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.

We performed calculations of electronic, optical, and transport properties of graphene on hexagonal boron nitride with realistic moiré patterns. The latter are produced by structural relaxation using a fully atomistic model. This relaxation turns out to be crucially important for electronic properties. We describe experimentally observed features such as additional Dirac points and the "Hofstadter butterfly" structure of energy levels in a magnetic field. We find that the electronic structure is sensitive to many-body renormalization of the local energy gap.

We investigate electronic band gap and transport in Fibonacci quasi-periodic graphene superlattice. It is found that such structure can possess a zero-k gap which exists in all Fibonacci sequences. Different from Bragg gap, zero-k gap associated with Dirac point is less sensitive to the incidence angle and lattice constants. The defect mode appeared inside the zero-k gap has a great effect on transmission, conductance and shot noise, which can be applicable to control the electron transport.Graphene, a monolayer of carbon atoms tightly packed into a honeycomb lattice, has attracted great interest in graphene-based nanoelectronic and optoelectronic devices [1], since it was fabricated by Novoselov and Geim et al. in 2004 [2]. In graphene, the unique band structure with the valance and conduction bands touching at Dirac point (DP) leads to the fact that electrons around the Fermi level can be described as the massless relativistic Dirac electrons, resulting in the linear energy dispersion relation. As a consequence, there are a great number of electronic properties, such as the half-integer quantum Hall effect [3][4][5], the minimum conductivity [3], and Klein tunneling [6]. In particular, Klein tunneling and perfect transmission are crucial for electron transport in various graphene heterostructures [7], i.e. single barrier [8] and n-p-n junctions [9].Motivated by the experimental realization of graphene superlattice (GSL) [10][11][12], electronic bandgap structures and transport properties in GSLs with electrostatic potential and magnetic barrier have been extensively investigated [13][14][15][16][17][18][19][20][21][22], since the conventional semiconductor superlattices are successful in controlling the electronic structures and the extension to graphene may give rise to different features and applications. For instance, DP appears in the GSL [14,15], and it is exactly located at the energy with the zero-k gap [17]. Interestingly, the zero-k gap associated with DP is insensitive to the lattice parameter changes in contrast with the behavior exhibited by Bragg gaps [17]. This gap is analogous to photonic zero-n gap in the photonic crystals containing negativeindex and positive-index materials [20], and originates from a zero total phase [23]. Accordingly, the zero-k gap is robust against the lattice constants, structural disorder [17], and external magnetic field [18], and thus is better to control the electron transport in GSL.In this Letter, we will investigate electronic band gap and transport in Fibonacci quasi-periodic GSLs in the fashion analogous to photonic crystal with metamaterials [23][24][25]. As we know, the quasi-periodic GSL is classified as intermediate between ordered and disordered systems [19,20], which has significant and common features like fractal spectrum and self-similar behavior [21,22]. How- * Corresponding author. Email: xchen@shu.edu.cn ever, what we concentrate on here is the electronic band gap and DP in such quasi-periodic system. We find that zero-k gap happens in all Fibonacci...

Aryl boronic acids and esters are important building blocks in API synthesis. The palladium-catalyzed Suzuki-Miyaura borylation is the most common method for their preparation. This paper describes an improvement of the current reaction conditions. By using lipophilic bases such as potassium 2-ethyl hexanoate, the borylation reaction could be achieved at 35 °C in less than 2 h with very low palladium loading (0.5 mol %). A preliminary mechanistic study shows a hitherto unrecognized inhibitory effect by the carboxylate anion on the catalytic cycle, whereas 2-ethyl hexanoate minimizes this inhibitory effect. This improved methodology enables borylation of a wide range of substrates under mild conditions.

We present a systematic study of the electronic, transport, and optical properties of disordered graphene, including the next-nearest-neighbor hopping. We show that this hopping has a nonnegligible effect on resonant scattering but is of minor importance for long-range disorder such as charged impurities, random potentials, or hoppings induced by strain fluctuations. Different types of disorders can be recognized by their fingerprints appearing in the profiles of dc conductivity, carrier mobility, optical spectroscopy, and Landau level spectrum. The minimum conductivity 4e 2 /h found in the experiments is dominated by long-range disorder and the value of 4e2 /π h is due to resonant scatterers only.