Perovskite Semiconductor Nanocrystals Research Articles

Breaking the Condon Approximation for Light Emission from Metal Halide Perovskite Nanocrystals.

The idea that the electronic transition dipole moment does not depend upon nuclear excursions is the Condon approximation and is central to most spectroscopy, especially in the solid state. We show a strong breakdown of the Condon approximation in the time-resolved photoluminescence from CsPbBr3 metal halide perovskite semiconductor nanocrystals. Experiments reveal that the electronic transition dipole moment increases on the 30 ps time scale due to structural dynamics in the lattice. Ab initio molecular dynamics calculations quantitatively reproduce experiments by considering excitation-induced structural dynamics.

(Invited) Ultrafast Spectroscopy Measurements of Lead-Halide Hybrid Semiconductor Nanocrystals for Photocatalysis

We are exploring the use of metal-halide perovskite semiconductor nanocrystals (PNCs) as efficient photocatalysis. Heterogeneous photocatalysis are less common than homogeneous but can provide unique avenues for inducing novel chemical transformations and can also be utilized for energy transductions, i.e., the energy in the photons can be captured in chemical bonds. Additional, earth abundant nanocrystal based photocatalyst are highly sought after as they can potentially eliminate expensive noble metal catalyst. A detailed understand of the underlying reaction mechanism could provide guidance in designing new systems that can activate a larger class of small molecules. Here, we discuss transient absorption spectroscopy to study a model C-C bond formation reaction, i.e., alkylation of aldehydes catalyzed by colloidal CsPbBr3 NCs. We find that both electrons and holes undergo ultrafast charge transfer from photoexcited PNCs. After charge transfer the charge separated state lives ~ 1 ns enabling radical mechanism to form C-C bonds. I will also discuss the use of PNCs to host a transition metal Cu-site incorporated at the surface of the PNCs to induce a to induce a N-N heterocyclization reaction. The reaction starts from surface coordinated diamine substrates and requires to subsequent photo-oxidation events. We established a photocatalytic cycle where photo-excited holes are funneled to the coordinated substrates.

Electrical Switching of Optical Gain in Perovskite Semiconductor Nanocrystals.

Perovskite semiconductor nanocrystals are promising for optical amplification and laser applications benefiting from efficient optical gain generation. Nevertheless, the pump threshold is limited by more than one exciton per nanocrystal required to generate population inversion in neutral nanocrystals due to the level degeneracy. Here, we show that by charging nanocrystals with current injection, the level degeneracy can be lifted to generate charged exciton gain with markedly low excitation density. On the basis of the scenario, we have demonstrated electrical switching of amplified spontaneous emission in films of CsPbBr3 nanocrystals sandwiched by two electrodes with over 50% threshold reduction owing to charged excitons. Our work provides an effective approach to electrically modulated optical gain in colloidal perovskite nanocrystals for potential applications in advanced laser and information technology.

Ultrafast dynamics of photoexcited carriers in perovskite semiconductor nanocrystals

Abstract Perovskite semiconductor nanocrystals have emerged as a promising family of materials for optoelectronic applications including light-emitting diodes, lasers, light-to-electricity convertors and quantum light emitters. The performances of these devices are fundamentally dependent on different aspects of the excited-state dynamics in nanocrystals. Herein, we summarize the recent progress on the photoinduced carrier dynamics studied by a variety of time-resolved spectroscopic methods in perovskite nanocrystals. We review the dynamics of carrier generation, recombination and transport under different excitation densities and photon energies to show the pathways that underpin the photophysics for light-emitting diodes and solar cells. Then, we highlight the up-to-date spin dynamics and coherent exciton dynamics being manifested with the exciton fine levels in perovskite semiconductor nanocrystals which are essential for potential applications in quantum information technology. We also discuss the controversial results and the possible origins yet to be resolved. In-depth study toward a comprehensive picture of the excited-state dynamics in perovskite nanocrystals may provide the key knowledge of the device operation mechanism, enlighten the direction for device optimization and stimulate the adventure of new conceptual devices.

Size-Dependent Hot Carrier Dynamics in Perovskite Nanocrystals Revealed by Two-Dimensional Electronic Spectroscopy.

The lifetimes of hot carriers have been predicted to be prolonged in small nanocrystals with an inter-level spacing larger than phonon energy. Nevertheless, whether such a phonon bottleneck is present in perovskite semiconductor nanocrystals remains highly controversial. Here we report compelling evidence of a phonon bottleneck in CsPbI3 nanocrystals with marked size-dependent relaxation of hot carriers by using broadband two-dimensional electronic spectroscopy (2DES). By combining high resolutions in both the time (<10 fs) and excitation energy domains, 2DES allows the clear disentanglement of the thermalization and cooling processes. The lifetime is over doubled for hot carriers when the average edge length of the nanocrystals decreases from 8.2 nm down to 4.6 nm. The confirmation of the phonon bottleneck effect suggests the feasibility of controlling hot carrier dynamics in perovskite semiconductors with nanocrystal size for potential applications of hot carrier devices.

Inhomogeneous Biexciton Binding in Perovskite Semiconductor Nanocrystals Measured with Two-Dimensional Spectroscopy.

Perovskite semiconductor nanocrystals have emerged as an excellent family of materials for optoelectronic applications, where biexciton interaction is essential for optical gain generation and quantum light emission. However, the strength of biexciton interaction remains highly controversial due to the entangled spectral features of the exciton- and biexciton-related transitions in conventional measurement approaches. Here, we tackle the limitation by using polarization-dependent two-dimensional electronic spectroscopy and quantify the excitation energy-dependent biexciton binding energy at cryogenic temperatures. The biexciton binding energy increases with excitation energy, which can be modeled as a near inverse-square size dependence in the effective mass approximation considering the quantum confinement effect. The spectral line width for the exciton-biexciton transition is much broader than that for the ground state to exciton transition, suggesting weakly correlated broadening between these transitions. These inhomogeneity effects should be carefully considered for the future demonstration of optoelectronic applications relying on coherent exciton-biexciton interactions.

Coherent exciton-phonon coupling in perovskite semiconductor nanocrystals studied by two-dimensional electronic spectroscopy

Perovskite semiconductor nanocrystals of lead halides exhibit excellent electronic and optical properties that are suitable for many optoelectronic applications. In this report, we investigate the underlying mechanism of the optical response of the material by probing coherent interaction between the exciton and lattice vibration by two-dimensional electronic spectroscopy. Coherent exciton-phonon coupling has been observed with time-domain oscillations in the dynamics of signals related to either ground or excited electronic states. Furthermore, the spectra of the coherent phonon derived from the dynamics of biexciton formation have larger amplitudes for higher frequency modes, which is attributed to the phonon bottleneck effect. Moreover, the composition-dependent coherent coupling between the exciton and lattice vibration in mixed halide samples reveals a critical role played by the anharmonicity and fluctuation of lattice vibration in the excited-state dynamics of perovskite nanocrystals.

Synthetic Evolution of Colloidal Metal Halide Perovskite Nanocrystals.

Metal halide perovskite semiconductor nanocrystals have emerged as a lucrative class of materials for many optoelectronic applications. By leveraging the synthetic toolboxes developed from decades of research into more traditional semiconductor nanocrystals, remarkable progress has been made across these materials in terms of their structural, compositional, and optoelectronic control. Here, we review this progress in terms of their underlying formation stages, synthetic approaches, and postsynthetic treatment steps. This assessment highlights the rapidly maturing nature of the perovskite nanocrystal field, particularly with regard to their lead-based derivatives. It further demonstrates that significant challenges remain around precisely controlling their nucleation and growth processes. In going forward, a deeper understanding of the role of precursors and ligands will significantly bolster the versatility in the size, shape, composition, and functional properties of these exciting materials.

Optical Gain from Biexcitons in CsPbBr3 Nanocrystals Revealed by Two-dimensional Electronic Spectroscopy.

Perovskite semiconductor nanocrystals (NCs) exhibit highly efficient optical gain, which is promising for laser applications. However, the intrinsic mechanism of optical gain in perovskite NCs, particularly whether more than one exciton per NCs is required, remains poorly understood. Here, we use two-dimensional electronic spectroscopy to resonantly probe the interplay between near-band-edge transitions during the buildup of optical gain in CsPbBr3 NCs. We find compelling evidence to conclude that optical gain in CsPbBr3 NCs is generated through stimulated emission from strongly interacting biexcitons. The threshold is largely determined by the competition between stimulated emission from biexcitons and excited-state absorption from single exciton to biexciton states. The findings in this work may guide future explorations of NC materials with low-threshold optical gain.

Composition-Dependent Energy Splitting between Bright and Dark Excitons in Lead Halide Perovskite Nanocrystals.

Perovskite semiconductor nanocrystals with different compositions have shown promise for applications in light-emitting devices. Dark excitonic states may suppress light emission from such nanocrystals by providing an additional nonradiative recombination channel. Here, we study the composition dependence of dark exciton dynamics in nanocrystals of lead halides by time-resolved photoluminescence spectroscopy at cryogenic temperatures. The presence of a spin-related dark state is revealed by magneto-optical spectroscopy. The energy splitting between bright and dark states is found to be highly sensitive to both halide elements and organic cations, which is explained by considering the effects of size confinement and charge screening, respectively, on the exchange interaction. These findings suggest the possibility of manipulating dark exciton dynamics in perovskite semiconductor nanocrystals by composition engineering, which will be instrumental in the design of highly efficient light-emitting devices.

Alkyl-Thiol Ligand-Induced Shape- and Crystalline Phase-Controlled Synthesis of Stable Perovskite-Related CsPb2Br5 Nanocrystals at Room Temperature.

Controlled synthesis of colloidal all-inorganic lead halide perovskite semiconductor nanocrystals, such as CsPbBr3, with tunable size, shape, composition, and crystalline phase have recently attracted wide interest for photonic and optoelectronic applications. Herein, we report a new strategy for using alkyl-thiols to induce the transformation of CsPbBr3 to perovskite-related cesium lead halide (CsPb2Br5) with controlled morphology and a crystalline phase at room temperature. By rational tuning the ratios of the alkyl-thiol ligands to alkyl-amines or to alkyl-acids, the as-synthesized colloidal nanocrystals can be rationally controlled from orthorhombic crystalline-phase CsPbBr3 to tetragonal-phase CsPb2Br5 nanosheets and nanowires with high yield. Significantly, the tetragonal CsPb2Br5 nanowires and nanosheets have high stability in high-temperature and high-humidity environments. These findings may open new directions for large-scale synthesis of shape- and crystalline phase-controlled perovskite nanocrystals for high-performance, low-cost optical electronic and optoelectronic devices.