Probing FranckâCondon-like Excitations in ... - ACS Publications

Subscriber access provided by BOSTON UNIV

Article

Probing Franck-Condon-Like Excitations in Anchoring of Phthalocyanine Molecules on Au(111) Yong Chan Jeong, Sang Yong Song, Hyo Won Kim, Hyung-Joon Shin, Joongoo Kang, and Jungpil Seo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06158 • Publication Date (Web): 24 Jul 2017 Downloaded from http://pubs.acs.org on July 28, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Probing Franck-Condon-like Excitations in Anchoring of Phthalocyanine Molecules on Au(111) Yong Chan Jeong1, Sang Yong Song1, Hyo Won Kim2, Hyung-Joon Shin3, Joongoo Kang1,* and Jungpil Seo1,* 1

333

2

Samsung Advanced Institute of Technology, Suwon 16676, Korea

Department of Emerging Materials Science, DGIST, Hyeonpung-Myun, Dalseong-Gun, Daegu 42988, Korea

3

Techno-Jungang-daero,

School of Materials Science and Engineering, Ulsan National Institute of Science and Technology, Ulsan 44919, Korea

1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT The nuclear motions coupled with electronic excitations of reactants play an essential role in electron-induced chemical reactions. Here, we study the vibrational-electronic (vibronic) coupling effects in the anchoring of Niphthalocyanine molecules (NiPCs) on Au(111) using scanning tunneling microscopy (STM). The anchoring occurs through the dehydrogenation of a CH bond in NiPC by tunneling electrons. By counting the number of anchored molecules, we measure the reaction rate as a function of the bias voltage. We find an unexpected dip feature in the reaction rate near the bias voltage of 4.8 V. To understand this, we employ density functional theory (DFT) calculations to study atomic force exerted on a NiPC by Franck-Condon-like excitations. We find the molecule anchoring is enhanced when the C-H bonds are stretched by the induced force, which is lacking for the bias voltage near 4.8 V and thus responsible for the anomalous dip in the reaction rate.

2


Page 2 of 26

Page 3 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION Manipulation of individual atoms and molecules using scanning tunneling microscopy (STM) has drawn wide interests owing to its unique ability to induce local mechanical motions and chemical reactions on a nanometer scale.1-11 In contrast to the conventional photo or thermal-induced chemical/mechanical transitions, tunneling electrons play an important role in the STM manipulation. When a large number of electrons travel through atoms or molecules, inelastic tunneling electrons, a small fraction of the electrons, transfer the energy to the nuclei of atoms or molecules giving rise to the vibration, hopping, dissociation and chemical reactions. Triggering mechanisms for the molecule chemical reactions induced by inelastic tunneling spectroscopy (IETS) have been studied in various systems over the past decades.6-23 The mechanisms can be divided into two categories depending on the electron energy. When the electron energy is relatively small compared to the electronic energy levels of a molecule, typically tens to hundreds of meV, the molecule in the tunneling junction plays as a part of the tunneling barrier.13 The electrons rapidly tunnel into the substrate through the molecule, and the tunneling electrons play as a weak perturbation in inducing vibrations of the molecule.6-9, 14-19 In comparison, the electrons tunnel through the electronic states of a molecule when the electron energy is in eV range, leading to chemical reactions.20-23 Systematic comparisons between two mechanisms are provided by Shen et al. in the desorption of H atoms from H-passivated Si(100)24, by Shin, Kim and Kawai et al. in the dissociation of water molecules on MgO(100)3, and more recently by Huang et al. in the breaking of C-I bonds of ortho-diiodobenzene on Cu(110)25. 3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Despite several studies which correlate the electronic excitations of molecules with the chemical reactions, however, the underlying mechanism of how the tunneling electrons affect the motions of nuclei in molecules has not been clearly identified. When a tunneling electron occupies a molecular orbital, atomic force acts on the nuclei in the systems depending on the type of the orbital. For example, if an anti-bonding orbital of a chemical bond is occupied, the force will act upon the bond to be dissociated. To demonstrate the way such electronic transitions couple with the motions of nuclei – vibronic excitations – in chemical reactions, we provide the experimental results of anchoring of Ni-phthalocyanine molecules (NiPCs) on Au(111) using scanning tunneling microscopy (STM). In the experiment, we observed the anchoring efficiency dropped near the bias voltage of 4.8 V. To understand this, we calculated atomic force exerting on a NiPC when an electron is injected into a molecular orbital by Franck-Condon-like excitations. We show the equilibrium structure of C-H bonds in NiPCs is shifted by the force depending on the electron energy. When the shift is in the stretching direction along the bond axes, the anchoring is promoted. On the other hand, when the shift is in the contracting direction, it rarely contributes to the NiPC anchoring, which explains the STM result for the decreased anchoring efficiency near the bias voltage of 4.8 V.

METHODS The STM experiment has been performed using an ultra-high vaccum (UHV) system with a base pressure of 8 x 10-11 Torr. Mechanically sharpened PtIr tip is used to acquire the topographic image and induce the molecule anchoring on Au(111). 4


Page 4 of 26

Page 5 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The Au(111) substrate is cleaned by the repeated cycles of sputtering in Ar pressure of 1.5 x 10-5 Torr (10 minutes) and annealing at 600 oC (15 minutes). NiPCs (Sigma Aldrich Ni(II) Phthalocyanine, P/N: 039453.06) were thermally evaporated onto the substrate of room temperature using a home-made Knudsen cell. The sample is then subsequently cooled down to measurement temperature of 77 K. We used the dimer method26, 27 combined with DFT calculations to identify the reaction pathway for the dehydrogenation of NiPC on the Au(111) surface. Total energies were calculated using nonlocal van der Waals density functional28 and optPBE29, 30 for the exchange functional. Our DFT calculations employed projectoraugmented wave potentials (PAW)31 with an energy cutoff of 400 eV for the plane wave part of the wave function, as implemented in the VASP code.32, 33 We used a 6layer Au(111) slab in a (6 × 6) supercell for the substrate and a (6 × 6) k-point mesh for Brillouin-zone sampling. Atomic structures were relaxed within 0.05 eV/Å. For the calculations of the projected atomic force in Eq. (1), a 4-layer Au(111) slab in a (9 × 9) supercell was used with an additional electron introduced to an excited energy level of the system.

RESULTS AND DISCUSSION We deposited NiPCs on Au(111) substrate at 300 K and subsequently cooled down to measurement temperature. All experiments in the paper were performed at 77 K where the molecules were diffusive. To calculate the density of the deposited NiPCs, we created a molecular fence made of anchored NiPCs on Au surface. We anchored the NiPCs in the fence and counted them to find the density as 0.25 5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

molecules per nm2 (Figure S1 in Supporting Information). A NiPC, which has a fourlobed structure (Figure 1a), possesses a weak electric dipole moment on Au(111), thus the molecules respond to the electric field in the STM tunneling junction. The top panel of Figure 1c shows the STM image obtained using a sample bias voltage of -1 V. The image shows typical herringbone structures of clean Au surface. The clean surface was obtained because the molecules, which diffuse easily on Au(111), escape from the tip position to lower the dipole energy. As a result, the tip scans the Au surface as if the molecules are absent. At the opposite bias polarity (positive voltages), the molecules appear in the image as they gather under the tip, resulting in the inverse contrast of the Au herringbone structures. This bias-dependent diffusivity of NiPCs is formerly reported in the literature.34, 35 Previously, we have shown that NiPCs are anchored on Au(111) through the dehydrogenation process in the molecule ligands when bias pulses of 4 V and 10 ms are applied to the molecules.35, 36 In the present paper, a line-drawing mode of STM is used instead of pulses to induce the molecule anchoring, which provides useful information for investigating the anchoring mechanism. The experiment is performed as follows. We first examine the clean Au surface by imaging with the bias voltage in negative polarity. Subsequently, the STM tip is moved along a pre-defined path (white dotted line in Figure 1c) on the surface with a flipped bias voltage in positive polarity. As the tip moves, NiPCs are anchored on the surface by the tunneling electrons with the reaction efficiency depending on the bias voltage (Vbias) and the tunneling current (It). The number of anchored NiPCs is confirmed by imaging the same area with the negative bias voltage again. An example of the procedure is 6


Page 6 of 26

Page 7 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


displayed in the bottom panel of Figure 1c with a parameter set of Vbias = 5.2 V and It = 62.5 pA. Figure 1d and 1e show the results when Vbias = 5.5 V and Vbias = 4.8 V are used in the anchoring, respectively. Figure 1b is a zoomed-in image of a typical NiPC anchored on Au surface. One of the ligands is shrunk in height as a result of dehydrogenation. The reaction rate (R) is calculated by R(Vbias, It) = N(Vbias, It) / t, where N is the number of anchored NiPCs, t is the elapsed time on tip moving. We performed five repeated measurements to determine the reaction rate for a given parameter set, which usually showed a standard error of less than 5 % for R on average. The result of the measurement of R as a function of bias voltage at a constant current (62.5 pA) is displayed in Figure 2a. We report two prominent features in the data. First, the R jumps near Vbias = 5.4 V and saturates by increasing the bias voltage (inset of Figure 2a). The rapid increase in R could be affected by the ionization of NiPCs at this bias voltage range. In this work, we show that it is related to the efficient elongations of the C-H bonds in NiPCs. Figure 1d shows the anchored NiPCs with a bias voltage of 5.5 V. The NiPCs are anchored along the tipmoving path in a close-packed way. Therefore, there is no space for further anchoring under the STM tip at the increased bias voltages. The anchoring at the positions deviated from the central path takes place less likely because tunneling current decreases exponentially with distance. This explains the saturation of R at high bias voltages. Secondly, we find a dip feature of R at Vbias = 4.8 V as marked with a red triangle in Figure 2a. To know how the dip feature is robust in the experiment, we measured the R near the bias voltage of 4.8 V at various tunneling 7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

conditions. Figure 2b shows the R measured by a different tip (Tip B) at the constant current of 104.1 pA. The result shows a clearer dip feature compared to Figure 2a, most likely due to the enhanced coupling between STM tip and NiPCs at high tunneling current. Figure 2c shows the R measured by another tip (Tip C) at two different constant powers, showing the consistent dip features at Vbias = 4.8 V. Therefore, we conclude that the observed dip feature is reliable and far from an experimental artifact. The anchoring of NiPCs occurs by the injection of tunneling currents. However, the STM image in Figure 3a shows that the positions of the anchored NiPCs are scattered around the central dashed line along which the STM tip has moved. The histogram of scattered NiPCs by the distance from the central line is represented in Figure 3b (vertical bars). We calculated the stray tunneling current depending on the distance from the central line in Figure 3b (red solid line), which exhibits an exponential decay with the distance. By comparing between the spatial distributions of the anchored NiPCs and the tunneling current, it is evident that the NiPCs are anchored where the tunneling current is absent. This strongly suggests that the NiPCs are excited by the tunneling current under the tip and diffuse until they are dehydrogenated and anchored on the Au surface. The distance of the anchored molecules deviated from the central line is up to several nanometers. By assuming the thermal speed of NiPC as = /2 = 13.4 nm/ns, where kB is the Boltzmann constant, T = 77 K and M is the mass of NiPC, the timescale for the dehydrogenation is a few hundreds of pico-seconds. To understand the anchoring process of NiPCs at an atomic scale, we 8


Page 8 of 26

Page 9 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


performed density-functional theory (DFT) calculations assuming a ground-state electronic structure of NiPCs. Although the STM-induced anchoring of NiPCs essentially involves inelastic electron tunneling and associated excited-state electrons, the ground-state reaction pathway still provides useful information for understanding the anchoring reaction. Figure 4 depicts the reaction pathway of the anchoring process, in which the H atom marked with a red circle in Figure 4a is detached from a benzene-like ligand of NiPC. The transition state in Figure 4b reveals that the dehydrogenation reaction is catalyzed by one of the surface Au atoms (marked by Au*), which protrudes from the Au (111) surface to form an H-Au* bond. Simultaneously, the Au atom forms an additional bond with the reactive C atom of the dehydrogenated ligand, stabilizing the damaged NiPC. As the reaction proceeds, the atomic H migrates to a neighboring Au atom, while the dehydrogenated NiPC remains anchored on Au* (Figure 4c). The energy barrier of this process is calculated to be 2.75 eV (Figure 4d). In our model, the dehydrogenated NiPC is anchored through a single Au*-C bond (Figures 4c and 4e). Hence, one may expect that the anchored NiPC is susceptible to Brownian rotation around Au* at 77 K. However, the STM images in Figure 1 clearly show that the individual anchored NiPCs exhibit no rotational motion. Indeed, our DFT calculations show that the anchored NiPC in our model cannot undergo such rotational motion. The Au*-C bond formation induces a large strain around the protruded Au* depending on the orientation of the anchored molecule (Figure 4e). Consequently, the associated strain energy substantially changes with the molecular orientation , as shown in the large energy change of E( ) in Figure 9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

4f. The thermal fluctuation in the angle at T = 77 K, which is related to the thermal average of E( ) by ⟨E( )⟩ = kBT/2, is estimated to be only ±1°. Therefore, the single Au*-C bond not only immobilizes the NiPC on Au* but also fixes its orientation at energetically preferred directions. When an electron is added from the STM tip, the injected electron first occupies an empty molecular orbital of the NiPC, giving rise to an atomic force on the nuclei of the molecule. The atomic force induced by the tunneling electron subsequently activates vibrations of NiPC. Among the 165 vibrational modes of NiPC, however, many of them are irrelevant to the necessary atomic motions for the dehydrogenation process. Most of the times the inelastic electron tunneling would thus trigger “non-productive” vibrations of NiPC (i.e., heating), rather than causing the NiPC anchoring. We identify two vibrational normal modes whose activation promotes the rotation and/or stretching of the C-H bonds of NiPC that are required for the dehydrogenation of NiPC. The atomic displacements of these “productive” normal modes are depicted in Figure 5a. In the left panel of Figure 5a, the C-H bonds of a ligand ring rotate either toward or outward from the Au surface (out-ofplane mode). Simultaneously, the C-H bonds are slightly elongated. As a result, one of the H atoms points to the Au* atom, and the H-Au* interaction can lead to the dehydrogenation. As presented in the right panel of Figure 5a, the in-plane vibrational mode involves the stretching of the C-H bonds in one of the ligands of NiPC. When combined with the bending of the flexible backbone of NiPC, the activation of the bond-stretching mode can also yield the anchored NiPCs. The vibrational energies of the out-of-plane and in-plane modes are calculated to be 0.12 10


Page 10 of 26

Page 11 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


eV and 0.39 eV, respectively. Figure 5b represents the proposed dehydrogenation process driven by the Franck-Condon-like excitation. Our theoretical analysis relies on the assumption that the most important vibronic interaction arises from a shift in the equilibrium structure of the C-H bonds of NiPC.4 The long-distance migration of excited NiPCs prior to the anchoring, which can be up to 4 nm from the tip position (Figure 1c-e), indicates that the injected electron remains in the molecule for sufficient time to induce the structural change (Figure 5b). The injected electron by the Franck-Condon-like excitation (step 1) induces an atomic force on the NiPC initially in its ground state. The resulting structural change of the NiPC can be decomposed by the normal-mode atomic displacements of the NiPC. To analyze how strongly the excited electron is coupled to the vibrational modes of NiPC, we introduce an equation that describes the atomic force projected to one of the normal-mode eigenvectors of the groundstate NiPC on Au. ∙ , = ∑ " / ,

(1)

is the atomic force on the atom k induced by the electron occupation where of the molecular orbital i at energy Ei, and ni is the number of electrons in the molecular state. Mk is the atomic mass of the atom k and " is the normal-mode eigenvector of the selected vibrational mode . The out-of-plane vibrational mode (left panel of Figure 5a) and the in-plane vibrational mode (right panel of Figure 5a) correspond to = 1 and = 2, respectively. The projected force in Eq. (1) is a slope of the excited energy surface along the corresponding atomic displacement, and it is 11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

also conceptually related to the electron-vibration coupling in Ref. 13 that mainly arises from a shift in the oscillator equilibrium point in proportional to the level occupation. A large magnitude of the projected force indicates that the injected electron in the molecular orbital effectively generates the corresponding atomic motions. For the out-of-plane vibrational mode ( = 1), the normal-mode vector is nearly perpendicular to the ground-state planar structure of NiPC. On the other hand, the induced force by the Franck-Condon-like excitation is nearly in-plane of the NiPC, because the effect of the Au substrate is weak. Therefore, the projected force Fproj(Ei, = 1) should be small, regardless of the molecular orbital i. When an electron is added from the STM tip to an excited molecular orbital i of NiPC, the excited-state energy surface for = 1 thus has an energy minimum close to the ground-state structure of NiPC (dashed line in Figure 5b). Therefore, the tunneling electron at an excited state of NiPC cannot directly activate the out-of-plane vibrational mode. From the calculated Fproj(Ei, = 1) for the electronic states within the energy range of 4.0 eV < Ei < 5.7 eV, we plot the density of states weighted by the projected force values (FDOS) as a function of the electron energy (Figure 5c). Here, the electron energy is referenced to the Fermi level of the Au substrate. As expected from the symmetry argument, we found that the FDOS for = 1 is small for the whole energy range (yellow dotted line in Figure 5c). In contrast, depending on the electron energy, the FDOS obtained for the inplane vibrational mode ( = 2) exhibits large values (red and blue solid lines in Figure 5c). The sign of the normal-mode eigenvector is selected so that a positive 12


Page 12 of 26

Page 13 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


projected force indicates a stretching force along the C-H bonds, while a negative value means a contracting force along the C-H bonds. Note that only the excitation with a positive projected force, corresponding to a negative slope of the excited-state energy surface in Figure 5b, can contribute to the dehydrogenation of NiPC. As the vibrational energy of = 2 is transferred to other vibrational modes through an anharmonic coupling, the molecular structure is relaxed along the reaction coordinate (step 2). If the system remains on the excited-state energy surface, the vibrational energy is then used to overcome a small energy barrier associated with the bending of the backbone of NiPC (step 3). Finally, the tunneling electron is transferred to the Au substrate (step 4), and the NiPC becomes dehydrogenated. The energy-dependent FDOS in Figure 5c provides useful insights into understanding the voltage dependence of the anchoring reaction rate. First, the FDOS for = 2 shows largely positive projected force for the electron energy larger than 5.4 eV, indicating that the dehydrogenation process is effectively driven by the high-energy electrons. Furthermore, from the FDOS for the anchored NiPC (Figure S2 in Supporting Information), we found that the tunneling electron near 5.3 eV promotes the stretching of the H-Au bond in Figure 4c, which in turn leads to the migration of the atomic H from the anchoring site. The migration of the reactive atomic H prevents the re-hydrogenation of the anchored NiPC. Therefore, our DFT calculations show that the anchoring reaction rate would be large at the bias voltage larger than 5.4 V, in consistent with the experimental results in Figure 2a. Second, the FDOS has a sharp dip at around 4.7 eV, indicating the existence of the molecular states having a negative projected force that leads to the C-H bond contraction. 13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

These molecular states only act as a channel for the electron tunneling at 4.7 eV, not contributing to the dehydrogenation of NiPC. Therefore, the existence of the nonproductive tunneling channels explicates the dip near Vbias = 4.8 V in the reaction rate in Figure 2. The reaction yield Y can be calculated by Y(Vbias, It) = R(Vbias, It) / (It / e), where e is the electron charge. Previously, Stipe et al. and Motobayashi et al. showed that the ∆ logY /∆V)*+ plot exhibited selective vibrational modes associated with chemical reactions, which provided a complementary method to measure ,- ./," - .38, 39 In our experiment, the force induced by the Franck-Condonlike excitations causes the Y to be changed depending on the bias voltage. Therefore, the ∆ logY /∆V)*+ should have a correlation with the FDOS we calculated. The inset of Figure 5c shows the FDOS plotted together with the calculated ∆ logY /∆V)*+ for various tunneling conditions. The result shows the experimental data qualitatively agree well with the FDOS, confirming the presence of the non-productive vibrational channels at Vbias = 4.7±0.1 V in the anchoring. The error range represents the resolution of ∆V)*+ . Our observation supports ∆ logY / ∆V)*+ is a general footprint of control factors which are associated with chemical reactions.

CONCLUSION In summary, we have studied the anchoring mechanism of NiPCs on Au(111) using STM. When the reaction rate of the NiPC anchoring is monitored as a function 14


Page 14 of 26

Page 15 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of bias voltage, we have observed a dip feature near the bias voltage of 4.8 V. Using DFT calculations, we have demonstrated that the dip feature is associated with nonproductive channels of NiPC anchoring. When tunneling electrons excite the vibronic modes of C-H bonds in the contracting direction, it rarely contributes to the NiPC anchoring. Our calculations show the non-productive channels are most intensive near 4.7 eV above the Fermi energy of the system, explaining the experimental results. Our experiment shows that vibronic excitations by tunneling electrons play a decisive role in STM-induced chemical reactions.

ASSOCIATED CONTENT Supporting Information Estimation of the density of NiPCs deposited on Au (111) surface, The FDOS of the H atom dehydrogenated from NiPC. AUTHOR INFORMATION Corresponding Author *E-mails: [email protected] or [email protected]

Author Contributions J.S and J.K. conceived and designed the experiment. Y.C.J. and S.Y.S performed STM measurements. J.K. carried out the theoretical calculations. Y.C.J., S.Y.S., H.W.K. and H.-J.S. analyzed the experimental data. All authors discussed the result and contributed to the writing of the manuscript.

15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We are grateful for the financial support from the Leading Foreign Research Institute Recruitment Program (2012K1A4A3053565) through the National Research Foundation (NRF) of Korea funded by the Ministry of Science, ICT & Future Planning. The theoretical work was supported by the DGIST R&D Program of the Ministry of Science, ICT and Future Planning (17-BT-02).

REFERENCES 1. Hla, S. W.; Bartels, L.; Meyer, G.; Rieder, K. H., Inducing All Steps of a Chemical Reaction with the Scanning Tunneling Microscope Tip: Towards Single Molecule Engineering. Phys. Rev. Lett. 2000, 85, 2777-2780. 2. Morgenstern, K.; Lorente, N.; Rieder, K. H., Controlled Manipulation of Single Atoms and Small Molecules Using the Scanning Tunnelling Microscope. Phys. Status Solidi B 2013, 250, 1671-1751. 3. Shin, H. J.; Jung, J.; Motobayashi, K.; Yanagisawa, S.; Morikawa, Y.; Kim, Y.; Kawai, M., State-Selective Dissociation of a Single Water Molecule on an Ultrathin MgO Film. Nat. Mater. 2010, 9, 442-447. 4. Gao, L.; Pal, P. P.; Seideman, T.; Guisinger, N. P.; Guest, J. R., Current-Driven Hydrogen Desorption from Graphene: Experiment and Theory. J. Phys. Chem. Lett. 2016, 7, 486-494. 5. Chen, L.; Li, H.; Wee, A. T. S., Nonlocal Chemical Reactivity at Organic-Metal Interfaces. ACS Nano 2009, 3, 3684-3690. 16


Page 16 of 26

Page 17 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6. Kim, H.; Chang, Y. H.; Jang, W. J.; Lee, E. S.; Kim, Y. H.; Kahng, S. J., Probing Single-Molecule Dissociations from a Bimolecular Complex NO-Co-Porphyrin. ACS Nano 2015, 9, 7722-7728. 7. Pascual, J. I.; Lorente, N.; Song, Z.; Conrad, H.; Rust, H. P., Selectivity in Vibrationally Mediated Single-Molecule Chemistry. Nature 2003, 423, 525-528. 8. Yang, K.; Liu, L. W.; Zhang, L. Z.; Xiao, W. D.; Fei, X. M.; Chen, H.; Du, S. X.; Ernst, K. H.; Goo, H. J., Reversible Achiral-to-Chiral Switching of Single MnPhthalocyanine Molecules by Thermal Hydrogenation and Inelastic Electron Tunneling Dehydrogenation. ACS Nano 2014, 8, 2246-2251. 9. Soukiassian, L.; Mayne, A. J.; Carbone, M.; Dujardin, G., Atomic-Scale Desorption of H Atoms from the Si(100)-2x1 : H Surface: Inelastic Electron Interactions. Phys. Rev. B 2003, 68, 035303-035307. 10. Chen, S.; Xu, H.; Goh, K. E. J.; Liu, L.; Randall, J. N., Patterning of Sub-1 nm Dangling-Bond Lines with Atomic Precision Alignment on H:Si(100) Surface at Room Temperature. Nanotechnology 2012, 23, 275301-275307. 11. Hla, S. W., Atom-by-Atom Assembly. Rep. Prog. Phys. 2014, 77, 056502-056517. 12. Hipps, K. W.; Mazur, U., 4A2→4T2 and 4A2→4T1 Electronic Transitions in Cobalt(II) Tetrachloride: An FT-IR and Inelastic Electron Tunneling Spectroscopy Study. J. Am. Chem. Soc. 1987, 109, 3861-3865. 13. Galperin, M.; Ratner, M. A.; Nitzan, A.; Troisi, A., Nuclear Coupling and Polarization in Molecular Transport Junctions: Beyond Tunneling to Function. Science 2008, 319, 1056-1060. 14. Stipe, B. C.; Rezaei, M. A.; Ho, W.; Gao, S.; Persson, M.; Lundqvist, B. I., SingleMolecule Dissociation by Tunneling Electrons. Phys. Rev. Lett. 1997, 78, 44104413. 15. Ueba, H.; Mii, T.; Lorente, N.; Persson, B. N., Adsorbate Motions Induced by Inelastic-Tunneling Current: Theoretical Scenarios of Two-Electron Processes. J. 17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chem. Phys. 2005, 123, 084707-084714. 16. Sainoo, Y.; Kim, Y.; Okawa, T.; Komeda, T.; Shigekawa, H.; Kawai, M., Excitation of Molecular Vibrational Modes with Inelastic Scanning Tunneling Microscopy Processes: Examination through Action Spectra of Cis-2-Butene on Pd(110). Phys. Rev. Lett. 2005, 95, 246102-246105. 17. Acharya, D. P.; Ciobanu, C. V.; Camillone, N.; Sutter, P., Mechanism of ElectronInduced Hydrogen Desorption from Hydroxylated Rutile TiO2 (110). J. Phys. Chem. C 2010, 114, 21510-21515. 18. Motobayashi, K.; Kim, Y.; Ohara, M.; Ueba, H.; Kawai, M., The Role of Thermal Excitation in the Tunneling-Electron-Induced Reaction: Dissociation of Dimethyl Disulfide on Cu(111). Surf. Sci. 2016, 643, 18-22. 19. Komeda, T.; Kim, Y.; Fujita, Y.; Sainoo, Y.; Kawai, M., Local Chemical Reaction of Benzene on Cu(110) via STM-Induced Excitation. J. Chem. Phys. 2004, 120, 5347-5352. 20. Dujardin, G.; Walkup, R. E.; Avouris, P., Dissociation of Individual Molecules with Electrons from the Tip of a Scanning Tunneling Microscope. Science 1992, 255, 1232-1235. 21. Sloan, P. A.; Palmer, R. E., Two-Electron Dissociation of Single Molecules by Atomic Manipulation at Room Temperature. Nature 2005, 434, 367-371. 22. Lastapis, M.; Martin, M.; Riedel, D.; Hellner, L.; Comtet, G.; Dujardin, G., Picometer-Scale Electronic Control of Molecular Dynamics Inside a Single Molecule. Science 2005, 308, 1000-1003. 23. Hla, S. W.; Meyer, G.; Rieder, K. H., Inducing Single-Molecule Chemical Reactions with a UHV-STM: A New Dimension for Nano-Science and Technology. Chemphyschem 2001, 2, 361-366. 24. Shen, T. C.; Wang, C.; Abeln, G. C.; Tucker, J. R.; Lyding, J. W.; Avouris, P.; Walkup, R. E., Atomic-Scale Desorption through Electronic and Vibrational18


Page 18 of 26

Page 19 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Excitation Mechanisms. Science 1995, 268, 1590-1592. 25. Huang, K.; Leung, L.; Lim, T.; Ning, Z. Y.; Polanyi, J. C., Vibrational Excitation Induces Double Reaction. ACS Nano 2014, 8, 12468-12475. 26. Henkelman, G.; Jonsson, H., A Dimer Method for Finding Saddle Points on High Dimensional Potential Surfaces Using Only First Derivatives. J. Chem. Phys. 1999, 111, 7010-7022. 27. Heyden, A.; Bell, A. T.; Keil, F. J., Efficient Methods for Finding Transition States in Chemical Reactions: Comparison of Improved Dimer Method and Partitioned Rational Function Optimization Method. J. Chem. Phys. 2005, 123, 224101224114. 28. Dion, M.; Rydberg, H.; Schroder, E.; Langreth, D. C.; Lundqvist, B. I., Van der Waals Density Functional for General Geometries. Phys. Rev. Lett. 2004, 92, 246401-246404. 29. Klimes, J.; Bowler, D. R.; Michaelides, A., Chemical Accuracy for the Van der Waals Density Functional. J. Phys.: Condens. Matter 2010, 22, 022201-022205. 30. Klimes, J.; Bowler, D. R.; Michaelides, A., Van der Waals Density Functionals Applied to Solids. Phys. Rev. B 2011, 83 195131-195143. 31. Blochl, P. E., Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 1795317979. 32. Kresse, G.; Furthmuller, J., Efficient Iterative Schemes for ab initio Total-Energy Calculations using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169-11186. 33. Kresse, G.; Joubert, D., From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758-1775. 34. Jiang, N.; Zhang, Y. Y.; Liu, Q.; Cheng, Z. H.; Deng, Z. T.; Du, S. X.; Gao, H. J.; Beck, M. J.; Pantelides, S. T., Diffusivity Control in Molecule-on-Metal Systems Using Electric Fields. Nano Lett. 2010, 10, 1184-1188. 19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

35. Jeong, Y. C.; Song, S. Y.; Kim, Y.; Oh, Y.; Kang, J.; Seo, J., Tip-Induced Molecule Anchoring in Ni Phthalocyanine on Au(111) Substrate. J. Phys. Chem. C 2015, 119, 27721-27726. 36. Song, S. Y.; Jeong, Y. C.; Kim, Y.; Kang, J.; Seo, J., Local Modification of Intermolecular Interactions at a Sub-Molecule Level. Nanotechnology 2016, 27, 415711-415717. 37. Borghetti, P.; El-Sayed, A.; Goiri, E.; Rogero, C.; Lobo-Checa, J.; Floreano, L.; Ortega, J. E.; de Oteyza, D. G., Spectroscopic Fingerprints of Work-FunctionControlled Phthalocyanine Charging on Metal Surfaces. ACS Nano 2014, 8, 12786-12795. 38. Stipe, B. C.; Rezaei, M. A.; Ho, W., Coupling of Vibrational Excitation to the Rotational Motion of a Single Adsorbed Molecule. Phys. Rev. Lett. 1998, 81, 1263-1266. 39. Motobayashi, K.; Kim, Y.; Ueba, H.; Kawai, M., Insight into Action Spectroscopy for Single Molecule Motion and Reactions through Inelastic Electron Tunneling. Phys. Rev. Lett. 2010, 105, 076101-076104.

20


Page 20 of 26

Page 21 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. (a) The schematic structure of intact NiPC and (b) the topographic image of anchored NiPC, with a bias voltage (Vbias) of -1 V. The position marked with a yellow triangle indicates the dehydrogenated benzene-like ring. (c) The topographic images of Au (111) before (top panel) and after (bottom panel) tip movement along the path defined by the white dotted line, scanned with a Vbias of -1 V. During the tip movement with 1 Å/s, Vbias and tunneling current (It) are set to 5.2 V and 62.5 pA, respectively. After the tip is moved along the path, NiPCs are anchored on the pathway. (d), (e) The results when the Vbias of 5.5 V and 4.8 V are used, respectively.

21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2. (a) The reaction rate for molecule anchoring (R) as a function of Vbias. The current is 62.5 pA. The red triangle indicates a dip feature in the reaction rate. The inset shows the reaction rate saturates when the anchoring bias voltage is increased. (b) The result when the current is increased to 104.1 pA. A tip of different condition from (a) is used for the measurement (Tip B). (c) The results in the consideration of constant powers of 400 pW and 500 pW. A tip condition for measurement is different from (a) and (b) (Tip C). All the measurements indicate the presence of a robust dip feature of reaction rate at Vbias = 4.8 V.

22


Page 22 of 26

Page 23 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. The spatial distribution of the anchored NiPCs and the tunneling current. (a) The STM image exhibits the anchored NiPCs are scattered around the central line (Vbias = 4.8 V and It = 62.5 pA). The variable x denotes the distance from the central line. (b) The histogram of the number of anchored NiPCs by the distance x (vertical bars). The division is 1 nm bin. When the tip is placed at the central line, the stray tunneling current depending on the distance x is calculated (red solid line) by I ∝ eV 2 exp 52√27∅/9 2 d, where e is the electron charge, V is applied bias voltage, m is the electron mass, ∅ is the work function, 9 is the Planck’s constant, d is the tip-sample distance which equals to (dv2 + x2)1/2, and dv is the vertical tipsample distance. In the calculation we used ∅ = 5.15 eV according to Ref. 37, and 2.2 nm vertical tip-sample distance which is determined based on the experiment.

23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4. The process for NiPC anchoring on Au(111) surface. (a) The initial, (b) transition, and (c) final state for chemisorption of NiPC on Au (111) surface, with (d) the reaction energy barrier. NiPC is anchored on Au(111) surface, due to the dehydrogenated C atom of benzene-like ring bonding to a surface Au atom (Au*). The reaction energy barriers of the forward and backward are 2.75 and 0.37 eV, respectively. (e) The top view of the atomic configuration of (c). (f) The energy variation as a function of the molecular orientation marked by θ.

24


Page 24 of 26

Page 25 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. (a) The behavior of H atoms induced by normal modes of vibration, in the benzene-like ring of NiPC. The left and right panels indicate out-of-plane and inplane modes (α = 1 and 2), respectively. (b) Reaction pathway of the dehydrogenation. The electron is injected into the molecule by Franck-Condon-like excitation. Only the excitation with a positive projected force (the curve of a magenta color) can contribute to the dehydrogenation of NiPC. (c) The density of states weighted by the projected force values (FDOS) as a function of voltage. The main panel shows the FDOS obtained for modes of α = 1 and 2 in (a). A yellow dotted line is the FDOS of α = 1 mode. Red and blue solid lines indicate the positive and negative FDOS in α = 2 mode, respectively. Note that the FDOS of α = 1 mode is much weaker than that of α = 2 mode in strength. The inset shows the FDOS of α = 2 mode and the ∆ logY /∆V)*+ obtained by the experimental data. The case of a constant current of 104.1 pA is representatively plotted with red vertical dotted lines.

25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

TOC Graphic

26


Page 26 of 26

Probing FranckâCondon-like Excitations in ... - ACS Publications

Recommend Documents

Probing FranckâCondon-like Excitations in ... - ACS Publications