A versatile and powerful method to create nanostructures on surfaces is the self-assembly of atoms and molecules. Here, the physical and chemical properties of the substrate and the adsorbate are the key that can be tuned to create nanostructures that fit specific needs . A research focus over the recent years has been the self-assembly of molecules or atoms into one-dimensional (1D) linear aggregates. They allow for electron transport along the long axes of the 1D aggregates, while a confinement effect is present along their short axes. Hence, they are considered as building blocks for new generations of devices for computing, photovoltaics, thermoelectrics, and energy storage . Furthermore, one-dimensional aggregates are also used for gas sensing and carbon-capturing materials .

A group of surfaces that is very appealing for the growth of 1D structures are vicinal surfaces because the step edges break the rotational symmetry of the surface further and add a periodic 1D grating, which can be used to direct the growth of nanostructures of adsorbates. The adsorption at step edges, as opposed to that on the terraces in between, is often favored because the additional interactions between the adsorbate and the atoms of the step edge contribute to the adsorption energy Eads. This step decoration can be exploited to grow 1D chain-like structures of adsorbates that otherwise tend to form two-dimensional (2D) domains on flat terraces. Examples are atoms forming 1D metallic chains and organic molecules forming 1D chains at step edges of vicinal metal surfaces at a low coverage, for example, PTCDA on Au(433) and Au(778) , a 1:1 mixture of PTCDI and 1,4-bis(2,4-diamino-1,3,5-triazine)benzene on Au(11 11 12) , and nickel-tetraphenyl-porphyrin on Au (788) .

Here, we report on the growth of an organic molecule, namely 5,12-dihydroquino[2,3-b]acridine-7,14-dione (quinacridone, QA), which forms 1D chains on flat metal surfaces already per se, on a vicinal Ag(100) surface. We describe how the presence of the step edges influences the azimuthal orientations of the chains. QA is a good candidate for such experiments because strong intermolecular hydrogen bonds (H-bonds) support the growth of long 1D chains of parallel oriented QA molecules, both in bulk crystals and on surfaces.

Figure 1: (a) Models of the five observed azimuthal QA chain orientations, A–E, relative to the unidirectional step edges of the Ag(35 1 1) surface. The four orientations A–D are identical to those that were already observed on the nominally flat Ag(100) surface . The chain directions A and C, as well as B and D, exhibit equivalent adsorption sites because of a mirror symmetry. Note that the adsorbed molecules in A(C) and B(D) are of the same chirality R(L). For the chain orientation E there exists the energetically equivalent chain with molecules of the opposite chirality. The color code of the atoms is: black = carbon, gray = hydrogen, blue = nitrogen, and red = oxygen. (b) Structural formula and hard-sphere model with dimensions of QA. The nominal van der Waals radii used for the molecular models were taken from .

In this work, we report the growth of QA on the vicinal Ag(35 1 1) surface. The central question of this work was whether the unidirectional step edges influence the azimuthal orientations of the QA chains with respect to those that were observed on the nominally flat Ag(100) surface . We found that the azimuthal orientations are identical to those on Ag(100), but, at elevated temperatures, the periodic step edges lead to a preferential growth of two distinct orientations out of four. Furthermore, we observed the growth of one, hitherto not described, azimuthal chain orientation parallel to the Ag step edges.

The experiments were conducted in an ultrahigh vacuum chamber with a base pressure of 2 × 10−10 mbar equipped with a beetle-type scanning tunneling microscope (STM, type UHV 300) from RHK Technology, a microchannel plate low-energy diffraction (MCP-LEED) instrument from OCI Vacuum Microenginneering Inc., and a quadrupole mass spectrometer (QMS) of the type PRISMA from Pfeiffer Vacuum. The vicinal Ag(35 1 1) crystal was obtained from MaTecK and cleaned by repeated cycles of sputtering and subsequent annealing at 700 K for 45 min. We tested whether a variation of the annealing temperatures between 500 and 800 K has an influence on the step distribution of the surface. However, no significant influence on the step distribution was observed. The purified QA was evaporated from a custom-built evaporator at a crucible temperature of 720 K, while the sample was held at different temperatures between 300 and 500 K in order to test the role of the growth temperature. The deposition process was monitored by the QMS, and the integrated QMS signal was used to calculate the QA coverage θQA. It is given in numbers of monolayers (ML) of the α-phase, as explained in detail in . A more detailed description of the experimental procedures can be found in .

The LEED measurements were performed at 300 K with beam currents below 10 nA and electron energies (E) between 30 and 200 eV. Note that the reported diffraction patterns are distorted due to the MCP geometry. All STM images were recorded at room temperature. As a tip, we used a self-cut Pt/Ir (90:10) wire. The bias voltage (UBias) refers to the sample, and the tunneling current (I) was at a constant value in the range of 10–50 pA. We usually adjusted the scanning plane parallel to the (35 1 1) plane of our sample in order to optimize the contrast of the images. The images were processed with the program SPIP , which included filtering out noise and an adjustment of the contrast. Additionally, to some images, a Prewitt or Roberts filter for edge enhancement was applied in order to enhance the visibility of the step edges.

The structure models were drawn with the free software Graphics Layout Engine . The shown hard-sphere models of QA use bond lengths and van der Waals radii from the literature.

Figure 2: QA on the Ag(35 1 1) surface deposited at a sample temperature of 300 K. (a) STM image (UBias = −1.55 V, I = 40 pA) showing many parallel QA chains with the two azimuthal orientations A and D. The left-hand side of a step edge corresponds to its upper terrace and the right-hand side to its lower terrace. The red and blue circles highlight cases in which there is a break in a chain at a step edge and where the chains appears to grow seamlessly over the step edge, respectively. The inset shows an image at a different sample position of three QA chains at a Ag step edge, which was chosen for the high molecular resolution. (b) Small-scale STM image (UBias = 1.5 V, I = 25 pA) showing two of the four azimuthal orientations A and D, as well as two chains in the new orientation E (red lines). The black dashed lines indicate the positions of the step edges. (c) Models of chain growth at Ag step edges. These illustrate that the chains that nucleated on the lower terrace may statistically end in two positions with respect to the step edge. The left-hand side shows the case where the gap between the molecules on the upper and lower terrace is too large to allow for the formation of H-bonds. The right-hand side shows the case where the chain is one Ag lattice constant closer to the step edge, which allows for the formation of H-bonds across the step.

Figure 3: (a) LEED image (E = 33 eV, T = 300 K) of the region around the specular spot of 0.65 ML QA on Ag(35 1 1) after deposition at a sample temperature of 300 K. Note that the sample was rotated with respect to the normal incidence geometry. The diffraction pattern is very similar to the one of the QA α-phase on Ag(100) but contains additional spots that correspond to the new chain orientation E. The black dashed lines indicate the five azimuthal chain orientations. (b) Schematic drawing of the LEED pattern in panel (a). It shows the observed spots and the unit cell vectors of orientations A and E. (c) Same as (a) but for θQA = 0.65 ML and a deposition at 400 K. The diffraction pattern is very similar to the one in panel (a). However, it is missing two sets of spots corresponding to orientations B and D. (d) Schematic drawing of the observed LEED pattern in panel (c) containing all the observed spots and the corresponding unit cells. Both LEED images were measured under an in-phase condition for the specular spot with respect to monoatomic Ag steps . This causes that the spots are sharp and not broadened because of the Ag steps. The scale bars for the LEED images are only approximately valid because of the distortion caused by the MCP geometry.

The formation of similar one-dimensional structures parallel to the step edges of a vicinal crystal has been observed before, for example, PTCDA on Au(433) and Au(778) , a 1:1 mixture of PTCDI and 1,4-bis(2,4-diamino-1,3,5-triazine)benzene on Au(11 11 12) , α-6T on Ag(441) , and nickel tetraphenylporphyrin on Au (788) .

Figure 4: (a) STM image (UBias = −1.5 V, I = 25 pA) of QA on the Ag(35 1 1) surface deposited at a sample temperature of 400 K. It shows an area with several step edges running vertically through the image, which are highlighted by the dashed black lines. (b) The same STM image as in panel (a) with a Prewitt filter for better visibility of the QA structures. The image shows domains of parallel QA chains with the three orientations A, C, and E. (c) Zoom-in onto the STM image in panel (b) showing the QA chains at step edges. (d) Structure model of chains in orientation C at a step corresponding to panel (c). The black dashed lines mark the Ag step edges and the red circles highlight small circular protrusions that are assigned to small Ag clusters.

Last, we discuss the structure of QA after deposition at 500 K. From our previous work regarding QA on Ag(100), we know that the elevated temperature induces an irreversible phase transition into a heterochiral and commensurate structure, the so-called β-phase . This phase is also obtained after deposition at 500 K and subsequent cooling to 300 K. This preparation was chosen here. The β-phase also consists of parallel molecular chains of QA dimers as the α-phase, but with periodic indents along the chains. The molecules in the domains of the β-phase exhibit the same azimuthal orientations as those in the α-phase. We find that the β-phase also forms on Ag(35 1 1), and the domains of the β-phase on Ag(35 1 1) will also be labeled A–D.

Figure 5: (a) LEED image (E = 33 eV, T = 300 K) of the β-phase of 0.65 ML QA on Ag(35 1 1) after deposition at a sample temperature of 500 K and subsequent cooling to 300 K. The diffraction pattern is very similar to the one of the QA β-phase on Ag(100) but exhibits a clear anisotropy. Roughly half of the spots are more intense than the other half. This LEED image was measured under an in-phase condition for the specular spot with respect to monoatomic Ag steps. (b) Simulated LEED pattern of the β-phase. The spots that are thicker and green correspond to the more intense spots in the diffraction pattern, which correspond to orientations A and C. The step direction is in the [10] direction, as before.

All spots of the LEED pattern are very sharp. Hence, the corresponding domains are significantly larger than the average terrace width of 50 Å. A coherent continuation of the domains across the Ag step is very unlikely. As stated above, the terrace width distribution of the Ag(35 1 1) surface is rather broad with terrace widths of up to 300 Å. Thus, we conclude that the β-phase predominantly forms on those large terraces. This is also supported by the STM results, which show that large terraces were completely covered by one of the four β-phase domains (not shown). Furthermore, in areas of the sample where many step edges are very close to each other and the terraces are very small (below 50 Å), no QA molecules were observed on the terraces. This indicates that the molecules diffused off the small terraces onto larger ones because the space on the small terraces is not sufficient to form the β-phase. One may also ask whether this preference of the β-phase for large terraces leads to a step bunching on the substrate surface in order to increase the size of the terraces. However, we could not find any evidence that the terrace width distribution changed significantly after the preparation of the β-phase.

As already postulated above, we assume that, upon adsorption at the step edges, the QA molecules replace Ag atoms in order to maximize their adsorption energy. The nuclei of orientations A and C are more stable; thus, the growth of the corresponding domains is favored compared to that of the orientations B and D. Nevertheless, the orientations B and D are still present on the surface to some minor extent, which can possibly be explained by domains that nucleate and grow on large terraces and not at steps.

As stated above, at 300 K, the QA chains are capable of growing continuously across Ag step edges. This brings forth some interesting aspects, which we discuss here. A chain across step edges without a significant break and intact intermolecular H-bonds requires some structural flexibility in the molecules and/or the intermolecular bonds. We suppose that QA molecules cannot bend in the plane of the π systems because of the annulated carbon rings. In contrast, H-bonds are mainly of electrostatic nature and, thus, have a pronounced flexibility in bond lengths and angles, which supports intermolecular bonds across step edges. Similar molecular chains that are connected by H-bonds and grow across monoatomic step edges without a break were observed before for NDCA on a vicinal Ag(110) surface by Schnadt et al. . They found that the slight distortion of the chains at step edges leads only to a minor energy loss (below 0.1 eV), which is energetically still more favorable than a scenario in which no H-bonds are formed across the step edges. We propose that a similar situation is present here.

Another interesting question concerns the angle between the QA chains and the step edges. Schnadt et al. found that the self-assembly across step edges crucially depends on the azimuthal orientations of the chains relative to the step edges . The chains of NDCA grow along the [10] direction of the substrate and are, thus, orthogonal to the direction of the step edges, which are oriented along the [01] direction. Density functional theory calculations have shown that the NDCA chains do not cross the step edges if they are rotated away from the close-packed rows . In contrast, in our case, no chains grow across the step edges at a 90° angle. The QA chains continuously grow across the step edges in the four azimuthal orientations A–D, resulting in angles of 32° and 58° between the chains and the Ag step edges. Hence, such a strict selection of the azimuthal angle of the chains, similar to the one found by Schnadt et al., does not apply to QA.

Last, we discuss some similarities and differences between our results and some that were published previously. One similarity to other molecular systems is the growth of the QA chains with orientation E, which grow along the Ag step edges. Such a behavior was found for a few other organic molecules on different vicinal metal surfaces . This shows that the contribution of the interactions between the atoms of the step edges and the organic molecules to the adsorption energy of the molecules is a common and important motif for the adsorption and self-assembly of organic molecules on stepped surfaces. We note, however, that the orientations of the QA molecules with respect to the steps at 300 K are unusual. Many examples from the literature that report on the decoration of steps by organic molecules found that the molecules adsorb in a flat-lying configuration with their long axis parallel to the step edges . In some cases, an adsorption of upright standing molecules at steps was also found . Here, the QA molecules are either oriented with their short edge parallel to the steps or at an angle of 32° or 58° with respect to the step direction. A stark difference to the examples described in the above publications is that, here, the Ag step edges also cause the preferential growth of QA chains or commensurate domains with distinct step-selected orientations at elevated temperatures. A similar phenomenon has, to the best of our knowledge, not been reported, so far.

Another phenomenon that was reported by Schmitt et al. for PTCDA on vicinal Ag(100) surfaces is the faceting of the Ag substrate surface that is induced by the adsorption of the molecules. They found that the adsorption of PTCDA leads to the formation of Ag facets, which are usually not stable on the bare surface and are induced by the adsorption of PTCDA. However, within the scope of the present work, we did not observe any faceting of the Ag(35 1 1) surface. This is because the adsorption energy of QA is likely significantly smaller than the one of PTCDA on Ag surfaces.

We found that, at room temperature (RT), the growth of QA on the vicinal Ag(35 1 1) surface is, in general, very similar to its growth on the nominally flat Ag(100) surface. QA forms molecular homochiral chains with four distinct symmetry-equivalent orientations that are connected via H-bonds (α-phase). At RT, the growth of the chains begins on the Ag terraces, and the unidirectional step edges do not have any influence on the azimuthal orientations of the chains. Furthermore, the QA chains grow across the steps. We also found one azimuthal chain direction that does not exist on Ag(100) and is parallel to the unidirectional step edges of the vicinal surface. This is explained by the fact that the growth in this direction is additionally stabilized by the dispersion interactions between the QA molecules and the atoms of the Ag step edges, which overcompensates the lower site energy of the molecules on the terraces in this orientation.

After deposition at 400 K, only two of the above four azimuthal orientations are observed. Furthermore, the deposition at 500 K leads to heterochiral chains in a commensurate structure (β-phase) that also exhibits a strong anisotropy, that is, two domains are favored over the other two. In both situations, the preferential growth of specific orientations at 400 and 500 K is caused by a growth start of the QA structures at the Ag step edges at elevated temperatures. Thus, the step edges have a strong influence on the azimuthal orientations of the QA structures because adsorption sites at the Ag step edges of molecules in two distinct azimuthal orientations out of four are favored.

Overall, the azimuthal orientations of one-dimensional QA chains can be influenced by step edges on vicinal surfaces. Our work is of importance for the general understanding of the interactions between organic molecules and metallic step edges and may be helpful for the preparation of one-dimensional organic structures with a global preferential orientation on surfaces.