Visibility of dichalcogenide nanolayers

Dichalcogenides with the common formula MX2 are layered materials with electrical properties that range from semiconducting to superconducting. Here, we describe optimal imaging conditions for optical detection of ultrathin, two-dimensional dichalcogenide nanocrystals containing single, double and triple layers of MoS2, WSe2 and NbSe2. A simple optical model is used to calculate the contrast for nanolayers deposited on wafers with varying thickness of SiO2. 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 MoS2, WSe2 and NbSe2 can be detected on 90nm and 270 nm SiO2 using optical means. By measuring contrast under broad-band green illumination we are also able to distinguish between nanostructures containing single, mono and triple layers of MoS2, WSe2 and NbSe2

Optoelectronics with electrically tunable PN diodes in a monolayer dichalcogenide

One of the most fundamental devices for electronics and optoelectronics is the PN junction, which provides the functional element of diodes, bipolar transistors, photodetectors, LEDs, and solar cells, among many other devices. In conventional PN junctions, the adjacent p- and n-type regions of a semiconductor are formed by chemical doping. Materials with ambipolar conductance, however, allow for PN junctions to be configured and modified by electrostatic gating. This electrical control enables a single device to have multiple functionalities. Here we report ambipolar monolayer WSe2 devices in which two local gates are used to define a PN junction exclusively within the sheet of WSe2. With these electrically tunable PN junctions, we demonstrate both PN and NP diodes with ideality factors better than 2. Under excitation with light, the diodes show photodetection responsivity of 210 mA/W and photovoltaic power generation with a peak external quantum efficiency of 0.2%, promising numbers for a nearly transparent monolayer sheet in a lateral device geometry. Finally, we demonstrate a light-emitting diode based on monolayer WSe2. These devices provide a fundamental building block for ubiquitous, ultra-thin, flexible, and nearly transparent optoelectronic and electronic applications based on ambipolar dichalcogenide materials.Comment: 14 pages, 4 figure

A comprehensive study of custom-made ceramic separators for microbial fuel cells: Towards "living" bricks

Towards the commercialisation of microbial fuel cell (MFC) technology, well-performing, cost-effective, and sustainable separators are being developed. Ceramic is one of the promising materials for this purpose. In this study, ceramic separators made of three different clay types were tested to investigate the effect of ceramic material properties on their performance. The best-performing ceramic separators were white ceramic-based spotty membranes, which produced maximum power outputs of 717.7 ± 29.9 µW (white ceramic-based with brown spots, 71.8 W·m−3) and 715.3 ± 73.0 µW (white ceramic-based with red spots, 71.5 W·m−3). For single material ceramic types, red ceramic separator generated the highest power output of 670.5 ± 64. 8 µW (67.1 W·m−3). Porosity investigation revealed that white and red ceramics are more porous and have smaller pores compared to brown ceramic. Brown ceramic separators under performed initially but seem more favourable for long-term operation due to bigger pores and thus less tendency of membrane fouling. This study presents ways to enhance the function of ceramic separators in MFCs such as the novel spotty design as well as fine-tuning of porosity and pore size

The Nature of Electronic States in Atomically Thin MoS2 Field-Effect Transistors

We present low temperature electrical transport experiments in five field effect transistor devices consisting of monolayer, bilayer and trilayer MoS2 films, mechanically exfoliated onto Si/SiO2 substrate. Our experiments reveal that the electronic states in all films are localized well up to the room temperature over the experimentally accessible range of gate voltage. This manifests in two dimensional (2D) variable range hopping (VRH) at high temperatures, while below \sim 30 K the conductivity displays oscillatory structures in gate voltage arising from resonant tunneling at the localized sites. From the correlation energy (T0) of VRH and gate voltage dependence of conductivity, we suggest that Coulomb potential from trapped charges in the substrate are the dominant source of disorder in MoS2 field effect devices, which leads to carrier localization as well.Comment: 10 pages, 5 figures; ACS Nano (2011

Optical signature of symmetry variations and spin-valley coupling in atomically thin tungsten dichalcogenides

Motivated by the triumph and limitation of graphene for electronic applications, atomically thin layers of group VI transition metal dichalcogenides are attracting extensive interest as a class of graphene-like semiconductors with a desired band-gap in the visible frequency range. The monolayers feature a valence band spin splitting with opposite sign in the two valleys located at corners of 1st Brillouin zone. This spin-valley coupling, particularly pronounced in tungsten dichalcogenides, can benefit potential spintronics and valleytronics with the important consequences of spin-valley interplay and the suppression of spin and valley relaxations. Here we report the first optical studies of WS2 and WSe2 monolayers and multilayers. The efficiency of second harmonic generation shows a dramatic even-odd oscillation with the number of layers, consistent with the presence (absence) of inversion symmetry in even-layer (odd-layer). Photoluminescence (PL) measurements show the crossover from an indirect band gap semiconductor at mutilayers to a direct-gap one at monolayers. The PL spectra and first-principle calculations consistently reveal a spin-valley coupling of 0.4 eV which suppresses interlayer hopping and manifests as a thickness independent splitting pattern at valence band edge near K points. This giant spin-valley coupling, together with the valley dependent physical properties, may lead to rich possibilities for manipulating spin and valley degrees of freedom in these atomically thin 2D materials

Evaluation of novel dendrimer-gold complex nanoparticles for theranostic application in oncology

To view the supplementary data that accompany this paper please visit the journal website at: www.tandfonline.com/doi/suppl/10.2217/nnm-2023-0355Aim: Despite some successful examples of therapeutic nanoparticles reaching clinical stages, there is still a significant need for novel formulations in order to improve the selectivity and efficacy of cancer treatment. Methods: The authors developed two novel dendrimerâ gold (Au) complex-based nanoparticles using twodifferent synthesis routes: complexation method (formulation A) and precipitation method (formulation B). Using a biomimetic cancer-on-a-chip model, the authors evaluated the possible cytotoxicity and internalization by colorectal cancer cells of dendrimerâ Au complex-based nanoparticles. Results: The results showed promising capabilities of these nanoparticles for selectively targeting cancer cells and delivering drugs, particularly for the formulation A nanoparticles. Conclusion: This work highlights the potential of dendrimerâ Au complex-based nanoparticles as a new strategy to improve the targeting of anticancer drugs.This work is financially supported by the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 952603 (http://sgabu.eu/). This article reflects only the author’s view. The Commission is not responsible for any use that may be made of the information it contains. For the experimental part of the study, we acknowledge BIONECA project – Biomaterials and advanced physical techniques for regenerative cardiology and neurology (CA16122) – and the FEDER funded project 2IQBIONEURO (0624 2IQBIONEURO 6 E). M Carvalho acknowledges her postdoctoral contract TERM RES Hub – Scientific infrastructure for Tissue Engineering and Regenerative Medicine Ref Norte-01-0145-FEDER-02219015. D Caballero acknowledges the financial support from the Portuguese Foundation for Science and Technology (FCT) under the program CEEC Individual 2017 (CEECIND/00352/2017), and the project 2MATCH (PTDC/BTM-ORG/28070/2017) funded by the Programa Operacional Regional do Norte supported by European Regional Development Funds (ERDF). This work was partially supported by IET A. F. Harvey Engineering Research Award 2018 (ENG The Cancer). The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed

Interlayer Registry Determines the Sliding Potential of Layered Metal Dichalcogenides: The case of 2H-MoS2

We provide a simple and intuitive explanation for the interlayer sliding energy landscape of metal dichalcogenides. Based on the recently introduced registry index (RI) concept, we define a purely geometrical parameter which quantifies the degree of interlayer commensurability in the layered phase of molybdenum disulphide (2HMoS2). A direct relation between the sliding energy landscape and the corresponding interlayer registry surface of 2H-MoS2 is discovered thus marking the registry index as a computationally efficient means for studying the tribology of complex nanoscale material interfaces in the wearless friction regime.Comment: 13 pages, 7 figure

Semiconducting Monolayer Materials as a Tunable Platform for Excitonic Solar Cells

The recent advent of two-dimensional monolayer materials with tunable optoelectronic properties and high carrier mobility offers renewed opportunities for efficient, ultra-thin excitonic solar cells alternative to those based on conjugated polymer and small molecule donors. Using first-principles density functional theory and many-body calculations, we demonstrate that monolayers of hexagonal BN and graphene (CBN) combined with commonly used acceptors such as PCBM fullerene or semiconducting carbon nanotubes can provide excitonic solar cells with tunable absorber gap, donor-acceptor interface band alignment, and power conversion efficiency, as well as novel device architectures. For the case of CBN-PCBM devices, we predict the limit of power conversion efficiencies to be in the 10 - 20% range depending on the CBN monolayer structure. Our results demonstrate the possibility of using monolayer materials in tunable, efficient, polymer-free thin-film solar cells in which unexplored exciton and carrier transport regimes are at play.Comment: 7 pages, 5 figure

Line Defects in Molybdenum Disulfide Layers

Layered molecular materials and especially MoS2 are already accepted as promising candidates for nanoelectronics. In contrast to the bulk material, the observed electron mobility in single-layer MoS2 is unexpectedly low. Here we reveal the occurrence of intrinsic defects in MoS2 layers, known as inversion domains, where the layer changes its direction through a line defect. The line defects are observed experimentally by atomic resolution TEM. The structures were modeled and the stability and electronic properties of the defects were calculated using quantum-mechanical calculations based on the Density-Functional Tight-Binding method. The results of these calculations indicate the occurrence of new states within the band gap of the semiconducting MoS2. The most stable non-stoichiometric defect structures are observed experimentally, one of which contains metallic Mo-Mo bonds and another one bridging S atoms

Switching Mechanism in Single-Layer Molybdenum Disulfide Transistors: an Insight into Current Flow across Schottky Barriers

In this article, we study the properties of metal contacts to single-layer molybdenum disulfide (MoS2) crystals, revealing the nature of switching mechanism in MoS2 transistors. On investigating transistor behavior as contact length changes, we find that the contact resistivity for metal/MoS2 junctions is defined by contact area instead of contact width. The minimum gate dependent transfer length is ~0.63 {\mu}m in the on-state for metal (Ti) contacted single-layer MoS2. These results reveal that MoS2 transistors are Schottky barrier transistors, where the on/off states are switched by the tuning the Schottky barriers at contacts. The effective barrier heights for source and drain barriers are primarily controlled by gate and drain biases, respectively. We discuss the drain induced barrier narrowing effect for short channel devices, which may reduce the influence of large contact resistance for MoS2 Schottky barrier transistors at the channel length scaling limit.Comment: ACS Nano, ASAP (2013