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Low-dimensional perovskites have—in view of their high radiative recombination rates—shown great promise in achieving high luminescence brightness and colour saturation. Here we investigate the effect of electron–phonon interactions on the luminescence of single crystals of two-dimensional perovskites, showing that reducing these interactions can lead to bright blue emission in two-dimensional perovskites. Resonance Raman spectra and deformation potential analysis show that strong electron–phonon interactions result in fast non-radiative decay, and that this lowers the photoluminescence quantum yield (PLQY). Neutron scattering, solid-state NMR measurements of spin–lattice relaxation, density functional theory simulations and experimental atomic displacement measurements reveal that molecular motion is slowest, and rigidity greatest, in the brightest emitter. By varying the molecular configuration of the ligands, we show that a PLQY up to 79% and linewidth of 20 nm can be reached by controlling crystal rigidity and electron–phonon interactions. Designing crystal structures with electron–phonon interactions in mind offers a previously underexplored avenue to improve optoelectronic materials' performance.

As the length scales of materials decrease, the heterogeneities associated with interfaces become almost as important as the surrounding materials. This has led to extensive studies of emergent electronic and magnetic interface properties in superlattices 1 – 9 . However, the interfacial vibrations that affect the phonon-mediated properties, such as thermal conductivity 10 , 11 , are measured using macroscopic techniques that lack spatial resolution. Although it is accepted that intrinsic phonons change near boundaries 12 , 13 , the physical mechanisms and length scales through which interfacial effects influence materials remain unclear. Here we demonstrate the localized vibrational response of interfaces in strontium titanate–calcium titanate superlattices by combining advanced scanning transmission electron microscopy imaging and spectroscopy, density functional theory calculations and ultrafast optical spectroscopy. Structurally diffuse interfaces that bridge the bounding materials are observed and this local structure creates phonon modes that determine the global response of the superlattice once the spacing of the interfaces approaches the phonon spatial extent. Our results provide direct visualization of the progression of the local atomic structure and interface vibrations as they come to determine the vibrational response of an entire superlattice. Direct observation of such local atomic and vibrational phenomena demonstrates that their spatial extent needs to be quantified to understand macroscopic behaviour. Tailoring interfaces, and knowing their local vibrational response, provides a means of pursuing designer solids with emergent infrared and thermal responses. The vibrational states emerging at the interface in oxide superlattices are characterized theoretically and at atomic resolution, showing the impact of material length scales on structure and vibrational response.

Reduced-dimensional perovskites are attractive light-emitting materials due to their efficient luminescence, color purity, tunable bandgap, and structural diversity. A major limitation in perovskite light-emitting diodes is their limited operational stability. Here we demonstrate that rapid photodegradation arises from edge-initiated photooxidation, wherein oxidative attack is powered by photogenerated and electrically-injected carriers that diffuse to the nanoplatelet edges and produce superoxide. We report an edge-stabilization strategy wherein phosphine oxides passivate unsaturated lead sites during perovskite crystallization. With this approach, we synthesize reduced-dimensional perovskites that exhibit 97 ± 3% photoluminescence quantum yields and stabilities that exceed 300 h upon continuous illumination in an air ambient. We achieve green-emitting devices with a peak external quantum efficiency (EQE) of 14% at 1000 cd m −2 ; their maximum luminance is 4.5 × 10 4  cd m −2 (corresponding to an EQE of 5%); and, at 4000 cd m −2 , they achieve an operational half-lifetime of 3.5 h. Reduced-dimensional halide perovskites are promising for light-emitting diodes but suffer from photo-degradation. Here Quan et al. identify the edge of the perovskite nanoplatelets as the degradation channels and use phosphine oxides to passivate the edges and boost device performance and lifetime.

Solar energy conversion is the process whereby sunlight is converted into a consumable source of energy. Semiconductors such as TiO2 can act as effective media for charge separation in photocatalytic and photovoltaic systems. The problem is that TiO2 and other metal oxide semiconductors do not absorb light in the visible wavelength region where the solar spectrum is most intense. One solution to this problem is to sensitize the semiconductor surface with strongly absorbing dye molecules whose absorption spectra lie within the visible wavelength region. The dye absorbs light placing it in a molecular excited state. From the excited state, electron transfer from the dye to the semiconductor can occur, effectively separating charge for photovoltaic or photocatalytic use. This work concerns the light absorption and the subsequent dynamical events of dye-sensitizer molecules. The class of dye sensitizers studied are rhodamine dyes with varying molecular features. Rhodamines are composed of a core chromophore and an aryl bridge moiety. When attached to TiO2, the chromophore absorbs light and acts as an electron donor to the TiO2 acceptor. The bridge group which separates the chromophore and TiO2 modulates the electronic coupling strength between the donor and acceptor and thus affects the electron transfer rate. Here we present the excited state dynamics of rhodamine dye sensitizers with phenylene and thiophene bridge groups. It is found that thiophene bridges can support excited state coplanarization with the chromophore’s π system which enhances electronic coupling to a TiO2 acceptor. Phenylene bridges, being larger and thus more rigid, do not coplanarize in the excited state and have smaller electronic coupling strengths. DFT and Fermi’s golden rule rate calculations show that dyes with either phenylene or thiophene bridges, near their ground state geometries, have fast electron transfer rates of < 6 ps. The coplanarization observed for thiophene substituted dyes results in a significantly reduced oxidation potential compared to the dye’s initial geometry. Although coupling strength is enhanced at coplanarity, the donor energy level falls below the conduction band edge and electron transfer is slowed significantly. Another structural aspect of rhodamines which is considered is chalcogen substitution on the core chromophore. Substituting the core chalcogen with oxygen, sulfur or selenium has little effect on the electron transfer rate. However, selenium derivatives have large triplet yields and fast intersystem crossing time constants. Electron transfer rates from the triplet states of selenium derivatives were calculated and it was found that, similar to coplanar dyes, the oxidation potential falls below the conduction band edge of TiO2. The triplet state of selenium substituted rhodamine dyes was thought to be beneficial to electron transfer due to its long lifetime. These results however suggest that the electron transfer process should be faster than intersystem crossing and that electron transfer from the triplet state should be much slower than from the initial singlet excited state. The conclusions of this work are used to interpret previous results of photocatalytic hydrogen generation and dye-sensitized solar cell experiments. Several new questions are generated regarding the effect of dye aggregation and solvent environment on the activity of each dye.

SiGe heterostructures integrated with Si via virtual substrate (VS) growth are promising hosts for spin qubits. While VS growth targets plastic relaxation, residual cross-hatch strain inhomogeneity propagates into heterostructure overgrowth. To quantify strain inhomogeneity's influence on interface structure and qubit properties, we measure strained-silicon (s-Si)/Si\\(_0.7\\)Ge\\(_0.3\\) heterostructures on 25 wafers processed via standard commercial chemical vapor deposition. Spatially-aligned images of strain (Raman microscopy) and interface structure (atomic force microscopy and cross-sectional scanning transmission electron microscopy) reveal strain-roughness interplay. A strain-driven surface diffusion model predicts the roughness and its temperature dependence. Measured strains suggest spurious double-dot qubit detunings of 0.1 meV over 100 nm distances may result. Modeling shows that interface roughness (atomic steps), when convolved with alloy disorder, only modestly reduces valley splitting (70\\(\\)13 vs. 77\\(\\)14 \\(\\)eV on average). Our findings point to thicker VS buffer layers beneath heterostructures and lower-temperature growth (T \\(\\) 700 \\(^\\)C) to limit roughening.

As the length-scales of materials decrease, heterogeneities associated with interfaces approach the importance of the surrounding materials. Emergent electronic and magnetic interface properties in superlattices have been studied extensively by both experiments and theory. \\(^1-6\\) However, the presence of interfacial vibrations that impact phonon-mediated responses, like thermal conductivity \\(^7,8\\), has only been inferred in experiments indirectly. While it is accepted that intrinsic phonons change near boundaries \\(^9,10\\), the physical mechanisms and length-scales through which interfacial effects influence materials remain unclear. Herein, we demonstrate the localized vibrational response associated with the interfaces in SrTiO\\(_3\\)-CaTiO\\(_3\\) superlattices by combining advanced scanning transmission electron microscopy imaging and spectroscopy and density-functional-theory calculations. Symmetries atypical of either constituent material are observed within a few atomic planes near the interface. The local symmetries create local phonon modes that determine the global response of the superlattice once the spacing of the interfaces approaches the phonon spatial extent. The results provide direct visualization and quantification, illustrating the progression of the local symmetries and interface vibrations as they come to determine the vibrational response of an entire superlattice; stated differently, the progression from a material with interfaces, to a material dominated by interfaces, to a material of interfaces as the period decreases. Direct observation of such local atomic and vibrational phenomena demonstrates that their spatial extent needs to be quantified to understand macroscopic behavior. Tailoring interfaces, and knowing their local vibrational response, provides a means of pursuing designer solids having emergent infrared and thermal responses.