Eduardo Solano

Atomic layer deposited second-order nonlinear optical metamaterial for back-end integration with CMOS-compatible nanophotonic circuitry

We report the fabrication of artificial unidimensional crystals exhibiting an effective bulk second-order nonlinearity. The crystals are created by cycling atomic layer deposition of three dielectric materials such that the resulting metamaterial is noncentrosymmetric in the direction of the deposition. Characterization of the structures by second-harmonic generation Maker-fringe measurements shows that the main component of their nonlinear susceptibility tensor is about 5 pm/V, which is comparable to well-established materials and more than an order of magnitude greater than reported for a similar crystal [Appl. Phys. Lett.107, 121903 (2015)APPLAB0003-695110.1063/1.4931492]. Our demonstration opens new possibilities for second-order nonlinear effects on CMOS-compatible nanophotonic platforms.

Chemically Triggered Formation of Two-Dimensional Epitaxial Quantum Dot Superlattices

Two dimensional superlattices of epitaxially connected quantum dots enable size-quantization effects to be combined with high charge carrier mobilities, an essential prerequisite for highly performing QD devices based on charge transport. Here, we demonstrate that surface active additives known to restore nanocrystal stoichiometry can trigger the formation of epitaxial superlattices of PbSe and PbS quantum dots. More specifically, we show that both chalcogen-adding (sodium sulfide) and lead oleate displacing (amines) additives induce small area epitaxial superlattices of PbSe quantum dots. In the latter case, the amine basicity is a sensitive handle to tune the superlattice symmetry, with strong and weak bases yielding pseudohexagonal or quasi-square lattices, respectively. Through density functional theory calculations and in situ titrations monitored by nuclear magnetic resonance spectroscopy, we link this observation to the concomitantly different coordination enthalpy and ligand displacement potency of the amine. Next to that, an initial ∼10% reduction of the initial ligand density prior to monolayer formation and addition of a mild, lead oleate displacing chemical trigger such as aniline proved key to induce square superlattices with long-range, square micrometer order; an effect that is the more pronounced the larger the quantum dots. Because the approach applies to PbS quantum dots as well, we conclude that it offers a reproducible and rational method for the formation of highly ordered epitaxial quantum dot superlattices.

Independent tuning of size and coverage of supported Pt nanoparticles using atomic layer deposition

Synthetic methods that allow for the controlled design of well-defined Pt nanoparticles are highly desirable for fundamental catalysis research. In this work, we propose a strategy that allows precise and independent control of the Pt particle size and coverage. Our approach exploits the versatility of the atomic layer deposition (ALD) technique by combining two ALD processes for Pt using different reactants. The particle areal density is controlled by tailoring the number of ALD cycles using trimethyl(methylcyclopentadienyl)platinum and oxygen, while subsequent growth using the same Pt precursor in combination with nitrogen plasma allows for tuning of the particle size at the atomic level. The excellent control over the particle morphology is clearly demonstrated by means of in situ and ex situ X-ray fluorescence and grazing incidence small angle X-ray scattering experiments, providing information about the Pt loading, average particle dimensions, and mean center-to-center particle distance.The performance of supported nanoparticle catalysts is closely related to their size, shape and interparticle distance. Here, the authors introduce an atomic layer deposition-based strategy to independently tune the size and coverage of platinum nanoparticles with atomic-level precision.

Neutron and X-ray diffraction study of ferrite nanocrystals obtained by microwave-assisted growth. A structural comparison with the thermal synthetic route. Corrigendum

Corrigendum to J. Appl. Cryst. (2014), 47, 414–420.

Mobile setup for synchrotron based in situ characterization during thermal and plasma-enhanced atomic layer deposition

We report the design of a mobile setup for synchrotron based in situ studies during atomic layer processing. The system was designed to facilitate in situ grazing incidence small angle x-ray scattering (GISAXS), x-ray fluorescence (XRF), and x-ray absorption spectroscopy measurements at synchrotron facilities. The setup consists of a compact high vacuum pump-type reactor for atomic layer deposition (ALD). The presence of a remote radio frequency plasma source enables in situ experiments during both thermal as well as plasma-enhanced ALD. The system has been successfully installed at different beam line end stations at the European Synchrotron Radiation Facility and SOLEIL synchrotrons. Examples are discussed of in situ GISAXS and XRF measurements during thermal and plasma-enhanced ALD growth of ruthenium from RuO4 (ToRuS™, Air Liquide) and H2 or H2 plasma, providing insights in the nucleation behavior of these processes.

Magnetic stability against calcining of microwave-synthesized CoFe2O4 nanoparticles

High quality CoFe2O4 nanoparticles were synthesized using a one-pot, microwave assisted method, that allows forming stable colloidal solutions in alcoholic solvents, as required for the preparation by Chemical Solution Deposition of hybrid nanocomposite ferromagnetic-high Tc YBa2Cu3O7 superconducting films or devices. We have investigated how the thermal process necessary for the preparation of such epitaxial nanocomposites, involving high temperatures (800 °C) and oxygen partial pressures (1 atm), affects the structure and magnetic properties of the isolated nanoparticles. The NPs were fully characterised by XRD, SQUID, STEM-EELS and XMCD at four different stages of the thermal process. Results show that, despite intermediate changes in the cation distribution occur during the process, the final NP magnetization is stable against the thermal treatment. This result opens up perspectives for the preparation of hybrid YBCO films with embedded magnetic NPs using low-cost chemical-solution methods.

ALD-Developed Plasmonic Two-Dimensional Au–WO3–TiO2 Heterojunction Architectonics for Design of Photovoltaic Devices

Electrically responsive plasmonic devices, which benefit from the privilege of surface plasmon excited hot carries, have supported fascinating applications in the visible-light-assisted technologies. The properties of plasmonic devices can be tuned by controlling charge transfer. It can be attained by intentional architecturing of the metal-semiconductor (MS) interfaces. In this study, the wafer-scaled fabrication of two-dimensional (2D) TiO2 semiconductors on the granular Au metal substrate is achieved using the atomic layer deposition (ALD) technique. The ALD-developed 2D MS heterojunctions exhibited substantial enhancement of the photoresponsivity and demonstrated the improvement of response time for 2D Au-TiO2-based plasmonic devices under visible light illumination. To circumvent the undesired dark current in the plasmonic devices, a 2D WO3 nanofilm (∼0.7 nm) was employed as the intermediate layer on the MS interface to develop the metal-insulator-semiconductor (MIS) 2D heterostructure. As a result, 13.4% improvement of the external quantum efficiency was obtained for fabricated 2D Au-WO3-TiO2 heterojunctions. The impedancometry measurements confirmed the modulation of charge transfer at the 2D MS interface using MIS architectonics. Broadband photoresponsivity from the UV to the visible light region was observed for Au-TiO2 and Au-WO3-TiO2 heterostructures, whereas near-infrared responsivity was not observed. Consequently, considering the versatile nature of the ALD technique, this approach can facilitate the architecturing and design of novel 2D MS and MIS heterojunctions for efficient plasmonic devices.

On the determination of Χ(2) in thin films: A comparison of one-beam second-harmonic generation measurement methodologies

The determination of the second-order susceptibility (χ(2)) of thin film samples can be a delicate matter since well-established χ(2) measurement methodologies such as the Maker fringe technique are best suited for nonlinear materials with large thicknesses typically ranging from tens of microns to several millimeters. Here we compare two different second-harmonic generation setups and the corresponding measurement methodologies that are especially advantageous for thin film χ(2) characterization. This exercise allows for cross-checking the χ(2) obtained for identical samples and identifying the main sources of error for the respective techniques. The development of photonic integrated circuits makes nonlinear thin films of particular interest, since they can be processed into long waveguides to create efficient nonlinear devices. The investigated samples are ABC-type nanolaminates, which were reported recently by two different research groups. However, the subsequent analysis can be useful for all researchers active in the field of thin film χ(2) characterization.

Size- and composition-controlled Pt–Sn bimetallic nanoparticles prepared by atomic layer deposition

Pt–Sn bimetallic nanoparticles (BMNPs) are used in a variety of catalytic reactions and are widely accepted as a model system for Pt-based bimetallics in fundamental catalysis research. Here, Pt–Sn BMNPs were prepared via a two-step synthesis procedure combining atomic layer deposition (ALD) and temperature programmed reduction (TPR). In situ X-ray diffraction measurements during TPR and ex situ X-ray absorption spectroscopy at the Pt LIII-edge revealed the formation of Pt–Sn bimetallic alloys with a phase determined by the Pt/(Pt + Sn) atomic ratio of the as-deposited bilayer. The size of the BMNPs could be tuned by changing the total thickness of the bilayers, while keeping the Pt/(Pt + Sn) atomic ratio constant. Due to the exceptional control over BMNP size and crystalline phase, the proposed method will enable highly systematic studies of the relation between the structure and the performance of Pt–Sn bimetallic catalysts.