Bruno Ullrich

Host matrix impact on Er3+ upconversion emission and its temperature dependence

By synthesizing Y(1.9−2x)Yb0.1Er2xO3, Y(0.95−x)Yb0.05ErxVO4 and Y(0.95−x)Yb0.05ErxPO4 phosphors, with phonon frequency maxima at 560, 826 and 1050 cm−1, respectively, we present the impact of phonon energy and crystal structure of the host matrix on upconversion and temperature sensing behavior. The spectral upconversion characteristics of all three phosphors reveal noticeable differences. The temperature sensing studies reveal that the phosphors have maximum sensitivity at ∼490 K, which is found to be highest (0.0105 K−1) in Y0.947Yb0.05Er0.003VO4 followed by Y1.894Yb0.1Er0.006O3 and Y0.947Yb0.05Er0.003PO4 phosphors. We found that the temperature sensitivity basically depends on the intensity ratio of two thermally coupled emission bands, 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2, of Er3+. Further, the intensity ratio depends on phonon energy of the host lattice, crystal structure, surface quenching centers and the temperature dependence of non-radiative decay rate.

Temperature dependence of the fundamental excitonic resonance in lead-salt quantum dots

The temperature dependences of the fundamental excitonic resonance in PbS and PbSe quantum dots fabricated by various technologies are experimentally determined. Above ∼150u2009K, sub-linearities of the temperature shifts and halfwidths are observed. This behavior is analyzed within the existing standard models. Concordant modeling, however, becomes possible only within the frame of a three-level system that takes into account both bright and dark excitonic states as well as phonon-assisted carrier redistribution between these states. Our results show that luminescence characterization of lead-salt quantum dots necessarily requires both low temperatures and excitation densities in order to provide reliable ensemble parameters.

Photocurrent limit in nanowires.

Square root photocurrent dependences of nanowires on light intensity were reported in the literature without clarification of the limiting effect. In this Letter, we derived a relation excellently fitting the observed nonlinearities and, intensifying the significance of the result, we demonstrated that the fit parameters involved can be employed to determine the impurity concentration and electronic response time of nano-sized semiconductors.

Inherent photoluminescence Stokes shift in GaAs.

The intrinsic photoluminescence Stokes shift, i.e., the energy difference between optical band gap and emission peak, of 350 μm thick semi-insulating GaAs wafers is found to be 4 meV at room temperature. The result is based on the determination of the optical bulk band gap from the transmission trend via modified Urbach rule whose result is confirmed with the transmission derivative method. The findings reveal the detailed balance of the optically evoked transitions and disclose the intrinsic link between Stokes shift and the Urbach tail slope parameter.

Magneto-optical controlled transmittance alteration of PbS quantum dots by moderately applied magnetic fields at room temperature

We observed changes of the transmitted monochromatic light passing through a colloidal PbS quantum dot film on glass owing to an applied moderate (smaller than 1 T) magnetic field under ambient conditions. The observed alterations show a square dependence on the magnetic field increase that cannot be achieved with bulk semiconductors. The findings point to so far not recognized application potentials of quantum dots.

Comment on 'Temperature dependent optical properties of PbS nanocrystals'.

We address erroneous statements made by Nordin et al (2012 Nanotechnology 23 275701) claiming the inadequacy of an expression we, Ullrich et al (2011 Appl. Phys. Lett. 99 081901), used to fit the temperature dependence of the photoluminescence and the absorption of PbS quantum dots. We further correct a quote by Nordin et al, who, when referring to our work, mistakenly claimed temperature invariance of the Stokes shift.

Photo-dynamic Burstein-Moss doping of PbS quantum dots in solution by single and two-photon optical pumping

We report photo-dynamically provoked photoluminescence blue shifts up to ∼8 meV of oleic acid capped 2.5 nm PbS quantum dots in toluene at room temperature. Exposing the solution to pulsed laser (26 ps, 10 Hz) emissions at 532 nm and 1064 nm, the photo-induced band gap increase is evoked by single and two-photon transitions, respectively. The emission peak blue shifts, recorded in reflection and transmission geometries, show a 2/3 power dependence on the optical stimulus gain, rendering the Burstein-Moss shift to be the underlying inherent n-type doping effect in the quantized colloid.

Photoluminescence limiting of colloidal PbS quantum dots

The exposure of colloidal 2u2009nm PbS quantum dots to growing continuous wave laser excitation at 532u2009nm increases the photoluminescence intensity with the square root of the optical stimulus. The results herein in conjunction with previous findings [B. Ullrich and H. Xi, Opt. Lett. 38, 4698 (2013)] advocate the square root trend to be the general limiting function for photo-carrier transport and emission of optically excited nano-sized materials. We further show that the excitation of one electron-hole pair per quantum dot defines the saturation threshold for photoluminescence intensity and dynamic band filling.

Magneto-optical reflectance and absorbance of PbS quantum dots

Reflectance and absorbance of colloidal 2.5 nm PbS quantum dots were coincidentally measured under the presence of moderate magnetic fields below one Tesla. The work provides further insight to the optical and magneto-optical properties of quantum dots by revealing disconnect of band gap data collected in different experimental geometries and by the demonstration of reflective magneto-optical devices addressable with weak magnetic fields.

Semiconductor band gap localization via Gaussian function

To determine the band gap of bulk semiconductors with transmission spectroscopy alone is considered as an extremely difficult task because in the higher energy range, approaching and exceeding the band gap energy, the material is opaque yielding no useful data to be recorded. In this paper, by investigating the transmission of industrial GaSb wafers with a thickness of 500 µm, we demonstrate how these obstacles of transmission spectroscopy can be overcome. The key is the transmission spectrums derivative, which coincides with the Gaussian function. This understanding can be used to transfer Beers law in an integral form opening the pathway of band gap determinations based on mathematical parameters only. The work also emphasizes the correlation between the thermal band gap variation and Debye temperature.