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

Does an intrinsic strain contribute to the effect of quantum confinement phenomenon? An alloyed transition metal dichalcogenide series, Mo(S1-xSex)2 as a case study

It is well known that the bandgap of 2D transition metal dichalcogenides (TMDs) in the quantum confinement regime increases with a decrease in the number of layers. In this work, we show the effect of lattice strain on the dependence of the gap. We have designed an ideal system in the form of common-cationic alloyed-TMDs, Mo(S1-xSex)2, for such studies. With a large difference between the ionic radii of the two chalcogens, the nanoflakes of the alloys possessed a lattice strain and have been found to yield a lower bandgap than those of both the end-members, MoS2 and MoSe2. More importantly, the dependence of the bandgap on the layer number in the nanoflakes of the alloys turned out to be steeper than in conventional binary TMDs. The experimental results imply that the lattice strain in 2D semiconductors has contributed to the effect of the quantum confinement phenomenon in addition to decreasing the bandgap, the latter being earlier predicted from a theoretical model. We have derived the electronic bandgap and the band-edge energies of the series of alloyed-TMDs in their nanoflake forms and the dependences on the number of layers from the density of states (DOS), as obtained from scanning tunneling spectroscopy (STS) recorded in a scanning tunneling microscope (STM) in an extremely localized manner.

Charge Transfer from Quantum-Confined 0D, 1D, and 2D Nanocrystals

The properties of colloidal quantum-confined semiconductor nanocrystals (NCs), including zero-dimensional (0D) quantum dots, 1D nanorods, 2D nanoplatelets, and their heterostructures, can be tuned through their size, dimensionality, and material composition. In their photovoltaic and photocatalytic applications, a key step is to generate spatially separated and long-lived electrons and holes by interfacial charge transfer. These charge transfer properties have been extensively studied recently, which is the subject of this Review. The Review starts with a summary of the electronic structure and optical properties of 0D-2D nanocrystals, followed by the advances in wave function engineering, a novel way to control the spatial distribution of electrons and holes, through their size, dimension, and composition. It discusses the dependence of NC charge transfer on various parameters and the development of the Auger-assisted charge transfer model. Recent advances in understanding multiple exciton generation, decay, and dissociation are also discussed, with an emphasis on multiple carrier transfer. Finally, the applications of nanocrystal-based systems for photocatalysis are reviewed, focusing on the photodriven charge separation and recombination processes that dictate the function and performance of these materials. The Review ends with a summary and outlook of key remaining challenges and promising future directions in the field.

Pressure-stimulus-responsive behaviors of core-shell InP/ZnSe nanocrystals: remarkable piezochromic luminescence and structural assembly

Piezochromic luminescence materials with optical properties can be adjusted (the colors most sensitive to the human eye range from red to green) to provide powerful means for information acquisition in various applications. Inorganic quantum dots, typically based on heavy metals such as cadmium and lead, have congenital advantages as luminescence materials, including strong inoxidizability and excellent photoelectric properties. However, small band-gap shifts under pressure have hindered the development of inorganic-based piezochromic materials. Herein, we combined in situ high-pressure photoluminescence (PL) and absorption measurements with synchrotron X-ray scattering spectra to elucidate the remarkable modulation of optical properties and morphologies by pressure, particularly that of the piezochromic luminescence, in all-inorganic core-shell InP/ZnSe nanocrystals (NCs). We observed a stepwise PL color change from red to green, and an ultrabroad bandgap tunability of 0.46 eV was observed from 1.99 to 2.45 eV in the pressure range of 14.2 GPa for InP/ZnSe NCs. Moreover, two-dimensional (2D) InP/ZnSe nanosheets were synthesized by the stress-driven attachment of nanoparticles. These results demonstrate the ability of the pressure-stimulus response to trigger remarkable piezochromic luminescence and 2D nanosheet assembly in InP/ZnSe NCs, which paves the way for new applications of all-inorganic InP-based semiconductor NCs.

Shedding Light on the Role of Misfit Strain in Controlling Core-Shell Nanocrystals

Heteroepitaxial modification of nanomaterials has become a powerful means to create novel functionalities for various applications. One of the most elementary factors in heteroepitaxial nanostructures is the misfit strain arising from mismatched lattices of the constituent parts. Misfit strain not only dictates epitaxy kinetics for diversifying nanocrystal morphologies but also provides rational control over materials properties. In recent years, advances in chemical synthesis along with the rapid development of electron microscopy and X-ray diffraction techniques have enabled a substantial understanding of strain-related processes, which offers theoretical foundation and experimental guidance for researchers to refine heteroepitaxial nanostructures and their properties. Herein, recent investigations on heterogeneous core-shell nanocrystals containing misfit strains are summarized, with a focus on the mechanistic understanding of strain and strain-induced effects such as tuning the epitaxial habit, modulating the optical emission, and enhancing the catalytic activity and magnetic coercivity.

InP Quantum Dots: Synthesis and Lighting Applications

InP quantum dots (QDs) are typical III-V group semiconductor nanocrystals that feature large excitonic Bohr radius and high carrier mobility. The merits of InP QDs include large absorption coefficient, broad color tunability, and low toxicity, which render them promising alternatives to classic Cd/Pb-based QDs for applications in practical settings. Over the past two decades, the advances in wet-chemistry methods have enabled the synthesis of small-sized colloidal InP QDs with the assistance of organic ligands. By proper selection of synthetic protocols and precursor materials coupled with surface passivation, the QYs of InP QDs are pushed to near unity with modest color purity. The state-of-the-art InP QDs with appealing optical and electronic properties have excelled in many applications with the potential for commercialization. This work focuses on the recent development of wet-chemistry protocols and various precursor materials for the synthesis and surface modification of InP QDs. Current methods for constructing light-emitting diodes using novel InP-based QDs are also summarized.

Piezochromic luminescence in all-inorganic core-shell InP/ZnS nanocrystals via pressure-modulated strain engineering

Piezochromic materials alter their photoluminescent (PL) colors in response to the action of external force. Such materials have attracted much attention owing to their promising applications in pressure-sensing, optoelectronic memory and anticounterfeiting. However, almost all the reported piezochromic materials were limited to the organic matters or compounds containing organic components. Here we present piezochromic materials and pressure-induced optical response based on all-inorganic core/shell InP/ZnS nanocrystals (NCs). The InP/ZnS NCs exhibit noticeable PL color changes, shifting from orange (2.08 eV) to green (2.25 eV), with the PL intensity showing slight enhancement below an applied pressure of 2.5 GPa. Further compressing to fluorescence quenching produces an ultrabroad energy tenability range up to 400 meV. Structural and time-resolved PL lifetime studies, together with first-principle calculations, reveal the weakening of strain-induced defect states in the low pressure regime, which contributes to effective excition recombination, thus ensuring high fluorescence emission of InP/ZnS NCs. This work provides a promising strategy to prepare piezochromic materials of all-inorganic semiconductors, thereby greatly increasing the choice of materials for new applications.

Pressure-Tuned Core/Shell Configuration Transition of Shell Thickness-Dependent CdSe/CdS Nanocrystals

Pressure is adopted as a "clean" tool to achieve a core/shell configuration transition of CdSe/CdS nanocrystals (NCs) from quasi-type II to type I. The pressure-dependent photoluminescence (PL) spectra demonstrate a sudden decrease in PL intensity, because of the enhanced rate of exciton-exciton annihilation of type I structured CdSe/CdS NCs. Likewise, the large decrease in the PL lifetime with pressure confirms that the electron wave function mainly localizes into the CdSe core, indicating the decreased separation of electrons and holes in type I band alignment. We propose that pressure increases the conduction band energy of the CdS shell but hardly changes that of the CdSe core with almost both unchanged valence band energies, thus ultimately increasing the conduction band offsets between the CdSe core and CdS shell to form the type I core/shell configuration. Our studies elucidate the significance of external pressure in determining the electronic and optical properties of core/shell nanomaterials.

Core–shell structured catalysts for thermocatalytic, photocatalytic, and electrocatalytic conversion of CO2

An in-depth assessment of properties of core–shell catalysts and their application in the thermocatalytic, photocatalytic, and electrocatalytic conversion of CO₂into synthesis gas and valuable hydrocarbons.

Large Band Edge Tunability in Colloidal Nanoplatelets

We study the impact of organic surface ligands on the electronic structure and electronic band edge energies of quasi-two-dimensional (2D) colloidal cadmium selenide nanoplatelets (NPLs) using density functional theory. We show how control of the ligand and ligand-NPL interface dipoles results in large band edge energy shifts, over a range of 5 eV for common organic ligands with a minor effect on the NPL band gaps. Using a model self-energy to account for the dielectric contrast and an effective mass model of the excitons, we show that the band edge tunability of NPLs together with the strong dependence of the optical band gap on NPL thickness can lead to favorable photochemical and optoelectronic properties.

Ultrafast charge carrier dynamics in CdSe/V2O5 core/shell quantum dots

Ultrafast transient absorption (TA) spectroscopy has been carried out to study the charge carrier dynamics of CdSe core and CdSe/V2O5 core/shell quantum dots (QDs). A significant redshift accompanied by broadening in the first excitonic peak was observed in the UV-Vis absorption spectra of the core/shell QDs as the shell thickness increases. This interesting observation is related to a quasi-type-II alignment characterized by the spatial separation of an electron into the core/shell and a hole into the core. The observed optical excitonic spectra have further been used to study the energetics of CdSe and charge separated states with the concept of Marcus theory and confirmed that electron transfer takes place in the Marcus inverted region (). Moreover, the growth kinetics of the CdSe core and CdSe/V2O5 core/shell QDs, studied with TA spectroscopy, exhibits slow electron cooling in core/shell QDs because of the de-coupling of the electronic wave functions with their hole counterpart. These exciting properties reveal a new paradigm shift from CdSe QDs to CdSe/V2O5 core/shell QDs for highly suitable applications in photovoltaics (PV) and optoelectronic devices.

Efficient Modeling of Optical Excitations of Colloidal Core-Shell Semiconductor Quantum Dots by Using Symmetrized Orbitals

An efficient method for the theoretical investigation of optical properties of semiconductor core-shell quantum dots (CSQDs) is introduced within the multiband k·p approach, which takes the advantage of the symmetry of the system. The heteroepitaxial strain and excitonic effect are included in the calculation of energy levels, envelope wave functions, exciton binding energy, and linear absorption coefficient. The adoption of symmetrized orbitals allows improvement of the computation time significantly. To avoid appearance of spurious solutions caused by imbalance of basis functions adopted, we consider an 8-band k·p model which is block-diagonalized into two conduction bands and six valence bands, that we call the 2 + 6-band model. The band nonparabolicity effect is modeled by an energy-dependent k·p term, such that the density of states obtained can mimic the actual density of states of a full-band model. The simulated absorption spectra of ZnTe/ZnSe CSQD are in good agreement with those observed experimentally, including the high rise of absorption at energies far above the absorption edge.

Strain-Driven Stacking Faults in CdSe/CdS Core/Shell Nanorods

Colloidal semiconductor nanocrystals are commonly grown with a shell of a second semiconductor material to obtain desired physical properties, such as increased photoluminescence quantum yield. However, the growth of a lattice-mismatched shell results in strain within the nanocrystal, and this strain has the potential to produce crystalline defects. Here, we study CdSe/CdS core/shell nanorods as a model system to investigate the influence of core size and shape on the formation of stacking faults in the nanocrystal. Using a combination of high-angle annular dark-field scanning transmission electron microscopy and pair-distribution-function analysis of synchrotron X-ray scattering, we show that growth of the CdS shell on smaller, spherical CdSe cores results in relatively small strain and few stacking faults. By contrast, growth of the shell on larger, prolate spheroidal cores leads to significant strain in the CdS lattice, resulting in a high density of stacking faults.

Piezoelectric Control of the Exciton Wave Function in Colloidal CdSe/CdS Nanocrystals

Using multiband k·p calculations, we show that strain-engineered piezoelectricity is a powerful tool to modulate the electron-hole spatial separation in a wide class of wurtzite CdSe/CdS nanocrystals. The inherent anisotropy of the hexagonal crystal structure leads to anisotropic strain and, consequently, to a pronounced piezoelectric field along the c axis, which can be amplified or quenched through a proper design of the core-shell structure. The use of large cores and thick shells promotes a gradual departure from quantum confined nanocrystals to a regime dominated by piezoelectric confinement. This allows excitons to evolve from the usual type-I and quasi-type-II behavior to a type-II behavior in dot-in-dots, dot-in-rods, rod-in-rods, and dot-in-plates. Piezoelectric fields explain experimental observations for giant-shell nanocrystals, whose time-resolved photoluminescence reveals long exciton lifetimes for large cores, contrary to the expectations of standard quantum confinement models. They also explain the large differences in exciton lifetimes reported for different classes of CdSe/CdS nanocrystals.

Geometry strategy for engineering the recombination possibility of excitons in nanowires

We proposed a geometry strategy to engineer the radiative recombination possibility and thus the lifetime of excitons in nanowires of some photovoltaic semiconductors by using theoretical analysis and first-principles calculations. We demonstrated that the shape can engineer the symmetry of the wave-functions of band-edge states and influence the radiative recombination possibility. The nanowires need to satisfy the following requirements to forbid the radiative recombination possibility of band-edge excitons: (i) wurtzite structure; (ii) pxy-characterized wave-function of VBM state and (iii) C3v-symmetry shape. The geometrical symmetry results in the pxy-characterized C3v-symmetry wave-function of VBM state and leads to forbidden radiative recombination of band-edge excitons. The geometry strategy offers a flexible proposal to prolong the exciton lifetime, leaving optical absorption impregnable.

Highly Efficient Copper-Indium-Selenide Quantum Dot Solar Cells: Suppression of Carrier Recombination by Controlled ZnS Overlayers

Copper-indium-selenide (CISe) quantum dots (QDs) are a promising alternative to the toxic cadmium- and lead-chalcogenide QDs generally used in photovoltaics due to their low toxicity, narrow band gap, and high absorption coefficient. Here, we demonstrate that the photovoltaic performance of CISe QD-sensitized solar cells (QDSCs) can be greatly enhanced simply by optimizing the thickness of ZnS overlayers on the QD-sensitized TiO2 electrodes. By roughly doubling the thickness of the overlayers compared to the conventional one, conversion efficiency is enhanced by about 40%. Impedance studies reveal that the thick ZnS overlayers do not affect the energetic characteristics of the photoanode, yet enhance the kinetic characteristics, leading to more efficient photovoltaic performance. In particular, both interfacial electron recombination with the electrolyte and nonradiative recombination associated with QDs are significantly reduced. As a result, our best cell yields a conversion efficiency of 8.10% under standard solar illumination, a record high for heavy metal-free QD solar cells to date.

Realization of a Strained Atomic Wire Superlattice

A superlattice of strained Au-Si atomic wires is successfully fabricated on a Si surface. Au atoms are known to incorporate into the stepped Si(111) surface to form a Au-Si atomic wire array with both one-dimensional (1D) metallic and antiferromagnetic atomic chains. At a reduced density of Au, we find a regular array of Au-Si wires in alternation with pristine Si nanoterraces. Pristine Si nanoterraces impose a strain on the neighboring Au-Si wires, which modifies both the band structure of metallic chains and the magnetic property of spin chains. This is an ultimate 1D version of a strained-layer superlattice of semiconductors, defining a direction toward the fine engineering of self-assembled atomic-scale wires.

Band structure engineering via piezoelectric fields in strained anisotropic CdSe/CdS nanocrystals

Strain in colloidal heteronanocrystals with non-centrosymmetric lattices presents a unique opportunity for controlling optoelectronic properties and adds a new degree of freedom to existing wavefunction engineering and doping paradigms. We synthesized wurtzite CdSe nanorods embedded in a thick CdS shell, hereby exploiting the large lattice mismatch between the two domains to generate a compressive strain of the CdSe core and a strong piezoelectric potential along its c-axis. Efficient charge separation results in an indirect ground-state transition with a lifetime of several microseconds, almost one order of magnitude longer than any other CdSe/CdS nanocrystal. Higher excited states recombine radiatively in the nanosecond time range, due to increasingly overlapping excited-state orbitals. k̇p calculations confirm the importance of the anisotropic shape and crystal structure in the buildup of the piezoelectric potential. Strain engineering thus presents an efficient approach to highly tunable single- and multiexciton interactions, driven by a dedicated core/shell nanocrystal design.

Quantum dots confine electrons to a nanometre length scale, and this gives rise to numerous quantum effects. Here, the authors directly control the excitonic structure of nanocrystal quantum dots by manipulating intra-particle piezoelectric fields.

Band gap modulation of Si-C binary core/shell nanowires by composition and ratio

Core/shell nanowires (CSNWs) composed of Si, C, and SiC are promising systems for optoelectronic devices. Through computational investigations, we find that the band gaps (Eg) of these nanowires can be controlled not only by changing their composition, but also by adjusting the core/shell thickness ratio. For Si/SiC or SiC/C CSNWs with a fixed total number of layers, the dependence of Eg on the core/shell thickness ratio shows a bowing effect. Eg can be tuned from a few eV all the way to zero. These investigations provide direction for designing optoelectronic devices based on Earth-abundant elements.

Size-dependent optical absorption modulation of Si/Ge and Ge/Si core/shell nanowires with different cross-sectional geometries

We present an atomic-level and quantitative study of the absorption properties in Si/Ge and Ge/Si core/shell nanowires (CSNWs) along [110] direction with different cross-sectional geometries using the atomic bond relaxation method. We find that the strain existing in self-equilibrium state of CSNWs and associated with elastic energy originating from interface mismatch and surface relaxation affect the band shift and absorption properties. Compared to the CSNWs with tetragonal, hexagonal and circular shapes, the triangular CSNWs have the largest band gap shift at a fixed strain and the smallest absorption coefficient at a determinate incident light wavelength. The tunable absorption property, realized by controlling the size and geometry structure, could be helpful for nanoelectronic applications.

ZnO/ITO core/shell nanostructure electrodes for future prototype solar cell devices

Crystalline ZnO/ITO core/shell nanostructures were developed and the devices fabricated with single NR showed excellent Ohmic characteristics under dark and light.

Understanding the opposite electrical responses of an individual ZnO nanowire under different bending deformations

Opposite electrical responses have been observed in the same individual ZnO nanowire during bending via changes in the contact modes.

Rainbow Emission from an Atomic Transition in Doped Quantum Dots

Although semiconductor quantum dots are promising materials for displays and lighting due to their tunable emissions, these materials also suffer from the serious disadvantage of self-absorption of emitted light. The reabsorption of emitted light is a serious loss mechanism in practical situations because most phosphors exhibit subunity quantum yields. Manganese-based phosphors that also exhibit high stability and quantum efficiency do not suffer from this problem but in turn lack emission tunability, seriously affecting their practical utility. Here, we present a class of manganese-doped quantum dot materials, where strain is used to tune the wavelength of the dopant emission, extending the otherwise limited emission tunability over the yellow-orange range for manganese ions to almost the entire visible spectrum covering all colors from blue to red. These new materials thus combine the advantages of both quantum dots and conventional doped phosphors, thereby opening new possibilities for a wide range of applications in the future.

Fabrication and photoactivity of a tunable-void SiO₂-TiO₂ core-shell structure on modified SiO₂ nanospheres by grafting an amphiphilic diblock copolymer using ARGET ATRP

SiO₂-based composites have important applications in various technological fields. In this work, a tunablevoid SiO₂-TiO₂ core-shell structure was successfully prepared for the first time using SiO₂-polymethyl methacrylate (PMMA)-polyoligo(ethylene glycol)methyl ether methacrylate (PO(EO)nMA) (n = 2, 5, and 8). An amphiphilic copolymer was used as the template, and calcination was performed using tetrabutyl titanate (TBT) as the titanium source. SiO₂-PMMA-b-PO(EO)nMA microspheres were first synthesized through activators regenerated by electron transfer-atom transfer radical polymerization. Methyl methacrylate and O(EO)nMA were grafted with different EO unit numbers onto the surface of the halogen functional group of SiO₂. TBT was hydrolyzed along with the PO(EO)nMA chain through hydrogen bonding, and then the SiO₂-TiO₂ core-shell structure was acquired through calcination to remove the polymer. Simultaneously, amorphous TiO₂ crystallized during calcination. A series of characterizations indicated that the amphiphilic block copolymer was grafted onto SiO₂ mesoparticle surfaces, the titania samples existed only in the anatase phase, and the prepared SiO₂-TiO₂ had hierarchically nanoporous structures. The gradient hydrophilicity of the PMMA-b-PO(EO)nMA copolymer template facilitated the hydrolysis of TBT molecules along the PO(EO)nMA to PMMA segments, thereby tuning the space between the core and the shell. In addition, the space was about 6 nm when the EO number was 2, and the space was about 10 nm when the EO numbers were 5 and 8. The photocatalytic activities of the SiO₂-TiO₂ materials were tested on the photodegradation of methyl orange.

Fabrication and photoresponse of ZnO nanowires/CuO coaxial heterojunction

The fabrication and properties of n-ZnO nanowires/p-CuO coaxial heterojunction (CH) with a photoresist (PR) blocking layer are reported. In our study, c-plane wurtzite ZnO nanowires were grown by aqueous chemical method, and monoclinic CuO (111) was then coated on the ZnO nanowires by electrochemical deposition to form CH. To improve the device performance, a PR layer was inserted between the ZnO buffer layer and the CuO film to serve as a blocking layer to block the leakage current. Structural investigations of the CH indicate that the sample has good crystalline quality. It was found that our refined structure possesses a better rectifying ratio and smaller reverse leakage current. As there is a large on/off ratio between light on and off and the major light response is centered at around 424 nm, the experimental results suggest that the PR-inserted ZnO/CuO CH can be used as a good narrow-band blue light detector.

Growth and large-scale assembly of InAs/InP core/shell nanowire: effect of shell thickness on electrical characteristics

InAs/InP core/shell nanowires with different shell thicknesses were grown by a two-step method, and large-scale assembly of single nanowire was realized by using dielectrophoresis alignment and patterned grooves. Thousands of single nanowire field-effect transistors were fabricated on a single chip. The effect of InP shell thickness on the electron mobility and density of InAs nanowires are experimentally investigated and discussed.

Intrinsic Charge Separation and Tunable Electronic Band Gap of Armchair Graphene Nanoribbons Encapsulated in a Double-Walled Carbon Nanotube

Recent synthesis of nanocomposite structures of graphene nanoribbons (GNRs) encapsulated in a carbon nanotube (CNT) has opened a new avenue for exploring new functionalities for applications in nanotechnology. This new class of carbon nanocomposites is expected to possess electronic properties beyond those offered by the constituent parts of nanotubes and nanoribbons; unveiling such new properties and understanding the underlying physics are among the most pressing issues in the study of these promising materials. Here, we report on first-principles calculations of the electronic properties of armchair GNRs encapsulated in a zigzag double-walled CNT. This unique structural configuration produces an intrinsic charge separation with electrons and holes localized in the outer tube and the ribbon, respectively, while the inner tube remains charge-neutral, forming an n-type/intrinsic/p-type semiconducting heterojunction due to the staggered lineup of the band structures of the constituent parts. The electronic band gap of the nanocomposite can be tuned sensitively by the changing width of encapsulated GNRs. Such intrinsic charge separation and widely tunable electronic properties without doping or an external field make this class of new carbon nanocomposites promising candidates for photovoltaic and electronics applications.

Crystal splitting and enhanced photocatalytic behavior of TiO2 rutile nano-belts induced by dislocations

Crystal splitting and enhanced photocatalytic activities caused by implied dislocations were observed in hierarchical TiO(2) nano-architectures prepared by one-pot hydrothermal synthesis in concentrated HCl. Microstructural observation revealed that the nanowires formed by continuous splitting of TiO(2) nano-belts, which is caused by a lattice misorientation of about 6°, were generated by an array of dislocations. In addition, the larger amount of dislocations implied in TiO(2) nano-architectures induces higher photocatalytic activities under ultra-violet illumination.

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.

Carrier Transport in Heterojunction Nanocrystals Under Strain

We present a theory for carrier transport in semiconducting nanoscale heterostructures that emphasizes the effects of strain at the interface between two different crystal structures. An exactly solvable model shows that the interface region, or junction, acts as a scattering potential that facilitates charge separation. As a case study, we model a type-II CdS/ZnSe heterostructure. After advancing a theory similar to that employed in model molecular conductance calculations, we calculate the electron and hole photocurrents and conductances, including nonlinear effects, through the junction at steady state.

First-principles studies on structural and electronic properties of GaN-AlN heterostructure nanowires

The structural and electronic properties of core-shell, eutectic, biaxial and superlattice GaN-AlN nanowires were studied through density functional theory computations. Due to more surface dangling bonds, nanowires with smaller diameters are energetically unfavorable. For the GaN-AlN heterostructure nanowires, their electronic properties highly depend on the GaN content, axial strain, configuration, and size. The valence bands are less affected by the GaN content, while the conduction bands depend on it. Hydrogen-passivated nanowires have much larger band gaps than their counterparts, since the surface states are removed by saturating the dangling bonds with hydrogen atoms. Moreover, due to multiple quantum-well structures, the confined electrons (holes) of superlattice nanowires become more localized and the difference of the mobility between the electron and hole becomes less apparent if the width of the barrier is larger. These findings are of value for better understanding heterostructure nanowires and their potential utilization.

Tuning semiconductor band edge energies for solar photocatalysis via surface ligand passivation

Semiconductor photocatalysts capable of broadband solar photon absorption may be nonetheless precluded from use in driving water splitting and other solar-to-fuel related reactions due to unfavorable band edge energy alignment. Using first-principles density functional theory and beyond, we calculate the electronic structure of passivated CdSe surfaces and explore the opportunity to tune band edge energies of this and related semiconductors via electrostatic dipoles associated with chemisorbed ligands. We predict substantial shifts in band edge energies originating from both the induced dipole at the ligand/CdSe interface and the intrinsic dipole of the ligand. Building on important induced dipole contributions, we further show that, by changing the size and orientation of the ligand's intrinsic dipole moment via functionalization, we can control the direction and magnitude of the shifts of CdSe electronic levels. Our calculations suggest a general strategy for enabling new active semiconductor photocatalysts with both optimal opto-electronic, and photo- and electrochemical properties.

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.

Spatially indirect emission in a luminescent nanocrystal molecule

Recent advances in the synthesis of multicomponent nanocrystals have enabled the design of nanocrystal molecules with unique photophysical behavior and functionality. Here we demonstrate a highly luminescent nanocrystal molecule, the CdSe/CdS core/shell tetrapod, which is designed to have weak vibronic coupling between excited states and thereby violates Kasha's rule via emission from multiple excited levels. Using single particle photoluminescence spectroscopy, we show that in addition to the expected LUMO to HOMO radiative transition, a higher energy transition is allowed via spatially indirect recombination. The oscillator strength of this transition can be experimentally controlled, enabling control over carrier behavior and localization at the nanoscale.

Probing the wave function delocalization in CdSe/CdS dot-in-rod nanocrystals by time- and temperature-resolved spectroscopy

Colloidal semiconductor quantum structures allow controlling the strong confinement of charge carriers through material composition and geometry. Besides being a unique platform to study fundamental effects, these materials attracted considerable interest due to their potential in opto-electronic and quantum communication applications. Heteronanostructures like CdSe/CdS offer new prospects to tailor their optical properties as they take advantage of a small conduction band offset allowing tunability of the electron delocalization from type-I toward quasi-type-II. Here, we report on a detailed study of the exciton recombination dynamics in CdSe/CdS heterorods. We observed a clear size-dependent radiative lifetime, which can be linked to the different degree of electron wave function (de)localization. Moreover, by increasing the temperature from 70 to 300 K, we observed a considerable increase of the radiative lifetime, clearly demonstrating a reduction of the conduction band offset at higher temperatures. Understanding and controlling electron delocalization in such heterostructures will be pivotal for realizing efficient and low-cost photonic devices.

Exciton Dissociation in CdSe/CdTe Heterostructure Nanorods

Type-II heterostructure nanorods hold good prospects for efficient charge separation in nano solar cells. Here we employed local density approximation (LDA) quality plane wave pseudopotential methods to study exciton dissociation in CdSe/CdTe collinear nanorods. We corrected the LDA band gap by approximating GW equations, and studied the correlation effect with configuration interaction methods. The calculated binding energy and radiative decay time of the charge transfer excitons agree well with experiments. The thermally activated escaping time is estimated to be shorter than the radiative recombination time, indicating the possibility of exciton dissociation if the nonradiative channel is ignored.