While liquid exfoliation is a powerful technique to produce defect-free nanosheets in large quantities, its usefulness is limited by broad nanosheet thickness distributions and low monolayer contents. Here we demonstrate liquid processing techniques, based on iterative centrifugation cascades, which can be designed to achieve either highly efficient nanosheet size-selection and/or monolayer enrichment. The resultant size-selected dispersions were used to establish quantitative metrics to determine monolayer volume fraction, as well as mean nanosheet size and thickness, from standard spectroscopic measurements. Such metrics allowed us to design and optimize centrifugation cascades to enrich liquid exfoliated WS2 dispersions up to monolayer contents of 75%. Monolayer-rich dispersions show relatively bright photoluminescence with narrow line widths (<35 meV) indicating the high quality of the nanosheets. The enriched dispersions display extinction spectra with distinct features, which also allow the direct estimation of monolayer contents.

A. Sample preparationThe YBa 2 Cu 3 O 6.5 (YBCO) single crystal used here was grown by top-seeded solution growth using a Ba 3 Cu 5 O solvent [1]. The as-grown single crystal was first annealed at 700°C for 70 h with flowing oxygen and quenched down to room temperature. The La 1.85 Sr 0.15 CuO 4 (LSCO) single crystal was synthesised by a travelling-solvent-floating-zone method utilizing infrared radiation furnaces (Crystal system, FZ-T-4000) and annealed in oxygen gas under ambient pressure at 600°C for 7 days[2]. B. Femtosecond pump-probe set-upA detailed description of the set-up used is found in [3]. In a "pump-probe" experiment, a "pump" pulse excites the sample and the induced change in transmission or reflection of a delayed probe pulse monitors the relaxation behaviour. In the linear approximation ∆R R directly tracks the electronic relaxation processes, and the time constants obtained from fits of its dynamics are the characteristic times of the underlying relaxation processes. In our data, this approximation is justified by two essential characteristics: (i) the ∆R R amplitude is linear in the excitation intensity (see Figure 1a for LSCO), and (ii) the same decay times appear independently of the probe wavelength, only with different spectral weights of the individual components.In order to resolve the dynamics of fast processes very short pulses are necessary, since the instrumental response function is given by the cross correlation between the pump and probe pulses. We use sub-10 fs probe pulses from an ultrabroadband (covering a spectral range from 500 to 700 nm) non-collinear optical parametric amplifier (NOPA) and ∼15 fs pump pulses from a narrower band (wavelength tunable, in our case centred at 530 nm) NOPA. The seed pulses for the NOPAs and the amplified pulses are steered and focussed exclusively with reflecting optics to avoid pulse chirping.A schematic of the experimental apparatus is shown in Fig. 1. The laser source is a regeneratively amplified modelocked Ti:sapphire laser (Clark-MXR Model CPA-1), delivering pulses at 1 kHz repetition rate with 780 nm center wavelength, 150 fs duration, and 500 µJ energy. Both NOPAs are pumped by the second harmonic of the Ti:sapphire laser, which is generated in a 1-mm-thick lithium triborate crystal (LBO), cut for type-I phase matching in the XY plane (θ = 90°, ϕ = 31.68°, Shandong Newphotons).The ultrabroadband visible NOPA that generates the probe pulses has been described in detail before[4]; a schematic of it is shown in Fig. 2. The white light continuum seed pulses are generated by a small fraction of the fundamental

3351wileyonlinelibrary.com for the monolayer. Hence, the monolayer, contrary to bi-and multilayers, behaves like a direct gap semiconductor and shows signifi cant fl uorescence. [ 11,12 ] The exciton binding energy for bulk MoS 2 has been determined to be 45 and 130 meV for the A and B excitons, respectively. [ 13 ] Both exciton binding energies increase upon decreasing the sample thickness, with estimates for monolayer [14][15][16] ranging from 0.4 to 0.9 eV. Despite this high exciton binding energy, monolayer MoS 2 shows a strong photovoltaic effect [ 17 ] and potential for high sensitivity photodetectors. [ 18 ] Both these functionalities require effi cient charge carrier photogeneration (CPG), either via direct excitation of mobile carriers or via exciton dissociation.The spectral signature of charge carriers has been identifi ed by absorption and fl uorescence spectroscopy of MoS 2 , where the charge concentration varies either via the gate voltage in an FET geometry [ 19 ] or via adsorption, [ 20 ] or substrate doping. [ 21 ] The absorption peaks of charges are red-shifted by about 40 meV compared with the ground-state absorption into the A and B excitons and have been attributed to optical transitions from a charged ground state to a charged exciton (trion). The possibility of alternative interpretations, such as polarons [ 22,23 ] or Stark effect in the local electric fi eld of the charges, [24][25][26] does The 2D semiconductor MoS 2 in its mono-and few-layer form is expected to have a signifi cant exciton binding energy of several 100 meV, suggesting excitons as the primary photoexcited species. Nevertheless, even single layers show a strong photovoltaic effect and work as the active material in high sensitivity photodetectors, thus indicating effi cient charge carrier photogeneration. Here, modulation spectroscopy in the sub-ps and ms time scales is used to study the photoexcitation dynamics in few-layer MoS 2 . The results suggest that the primary photoexcitations are excitons that effi ciently dissociate into charges with a characteristic time of 700 fs. Based on these fi ndings, simple suggestions for the design of effi cient MoS 2 photovoltaic and photodetector devices are made.

We use femtosecond spectroscopy to investigate the quasiparticle relaxation and low-energy electronic structure in a nearly optimally doped pnictide superconductor with T{c}=49.5 K. Multiple relaxation processes are evident, with distinct superconducting state quasiparticle recombination dynamics exhibiting a T-dependent superconducting gap, and a clear "pseudogaplike" feature with an onset above 180 K indicating the existence of a temperature-independent gap of magnitude Delta{PG}=61+/-9 meV above T{c}. Both the superconducting and pseudogap components show saturation as a function of fluence with distinct saturation fluences 4 and 40 microJ/cm{2}, respectively.

We excite and detect coherent phonons in semiconducting (6,5) carbon nanotubes via a sub-10-fs pump-probe technique. Simulation of the amplitude and phase profile via time-dependent wave packet theory yields excellent agreement with experimental results under the assumption of molecular excitonic states and allows determining the electron-phonon coupling strength for the two dominant vibrational modes.

The fabrication of organic light‐emitting devices (OLEDs) from semiconducting polymer nanospheres (SPNs) deposited from aqueous dispersions is described. It is found that the active device layer consists of a homogeneous single layer of light‐emitting SPNs. The OLEDs exhibit an electroluminescence onset at the SPN energy gap, which can be attributed to field‐enhanced charge‐carrier injection at the nanostructured Al cathode.

Sub-ps three-pulse transient differential transmission spectroscopy using two excitation pulses is used to directly investigate the generation of charge carriers in ladder-type poly(para)phenyl in bulk film. The role of higher excited singlet states of both even and odd symmetry is examined and the dynamics of the major processes involved is described quantitatively. The charge generation efficiency is found to depend strongly on the delay between the two excitation pulses. This is explained by the interplay between internal conversion, excitation energy migration, and on-site vibronic relaxation.

While liquid phase exfoliation can be used to produce nanosheets stabilized in polymer solutions, very little is known about the resultant nanosheet size, thickness or monolayer content. Here we use semi-quantitative spectroscopic metrics based on extinction, Raman and photoluminescence (PL) spectroscopy to investigate these parameters for WS2 nanosheets exfoliated in aqueous polyvinylalcohol (PVA) solutions. By measuring Raman and PL simultaneously, we can track the monolayer content via the PL/Raman intensity ratio while varying processing conditions. We find the monolayer population to be maximized for a stabilizing polymer concentration of 2 g/L. In addition, the monolayer content can be controlled via the centrifugation conditions, exceeding 5% by mass in some cases. These techniques have allowed us to track the ratio of PL/Raman in a droplet of polymer-stabilized WS2 nanosheets as the water evaporates during composite formation. We find no evidence of nanosheet aggregation under these conditions although the PL becomes dominated by trion emission as drying proceeds and the balance of doping from PVA/water changes. Finally, we have produced bulk PVA/WS2 composites by freeze drying where >50% of the monolayers remain unaggregated, even at WS2 volume fractions as high as 10%.2