van der Waals Solids from Self-Assembled Nanoscale Building Blocks

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van der Waals Solids from Self-Assembled Nanoscale Building Blocks Bonnie Choi,† Jaeeun Yu,† Daniel W. Paley,† M. Tuan Trinh,† Maria V. Paley,‡ Jessica M. Karch,† Andrew C. Crowther,‡ Chul-Ho Lee,§ Roger A. Lalancette,∥ Xiaoyang Zhu,† Philip Kim,⊥ Michael L. Steigerwald,*,† Colin Nuckolls,*,† and Xavier Roy*,† †

Department of Chemistry, Columbia University, New York, New York 10027, United States Department of Chemistry, Barnard College, New York, New York 10027, United States § KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul 136-701, Korea ∥ Department of Chemistry, Rutgers University-Newark, Newark, New Jersey 07102, United States ⊥ Department of Physics, Harvard University, Cambridge, Massachusetts 02138, United States ‡

S Supporting Information *

ABSTRACT: Traditional atomic van der Waals materials such as graphene, hexagonal boron-nitride, and transition metal dichalcogenides have received widespread attention due to the wealth of unusual physical and chemical behaviors that arise when charges, spins, and vibrations are confined to a plane. Though not as widespread as their atomic counterparts, molecule-based two-dimensional (2D) layered solids offer significant benefits; their structural flexibility will enable the development of materials with tunable properties. Here we describe a layered van der Waals solid self-assembled from a structure-directing building block and C60 fullerene. The resulting crystalline solid contains a corrugated monolayer of neutral fullerenes and can be mechanically exfoliated. The absorption spectrum of the bulk solid shows an optical gap of 390 ± 40 meV that is consistent with thermal activation energy obtained from electrical transport measurement. We find that the dimensional confinement of fullerenes significantly modulates the optical and electronic properties compared to the bulk solid. KEYWORDS: Self-assembly, two-dimensional materials, molecular clusters, fullerene

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into neutral, corrugated monolayers. The director- and fullerene-layers form a 2D van der Waals solid whose intralayer bonding is strong enough and whose interlayer bonding is weak enough that the material may be mechanically exfoliated (Figure 1). The reaction of the diethyl(9-ethynylphenanthrene)phosphine (PEt2phen) ligand, elemental Se, and Co2(CO)8 in refluxing toluene provides the nanoscale director, Co6Se8(PEt2phen)6. Single crystal X-ray diffraction confirms that the inorganic core of Co6Se8(PEt2phen)6 is isostructural with that of the parent cluster Co6Se8(PEt3)6.18 The similarity between the two compounds is further supported by electronic absorption spectroscopy where the spectrum (Figure S1) shows three longer-wavelength absorptions characteristic of the Co6Se8 core18 in addition to the absorptions due to the phenanthrene groups in the UV region. The electrochemical properties of Co6Se8(PEt2phen)6 (Figure S5) also show three [Co6Se8]-centered reversible oxidations but our director is less reducing than Co6Se8(PEt3)6 due to the diminished electrondonating ability of the PEt2phen ligand compared to that of PEt3.

he development of facile methods for isolation and manipulation of two-dimensional (2D) materials has fueled an explosion of interest in layered van der Waals structures.1−4 These solids exhibit strong in-plane bonding and comparatively weak interactions between the layers, allowing them to be exfoliated. Fullerenes are attractive molecular building blocks for assembling functional materials,5,6 yet C60 crystallizes into a three-dimensional solid held together by isotropic van der Waals interactions.7 As a result, C60 cannot be exfoliated into discrete layers, unlike other nanostructured forms of carbon such as graphene8 and carbon nanotubes.9 Ionic solids of C60 with layered structures have been reported,10,11 but these solids also cannot be exfoliated because the layers are held together by electrostatic interactions. While theory suggests that neutral C60 could form mechanically stable, free-standing monolayers,12 these structures do not assemble spontaneously from solution and have only been prepared as epitaxial layers through carefully controlled vapor growth or on chemically modified surfaces.13 As with many other 2D materials, unique emergent properties may arise in lowdimensional C60 arrays.14−17 Here we describe the self-assembly of fullerenes into a layered van der Waals material. We employ a structurally adaptable transition metal chalcogenide cluster that associates with C60 and thereby directs the spontaneous assembly of C60 © XXXX American Chemical Society

Received: December 10, 2015 Revised: January 21, 2016

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DOI: 10.1021/acs.nanolett.5b05049 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 1. Schematic of a nanoscale director organizing a monolayer of fullerenes. Additional fullerenes that are not held in the ligand sphere have been omitted from the schematic. Optical microscope images show a crystal of the layered van der Waals material before and after exfoliation.

Figure 2. AFM topographic images of (a) as-grown single crystal and (b) mechanically exfoliated crystal of [Co6Se8(PEt2phen)6][C60]5. The optical microscope image of the respective crystals are shown in the inset. The thickness of the exfoliated crystal is 139 nm.

calix[n]arenes,23,24 cyclic paraphenyleneacetylenes,25 and contorted hexabenzocoronenes26,27 that have concave surfaces. We reasoned that the octahedral geometry of the Co6Se8 cluster provides a simple yet versatile platform upon which we could build a flexible unit that can recognize C60. While the phenanthrene ring on each phosphine is flat and provides only weak van der Waals interactions with C60, three ethynylphenanthrene groups, one from each of three adjacent phosphines, can rotate in concert to form a “buckybowl” (Figure 3). The disposition of the ligands is different in isolated Co6Se8(PEt2phen)6 (Figure 3a) than in the cocrystal of [Co6Se8(PEt2phen)6][C60]5 (Figure 3b). In the presence of C60, the phenanthrenes decorating the Co6Se8 core extend and organize themselves to form two antipodal buckybowls, each capable of interacting with a guest C60. This supramolecular “dumbbell” is the template that forms a basic unit in the extended structure. An unanticipated consequence of the buckybowl formation is the exposure of the equator of the Co6Se8 core, leaving a void that is filled by additional fullerenes (Figure 4a). Figure 4a shows the extended molecular structure of the dumbbell with the other three equivalents of C 60 in [Co6Se8(PEt2phen)6][C60]5. The structure is composed of two layers (identified as Layers CF and FL) stacked along the hexagonal c-axis. Layer CF is trigonal with stoichiometry [Co6Se8(PEt2phen)6][C60]2 in which the molecular cluster is surrounded by a flattened octahedron of fullerenes that are 3.3 Å above and below the mean plane of the layer (Figure 4b). Layer FL is a slightly corrugated trigonal array of fullerenes (Figure 4c). For each cluster, there are five C60s that occupy three crystallographically distinguishable positions: two symmetry-related C60s (gray) are sitting in the buckybowls, and

We have previously reported that Co6Se8(PEt3)6 forms hierarchical solids with C60;19 however, while Co6Se8(PEt3)6 is functionally spherical, Co6Se8(PEt2phen)6 can be anisotropic. The ligands on Co6Se8(PEt2phen)6 can reorganize to a thermodynamically stable unit with a shape that is complementary to C60. When we combine Co6Se8(PEt2phen)6 and five equivalents of C60 in toluene, thin, flat, and hexagonal brown crystals of [Co6Se8(PEt2phen)6][C60]5 form. Mechanical evaluation in tandem with crystallography show that this material is not only structurally two-dimensional but also behaves physically like a layered solid. Similar to more traditional 2D van der Waals materials, [Co6Se8(PEt2phen)6][C60]5 crystals can be exfoliated. Figure 2a shows the atomic force microscope (AFM) image of a [Co6Se8(PEt2phen)6][C60]5 single crystal grown from solution. We observe a step-and-terrace structure on the surface of the [Co6Se8(PEt2phen)6][C60]5 crystals with ∼3 nm steps. We mechanically exfoliated single crystals of [Co6Se8(PEt2phen)6][C60]5, thereby isolating multilayer samples that we transfer onto Si wafers (Figure 2b). Under an optical microscope, exfoliated crystals display various colors (Figure S7) that we interpret as interference fringes, suggesting that flakes of different thickness are produced using this technique. We are able to produce samples that are as thin as ∼130 nm and have smooth surfaces (average roughness Rq ∼ 0.6 nm). This surface roughness is comparable to self-assembled C60 monolayers on Au,20 and C60 films deposited and annealed on graphite.21 In some exfoliated samples, we also observe small, ∼3 nm-high islands on the surfaces (Figure S8). Flexible molecular recognition elements in the ligand sphere create this self-assembled van der Waals material. Fullerenes are known to bind to molecular receptors such as corannulenes,22 B

DOI: 10.1021/acs.nanolett.5b05049 Nano Lett. XXXX, XXX, XXX−XXX

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monolayer (Layer CF + Layer FL) calculated from the crystal structure (1/3 of the c-axis length, 2.64 nm). Therefore, when we exfoliate the flakes, they unzip along the 2D sheets of fullerenes. Micro-Raman spectroscopy of the layered van der Waals material indicates that no charge is transferred between the constituent clusters in [Co6Se8(PEt2phen)6][C60]5 (Figure S6) and that the structure is held together entirely through van der Waals interactions. SQUID magnetometry reveals that the material is diamagnetic and supports the observation that there is no measurable charge transfer. We have collected structural data on a large library of Co6Se8(PR3)6 clusters in known oxidation states (0, +1, and +2; Figure S2), and we can assess the oxidation state of the cluster by comparing its Co−P and Co−Cotrans distances to this database. This analysis also indicates that the cluster bears no charge in the layered solid. We performed two-probe electrical resistivity measurements on single crystals of [Co6Se8(PEt2phen)6][C60]5. As shown in Figure 5a, the layered solid exhibits in-plane activated electrical transport behavior and we observe an exponential decrease of the conductance (G) with decreasing temperature. This thermally activated transport displays an Arrhenius behavior with an activation energy Ea of ∼400 meV. This energy barrier is higher than the transport barrier of the parent, ionic solid [Co6Se8(PEt3)6][C60]2 (Ea ∼ 150 meV).19 Figure 5b shows the optical absorption spectrum for [Co6Se8(PEt2phen)6][C60]5 using Fourier transform infrared (FT-IR) spectroscopy. The measurement was performed in microscopic transmission mode on a single flake of the crystal at room temperature. The spectrum is characteristic of that of a direct bandgap semiconductor28 and the enhanced absorbance near the threshold is similar to that of an excitonic resonance.29 We fit the experimental data with the bulk term of Elliot’s theory describing band-to-band optical transition for direct bandgap semiconductors, which we augment with a Gaussian (see

Figure 3. Molecular structure of the nanoscale director built from Co6Se8(PEt2phen)6 (a) before and (b) after self-assembly. The PEt2phen ligands on each cluster reorganize to form two buckybowls that host C60s in the solid state.

these are a part of the Layer FL. Two symmetry-related C60s (light blue) are located in Layer CF, close to the inorganic cluster; the fifth C60 (green), positioned within Layer FL, is not sitting in a buckybowl but is in contact with two interstitial coplanar toluene solvent molecules. Within Layer FL, the C60s are in close contact having centroid-to-centroid distance ranging from 9.65 to 9.94 Å. The step height in the layers (Figure 2) is consistent with the height of a superatomic

Figure 4. (a) Edge-on view of the molecular structure of [Co6Se8(PEt2phen)6][C60]5 looking down the ab plane. Layer CF consists of [Co6Se8(PEt2phen)6][C60]2, and Layer FL is a corrugated monolayer of C60. Cobalt, blue; selenium, light green; phosphorus, orange; carbon, black. The fullerenes in Layer CF are shown in light blue; fullerenes that are sitting in the buckybowls within Layer FL are gray; fullerenes that are not associated with the buckybowls within FL are green. Hydrogen atoms and toluene molecules of solvation were removed to clarify the view. (b) Space-filled view of a single Layer CF looking down the c-axis. (c) Space-filled view of a single Layer FL looking down the c-axis. C

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Figure 5. (a) Log plot of conductance (G) versus 1/T for [Co6Se8(PEt2phen)6][C60]5. The two-probe conductance measurements were done on a single crystal. Using the Arrhenius fit, solid red line, we obtained an activation energy of 400 meV. (b) Mid-IR absorption spectrum of [Co6Se8(PEt2phen)6][C60]5. The red dashed curve was obtained from a fit using band-to-band transition for semiconductor (blue) with an additional Gaussian peak (green) near the band-edge. The obtained bandgap is 390 ± 40 meV.

Supporting Information).29 The fit yields a bandgap of 390 ± 40 meV, which is in excellent agreement with the activation energy obtained from the electrical transport measurements. The exact origin of the resonant enhancement near the bandgap (peak at ∼350 meV, green curve in Figure 5b) is not known and remains a subject for future mechanistic study. In conclusion, we describe here a 2D van der Waals solid selfassembled from a nanoscale building block and C60. The resulting crystalline solid features a corrugated monolayer of fullerenes that is separated by a layer comprised of a combination of the structure-directing inorganic cluster and C60. The crystal can be mechanically exfoliated into thinner sheets with smooth surfaces. Because we see a consistent step size of ∼3 nm, we believe that the exfoliation results in “unzipping” of the fullerene monolayer. The absorption spectrum of the bulk crystal shows what is consistent with an excitonic feature at room temperature and a bulk optical gap of 390 ± 40 meV that matches well with the thermal activation energy obtained from transport measurement. These properties are decidedly different from those of bulk fullerene and thus make this system exciting for future study of dimensionally confined neutral C60s. Furthermore, we aim to generalize the use of structure-directing molecular clusters to reorganize canonical materials in new and exciting ways.

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ACKNOWLEDGMENTS

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REFERENCES

We thank Ryan Hastie for her help in making the figures and Timothy Atallah for his help with FT-IR measurement. Funding for this research was provided by the Center for Precision Assembly of Superstratic and Superatomic Solids, an NSF MRSEC (Award Number DMR-1420634) and in part by FAME, one of six centers of STARnet, a Semiconductor Research Corporation program sponsored by MARCO and DARPA. B.C. is supported by the NSF Graduate Research Fellowship under Grant DGE 11-44155. J.Y. thanks the Kwanjeong Educational Foundation for support. X-ray diffraction, AFM and SQUID measurements were performed in the Shared Materials Characterization Laboratory at Columbia University. Use of the Shared Materials Characterization Laboratory was made possible by funding from Columbia University. We also thank the Dean Lab in the Department of Physics at Columbia University for the use of their optical microscope.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b05049. Synthetic details and characterization data for all compounds and experimental and data analysis details. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: (M.L.S.) [email protected]. *E-mail: (C.N.) [email protected]. *E-mail: (X.R.) [email protected]. Notes

The authors declare no competing financial interest. D

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DOI: 10.1021/acs.nanolett.5b05049 Nano Lett. XXXX, XXX, XXX−XXX