Femtosecond spectroscopy of carrier relaxation dynamics in type II CdSe/CdTe tetrapod heteronanostructures

Electronic Structure and Excited State Dynamics of Cadmium Chalcogenide Nanorods

The cylindrical quasi-one-dimensional shape of colloidal semiconductor nanorods (NRs) gives them unique electronic structure and optical properties. In addition to the band gap tunability common to nanocrystals, NRs have polarized light absorption and emission and high molar absorptivities. NR-shaped heterostructures feature control of electron and hole locations as well as light emission energy and efficiency. We comprehensively review the electronic structure and optical properties of Cd-chalcogenide NRs and NR heterostructures (e.g., CdSe/CdS dot-in-rods, CdSe/ZnS rod-in-rods), which have been widely investigated over the last two decades due in part to promising optoelectronic applications. We start by describing methods for synthesizing these colloidal NRs. We then detail the electronic structure of single-component and heterostructure NRs and follow with a discussion of light absorption and emission in these materials. Next, we describe the excited state dynamics of these NRs, including carrier cooling, carrier and exciton migration, radiative and nonradiative recombination, multiexciton generation and dynamics, and processes that involve trapped carriers. Finally, we describe charge transfer from photoexcited NRs and connect the dynamics of these processes with light-driven chemistry. We end with an outlook that highlights some of the outstanding questions about the excited state properties of Cd-chalcogenide NRs.

Optoelectronic properties and applications of two-dimensional layered semiconductor van der Waals heterostructures: perspective from theory

Van der Waals heterostructures (vdWHs) which combine two different materials together have attracted extensive research attentions due to the promising applications in optoelectronic and electronic devices, the investigations from theoretical simulations can not only predict the novel properties and the interfacial coupling, but also provide essential guidance for experimental verification and fabrications. This review summarizes the recent theoretical studies on electronic and optical properties of two-dimensional semiconducting vdWHs. The characteristics of different band alignments are discussed, together with the optoelectronic modulations from external fields and the promising applications in solar cells, tunneling field-effect transistors and photodetectors. At the end of the review, the further perspective and possible research problems of the vdWHs are also presented.

Insight into morphology dependent charge carrier dynamics in ZnSe-CdS nanoheterostructures

Semiconductor nanoheterostructures (NHSs) are being increasingly used for the photocatalytic conversion of solar energy in which photo-induced charge separation is an essential step and hence it is necessary to understand the effect of various factors such as size, shape, and composition on the charge transfer dynamics. Ultrafast transient absorption spectroscopy is used to investigate the nature and dynamics of photo-induced charge transfer processes in ZnSe-CdS NHSs of different morphologies such as nanospheres (NSs), nanorods (NRs), and nanoplates (NPs). It demonstrates the fast separation of charge carriers and localization of both charges in adjacent semiconductors, resulting in the formation of a charge-separated (CS) state. The lifetime of the charge-separated state follows the order of NSs < NPs < NRs, emphasizing the effect of morphology on the enhancement of photo-induced charge separation and suppression of backward recombination. The separated charge carriers have been utilized in visible light driven hydrogen production and the hydrogen generation activity follows the same order as that for the lifetime of the CS state, underlining the role of charge separation efficiency. Therefore, the variation of the morphology of NHSs plays a significant role in their charge carrier dynamics and hence the photocatalytic hydrogen production activity.

Structural rigidity accelerates quantum decoherence and extends carrier lifetime in porphyrin nanoballs: a time domain atomistic simulation

Nonradiative electron-hole (e-h) recombination is the primary source of energy loss in photovoltaic cells and inevitably, it competes with the charge transfer process, leading to poor device performance. Therefore, much attention has to be paid for delaying such processes; increasing the excitonic lifetime may be a solution for this. Using the real-time, density functional tight-binding theory (DFTB) combined with nonadiabatic molecular dynamics (NAMD) simulations, we demonstrate the exciton relaxation phenomena of different metal-centered porphyrin nanoballs, which are supposed to be very important for the light-harvesting process. It has been revealed that the carrier recombination rate gradually decreases with the increase in the molecular stiffness by introducing metal-coordinating templating agents into the nanoball. Our simulation demonstrates that the lower atomic fluctuations lead to poorer electron-phonon nonadiabatic coupling in association with weak phonon modes and these as a whole are responsible for shorter quantum coherence and hence delayed recombination events. Our analysis is in good agreement with the recent experimental observation. By replacing the Zn metal center with a heavier Cd atom, a similar trend is observed; however, the rate slows down abruptly. The present simulation study provides the fundamental mechanism in detail behind the undesired energy loss during exciton recombination and suggests a rational design of impressive nanosystems for future device fabrication.

Ternary Composite of Co-Doped CdSe@electrospun Carbon Nanofibers: A Novel Reusable Visible Light-Driven Photocatalyst with Enhanced Performance

In this work, flexible ternary composites of cobalt-doped cadmium selenide/electrospun carbon nanofibers (Co-CdSe@ECNFs) for photocatalytic applications were fabricated successfully via electrospinning, followed by carbonization. For the fabrication of the proposed photocatalysts, Co-CdSe nanoparticles were grown in situ on the surface of ECNFs during the carbonization of precursor electrospun nanofibers obtained by dispersing Se powder in the electrospinning solution of polyacrylonitrile/N,N-Dimethylformamide (PAN/DMF) containing Cd2+ and Co2+. The photocatalytic performance of synthesized samples is investigated in the photodegradation of methylene blue (MB) and rhodamine B (RhB) dyes. Experimental results revealed the superior photocatalytic efficiency of Co-CdSe@ECNFs over undoped samples (CdSe@ECNFs) due to the doping effect of cobalt, which is able to capture the photogenerated electrons to prevent electron–hole recombination, thereby improving photocatalytic performance. Moreover, ECNFs could play an important role in enhancing electron transfer and optical absorption of the photocatalyst. This type of fabrication strategy may be a new avenue for the synthesis of other ECNF-based ternary composites.

Semiconductor nanocrystals for small molecule activation via artificial photosynthesis

The protocol of artificial photosynthesis using semiconductor nanocrystals shines light on green, facile and low-cost small molecule activation to produce solar fuels and value-added chemicals.

Wet-Chemical Synthesis and Applications of Semiconductor Nanomaterial-Based Epitaxial Heterostructures

Semiconductor nanomaterial-based epitaxial heterostructures with precisely controlled compositions and morphologies are of great importance for various applications in optoelectronics, thermoelectrics, and catalysis. Until now, various kinds of epitaxial heterostructures have been constructed. In this minireview, we will first introduce the synthesis of semiconductor nanomaterial-based epitaxial heterostructures by wet-chemical methods. Various architectures based on different kinds of seeds or templates are illustrated, and their growth mechanisms are discussed in detail. Then, the applications of epitaxial heterostructures in optoelectronics, catalysis, and thermoelectrics are described. Finally, we provide some challenges and personal perspectives for the future research directions of semiconductor nanomaterial-based epitaxial heterostructures.

Energetics at the Surface: Direct Optical Mapping of Core and Surface Electronic Structure in CdSe Quantum Dots Using Broadband Electronic Sum Frequency Generation Microspectroscopy

Understanding and controlling the electronic structure of nanomaterials is the key to tailoring their use in a wide range of practical applications. Despite this need, many important electronic states are invisible to conventional optical measurements and are typically identified indirectly based on their inferred impact on luminescence properties. This is especially common and important in the study of nanomaterial surfaces and their associated defects. Surface trap states play a crucial role in photophysical processes yet remain remarkably poorly understood. Here we demonstrate for the first time that broadband electronic sum frequency generation (eSFG) microspectroscopy can directly map the optically bright and dark states of nanoparticles, including the elusive below gap states. This new approach is applied to model cadmium selenide (CdSe) quantum dots (QDs), where the energies of surface trap states have eluded direct optical characterization for decades. Our eSFG measurements show clear signatures of electronic transitions both above the band gap, which we assign to previously reported one- and two-photon transitions associated with the CdSe core, as well as broad spectral signatures below the band gap that are attributed to surface states. In addition to the core states, this analysis reveals two distinct distributions of below gap states, providing the first direct optical measurement of both shallow and deep surface states on this system. Finally, chemical modification of the surfaces via oxidation results in the relative increase in the signals originating from the surface states. Overall, our eSFG experiments provide an avenue to directly map the entirety of the QD core and surface electronic structure, which is expected to open up opportunities to study how these materials are grown in situ and how surface states can be controlled to tune functionality.

Type-I van der Waals heterostructure formed by MoS2 and ReS2 monolayers

We report a van der Waals heterostructure formed by monolayers of MoS₂ and ReS₂ with a type-I band alignment. First-principle calculations show that in this heterostructure, both the conduction band minimum and the valence band maximum are located in the ReS₂ layer. This configuration is different from previously accomplished type-II van der Waals heterostructures where electrons and holes reside in different layers. The type-I nature of this heterostructure is evident by photocarrier dynamics observed by transient absorption measurements. We found that carriers injected in MoS₂ transfer to ReS₂ in about 1 ps, while no charge transfer was observed when carriers are injected in ReS₂. The carrier lifetime in the heterostructure is similar to that in monolayer ReS₂, further confirming the lack of charge separation. We attribute the slower transfer time to the incoherent nature of the charge transfer due to the different crystal structures of the two materials forming the heterostructure. The demonstrated type-I semiconducting van der Waals heterostructure provides new ways to utilize two-dimensional materials for light emission applications, and a new platform to study light-matter interaction in atomically thin materials with strong confinement of electrons and holes.

Quasi-type II CuInS2/CdS core/shell quantum dots

Ternary chalcopyrite CuInS2 quantum dots (QDs) have been extensively studied in recent years as an alternative to conventional QDs for solar energy conversion applications. However, compared with the well-established photophysics in prototypical CdSe QDs, much less is known about the excited properties of CuInS2 QDs. In this work, using ultrafast spectroscopy, we showed that both conduction band (CB) edge electrons and copper vacancy (VCu) localized holes were susceptible to surface trappings in CuInS2 QDs. These trap states could be effectively passivated by forming quasi-type II CuInS2/CdS core/shell QDs, leading to a single-exciton (with electrons delocalized among CuInS2/CdS CB and holes localized in VCu) half lifetime of as long as 450 ns. Because of reduced electron-hole overlap in quasi-type II QDs, Auger recombination of multiple excitons was also suppressed and the bi-exciton lifetime was prolonged to 42 ps in CuInS2/CdS QDs from 10 ps in CuInS2 QDs. These demonstrated advantages, including passivated trap states, long single and multiple exciton lifetimes, suggest that quasi-type II CuInS2/CdS QDs are promising materials for photovoltaic and photocatalytic applications.

Ultrafast Carrier Dynamics and Hot Electron Extraction in Tetrapod-Shaped CdSe Nanocrystals

The ultrafast carrier dynamics and hot electron extraction in tetrapod-shaped CdSe nanocrystals was studied by femtosecond transient absorption (TA) spectroscopy. The carriers relaxation process from the higher electronic states (CB2, CB3(2), and CB4) to the lowest electronic state (CB1) was demonstrated to have a time constant of 1.04 ps, resulting from the spatial electron transfer from arms to a core. The lowest electronic state in the central core exhibited a long decay time of 5.07 ns in agreement with the reported theoretical calculation. The state filling mechanism and Coulomb blockade effect in the CdSe tetrapod were clearly observed in the pump-fluence-dependent transient absorption spectra. Hot electrons were transferred from arm states into the electron acceptor molecules before relaxation into core states.

Efficient and ultrafast formation of long-lived charge-transfer exciton state in atomically thin cadmium selenide/cadmium telluride type-II heteronanosheets

Colloidal cadmium chalcogenide nanosheets with atomically precise thickness of a few atomic layers and size of 10-100 nm are two-dimensional (2D) quantum well materials with strong and precise quantum confinement in the thickness direction. Despite their many advantageous properties, excitons in these and other 2D metal chalcogenide materials are short-lived due to large radiative and nonradiative recombination rates, hindering their applications as light harvesting and charge separation/transport materials for solar energy conversion. We showed that these problems could be overcome in type-II CdSe/CdTe core/crown heteronanosheets (with CdTe crown laterally extending on the CdSe nanosheet core). Photoluminesence excitation measurement revealed that nearly all excitons generated in the CdSe and CdTe domains localized to the CdSe/CdTe interface to form long-lived charge transfer excitons (with electrons in the CdSe domain and hole in the CdTe domain). By ultrafast transient absorption spectroscopy, we showed that the efficient exciton localization efficiency could be attributed to ultrafast exciton localization (0.64 ± 0.07 ps), which was facilitated by large in-plane exciton mobility in these 2D materials and competed effectively with exiton trapping at the CdSe or CdTe domains. The spatial separation of electrons and holes across the CdSe/CdTe heterojunction effectively suppressed radiative and nonradiative recombination processes, leading to a long-lived charge transfer exciton state with a half-life of ∼ 41.7 ± 2.5 ns, ∼ 30 times longer than core-only CdSe nanosheets.

Assessment of Hot-Carrier Effects on Charge Separation in Type-II CdS/CdTe Heterostructured Nanorods

Charge separation in semiconducting materials is an essential process that determines the efficiency of photovoltaic devices and photocatalysts. Herein, we report the charge-separation dynamics in type-II CdS/CdTe heterostructured nanorods revealed by femtosecond transient-absorption (TA) measurements with a broad-band white-light probe. Under selective excitation of the CdTe segment, bleaching signals at the band gap energy of CdS were clearly observed with a rise component on a subpicosecond time scale, which indicates efficient electron transfer from CdTe to CdS. The pump-energy dependence of the TA dynamics shows that hot electrons rapidly relax to the bottom of the conduction band of CdTe, and then the electrons transfer to the CdS segment.

Broadband optical non-linearity induced by charge-transfer excitons in type-II CdSe/ZnTe nanocrystals

Benefiting from excitonic charge-transfer states, an efficient non-linear optical response is observed in type-II nanocrystals by four-wave mixing when the incident photon energy lies below the bandgaps of constituent semiconductors. The non-linear optical properties in nanocrystals can be manipulated by the band alignment of the core-shell components, which serves as a promising platform to design broadband non-linear optical devices.

Theory of coherent phonons in carbon nanotubes and graphene nanoribbons

We survey our recent theoretical studies on the generation and detection of coherent radial breathing mode (RBM) phonons in single-walled carbon nanotubes and coherent radial breathing like mode (RBLM) phonons in graphene nanoribbons. We present a microscopic theory for the electronic states, phonon modes, optical matrix elements and electron-phonon interaction matrix elements that allows us to calculate the coherent phonon spectrum. An extended tight-binding (ETB) model has been used for the electronic structure and a valence force field (VFF) model has been used for the phonon modes. The coherent phonon amplitudes satisfy a driven oscillator equation with the driving term depending on the photoexcited carrier density. We discuss the dependence of the coherent phonon spectrum on the nanotube chirality and type, and also on the graphene nanoribbon mod number and class (armchair versus zigzag). We compare these results with a simpler effective mass theory where reasonable agreement with the main features of the coherent phonon spectrum is found. In particular, the effective mass theory helps us to understand the initial phase of the coherent phonon oscillations for a given nanotube chirality and type. We compare these results to two different experiments for nanotubes: (i) micelle suspended tubes and (ii) aligned nanotube films. In the case of graphene nanoribbons, there are no experimental observations to date. We also discuss, based on the evaluation of the electron-phonon interaction matrix elements, the initial phase of the coherent phonon amplitude and its dependence on the chirality and type. Finally, we discuss previously unpublished results for coherent phonon amplitudes in zigzag nanoribbons obtained using an effective mass theory.

The effect of the charge-separating interface on exciton dynamics in photocatalytic colloidal heteronanocrystals

Ultrafast transient absorption spectroscopy was used to investigate the nature of photoinduced charge transfer processes taking place in ZnSe/CdS/Pt colloidal heteronanocrystals. These nanoparticles consist of a dot-in-a-rod semiconductor domain (ZnSe/CdS) coupled to a Pt tip. Together the three components are designed to dissociate an electron-hole pair by pinning the hole in the ZnSe domain while allowing the electron to transfer into the Pt tip. Separated charges can then induce a catalytic reaction, such as the light-driven hydrogen production. Present measurements demonstrate that the internal electron-hole separation is fast and results in the localization of both charges in nonadjacent parts of the nanoparticle. In particular, we show that photoinduced holes become confined within the ZnSe domain in less than 2 ps, while electrons take approximately 15 ps to transition into a Pt tip. More importantly, we demonstrate that the presence of the ZnSe dot within the CdS nanorods plays a key role both in enabling photoinduced separation of charges and in suppressing their backward recombination. The implications of the observed exciton dynamics to photocatalytic function of ZnSe/CdS/Pt heteronanocrystals are discussed.

Effects of Lattice Strain and Band Offset on Electron Transfer Rates in Type-II Nanorod Heterostructures

Type-II nanorod heterostructures (NRHs) exhibit efficient directional charge separation and provide the potential to control this flow of charges through changes in structure and composition. We use transient-absorption spectroscopy to investigate how the magnitude of band offset and lattice strain alters dynamics of photogenerated electrons in CdSe/CdTe type-II NRHs. In the absence of alloying and strain effects, electron transfer occurs in ∼300 fs. Reducing the conduction band offset by means of alloying leads to an even shorter charge-separation time (<200 fs), whereas curved NRHs with pronounced strain exhibit a longer charge-separation time of ∼700 fs.

Measuring electron and hole transfer in core/shell nanoheterostructures

Using femtosecond transient absorption and time-resolved photoluminescence spectroscopy, we studied the electron versus hole dynamics in photoexcited quasi-type-II heterostructured nanocrystals with fixed CdTe core radii and varying CdSe shell coverage. By choosing the pump wavelength in resonance with the core or the shell states, respectively, we were able to measure the excited electron and hole dynamics selectively. Both, the core- and the shell-excited CdTe/CdSe nanocrystals showed the same spectral emission and photoluminescence lifetimes, indicating that ultrafast electron and hole transfer across the core/shell interface resulted in the identical long-lived charge transfer state. Both charge carriers have subpicosecond transfer rates through the interface, but the subsequent relaxation rates of the hole (τ(dec) ∼ 800 ps) and electron (τ(avg) ∼ 8 ps) are extremely different. On the basis of the presented transient absorption measurements and fitting of the steady-state spectra, we find that the electron transfer occurs in the Marcus inverted region and mixing between the CdTe exciton and charge transfer states takes place and therefore needs to be considered in the analysis.

Suppression of the plasmon resonance in Au/CdS colloidal nanocomposites

The nature of exciton-plasmon interactions in Au-tipped CdS nanorods has been investigated using femtosecond transient absorption spectroscopy. The study demonstrates that the key optoelectronic properties of composite heterostructures comprising electrically coupled metal and semiconductor domains are substantially different from those observed in systems with weak interdomain coupling. In particular, strongly coupled nanocomposites promote mixing of electronic states at semiconductor-metal domain interfaces, which causes a significant suppression of both plasmon and exciton excitations of carriers.

Emergent properties resulting from type-II band alignment in semiconductor nanoheterostructures

The development of elegant synthetic methodologies for the preparation of monocomponent nanocrystalline particles has opened many possibilities for the preparation of heterostructured semiconductor nanostructures. Each of the integrated nanodomains is characterized by its individual physical properties, surface chemistry, and morphology, yet, these multicomponent hybrid particles present ideal systems for the investigation of the synergetic properties that arise from the material combination in a non-additive fashion. Of particular interest are type-II heterostructures, where the relative band alignment of their constituent semiconductor materials promotes a spatial separation of the electron and hole following photoexcitation, a highly desirable property for photovoltaic applications. This article highlights recent progress in both synthetic strategies, which allow for material and architectural modulation of novel nanoheterostructures, as well as the experimental work that provides insight into the photophysical properties of type-II heterostructures. The effects of external factors, such as electric fields, temperature, and solvent are explored in conjunction with exciton and multiexciton dynamics and charge transfer processes typical for type-II semiconductor heterostructures.

Spectral and dynamic properties of excitons and biexcitons in type-II semiconductor nanocrystals

Using time-resolved photoluminescence spectroscopy, we analyze single and multiexciton emission energies and decay dynamics of CdS/ZnSe core/shell nanocrystals (NCs). The NCs exhibit a characteristic type-II band alignment that leads to spatially separated charges; this effect is at the origin of long radiative exciton lifetimes, repulsive biexciton interaction energies, and reduced Auger recombination efficiencies. We determine these properties as a function of ZnSe shell thickness and find that the exciton emission energies and the decay times depend little on this parameter. In contrast, the spectral and dynamic properties of biexcitons vary strongly with the shell thickness. The considerable shell dependence of the biexciton decay lets us conclude that the associated Auger process involves the excitation of holes localized in the ZnSe shell. In NCs with thick shells, the large blue shift of the biexciton emission energy is mainly caused by Coulomb repulsion between electrons localized in the CdS core. The different sensitivity of exciton and multiexciton characteristics on the ZnSe shell thickness provides a unique opportunity to tune them independently.

Strain-induced band gap modification in coherent core/shell nanostructures

Using first-principles calculations within density functional theory, we study the relative impacts of quantum confinement and strain on the electronic structure of two II-VI semiconductor compounds with a large lattice-mismatch, CdSe and CdTe, in core/shell nanowire geometries with different core radii and shell thicknesses. For fixed CdSe core radius, we find that the electronic band gap in the core is significantly reduced with increasing CdTe shell thickness, by an amount comparable to that expected from quantum confinement, due to the development of a large and highly anisotropic strain throughout the heterostructure. A straightforward analysis allows us to separate quantitatively changes in band gap due to quantum confinement and strain. Our studies elucidate and quantify the importance of strain in determining the electronic and optical properties of core/shell nanostructures.

Ultrafast carrier dynamics in type II ZnSe/CdS/ZnSe nanobarbells

We employ femtosecond transient absorption spectroscopy to get an insight into ultrafast processes occurring at the interface of type II ZnSe/CdS heterostructured nanocrystals fabricated via colloidal routes and comprising a barbell-like arrangement of ZnSe tips and CdS nanorods. Our study shows that resonant excitation of ZnSe tips results in an unprecedently fast transfer of excited electrons into CdS domains of nanobarbells (<0.35 ps), whereas selective pumping of CdS components leads to a relatively slow injection of photoinduced holes into ZnSe tips (tau(h)= 95 ps). A qualitative thermodynamic description of observed electron processes within the classical limit of Marcus theory was used to identify a specific charge transfer regime associated with the ultrafast electron injection into CdS. Potential photocatalytic applications of the observed fast separation of carriers along the main axis of ZnSe/CdS barbells are discussed.

Phosphine-free synthesis of high quality ZnSe, ZnSe/ZnS, and Cu-, Mn-doped ZnSe nanocrystals

High quality zinc blende ZnSe and ZnSe/ZnS core/shell nanocrystals have been synthesized by two converse injection methods (i.e. zinc precursor injection or selenium precursor injection) when Se-ODE complex was chosen as the phosphine-free selenium precursor. Absorption spectroscopy, fluorescence spectroscopy, X-ray diffraction (XRD), and transmission electron microscopy (TEM) were used to characterize the as-synthesized ZnSe and ZnSe/ZnS nanocrystals. The quality of the as-prepared ZnSe nanocrystals reached the same high level compared with the method using phosphine selenium precursors since the quantum yields were between 40 and 60% and photoluminescence (PL) full width at half-maximum (FWHM) was well controlled between 14 and 17 nm. The parameter window for the growth of high quality ZnSe nanocrystals was found to be much broader and monodisperse ZnSe nanocrystals were synthesized successfully even when the reaction temperature was set as low as 240 degrees C. As cores, such zinc blende ZnSe nanocrystals were also used to synthesize ZnSe/ZnS core/shell nanocrystals with high fluorescence quantum yields of 70%. Cu(2+) or Mn(2+) doped ZnSe nanocrystals were also synthesized by simply modifying this phosphine-free method. The emission range has been extended to 500 and 600 nm with the use of Cu(2+) and Mn(2+) dopants compared with the emission coverage of ZnSe at around 400 nm. This is the first totally "green approach" (i.e. phosphine-free synthesis) for the synthesis of high quality ZnSe, ZnSe/ZnS, and Cu(2+) or Mn(2+) doped ZnSe nanocrystals.

Tetrapod-shaped colloidal nanocrystals of II-VI semiconductors prepared by seeded growth

We report a general synthetic approach to tetrapod-shaped colloidal nanocrystals made of various combinations of II-VI semiconductors. Uniform tetrapods were prepared using preformed seeds in the sphalerite structure, onto which arms were grown by coinjection of the seeds and chemical precursors into a hot mixture of surfactants. By this approach, a wide variety of core materials could be chosen (in practice, most of the II-VI semiconductors that could be prepared in the sphalerite phase, namely, CdSe, ZnTe, CdTe); in contrast, the best materials for arm growth were CdS and CdTe. The samples were extensively characterized with the aid of several techniques.

Photocatalysis with CdSe nanoparticles in confined media: mapping charge transfer events in the subpicosecond to second timescales

Photoinduced charge transfer events between 3 nm diameter CdSe semiconductor nanocrystals and an electron acceptor, MV2+, have been probed in the subpicosecond-microseconds-seconds time scale by confining the reactants in an AOT/heptane reverse micelle. The probe molecule, methyl viologen (MV2+) interacts with the excited CdSe nanoparticle and quenches its emission effectively. The ultrafast electron transfer to MV2+, as monitored from the exciton bleaching recovery of CdSe and the formation of MV+* radical, is completed with an average rate constant of 2.25x10(10) s(-1). Under steady state irradiation (450 nm) the accumulation of MV+* is seen with a net quantum yield of 0.1. Mediation of the electron transfer through TiO2 nanoparticles is achieved by coupling them with the CdSe-MV2+ system within the reverse micelle. This coupling of two semiconductor nanoparticles increases the quantum yield of MV2+ reduction by a factor of 2. The dual roles of TiO2 as an electron shuttle and a rectifier are elucidated by transient absorption spectroscopy and steady state photolysis. The presence of both TiO2 and MV2+ in the reverse micelle creates a synergistic effect to enhance the electron transfer rate constant by an order of magnitude. The time-resolved events that dictate the production and stabilization of electron transfer product provide an insight into the photocatalytic systems that are potentially important in solar hydrogen production and photocatalytic remediation.

Control of exciton spin relaxation by electron-hole decoupling in type-II nanocrystal heterostructures

The electron spin flip relaxation dynamics in type II CdSe/CdTe nanorod heterostructures are investigated by an ultrafast polarization transient grating technique. Photoexcited charge separation in the heterostructures suppresses the electron-hole exchange interaction and their recombination, which reduces the electron spin relaxation rate in CdSe nanocrystals by 1 order of magnitude compared to exciton relaxation. The electron orientation is preserved during charge transfer from CdTe to CdSe, and its relaxation time constant is found to be approximately 5 ps at 293 K in the CdSe part of these nanorods. This finding suggests that hole spin relaxation determines the exciton fine structure relaxation rate and therefore control of exciton spin relaxation in semiconductor nanostructures is possible by delocalizing or translating the hole density relative to the electron.

Synthesis and characterization of CdSe nanorods using a novel microemulsion method at moderate temperature

CdSe nanoparticles have been successfully synthesized using a novel microemulsion method at moderate temperature. It is found that with a combination of the surfactant AOT and hydrazine hydrate, it is possible to control the morphology of the nanoparticles. The hydrazine hydrate acts as both a reducing agent and a templating agent that favors the formation of a rodlike structure. The composition, morphology and optical properties of the CdSe nanoparticles were investigated using powder X-ray diffraction (XRD), transmission electron microscopy (TEM), ultraviolet-visible (UV-vis) absorption spectroscopy, photoluminescence (PL) spectroscopy, energy dispersive X-ray spectroscopy (EDX) and Fourier transform infrared (FT-IR) spectroscopy. The nucleation and growth mechanism for this system is also proposed based on a time-dependent study. This synthesis route provides a moderate temperature (100 degrees C) method for synthesizing rodlike CdSe, hence reducing the possibility of oxidation of this chalcogenide compound.