Revealing the Presence of Mobile Molecules on the Surface

Article pubs.acs.org/JPCC

Revealing the Presence of Mobile Molecules on the Surface G. Antczak,*,† K. Boom,‡ and K. Morgenstern§ †

Institute of Experimental Physics, University of Wrocław, Wrocław 50-204, Poland Leibniz University Hannover, 30167 Hannover, Germany § Chair for Physical Chemistry I, Ruhr-Universität Bochum, 44801 Bochum, Germany ‡

ABSTRACT: Mobile molecules crossing freely underneath the scanning tip of a scanning tunneling microscope create a uniform diffusive noise, making the identification of single molecules on the surface a challenge. We demonstrate the possibility of detecting mobile molecules on a surface by scanning tunneling microscopy and reveal how the diffusive noise is created. Additionally, we show that a molecule caught in the tip−sample junction allows us to explore the potential energy surface of the system. Finally, voltage pulses disturb the mobile molecules, causing the loss of that ability. They also allow the creation of islands on the surface. Most of the investigations were done for Co- and Cuphthalocyanine (Pc) on Ag(100). However, the concept is limited to neither Pc molecules nor Ag(100), as shown for a different organic molecule, astraphloxin, on Cu(111).

promising properties for sensors,20 quantum computing,21 and other electronic devices. 22 With respect to molecular electronics, there is a whole branch of nanoscience exploring the ability of small molecules to undergo reversible isomerization reactions to play the role of a molecular switch.23,24 The cyanine dye commonly named astrophloxin (C25H29ClN2) was already investigated in this direction.25 It is important to detect the presence of such molecules when they are mobile and to know conditions of molecules immobilization on the surface to utilize them in future electronic devices. We present a route how to identify the existence of the mobile Pc molecules on the Ag(100) surface and explore the creation of diffusive noise. Additionally, we probe the potential energy landscape by scanning with a tip modified by a Pc molecule. We proof our procedure by application to a different system, a cyanine dye on Cu(111).

1. INTRODUCTION Scanning tunneling microcopy (STM) has become over the years one of the most powerful techniques to explore surface processes.1−3 It was extensively used to characterize structural arrangements and adsorption units of molecules and other nanosized objects.4 The technique, however, is relatively difficult to apply for mobile species on the surface. In many investigations, mobile species are rather a hindrance to the proper characterization. A molecule passing multiple times under the scanning tip, while the current is integrated at one pixel, changes the nature of the junction and thus the measured tunneling current. This uniform diffusive noise is not easy to interpret. Nonetheless, such noise was used to determine diffusion characteristics.5−7 A Monte Carlo method is widely used to explore the influence of repulsive and attractive adsorbate−adsorbate interactions,8−10 the proximity of a stepedge,11 and the electric field12 on the atomic diffusion inside STM junctions. Mobile molecules can also cause a polaritydependent contrast reversal when the tip−molecule interaction is increased.13 Depending on the tip−molecule distance, the tip−molecule interaction can be tuned and the STM can be operated in the manipulation regime, which is characteristic of the specific adsorbate−substrate system.14 The tip can induce the motion of adatoms and molecules, as studied on various surfaces.14−17 The different mechanisms of manipulations (e.g., pulling, pushing, sliding, and push pulling) were identified.17,18 In this article, we mostly investigate the pure molecular thermal diffusive noise, except for Section 3.3, where we explore the potential energy surface (PES) through an organic molecule, similarly to the observation of Böhringer et al.14 for anthracene molecule on Ag(110). On the contrary, organic molecules, in particular, phthalocyanine (Pc) molecules, are widely investigated19 due to their © XXXX American Chemical Society

2. EXPERIMENTAL SECTION We use two types of STMs for this investigation. The first one is a home-built low-temperature (LT) STM with a bandwidth of 1.5 kHz,26 which allows deposition of molecules on a heliumcooled manipulator between 20 K and room temperature (RT). This STM can be used for imaging in the temperature range of 5 to 100 K. The second type of STM is a commercial variable temperature (VT) STM of the Aarhus type with a bandwidth of 1 kHz, which operates in the temperature range of 100 to 350 K. The molecules here can be adsorbed between 120 K and RT. In the VT STM the image is created by scanning from bottom to top of the image, whereas in the LT STM it is created by Received: November 8, 2016 Revised: December 19, 2016

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DOI: 10.1021/acs.jpcc.6b11246 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

After adsorption of Pc molecules at 120 K the molecules adsorbed at step-edges are immobile on the time scale of the experiment; see Figure 2a,b. The immobile Pc molecules are

scanning from top to bottom. We name this direction the slow scan direction. The fast scan direction is in both STMs from the left side to the right side of the image. The time needed to scan one image in the LT STM is 3 min, while in the VT STM it is in the range of 1 min. The time scale available for experiment is in the range of minutes, whereas the time for scanning one line of an image is less than 1 s. CuPc and CoPc adsorbed on Ag(100) were investigated by both STMs, and astrophloxin on Cu(111) was investigated by the VT STM. The Ag(100) sample was cleaned by a combination of sputtering (energy 1.3 keV, sputtering current 8 μA, for 1 h) and annealing at 903 K, repeated twice. The Cu and Co phthalocyanine molecules were deposited on the sample from a Knudsen cell kept between 663 and 700 K. We adsorbed CuPc and CoPc molecules at 20 K using the liquidhelium-cooled manipulator in the LT STM and at 200 K or RT in the VT STM. Prior to deposition, the cell was carefully outgassed for 3 days. The Cu(111) sample was cleaned by a combination of sputtering and annealing. Sputtering consisted of three runs (separated by annealing cycles) with an energy of 0.6 keV (sputtering current 6 μA, time 15 min). The first run of annealing was at 905 K for 30 min, the second run at 675 K for 10 min, and the third and final run at 875 K for 20 min. Astrophloxin was deposited from a glass tube connected via a leak valve to the UHV system, and aluminum foil protected the molecule from light. The cleaning and calibration of the source was described in ref 25. The molecules were deposited on Cu(111) at 120 K. The STM images are analyzed using the WSxM program.27

3. RESULTS 3.1. Phthalocyanines on Ag(100) - Uniform Noise on Terraces. We investigated the CuPc and CoPc molecules adsorbed on Ag(100) with the LT-STM. Both molecules (when immobile) are imaged as a four-lobe structure (Figure 1b) consistent with previous work.28−31 This shape reflects the

Figure 2. Formation of black rim for Pc molecules close to steps edges on Ag(100). (a) Low coverage of CuPc adsorbed at 200 K with no black contour. STM imaging conditions: V = 312 mV; I = 70 pA, T = 120 K. (b) Larger coverage of CoPc molecules adsorbed at 200 K with black contour. STM image conditions: V= 525 mV, I = 70 pA, T = 120 K. (c) Formation of small-molecule cluster close to step-edge upon adsorption at RT due to Ag adatom. STM imaging conditions: V = 442 mV, I = 80 pA, T = 120 K; zoom into images is marked and presented in the right column. (d) Height profiles along lines as indicated in zoomed STM images of panels a−c.

Figure 1. Immobile CuPc on Ag(100): (a) Ball-and-stick model of CuPc molecule. (b) STM image of CuPc molecule. Molecule in configuration 1 (left) and in configuration 2 (right). STM imaging conditions: V = 289 mV, I = 51pA, T = 75 K.

symmetry of the molecule, shown in Figure 1a. It has been shown previously that both CoPc and CuPc adsorb on Ag(100) with two, equally probable, mirror-like configurations with respect to the [110] direction, as shown in Figure 1b, named configuration 1 and configuration 2.28−31 At 5 K, both molecules are immobile on the surface. In the temperature range of 43 to 49 K, CoPc diffuses over the Ag(100) surface by a combination of translational and rotational movements with an activation energy of 0.15 eV.31 The activation energy for CuPc molecule diffusion was estimated by us to be ∼0.20 eV.32 With respect to this study the CoPc and CuPc molecules behave in the same fashion; that is why images of both molecules are shown interchangeably.

adsorbed across step-edges, which provide stable adsorption sites. We expect that a coverage in the range of 0.01 ML is sufficient to decorate the step-edges of our Ag(100) surface. Higher coverages manifest themselves by a dark contour around the molecules that are immobilized at the step-edges; see zoom of Figure 2b. We associate this dark contour with the presence of mobile molecules on the terrace. For molecules deposited at 200 K (and imaged at 120 K), we hardly observe attachment of further molecules to the molecules immobilized at the step-edges; see Figure 2a,b. Only occasionally another molecule is attached to molecules adsorbed across step-edges. B


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The Journal of Physical Chemistry C We utilized those molecules for the determination of the depth of the rim; see Figure 2d. The apparent depth of the black contour (rim) measured from the STM image is 40−65 pm. There are, however, more molecules adsorbed on the surface than those that are visible at step edges. The diffusive noise is created because the mobile molecules are crossing multiple times below the scanning tip, while it scans the terrace far away from step edges but not close to the molecules immobilized at the step edges. The dark contour develops because the repulsive interactions between molecules hinder the mobile molecules to approach to the immobile ones, leaving some space. The CoPc/CuPc molecules do not attract each other in the submonolayer regime, as deduced from the fact that no island formation was observed at 120 K and from reported LEED patterns, which are characteristic for the presence of a 2D gas on the surface.33,34 Molecules that approach the stepedges do not attach to the immobilized molecules but are reflected and continue their motion on the terrace. They can thus not cross under the tip, while it is close to the immobilized molecules, causing the dark rim. The uniform noise is independent of the tip polarity; see Figure 3. Furthermore, it

Figure 4. Determination of molecule distance for CuPc on Ag(100): (a) STM image of immobile molecules with similar depression between two of them as size of the rim. (b) Stick-and-ball models of Pc molecules overlapped on STM image. STM imaging conditions: V = 450 mV, I = 51pA, T = 5 K. (c) Height profile along the line indicated in panel a.

molecules) for two CuPc molecules immobilized at low temperature. In the case of such immobile molecules we can identify the molecule−molecule distance, clearly, which is 0.53 nm, at FWHM, in the case shown. Figure 4b shows the overlap of a stick-and-ball models of Pc molecules on the STM image. The experiments show that the repulsion is not long-range, only around two Ag(100) surface distances between the hydrogen atoms. Note also that hydrogen atoms at the periphery of the CuPc molecule are not imaged by STM. Likely, there is only Pauli repulsion between hydrogen atoms at this distance. The presence of the black contour demands mobile molecules on the surface. To observe the black contour the coverage has to be high enough so that molecules on the terrace frequently approach the immobile molecules to create the diffusive noise around immobile Pc molecules. We observe the diffusive noise on the surface for coverages in the range of 0.4 to 0.8 ML. The coverage has to be smaller than 1 monolayer (ML) because the Pc molecules are known, from low electron energy diffraction (LEED) and STM investigations, to be arranged in ordered stable structures at 1 ML.35,36 3.2. Phthalocyanines on Ag(100) - Effect of Voltage Pulses during the Presence of a Pc Gas. The larger apparent height of the terrace at temperatures around 120 K as compared with low temperature imaging is a strong indication for mobile molecules. To proof their existence, we apply a voltage pulse between 1 and 5 V to the scanning tip. In the bottom of Figure 5a the bare surface is imaged with some stripes, which are another indication of mobile molecules. We applied a voltage pulse of ∼5 V after having scanned ∼30% of the image from the bottom, at the marked position in Figure 5a. Immediately after the pulse, an island appears on the surface that is imaged in the upper part of the image. The complete

Figure 3. Dependence on polarity for black rim: CoPc on Ag(100). STM imaging conditions: (−) V = −441 mV, I = 80 pA, T = 120 K; (+) V = 525 mV, I = 70 pA, T = 120 K.

exists after deposition of the Pc molecules at both 200 K and room temperature. However, there is a difference with respect to nucleation at the immobilized molecules. After adsorption at RT (and imaged at 120 K), more additional molecules are attached because Ag adatoms are present close to the stepedges; see Figure 2c. This observation is consistent with the creation of a CuPc−Ag network, previously observed, after adsorption of CuPc molecules at RT.32 In this network, the Ag atoms, which are present on the surface due to step fluctuations, interconnect the CoPc molecules. Also, around these networks a black contour, similar to the one observed after 200 K adsorption, exists. In Figure 2d we present height profiles across the molecules attached to the step-edges. Line (a) (black squares) illustrates the changes in the apparent height when the molecule has no black contour in the low-coverage regime. Lines (b) (red dots) and (c) (green triangles) show changes in the apparent height when the black contour is present, for both 200 K and RT adsorption, respectively. The apparent heights of the Pc molecule and the diffusive noise are in a comparable range. Typically, the apparent height of the diffusive noise is at least 80% of the apparent height of the molecule, which means that a molecule must be below the tip during a similar percentage of time, while the value for a certain pixel of the STM image is integrated. To get a feeling how close the mobile molecules approach the immobile ones, we present in Figure 4 a height profile with comparable depth (60 pm between ligands of C


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Figure 5. Formation of molecular islands by voltage pulse, CoPc on Ag(100): STM images recorded (a) during and (b) after voltage pulse of ∼5 V. Star indicates the place of applied voltage pulse of 5 V for 0.256 s. STM imaging conditions: V = 465 mV, I = 70 pA, T = 120 K. (c) Height profile along red line indicated in panel b.

Figure 6. Formation of molecular islands by voltage pulse, CoPc on Ag(100): STM images recorded (a) during and (b) after voltage pulse of ∼1 V. Star indicates the place of applied voltage pulse of 1 V for 0.256 s. STM imaging conditions: V = 465 mV, I = 70 pA, T = 120 K. (c) Height profile along red line indicated in panel b.

island produced in this way is shown in Figure 5b. It is stable on the time scale of minutes under imaging conditions (∼500 mV, 70 pA). While the island is disordered at the point of the voltage pulse, other parts of the island show an ordered structure, consistent with the (5 × 5) structure, observed previously with by LEED and STM at monolayer coverage.36 Possible reasons for the creation of such an island are the local creation of surface defects combined with a modification of the molecules during the pulse. The modification of molecules can be deduced from the fact that on the periphery of the island the molecules do not have the characteristic four-lobe structure of an intact molecule. It is likely that they are dissociated. It is also worth noticing that there is almost no diffusive noise around this island. The apparent height of the island with respect to the surrounding area is shown in Figure 5c. The apparent height of the island is comparable to the apparent height of Pc molecule adsorbed on Ag(100) at low temperature. Thus the lack of uniform diffusive noise around the island suggests that all mobile molecules from the area around the island were accumulated in it. However, we cannot rule out completely that some of the molecules embedded in the island were deposited from the scanning tip. The effect is less pronounced after applying a lower voltage pulse of the same length presented in Figure 6. After scanning 75% of the image with diffusive noise, we apply a voltage pulse of ∼1 V. Again an island appears immediately, as imaged in the upper part of Figure 6a. This island is much smaller (Figure 6b) than the island created by a 5 V pulse, a black rim surrounds this island, and the apparent height of the terrace indicates diffusive noise. The height profile, shown in Figure 6c, again indicates that the apparent heights of molecule and diffusive noise are in a comparable range. It is well established that voltage pulses of this size do not create defects in the surface. This confirms that the creation of islands is related to a modification of the molecules underneath the tip, which, in turn, influences their mobility. Indeed, a sensitivity of the CoPc molecule to electron bombardment was shown by recording

LEED patterns after various lengths of electron beam exposition in ref 35. 3.3. Phthalocyanines on Ag(100) - Probing of the Potential Energy Landscape through a Pc Molecule. After having proven the existence of a mobile gas on the terraces at 120 K, we now demonstrate how molecules of this gas can be used for contrast enhancement of the surface. It has been shown before that organic molecules from a diffusive gas that are trapped below the tip during scanning can be used to enhance the resolution of the tip and in particular image the geometrical structure of the surface with enhanced contrast.13,14 Indeed, we are able to achieve atomic resolution of the Ag(100) surface, with the expected lattice spacing of 0.29 nm, at a tip− sample distance that is far too large for conventional atomic resolution (Figure 7a). As expected for true atomic resolution, the unit cell is independent of image size and orientation. Böhringer et al.14 also obtained atomic resolution of Ag(110) by dragging anthracene molecules below scanning tip. In contrast with data presented in ref 13, the enhanced corrugation can be observed here at both polarities. Consistent with the explanation in ref 13, the molecule is not stochastically crossing under the tip but is trapped below it. The tip probes the PES of the CoPc-Ag(100) system through this molecule. Because the CoPc molecule has only one stable adsorption site, which is the hollow site,28,31 the PES has the same symmetry as atomic resolution of the bare Ag(100) surface. Note that for molecules with more than one stable adsorption site, such an image could be easily misinterpreted. As the tip is unable to drag the molecule across the step-edges due to the trapping ability of steps,37 the dragged molecule is released close to the step-edges and the atomic resolution is lost. As shown in Figure 7a, during the scan across the step-edge a new molecule is picked up at the other side of the step-edge and the atomic resolution is regained. The stochastic distribution of moving molecules is reflected in an irregular shape of the border of atomically resolved regions of the upper terrace, Figure 7a (right side of step-edge). The height profile shown in Figure 7b reveals that the apparent heights of molecule and diffusive noise D


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Figure 8. Origin of diffusive noise: CuPc on Ag(100). Motion of molecule during imaging at 75 K; (a) molecules in configuration 1; (b) molecules in configuration 2; and (c) molecule changing configuration during scanning. STM imaging conditions: V = 289 mV, I = 51pA, T = 75 K. In schematics red contour illustrates the position of molecule before and green after jump/configuration change. The yellow line indicates the scan line during which the molecule jumped or changed configuration.

Figure 7. Diffusing molecules as contrast enhancers: (a) STM image recorded by dragging the CoPc molecule below the tip on Ag(100) with zooms into indicated regions. STM imaging conditions: V = −442 mV, I = 70 pA, T = 120 K. (b) Height profile along the line indicated in panel a.

of scanning its area. In Figure 8b-ii we sketched the molecule before and after the jump. In Figure 8b-iii the molecule jumps three times. In Figure 8b-iv it is difficult to distinguish the exact steps of motion. In principle, the Pc molecules not only translate but also change their configuration during diffusion, with the change of configuration occurring less frequently.31 An example for changes in molecule configuration during scanning the molecule is illustrated in Figure 8c. In Figure 8c-i and c-ii, the molecule is first in configuration 1 and changes during scanning to configuration 2, whereas in Figure 8c-iii and c-iv the molecule is first in configuration 2 and changes to configuration 1. The process is well visible in Figure 8c-i because the jump occurred after scanning of ∼50% of configuration 1 (we see two upper ligands of the molecule marked in red) and then we observe ∼70% of the molecule in configuration 2 in its new position (marked in green). Both configurations here can be easily recognized. Clearly the molecule does not only change configuration but also translates downward in this image. If the molecule only rotated (change of configuration), then we would see 50% of the molecule in configuration 1 and 50% in configuration 2. In Figure 8c-iii and c-iv the molecule experiences in addition to rotation a jump upward in the image; that is why we see the molecule only partially in these images. At the temperature of 75 K we have a chance to get insight into separate jumps during scanning. It is widely known that thermal jump rates increase exponentially with temperature.37 That is the reason why above 75 K the jumps are so frequent that it is impossible to recognize where the molecule is in the STM image. Sometimes this is already the case at 75 K, as it was demonstrated in the images presented in Figure 8a-iv and b-iv. Clearly the position of the molecule cannot be tracked; only its approximated position can be identified. With increasing temperature, a molecule will do many jumps during scanning of only one line of an image, so the same molecule may be probed by the scanning tip multiple times but not at all times during the integration at one pixel. This explains why the apparent height of the diffusive noise observed at 120 K is less

are in a comparable range, as above. The modification of the tip clearly enhances the contrast in STM images, and atomic resolution is routinely obtained with such tips.14,38,39 3.4. Phthalocyanines on Ag(100) - Origin of Diffusive Noise. To clarify the origin of the diffusive noise, we investigate the molecules at lower temperatures. In our study we first find conditions for which the tip does not influence the position of the molecule and all noise measured results from thermal motion. In this case, the displacement of the CuPc molecule is correlated with the close-packed Ag(100) directions but not correlated with direction of scanning. In Figure 8, we follow the motion of a CuPc molecule at a temperature of 75 K. At this temperature, the CuPc molecule moves rapidly enough to observe jumps in one STM image without losing the ability of tracking the position of the molecule over the surface. The time scale of measurements is an important factor in this investigation. To scan one line of the STM image, we need 0.37 s. At the chosen resolution and image orientation, the tip needs 42 scan lines to scan configuration 1 and 52 scan lines to scan configuration 2. To scan the whole molecule in configurations 1 and 2 (as shown in Figures 1b), the molecule needs to stay immobile for 15.53 and 19.35 s, respectively. However, this is rarely the case at 75 K. In the panels of Figure 8a we present a CuPc molecule in configuration 1, which jumps while scanning the area of the molecule. In Figure 8a-i the CuPc molecule jumps once by one Ag(100) lattice spacing. In Figure 8a-ii we sketch the molecule before (red) and after the jump (green) and mark the line when molecule jumped in yellow. In Figure 8a-iii the molecule jumps at least four times. Figure 8a-iv presents an image where it is even impossible to count the number of jumps. In the series of Figure 8a the molecule is mostly in configuration 1. A similar example for CuPc in configuration 2 is presented in Figure 8b. In Figure 8b-i, the CuPc molecule, in configuration 2, jumps once during the time E


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The Journal of Physical Chemistry C than the one of an immobile molecule. At ∼120 K the possibility to track both CoPc and CuPc molecules on Ag(100) is lost. Instead we observe a uniform diffusive noise due to multiple crossing of molecules underneath the scanning tip. 3.5. Astraphloxin on Cu(111). To proof the general applicability of our results obtained for CuPc and CoPc on Ag(100), we present results for another organic molecule, astraphloxin, on a different surface, Cu(111). The industrial dye (Figure 9a) is, in contrast with the Pc molecules, not planar. A multitude of conformers exist, but a previous study25 identified only two conformers, dd and dl of the 10-cis-isomer, after adsorption on Au(111) and Ag(111) surfaces (Figure 9b,c). Both conformers are imaged in low-temperature STM as single

protrusions with different apparent heights.25 On the Cu(111) surface the astraphloxin molecules nicely decorate the step edges at 110 K, similar to the Pc molecules on Ag(100); see Figure 9d. Also, in similarity to the Pc/Ag(100) case, we do not observe the growth of molecular islands from the steps but a black contour around the molecules that is immobilized at the steps. In contrast with the Pc/Ag(100) case, some molecules are adsorbed and immobile on the terrace. We also observed black region around those molecules (Figure 9e). Figure 9f presents the height profile across such a molecule and its black contour around it. On Cu(111) we have an apparent height of 280 pm above the black rim. It agrees with the apparent height of the dl-conformer of the molecule imaged by LT-STM on Au(111) at 5 K at a comparable voltage, at 270 pm (see ref 25). However, the molecule at 5 K has no black contour.25 We conclude than that molecule on the terrace is the dl-conformer of astraphloxin and that the diffusive noise results from a mobile molecular gas. Surprisingly, the apparent height of the diffusive noise is only half of the apparent height of the astraphloxin molecule. Recall that for Pc on Ag(100) the diffusive noise has ∼80% of the apparent height of the Pc molecule. However, the dd-conformer, identified in ref 25, has a lower apparent height than the dl-conformer. The apparent height of this conformer varies with the surface. It amounts to 210 pm on Au(111) and to 160 pm on Ag(111).25 The diffusive noise on Cu(111) has a slightly lower apparent height than the apparent height of dd-conformer on Ag(111). This suggests that it is the dd-conformers that are present in the mobile gas on Cu(111). Additionally, gas-phase calculations predict the presence of a third stable conformer, the llconformer.25,40 This conformer was not detected by LT STM on Ag(111) and, Au(111)25 but could exist on Cu(111). From a geometrical point of view, such a conformer (if it existed) should have an even lower apparent height. So it is likely that at 110 K the astraphloxin on the Cu(111) surface is immobile as dl-conformer and mobile as a dd- or ll-conformer. Note that the black contour observed around the immobile molecule is wider than the one observed for Pc molecules. Wider contours are likely associated with longer range repulsion between the molecules. Apart from these specific differences in detail, this example shows that the diffusive noise is likely to exist in more systems, which show no tendency to nucleate or to form islands in the submonolayer regime.

4. CONCLUSIONS A molecular gas leads to a uniform noise on the terraces accompanied by a black contour around immobilized molecules. The former one results from molecules moving multiple times below the tip during imaging as proven by measurements at 75 K, at which the molecules are slowed down such that they hardly move on the time scale of imaging; the latter one results from repulsion between the mobile and immobile molecules, as is clear from fact that immobilized molecules do not play the role of nucleation centers. The existence of a molecular gas is proven by voltage-induced manipulation that leads to island formation. By trapping a molecule from the molecular gas, its potential energy landscape on the surface is probed resulting, in the case of Pc on Ag(100), in an STM imaging reflecting the atomic structure of the surface. As confirmed by similar results for a different molecule on another surface, the concept of diffusive noise is general and

Figure 9. Diffusive noise for astraphloxin on Cu(111): (a) Chemical structure of astraphloxin molecules. (b,c) Conformers of astraphloxin as calculated in THE gas phase by Argus Lab:40 (b) dd-conformer and (c) dl-conformer. (d,e) STM images: (d) Steps covered by astraphloxin molecules; Imaging conditions: I = 0.15 nA, V = −743 mV, T = 110 K. (e) Image of molecules immobilized on the terrace; imaging conditions: I = 0.16 nA, V = −2102 mV, T = 110 K. (f) Height profile along red line shown in panel e. F


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thus applicable to other systems of organic molecules adsorbed in submonolayer coverage.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

G. Antczak: 0000-0001-6828-6576 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The project was supported by the Humboldt Stiftung as part of G.A. Humboldt Fellowship stay at the Leibniz University Hanover and by the project number 1010/S/IFD/16. K.M. acknowledges support by the German Israeli foundation. We thank St. Ernst for discussions about this paper.

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