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Evanston, IL, United States

Liu X.,Graduate Program in Applied Physics | Hersam M.C.,Northwestern University
ACS Nano

Solution dispersions of two-dimensional (2D) black phosphorus (BP)-often referred to as phosphorene-are achieved by solvent exfoliation. These pristine, electronic-grade BP dispersions are produced with anhydrous organic solvents in a sealed-tip ultrasonication system, which circumvents BP degradation that would otherwise occur via solvated O2 or H2O. Among conventional solvents, N-methylpyrrolidone (NMP) is found to provide stable, highly concentrated (∼0.4 mg/mL) BP dispersions. Atomic force microscopy, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy show that the structure and chemistry of solvent-exfoliated BP nanosheets are comparable to mechanically exfoliated BP flakes. Additionally, residual NMP from the liquid-phase processing suppresses the rate of BP oxidation in ambient conditions. Solvent-exfoliated BP nanosheet field-effect transistors exhibit ambipolar behavior with current on/off ratios and mobilities up to ∼104 and ∼50 cm2 V-1 s-1, respectively. Overall, this study shows that stable, highly concentrated, electronic-grade 2D BP dispersions can be realized by scalable solvent exfoliation, thereby presenting opportunities for large-area, high-performance BP device applications. © 2015 American Chemical Society. Source

Wang D.,Graduate Program in Applied Physics | Yang A.,Northwestern University | Hryn A.J.,Northwestern University | Schatz G.C.,Graduate Program in Applied Physics | And 3 more authors.
ACS Photonics

Periodic metal nanoparticle (NP) arrays support narrow lattice plasmon resonances that can be tuned by changing the localized surface plasmons of the individual NPs in the array, NP periodicity, and dielectric environment. In this paper, we report superlattice plasmons that can be supported by hierarchical Au NP arrays, where finite arrays of NPs (patches) are organized into arrays with larger periodicities. We show that superlattice plasmons can be described by the coupling of single-patch lattice plasmons and Bragg modes defined by the patch periodicity. Superlattice plasmon resonances are often significantly narrower than that of single-patch lattice plasmon resonances and exhibit stronger local peak fields. By varying the periodicity of the patches, we demonstrated that the number and spectral location of superlattice plasmon resonances can be tailored in hierarchical Au NP arrays. These narrow superlattice plasmon resonances open prospects in ultrasensitive sensing and energy transfer and plasmon amplification in plasmonic cavities. © 2015 American Chemical Society. Source

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