Heterostructured Nanocrystals Research Articles

Synthetic Approaches to Colloidal Nanocrystal Heterostructures Based on Metal and Metal-Oxide Materials.

Composite inorganic nanoarchitectures, based on combinations of distinct materials, represent advanced solid-state constructs, where coexistence and synergistic interactions among nonhomologous optical, magnetic, chemical, and catalytic properties lay a basis for the engineering of enhanced or even unconventional functionalities. Such systems thus hold relevance for both theoretical and applied nanotechnology-based research in diverse areas, spanning optics, electronics, energy management, (photo)catalysis, biomedicine, and environmental remediation. Wet-chemical colloidal synthetic techniques have now been refined to the point of allowing the fabrication of solution free-standing and easily processable multicomponent nanocrystals with sophisticated modular heterostructure, built upon a programmed spatial distribution of the crystal phase, composition, and anchored surface moieties. Such last-generation breeds of nanocrystals are thus composed of nanoscale domains of different materials, assembled controllably into core/shell or heteromer-type configurations through bonding epitaxial heterojunctions. This review offers a critical overview of achievements made in the design and synthetic elaboration of colloidal nanocrystal heterostructures based on diverse associations of transition metals (with emphasis on plasmonic metals) and transition-metal oxides. Synthetic strategies, all leveraging on the basic seed-mediated approach, are described and discussed with reference to the most credited mechanisms underpinning regioselective heteroepitaxial deposition. The unique properties and advanced applications allowed by such brand-new nanomaterials are also mentioned.

Defect-Mediated Electron Transfer in Pt-CuInS2/CdS Heterostructured Nanocrystals for Enhanced Photocatalytic H2 Evolution

The effective separation of photogenerated electrons and holes is the key to improve photocatalytic activity in hydrogen evolution reaction (HER). However, the mechanism of photoinduced charge transfer between the cocatalyst and the semiconducting heterojunction in a single nanocrystal is vague at present. Herein, the relationships between the crafted cocatalyst and the constructed semiconducting heterojunction in a single nanocrystal were verified. Manipulating these relationships will improve the performance of visible light-driven hydrogen evolution. As a model, we used CuInS2 (CIS) nanocrystals (NCs) modified by the cocatalyst platinum (Pt) NCs and the semiconductor cadmium sulfide (CdS), forming the ternary heterostructured NCs (HNCs). Under visible light irradiation, the photocatalytic activity in hydrogen evolution of Pt-CIS/CdS HNCs is 6.38 and 1.76 times higher than those of pristine CIS NCs and CIS/CdS HNCs, respectively. Transient absorption measurements certified that the high photocatalytic activity in the HER was attributed to the photoinduced electron transfer from the defect-mediated trap state in CIS to the cocatalyst Pt, which was superior to the direct Z-scheme heterojunction between CIS and CdS. This research paves a route for the design of a suitable cocatalyst and the formation of heterostructured semiconducting NCs for boosting the photocatalytic activity in solar energy conversion.

Studies on Electron Escape Condition in Semiconductor Nanomaterials via Photodeposition Reaction.

In semiconductor material-driven photocatalysis systems, the generation and migration of charge carriers are core research contents. Among these, the separation of electron-hole pairs and the transfer of electrons to a material’s surface played a crucial role. In this work, photodeposition, a photocatalysis reaction, was used as a “tool” to point out the electron escaping sites on a material’s surface. This “tool” could be used to visually indicate the active particles in photocatalyst materials. Photoproduced electrons need to be transferred to the surface, and they will only participate in reactions at the surface. By reacting with escaped electrons, metal ions could be reduced to nanoparticles immediately and deposited at electron come-out sites. Based on this, the electron escaping conditions of photocatalyst materials have been investigated and surveyed through the photodeposition of platinum. Our results indicate that, first, in monodispersed nanocrystal materials, platinum nanoparticles deposited randomly on a particle’s surface. This can be attributed to the abundant surface defects, which provide driving forces for electron escaping. Second, platinum nanoparticles were found to be deposited, preferentially, on one side in heterostructured nanocrystals. This is considered to be a combination result of work function difference and existence of heterojunction structure.

Tailoring defects and nanocrystal transformation for optimal heating power in bimagnetic CoyFe1-yO@CoxFe3-xO4 particles.

The effects of cobalt incorporation in spherical heterostructured iron oxide nanocrystals (NCs) of sub-critical size have been explored by colloidal chemistry methods. Synchrotron X-ray total scattering methods suggest that cobalt (Co) substitution in rock salt iron oxide NCs tends to remedy their vacant iron sites, offering a higher degree of resistance to oxidative conversion. Self-passivation still creates a spinel-like shell, but with a higher volume fraction of the rock salt Co-containing phase in the core. The higher divalent metal stoichiometry in the rock salt phase, with increasing Co content, results in a population of unoccupied tetrahedral metal sites in the spinel part, likely through oxidative shell creation, involving an ordered defect-clustering mechanism, directly correlated to core stabilization. To shed light on the effects of Co-substitution and atomic-scale defects (vacant sites), Monte Carlo simulations suggest that the designed NCs, with desirable, enhanced magnetic properties (cf. exchange bias and coercivity), are developed with magnetocrystalline anisotropy which increases due to a relatively low content of Co ions in the lattice. The growth of optimally performing candidates combines also a strongly exchange-coupled system, secured through a high volumetric ratio rock salt phase, interfaced by a not so defective spinel shell. In view of these requirements, specific absorption rate (SAR) calculations demonstrate that the rock salt core sufficiently protected from oxidation and the heterostructure preserved over time, play a key role in magnetically mediated heating efficacies, for potential use of such NCs in magnetic hyperthermia applications.

Elastic forces drive nonequilibrium pattern formation in a model of nanocrystal ion exchange

Chemical transformations, such as ion exchange, are commonly employed to modify nanocrystal compositions. Yet the mechanisms of these transformations, which often operate far from equilibrium and entail mixing diverse chemical species, remain poorly understood. Here we explore an idealized model for ion exchange in which a chemical potential drives compositional defects to accumulate at a crystal's surface. These impurities subsequently diffuse inward. We find that the nature of interactions between sites in a compositionally impure crystal strongly impacts exchange trajectories. In particular, elastic deformations which accompany lattice-mismatched species promote spatially modulated patterns in the composition. These same patterns can be produced at equilibrium in core/shell nanocrystals, whose structure mimics transient motifs observed in nonequilibrium trajectories. Moreover, the core of such nanocrystals undergoes a phase transition-from modulated to unstructured-as the thickness or stiffness of the shell is decreased. Our results help explain the varied patterns observed in heterostructured nanocrystals produced by ion exchange and suggest principles for the rational design of compositionally patterned nanomaterials.

Au-Cu2-xTe Plasmonic Heteronanostructure Photoelectrocatalysts.

Semiconductor nanocrystals coupled with plasmonic Au nanoparticles have been widely studied as photoelectrocatalysts for solar water splitting. Among these, heterostructures of copper chalcogenides with Au remained a unique category for their dual plasmon character. However, while sulfides and selenides of copper have been extensively reported, heterostructures of copper tellurides with Au have not been explored. Herein, the plasmonic semiconductor Cu2-xTe disks grown on Au nanoparticles (disk-on-dot) were explored as efficient photoelectrocatalysts for hydrogen evolution reactions (HER). This has been successfully designed by growing Cu2-xTe disks on presynthesized Au nanoparticles under optimized reaction conditions. The resulting heterostructured nanocrystals acted as efficient photoelectrocatalysts for the H2 evolution reaction with a low Tafel slope and less cathodic overpotential in the presence of light. Details of their synthesis, characterization, optical properties, and electrocatalytic activities are studied and reported in this letter.

Interface polarization in heterovalent core-shell nanocrystals.

The potential profile and the energy level offset of core-shell heterostructured nanocrystals (h-NCs) determine the photophysical properties and the charge transport characteristics of h-NC solids. However, limited material choices for heavy metal-free III-V-II-VI h-NCs pose challenges in comprehensive control of the potential profile. Herein, we present an approach to such a control by steering dipole densities at the interface of III-V-II-VI h-NCs. The controllable heterovalency at the interface is responsible for interfacial dipole densities that result in the vacuum-level shift, providing an additional knob for the control of optical and electrical characteristics of h-NCs. The synthesis of h-NCs with atomic precision allows us to correlate interfacial dipole moments with the NCs' photochemical stability and optoelectronic performance.

Stable CsPbX3/ZnO Heterostructure Nanocrystals for Light-Emitting Application.

Lead halide perovskite nanocrystals (PNCs) are attractive in light-emitting applications with their extraordinary photoluminescent property. However, the inherent instability of mixed Br/I ions poses a major hurdle to the practical application of yellow-red emitting PNCs. In this work, we report the growth of crystalline ZnO on the surface of CsPb(Br/I)3 nanocrystals through a ligand-mediated in situ surface reaction route, which forms monodispersed CsPb(Br/I)3/ZnO heterostructure nanocrystals (PZNCs). The unique heterostructure endows the PZNCs with largely enhanced stability against air, moisture, and polar solvents. Notably, the red-emitting PZNCs exhibit high color quality and superior stability compared with classical CsPb(Br/I)3 PNCs. The PZNCs retain 95% of initial luminescence after 500 h of illumination, and the PZNC-converted red-light LEDs show no obvious change in the peak position and light intensity after continuous operation for 100 h, demonstrating the promising prospect of the materials in light-emitting applications.

Upconversion Perovskite Nanocrystal Heterostructures with Enhanced Luminescence and Stability by Lattice Matching.

Lead halide perovskite quantum dots (PQDs) exhibit excellent photoelectric and optical properties, but their poor stability and low multiphoton absorption efficiency greatly limit their biological applications. Efforts have been made to combine upconversion nanoparticles (UCNPs) with PQDs to produce a composite material that is NIR-excitable, upconverting, and emission-tunable due to the unique optical properties of UCNPs, which converts tissue-penetrating near-infrared light into visible light based on an upconversion multiphoton excitation process. However, it is challenging to make such a nanocrystal heterostructure and maintain good optical properties and stability of both UCNPs and PQDs because they have different crystal structures. Here, we report the synthesis of heterostructured UCNP-PQD nanocrystals to bring hexagonal-phase NaYF4 UCNPs and cubic-phase CsPbBr1X2 PQDs in close proximity in a single nanocrystal, leading to efficient Förster resonance energy transfer (FRET) from the UCNP to the PQD under NIR excitation, as compared to their counterparts in solution. Moreover, by further improving the lattice matching between the UCNP and PQD using Gd to replace Y, heterostructured CsPbBr3-NaGdF4:Yb,Tm nanocrystals are successfully synthesized, with much enhanced luminescence and stability at high temperatures or in polar solvents or under continuous ultraviolet light excitation as compared to those of the CsPbBr3-NaYF4:Yb,Tm nanocrystals and pure PQDs.

One-Pot Synthesis and Structural Evolution of Colloidal Cesium Lead Halide-Lead Sulfide Heterostructure Nanocrystals for Optoelectronic Applications.

Heterostructures, combining perovskite nanocrystals (PNC) and chalcogenide quantum dots, could pave a path to optoelectronic device applications by enabling absorption in the near-infrared region, tailorable electronic properties, and stable crystal structures. Ideally, the heterostructure host material requires a similar lattice constant as the guest which is also constrained by the synthesis protocol and materials selectivity. Herein, we present an efficient one-pot hot-injection method to synthesize colloidal all-inorganic cesium lead halide-lead sulfide (CsPbX3 (X = Cl, Br, I)-PbS) heterostructure nanocrystals (HNCs) via the epitaxial growth of the perovskite onto the presynthesized PbS nanocrystals (NCs). Optical and structural characterization evidenced the formation of heterostructures. The embedding of PbS NCs into CsPbX3 perovskite allows the tuning of the absorption and emission from 400 to 1100 nm by tuning the size and composition of perovskite HNCs. The CsPbI3-PbS HNCs show enhanced stability in ambient conditions. The stability, tunable optical properties, and variable band alignments accessible in this system would have implications in the design of novel optoelectronic applications such as light-emitting diodes, photodetectors, photocatalysis, and photovoltaics.

PtPd‐decorated Multibranched Au@Ag Nanocrystal Heterostructures for Enhanced Hydrogen Evolution Reaction Performance in Visible to Near Infrared Light

AbstractPtPd‐decorated multibranched Au@Ag nanocrystals (NCs) were synthesized to form heterostructures with enhanced electrochemical performance for water splitting in the visible/near infrared (vis‐NIR) light region. Three kinds of heterostructures including PtPd‐covered, PtPd‐edged, and PtPd‐tipped Multi‐branched Au@Ag NCs were fabricated by controlling the growth process of PtPd alloy nanoparticles on Au@Ag NCs. PtPd‐covered Multi‐branched Au@Ag nanocrystal exhibited the best photo‐assisted electrocatalytic activity. The initial overpotential under vis‐NIR light is 33 mV with a Tatel slope of 101 mV/dec while the overpotential is 150 mV at 10 mA cm−2. The enhanced catalyst activity is ascribed to samples with heterogeneous structures and changed morphology and composition. In addition, localized surface Plasmon resonance (LSPR) effect from the surface of samples resulted in the effective separation of electron‐hole pairs (photoelectric effect). Finally the photothermal effect generated from the absorbance of Au nanostructures in near‐infrared light increased the temperature of the catalyst. The synergy of structure, photoelectricity, and photothermal effect endows the catalyst with excellent photo‐assisted electrocatalytic activity.

ARTICLE WITHDRAWN] UV-Assisted Room Temperature Gas Sensing with ZnO-Ag Heterostructure Nanocrystals Studied by Photoluminescence.

THIS ARTICLE WAS WITHDRAWN BY THE PUBLISHER IN APR 2021

Synergistic Engineering of Defects and Heterostructures Enhance Lithium/Sodium Storage Properties of F-SnO2-x -SnS2-x Nanocrystals Supported on N,S-Graphene.

Phase engineering of the electrode materials in terms of designing heterostructures, introducing heteroatom and defects, improves great prospects in accelerating the charge storage kinetics during the repeated Li+ /Na+ insertion/deintercalation. Herein, a new design of Li/Na-ion battery anodes through phase regulating is reported consisting of F-doped SnO2 -SnS2 heterostructure nanocrystals with oxygen/sulfur vacancies (VO /VS ) anchored on a 2D sulfur/nitrogen-doped reduced graphene oxide matrix (F-SnO2-x -SnS2-x @N/S-RGO). Consequently, the F-SnO2-x -SnS2-x @N/S-RGO anode demonstrates superb high reversible capacity and long-term cycling stability. Moreover, it exhibits excellent great rate capability with 589 mAh g-1 for Li+ and 296 mAh g-1 at 5 A g-1 for Na+ . The enhanced Li/Na storage properties of the nanocomposites are not only attributed to the increase in conductivity caused by VO /VS and F doping (confirmed by DFT calculations) to accelerate their charge-transfer kinetics but also the increased interaction between F-SnO2-x -SnS2-x and Li/Na through heterostructure. Meanwhile, the hierarchical F-SnO2-x -SnS2-x @N/S-RGO network structure enables fast infiltration of electrolyte and improves electron/ion transportation in the electrode, and the corrosion resistance of F doping contributes to prolonged cycle stability.

Ion-exchange strategy of CoS2/Sb2S3 hetero-structured nanocrystals encapsulated into 3D interpenetrating dual-carbon framework for high-performance Na+/K+ batteries

Combining the advantages of cobalt and zinc-based imidazolate frameworks (ZIF-67 and ZIF-8), CoS2/Sb2S3 hetero-structured nanocrystals encapsulated into interpenetrating dual-carbon framework of nitrogen-doped carbon and carbon nanotubes (CoS2/Sb2S3@NC/CNT) were successfully prepared by ion-exchange reaction between Co and Sb3+ and subsequent sulfidation reaction process. The supported dual carbon framework derived from ZIF-8 and ZIF-67 not only buffers volume expansion during repeated discharge/charge to prevent polyhedron from collapsing but also significantly enhances electronic conductivity which is beneficial to optimize rate performance and cycle stability. Besides, introducing of Sb2S3 to form heterostructure greatly improves electrochemical properties. As a result, the CoS2/Sb2S3@NC/CNT heterostructure exhibits favorable rate capability and cycle stability in both sodium-ion battery (SIBs) and potassium-ion battery (PIBs). The CoS2/Sb2S3@NC/CNT anode in SIBs delivers the reversible specific capacity of 360.1 mAh g−1 at the current density of 0.5 A g−1 after 200 cycles, while the CoS2/Sb2S3@NC/CNT anode in PIBs exhibits the specific capacity of 453.5 mAh g−1 at the current density of 0.2 A g−1 after 50 cycles. The complex heterostructure consisting of CoS2/Sb2S3 core and coated carbon shell easily promotes electrolyte penetration and further accelerates ions diffusion and electrochemical reaction kinetics. This material will provide a novel alternative method for the preparation of heterogeneous structure anode materials, paving the way for being applied in high-efficiency energy storage devices.

Why Is Making Epitaxially Grown All Inorganic Perovskite–Chalcogenide Nanocrystal Heterostructures Challenging? Some Facts and Some Strategies

Nanocrystal heterostructures are one of the frontline energy materials widely known for enhancing the rate of photocatalysis and tuning optical properties and are also used in photovoltaics. These nanostructures where two crystalline nanomaterials are placed together in a single building block and share lattices are extensively studied for chalcogenide semiconductors and metal oxides but limited for recently emerged perovskite nanocrystals. Due to differences in the crystal bonding nature, interface chemistry, and also the formation mechanism, some constraints are present in designing common reaction pathways for the simultaneous formation of perovskites and chalcogenides or similar nanocrystals sharing common lattice planes. Hence, more fundamental understanding of both nucleation and growth of both materials is required for inducing heteronucleations of one on the surface of another. Literature reports also revealed that epitaxial growth of lead halide perovskites with nonhalide or chalcogenide colloidal 0D nanocrystals could not be established yet. Hence, the field remains challenging, and more investigations on both experimental as well as theoretical studies are timely required. Keeping these in mind, in this perspective the state-of-the-art issues related to the formation of all inorganic halides and nonhalide heterostructures formed with epitaxial relations are discussed. At the beginning, the chemistry of these nanocrystals with similarities and dissimilarities in their nature and examples of different heterostructures are summarized, and then, different synthetic possibilities for overtaking the hurdles and designing such heterostructures are proposed.

Tuning Exchange Coupling in NiO-Based Bimagnetic Heterostructured Nanocrystals.

A series of bimagnetic heterostructured nanocrystals having an antiferromagnetic NiO core and a ferrimagnetic MnxNi1-xO and/or FiM Mn3O4 island nanophase overgrowth has been synthesized under varying aqueous solution pH conditions. The two-step self-assembly process employs a thermal decomposition method to synthesize NiO nanoparticles, followed by growth of the MnxNi1-xO and/or Mn3O4 nanophase over the NiO core using hydrothermal synthesis at pH values ranging from 2.4-7.0. The environmentally benign hydrothermal process involves pH control of the protonation vs hydroxylation reactions occurring at the nanoparticle surface. TEM analysis and Rietveld refinement of XRD data show that three distinct types of heterostructured nanocrystals occur: NiO/MnxNi1-xO core-shell-like heterostructures at the pH of 2.4, mixed NiO/MnxNi1-xO and/or/Mn3O4 core-overgrowth structures for 2.4 < pH < 4.5, and predominantly NiO/Mn3O4 core-island structures for pH > 4.5. The magnetic coercivity and exchange bias of the heterostructured nanocrystals vary systematically with the pH of the aqueous solution used to synthesize the samples. The temperature-dependent magnetization and hysteresis loop data are consistent with the nature of overlayer coverage of the NiO core. Our DFT based calculations show that the MnxNi1-xO phase has ferrimagnetic properties with a stable spin orientation along the ⟨111⟩ orientation. Furthermore, the calculations show that the magnetic anisotropy constant (K1) of the Mn3O4 phase is considerably larger than that of the MnxNi1-xO phase, which is confirmed by our experimental results. The coercivity and exchange bias field are the largest for the NiO/Mn3O4 core-island nanocrystals, synthesized at a pH value of 5.0, with robust values of nearly 6 kOe and 3 kOe, respectively. This work demonstrates the tunability of hydrothermal deposition, and concomitant magnetic coercivity and exchange bias properties, of MnxNi1-xO and/or Mn3O4 nanophase overgrowth over a NiO core with pH, that makes these heterostructured nanocrystals potentially useful for magnetic device, biomedical, and other applications.

Halide Perovskite-Lead Chalcohalide Nanocrystal Heterostructures.

We report the synthesis of colloidal CsPbX3–Pb4S3Br2 (X = Cl, Br, I) nanocrystal heterostructures, providing an example of a sharp and atomically resolved epitaxial interface between a metal halide perovskite and a non-perovskite lattice. The CsPbBr3–Pb4S3Br2 nanocrystals are prepared by a two-step direct synthesis using preformed subnanometer CsPbBr3 clusters. Density functional theory calculations indicate the creation of a quasi-type II alignment at the heterointerface as well as the formation of localized trap states, promoting ultrafast separation of photogenerated excitons and carrier trapping, as confirmed by spectroscopic experiments. Postsynthesis reaction with either Cl– or I– ions delivers the corresponding CsPbCl3–Pb4S3Br2 and CsPbI3–Pb4S3Br2 heterostructures, thus enabling anion exchange only in the perovskite domain. An increased structural rigidity is conferred to the perovskite lattice when it is interfaced with the chalcohalide lattice. This is attested by the improved stability of the metastable γ phase (or “black” phase) of CsPbI3 in the CsPbI3–Pb4S3Br2 heterostructure.

NIR-excitable heterostructured upconversion perovskite nanodots with improved stability

There is a great need to develop heterostructured nanocrystals which combine two or more different materials into single nanoparticles with combined advantages. Lead halide perovskite quantum dots (QDs) have attracted much attention due to their excellent optical properties but their biological applications have not been much explored due to their poor stability and short penetration depth of the UV excitation light in tissues. Combining perovskite QDs with upconversion nanoparticles (UCNP) to form hybrid nanocrystals that are stable, NIR excitable and emission tunable is important, however, this is challenging because hexagonal phase UCNP can not be epitaxially grown on cubic phase perovskite QDs directly or vice versa. In this work, one-pot synthesis of perovskite-UCNP hybrid nanocrystals consisting of cubic phase perovskite QDs and hexagonal phase UCNP is reported, to form a watermelon-like heterostructure using cubic phase UCNP as an intermediate transition phase. The nanocrystals are NIR-excitable with much improved stability.

Pushing the Band Gap Envelope of Quasi-Type II Heterostructured Nanocrystals to Blue: ZnSe/ZnSe 1- X Te X /ZnSe Spherical Quantum Wells

Quasi-type II heterostructured nanocrystals (NCs) have been of particular interest due to their great potential for controlling the interplay of charge carriers. However, the lack of material choices for quasi-type II NCs restricts the accessible emission wavelength from red to near-infrared (NIR), which hinders their use in light-emitting applications that demand a wide range of visible colors. Herein, we demonstrate a new class of quasi-type II nanoemitters formulated in ZnSe/ZnSe 1- X Te X /ZnSe seed/spherical quantum well/shell heterostructures (SQWs) whose emission wavelength ranges from blue to orange. In a given geometry, ZnSe 1- X Te X emissive layers grown between the ZnSe seed and the shell layer are strained to fit into the surrounding media, and thus, the lattice mismatch between ZnSe 1- X Te X and ZnSe is effectively alleviated. In addition, composition of the ZnSe 1- X Te X emissive layer and the dimension of the ZnSe shell layer are engineered to tailor the distribution and energy of electron and hole wave functions. Benefitting from the capabilities to tune the charge carriers on demand and to form defect-free heterojunctions, ZnSe/ZnSe 1- X Te X /ZnSe/ZnS NCs show near-unity photoluminescence quantum yield ( PL QY &gt; 90 % ) in a broad range of emission wavelengths (peak PL from 450 nm to 600 nm). Finally, we exemplify dichromatic white NC-based light-emitting diodes (NC-LEDs) employing the mixed layer of blue- and yellow-emitting ZnSe/ZnSe 1- X Te X /ZnSe/ZnS SQW NCs.

(Invited) Ultrafast Exciton Dynamics in Semiconductor Nanocrystals: Effects on Single Vs Dual Emission and on Optical Gain

A wide variety of materials with nanometre dimensions are increasingly explored as light emitters. Among them, semiconductor nanocrystals (NCs) are very promising for a variety of applications, including light emission devices (LEDs), lasers, detectors, photovoltaic cells, biological labelling and sensing [1]. Key advantage of NCs is the possibility to tailor their optical response by controlling the electronic structure (“wave function engineering”) through the choice of composition, size and shape. Significant results have been mostly obtained with CdSe/CdS and PbS/CdS heterostructured NCs. Beyond single wavelength tuneable emission, other regimes have been demonstrated such as simultaneous emission on two different wavelengths, amplified spontaneous and laser emission.The luminescent properties arise from exciton decay, which can proceed through radiative or nonradiative pathways, following different routes. The study of exciton dynamics can thus allow elucidating the processes connected to single or dual emission and to optical gain. This, in turn, can lead to the identification of the functional and structural characteristics that are responsible for these behaviors. Exciton relaxation occurs on picosecond timescales, so ultrafast optical techniques are required to perform these studies.In this talk, we present studies carried out by ultrafast pump-probe spectroscopy techniques, with 100-fs time resolution, on CdSe/CdS and PbS/CdS NCs, with different structures (core/shell, dot-in-rod, dot-in-bulk, with sharp or graded interface) [2-6]. These NCs are optically active in the visible and near-infrared spectral region, show single and dual colour photoluminescence emission, optical gain and laser emission [2-7]. The analysis of the experimental data allowed us to unravel the decay processes. Exciton relaxation between core and shell states takes place typically in a few ps, but the ultimate band-edge lifetime can extend to hundreds of ps to few ns, allowing for efficient luminescence and optical gain.Moreover, our data allowed us to clarify the role of the volume and of the shape of the outer component and the effect of the interface in the heterostructure [2-4]. We found that dual emission is possible for both thick and thin quantum-confined shells, and for different interfaces. We studied the decoupling of excitons lying in the two different component of the NC (core exciton and shell exciton) and we revealed the evolution of the exciton barrier known as dynamic hole-blockade effect. We showed that these phenomena are strictly connected to dual emission and optical gain and we identified the condition for their maximum efficiency, in term of band alignment and band transitions. Our results provide a comprehensive understanding of the physical phenomena governing dual-emission mechanisms, suppression of Auger recombination, optical gain and laser emission in heterostructured NCs.In conclusion, the study of the exciton dynamics in different, but similar, heterostructured NCs allowed us to elucidate the relation between structural-morphological characteristics (shape, volume, and interface) and unconventional emission capabilities (dual emission and optical gain). This knowledge is very important to control NC functionalities in applications such as lasers [7], photoelectrochemical (PEC) cells [8], white light emission [9], ratiometric sensing [10].[1] P. V. Kamat and G. D. Scholes, J. Phys. Chem. Lett. 7, 584 , (2016)[2] G. Sirigu et al., Phys. Rev. B 96, 155303 (2017)[3] V. Pinchetti et al., ACS Nano 10, 6877-6887 (2016)[4] H. Zhao et al., Nanoscale 8, 4217-4226 (2016)[5] M. Zavelani-Rossi et al., Nano Lett. 10, 3142-3150 (2010)[6] R. Krahne et al., Appl. Phys. Lett. 98, 063105 (2011)[7] M. Zavelani-Rossi et al., Laser & Photonics Reviews 6, n. 5, 678-683, (2012)[8] L. Jin et al., Nano Energy 30, 531-541 (2016)[9] S. Sapra et al., Adv. Mater. 19, 569 (2007)[10] J. Liu et al., ACS Photonics, 2479 (2019)