Optimizing infrared to near infrared upconversion quantum yield of β-NaYF₄:Er³⁺ in fluoropolymer matrix for photovoltaic devices

The present study reports for the first time the optimization of the infrared (1523 nm) to near-infrared (980 nm) upconversion quantum yield (UC-QY) of hexagonal trivalent erbium doped sodium yttrium fluoride (β-NaYF4:Er3+) in a perfluorocyclobutane (PFCB) host matrix under monochromatic excitation. Maximum internal and external UC-QYs of 8.4% ± 0.8% and 6.5% ± 0.7%, respectively, have been achieved for 1523 nm excitation of 970 ± 43 Wm−2 for an optimum Er3+ concentration of 25 mol% and a phosphor concentration of 84.9 w/w% in the matrix. These results correspond to normalized internal and external efficiencies of 0.86 ± 0.12 cm2 W−1 and 0.67 ± 0.10 cm2 W−1, respectively. These are the highest values ever reported for β-NaYF4:Er3+ under monochromatic excitation. The special characteristics of both the UC phosphor β-NaYF4:Er3+ and the PFCB matrix give rise to this outstanding property. Detailed power and time dependent luminescence measurements reveal energy transfer upconversion as the dominant UC mechanism

Beyond the energy gap law : the influence of selection rules and host compound effects on nonradiative transition rates in boltzmann thermometers

P.N. and M.H. contributed equally to this work. H.A.H., P.N., M.H., and E.T. thank the Deutsche Forschungsgemeinschaft (DFG) for generous support (Project HO 4503/5-1). Open access funding enabled and organized by Projekt DEAL.Apart from the energy gap law, control parameters over nonradiative transitions are so far only scarcely regarded. In this work, the impact of both covalence of the lanthanoid–ligand bond and varying bond distance on the magnitude of the intrinsic nonradiative decay rate between the excited 6P5/2 and 6P7/2 spin–orbit levels of Gd3+ is investigated in the chemically related compounds Y2[B2(SO4)6] and LaBO3. Analysis of the temperature-dependent luminescence spectra reveals that the intrinsic nonradiative transition rates between the excited 6PJ (  J = 5/2, 7/2) levels are of the order of only 10 ms−1 (Y2[B2(SO4)6]:Gd3+: 8.9 ms−1; LaBO3:Gd3+: 10.5 ms−1) and differ due to the different degree of covalence of the Gd—O bonds in the two compounds. Comparison to the established luminescent Boltzmann thermometer Er3+ reveals, however, that the nonradiative transition rates between the excited levels of Gd3+ are over three orders of magnitude slower despite a similar energy gap and the presence of a single resonant phonon mode. This hints to a fundamental magnetic dipolar character of the nonradiative coupling in Gd3+. These findings can pave a way to control nonradiative transition rates and how to tune the dynamic range of luminescent Boltzmann thermometers.Publisher PDFPeer reviewe

Electrical control of spin-polarized topological currents in monolayer WTe2

Altres ajuts: J.Y., P.K., M.G.M., and Z.Z. acknowledge the computer resources at MareNostrum and the technical support provided by the Barcelona Supercomputing Center through Red Española de Supercomputación (Grants No. RES-FI-2020-1-0018, No. RES-FI-2020-1-0014, and No. RES-FI-2020-2-0039). ICN2 is funded by the Generalitat de Catalunya (CERCA Programme). We acknowledge a PRACE award granting access to MareNostrum4 at Barcelona Supercomputing Center (BSC), Spain (OptoSpin project id. 2020225411).We evidence the possibility for coherent electrical manipulation of the spin orientation of topologically protected edge states in a low-symmetry quantum spin Hall insulator. By using a combination of ab initio simulations, symmetry-based modeling, and large-scale calculations of the spin Hall conductivity, it is shown that small electric fields can efficiently vary the spin textures of edge currents in monolayer 1T'-WTe2 by up to a 90-degree spin rotation, without jeopardizing their topological character. These findings suggest a new kind of gate-controllable spin-based device, topologically protected against disorder and of relevance for the development of topological spintronics

Upconversion solar cell measurements under real sunlight

The main losses in solar cells result from the incomplete utilization of the solar spectrum. Via the addition of an upconverting layer to the rear side of a solar cell, the otherwise-unused sub-bandgap photons can be utilized. In this paper, we demonstrate an efficiency enhancement of a silicon solar cell under real sunlight due to upconversion of sub-bandgap photons. Sunlight was concentrated geometrically with a lens with a factor of up to 50 suns onto upconverter silicon solar cell devices. The upconverter solar cell devices (UCSCDs) were also measured indoors using a solar simulator. To correct for differences in the spectral distribution between real sunlight and the solar simulator a spectral mismatch correction is required and is especially important to properly predict the performance when a non-linear response (e.g. upconversion) is involved. By applying a spectral mismatch correction, good agreement between the solar simulator measurements and the outdoor measurements using real sunlight was achieved. The method was tested on two different upconverter powders, β-NaYF4: 25% Er3+ and Gd2O2S: 10% Er3+, which were both embedded in a polymer. We determined additional photocurrents due to upconversion of 9.4 mA/cm2 with β-NaYF4 and 8.2 mA/cm2 with Gd2O2S under 94-suns concentration. Our results show i) the applicability of measurements using standard solar cell characterization equipment for predicting the performance of non-linear solar devices, and ii) underline the importance of applying proper mismatch corrections for accurate prediction of the performance of such non-linear devices

Modeling and Assessment of Afterglow Decay Curves from Thermally Stimulated Luminescence of Complex Garnets

Post-print (lokagerð höfundar)Afterglow is an important phenomenon in luminescent materials and can be desired (e.g., persistent phosphors) or undesired (e.g., scintillators). Understanding and predicting afterglow is often based on analysis of thermally stimulated luminescence (TSL) glow curves, assuming the presence of one or more discrete trap states. Here we present a new approach for the description of the time-dependent afterglow from TSL glow curves using a model with a distribution of trap depths. The method is based on the deconvolution of the energy dependent density of occupied traps derived from TSL glow curves using Tikhonov regularization. To test the validity of this new approach, the procedure is applied to experimental TSL and afterglow data for Lu1Gd2Ga3Al2O12:Ce ceramics codoped with 40 ppm of Yb3+ or Eu3+ traps. The experimentally measured afterglow curves are compared with simulations based on models with and without the continuous trap depth distribution. The analysis clearly demonstrates the presence of a distribution of trap depths and shows that the new approach gives a more accurate description of the experimentally observed afterglow. The new method will be especially useful in understanding and reducing undesired afterglow in scintillators.I.I.V, R.G.P and I.A.S. acknowledge support support from the Projects 14.Y26.31.0015 and 3.8884.2017/8.9 of the Ministry of Education and Science of the Russian Federation and Horizon2020 RISE project CoExAN.Peer Reviewe

Unraveling Heat Transport and Dissipation in Suspended MoSe 2 from Bulk to Monolayer

Understanding heat flow in layered transition metal dichalcogenide (TMD) crystals is crucial for applications exploiting these materials. Despite significant efforts, several basic thermal transport properties of TMDs are currently not well understood, in particular how transport is affected by material thickness and the material's environment. This combined experimental-theoretical study establishes a unifying physical picture of the intrinsic lattice thermal conductivity of the representative TMD MoSe. Thermal conductivity measurements using Raman thermometry on a large set of clean, crystalline, suspended crystals with systematically varied thickness are combined with ab initio simulations with phonons at finite temperature. The results show that phonon dispersions and lifetimes change strongly with thickness, yet the thinnest TMD films exhibit an in-plane thermal conductivity that is only marginally smaller than that of bulk crystals. This is the result of compensating phonon contributions, in particular heat-carrying modes around ≈0.1 THz in (sub)nanometer thin films, with a surprisingly long mean free path of several micrometers. This behavior arises directly from the layered nature of the material. Furthermore, out-of-plane heat dissipation to air molecules is remarkably efficient, in particular for the thinnest crystals, increasing the apparent thermal conductivity of monolayer MoSe by an order of magnitude. These results are crucial for the design of (flexible) TMD-based (opto-)electronic applications

Ubiquitin ligase STUB1 destabilizes IFNγ-receptor complex to suppress tumor IFNγ signaling

The cytokine IFNγ differentially impacts on tumors upon immune checkpoint blockade (ICB). Despite our understanding of downstream signaling events, less is known about regulation of its receptor (IFNγ-R1). With an unbiased genome-wide CRISPR/Cas9 screen for critical regulators of IFNγ-R1 cell surface abundance, we identify STUB1 as an E3 ubiquitin ligase for IFNγ-R1 in complex with its signal-relaying kinase JAK1. STUB1 mediates ubiquitination-dependent proteasomal degradation of IFNγ-R1/JAK1 complex through IFNγ-R1K285 and JAK1K249. Conversely, STUB1 inactivation amplifies IFNγ signaling, sensitizing tumor cells to cytotoxic T cells in vitro. This is corroborated by an anticorrelation between STUB1 expression and IFNγ response in ICB-treated patients. Consistent with the context-dependent effects of IFNγ in vivo, anti-PD-1 response is increased in heterogenous tumors comprising both wildtype and STUB1-deficient cells, but not full STUB1 knockout tumors. These results uncover STUB1 as a critical regulator of IFNγ-R1, and highlight the context-dependency of STUB1-regulated IFNγ signaling for ICB outcome

Connecting the dots: shedding light on the self-assembly of semiconductor nanocrystals with synchrotron X-ray scattering techniques

We studied the formation of two-dimensional crystals from nanocrystals using X-ray scattering techniques. Inside these nanocrystals, with sizes between 5-10 nm, the atoms are ordered in an atomic lattice. We use the nanocrystals as building blocks to create larger lattices in two dimensions. By adsorbing them at a liquid-air interface we are able to grow the crystal in only two dimensions, instead of the usual three dimensions. The particles and the resulting superlattices are still too small to visualize them through conventional microscopy. To study their self organization in real time, we use synchrotron based X-ray scattering. Any periodic structure will reflect the X-ray photons in directions governed by the crystal lattice. By looking at the position of these reflections at each point in time, we can calculate back what the particles look like at the liquid-air interface. For example, we can follow the distance between the particles, how they rotate and how they fuse together all in real time. We also studied the organization of novel perovskite nanocrystals into larger three-dimensional ordered structures, and show that we can alter their optical properties by exchanging the cations in these lattices

Exciton Dynamics in InP Quantum Dots: Fine Structure and Radiative Recombination Processes

Colloidal quantum dots (QDs) are semiconductor nanocrystals with size- and shape- tunable optical properties, fundamentally different from those of the bulk, and of significant interest in optoelectronics. For decades, CdSe-based QDs have been the workhorses in this field and they have reached a very mature level with a bright and size-tunable photoluminescence in the visible range, making them very suitable for applications in LEDs, displays and TV screens. However, Europa’s legislation does not allow the use of Cd-containing materials for opto-electronic applications. For this reason, the implementation of QDs in commercial devices requires to replace these QDs by toxicologically harmless materials, which should have a similar optical performance. In this respect, the development of III-V QDs, especially of InP, has received increasing attention in recent years. Colloidal hetero-nanocrystals composed of InP cores show a reasonable photoluminescence quantum yield, a size-tunable emission spectrum in the visible range and a good chemical stability, provided that they are coated with a suitable inorganic shell material. At present, suspensions of well-passivated InP core/shell QDs are prepared by wet-chemical synthesis and this synthesis can be scaled up to industrial production. They show bright emission ranging from the near-IR (1.5 eV) to the green (2.5 eV) spectral range with near-unity photoluminescence quantum yields. It is thus very important to investigate the energy-level structure and optical properties of this promising class of nanocrystals. The optoelectronic properties of InP core/shell QDs are determined by the chemical nature of both the core and the shell materials, the strain induced on the core due to core/shell epitaxy, and the relative offsets between the conduction and valence band edges of the core and shell materials. The electron and hole can be both localized within the core, or one carrier can be localized in the core and the other in the shell. For all these reasons, the nature of the shell has a strong influence on the radiative processes. This thesis aims to investigate these aspects in detail, providing a fundamental understanding of the physical mechanisms of spontaneous emission by unravelling the exciton fine-structure of InP core/shell nanocrystal quantum dots, and by studying how the exciton recombination depends on the size of the nanocrystals, the shell composition and core/shell structure, with an emphasis on the active role of the different types of phonons interacting with dark exciton states

Explorations in nanoscale copper indium sulfide: Synthesis, structural analysis, and optical properties of CuInS2 nanocrystals

The work presented in the thesis focusses on the synthesis and optical properties of colloidal Cu-chalcogenide nanocrystals and CuInS2 nanocrystals in particular. Novel synthesis approaches that yield nanocrystals with unprecedented sizes, shapes and/or hetero-architectures are developed, and the properties of these new materials are investigated. Another theme in this thesis are the remarkable opto-electronic properties of nanocrystals of CuInS2 and other I-III-VI2 materials, and their origin, which is still under intense debate