Pedro J. Rodríguez-Cantó

Nanotexturing To Enhance Photoluminescent Response of Atomically Thin Indium Selenide with Highly Tunable Band Gap.

Manipulating properties of matter at the nanoscale is the essence of nanotechnology, which has enabled the realization of quantum dots, nanotubes, metamaterials, and two-dimensional materials with tailored electronic and optical properties. Two-dimensional semiconductors have revealed promising perspectives in nanotechnology. However, the tunability of their physical properties is challenging for semiconductors studied until now. Here we show the ability of morphological manipulation strategies, such as nanotexturing or, at the limit, important surface roughness, to enhance light absorption and the luminescent response of atomically thin indium selenide nanosheets. Besides, quantum-size confinement effects make this two-dimensional semiconductor to exhibit one of the largest band gap tunability ranges observed in a two-dimensional semiconductor: from infrared, in bulk material, to visible wavelengths, at the single layer. These results are relevant for the design of new optoelectronic devices, including heterostructures of two-dimensional materials with optimized band gap functionalities and in-plane heterojunctions with minimal junction defect density.

Polymer/QDs nanocomposites for waveguiding applications

In this paper we review our recent progress in a still young type of active waveguides based on hybrid organic (polymer)-- inorganic (semiconductor quantum dots) materials. They can be useful for the implementation of new photonic devices, because combining the properties of the semiconductor nanostructures (quantum size carrier confinement and temperature independent emission) with the technological capabilities of polymers. These optical waveguides can be easily fabricated by spin-coating and UV photolithography on many substrates (SiO2/Si, in the present work). We demonstrate that it is possible to control the active wavelength in a broad range (400-1100 nm), just by changing the base quantum dotmaterial (CdS, CdSe, CdTe and PbS, but other are possible), without the necessity of changing fabrication conditions. Particularly, we have determined the optimum conditions to produce multi-color photoluminescence waveguiding by embedding CdS, CdSe and CdTe quantum dots into Poly(methyl methacrylate). Finally, we show new results regarding the incorporation of CdSe nanocrystals into a SU-8 resist, in order to extrapolate the study to a photolithographic and technologically more important polymer. In this case ridge waveguides are able to confine in 2D the light emitted by the quantum dots.

Quantum-Dot Double Layer Polymer Waveguides by Evanescent Light Coupling

In this work we analyze numerically and experimentally new active waveguides based on a bilayer structure composed by a passive polymer and an active poly(mehtyl methacrylate) (PMMA) film doped with CdSe colloidal quantum dots (QDs), namely a nancomposite. In a first bilayer structure a planar PMMA layer is deposited on top of the nanocomposite, where the signal beam intensity is enhanced because this cladding layer is able to collect radiated emission of QDs. Moreover, the pump beam is also propagating through the cladding without limitation by QD absorption. These results are extended to a second bilayer structure, where ridge patterns of a commercially available resist (SU-8) are deposited on the top of the nanocomposite active layer. These SU-8 patterns are also able to guide with low absorption losses both pump and signal beams. The optimum geometrical parameters of the bilayer structures were properly designed to optimize the light waveguiding, previous to their fabrication and optical characterization. For this purpose, a spontaneous emission model has been developed and programmed into an active beam propagation method. This technology can be the base for developing integrated photonics on silicon at visible and telecom wavelengths.

Plasmonic optical sensors printed from Ag–PVA nanoinks

In this paper we report on the use of a nanocomposite based on silver nanoparticles embedded in PVA as a plasmonic optical sensor to detect and quantify trace amounts of amines in gas and water, respectively. The transduction mechanism of the sensor is based on the changes of the LSPR band of Ag NPs when analyte molecules are chemisorbed on their surface. The Ag–PVA sensors are fabricated by means of a high-precision microplotter, a direct-write technology developed for printing materials from solution. The nanoink is formulated with a metal precursor (AgNO3) and a polymer (PVA) using an adequate mixture of solvents to meet the rheological requirements for the fluid dispensing process. The LSPR intensity is the most sensitive magnitude to follow the interaction between Ag NPs embedded in PVA and amines. Ag–PVA patterns are tested as a plasmonic optical sensor for the detection of ethylenediamine in solution showing a limit of detection as low as 0.1 nM. Moreover Ag nanocomposite patterns are also used for sensing vapours of several biogenic (cadaverine and putrescine) and synthetic (ethylenediamine and methylenediamine) amines, where shorter amines exhibit the largest sensor response. This plasmonic optical sensor is also tested in real-time monitoring of chicken meat spoilage at room temperature. We believe that the Ag–PVA nanocomposite can be the basis for the development of sensor spots, bar-codes and other labels for smart packaging technology, among other sensing applications.

Color Tuning and White Light by Dispersing CdSe, CdTe, and CdS in PMMA Nanocomposite Waveguides

In this paper, active nanocomposite waveguides based on the dispersion of CdS, CdTe, and CdSe colloidal quantum dots (QDs) in PMMA are proposed. Their propagation properties are studied as a function of the concentration of nanoparticles in the polymer using the variable length stripe method. When the three nanostructures are dispersed in the same film, the structure is able to waveguide the three basic colors: red (CdSe), green (CdTe), and blue (CdS), it being possible to engineer any waveguided color by an appropriate choice of the filling factor of each QD in the PMMA matrix. For this purpose, it is important to take into account reabsorption effects and the Förster energy transfer between the different QDs families. As a final application, white waveguided light at the output of the structure is demonstrated. This energy transfer can be also the origin of the surprising observation that initial gain (losses) are much higher (smaller) in these active multinanopaticle waveguides than in single-loaded ones.

Efficient excitation of photoluminescence in a two-dimensional waveguide consisting of a quantum dot-polymer sandwich-type structure

In this Letter, we study a new kind of organic polymer waveguide numerically and experimentally by combining an ultrathin (10-50 nm) layer of compactly packed CdSe/ZnS core/shell colloidal quantum dots (QDs) sandwiched between two cladding poly(methyl methacrylate) (PMMA) layers. When a pumping laser beam is coupled into the waveguide edge, light is mostly confined around the QD layer, improving the efficiency of excitation. Moreover, the absence of losses in the claddings allows the propagation of the pumping laser beam along the entire waveguide length; hence, a high-intensity photoluminescence (PL) is produced. Furthermore, a novel fabrication technology is developed to pattern the PMMA into ridge structures by UV lithography in order to provide additional light confinement. The sandwich-type waveguide is analyzed in comparison to a similar one formed by a PMMA film homogeneously doped by the same QDs. A 100-fold enhancement in the waveguided PL is found for the sandwich-type case due to the higher concentration of QDs inside the waveguide.

Colloidal Quantum Dots-PMMA Waveguides as Integrable Microwave Photonic Phase Shifters

A novel scheme for the control of microwave signals carried at optical wavelengths by use of PbS colloidal quantum dots embedded in PMMA waveguides is presented. When these structures are pumped at wavelengths where PbS has efficient absorption (980 or 1310 nm), a phase shift in a signal carried at 1550 nm is induced. Optimal conditions have been analyzed by studying the influence of the microwave signal and the waveguide structure. In a proof-of-concept experiment, a continuous phase shift up to 35 ° at 25 GHz has been demonstrated, with good thermal stability ( at 25 GHz) when the samples are heated 20 °C above room temperature. The potential benefits of the use of this active-waveguide technology in microwave photonics are due to the continuous scan of the phase delay, its high tuning speed, and its small size, which leads to the possibility of integration.

Strongly-coupled PbS QD solids by doctor blading for IR photodetection

Solution-processed QD solids are emerging as a novel concept for high-performance optoelectronic devices. In this work, doctor blading is proposed for the fabrication of strongly-coupled QD solids from a PbS nanoink for photodetection at telecom wavelengths. The key step of this procedure is the solid-state ligand exchange, which reduces the interparticle distance and increases the carrier mobility in the resulting strongly-coupled QD solid. This is accomplished by replacing the original long oleylamine molecules by shorter molecules like 3-mercaptopropionic acid, as confirmed by FTIR, TGA and XPS. Further, a detailed investigation with XPS confirms the air-stability of the QD solids and the extreme reduction of the principal oxidation product, PbSO3, from ligand exchange times of 60 s. Finally, the QD solid is tested as an active layer for the fabrication of a Schottky NIR photodetector. The device shows a maximum responsivity of 0.26 A W−1 that corresponds to an internal quantum efficiency higher than 30% at 1500 nm and detectivity around 1011 jones.

Polymer waveguide couplers based on metal nanoparticle-polymer nanocomposites.

In this work Au nanoparticles (AuNPs) are incorporated into poly(methyl methacrylate) (PMMA) waveguides to develop optical couplers that are compatible with planar organic polymer photonics. A method for growing AuNPs (of 10 to 100 nm in size) inside the commercially available Novolak resist is proposed with the intention of tuning the plasmon resonance and the absorption/scattering efficiencies inside the patterned structures. The refractive index of the MNP-Novolak nanocomposite (MNPs: noble metal nanoparticles) is carefully analysed both experimentally and numerically in order to find the appropriate fabrication conditions (filling factor and growth time) to optimize the scattering cross section at a desired wavelength. Then the nanocomposite is patterned inside a PMMA waveguide to exploit its scattering properties to couple and guide a normal incident laser light beam along the polymer. In this way, light coupling is experimentally demonstrated in a broad wavelength range (404-780 nm). Due to the elliptical shape of the MNPs the nanocomposite demonstrates a birefringence, which enhances the coupling to the TE mode up to efficiencies of around 1%.

UV-patternable nanocomposite containing CdSe and PbS quantum dots as miniaturized luminescent chemo-sensors

In this study, a novel multifunctional hybrid polymer-based luminescent material, particularly formulated for photolithography, was developed, fabricated and tested as a miniaturized chemosensor. This nanocomposites were formulated with either luminescent CdSe (for the visible) or PbS (for the near-IR) colloidal QDs embedded in a polyisoprene-based photoresist (PIP). We checked the sensing capability of the nanocomposite by exposing 1 cm2 CdSe nanocomposite patterns to vapours of some analyte solutions such as 2-mercaptoethanol (MET) and ethylenediamine (EDA). The transduction mechanism of the sensor is based on changes of the QD photoluminescence (PL) when molecules are adsorbed on the QD surface. Because the polymer used suffered from swelling during the developmental step of the sensor fabrication, the diffusion of the analytes through the matrix was rather high. As a result, the sensor response to the analyte–QD interactions was considerably short and sensitive. We observed shorter sensor response times for MET than EDA. Moreover, we found a limit of detection of MET and EDA of 0.1 pg and 15 ng, respectively. The linear detection range for MET and EDA was determined to be over an analyte concentration of 6 and 5 orders of magnitude, respectively. We also tested the PbS-based nanocomposite response to MET and EDA and found very different responses. Although EDA quenched the PbS PL, exposure to MET molecules resulted in a 6.5-fold enhancement of the PL. The mechanisms of the observed effects are discussed in detail.