70 research outputs found
Fully inorganic oxide-in-oxide ultraviolet nanocrystal light emitting devices
The development of integrated photonics and lab-on-a-chip platforms for environmental and biomedical diagnostics demands ultraviolet electroluminescent materials with high mechanical, chemical and environmental stability and almost complete compatibility with existing silicon technology. Here we report the realization of fully inorganic ultraviolet light-emitting diodes emitting at 390 nm with a maximum external quantum efficiency of ~0.3%, based on SnO(2) nanoparticles embedded in SiO(2) thin films obtained from a solution-processed method. The fabrication involves a single deposition step onto a silicon wafer followed by a thermal treatment in a controlled atmosphere. The fully inorganic architecture ensures superior mechanical robustness and optimal chemical stability in organic solvents and aqueous solutions. The versatility of the fabrication process broadens the possibility of optimizing this strategy and extending it to other nanostructured systems for designed applications, such as active components of wearable health monitors or biomedical devices
Reversed oxygen sensing using colloidal quantum wells towards highly emissive photoresponsive varnishes
Colloidal quantum wells combine the advantages of size-tunable electronic properties with vast reactive surfaces that could allow one to realize highly emissive luminescent-sensing varnishes capable of detecting chemical agents through their reversible emission response, with great potential impact on life sciences, environmental monitoring, defence and aerospace engineering. Here we combine spectroelectrochemical measurements and spectroscopic studies in a controlled atmosphere to demonstrate the 'reversed oxygen-sensing' capability of CdSe colloidal quantum wells, that is, the exposure to oxygen reversibly increases their luminescence efficiency. Spectroelectrochemical experiments allow us to directly relate the sensing response to the occupancy of surface states. Magneto-optical measurements demonstrate that, under vacuum, heterostructured CdSe/CdS colloidal quantum wells stabilize in their negative trion state. The high starting emission efficiency provides a possible means to enhance the oxygen sensitivity by partially de-passivating the particle surfaces, thereby enhancing the density of unsaturated sites with a minimal cost in term of luminescence losses
Efficient Luminescent Solar Concentrators Based on Environmentally Friendly CdâFree Ternary AIS/ZnS Quantum Dots
Luminescent solar concentrators (LSC) allow to obtain renewable energy from building integrated photovoltaic systems. As promising efficient and long-term stable LSC fluorophores semiconductor nanocrystals like quantum dots (QDs) with size and composition tunable optoelectronic properties have recently emerged. The most popular II/VI or IV/VI semiconductor QDs contain, however, potentially hazardous cadmium or lead ions, which is a bottleneck for commercial applications. A simple aqueous based, microwave-assisted synthesis for environmentally friendly and highly emissive AgInS2/ZnS QDs is developed using 3-mercaptopropionic acid (MPA) and glutathione (GSH) and their incorporation into polylaurylmethacrylate (PLMA) polymer slabs integrable in LSC devices (10.4 Ă 10.4 Ă 0.2 cm3, G = 12.98). With this simple approach, optical power efficiencies (OPE) of 3.8% and 3.6% and optical quantum efficiencies (OQE) of 24.1% and 27.4% are obtained, which are among the highest values yet reported.German Research CouncilEuropean Union's Horizon 2020Marie SklodowskaâCurieDeutsche Forschungsgemeinschaft
http://dx.doi.org/10.13039/501100001659Peer Reviewe
High-efficiency all-solution-processed light-emitting diodes based on anisotropic colloidal heterostructures with polar polymer injecting layers
Colloidal quantum dots (QDs) are emerging as true candidates for light-emitting diodes with ultrasaturated colors. Here, we combine CdSe/CdS dot-in-rod hetero-structures and polar/polyelectrolytic conjugated polymers to demonstrate the first example of fully solution-based quantum dot light-emitting diodes (QD-LEDs) incorporating all-organic injection/transport layers with high brightness, very limited roll-off and external quantum efficiency as high as 6.1%, which is 20 times higher than the record QD-LEDs with all-solution processed organic interlayers and exceeds by over 200% QD-LEDs embedding vacuum-deposited organic molecules
Evidence for the Band-Edge Exciton of CuInS2 Nanocrystals Enables Record Efficient Large-Area Luminescent Solar Concentrators
AbstractTernary IâIIIâVI2 nanocrystals (NCs), such as CuInS2, are receiving attention as heavyâmetalsâfree materials for solar cells, luminescent solar concentrators (LSCs), LEDs, and bioâimaging. The origin of the optical properties of CuInS2 NCs are however not fully understood. A recent theoretical model suggests that their characteristic Stokesâshifted and longâlived luminescence arises from the structure of the valence band (VB) and predicts distinctive optical behaviours in defectâfree NCs: the quadratic dependence of the radiative decay rate and the Stokes shift on the NC radius. If confirmed, this would have crucial implications for LSCs as the solar spectral coverage ensured by lowâbandgap NCs would be accompanied by increased reâabsorption losses. Here, by studying stoichiometric CuInS2 NCs, it is revealed for the first time the spectroscopic signatures predicted for the free bandâedge exciton, thus supporting the VBâstructure model. At very low temperatures, the NCs also show darkâstate emission likely originating from enhanced electronâhole spin interaction. The impact of the observed optical behaviours on LSCs is evaluated by Monte Carlo rayâtracing simulations. Based on the emerging device design guidelines, opticalâgrade largeâarea (30Ă30 cm2) LSCs with optical power efficiency (OPE) as high as 6.8% are fabricated, corresponding to the highest value reported to date for largeâarea devices
Band structure engineering via piezoelectric fields in strained anisotropic CdSe/CdS nanocrystals
Strain in colloidal heteronanocrystals with non-centrosymmetric lattices presents a unique opportunity for controlling optoelectronic properties and adds a new degree of freedom to existing wavefunction engineering and doping paradigms. We synthesized wurtzite CdSe nanorods embedded in a thick CdS shell, hereby exploiting the large lattice mismatch between the two domains to generate a compressive strain of the CdSe core and a strong piezoelectric potential along its c-axis. Efficient charge separation results in an indirect ground-state transition with a lifetime of several microseconds, almost one order of magnitude longer than any other CdSe/CdS nanocrystal. Higher excited states recombine radiatively in the nanosecond time range, due to increasingly overlapping excited-state orbitals. kËp calculations confirm the importance of the anisotropic shape and crystal structure in the buildup of the piezoelectric potential. Strain engineering thus presents an efficient approach to highly tunable single- and multiexciton interactions, driven by a dedicated core/shell nanocrystal design.F.R., J.I.C. and J.P. acknowledge financial support from MINECO project CTQ2011-27324 and UJI-Bancaixa P1-1B2011-01. S.B. and F.M. acknowledge support from the Cariplo Foundation (2012-0844). S.B. thanks the European Communityâs Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 324603 for financial support (EDONHIST). The present publication is further realized with the support of the Ministero degli Affari Esteri e della Cooperazione Internazionale (IONX-NC4SOL, I.M.). K. Miszta (IIT, Italy) is acknowledged for initial discussions on the RIR samples, and W. Langbein (Cardiff University, UK) for enlightening conversations on strain and the Stark effect in CdSe/CdS superlattices
Tuning Radiative Recombination in Cu-Doped Nanocrystals via Electrochemical Control of Surface Trapping
The incorporation of copper dopants into IIâVI
colloidal
nanocrystals (NCs) leads to the introduction of intragap electronic
states and the development of a new emission feature due to an optical
transition which couples the NC conduction band to the Cu-ion state.
The mechanism underlying Cu-related emission and specifically the
factors that control the branching between the intrinsic and impurity-related
emission channels remain unclear. Here, we address this problem by
conducting spectro-electrochemical measurements on Cu-doped core/shell
ZnSe/CdSe NCs. These measurements indicate that the distribution of
photoluminescence (PL) intensity between the intrinsic and the impurity
bands as well as the overall PL efficiency can be controlled by varying
the occupancy of surface defect sites. Specifically, by activating
hole traps under negative electrochemical potential (the Fermi level
is raised), we can enhance the Cu band at the expense of band-edge
emission, which is consistent with the predominant Cu<sup>2+</sup> character of the dopant ions. Furthermore, we observe an overall
PL âbrighteningâ
under negative potential and âdimmingâ under positive
potential, which we attribute to changes in the occupancy of the electron
trap sites (that is, the degree of their electronic passivation) that
control nonradiative losses due to electron surface trapping
Two-Color Quantum and Classical Light Using Dot-in-Bulk Semiconductor Nanocrystals
Using novel dot-in-bulk nanocrystals comprising a quantum-confined CdSe core overcoated with a thick, bulk-like CdS shell, we realize a new antibunching mechanism, which allows for simultaneous generation of quantum and classical light from the same nanostructure
- âŠ