Engineering the spectral properties of fluorophores, such as the enhancement of luminescence intensity, can be achieved through coupling with surface plasmons in metallic nanostructures. This ...process, referred to as metal-enhanced fluorescence, offers promise for a range of applications, including LEDs, sensor technology, microarrays and single-molecule studies. It becomes even more appealing when applied to colloidal semiconductor nanocrystals, which exhibit size-dependent optical properties, have high photochemical stability, and are characterized by broad excitation spectra and narrow emission bands. Other approaches have relied upon the coupling of fluorophores (typically organic dyes) to random distributions of metallic nanoparticles or nanoscale roughness in metallic films. Here, we develop a new strategy based on the highly reproducible fabrication of ordered arrays of gold nanostructures coupled to CdSe/ZnS nanocrystals dispersed in a polymer blend. We demonstrate the possibility of obtaining precise control and a high spatial selectivity of the fluorescence enhancement process.
By dispersing molten carnauba wax in warm isopropyl alcohol or ethanol with subsequent mild ultrasonic mixing, highly stable surfactant-free natural wax-in-alcohol emulsions were prepared for the ...first time. These emulsions can easily suspend hydrophobic colloidal organics or solids. We demonstrate fabrication of superhydrophobic films from sub-micron polytetrafluoroethylene (PTFE) particle-dispersed emulsions by a simple drop and/or spray casting procedure and subsequent thermal annealing above the melting point of carnauba wax. We investigate effect of wax and Teflon concentration and thermal annealing on the degree of superhydrophobicity. Moreover, the films display superior resistance to solvent etching against aggressive solvents such as chloroform, toluene, acetone and alcohols upon immersion into such solvent baths for one hour. Composite films remain superhydrophobic after solvents evaporate from their surfaces following their removal from the solvent baths. Moreover, detailed contact angle hysteresis measurements revealed that solvent bath immersion does not downgrade "self-cleaning" superhydrophobicity to "sticky" superhydrophobicity.
In this work we demonstrate the possibility of obtaining a significant increase of the photoluminescence of colloidal semiconductor nanocrystals (NCs) by means of metallic nanopatterns. Highly ...ordered triangular-shaped gold nanopatterns (typical dimensions 200
nm) were fabricated on planar substrates by electron beam lithography (EBL). Colloidal semiconductor nanocrystals (core/shell CdSe/ZnS quantum dots or CdSe nanorods) dispersed in a polymer matrix (PMMA) were subsequently deposited on the substrates by spin-coating. The coupling between the surface plasmons (SPs) resonance band of the metallic nanostructures and the excitation/emission bands of the nanocrystals resulted in a strong enhancement of the fluorescence from the quantum emitters, as probed by confocal microscopy analyses. Importantly, the proposed approach allows a precise control of the shape and dimensions of the single metallic nanostructure (and consequently of the SPs resonances), thanks to the nanometer resolution of the EBL. Moreover, the concentration of the NCs dispersed in the blend, as well as the thickness of the active layer, can be finely tuned. These results may open interesting perspectives for a wide range of applications, such as photonic devices, LEDs, sensor technology, microarrays, single/few molecules experiments, and biochemical/biophysical investigations.
In this work, we show the fabrication of colloidal nanocrystals (NCs) based waveguide photonic devices by exploiting a new lithographic approach for the nano-positioning of NCs. Our approach relies ...on the dispersion of CdSe/ZnS core/shell NCs into a layer of electro-sensitive resist (poly-methyl methacrylate, PMMA), which is subsequently patterned by means of electron beam lithography (EBL). This localization technique has been demonstrated by detecting high-resolution photoluminescence (PL) maps on an e-beam nano-patterned PMMA/NCs film through a confocal microscope.
This technique has been exploited for the fabrication of distributed feedback (DFB) structures and distributed Bragg reflectors (DBR) suitable for the realization of in-plane waveguide lasers. A DFB structure has been obtained by patterning a periodical grating on a PMMA/NCs ridge waveguide. Two DBR have been similarly fabricated by creating quarter-wavelength thick alternated stripes of air and active blend; these structures have been exploited as front and back mirrors of an in-plane PMMA/NCs ridge cavity. In both cases, no following etching processes have been required, thus simplifying the steps needed for the fabrication of NCs based active photonic devices.
Single colloidal quantum dots (QDs) are increasingly exploited as triggered sources of single photons. This review reports on recent results on single photon sources (SPS) based on colloidal quantum ...dots, whose size, shape and optical properties can be finely tuned by wet chemistry approach. First, we address the optical properties of different colloidal nanocrystals, such as dots, rods and dot in rods and their use as single photon sources will be discussed. Then, we describe different techniques for isolation and positioning single QDs, a major issue for fabrication of single photon sources, and various approaches for the embedding single nanocrystals inside microcavities. The insertion of single colloidal QDs in quantum confined optical systems allows one to improve their overall optical properties and performances in terms of efficiency, directionality, life time, and polarization control. Finally, electrical pumping of colloidal nanocrystals light emitting devices and of NC-based single photon sources is reviewed.
A semiconductor laser containing seven InAs-InGaAs stacked quantum-dot (QD) layers was grown by molecular beam epitaxy. Shallow mesa ridge-waveguide lasers with stripe width of 120 mum were ...fabricated and tested. A high modal gain of 41 cm -1 was obtained at room temperature corresponding to a modal gain of ~6 cm -1 per QD layer, which is very promising to enable the realization of 1.3-mum ultrashort cavity devices such as vertical-cavity surface-emitting lasers. Ground state laser action was achieved for a 360-mum-cavity length with as-cleaved facets. The transparency current density per QD layer and internal quantum efficiency were 13 A/cm 2 and 67%, respectively
In this work we show the fabrication of disordered structures for organic random lasing devices by means of the combined use of two high resolution techniques, namely electron beam lithography (EBL) ...and nano imprint lithography (NIL). We have developed a model for random lasing disordered structures by introducing a well defined degree of disorder in a regular photonic crystal (PC) pattern. Before the fabrication of the device, a preliminary modelling of the ordered 2D-PC has been realized by a finite difference time domain (FDTD) based simulation software. Subsequently, random point defects were introduced in the ordered pattern in order to study the influence of disorder on the reflection spectra and to establish the presence of localized modes. The engineered random pattern was transferred through EBL and reactive ion etching (RIE) onto a Si substrate, thus obtaining an hard master used in the following imprint process of polyfluorene (PFO). Preliminary optical characterization collected from one edge of the structure show the occurrence of localized modes which have been attributed to random scattering in the patterned organic layer.
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