TY  - JOUR
A1  - Wyborski, Paweł
A1  - Podemski, Paweł
A1  - Wroński, Piotr Andrzej
A1  - Jabeen, Fauzia
A1  - Höfling, Sven
A1  - Sęk, Grzegorz
T1  - Electronic and optical properties of InAs QDs grown by MBE on InGaAs metamorphic buffer
JF  - Materials
N2  - We present the optical characterization of GaAs-based InAs quantum dots (QDs) grown by molecular beam epitaxy on a digitally alloyed InGaAs metamorphic buffer layer (MBL) with gradual composition ensuring a redshift of the QD emission up to the second telecom window. Based on the photoluminescence (PL) measurements and numerical calculations, we analyzed the factors influencing the energies of optical transitions in QDs, among which the QD height seems to be dominating. In addition, polarization anisotropy of the QD emission was observed, which is a fingerprint of significant valence states mixing enhanced by the QD confinement potential asymmetry, driven by the decreased strain with increasing In content in the MBL. The barrier-related transitions were probed by photoreflectance, which combined with photoluminescence data and the PL temperature dependence, allowed for the determination of the carrier activation energies and the main channels of carrier loss, identified as the carrier escape to the MBL barrier. Eventually, the zero-dimensional character of the emission was confirmed by detecting the photoluminescence from single QDs with identified features of the confined neutral exciton and biexciton complexes via the excitation power and polarization dependences.
KW  - molecular beam epitaxy
KW  - quantum dot
KW  - metamorphic buffer layer
KW  - band structure
KW  - photoluminescence
KW  - photoreflectance
Y1  - 2022
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-297037
SN  - 1996-1944
VL  - 15
IS  - 3
ER  - 
TY  - JOUR
A1  - Straßer, Marion
A1  - Schrauth, Joachim H. X.
A1  - Dembski, Sofia
A1  - Haddad, Daniel
A1  - Ahrens, Bernd
A1  - Schweizer, Stefan
A1  - Christ, Bastian
A1  - Cubukova, Alevtina
A1  - Metzger, Marco
A1  - Walles, Heike
A1  - Jakob, Peter M.
A1  - Sextl, Gerhard
T1  - Calcium fluoride based multifunctional nanoparticles for multimodal imaging
JF  - Beilstein Journal of Nanotechnology
N2  - New multifunctional nanoparticles (NPs) that can be used as contrast agents (CA) in different imaging techniques, such as photoluminescence (PL) microscopy and magnetic resonance imaging (MRI), open new possibilities for medical imaging, e.g., in the fields of diagnostics or tissue characterization in regenerative medicine. The focus of this study is on the synthesis and characterization of CaF\(_{2}\):(Tb\(^{3+}\),Gd\(^{3+}\)) NPs. Fabricated in a wet-chemical procedure, the spherical NPs with a diameter of 5–10 nm show a crystalline structure. Simultaneous doping of the NPs with different lanthanide ions, leading to paramagnetism and fluorescence, makes them suitable for MR and PL imaging. Owing to the Gd\(^{3+}\) ions on the surface, the NPs reduce the MR T\(_{1}\) relaxation time constant as a function of their concentration. Thus, the NPs can be used as a MRI CA with a mean relaxivity of about r = 0.471 mL·mg\(^{−1}\)·s\(^{−1}\). Repeated MRI examinations of four different batches prove the reproducibility of the NP synthesis and determine the long-term stability of the CAs. No cytotoxicity of NP concentrations between 0.5 and 1 mg·mL\(^{−1}\) was observed after exposure to human dermal fibroblasts over 24 h. Overall this study shows, that the CaF\(_{2}\):(Tb\(^{3+}\),Gd\(^{3+}\)) NPs are suitable for medical imaging.
KW  - calcium fluoride nanoparticles
KW  - magnetic resonance imaging (MRI)
KW  - multifunctional nanoparticles
KW  - multimodal imaging
KW  - photoluminescence
Y1  - 2017
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-170657
VL  - 8
ER  - 
TY  - JOUR
A1  - Sung, Jooyoung
A1  - Kim, Pyosang
A1  - Fimmel, Benjamin
A1  - Würthner, Frank
A1  - Kim, Dongho
T1  - Direct observation of ultrafast coherent exciton dynamics in helical π-stacks of self-assembled perylene bisimides
JF  - Nature Communications
N2  - Ever since the discovery of dye self-assemblies in nature, there have been tremendous efforts to exploit biomimetic supramolecular assemblies for tailored artificial photon processing materials. This feature necessarily has resulted in an increasing demand for understanding exciton dynamics in the dye self-assemblies. In a sharp contrast with pi-type aggregates, however, the detailed observation of exciton dynamics in H-type aggregates has remained challenging. In this study, as we succeed in measuring transient fluorescence from Frenkel state of π-stacked perylene tetracarboxylic acid bisimide dimer and oligomer aggregates, we present an experimental demonstration on Frenkel exciton dynamics of archetypal columnar π-π stacks of dyes. The analysis of the vibronic peak ratio of the transient fluorescence spectra reveals that unlike the simple π-stacked dimer, the photoexcitation energy in the columnar π-stacked oligomer aggregates is initially delocalized over at least three molecular units and moves coherently along the chain in tens of femtoseconds, preceding excimer formation process.
KW  - systems
KW  - molecules
KW  - J-aggregate behavior
KW  - absorption
KW  - spectroscopy
KW  - photoluminescence
KW  - diffusion
KW  - fluorescence
KW  - excimer formation
KW  - organic semiconductors
Y1  - 2015
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-148157
VL  - 6
IS  - 8646
ER  -