ABSTRACT
Since ZnO nanoparticles (NPs) possess a variety of intrinsic defects, they can provide a wide spectrum of visible emission, without adding any impurity or any doping atoms. They are attracting more and more interest as a material for light sources and energy downshifting systems. However, defect emission with a high luminescence quantum efficiency (PL QY) is difficult to obtain. Here, we present the co-precipitation synthesis parameters permitting to attain ZnO NPs with highly visible PL QYs. We found that the nature of zinc precursors and alkaline hydroxide (KOH or LiOH) used in this method affects the emission spectra and the PL QY of the as-grown ZnO NPs. LiOH is found to have an advantageous effect on the visible emission efficiency when added during the synthesis of the ZnO NPs. More precisely, LiOH permits to increase the emission efficiency in the visible up to 13%. We discuss the effects of the nanoparticle size, the morphology and the surface stabilization on the enhancement of the luminescent emission efficiency. Various spectral contributions to the luminescent emission were also examined, in order to achieve a control of the defect emission to increase its efficiency.
ABSTRACT
We report the enhancement of the optical emission between 850 and 1400 nm of an ensemble of silicon mono-vacancies (VSi), silicon and carbon divacancies (VCVSi), and nitrogen vacancies (NCVSi) in an n-type 4H-SiC array of micropillars. The micropillars have a length of ca. 4.5 µm and a diameter of ca. 740 nm, and were implanted with H+ ions to produce an ensemble of color centers at a depth of approximately 2 µm. The samples were in part annealed at different temperatures (750 and 900 °C) to selectively produce distinct color centers. For all these color centers we saw an enhancement of the photostable fluorescence emission of at least a factor of 6 using micro-photoluminescence systems. Using custom confocal microscopy setups, we characterized the emission of VSi measuring an enhancement by up to a factor of 20, and of NCVSi with an enhancement up to a factor of 7. The experimental results are supported by finite element method simulations. Our study provides the pathway for device design and fabrication with an integrated ultra-bright ensemble of VSi and NCVSi for in vivo imaging and sensing in the infrared.
ABSTRACT
A new in situ technique for temperature and pressure measurement within dynamic thin-film flows of liquids is presented. The technique is based on the fluorescence emission sensitivity of CdSe/CdS/ZnS quantum dots to temperature and pressure variations. In this respect, the quantum dots were dispersed in squalane, and their emission energy dependence on temperature and pressure was calibrated under static conditions. Temperature calibration was established between 295 K and 393 K showing a temperature sensitivity of 0.32 meV K-1. Pressure calibration was, in turn, conducted up to 1.1 GPa using a diamond anvil cell, yielding a pressure sensitivity of 33.2 meV GPa-1. The potential of CdSe/CdS/ZnS quantum dots as sensors to probe temperature and pressure was proven by applying the in situ technique to thin films of liquids undergoing dynamic conditions. Namely, temperature rises have been measured in liquid films subjected to shear heating between two parallel plates in an optical rheometer. In addition, pressure rises have been measured in a lubricated point contact under pure rolling and isothermal conditions. In both cases, the measured values have been successfully compared with theoretical or numerical predictions. These comparisons allowed the validation of the new in situ technique and demonstrated the potential of the quantum dots for further mapping application in more complex and/or severe conditions.
ABSTRACT
Silicon nanoparticles (Si NPs) with a diameter size ranging from 4 to 8 nm were successfully fabricated. They exhibit a visible photoluminescence (PL) due to the quantum confinement effect. Chemical functionalization of these Si NPs with alkyl groups allowed to homogeneously disperse them in nonpolar liquids (NPLs). In comparison to most of literature results for Si NPs, an important PL peak position variation with temperature (almost 1 meV/K) was obtained from 303 to 390 K. The influence of the liquid viscosity on the peak positions is also presented. These variations are discussed considering energy transfer between nanoparticles. The high PL thermal sensitivity of the alkyl-capped Si NPs paves the way for their future application as nanothermometers.
ABSTRACT
A comparative photoluminescence analysis of as-prepared and chemically modified (by alkyl chains -C18H37) silicon and carbon nanoparticles dispersed in low-polar liquids is reported. Influence of the low-polar liquid nature and ambient temperature on photoluminescence of the nanoparticles has been investigated from the point of view of their possible application as thermal nanoprobes.
