Two-dimensional nanomaterials such as MoS 2 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. Here we demonstrate a method to simultaneously determine mean values of these properties from an optical extinction spectrum measured on a liquid dispersion of MoS 2 nanosheets. The concentration measurement is based on the size-independence of the low-wavelength extinction coefficient, while the size and thickness measurements rely on the effect of 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 A-exciton MoS 2 luminescence in liquid suspensions. These techniques are general and can be applied to a range of two-dimensional materials including WS 2 , MoSe 2 and WSe 2 .

Here we demonstrate inkjet printing of nanosheets of both graphene and MoS2 prepared by liquid exfoliation. We describe a protocol for the preparation of inks of nanosheets with well-defined size distribution and concentration up to 6 mg/ml. Graphene traces were printed at low temperature (<70 °C) with no subsequent thermal or chemical treatment. Thin traces displayed percolation effects while traces with thickness above 160 nm displayed thickness-independent conductivity of 3000 S/m. We also demonstrate the printing of semiconducting traces using solvent exfoliated MoS2. Such traces can be combined with inkjet-printed graphene interdigitated array electrodes to produce all-printed photodetectors.Keywords: suspension, dispersion, printing, exfoliation, layered compound, photoconductivity. ToC figLow temperature inkjet printing of exfoliated nanosheets has been demonstrated leading to conductive graphene traces and all-inkjet printed devices.2

We study the ability of a plasmonic structure under illumination to release heat and induce fluid convection at the nanoscale. We first introduce the unified formalism associated with this multidisciplinary problem combining optics, thermodynamics, and hydrodynamics. On this basis, numerical simulations were performed to compute the temperature field and velocity field evolutions of the surrounding fluid for a gold disk on glass while illuminated at its plasmon resonance. We show that the velocity amplitude of the surrounding fluid has a linear dependence on the structure temperature and a quadratic dependence on the structure size (for a given temperature). The fluid velocity remains negligible for single nanometer-sized plasmonic structures (<1 nm/s) due to a very low Reynolds number. However thermal-induced fluid convection can play a significant role when considering either micrometer-size structures or an assembly of nanostructures.

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 MPa 1/2 . 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.

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 spectra.

Solution-exfoliated MoS 2 nano-platelets were formed into thin films by deposition onto a water surface followed by transfer to indium tin oxide coated glass. After drying, a gold electrode was evaporated on top to give a sandwich structure with quasi-Ohmic contacts. Illumination of this device with broadband light of ~1 kW/m 2 intensity gave a fourfold increase in conductivity. The photocurrent increased sub-linearly with intensity and exponentially with time indicating the presence of traps. The photo-responsively was ~10 -4 A/W at 15 V, competitive with other 2-dimensional photoconductors. This work demonstrates the potential for liquidexfoliated, inorganic nanosheets to be fabricated into low-cost optoelectronic devices. ToC entryWe have prepared solution-processed thin films of MoS 2 nano-platelets which show four-fold conductivity increase under 1-sun illumination.2

Optically transparent photodetectors are crucial in next-generation optoelectronic applications including smart windows and transparent image sensors. Designing photodetectors with high transparency, photoresponsivity, and robust mechanical flexibility remains a significant challenge, as is managing the inevitable trade-off between high transparency and strong photoresponse. Here we report a scalable method to produce flexible crystalline Si nanostructured wire (NW) networks fabricated from silicon-on-insulator (SOI) with seamless junctions and highly responsive porous Si segments that combine to deliver exceptional performance. These networks show high transparency (∼92% at 550 nm), broadband photodetection (350 to 950 nm) with excellent responsivity (25 A/W), optical response time (0.58 ms), and mechanical flexibility (1000 cycles). Temperature-dependent photocurrent measurements indicate the presence of localized electronic states in the porous Si segments, which play a crucial role in light harvesting and photocarrier generation. The scalable low-cost approach based on SOI has the potential to deliver new classes of flexible optoelectronic devices, including next-generation photodetectors and solar cells.

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