Search results for "Blueshift"
showing 10 items of 33 documents
Depth profiling of optical and vibrational properties in GaN/AlN quantum dot superlattices
2009
Spatially resolved confocal μ-Raman and μ-photoluminescence experiments were performed to analyze the vibrational and optical properties of GaN/AlN quantum dots as a function of depth. Two approaches have been followed. First, spectra were taken by defocusing the microscope objective at various depths on the sample surface. In a second set of experiments a bevel at an angle of 20° with respect to the surface normal was prepared by mechanical polishing of the surface, and spectra were taken across the bevel. The E2h vibrational modes ascribed to the GaN QDs and the AlN spacer redshift towards the surface, indicating the progressive relaxation of the QDs and a considerable increase of the ten…
Luminescence properties of wurtzite AlN nanotips
2006
The optical properties of aluminum nitride nanotips (AlNNTs) synthesized via vapor transport and condensation process have been studied by cathodoluminescence, photoluminescence (PL), thermoluminescence (TL), and UV absorption measurements. Two defect related transitions around 2.1 and 3.4eV and an excitonic feature at 6.2eV were identified. Compared to the AlN macropowders, the AlNNTs showed a blueshift (+0.2eV) of the ∼3.2eV peak. Analysis of both PL and TL excitation measurements indicated the existence of subband gap multiple energy levels in AlNNTs. A significant TL intensity even at 145°C suggests possible ultraviolet detector and dosimetric applications of these AlNNTs.
Temperature Sensor Based on Colloidal Quantum Dots PMMA Nanocomposite Waveguides
2012
In this paper, integrated temperature sensors based on active nanocomposite planar waveguides are presented. The nanocomposites consist of cadmium selenide (CdSe) and cadmium telluride (CdTe) quantum dots (QDs) embedded in a polymethylmethacrylate (PMMA) matrix. When the samples are heated in a temperature range from 25$^{circ}{rm C}$ to 50 $^{circ}{rm C}$, the waveguided photoluminescence of QDs suffers from a strong intensity decrease, which is approximately quadratic dependent on temperature. Moreover, the wavelength peak of the waveguided emission spectrum of CdTe-PMMA shows a blue shift of 0.25 ${rm nm}/^{circ}{rm C}$, whereas it remains constant in the case of CdSe-PMMA. A temperature…
Vibrational modes and strain in GaN/AlN quantum dot stacks: dependence on spacer thickness
2007
We have investigated the influence of spacer thickness on the vibrational and strain characteristics of GaN/AlN quantum dot multilayers (QD). The Raman shift corresponding to the E2h vibrational mode related to the QDs has been analyzed for AlN thicknesses ranging from 4.4 nm to 13 nm, while the amount of GaN deposited in each layer remained constant from sample to sample. It is shown that there is a rapid blue shift of the GaN vibrational mode with spacer thickness when its value is smaller than 7 nm while it remains almost constant for thicker spacers. A rapid increase of the Raman line-width in the thicker samples is also observed. The experimental behavior is discussed in comparison wit…
Characterization of used mineral oil condition by spectroscopic techniques
2004
Optical absorption, fluorescence, and quantitative 13C NMR spectroscopy have been used to study the degradation of mineral gearbox oil. Samples of used oil were collected from field service. Measured absorption, fluorescence, and quantitative 13C NMR spectra of used oils show characteristic changes from the spectra of a fresh oil sample. A clearly observable, approximately 20-nm blueshift of the fluorescence emission occurs during the early stages of oil use and correlates with changes in intensity of some specific 13C NMR resonance lines. These changes correlate with oil age because of the connection between the blueshift and breaking of the larger conjugated hydrocarbons of oil as a resul…
Tuning of High-Order Harmonics for Soft X-Ray Laser Seeding
2011
Within this work we present results of an experimental campaign studying the influence of the adiabatic and non-adiabatic blue shift on high-order harmonic (HH) radiation spectra. The results demonstrate that we are able to spectrally tune the HH radiation to cover more than 50% of the spectrum between 17 nm and 35 nm, paving the way to performing injection-seeded soft X-ray laser (SXRL) experiments with Mo, Zr and Y.
Investigations of the emission properties of single CdS-nanocrystallites
1997
Semiconductor nanocrystallites (NCs) with a radius in the range of the bulk exciton radius exhibit a strong quantum size effect. We investigated CdS-NCs, that were spin coated on a glass cover slip, with a low temperature confocal microscope. At low coverage we could image the fluorescence light from single NCs. The typical emission spectra of single NCs show four peaks. Besides the main peak related to near bandgap recombination, up to two LO-phonon satellites and one blue shifted peak were observed. It could be shown that the absorption can be polarization dependent. From the images as well as from the intensity autocorrelation function it could be concluded that the fluorescence emission…
Room-temperature efficient light detection by amorphous Ge quantum wells
2013
In this work, ultrathin amorphous Ge films (2 to 30 nm in thickness) embedded in SiO2 layers were grown by magnetron sputtering and employed as proficient light sensitizer in photodetector devices. A noteworthy modification of the visible photon absorption is evidenced due to quantum confinement effects which cause both a blueshift (from 0.8 to 1.8 eV) in the bandgap and an enhancement (up to three times) in the optical oscillator strength of confined carriers. The reported quantum confinement effects have been exploited to enhance light detection by Ge quantum wells, as demonstrated by photodetectors with an internal quantum efficiency of 70%. © 2013 Cosentino et al.
Pressure and temperature dependence of the band-gap in CdTe
2003
In this paper we report on isothermal compression measurements (up to 5 GPa and 500 K) of the optical absorption edge of 1 μm epitaxial layers of CdTe growth by metalorganic chemical vapor deposition (MOCVD) on GaS substrates. The isothermal blue shift under pressure of the direct energy gap (Γ v 15 → Γ c 1 ) in the zinc-blende phase is about 7.1 × 10 -2 eV GPa -1 and is found to be independent of temperature within the experimental errors. The isobaric red shift in the stability range of the zinc-blende phase is about -3.76 × 10 -4 eV K -1 . Regarding the phase transitions, no discontinuity in the energy gap has been found in the narrow pressure range where the cinnabar phase can be presen…
Temperature Dependence of Electronic Transitions of Single-Wall Carbon Nanotubes: Observation of an Abrupt Blueshift in Near-Infrared Absorption
2007
Near-infrared (NIR) absorption spectra of single-wall carbon nanotube (SWNT) films are studied between 10 and 293 K. The most prominent effect is the shift of bands with temperature. Some nanotubes show a redshift of transition upon increasing temperature while some show blueshift and others show no shift. The shift is interpreted to originate mainly from the effect of strain induced in the tubes because of interaction with the environment. In particular, at temperatures T = 175−225 K, for some bands, there is an abrupt large blueshift, which is interpreted to originate from interaction of the nanotubes with water. Two models could be considered to explain the effect: (1) strain induced by…