Suitable postsynthesis surface modification of lead-chalcogenide quantum dots (QDs) is crucial to enable their integration in photovoltaic devices. Here we exploit arenethiolate anions to completely replace pristine oleate ligands on PbS QDs in the solution phase, thus preserving the colloidal stability of QDs and allowing their solution-based processability into photoconductive thin films. Complete QD surface modification relies on the stronger acidic character of arenethiols compared to that of alkanethiols and is demonstrated by FTIR and UV−vis−NIR absorption spectroscopy analyses, which provide quantitative evaluation of stoichiometry and thermodynamic stability of the resulting system. Arenethiolate ligands induce a noticeable reduction of the optical band gap of PbS QDs, which is described and explained by charge transfer interactions occurring at the organic/inorganic interface that relax exciton confinement, and a large increase of QD molar absorption coefficient, achieved through the conjugated moiety of the replacing ligands. In addition, surface modification in the solution phase promotes switching of the symmetry of PbS QD self-assembled superlattices from hexagonal to cubic close packing, which is accompanied by further reduction of the optical band gap, ascribed to inter-QD exciton delocalization and dielectric effects, together with a drastic improvement of the charge transport properties in PbS QD solids. As a result, smooth dense-packed thin films of arenethiolatecapped PbS QDs can be integrated in heterojunction solar cells via a single solution-processing step. Such single PbS QD layers exhibit abated cracking upon thermal or chemical postdeposition treatment, and the corresponding devices generate remarkable photocurrent densities and overall efficiencies, thus representing an effective strategy toward low-cost processing for QD-based photovoltaics.
The future smart cities vision can be developed through leveraging the potentials of Internet of Things (IoT) and wireless sensor network (WSN) technologies. WSN is a resource constrained network where network nodes are tiny devices that are run on battery power. Diverse types of applications such as environmental and habitual monitoring, detection, and tracking, use WSNs. The invention of new network protocols, the establishment of new models for communications, and testing the available solutions in real world environment are some of the current research issues in WSNs. Main challenges in such networks include energy conservation in an efficient way, dealing with variable channel capacity, and the resource constrained nature of such networks. The design of architecture for such networks has a vital role in solving the issues to some extent, i.e., the cross layer design approach is an architectural technique that offers the interaction of different layers together to enhance the performance, minimize the energy consumption, enhance the network life time, and provide Quality of Service (QoS) in real time communications. These are some of the current areas where cross-layer design approaches are being used. This paper presents different types of cross-layer design techniques in wireless multimedia sensor networks. Using such architectural techniques, different state of the art cross-layer optimization approaches are discussed while giving the reader an insight on prominent challenges and issues along with future directions.
We report on the first dielectric investigation of high-k yttrium copper titanate thin films, which were demonstrated to be very promising for nanoelectronics applications.
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