Iñaki López

Critical behavior of ionic micellar systems at different salt concentrations

The coexistence curves of the system dodecylammonium chloride+water+KCl have been measured at different salt concentrations. The results can be described with the usual Ising 3‐D value for the critical exponent β=0.325. The analysis of the diameter indicates that the correct order parameter is defined in terms of an effective concentration calculated according to Eq. (4). Both the order parameter and the diameter of the coexistence curve point out that the range of validity of simple scaling decreases with the KCl concentration, i.e., as the system approaches a critical end point. The critical line, and an estimation of the Krafft temperatures have allowed us to estimate the position of the critical end point within the T‐w‐[KCl] space, w indicating the weight fraction of the surfactant. The pressure dependence of the critical temperature has been measured and found to be independent of the salt content for the present range of concentrations.

Crossed Ga2O3/SnO2 multiwire architecture: a local structure study with nanometer resolution.

Crossed nanowire structures are the basis for high-density integration of a variety of nanodevices. Owing to the critical role of nanowires intersections in creating hybrid architectures, it has become a challenge to investigate the local structure in crossing points in metal oxide nanowires. Thus, if intentionally grown crossed nanowires are well-patterned, an ideal model to study the junction is formed. By combining electron and synchrotron beam nanoprobes, we show here experimental evidence of the role of impurities in the coupling formation, structural modifications, and atomic site configuration based on crossed Ga2O3/SnO2 nanowires. Our experiment opens new avenues for further local structure studies with both nanometer resolution and elemental sensitivity.

Resonant cavity modes in gallium oxide microwires

Fabry Perot resonant modes in the optical range 660-770 nm have been detected from single and coupled Cr doped gallium oxide microwires at room temperature. The luminescence is due to chromium ions and dominated by the broad band involving the T-4(2)-(4)A(2) transition, strongly coupled to phonons, which could be of interest in tunable lasers. The confinement of the emitted photons leads to resonant modes detected at both ends of the wires. The separation wavelength between maxima follows the Fabry-Perot dependence on the wire length and the group refractive index for the Ga2O3 microwires.

β-Ga 2 O 3 nanowires for an ultraviolet light selective frequency photodetector

The behaviour of β-Ga2O3 nanowires as photoconductive material in deep ultraviolet photodetectors to operate in the energy range 3.0–6.2 eV has been investigated. The nanowires were grown by a catalyst-free thermal evaporation method on gallium oxide substrates. Photocurrent measurements have been carried out on both undoped and Sn-doped Ga2O3 nanowires to evidence the influence of the dopant on the photodetector performances. The responsivity spectrum of single nanowires show maxima in the energy range 4.8–5.4 eV and a strong dependence on the pulse frequency of the excitation light has been observed for undoped nanowires. Our results show that the responsivity of β-Ga2O3 nanowires can be controlled by tuning the chopper frequency of the excitation light and/or by doping of the nanowires. Non-linear behaviour in characteristic current–voltage curves has been observed for Ga2O3 : Sn nanowires. The mechanism leading to this behaviour has been discussed and related to space-charged-limited current effects. In addition, the responsivity achieved by doped nanowires at lower bias is higher than for undoped ones.

Shape engineering driven by selective growth of SnO2 on doped Ga2O3 nanowires

Tailoring the shape of complex nanostructures requires control of the growth process. In this work, we report on the selective growth of nanostructured tin oxide on gallium oxide nanowires leading to the formation of SnO2/Ga2O3 complex nanostructures. Ga2O3 nanowires decorated with either crossing SnO2 nanowires or SnO2 particles have been obtained in a single step treatment by thermal evaporation. The reason for this dual behavior is related to the growth direction of trunk Ga2O3 nanowires. Ga2O3 nanowires grown along the [001] direction favor the formation of crossing SnO2 nanowires. Alternatively, SnO2 forms rhombohedral particles on [110] Ga2O3 nanowires leading to skewer-like structures. These complex oxide structures were grown by a catalyst-free vapor-solid process. When pure Ga and tin oxide were used as source materials and compacted powders of Ga2O3 acted as substrates, [110] Ga2O3 nanowires grow preferentially. High-resolution transmission electron microscopy analysis reveals epitaxial relationship lattice matching between the Ga2O3 axis and SnO2 particles, forming skewer-like structures. The addition of chromium oxide to the source materials modifies the growth direction of the trunk Ga2O3 nanowires, growing along the [001], with crossing SnO2 wires. The SnO2/Ga2O3 junctions does not meet the lattice matching condition, forming a grain boundary. The electronic and optical properties have been studied by XPS and CL with high spatial resolution, enabling us to get both local chemical and electronic information on the surface in both type of structures. The results will allow tuning optical and electronic properties of oxide complex nanostructures locally as a function of the orientation. In particular, we report a dependence of the visible CL emission of SnO2 on its particular shape. Orange emission dominates in SnO2/Ga2O3 crossing wires while green-blue emission is observed in SnO2 particles attached to Ga2O3 trunks. The results show that the Ga2O3-SnO2 system appears to be a benchmark for shape engineering to get architectures involving nanowires via the control of the growth direction of the nanowires.

Doped gallium oxide nanowires for photonics

Monoclinic gallium oxide, β-Ga2O3, is a transparent conducting oxide (TCO) that presents one of the widest band gaps among this family of materials. Its characteristics make it highly interesting for applications in UV - visible - IR optoelectronic and photonic devices. On the other hand, the morphology of nanowires made of this oxide presents specific advantages for light emitting nanodevices, waveguides and gas sensors. Control of doping of the nanostructures is of the utmost importance in order to tailor the behavior of these devices. In this work, the growth of the nanowires is based on the vapor-solid (VS) mechanism during thermal annealing treatment while the doping process was carried out in three different ways. In one of the cases, doping was obtained during the growth of the wires. A second method was based on thermal diffusion of the dopants after the growth of undoped nanowires, while the third method used ion implantation to introduce optically active ions into previously grown nanowires. The study of the influence of the different dopants on the luminescence properties of gallium oxide nanowires is presented. In particular, transition metals and rare earths such as Cr, Gd, Er or Eu were used as optically active dopants that allowed selection of the luminescence wavelength, spanning from the UV to the IR ranges. The benefits and drawbacks of the three different doping methods are analyzed. The waveguiding behavior of the doped nanowires has been studied by room temperature micro-photoluminescence.

Doping of Ga2O3 bulk crystals and NWs by ion implantation

Ga2O3 bulk single crystals have been implanted with 300 keV Europium ions to fluences ranging from 1×1013 to 4×1015 at/cm2. The damage build-up and Eu-incorporation was assessed by Rutherford Backscattering Spectrometry in the channeling mode (RBS/C). RBS/C results suggest that implantation causes a mixture of defect clusters and extended defects such as dislocations. Amorphisation starts at the surface for fluences around 1×1015 at/cm2 and then proceeds to deeper regions of the sample with increasing fluence. Amorphous regions and defect clusters are efficiently removed during rapid thermal annealing at ~1100 °C; however, Eu diffuses towards the surface. Nevertheless, Eu ions are optically activated and show cathodoluminescence at room temperature. Results in bulk samples are compared to those in Eu-implanted Ga2O3 nanowires and despite strong similarities in the structural properties differences were found in the optical activation. Furthermore, damage and dopant incorporation studies were performed using the Perturbed Angular Correlation technique, which allows probing the immediate lattice surroundings of an implanted radioactive probe at the atomic level.

Hierarchical ZnGa2O4 and Cr doped Zn1 − x Mn x Ga2O4 nanostructures for room temperature light-emitting devices

Branched, hierarchically grown ZnGa2O4 and Zn1 − x Mn x Ga2O4 (0.1 < x < 0.21) micro- and nanostructures have been fabricated by a thermal evaporation method. Comparison of both materials shows that the presence of Mn favours the formation of the branched morphology, with oriented nanowires of high crystalline quality. The origin of the growth of these nanostructures is discussed. Raman peaks are observed to broaden and shift as a function of Mn content in the alloy. Cathodoluminescence analysis shows that ZnGa2O4 structures emit the characteristic defect-related UV-blue band and Zn1 − x Mn x Ga2O4 nanowires show efficient green emission due to intraionic Mn2+ transitions. Zn1 − x Mn x Ga2O4 structures have been doped with Cr3+ during thermal treatments in presence of Cr. This leads to structures with additional red emission related to Cr3+. The excitation conditions allow selection of the dominant luminescence band. Photocurrent measurements have also been carried out in ZMGO structures, showing the characteristic intraionic Mn2+ transitions.

3D and 2D growth of SnO2 nanostructures on Ga2O3 nanowires: synthesis and structural characterization

In this work, a simple thermal evaporation method has been used to obtain a variety of Ga2O3/SnO2 nano-assemblies with different shapes and dimensionalities, which may affect their physical properties, especially those influenced by surface properties. The obtained nanostructures have been characterized using electron microscopy-related techniques in order to understand their growth mechanisms. By using both metallic gallium and tin oxide powders as precursors, Ga2O3 nanowires (straight or branched) decorated with SnO2 nanoparticles or SnO2 quasi-two dimensional plates have been produced after dynamic thermal annealing for 2.5, 8.0 and 15.0 hours. For shorter treatments, accumulation of Sn atoms at the Ga2O3 nanowire surface or defect planes has been observed by high resolution TEM, which suggests that they could act as nucleation sites for the further growth of SnO2. On the other hand, longer treatments promote the formation of Ga-doped SnO2 belts, from which SnO2 nanowires eventually emerge. High-resolution TEM imaging and microanalysis reveal that Ga accumulation at (200) SnO2 planes could stabilize some non-stoichiometric or intermediate tin oxide phases, such as Sn2O3, at local areas in the belts. The presence of non-stoichiometric tin oxide is relevant in applications, since surface states affect the physical–chemical behavior of tin oxide.

Waveguiding and confinement of light in semiconductor oxide microstructures

Interest on the control of light at the nano- and microscale has increased in the last years because of the incorporation of nanostructures into optical devices. In particular, semiconductor oxides microstructures emerge as important active materials for waveguiding and confinement of light from UV to NIR wavelengths. The fabrication of high quality and quantity of nano- and microstructures of semiconductor oxides with controllable morphology and tunable optical properties is an attractive challenge in this field. In this work, waveguiding and optical confinement applications of different micro- and nanostructures of gallium oxide and antimony oxide have been investigated. Structures with morphologies such as nanowires, nanorods or branched nanowires as elongated structures, but also triangles, microplates or pyramids have been obtained by a thermal evaporation method. Light waveguide experiments were performed with both oxides, which have wide band gap and a rather high refractive index. The synthesized microstructures have been found to act as optical cavities and resonant modes were observed. In particular, photoluminescence results showed the presence of resonant peaks in the PL spectra of Ga2O3 microwires and Sb2O3 micro-triangles and rods, which suggest their applications as optical resonators in the visible range.