Heterostructured Nanocrystals Research Articles

(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)

Modulation of the magnetic properties of gold-spinel ferrite heterostructured nanocrystals

The rational design of complex nanostructures is of paramount importance to gain control over their chemical and physical properties. Recently, magnetic-plasmonic heterostructured nanocrystals have been recognized as key players in nanomedicine as multifunctional therapeutic-diagnostic tools and in catalysis. Here we show how the properties of gold-iron oxide heterostructured nanocrystals can be tuned by chemical doping of the magnetic subunit. The divalent cations in the iron oxide were substituted with cobalt and manganese to obtain a general formula Au-MFe2O4 (M = Fe, Co, Mn). Magnetic properties of the heterostructures could be tuned, while maintaining well-defined plasmon resonance signatures, confirming the dual magnetic-plasmonic functional capability of these nanostructures.

Screwdriver-like Pd-Ag heterostructures formed via selective deposition of Ag on Pd nanowires as efficient photocatalysts for solvent-free aerobic oxidation of toluene

Heterostructured bimetal nanocrystals with a component having localized surface plasmon resonance (LSPR) property are promising photocatalysts for a series of reactions. In this work, kinetic products of Pd-Ag with a screwdriver-like heterostructure have been successfully fabricated via the selective epitaxial growth of Ag on Pd nanowires (NWs). It was confirmed that the deposition rate (Vdeposition) of Ag is much more sensitive to the temperature, compared to the surface diffusion rate (Vdiffusion) which can be effectively reduced by the binding of poly(vinylpyrrolidone) (PVP) molecules. Then the magnitude of Vdeposition/Vdiffusion has been well tailored for the formation of a kinetic growth environment. The interactions between the components of the as-prepared Pd-Ag heterostructures resulted in intensified LSPR effects. As a result, they gained better photocatalytic performance toward solvent free aerobic oxidation of toluene than Pd NWs, Ag NWs and the mixture of them. Additionally, the Pd-Ag heterostructured nanocrystals exhibited excellent catalytic stability for recycling. This work not only presents an idea for realizing kinetic growth but also supports that LSPR effect is a good tool for improving the photocatalytic activity.

Manipulation of Precursor Reactivity for the Facile Synthesis of Heterostructured and Hollow Metal Selenide Nanocrystals

We present a one-pot synthesis of nanocrystal heterostructures containing metal selenide cores shelled with tungsten-based metal selenides. This synthesis is enabled by the use of oleic acid as a W...

Electrochemical reaction mechanism of amorphous iron selenite with ultrahigh rate and excellent cyclic stability performance as new anode material for lithium-ion batteries

Metal selenite materials have unique advantages from forming metal oxide and selenide heterostructure nanocrystals, which assist in accelerating electron and lithium-ion transportation and providing more active sites via interfacial coupling, during the first cycle. In this study, synthesis of amorphous iron selenite materials derived via oxidation at a low temperature of 250 °C of crystalline iron selenide was firstly researched in detail, and their composite (FeSeO–C–CNT) with carbon materials was applied as an anode material for lithium-ion batteries. The reversible reaction mechanism of iron selenite with Li ions is described by the reaction: Fe2O3 + FeSe2 + xSeO2 + (1 − x)Se + (4x + 12)Li+ + (4x + 12)e− ↔ 3Fe + (2x + 3)Li2O + 3Li2Se. FeSeO–C–CNT composite electrode showed high reversible capacities of 617 mA h g−1 for the 1800th cycle even at an extremely high current density of 30 A g−1, which surprisingly indicated that FeSeO–C–CNT is enabled to fully charge in a very short time of 72 s. This study demonstrated that amorphous iron selenite materials could be excellent candidates for new anode compositions with high capacities and fast electrochemical kinetics properties.

Correction: Highly stable hetero-structured green-emitting cesium lead bromide nanocrystals via ligand-mediated phase control.

Correction for 'Highly stable hetero-structured green-emitting cesium lead bromide nanocrystals via ligand-mediated phase control' by G. Krishnamurthy Grandhi et al., Nanoscale, 2019, 11, 21137-21146.

Seed-mediated growth of heterostructured Cu1.94S-MS (M = Zn, Cd, Mn) and alloyed CuNS2 (N = In, Ga) nanocrystals for use in structure- and composition-dependent photocatalytic hydrogen evolution.

Multinary copper-based chalcogenide nanocrystals (NCs) as light-driven photocatalysts have attracted extensive research interest due to their great potential for generating sustainable energy without causing environmental concerns. However, systematic studies on the growth mechanism and related photocatalytic activities involving different valent metal ions (either M2+ or N3+) as foreign cations and monoclinic Cu1.94S NCs as the 'parent lattice' have rarely been carried out. In this work, we report an effective seed-mediated method for the synthesis of heterostructured Cu1.94S-MS NCs (M = Zn, Cd and Mn) and alloyed CuNS2 NCs (N = In and Ga). A typical cation exchange process took place prior to the growth of heterostructured NCs, while further inter-cation diffusion occurred only for the alloyed NCs. When compared with Cu1.94S NCs, all the heterostructured Cu1.94S-MS NCs and CuGaS2 NCs showed enhanced photocatalytic activities toward hydrogen production by water splitting, owing to their tailored optical band gaps and energy level alignments. Although optically favored, CuInS2 ANCs were not comparable to others due to their low conduction band minimum for the reduction of H2O to H2.

Investigation of Binary Metal (Ni, Co) Selenite as Li-Ion Battery Anode Materials and Their Conversion Reaction Mechanism with Li Ions.

Highly efficient anode materials with novel compositions for Li-ion batteries are actively being researched. Multicomponent metal selenite is a promising candidate, capable of improving their electrochemical performance through the formation of metal oxide and selenide heterostructure nanocrystals during the first cycle. Here, the binary nickel-cobalt selenite derived from Ni-Co Prussian blue analogs (PBA) is chosen as the first target material: the Ni-Co PBA are selenized and partially oxidized in sequence, yielding (NiCo)SeO3 phase with a small amount of metal selenate. The conversion mechanism of (NiCo)SeO3 for Li-ion storage is studied by cyclic voltammetry, in situ X-ray diffraction, ex situ X-ray photoelectron spectroscopy, in situ electrochemical impedance spectroscopy, and ex situ transmission electron microscopy. The reversible reaction mechanism of (NiCo)SeO3 with the Li ions is described by the reaction: NiO + CoO + xSeO2 + (1 - x)Se + (4x + 6)Li+ + (4x + 6)e- ↔ Ni + Co + (2x + 2)Li2 O + Li2 Se. To enhance electrochemical properties, polydopamine-derived carbon is uniformly coated on (NiCo)SeO3 , resulting in excellent cycling and rate performances for Li-ion storage. The discharge capacity of C-coated (NiCo)SeO3 is 680 mAh g-1 for the 1500th cycle when cycled at a current density of 5 A g-1 .

Light-Activated Sub-ppm NO2 Detection by Hybrid ZnO/QD Nanomaterials vs. Charge Localization in Core-Shell QD

New hybrid materials – photosensitized nanocomposites containing nanocrystal heterostructures with spatial charge separation, show high response for practically important sub-ppm level NO2 detection at room temperature. Nanocomposites ZnO/CdSe, ZnO/(CdS@CdSe), ZnO/(ZnSe@CdS) were obtained by the immobilization of nanocrystals – colloidal quantum dots (QDs), on the matrix of nanocrystalline ZnO. The formation of crystalline core-shell structure of QDs was confirmed by HAADF-STEM coupled with EELS mapping. Optical properties of photosensitizers have been investigated by optical absorption and luminescence spectroscopy combined with spectral dependences of photoconductivity, which proved different charge localization regimes. Photoelectrical and gas sensor properties of nanocomposites have been studied at room temperature under green light (λmax = 535 nm) illumination in the presence of 0.12 – 2 ppm NO2 in air. It has been demonstrated that sensitization with type II heterostructure ZnSe@CdS with staggered gap provides the rapid growth of effective photoresponse with the increase in the NO2 concentration in air and the highest sensor sensitivity toward NO2. We believe that the use of core-shell QDs with spatial charge separation opens new possibilities in the development of light-activated gas sensors working without thermal heating.

Eco-friendly synthesis of glutathione-capped CdTe/CdSe/ZnSe core/double shell quantum dots: their cytotoxicity and genotoxicity effects on Chinese hamster ovary cells

In this work, we report green one-pot synthesis, cytotoxicity and genotoxicity of glutathione-capped CdTe/CdSe/ZnSe heterostructured quantum dots (QDs) using a label-free xCELLigence RTCA system as well as the Cytokinesis Blocked Micronucleus assay. The as-synthesised nanocrystals displayed good optical properties and were spherical in shape with an average particle diameter of 5.9 ± 1.13 nm. The intracellular uptake study showed that most of the as-synthesised glutathione stabilized QDs penetrated the cell membranes and were found randomly localized in the cytoplasm of Chinese Hamster Ovary (CHO) cells even at a lower concentration of 0.5 μg ml-1. The QDs showed no cytotoxicity to Chinese Hamster Ovary (CHO) cells at six concentrations tested (0.5, 1.0, 2.5, 5.0, 10, and 25 μg ml-1). However, at 50 and 100 μg ml-1 the material was cytotoxic at significant p values of 3.1 × 10-4 and 9.47 × 10-10, respectively. Likewise, the material was found to be genotoxic at almost all concentrations tested. The genotoxicity of the nanocrystals in question confers unfavorable potential to all complex heterostructured nanocrystals. Hence, more studies are needed to negate the prevailing assumption that multishell passivation provides enough protection against intracellular QD core dissolution or the production of reactive oxygen species (ROS) before these nanomaterials can be used in vivo for human health applications.

Design Rules for One-Step Seeded Growth of Nanocrystals: Threading the Needle between Secondary Nucleation and Ripening

The heterogeneous growth of inorganic shells on seed nanocrystals is used to synthesize heterostructured nanocrystals such as core@shell quantum dots for applications ranging from biological imagin...

(Invited) Novel Plasmonic Nanomaterials for Near Infrared Light Energy Conversion

The structure of nanomaterial determines their individual properties and assembled structures they can form [1,2].Highly efficient photoenergy conversion in heterostructured nanocrystals (HNCs) requires the formation of epitaxial heterointerfaces and band alignment engineering.One of the phases in HNCs should hold the light absorbing ability, such as narrow band gap semiconductor [3-6] and plasmonic material [7-9].Special attention has been given to the HNCs with the combination of plasmonic semiconductor/semiconductor [7-9], where the near infrared plasmon-induced carrier transfer takes place to realize the long-lived charge separation and near infrared light energy conversion into electricity [8] and chemical energy [7,9]. [1] Wu, H.-L.; Teranishi, T. et al. Science 2016, 351, 1306. [2] Kanehara, M.; Arakawa, H.; Honda, T.; Saruyama, M.; Teranishi, T. Chem. Eur. J. 2012, 18, 9230. [3] Saruyama, M.; So, Y.-G.; Kimoto, K.; Taguchi, S.; Kanemitsu, Y.; Teranishi, T. J. Am. Chem. Soc. 2011, 133, 17598. [4] Sakamoto, M.; Teranishi, T. et al. Chem. Sci. 2014, 5, 3831. [5] Teranishi, T.; Samamoto, M. J. Phys. Chem. Lett. 2013, 4, 2867. (Perspective) [6] Sakamoto, M.; Teranishi, T. et al. Nanoscale 2016, 8, 9517. [7] Lian, Z.; Sakamoto, M.; Teranishi, T.et al. Nat. Commun. 2018, 9, 2314. [8] Sakamoto, M.; Teranishi, T.et al. Nat. Commun., accepted. [9] Lian, Z.; Sakamoto, M.; Teranishi, T. et al. J. Am. Chem. Soc., accepted.

Fabricating CsPbX3-Based Type I and Type II Heterostructures by Tuning the Halide Composition of Janus CsPbX3/ZrO2 Nanocrystals.

Fabricating CsPbX3-based heterostructures has proven to be a feasible way to tune their photophysical properties. Here, we report the successful fabrication of Janus CsPbX3/ZrO2 heterostructure nanocrystals (NCs), in which each CsPbX3 NC is partially covered by ZrO2. According to the band alignment, CsPbBr3/ZrO2 and CsPbI3/ZrO2 can be indexed as type I and type II composites, respectively. The type I composites display great enhancement in photoluminescence quantum yield (from 63 to 90%) and photoluminescence lifetime (from 12.9 to 66.1 ns) because of the charge carrier confinement and passivation effect provided by ZrO2. In contrast, the type II composites can be used in photocatalytic reduction of CO2 because electrons and holes are effectively separated and accumulated in ZrO2 and CsPbI3, respectively, under irradiation. Janus CsPbBr3/ZrO2 NCs showed a stability much higher than that of pristine CsPbBr3 against polar solvent treatment. A stable and highly efficient light-emitting device with luminous efficiency up to 55 lm W-1 is fabricated by using CsPbBr3/ZrO2 NCs as the green light source. This work may not only enrich the family of surface-passivated perovskite materials but also provide a good example for the rational design of specific composites in the metal halide perovskite field.

Strain-controlled shell morphology on quantum rods

Semiconductor heterostructure nanocrystals, especially with core/shell architectures, are important for numerous applications. Here we show that by decreasing the shell growth rate the morphology of ZnS shells on ZnSe quantum rods can be tuned from flat to islands-like, which decreases the interfacial strain energy. Further reduced growth speed, approaching the thermodynamic limit, leads to coherent shell growth forming unique helical-shell morphology. This reveals a template-free mechanism for induced chirality at the nanoscale. The helical morphology minimizes the sum of the strain and surface energy and maintains band gap emission due to its coherent core/shell interface without traps, unlike the other morphologies. Reaching the thermodynamic controlled growth regime for colloidal semiconductor core/shell nanocrystals thus offers morphologies with clear impact on their applicative potential.

Highly stable hetero-structured green-emitting cesium lead bromide nanocrystals via ligand-mediated phase control.

Green-emissive Cs4PbBr6 shows promise for light-emitting diode devices superior to that of CsPbBr3 NCs owing to their stability and high photoluminescence efficiency. Nevertheless, there is still no consensus regarding the basis of their green emission, which decelerates their advance in light-emitting applications. Herein, a systematic investigation on the concentration of capping ligands (oleylamine and oleic acid), which determines the predominant phase between CsPbBr3 and Cs4PbBr6 for a given Cs to Pb feed ratio, is conducted. This study deduces that oleylamine to oleic acid ratio plays a crucial role in obtaining either green-emissive or non-emissive Cs4PbBr6 NCs. Scrutiny of Cs4PbBr6 microscopic and optical data in addition to their emission quenching study with a hole-withdrawing molecule reveals that the green emission originates from the CsPbBr3 impurity phase. Furthermore, stable green emission is observed for CsPbBr3/Cs4PbBr6 nanocrystals when CsPbBr3 particles are well protected by the Cs4PbBr6 matrix. These CsPbBr3/Cs4PbBr6 films remained highly luminescent even after UV exposure for hours or annealing at ∼150 °C for days in addition to their long-term stability under an ambient atmosphere, which are the desirable properties for various practical applications.

Co-CoO/MnO Heterostructured Nanocrystals Anchored on N/P-Doped 3D Porous Graphene for High-Performance Pseudocapacitive Lithium Storage

Lithium ion batteries (LIBs) are extensively used in numerous applications due to their impressive energy density and low memory effect. However, the inherently diffusion-controlled lithium storage limits their high-power applications. Herein, a novel surface redox capacitive lithium storage which originates from the deep oxidation of oxides during cycling of a hybrid consisting of nitrogen/phosphorous co-doped 3D graphene networks and Co-CoO/MnO nanoparticles (NPGCM), was employed to enhance the rate performance of anode for Li-ion batteries. The combination of surface pseudocapacitance with the diffusion-related lithium storage led to an extraordinary rate capability (1170.7 mAh g−1 at 0.2 A g−1 and 258.3 mAh g−1 at 6 A g−1). Furthermore, the NPGCM hybrid exhibited excellent cyclic stability, showing a capacity of 397 mAh g−1 after 1300 cycles at 4 A g−1. Electron microscopy and spectroscopic investigations suggested the pseudocapacitance is originated from Mn3O4 generated during cycling.

Two-Dimensional Morphology Enhances Light-Driven H2 Generation Efficiency in CdS Nanoplatelet-Pt Heterostructures.

Light-driven H2 generation using semiconductor nanocrystal heterostructures has attracted intense recent interest because of the ability to rationally improve their performance by tailoring their size, composition, and morphology. In zero- and one-dimensional nanomaterials, the lifetime of the photoinduced charge-separated state is still too short for H2 evolution reaction, limiting the solar-to-H2 conversion efficiency. Here we report that using two-dimensional (2D) CdS nanoplatelet (NPL)-Pt heterostructures, H2 generation internal quantum efficiency (IQE) can exceed 40% at pH 8.8-13 and approach unity at pH 14.7. The near unity IQE at pH 14.7 is similar to those reported for 1D nanorods and can be attributed to the irreversible hole removal by OH-. At pH < 13, the IQE of 2D NPL-Pt is significantly higher than those in 1D nanorods. Detailed time-resolved spectroscopic studies and modeling of the elementary charge separation and recombination processes show that, compared to 1D nanorods, 2D morphology extends charge-separated state lifetime and may play a dominant role in enhancing the H2 generation efficiency. This work provides a new approach for designing nanostructures for efficient light-driven H2 generation.

Near infrared light induced plasmonic hot hole transfer at a nano-heterointerface

Localized surface plasmon resonance (LSPR)-induced hot-carrier transfer is a key mechanism for achieving artificial photosynthesis using the whole solar spectrum, even including the infrared (IR) region. In contrast to the explosive development of photocatalysts based on the plasmon-induced hot electron transfer, the hole transfer system is still quite immature regardless of its importance, because the mechanism of plasmon-induced hole transfer has remained unclear. Herein, we elucidate LSPR-induced hot hole transfer in CdS/CuS heterostructured nanocrystals (HNCs) using time-resolved IR (TR-IR) spectroscopy. TR-IR spectroscopy enables the direct observation of carrier in a LSPR-excited CdS/CuS HNC. The spectroscopic results provide insight into the novel hole transfer mechanism, named plasmon-induced transit carrier transfer (PITCT), with high quantum yields (19%) and long-lived charge separations (9.2 μs). As an ultrafast charge recombination is a major drawback of all plasmonic energy conversion systems, we anticipate that PITCT will break the limit of conventional plasmon-induced energy conversion.

Controlled Synthesis, Formation Mechanism, and Applications of Colloidal Ag8SnS6 Nanoparticles and Ag8SnS6/Ag2S Heterostructured Nanocrystals

Semiconductor nanocrystals (SC NCs) have attracted scientists’ attention because of their size- and shape-dependent optical properties. Now, many scientists’ research is focused on the synthesis of the earth-abundant and environmentally compatible SC NCs and their applications for solar energy conversion. Herein, colloidal silver-based I–IV–VI SC NCs, Ag8SnS6 nanoparticles (NPs) were synthesized. Ag8SnS6 NPs showed a strong quantum size confinement effect from optical measurement. The photocurrent response measurement showed a typical n-type property for Ag8SnS6 NPs. It is very interesting that colloidal Ag8SnS6/Ag2S heterostructured nanocrystals (HSNCs), containing drop-shaped and little bowtie-shaped HSNCs, can also be synthesized via changing the adding process of precursors in the beginning of the experiment. The first formation of Ag NPs is very important to form Ag8SnS6/Ag2S HSNCs. A seeded growth mechanism was proposed to explain the formation of Ag8SnS6/Ag2S HSNCs. The polycrystalline structure in...

Enhanced Photocatalytic Activity of Zn–Ag–In–S Semiconductor Nanocrystals with a Dumbbell-Shaped Heterostructure

Nanocrystals with a heterojunction of type-II band alignment have attracted much interest for the construction of solar energy conversion systems because photogenerated electrons and holes can be effectively separated at the heterojunction. Here, we report the preparation of heterostructured multinary nanocrystals composed of ZnS–AgInS2 solid solution (ZAIS), which have tunability of the electronic energy structure depending on their chemical composition. Ellipsoidal ZAIS nanocrystal domains with various compositions were epitaxially grown on both termini of preformed ZAIS nanorods as seeds. The resulting nanocrystals had a dumbbell shape composed of two ellipsoidal nanocrystals (ca. 4–6 nm in width × 7–11 nm in length) connected by a nanorod (ca. 4 nm in width × 16–23 nm in length). Because the Zn fraction in the chemical composition was larger in the rod part than in the two tip parts in the nanodumbbell, heterojunctions of type-II or quasi-type-II were formed at the interface between the rod and tip pa...