Implementing Functionality in Molecular Self-Assembled Monolayers

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Implementing functionality in molecular self-assembled monolayers Nemanja Kocic, Dominik Blank, Paula Abufager, Nicolas Lorente, Silvio Decurtins, Shi-Xia Liu, and Jascha Repp Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b03960 • Publication Date (Web): 01 Apr 2019 Downloaded from http://pubs.acs.org on April 1, 2019

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Implementing functionality in molecular self-assembled monolayers †,k

Nemanja Koci¢,

†

Dominik Blank,

§

Decurtins,

Paula Abufager,

§

Shi-Xia Liu,

‡

Nicolas Lorente,

and Jascha Repp

¶

Silvio

∗,†

†Department of Physics, University of Regensburg, 93040 Regensburg, Germany ‡Instituto de Física de Rosario, Consejo Nacional de Investigaciones Cientícas y Técnicas

(CONICET), and Universidad Nacional de Rosario, Bv. 27 de Febrero 210 Bis, 2000 Rosario, Argentina ¶Centro de Física de Materiales CFM/MPC (CSIC-UPV/EHU), Paseo Manuel de

Lardizabal 5 and Donostia International Physics Center (DIPC), Paseo Manuel de Lardizabal 4, 20018 Donostia-San Sebastián, Spain §Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012

Bern, Switzerland kCurrent address: Faculty of Physics, Friedrich-Alexander-Universität Erlangen-Nürnberg,

91058 Erlangen, Germany E-mail: [email protected]

Abstract The planar heterocyclic molecules 1,6,7,12-tetraazaperylene on an Ag(111) metal substrate show dierent charging characteristics depending on their local environment: next to vacancies in self-assembled islands, molecules can be charged by local electric elds, whereas their charge state is xed otherwise. This enables the activation of

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selected molecules inside islands by vacancy creation from scanning-probe based manipulation. This concept allows for combining the precise mutual atomic-scale alignment of molecules by self-assembly, on the one hand, and the implementation of specic functionality into otherwise homogeneous monolayers, on the other. Activated molecules in direct neighborhood inuence each other in their charging characteristics, suggesting their use as molecular quantum cellular automata. Surprisingly, only very few interacting molecules exhibit a rich spectroscopic signature, which oers the prospect of implementing complex functionality in such structures in the future.

Keywords quantum cellular automata, single molecule, self-assembly, scanning tunneling microscopy, density-functional theory During the last decades several dierent approaches have been followed to realize alternative data processing based on interacting cells on nanometer dimensions or even at molecular scales. Logic gates were realized, based on coupled nanometer-sized magnets 1,2 and even as molecular cascades, 3 interacting mechanically. Quantum cellular automata (QCA) based on electrostatics were proposed by Lent and coworkers 4,5 exploiting the arrangement of charges for processing information. In the original concept, a basic cell is usually envisioned as a rectangular ensemble of four sites occupied by two extra electrons that can tunnel between the sites. Because of electrostatic repulsion, the two electrons will always be located at diagonally opposing sites, such that an isolated cell has two degenerate ground states, representing logic 0 and 1. Neighboring cells can inuence each other, which is the basis for data processing. Although in its conception a cellular automaton implies a clocked operation, the latter is yet absent in its quantum realizations. 6,7 Experimentally, the rst functional electrostatic QCA cell was demonstrated back in 1997. 8 This was followed by realization of the elementary circuit elements, a digital logic gate, 9 small binary wire and shift register. 10 In the aforementioned experiments the sites were realized by quantum dots and, owing to their very 2


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small charging energies, the experiments had to be performed at sub-kelvin temperatures. In order to achieve a room-temperature QCA device, the energy spacing has to be increased by reducing the cell size to the molecular or even atomic scale. To this end, molecular-based QCA were proposed 1113 and molecular units that could serve as cells were developed. 1419 The recently realized QCA, based on the controlled formation of dangling bonds on a silicon surface, represents a leap in this development. 2023 Also, the well-dened multistability in assembled atomic scale structures suggests itself for the use as QCA. 24 The smaller the cell, however, the more critical the correct alignment and spacing of neighboring cells is for a correct implementation of its function. 2527 This poses a severe challenge in implementing molecular-based QCA, since all cells must be positioned with atomic-scale precision. Self-assembled monolayers of molecules allow for such a precise mutual atomic-scale alignment of zillions of molecules, 28,29 however, implementing a function in QCA requires particular structures of aligned cells and not just extended monolayers thereof. This major obstacle in realizing molecular QCA can be solved by implementing an activation of the molecules to become an active cell. This way, one can still make use of the precision and reproducibility in the molecular alignment from self-assembly, but at the same time realize specic QCA structures. In the realization demonstrated here, the activation of molecules is achieved by the creation of vacancies next to the molecule to be activated. These molecules next to vacancies can be inuenced in their charge state by gating, whereas all other molecules are stable in only one single charge state. This concept of activation is much more general and could also be implemented dierently, e. g. by addition or removal of a ligand on a functional molecule. 3033 Here, the two states of each cell are not implemented as usual by a charge rearrangement inside the cell, but by an exchange of charge between the substrate and the molecule. 34 Further, our work demonstrates that even only few interacting molecules exhibit a rich spectroscopic signature, which oers the prospect of implementing complex functionality in the future. Our experiments were performed with a combined scanning tunneling and frequency3


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modulation atomic force microscope (STM/AFM) at a temperature of about 5 K. For further details we refer to the methods section below. 1,6,7,12-tetraazaperylene molecules (TAPE) 35 (see Fig. 1a for its chemical structure) form self-assembled monolayers on a monocrystalline Ag(111) substrate. 34 This self-assembly is guided by hydrogen bonds of C-H· · ·N contacts, and it results in a regular array of molecules, in which all molecules experience an identical environment. Contrary to that, along the periphery of the island, the molecules dier in the number of nearest neighbors. Those of the molecules along the island edge that have their two lone-pairs of the nitrogen atoms unmasked along the edges, are found in a charge state dierent from the one of all other molecules in equilibrium, and they can be charged by means of the electrostatic eld brought about by a scanning tunneling microscope tip. 34,36 Such molecules that facilitate charge transitions are henceforth referred to as Q, all other molecules as A. Due to their dierent charge state, Q-type molecules appear brighter in STM images at low bias voltage and they exhibit a pronounced negative dierential conductance (dI /dV ) at a negative sample voltage of

V ≈ −0.9 V (see Fig. 1). This dip in dI /dV marks the charge state transition induced by the electric eld of the tip, as explained previously. 34,3638 The control of the charge state of Qtype molecules is associated to an exchange of charge between the molecule and the substrate and represents an outer-sphere electron transfer. 18,19,39,40 The two possible charge states suggest their use for implementing QCA. Unmasking the lone-pairs of the nitrogen atoms of a specic molecule deep inside an island is also possible in a controlled fashion by selected removal of one of the neighboring molecules. We employed STM manipulation to remove individual TAPE molecules from the array (see above and Fig. 1a) creating vacancies 41,42 (for details of the manipulation procedure see methods section). Indeed, two molecules next to a vacancy are transformed from A to Q as evidenced by their larger apparent height in low-voltage STM images, see Fig. 2a and b. To investigate the electronic properties of the molecular structures around a single vacancy, we measured a set of dierential conductance (dI /dV ) spectra at dierent positions as 4


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Figure 1: (a) Schematic representation of the experimental setup. At island edges (in the background) in every second molecule the lone pairs of nitrogen atoms are unmasked resulting in a dierent charge state of these molecules (referred to as Q-type molecules). This enables the activation of pairs of Q-type molecules in the interior of molecular islands by removing selected molecules with the STM tip (foreground). (b) STM image of an actual structure of TAPE molecules similar to the one depicted in (a). In the island edge every second molecule is of Q-type and appears brighter. Next to a vacancy two molecules are also of Q-type. Imaging parameters: V = 0.16 V; I = 2 nA. A constant-height AFM image (inset) of four molecules conrms their alignment. (c) Current I (red) and dierential conductance dI /dV (blue) as a function of voltage V acquired above one of the Q-type molecules next to the vacancy. The spectra reveal a pronounced negative dierential conductance at around -0.9 V.

Figure 2: (a and b) Experimental constant-current STM image of the interior of an island before (a) and after (b) one molecule is removed. The red circle marks the position, at which the tip was approached to remove the molecule. The removal of the molecule results in the activation of two neighboring molecules to Q-type molecules. Imaging parameters: I = 0.9 nA, V = 0.2 V. Scale bar 1 nm. (c) A set of 43 dierential conductance (dI /dV ) spectra at dierent lateral positions along the yellow line shown in (b) are combined to a two-dimensional map, in which the position is displayed laterally and the voltage vertically. The dark contrast is associated to a negative dI /dV indicating a charge state transition in the Q-type molecule beneath the tip. White labels indicate the charge state. The greyscale is in arbitrary units, where the yellow line marks zero. 5


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Figure 3: (a) Simulated STM image of the self-assembled structure without vacancy. The image closely resembles the experimental counterpart shown in Fig. 2a. (b) For Q-type molecules, at sucient negative bias voltage, the molecule's LUMO, initially located just above the Fermi level (dashed line) can be pulled below the Fermi level of the substrate, such that it becomes singly occupied (full line and dot). (c) To study the role of a vacancy, a large supercell (dotted diamond) consisting of seven molecules on Ag(111) (not shown) and one vacancy was simulated. (d) Projected density of states of molecules in a full monolayer (black dashed line), of molecules next to a vacancy (blue and red, corresponding to the marked molecules in c), and of all other molecules of the supercell shown in b (grey dashed lines). The inset represents a zoom to the energy range around the Fermi level. In all cases, the peak close to the Fermi level derives from the molecule's LUMO. For those molecules having two of their nitrogen atoms exposed to the vacancy, the LUMO is shifted to higher energy by ∼ 20 meV (blue and red). shown in Fig. 2c. Directly above the two Q-type molecules next to the vacancy, the spectra show a single sharp dark feature, associated to the dip in dI /dV at V ≈ −0.9 V as described above. In these and following measurements the tip acts simultaneously as a sensor, sensing charge state transitions and as a gate 4349 for molecules in the surrounding. When the tip is moved slightly laterally away from a Q-type molecule, a larger negative bias voltage (more negative) is required to have the same gating eect onto the specic molecule, as can be seen as the downward bending of the dark feature in Fig. 2c. If the tip is laterally too far away, the charge state transition cannot be detected any more and the signal gradually vanishes. That is, the molecule may still change its charge state, but this will not aect the tunneling current at the position of the tip, where only an A-type molecule is situated, which is stable in its negative charged state. The voltage threshold for the charge transition also depends on the vertical tip position, and it slightly varies with variations of the tip apex, most probably determined by its exact shape and work function. 36,50 6


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To better understand the charge state transition and the level alignment in these structures we performed DFT calculations (for technical details see methods section) of a full monolayer as well as single-vacancy structures of TAPE/Ag(111). For the full monolayer the alignment of molecules is consistent with a self-assembled structure being stabilized by C-H· · ·N inter-molecular interactions. We nd that the molecule's lowest unoccupied molecular orbital (LUMO) is located energetically slightly (∼ 50 meV) above the Fermi level EF of the system. Due to hybridization-induced level broadening, the LUMO extends over EF , indicating a partial occupation. A simulated STM image of the structure, see Fig. 3a, is in very good agreement with the experimental one (Fig. 2a). Introducing a single vacancy per unit cell consisting of seven remaining molecules did not lead to appreciable geometric dierences for the remaining molecules conrming the structural stability of the structure upon vacancy formation (see Fig. 3c). The two molecules that have two of their nitrogen atoms exposed to the vacancy show a clear shift of their LUMO towards higher energy, as can be seen from the projected density of states shown in Fig. 3d. In general, the close alignment of the LUMO with EF is in qualitative agreement with the assumed mechanism of the charging, in which the electric eld of the tip can shift a molecular level across EF and thereby change the charge state, as depicted in Fig. 3b and explained previously. 34,3638,49 Note that, the experiment suggests that the molecules within the islands are already singly charged, which is not fully captured by the simulations. In experiment, the presence of the tip, which is not included in the calculations, may also shift down the LUMO in energy due to its additional screening. Further, we believe that a self-stabilization of charge (e. g. a polaronic shift) has to be in place, to explain the relatively abrupt change in charge state observed in the experiment. Most importantly, the observed shift to higher energies captures the trend that the molecules next to vacancies are initially not charged, but can be charged by shifting their LUMO across EF by gating with the tip. Considering the lever arm of this mechanism, the small energetic shift of 20 meV can lead to a large voltage required for charging. 36,49 7


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Figure 4: (a-h) Experimental STM images of various realizations of structures containing two or more activated molecules. (i) Schematic representations of several basic patterns. Removed molecules are represented in black, activated and coupled molecules in green, blue and red for dimeric, trimeric and tetrameric structures, respectively.

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In contrast to island edges, where only every second molecule is of Q-type, the articial and controlled vacancy creation inside islands facilitates the formation of Q-type molecules in direct vicinity to each other. Various experimental realizations of such structures of dierent complexity are exemplied in Fig. 4. In the low-bias images next to vacancies certain molecules show a distinctly brighter appearance than all others in analogy to the behavior at step edges. 34 In conjunction to the previous ndings at the step edges, 34 these molecules can be identied as being of Q-type and neutral (in absence of an electric eld), whereas all other A-type molecules are singly negatively charged. We note that all the patterns shown were stable at the temperature of our experiments. The simplest structures consist only of two molecules interacting with each other. As the self-assembled islands exhibit low symmetry 34 (see also inset Fig. 1b) there exist several non-equivalent types of dimeric cell structures, four of which are depicted in Fig. 4i.

Figure 5: (a and b) Schematics and constant-current STM images of two dierent dimeric structures. Imaging parameters I = 0.9 nA, V = 0.2 V. (c) dI /dV spectra recorded above the center of the molecules as indicated by the arrows. Set point parameters prior to spectra acquisition I = 0.9 nA, V = 0.2 V. (d-h) Schematic representation and constant-current (I = 0.9 nA) STM images of a windmill structure at dierent bias voltages of (e) V = 0.16 V, (f) V = −0.1 V, (g) V = −0.2 V, (h) V = −1.0 V. Fig. 5a and b show constant current images of two dierent dimeric structures. Above the centers of Q-type molecules dI /dV spectra were recorded. The outermost Q-type molecules, 9


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which have no other Q-type molecules as direct neighbors, show the characteristic dI /dV -dip at a negative voltage of about V ' −0.75 V. The Q-type molecules in a dimeric structure exhibit similar dI /dV -dips but shifted to considerably more positive bias voltages. This shows that the local environment inuences the threshold for charging of Q-type molecules. In the dimer shown in Fig. 5b, the Q-type molecules do not experience an equivalent environment, and consequently exhibit a distinctly dierent charging threshold. Windmill structures of four coupled molecules (see Fig. 5d) exhibit an even more pronounced shift toward positive voltages, such that their charging threshold is close to zero voltage, as can be seen from voltage-dependent imaging in Fig. 5(e - h). Corresponding dI /dV -spectra are provided as Supporting Information. We note that equivalent structures, that is, those that can be transformed into each other by symmetry operations show spectroscopic features that are very similar. The data presented above shows that the charging threshold critically depends on the environment of a given Q-type molecule. Hence, it can be expected that charging one molecule is going to aect neighboring molecules. 46,47,5154 Such mutual interaction of neighboring cells 11,55 is a most important prerequisite of cellular automata and will be demonstrated for a structure consisting of three activated molecules, as shown in Fig. 6. The two panels in Fig. 6c show spectral maps measured along the lines indicated in the topographic images (Fig. 6b). Instead of single curved black lines, as seen for isolated Q-type molecules and corresponding to the charging, a very puzzling pattern emerges, suggesting a much more complex switching behavior. In particular, the dark and bright features suggest that a given molecule, once charged, can be discharged due to the electric eld from a neighboring molecule, and be recharged again at more negative voltages. The dierent progression of the curved dark and bright features indicate that they originate from charge transitions in dierent neighboring molecules. This suggests that the coupled Q-type molecules represent a few-quantum-dot system. 5658 Research has made great strides towards achieving well organized and even intricate molecular assemblies on a monolayer basis, however there is still ample room for improvement 10


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Figure 6: (a) Schematic representation and (b) constant current STM image of a trimeric structure. Imaging parameters I = 0.9 nA, V = 0.2 V. Scale bar 1 nm. (c) Two sets of dI /dV spectra acquired along two directions indicated in the STM images. In contrast to the situation of isolated Q-type molecules (cf. Fig. 2c), here, complex patterns of dark and bright features in the dI /dV spectra can be observed. The dark and bright features suggest that a given molecule, once charged, can be discharged due to the electric eld from a neighboring molecule and recharged again at more negative voltages. The dierent progression of the curved dark and bright features indicate that they originate from charge transitions in dierent neighboring molecules. The tunneling gap was set with I = 0.9 nA, V = 0.2 V before the feedback was turned o. The greyscale is in arbitrary units, where the yellow line marks zero.

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on how to add functions to them. Up to now, steps in this direction prominently emerge from studies addressing the spin states of the molecular entities within such assemblies, hence lying in the eld of magnetism. 59,60 In this context, the actual study gives another striking example of how complex functional properties can be found and explored in extended monolayer assemblies, even when they at rst sight may look like a totally homogeneous and non-functional structure. We note that, besides the prospect of mutual coupling of molecules to functional structures as described here, this system also allows for a precise patterning of a surface with local charges since in absence of a gating tip, A and Q-type molecules are in dierent charge states. In the context of cellular automata, it will be desirable to electronically decouple the cells from any substrate, which would enable to stabilize out-of-equilibrium charge states. 61,62 We therefore envision the application of the concept proposed here for self-assembled molecular layers in an insulating environment. 61,62 In addition, in such structures a much stronger electronic coupling between adjacent cells can be expected due to reduced screening. 53,54 In conclusion, we addressed two critical issues in the realization of molecular quantum cellular automata, namely the simultaneous exact alignment and spacing of neighboring cells, on the one hand, and the programmability of automata, on the other. This is achieved by the combination of self-assembly as a rst step and the selective and controlled activation of molecules as a second. Whereas here the activation was achieved by vacancy formation, in the future, other mechanisms such as the selective removal of ligands are envisioned. We demonstrate that structures consisting of only few cells already show rich switching behavior.

Methods Our experiments were performed with a custom-built combined scanning tunneling and frequency-modulation atomic force microscope (STM/AFM) operating at ultrahigh vacuum of < 5 × 10−11 mbar and a base temperature of about 5 K. The combined STM/AFM system

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is equipped with a qPlus 63 sensor that allows to measure the current and frequency shift (∆f ) simultaneously. A monocrystalline Ag(111) substrate was cleaned by repetitive cycles of annealing and sputtering at temperatures of (650 ± 25) ◦ C. Molecules were deposited from the gas phase onto the cold sample, followed by annealing to room temperature. Vacancy creation was performed by positioning the tip close to the center of the molecule to be removed (set point parameters: 2 nA, 160 mV), switching o the feedback loop, and lowering the tip until a contact was observed as a sudden increase in current versus distance as well as a sudden change of ∆f . At a bias voltage of 30 mV the typical tip approach was 2.9 - 3.3 Å from the set point. The molecules extracted in this manner were dropped o to the sample surface outside the area of interest. The eciency of the manipulation procedure depends on the lateral tip position with respect to molecule and the tip apex geometry. In total more than 250 molecules were extracted with a success rate larger than 90 %. All calculations were carried out with the VASP 64 code by solving the one-electron KohnSham equation within the generalized gradient approximation proposed by Perdew, Burke, and Ernzerhof (PBE) 65 to treat electronic exchange and correlation. A plane wave basis set and the projected augmented wave (PAW) method 66 with an energy cut-o of 400 eV was used. To improve the description of dispersion forces two dierent schemes were used, the one proposed by Tkatchenko and Scheer 67 and the non-local correlation functional optB86bvdW introduced by Klime² et al. 68 Since both methods yield very similar adlayer structures, indistinguishable STM images and similar qualitative electronic structure, the presented results are the ones obtained with the optB86b-vdW scheme. The surface was represented by a three-layer slab and in all calculations we have allowed the relaxation of the substrate atoms in the top-most metal layer as well as all the atoms of the adsorbates. We have assumed that adsorption takes place on the perfect Ag(111) surface. The supercell corresponds to a

12 2 0

8

!

unit cell with respect to unit cell of the silver (111) plane. We used 8 and 7

molecules per cell to describe the full monolayer and a monolayer where a single molecule was removed to mimick a monolayer with a vacancy, respectively. Geometry relaxations were 13


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carried out for a single k-point (Γ). STM topographic images have been simulated applying the Terso and Hamann theory 69,70 using the method described by Bocquet et al. 71

Supporting Information Available Experimental data for two dierent but equivalent quadrumeric structures is available as Supporting Information showing that the complex spectroscopic features are ngerprints of the specic combination of interacting cells.

Acknowledgement The authors thank Ferdinand Evers for discussions and Hermann Angerer for help. Financial support from the Deutsche Forschungsgemeinschaft (GRK 1570), the European Commission (EC) FP7 ITN MOLESCO (Project No. 606728), by MINECO (MAT2015-66888-C3-2-R), and FEDER funds are gratefully acknowledged. We acknowledge computer time provided by the CCT-Rosario Computational Center and the Computational Simulation Center (CSC) for Technological Applications, members of the High Performance Computing National System (SNCAD, MincyT-Argentina).

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Table-Of-Contents Figure

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Implementing Functionality in Molecular Self-Assembled Monolayers

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