Growth of pentacene molecules on Tsai-type quasicrystals and related crystal surfaces

Quasicrystals are physical systems that possess long-range order but without translational symmetry. First found in a metastable binary metallic alloy,¹ quasicrystallinity has been found in a large variety of systems, including dendritic liquid crystals,² ABC-star polymers,³ binary nanoparticle superlattices,⁴ colloids,⁵ mesoporous silica,⁶ and oxide thin films.⁷ 

When stable quasicrystals were discovered and produced in large enough samples,^8,9 surface science studies of these materials became possible. The initial goal was to produce and characterize clean quasicrystal surfaces. Experience and know-how accumulated in the surface science community over several decades was focused on this problem, and in a short period of time, the surfaces of Al-based quasicrystals were well-studied and generally understood to be unreconstructed terminations of the bulk structure.^10–13 Initially, the most investigated surfaces were the fivefold surfaces of icosahedral Al–Pd–Mn and Al–Cu–Fe, and the tenfold surface of Al–Ni–Co.¹³ 

After the discovery of the Cd–Yb binary quasicrystal and its structural equivalent i–Ag–In–Yb,¹⁴ the surfaces of the latter began to receive attention.^15–17 The i–Ag–In–Yb quasicrystal consists of a quasiperiodic arrangement of “Tsai” type clusters, a hierarchical system of atomic shells with icosahedral symmetry that are joined by rhombohedral “glue” units.¹⁴ The quasicrystal structure can then be described in terms of these 3D clusters or, equivalently, in terms of the 2D planes of atoms, which are formed along the high symmetry directions of the icosahedral structure (twofold, threefold, and fivefold).

Once excellent clean surfaces of the quasicrystals described above could be prepared and characterized, the surface science focus turned to adsorption studies and to the structure of the overlayers and thin films which resulted.^12,18,19 A wide range of metallic and nonmetallic elemental adsorbates were employed. This was in large part curiosity driven: Would thin films adopt the quasicrystalline order of the substrate or order as their bulk structure? This question has been answered, in part: several elements do adopt the structure of the substrate, at least in the first layer; indeed, for some elements the quasicrystalline architecture persists beyond a single layer.²⁰ 

Interest in molecular ordering on quasicrystals surfaces was also present from an early stage. Initial efforts in our group were centered on the use of C$60$ as an adsorbing molecule, partly, because of the widespread general interest at that time in fullerenes and carbon nanotubes but also because of the ease with which it is imaged in scanning tunneling microscopy (STM). Indeed under favorable conditions, it is possible to image the internal molecule yielding information on its bonding orientation. Our study of C$60$ adsorption on the fivefold surface of i–Al-Pd-Mn was published in 2001;²¹ this was followed by a paper on C$60$ adsorption on the tenfold surface of Al–Ni–Co.²² Both papers indicated some local ordering of C$60$⁠; however, long-range ordering of C$60$ enhanced by thermal activation of the adsorbed molecules and was not achieved until later.^23,24

Attention also turned to adsorption of molecules with other symmetries. Fivefold symmetric corannulene (C$20$H$10$⁠) was found to form a quasiperiodic overlayer with a decagonal motif on the fivefold surfaces of i–Ag–In–Yb,²⁵ and most recently, sixfold symmetric coronene (C$24$H$12$⁠) was observed to form an ordered quasiperiodic structure on the fivefold surface of i–Ag–In–Yb. However, in this paper, we restrict our focus to a molecule with simpler symmetry: pentacene (Pn, C$22$H$14$⁠).

Pentacene is a flat polycyclic aromatic hydrocarbon molecule consisting of five fused benzene rings; it is an organic semiconductor. We concentrate on the adsorption of this linear, easily-imaged molecule on three high symmetry surfaces of the Tsai-type quasicrystal Ag–In–Yb and on the related Au–Al–Tb 1/1 approximant. We review previously published work on the adsorption of Pn on the fivefold and twofold surfaces of i–Ag–In–Yb,^23,26 and on the (111) surface of the related Au–Al–Tb 1/1 approximant, and present new results on Pn adsorption on the threefold surface of i–Ag–In–Yb. We compare and contrast the adsorption behavior on all four of these surfaces and discuss what we have learned about the reactivity of these surfaces and their use as templates for ordered adsorption.

All the work presented in this paper was carried out under ultrahigh vacuum conditions. As mentioned in the Introduction, we present results of the Pn growth on four different surfaces: the twofold, threefold, and fivefold surfaces of the i–Ag–In–Yb quasicrystal, and the (111) surface of Au–Al–Tb approximant. The surfaces of all four systems were prepared under repeated cycles of sputtering and annealing. The details of the surface preparation can be found in earlier publications.^15–17,27 The Pn molecules were evaporated from homemade sources, consisting of a Pyrex tube wrapped with a Ta filament. Substrates were kept at room temperature during deposition. The surfaces were characterized by an Omicron RT-STM 1 and an Omicron VT-STM system.

The Tsai-type cluster, the aforementioned basic building block of i–Ag–In–Yb, consists of five successive shells, shown in Fig. 1.¹⁴ The innermost shell has Ag/In atoms forming a tetrahedron (gray). The second shell is a dodecahedron of 20 Ag/In atoms (yellow). Twelve Yb atoms in the third shell make an icosahedron (green). The fourth shell is an icosidodecahedron consisting of 30 Ag/In atoms (blue). Finally, the outermost shell is a rhombic triacontahedron with 92 Ag/In atoms located on the vertices and midedges (red). The Au–Al–Tb 1/1 approximant is a periodic crystal that is formed by the same Tsai clusters, located at a body center cubic lattice with lattice parameter 1.475 81 nm.²⁷ Here, Yb is replaced by Tb and Ag/In by Au/Al.

The surfaces of the i–Ag–In–Yb quasicrystal can be prepared by sputtering and annealing to form atomically flat terraces. The surface structure can be explained by bulk planes terminated at the center of the Tsai clusters. This is true for all three high symmetry surfaces: two-, three-, and fivefold surfaces of i–Ag–In–Yb quasicrystal,^15–17 where each of the surface planes contain Yb atoms. The 1/1 Au–Al–Tb (111) surface also terminates at bulk planes containing Tb atoms.²⁷ As we will show, the Yb and Tb atoms provide active sites for adsorption of Pn molecules.

Figure 2(a) shows an STM image taken from the fivefold surface after the deposition of Pn.²⁸ The rod-features represent single Pn molecules. The molecules are aligned along the high-symmetry directions of the substrate such that pentagonal arrangements of molecules are observed. The most common features seen are highlighted in Fig. 2(b).²³ 

The autocorrelation function [Fig. 2(c)] and Fourier transform (not shown) of the distribution of Pn molecules show maxima distributed in tenfold symmetry and located at $τ$–scaling distances (where $τ$ is the golden mean), indicating quasicrystalline order of the molecules.

To discuss the adsorption sites of the molecules, we show the model structure of the fivefold surface in Fig. 2(d). The surface is formed at bulk planes that bisect the RTH cluster such that the cross-section of the fourth shell appears as a Ag/In decagon formed at the surface [blue ring in Fig. 2(d)]. The Ag/In decagon is surrounded by five Yb pentagons (green atoms.) All features observed by STM can be explained if terminal benzene rings of the Pn bond with two Yb atoms. The distance between the Yb atoms is 1.14 nm, which is close to the distance between outermost C-atoms along the longer axis of the molecule (1.22 nm).

The Pn molecule consists of five benzene rings fused along C–C bonds. However, Pn molecules appear as four-lobed features [Fig. 2(b)]. This can be explained if there is some electronic disruption of the molecules induced by the substrate.²³ 

Figure 3(a) shows an STM image from the threefold Ag–In–Yb surface after deposition of $∼$ monolayer (ML) of Pn. Most of the Pn molecules are aligned along the three high symmetry directions of the substrate [indicated by white arrows in Fig. 3(a)], which are 120$°$ apart. A few molecules have orientations in between the high symmetry directions, 30$°$ off from the high symmetry directions [marked by green rectangles in Fig. 3(a)]. A magnified image is shown in Fig. 3(b). Two types of features are predominantly observed: triangles and rods. The size of the rod is $∼1.0$–1.4 nm, which is close to the size of a Pn molecule (H–H distance along longer axis of the molecule, 1.38 nm), indicating that the rods correspond to single Pn molecules.

The threefold surface is comparatively rough compared to the other high symmetry surfaces, characterized by a low atomic density at the surface plane.²⁹ As such, isolating adsorbate and substrate contributions can be difficult. However, we conclude that the triangular features are formed by Pn molecules due to the following reasons: the triangular features have identical sizes and orientations; the orientation of the triangles is the same as the isolated Pn molecules; the edge length of the triangles is 1.2–1.3 nm, and the triangles are observed at $∼0.35$ nm height from the substrate (darker area of STM image). Triangles of this height, size, and singular orientation were not observed on the clean surface.¹⁶ Likewise, the edge length of the triangles is close to the size of Pn molecules.

Figures 3(c) and 3(d) show the autocorrelation function and fast Fourier transform of Pn molecule positions extracted from the substrate. The maxima in both appear at $τ$–scaling distances, confirming quasicrystalline ordering of the molecules.

Both the triangular and rod features can be explained if the two terminal benzene rings of Pn adsorb between Yb atoms, similar to the fivefold surface. The distance between the two Yb atoms is 0.97 nm, which is close to the center-to-center distance of the two terminal benzene rings, 0.98 nm. Figure 3(e) shows a model schematic of the adsorption scheme. This model produces the triangular features of identical size and orientation as observed by STM. The distance between adjacent triangles in the model is 2.53 nm, which is also consistent with the value observed by STM (⁠$2.52±0.02$ nm). Furthermore, we occasionally observe two parallel molecules separated by $0.96±0.04$ nm, which is also reproduced in the model with a separation of 0.84 nm apart [features marked by circles in Figs. 3(a), 3(b), and 3(e)].

The Pn molecules aligned in-between the high symmetry directions of the substrate can be modeled considering different adsorption sites. In this case, a Pn molecule is pinned between a Yb-triangle; a terminal benzene ring is adsorbed on top of an atom of the Yb-triangle, and two hydrogen atoms are attached to the other two Yb atoms of the triangle [see the magnified view in Fig. 3(f)]. The Yb–Yb distance is 0.60 nm, which is close to the H–H distance, 0.47 nm.

The twofold surface of Ag–In–Yb is atypical with respect to the other high symmetry surfaces, in that each shell of the bisected Tsai clusters at the surface “donate” atoms to the surface structure: at the threefold and fivefold surfaces, only the third and fourth shells contribute. As a consequence, the twofold surface structure is comparatively dense and, therefore, chemically more complex, as each Ag/In shell has a certain amount of chemical disorder. However, as we will show, this additional complexity does not prevent the formation of a well-ordered, quasiperiodic Pn film.²⁶ 

Figure 4(a) shows an STM image taken of the twofold surface after deposition of approximately 0.37 ML of Pn. The high-symmetry directions of the surface are shown at the bottom right. Individual molecules are found to align their length along three directions with respect to the horizontal axis (which is the shorter of the 2 twofold axes of the surface): $57.0±0.4°$⁠, $122.1±0.4°$⁠, and $90.2±0.5°$⁠, corresponding to the 2 fivefold and remaining twofold direction, respectively. The majority of molecules are aligned along either of the fivefold directions [Fig. 1(b) of Ref. 26].

Analysis in Fourier space confirms the long-range order of the Pn molecules: Fig. 4(b) shows an FFT taken after isolating the molecular signal from Fig. 4(a) via a flooding algorithm.³⁰ High intensity spots are highlighted by white or green circles, which are aligned along the twofold and fivefold directions of Fourier space, respectively, as indicated by the axes in the bottom left of the figure. The separation of the white spots from the origin along the horizontal and vertical directions, calculated in real-space, are $1.31±0.02$ nm and $2.13±0.03$ nm, respectively. These values correspond to a rectangular rowlike structure, which is indicative of the underlying substrate.^17,31 Likewise, the real-space separation of the green dots along each of the fivefold symmetric axes in Fig. 4(b) is $0.71±0.03$ nm and $1.16±0.02$ nm. These values correspond to the perpendicular separations of molecular rows aligned along the fivefold axes of the surface, with an example indicated in Fig. 4(c). Here, blue schematics of Pn molecules highlight rows that are aligned along the fivefold axis, marked by black lines. The perpendicular separations of the lines measured from the green spots of the FFT are shown as S and L, where L/S = $τ$⁠. These rows follow sections of the Fibonacci sequence as expected from the quasicrystalline nature of the FFT and underlying substrate.

The potential adsorption sites of the Pn molecules were considered in terms of geometry and chemistry, with a similar conclusion to the threefold and fivefold surfaces: that Yb atoms are responsible for the Pn structure. Geometrically, to grow a quasicrystalline layer, the adsorption sites must have a distinct arrangement. Considering this, it is trivial to state that the distribution of Yb atoms among the surface plane is sparser than the Ag/In atoms. Furthermore, there are Yb–Yb separations in the model that are similar to the distances between the internal benzene rings of the Pn molecule. Chemically, Pn is an electron donor³² and, it has been shown that the unoccupied states of the twofold surface are dominated by Yb atoms.³¹ 

Figure 4(d) shows a model schematic of the proposed adsorption scheme. Green circles indicate Yb atoms, and red circles indicate fifth shell atoms. The Yb–Yb atom separation closest to the length of a Pn molecule (⁠$∼1$ nm) along the fivefold direction is 1.27 nm, with an example highlighted along one of the fivefold axes so that the outer ends of the molecule sit directly on top each atom, with some small offset. Indeed, the separation of the rectangular rowlike structure of the Pn molecules along the twofold axes can be replicated considering the central position between pairs of these atoms (i.e., where the center of the molecule resides)—as shown by the black rectangle marked in Fig. 4(d). These separations, 1.26 nm and 2.05 nm, are good matches to the experimental data.

Similarly, molecules aligned along the twofold axes adsorb atop two Yb atoms: the vertical molecules have two benzene rings sitting directly on top of two Yb atoms separated by 0.97 nm, while the horizontal molecules have two inner benzene rings sitting on Yb atoms separated by 0.6 nm. The increased density of fivefold aligned molecules is attributed to the local environment of the described adsorption sites: along the fivefold Yb–Yb separation, there are three fifth shell atoms, as marked, which forms a linear chain of five atoms. This chain is much less complex than some of the possible arrangements along the twofold axes. The assignment of these adsorption sites is given further weight by additional Pn coverage. At 0.6 ML, we see the construction of several molecular motifs that replicate certain Yb motifs in the surface model, including squarelike, diamond, and triangular designs [Figs. 3 and 4(b) of Ref. 26]. An example of the triangle motif is shown in Fig. 4(d) by the three Pn molecules.

Periodic Tsai-type approximant systems share the same basic Tsai cluster building block as their quasicrystalline cousins. While 2/1 approximants also exhibit the “glue” units of the Cd–Yb model, the 1/1 structure simply consists of a body-centered packing of the Tsai cluster unit.¹⁴ Despite the structural similarities, approximants are indeed separate systems and, therefore, exhibit individualistic behavior.

We have recently shown that the (111) surface of the Au–Al–Tb 1/1 approximant crystal (analogous to the threefold surface of i–Ag–In–Yb) forms a partial reconstruction, the first reported surface reconstruction of a Tsai-type material.²⁷ Here, the Au/Al atoms create rows while the Tb atoms follow the expected bulk structure: the result is a surface that is a combination of twofold (Au/Al) and threefold (Tb) symmetry. Figure 5(a) shows a model of the surface structure, where atoms are color-coded according to their shell: the first is gray, the second is yellow, the third is green, the fourth is blue, and the fifth is red. The row structure of the Au/Al atoms is evident, while Tb triangles are highlighted and enclosed by red, blue, and yellow triangles. The Tb atoms of the yellow triangles occupy the subsurface layer. Each of the Tb triangles can be considered as a basis decorating three, large, triangular sublattices. Likewise, a dashed rhombus indicates a smaller quasicell that can link each of the triangles together across the surface, with a nominal “lattice” parameter of $a=b=1.04$ nm. The juxtaposition of these two chemical symmetries makes for potentially intriguing adsorption studies, especially when considering any similarities or differences compared to other Tsai-type systems. Indeed, our deposition of Pn on the (111) surface shows adsorption behavior that is both commensurate with the studies discussed and dependent on the unique landscape afforded by the reconstruction.³³ 

Figure 5(b) shows an STM image of the surface after depositing $∼0.3$ ML of Pn with the substrate held at room temperature. The inset shows the autocorrelation function calculated after extracting the centers of the molecular positions. According to the autocorrelation function, the Pn molecules adsorb at positions that form a rhombohedral lattice, with parameters $a=1.26±0.08$ nm, $b=1.28±0.05$ nm. This is a match to the distribution of the Tb triangles across the surface, so that the Pn molecules appear to adsorb at Tb atomic sites—similar to Pn adsorption on i–Ag–In–Yb surfaces that show a strong rare-earth atom/molecule bond. Close analysis of the sites and orientations of the molecules indicates that there are a range of viable adsorption sites (Fig. 3 of Ref. 33). However, each site has at least one benzene ring sitting directly on the top of a Tb atom, or, at least one ring pinned between two Tb atoms (or both). Critically, certain sites and orientations appear more favorable than others. Molecules aligned along the [011] direction i.e., perpendicular to the Au/Al row direction, are observed more frequently: several examples are highlighted with black circles in Fig. 5(b). These vertical molecules were found to only occupy positions at either the red or blue Tb triangles in Fig. 5(a).

Higher Pn coverage (⁠$∼0.7$ ML) data at room temperature leads to a saturation of the viable adsorption sites, creating a film which exhibits structural order, yet rotational or orientational disorder [Fig. 5(a) of Ref. 33]. Figure 5(c) shows the effect of postannealing this film to 600 K. The majority of the Pn molecules are now oriented either vertically or horizontally i.e., perpendicular or parallel to the Au/Al row direction. The molecules are marked with colored arrows or bars to indicate their adsorption site: eight possible sites were identified. The majority of the molecules occupy red or blue “down” sites, which correspond to the vertical sites observed in Fig. 5(b). The remaining arrow or bar sites in Figs. 5(c) and 5(d) correspond to adsorption positions not previously observed, indicating that the postdeposition annealing promotes new adsorption sites. The adsorption scheme for each site is shown schematically with respect to the underlying substrate in Fig. 5(d), where green circles are Tb, and gray circles are Au/Al.

We analyzed the Pn adsorption not only from a geometric standpoint but from an electronic one. The STM signal observed for each molecule showed a double-lobe structure consistent with relatively poor molecular resolution of Pn on metal surfaces.^34–36 At a local level, the Pn molecules often exhibited a difference in contrast between the two lobes (Fig. 4 of Ref. 33). The occurrence of brighter lobes of the Pn molecules appears linked to the underlying Tb atom concentration, suggesting a higher flow of electrons at these locations.

We have presented the adsorption behavior of Pn molecules on four different surfaces: fivefold, threefold, and twofold surfaces of i–Ag–In–Yb quasicrystal and the (111) surface of the Au–Al–Tb 1/1 approximant. We find that Pn molecules preferentially adsorb on Yb (Tb) sites, reflecting the symmetry of the substrates. Autocorrelation and Fourier transform analysis of STM images confirm quasicrystalline order of the Pn molecules on the i–Ag–In–Yb surfaces.

We conclude that adsorption of Pn on selective sites is due to both geometrical and electronic factors. The Yb–Yb (Tb–Tb) distances on the substrate are comparable to the C–C or H–H separations in Pn. Likewise, as Pn is an electron donor and the unoccupied electronic states of the substrate are dominated by the rare earth atoms, Pn is most likely to bond to Yb (Tb). Adsorption of molecules on selective species of the alloy substrate is not uncommon. C$60$ molecules on the Al-based quasicrystal surfaces are found to adsorb preferably on Mn or Fe atoms, where electrons transfer from the adatom to C$60$⁠.^23,24

The authors wish to acknowledge the outstanding leadership of Pat Thiel in the field of quasicrystal surfaces. Many of our group (academics, postdocs, and Ph.D. students) benefited greatly from her inspiration, mentorship, and generosity. She is sorely missed by us; however, her legacy and her memory will live on in Liverpool. Amnah Alofi is grateful to Albaha University and Albaha, Saudi Arabia for funding for her Ph.D. study, that contributed to a part of this work.

The data that support the findings of this study are available from the corresponding author upon reasonable request.