II-VI semiconductors have been known to favour three-dimensional crystal structures. It is therefore a surprise that they form two-dimensional platelets at the few monolayers limit. Considering nanoplatelets of CdSe, we examine both the bulk limit where a zinc-blende crystal structure is favoured as well as the few monolayers limit considering nanoplatelets with varying thicknesses, within density functional theory based calculations. In agreement with experiment, we find that the formation energy is less for the nanoplatelets for few layers than for the bulk structure, indicating that it is energetically more favourable for them to form. Different surface terminations for the nanoplatelets as well as different atoms at the surface are explored in order to understand the energetics. Here again, we find the Cd-terminated (100) surface facets to be most stable. The possible energetics allowing the nanoplatets to form as well as their electronic structure will be discussed.

The anion and cation exchanges in nanocrystals are common synthetic strategies to tune their optoelectronic properties. The process of ion exchanges often leads to the formation of heterostructures inside the nanocrystals - that may control their stability and other properties. A fundamental understanding of ion exchange processes and ion exchange induced heterostructures is necessary.

In this talk, we will be discussing the anion exchange in CsPbX₃ nanocrystals and how it can lead to the formation of internal heterostructures. It is generally believed that the anion-exchanged nanocrystals possess a homogeneously alloyed composition. We have employed variable energy hard X-ray photoelectron spectroscopy to determine the internal heterostructure of anion-exchanged nanocrystals - which has a gradient-alloyed heterostructure. We will also discuss the mechanism of the anion exchange in pre-synthesized CsPbX₃ nanocrystals.

Colloidal semiconductor heteronanocrystals (HNCs) exhibit unique optoelectronic properties that are inaccessible to single-component NCs, making them promising materials for a wide range of applications. The optoelectronic properties of HNCs are determined not only by the bandgap and band alignment of its constituent materials but also by their morphology and heteroarchitecture [1]. Applications requiring efficient charge separation (e.g., photovoltaics and photocatalysis) greatly benefit from anisotropic morphologies, such as heteronanorods. Most of the work on heteronanorods has focused on Cd-chalcogenide-based HNCs. However, given the toxicity of Cd, the potential of these materials for large scale applications is limited.

Copper-chalcogenide based NCs have attracted increasing attention as promising alternatives for Cd- and Pb-chalcogenide NCs due to their low toxicity, large absorption cross-sections across a broad spectral range, composition-dependent band gaps in the 1 to 2.5 eV range, and photoluminescence tunability, spanning a spectral window that extends from the UV to the NIR depending on the NC size and composition [2,3].

Several synthetic strategies have been used in the quest for high-quality colloidal copper chalcogenide based HNCs. The most promising ones are based on the multistage approach, which allows the combination of different synthesis techniques (e.g., cation exchange [4,5] or seeded growth [6-9]) in a sequential manner in order to achieve the targeted preparation of colloidal HNCs. In this talk, I will discuss a selection of recent examples, chosen in order to provide an overview of the current status of the quest for Cd- and Pb-free HNCs and to illustrate specific synthesis strategies: CuInSe₂/CuInS₂ dot core/rod shell heteronanorods [4], Cu_1.8S-based multicomponent axially segmented heteronanorods [5], CuInS₂/ZnS dot core/rod shell heteronanorods [6], and Janus-type Cu_2‒xS/CuInS₂ and Cu_2−xS/ZnS heteronanorods [8,9]. I will show that by combining the design principles of post-synthetic heteroepitaxial seeded growth and nanoscale cation exchange into multistage synthesis strategies one can potentially gain access to a plethora of Cu-chalcogenide-basedmulticomponent heteronanorods with diameters in the quantum confinement regime.

Benefiting from their large compositional space, direct bandgap nature, and outstanding structural stability,^[1–4] vacancy-ordered layered double perovskites with formula Cs₄M(II)M(III)₂X₁₂ (M(II): Cu²⁺, Mn²⁺; M(III): Bi³⁺, Sb³⁺, In³⁺; X: Cl^-, Br^-, I^-), have recently attracted increasing attention as substitutive materials of lead-based halide perovskites for potential commercial optoelectronic applications. The layered double perovskite structure comprises one layer of [M(II)X₆]^4- octahedra inserted in between two layers of [M(III)X₆]^3- octahedra. As a representative, Cs₄CuSb₂Cl₁₂ has been successfully synthesized both in the form of single-crystalline powder^[2] and NCs,^[3,5] exhibiting a narrow direct bandgap (1.0–1.8 eV) and impressive stability, which however still suffers from the high toxicity of Sb element and the absence of emission at room temperature. To overcome these drawbacks, Cs₄CuIn₂Cl₁₂, with toxic Sb³⁺ substituted by relatively non-toxic In³⁺, could be a promising candidate to fulfill the requirement of compositional engineering and optical tunability particularly in the UV range. Yet, to date, the synthesis of Cs₄CuIn₂Cl₁₂ layered double perovskites has not yet been reported for neither bulk film nor NCs.

Herein, we report the first-ever colloidal synthesis of lead-free Cs₄CuIn₂Cl₁₂ layered double perovskite NCs using a modified hot-injection method.^[5,6] The synthetic details are described in the Supporting Information (SI). While a standard hot-injection reaction in moisture-free environment resulted in Cs₄CuIn₂Cl₁₂ with a very low photoluminescence quantum yield (PLQY) of 0.12%, we found that the handling of synthesis precursors in the presence of moisture (RH~40%) enhances PLQY by more than one order of magnitude up to 1.70 %. Water-assisted in-situ synthesis has been recognized as an effective strategy to tune the optical properties and stability for both lead-based^[7,8] and lead-free halide perovskite NCs^[9,10] via the controlling of NCs size, shape, and crystallinity. Nevertheless, there is still a lack of deep understanding of how water influences NCs growth regime and corresponding PL property, especially for lead-free double perovskite NCs. The introduction of moisture in the precursor (namely “wet” precursor conditions) of Cs₄CuIn₂Cl₁₂ NCs (w-Cs₄CuIn₂Cl₁₂) induces the morphological transformation from 3D nanocubes (NCus) to 2D nanoplatelets (NPLs), driven by the ionized H₃O⁺ and OH^- from the water content as additional capping ligands. The ultrafast transient absorption studies suggest a strengthened self-trapped exciton (STE) effect for w-Cs₄CuIn₂Cl₁₂ NCs compared to the NCs synthesized in “dry” conditions (d-Cs₄CuIn₂Cl₁₂ NCs), resulting in the conversion of dark transitions into radiative transitions, which directly contributes to the PLQY.

Investigating halide perovskites’ optoelectronics properties for stimuli-responsive materials (SRMs) opens new avenues in smart chromic windows, switchable optoelectronics, and information display applications. Here, we carried out an in-depth study of the structural transformation of FAn+2PbnBr3n+2 (2≤ n ≤∞) layered halide perovskite using excess organic cation (FA/Pb>1), strain, optical and thermal input. Initially as FA/Pb ratio increases, three-dimensional (3D, FA/Pb=1, n=∞) (FAPbBr3) perovskite transform to two-dimensional (2D, FA/Pb=1, n=2) (FA2PbBr4) layered halide perovskite and is aligned in <110> orientation. The intermediate n phases (2≤ n ≤∞) in FAn+2PbnBr3n+2 system stabilize in metastable states and are accessible via strain, optical or thermal input. These results were supported by X-ray diffraction, solid-state nuclear magnetic response (ssNMR), in-situ temperature-dependent Raman spectroscopy, optical spectroscopy, and ab initio simulation techniques. Furthermore, upon photo-decomposition of FABr, the <110> oriented 2D perovskite transforms to 3D perovskite.  Therefore, a metal oxide mesoporous substrate has been introduced to prevent FA decomposition and impart reversibility (2D to 3D and vice versa), which facilitate as a reservoir for FA cations. Shuttling of FABr between the reservoir and the HP film under light and moisture ensure reversible chromism.  We realised that the transformation in the FAn+2PbnBr3n+2 structure is governed by the H-bonds strength of the intermediate n phase (lower n phases to higher n phases or vice versa). Additionally, the remnant organic solvent in the film (observed using proton transfer reaction-mass spectroscopy (PTR-MS)) is crucial to realize structural transformation and, therefore, the reversible chromism. Finally, we relate photo-induced reconfigurability with photo-adaptable intelligent sensors. The realization of structural configurability using just one cation is not only employable in solar cells and LEDs but simultaneously can be utilized in artificially intelligent sensors by optimizing their operating parameters according to the ambient environment.

Ternary I-III-VI₂ NCs, such as CuInS₂ and AgInS₂ are receiving attention as heavy-metals-free materials for solar cells, luminescent solar concentrators (LSCs), LEDs, and bio-imaging. The origin of the optical properties of NCs are however not fully understood. A recent theoretical model suggests that their characteristic Stokes-shifted and long-lived luminescence arises from the structure of the valence band (VB) and predicts distinctive optical behaviors in defect-free NCs: the quadratic dependence of the radiative decay rate and the Stokes shift on the NC radius. If confirmed, this would have crucial implications for LSCs as the solar spectral coverage ensured by low-bandgap NCs would be accompanied by increased re-absorption losses. In this work, we performed spectroscopic studies on stoichiometric CuInS₂ and AgInS₂ NCs, and in the process it was revealed for the first time, the spectroscopic signatures predicted for the free band-edge exciton, thus supporting the VB-structure model. At very low temperatures, we also observed dark-state emission from these NCs likely originating from enhanced electron-hole spin interaction. The impact of the observed optical behaviors on LSCs was evaluated by Monte Carlo ray-tracing simulations. Based on the emerging device design guidelines, optical-grade large-area (30×30 cm²) LSCs with optical power efficiency (OPE) as high as 6.8% were fabricated, corresponding to the highest value reported to date for large-area devices.

While initial theories on quantum confinement in colloidal quantum dots (QDs) led to analytical band-gap/size relations or sizing functions, numerical methods proofed more accurate to describe size quantization. However, for lack of reliable sizing functions, researchers fit experimental band-gap/size datasets using models with redundant, physically meaningless parameters that break down upon extrapolation. Here, we propose a new sizing function based on a proportional correction for non-parabolic bands. Using known bulk semiconductor parameters, we accurately predict size quantization for group IV, III-V, II-VI, IV-VI and metal halide perovskite semiconductors, including straightforward adaptations for negative-gap semiconductors and non-spherical QDs. Refinement with respect to experimental data is possible using the Bohr diameter as a fitting parameter, by which we show a statistically relevant difference in band-gap/size relation for wurtzite and zinc blende CdSe. The general sizing function proposed here unifies QD size calibration, and enable researchers to assess bulk semiconductor parameters and predict size quantization in unexplored materials

Two-dimensional nanoplatelets (NPLs) of II-VI semiconductors exhibit the narrowest optical features among nanocrystals. Despite the great maturity of cadmium based NPLs, their optical properties are limited by the band gap of the bulk material. To push toward infra-red wavelength, mercury-based NPLs are good candidates as they keep narrow optical properties^[1].

To obtain such material, only cation exchange is yet available. However, this procedure leads to the creation of structure defects and to the degradation of the optical properties. In this context, we developed an optimized procedure for cation exchange and rationalized the role of the solvent, the temperature and the ligands during this process. This optimized procedure has then been applied to Cd-based alloys of CdSe_xTe_1-x to obtain HgSe_xTe_1-x NPLs of 3MLs with photoluminescence for every composition. Using UV-visible and X-ray spectroscopy, the evolution of the band alignment according to the composition has been established revealing a crossover from n- to p-type behavior as the Te content increases in HgSe_xTe_1-x NPLs. In parallel, this trend has also been observed in time-resolved photoemission measurements.

Quasi-Type II CdSe@CdS dot-in-rod nanorods (NRs) exhibit high absorption coefficients (~10⁷ M^-1 cm^-1) in the UV/Vis and efficient charge-separation following photoexcitation, with the photogenerated hole localizing to the CdSe core and the electron delocalized over the entire structure. These properties render them attractive materials as photosensitizers in photocatalytic reactions such as light-driven water splitting, especially when coupled to a reaction centre such as metal nanoparticles.¹ On the other hand, some drawbacks of these structures have to be considered when designing a catalytic system. For instance, molecular functionalization of NRs is limited to specific anchoring groups such as thiols and amines.² Additionally, the catalytic efficiency is limited by slow (> 100 ps) hole removal, whereas the electron transfer steps are usually very fast (<10s of ps). Last, these structures are prone to photo-oxidation, which limits their long-term usage.

Recently, we demonstrated a photocatalytically able system consisting of polydopamine (PDA) coated NRs.³ The PDA-coating tried to counteract the above-mentioned drawbacks of NRs: it served as a scaffold for a molecular catalyst and acted as a charge-mediator between NRs and catalyst, potentially improving long-term stability. However, the underlying charge transfer processes have not been fully resolved yet.

Here, the photoinduced charge transfer processes between NRs and PDA are investigated. First, as a model system, the interaction between water-soluble NRs and molecular dopamine is investigated at different pH values and dopamine concentrations using steady-state and time-resolved absorption and photoluminescence spectroscopies. Insights gained from these quenching experiments serve as a basis for explaining the charge transfer processes in PDA-coated nanorods. Last, implications of these fundamental processes on photocatalysis are considered.

Chemical transformations reactions, such as the cation exchange reaction, have been one of the most exciting means of studying and post-synthetically modifying nanocrystals. These reactions can create atomic arrangements that are impossible to reach in bulk materials due to kinetic limitations, and these reactions are believed to enhance diffusion beyond bulk-derived limits. In this talk I’ll first discuss cation exchange induced accelerated diffusion and then cation exchange to form a dual-interfaced heteroepitaxy.

The phenomenology of solid-state transformations and diffusion at short length scales remains poorly understood but is increasingly important for nanostructured devices that utilize “nano properties”. Using in-situ synchrotron x‑ray diffraction (XRD), we directly interrogate the structure and reaction kinetics of lead sulfide (PbS) nanocrystals transforming into cadmium sulfide (CdS) through cation exchange. The epitaxial relationship of zincblende CdS to rocksalt PbS breaks the overall symmetry of the core-shell nanocrystal without requiring the loss of unit cell symmetry, leading to anomalous peak shifts in the diffraction pattern. The magnitude of the interdiffusion coefficient, D̃, is larger by four orders of magnitude or more compared to the slowest diffusing species in our system (self-diffusion of Cd in CdS). This surprising result suggests interdiffusion is enhanced in nanocrystals. These results illustrate that the distinction between chemical diffusion in a potential gradient and diffusion at thermodynamic equilibrium has not been fully appreciated.

Additionally, through a cation exchange reaction on copper sulfide nanoparticles we have created dual interface Cu_2-xS-ZnS heterostructures, with a metastable Cu_2-xS layer. The copper sulfide phase region can be tuned to form two-dimensional (2D), single atomic layers (<1 nm). As the nanoparticles transform, we observe a solid-solid phase transformation of the copper sulfide phase from the initial low-copper phase Cu_1.8S into a higher copper phase djurleite (Cu_1.94S), but as the epitaxial strain increases a second phase transformation back to roxbyite Cu_1.8S occurs to minimize strain energy. This work demonstrates novel routes to metastable phases through strain stabilization. The copper sulfide can be etched with phosphines in oxidizing conditions. Importantly, this etching reaction is capable of removing Cu_2-xS from Cu_2-xS-ZnS epitaxial heterostructures with perfect selectivity, that is, the phosphines completely remove the Cu_2-xS without disturbing the ZnS. The etching reaction is preceded by abstraction of sulfur from the particles, destabilizing the Cu_1.81S roxbyite phase. 

Heterojunction nanocrystals (HNCs) consisting of two or more semiconductor components contain junctions at the interface of the constituent blocks. Unlike the electronic property tuning by changing the size or shape of mono-component nanocrystal, HNCs rely on controlling energy states via band-offset engineering at the material interface. We demonstrate the usefulness of band-offsets by designing component size modulated type-II coupled quantum dots (CQDs-HNCs) composed of a fixed sized ZnSe quantum dots and size-tuned CdS quantum dots. The interface of single CQDs-HNCs is probed by using scanning tunneling microscopy (STM) at the local scale. Furthermore, we demonstrate fabrication of a differentiator using a single Cu₂S-CdS HNC. The differentiator is capable for conversion of variety of waveforms demonstrating the possibility of fabrication of single HNC electronic component. 

Colloidal semiconductor Quantum Dots (CQDs) that contain hundreds to thousands of atoms manifest size dependent tunable properties and have reached an exquisite level of control, leading to their technological applications in optoelectronics and bioimaging. Considering them as artificial atoms, CQD molecules connected with molecular linkers such as DNA strands were studied. Yet, in these structures the presence of a high potential barrier limits coupling between the quantum states of neighboring CQDs limiting their full potential as coupled systems. Although coupled quantum dots were prepared by means of molecular beam epitaxy (MBE), their typical dimensions limit coupling to small energy scales suitable to low temperature operation. Furthermore, while MBE grown structures are inherently buried within a host semiconductor, CQDs are free in solution and accessible for wet-chemical manipulations.

Herein, we introduce a facile and powerful strategy for programmed synthesis of coupled CQDs molecules with precise control over the composition and size of the barrier in between the artificial atoms to allow for tuning the electronic coupling characteristics and their optical properties.¹ Our approach entails fusing two CdSe/CdS core/shell CQDs via constrained oriented attachment.² This yields a dimer with a tailored barrier dictated by the shell composition, thickness and fusion reaction conditions. The fusion reaction also enables tuning the neck barrier width.³ The possible nanocrystal facets in which such fusion takes place are analysed with atomic resolution revealing the distribution of possible crystal fusion scenarios. Coherent coupling is revealed by ensemble and single particle spectroscopic signatures, in agreement with quantum mechanical simulations.⁴ Cryogenic single nanocrystal spectroscopy accompanied by the simulations, unravels the complete tracking of fluorescence and charging events in such coupled colloidal quantum dot molecules.

This sets the stage for nanocrystals chemistry to yield a diverse selection of coupled CQD molecules utilizing the rich collection of ubiquitous artificial atom core/shell CQD building blocks. Such CQD molecules are of direct relevance for numerous applications including in displays, sensing, biological tagging and emerging quantum technologies.

Lead halide perovskite nanocrystals (NCs) have drawn tremendous research interest in the past five years fueled by their outstanding optical and electronic properties.[1, 2] In particular CsPbBr₃ NCs exhibit excellent photoluminescence with a very narrow peak and high quantum yield. They can be conveniently synthesized in high yield by means of hot-injection methods.[3-5]  However, due to their ionic nature, perovskite NCs are unstable in polar solvents, and when dispersed in water they quickly lose their luminescence properties and chemical integrity.

            Many efforts have been undertaken to enhance aqueous stability, encompassing changes in stoichiometry (synthesis under Pb-poor conditions)[6] and encapsulation with appropriate organic compounds or oxides (e.g., block copolymers,[7] SiO₂[8]). On the other hand, the growth of an inorganic shell, as generally applied in the case of conventional nanocrystals/quantum dots, has turned out to be challenging: upon prolonged heating of perovskite NCs, uncontrolled ripening leads to a variety of sizes and shapes. Furthermore, classic shell materials crystallize in different structures and exhibit large lattice mismatches.

            Here, we present novel approaches for the growth of metal sulfide and metal oxide shells on CsPbBr₃ nanocrystals, keeping a low size distribution and high fluorescence quantum yield. In the case of metal sulfide shells, aqueous phase transfer of the core/shell NCs could be achieved while maintaining the photoluminescence in water for extended periods. The improved stability of the core/shell perovskite NCs paves the way for their use in various applications such as single-photon emitters and photocatalysis.

Semi-conducting nanoplatelets (NPL) exhibit outstanding optical properties but contain toxic heavy metals such as cadmium, lead or mercury. Indium sulfide is believed to be a credible alternative for semi-conductor based applications. We report the synthesis of ultrathin indium sulfide In₂S₃ nanoplatelets which display a giant aspect ratio using a simple and fast solvothermal method. We show that these NPL display a thickness controlled at the atomic level below the nanometer, a width of around 9 nm and a length which can reach several micrometers. We determined the atomic composition of the inorganic core by Rutherford backscattering spectrometry (RBS) and measured by X-ray photoelectron spectrometry (XPS) an oleylamine surface coverage of 2.3 ligands per nm². X-ray diffraction experiments and simulations as well as high-resolution dark-field STEM point towards a trigonal crystallographic structure (gamma phase). Depending on the dispersion solvent, these long ribbon-like nanoplatelets can form well dispersed colloids or bundles in which they stack face to face. Their large aspect ratio induces the formation of gels. Their lateral dimensions can be tuned by the amount of water present in the reaction media: anhydrous synthesis conditions lead to hexagonal nanoplates while controlled addition of water induces a symmetry break yielding long NPL with a rectangular

Transition Metal Dichalcogenide (TMDC) monolayers exhibit a direct band gap in their semiconducting crystal phase and have great potential for future photonics through spin- and valleytronics and their ultrafast response to external stimuli, which is essential for fast optoelectronic components [1, 2]. The materials are typically obtained by exfoliation and chemical vapor deposition methods that yield samples of high quality for fundamental research but lack potential scalability. Colloidal wet-chemistry represents a challenging yet promising approach to synthesize TMDCs from precursors in a solution-processed and flexible manner. We show how fundamental properties of MoS₂ nanosheets including the layer count, the lateral dimension and the crystal phase is controlled. The initially forming a metallic crystal phase, attractive for catalytic hydrogen evolution, is kinetically preferred and transforms into the thermodynamically stable semiconducting crystal phase throughout the reaction. In order to show that the colloidal approach is able to compete with conventional manufacturing methods we investigate photoluminescence of ultrathin colloidal WS₂ nanosheets synthesized with WCl₆ and elemental sulfur in oleic acid and oleylamine at 320 °C, finding photoemission for the first time [3]. We observe mono- as well as multilayer photoluminescence exhibiting comparable characteristics to exfoliated TMDC monolayers and underpinning the high quality of colloidal WS₂ nanosheets. In addition, the observed monolayer emission has a narrow linewidth, which qualifies colloidal WS₂ as a potential single photon emitter complementing epitaxially grown WS₂ quantum emitters [4]. Our results render colloidal WS₂ as straightforwardly synthesized and highly promising 2D semiconductors with optical properties competitive with conventionally fabricated ultrathin WS₂.

Among semiconductor colloidal nanocrystals, 2D nanoplatelets (NPLs) are geometrically seen as well-defined flexible substrates for the self-assembly of molecules. In the presence of stress brought by surface stabilizers, helical structures are formed according to the parameters of the initial material ^[1,2].

Here, we demonstrate the control of the NPLs helices radii through the organic ligands, described as an anchoring group and an aliphatic chain of a given length. A perfect control in surface chemistry allows the tuning of the morphological feature of these nanohelices. Nonetheless, their optical properties are well-preserved upon surface modification. Structural studies done on these anisotropic nanoparticles unveil a preferential orientation effect on the  resulting X-ray scattering patterns.

A mechanical model accounting for the misfit strain between the inorganic core and the surface ligands, enables to predict the nanohelices radii. The model treats the substrate layer, anchoring group and aliphatic chain contributions individually and demonstrates good agreement for all studied homo- and hetero-structure NPLs. It reveals ultimately that the self-assembly of organic ligands is equivalent to a layer, of Young modulus in lateral compression estimated close to 0.9 GPa. Furthermore, the chirality of the nanohelices shown in this work can be tuned by the ligands anchoring group and inverted from one population to another. 

Colloidal nanocrystals (QDs) have been developed as excellent opto-electronic materials over the past 3 decades. Being used in applications as diverse as light emission, conversion and detection, they found their way mostly in those requiring interaction with visible or near-infrared radiation. Going further into the mid-infrared spectrum however, a plethorum of applications await in fields as defense, security, trace gas detection, imaging, etc. Over the past decade, giant leaps have been made using inter – and intraband processes in QDs to address this exciting wavelength range, resulting in solution processable photo-detectors with state-of-the-art performance at a fraction of the cost. [1]

As important as light detection is the ability to develop light emitting devices in the mid-infrared. Sources with broadband or coherent narrowband radiation would be disruptive in several fields. However, many properties of excitons and multi-excitons that enable these devices remain unexplored for these narrow gap materials, such as exciton cooling, multi-exciton interactions (shifts, Auger processes, …) and the fate of excitations in an environment full of quenching sites, i.e. non-radiative transfer of energy to surface ligands.

In this contribution, we show our recent work on applying ultra-broadband 1D mid-infrared spectroscopy to n-doped HgSe QDs. These advanced measurements provide for the first time a complete overview of the ultrafast time dynamics of these materials over the full spectral range, revealing intraband cooling, photo-induced absorption and multi-exciton shifts. Our results pave the way towards a full understanding of the (multi-) exciton physics of intraband colloidal nanomaterials.

During the past decade, nanocrystals (NCs) have shown that they were an efficient and low-cost photodetection technology to address the infrared range. In particular, in the short-wave infrared by using interband transitions in narrow band gap semiconductors such as HgTe NCs [1]. Here, to go beyond and access the mid-wave infrared range, we use an interesting alternative that are intraband transitions of HgSe doped NCs [2][3]. Throughout a dye sensitized approach which couples an intraband absorber to an undoped material driving the charge conduction, the limitations observed from intraband materials (high dark current, slow response, and low activation energy) are ruled out [3]. Using mid-infrared transient reflectivity (TR) measurement, we unveil the coupling between both material in this heterostructure. The latter displays a unique feature in the TR signal which dynamic matches the hopping time and therefore is an evidence of a charge transfer. Moreover, we develop a strategy to enhance the hybrid material’s photodetection performance by coupling it to an optical nano-resonator. Finally, we demonstrate that the final structure has an enhanced absorption of a factor 4 allowing for an 80 K increase of the operating temperature. The resulting device has among the highest intraband-based photodetection performances.

Nanostructured semiconductors are heavily investigated for their applications in light emission such as light emitting diodes and, more challenging, lasers[1-4]. Using quantum confined Cd-based QDs, several groups have shown light amplification and ensuing lasing action in the red part of the spectrum. Although further work is necessary to reduce gain threshold densities for efficient lasing action, there has been some push toward moving away from the current red gain band region, toward green and near-infrared stimulated emission.

In this work, we take a look at weakly confined “giant” CdS and CdSe Quantum Dots which display disruptive optical gain metrics in the green and near-infrared spectrall region. While showing similar gain thresholds compared to state-of-the-art materials, the gain window, amplitude (up to 50000/cm) and gain lifetime (up to 3ns) outpace other materials in the same spectral region.

These remarkable results are explained by using a bulk semiconductor gain model, which can be done due to the large size of the quantum dots (8-12 nm). We can quantitatively reconstruct the gain spectrum with this model, yet only by including large bandgap renormalizations (up to 70 meV). This inclusion helps us to understand the gain mechanism in these particles. Our results indicate a paradigm shift towards weakly confined photo-physics as a means to push quantum dots towards efficient solution processable lasers.

Optical properties tunability is one of the essential characteristics of nanocrystals (NCs). Nonetheless, obtaining reconfigurable spectral response in NC-based optoelectronic devices remains a hardly explored domain. This is because the conventional strategies, such as exploiting Stark effect or integrating NCs into MEMS structures, are not straightforward to apply for NC-based devices due to high electrostatic field requirement and fabrication incompatibility. Here, we demonstrate optical tunability with bias in a device coupling HgTe¹ NCs with a plasmonic structure. Bias tunability can be obtained thanks to the interplay between absorption inhomogeneity due to the plasmonic cavity² and the NC hopping transport. As a result, 15 meV blueshift can be observed under the application of 3 V bias voltage for a device working in the extended short-wave infrared³. We show that hopping transport, which is usually seen as a limitation of NCs, can be combined with inhomogeneous absorption to obtain an active photonic NC-based device.

Metal oxide (MO) doped semiconductor nanocrystals (NCs) are characterized by tunable optoelectronic properties, which can be modified by appropriate engineering of the NCs geometry, structure and carrier density profile [1]. Their versatility makes them tailorable for specific applications, ranging from electronic [2] and optical devices [3] to energy storage[4].

We report a semi-classical multi-layer optical model that can effectively describe the response of colloidal NCs with different doping profiles in core-shell structures. In particular, we applied our model to the investigation of the multi-peak absorption of Sn-doped Indium Oxide (ITO) NCs. We demonstrate that our model is suitable for predicting the experimentally observed absorption peak splitting, both with variable NCs doping profiles and, post-synthetically, via photodoping. In both cases, we found that depletion layer is fundamental in order to correctly describe the spectrum evolution. We foresee that our model can be employed as a useful tool to design the optoelectronic properties of core-shell NCs systems in the framework of energy band and depletion layer engineering.

Efficient transport of excited state energy in nanocrystal thin-films is essential in the operation of light harvesting and light emitting devices. Transient Photoluminescence Microscopy has emerged as a powerful tool to characterize energy transport, allowing for a direct visualization of the transport in both space and time.[1] 

In this talk, I will present past and current work of our use of Transient Photoluminescence Microscopy in characterizing energy transport in different nanocrystal solids.[2,3] I will specifically highlight the role of energetic disorder in these materials, which greatly impacts the transport characteristics. For example, size-polydispersity in colloidal quantum-dot solids leads to inhomogeneous energy landscapes that cause disitinct non-equilibrium effects in the transport of energy carriers. Similarly, statistical fluctuations in the doping levels of perovskite nanocrystals casue local variations in the energy landscape. Using Transient Photoluminescence Microscopy, we provide a direct visualization of the impact of disorder on the transport characteristics in these materials, and allow us to start formulating desing-rules for optimized energy landscapes. 

Understanding exciton transport in quantum dot (QD) solids is crucial to their broad applications in emerging devices. Here we reveal the early-time (femtosecond) dynamics of exciton in QD solids by transient absorption microscopy [1]. We find unusually high exciton diffusivities (~10² cm² s^-1 ) in lead chalcogenide QDs within the first few hundred femtoseconds after photoexcitation, followed by a transition to a slower transport regime (10^-1~1 cm² s^-1). Counterintuitively, the initial diffusivity is higher in QD solids with larger interdot distances, and the transport phase also lasts longer. This initial fast transport only occurs in materials with exciton Bohr radii much larger than the QD sizes, suggesting this regime is based on delocalized excitons, and the transition to slower transport is related to the process of exciton localization. Both higher QD packing density and heterogeneity accelerate the transition. These results suggest new principals to control the optoelectronic properties of QD solids.Understanding exciton transport in quantum dot (QD) solids is crucial to their broad applications in emerging devices. Here we reveal the early-time (femtosecond) dynamics of exciton in QD solids by transient absorption microscopy. We find unusually high exciton diffusivities (~10² cm² s^-1 ) in lead chalcogenide QDs within the first few hundred femtoseconds after photoexcitation, followed by a transition to a slower transport regime (10^-1~1 cm² s^-1). Counterintuitively, the initial diffusivity is higher in QD solids with larger interdot distances, and the transport phase also lasts longer. This initial fast transport only occurs in materials with exciton Bohr radii much larger than the QD sizes, suggesting this regime is based on delocalized excitons, and the transition to slower transport is related to the process of exciton localization. Both higher QD packing density and heterogeneity accelerate the transition. These results suggest new principals to control the optoelectronic properties of QD solids.

[1] Zhang et al., Nature Materials, 2022, In Press

In the past 30 years, material scientists have largely capitalized on the grand appeal of utilizing quantum confinement to obtain size-tunable inter-band optical transitions and implement colloidal quantum dots (CQDs) in optoelectronic applications throughout the electromagnetic spectrum. The infrared region is particularly exciting with applications in telecommunications, night-time surveillance, and satellite imaging for agricultural water conservation. While most progress with CQDs in the infrared (IR) has been achieved using inter-band transitions in Pb- and Hg-based heavy metal compounds, intra-band optical transitions originating from external- or self- dopants can potentially expand the library of materials to generate IR-optoelectronic devices with non-toxic materials. In this talk, I will focus on my group’s work on silver chalcogenide (Ag₂Se) quantum dots that exhibit distinct optical absorption in the mid-wave IR wavelength spectrum. These CQDs demonstrate a narrow bandgap metastable tetragonal phase, not available in bulk, and contain excess electrons in the lowest level of the conduction band. This allows for intra-band optical transitions between the first and the second conduction energy level which can potentially decrease Auger recombination rates and avoid the need for cryogenic cooling. I will present a detailed study of the size-dependent inter-band to intra-band optical transition and compare the competing effects of quantum confinement, environmental Fermi level and particle stoichiometry to provide guidelines for stable electron occupation of the 1S_e state and obtaining tunable mid-wave IR absorption. Finally, I will also discuss some of the challenges with Ag₂Se quantum dot devices and potential strategies to overcome these issues.

Colloidal 2D semiconductors (nanosheets (NSs) and nanoplatelets (NPLs)) are only a few atom layers thick and strongly quantum-confined in their thickness dimension. This leads to increased exciton binding energies in the structures and optical properties that are wet-chemically tunable from visible to infrared wavelengths. Synthetic fine-tuning of the NS and NPL to control their narrow absorption and efficient emission is highly interesting for photonic and optoelectronic applications.[1,2]

In the first part of my talk, I will touch on our recent results on the synthesis of colloidal 2D transition metal dichalcogenides (WS2 and MoS2). By adjusting the Mo- and W-precursor concentration and reaction times, a control on the formation of the semiconducting vs. the metallic TMDC crystal phase during the reaction is gained and followed by XPS. We applied micro-photoluminescence spectroscopy to study semiconducting WS2 mono- and multilayer photoluminescence comparable to exfoliated WS2 for the first time.[3]

The second part of my talk is dedicated to our results on the direct synthesis of infrared-emitting 2D PbSe NPLs. By synthetically tuning the lateral size of the NPLs through the addition of small amounts of octylamine to the reaction, we obtain efficient emission of the NPLs covering the telecom O-, E- and S-band, respectively.[4] Infrared emitting NPLs are highly interesting for emerging photonic quantum technologies, e.g. single photon emission at technologically relevant wavelengths.

Carriers confined in colloidal nanoplatetelets feel strong Coulomb interactions, enhanced by dielectric confinement and quasi-planar geometry.  We review from a theoretical perspective how these interactions make the optoelectronic response diverge from that of quantum dots and even that of quantum wells, thus providing nanoplatelets with characteristic properties. Large exciton[1] (and trion[2]) binding energies, Giant (and Dwarf) Oscillator Strength[3] and radiative Auger processes[4] are some of the effects that can be observed with due material engineering. Special attention is paid to the role of Coulomb repulsions, which make biexcitons behave differently from simpler species, and stimulate the formation of spontaneous magnetic phases in few-electron nanoplatelets.

Further, prospects of exploiting topological effects in colloidal systems are addressed in two systems:

(i) Core/crown nanoplatelets, where carriers localized in the crown are shown to be susceptible of displaying Aharonov-Bohm phenomena;

(ii) Mercury chalcogenide nanoplatelets. Using multi-band k·p theory, we explain why recent experiments with such structures show absorption spectra which are reminiscent of cadmium-based ones, in spite of the inverted band gap these materials present in bulk. Predictions are made on the structural conditions which will permit the formation of topological (surface) states in such systems.

Thermoelectricity enables the direct and reversible conversion between heat and electricity through solid-state devices. However, despite many potential applications, their extended use has been seriously hampered by the relatively high production cost and low efficiency of thermoelectric materials. The problem is that thermoelectric materials require high electrical conductivity (s), high thermopower (S), and low thermal conductivity (k), three strongly interdependent properties.

Thermoelectric materials are often dense, polycrystalline inorganic semiconductors. Among the various strategies to produce them, the use of solution-processed nanoparticles as precursors allows their production with mild synthesis conditions, inexpensive equipment, and the possibility of high-throughput processing. However, solution synthesis generally involves the presence of additional molecules or ions belonging to the precursors or added to enable solubility and/or regulate nucleation and growth. These molecules or ions can end up in the particles either as impurities within the crystal lattice or as surface adsorbates and, therefore, interfere in the material properties.

Herein, we demonstrate the unavoidable but generally overlooked presence of ionic adsorbates in solution-processed surfactant-free synthesis and their importance in the transport properties. Furthermore, we explain the rationale behind its presence based on the fundamentals of colloidal science. These findings highlight the importance of evaluating possible unintentional impurities and their origin to i) establish the proper structure-property relationships and ii) redefine synthetic protocols to tune material properties controllably [1].