Colloidal perovskite nanocrystals exhibit many intriguing properties, one of which is collective behavior. Assemblies of close-packed nanocrystals in the form of continuous films and individual mesocrystals (superlattices) attract attention because of reported enhancements in exciton diffusion, energy transport, light amplification, and quantum phenomena. To deepen our understanding of these materials, it is important to study changes in the photophysics of perovskite nanocrystals as they transition from a liquid dispersion to a disordered glassy film and an ordered superlattice.

In this contribution, I will discuss our efforts to study the optical properties of single-component all-inorganic and hybrid organic-inorganic perovskite nanocrystal superlattices. First, we will consider the possibility of collective response in CsPbBr₃ nanocrystals from a theoretical perspective of single-excitation superradiance, discussing factors that may weaken or strengthen it. Second, we will examine experimentally achievable CsPbBr₃ nanocrystal superlattices, focusing on changes in their steady-state and time-resolved spectroscopic observables in response to nanocrystal synthesis origin and environmental conditions such as sample aging. Lastly, we will highlight other compositions of single-component perovskite nanocrystal superlattices and the prospects of achieving a tunable collective response.

Absorption of light via interband optical transitions constitutes a primary process in nature, e.g., in photosynthesis, as well as in applied technologies, e.g., in solar photovoltaic cells, photodetectors, or (quantum) light-matter interfaces. In cavity-free systems, engineerability of the rate of absorption has thus far been limited, consistent with the wide-spread belief that the coupling strength between initial and final state (described by the square of the matrix element of the light-matter interaction in Fermi’s golden rule) is an intrinsic parameter of the employed material. However, enhanced absorption rates could be realized via giant-oscillator-strength (GOS) transitions, leveraging coherent oscillations of the electron polarization in a volume significantly larger than a unit cell.[1] While experimental evidence for such a superradiance phenomenon has indeed already been provided in emission processes, realizations in an absorption process, i.e., in the form of “superabsorption”, have been sparse and/or require complicated excited-state engineering approaches.[2]

Here, employing colloidal CsPbBr₃ perovskite QDs, we demonstrate a robust and straightforward implementation of superabsorption as a time-reversal process of single-photon superradiance.[3][4] Optical spectroscopy reveals that the band-edge absorption in large CsPbBr₃ perovskite QDs exhibits a superlinear increase with QD volume, consistent with the 3D delocalization of a giant exciton wavefunction. Calculations based on the configuration-interaction framework attribute this behavior to strong electronic correlations, and fully corroborate the experimental findings. Our results shed light on a process as fundamental as light absorption, in a new class of commercially relevant direct semiconductors, and may facilitate the development of more efficient optoelectronic devices and new quantum light-matter interfaces.

Indium phosphide (InP)-based quantum dots (QDs) are the most industrially relevant Cd- and Pb-free QDs for photonic applications due to their excellent optoelectronic properties, including a tunable band gap, tolerance to dopants, high absorption coefficients, near-unity photoluminescent (PL) quantum yields (QYs), and narrow PL linewidths.[1] In recent years, research efforts have successfully improved the quality of InP-based QDs.[1] However, their long-term stability and the deterioration of their optoelectronic properties are not yet well understood. In this work, we systematically investigate how the optoelectronic properties of colloidal InP/ZnSe/ZnS core/shell QDs in solution are affected by continuous white LED light exposure under a nitrogen atmosphere (< 0.1 ppm O₂, < 0.1 ppm H₂O).

Characterization of the absorption spectrum over time shows that the QDs are colloidally stable in solution and do not photodegrade. Remarkably, the PLQY of the InP/ZnSe/ZnS QDs decreased rapidly over time and did not recover after light exposure, indicating irreversible photodarkening of the QDs. A control experiment conducted in the dark demonstrated no degradation of the QDs or reduction in their PLQY throughout the duration of the experiment. The photodarkening effect was mitigated by reducing the incoming photon flux. However, when the changes in QY are plotted against the absorbed photon dose, the data collapses in a master curve with a logarithmic dependence on the dose. These ensemble-level results are further complemented by single-particle measurements for a comprehensive interpretation. In summary, our findings provide new insights into the photodarkening of InP-based core/shell QDs relevant for photonic applications.

Colloidal semiconductor nanocrystals have long been considered a promising source of time-correlated and entangled photons via the cascaded emission of multiexcitonic states. The realization and spectroscopy of such cascaded emission, however, is strongly hindered by the highly-efficient, nonradiative Auger process, which renders multiexcitonic states non-emissive. Here we present a room-temperature heralded spectroscopy study of three-photon cascades from triexcitons in giant CsPbBr₃ nanocrystals. Single particle heralded spectroscopy combines the power of the temporal correlation of photon detections with an added information of spectral resolution. In this technique the photoluminescence of a single nanocrystal is collected through a spectrometer coupled to a single-photon avalanche diode array detector, so that each detected photon is time-stamped according to its arrival time, and energy-stamped according to the array pixel it was detected in. This is the first study to fully characterize these types of cascades from quantum dots at room temperature, a task which is difficult due to the significant broadening of emission lines as well as due to temporal fluctuations in the emission. Our results show that the emission pathway of triplets of photons in these particles is dominated by the lowest excited state, and that multiexcitons (biexcitons and triexcitons) are extremely weakly bound, in contrast with low temperature observations. In addition, we aim at elucidating the underlying properties or processes that can lay in the basis of observed differences in blinking statistics and try to correlate these with either intrinsic or surface related properties. This presents interesting opportunities in using emission cascades as deterministic few-photon sources at room temperature that could have important consequences in the development of colloidal quantum light sources.

The ability to efficiently up-convert broadband, low-intensity light would be an enabling technology for volumetric 3D printing, background-free biomedical imaging, and sensitizing silicon-based cameras to the short-wave infrared. Our approach uses colloidal quantum dots to absorb low-energy photons and sensitize the spin-triplet excitonic states of nearby conjugated molecules.[1-3] Once there, pairs of these long-lived excitations can combine via triplet fusion to generate shorter-wavelength fluorescence.

We recently harnessed high-quality, ultra-small (d:1.7-2.8 nm) PbS quantum dots[4] to generate photochemically active blue light (λ~420nm) from continuous-wave red (λ: 635nm) excitation.[5] However, this performance was somewhat unanticipated, because the large ‘Stokes shift’ in most ultra-small nanocrystals appears to herald an unacceptable loss of incident photon energy. Intriguingly, we inferred from the quasi-equilibrium dynamics of triplet energy transfer that the chemical potential of photoexcited, ultra-small PbS quantum dots is surprisingly high—completing an advantageous suite of photophysical properties, but reinforcing fundamental questions regarding their emissive state(s).[5,6]

Accordingly, I will present a photophysical effort to relate these anomalous behaviours to the long-discussed, surface-linked ‘trap’ emission from canonical Cd-chalcogenide quantum dots. We show that this non-ideal emission can be observed, intermittently, from individual nanocrystals, and is consistent with the occasional formation of defect states, shallow within the gap, on timescales that align with rare, photoinduced structural reorganization.

Semiconductor nanocrystals have promise in many optoelectronic and electronic technologies due to their tunable electronic structure and facile colloidal processing. However, one major factor limiting their widespread adoption is inherent heterogeneity within an ensemble. For this reason, magic sized clusters, with atomically precise structures and negligible heterogeneous broadening, have drawn significant attention as a potential system with ensemble level homogeneity. However, many of the most common nanocrystal materials, such as Cd-chalcogenides and In-pnictides, form clusters with bandgaps far to the blue, limiting potential applications. We illustrate the colloidal synthesis of nontoxic, earth abundant, iron sulfide clusters with narrow and invariant absorption features and a ~700 nm band-gap which allows for absorption across the visible spectrum. These ~2nm diameter particles exhibit quantum confinement, with a blue shift from the expected 0.95 eV bandgap. Furthermore, through controlled surface coordination via ligand exchange, we can promote band-edge photoluminescence. These results suggest surface defects play a major role in determining the nonradiative processes present in the system. Further growth in polymerizing media (e.g. oleyl amine) facilitates the formation of long-range order into quasi-1D fibrils. The formation of these fibrils coincides with a red-shift to the absorption spectrum while maintaining the apparent morphology of the individual nanocrystals. These iron sulfide clusters show promise as a platform for future device engineering due to the unique combination of optical properties and material availability and safety.

Photoluminescence (PL) down conversion has been a major success for wide band gap colloidal material, now being commercially available. More recently large efforts have been devoted to infrared (IR) detection using nanocrystal as active material [1]. This combination of cost-effective production and high performances further raise the interest for using the PL of such nanocrystals in the IR range. In particular, incoherent sources are missing beyond telecom wavelengths. Targeted applications relate to active imaging (machine vision for material sorting, damage inspection on food), LIDAR, or airfield lightning. The main challenge relates to the dop of PL efficiency as wavelength is increased. Part of this drop relates to intrinsic behaviour (lengthening of radiative lifetime) and some to the presence of non-radiative process that have to be correlated with the less mature growth of heterostructure using narrow band gap core. In this talk I will review our recent effort to push the electroluminescence of such IR nanocrystals beyond 2 µm [2] and then to further address limitation relates to their surface passivation. Another direction of interest is the coupling of such IR material to photonic structure. This effort has been very successful for photodetection to enhance the light absorption. Here, I will show that the coupling to plasmonic cavity or dielectric cavity is highly effective to shape the PL spectrum and enhance the PL directivity [3-5]. Last, I will show that the benefit of such cavity can also be applied to heavy metal free IR nanocrystals.

Monolithic integration of thin-film photodiode with the Si read-out circuit (ROIC) offers a low-cost alternative to the conventional flip-chip bonded III-V near-infrared (NIR) and short-wave infrared (SWIR) imagers. Thin film absorber materials have already enabled advanced SWIR imagers featuring the smallest pixel pitch and highest resolution. Attractive optoelectronic properties such as spectral tunability, quantum confinement, large area solution processability, ligand dependent energy band structure and electronic properties tunability make colloidal quantum-dot (CQD) as one of the most promising candidates for thin film NIR and SWIR photodetector absorber materials. Thanks to the massive advancement towards synthesis of the CQDs, these materials now include a wide range of semiconductors to choose from for applications like photodetectors, phototransistors, light-emitting diodes and solar cells.

To engineer thin-film CQD based optoelectronic devices it is crucial to obtain a complete energy band structure of CQD with various ligand types, exchange methods, and sizes of these quantum confined nanocrystals. However, the effort to design and simulate efficient devices is constrained by the lack of systematic studies and the inconsistencies found in the literature regarding reported energy band structures of CQDs. We demonstrate that the accurate characterization of energy band structure by ultraviolet photoelectron spectroscopy lies at the heart of the film preparation process and largely depends on the distribution and packing density of the deposited CQDs. Transmission electron microscopy images confirm that our proposed multi-step coating technique ensures over 90% CQD coverage (both PbS and InAs) within probing area. This is well supported by X-ray photoelectron spectroscopy, atomic force microscopy and variable angle spectroscopic ellipsometry measurements. Extracted energy band structures are further validated by fabricating SWIR PbS and InAs CQD thin film photodiodes.

Our comprehensive energy band characterization of SWIR PbS and InAs CQDs with various ligands by both solid state and liquid phase ligand exchange processes showcases an accurate and reproducible scheme to achieve complete Fermi reference energy band structure modelling of thin film photodiode stack. The insight on the overestimation of Fermi and valence band maximum |E_F-E_VBM| due to poor packing density and the summary of the energy band structures of both PbS and InAs CQDs for various ligands will advance the energy landscaping of thin film CQDs. With this methodology, we can better choose the proper ligand and size of the CQDS to efficiently design thin film devices.

Combining integrated optical platforms with solution processable semiconductor materials offers a clear path towards miniaturized and robust light sources, including lasers. Semiconducting colloidal quantum dots present a unique platform to realize this by combining tunable properties and high luminescence efficiency with solution processing. A limiting aspect for both red and green emitting materials remains the drop in efficiency at high excitation density due to non-radiative quenching pathways, such as Auger recombination. Next to this, lasers based on such materials remain ill characterized, leaving questions on their ultimate performance.

Here, we show that weakly confined ‘bulk’ colloidal quantum dots offer a unique solution processable materials platform to circumvent the long-standing material issues. First, we demonstrate that optical gain in such systems is mediated by a 3D plasma state of unbound electron-hole pairs which gives rise to broadband and sizable gain across the full red spectrum with record gain lifetimes and low threshold. As proof of concept, the nanocrystals are integrated on a silicon nitride platform enabling high spectral contrast, surface emitting and TE polarized PCSEL – type lasers with ultra-narrow beam divergence across the visible (green, red) spectrum from a small surface area. Our results prime these 'bulk' nano-materials as excellent materials platform to realize highly performant and compact on-chip light sources.

Colloidal nanoplatelets (NPLs) are promising materials for lasing applications. The properties are usually discussed in the framework of 2D materials, where strong excitonic effects dominate the optical properties near the band edge. At the same time, NPLs have finite lateral dimensions such that NPLs are not true extended 2D structures. Here we study the photophysics and gain properties of CdSe/CdS/ZnS core–shell–shell NPLs upon electrochemical n doping and optical excitation. Steady-state absorption and PL spectroscopy show that excitonic effects are weaker in core–shell–shell nanoplatelets due to the decreased exciton binding energy. Transient absorption studies reveal a gain threshold of only one excitation per nanoplatelet. Using electrochemical n doping, we observe the complete bleaching of the band edge exciton transitions. Combining electrochemical doping with transient absorption spectroscopy, we demonstrate that the gain threshold is fully removed over a broad spectral range and gain coefficients of several thousand cm–1 are obtained. These doped NPLs are the best performing colloidal nanomaterial gain medium reported to date, with the lowest gain threshold and broadest gain spectrum and gain coefficients that are 4 times higher than in n-doped colloidal quantum dots. The low exciton binding energy due to the CdS and ZnS shells, in combination with the relatively small lateral size of the NPLs, results in excited states that are effectively delocalized over the entire platelet. Core–shell NPLs are thus on the border between strong confinement in QDs and dominant Coulombic effects in 2D materials. We demonstrate that this limit is in effect ideal for optical gain and that it results in an optimal lateral size of the platelets where the gain threshold per nm2 is minimal.

Colloidal cadmium chalcogenide-based 2D nanoplatelets (NPLs) display exceptionally narrow absorption and photoluminescence bands, large one- and two-photon absorption cross sections [1] as well as low Auger recombination rates [2] and low gain thresholds [3]. Following the major strides made in the colloidal synthesis of tailored NPLs and related heterostructures, the current research focus has shifted to the incorporation of such NPLs into optical setups and devices, e.g. LEDs and lasing [4]. To date, such application-oriented setups often rely on NPL thin films [5]. Recently, however, an alternative approach using colloidal NPLs in solution gained traction, e.g. in short capillaries with the promise of higher photostability and integration into cavities [6].

Here, we demonstrate optical gain in hollow fused silica liquid-core fibers (LCFs, 20 µm core diameter) filled with a colloidal solution of 4.5 monolayer thick core-crown CdSe/CdS NPLs. The fibers are transversally excited at 480 nm in a stripe geometry (ca. 60 mm) by a 4 ns optical parametric oscillator. Aside from monoexcitonic spontaneous emission, we also observe amplified spontaneous emission (ASE), showing a characteristic bathochromic shift and peak sharpening due to its biexcitonic nature. Importantly, the arising ASE (pump energy threshold of 65 µJ) could only be observed when enabling the LCF waveguiding properties by utilizing a high refractive index solvent, like tetrachloroethylene. If a solvent with a lower refractive index than fused silica is used, e.g. hexane, which suppresses waveguiding, no ASE threshold is reached.

In conclusion, our findings indicate that NPL-filled LCFs offer a viable and efficient approach to achieving visible lasing from fused silica fibers. Incorporating colloidal semiconductor nanostructures into LCFs enables a pathway towards visible-range fiber lasers and offers integrability and flexibility, including tunable optical properties by simple replacement of the lasing medium.

The development of high-quality QDs with tunable visible emission started with CdSe. QD designs containing Cd or Pb remain those with the brightest emission and most precise control over properties. However, as consumer applications demand QDs free of toxic elements, alternative materials have drawn considerable attention in recent years. InP-based QDs now offer photoluminescence efficiencies and a color tunability that are on par with those of high-quality Cd- and Pb-containing designs. This makes InP-based QDs an ideal phosphor for displays and lighting.

In stark contrast to these successes, InP-based QDs struggle in more demanding optical applications. In particular, the development of QD lasers using InP-based QDs lags behind other QD materials by more than two decades. Lasing from InP-based QDs has been reported only once, while the vast majority of the QD lasing literature has successfully used Cd-based or Pb-halide perovskite QDs. The near-total absence of InP-based QDs in the lasing literature is consistent with spectroscopic measurements, which have highlighted difficulties to achieve population inversion and gain from an ensemble of InP-based QDs.

In this presentation, I will show ensemble and single-particle experiments to investigate why InP-based QDs are not yet suitable for lasing applications, despite their high brightness, and how their performance may be improved. On the ensemble level, we find a correlation between the magnitude of charge-carrier losses on the sub-ps timescales and slow delayed emission on the ns-to-μs timescales. From single-particle measurements, we find a cause–effect relationship between hot-carrier trapping and delayed trap emission. Based on the characteristics of the trap-related emission, we propose that hot-carrier traps are most likely internal defects, for example located on the InP/ZnSe interface. This highlights the direction into which InP-based QDs should be improved for next-generation applications.