Two-dimensional semiconductors such as MoS 2 are promising for future electrical devices. The interface to metals is a crucial and critical aspect for these devices because undesirably high resistances due to Fermi level pinning are present, resulting in unwanted energy losses. To date, experimental information on such junctions has been obtained mainly indirectly by evaluating transistor characteristics. The fact that the metal− semiconductor interface is typically embedded, further complicates the investigation of the underlying physical mechanisms at the interface. Here, we present a method to provide access to a realistic metal−semiconductor interface by large-area exfoliation of single-layer MoS 2 on clean polycrystalline gold surfaces. This approach allows us to measure the relative charge neutrality level at the MoS 2 −gold interface and its spatial variation almost directly using Kelvin probe force microscopy even under ambient conditions. By bringing together hitherto unconnected findings about the MoS 2 −gold interface, we can explain the anomalous Raman signature of MoS 2 in contact to metals [ACS Nano. 7, 2013, 11350] which has been the subject of intense recent discussions. In detail, we identify the unusual Raman mode as the A 1g mode with a reduced Raman shift (397 cm −1 ) due to the weakening of the Mo−S bond. Combined with our X-ray photoelectron spectroscopy data and the measured charge neutrality level, this is in good agreement with a previously predicted mechanism for Fermi level pinning at the MoS 2 −gold interface [Nano Lett. 14, 2014, 1714. As a consequence, the strength of the MoS 2 −gold contact can be determined from the intensity ratio between the reduced A 1g reduced mode and the unperturbed A 1g mode.

Networks of ligand-free semiconductor nanocrystals (NCs) offer a valuable combination of high carrier mobility and optoelectronic properties tunable via quantum confinement. In principle, maximizing carrier mobility entails crossing the insulator-metal transition (IMT), where carriers become delocalized. A recent theoretical study predicted that this transition occurs at nρ ≈ 0.3, where n is the carrier density and ρ is the interparticle contact radius. In this work, we satisfy this criterion in networks of plasma-synthesized ZnO NCs by using intense pulsed light (IPL) annealing to tune n and ρ independently. IPL applied to as-deposited NCs increases ρ by inducing sintering, and IPL applied after the NCs are coated with AlO by atomic layer deposition increases n by removing electron-trapping surface hydroxyls. This procedure does not substantially alter NC size or composition and is potentially applicable to a wide variety of nanomaterials. As we increase nρ to at least twice the predicted critical value, we observe conductivity scaling consistent with arrival at the critical region of a continuous quantum phase transition. This allows us to determine the critical behavior of the dielectric constant and electron localization length at the IMT. However, our samples remain on the insulating side of the critical region, which suggests that the critical value of nρ may in fact be significantly higher than 0.3.

A facile method is described to couple flash sintering (FS) and spark plasma sintering (SPS). Flash spark plasma sintering (FSPS) combines advantages of both techniques: the use of pellet-shaped samples under mechanical load with the controlled passage of electric current through the sample. FSPS is realized by partially replacing graphite pressing tools (two punches and one matrix) used in standard SPS. An insulating boron nitride matrix substitutes the conducting graphite matrix to force the electric current through the sample. Additionally, external heating of the boron nitride matrix is implemented. Microstructures of standard and flash-SPS are compared using aluminum doped zinc oxide as an example. Scanning electron microscopy reveals that different microstructures are generated for SPS and FSPS. The new setups provide novel processing routes for different current sintering methods of materials under mechanical load and assist in identifying the role of the electric current or field in the microstructure.

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