In recent years, due to its inherent flexibility, magnetron-sputtering has been widely used to synthesise bi-metallic nanoparticles (NPs) via subsequent inert-gas cooling and gas-phase condensation of the sputtered atomic vapour. Utilising two separate sputter targets allows for good control over composition. Simultaneously, it involves fast kinetics and non-equilibrium processes, which can trap the nascent NPs into metastable configurations. In this study, we observed such configurations in immiscible, bi-metallic Ag-Cu NPs by scanning transmission electron microscopy (S/TEM) and electron energy-loss spectroscopy (EELS), and noticed a marked difference in the shape of NPs belonging to Ag- and Cu-rich samples. We explained the formation of Janus or Ag@Cu core/shell metastable structures on the grounds of in-flight mixed NP coalescence. We utilised molecular dynamics (MD) and Monte Carlo (MC) computer simulations to demonstrate that such configurations cannot occur as a result of nanoalloy segregation. Instead, sintering at relatively low temperatures can give rise to metastable structures, which eventually can be stabilised by subsequent quenching. Furthermore, we compared the heteroepitaxial diffusivities along various surfaces of both Ag and Cu NPs, and emphasised the differences between the sintering mechanisms of Ag- and Cu-rich NP compositions: small Cu NPs deform as coherent objects on large Ag NPs, whereas small Ag NPs dissolve into large Cu NPs, with their atoms diffusing along specific directions. Taking advantage of this observation, we propose controlled NP coalescence as a method to engineer mixed NPs of a unique, patterned core@partial-shell structure, which we refer to as a "glass-float" (ukidama) structure.

Controlling the polarity of ZnO nanowires in addition to the uniformity of their structural morphology in terms of position, vertical alignment, length, diameter, and period is still a technological and fundamental challenge for real-world device integration. In order to tackle this issue, we specifically combine the selective area growth on prepatterned polar c-plane ZnO single crystals using electron-beam lithography, with the chemical bath deposition. The formation of ZnO nanowires with a highly controlled structural morphology and a high optical quality is demonstrated over large surface areas on both polar c-plane ZnO single crystals. Importantly, the polarity of ZnO nanowires can be switched from O- to Zn-polar, depending on the polarity of prepatterned ZnO single crystals. This indicates that no fundamental limitations prevent ZnO nanowires from being O- or Zn-polar. In contrast to their catalyst-free growth by vapor-phase deposition techniques, the possibility to control the polarity of ZnO nanowires grown in solution is remarkable, further showing the strong interest in the chemical bath deposition and hydrothermal techniques. The single O- and Zn-polar ZnO nanowires additionally exhibit distinctive cathodoluminescence spectra. To a broader extent, these findings open the way to the ultimate fabrication of well-organized heterostructures made from ZnO nanowires, which can act as building blocks in a large number of electronic, optoelectronic, and photovoltaic devices.

Crystal defects in unintentionally doped ZnO nanowires grown by chemical bath deposition (CBD) play a capital role on their optical and electrical properties, governing the performances of many nanoscale engineering devices. However, the nature of these crystal defects is still highly debated. In particular, the hydrogen-related defects have not been explored in detail yet although the growth medium operates in aqueous solution. By using four-point probe resistivity measurements, we show that ZnO nanowires grown by CBD using zinc nitrate and hexamethylenetetramine exhibit a high electrical conductivity with electron densities ranging from 2.7 x 10 18 to 3.1 x 10 19 cm -3 . Most of them have a metallic electrical conduction. By combining density-functional theory calculations with cathodoluminescence and Raman spectroscopy, we reveal that the high electrical conductivity mostly originates from the formation of interstitial hydrogen in bond-centered sites (HBC) and of zinc vacancy -hydrogen (VZn-nH) complexes. In particular, the HBC and (VZn-3H) complex are found to act as two shallow donors with a very low formation energy, for which the most stable configurations are reported. Additionally, this combined theoretical and experimental approach allows us to revisit the highly debated origin of the visible and ultra-violet emission bands in the luminescence spectra. They are found to be mostly related to VZn and (VZn-nH) complexes located in the bulk and on the surfaces of ZnO nanowires. These findings represent an important step forward in the identification of the predominant native and extrinsic defects driving the electronic structure properties of ZnO nanowires grown by CBD. They further reveal the significance of hydrogen engineering to tune the source of crystal defects for optimizing the physical properties of ZnO nanowires.

The coalescence of nanoclusters fabricated in the gas phase is a fundamental growth mechanism determining cluster shapes, sizes, compositions, and structures, with resultant effects on practically all of their physical and chemical properties. Furthermore, coalescence can affect properties of larger structures that consist of nanoparticles as their elementary building blocks, such as the fractal dimension of cluster aggregates and the porosity and conductance of thin films. Therefore, it comes as no surprise that a great body of research, both experimental and theoretical, has focused on nanoparticle coalescence over the course of the past few decades. This review attempts to summarize the most important recent results from computational studies on nanoparticle coalescence and draw parallels between theoretical and experimental findings. The approach used here aspires to explain nanoparticle coalescence within the framework of a single intuitive narrative by integrating previous results obtained using various methods by the authors and others. Simultaneously, it is discussed where understanding and controlling (i.e., enhancing or inhibiting) nanoparticle coalescence can have great technological interest.

The structural properties and the strain state of InGaN/GaN superlattices embedded in GaN nanowires were analyzed as a function of superlattice growth temperature, using complementary transmission electron microscopy techniques supplemented by optical analysis using photoluminescence and spatially resolved microphotoluminescence spectroscopy. A truncated pyramidal shape was observed for the 4 nm thick InGaN inclusions, where their (0001¯) central facet was delimited by six-fold {101¯l} facets towards the m-plane sidewalls of the nanowires. The defect content of the nanowires comprised multiple basal stacking faults localized at the GaN base/superlattice interface, causing the formation of zinc-blende cubic regions, and often single stacking faults at the GaN/InGaN bilayer interfaces. No misfit dislocations or cracks were detected in the heterostructure, implying a fully strained configuration. Geometrical phase analysis showed a rather uniform radial distribution of elastic strain in the (0001¯) facet of the InGaN inclusions. Depending on the superlattice growth temperature, the elastic strain energy is partitioned among the successive InGaN/GaN layers in the case of low-temperature growth, while at higher superlattice growth temperature the in-plane tensile misfit strain of the GaN barriers is accommodated through restrained diffusion of indium from the preceding InGaN layers. The corresponding In contents of the central facet were estimated at 0.42 and 0.25, respectively. However, in the latter case, successful reproduction of the experimental electron microscopy images by image simulations was only feasible, allowing for a much higher occupancy of indium adatoms at lattice sites of the semipolar facets, compared to the invariable 25% assigned to the polar facet. Thus, a high complexity in indium incorporation and strain allocation between the different crystallographic facets of the InGaN inclusions is anticipated and supported by the results of photoluminescence and spatially resolved microphotoluminescence spectroscopy.

Using first-principles calculations based on density functional theory, we study the magnetic and electronic properties of hole-doped two-dimensional InSe. Our simulations reveal that although 2D InSe is intrinsically non-magnetic, a stable ferromagnetic phase appears for a wide range of hole densities. Interestingly, hole doping not only induces spontaneous magnetization but also half-metallicity, and hole-doped InSe, presenting one conducting and one insulating spin channel, could be highly promising for next generation spintronic nanodevices. The possibility to induce hole doping and a subsequent ferromagnetic order by intrinsic and extrinsic defects was also investigated. We found that In vacancy creates spin-polarized states close to the valence band and leads to a p-type behavior. Similar to In vacancies, group-V atoms replacing Se atoms lead to a p-type behavior, potentially stabilizing a ferromagnetic order in 2D InSe.

Indium tin oxide nanowires were grown by the reaction of In and Sn with O2 at 800 °C via the vapor-liquid-solid mechanism on 1 nm Au/Si(001). We obtain Sn doped In2O3 nanowires having a cubic bixbyite crystal structure by using In:Sn source weight ratios > 1:9 while below this we observe the emergence of tetragonal rutile SnO2 and suppression of In2O3 permitting compositional and structural tuning from SnO2 to In2O3 which is accompanied by a blue shift of the photoluminescence spectrum and increase in carrier lifetime attributed to a higher crystal quality and Fermi level position.

No abstract