Dichalcogenides with the common formula MX 2 are layered materials with electrical properties that range from semiconducting to superconducting. Here, we describe optimal imaging conditions for the optical detection of ultrathin, two-dimensional dichalcogenide nanocrystals containing single, double and triple layers of MoS 2 , WSe 2 and NbSe 2 . A simple optical model is used to calculate the contrast for nanolayers deposited on wafers with varying thicknesses of SiO 2 . The model is extended for imaging using the green channel of a video camera. Using AFM and optical imaging we confirm that single layers of MoS 2 and WSe 2 can be detected on 90 and 270 nm SiO 2 using optical means. By measuring contrast under broadband green illumination we are also able to distinguish between nanostructures containing single, double and triple layers of MoS 2 and WSe 2. We observe and discuss discrepancies in the case of NbSe 2 .(Some figures in this article are in colour only in the electronic version)The family of transition metal dichalcogenides with the common formula MX 2 , where M stands for transition metal (M = Mo, W, Nb, Ta, Ti) and X for Se, S or Te displays a rich variety of physical properties. Depending on the metal and the chalcogen involved, their electrical properties span the range from semiconducting to superconducting. Bulk dichalcogenide crystals are composed of vertically stacked layers bound together by weak van der Waals interaction. Just as in the case of graphene [1], single dichalcogenide layers can be extracted from bulk crystals [2,3] and deposited on substrates for further studies. Single MX 2 layers present a wide range of systems for studying mesoscopic transport in 2D and could find practical applications complementary to those of graphene. Bulk WSe 2 has, for example, been used in the past for fabrication of photovoltaic cells [4], whereas MoS 2 nanotubes [5] and nanowires [6] show confinement effects in their electronic and optical properties. Semiconducting dichalcogenides could also be interesting for fabrication of nanoscale field effect transistors [3, 7-9] while superconducting NbSe 2 could be a model for studying superconductivity in low-dimensional systems at mesoscopic scales [10,11].Locating and identifying single nanolayers of materials such as graphite [1] or semiconducting transition metal dichalcogenides [3] such as MoS 2 or WSe 2 is the first, enabling step in the study and practical applications of these materials. Atomic force microscopy (AFM) can be used to accurately determine both the vertical and lateral dimensions of nanolayers deposited on insulating substrates such as SiO 2 . AFM imaging is, however, time-consuming and the relatively slow throughput of the technique is a serious drawback. Scanning electron microscopy (SEM) or transmission electron microscopy (TEM) could also be used here, but contamination [12] due to electron-beam-induced deposition or knock-on damage in TEM due to electron-beam radiation-induced displacement of atoms could be a serious problem ...

We report the measurement of thermal conductance of silicon nanowires at low temperature. It is demonstrated that the roughness at the nanometer scale plays a crucial role for the phonon transport in low-dimensional samples. To this end, using e-beam lithography, nanowires of size 200 nm by 100 nm and 10 microm long have been nanofabricated. Their thermal properties have been measured using the 3 omega method between 0.3 and 6 K. The change in the temperature behavior of the thermal conductance (quadratic temperature dependence of K(T)) is a signature of an intermediate regime lying between the classical Casimir regime and the quantum regime. The Casimir-Ziman model is used to show that this specific behavior originates in mesoscopic samples where the dominant phonon wavelength becomes commensurate to the characteristic length of the roughness of the nanowire surfaces.