Dot-in-rod Research Articles

Temperature-activated polarization of single photon emitters

Controlled activation of the polarization of single photon emitters is a challenge. We use CdSe/CdS dot-in-rods (DRs) confined and oriented in smectic topological grain boundaries to activate the fluorescence polarization through temperature variation. We show that temperature acts as a knob to switch on/off the polarization of DR emitted light between smectic and isotropic phase. This occurs through the orientational motion of the DR assemblies, which is induced in isotropic phase due to the disappearing of the defects. In addition, we evidence a significant improvement of DR emitted light polarization after cooling back from the isotropic phase. It is measured by the increase in polarization degree from 0.28 to 0.36 on average for DR assemblies. This improvement is managed by the smectic phase diagram near the smectic/nematic transition. Near the smectic/nematic transition, the smectic grain boundaries entirely cover the sample surface and allow for a reorientation of all DRs, even of those initially not confined in smectic grain boundaries.

Linearly polarized emission from CdSe/CdS core-in-rod nanostructures: Effects of core position.

Semiconductor nanocrystals with an anisotropic morphology exhibit unique properties, most notably their linear polarization. The colloidal growth of semiconductor nanorods with core dots inside, also referred to as dot-in-rod (DIR) structure, has enabled the synthesis of anisotropic nanocrystals with better stability and controllable fluorescence polarization. In this study, we synthesize CdSe/CdS DIR nanocrystals, in which the position of the CdSe core particle can be controlled by using different ligand compositions during the CdS growth. Varying the core position within the DIR structure, e.g., from the center to the end of the DIR particles, results in a change in the degree of linear polarization. When the core is positioned at the center of the nanorod, the linear polarization turns out to be higher compared with tip-core DIRs. Time-resolved photoluminescence analysis reveals that the center-core DIRs have higher electron-hole interaction than tip-core DIRs because of weak uniaxial strain in center-core DIR that arises from lattice dislocations at the interface to relieve accumulated strain.

Enantioselective theranostics of brain glioma using chiral quantum structures

CdSe/CdS dot-in-rods (DRs) tailored by chiral biomolecules are promising candidates for biological theranostics. The authors of this study synthesised chiral CdSe/CdS DRs with different sizes using the post-ligand-exchange method. Their high-resolution imaging and cytotoxicity performances were investigated, respectively. The results suggested that the chiral cysteine coverage strategy could significantly enhance the biocompatibility of CdSe/CdS DRs in bioimaging and bioimaging-guided therapies. Numerous photodynamic and chemodynamic therapy studies for brain glioma cells demonstrated enantioselective efficacy, wherein chiral DRs could exhibit improved oxygen species and hydroxyl radical generations under corresponding circularly polarised light radiation. d-DRs with right-handed circularly polarised light activation could generally present 3.14/1.5 times higher efficacy than left-handed or linearly polarised light irradiated samples. Wound-healing assay/migration and invasion assays asserted that the applications of chiral DRs could suppress the infiltrative growth and metastasis of glioma cells by reducing the migration and invasion ability by nearly 50%. In vivo experiments also confirmed that d-DRs exerted more substantial enhanced permeability and retention (EPR) effects in the tumour tissue, leading to better bioimaging ability and efficient tumour ablation. Such adjuvant approaches would be a viable alternative to cancer treatments with a high possibility of recurrence and poor prognosis.

Rational Design of Dot-on-Rod Nano-Heterostructure for Photocatalytic CO2 Reduction: Pivotal Role of Hole Transfer and Utilization.

Inspired by green plants, artificial photosynthesis has become one of the most attractive approaches toward carbon dioxide (CO2 ) valorization. Semiconductor quantum dots (QDs) or dot-in-rod (DIR) nano-heterostructures have gained substantial research interest in multielectron photoredox reactions. However, fast electron-hole recombination or sluggish hole transfer and utilization remains unsatisfactory for their potential applications. Here, the first application of a well-designed ZnSe/CdS dot-on-rods (DORs) nano-heterostructure for efficient and selective CO2 photoreduction with H2 O as an electron donor is presented. In-depth spectroscopic studies reveal that surface-anchored ZnSe QDs not only assist ultrafast (≈2ps) electron and hole separation, but also promote interfacial hole transfer participating in oxidative half-reactions. Surface photovoltage (SPV) spectroscopy provides a direct image of spatially separated electrons in CdS and holes in ZnSe. Therefore, ZnSe/CdS DORs photocatalyze CO2 to CO with a rate of ≈11.3µmol g-1 h-1 and ≥85% selectivity, much higher than that of ZnSe/CdS DIRs or pristine CdS nanorods under identical conditions. Obviously, favored energy-level alignment and unique morphology balance the utilization of electrons and holes in this nano-heterostructure, thus enhancing the performance of artificial photosynthetic solar-to-chemical conversion.

Hole Scavenging and Electron-Hole Pair Photoproduction Rate: Two Mandatory Key Factors to Control Single-Tip Au-CdSe/CdS Nanoheterodimers.

Metal/semiconductor hetero-nanostructures are now considered as benchmark functional nanomaterials for many light-driven applications. Using laser-driven photodeposition to control growth of gold nanodots (NDs) onto CdSe/CdS dot-in-rods (DRs), we show that the addition of a dedicated hole scavenger (MeOH) is the cornerstone to significantly reduce to less than 3.5% the multiple-site nucleation and 2.5% the rate of gold-free DRs. This means, from a synthetic point of view, that rates up to 90% of single-tip DRs can be reproducibly achieved. Moreover, by systematically varying this hole scavenger concentration and the Au/DRs ratio on the one hand, and the irradiation intensity and the time exposure on the other hand, we explain how gold deposition switches from multisite to single-tipped and how the growth and final size of the single photodeposited ND can be controlled. A model also establishes that the results obtained based on these different varying conditions can be merged onto a single "master behavior" that summarizes and predicts the single-tip gold ND growth onto the CdSe/CdS DRs. We eventually use data from the literature on growth of platinum NDs onto CdS nanorods by laser-deposition to extend our investigation to another metal of major interest and strengthen our modeling of single metallic ND growth onto II-VI semiconducting nanoparticles. This demonstrated strategy can raise a common methodology in the synthesis of single-tip semiconductor-metal hybrid nanoheterodimers (NHDs), leading to advanced nanoparticles architectures for applications in areas as different as photocatalysis, hydrogen production, photovoltaics, and light detection.

Selective Excitation of CdSe/CdS Dot‐in‐Rod Nanocrystals and Distinct Roles of Electrons and Holes in Photophysical and Photochemical Interactions with Ambient Air

CdSe/CdS dot‐in‐rod (DiR) nanocrystals are an advantageous heterostructured semiconductor system with a quasi‐type‐II electronic band structure, which enables a rich diversity of optical manipulation of carrier excitations and interactions. Herein, the selective excitation of respective CdSe quantum dots (QDs) and CdS nanorods of the CdSe/CdS DiR nanostructure is reported, which results in the distinct presence of different types of carriers at the nanorod surface. The presence of only electrons at the surface upon CdSe QD excitation leads to photophysical interactions with ambient air, which is manifested as reversible photoluminescence (PL) of the CdSe QDs. The excitation of CdS nanorods produces holes at the surface, which induces photooxidation of the CdS surface and irreversible change in PL intensity and spectral lineshape. Benefitting from unambiguously relating carrier types to photophysical and photochemical interactions, this research also verifies that trap states at the CdS surface do not act as nonradiative recombination centers, which can be removed by photochemical reactions. Herein, distinct roles of electrons and holes in the photophysical and photochemical interactions with ambient air are identified, and a more precise understanding of the interacting mechanisms is achieved.

Coulomb Barrier for Sequential Two-Electron Transfer in a Nanoengineered Photocatalyst.

Multielectron photocatalysis requires sequential, multiple charge transfer from the light absorber to the catalytic site. As a result, many-body effects induced by charge accumulation play a fundamental role in these reactions, especially when photocatalysts are miniaturized to the nanoscale. Here, we study sequential two-electron transfer in a state-of-the-art nanophotocatalyst, CdSe@CdS dot-in-rod (DIR) decorated with Pt tips, using pump-pump-probe transient absorption spectroscopy. Following the first electron transfer (ET) from DIR to the Pt tip, the second ET needs to not only compete with Auger recombination of a positively charged exciton but also experience a large Coulomb barrier exerted by two holes. As a result, both the ET rate and efficiency decrease by an order of magnitude. Analysis using a dissociation-limited long-range charge transfer model reveals that the Coulomb barrier of the second ET is ∼60 meV higher than that of the first one. This study not only uncovers the mechanism and efficiency bottleneck of a real multielectron photocatalyst but also provides general guidelines for the design of multielectron photocatalytic systems.

Investigating the Kinetic Competency of CrHydA1 [FeFe] Hydrogenase Intermediate States via Time-Resolved Infrared Spectroscopy.

Hydrogenases are metalloenzymes that catalyze the reversible oxidation of H2. The [FeFe] hydrogenases are generally biased toward proton reduction and have high activities. Several different catalytic mechanisms have been proposed for the [FeFe] enzymes based on the identification of intermediate states in equilibrium and steady state experiments. Here, we examine the kinetic competency of these intermediate states in the [FeFe] hydrogenase from Chlamydomonas reinhardtii (CrHydA1), using a laser-induced potential jump and time-resolved IR (TRIR) spectroscopy. A CdSe/CdS dot-in-rod (DIR) nanocrystalline semiconductor is employed as the photosensitizer and a redox mediator efficiently transfers electrons to the enzyme. A pulsed laser induces a potential jump, and TRIR spectroscopy is used to follow the population flux through each intermediate state. The results clearly establish the kinetic competency of all intermediate populations examined: Hox, Hred, HredH+, HsredH+, and Hhyd. Additionally, a new short-lived intermediate species with a CO peak at 1896 cm-1 was identified. These results establish a kinetics framework for understanding the catalytic mechanism of [FeFe] hydrogenases.

Influence of surface charges on the emission polarization properties of single CdSe/CdS dot-in-rods

We report an experimental investigation of the influence of surface charges on the emission polarization properties of single CdSe/CdS dot-in-rods (DRs), which is important for their polarization-based practical applications. By covering the single DRs with N-type semiconductor indium tin oxide (ITO) nanoparticles, the surface of single DRs is charged by ITO through interfacial electron transfer. This is confirmed by the experimental observations of the reduced photoluminescence intensities and lifetimes as well as the suppressing blinking. It is found that the full width at half maximum of histogram of polarization degrees of the single DRs is broadened from 0.24 (on glass) to 0.41 (in ITO). In order to explain the exprimental results, the band-edge exciton fine structure of single DRs is calculated by taking into account the sample parameters, the emission polarization, and the surface charges. The calculation results show that the level ordering of the emitting states determines the polarization degrees tending to increase or decrease under the influence of surface electrons. The surface electrons can induce an increase in the spacing between the emitting levels to change the populations and thus change the polarization degrees. In addition, different numbers of surface electrons may randomly distribute on the long CdSe/CdS rods, leading to the heterogeneous influences on the single DRs causing the broadening of polarization degrees also.

Giant-Shell CdSe/CdS Nanocrystals: Exciton Coupling to Shell Phonons Investigated by Resonant Raman Spectroscopy.

The interaction between excitons and phonons in semiconductor nanocrystals plays a crucial role in the exciton energy spectrum and dynamics, and thus in their optical properties. We investigate the exciton-phonon coupling in giant-shell CdSe/CdS core-shell nanocrystals via resonant Raman spectroscopy. The Huang-Rhys parameter is evaluated by the intensity ratio of the longitudinal-optical (LO) phonon of CdS with its first multiscattering (2LO) replica. We used four different excitation wavelengths in the range from the onset of the CdS shell absorption to well above the CdS shell band edge to get insight into resonance effects of the CdS LO phonon with high-energy excitonic transitions. The isotropic spherical giant-shell nanocrystals show consistently stronger exciton-phonon coupling as compared to the anisotropic rod-shaped dot-in-rod (DiR) architecture, and the 2LO/LO intensity ratio decreases for excitation wavelengths approaching the CdS band edge. The strong exciton-phonon coupling in the spherical giant-shell nanocrystals can be related to the delocalization of the electronic wave functions. Furthermore, we observe the radial breathing modes of the GS nanocrystals and their overtones by ultralow frequency Raman spectroscopy with nonresonant excitation, using laser energies well below the band gap of the heteronanocrystals, and highlight the differences between higher-order optical and acoustic phonon modes.

Carrier-doping as a tool to probe the electronic structure and multi-carrier recombination dynamics in heterostructured colloidal nanocrystals.

Heterostructured colloidal nanocrystals, such as core/shells and dot-in-rods, enable new spectral and dynamic properties otherwise unachievable with single-component nanocrystals or quantum dots (QDs). For example, the electron and hole wavefunctions can be engineered such that they are either both confined in the same domain or (partially) separated over different domains in the heterostructures, which are the so-called type I or (quasi-) type II localization regimes, respectively. A critical factor dictating the carrier localization regime is the band alignment or electronic structure of the heterostructure, which, however, is difficult to measure and hence is often ambiguous. In this work, using CdSe@CdS dot-in-rods (DIRs) as a model system, we show that band edge carrier-doping is a simple-yet-powerful tool to probe the electronic structure of heterostructures. By doping an electron into the CdSe core and then observing whether the doped electron bleaches band edge absorption of only the core or those of both the core and shell, we can easily differentiate the type I and quasi-type II structures. A systematic study of DIRs with various dimensions shows that the extent of electron wavefunction delocalization can be tuned by the core sizes and rod diameters. Comparison with the electronic structure determined from transient absorption measurements also reveals the important role of electron-hole binding in affecting the delocalization of electron wavefunction. In addition to probing the electronic structure, the doped electron allows for studying multi-carrier recombination dynamics in these heterostructures which plays a vital role in their many optical and optoelectronic applications. Specifically, by comparing the band edge exciton recombination kinetics of the doped and neutral DIRs, we can extract the negative trion lifetime, which can be further used to derive the positive trion lifetime when combined with biexciton lifetime measurements. These lifetimes also depend sensitively on the core sizes and rod diameters of the DIRs.

Balancing electron transfer rate and driving force for efficient photocatalytic hydrogen production in CdSe/CdS nanorod–[NiFe] hydrogenase assemblies

We describe a hybrid photocatalytic system for hydrogen production consisting of nanocrystalline CdSe/CdS dot-in-rod (DIR) structures coupled to [NiFe] soluble hydrogenase I (SHI) fromPyrococcus furiosus.

Photocatalytic Hydrogen Generation by CdSe/CdS Nanoparticles.

The photocatalytic hydrogen (H2) production activity of various CdSe semiconductor nanoparticles was compared including CdSe and CdSe/CdS quantum dots (QDs), CdSe quantum rods (QRs), and CdSe/CdS dot-in-rods (DIRs). With equivalent photons absorbed, the H2 generation activity orders as CdSe QDs ≫ CdSe QRs > CdSe/CdS QDs > CdSe/CdS DIRs, which is surprisingly the opposite of the electron-hole separation efficiency. Calculations of photoexcited surface charge densities are positively correlated with the H2 production rate and suggest the size of the nanoparticle plays a critical role in determining the relative efficiency of H2 production.

34-4: Simultaneous Scanning Transmission Electron Microscopy, Cathodoluminescence Imaging and EELS of Quantum Dot in Rods

We are grateful to the EPSRC and Technology Strategy Board (TSB) for funding the PURPOSE (TP11/MFE/6/1/AA129F; EP-SRC TS/G000271/1) and CONVERTED (JeS no. TS/1003053/1), PRISM (EP/N508974/1), HTRaD (EP/L504671/1) and FAB3D programs. We are also grateful to the TSB for funding the CONVERT program.

Paper No S10.2: Cathodoluminescence Imaging and EELS of Quantum Dot in Rods Excited in a Field Emission Transmission Electron Microscope

AbstractRed light filtered cathodoluminescent (CL) imaging and chemical spectroscopy are combined with STEM imaging to analyze CdSe/CdS core/shell quantum dot in rods. Shape and uniformity is shown in conjunction with the location of the selenide containing cores.

Exciton spin dynamics and photoluminescence polarization of CdSe/CdS dot-in-rod nanocrystals in high magnetic fields

The exciton spin dynamics and polarization properties of the related emission are investigated in colloidal CdSe/CdS dot-in-rod (DiR) and spherical core/shell nanocrystal (NC) ensembles by magneto-optical photoluminescence (PL) spectroscopy in magnetic fields up to 15 T. It is shown that the degree of circular polarization (DCP) of the exciton emission induced by the magnetic field is affected by the NC geometry as well as the exciton fine structure and can provide information on nanorod orientation. A theory to describe the circular and linear polarization properties of the NC emission in magnetic field is developed. It takes into account phonon mediated coupling between the exciton fine structure states as well as the dielectric enhancement effect resulting from the anisotropic shell of DiR NCs. This theoretical approach is used to model the experimental results and allows us to explain most of the measured features. The spin dynamics of the dark excitons is investigated in magnetic fields by time-resolved photoluminescence. The results highlight the importance of confined acoustic phonons in the spin relaxation of dark excitons. The bare core surface as well as the core/shell interface give rise to an efficient spin relaxation channel, while the surface of core/shell NCs seems to play only a minor role.

Photon correlations for colloidal nanocrystals and their clusters

Images of semiconductor "dot-in-rods" and their small clusters are studied by measuring the second-order correlation function with a spatially resolving intensified CCD camera. This measurement allows one to distinguish between a single dot and a cluster and, to a certain extent, to estimate the number of dots in a cluster. A more advanced measurement is proposed, based on higher-order correlations, enabling more accurate determination of the number of dots in a small cluster. Nonclassical features of the light emitted by such a cluster are analyzed.

Production and biofunctionalization of elongated semiconducting nanocrystals for ex-vivo applications

ABSTRACTHereby, we present a synthetic route for the production of wurtzite (WZ) CdSe nanocrystals (NCs), which are essential for further shell growing reaction (e.g. CdSe/CdS dot-in-rod (DRs) nanoheterostructures). Our continuous flow reactor set-up consists of a separate nucleation chamber and growth oven. Both components can be heated up to temperatures above 350 °C to guarantee WZ crystal structure.Furthermore, we introduce DRs as the next powerful tool concerning biological imaging and assay detection. Using DRs in cell imaging results in an increased sensitivity due to the higher brightness compared to spherical core/shell/shell (CSS) nanocrystals.

Beyond Band Alignment: Hole Localization Driven Formation of Three Spatially Separated Long-Lived Exciton States in CdSe/CdS Nanorods

Colloidal one-dimensional semiconductor nanoheterostructures have emerged as an important family of functional materials for solar energy conversion, although the nature of the long-lived exciton state and their formation and dissociation dynamics remain poorly understood. In this paper we study these dynamics in CdSe/CdS dot-in-rod (DIR) NRs, a representative of 1D heterostructures, and DIR-electron-acceptor complexes by transient absorption spectroscopy. Because of a quasi-type II band alignment of CdSe and CdS, it is often assumed that there exists one long-lived exciton state with holes localized in the CdSe seed and electrons delocalized among CdSe and CdS. We show that excitation into the CdS rod forms three distinct types of long-lived excitons that are spatially localized in the CdS rod, in and near the CdSe seed and in the CdS shell surrounding the seed. The branching ratio of forming these exciton states is controlled by the competition between the band offset driven hole localization to the CdSe seed and hole trapping to the CdS surface. Because of dielectric contrast induced strong electron-hole interaction in 1D materials, the competing hole localization pathways lead to spatially separated long-lived excitons. Their distinct spatial locations affect their dissociation rates in the presence of electron acceptors, which has important implications for the application of 1D heterostructures as light-harvesting materials.

Recent advances on single photon sources based on single colloidal nanocrystals

AbstractSingle colloidal quantum dots (QDs) are increasingly exploited as triggered sources of single photons. This review reports on recent results on single photon sources (SPS) based on colloidal quantum dots, whose size, shape and optical properties can be finely tuned by wet chemistry approach. First, we address the optical properties of different colloidal nanocrystals, such as dots, rods and dot in rods and their use as single photon sources will be discussed. Then, we describe different techniques for isolation and positioning single QDs, a major issue for fabrication of single photon sources, and various approaches for the embedding single nanocrystals inside microcavities. The insertion of single colloidal QDs in quantum confined optical systems allows one to improve their overall optical properties and performances in terms of efficiency, directionality, life time, and polarization control. Finally, electrical pumping of colloidal nanocrystals light emitting devices and of NC-based single photon sources is reviewed.