Article pubs.acs.org/Langmuir
Self-Assembly of LiMo3Se3 Nanowire Networks from Nanoscale Building-Blocks in Solution John G. Sheridan,† Andreas Heidelberg,† Dermot F. Brougham,‡ Peter D. Nellist,§ Richard M. Langford,∥ and John J. Boland*,† †
School of Chemistry and the Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN), Trinity College Dublin, Ireland ‡ School of Chemical Sciences, Dublin City University, Ireland § Department of Materials, University of Oxford, Oxford, United Kingdom ∥ School of Physics and the Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN), Trinity College Dublin, Ireland S Supporting Information *
ABSTRACT: LiMo3Se3 is a highly anisotropic solid comprised of a regular pattern of quasi-1-D wire-like structures. Solutions of LiMo3Se3 deposited on substrates and TEM grids reveal the presence of two-dimensional network morphologies. High resolution STEM imaging reveals that the junctions within these networks are not formed by discrete overlying LiMo3Se3 fibers or wires. Rather the junctions are continuous in that the wires are seamlessly interwoven from one bundle to the next. We investigated network formation by dynamic light scattering and AFM and demonstrate that the networks are not pre-existent in solution but rather form via self-assembly of nanoscale building blocks that is driven by solvent evaporation.
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INTRODUCTION Self-assembly is a fundamental principle of nanotechnology and relies on the presence of weak interactions that can be made and broken to enable assembling species to find the optimum bonding arrangement by sampling a variety of possible configurations. This principle has been demonstrated at a molecular level and exemplified by the numerous self-assembled 2-D monolayer films that have been reported.1,2 This same approach has also been extended to 3-D systems using recent advances in supramolecular chemistry.3 Self-assembly of 1-D structures has attracted particular attention, in part due to biologically inspired systems such as DNA4 and amyloid fibrils.5 Much of the current interest stems from the potential for low cost fabrication of quasi-1-D channels and interconnects for device applications. Although it has proven possible to assemble molecules6 and nanoparticles7 into quasi-1-D structures on surfaces, their electrical proper2ties have often proven to be unreliable. Here, we demonstrate that solutions of the simple inorganic solid LiMo3Se3 actually contain buildingblocks that are sub-100 nm in size and that these building-blocks self-assemble to form conductive networks. We focus on the structure and composition of the network junctions since for all wires network systems the global behavior is determined by the junction properties.8 The potential of these and similar materials systems is also discussed. The solids XMo3Se3 have a 1-D structural motif and were first reported by Potel et al.9 Tarascon et al.10 showed © 2012 American Chemical Society
LiMo3Se3 to be soluble in polar solvents and that nanowires could be cast from solutions, and following which there has been considerable interest in these materials as sources of 1-D wires. DiSalvo et al.11 performed high resolution scanning transmission electron microscopy (STEM) measurements on these materials in which they showed overlapping Mo3Se3 wires but no high resolution images of the network or the junctions between wires. Similarly, the Lieber group12 made use of scanning tunneling microscopy (STM) to image individual Mo3Se3 wires and bundles but not networks or junctions of this material. More recently the group of Osterloh13−16 has fabricated films of LiMo3Se3 deposited from solution for sensor applications. Once again, large scale networks have been reported, but the mechanism of wire formation or the nature of the junctions between the wires was not studied.
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EXPERIMENTAL SECTION
We synthesized LiMo3Se3 using a method that has been described elsewhere.17 Briefly, we started by preparing InMo3Se3 by reacting stoichiometric appropriate amounts of In, Mo, and Se (4N, Sigma Received: May 10, 2012 Revised: September 14, 2012 Published: September 25, 2012 15344
dx.doi.org/10.1021/la301918x | Langmuir 2012, 28, 15344−15349
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Aldrich), at 1050 °C in an evacuated quartz ampule. 1050°C
In + 3Mo + 3Se ⎯⎯⎯⎯⎯⎯→ InMo3Se3
(1)
LiI (Sigma Aldrich, >98% anhydrous) was then reacted (in a 10% molar excecss) with the InMo3Se3 in an evacuated quartz ampule. 550°C
InMo3Se3 + LiI ⎯⎯⎯⎯⎯→ LiMo3Se3 + InI
(2)
As the LiMo3Se3 compound is prone to oxidation in air, the final material is stored in an inert argon glovebox and all further preparation is performed in this environment (unless noted otherwise). PXRD and EDX spectroscopy was used to evaluate the quality of the material (see the Supporting Information, Figures S2 and S3). Initially LixIn1‑xMo3Se3 is obtained but subsequent “refirings” of the material with additional LiI yielded a significantly purer product. Solutions of LiMo3Se3 in DMSO or NMF (Sigma Aldrich, anhydrous) were prepared in a 1 mg per 1 mL material/solvent ratio, yielding a concentration of ∼1.8 mM, colloquially referred to as 10−3 M. Lower concentrations (10−4 M, 10−5 M, etc.) were obtained following dilution of the 10−3 M solution. Filtration when necessary was achieved using PALL Acrodisc PTFE (0.45 μm) syringe filters. Samples for AFM and SEM were typically prepared by drop casting or spin-coating (∼1500 rpm) 50 μL of solution on clean flat substrates (SiO2 and HOPG), and samples for TEM were prepared by drop casting microliter amounts on lacey carbon TEM grids (unless noted otherwise). As solutions of LiMo3Se3 in the solvents used are nonvolatile, these samples were transferred to the glovebox antechamber and pumped overnight (scroll pump) to remove the solvent. STEM imaging was carried out at the SuperSTEM1 facility at Daresbury Laboratories, U.K. (a VG HB 501 system with Mark II Nion CS corrector). A dual beam FEI Strata 235 was used for SEM imaging and a Veeco Dimension 3100 was used for AFM imaging. Light scattering studies used a Malvern HPPS system.
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RESULTS AND DISCUSSION Here, we focus on the properties of LiMo3Se3 solutions and solution-cast networks. When a 10−3 M solution is drop cast onto a substrate two distinctly different types of features are observed. Figure 1 shows electron micrographs of LiMo3Se3 nanowire fibers that result from the break-up of the original crystallites in the solid. Figure 1a is a TEM image of a LiMo3Se3 crystallite disassembling into smaller nanowire fibers whose lengths appear constant and controlled by that of the original crystallite (see inset in Figure 1a). As these fibers exfoliate they overlap, and although there is some entanglement, these fibers are clearly visible as discrete independent objects. Figure 1b is a FIB prepared cross-section of a large LiMo3Se3 fiber. Of interest here is the fact that these large fibers have noncylindrical cross sections, with the existence of voids between the fiber and the supporting substrate. Panels c and d of Figure 1 show HAADF STEM images (obtained at SuperSTEM1, Daresbury, U.K.) of smaller (
