Rachel Fainblat

Unprecedented insight into the lanthanide dispersion in mixed-metal Eu _x Tb _{1− x} BTC using high-field ¹ H MAS NMR spectroscopy. Isolation of Eu sites in the TbBTC matrix leads to efficient Tb-to-Eu energy transfer and high temperature sensitivity near RT.

At cryogenic temperatures, the photoluminescence spectrum of CdSe nanoplatelets (NPLs) usually consists of multiple emission lines, the origin of which is still under debate. While there seems to be consensus that both neutral excitons and trions contribute to the NPL emission, the prominent role of trions is rather puzzling. In this work, we demonstrate that Förster resonant energy transfer in stacks of NPLs combined with hole trap states in specific NPLs within the stack trigger trion formation, while single NPL spectra are dominated by neutral excitonic emission. This interpretation is verified by implementing copper (Cu⁺) dopants into the lattice as intentional hole traps. Trion emission gets strongly enhanced, and due to the large amount of hole trapping Cu⁺ states in each single NPL, trion formation does not necessarily require stacking of NPLs. Thus, the ratio between trion and neutral exciton emission can be controlled by either changing the amount of stacked NPLs during sample preparation or implementing copper dopants into the lattice which act as additional hole traps.

The chemical synthesis of (CdSe)13 magic-sized clusters (MSCs) allows the replacement of host atoms by individual transition metals such as Mn. By analyzing the spectral fingerprints of the Mn2+ photoluminescence (PL) in MSCs with different dopant concentrations, we are able to distinguish between single Mn2+ ions and coupled Mn2+ pairs. In case of Mn2+ pair emission, temperature-dependent studies show a pronounced red shift, followed by a distinct blue shift of the PL energy upon heating. This is related to a spin ladder formation of the ground and excited states due to Mn2+–Mn2+ exchange interaction at cryogenic temperatures, which is assumed to vanish at higher temperatures. In contrast, single Mn2+ ion PL exhibits a unique redshift with increasing temperature, which can be attributed to a particularly strong coupling to vibronic modes due to the ultimate small size of the MSCs.

The ability to molecularly engineer luminescent metal-organic frameworks is a powerful tool for the design of better performing rational temperature sensors. Lanthanide based MOF stand out as luminescent temperature sensors due to the high luminescence intensity and sharp emission lines of the lanthanides. The use of two different lanthanide cations incorporated into the same MOF structure is supposed to allow for a rational, that is self-referencing, temperature sensing. Here, we present series of mixed EuxTb(1-x)BTC, which were designed as nanoparticles. The EuxTb(1-x)BTC series shows controllable luminescent properties, which depend on the solvation of the lanthanide. The two MOFs in the series with the lowest Eu content, i.e. Eu0.04Tb0.96BTC and Eu0.02Tb0.98BTC, are suitable candidates for rational temperature sensing in the range between 200 and 270 K and above 300 K.

A Fano resonance, as often observed in scattering, absorption, or transmission experiments, arises from quantum interference between a discrete optical transition and a continuous background. Here, we present a temperature-dependent study on Fano resonances observed in photoluminescence from flakes of the layered semiconductor antiferromagnet chromium thiophosphate (CrPS₄). Two Fano resonances with a distinctly different temperature dependence were identified. The continuous background that is responsible for the Fano resonances is attributed to the d–d transition of the optically active Cr³⁺ center, predominantly the spin-forbidden ²E_g → ⁴A_2g transition with contributions of the broad-band ⁴T_2g → ⁴A_2g transition. The discrete states that interfere with this continuous background are suggested to arise from localized atomic phosphorus. A model idea for explaining the individual temperature dependence of the Fano resonances is presented.

The polarization of photoluminescence emitted from anisotropic nanocrystals directly reflects the symmetry of the eigenstates involved in the recombination process and can thus be considered as a characteristic feature of a nanocrystal. We performed polarization resolved magneto-photoluminescence spectroscopy on single colloidal Mn²⁺:CdSe/CdS core–shell quantum dots of wurtzite crystal symmetry. At zero magnetic field, a distinct linear polarization pattern is observed, while applying a magnetic field enforces circularly polarized emission with a characteristic saturation value below 100%. These polarization features are shown to act as a specific fingerprint of each individual nanocrystal. A model considering the orientation of the crystal c⃗ axis with respect to the optical axis and the magnetic field and taking into account the impact of magnetic doping is introduced and quantitatively explains our findings. We demonstrate that a careful analysis of the polarization state of single nanocrystal emission using the full set of Stokes parameters allows for identification of the complete three-dimensional orientation of the crystal anisotropy axis of an individual nanoobject in lab coordinates.

Giant band-splitting observed in EuS and EuSe nanocrystals by magnetic circular dichroism.

One of the most prominent signatures of transition metal doping in colloidal nanocrystals is the formation of charge carrier induced magnetization of the dopant spin sublattice, called exciton magnetic polaron (EMP). Understanding the direction of EMP formation, however, is still a major obstacle. Here, we present a series of temperature-dependent photoluminescence studies on single colloidal Mn²⁺:CdSe/CdS core/shell quantum dots (QDs) performed in a vector magnetic field providing unique insight into the interaction between individual excitons and numerous magnetic impurities. The energy of the QD emission and its full width at half maximum (FWHM) are controlled by the interplay of EMP formation and statistical magnetic fluctuations, in excellent agreement with theory. Most important, we give the first direct demonstration that anisotropy effects – hypothesized for more than a decade – dominate the direction of EMP formation. Our findings reveal a pathway for directing the orientation of optically induced magnetization in colloidal nanocrystals.