Article pubs.acs.org/JPCC
Steering On-Surface Supramolecular Nanostructures by tert-Butyl Group Kai Sheng, Qiang Sun, Chi Zhang, and Qinggang Tan* College of Materials Science and Engineering, Key Laboratory for Advanced Civil Engineering Materials (Ministry of Education), Tongji University, Caoan Road 4800, Shanghai 201804, P. R. China S Supporting Information *
ABSTRACT: Molecular self-assembly is an efficient approach to fabricate supramolecular nanostructures on well-defined surfaces. The nanostructures can be regulated through functionalizing the molecular precursors with different functional groups. Here, from an interplay of high-resolution scanning tunneling microscopy imaging and density functional theory calculations, we have at the atomic scale investigated the influence of tert-butyl groups on the on-surface self-assembled behaviors of the organic molecules where intermolecular interactions mainly originate from relatively weak van der Waals interactions. Our results demonstrate that the tert-butyl groups can not only affect the adsorption geometry but also change the self-assembled properties of organic molecules on surfaces due to the enhanced intermolecular interactions.
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INTRODUCTION Supramolecular architectures from molecular self-assembly on solid surfaces have aroused significant attention since the selfassembled nanostructures can not only afford new functionality to the surface but also provide desired nanopatterns leading to a multitude of potential applications in biochemical sensing, chiral catalysis, organic electronics, etc.1−3 The formation of the nanostructures on well-defined solid surfaces depends significantly on a subtle balance between the molecule−molecule interactions (e.g., hydrogen bonding,4,5 van der Waals forces,6,7 and π−π interactions8,9) within the molecular adlayers and the molecule−substrate interactions.10−14 It should, however, be noted that one prerequisite for molecular self-assembly to occur is that the molecule can be anchored on the surface, which is determined by the molecule−substrate interactions.15 Furthermore, despite that the surfaces hold great promise in affecting the molecular adsorption geometries (the so-called templating effect), the specified functional groups can also affect the molecular adsorption and self-ordering properties.16 To date, main efforts have been devoted in this issue to functionalizing the molecular precursors by functional groups with relatively strong and directional characteristics, e.g., hydrogen donor/ acceptor and metal/ligand groups.17,18 The tert-butyl group, which was believed to yield only weak intermolecular interactions (van der Waals forces), has recently been found that it can not only regulate the diffusion behaviors but also control the molecule−substrate geometry.19 Thus, it is of great interest to explore the role that the tert-butyl groups within the organic molecule play in the formation of surface-supported supramolecular nanostructures and further regulate the subtle balance between intermolecular interactions and molecule− substrate interactions toward controllable fabrication of functional surfaces or nanostructures. © 2014 American Chemical Society
In this work, we have designed two aromatic molecules, di(tert-butyl)terphenyl and dinaphthalenyl−benzene, shortened as DTBT and DNYB, respectively (chemical structures shown in Scheme 1). The DTBT has similar molecular structure and Scheme 1. Chemical Structures of DNYB and DTBT Molecules
molecular weight as DNYB but with different functional groups (the tert-butyl-phenyl groups replacing the naphthyl groups). Both molecules are only expected to yield van der Waals interactions between the molecules.7,16,20 From an interplay of high-resolution scanning tunneling microscopy (STM) imaging and density functional theory (DFT) calculations, we have investigated the self-assembly of the two organic molecules on different metal surfaces (Cu(110) and Au(111)) under ultrahigh vacuum (UHV) conditions. The experimental and theoretical findings show that on Cu(110) surface , the DTBT molecules form two kinds of well-ordered superstructures coexisting on the surface, while the DNYB molecules form Received: November 15, 2013 Revised: January 24, 2014 Published: January 27, 2014 3088
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Figure 1. (a) STM image after deposition of DNYB molecules on Cu(110) at RT. The three colored circles overlaid on three protrusions present three DNYB molecules as indicated in the corresponding DFT calculated periodic structural model in panel b. (c) Adsorption geometry of a single DNYB molecule on Cu(110) extracted from structural models of panel b. The unit cell of the experimental and theoretical structure has been indicated by blue and yellow arrows, respectively (scanning parameters: It = 0.70 nA and Vt = −2500 mV).
the molecular adsorption geometry and intermolecular interactions. From the comparison of experimental STM image and the DFT calculations, we can identify one DNYB molecule as one bright protrusion as marked by colored circles and the structure has a unit cell with the side length a = 11.1 ± 1 Å, b = 23.0 ± 1 Å, and angle α = 75 ± 2°. From the corresponding structural models (cf. Figure 1b), we can see that the DNYB molecules adopt an antiparallel arrangement in the ribbon structure. Figure 1c shows the adsorption geometry of DNYB molecules on Cu(110) extracted from the periodic models of Figure 1b. From the top and side views,we can clearly see that one of its naphthyl groups lifts upward, whereas the other naphthyl group lies flat on the surface, which has resulted in the one-protrusion shape of the STM image of a single DNYB.29 In addition, all the molecules orientated in the same direction with the symmetry axis of the molecule aligns (marked by the blue dotted line) along [001] direction of the substrate as indicated in Figure 1c. To investigate the role that the tert-butyl groups play in the formation of self-assembled surface nanostructures, we employ the DTBT molecule in which the naphthyl moieties are replaced by the tert-butyl-phenyl groups in comparison with the DNYB molecule. As shown in Figure 2a, deposition of DTBT molecules on Cu(110) at RT leads to long-range ordered selfassembled nanostructures where two different structures (denoted as phase I and phase II, respectively) coexist. Close inspections reveal that phase I shows a ladder-shaped structure, whereas phase II shows a honeycomb network structure. After annealing the sample at about 370 K, phase I vanishes completely and phase II still exists on the surface (cf. Figure 2b), which indicates that the phase II is energetically more favorable than phase I. By combining the close-up STM images (Figure 2c,d) and the corresponding DFT calculated structural models (Figure 2e,f) of two self-assembled nanostructures, we can identify a single DNYB molecule as two bright protrusions (double-lobed features) in both phases as we have marked with the ovals in Figure 2. The bright protrusions are assigned to the tert-butyl moieties of the molecule, and the phenyl moiety cannot be resolved due to the high contrast of the bright protrusions. Figure S1, Supporting Information, shows the adsorption geometries of DTBT molecules on Cu(110) in the periodic models shown in Figure 2f. From the top view, we can see that the symmetry axis of the two molecules align ±7° offset
ribbon nanostructures. When we move to a more inert surface, Au(111), we found that the self-assembled structures formed by DTBT are not long-range ordered anymore, and several structural motifs can be observed. However, no ordered structure formed by DNYB can be found even at relatively low temperatures (about 100 K). The different self-assembled behaviors of DTBT and DNYB on the same surfaces are attributed to the different intermolecular interactions resulted from the functionalized tert-butyl groups. The results thus provide further insight into the understanding on how the tertbutyl groups could influence the molecular adsorption behaviors and moreover the molecular patterns on the surface, and also indicate that the tert-butyl groups could be potential candidates for delicately controlling self-assembled nanostructures on surfaces.
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EXPERIMENTAL SECTION The STM experiments were performed in a UHV chamber (base pressure 1 × 10−10 mbar) equipped with a variable temperature “Aarhus-type” scanning tunneling microscope purchased from SPECS,21,22 a molecular evaporator, and standard facilities for sample preparation. After the system was thoroughly degassed, the molecules were deposited by thermal sublimation onto the substrate. The sample was thereafter transferred within the UHV chamber to the microscope, where measurements were carried out at about 110−130 K. All of the calculations were carried out in the framework of DFT by using the Vienna ab initio simulation package (VASP).23,24 The projector augmented wave method was used to describe the interaction between ions and electrons.25,26 We employed the Perdew−Burke−Ernzerhof generalized gradient approximation exchange−correlation functional,27 and van der Waals (vdW) interactions were included using the dispersion corrected DFT-D2 method of Grimme.28 The atomic structures were relaxed using the conjugate gradient algorithm scheme as implemented in VASP until the forces on all unconstrained atoms were ≤0.03 eV/Å.
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RESULTS AND DISCUSSION As shown in Figure 1a, we first deposit DNYB molecules on Cu(110) at room temperature (RT), and a close-packed ribbon superstructure can be observed. DFT calculations of the superstructure (cf. Figure 1b) have been performed to unravel 3089
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patterning for DTBT molecules are attributed to the replacement of phenyl groups by the tert-butyl groups. To further regulate the balance between the molecule− substrate interactions and the intermolecular interactions we move to a more inert surface, Au(111), to weaken the molecule−substrate interactions. After depositing the DNYB molecules on Au(111) at RT, no ordered nanostructure or single molecule could be observed even when scanning at a relatively low temperature of about 110 K, indicating the high mobility of DNYB molecules on the surface and the relatively weak intermolecular interactions of DNYB molecules. However, unlike the scenario of DNYB on Au(111), the DTBT molecules can form a supramolecular structure at RT on Au(111) as shown in Figure 3a. The DTBT molecules mainly
Figure 2. Large-scale STM images of DTBT molecules on Cu(110) (a) before and (b) after annealing process, respectively. (c,d) Close-up STM images of the two phases observed in panel a. Two ellipses with the different colors superimposed on the STM image are assigned to two DTBT molecules as also indicated in the calculated structural models of (e) phase I and (f) phase II. The unit cells of the experimental and theoretical structures of two phases have been indicated by the blue and yellow arrows, respectively (scanning parameters: It = 0.80 nA and Vt = 1500 mV).
Figure 3. (a) STM image shows the nanostructure (dominated by ribbon motifs) formed by DTBT molecules on Au(111). Two ellipses with different colors superimposed on the STM image represent two DTBT molecules. (b) DFT calculated models of the ribbon structure. (c) Adsorption geometry of a single DTBT molecule on Au(111) extracted from structural models shown in panel b. White arrows in panel c represent the close-packed direction of the substrate (scanning parameters: It = 0.80 nA and Vt = 1.53 V).
with respect to [11̅0] direction of the substrate. From the side view, we can clearly see that the DTBT molecule tends to adsorb with a nonplanar configuration in which the tert-butylphenyl groups are titled upward. Close inspection of phase I (cf. Figure 2c) shows that ladder-shaped structures are composed of quartet-like units assembled by two DTBT molecules as indicated by colored circles, and the structure has a unit cell with the side length a = 21.8 ± 1 Å, b = 14.7 ± 1 Å, and angle β = 67 ± 2°. Close inspection of phase II details that the structure has a unit cell with the side length a = 25.3 ± 1 Å, b = 34.2 ± 1 Å and angle γ = 42 ± 2°. Further DFT calculations of the binding energies of the two self-assembled structures also show that the phase II is more stable than phase I, which is in good agreement with our experimental findings. We have also performed a statistical analysis on the two phases in STM experiments, and the molecular densities of the two phases are almost the same (approximately 1.2 molecule/nm2). In comparison with DNYB molecules on Cu(110), the differences of the adsorption geometry and the self-assembled nano-
self-assembled into the ribbon structure, which differs greatly from that on Cu(110), most likely arising from variations in the electronic nature and lattice spacing of the two substrates. Note that the supramolecular structure of DTBT molecules on Au(111) lacks long-range order and that other structural motifs could also exist on the surface. From the STM image (cf. Figure 3a) we can identify that one tert-butyl group of the molecule appears as a bright protrusion, whereas the phenyl group is difficult to be clearly resolved due to the high contrast of the bright protrusions, which is consistent with DTBT on Cu(110). DFT calculations of the ribbon structure (Figure 3b) details that the ribbon structure has a unit cell with the side length a = 15.0 ± 1 Å, b = 22.5 ± 1 Å, and angle δ = 56 ± 2°. For the ribbon structure, the DTBT molecules within the neighboring two rows pack in a staggered arrangement with the tert-butyl groups interlocked with each other as shown in the STM image and DFT model. Furthermore, the miss-packed structural 3090
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interactions. Since the DTBT molecules have higher intermolecular interactions than DNYB due to the higher van der Waals forces caused by tert-butyl groups, they could form selfassembled nanostructures, while no self-assembled nanostructures formed by the DNYB molecules are observed.
motifs, which are different from the ribbon motif (denoted as zigzag motif and six-membered ring motif), have also been addressed by superimposing their molecular models on the close-up image (shown in Figure S2, Supporting Information). Figure 3c clearly details the nonplanar adsorption geometry of DTBT on Au(111). Even in the self-assembled structures, the DTBT molecules were found to be mobile and switchable among different structural motifs as shown in Figure 4.
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CONCLUSIONS From an STM-DFT study, we have investigated the adsorption and self-assembly of two organic molecules with and without tert-butyl groups on two intrinsically different substrates (Cu(110) and Au(111)) and explored the influence of the tert-butyl groups on the assembled behaviors on surfaces. Our findings suggest that the tert-butyl groups can not only affect the adsorption geometry but also change the self-assembled properties of organic molecules on surfaces due to the enhanced intermolecular interactions. Such results would supplement the fundamental understandings of the on-surface supramolecular self-assembly process, which is directed by relatively weak van der Waals forces and may also shed light on the rational design of desired supramolecular architectures on solid surfaces.
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ASSOCIATED CONTENT
S Supporting Information *
Adsorption geometry of DTBT molecules on Cu(110), closeup STM image of DTBT molecules on Au(111) superimposing with molecular models on the different structural motifs, DFT calculated adsorption energies of DNYB and BTBT molecules on Au(111) and Cu(110) surfaces. This material is available free of charge via the Internet at http://pubs.acs.org.
Figure 4. Overlay of two sequential STM images shows the diffusion behavior of DTBT molecules in the supramolecular structure. Purple/ green colors correspond to the initial/final positions, respectively, whereas the cyan molecules correspond to the stationary ones (It = 0.80 nA, Vt = 1.53 V, and Tscan = 110−130 K).
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Corresponding Author
Through overlap of two sequential STM images (at a time interval of about 5 s) scanning at the same area, the shift in position of the molecules is determined. The cyan protrusions represent the stationary ones, whereas the purple/green ones represent the initial/final positions of the protrusions that have moved. This phenomenon indicates the higher mobility of DTBT on Au(111) than that on Cu(110), which is attributed to the weakened molecule−substrate interactions. Considering the distinct assembled behaviors of DNYB and DTBT molecules on Cu(110) and Au(111), we have also calculated the adsorption energies of DNYB and DTBT molecules on Au(111) and Cu(110) to elucidate the molecule−substrate interactions and make a comparison between the two molecules as shown in Table 1. On the
*(Q.T.) E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the National High Technology Research and Development Program of China 863 (2012AA022606) and the National Science Foundation of China (NSFC-50603019). Aiguo Hu is gratefully acknowledged for providing the molecules.
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Au(111)
Cu(110)
Ead = 1.943 eV Ead = 2.061 eV
Ead = 3.468 eV Ead = 3.035 eV
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Table 1. DFT Calculated Adsorption Energies of DNYB and BTBT Molecules on Au(111) and Cu(110) DNYB DTBT
AUTHOR INFORMATION
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