Spin Polarization Dynamics of Free Charge Carriers in CsPbI3 Nanocrystals.

Lead halide perovskites (LHPs) exhibit large spin-orbit coupling (SOC), leading to only twofold-degenerate valence and conduction bands and therefore allowing for efficient optical orientation. This makes them ideal materials to study charge carrier spins. With this study we elucidate the spin dynamics of photoexcited charge carriers and the underlying spin relaxation mechanisms in CsPbI3 nanocrystals by employing time-resolved differential transmission spectroscopy (DTS). We find that the photoinduced spin polarization significantly diminishes during thermalization and cooling toward the energetically favorable band edge. Temperature-dependent DTS reveals a decay in spin polarization that is more than 1 order of magnitude faster at room temperature (3 ps) than at cryogenic temperatures (32 ps). We propose that spin relaxation of free charge carriers in large-SOC materials like LHPs occurs as a result of carrier-phonon scattering, as described by the Elliott-Yafet mechanism.

Boosting Tunable Blue Luminescence of Halide Perovskite Nanoplatelets through Postsynthetic Surface Trap Repair.

The easily tunable emission of halide perovskite nanocrystals throughout the visible spectrum makes them an extremely promising material for light-emitting applications. Whereas high quantum yields and long-term colloidal stability have already been achieved for nanocrystals emitting in the red and green spectral range, the blue region currently lags behind with low quantum yields, broad emission profiles, and insufficient colloidal stability. In this work, we present a facile synthetic approach for obtaining two-dimensional CsPbBr3 nanoplatelets with monolayer-precise control over their thickness, resulting in sharp photoluminescence and electroluminescence peaks with a tunable emission wavelength between 432 and 497 nm due to quantum confinement. Subsequent addition of a PbBr2-ligand solution repairs surface defects likely stemming from bromide and lead vacancies in a subensemble of weakly emissive nanoplatelets. The overall photoluminescence quantum yield of the blue-emissive colloidal dispersions is consequently enhanced up to a value of 73 ± 2%. Transient optical spectroscopy measurements focusing on the excitonic resonances further confirm the proposed repair process. Additionally, the high stability of these nanoplatelets in films and to prolonged ultraviolet light exposure is shown.

Dual-comb coherent Raman spectroscopy with lasers of 1-GHz pulse repetition frequency.

We extend the technique of multiplex coherent Raman spectroscopy with two femtosecond mode-locked lasers to oscillators of a pulse repetition frequency of 1 GHz. We demonstrate a spectra of liquids, which span 1100 cm-1 of Raman shifts. At a resolution of 6 cm-1, their measurement time may be as short as 5 µs for a refresh rate of 2 kHz. The waiting period between acquisitions is improved 10-fold compared to previous experiments with two lasers of 100-MHz repetition frequencies.

From Precursor Powders to CsPbX3 Perovskite Nanowires: One-Pot Synthesis, Growth Mechanism, and Oriented Self-Assembly.

The colloidal synthesis and assembly of semiconductor nanowires continues to attract a great deal of interest. Herein, we describe the single-step ligand-mediated synthesis of single-crystalline CsPbBr3 perovskite nanowires (NWs) directly from the precursor powders. Studies of the reaction process and the morphological evolution revealed that the initially formed CsPbBr3 nanocubes are transformed into NWs through an oriented-attachment mechanism. The optical properties of the NWs can be tuned across the entire visible range by varying the halide (Cl, Br, and I) composition through subsequent halide ion exchange. Single-particle studies showed that these NWs exhibit strongly polarized emission with a polarization anisotropy of 0.36. More importantly, the NWs can self-assemble in a quasi-oriented fashion at an air/liquid interface. This process should also be easily applicable to perovskite nanocrystals of different morphologies for their integration into nanoscale optoelectronic devices.

Near-Field Imaging of Phased Array Metasurfaces.

Phased-antenna metasurfaces can impart abrupt, spatially dependent changes to the amplitude, phase, and polarization of light and thus mold wavefronts in a desired fashion. Here we present an experimental and computational near-field study of metasurfaces based on near-resonant V-shaped antennas and connect their near- and far-field optical responses. We show that far fields can be obtained from limited, experimentally obtained knowledge of the near fields, paving the way for experimental near-field characterization of metasurfaces and other optical nanostructures and prediction of their far fields from the near-field measurements.

Thickness-Dependence of Exciton-Exciton Annihilation in Halide Perovskite Nanoplatelets.

Exciton-exciton annihilation (EEA) and Auger recombination are detrimental processes occurring in semiconductor optoelectronic devices at high carrier densities. Despite constituting one of the main obstacles for realizing lasing in semiconductor nanocrystals (NCs), the dependencies on NC size are not fully understood, especially for those with both weakly and strongly confined dimensions. Here, we use differential transmission spectroscopy to investigate the dependence of EEA on the physical dimensions of thickness-controlled 2D halide perovskite nanoplatelets (NPls). We find the EEA lifetimes to be extremely short on the order of 7-60 ps. Moreover, they are strongly determined by the NPl thickness with a power law dependence according to τ2 ∝ d5.3. Additional measurements show that the EEA lifetimes also increase for NPls with larger lateral dimensions. These results show that a precise control of the physical dimensions is critical for deciphering the fundamental laws governing the process especially in 1D and 2D NCs.

Fast Electron and Slow Hole Relaxation in InP-Based Colloidal Quantum Dots.

Colloidal InP-based quantum dots are a promising material for light-emitting applications as an environment friendly alternative to their Cd-containing counterparts. Especially for their use in optoelectronic devices, it is essential to understand how charge carriers relax to the emitting state after injection with excess energy and if all of them arrive at this desired state. Herein, we report time-resolved differential transmission measurements on colloidal InP/ZnS and InP/ZnSe core/shell quantum dots. By optically exciting and probing individual transitions, we are able to distinguish between electron and hole relaxation. This, in turn, allows us to determine how the initial excess energy of the charge carriers affects the relaxation processes. According to the electronic level scheme, one expects a strong phonon bottleneck for electrons, whereas holes should relax easier as their energy levels are more closely spaced. On the contrary, we find that electrons relax faster than holes. The fast electron relaxation occurs via an efficient Auger-like electron-hole scattering mechanism. On the other hand, a small wave function overlap between core and shell states slows the hole relaxation. Additionally, holes can be trapped at the core/shell interface, leading to either slow detrapping or nonradiative recombination. Overall, these results demonstrate that it is crucial to construct devices enabling the injection of charge carriers energetically close to their emitting states in order to maximize the radiative efficiency of the system.

Oriented Thin Films of Electroactive Triphenylene Catecholate-Based Two-Dimensional Metal-Organic Frameworks.

Two-dimensional triphenylene-based metal-organic frameworks (TP-MOFs) attract significant scientific interest due to their long-range order combined with significant electrical conductivity. The deposition of these structures as oriented films is expected to promote their incorporation into diverse optoelectronic devices. However, to date, a controlled deposition strategy applicable for the different members of this MOF family has not been reported yet. Herein, we present the synthesis of highly oriented thin films of TP-MOFs by vapor-assisted conversion (VAC). We targeted the M-CAT-1 series comprising hexahydroxytriphenylene organic ligands and metal-ions such as Ni2+, Co2+, and Cu2+. These planar organic building blocks are connected in-plane to the metal-ions through a square planar node forming extended sheets which undergo self-organization into defined stacks. Highly oriented thin Ni- and Co-CAT-1 films grown on gold substrates feature a high surface coverage with a uniform film topography and thickness ranging from 180 to 200 nm. The inclusion of acid modulators in the synthesis enabled the growth of films with a preferred orientation on quartz and on conductive substrates such as indium-doped tin oxide (ITO). The van der Pauw measurements performed across the M-CAT-1 films revealed high electrical conductivity values of up to 10-3 S cm-1 for both the Ni- and Co-CAT-1 films. Films grown on quartz allowed for a detailed photophysical characterization by means of UV-vis, photoluminescence, and transient absorption spectroscopy. The latter revealed the existence of excited states on a nanosecond time scale, sufficiently long to demonstrate a photoinduced charge generation and extraction in Ni-CAT-1 films. This was achieved by fabricating a basic photovoltaic device with an ITO/Ni-CAT-1/Al architecture, thus establishing this MOF as a photoactive material. Our results point to the intriguing capabilities of these conductive M-CAT-1 materials and an additional scope of applications as photoabsorbers enabled through VAC thin-film synthesis.