In recent years, InAs colloidal quantum dots have gained significant interest due to their potential role as foundations for cutting-edge Short-Wave Infrared (SWIR) optoelectronic devices. Despite the promise these materials hold, the progress in synthetic development and the advancement in material design of InAs nanocrystals has been hindered by two major factors: the limited choices of synthetic precursors available for use and a prevailing lack of comprehensive understanding regarding the underlying reaction pathways that form these nanocrystals. In this presentation, we will navigate through the field of precursor chemistry, employing widely used Arsenic precursors to broaden the window of reaction conditions. Our novel approach, involving the systematic exploration and adoption of these Arsenic precursors, has enabled us to achieve a major breakthrough: the synthesis of size-tunable InAs nanocrystals exhibiting remarkable band-edge absorption extending up to 1700 nm. Furthermore, we will unveil the groundbreaking results of our relentless efforts in synthesizing Zinc-doped InAs nanocrystals using precursor chemistry. This achievement, a significant milestone, advances the quest for creating unprecedentedly highly doped p-type materials.

Despite significant progress in recent years in understanding the chemical reactions occurring on the surfaces of II-VI, III-V, and lead halide perovskite quantum dots (QDs), there are still fundamental questions that remain unanswered regarding the nature of QD surfaces, QD-ligand interactions, and the formation of trap states. Addressing these aspects is crucial for enhancing the optoelectronic efficiency of QDs.

To tackle these challenges, an essential step is the utilization of first principle simulations to analyze QD surfaces. Traditional simulations have been limited by their restricted system size, typically confined to a few hundred atoms, and their focus on static properties without considering dynamic effects.

In this study, we present a pioneering multiscale modeling approach that combines Density Functional Theory and Molecular Dynamics simulations. Our approach encompasses QDs ranging from small to real-sized QDs passivated with oleate ligands and immersed in organic solvents. Through this methodology, we gain invaluable insights into the surface characteristics and the binding energies of ligands under different experimental conditions. This groundbreaking methodology not only provides a deeper understanding of the intricate behavior of colloidal semiconductor nanocrystals but also paves the way for future advancements in their diverse applications.

The presentation will review some recent combined theoretical and experimental results on the interplay between the optoelectronic properties of 3D perovskites (and their nanostructures), and lattice dynamics and polarizability.

Acknowledgments

[1] B. Hehlen, P. Bourges, B. Rufflé, S. Clément, R. Vialla, A. C. Ferreira, C. Ecolivet, S. Paofai, S. Cordier, C. Katan, A. Létoublon, J. Even, Pseudospin-phonon pretransitional dynamics in lead halide hybrid perovskites, Phys. Rev. B (2022)

[2] P. Tamarat, E. Prin, Y. Bezovska, A. Moskalenko, T. Nguyen, C. Xia, L. Hou, J.-B. Trebbia, M. Zacharias, L.  Pedesseau, C.  Katan, M. Bodnarchuk, M. Kovalenko, J.  Even, B. Lounis, Universal scaling laws for charge-carrier interactions with quantum confinement in lead-halide perovskites , Nature Comm. (2023)

[3] C. Zhu, T. Nguyen, S. Böhme, A. Moskalenko, D. Dirin, M. Bodnarchuk, C Katan, J. Even, G. Rainò and M. Kovalenko*, Many-body Correlations and Bound Exciton Complexes in CsPbX3 (X=Br, Br/Cl) Quantum Dots, Advanced Materials (2023)

[4] M. Zacharias, G. Volonakis, F. Giustino, J. Even, Anharmonic electron-phonon coupling in ultrasoft and locally disordered perovskites, NPG Comput. Mat. (2023)

Optoelectronic properties of ultrasmall semiconductor nanocrystals are strongly related to their structural and microstructural features. However, due to the complexity of these materials, this intermingled relationship remains mostly elusive.

Over the past decades, total scattering methods, in particular the ones based on the Debye Scattering Equation (DSE) and operating in reciprocal space, have been established as essential tools for characterizing the structure, microstructure, and morphology of nanocrystals, including ultrasmall Quantum Dots (QDs).[1–5]

Although wide-angle scattering-based techniques are primarily sensitive to the atomic-scale structures of materials, reciprocal space total scattering methods provide robust information on multiple length scales, in particular if nanocrystalline materials are considered.

Nevertheless, constructing reliable, material-oriented atomistic models, to be optimized against the experimental data in order to extract structural and microstructural parameters remains a highly challenging task and often poses a bottleneck for scattering-based methods.[6–9]

To overcome this limitation, we tackle the challenge of developing reliable, efficient, and user-friendly methods for determining the average size of colloidal QDs, with a combination of reciprocal space total scattering methods based on DSE and an all-convolutional neural network (all-CNN) that provides physically interpretable results.

In this talk, I will present the development and first application of this novel tool to a selected class of lead-chalcogenide binary QDs that serves as a benchmark system. Indeed, they have been extensively characterized within the DSE approach, which has provided well-established knowledge about their structural and morphological features.[1]

The presented automated tool can be readily employed for real-time size classification of PbS QDs, even from diluted colloidal suspensions, within the limitations of the Q-range and signal-to-noise ratio typically encountered in in-situ and in-operando diffraction experiments.  Additionally, it may serve as a rapid screening tool for the optimization of synthetic protocols.

Furthermore, the proposed method can be easily extended to other classes of nanocrystals, allowing non-experts in crystallography and X-ray diffraction to utilize the automated workflow for creating DSE pattern libraries used for training the all-CNN.

The coupling of phonons to the elementary excitations of a material determines its excited state properties and the rates of dynamical processes. The strength of Electron-Phonon Coupling (EPC) therefore dictates the performance metrics of applications utilizing a material. Stronger EPC increases the rate of non-radiative recombination of charge carriers, ultimately limiting the efficiency of semiconductor devices. For applications utilizing luminescence, stronger EPC leads to broadening of the emission spectrum reducing its spectral purity, and drives decoherence and loss of information in single coherent photon sources. Strong electron-phonon coupling, however, is not always detrimental, as exemplified by the case of superconductivity. To understand the intrinsic limitations or aptitude of a material for a given application, precise determination of its phonons and how they couple to transitions is required. Additionally, through material engineering, one can aim to tune EPC in a material system in order to improve performance metrics. To achieve this, detailed understanding of the mechanistic origins of EPC in the material is required.

In this talk I will review recent studies we have performed to quantify EPC in lead-halide perovskites. Ab-initio calculations indicate strong deformation potential type coupling to low energy optical phonons, vibrational modes which drive octahedral tilting in lead-halide perovskites. Optical-pump diffraction-probe, single dot luminescence, and time resolved emission measurements on lead-halide perovskite nanocrystals corroborate this finding, and confirm that deformation potential coupling to octahedral tilting is the dominant coupling to the excitation/recombination of excitons in LHPs. The dependence of strength of this EPC on temperature and on the LHP composition (and phase) point to strong enhancement of the EPC as a result of dynamic structural disorder and phonon-phonon scattering.

Metal-halide perovskite materials demonstrated extraordinary performance in solar cells and light emission in recent years, and their layered low-dimensional counterparts promise increased stability and even greater tunability due to the huge variety of molecules available for the organic phase. [1-3] Single or double octahedra-layer (1L and 2L) structures show strong confinement and large exciton binding energies, and the band gap and light emission depends sensitively on the lattice distortions induced by the organic cations.[4-7] In particular, the broadband emission from trapped excitons in combination with the band-edge emission can lead to white light emission from a single material with tunable color temperature. [8]

We investigate the emission from several Ruddlesden-Popper metal-halide perovskites with single octahedra layers (n=1) that differ by their organic cations and lattice distortions.[9, 10] The impact of phonons on the optical emission dynamics can be assessed by the temperature-dependence of the emission linewidth broadening.[11, 12] However, only in some cases this analysis allows to identify the contribution of specific phonon modes.[9, 10] We therefore combine angle-dependent polarized Raman and photoluminescence spectroscopy on single microcrystalline flakes that reveal a strong directionality of the signal along the major axes of the octahedra lattice, to get a deeper insight into the photophysics of these materials. With non-resonant Raman spectroscopy in the ultralow frequency range (from 10 to 200 cm-1), we observe a very large number of phonon bands with different vibrational symmetries, which makes it plausible that for certain materials not only one single dominant phonon mode impacts the emission process.

Interestingly, also the light emission of single two-dimensional metal halide perovskite flakes shows a strong angle-dependence in polarization. Here the band-edge emission along one major axis of the octahedra lattice is significantly stronger than along the perpendicular one, which indicates that the in-plane distortions of the lattice lead to emission polarization. Therefore, the directional behavior of the emission of single perovskites flakes evidences the impact of the lattice strain on the excitonic properties and opens new opportunities for the design of light-emitting devices, for example by tailoring optical cavities along these directions or optimizing the structures for efficient outcoupling of the light.

Metal halide perovskites (MHPs) have shown excellent results in many optoelectronic applications. Shaping MHPs in the form of nanowires could offer advantages over thin films such as control over charge carrier transport and nanophotonic light guiding. However, present methods to obtain free-standing vertically aligned MHP nanowire arrays and heterostructures lack the scalability needed for applications. We have previously reported use a one-step solution low-temperature method to grow arrays of long single-crystalline CsPbBr₃ nanowires in an anodized aluminum oxide (AAO) template [1]. Surprisingly, we find that free-standing CsPbBr₃ nanowires can be obtained by the same method [2]. The length of the vertically aligned nanowires is controlled from 1 to 20 μm with the precursor amount. The nanowires are single-crystalline and exhibit excellent photoluminescence and clear light guiding.

Lithography methods for MHPs have been limited because of their solubility in polar solvents. However, we show that o-xylene can be used in a perovskite-compatible electron beam lithography (EBL) process based on nonpolar solvents [3]. Features down to 50 nm size are created, and photoluminescence of CsPbBr3 nanowires exhibits no degradation. We use the EBL method fabricate metal contacts to single CsPbBr3 nanowires and create single nanowire devices.

Recently, we have developed a gas-phase halide exchange method based on HCl or Cl₂ to convert the green-emitting CsPbBr₃ to blue-emitting CsPb(Br_xCl_1-x)₃. By combining the Cl₂ exchange process with the aforementioned EBL process, heterojunction NWs with varying halide compositions are produced, including complex barcode-like NWs with segment lengths as short as 500 nm. Using, we selectively convert segments of nanowires to CsPb(Br_xCl_1-x)₃ and create axially heterostructured nanowires [4]. Both the new EBL process and the gas phase halide exchange are compatible with any MHP morphology.

Halide perovskites are of immense importance due to their excellent light-absorbing and emitting properties, holding great promise in the field of advanced and clean energy technologies. One of the main mechanisms governing their unique properties at finite temperatures is the effect of electron-phonon coupling. However, achieving accurate simulations of electron-phonon coupling in these compounds requires understanding the effects of anharmonicity and polymorphism, as well as the intricate nature of their potential energy surface. In this talk, I will show how the anharmonic special displacement method (A-SDM) [1] allows for exploring anharmonic electron-phonon coupling in both layered and bulk halide perovskites. I will first demonstrate the importance of polymorphism in these compounds leading to strongly-coupled vibrational dynamics and strong modifications in the electron-phonon matrix elements [2]. Then, I will discuss that polymorphism holds the answer to elucidating the smooth evolution of the band gap with temperature around phase transitions [2]; a behavior that has proven challenging to understand thus far. Our work establishes a comprehensive framework that enables precise simulations of halide perovskites' carrier mobilities, excitonic spectra, and polaron physics.

Two-dimensional layered metal-halide perovskites (2DLPs) represent an emerging class of materials where a semiconducting metal-halide octahedral layer is sandwiched between two layers of bulky organic cations. The distinctive structural characteristic of these materials results in high in-plane mobility of excitons and charge carriers, but at the same time, hinders the out-of-plane mobility due to the presence of mostly isolating organic cations [1], [2]. This limits the possibility to study charge and energy transfer processes in vertical heterostructures of this material class. Instead, lateral heterostructures, wherein the composition changes along the in-plane direction, offer an intriguing approach for investigating potential transfer processes at the junction region.

We studied the formation of lateral heterostructures in the system PEA₂PbBr₄-PEA₂PbI₄ by developing a facile room-temperature anion exchange method in solution. Ion exchange methods are a common tool used to tune the optical properties in 3D metal-halide perovskites but are underrepresented in the case of 2DLPs [3]. The approach takes advantage of the highly anisotropic structure of 2DLPs, where the bulky organic cations suppress the vertical diffusion of the ions while lateral diffusion is preferred [4]. We observed that 2DLPs can be stabilized in polar solvents such as octanol by exposing them to the corresponding halide salt of the organic cation. Introducing a different halide salt of the cation compared to the parent 2DLP leads to the formation of lateral heterostructures, which initiate at the edges of the microcrystals and propagate toward the center. This process ultimately results in a core-crown-like microstructure containing 2DLPs of two different halides, each contributing to a distinct emission profile. We demonstrate the influence of different processing parameters like the type of ion source and the solvent on the microstructure and optical properties of the resulting heterostructure. The formation of such a heterojunction in the in-plane direction of the semiconducting layer provides an opportunity to control the directionality of the charge carrier or energy flow toward the edges or the center of these microstructures. This, in turn, contributes significantly to a better understanding of the optoelectronic properties in heterostructured 2DLPs.

Lead halide perovskites open great prospects for optoelectronics and a wealth of potential applications in quantum optical and spin-based technologies. Precise knowledge of the fundamental optical and spin properties of charge-carrier complexes at the origin of their luminescence is crucial in view of the development of these applications. On nearly bulk Cesium-Lead-Bromide single perovskite nanocrystals, which are the test bench materials for next-generation devices as well as theoretical modeling, we perform low temperature magneto-optical spectroscopy to reveal their entire band-edge exciton fine structure and charge-complex binding energies. We demonstrate that the ground exciton state is dark and lays several millielectronvolts below the lowest bright exciton sublevels, which settles the debate on the bright-dark exciton level ordering in these materials. More importantly, combining these results with spectroscopic measurements on various perovskite nanocrystal compounds, we show evidence for universal scaling laws relating the exciton fine structure splitting, the trion and biexciton binding energies to the band-edge exciton energy in lead-halide perovskite nanostructures, regardless of their chemical composition. These scaling laws solely based on quantum confinement effects and dimensionless energies offer a general predictive picture for the interaction energies within charge-carrier complexes photo-generated in these emerging semiconductor nanostructures. Finally, using a new class of CsPbBr3 nanostructures, we observe direct evidence of quantum interference between single photons emitted by single nanocrystals, demonstrating their potential as colloidal sources of indistinguishable single photons.

This talk will describe our group's recent work on controlling the spin-photonic properties of magnetic van der Waals materials through synthesis, doping, and dimensional reduction. In one approach, the CrX₃ (X = Cl, Br, I) family of two-dimensional (2D) ferromagnets have each been doped with the optical impurity Yb³⁺ to yield new, narrow-line lanthanide photoluminescence (PL) sensitized by CrX₃. Magneto-PL is used to probe Yb³⁺ magnetization and its relationship to that of the host CrX₃, and reveals extremely large exchange fields arising from strong in-plane Yb³⁺-Cr³⁺ superexchange coupling. Spectral analysis indicates anomalously high covalency in Yb³⁺-doped CrI₃, providing insight into the microscopic origins of the strong Yb³⁺-Cr³⁺ superexchange coupling. In a second approach, CrX₃ is prepared as colloidal nanoplatelets to probe the effects of dimensional reduction on T_C and magneto-optical response, revealing robust single-domain ferromagnetism down to domain diameters of ca. 20 nm x 3 nm, along with dramatically enhanced coercivity relative to bulk. These results illustrate the use of optical impurities and dimensional reduction as tools for manipulating the spin-photonic properties of metal-halide 2D magnets.

Lead halide perovskite nanocrystals are promising candidates for applications in light-emitting devices due to their exceptionally high quantum efficiency. Single nanocrystals are even considered as efficient single photon sources. For fully exploiting this application potential, a detailed understanding of the recombination mechanism - in particular of the exchange-split excitonic fine structure - is essential. Here, we demonstrate the strength of polarization-resolved micro-photoluminescence (PL) spectroscopy on a single nanocrystal level for getting insight into exciton fine structure states and their relation to crystal symmetry and shape anisotropy.

The specific band structure of lead halide perovskites – s-states forming the valence band and p-states that form the conduction band - leads to an exciton fine structure consisting of a singlet and a triplet state. These can be energetically split depending on crystal symmetry and shape anisotropy [1]. In nearly cubic FAPbBr3 nanocrystals, the degeneracy of the bright exciton triplet is lifted leading to three bright states with transition dipoles oriented along the orthorhombic crystal symmetry at cryogenic temperatures. Depending on the orientation of the nanocrystals with respect to the optical axis between one and three polarized emission lines are visible [2]. Magneto-PL and time-resolved-PL experiments on single nanocrystals demonstrate that the dark singlet exciton is energetically below the bright one, shifted by about 2.6 meV.

In highly anisotropic CsPbBr3 nanoplatelets (NPLs) either one, two or three resolvable emission lines (case I, II and III, respectively) with significantly different polarization patterns are found. The polarization of the two emission lines of case II NPLs are oriented orthogonally with respect to each other. In case III NPLs, in contrast, the lowest and highest energy peaks are polarized collinearly, while the central emission line is polarized in a direction orthogonal to the former two. This quite characteristic polarization pattern can be explained by the occurrence of orthorhombic CsPbBr3 NPLs with two different orientations of the crystal axes [3]. Interestingly, we found that the occupation of the fine structure split states cannot be explained by a simple Boltzmann distribution, indicating an inhibited transfer of excitons between the exchange-split states [4]. We will discuss the potential role of the distinctly different band structure of lead halide perovskites compared to II-VI nanocrystals on this finding.

References

[1] Tamarat et al. (2019): Nature Materials 18, 717. DOI: 10.1038/s41563-019-0364-x

[2] Pfingsten et al. (2018): Nano Letters 18, 4440. DOI: 10.1021/acs.nanolett.8b01523.

[3] Bertolotti et al. (2019): ACS Nano 13, 14294. DOI: 10.1021/acsnano.9b07626.

[4] Schmitz et al. (2021): Nano Letters 21, 9085. DOI: 10.1021/acs.nanolett.1c02775.

Among the transition metal oxides (TMOs), spinel Co3O4 has recently emerged as promising material for photovoltaic and photocatalytic applications. Spinel Co3O4 contains Co[3+] and Co[2+] centers respectively occupying octahedral and tetrahedral sites surrounded by O[2-] anions. The two nonequivalent Cobalt centers are characterized by paired and unpaired d-orbital electronic configurations, determining the presence of Mott-Hubbard and charge transfer gaps close in energy. Because of its complex physics involving the correlated interaction of multiple degrees of freedom, the investigation of its relaxation dynamics calls for experimental approaches able to disentangle electronic, spin and lattice photo-responses with ultrafast time resolution. With ultrafast X-ray emission spectroscopy (XES) at the FXE instrument of the European X-ray free electron laser (EXFEL), we studied the temporal evolution of metal-centered transient electronic configurations, which are degenerate in the optical domain. Specifically, we excited the ligand-to-metal charge transfer (LMCT) and metal-to-metal charge transfer (MMCT) transitions of Co3O4 by pumping a 27 nm thin film at 400 nm and 800 nm, respectively. Our results rule out a stepwise relaxation cascade from the highest LMCT to the lowest d-d gap through the intermediate MMCT state, presenting a radically different picture compared to previous time-resolved optical studies on Co3O4. These results establish correlative time-resolved X-ray emission and optical spectroscopy as a novel strategy for the investigation of condensed matter systems.

Halide perovskites are promising semiconductors for next-generation optoelectronic and spintronic applications. Yet, we still don’t fully understand what governs the charge and spin dynamics in these materials. This is especially true when studying device-relevant thin films of halide perovskites, which lack single-crystalline perfection. In this talk, I will give an overview of our recent efforts to understand the spin-optoelectronic performance of these films better by using time-, space- and polarization-resolved optical spectroscopy and microscopy. We will find that the energetically heterogeneous energy landscape in mixed-halide perovskites can lead to the local accumulation of charges, with unexpected consequences for devices [1]; how despite strong differences in vertical diffusivity and across grains charge extraction can remain very efficient [2], and how locally varying degrees of symmetry-breaking drive spin domain formation [3,4] in this fascinating class of solution-processable semiconductors.

[1] Nature Photonics 14, 123 (2020)

[2] Nature Materials 21, 1388 (2022)

[3] Nature Materials https://doi.org/10.1038/s41563-023-01550-z (2023)

[4] Nature Reviews Materials 8, 365 (2023)

The emerging field of lead halide perovskite semiconductors offers a plethora of promising material compositions for applications. Experimentally exploring the parameter space for all these combinations, especially in view of the increasing number of tandem devices, is challenging. Theoretical models help predict suitable candidates, but require a fundamental understanding of the perovskite band structure. This is where our experimental methods come in. Experimental spin physics helps improve, validate, and support these models and provide insight into the underlying physics. The experimental method uses the optically oriented spin property of resident and photoexcited charge carriers as a probe. The spin reveals the distinct interactions between the charge carriers and its environment and is sensitive to band mixing, which allows, for example, to determine the significance of distant bands as well as to study the effective mass of the charge carriers. The methodological toolkit includes time-resolved pump-probe Kerr spectroscopy, spin-flip Raman scattering, and optical orientation, which we have successfully applied to macroscopic perovskite crystals [4,5], nanocrystals [1,2], and 2D films [3].

Colloidal quantum dots (QDs) are nanocrystals with size-dependent properties that undergo quantum confinement at the nanoscale. However, accurately describing the band alignment of core and shell materials when their size is still in the quantum confinement regime is challenging, especially if one should consider also interface and surface effects. Traditional computational methodologies to describe QDs, like the effective mass approximation, overlook the complexities of real systems. In this regard, Density Functional Theory (DFT) can describe accurately the atomistic composition of QDs, along with its electronic structure, including band gap. However, DFT models are restricted to core and shell sizes that are not aligned with those found experimentally. Furthermore, it is also common to design atomistic models of QDs that artificially introduce superficial intra-gap states that localize carriers, narrowing the band gap, and making the construction of accurate QD models more complicated.

In this work, we explore various cases of core-shell QD systems based on II-VI and III-V materials, which are comparable in size to those observed experimentally. Here, we investigate different sizes and shell thicknesses. Our aim is to examine the evolution of the band gap energy and band alignment in nanocrystals within the quantum confinement regime, considering interface and surface effects. The latter effects being mainly modulated by the use of the surface reconstruction technique. Through this work, we will demonstrate that atomistic models can effectively describe large nanocrystals when certain structural criteria are met.

Lead-free perovskite nanocrystals are of interest due to their nontoxicity and potential application in the display industry. However, engineering their optical properties is nontrivial and demands an understanding of emission from both self-trapped and free excitons. Here, we focus on tuning silver-based double perovskite nanocrystals' optical properties via two iso-valent dopants, Bi and Sb. The photoluminescence quantum yield of the intrinsic Cs₂Ag_1-yNayInCl₆ perovskite increases dramatically upon doping. However, the two dopants affect the optical properties very differently. We hypothesize that the differences arise from their differences in electronic level contributions and ionic sizes. This hypothesis is validated through absorption and temperature dependence photoluminescence measurements, namely by employing the Haung-Rhys factor, which indicates the coupling of the exciton to the lattice environment. Synchrotron measurements reveal the microstaining different effects of the two sizes of dopants, demonstrating that the octahedral tilting is larger in the case of Bi due to its size.  These differences make Sb more sensitive to doping concentration (optimum ~10%) and sodium allowing (optimum ~100%). Such understanding is important for the engineering of optical properties in double perovskite, especially in light of recent achievements in boosting the photoluminescence quantum yield.

Two-dimensional quantum dot (QD) arrays are considered promising candidates for a wide range of applications that heavily rely on their transport properties [1] [2]. Existing QD films, however, were used to be made of either toxic or heavy-metal-based materials, limiting their applications and the commercialization of devices. As a result, there is an increasing trend in looking for non-toxic alternatives, in an effort to replace the toxic ones and enhance their commercial applications [3]. This theoretical study, provided a detailed analysis of the transport properties of environmentally-friendly colloidal QD films (In-based and Ga-based), identifying possible alternatives to their currently used toxic counterparts. Specifically, 2D colloidal QD films based on InAs QDs were modeled and compared with existing relevant experimental work, highlighting how changing the composition, stoichiometry, and the distance between the QDs in the array affects the resulting carrier mobility for different operating temperatures. Additionally, this work showed that by engineering the QD stoichiometry, it is possible to enhance the film’s transport properties, paving the way for the synthesis of higher-performance devices. The mobility of the films was calculated for temperatures ranging between 50 and 350 K (corresponding to a realistic range for device operation).

The isolated QDs were generated following the state-of-the-art semiempirical pseudopotential method, forming a periodic array when placed in a square lattice. Following a tight-binding model, the electronic structure of the system was calculated, i.e., the QD film miniband structure, from which the transport properties (e.g. carrier mobility) were extracted. The main scattering mechanism was considered due to the presence of impurity dots (smaller dots compared to the periodic dots), and thus a 1% density of impurities was assumed as a realistic value in accordance with experimental samples [4].

The results of this study provided a strong indication that 2D films based on InAs colloidal QDs can become a potential replacement for existing CdSe, and Pb-based films while having similar temperature decay trends with other experimental results on InAs QD films.

InP-based quantum dots are a bright and non-toxic alternative for lighting applications. However­­––unlike for quantum dots based on cadmium or perovskite materials––achieving lasing is difficult. Previously, the relatively low gain has been suggested to be caused by a high band-edge degeneracy for holes.[1] While this explains the inability of single excitons to counteract absorption by stimulated emission, it does not explain that multiexcitons contribute so little to gain.

In this work, we suggest that hot-carrier trapping might limit gain in red-emitting InP/ZnSe/ZnS core–shell–shell quantum dots.[2] Using pump–push–probe spectroscopy, we find that a fraction of the hot carriers created by intraband absorption of a push pulse are lost due to ultrafast trapping. Interestingly, single-QD measurements reveal that these hot-carrier losses do not lower the quantum yield but instead introduce broad and redshifted trap-state emission with a low oscillator strength compared to band-edge emission. Our results show that combining spectroscopic techniques is necessary to evaluate quantum-dot performance in more advanced applications.

Materials that emit either white light or near-infrared (NIR) are known for indoor/ outdoor room lighting^[1] or NIR spectroscopic applications like food inspection, remote sensing, bioimaging etc. However, simultaneous white light and broad NIR radiation from a single material can provide both information viz. visual inspection (color/ overall appearance) and early signs of rotting of food products. The broad NIR emission near 1000 nm can effectively absorb by the vibrational overtones of water molecules (-OH) present in food items, providing the non-invasive image contrast to assess the food freshness. Upon single excitation a material that can provide dual emissions covering white light and broad NIR is desired and challenging to design. Here I will be discussing about our recently designed such broad dual emitter, Cr³⁺-Bi³⁺-codoped Cs₂Ag_0.6Na_0.4InCl₆.

Octahedral site of perovskites lattice is suitable for wide range of ions to dope and hence allows to tune optical properties^[2]. Codoping Cr³⁺ and Bi³⁺ in the lattice of Cs₂Ag_0.6Na_0.4InCl₆ halide double perovskite can simultaneously emit warm white light and broad NIR (λ_max≈1000 nm) radiation with quantum yield 27%. This dual emitter is designed by combining the features of s²-electron (Bi³⁺) and d³-electron (Cr³⁺) doping. Importantly, host lattice Cs₂Ag_0.6Na_0.4InCl₆ provides weak crystal field to Cr³⁺ enabling to get broadness in the NIR PL. Other hand, Bi³⁺ via 6s²→6s¹6p¹ excitation allows to use a commercial 370 nm ultraviolet light-emitting-diodes (UV-LED), yielding both emissions. A fraction of the excited Bi³⁺ dopants emit warm white light, and the other fraction transfers its energy non-radiatively to codopants Cr³⁺. Then the Cr³⁺ de-excites emitting broad NIR emission. Temperature dependent (6.4–300 K) photoluminescence in combination with Tanabe-Sugano diagram helps to understand the photophysics of the system. It reveals that Cr³⁺ experiences a weak crystal field (Dq∕B=2⋅2), yielding the ⁴T₂→ ⁴A₂ (d-d transition) NIR emission. As a proof of concept, we fabricated a panel of 122 phosphor-converted LEDs with dimensions 22 × 17 cm², demonstrating its capability to inspect food products.^[3]

At the most fundamental level, transport of energy carriers in the solid state is determined by their wavefunctions and the interactions with the environment. The transport of excitons in colloidal quantum dot (QD) solids plays a pivotal role for their optoelectronic and quantum information applications. However, the investigations of exciton transport in colloidal QD solids thus far primarily exist in the classical diffusive regime. In this study, we demonstrate unambiguous signatures of quantum transport in perovskite quantum dots superlattices with delocalized excitons. By directly imaging exciton propagation with high spatial and temporal resolutions over a wide temperature range, we elucidated the interplay between coupling, Anderson localization, and dephasing. Our experimental results directly confirm two important theoretical predictions. Firstly, we observed ballistic motions within the coherence length at low temperatures, highlighting the role of quantum effects in disordered systems. Secondly, we provided direct evidence for environment-assisted quantum transport by identifying a peak in the long-time diffusion constant at a temperature where disorder and environmental dephasing are perfectly balanced. These results provide a fundamental understanding of quantum transport in disordered systems and offer guidelines for designing quantum materials using perovskite QDs as building blocks.

Lead halide perovskite nanocrystals (NCs) have recently emerged as promising materials for classical and quantum light emission applications thanks to their outstanding optoelectronic properties, such as efficient fluorescence tunable over the entire visible spectral region, minimal inhomogeneous broadening of emission lines, fast radiative decay, high oscillator strength of bright triplet excitons, and long exciton coherence times. While compositional engineering provides an avenue to tuning optical bandgap, NCs shape engineering offers an additional tool for controlling the properties of NCs. It enables, for example, directional emission, spatial confinement of excitons in one or two dimensions, tuning of exciton fine structure and radiative decay. There are many detailed studies on the optoelectronic properties of two-dimensional nanoplatelets with different thicknesses. In order to fill the gap and systematically study shape-dependent properties of one-dimensional perovskite structures, a synthetic approach toward stable, size- and shape-uniform nanorods with tunable thickness and aspect ratio is desired. By exploiting the difference between {110} and {001} facets of the orthorhombic perovskite structure (Pbnm space group), we present a facile synthesis of CsPbBr3 nanorods with tunable size (5-24 nm in thickness) and aspect ratio (1-16, larger for thinner nanorods). The utilization of ligands capable of ensuring sufficient stability will allow a thorough optical characterization of the synthesized nanorods.

Significant attentions directed toward tin-based perovskites as potential replacements for lead halide perovskites in the production of optoelectronic devices. The primary issue associated with tin-based perovskites pertains to their stability. Additionally, other factors such as the cost of precursor materials for synthesis and the operational stability of LED devices are also important considerations. In this context, this study introduces an efficient and straightforward method for synthesizing 2D perovskite microcrystal powder, specifically (4-fluorophenethylammonium)₂SnX₄ (X=I/Br). This method employs affordable starting materials and addresses both adequate ambient stability and the extended inert atmosphere storage of the microcrystal powder. Furthermore, these perovskite powders are further processed into thin films, which are then utilized in the fabrication of LED devices. The two-step recrystallized devices exhibit superior performance and operational stability when compared to reference devices produced using conventional methods and commercial precursors. The established synthesis approach is considered a versatile method for producing various hybrid tin-based perovskite microcrystal powders, which can be used in the manufacturing of LED devices.

Chemically synthesized metal halide perovskite nanocrystals have recently emerged as a new class of efficient light emitting materials which are particularly promising for development of light-emitting diodes (LEDs). Stability of perovskite-based LEDs is still an issue, which can be partially mitigated by proper interface design, such as the use of inter-layer amine terminated carbon dots. As for many other colloidal nanocrystals, proper surface passivation is a key to ensure high colloidal stability and processability of perovskites; this can be achieved by employment of multi-amine chelating ligands. We also show how water-stable perovskite nanocrystals with a mixed fluoropolymer shell can be applied for optical temperature sensing. The use of the lead-based metal halide perovskites is also considered as an issue because of the toxicity concerns related to the lead component. To avoid using lead in light-emitting perovskites, co-doping of cerium and bismuth, as well as tellurium and bismuth into lead-free double perovskite nanocrystals is a useful strategy resulting in their improved photoluminescence efficiency.

The aligned quantum rods emit polarized light that could potentially improve the efficiency of the LCD backlights. Semiconductor quantum rods (QRs), due to their unique shape-dependent emission properties, have been a center of attraction for the last decade [1-8]. These materials not only exhibit the quantum confinement effect due to their size, but also show polarized photoluminescence properties. In this talk, we will discuss the High-quality alignment of the QRs showing a high polarization ratio for the PL. We developed a photoaligned quantum rod enhancement films (QREF) containing red and green QRs, for their application in LCD backlights. A 6-inch QREF was printing using Inkjet printing and later deployed to the LCD [5-8]. The polarization ratio of the emitted light for the printed QREFs is measured to be ~ 7:1 (see fig. 1 (a-c)) [8]. Later, we developed the LCD backlight. The intensity of the QREF backlight is measured at ~ 7200 nits, which becomes ~561 nits after the LCD panel shows an optical efficiency of ~7.8%, which is approximately 1.7 times higher than conventional LCD. In conclusion, we can say that the photoaligned quantum rods can double the efficiency of modern display devices. In this talk, we will explore the detail of photoalignment of QRs and their application in displays.  

The use of scintillators for the detection of ionising radiation is a critical aspect in many fields, including medicine, nuclear monitoring, and homeland security. Lead halide perovskite nanocrystals (LHP-NCs) are emerging as promising scintillator materials. However, the difficulty of affordably upscaling synthesis to the multi-gram level and embedding NCs in optical-grade nanocomposites without compromising their optical properties still limits their widespread use. In addition, fundamental aspects of the scintillation mechanisms are not fully understood, leaving the scientific community without suitable fabrication protocols and rational guidelines for the full exploitation of their potential. In this talk I will present our recent progress in the fabrication of nanocomposite scintillators based on CsPbBr₃ NCs synthesised via high throughput approaches. Our data show that the interaction between NCs and polyacrylate chains strengthens the scintillator structure, homogenises the particle size during ripening and is capable of passivating defects on the NC surfaces, resulting in nanocomposite prototypes with high luminescence efficiency, exceptional radiation hardness, competititive scintillation yield even at low NC loading, and ultrafast response time, with a large fraction of the scintillation occurring in the first 80 ps, which is highly promising for fast-time applications in precision medicine and high-energy physics. Additonally, the combination of ultrafast radioluminescence and optical spectroscopies disambiguates the origin of the scintillation kinetics as the result of charged- and multi-exciton recombination formed under ionising excitation. This highlights the role of non-radiative Auger decay, whose potential impact on fast timing applications I will discuss via a kinetic model.

Perovskite semiconductors are appealing for optoelectronic and photonic applications. The knowledge currently available about the energy structure of photoexcited electrons, holes and exciton complexes, their interaction, binding energy, and relaxation dynamics are far from being complete. Conventional time-integrated reflectivity or photoluminescence techniques alone often do not allow one to make unambiguous conclusions about the energy structure due to inhomogeneous broadening of optical transitions and complex dynamics of photoexcited carriers. Here, nonlinear optical techniques based on photon echoes or two-dimensional Fourier spectroscopy provide unique access to the energy structure of perovskite semiconductors.

We investigate the coherent dynamics of excitons in halide perovskites materials of different composition and dimensionality. Most importantly, the exciton itself serves as a probe for the interaction with the crystal lattice, local potential fluctuations, other excitons and charge carriers in our studies. First, the results on single bulk crystals are presented. Here, the magnitude of fluctuations of the energy bandgap is evaluated. We observe exceptionally long exciton coherence time up to 80 ps at low temperature of 1.5K in mixed mixed-halide perovskite crystals due to the localization of excitons at the scale of tens to hundreds of nanometers [1]. Next, the role of exciton-exciton interactions in bulk crystals is discussed. In particular, polarization resolved transient signals provide rich information about the biexciton binding energy and spin dependent interactions in dense exciton ensembles [2]. Finally, we study coherent optical response from lead-halide nanocrystals where quantum beats in the photon echo signal are observed due to excitons fine structure and strong interaction with optical phonons.

Highly ordered nanocrystal (NC) assemblies, namely superlattices, have been investigated as a building block of novel bright (quantum) light sources because of their unique collective superfluorescent emission based on enhanced inter-NC interactions[1,2]. Thus far, the primary preparation method for perovskite NC superlattices has been drying-mediated self-assembly, in which the NCs spontaneously assemble into superlattices while the solvent evaporates. However, a drawback of this method is the lack of controllability on the position and size of assemblies, making it difficult to place NC assemblies in photonic structures like microcavities and realize NC assembly-based devices.

Here, we demonstrate template-assisted self-assembly of CsPbBr₃ NCs to achieve precise control of the geometrical features of NC assemblies. NC assemblies are formed using drying-mediated assembly on a substrate with hollow, lithographically-defined template structures made from SiO₂. The templates have lateral sizes of a few micrometers and 220 nm height and are fabricated with a similar method as developed for templates in inorganic-semiconductor growth [3]. After drop-casting a solution of NCs, we allow slow evaporation of the solvent and remove excess NCs from the substrate surface afterwards. Thus, NCs only remain in the templates, and position and size of these NC assemblies can be controlled by changing the design of the hollow structures. Furthermore, we find that the yield of successful assemblies depends on the ligands and solvents used, as well as on the geometry and number of openings of the hollow templates.

We investigate the photoluminescence (PL) properties of these template-assisted NC assemblies. Micro-PL studies at cryogenic temperature allow to evaluate the homogeneity and quality of the assemblies. Moreover, we perform time-resolved PL measurements where we observe signatures of cooperative photon emission. Lastly, we assess the stability and robustness of the assemblies. Our results provide an important step forward for the development of optical devices that harness embedded perovskite NC assemblies.

Colloidal quantum dots (QDs) are heavily investigated for their applications in light emission such as light emitting diodes and, more challenging, lasers. This is due to their appealing processing conditions, compared to e.g. epitaxy, resulting in lowering cost. They can also be patterned and their optical properties can be tuned. Using quantum confined Cd-based QDs, several groups have shown light amplification and ensuing lasing action in the red part of the spectrum. Although impressive milestones were achieved, there is to date no single material that can provide the demanding combination of gain metrics to be truly competitive with existing epitaxial growth approaches.

In this work, we take a look at CdS/Se nanocrystals in the regime of vanishing quantum confinement, so-called ‘bulk nanocrystals’.  We show that these unique materials display disruptive optical gain metrics in the green optical region. Indeed, while showing similar gain thresholds compared to state-of-the-art QD materials, the gain window (440-600 nm, 640-750 nm), amplitude (up to 50.000/cm) and gain lifetime (up to 3 ns) vastly outpace other QD materials.

Using these novel gain materials, we demonstrate lasing in the highly demanded green spectral region (480 – 530 nm) and in the red (650 – 740 nm) both with pulsed and quasi-CW optical excitation. These lasers are made using a Photonic Crystal Surface Emitting Laser (PCSEL) type cavity. As a final step, we attempt to further optimize the lasing properties, be it either narrow linewidth lasers, or high power output, based on in-depth understanding of the hybrid QD-PCSEL laser system.

Self-assembly of colloidal nanocrystals (NCs) into highly ordered structures – superlattices (SLs) – became feasible a few decades ago with the first successful syntheses of monodisperse and shape-uniform colloidal NCs [1]. Close NCs' proximity in the SLs with a long-range positional and orientational ordering facilitate the emergence of diverse synergistic and collective effects different from ensemble-average properties. In this regard, monodisperse and size-tunable lead halide perovskite NCs have drawn much attention owing to their spectrally tunable and narrow, fast and optically coherent (at low temperature) fluorescence. In particular, CsPbBr₃ NCs assembled into the single-component SLs were shown to exhibit superfluorescent emission [2]. This stimulated the research into their multicomponent SLs, where lead halide perovskite NCs are co-assembled with dielectric NCs acting as spacers between fluorescent NCs. Consequently, a plethora of novel SL types has become accessible, displaying the superfluorescence phenomena as well [3]. An interesting avenue is to devise superlattices comprising several distinct kinds of light emitters. To this end, we succeeded to co-assemble differently sized CsPbBr₃ NCs into binary SLs with a high degree of ordering and a large domain area. This enabled us to reveal, for the first time, a highly efficient Foster-like energy transfer from strongly confined NCs to weakly confined ones in multicomponent CsPbBr₃ NC SLs. The strong excitonic coupling between NCs was also manifested by the accelerated exciton diffusion.

Long-range ordered nanocrystal superlattices (NC SLs) can combine the attractive properties of the individual nanocrystal constituents with new collective optoelectronic phenomena emerging through the periodic NC assembly. Perovskite NCs have emerged as highly attractive SLs building blocks, based on their facile fabrication as sharp, monodisperse cubes and the appearance of new collective effects such as superfluorescence [1]. 

Binary superlattices of the ABO₆ type produced via self-assemblies of CsPbBr₃ NCs, with sizes in the strong- (~5 nm) and weak- confinement (~18 nm) regime, are being demonstrated for the first time. The presented work discusses the optical properties of such binary SLs and the interactions between the strongly and weakly confined excitons. Transient photoluminescence and absorption measurements indicate efficient depletion of the small NC exciton population at the timescale of few to tens of picoseconds. Simultaneously a delay in the transient absorption rise time signal of the large NC exciton bleaching is observed. Such results support the presence of efficient energy funneling from the small to the large NCs within the binary superstructure. On-going experimental investigations are underway to provide further understanding on the impact of the energy transfer on the superfluorescence process as well as its competition with hot carrier transfer between the small and large NCs in the binary SLs.