Mechanical Conformation Switching of a Single Pentacene Molecule

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Mechanical Conformation Switching of a Single Pentacene Molecule on Si(100)-(2 × 1) O. A. Neucheva,*,† F. Ample,† and C. Joachim†,‡ †

IMRE (Institute of Materials Research and Engineering), A*STAR (Agency for Science, Technology and Research), 3 Research Link, 117602 Singapore ‡ GNS-CEMES & MANA Satellite, CNRS, 29 rue J. Marvig, 31055 Toulouse Cedex, France ABSTRACT: The mechanical switching of a single pentacene molecule chemisorbed in a planar configuration along a dimer row of the Si(100)-(2 × 1) surface was performed experimentally using the tip apex of a scanning tunneling microscope. The mechanical switching reaction path was identified theoretically on the ground state potential energy surface of the pentacene/Si(100)-(2 × 1) system. A low-temperature scanning tunneling microscope as well as semiempirical ASED+ molecular mechanical and elastic scattering quantum chemistry (ESQC) calculations were employed to perform the studies. Pushing with the STM tip apex and at zero bias voltage exactly at the center of the chemisorbed pentacene molecule induces a mechanical conformation change of the pentacene from its metastable to its surface stable conformation on the Si(100)-(2 × 1) surface.

I. INTRODUCTION A conjugated molecule anchored on a reactive and open Si(100)-(2 × 1) surface can adopt different chemisorbed stable surface configurations and conformations. In principle, they are all accessible depending on the molecule sublimation rate and on semiconductor surface temperature during the sublimation.1,2 In recent years, dedicated conformation switching of one to a few molecules on the Si(100)-(2 × 1) surface was observed by applying a large bias voltage pulse to the tip apex of an STM.2,3 On the electronic ground and first excited states potential energy surfaces of the corresponding molecule− semiconductor surface chemical system, the reaction path for a given conformation change is controlled by (i) the adsorbed molecule electronic structure, (ii) its spatial extension relative to the semiconductor surface lattice constant, (iii) the semiconductor surface electronic structure, and (iv) its reconstructions consequent to the molecule adsorption. It was recently proposed to play with the control conformation change of a conjugated molecule to latch surface dangling bond atomic scale Boolean logic gates.4 Here, the bonding (debonding) of a part of the adsorbed molecule can locally saturate (desaturate) a small number of the surface dangling bonds (DBs) if the molecules are deposited on the passivated Si(100) surface. Along an atomic scale circuit, this will nicely control through DB electronic interference effects,4 leading to very efficient atomic scale latches. The conformation change of a conjugated molecule on a Si(100)-(2 × 1) surface can be performed laterally or vertically with the help of the STM tip. On Si(100)-(2 × 1), the lateral rotation of a biphenyl molecule around one phenyl group was actuated very precisely using inelastic tunneling current effects.3,5−7 A vertical change of a phenyl position relative to © 2013 American Chemical Society

the surface can also be triggered using the acetone pivot of an acetophenone molecule.8,9 In those examples, the STM bias voltage pulse required to induce the conformation change is at the threshold of the bias voltages values used to extract single atoms from the surface10 which can be distractive for both Si(100) or Si(100)H surface and the molecule.11,12 Although for the case of DB atomic wires, the hydrogen desorption is initiated by positive voltage pulse whereas the molecular manipulations usually performed with the voltages of opposite polarity13 on the n-type Si(100)H, the created DB wires are unstable under scanning at the voltages lower than −2 V when the single DBs start to hop between the two positions within the same dimer.14 Therefore, it is important to find a molecule that can be switched either by the low voltage pulse or by mechanical manipulation as well as to be able to study its properties without destruction of the atom by atom created DB circuit after its construction. From a more basic point of view, the interpretation of an STM bias voltage pulse conformation change of a molecule adsorbate is often not complete, missing the change of the underneath atomic structure of the supporting semiconductor surface.2 In this work, the molecular conformation change after manipulation is taken into account together with the surface respond to such switching of molecular geometry. Small or zero bias voltage mechanical atomic scale manipulations are well-known since the first LT-STM atom by atom Ge manipulation on a Ge(111) surface using a gentle mechanical touch of the Ge(111) surface with the tip apex of an Received: July 26, 2013 Revised: September 16, 2013 Published: November 19, 2013 26040

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Figure 1. STM images of pentacene molecules on Si(100), −2.5 V, 50 pA. (a) 100 × 100 nm, pentacene molecules appear bright and surface defects dark. (b) 10 × 10 nm, pentacene molecules of two parallel alignments (A4 and A2). (c) 10 × 10 nm, two A4 pentacene molecules separated by one Si dimer row. (d−f) 2 × 2 nm, single pentacene molecules of A4, A2, and B2 configurations, respectively. (g) Temperature dependence of the adsorption geometries of the pentacene molecules.

STM.15 Intramolecular conformation changes have also been reported on a metal surface by gently touching the switchable part of the molecular adsorbate with the tip apex of an LTSTM.16 Hereafter, we describe how to switch mechanically the conformation of a single pentacene molecule adsorbed planar on Si(100)-(2 × 1) surface without changing the adsorption position. The pentacene molecule was selected because it is known to adsorb mainly planar and along the dimer row of the Si(100)-(2 × 1) reconstruction. This opens the possibility to chemically bind a different number of the pentacene phenyl to the surface starting for example from a butterfly-like adsorption the pentacene molecule on Si(100)-(2 × 1) with the two silicon dangling bonds saturated by the molecule. In section II, the details of the experimental setup, the preparation conditions, and the calculation technique used to interpret the experimental results are provided. In section III, experimental and theoretical images of the two pentacene conformations are discussed together with electronic properties of the interfaces. In section IV, the mechanical conformation switching of a single pentacene molecule is presented and characterized. In conclusion, we anticipate for the next step toward the molecular latching of a DB atomic scale circuit.

cleaning in deionized water for 15 min. After introducing the sample into the UHV chamber, it was outgassed at 600 °C overnight followed by flashing to 1200 °C several times during 10 s in order to obtain large ultraclean Si(100)-(2 × 1) reconstructed surface terraces (see the background LT-STM image Figure 1a). Subsequently, the sample was slowly cooled to 800 °C, held at this temperature for a few minutes, and finally cooled to the room temperature at a rate of 2 °C/s to minimize the number of Ni contamination on the surface.17 An infrared optical pyrometer was used to monitor the substrate temperatures during the preparation process. The pentacene molecules powder was obtained from an ultra pure Sigma-Aldrich delivery and placed in a small stainless steel deposition crucible inside a one-pocket e-beam evaporator. The molecules were deposited on the Si(100)-(2 × 1) at 25, 50, and 70 °C (substrate temperature) using the e-beam to heat the molecular powder. There was no thermocouple attached to the molecular source; thus, the exact sublimation temperature was not known. Prior to the molecular sublimation process, the source was degassed by heating for 2 h to remove the H2O vapor. The pressure in the preparation chamber during deposition reached 1 × 10−9 mbar. The surface molecular coverage was controlled by the deposition time, giving very low submonolayer coverage after 10 s of sublimation (see Figure 1a). The distance between the silicon sample and the molecular source was about 15 cm. On the calculation side, the adsorption geometries for a pentacene molecule on the Si(100) surface were calculated using the semiempirical atom superposition and electronic delocalization molecule orbital (ASED-MO) approach18 by means of the ASED+ software19 since the chemisorption of polyacene molecules on Si(100) were well described by ASED+ as shown in ref 19. The geometries were optimized until a threshold of 0.01 eV/Å was reached for the individual forces on each atom. On the ASED+ calculated ground state potential energy surface of the pentacene/Si(100) surface system, the nudged elastic band (NEB) algorithm20 was applied after its introduction in the ASED+ source code. It serves to determine

II. EXPERIMENTS The experiments were carried out in an ultrahigh vacuum (UHV) environment with a base pressure lower than 5 × 10−10 mbar essential to avoid any contamination during the molecule chemisorption process. The STM measurements were performed with an Omicron low-temperature (4.2 K) scanning tunneling microscope (LT-STM) to stabilize each molecular configuration enough to determine their adsorption structure. The STM tips were electrochemically etched tungsten wire with a diameter of 0.25 mm. The tips were cleaned in situ by current heating in order to remove the oxide layer. The silicon crystal is a 10 × 3 mm2 Si(100) sample with a bulk resistance of less than 0.1 ohm/cm. Both p-type (boron doped) and n-type (phosphorus doped) silicon were used for the experiments. The crystal was treated in a 1% HF solution for 5 min followed by 26041

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the minimum energy path for the switching of the adsorbed pentacene molecule between its different stable chemisorption conformations on the Si(100) surface. We used the elastic scattering quantum chemistry (ESQC) technique21 to calculate constant height and constant current STM images. The ESQC technique is based on the transfer matrix approach to solve exactly the corresponding electronic scattering problem and on the effective Hamiltonian technique to describe the electronic structure of the “STM tip−pentacene−Si(100) surface−silicon bulk” tunneling junction including the pentacene/Si(100) surface electronic interaction, the weak pentacene−tip apex interactions, and the detailed band structure of the Si(100)-(2 × 1) surface and of the silicon bulk. WSxM software was used for image processing.22

Figure 2. Some possible optimized structures of pentacene adsorbed on Si(100): (a) pentacene chemisorbed on two Si dangling bonds in a butterfly structure; (b) pentacene chemisorbed on four Si dangling bonds in a bridge structure; this structure is different from found experimentally tagged as A4 (compare with Figure 3a); (c) pentacene chemisorbed on eight Si dangling bonds perpendicular to the silicon dimer rows; (d) free pentacene 4 Å above a Si(100) surface and its corresponding calculated STM image at the HOMO energy resonance (inset).

III. LT-STM IMAGES AND dI/dV SPECTRA FOR THE DIFFERENT PENTACENE/SI(100) SURFACE ADSORPTION CONFIGURATIONS Typical LT-STM images of pentacene molecules adsorbed on the Si(100)-(2 × 1) surface are presented in Figure 1a,b. At this very low coverage, those images reveal a random distribution of the pentacene molecules on the Si(100)-(2 × 1) surface with the molecules mostly adsorbed parallel to the silicon dimer rows (A-type molecules following the classification proposed in refs 1 and 2). Molecules tend often to occupy a position next to a surface defect like missing Si dimer or Ni contamination. Pentacene molecules on defect free Si(100)-(2 × 1) reconstructed area can be also found. In most cases, the pentacene molecules are isolated from each other, implying that there is no surface diffusion taking place on the surface after the deposition and that the molecules are strongly chemisorped on the Si(100) surface, in contrast to their physisorption on Si(100)H surface.19 Very rarely (≪0.1%), two molecules located next to each other were also observed (Figure 1c). Pentacene molecules adsorbed in a configuration perpendicular to the silicon dimer rows (B-type following refs 1 and 2) were also found but in a smaller amount (Figure 1f). However, no pentacene molecules adsorbed in between the dimer rows were observed even if reported in ref 2. Also found by others,1,2 quite exclusive asymmetrical adsorption conformation geometries were sometimes observed but will not be discussed in the following because they are a minute number and not in a good orientation to be switched. Furthermore, they seem to correspond to adsorption geometries where the underlying Si(100)-(2 × 1) surface lattice is not atomically well-defined. A closer look at the LT-STM images reveals the adsorbed pentacene molecular structure in more detail. Thanks to the excellent stability of our instrument, very small scan size LTSTM images were recorded for the already identified A and B conformations. The A-type conformation shows two different structures one with four (A4) and one with two lobes (A2) in the STM images (Figures 1d and 1e, respectively). The B-type conformation of pentacene was found to have only one configuration (B2). For the same tunneling conditions, the B2type STM image corrugation appears always laterally less extended than the A type. Our experiments on B2-type molecules reveal no disagreement with the previous studies.1,2 Therefore, in the following text, we only discuss the A-type. The ASED+ B2 adsorption configuration is presented in Figure 2c and does not differ from the one proposed in refs 1 and 2. A2 was already identified as the most stable pentacene chemisorption conformation on Si(100)-(2 × 1) using density functional theory (DFT) calculations.1 Here, the pentacene

molecule is chemisorbed to the surface with four pairs of C−Si bonds along the dimer row in a generalization of the wellknown butterfly-like conformation of a benzene molecule on Si(100)-(2 × 1) (see Figures 3f and 2a, respectively). Calculated using ASED+, the adsorption energy of this configuration is quite large, 4.29 eV, in agreement with the calculated adsorption energy of 4.53 eV in ref 2. This adsorption configuration was confirmed by comparing experimental and ESQC calculated STM images for both negative and positive bias voltages (see Figures 3g,h and Figures 3j,k, respectively). One A4 4-lobe-like possible adsorption configuration was also reported previously.2 But due to the two small lateral protrusions observed in its STM image (see Figure 1c) as compared to A2, it was argued that the corresponding adsorbate is not pentacene but its main synthesis intermediate the pentacenequinone molecule (C22H14O2). With our deposition conditions (slightly elevated substrate temperature during sublimation), a great majority of the STM images have a Figure 1c like STM contrast which excludes the suggested impurity explanation. The same deposition conditions on the different samples as well as using p- or n-type Si(100) found to have no influence on the conformations observed. Furthermore, using the ASED+ optimization technique, other possible stable adsorption geometries of the pentacene molecule on Si(100)-(2 × 1) can be found (Figure 3a), but always less stable than A2. Therefore, by calculating using the ESQC technique STM images of all those possible pentacene conformations found by ASED+ and comparing them with the A4 experimental STM images, we identified the A4 configuration as presented in Figure 3a. The corresponding ESQC calculated STM images are presented in Figure 3c and Figure 3e, respectively, for both negative and positive bias voltages. Here, pentacene is bonded to the surface only via four C−Si bonds at the edge sides of the outer rings of the molecule. The A4 inverse central curvature makes the center of the molecule further away from the Si(100)-(2 × 1) surface. This explains the presence of the two lateral additional lobes for the A4 in the STM image as compared to an A2 conformation. Those two small lobes come from the central lobe of the pentacene HOMO because in this 26042

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Figure 3. Structure of the pentacene chemisorbed on the clean Si(100): (a) the mestastable configuration called A4 (the pentacene molecule chemisorbed on four silicon dangling bonds) and (f) the most stable configuration called A2 (the pentacene molecule chemisorbed on four silicon dangling bonds). STM images for the A4 configurations at negative voltage are (b) and (c) for the experimental and calculated one, respectively, and (d) and (e) for the same configuration at a positive voltage. For configuration A2, the experimental and calculated STM image corresponds to (g) and (h) at negative voltages and (j) and (k) at positive voltages. The adsorption energies are A4, 1.70 eV, and A2, 4.29 eV, respectively. Imaging conditions: (b, g) −2.5 V, 50 pA; (d, j) 3.2 V, 50 pA.

conformation, the pentacene central part is preserved from hybridization with the surface states. An example of a ESQC calculated image forcing the pentacene to be flat and artificially raised up by 2 Å as compared to its A4 adsorption height is presented in Figure 2d. The standard STM contrast of a pentacene molecule recorded for example when the pentacene is physisorbed on Au(111)23 or Si(100)H5 is almost recovered. In agreement with this hypothesis, A2 type of molecules reveals no central lobes feature when imaged at the same voltages. The change in the appearance of the image between A2 and A4 is due to the four new chemical bonds which are created between the molecule and the surface. The molecule and the surface have to adapt to a new conformation both mechanically and electronically. Since the A2 molecule has double the number of the covalent bonds with the silicon substrate, it is stronger bound to the surface and the electronic density of the system is greatly modified, resulting in a different appearance in the STM images. The ASED+ calculated adsorption energies of the A4 and A2 conformations are 1.70 and 4.29 eV, respectively. On the ground state potential energy surface, the goal is to find a mechanical activated reactive path for the pentacene/Si(100) system to pass from the metastable A4 to its most stable A2 conformations. This will be discussed in the next section. Since in the previous works it was not common to find A4 on Si(100)-(2 × 1), we have checked the influence of the substrate temperature during the deposition on the molecular orientation and conformations by using three different substrate temperatures during the sublimation: 25, 50, and 70 °C. At room temperature, a large majority of molecules adsorb in a conformation leading the two-lobe A2 STM images agreeing with ref 2. However, at this temperature, there are more A4 molecules than B-type molecules (Figure 1a) which contradict with refs 1 and 2. This seems to be due to the very slow deposition rate used during our sublimation procedure. There is a competition between mechanical shrinking of the molecular skeleton and formation of the chemical bonds on the Si(100)(2 × 1) surface. This happens due to the mismatch between the pentacene molecular geometry and Si(100)-(2 × 1) surface lattice. The pentacene molecule and the Si(100)-(2 × 1) surface need to adopt a compromised geometry and the A4 is also one of them. With the temperature is raised to approximately 50 and 70 °C, the percentage of A4 and A2

conformation changes toward more molecules having A4 configuration. At 70 °C temperature deposition, the A4 molecules take a lead on the surface (Figure 1g). In order to further understand the pentacene molecular electronic states involved in the tunneling imaging process, dI/ dV STM spectra and constant current dI/dV mapping were performed using a Stanford Research lock-in amplifier with modulation amplitude of 30 mV. Since on bare Si(100)-(2 × 1) surface, molecular electronic states are not decoupled from the surface, the dI/dV spectra reveal the hybridized electronic states of the pentacene/Si(100)-(2 × 1) system. STM images at a negative bias voltages reveal the filled states of the molecules with the pentacene HOMO being positioned in energy beyond the Si(100) bulk band gap (Figure 3a,e). The HOMO electronic resonance is wide, and for example, the four STM lobes of the A4 can be captured over a wide range of negative bias voltages from −1.4 to −3 V. The pentacene LUMO lies within the Si bulk band gap. Therefore, STM images recorded at positive bias voltages reveal the consequence of the LUMO electronic structure on the Si(100)-(2 × 1) surface due to the pentacene chemisorption but are not able to capture its related contrast (Figure 3b,d). In this case, a nice depletion around the pentacene can be observed for both A4 and A2 showing the relaxation of the surface in the vicinity of the molecule. Although at low temperatures the Si bulk band gap hides some of the electronic tunnel junction resonances near the Fermi level, dI/dV spectra recorded on a single pentacene with a negative bias voltage succeed to capture the main electronic adsorption feature of each of the A4, A2, and B2 adsorption configurations on the n-type Si(100) (Figure 4a,c,d, respectively). For example, the very prominent resonance at −2.0 V is very intense over the small central lobes and in the center of the A4 molecule and vanishing at the outer big lobes. This resonance can be defined as the highest occupied molecular orbitals (HOMO) of the pentacene molecule and is observed only in the middle part of the molecule because it is the most distant from the underlying silicon surface and thus most decopled. A similar resonance at the same energy range was reported as HOMO on Si(100)H surface where the pentacene molecule is physisorbed. Once the molecule is switched from A4 to A2 configuration, the position of the peak observed in the center of the molecule changes and a new peak appears further 26043

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the resonance at −2.2 V with the HOMO−1 state of the molecule whereas HOMO state is located in the band gap of silicon and cannot be accessed. For the p-type sample, electronic properties of the A4 molecule are studied. Similarly to the previous case, an additional peak appears at 2.9 V when measuring spectroscopy in the center of the molecule and vanishes at the position of outer lobe (Figure 4b). Although the lowest unoccupied molecular orbital (LUMO) of the pentacene molecule cannot be detected due to its position in the band gap of Si(100), this peak might be a shoulder of one of the next order LUMO, such as LUMO+1 or LUMO+2 resonances. Notice that all the presented spectra were not recorded with the same tip leading to some diviations in the Si(100) spectra additional resonances. In order to obtain similar tip conditions a characteristic peak of Si(100) at −1.5 V was used as a standard.24 Using lock-in detection and selecting the −2.0 V resonance, constant height dI/dV maps were recorded on A4 and B2 molecules (Figure 4e,f, respectively). Those maps provide the location of the maximum tunnel conductance spots over a chemisorbed pentacene. The print of recorded dI/dV molecule maps appears very similar to constant current images. For B-2 molecules, the vanishing dI/dV contrast relative to the Si(100) STM apparent surface corrugation is the signature of an out-ofphase electronic coupling between the pentacene π system and the surface DBs. An opposite behavior occurs for the A4 dI/dV maps which are very bright, revealing a good in phase electronic coupling. For this A4, the two weak lateral lobes are coming from the central part of the pentacene molecule where the central curved part of the pentacene is not hybridized with the Si(100)-(2 × 1) surface states. Nevertheless, the distance between the central curved phenyl and the Si surface is small enough to offer still a good electronic coupling for a conductance channel to be active at this location. The native nodal plane of the pentacene HOMO brings the contrast laterally for this central location. This follows the previous discussion (see Figure 2d) showing that at a surface distance close to the actual A4 central phenyl distance the full HOMO

Figure 4. dI/dV spectroscopy at the different points of a pentacene molecules compared to the Si(100) surface. Inset: STM images of pentacene molecules. (a−d) Spectra measured over A4-, A2-, and B2type molecules, on the p-type (b) and n-type (a, c, d, respectively) Si(100) surfaces. (e, f) dI/dV maps of the A4- and B-type pentacene molecule.

away from the Fermi level (−2.2 V). Since one would expect the HOMO−LUMO gap to shrink when the molecule establishes more chemical bonds with a surface, we associate

Figure 5. Switching of a single pentacene molecule from A4 to A2 configuration performed with an STM tip. (a) I(z) curve recorded over the outer bright lobe of the pentacene molecule (results in no switching). (b) I(z) curve recorded over the center of the molecule; current jerk corresponds to switching of the molecule from A4 to A2 conformation. (c, d) STM images of the switching done using vertical manipulation of an STM tip. The tip is approached toward the center of the molecule. (e, f) STM images of the switching done using lateral manipulation of an STM tip. Tip manipulation started from the silicon toward the center of a molecule laterally. 26044

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manipulations (see double-tip image in Figure 5f). Therefore, vertical manipulation was used for a reproducible and nondisruptive switching of the molecule. Less than 1% of the molecules were destroyed while using the vertical manipulation technique. The results of those two manipulation modes are presented in Figures 5a,b and 5c,d, respectively. A pentacene molecule switched by a vertical manipulation from A4 to A2 has generally the same dI/dV spectrum as a native A2 molecule. This A4 to A2 mechanical configuration change is irreversible, confirming that A2 is more stable than A4. Furthermore, in the event of an A4 to A2 conformation change performed by a lateral mode of manipulation, there is no lateral shift of the molecular position with respect to the underlying silicon lattice after the conformation changes, and the A2 dI/dV spectrum is recovered. The detailed analysis of Si(100)-(2 × 1) surface lattice organization around the molecule provides its exact position before and after the A4 to A2 switching. The pentacene center is located at the hollow side in between two silicon dimers. This excludes the possibility of the butterfly-like conformation presented Figure 2a as was proposed earlier.1,2,19 Notice that a 3D view of all possible symmetrical conformations is given in Figure 2a−d for reference exclusive of the A4 and A2 conformations which are presented on Figures 3a and 3f, respectively. To understand better the mechanics occurring during a switch, the minimum-energy path between configurations A4 and A2 was calculated by means of the NEB algorithm implemented in the ASED+ program. As presented in Figure 6, both A4 and A2 are located at a minimum of energy with adsorption energies of 1.70 and 4.29 eV, respectively. The barrier height to pass from A4 to A2 following this minimumenergy path is only 0.13 eV. This small energy barrier results from two competitive energies. One is the energy needed to

contrast can be recovered by restoring an artificial planar conformation of the pentacene.

IV. MECHANICAL SWITCHING OF A PENTACENE MOLECULE As described above, the A4 pentacene adsorption configuration is metastable on a Si(100)-(2 × 1) surface. To explore this metastability, a single pentacene can be manipulated in a vertical or a lateral mode at low temperature on the Si(100)-(2 × 1) surface. In a vertical mode of manipulation, the tip is stabilized over the molecule at a given STM tunneling set point (−2.5 V, 50 pA for the n-type samples). Then feedback loop is open, and the tip is approached toward the center of the molecule by ∼5 Å in the I(z) mode until there is a jump in the current occurs corresponding to the switching of this molecule from the A4 to the A2 configuration (Figure 5b). Low bias voltage (few millivolts) is used to monitor the tunneling current during manipulations. Interestingly, the I(z) spectrum curvature changes when the current level reaches ∼12 nA every time the STM tip approaches to the molecule independently of the switching outcome (Figure 5a,b for not switched and switched cases, respectively). This value is in order of the current used in the lateral manipulation mode (see below). Change in the I(z) slope happens in both directions (approach and retraction of the tip) and has no impact on the molecule configuration. From this observation we can assume that it is the point when the tip starts to repulsively interact with the molecule. The switching can be realized on both n- and p-type Si(100); we found no influence of the doping nature on the switching success. Thus, quantitative studies are done only for the n-type surface due to better stability of the tip while imaging and spectroscopy. The success rate of the switch is close to 60% and greatly depends on the tip conditions. After some manipulations clusters of Si from the tip are released to the surface. Approaching the tip to the other parts of the molecule also leads to the switch; however, more attempts are required to switch the molecule. Since no voltage pulse is needed here to trigger this switching, such a manipulation becomes unique in terms of the intramolecular mechanics involved. To the best of our knowledge only a Cu−TBPP molecule was previously internally switched on the Cu(111) surface using a NC-AFM technique,25 whereas a majority of the manipulations require the pulsing of the STM bias voltage.2,16,26 For example, at room temperature and on a Si(100)-(2 × 1) surface, a single pentacene molecule was rotated by applying voltage pulse of few volts to the STM tip.2 However, those experiments lead very often to the decomposition of the molecule or to a change of the surface atomic scale structure. In the lateral mode of manipulation, the STM tip apex is stabilized very near to the surface (−2.5 V, 10 nA) over the middle of the silicon dimer row next to the right or left side of the molecule (along long axis of the molecule). Then, the feedback loop is switched off and the tip is moved laterally along the middle of silicon dimer row toward the middle of the molecule (the trajectory is shown as red arrow on Figure 5e) until a change in the tunneling current is detected. A few millivolts voltage is applied to monitor the tunneling current during manipulations. In contrast to the vertical mode of manipulations, this method is distructive and great majority of the manipulated molecules are destroyed. Also bringing the tip in vicinity of the silicon substrate makes it unstable, and usually the tip shape is changed after performing the lateral

Figure 6. Energy profile for the minimum-energy pathway from the free molecule to the adsorption in the metastable configuration A4 toward the most stable configuration A2. Inset: energy profile for the A4 to A2 transition with a tip in the range 2.8−3.2 Å above the molecule. The system has a minimum energy when the molecule is 2.7 Å away from the silicon surface (corresponds to reaction coordinate 3 Å). 26045

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deform the molecule to reach A2 from A4. The other is the stabilization energy of the molecule interacting with the Si dangling bonds below. This stabilization energy is large, making the barrier for the pass from A4 to A2 relatively small. A4 is a metastable state since more than 2.7 eV is required to return to A4 from A2. An excitation of the low-lying pentacene excited states is necessary to escape from this 2.7 eV deep potential energy valley and switch back to A4. The nearest electronic states are the T1 (triplets) and S1 (singlet) first excited states which can be described mainly by their LUMO component as performed for example in ref 26 on a biphenyl molecule. But those LUMO like states can hardly be accessed on Si(100)-(2 × 1) because they are positioned within the band gap of the silicon bulk supporting the Si(100) 2 × 1 surface. Instead, we have tried without success to access to higher in energy excited states (e.g., LUMO+1), but it requires a very large STM bias voltage pulse incompatible with the local stability of the Si(100)-(2 × 1) surface as discussed in the Introduction.

REFERENCES

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V. CONCLUSION In summary, the adsorption orientations and geometries of pentacene molecules on a Si(100)-(2 × 1) surface were presented together with a detail investigation of their electronic properties. Three major configurations of the molecules were found on the surface: two parallel to the Si(100)-(2 × 1) dimer rows (A2 and A4) and one perpendicular (B-2) which was supported by ESQC image calculations. A2 and B-2 molecules have eight covalent bonds to the surface, whereas A4 has four bonds. A2 is the more stable chemisorbed conformation of a pentacene molecule on the Si(100)-(2 × 1) surface which was also confirmed by ASED+ calculations. The conductance spectra of the molecules reveal characteristic features of each type of molecule. The dI/dV image of the A4 and B-2 molecules was recorded at the peak of the tunneling spectra of the A4 molecule mapping the electron transparency of the interface. Switching of a single pentacene molecule from A4 to A2 conformations was demonstrated. The mechanical switching can be realized with vertical or lateral manipulation of an STM tip with no bias voltage applied. Switching is irreversible from A4 to A2. This switching involves a change in the electronic bonding configuration of the pentacene which is the same for both a vertical or lateral manipulation sequence. The calculated double-well energy potential profile along the minimum-energy path of this mechanical switching demonstrates that the barrier height to switch the molecule back form A2 to A4 is very high. The excited electronic states must be brought into play in order to have reversible manipulation of a single pentacene molecule on a Si(100)-(2 × 1) surface.

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*E-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the A*STAR’s VIP Atom Technology project 1021100972 and EU ATMOL project Contract No. 270028 for the financial support and the A*STAR Computational Resource Centre (A*CRC) for the computational resources and support. 26046

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dx.doi.org/10.1021/jp407445u | J. Phys. Chem. C 2013, 117, 26040−26047