2 In addition to this, developments in real-space imaging of periodic 2D-networks via the scanning tunneling microscope (STM) coupled with other complementary techniques allowed a deeper understanding of molecule-molecule and molecule-substrate interactions.1-3 Due to these, new techniques to induce the formation of surface supramolecular assemblies have recently been developed. These include: (1) chemical modification to control molecule-molecule and molecule-surface interactions;2-3 (2) co-deposition of metal ions that could reorient molecules through dipole interactions;4 and (3) surface annealing that gives molecules enough energy to reconfigure and form assemblies.5-6 Aside from being a powerful tool to precisely address molecules and their interactions, the STM may also induce an electric field between the tip and the scanned surface. This has been shown to play a function in the rotation of molecular networks7-9 and chemical reactions.10 Meanwhile, in solution chemistry, it is well-known that due to the polarizability of polar molecules in a nonpolar solvent, they realign their dipole moment along field lines upon application of an external electric field. From the works of Debye and Guggenheim,11-12 experimental methods are


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available in order to calculate the dipole moment of a molecule.13-14 The electric field applied by the tip of a scanning tunneling microscope (STM) therefore can be a potential trigger to induce the ordering of molecules on a surface. External electric field-induced molecular reorientation in a molecular junction was first predicted in 2002.15 Thereafter, subsequent studies focused on consequent conductance modulation to demonstrate single molecular switching capabilities.16-17 To the best of our knowledge, this electric field has not been applied yet on large numbers of molecules which could realign in one direction, allowing for intermolecular forces to take over and form a stable supramolecular network. A couple of recent works come close to this but the ordering of assemblies were induced by hot tunneling electrons. In one, the hot electrons coming from the STM tip to the surface were shown to induce surface phase changes of an assembly of a dipolar perylene derivative18 while in another, transition from disordered to an ordered assembly was induced by injecting hot electrons onto the surface.19 In this study, we investigate the formation of on-surface molecular networks of a diarylethene derivative. Diarylethenes are promising switching materials for molecular opto-electronic devices because of their bistability, high fatigue resistance, and sensitivity.20 Assemblies of diarylethenes21-27 and its derivative terarylene28-29 have already been reported but they remain at the solid-liquid interface. Meanwhile, under ultra-high vacuum conditions, diarylethenes were assembled only through dipole-dipole interactions30-31 and ion-dipole interactions4. 2,3-Dithiazolylbenzothiophene has been selected because of its promising switching sensitivity to light-stimulus in solution and solid states.32 Figure 1 shows the molecular structure and the gas-phase DFT-optimized geometry (see SI2 for computational details). Due to several intramolecular forces that lock it in its reactive conformation, the molecule displayed photon quantitative reaction with photocyclization quantum yield close to unity in solution.32


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Figure 1. Structure of the terarylene used in this study. On the left is the chemical structure showing intramolecular stabilizing interactions while on the right is the CPK model of the DFToptimized structure. Characteristic distances and direction of the dipole moment are shown. (The dipole moment is centered on the photoreactive 6π-system but shifted to the side for clarity.)

Furthermore, the combination of two thiazoles and one benzothiophene subunit (Fig. 1) are most likely at the origin of an important intrinsic dipole moment (2.1 D) which could be manipulated with an external electric field. We show in this letter that the application of an external electric field through a bias pulse from an STM-tip induced the spontaneous formation of assemblies of molecules without any solvent under ultra-high vacuum conditions.

METHODS The terarylene derivative was synthesized as previously reported.33 STM Experiments were performed in an Omicron low-temperature scanning tunneling microscope (LT-STM) working at a base pressure of 1.5 x 10-11 mbar on single-crystalline Cu(111) substrates. A dichloromethane solution of the terarylene was first deposited on a W wire 0.15 mm in diameter and then subjected to high-temperature degassing under UHV. The molecules present on the filament were sublimated at about 200 oC while the Cu substrate was kept at 77 K. These special


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conditions of sublimation usually minimize the decomposition of molecules. STM-tips were 250 µm diameter tungsten wires prepared by electrochemical etching. Tip quality was confirmed by observing the characteristic surface state of Cu(111) at 400 meV below the Fermi level through tunneling spectroscopy. All bias voltages reported refer to the surface relative to the tip. All images were taken at 77 K under constant current mode and processed by the WSxM software.34

RESULTS and DISCUSSION Deposition of molecules (up to ~0.2 monolayer coverage) resulted in STM images with fuzzy objects moving below the tip particularly close to the step edges. (Fig. 2a) The streaking patterns show the movement of molecules during scans. Although a couple of columns of seemingly ordered structure could be found (Fig. 2a gray arrow), line scans of the island of these fuzzy objects show generally no long-range order. (SI3) Deposition was repeated under similar conditions several times and no long-range order was found.


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Figure 2. a) Islands of fuzzy objects moving on the surface or close to the tip. The yellow arrows point onto an adatom as a landmark. White arrows point to partially formed rows with no longrange order. b) After application of a bias pulse (position of the tip indicated as a white cross), a new ordered island appeared. Some order was also induced in the island above, but the disordered state seemed to be too much for assembly formation to happen c) Extract of an island formed from 2b, showing two apparent orientations of the ordered regions rotated 60o from each other. The disorder between the two regions seemed to be due to the completion between the two areas to extend and establish order of a longer-range. d) Experiment repeated on another region (position of the tip indicated as a white cross) showing the other 60o rotation, indicating the influence of the three-fold symmetry of the Cu(111) surface on orienting the assemblies. Imaging conditions: -1.0 V at 1pA.


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Then, the STM tip was retracted 1 nm from the surface and a controlled bias pulse of 3.0 V for 100ms was applied, then the tip potential was settled back to 0V. Because of this tip to surface distance upon the bias pulse, no appreciable net current was derived. Subsequent approach of the tip to the surface and imaging at -1.0 V and 1pA shows the formation of islands with welldefined rows of molecules. (Fig. 2b) The adatom (Fig. 2a-b, yellow arrow) served as a landmark to ensure that the same region was imaged. Between the dashed gray lines on Fig 2b, some molecules seemed to rearrange to form two ordered rows of molecules. However, the surrounding disordered molecules seemed to prevent the formation of long-range order. Some new molecules appeared at the lower part of the image to form another more ordered island. The new island has two regions A and B rotated 60o relative to each other. (Fig. 2c) This angle value suggests that the three-fold symmetry of the Cu(111) surface plays a role in orienting the assemblies of molecules. This was confirmed by finding a orientation in the opposite rotational direction relative to region A. Repeating the experiment in other regions yielded exactly this expected outcome. Region C in Figure 2d is just exactly the rotational orientation of A 60o in the clockwise direction. Only these three orientations were found in all assemblies observed. We found almost equal probabilities of forming orientations A, B, and C. The disorder between two regions in Fig. 2c,d seems to be the direct effect of competition between them attempting to expand for longer range order. When perfectly-ordered islands were formed so that only one orientation of rows were observed, no such disorder was observed over the whole island. (Fig. 3a, SI 4)


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Figure 3. a-c) Evolution of the assemblies during consecutive scans. The assemblies are stable for several hours even without scanning. d) Orientation of the molecules as overlayed over the assemblies. Imaging conditions: -1.6 V, 1pA, T = 77 K.

The duration of the bias pulse suggests that the self-assembly formation was very fast relative to the timescale of STM scans. We did not attempted to apply the voltage at longer time intervals nor lower tip-surface distances for fear of other events happening (tip decoration, distortion of the tip, bond breaking, etc). The large tip-surface distance shows that only the applied electric field and not the STM current causes the formation of assemblies. Bias pulses of lower voltages could not induce network formation while at 3 V, the assembly could be induced 90 % of the time. The tunneling current during scanning does not seem to affect the stable assemblies as there are no spontaneous changes in the assembly during scanning of the islands (Fig 3a-c). This is unlike the observation by Niederhausen et al. in which a dipolar perylene derivative (with a large dipole moment of 12 D) undergoes phase changes while scanning18 nor that of Li et.al. in which hot electrons induce the formation of assemblies.19 The dipole moment (2.1 D) of the terarylene is large enough to be influenced by the 3 V electric field induced 1 nm away but weak enough not to be affected during scans at -1 V when the tip is close to the surface. To compare this to another work, electric-field induced rotational switching of a molecule with a higher dipole moment (8 D) required a bias pulse of only 0.93 V.7 This is


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unsurprising as a lower dipole moment would require higher voltages to be influenced by an electric field. Interestingly, application of a negative bias pulse did not resulted to any distortion of the networks. This method for inducing the formation of assembly is also different from that reported by Rabe, Müllen and co-workers in which an assembly was induced by application of an external electric field parallel to the surface during liquid deposition and subsequent drying.35 This uses the same well-known phenomena as electric-field alignment in a non-polar solvent.11-14 In our system, the assemblies were formed without any solvent under ultra-high vacuum conditions. The stability of the images suggests that the intermolecular forces among molecules are strong enough to hold the assemblies at this temperature. However, the streaking patterns at the peripheries indicate that molecules in these regions are more mobile than those in the interior of the island. (Fig. 3a) As expected, subsequent scans show the disappearance of some molecules at the edge of the islands due to less neighbors that contribute to the stability. (Fig. 3a-c) It must be noted however, that consecutive scans are not necessary for the stability of the islands as the tunneling current do not have an effect on the assemblies. The assemblies could be reformed upon reapplication of a bias pulse. This is a clear sign that between the assembly and the surface, there is only weak interaction and no covalent bond nor charge transfer is happening upon deposition. Further, we discount the possibility of degradation of molecules because we did not observe any fragments of molecules before and after the bias pulse. Such fragmentation would have cause heterogeneity in the network and prevent the formation of regular assemblies which is not the case here. Also, it has been shown that after a few hours, the network slowly become disordered with some molecules


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diffusing on the surface. Then, application of a new pulse regenerates the self-assembled monolayer. This reproducibility would have been impossible to achieve if the molecule has been previously destroyed by the pulse. The molecular network has the following dimensions: a = 1.28 ± 0.04 and b = 0.35 ± 0.01 nm with an oblique lattice. (Fig. 3d) These distances correspond well, respectively, with the distance between the terminal hydrogens in Fig. 1 and typical π-π stacking interaction distances. The a vector is aligned to the [1ത10] direction on the Cu(111) surface while b is rotated 25 ± 3o relative to the [11ത0] direction. A typical pulse produces islands with an average size of 300 ± 80 nm2 stable for several hours. In the solid-liquid interface, typical distance between neighbouring diarylethenes stabilized by van der Waals forces between long alkyl groups are in the 1.0 – 1.5 nm range.21-27 Since the observed intermolecular distance in this supramolecular network (distance b in Fig 3d) is far lower (0.35 nm instead of 1.0 – 1.5 nm) and the molecules do not harbor these long alkyl groups but only aromatic rings, we postulated π-stacking interactions as the main intermolecular interactions stabilizing the assemblies. This was confirmed by a molecular mechanics calculation (Materials Studio 7.0) of π stacking conformation between two molecules predicting a stacking distance of 0.34 nm. (SI5) Due to the S-N and N-H interactions that contribute to the rigid structure of the molecule32, the planes of the side 5-methyl-2-phenyl-thiazol-4-yl moieties are almost parallel to each other so they could stack neatly to neighbouring molecules. Dihedral angles between the benzothiophene and the two left and right thiazolyl moieties have been calculated to be 45° and 55° respectively. The co-planarity of the benzothiophene moiety also contributes to these π-stacking interactions.


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Meanwhile, the average height of the islands is found to be around 0.91 ± 0.04 nm, which is in accordance with the distance between the topmost hydrogen of the benzothiophene moiety and methyl hydrogens in Fig. 1. With these, the terarylenes seem to stand on the surface through its side phenylthiazole moieties with the benzothiophene moieties sticking out of the surface. (Fig. 3d) As expected, the topographically higher and electron-richer heteroaromatic rings contribute more to the contrast of the central part of the molecules than the side phenyl rings. Finally, we propose a mechanism by which the molecules assemble under the influence of the STM tip. During deposition, the molecule is first sublimated at about 200°C before it lands on the substrate kept at 77 K. DFT calculations have been performed by doing torsion angle scan on the relevant dihedral angle and found out that, other than the conformation C0 shown on Fig. 1 where both N-H and S-N interactions are present, the molecule has two other stable conformations CN-H and CS-N where only one interaction is conserved, the N-H or the S-N interaction respectively (see SI6). By using the Boltzmann equation, we found out that these conformations increase in population at the high temperatures of sublimation similar to our previous report on a related molecule.33 (Table SI1) To add to this disorder from the flexibility of the single bonds connecting the aromatic rings, each respective conformation would have different surface-induced conformations depending on their orientation once they land on the surface. Also, the molecules seem to have enough energy to allow their random motion and slight diffusion. This is evidenced by the streaking patterns in some images before the application of a bias pulse (SI3). At 77 K, the molecules seem to have enough degree of freedom to orient to the electric field lines. Further, once stabilized on the surface, the molecules are hindered from rotation not only geometrically (by the surface and the network) but also


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energetically, as the rotation barriers are 1 to 2 orders of magnitude greater than the thermal energy at this temperature. (Table SI2) In our system, bias voltage refers to the potential with respect to the tip, which means that the electric field points to the tip when a positive bias voltage is applied. (Fig. 4) Meanwhile, the dipole moment of the molecule is mainly due to the aromatic thiazole rings with the benzothiophene ring having almost no net charge (see SI2 for the Mulliken charges). Then, the negative end of the dipole moment is located around the center of the triangular molecule coplanar with the nitrogen atoms of the thiazole rings. The diffusive mobility of the molecules on top of the surface is hindered by the electric field; the dipole moments orient themselves parallel to the electrical field and point upwards. In this way, the electric field lines and dipole moments realign in the same direction. Induced dipole moment under the applied field seems also to associate cooperatively with the static dipole moment in this field induced orientation. The extended π-conjugation system of the triangle terarylene structure may contribute to the enhanced induced polarizability. With enough molecules aggregating in a small amount of space, intermolecular forces such as π-π stacking take over so that assemblies are extended and stabilized. The electric field is therefore instrumental in inducing a change in the system from a disordered state to an ordered one stabilized by intermolecular forces of attraction, similar to electric field-induced crystallization for the realignment of polymer fibers,36 tuning of molecular packing in thin films,37 and purification from a liquid suspension38.


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Figure 4. Description of the process: a) Molecules deposited on the surface are disordered. b) When a positive bias voltage pulse is applied, the STM tip is negatively charged while the Cu(111) substrate is positively charged and the molecules align their dipole following the lines of the electric field. c) The supramolecular assembly is maintained after the pulse.

The

calculated dipole moment of the molecule has a magnitude of 2.1 Debye.

CONCLUSION By taking advantage of the electric field induced by the bias voltage between the STM-tip and the surface under ultra-high-vacuum conditions, we succeeded to induce the formation of a supramolecular self-assembled network from a disordered system. To the best of our knowledge, this is the first report utilizing the well-known electric-field-induced realignment of molecules to form assemblies in ultra-high vacuum conditions without solvent. The local nature of the STM probe further brings this control of molecular alignment down to the nanoscale as a bottom-up approach to assemble 2D systems. This expands the repertoire of bottom-up approaches that could be exploited to form supramolecular assemblies of functional molecules for future applications.


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SUPPORTING INFORMATIONS Details of DFT calculations, supplementary images, and molecular mechanics calculation are provided in the Supporting Information. The following files are available free of charge. Supporting Information.pdf

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] and [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the MEXT Program for Promoting the Enhancement of Research Universities in NAIST, the CNRS, the University Paul Sabatier (Toulouse). This research was also partly supported by the JSPS KAKENHI Grant Number JP26107006 in Scientific Research on Innovative Areas “Photosynergetics” and the Programme Investissements d’Avenir ANR-11IDEX-0002-02, reference ANR-10-LABX-0037-NEX.

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(22) Yokoyama, S.; Hirose, T.; Matsuda, K. Effects of alkyl chain length and hydrogen bonds on the cooperative self-assembly of 2-thienyl-type diarylethenes at a liquid/highly oriented pyrolytic graphite interface. Chem. Eur. J., 2015, 21, 13569-13576. (23) Yokoyama, S.; Hirose, T.; Matsuda, K. Phototriggered formation and disappearance of surface-confined self-assembly composed of photochromic 2-thienyl-type diarylethene: a cooperative model at the liquid/solid interface. Chem. Commun., 2014, 50, 5964-5966. (24) Sakano, T.; Imaizumi, Y.; Hirose, T.; Matsuda, K. Formation of Two-dimensionally ordered diarylethene annulated isomer at the liquid/HOPG interface upon in situ UV irradiation. Chem. Lett., 2013, 42, 1537-1539. (25) Bonacchi, S.; El Garah, M.; Ciesielski, A.; Herder, M.; Conti, S.; Cecchini, M.; Hecht, S.; Samori, P. Surface-induced selection during in situ photoswitching at the solid/liquid interface. Angew. Chem. Int. Ed., 2015, 54, 4865-4869. (26) Maeda, N.; Hirose, T.; Yokoyama, S.; Matsuda, K. Rational design of highly photoresponsive surface-confined self-assembly of diarylethenes: reversible three-state photoswitching at the liquid/solid interface. J. Phys. Chem. C, 2016, 120, 9317-9325. (27) Maeda, N. Hirose, T. Matsuda, K. Discrimination between conglomerates and pseudoracemates using surface coverage plots in 2-D self-assemblies at the liquid/graphite interface. Angew. Chem. Int. Ed., 2017, 56, 2371-2375. (28) Snegir, S.V.; Marchenko, A.; Yu, P.; Maurel, F.; Kapitanchuk, O.; Mazerat, S.; Lepeltier, M; Léaustic, A.; Lacaze, E. STM observation of open- and closed-ring forms of functionalized diarylethene molecules self-assembled on a Au(111) surface. J. Phys. Chem. Lett. 2011, 2, 2433-2436. (29) Snegir, S.V.; Yu, P.; Maurel, F.; Kapitanchuk, O.; Marchenko, A.; Lacaze, E. Switching at the nanoscale : light- and STM-tip-induced switch of a thiolated diarylethene self-assembly on Au(111). Langmuir, 2014, 30, 13556-13563.


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(30) Wirth, J.; Hatter, N.; Drost, R.; Umbach, T.R.; Barja, S.; Zastrow, M.; Rück-Braun, K.; Pascual, J.I.; Saalfrank, P.; Franke, K.J. Diarylethene molecules on a Ag(111) surface: stability and electron-induced switching. J. Phys. Chem. C, 2015, 119, 4874-4883. (31) Reecht, G.; Lotze, C.; Sysoiev, D.; Huhn, T.; Franke, K. Visualizing the role of molecular orbitals in charge transport through individual diarylethene isomers. ACS Nano, 2016, 10, 10555-10562. (32) Fukumoto, S.; Nakashima, T.; Kawai, T. Photon-quantitative reaction of a dithiazolylarylene in solution. Angew. Chem. Int. Ed. 2011, 50, 1565-1568. (33) Calupitan, J.P.D.C.; Galangau, O.; Guillermet, O.; Coratger, R.; Nakashima, T.; Rapenne, G.; Kawai, T. Synthesis and photochromism of chloro- and tert-butyl functionalized terarylene derivatives for surface deposition. Eur. J. Org. Chem. 2017, 17, 2451-2461. (34) Horcas, R. Fernandez, J.M. Gomez-Rodriguez, J. Colchero, J. Gomez-Herrero and A. M. Baro. WSXM: A software for scanning probe microscopy and a tool for nanotechnology. Rev. Sci. Instrum., 2007, 78, 013705. (35) Cristadoro, A.; Ai, M.; Räder, H.J.; Rabe, J.P.; Müllen, K. Electrical field-induced alignment of nonpolar hexabenzocoronene molecules into columnar structures on highly oriented pyrolytic graphite investigated by STM and SFM. J. Phys. Chem. C, 2008, 112, 55635566. (36) Xi, Y.; Pozzo, L. D. Electric field directed formation of aligned conjugated polymers. Soft Matter, 2017, 13, 3894-3908. (37) Molina-Lopez, F.; Yan, H.; Gu, X.; Kim, Y.; Toney, M. F.; Bao, Z. Electric field tuning molecular packing and electrical properties of solution-shearing coated organic semiconducting thin films. Adv. Funct. Mater. 2017, 27, 1605503. (38) Li, W. W.; Radasci, N.; Kramer, H. J. M.; van der Heijden, A. E. D. M.; ter Horst J. H. Solid separation from a mixed suspension through electric-field-enhanced crystallization. Angew. Chem. 2016, 128, 16322-16325.


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TOC Graphic.


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Figure 1. Structure of the terarylene used in this study. On the left is the chemical structure showing intramolecular stabilizing interactions while on the right is the CPK model of the DFT-optimized structure. Characteristic distances and direction of the dipole moment are shown. (The dipole moment is centered on the photoreactive 6π-system but shifted to the side for clarity.) 253x98mm (144 x 144 DPI)



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Figure 2. a) Islands of fuzzy objects moving on the surface or close to the tip. The yellow arrows point onto an adatom as a landmark. White arrows point to partially formed rows with no long-range order. b) After application of a bias pulse (position of the tip indicated as a white cross), a new ordered island appeared. Some order was also induced in the island above, but the disordered state seemed to be too much for assembly formation to happen c) Extract of an island formed from 2b, showing two apparent orientations of the ordered regions rotated 60o from each other. The disorder between the two regions seemed to be due to the completion between the two areas to extend and establish order of a longer-range. d) Experiment repeated on another region (position of the tip indicated as a white cross) showing the other 60o rotation, indicating the influence of the three-fold symmetry of the Cu(111) surface on orienting the assemblies. Imaging conditions: -1.0 V at 1pA. 403x246mm (144 x 144 DPI)


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Figure 3. a-c) Evolution of the assemblies during consecutive scans. The assemblies are stable for several hours even without scanning. d) Orientation of the molecules as overlayed over the assemblies. Imaging conditions: -1.6 V, 1pA, T = 77 K. 414x101mm (144 x 144 DPI)



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Figure 4. Description of the process: a) Molecules deposited on the surface are disordered. b) When a positive bias voltage pulse is applied, the STM tip is negatively charged while the Cu(111) substrate is positively charged and the molecules align their dipole following the lines of the electric field. c) The supramolecular assembly is maintained after the pulse. The calculated dipole moment of the molecule has a magnitude of 2.1 Debye. 404x103mm (144 x 144 DPI)


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Table of content graphic 359x180mm (144 x 144 DPI)


Scanning Tunneling Microscope Tip-Induced ... - ACS Publications

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