Preparation of nanomaterials with controllable size and shape at ambient conditions, without heating or cooling, is extremely attractive from the perspective of cost and energy efficiency. However, highly reactive precursors must be used to obtain NCs at ambient conditions, and this can make the control of particle formation extremely challenging. Degenerately p-doped copper sulfide NCs have attracted much recent interest based on the observation of localized surface plasmon resonance (LSPR) in these materials. These earth-abundant semiconductor NCs have potential applications ranging from photovoltaics to biomedical imaging. Here, we provide the first report of ambient-temperature preparation of covellite nanoplatelets. The lateral dimensions of these are controllable over a wide range while maintaining a constant thickness of 4 nm. The crystalline phase of the NCs is shown here to be controlled by the oxidation state of copper reagent, with a Cu(II) precursor required to prepare phase-pure covellite NCs. The NCs exhibit LSPR absorbance that depends upon their aspect ratio (their lateral dimension, at fixed thickness) and can be tuned over a range of more than 600 nm. Their optical absorbance was modelled quantitatively to extract consistent values of free carrier concentration and background polarizability that apply over a wide range of NC sizes.

Magnetic levitation has been used to implement low-cost and maintenance-free electromagnetic energy harvesting. The ability of levitation-based harvesting systems to operate autonomously for long periods of time makes them well-suited for self-powering a broad range of technologies. In this paper, a combined theoretical and experimental study is presented of a harvester configuration that utilizes the motion of a levitated hard-magnetic element to generate electrical power. A semi-analytical, non-linear model is introduced that enables accurate and efficient analysis of energy transduction. The model predicts the transient and steady-state response of the harvester a function of its motion (amplitude and frequency) and load impedance. Very good agreement is obtained between simulation and experiment with energy errors lower than 14.15% (mean absolute percentage error of 6.02%) and cross-correlations higher than 86%. The model provides unique insight into fundamental mechanisms of energy transduction and enables the geometric optimization of harvesters prior to fabrication and the rational design of intelligent energy harvesters.

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A method is proposed for controlling the assembly of colloidal magnetic core-shell nanoparticles into patterned monolayer structures with nanoscale feature resolution. The method is based on magnetic field-directed self-assembly that is enhanced using soft-magnetic template elements. The elements are embedded in a nonmagnetic substrate and magnetized using a uniform bias field. A key feature of this approach is the combined use of a uniform field with induced gradient-fields produced by the template elements. This enables the customization of a force field with localized regions of attractive and repulsive magnetic forces that provide extraordinary control of particle motion during assembly. The method is demonstrated using a computational model that simulates the assembly process taking into account magnetic and hydrodynamic forces including interparticle interactions, Brownian diffusion, van der Waals force and effects of surfactants. The analysis shows that extended geometric patterns of particles can be assembled with nanoscale resolution, beyond that of the template elements, within milliseconds. This is achieved by tailoring key parameters including the template geometry to produce a force field that focuses the particles into prescribed patterns; the thickness of the dielectric particle shell to control the magnetic dipole-dipole force upon contact and the particle volume fraction to suppress undesired aggregation during assembly. The proposed method broadly applies to arbitrary template geometries and multi-layered core-shell particles with at least one magnetic component. It can enable the self-assembly of complex patterns of nanoparticles and open up opportunities for the scalable fabrication of multifunctional nanostructured materials for a broad range of applications.