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PUBLISHED ; Cited By :4 Export Date: 15 September 2016 ; Liquid phase exfoliation is a powerful and scalable technique to produce defect-free mono- and few-layer graphene. However, samples are typically polydisperse and control over size and thickness is challenging. Notably, high throughput techniques to measure size and thickness are lacking. In this work, we have measured the extinction, absorption, scattering and Raman spectra for liquid phase exfoliated graphene nanosheets of various lateral sizes (90 ≤ 〈L〉 ≤ 810 nm) and thicknesses (2.7 ≤ 〈N〉 ≤ 10.4). We found all spectra to show well-defined dependences on nanosheet dimensions. Measurements of extinction and absorption spectra of nanosheet dispersions showed both peak position and spectral shape to vary with nanosheet thickness in a manner consistent with theoretical calculations. This allows the development of empirical metrics to extract the mean thickness of liquid dispersed nanosheets from an extinction (or absorption) spectrum. While the scattering spectra depended on nanosheet length, poor signal to noise ratios made the resultant length metric unreliable. By analyzing Raman spectra measured on graphene nanosheet networks, we found both the D/G intensity ratio and the width of the G-band to scale with mean nanosheet length allowing us to establish quantitative relationships. In addition, we elucidate the variation of 2D/G band intensities and 2D-band shape with the mean nanosheet thickness, allowing us to establish quantitative metrics for mean nanosheet thickness from Raman spectr ; The research leading to these results has received funding from the European Union Seventh Framework Programme under grant agreement no 604391 Graphene Flagship. In addition, we acknowledge Science Foundation Ireland (11/PI/ 1087), the European Research Council (SEMANTICS and POC grant UP2DM) and Thomas Swan & Co. Ltd for financial support. CB acknowledges the German research foundation DFG (BA 4856/1-1). We thank Asbury Carbons and Imerys Graphite and Carbon for suppling parent graphite materials free of ch

PUBLISHED ; Export Date: 15 September 2016 ; Here we demonstrate that liquid phase exfoliation can be used to convert layered crystals of nickel hydroxide into Ni(OH)2 nanosheets in relatively large quantities and without the need for ion intercalation. While other procedures require harsh synthesis conditions and multiple reaction steps, this method involves ultrasonication of commercially available powders in aqueous surfactant solutions and so is relatively mild and potentially scalable. Such mild exfoliation is possible because the surface energy of Ni(OH)2, as measured by inverse gas chromatography, is relatively low at ?70 mJ m?2, similar to other layered materials. TEM, AFM, XPS and Raman spectroscopy show the exfoliated nanosheets to be relatively thin (mean ?10 monolayers thick) and of good quality. Size selection by liquid cascade centrifugation allowed the production of samples with mean nanosheet lengths ranging from 55 to 195 nm. Optical measurements on dispersions showed the optical absorption coefficient spectra to be relatively invariant with nanosheet size while the scattering coefficient spectra varied strongly with size. The resultant size-dependence allows the extinction spectra to be used to estimate nanosheet size as well as concentration. We used the exfoliated nanosheets to prepare thin film electrodes for use in supercapacitors and as oxygen evolution catalysts. While the resultant capacitance was reasonably high at ?1200 F cm?3 (20 mV s?1), the catalytic performance was exceptional with currents of 10 mA cm?2 observed at overpotentials as low as 297 mV, close to the state of the art. ; This work was primarily funded by Science Foundation Ireland through the PI program (11/PI/1087). The research leading to these results has also received funding from the European Union Seventh Framework Program under grant agreement no. 604391 Graphene Flagship, the European Research Council (SEMANTICS) and the Science Foundation Ireland (SFI) funded centre AMBER (SFI/12/RC/2278)

PUBLISHED ; Export Date: 5 August 2015 ; Here we demonstrate the production of large quantities of gallium sulfide (GaS) nanosheets by liquid exfoliation of layered GaS powder. The exfoliation was achieved by sonication of the powder in suitable solvents. The variation of dispersed concentration with solvent was consistent with classical solution thermodynamics and showed successful solvents to be those with Hildebrand solubility parameters close to 21.5 MPa1/2. In this way, nanosheets could be produced at concentrations of up to ~0.2 mg/ml with lateral sizes and thicknesses of 50-1000 nm and 3-80 layers, respectively. The nanosheets appeared to be relatively defect free although oxygen was observed in the vicinity of the edges. Using controlled centrifugation techniques, it was possible to prepare dispersions containing size-selected nanosheets. Spectroscopic measurements showed the optical properties of the dispersions to vary strongly with nanosheet size, allowing the elucidation of spectroscopic metrics for in-situ estimation of nanosheet size and thickness. These techniques allow the production of nanosheets with controlled sizes which will be important for certain applications. To demonstrate this, we prepared films of GaS nanosheets of three different sizes for use as hydrogen evolution electrocatalysts. We found a clear correlation between performance and size showing small nanosheets to be more effective. This is consistent with the catalytically active sites residing on the nanosheet edges. ; The research leading to these results has received funding from the European Union Seventh Framework Programme under grant agreement n°604391 Graphene Flagship. We have also received support from the Science Foundation Ireland (SFI) funded centre AMBER (SFI/12/RC/2278). In addition, JNC acknowledges the European Research Council (SEMANTICS) and SFI (11/PI/1087) for financial support. CB acknowledges the German research foundation DFG (BA 4856/1-1). HCN, EMcG, AS-A and VN acknowledge ERC 2DNanoCaps, SFI PIYRA and FP7 MoWSeS. GSD, NCB and SW acknowledge SFI for PI_10/IN.1/I3030. NMcE acknowledges SFI (14/TIDA/2329). STEM experiments were performed at SuperSTEM, the EPSRC UK national facility for aberration-corrected STEM.

PUBLISHED ; cited by 1 ; Two-dimensional nanomaterials such as MoS2 are of great interest both because of their novel physical properties and their applications potential. Liquid exfoliation, an important production method, is limited by our inability to quickly and easily measure nanosheet size, thickness or concentration. Herein, we demonstrate a method to simultaneously determine mean values of these properties from an optical extinction spectrum measured on a liquid dispersion of MoS2 nanosheets. The concentration measurement is based on the size-independence of the low-wavelength scattering coefficient while the size and thickness measurements rely on the effect on edges and quantum confinement on the optical spectra. The resultant controllability of concentration, size and thickness facilitates the preparation of dispersions with pre-determined properties such as high monolayer-content, leading to first measurement of direct-gap MoS2 luminescence in liquid suspensions. These techniques are general and can be applied to a range of 2-dimensional materials including WS2, MoSe2 and WSe2. ; The research leading to these results has received funding from the European Union Seventh Framework Programme under grant agreement n°604391 Graphene Flagship. In addition, JNC acknowledges the European Research Council grant SEMANTICS and the SFI grant number 17 11/PI/1087 for financial support. CB acknowledges the German research foundation DFG (BA 4856/1-1). NS and JM acknowledge support from DFG SPP 1459 (MA 4079/7-1) and European Research Council (grant number 259286). GSD and NB acknowledge SFI for PI_10/IN.1/I3030. NMcE thanks FP7-2010-PPP Green Cars Electrograph (266391).

We incorporate thermal effects for injection currents ranging up to 150 mA in order to model the tuning behavior of a two-section, all-active distributed-Bragg-reflector (DBR),ridge-wave guide semiconductor laser utilized for a single-mode operation. In particular, we investigate wavelength tuning as a function of injected currents within the grating andphase/gain sections of the laser cavity and examine how any athermal lasing conditions may arise. The effect of thermal drift on the resonant wavelength due to a change in refractive index as well as thermal expansion of the laser cavity is included within a traveling wave analysis (TWA). From the TWA, the spatial distribution of gain along the active region of the laser is also derived in order to help describe the tuning behavior for a high-order (37th) grating previously optimized to minimize line width. A comparative analysis with a singlemirrored, active-passive DBR laser is also included. Results show a good agreement withreported experimental data and compare well with the wavelength stability of other laser devices. ; This work was supported in part by the ScienceFoundation of Ireland under Grants 15/IFB/3317, 15/IA/2854, and CONNECT 13/RC/2077; and in partby the European Union?s Horizon 2020 research and innovation program under the Marie Sk?odowska-Curie Grant agreement 713567.