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The lack of mirror symmetry in binary semiconductor compounds turns them into polar materials, where two opposite orientations of the same crystallographic direction are possible. Interestingly, their physical properties (e.g., electronic or photonic) and morphological features (e.g., shape, growth direction, and so forth) also strongly depend on the polarity. It has been observed that nanoscale materials tend to grow with a specific polarity, which can eventually be reversed for very specific growth conditions. In addition, polar-directed growth affects the defect density and topology and might induce eventually the formation of undesirable polarity inversion domains in the nanostructure, which in turn will affect the photonic and electronic final device performance. Here, we present a review on the polarity-driven growth mechanism at the nanoscale, combining our latest investigation with an overview of the available literature highlighting suitable future possibilities of polarity engineering of semiconductor nanostructures. The present study has been extended over a wide range of semiconductor compounds, covering the most commonly synthesized III–V (GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb) and II–VI (ZnO, ZnTe, CdS, CdSe, CdTe) nanowires and other free-standing nanostructures (tripods, tetrapods, belts, and membranes). This systematic study allowed us to explore the parameters that may induce polarity-dependent and polarity-driven growth mechanisms, as well as the polarity-related consequences on the physical properties of the nanostructures.

Phonon surface scattering has been at the core of heat transport engineering in nanoscale devices. Herein, we demonstrate that this phonon pathway can be the sole mechanism only below a critical, size-dependent temperature. Above this temperature, the lattice phonon scattering coexists along with surface effects. By tailoring the mass disorder at the atomic level, the lattice dynamics in nanowires was artificially controlled without affecting morphology, crystallinity, chemical composition, or electronic properties, thus allowing the mapping of the temperature-thermal conductivity-diameter triple parameter space. This led to the identification of the critical temperature below which the effects of lattice mass disorder are suppressed to an extent that phonon transport becomes governed entirely by the surface. This behavior is discussed based on a modified Landauer-Datta-Lundstrom near-equilibrium transport model. Besides disentangling the main phonon scattering mechanisms, the established framework also provides the necessary input to further advance the design and modeling of heat transport in semiconductor nanoscale systems.

In the past decade, profuse research efforts explored the uses of iron oxide particles in nanomedicine. To a great extent, the efficiency and fate of those magnetic nanoparticles depend on how their surfaces interface with the proteins in a physiological environment. It is well reported how an ungoverned protein corona can be detrimental to cellular uptake and targeting efficiency and how it can modify the nanoparticles biodistribution. Novel strategies are emerging to achieve enhanced and more reproducible performances of engineered nanoparticles with a custom-built protein corona. Here we report on a generalized protocol to preform a monolayer of human serum albumin (HSA) on superparamagnetic iron oxide nanoparticles (SPIONs) of different sizes. The resulting molecular structures are described by molecular dynamics simulations of the hybrid nanoconjugates. The simulations outcomes regarding the number of proteins in the corona and their monolayer arrangement on the particle surface are in agreement with the results obtained from dynamic light scattering and electronic microscopy analysis. Using tryptophan fluorescence quenching, we revealed the existence of a strong interaction between the SPIONs and the HSA which endorses the robustness of the protein−nanoparticle conjugates in this system. Moreover, we evaluated the effect of the HSA corona on the SPIONs efficiency as magnetic resonance imaging (MRI) contrast agents in water, human serum, and saline media. The protein corona did not affect the efficiency of the SPIONs as T2 contrast agents but reduce their T1 efficiency. In addition, we observed a greater stability for HSA-SPIONs nanoconjugates in saline and in acid media, preventing nanoparticle dissolution in extreme gastric conditions.

Methodologies that involve the use of nanoparticles as “artificial atoms” to rationally build materials in a bottom-up fashion are particularly well-suited to control the matter at the nanoscale. Colloidal synthetic routes allow for an exquisite control over such “artificial atoms” in terms of size, shape, and crystal phase as well as core and surface compositions. We present here a bottom-up approach to produce Pb–Ag–K–S–Te nanocomposites, which is a highly promising system for thermoelectric energy conversion. First, we developed a high-yield and scalable colloidal synthesis route to uniform lead sulfide (PbS) nanorods, whose tips are made of silver sulfide (Ag2S). We then took advantage of the large surface-to-volume ratio to introduce a p-type dopant (K) by replacing native organic ligands with K2Te. Upon thermal consolidation, K2Te-surface modified PbS–Ag2S nanorods yield p-type doped nanocomposites with PbTe and PbS as major phases and Ag2S and Ag2Te as embedded nanoinclusions. Thermoelectric characterization of such consolidated nanosolids showed a high thermoelectric figure-of-merit of 1 at 620 K.

Nanowires are a versatile platform to investigate and harness phonon and thermal transport phenomena in nanoscale systems. With this perspective, we demonstrate herein the use of crystal phase and mass disorder as effective degrees of freedom to manipulate the behavior of phonons and control the flow of local heat in silicon nanowires. The investigated nanowires consist of isotopically pure and isotopically mixed nanowires bearing either a pure diamond cubic or a cubic-rhombohedral polytypic crystal phase. The nanowires with tailor-made isotopic compositions were grown using isotopically enriched silane precursors 28SiH4, 29SiH4, and 30SiH4 with purities better than 99.9%. The analysis of polytypic nanowires revealed ordered and modulated inclusions of lamellar rhombohedral silicon phases toward the center in otherwise diamond-cubic lattice with negligible interphase biaxial strain. Raman nanothermometry was employed to investigate the rate at which the local temperature of single suspended nanowires evolves in response to locally generated heat. Our analysis shows that the lattice thermal conductivity in nanowires can be tuned over a broad range by combining the effects of isotope disorder and the nature and degree of polytypism on phonon scattering. We found that the thermal conductivity can be reduced by up to ∼40% relative to that of isotopically pure nanowires, with the lowest value being recorded for the rhombohedral phase in isotopically mixed 28Six30Si1–x nanowires with composition close to the highest mass disorder (x ∼ 0.5). These results shed new light on the fundamentals of nanoscale thermal transport and lay the groundwork to design innovative phononic devices.

Van der Waals epitaxy is an attractive alternative to direct heteroepitaxy where the forced coherency at the interface cannot sustain large differences in lattice parameters and thermal expansion coefficients between the substrate and the epilayer. Herein, the growth of monocrystalline InP on Ge and SiO2/Si substrates using graphene as an interfacial layer is demonstrated. Micrometer‐sized InP crystals are found to grow with interfaces of high crystalline quality and with different degrees of coalescence depending on the growth conditions. Some InP crystals exhibit a polytypic structure, consisting of alternating zinc‐blende and wurtzite phases, forming a type‐II homojunction with well (barrier) width of about 10 nm. The optical properties, investigated using room temperature nano‐cathodoluminescence, indicate the signatures of the direct optical transitions at 1.34 eV across the gap of the zinc‐blende phase and the indirect transitions at ≈1.31 eV originating from the alternating zinc‐blende and wurtzite phases. Additionally, the InP nanorods, found growing mainly on the graphene/SiO2/Si substrate, show optical transition across the gap of the wurtzite phase at ≈1.42 eV. This demonstration of InP growth on graphene and the correlative study between the structure and optical properties pave the way to develop hybrid structures for potential applications in integrated photonic and optoelectronic devices.

Colloidal Pd2Sn and Au–Pd2Sn nanorods (NRs) with tuned size were produced by the reduction of Pd and Sn salts in the presence of size- and shape-controlling agents and the posterior growth of Au tips through a galvanic replacement reaction. Pd2Sn and Au–Pd2Sn NRs exhibited high catalytic activity toward quasi-homogeneous hydrogenation of alkenes (styrene and 1-octene) and alkynes (phenylacetylene and 1-octyne) in dichloromethane. Au–Pd2Sn NRs showed higher activity than Pd2Sn for 1-octene, 1-octyne, and phenylacetylene. In Au–Pd2Sn heterostructures, X-ray photoelectron spectroscopy evidenced an electron donation from the Pd2Sn NR to the Au tips. Such heterostructures showed distinct catalytic behavior in the hydrogenation of compounds containing a triple bond such as tolan. This can be explained by the aurophilicity of triple bonds. To further study this effect, Pd2Sn and Au–Pd2Sn NRs were also tested in the Sonogashira coupling reaction between iodobenzene and phenylacetylene in N,N-dimethylformamide. At low concentration, this reaction provided the expected product, tolan. However, at high concentration, more reduced products such as stilbene and 1,2-diphenylethane were also obtained, even without the addition of H2. A mechanism for this unexpected reduction is proposed.

An optical in situ strategy for the analysis of oxygen diffusion in ultrathin ceria layers with a thickness of 2–10 nm at temperatures between 50 and 200 °C is presented, which allows for the determination of diffusion coefficients. This method is based on the sensitivity of the photoluminescence (PL) intensity of InGaN nanowires to adsorbed oxygen. The oxygen diffusion through an ultrathin CeO2 coating deposited on the InGaN nanowires is monitored by analyzing the transient PL behavior of the InGaN nanowires, which responds to changes of the oxygen concentration in the environment when the corresponding oxygen concentration is established at the CeO2/InGaN interface due to diffusion through the coating. Quantitative evaluation of the oxygen diffusion in CeO2 based on a model considering Langmuir Adsorption and recombination yields a diffusion coefficient D of (2.55 ± 0.05) × 10−16 cm2 s−1 at a temperature of 100 °C. Temperature‐dependent measurements reveal an Arrhenius type behavior of D with an activation energy of (0.28 ± 0.04) eV. In contrast, no oxygen diffusion is detected for an ultrathin layer (≥5 nm) of Al2O3, which is known as a poor oxygen ion conductor within the analyzed temperature regime.

Axial Si/Ge heterostructure nanowires, despite their promise in applications ranging from electronics to thermal transport, remain notoriously difficult to synthesize. Here, we grow axial Si/Ge heterostructures at low temperatures using a Au catalyst with a combination of trisilane and digermane. This approach yields, as determined with detailed electron microscopy characterization, arrays of epitaxial Si/Ge nanowires with excellent morphologies and purely axial composition profiles. Our data indicate that heterostructure formation can occur via the vapor-liquid-solid or vapor-solid-solid mechanism. These findings highlight the importance of precursor chemistry in semiconductor nanowire synthesis and open the door to Si/Ge nanowires with programmable quantum domains.

Self-assembling approaches based on chemical solution deposition (CSD) are ideal methods for the cost-effective production of epitaxial nanostructures with high throughput. Therefore, an in-depth investigation of the nucleation and coarsening processes involved in the self-assembly of nanostructures is mandatory to achieve a good control over nanostructure shape, dimensions, and orientation. Heteroepitaxial Ce0.9Gd0.1O2-y (CGO) is an ideal model system to unveil the underlying nanostructure development mechanisms in addition to their promising properties for catalysis, gas sensors, and ionic conductivity. Rapid thermal annealing furnaces have been used to study separately the thermodynamic and kinetic nucleation and coarsening mechanisms of self-assembled CGO isotropic and anisotropic nanostructures based on strain-engineering and surface energies control. Different CGO nanoislands are obtained: isotropic (001)CGO nanodots are grown on (001)-oriented Y2O3:ZrO2 (YSZ) and LaAlO3 (LAO) substrates, whereas (011)LAO substrates promote the growth of elongated (011)CGO nanowires. HRTEM and RHEED analyses are used to study the early stages of nucleation, as well as the shape and interfacial structure of CGO nanostructures. A systematic study with the heating ramp, annealing temperature and time, and strain in combination with thermally activated theoretical models provides information on the nucleation behavior, nucleation barriers, and atomic diffusion coefficients along in-plane and out-of-plane island orientations. Highly anisotropic atomic diffusion constants have been shown to be at the origin of the high aspect ratios of some of the nanostructures. Overall, our study provides a general method for the evaluation of nucleation and coarsening of multiple CSD-derived oxide nanostructures and understanding the shape development by combining thermodynamic and kinetic approaches.

Understanding the growth mechanisms of nanostructures obtained from chemical solutions, a high-throughput production methodology, is essential to correlate precisely the growth conditions with the nanostructures' morphology, dimensions and orientation. It is shown that self-organized (011)-oriented CeGdO (CGO) nanowires having a single in-plane orientation are achieved when an anisotropic (011)-LaAlO (LAO) substrate is chosen. STEM and AFM images of the epitaxial nanowires reveal the (001)CGO[0-11](011)LAO[100] growth orientation, with the enlargement occurring along the [0-11]CGO direction with (111) lateral facets. The chosen substrate allowed us to study a unique case where the resulting biaxial strain is isotropic, while the dissimilar lateral surface energies are the key factor to obtain an energetically imbalanced and non-degenerated nanowire configuration. Rapid Thermal Annealing (RTA) has allowed sorting of experimental nucleation from coarsening and analysis of the kinetic phenomena of the nanowires. A thermodynamic driving force is shown to exist for a continuous elongation of the nanowires while the coarsening rates are found to be strongly temperature dependent and so kinetic effects are the key factors to control the size and density of the self-organized nanowire system. A remarkably fast nanowire growth rate (14-40 nm min) is observed, which we associate with a high atomic mobility probably linked to a high concentration of oxygen vacancies, as detected by XPS. These nanowires are envisaged as model systems pushing forward the study of low energetic and highly oxygen deficient {111} lateral facets useful for catalysis, gas sensors and ionic conductivity applications.

The control over crystalline defects is of extreme importance when growing functional materials, since their presence may alter the final behavior (opto-electronic properties) and evolution of the growing system (orientation and shape). In the present work we address both phenomena in the particular case of InSb related nanostructures. On one hand, heterostructured architectures may show interfacial dislocations if reaching the coherency limit, which is dependent on geometrical constrictions of the systems in addition to the mismatch between phases. In this context, it is usually assumed that the mismatch strain in axial heterostructured nanowires is mainly relaxed by elastic distortion of the lattice, although theoretical calculations predict the formation of misfit dislocations. It is remarkably important unveiling whether the partial/total lattice relaxation takes place through elastic and/or plastic mechanisms since both will affect the material performance. Interestingly, the location and shape of the heterointerfaces, as well as possible diffusion phenomena involved, may hinder the understanding of the actual relaxation mechanism, as we show in the case of axial InAs/InSb nanowires. Contrary to most reported works, we find out the presence of misfit dislocations at the core of the system while there is a huge plane bending through the edges of the nanowires. On the other hand, we correlate the systematic observation of a lateral twin boundary with the morphological transition from nanowires to membrane-like systems, called nanosails. Based on the experimental data gathered, consisting on SEM and aberration-corrected STEM measurements, including polarity determination, we are able to establish the underlying defect-driving growth mechanism leading to the formation of membrane-like structures growing aside InAs/InSb nanowires, showing excellent transport properties. Possible instabilities during the growth may promote the sinking of the catalytic droplet to wet

Honeycomb-like hematite nanoflakes/branched polypyrrole nanoleaves heterostructures with a 3D complex structure have been synthesized and employed as high-performance negative electrodes for asymmetric supercapacitors application. The detailed TEM-STEM characterization and deep EELS chemical analysis at the nanoscale has been combined to elucidate the mechanisms underlying the formation and morphology evolution of core-branch Fe2O3@PPy heterostructures.[1] In addition, we have studied the mechanism of converting MoO2 nanoflakes into 2D free-standing MoS2 electrode by sulfurization process for water splitting. In this way, the atomic resolution aberration corrected HAADF STEM reveals the sulfurization mechanism in an unprecedented detail, together with EELS chemical maps. Furthermore, in-situ electrical measurements have been performed by means of c-AFM on the MoO2/MoS2 nanoflakes surfaces in order to detect their electronic active sites.

A wide variety of metal oxide manganite and ferrite core-shell nanoparticles (NPs) with adjustable composition were characterized by means of Transmission Electron Microscopy related tools in order to precisely understand their composition, the arrangement of the different chemical species and cation valence state variation along the nanoparticles. The nanoparticles were produced at low temperature and ambient atmosphere using a one-pot two-step synthesis protocol involving the cation exchange of Mn or Fe by Co or Ni in preformed Mn3O4 or Fe3O4 NPs, allowing the formation of a core shell structure. These nanoparticle systems present a complex 3D structure, which has been modelled with Rhodius software and simulated with the STEM-CELL packages, giving rise to a fine determination of the studied systems. Special attention has been paid on manganite – cobalt systems. By selecting the proper cobalt precursor, CoO crystallites could be simultaneously nucleated on the NP surface to form Mn3O4@CoMn2O4–CoO (Figure 1). In this latest case, heterostructured NPs exhibited improved performance and durability as bifunctional catalysts for the oxygen reduction and evolution reactions (ORR, OER) over commercial Pt and IrO2-based catalysts and over previously reported spinel electrocatalysts in alkaline solution.

Ultra narrow bandgap III-V semiconductor nanomaterials provide a unique platform for realizing advanced nanoelectronics, thermoelectrics, infrared photodetection, and quantum transport physics. In this work we employ molecular beam epitaxy to synthesize novel nanosheet-like InSb nanostructures exhibiting superior electronic performance. Through careful morphological and crystallographic characterization we show how this unique geometry is the result of a single twinning event in an otherwise pure zinc blende structure. Four-terminal electrical measurements performed in both the Hall and van der Pauw configurations reveal a room temperature electron mobility greater than 12 000 cm·V·s. Quantized conductance in a quantum point contact processed with a split-gate configuration is also demonstrated. We thus introduce InSb >nanosails> as a versatile and convenient platform for realizing new device and physics experiments with a strong interplay between electronic and spin degrees of freedom.

Vapor-liquid-solid nanowire growth below the bulk metal-semiconductor eutectic temperature is known for several systems; however, the fundamental processes that govern this behavior are poorly understood. Here, we show that hydrogen atoms adsorbed on the Ge nanowire sidewall enable AuGe catalyst supercooling and control Au transport. Our approach combines in situ infrared spectroscopy to directly and quantitatively determine hydrogen atom coverage with a >regrowth> step that allows catalyst phase to be determined with ex situ electron microscopy. Maintenance of a supercooled catalyst with only hydrogen radical delivery confirms the centrality of sidewall chemistry. This work underscores the importance of the nanowire sidewall and its chemistry on catalyst state, identifies new methods to regulate catalyst composition, and provides synthetic strategies for subeutectic growth in other nanowire systems.

We have characterized the photodetection capabilities of single GaN nanowires incorporating 20 periods of AlN/GaN:Ge axial heterostructures enveloped in an AlN shell. Transmission electron microscopy confirms the absence of an additional GaN shell around the heterostructures. In the absence of a surface conduction channel, the incorporation of the heterostructure leads to a decrease of the dark current and an increase of the photosensitivity. A significant dispersion in the magnitude of dark currents for different single nanowires is attributed to the coalescence of nanowires with displaced nanodisks, reducing the effective length of the heterostructure. A larger number of active nanodisks and AlN barriers in the current path results in lower dark current and higher photosensitivity and improves the sensitivity of the nanowire to variations in the illumination intensity (improved linearity). Additionally, we observe a persistence of the photocurrent, which is attributed to a change of the resistance of the overall structure, particularly the GaN stem and cap sections. As a consequence, the time response is rather independent of the dark current.

Uniform silicon nanocrystals were synthesized with cuboctahedral shape and passivated with 1-dodecene capping ligands. Transmission electron microscopy, electron diffraction, and grazing incidence wide-angle and small-angle X-ray scattering show that these soft cuboctahedra assemble into face-centered cubic superlattices with orientational order. The preferred nanocrystal orientation was found to depend on the orientation of the superlattices on the substrate, indicating that the interactions with the substrate and assembly kinetics can influence the orientation of faceted nanocrystals in superlattices.

MnO@CoMnO nanoparticles (NPs) were produced at low temperature and ambient atmosphere using a one-pot two-step synthesis protocol involving the cation exchange of Mn by Co in preformed MnO NPs. Selecting the proper cobalt precursor, the nucleation of CoO crystallites at the MnO@CoMnO surface could be simultaneously promoted to form MnO@CoMnO-CoO NPs. Such heterostructured NPs were investigated for oxygen reduction and evolution reactions (ORR, OER) in alkaline solution. MnO@CoMnO-CoO NPs with [Co]/[Mn] = 1 showed low overpotentials of 0.31 V at -3 mA·cm and a small Tafel slope of 52 mV·dec for ORR, and overpotentials of 0.31 V at 10 mA·cm and a Tafel slope of 81 mV·dec for OER, thus outperforming commercial Pt-, IrO-based and previously reported transition metal oxides. This cation-exchange-based synthesis protocol opens up a new approach to design novel heterostructured NPs as efficient nonprecious metal bifunctional oxygen catalysts.

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 FeO@NiFeO 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 FeO nanocubes with a defective shell and the posterior diffusion of nickel cations within this defective shell. FeO@NiFeO NPs were subsequently spin-coated over ITO-covered glass and their electrocatalytic activity toward water oxidation in carbonate electrolyte was characterized. FeO@NiFeO catalysts reached current densities above 1 mA/cm with a 410 mV overpotential and Tafel slopes of 48 mV/dec, which is among the best electrocatalytic performances reported in carbonate electrolyte.

Bismuth telluride based thermoelectric materials show great promise in electricity generation from waste heat and solid-state refrigeration, but improving their conversion efficiency with economical approaches for widespread use remains a challenge. An economical facile bottom-up approach has been developed to obtain nanostructured powders, which are used to build bulk thermoelectric materials. Using excess tellurium as sacrificial additive to enable liquid-phase sintering in the spark plasma sintering process, the lattice and bipolar contributions to the thermal conductivity are both greatly reduced without compromising too much the power factor, which leads to the achievement of high figure of merit (ZT) in both n-type and p-type bismuth telluride based nanocomposites. The ZT values are 1.59±0.16 for p-type BiSbTe and 0.98±0.07 for n-type BiTeSe at 370 K, which are significantly high for bottom-up approaches. These results demonstrate that solution-chemistry approaches as facile, scalable and low-energy-intensive ways to achieve nanopowders, combined with liquid-phase sintering process, can open up great possibilities in developing high-performance low-price thermoelectric bulk nanocomposites.

This study investigates the mechanisms of epitaxial development and functional properties of oxide thin films (Ce0.9Zr0.1O2−y, LaNiO3, and Ba0.8Sr0.2TiO3) grown on single crystal substrates (Y2O3:ZrO2, LaAlO3, and SrTiO3) by the chemical solution deposition approach. Rapid thermal annealing furnaces are very powerful tools in this study providing valuable information of the early stages of nucleation, the kinetics of epitaxial film growth, and the coarsening of nanocrystalline phases. Advanced transmission electron microscopies, X-ray diffraction, and atomic force microscopy are employed to investigate the film microstructure and morphology, microstrain relaxation, and epitaxial crystallization. This study demonstrates that the isothermal evolution toward epitaxial film growth follows a self-limited process driven by atomic diffusion, and surface and interface energy minimization. All investigated oxides experience a transformation from the polycrystalline to the epitaxial phase. This study unequivocally evidences that the film thickness highly influences the epitaxial crystallization rate due to the competition between heterogeneous and homogeneous nucleation barriers and the fast coarsening of polycrystalline grains as compared to epitaxial growth. The investigated films possess good functional properties, and this study successfully confirms an improvement at long annealing times that can be correlated with grain boundary healing processes. Thick epitaxial films can be crystallized by growing sequential individual epitaxial layers.

In this work, BaSrTiO (BST) films on LaNiO-buffered SiO/Si (LNO/SiO/Si) substrates were crystallized by pulsed laser irradiation. Solution-derived amorphous barium-strontium-titanate precursor layers were crystallized with a KrF excimer laser in oxygen ambient at fluences ranging from 50 to 75 mJ cm. With the substrate temperature set to 500 °C, the number of pulses and film thickness were varied until high-quality crystallinity could be achieved. It was found that films with a thickness of 40 nm are fully crystallized with a uniaxial {00l} orientation which is predetermined by the LaNiO orientation. On the other hand, for 160 nm thick films, crystallization was observed after 12,000 pulses in the 70 nm close to the surface, while the rest of the film remained amorphous. The large temperature difference between the film surface and interface due to the low thermal conductivity of the amorphous BST is suggested as the origin of this behavior. Films thicker than 80 nm cracked on crystallization due to the stress caused by the different thermal expansion coefficients of film and substrate, as well as the large temperature variations within the BST film.

© 2015 AIP Publishing LLC. Highly crystalline epitaxial Ba0.8Sr0.2TiO3 (BST) thin-films are grown on (001)-oriented LaNiO3-buffered LaAlO3 substrates by pulsed laser irradiation of solution derived barium-zirconium-titanium precursor layers using a UV Nd:YAG laser source at atmospheric conditions. The structural analyses of the obtained films, studied by X-ray diffractometry and transmission electron microscopy, demonstrate that laser processing allows the growth of tens of nm-thick BST epitaxial films with crystalline structure similar to that of films obtained through conventional thermal annealing methods. However, the fast pulsed nature of the laser employed leads to crystallization kinetic evolution orders of magnitude faster than in thermal treatments. The combination of specific photothermal and photochemical mechanisms is the main responsible for the ultrafast epitaxial laser-induced crystallization. Piezoresponse microscopy measurements demonstrate equivalent ferroelectric behavior in laser and thermally annealed films, being the piezoelectric constant ∼25 pm V-1.

In this work the position-controlled growth of GaN nanowires (NWs) on diamond by means of molecular beam epitaxy is investigated. In terms of growth, diamond can be seen as a model substrate, providing information of systematic relevance also for other substrates. Thin Ti masks are structured by electron beam lithography which allows the fabrication of perfectly homogeneous GaN NW arrays with different diameters and distances. While the wurtzite NWs are found to be Ga-polar, N-polar nucleation leads to the formation of tripod structures with a zinc-blende core which can be efficiently suppressed above a substrate temperature of 870 °C. A variation of the III/V flux ratio reveals that both axial and radial growth rates are N-limited despite the globally N-rich growth conditions, which is explained by the different diffusion behavior of Ga and N atoms. Furthermore, it is shown that the hole arrangement has no effect on the selectivity but can be used to force a transition from nanowire to nanotube growth by employing a highly competitive growth regime.

GaN/AlN nanowire heterostructures can display photoluminescence (PL) decay times on the order of microseconds that persist up to room temperature. Doping the GaN nanodisk insertions with Ge can reduce these PL decay times by two orders of magnitude. These phenomena are explained by the three-dimensional electric field distribution within the GaN nanodisks, which has an axial component in the range of a few MV/cm associated to the spontaneous and piezoelectric polarization, and a radial piezoelectric contribution associated to the shear components of the lattice strain. At low dopant concentrations, a large electron-hole separation in both the axial and radial directions is present. The relatively weak radial electric fields, which are about one order of magnitude smaller than the axial fields, are rapidly screened by doping. This bidirectional screening leads to a radial and axial centralization of the hole underneath the electron, and consequently, to large decreases in PL decay times, in addition to luminescence blue shifts.

GaAs nanowire arrays on silicon offer great perspectives in the optoelectronics and solar cell industry. To fulfill this potential, gold-free growth in predetermined positions should be achieved. Ga-assisted growth of GaAs nanowires in the form of array has been shown to be challenging and difficult to reproduce. In this work, we provide some of the key elements for obtaining a high yield of GaAs nanowires on patterned Si in a reproducible way: contact angle and pinning of the Ga droplet inside the apertures achieved by the modification of the surface properties of the nanoscale areas exposed to growth. As an example, an amorphous silicon layer between the crystalline substrate and the oxide mask results in a contact angle around 90°, leading to a high yield of vertical nanowires. Another example for tuning the contact angle is anticipated, native oxide with controlled thickness. This work opens new perspectives for the rational and reproducible growth of GaAs nanowire arrays on silicon.

The introduction of stable isotopes in the fabrication of semiconductor nanowires provides an additional degree of freedom to manipulate their basic properties, design an entirely new class of devices, and highlight subtle but important nanoscale and quantum phenomena. With this perspective, we report on phonon engineering in metal-catalyzed silicon nanowires with tailor-made isotopic compositions grown using isotopically enriched silane precursors 28SiH4, 29SiH4, and 30SiH4 with purity better than 99.9%. More specifically, isotopically mixed nanowires 28Six30Si1–x with a composition close to the highest mass disorder (x ∼ 0.5) were investigated. The effect of mass disorder on the phonon behavior was elucidated and compared to that in isotopically pure 29Si nanowires having a similar reduced mass. We found that the disorder-induced enhancement in phonon scattering in isotopically mixed nanowires is unexpectedly much more significant than in bulk crystals of close isotopic compositions. This effect is explained by a nonuniform distribution of 28Si and 30Si isotopes in the grown isotopically mixed nanowires with local compositions ranging from x = ∼0.25 to 0.70. Moreover, we also observed that upon heating, phonons in 28Six30Si1–x nanowires behave remarkably differently from those in 29Si nanowires suggesting a reduced thermal conductivity induced by mass disorder. Using Raman nanothermometry, we found that the thermal conductivity of isotopically mixed 28Six30Si1–x nanowires is ∼30% lower than that of isotopically pure 29Si nanowires in agreement with theoretical predictions.