Colloidal lead halide perovskite nanocrystals (LHP NCs_, APbX3, where A=Cs⁺, FA⁺, FA=formamidinium; X=Cl, Br, I) have become a research spotlight owing to their spectrally narrow (~100 meV) fluorescence, tunable over the entire visible spectral region of 400-800 nm, as well as facile colloidal synthesis. These NCs are attractive single-photon emitters and building blocks for creating controlled, aggregated states exhibiting collective luminescence phenomena. Attaining such states through the spontaneous self-assembly into long-range ordered superlattices (SLs) is a particularly attractive avenue. The atomically flat, sharp cubic shape of LHP NCs is also of interest because the vast majority of prior work had invoked NCs of spherical shape. Long-range ordered SLs with the simple cubic packing of cubic perovskite NCs exhibit sharp red-shifted lines in their emission spectra and superfluorescence (a fast collective emission resulting from coherent multi-NCs excited states).

When CsPbBr₃ NCs are combined with spherical dielectric NCs, perovskite-type ABO₃ binary NC SLs form, wherein CsPbBr₃ nanocubes occupy B- and/or O-sites, while spherical dielectric Fe₃O₄ or NaGdF₄ NCs reside on A-sites. When truncated-cuboid PbS NCs are added to these systems, ternary ABO₃-phase form (PbS NCs occupy B-sites). Such ABO₃ SLs, as well as other newly obtained SL structures (binary NaCl, AlB₂- and ABO₆ types, columnar assemblies with disks, etc.), exhibit a high degree of orientational ordering of CsPbBr₃ nanocubes. These mesostructures also exhibit superfluorescence, characterized, at high excitation density, by emission pulses with ultrafast (22 ps) radiative decay and Burnham-Chiao ringing behavior with a strongly accelerated build-up time. Co-assembly of steric-stabilized CsPbBr₃ nanocubes with disk-shaped LaF₃ NCs yields six columnar structures with AB, AB₂, AB₄, and AB₆ stoichiometry, not observed before with NC systems comprising spheres and disks. Combining CsPbBr₃ nanocubes with large and thick NaGdF₄ nanodisks results in the orthorhombic SL resembling CaC₂ structure with pairs of CsPbBr₃ NCs on one lattice site. Additionally, we have also implemented two substrate-free methods of SL formation. The first method involves oil-in-oil templated assembly that leads to the formation of binary supraparticles, while the second method utilizes self-assembly at the liquid-air interface.

Semiconductor colloidal nanoplatelets are quasi-2D nanocrystals, which are often considered to be the
wet chemistry analogous of epitaxial quantum wells. They show however a few distinct features that lead to
characteristic properties: (i) the presence of weak, but finite lateral confinement; (ii) the presence of strong
dielectric confinement posed by organic ligands surrounding the inorganic platelets; (iii) the possibility of
building radial heterostructures.

In this presentation we give an overview on our group’s attempts to provide insight into the photophysics
of these systems using effective mass Hamiltonians. The electronic structure of CdSe-based nanoplatelets
(homo and heterostructures) is reported for excitons, charged excitons and biexcitons. It is found that
Coulomb interactions, boosted by dielectric confinement, play a prominent role in determining the physical
response of these systems.[1-5]

Excitons in metal halide perovskite platelets are also investigated. Here, the soft lattice leads to a sizable

polaron radius (~1 nm), which we incorporate to our model by means of Haken's potential. We find that 

dielectric screening is weakened in quasi-2D structures because the exciton radius becomes comparable 

to the polaron one. The resulting exciton binding energies and self-energy potentials agree well with 

experiments and atomistic calculations for layered perovskites.[6]

REFERENCES:

[1] F. Rajadell et al. Phys. Rev. B  2017, 96, 035307.

[2] D. Macias-Pinilla et al. J. Phys. Chem. C 2021, 125, 15614.

[3] J. Llusar et al. J. Phys. Chem. C 2022, 126, 7152.

[4] J. Llusar et al. Phys. Rev. Lett. 2022, 129, 066404.

[5] D. Macias-Pinilla et al. Nanoscale 2022, 14, 8493.

[6] J.L. Movilla et al. Nanoscale Adv. 2023, DOI: 10.1039/D3NA00592E.

The coupling of elementary excitations to phonons in a material plays a critical role in determining its excited state properties and rates of dynamical processes. Performance metrics of applications utilizing a material are therefore intimately linked with the strength of Electron-Phonon Coupling (EPC). 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.

Our recent works,^1,2 along with the reports of few other groups from Columbia University³ and MIT^4–6 shed a new light on a hybrid quantum well material platform based on metal organic chalcogenides (MOCs) that allow the manipulation of stable 2D Wannier-type excitons at room temperature, in a bulk solid.

Layered metal organic chalcogenides (MOCs) with molecular formula [MEPh]_∞ (with M metal, and E chalcogen) form hybrid multi-quantum-well nanostructures. These materials can host tightly bound 2D excitons in a 3D crystal. MOCs show high optoelectronic quality, they are chemically tunable, and may therefore offer an air-stable alternative to the moisture-sensitive layered (so-called “2D”) metal halide perovskites in future optoelectronic applications.

A key aspect of hybrid inorganic-organic material systems is that their “soft” lattice structures render the excitonic properties particularly susceptible to exciton-phonon interactions. In this talk, I will present an investigation of the complex optical transitions of the prototypical MOC [AgSePh]_∞. We visualized the excitons dynamics with transient absorption spectroscopy whose temporal oscillations reveal coherent exciton-phonon coupling. Steady state absorption and Raman spectroscopies revealed a strong exciton-phonon coupling and its anharmonicity, manifested as a nontrivial temperature-dependent Stokes shift. Our work highlight the general importance of exciton coupling to optical phonons hybrid quantum wells and it finally suggests that the peculiar spatial symmetry of the excitonic states and phonon modes needs to be accounted for in a proper treatment of exciton−phonon coupling.⁷

Transparent conductive oxides (TCOs) are materials of particular interest in the electronics industry. For instance, in the manufacture of electronic devices such as high-resolution flat screens, electrochromic windows, solar cells and gas sensors. These materials simultaneously present optimal electrical (resistivity lower than 10^-3 Ohm-cm) and optical properties (average transmittance greater than 80 % in the visible spectral range and a bandgap greater than 3.3 eV) [1-2]. The structural, optoelectronic and electrical properties of TCOs can be tuned by the incorporation of transition metal ions as dopants. In particular, Al-doped ZnO (ZnO:Al or AZO) is a material that has generated interest in recent years due to its potential to replace ITO, the most widely used conductive transparent oxide.

This work aims to identify the effects of aluminum doping and the post-deposition annealing treatment conditions on the structural, optical, and electrical properties of ZnO sputtered thin films. The films were deposited on silicon and fused silica (FS) substrates by radiofrequency magnetron sputtering using AZO targets (ZnO doped with 2 wt.% and 3 wt.% Al). For both Al concentrations, active cooling and non-cooling conditions were used independently for each set of substrates during the deposition process. Subsequently, post-deposition shock thermal treatments were carried out at 100, 200, 300, 400, 500 and 600 °C in an inert atmosphere of ultra-high purity argon.

Optical properties of the films were investigated through variable angle spectroscopic ellipsometry (VASE) and Ultraviolet-Visible-Near-Infrared (UV/VIS/NIR) spectrophotometry. The thickness, extinction coefficient (k), refractive index (n) and optical bandgap (Eg) of the films were determined using Drude and Tauc-Lorentz models. Point by point analysis was also used as an alternative method to determine n and k. Charge carrier density and mobility were estimated from the optical analysis.

Electrical resistivity, mobility, and charge carrier density of the films were estimated by the Van der Pauw method and Hall effect. The experimental and optically estimated values for the charge carrier density and mobility exhibit good agreement.

Colloidal nanoparticles are of growing importance in a broad range of practical applications, from medicine and biodiagnostics through to energy and environmental science. However, the fact that they are non-discreet reaction products, and their properties vary extremely sensitively as a function of size, shape and composition, makes materials discovery, characterization and optimization a daunting task in the face of an extensive reaction parameter space.

Here we present our experience in the development and application of segmented-flow microfluidic systems for the advanced synthesis, analysis and optimization of colloidal nanoparticles [1]. These systems allow rapid and efficient exploration of the mentioned extensive reaction parameter space. Intrinsic advantages include rapid heat and mass transfer in micro-isolated reaction volumes, leading to highly uniform and reproducible reaction conditions. Incorporation of in-line analytics (e.g. photoluminescence and absorption spectroscopy) allows real-time characterization and data feedback for powerful real-time reaction optimization. We discuss how this approach interfaces with new opportunities in data science, including machine learning, and how we are pushing to move materials chemistry into a new ‘big data’ regime. We present examples from our lab of the application of such systems in nanomaterials investigations, with a particular focus on the synthesis and characterization of cesium lead bromide perovskite nanocrystals. Here, we have used multiparametric reaction mapping to elucidate how nanocrystal optical and morphological properties vary with ligand type, concentrations, and blend ratios.

There are many opportunities and challenges in the use of automated microfluidic systems in the future of colloidal nanocrystal synthesis, and there is vast potential for such systems to be developed and applied by a variety of research groups for wide impact.

Perovskite nanocrystal superlattices (NC SLs), made from millions of ordered crystals, support collective optoelectronic phenomena such as superfluorescent (SF) emission. In these SL collectives, the coupled NC emitters are highly sensitive to structural and spectral inhomogeneities of the NC ensemble. We use Free electrons in scanning electron microscopy (SEM) to probe the cathodoluminescence (CL) properties of CsPbBr₃ SLs with a ~20nm spatial resolution. Correlated CL-SEM measurements allow simultaneous characterization of structural and spectral heterogeneities of the SLs. Hyperspectral CL mapping, in which a full CL spectrum is acquired from every pixel, shows multipole emissive domains within a single SL. Consistently, the edges of the SLs are blue-shifted relative to the central domain by up to 65meV. Residual uniaxial compressive strains, indicated by structural characterizations of the SLs, accompanying SL formation are contributors to these emission shifts.  We discover a relation between NC building block colloidal softness and the extent of the CL shift. In SLs made from smaller NC building blocks, the CL shift is much higher due to the high colloidal softness which influence the residual strains in the SL from the assembly process. Therefore, precise control over the NC building blocks colloidal softness is critical for SL engineering. 

Colloidal quantum dots (QDs) are promising photoredox catalysts that offer broadly tunable potentials, high absorption coefficients and regenerability. These properties have prompted QDs to be examined for various photocatalytic reactions, from water splitting and CO2 reduction to various organic transformations [1]. An even wider parameter space emerges upon coupling QDs with other homogeneous catalysts – transition metal complexes or organic dyes – in hybrid nanoassemblies exploiting energy transfer (ET). Such nanoassemblies could significantly exceed the performance of the individual constituents thanks to the very large absorption cross section of QDs combined with the long-lived triplet states of co-catalysts [2]. However, understanding the complex behavior arising in hybrid nanoassemblies requires methods with high spatio-temporal resolution [3]. Here, we probe ET from single lead halide perovskite QDs to organic dye molecules employing single-particle photoluminescence spectroscopy with single-photon resolution. We identify ET by spatial, temporal, and photon-photon correlations in the QD and dye emission. Exploiting the high temporal resolution of our experiment as well as the discrete quenching steps due to photobleaching of individual organic dyes, we observe a characteristic Förster-type ET, with efficiencies higher than 70% in the sole case of a strong donor-acceptor spectral overlap. Our work sheds light on the processes occurring at the QD/molecule interface and demonstrates the feasibility of sensitizing organic catalysts with QDs.

Metal halide perovskite (MHP) nanocrystals (NCs) are newcomer materials with excellent features to be exploited as luminescent labels in immunochemistry and biosensing, such as high emission yields (~100%), easy synthesis, narrow emission bands, and wide color gamut. They also exhibit giant multiphoton absorption, which makes them promising NIR-label biosensing. Thus, we envision that shortly perovskite NCs will enable the manufacture of high-throughput analytical platforms for the simultaneous and sensitive detection of multiple analytes, reducing the costs of the current detection kits. However, their future relies on addressing several challenges, such as their stabilization in water, the media of serological and biological samples, the obtention of monodispersed colloidal NCs, and their size reduction. ^[1]

This work presents different strategies to prepare metal halide perovskite NCs stable in water and with suitable properties as luminescent tags. The approaches tackled include (A) an adaptation of the sol-gel process consisting of a post-synthetic chemical transformation of pre-synthesized NCs, either CsPbBr₃ or Cs₄PbX₆, in the presence of silica precursors, ^[2-3] (B) in situ growth of perovskite NCs in silica mesoporous nanoparticles, (C) polymeric nanobeads produced by coprecipitation methods ^[4] and (D) adaptation of liquid atomization methods. The stabilized materials were successfully probed as fluorescent agents for staining mouse cell cultures and in immunosensing assays. Finally, we discuss open research challenges and future directions toward their final use as a luminescent tag.

References

[1] C. Collantes, W. Teixeira; et al. Appl. Mater. Today. 2023, 31, 101775

[2] C. Collantes, V. González-Pedro, et al. ACS Appl. Nano Mater. 2021, 4, 2011−2018.

[3] C. Collantes, W. Teixeira et al. J. Mater. Chem. B. Submitted.

[4] Avugadda, S. K., Castelli A. et al. ACS Nano. 2022, 16(9), 13657-13666.

Climate change and environmental degradation, mainly associated to the use of fossil fuels, are existential threats to the world. So, new and more advanced systems with reduced energy consumption have to be developed in order to allow real energy savings. In this context, the fabrication of low-cost, environmentally-friendly and high-efficiency photovoltaic and optoelectronic devices such as solar cells, LEDs, photodetectors, sensors or luminescent solar concentrators, is now more than ever a need to be addressed. Lead-halide perovskites have emerged as promising candidates for solid-state lighting applications. Perovskite light-emitting diodes (PeLEDs), akin to solar cells, have witnessed significant advancements.

This study presents a novel approach to address this challenge by utilizing CsPbBr₃ colloidal perovskite nanocrystals (PeNCs) synthesized through a microwave-assisted method. The PeNCs exhibit a photoluminescence emission peak centered at 514 nm, with a narrow full width at half-maximum of 20.4 nm, resulting in an impressive photoluminescence quantum yield of 66.8%. Consequently, these PeLEDs demonstrate exceptional external quantum efficiency of 23.0% and power luminance levels surpassing 40000 cd m^-2, meeting the requirements for various applications. These findings highlight the tremendous promise of PeNCs for LED lighting and displays, offering feasible levels of device performance.

Thermoelectricity is the phenomenon of converting heat directly into electricity and vice versa, offering a sustainable path to produce electricity from waste heat. To maximize the efficiency of this process, complex materials where not only the crystal structure but also other structural features such as defects, grain size, orientation, and interfaces must be controlled. To date, conventional solid-state techniques cannot provide this level of control. Herein, we present a synthetic approach in which dense inorganic thermoelectric materials are produced by the consolidation of nanoparticle powders produced in solution. This synthetic approach can provide a significant degree of control over the nanoparticle features and hence a unique opportunity to control the final characteristics of the inorganic solid through nanoparticle design. 

In particular, in this talk, we will focus on Ag2Se, an important thermoelectric material for the use of thermoelectricity near room temperature, where the library of high-performing materials is minimal. Despite Ag2Se being a promising candidate, the main problems are the large discrepancy in the reported thermoelectric properties and the struggles to reproduce the high performance achieved. Such divergence appears to arise from the difficulty of controlling the defects present in the material, such as vacancies, interstitial atoms, dislocations, grain boundaries, precipitates, etc. We will show that our solution synthesis allows for precise control of such defects, especially avoiding fluctuations in stoichiometry. Furthermore, we show how we can tune microstructural defects, such as strain, dislocations, and grain boundary density, utilizing the characteristic phase transition of Ag2Se during the sintering process. Overall, our results will highlight that besides stoichiometry, the microstructure is crucial for tuning Ag2Se transport properties and how this control can be provided by our novel synthetic route.

In the last decade, metal halide perovskites (MHPs) have rapidly emerged as one of the most interesting classes of materials, with strong light-matter interactions, tunable luminescence over the entire visible spectrum and large electron-hole diffusion lengths, which have proven extremely useful in a wide range of scientific and technological disciplines. The subsequent development of material preparation using low-temperature solution-based strategies has allowed the same material to be obtained as nanocrystals (NCs) with high control over size and shape and further increase their optical properties tunability via dimensional confinement. Also importantly, this novel material preparation approach further extended LHPs applicability thanks to the possibility of processing directly from solution to realize functional thin films, nanocomposites or simple solution-based devices. In their NCs form, LHP have been widely applied as efficient emitters in artificial light sources, overcoming the limits of resolution and color purity of single crystals, or as high-Z materials efficiently interacting with ionizing radiation for scintillators, showing performances comparable or even higher than their large macroscopic crystal counterparts. In this talk, I will focus on our recently developed strategies for engineering LHP-NCs for different application: from high-power direct and down-converting light emitting devices (LEDs) to potential biomedical applications. I will present the different approaches that have been adopted to specifically tune the spectroscopical, chemical and physical properties of LHP NCs for these application, pursued via chemical modification (compositional tuning, post-synthesis surface treatments) of physical confinement (encapsulation into mesoporous silica nanoparticles).

Near-infrared to visible photon upconversion holds great promise for a diverse range of applications. Current photosensitizers for triplet-fusion upconversion across this spectral window often contain either precious or toxic elements, and have relatively low efficiencies. Although colloidal nanocrystals have emerged as versatile photosensitizers, the only family of nanocrystals discovered for near-infrared upconversion is the highly-toxic lead chalcogenides. Here we report zinc-doped CuInSe2 nanocrystals as a low-cost and lead-free alternate, allowing for near-infrared to yellow upconversion with an external quantum efficiency reaching 16.7%. When directly merged with photoredox catalysis, this system enables efficient near-infrared-driven photoreducation, oxidation, C-O couplong and polymerization, which in turn solves the issue of reabsorption loss for nanocrystal-sensitized upconversion. Moreover, the broadband light capturing of these nanocrystals allows for very rapid reactions under indoor sunlight, an especially remarkable example among which is the polymerization of acrylates within just 30 seconds. Extending the reach of "solar synthesis" into the near-infrared may realize the century-long dream of conducting high added-value chemical transformations using sunlight.

Mercury is a peculiar metal, which can be found as a liquid in its native form in the nature. Its distinct properties arise from the strong relativistic effects and the ensuing lanthanide contraction.[1] Mercury chalcogenides, such as HgTe, manifest the characteristic physics of mercury by having the Gamma₆  band (arising from Hg 6s orbitals) lower than the Gamma₈ one (arising from 5p orbitals of Te). This is contrary to other metal chalcogenides such as CdSe, and makes HgTe have a so-called negative band gap (i.e. a semi-metal). Because quantum confinement restores the usual band order, it has been used to open gaps controllably from the THz regime up to the near infrared, which makes HgTe nanocrystals the most widely tunable colloidal material.[2]

In this presentation we explain the electronic structure and optical properties of HgTe nanoplatelets.[3,4] Optical measurements carried out on these systems reveal scarce qualitative differences with respect to their CdTe counterparts. We show this is because the small thickness (3.5 monolayers) suppresses any
trace of band inversion.[4] Next we investigate, by means of an eight-band k·p Hamiltonian and configuration interaction methods, how the response should change when increasing the thickness.

We predict that for thicknesses beyond 2 nm, the physics of HgTe starts being governed by the Gamma₆-Gamma₈ band mixing, which leads to important departures with respect to the expectations of quantum confinement. A prominent example is the formation of hybrid states in the conduction band, where the charge density is not localized in the volume, but partly migrates towards the surface of the platelet. Because this does not happen to the valence band states, we foresee the possibility of building indirect excitons without the need of using type-II heterostructures.

Around 6 nm the Gamma₆-Gamma₈ band inversion takes place, which is indeed close to the critical thickness reported in epitaxial HgTe/CdTe quantum wells.[5] At this point a quantum phase transition from regular to topological insulator takes place. A few hints are given on the topological magnetoelectric effect that could be achieved in this regime. Namely, we show through axion electrodynamics equations that electrical charges could induce magnetic fields in the nanostructure. These could be modulated through the vertical and lateral confinement of the platelet.[6]

We provide a survey of our experimental studies of spin phenomena involving electrons, holes and nuclei in lead halide perovskite nanocrystals [1-6] and 2D structures [7,8]. Several experimental techniques are used: polarized emission in strong magnetic fields, time-resolved Faraday/Kerr rotation and spin-flip Raman scattering. Perovskite nanocrystals for these studies are grown by colloidal chemistry and synthesized in glass matrix. We measure spin relaxation and spin coherence times, evaluate electron and hole Lande g-factors, analyze the effects of the quantum confinement and perovskite composition, which tune the band gap, on the g-factors and compare that with model predictions. Electron spin coherence is detected even at room temperature, and unusual temperature dependence of the electron g-factor is found. Combination of the pump-probe Faraday rotation with radiofrequency allow us to use this spin resonance technique for detecting spin relaxation times of hundreds microseconds. Spin mode locking effect based on spin synchronization under periodic laser excitation is found in nanocrystals in glass. Experimental approaches of spin physics give reach information about these materials.

References

[1] D. Canneson, et al., Negatively charged and dark excitons in CsPbBr₃ perovskite nanocrystals revealed by high magnetic fields, Nano Letters 17, 6177 (2017).

[2] P. S. Grigoryev, et al., Coherent spin dynamics of electrons and holes in CsPbBr₃ colloidal nanocrystals, Nano Letters 21, 8481 (2021).

[3] V. V. Belykh, et al., Submillisecond spin relaxation in CsPb(Cl,Br)₃ perovskite nanocrystals in a glass matrix, Nano Letters 22, 4583 (2022).

[4] E. Kirstein, et al., Mode locking of hole spin coherences in CsPb(Cl,Br)₃ perovskite nanocrystals, Nature Communications 14, 699 (2023).

[5] M. O. Nestoklon, et al., Tailoring the electron and hole Lande factors in lead halide perovskite nanocrystals by quantum confinement and halide exchange, Nano Letters 23, 8218 (2023).

[6] S. R. Meliakov, et al., Coherent spin dynamics of electrons in CsPbBr₃ perovskite nanocrystals at room temperature, Nanomaterials 13, 2454 (2023).

[7] E. Kirstein, et al., Coherent spin dynamics of electrons in two-dimensional (PEA)₂PbI₄ perovskites, Nano Letters 23, 205 (2023).

[8] C. Harkort, et al., Spin-flip Raman scattering on electrons and holes in two-dimensional (PEA)₂PbI₄ perovskites, Small 2300988 (2023).

Colloidal nanocrystals underwent a tremendous development with full control over dimensions and surface chemistry, resulting in vast opto-electronic applications. Can they also form a platform for quantum materials, in which electronic coherence is key? We use colloidal, two-dimensional Bi₂Se₃ crystals, uniform in thickness and with limited lateral dimensions, as a model system to study the evolution of a three-dimensional topological insulator to the technologically important case of two-dimensions and limited crystal domains.

Individual Bi₂Se₃ platelets with diameter in the 100-200 nm range and well-defined thickness (1-6 quintuple layers) with cryogenic scanning tunneling microscopy and spectroscopy. For 4-6 Bi₂Se₃ quintuple layers, we observe an edge state, 8 nm wide, around the entire crystal. The edge state is faint or absent for thinner (1-2 QLs) Bi₂Se₃ platelets. The edge states are resilient under a perpendicular magnetic field. Ab-initio calculations confirm that crystals with 3 QLs or more have a non-trivial band structure with a one-dimensional quantum channel at the edge. The quantum channel consists of 2 counter propagating states with momentum-spin locking, key for non-dissipative information transfer and quantum computing.

We've performed optical spectroscopy in the high energy region (1-3 eV). We coud classify the optical transitions as (1) transitions due to the surface (outer QLs) or (2) due to the inner QLs. By comparison with GW simulations, we identified all transitions in a (energy, momentum in x, momentum in y) two-dimensional Brillouin zone frame. Some transitions show electron and hole cooling in which the carriers separate in momentum space.

Colloidal Bi2Se3 platelets are not only a model system for a two-dimensional toplogical insulator, but also a layer semi-metal with exotic optical transitions. The processability and dimensional control of topological insulator colloidal nanocrystals opens a unique window to devices with a large density of addressable quantum states. 

REFERENCES

J. Moes et al., manuscript in preparation

Swart, I., Liljeroth, P. & Vanmaekelbergh, D. Scanning probe microscopy and spectroscopy of colloidal semiconductor nanocrystals and assembled structures. Chem. Rev. 116, 11181-11219 (2016).

Kane, C. L. & Mele, E. J. Z(2) topological order and the quantum spin Hall effect. Physical Review Letters 95 (2005).

Zhang, H. J. et al. Topological insulators in Bi2Se3, Bi2Te3 and Sb2Te3 with a single Dirac cone on the surface. Nature Physics 5, 438-442 (2009).

Yazyev, O. V. et al.   Spin Polarization and Transport of Surface States in the Topological Insulators Bi2Se3 and Bi2Te3 from First Principles. Physical Review Letters 105 (2010).

Zhang, Y. et al. Crossover of the three-dimensional topological insulator Bi2Se3 to the two-dimensional limit.  Nature Physics 6, 584-588 (2010).

Liu, C. X. et al. Oscillatory crossover from two-dimensional to three-dimensional topological insulators. Physical Review B 81 (2010).

Neupane, M. et al. Observation of quantum-tunnelling-modulated spin texture in ultrathin topological insulator Bi2Se3 films. Nature Communications 5 (2014).

Chiatti, O. et al. 2D layered transport properties from topological insulator Bi2Se3 single crystals and micro flakes. Sci Rep 6 (2016).

Halide perovskite nanocrystals (NCs) have emerged as an intriguing material for optoelectronic applications, most notably for light-emitting diodes (LEDs), lasers, and solar cells. Despite impressive advances, halide perovskite nanocrystals have not yet been commercialized due to certain inherent limitations. Importantly, they are highly prone to environmentally induced degradation. Additionally, the incorporation of chloride, necessary to achieve blue emission, renders the perovskite nanocrystals defect intolerant.

In this talk, I will explore our recent results on specifically tailoring halide perovskite nanocrystals to improve their overall stability and performance in the blue spectral region. To achieve the latter we produce anisotropically quantum confined nanocrystals, in the form of two-dimensional (2D) nanoplatelets (NPLs). These colloidal quantum wells have additional appeal for light emission, as the one-dimensional quantum confinement enhances their radiative rates and enables directional outcoupling. On top of this, due to a monolayer-precise control over their thickness, they constitute an intriguing system for spectroscopic studies on their fundamental optical, phononic, and energetic properties. To achieve a high degree of stability, we introduce block copolymer micelles, which serve as nanoreactors and shield the encapsulated nanocrystals from the environment. Importantly, the hydrophobic shell prevents moisture from degrading the nanocrystals and prohibits ion migration. It is thus possible to fabricate halide perovskite heterolayers.

The generation, identification, and utilization of excited states is of paramount importance for optimizing semiconductor nanocrystals for optoelectronic applications. Of particular interest is characterizing the number and type of excitations present in a nanocrystal to understand the efficiency of processes such as Auger recombination, carrier multiplication, or charge transfer. While charged (trion) and multiexciton states are functionally quite different from single excitons states with respect to their excited state lifetime, they are spectrally quite similar, resulting in challenges in conclusively identifying these states. Through power dependent transient absorption measurements and globally fitting the exciton and multiexciton component spectra, we directly identify both the recombination dynamics, spectral form, and relative intensities affiliated with each state in CdSe quantum dots. Through these measurements we illustrate that the biexciton transient absorption spectra is < 1.6 times as intense as the exciton spectrum at the band-edge across a range of CdSe quantum dot sizes. This is in contrast to the oft-assumed factor of 2 between the exciton and biexciton absorption due to the degeneracy of the state. We identify that differences in the spectral intensity between samples is most sensitive to the difference in peak energies, which is correlated with the surface termination rather than quantum dot size. Surface effects also have a large impact on the excited state properties of CdS nanocrystals. One such example is surface/ligand mediated photoreduction which we identify through both steady-state absorption and photoluminescence measurements [1]. Of particular importance is the ubiquitous nature of this ligand-mediated photoreduction, which can have an enormous impact on optical measurements and device functionality for a variety of applications of CdS nanocrystals. While problematic for spectroscopic measurements of exciton properties, these photogenerated charged states can be long-lived (minutes to hours), opening avenues for highly efficient charge transfer to enable photocatalytic applications.

The advent of SPAD-array based parallelized detection of time-stamped single photons opens a pathway for extracting previously inaccessible spectroscopic information from dim sources of quantum light. In particular, it enables to multiplex detection both in time and in additional dimensions such as space, frequency or spatial frequency. We use this to study the interactions between pairs of excitons in doubly excited colloidal semiconductor nanocrystals, revealing weak multielectron effects at room temperature against a strong temperature broadened background.

We present heralded spectroscopy [1], the unique identification and post-selection of pair emission events from single nanocrystals while performing a spectral measurement (using a line SPAD array [1]) or a defocused imaging measurement (using a 2D SPAD array [2]). Using this method we can characterize subtle differences between the first emitted photon (representing emission from the doubly excited state) and the second emitted photon (representing the singly excited state). Several examples for this will be given, including interaction effects on emission anisotropy from semiconductor nanorods [2] and identification of multiexcitonic states in quantum dot molecules. The utility of our method for study of higher excited states via three-photon correlation will also be discussed.