Raman Spectroscopy is able to probe disorder in graphene through defect-activated peaks. It is of great interest to link these features to the nature of disorder. Here we present a detailed analysis of the Raman spectra of graphene containing different type of defects. We found that the intensity ratio of the D and D' peak is maximum (~ 13) for sp 3 -defects, it decreases for vacancy-like defects (~ 7) and reaches a minimum for boundaries in graphite (~3.5).

We have developed a method to separate metallic from semiconducting single-walled carbon nanotubes from suspension using alternating current dielectrophoresis. Our method takes advantage of the difference of the relative dielectric constants of the two species with respect to the solvent, resulting in an opposite movement of metallic and semiconducting tubes along the electric field gradient. Metallic tubes are attracted toward a microelectrode array, leaving semiconducting tubes in the solvent. Proof of the effectiveness of separation is given by a comparative Raman spectroscopy study on the dielectrophoretically deposited tubes and on a reference sample.

We detect electroluminescence in single layer molybdenum disulphide (MoS2) field-effect transistors built on transparent glass substrates. By comparing absorption, photoluminescence, and electroluminescence of the same MoS2 layer, we find that they all involve the same excited state at 1.8eV. The electroluminescence has pronounced threshold behavior and is localized at the contacts. The results show that single layer MoS2, a direct band gap semiconductor, is promising for novel optoelectronic devices, such as 2-dimensional light detectors and emitters. Here, we report electrically excited luminescence in 1L-MoS 2 FETs, and study the underlying emission mechanism. We find that the electroluminescence occurs via hot carriers and is localized in the contacts region. The observed photoluminescence and electroluminescence arise from the same excited state at 1.8eV. Molybdenum disulphide (MoS1L-MoS 2 crystals are produced by micromechanical cleavage of bulk MoS 2 (Structure Probe Inc.-SPI, Natural Molybdenite) on 100nm SiO 2 . As for the case of graphene [20], interference allows visibility and counting the number of layers, Fig.1a. Due to the different dielectric properties, an optimum thickness of 100nm SiO 2 is well suited for MoS 2 [21]. The presence of monolayers is then confirmed by performing PL measurements, Fig.1b. The PL spectrum of 1L-MoS 2 exhibits two bands at 2eV and 1.8eV (Fig.1b) associated with excitonic transitions at the K point of the Brillouin zone[4]. The energy difference of 0.2eV has been attributed to the degeneracy breaking of the valence band due to spin-orbit coupling [4,7,8,22]. As compared to bulk MoS 2 , Fig.1b, 1L-MoS 2 does not have a peak at 1.4eV [3,4], associated with the indirect band gap [12]. In addition 1L-MoS 2 exhibits a stronger PL intensity compared to bulk MoS 2 [3, 4] due to the direct band gap. Another evidence for 1L-MoS 2 comes from the analysis of the Raman spectrum, Fig.1d. The peak at∼385cm −1 corresponds to the in plane (E 1 2g ) mode [23], while that at ∼404 cm −1 is attributed to the out of plane (A 1g ) mode [23]. The E 1 2g mode softens and A 1g mode stiffens with increasing layer thickness[23], similar to what happens for other layered materials, where the bond distance changes with number of layers [24]. The frequency difference between these two modes can be used as a signature of 1L-MoS 2 [23].1L-MoS 2 flakes are then transferred onto glass substrates by using a poly(methyl methacrylate) (PMMA) based transfer technique, similar to that previously used to transfer graphene onto optical fibre cores [25]. This process involves spin coating two layers of 495K PMMA and one layer of 950K PMMA on the substrate where flakes are deposited. The samples are subsequently immersed in de-ionized (DI) water at 90 • C for 1h, resulting in the detachment of the polymer film, due to the intercalation of water at the polymer-SiO 2 interface. MoS 2 flakes stick to the PMMA, and can thus be removed from the original substrate and mechanically transferred onto glass substrates [2...

One of the biggest limitations of conventional carbon nanotube device fabrication techniques is the inability to scale up the processes to fabricate a large number of devices on a single chip. In this report, we demonstrate the directed and precise assembly of single-nanotube devices with an integration density of several million devices per square centimeter, using a novel aspect of nanotube dielectrophoresis. We show that the dielectrophoretic force fields change incisively as nanotubes assemble into the contact areas, leading to a reproducible directed assembly which is self-limiting in forming single-tube devices. Their functionality has been tested by random sampling of device characteristics using microprobes.