Two-Dimensional Hierarchical Semiconductor with Addressable

Jul 19, 2018 - Surfaces play a key role in determining material properties, and their importance is further magnified in the two-dimensional (2D) limi...
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Two-Dimensional Hierarchical Semiconductor with Addressable Surfaces Bonnie Choi,† Kihong Lee,† Anastasia Voevodin,† Jue Wang,† Michael L. Steigerwald,† Patrick Batail,*,†,‡ Xiaoyang Zhu,*,† and Xavier Roy*,† †

Department of Chemistry, Columbia University, New York, New York 10027, United States Laboratoire MOLTECH, CNRS UMR 6200, Université d’Angers, 49045 Angers, France

‡

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S Supporting Information *

Here we describe a single-layer 2D semiconductor with a unique hierarchical structure and a surface that can be chemically modified through ligand substitution. We demonstrate the first example of such tunability by installing cyanide ligands on the surface of the nanosheets and show that the structural integrity is maintained. Figure 1a shows the layered van der Waals structure of the parent compound from which the 2D semiconductor is

ABSTRACT: Surfaces play a key role in determining material properties, and their importance is further magnified in the two-dimensional (2D) limit. Though monolayers are entirely composed of surfaces, there is no chemical approach to covalently address them without breaking intralayer bond. Here, we describe a 2D semiconductor that offers two unique features among 2D materials: structural hierarchy within the monolayer and surface reactive sites that enable functionalization. The 2D semiconductor is composed of a single layer of strongly interconnected Re6Se8 clusters arranged in an oblique lattice capped by substitutionally labile Cl atoms. We show that a simple ligand substitution strategy borrowed from traditional coordination chemistry can be used to modify the surface of the 2D material while preserving its internal structure. The potential generality of this approach establishes a promising route toward multifunctional 2D materials with tunable physical and chemical properties and may also facilitate better electrical top contact to 2D semiconductors.

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he excitement generated by 2D materials has been fueled by the endless possibilities they offer for fundamental research,1−6 and for novel technologies in nanoelectronics,7,8 catalysis,9−12 sensing,13 and energy production and storage.14,15 The realization of many of these promises, however, hinges on the development of effective strategies to control their surface chemistry, which plays a prominent role in determining their physical and chemical properties. This remains a major challenge for nearly all existing 2D materials. Graphene is unique in this respect: its surface can be functionalized, opening up many new technological frontiers, but this process invariably disrupts its network of CC bonds and introduces defects in both its atomic and electronic structures.3 Transition metal dichalcogenides (TMDCs), by contrast, have nonreactive surfaces: their atomic structure is constituted of chemically inaccessible metal atoms, sandwiched between inert layers of bridging chalcogens.4 The few methods for TMDC surface functionalization, which were primarily developed for MoS2, are severely limited because they typically yield low surface coverage density, break metal−chalcogen bonds as a result of the functionalization, and/or rely on weak noncovalent interactions.5,6,16,17 © XXXX American Chemical Society

Figure 1. (a) Edge-on view of Re6Se8Cl2. (b) Top-view of a single layer. Color code: Re, blue; Se, red; Cl, green. (c) Optical microscope image of Re6Se8Cl2 crystals. (d) AFM image of mechanically exfoliated flakes with a thickness of 60−150 nm.

isolated, Re6Se8Cl2, as determined by single crystal X-ray diffraction (SCXRD). This compound is a 2D structural analogue of the 3D superconductor Chevrel phase.18,19 In the basal plane, neighboring Re6Se8 clusters assemble into an array of equilateral parallelograms, with each cluster being connected to each of its four in-plane neighbors by two Re−Se linkages (Figure 1b). Each cluster is made of a Re6 octahedron enclosed in a Se8 cube and is capped by two terminal Cl ligands in the trans positions. The weak van der Waals contacts between the Received: May 13, 2018

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DOI: 10.1021/jacs.8b05010 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Communication

Journal of the American Chemical Society

Figure 2. (a) Schematic of the chemical intercalation/liquid exfoliation process. Colors as in Figure 1; Li, orange. (b) PXRD pattern of Re6Se8Cl2 (black); Li1.6Re6Se8Cl2 (red); and deintercalated Re6Se8Cl2 (blue). (c) Solutions obtained when pristine Re6Se8Cl2 (left) and Li1.6Re6Se8Cl2 are soaked in N-methylformamide (NMF). Tyndall scattering using a green laser beam indicates the solutes are nanoparticles. When pristine Re6Se8Cl2 crystals are soaked in NMF for 8 months, the solvent is colorless and shows no Tyndall scattering. (d) Solid state electronic absorption spectrum of a Re6Se8Cl2 single crystal (black) (from ref 21) and solution phase electronic absorption spectrum of monolayers in NMF (red).

energy-dispersive X-ray spectroscopy (EDS) reveals that the Re6Se8Cl2 composition remains unchanged (Table S2). Inductively coupled plasma optical emission spectrometry (ICP-OES) is used to determine the Li composition in the intercalated compound, Li1.6Re6Se8Cl2. The intercalation is reversible: when we treat bulk Li1.6Re6Se8Cl2 crystals with Br2, the (001) peak shifts back to its original position and the peaks in the range 13−16° reappear, as anticipated for the deintercalated structure. The intercalation compound Li1.6Re6Se8Cl2 dissolves readily in N-methylformamide (NMF). Dissolution results in exfoliation of the ionic structure, forming a dark brown solution containing solvated Re6Se8Cl2 monolayers, which are presumably associated with Li+ cations (Figure 2c). For reference, pristine Re6Se8Cl2 does not dissolve in NMF, even after soaking for more than 8 months. To prepare a solution of monolayers, Li1.6Re6Se8Cl2 crystals are suspended in NMF; a dark brown solution is obtained after a few hours, which is centrifuged to remove undissolved solid. Sonication, which can damage the structure, is unnecessary, and the solution is stable for months when kept in an inert atmosphere. Tyndall scattering demonstrates that the monolayers are at least on the scale of tens of nanometers (Figure 2c). Moreover, while scattering and solvation effects preclude the precise determination of the band gap energy, the electronic spectrum of the solution (Figure 2d, red line) agrees well with the solid state spectrum of the parent compound Re6Se8Cl2 (Figure 2d, black line).21 To verify the structure of the monolayers, an aliquot of the solution is dropcast onto a sapphire substrate, and the solvent is allowed to evaporate. A representative AFM image of the deposited material (Figure 3a) reveals a high coverage density of plate-like micrometer-sized nanosheets (see also Figure S2). A closer examination of these nanosheets (Figure 3b) shows that the thickness of the first layer is ∼1.7 nm; the step height histograms (Figure S3) of the AFM images indicates that

layers and strong in-plane bonding give this material a robust 2D character. First reported by Sergent and co-workers,20 microcrystalline Re6Se8Cl2 is synthesized by heating a stoichiometric mixture of Re, Se, and ReCl5 to 1100 °C in a fused silica tube sealed under vacuum. Millimeter-sized single crystals, shown in Figure 1c, are grown by chemical vapor transport in a temperature gradient of 970−920 °C using ReCl5 as the transporting agent (see Supporting Information). We use SCXRD to determine the structure of the compound and powder X-ray diffraction (PXRD) to confirm that each sample is phase pure (Figure S1, Table S1). Consistent with their layered van der Waals nature, crystals of Re6Se8Cl2 can be mechanically exfoliated. Monolayers are especially attractive because they offer exciting prospects for novel material properties, surface modification and fabrication of van der Waals heterostructures. To date, however, the thinnest flakes we have produced using mechanical exfoliation are ∼10 nm in height. Figure 1d displays an atomic force microscopy (AFM) image of mechanically exfoliated multilayer flakes whose electronic properties we have recently shown closely resemble those of the bulk solid.21 To reach the 2D limit and isolate Re6Se8Cl2 monolayers, we report in this work a new technique for exfoliation of these compounds. As is the case with many TMDCs,22,23 alkali metals can be intercalated into Re6Se8Cl2. Results for the intercalation of Li+ ions are presented in Figure 2a. Crystals of Re6Se8Cl2 are suspended in a ∼0.1 M solution of n-butyllithium (n-BuLi) in hexanes at −40 °C. After 24 h, the suspension is decanted, and the solid is washed with hexanes and dried under vacuum. The intercalation process is monitored by PXRD (Figure 2b). For pristine Re6Se8Cl2, the interlayer spacing, calculated from the position of the (001) diffraction peak, is d001 ∼ 8.1 Å. After intercalation, d001 ∼ 8.4 Å, indicating a ∼0.3 Å increase of the interlayer spacing. This is consistent with the insertion of Li+ ions between the Re6Se8Cl2 layers, which has recently been demonstrated electrochemically.24 Elemental analysis using B

DOI: 10.1021/jacs.8b05010 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society

Figure 3. (a) AFM image of monolayers dropcast on a sapphire substrate. (b) AFM image of a few monolayers and height profile along the black line. (c) High resolution HAADF TEM micrograph of a monolayer and (d) corresponding SAED pattern. (e) High resolution HAADF TEM micrograph showing a tear in the edge of a monolayer. (f−h) EDS mapping of Cl, Re, and Se in the monolayer, respectively.

Figure 4. (a) Schematic of the surface functionalization reaction of monolayers supported on a substrate. Color as in Figure 1; C, gray; N, aqua. (b) Raman spectra (2000−2400 cm−1 region) of monolayers on sapphire substrate (green), monolayers after reaction with TMSCN (red), and pristine sapphire substrate exposed to TMSCN in air (blue). The signal from the sapphire background has been subtracted in all the spectra. (c) AFM image and (d) TEM micrograph (inset: corresponding SAED pattern) of monolayers after surface functionalization.

subsequent layers are ∼1.5 nm thick, corresponding to the height of one monolayer. We attribute the difference between the height of the monolayer measured by AFM and that calculated from the SCXRD data (∼1 nm) to the AFM tip− surface interactions, which inherently overestimate the step height, as well as to the presence of cations and solvent molecules on the surface of the monolayers. Comparable differences in height have been reported for other 2D materials prepared by liquid exfoliation.25−27 The structure of the monolayer is investigated by transmission electron microscopy (TEM). Figure 3c,d presents a high resolution TEM micrograph and corresponding selected

area electron diffraction (SAED) pattern of a dropcast monolayer, respectively. Figure 3c reveals the ordered oblique array of individual Re6 octahedra, which appear as white dots in the high resolution high angle annular dark field (HAADF) micrograph. The diffraction peaks in the SAED pattern correspond to the (010) and (100) reflections when the electron beam is aligned along the c-axis. The interplane spacings calculated from the SAED pattern (d010 = 6.4 Å and d100 = 6.3 Å) match well with those obtained from PXRD and SCXRD (Figure S1). Figure 3e presents a high resolution HAADF TEM micrograph of a tear at the edge of a monolayer. EDS elemental mapping of an exfoliated flake shows uniform C

DOI: 10.1021/jacs.8b05010 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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ligands using the traditional methods of coordination chemistry.

distribution of Re, Se, and Cl on the nanosheet (Figure 3f−h). The Re:Se:Cl mass ratio, 62:35:3, matches well with the composition of the parent compound (theoretical: 61:35:4). From the crystal structure of Re6Se8Cl2, we can infer that Cl atoms cap the surface of the 2D semiconductor. This is corroborated by recent scanning tunneling microscopy (STM) measurements performed by our team on mechanically exfoliated Re6Se8Cl2 crystals.21 Previous studies on isolated Re6Q8L6 (Q = S, Se, Te) molecular clusters have shown that the clusters can be decorated with a wide range of ligands using ligand substitution chemistry.28,29 This observation is crucial to the prospect of functionalizing the surface of Re6Se8Cl2 monolayers. As a first demonstration, we install cyanide groups, which have a distinctive and easily identifiable vibrational signature, on the surface of the monolayers (Figure 4a). The monolayers, supported on a sapphire substrate, are reacted with a solution of trimethylsilyl cyanide (TMSCN), which is expected to substitute −CN for −Cl.30,31 Comparing the micro-Raman spectra of the monolayers before and after the reaction attests to the surface functionalization. The Raman spectrum of the pristine monolayers (Figure 4b, green) contains no peaks in the spectral region 2000−2400 cm−1, where we expect the CN stretching vibrational modes. The monolayers after substitution show two broad peaks at ∼2140 and ∼2230 cm−1 (Figure 4b, red). The main peak at 2140 cm−1 is a clear signature of cyanide attachment to the monolayers as it agrees well with reported νCN frequencies for [Re6Se8]2+ molecular clusters decorated with −CN.32,33 TMSCN, which has a strong, sharp peak at 2190 cm−1, is not detected on the surface (see Supporting Information for additional discussion). The second broad peak at 2230 cm−1 closely resembles the one observed in the Raman spectrum of a pristine sapphire substrate exposed to TMSCN in air (Figure 4b, blue). Crucially, the surface functionalization reaction preserves the [Re6Se8] structure of the monolayer. Figure 4c shows an AFM image of a few monolayers after the TMSCN treatment. The step height histogram (Figure S4) confirms that the thickness of the monolayers is similar to that of the pristine monolayers. The monolayer Raman modes in the 100−400 cm−1 spectral region, which are characteristic of the [Re6Se8] bonding, are essentially unchanged before and after functionalization (Figure S5). Carrying out the reaction on monolayers supported on a TEM grid enables direct imaging and diffraction of the resulting nanosheets (Figure 4d). The SAED pattern of the functionalized 2D semiconductor matches that of the pristine monolayer, confirming the retention of the crystalline [Re6Se8] structure. Furthermore, we note that the surface functionalization of monolayers with −CN ligands results in a 30−40 meV redshift of the lowest energy absorption peak (Figure S6). Larger shifts should be achievable with more strongly interacting ligands. The ability to chemically functionalize atomically thin semiconductors opens up a host of new opportunities for 2D materials, both as a way to tune their physical properties and as a platform to build novel structures and devices. Traditional 2D materials are limited in this respect because their covalent modification necessitates breaking intralayer bonds. The unique hierarchical structure of the Re6Se8Cl2 monolayers enables a new approach to functionalize the surface of this 2D material through ligand substitution. Importantly, the substitution reaction does not affect the intralayer bonding. These results chart a clear path to attaching a wide range of functional

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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/jacs.8b05010. Experimental and spectroscopic details (PDF) Data for Re6Se8Cl2 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] *[email protected] ORCID

Jue Wang: 0000-0001-6843-9771 Patrick Batail: 0000-0001-7125-5009 Xiaoyang Zhu: 0000-0002-2090-8484 Xavier Roy: 0000-0002-8850-0725 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Crystal synthesis and structural characterizations were supported by the NSF MRSEC program through the Center for Precision Assembly of Superstratic and Superatomic Solids (DMR-1420634). Optical characterization was supported by the U.S. Air Force Office of Scientific Research (AFOSR) grant FA9550-18-1-0020. B.C. (DGE 11-44155) and A.V. (DGE 1644869) are supported by the NSF graduate research fellowship. The authors acknowledge the Imaging Facility of CUNY Advanced Science Research Center for help with EDS, the Columbia University Shared Materials Characterization Lab (SMCL) and the Electron Microscopy Lab, and Luis Campos, Cory Dean, Colin Nuckolls, Daniel Paley and Amirali Zangiabadi for use of their instruments and helpful discussions.

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REFERENCES

(1) Novoselov, K. S.; Mishchenko, A.; Carvalho, A.; Castro Neto, A. H. Science 2016, 353, aac9439. (2) Gong, C.; Li, L.; Li, Z. L.; Ji, H. W.; Stern, A.; Xia, Y.; Cao, T.; Bao, W.; Wang, C. Z.; Wang, Y. A.; Qiu, Z. Q.; Cava, R. J.; Louie, S. G.; Xia, J.; Zhang, X. Nature 2017, 546, 265−275. (3) Georgakilas, V.; Otyepka, M.; Bourlinos, A. B.; Chandra, V.; Kim, N.; Kemp, K. C.; Hobza, P.; Zboril, R.; Kim, K. S. Chem. Rev. 2012, 112, 6156−6214. (4) Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L. J.; Loh, K. P.; Zhang, H. Nat. Chem. 2013, 5, 263−275. (5) Voiry, D.; Goswami, A.; Kappera, R.; Silva, C. D. C. E.; Kaplan, D.; Fujita, T.; Chen, M. W.; Asefa, T.; Chhowalla, M. Nat. Chem. 2015, 7, 45−49. (6) Lei, S. D.; Wang, X. F.; Li, B.; Kang, J. H.; He, Y. M.; George, A.; Ge, L. H.; Gong, Y. J.; Dong, P.; Jin, Z. H.; Brunetto, G.; Chen, W. B.; Lin, Z. T.; Baines, R.; Galvao, D. S.; Lou, J.; Barrera, E.; Banerjee, K.; Vajtai, R.; Ajayan, P. Nat. Nanotechnol. 2016, 11, 465−471. (7) Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S. Nat. Nanotechnol. 2012, 7, 699−712. (8) Fiori, G.; Bonaccorso, F.; Iannaccone, G.; Palacios, T.; Neumaier, D.; Seabaugh, A.; Banerjee, S. K.; Colombo, L. Nat. Nanotechnol. 2014, 9, 768−779. (9) Jaramillo, T. F.; Jorgensen, K. P.; Bonde, J.; Nielsen, J. H.; Horch, S.; Chorkendorff, I. Science 2007, 317, 100−102.

D

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Journal of the American Chemical Society (10) Deng, D. H.; Novoselov, K. S.; Fu, Q.; Zheng, N. F.; Tian, Z. Q.; Bao, X. H. Nat. Nanotechnol. 2016, 11, 218−230. (11) Voiry, D.; Yang, J.; Chhowalla, M. Adv. Mater. 2016, 28, 6197− 6206. (12) Lu, Q. P.; Yu, Y. F.; Ma, Q. L.; Chen, B.; Zhang, H. Adv. Mater. 2016, 28, 1917−1933. (13) Perkins, F. K.; Friedman, A. L.; Cobas, E.; Campbell, P. M.; Jernigan, G. G.; Jonker, B. T. Nano Lett. 2013, 13, 668−673. (14) Bonaccorso, F.; Colombo, L.; Yu, G. H.; Stoller, M.; Tozzini, V.; Ferrari, A. C.; Ruoff, R. S.; Pellegrini, V. Science 2015, 347, 1246501. (15) Pomerantseva, E.; Gogotsi, Y. Nat. Energy 2017, 2, 17089. (16) Presolski, S.; Pumera, M. Mater. Today 2016, 19, 140−145. (17) Nguyen, E. P.; Carey, B. J.; Ou, J. Z.; van Embden, J.; Della Gaspera, E.; Chrimes, A. F.; Spencer, M. J. S.; Zhuiykov, S.; Kalantarzadeh, K.; Daeneke, T. Adv. Mater. 2015, 27, 6225−6229. (18) Fischer, O. Appl. Phys. 1978, 16, 1−28. (19) Chevrel, R.; Hirrien, M.; Sergent, M. Polyhedron 1986, 5, 87− 94. (20) Leduc, L.; Perrin, A.; Sergent, M. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1983, 39, 1503−1506. (21) Zhong, X.; Lee, K.; Choi, B.; Meggiolaro, D.; Liu, F.; Nuckolls, C.; Pasupathy, A.; De Angelis, F.; Batail, P.; Roy, X.; Zhu, X. Nano Lett. 2018, 18, 1483−1488. (22) Nicolosi, V.; Chhowalla, M.; Kanatzidis, M. G.; Strano, M. S.; Coleman, J. N. Science 2013, 340, 1226419. (23) Zheng, J.; Zhang, H.; Dong, S. H.; Liu, Y. P.; Nai, C. T.; Shin, H. S.; Jeong, H. Y.; Liu, B.; Loh, K. P. Nat. Commun. 2014, 5, 2995. (24) Bruck, A. M.; Yin, J.; Tong, X.; Takeuchi, E. S.; Takeuchi, K. J.; Szczepura, L. F.; Marschilok, A. C. Inorg. Chem. 2018, 57, 4812− 4815. (25) Eda, G.; Yamaguchi, H.; Voiry, D.; Fujita, T.; Chen, M. W.; Chhowalla, M. Nano Lett. 2011, 11, 5111−5116. (26) Shearer, C. J.; Slattery, A. D.; Stapleton, A. J.; Shapter, J. G.; Gibson, C. T. Nanotechnology 2016, 27, 125704. (27) Backes, C.; Smith, R. J.; McEvoy, N.; Berner, N. C.; McCloskey, D.; Nerl, H. C.; O’Neill, A.; King, P. J.; Higgins, T.; Hanlon, D.; Scheuschner, N.; Maultzsch, J.; Houben, L.; Duesberg, G. S.; Donegan, J. F.; Nicolosi, V.; Coleman, J. N. Nat. Commun. 2014, 5, 4576. (28) Kim, Y.; Fedorov, V. E.; Kim, S. J. J. Mater. Chem. 2009, 19, 7178−7190. (29) Gabriel, J. C. P.; Boubekeur, K.; Uriel, S.; Batail, P. Chem. Rev. 2001, 101, 2037−2066. (30) Suginome, M.; Yamamoto, A.; Sasaki, T.; Murakami, M. Organometallics 2006, 25, 2911−2913. (31) Hussain, M.; Kohser, S.; Janssen, K.; Wartchow, R.; Butenschon, H. Organometallics 2009, 28, 5212−5221. (32) Bennett, M. V.; Beauvais, L. G.; Shores, M. P.; Long, J. R. J. Am. Chem. Soc. 2001, 123, 8022−8032. (33) Gray, T. G.; Holm, R. H. Inorg. Chem. 2002, 41, 4211−4216.

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DOI: 10.1021/jacs.8b05010 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX