Zinc Blende Core Research Articles

Spatially ordered NiOOH-ZnS/CdS heterostructures with an efficient photo-carrier transmission channel for markedly improved H2 production.

Spatially-ordered 1D nanocrystal-based semiconductor nanostructures possess distinct merits for photocatalytic reaction, including large surface area, fast carrier separation, and enhanced light scattering and absorption. Nevertheless, establishing a valid photo-carrier transmission channel is still crucial yet challenging for semiconductor heterostructures to realize efficient photocatalysis. In this work, spatially ordered NiOOH-ZnS/CdS heterostructures were constructed by sequential ZnS coating and NiOOH photo-deposition on multi-armed CdS, which consists of {112̄0}-faceted wurtzite nanorods grown epitaxially on {111}-faceted zinc blende core. Intriguingly, the surface photovoltage spectroscopy and PbO2 photo-deposition results suggest that the photogenerated holes of CdS were first transferred to the Zn-vacancy level of ZnS and then to NiOOH, as driven by the built-in electric field between ZnS and CdS and the hole-extracting effect of the NiOOH cocatalyst, leading to the efficient charge separation of NiOOH-ZnS/CdS. With visible-light (λ > 420 nm) irradiation, NiOOH-ZnS/CdS exhibited a distinguished H2-evolution rate of 152.20 mmol g-1 h-1 (apparent quantum efficiency of 40.9% at 420 nm), approximately 18 folds that of 3 wt% Pt-loaded CdS and much higher than that of ZnS/CdS and NiOOH-CdS counterparts as well as the most reported CdS-containing photocatalysts. Moreover, the cycling and long-term H2 generation tests manifested the outstanding photocatalyst stability of NiOOH-ZnS/CdS. The study results presented here may propel the controllable design of highly-active nanomaterials for solar conversion and utilization.

Colloidal InAs Tetrapods: Impact of Surfactants on the Shape Control.

We have approached the synthesis of colloidal InAs nanocrystals (NCs) using amino-As and ligands that are different from the commonly employed oleylamine (OA). We found that carboxylic and phosphonic acids led only to oxides, whereas tri-n-octylphosphine, dioctylamine, or trioctylamine (TOA), when employed as the sole ligands, yielded InAs NCs with irregular sizes and a broad size distribution. Instead, various combinations of TOA and OA delivered InAs NCs with good control over the size distribution, and the TOA:OA volume ratio of 4:1 generated InAs tetrapods with arm length of 5-6 nm. Contrary to tetrapods of II-VI materials, which have a zinc-blende core and wurtzite arms, these NCs are entirely zinc-blende, with arms growing along the ⟨111⟩ directions. They feature a narrow excitonic peak at ∼950 nm in absorption and a weak photoluminescence emission at 1050 nm. Our calculations indicated that the bandgap of the InAs tetrapods is mainly governed by the size of their core and not by their arm lengths when these are longer than ∼3 nm. Nuclear magnetic resonance analyses revealed that InAs tetrapods are mostly passivated by OA with only a minor fraction of TOA. Molecular dynamics simulations showed that OA strongly binds to the (111) facets whereas TOA weakly binds to the edges and corners of the NCs and their combined use (at high TOA:OA volume ratios) promotes growth along the ⟨111⟩ directions, eventually forming tetrapods. Our work highlights the use of mixtures of ligands as a means of improving control over InAs NCs size and size distribution.

Synthesis and Characterization of CuZnSe2 Nanocrystals in Wurtzite, Zinc Blende, and Core–Shell Polytypes

CuZnSe2 (CZSe) is an important ternary semiconductor comprised of earth-abundant elements with a suitable bandgap for visible light absorption and structural/stoichiometric versatility that make it a promising candidate for photovoltaic applications. Here we report the controlled synthesis of the compound copper chalcogenide in nanocrystal form using a colloidal hot injection approach. Furthermore, we demonstrate control over the crystal phase to occur as either wurtzite (WZ) or zinc blende (ZB) as a function of the presence and absence of phosphine-based ligands. A major emission peak was observed at ∼1.7 eV using low-temperature photoluminescence (PL), ranging from 30 to 200 K. Additionally, we demonstrate the ability to extend this synthetic protocol to form a polytype structure comprised of a ZB core with a WZ shell.

Effect of Core/Shell Interface on Carrier Dynamics and Optical Gain Properties of Dual-Color Emitting CdSe/CdS Nanocrystals.

Two-color emitting colloidal semiconductor nanocrystals (NCs) are of interest for applications in multimodal imaging, sensing, lighting, and integrated photonics. Dual color emission from core- and shell-related optical transitions has been recently obtained using so-called dot-in-bulk (DiB) CdSe/CdS NCs comprising a quantum-confined CdSe core embedded into an ultrathick (∼7-9 nm) CdS shell. The physical mechanism underlying this behavior is still under debate. While a large shell volume appears to be a necessary condition for dual emission, comparison between various types of thick-shell CdSe/CdS NCs indicates a critical role of the interface "sharpness" and the presence of potential barriers. To elucidate the effect of the interface morphology on the dual emission, we perform side-by-side studies of CdSe/CdS DiB-NCs with nominally identical core and shell dimensions but different structural properties of the core/shell interface arising from the crystal structure of the starting CdSe cores (zincblende vs wurtzite). While both structures exhibit dual emission under comparable pump intensities, NCs with a zincblende core show a faster growth of shell luminescence with excitation fluence and a more readily realized regime of amplified spontaneous emission (ASE) even under "slow" nanosecond excitation. These distinctions can be linked to the structure of the core/shell interface: NCs grown from the zincblende cores contain a ∼3.5 nm thick zincblende CdS interlayer, which separates the core from the wurtzite CdS shell and creates a potential barrier for photoexcited shell holes inhibiting their relaxation into the core. This helps maintain a higher population of shell states and simplifies the realization of dual emission and ASE involving shell-based optical transitions.

Position-controlled growth of GaN nanowires and nanotubes on diamond by molecular beam epitaxy.

In this work the position-controlled growth of GaN nanowires (NWs) on diamond by means of molecular beam epitaxy is investigated. In terms of growth, diamond can be seen as a model substrate, providing information of systematic relevance also for other substrates. Thin Ti masks are structured by electron beam lithography which allows the fabrication of perfectly homogeneous GaN NW arrays with different diameters and distances. While the wurtzite NWs are found to be Ga-polar, N-polar nucleation leads to the formation of tripod structures with a zinc-blende core which can be efficiently suppressed above a substrate temperature of 870 °C. A variation of the III/V flux ratio reveals that both axial and radial growth rates are N-limited despite the globally N-rich growth conditions, which is explained by the different diffusion behavior of Ga and N atoms. Furthermore, it is shown that the hole arrangement has no effect on the selectivity but can be used to force a transition from nanowire to nanotube growth by employing a highly competitive growth regime.

Engineering the Effective p-Type Dopant in GaAs/InAs Core–Shell Nanowires with Surface Dangling Bonds

Using first-principles calculation based on density-functional theory, we investigated the effect of surface dangling bond on p-type doping mechanism and the electronic structures in wurtzite (WZ) and zinc blende (ZB) GaAs/InAs core–shell nanowires (NWs) along the [0001] and [111] directions, respectively. The results of the formation energies show that the surface dangling bond of the In atom is a kind of stable defect. Both in WZ and ZB core–shell NWs, we found it is easier and more stable to realize dopant in the GaAs core. Moreover, the position of Cd impurity plays a key role in the formation of p-type nanowires. The farther the distance between the impurity and the surface dangling In atom, the easier it is to form the p-type characteristic of the nanowires. In particular, it shows an intrinsic behavior when doping the Cd impurity near the surface dangling bond. The surface dangling bonds have an ability to capture the holes from the neighbor doping impurity, resulting in the deactivation of dopants. Meanwhile, the transfer of hole moves the valence band down to the lower energy levels and even can lead to a band anticrossing phenomenon in the conduction band. Our results highlight a new physical coupling between the doped state and surface dangling bonds in GaAs/InAs core–shell NWs, and open a new opportunity for the development of tailoring nanoscale electronic properties.

Thermally Induced Structural and Morphological Changes of CdSe/CdS Octapods

Branched nanostructures are of great interest because of their promising optical and electronic properties. For successful and reliable integration in applications such as photovoltaic devices, the thermal stability of the nanostructures is of major importance. Here the different domains (CdSe cores, CdS pods) of the heterogeneous octapods are shown to have different thermal stabilities, and heating is shown to induce specific shape changes. The octapods are heated from room temperature to 700 °C, and investigated using (analytical and tomographic) transmission electron microscopy (TEM). At low annealing temperatures, pure Cd segregates in droplets at the outside of the octapods, indicating non-stochiometric composition of the octapods. Furthermore, the tips of the pods lose their faceting and become rounded. Further heating to temperatures just below the sublimation temperature induces growth of the zinc blende core at the expense of the wurtzite pods. At higher temperatures, (500-700 °C), sublimation of the octapods is observed in real time in the TEM. Three-dimensional tomographic reconstructions reveal that the four pods pointing into the vacuum have a lower thermal stability than the four pods that are in contact with the support.

Ligand-Controlled Polytypism of Thick-Shell CdSe/CdS Nanocrystals

We report the synthesis of CdSe/CdS semiconductor core/shell nanocrystals with very thick (5 nm) CdS shells. As in the case of core CdSe nanocrystals, we show that a thick-shell CdSe/CdS core/shell structure can be synthesized in either a pure wurtzite (W) or a zinc-blende (ZB) crystal structure. While the growth of thick-shell wurtzite CdSe/CdS is quite straightforward, we observe that the growth of a CdS shell on zinc-blende CdSe cores is more difficult and leads to wurtzite/zinc-blende polytypism when primary amines are present during the shell formation. Using absorption spectra analysis to differentiate zinc blende from wurtzite CdSe, we show that primary amines can induce a nearly complete structural transformation of CdSe ZB cores into W cores. This better understanding of the CdSe ligand-dependent crystal structural evolution during shell growth is further used to grow large (10 nm)-diameter perfect zinc-blende CdSe core crystals emitting above 700 nm, and perfect ZB thick-shell CdSe/CdS nanocrystals. We observed that all thick-shell CdSe/CdS QDs have extremely reduced blinking events compared to thin-shell QDs, without any significant influence of crystalline structure and polytypism.

A hierarchical lattice structure and formation mechanism of ZnO nano-tetrapods

The existence of characteristic longitudinal optical and transverse optical phonons of cubicZnO in ZnO nano-tetrapods is determined by Raman spectroscopy and first-principlescalculations. Stacking sequence change at the boundary of the core and legs isalso identified by high-resolution transmission electron microscopy. Based on thisexperimental and theoretical evidence, we demonstrate that the lattice structure of ZnOnano-tetrapods is hierarchical with a zinc blende core connecting to four wurtzite legs.Furthermore, we establish the atomic configuration and propose a formation mechanisminduced by Laplace pressure in the initial growth stage of ZnO nano-tetrapods.

Growth mechanism and optical properties of ZnS nanotetrapods

ZnS nanotetrapods have been synthesized through the directreaction of Zn with S via a thermal evaporation method in Ar at800 °C without catalyst. Studies on the microstructures give experimental evidence for the growthmechanism of ZnS nanotetrapods. A zinc blende (ZB) ZnS core nucleates in the first stageduring the formation of ZnS nanotetrapods; wurtzite twins based on the ZB core then formand lead to the assembly of ZnS nanotetrapod structures. Room-temperaturephotoluminescence properties were also examined, showing a strong green emissioncentered at 495.2 nm.

Zinc-blende ZnO and its role in nucleating wurtzite tetrapods and twinned nanowires

In this letter, the authors directly observed the zinc-blende (ZB) ZnO core in the initial formation of wurtzite (WZ) ZnO tetrapods. The formation of the wurtzite (011¯3) twined nanowires is proposed based on the ZB core. Simple bonding density calculation shows that the wurtzite nanowires with {011¯0} side surfaces have the lowest surface energy. A favorable choice of WZ phase over ZB when forming nanostructures is likely to be a result of surface energy minimization. This could be the reason that ZnS nanowires take WZ rather than ZB.

CdSe Nano-tetrapods: Controllable Synthesis, Structure Analysis, and Electronic and Optical Properties

Tetrapod-shaped CdSe nanocrystals have been successfully synthesized in high yields using a simple method of controlling the protonic acidity of the reaction system. A possible growth mechanism is discussed on the basis of the surface modification of H+ present. The crystal structure of CdSe tetrapods is determined by high-resolution transmission electron microscopy analysis to have a zinc blende core at the center with four wurtzite arms growing out from the core along four [111] directions. The electronic structure of CdSe tetrapods is studied theoretically in comparison with that of spherical dots.

Shape-Controlled Synthesis of CdS Nanocrystals in Mixed Solvents

The formation of CdS nanotetrahedrons, pencil-shaped nanorods, tetrapods, prickly spheres, and high aspect-ratio hexagonal nanoprisms has been, respectively, achieved by adjusting the ratio of two solvents ethylenediamine and ethylene glycol under the solvothermal condition. None of the surfactants or other templates was needed in the process. The reaction time and temperature can be used as the additional means to control the size and morphology. By closely inspecting the growing process of the tetrapod structure and the crystallographic analysis of the products, we proposed that the lattice space match and the location match of ions at the interface of the zinc blende core and the wurtzite arms are two important structural factors to guide this furcate growth. Also, the anisotropic adsorption ability of ethylenediamine at the different surfaces of wurtzite CdS crystals results in the 1D growth of the arms. The possible growing mechanisms of CdS with other shapes were also discussed.

Surfactant–ligand co-assisted solvothermal technique for the synthesis of different-shaped CdS nanorod-based materials

1-D nanorods, twinrods, golfclubs, and tripods of CdS were prepared via a surfactant–ligand co-assisted solvothermal method at 160°C. The surfactant of S-dodecylisothiounium bromide (C 12) used in the process was favorable for synthesis of different-shaped CdS nanorod with high aspect ratio. X-ray diffraction (XRD) and TEM images showed that the 1-D nanorods had wurtzite phase and others had a zinc blende core and wurtzite arms. The morphologies of CdS prepared under different conditions suggested the “template-assistance” of the surfactant and that the nonaqueous organic media are important for the self-assembling of inorganic components at atomic level.