A strong enhancement of the 1.54 μm fluorescence of
Er3+ has been achieved in highly
concentrated II−VI semiconductor quantum dot environments. A new
preparation strategy
allowed to incorporate up to 20 at. % Er3+ into ZnS,
CdS, and CdSe as well as ZnO
semiconductor clusters and nanocrystals (sizes 1.5−5 nm). All
clusters investigated contain
OH groups that serve as bridging ligands for the lanthanide attachment.
Er3+ in ethanolic
cluster solutions is fluorescing by 3 orders of magnitude more strongly
than in pure ethanol,
which can only be explained by a cagelike architecture of these
clusters offering a large
intake capacity. With this new material concept, the two
well-known radiationless
recombination channels related to electron−phonon coupling and
Er−O−Er clustering can
be controlled. First, with decreasing number of erbium ions per
nanoparticle, the fluorescence
intensity increases, approaching its maximum at 2 at. %
Er3+. Second, it is shown that the
fluorescence intensity increases with decreasing energy of phonons
produced by lattice
vibrations of the surrounding cluster carrier. For example,
ethanolic molecular erbium/(aminopropyl)trialkoxysilane (AMEO) complexes exhibit the lowest
fluorescence intensity
of all samples employed, due to the presence of high-energy OH and NH
vibrations (between
3000 and 3500 cm-1). Ethanolic Er/ZnO
colloids, however, fluoresce 100 times more intense,
which can be interpreted in terms of the lower phonon energy of the ZnO
lattice vibrations
(between 500 and 1000 cm-1). The
AMEO-capped 1.6 nm CdSe/Er3+ clusters in
ethanol
fluoresce 1000 times more strongly than ethanolic
AMEO/Er3+ complexes (CdSe phonon
energies around 200 cm-1).
Free-standing wire arrays prepared by holographic exposure and wet chemical deep etching on a vertically arranged GaAs/GaInAs/GaAs[001] single quantum well structure were characterized by x-ray grazing incidence diffraction using synchrotron radiation. Using a grazing angle of αi≈0.05° the diffracted intensity stems primarily from the surface grating. It’s periodicity (D≈480 nm) was determined close to the (−220) and (220) Bragg reflection being parallel and perpendicular to the orientation of wires, respectively. The average wire width [(21.6±1.5) nm and (96.6±1.5) nm, respectively] and the coherence length of the grating (ξ≈2 μm) were obtained via Fourier transformation of the (220) shape function.
We have used a simple approach to fabricate buried InGaAs/InP quantum wires with widths down to 15 nm. Combining high resolution electron beam lithography and selective wet chemical etching only the InP cap layer of an InGaAs/InP quantum well is locally removed. InGaAs surface quantum wells are formed in the etched parts of the samples, where the energy band discontinuity of the quantum well is replaced by the high vacuum barrier. Therefore a lateral potential barrier is induced, which confines the carriers to the InP covered wire regions. In addition, the lateral potential can be strongly increased by a selective thermal intermixing step. The luminescence spectra of the wires show significant lateral quantization effects with energy shifts up to 13 meV and high quantum efficiencies up to room temperature.
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