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Molecular Resonance Imaging and Manipulation of Hexabenzocoronene on NaCl(001) and KBr(001) on Ag(111) Thibault Ardhuin, Olivier Guillermet, André Gourdon, and Sébastien Gauthier J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02959 • Publication Date (Web): 14 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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The Journal of Physical Chemistry

Molecular Resonance Imaging and Manipulation of Hexabenzocoronene on NaCl(001) and KBr(001) on Ag(111) Thibault Ardhuin, Olivier Guillermet, André Gourdon and Sébastien Gauthier* CEMES CNRS UPR 8011 and Université de Toulouse 29 rue Jeanne Marvig, 31055 Toulouse, France Abstract: The adsorption of hexa-peri-hexabenzocoronene (HBC) on NaCl and KBr bilayers deposited on Ag(111) is studied by Scanning Tunneling Microscopy (STM) and spectroscopy at low temperature (5K). HBC tends to move under the influence of the STM tip on NaCl/Ag(111), even in the mildest imaging conditions, preventing the imaging of its molecular electronic resonances (MERs). It is more stable on KBr, due to a higher diffusion barrier, as confirmed by a force-field based calculation of its adsorption on both surfaces. The MER associated with the Lowest Unoccupied Molecular Orbital (LUMO) of HBC is imaged and analyzed in detail on KBr/Ag(111). Assemblies of two to four HBC could be built on NaCl by lateral manipulations with the STM tip. These objects present a higher stability than single molecules making them more amenable to MER imaging in a large bias voltage range. While the constituting molecules are too far apart to interact chemically, their electronic clouds overlap, producing in some cases complex images of the MERs that are difficult to disentangle to extract the single molecule contributions. This problem is examined by comparing the images of a two HBC assembly to those of a single molecule on KBr. A combination rule is proposed, that could be extended to extract single molecule contributions from larger assemblies.

* Corresponding author: [email protected]: +33 5 62 25 79 80

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Introduction The adsorption of a molecule on a metal usually strongly perturbs its electronic structure. When it is important to preserve the molecular properties in scanning tunneling microscopy (STM) experiments, the electronic coupling to the metal can be reduced by using an ultrathin insulating film deposited on the metal substrate, such as a bilayer of NaCl(001) on Cu(111). This strategy has been used in a large number of studies, leading to major advances such as the imaging of MERs, which carry a strong similarity with the electronic probability density of the molecular orbitals of the isolated molecule,1-3 the control of the charge state of atoms

4,5

and

molecules,6 and more recently, the on-surface synthesis and observation of molecules that cannot be stabilized otherwise.7 One limitation of this approach is that the molecule is generally much less strongly bounded to the insulator surface than on a metal. It very often tends to diffuse or is prone to unintentional manipulation by the tip of the STM even when these studies are restricted to low temperature (5K) and very low imaging currents (in the pA range). Hexa-peri-hexabenzocoronene (HBC) is an emblematic molecule, which can be considered either as a graphene building block 8 or as a super benzene molecule 9. It is a representative of the family of polyaromatic hydrocarbons (PAH), which are believed to play an important role in the interstellar medium 10 and is considered as a potential material for organic electronics. 8 It has been studied by STM on different metallic substrates 11-14 but rarely on ultrathin insulating films.13 It was possible to image some of its MERs on Au(111), because the molecule is only weakly physisorbed on this surface, 14 but not on NaCl thin films, as mentioned in ref. 13. Here we follow another strategy by working on a KBr(001) thin film on Ag(111). We first describe our methods, then expose our results on the adsorption of HBC on NaCl/Ag(111). We show that imaging the MERs of a single molecule is difficult because it tends to move under the tip with our experimental conditions. Nevertheless, it is possible to build by lateral manipulation assembly of 2 2


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to 4 molecules on which MER images can be recorded without unintentional manipulation. We then change our substrate to KBr/Ag(111) and show that HBC is more stable on this substrate than on NaCl, allowing to image the MERs of single molecules. Our data on KBr allow to investigate the relation between the MER images of assemblies and of single molecules. Finally, we suggest a method to disentangle these images of assemblies, which could be used on HBC/NaCl/Ag(111).

Experimental methods The experiments were performed in a UHV system (pressure in the 10-11 mbar range) including a preparation chamber and a low temperature (5K) STM/nc-AFM from Scienta Omicron. The control electronics were provided by SPECS. All images, spectra, and manipulation experiments were performed at a temperature of 5K. The STM tips were in platinum. The Cu(111) and Ag(111) samples were cleaned by repeated Ar sputtering followed by annealing at 750 K. NaCl and KBr were deposited from a tantalum crucible on the substrate at room temperature. HBC has been synthesized by cyclodehydrogenation of hexaphenylbenzene and purified by double sublimation in vacuum.15 The molecules were deposited directly on the sample, maintained at low temperature (T below 10K) in the microscope, from a heated crucible containing the molecular powder.

Experimental results Imaging of HBC on NaCl/Ag(111) The morphology of room temperature NaCl deposits on Ag(111) has been analyzed previously.

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The image of Figure 1(a) confirms the trends observed in this work: NaCl grows in the form of bilayer high rectangular islands bordered by non-polar atomic rows. These islands do not adopt a specific epitaxial relation with the Ag(111) lattice, but their orientation is strongly influenced by the Ag(111) step edges. HBC molecules appear in the image of Figure 1 as small dots, evenly dispersed on the surface. In the following, we focus on HBC on bilayer islands. 3



Figure 1: (a) STM image of a low coverage of HBC molecules on NaCl/Ag(111). I=1pA, V=1V. (b) Profile along the lines drawn in (a). The images of Figure 2 show an HBC molecule on a NaCl bilayer on Ag(111) recorded with the smallest current that our microscope can control (I=0.6 pA) with different bias voltages. At V=1.6V, the molecule is stably imaged as a circular dot with a diameter and a height of approximately 1.5 and 0.17 nm. At V=1.7V, the image is larger (2.3 nm) and higher (0.33 nm) and reveals submolecular features, that are characteristic of a MER. But the molecule can no more be stably imaged, it is laterally manipulated [see the green arrow in Figure 2(b)]. This sensitivity when the bias voltage is close to a MER is precisely what is exploited when it is desired to displace the molecule on the insulator. But in the present case, it prevents the study of these resonances for single HBC molecules on NaCl.

Figure 2: HBC on NaCl bilayer on Ag(111): Z scale = 0.38 nm, I=0.6pA, (a) 1.6V, (b) 1.7V. Insert in (a): model of the HBC molecule. It was nevertheless possible to image single HBC when adsorbed on defects (Cl vacancies or step edges), or, as shown in the following, when interacting with other HBC in molecular assemblies of 2 to 4 molecules.

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Molecular assemblies

Figure 3: Assembling of HB3 by lateral manipulation on NaCl/Ag(111). 1.3V, 2pA. The arrows indicate which molecule was moved to build HBC3. The assembly of 3 HBC molecules on a bilayer of NaCl by lateral manipulation is illustrated by the sequence of images of Fig 3. STM lateral manipulation techniques have been developed on metal surfaces for a long time, 17-18 but are much less advanced on insulating thin films. The protocol was the following: the tip is positioned in the normal imaging mode with a bias voltage below the first positive MER near the border of the molecule. The voltage is then increased above this resonance (1.7V for HBC/NaCl). Shortly after this change, Z increases, indicating that the molecule moves under the tip. The tip is then manually displaced with the mouse of the computer along the desired trajectory to reach the final position, slowly enough to keep the molecule trapped under the tip, as indicated by the Z signal. This method is very efficient and allows to move reliably the molecule on large distances, for instance more than 30 nm in Figure 3. It is similar to a “sliding” mode according to the classification proposed in ref. 19. It works only when the bias voltage is close to or above a MER, confirming that it is based on the vibrational excitation of the molecule by inelastic tunneling of electrons through the MER, as reported previously. 20 Our technique is similar to that reported in ref.20, except that in our case the distance regulating feedback loop is not disabled before the voltage step. Up to 4 molecules could be assembled in this way, as shown in Supporting Information.

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Figure 4: STM images of an assembly of 3 HBC on NaCl. I= 600fA. The imaging bias voltages are indicated below the images. Figure 4 shows the evolution of the image of an assembly of 3 HBC on NaCl. MERs can now be imaged in a large range of bias voltage (-4V to 4V) in contrast to isolated HBC [Figure 2(b)]. All these images except at -1.6V exhibits features that are characteristic of MERs. The bias voltage at which they appear can be estimated from the images at -2.8V, +1.7V and +2.6V. A close examination of Figure 4 suggests that the MER images could be “separated” into single HBC contributions. But a method is needed to do that rigorously. A first step in the elaboration of such a procedure is to understand how images of MER of single molecules contribute to the MER image of an assembly of molecules. Our experimental results on HBC/KBr/Ag(111) presented in the following will be used to propose a simple model to elaborate and to test a suitable “combination” rule to calculate the MER image of an assembly of molecules from the MER image of a single molecule.

Imaging of HBC on KBr/Ag(111) The images of Figure 5 show a KBr bilayer on the left and the Ag(111) substrate on the right. Two HBC molecules are adsorbed on the metal and five on the KBr bilayer. The images are recorded with I=1 pA and a bias voltage of 1.4V [Fig.5(a)] and 1.8 V [Fig.5(b)]. At V=1.8V, a MER is clearly imaged in the form of a six lobes pattern with a symmetry axis aligned on the polar directions of the KBr substrate.

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Figure 5: HBC on KBr bilayer on Ag(111). I=1pA. (a) V=1.4, (b) V=1.8V. Molecules labelled (1) are rotated by 90° relative to molecules labelled (2). Force-field based calculations have been performed in order to understand this increased stability relative to HBC/NaCl. The COMPASS (Condensed-phase Optimized Molecular Potentials for Atomistic Simulation Studies) force-field

21

was used with Materials Studio

22

to find the optimal

adsorption configuration of HBC on NaCl(001) and KBr(001). This force-field is well suited to calculate the adsorption of organic molecules on the surface of inorganic materials.23 Previous studies have shown that the interaction of organic molecules with ionic surfaces is dominated by van der Waals and electrostatic forces. In addition, these works indicate that the charge transfer between the substrate and the molecule is generally negligible for this type of surfaces,

24-25

ensuring that force-field based calculations are sufficient for a realistic description of these systems. HBC is a flat, rigid molecule.26 To find its optimal adsorption configuration and estimate its diffusion barrier on NaCl(001) and KBr(001), we calculate the total energy of the system with the molecule positioned parallel to the substrate surface as a function of its lateral position (X,Y), altitude Z and angular position θ, without any structural optimization. We then calculate the energy minimum for each (X,Y) position and determine the minimum energy site and the corresponding molecule altitude and orientation. The details of the calculation are given in Supporting Information. The result of this estimation is shown in Figure 6 where the minimum energy configuration are presented.

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Figure 6: Optimal adsorption configuration of HBC on (a) NaCl(001) and (b) KBr(001). Cl ions are in green, Na in pink, Br in red and K in pink. In both cases, HBC is oriented along a polar direction of the substrate (arrows in Figure 6). This result is in agreement with the observation made on the image of Figure 5(b). The molecule is stabilized by the interaction of its lateral hydrogen atoms, which carry a positive partial charge, with the negative ions of two polar rows of the substrate. The calculation predicts similar adsorption energies, of 0.85eV for NaCl and 0.74eV for KBr, but a much higher diffusion barrier on KBr (60 meV instead of 15 meV on NaCl) explaining why HBC is more easily imaged on KBr by STM.

Spectroscopy and molecular resonance imaging of HBC on KBr/Ag(111)

Figure 7: (a) I(V) (black curve) and dI/dV(V) (red curve) spectra recorded above the center of a HBC molecule on KBr/Ag(111). (b) Constant current images measured at the bias voltages indicated by green lines.

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An I(V) spectrum and its derivative measured on HBC on KBr are presented in Figure 7. A strong increase of the tunneling current is seen near 1.5V. The maximum of the corresponding peak on the derivative spectrum appears at 1.67V. Images measured at the bias voltages indicated by green lines are displayed in Figure 7(b). At 1.4V, below the beginning of the dI/dV peak, the molecule is imaged as a circular dot with a diameter and a height of approximately 1.5 and 0.16 nm. In the 1.45V to 1.7V range, which spans the rising front of the peak, the diameter and the height of the image increase and submolecular features appear. These observations clearly characterize a MER. Interestingly, the noise on I(V) and dI/dV(V) in Figure 7 increases significantly for bias voltages above the peak at 1.67 V. We believe that this observation results from inelastic tunneling of electrons through the resonance inducing molecular fluctuations of the molecule under the tip. It has been demonstrated that the MERs observed by STM do not correspond in general to the molecular orbitals of the isolated molecule.14,27 The interaction of the molecule with the substrate and the tip, but also the tunneling process itself lead to a mixing of the molecular states, following rules that are not yet clearly established.28 Nevertheless, in most cases, the resonances that are the closest to the Fermi level can be approximately related to the HOMO and the LUMO of the molecule. It is then natural to attribute the 1.67V peak to the LUMO of HBC. The position of this orbital was measured at 2.2 V on a single HBC on Au(111) 14 and at 1.8 V on a HBC monolayer on Au(111).29 With the 5.1 eV work function of Au(111), the LUMO is then located between -3.3 and 2.9 eV relative to the vacuum level on this substrate. The work function of Ag(111) is around 4.5 eV. A bilayer of KBr is expected to lower the work function of the combined system by approximately 1 eV,30 resulting in a value of 3.5 eV and a LUMO position relative to the vacuum level of -1.8 eV. These values are consistent with a larger shift of the LUMO toward the Fermi level on the metal substrate, as expected due to the larger image interaction with the metal.31 The LUMO of the free HBC molecule is degenerate and includes two levels that will be called La and Lb in the following. When adsorbed on KBr(001) with the configuration suggested by the calculation (Figure 6), the symmetry is lowered from the 6-fold symmetry of the free molecule to a 9



2-fold symmetry. One then expects the molecule substrate interaction to lift the degeneracy and the LUMO peak to split in two. Isolating closely spaced individual MER in STM is usually performed by dI/dV imaging with a lock-in amplifier. But this method requires high tunneling current to get a reasonable S/N ratio and is then difficult to use on insulating thin films. An alternative consists in calculating image differences from a set of images measured at small bias voltage interval. This procedure was applied to the images of Figure 7, resulting in Figure 8. These images differences are in fact close to dZ/dV images since the images of Figure 7 are constant current images. But the voltage increment of 50 mV is small enough to consider the I(Z) as locally linear, making them equivalent to dI/dV images.

Figure 8: (a) Image difference 1.45V – 1.40 V, (b) Image difference 1.55V – 1.50V, (c) Normal image at V=1.65V. Insert in (a): Calculated La image, in (b) calculated Lb image and in (c) calculated sum of La and Lb. The arrow in (a) gives the orientation of HBC for the calculation (see the insert in Figure 2(a)).

Figure 8 shows that the symmetry of the MER image differences changes with the bias voltage: Figure 8(a) exhibit two nodal planes at a 60° angle, suggesting that the La level is the dominant contribution to the image in the corresponding bias voltage range while Figure 8(b) exhibit two dark lines at a 90° angle corresponding to the Lb level. In Figure 8(c), the image recovers approximately the 6-fold symmetry of the degenerate LUMO orbital of the free molecule. These images are compared with images simulated with the method presented in ref.32, which considers that the image of a MER can be approximated by the square of the overlap energy between the wave function of a molecular orbital of the free molecule and a 4s wave function of adjustable diameter representing the tip. The calculated image in Fig.8(a) corresponds to La. It exhibits two nodal planes at 60°. In contrast, the calculated image of Lb [Fig.8(b)] shows two nodal planes at 90°. These features are reminiscent of the doubly degenerate LUMO of benzene when 10


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using the 2pz atomic orbitals of C as a basis, a consequence of the similarity of HBC with benzene [9]. The good qualitative agreement between the experimental MER image differences and the calculated orbital images of Figure 8 suggests that the La contribution is lower in energy than the Lb contribution, confirming our hypothesis of a degeneracy lifting of the LUMO by the interaction of HBC with Kbr. In Figure 8(c), the image recovers approximately the 6-fold symmetry of the degenerate LUMO orbital of the free molecule. In the following, we use our experimental results on HBC on KBr to propose a simple model to understand how MER images of single molecules should be combined to recover MER images of an assembly of molecules.

Figure 9: (a) STM image of 3 HBC molecules on KBr/Ag(111). V=1.5V, I=1pA. (b) Profiles on the lines drawn in (a) on HBC2 (black), on HBC (green) and calculated profile on HBC2 (red). The starting idea is to use KBr to image in the same condition the single molecule and an assembly, as for example in Figure 9(a) in order to clarify the relation between the single molecule image and the assembly image. This assembly of two HBC was built by lateral manipulation, using the same technique as on NaCl. Lateral manipulation was more difficult on KBr, where tunneling current of the order of 700 pA were needed, resulting often in damages of the molecule or the substrate. This observation is consistent with the higher diffusion barrier of HBC on KBr. The image of the upper two molecules (HBC2) is clearly constituted by a “superposition” of the images of two single molecules. Deviations from single molecule images appear in the region of overlap of these two images, in the form of a height increased by approximately 0.05 nm [Figure

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9(b) black curve]. The images are not modified otherwise, suggesting that the molecules are not strongly perturbed by their proximity. They are far too distant (1.84 nm center-to-center, whereas the van der Waals diameter of HBC is ca 1.52 nm,) to interact chemically. Nevertheless, the tunneling current participates in the image of the two molecules at the same time and one could ask if this has measurable consequences on the image of HBC2. To answer this question, we propose to “deconvolute” the image of HBC2 in a simple way. Let's consider that the tunneling current above the molecule is given by , = exp− . When imaging a molecule in the constant current mode, with = , this relation gives

= exp [− ], where is the profile scanned over the molecule. We choose the origin of the Z scale by imposing = 0 on the substrate. Then, = on the substrate. When the two images overlap, we assume that the contribution to the tunneling current of each molecule can be added [31], giving [ − ] + [ − − ] + = exp where d is the molecule center-to center distance and the profile above the dimer. Combining

these equations gives finally: = ln !"# $ + !"# − $ − 1&

(1)

To check this equation we use the profiles drawn on the isolated molecule in Figure 9(a) to calculate the profile on HBC2 using equation (1). The only free parameter is β, since d can be measured directly on the image. The result is shown in Figure 9(b). The agreement between the experimental and calculated profile is excellent, confirming that the assumption of additivity of the tunneling current is fully justified, as predicted in ref. 33, when the two molecules are in positioned in parallel in the tunnel junction, without sharing a molecular node. Another example using this procedure is presented in Supplementary information.

Conclusions 12


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Images of MERs of single HBC molecules have been obtained on KBr bilayers on Ag(111), but not on NaCl bilayers due to a lack of stability during imaging. Force-field based calculation of the potential energy surfaces of the molecule-substrate system allow to identify an adsorption site of the molecule and point to a much higher diffusion barrier on KBr than on NaCl. These findings are in good agreement with the experimental observations. Stable MER imaging can be achieved on NaCl on assemblies of molecules built by controlled lateral manipulation. A simple formula, based on the additivity of the tunneling currents when imaging two overlapping -but not significantly interacting- objects is proposed to calculate the profile on an assembly from the profile on a single molecule. It is successfully tested by comparing the profiles on an assembly of two HC and on a single HBC on KBr. This method will be useful to identify situations where molecules do interact (see e.g. ref. 34), for instance by charge transfer or chemical bonding. In that case, the difference between the profile calculated with the assumption of a negligible interaction will deviate from the experimental profile in a way that could reveal the nature of this interaction. Furthermore, the method is not restricted to profiles, but can be generalized to images. It could then be used to extract a single molecule image from an assembly image by Fourier techniques, as it will be shown in a future publication.

Supporting Information Description 1. HBC optimal adsorption configuration on NaCl(001) and Kbr(001) 2. Observation of two molecular configurations on KBr 3. HBC assemblies A. Profiles on an assembly of 2 HBC B. Images of a linear assembly of 4 HBC

Acknowledgements 13



We thank C. Joachim for useful discussions. T. A. acknowledges the French Ministry of Higher Education and Research (MESR) for a PhD fellowship. The images have been processed with the WSXM software. 35

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