p‑GaN/n-ZnO Heterojunction Nanowires: Optoelectronic Properties and the Role of Interface Polarity

In this work, simulations of the electronic band structure of a p-GaN/n-ZnO heterointerface are presented. In contrast to homojunctions, an additional energy barrier due to the type-II band alignment hinders the flow of majority charge carriers in this heterojunction. Spontaneous polarization and piezoelectricity are shown to additionally affect the band structure and the location of the recombination region. Proposed as potential UV-LEDs and laser diodes, p-GaN/n-ZnO heterojunction nanowires were fabricated by plasma-assisted molecular beam epitaxy (PAMBE). Atomic resolution annular bright field scanning transmission electron microscopy (STEM) studies reveal an abrupt and defect-free heterointerface with a polarity inversion from N-polar GaN to Zn-polar ZnO. Photoluminescence measurements show strong excitonic UV emission originating from the ZnO-side of the interface as well as stimulated emission in the case of optical pumping above a threshold of 55 kW/cm²

Extending the Nanocrystal Synthesis Control to Quaternary Compositions

The ample chemical and structural freedom of quaternary compounds permits engineering materials that fulfill the requirements of a wide variety of applications. In this work, the mechanisms to achieve unprecedented size, shape, and composition control in quaternary nanocrystals are detailed. The described procedure allows obtaining tetrahedral and penta-tetrahedral quaternary nanocrystals with tuned size distributions and controlled compositions from a plethora of I₂–II–IV–VI₄ semiconductors

Metal Ions To Control the Morphology of Semiconductor Nanoparticles: Copper Selenide Nanocubes

Morphology is a key parameter in the design of novel nanocrystals and nanomaterials with controlled functional properties. Here, we demonstrate the potential of foreign metal ions to tune the morphology of colloidal semiconductor nanoparticles. We illustrate the underlying mechanism by preparing copper selenide nanocubes in the presence of Al ions. We further characterize the plasmonic properties of the obtained nanocrystals and demonstrate their potential as a platform to produce cubic nanoparticles with different composition by cation exchange

Cu₂ZnGeSe₄ Nanocrystals: Synthesis and Thermoelectric Properties

A synthetic route for producing Cu₂ZnGeSe₄ nanocrystals with narrow size distributions and controlled composition is presented. These nanocrystals were used to produce densely packed nanomaterials by hot-pressing. From the characterization of the thermoelectric properties of these nanomaterials, Cu₂ZnGeSe₄ is demonstrated to show excellent thermoelectric properties. A very preliminary adjustment of the nanocrystal composition has already resulted in a figure of merit of up to 0.55 at 450 °C

Crystallographic Control at the Nanoscale To Enhance Functionality: Polytypic Cu₂GeSe₃ Nanoparticles as Thermoelectric Materials

The potential to control the composition and crystal phase at the nanometer scale enable the production of nanocrystalline materials with enhanced functionalities and new applications. In the present work, we detail a novel colloidal synthesis route to prepare nanoparticles of the ternary semiconductor Cu₂GeSe₃ (CGSe) with nanometer-scale control over their crystal phases. We also demonstrate the structural effect on the thermoelectric properties of bottom-up-prepared CGSe nanomaterials. By careful adjustment of the nucleation and growth temperatures, pure orthorhombic CGSe nanoparticles with cationic order or polytypic CGSe nanoparticles with disordered cation positions can be produced. In this second type of nanoparticle, a high density of twins can be created to periodically change the atomic plane stacking, forming a hexagonal wurtzite CGSe phase. The high yield of the synthetic routes reported here allows the production of single-phase and multiphase CGSe nanoparticles in the gram scale, which permits characterization of the thermoelectric properties of these materials. Reduced thermal conductivities and a related 2.5-fold increase of the thermoelectric figure of merit for multiphase nanomaterials compared to pure-phase CGSe are systematically obtained. These results are discussed in terms of the density and efficiency of phonon scattering centers in both types of materials

Fe₃O₄@NiFe_<i>x</i>O_<i>y</i> Nanoparticles with Enhanced Electrocatalytic Properties for Oxygen Evolution in Carbonate Electrolyte

The design and engineering of earth-abundant catalysts that are both cost-effective and highly active for water splitting are crucial challenges in a number of energy conversion and storage technologies. In this direction, herein we report the synthesis of Fe₃O₄@NiFe_<i>x</i>O_<i>y</i> core–shell nanoheterostructures and the characterization of their electrocatalytic performance toward the oxygen evolution reaction (OER). Such nanoparticles (NPs) were produced by a two-step synthesis procedure involving the colloidal synthesis of Fe₃O₄ nanocubes with a defective shell and the posterior diffusion of nickel cations within this defective shell. Fe₃O₄@NiFe_<i>x</i>O_<i>y</i> NPs were subsequently spin-coated over ITO-covered glass and their electrocatalytic activity toward water oxidation in carbonate electrolyte was characterized. Fe₃O₄@NiFe_<i>x</i>O_<i>y</i> catalysts reached current densities above 1 mA/cm² with a 410 mV overpotential and Tafel slopes of 48 mV/dec, which is among the best electrocatalytic performances reported in carbonate electrolyte