Complications in silane-assisted GaN nanowire growth.

Understanding the growth mechanisms of III-nitride nanowires is of great importance to realise their full potential. We present a systematic study of silane-assisted GaN nanowire growth on c-sapphire substrates by investigating the surface evolution of the sapphire substrates during the high temperature annealing, nitridation and nucleation steps, and the growth of GaN nanowires. The nucleation step - which transforms the AlN layer formed during the nitridation step to AlGaN - is critical for subsequent silane-assisted GaN nanowire growth. Both Ga-polar and N-polar GaN nanowires were grown with N-polar nanowires growing much faster than the Ga-polar nanowires. On the top surface of the N-polar GaN nanowires protuberance structures were found, which relates to the presence of Ga-polar domains within the nanowires. Detailed morphology studies revealed ring-like features concentric with the protuberance structures, indicating energetically favourable nucleation sites at inversion domain boundaries. Cathodoluminescence studies showed quenching of emission intensity at the protuberance structures, but the impact is limited to the protuberance structure area only and does not extend to the surrounding areas. Hence it should minimally affect the performance of devices whose functions are based on radial heterostructures, suggesting that radial heterostructures remain a promising device structure.

Structural investigations into colour-tuneable fluorescent InZnP-based quantum dots from zinc carboxylate and aminophosphine precursors.

Fluorescent InP-based quantum dots have emerged as valuable nanomaterials for display technologies, biological imaging, and optoelectronic applications. The inclusion of zinc can enhance both their emissive and structural properties and reduce interfacial defects with ZnS or CdS shells. However, the sub-particle distribution of zinc and the role this element plays often remains unclear, and it has previously proved challenging to synthesise Zn-alloyed InP-based nanoparticles using aminophosphine precursors. In this report, we describe the synthesis of alloyed InZnP using zinc carboxylates, achieving colour-tuneable fluorescence from the unshelled core materials, followed by a one-pot ZnS or CdS deposition using diethyldithiocarbamate precursors. Structural analysis revealed that the "core/shell" particles synthesised here were more accurately described as homogeneous extended alloys with the constituent shell elements diffusing through the entire core, including full-depth inclusion of zinc.

Enhancing the energy storage performances of metal-organic frameworks by controlling microstructure.

Metal-organic frameworks (MOFs) are among the most promising materials for next-generation energy storage systems. However, the impact of particle morphology on the energy storage performances of these frameworks is poorly understood. To address this, here we use coordination modulation to synthesise three samples of the conductive MOF Cu3(HHTP)2 (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) with distinct microstructures. Supercapacitors assembled with these samples conclusively demonstrate that sample microstructure and particle morphology have a significant impact on the energy storage performances of MOFs. Samples with 'flake-like' particles, with a pore network comprised of many short pores, display superior capacitive performances than samples with either 'rod-like' or strongly agglomerated particles. The results of this study provide a target microstructure for conductive MOFs for energy storage applications.

Elucidating heterogeneous iron biomineralization patterns in a denitrifying As(iii)-oxidizing bacterium: implications for arsenic immobilization.

Anaerobic nitrate-dependent iron(ii) oxidation is a process common to many bacterial species, which promotes the formation of Fe(iii) minerals that can influence the fate of soil and groundwater pollutants, such as arsenic. Herein, we investigated simultaneous nitrate-dependent Fe(ii) and As(iii) oxidation by Acidovorax sp. strain ST3 with the aim of studying the Fe biominerals formed, their As immobilization capabilities and the metabolic effect on cells. X-ray powder diffraction (XRD) and scanning transmission electron microscopy (STEM) nanodiffraction were applied for biomineral characterization in bulk and at the nanoscale, respectively. NanoSIMS (nanoscale secondary ion mass spectrometry) was used to map the intra and extracellular As and Fe distribution at the single-cell level and to trace metabolically active cells, by incorporation of a 13C-labeled substrate (acetate). Metabolic heterogeneity among bacterial cells was detected, with periplasmic Fe mineral encrustation deleterious to cell metabolism. Interestingly, Fe and As were not co-localized in all cells, indicating delocalized sites of As(iii) and Fe(ii) oxidation. The Fe(iii) minerals lepidocrocite and goethite were identified in XRD, although only lepidocrocite was identified via STEM nanodiffraction. Extracellular amorphous nanoparticles were formed earlier and retained more As(iii/v) than crystalline "flakes" of lepidocrocite, indicating that longer incubation periods promote the formation of more crystalline minerals with lower As retention capabilities. Thus, the addition of nitrate promotes Fe(ii) oxidation and formation of Fe(iii) biominerals by ST3 cells which retain As(iii/v), and although this process was metabolically detrimental to some cells, it warrants further examination as a viable mechanism for As removal in anoxic environments by biostimulation with nitrate.

Origin(s) of Anomalous Substrate Conduction in MOVPE-Grown GaN HEMTs on Highly Resistive Silicon.

The performance of transistors designed specifically for high-frequency applications is critically reliant upon the semi-insulating electrical properties of the substrate. The suspected formation of a conductive path for radio frequency (RF) signals in the highly resistive (HR) silicon substrate itself has been long held responsible for the suboptimal efficiency of as-grown GaN high electron mobility transistors (HEMTs) at higher operating frequencies. Here, we reveal that not one but two discrete channels distinguishable by their carrier type, spatial extent, and origin within the metal-organic vapor phase epitaxy (MOVPE) growth process participate in such parasitic substrate conduction. An n-type layer that forms first is uniformly distributed in the substrate, and it has a purely thermal origin. Alongside this, a p-type layer is localized on the substrate side of the AlN/Si interface and is induced by diffusion of group-III element of the metal-organic precursor. Fortunately, maintaining the sheet resistance of this p-type layer to high values (∼2000 Ω/□) seems feasible with particular durations of either organometallic precursor or ammonia gas predose of the Si surface, i.e., the intentional introduction of one chemical precursor just before nucleation. It is proposed that the mechanism behind the control actually relies on the formation of disordered AlSiN between the crystalline AlN nucleation layer and the crystalline silicon substrate.

Synthesis of IR-emitting HgTe quantum dots using an ionic liquid-based tellurium precursor.

New scalable precursor chemistries for quantum dots are highly desirable and ionic liquids are viewed as an attractive alternative to existing solvents, as they are often considered green and recyclable. Here we report the synthesis of HgTe quantum dots with emission in the near-IR region using a phosphonium based ionic liquid, and without standard phosphine capping agents.

Crystalline Interlayers for Reducing the Effective Thermal Boundary Resistance in GaN-on-Diamond.

Integrating diamond with GaN high electron mobility transistors (HEMTs) improves thermal management, ultimately increasing the reliability and performance of high-power high-frequency radio frequency amplifiers. Conventionally, an amorphous interlayer is used before growing polycrystalline diamond onto GaN in these devices. This layer contributes significantly to the effective thermal boundary resistance (TBReff) between the GaN HEMT and the diamond, reducing the benefit of the diamond heat spreader. Replacing the amorphous interlayer with a higher thermal conductivity crystalline material would reduce TBReff and help to enable the full potential of GaN-on-diamond devices. In this work, a crystalline Al0.32Ga0.68N interlayer has been integrated into a GaN/AlGaN HEMT device epitaxy. Two samples were studied, one with diamond grown directly on the AlGaN interlayer and another incorporating a thin crystalline SiC layer between AlGaN and diamond. The TBReff, measured using transient thermoreflectance, was improved for the sample with SiC (30 ± 5 m2 K GW-1) compared to the sample without (107 ± 44 m2 K GW-1). The reduced TBReff is thought to arise from improved adhesion between SiC and the diamond compared to the diamond directly on AlGaN because of an increased propensity for carbide bond formation between SiC and the diamond. The stronger carbide bonds aid transmission of phonons across the interface, improving heat transport.

Rapid and Low-Temperature Molecular Precursor Approach toward Ternary Layered Metal Chalcogenides and Oxides: Mo1-x W x S2 and Mo1-x W x O3 Alloys (0 ≤ x ≤ 1).

Metal sulfide and metal oxide alloys of the form Mo1-x W x S2 and Mo1-x W x O3 (0 ≤ x ≤ 1) are synthesized with varying nominal stoichiometries (x = 0, 0.25, 0.50, 0.75, and 1.0) by thermolysis of the molecular precursors MoL4 and WS(S2)L2 (where L = S2CNEt2) in tandem and in various ratios. Either transition-metal dichalcogenides or transition-metal oxides can be produced from the same pair of precursors by the choice of reaction conditions; metal sulfide alloys of the form Mo1-x W x S2 are produced in an argon atmosphere, while the corresponding metal oxide alloys Mo1-x W x O3 are produced in air, both under atmospheric pressure at 450 °C and for only 1 h. Changes in Raman spectra and in powder X-ray diffraction patterns are observed across the series of alloys, which confirm that alloying is successful in the bulk materials. For the oxide materials, we show that the relatively complicated diffraction patterns are a result of differences in the tilt angle of MO6 octahedra within three closely related unit cell types. Alloying of Mo and W in the products is characterized at the microscale and nanoscale by scanning electron microscopy-energy-dispersive X-ray spectroscopy (EDX) and scanning transmission electron microscopy-EDX spectroscopy, respectively.

Characterising porosity in platinum nanoparticles.

Accurately determining the morphology and hence the true surface areas of catalytic nanoparticles remains challenging. For many chemically synthesised nanoparticle suspensions conventional BET surface area measurements are often not feasible due to the large quantities of material required. For platinum, a paradigmatic catalyst, this issue is further complicated by the propensity of this metal to form porous aggregate structures comprised of smaller (ca. 2-5 nm) crystallites as opposed to continuous solid structures. This dendritic/porous particulate morphology leads to a large but poorly defined 'active' surface which is difficult to measure accurately. Here we compare, single nanoparticle electrochemistry with three dimensional (3D) electron tomography and quantitative 2D high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) analysis to yield insights into the porosity and chemically accessible surface area of a 30 nm diameter commercial Pt nanoparticle catalyst. Good quantitative agreement is found between 2D and 3D STEM-based measurements of the particle morphology, density and size distribution. Both 3D STEM tomography and single nanoparticle electrochemical measurements allow quantification of the surface area but the electrocatalytic surface area is found to be 2.8× larger than is measured in STEM; indicating the importance of the atomic scale roughness and structure (<2 nm) in contributing to the total catalytic surface area of the nanomaterial.

Optimizing hot carrier effects in Pt-decorated plasmonic heterostructures.

Hot carrier generation by light in various semiconductors and metallic nanostructures is important for many photocatalytic and photochemical processes, including water and hydrogen splitting. Here, we report on investigations of hot electron generation and extraction from Pt decorated SiO2-Au nanoparticles using the degradation of methylene blue dye as a test-bed. Enhanced catalytic activity was found with an increase of Pt loading on the surface of the heterostructures. The small size of the Au nanoparticles (∼12 nm) decorating the silica nanoparticles reduces hot electron collisions and related thermalization processes, since charge carriers have short paths to the surface where reactions take place and where Pt is situated. The heterostructures exhibit a broad plasmonic resonance in the visible wavelength range from 500 to 700 nm and hot carrier generation predominately takes place under resonant excitation. Electron-microscopy characterization and numerical modelling have allowed the optimization of Pt coverage for hot-electron transfer, consisting of a thin Pt shell covering the Au nanoparticle with Pt nanoparticles additionally placed on top. This geometry provides an increased number of active sites for methylene blue degradation and promotes separation of charge carriers generated by plasmonic excitations in Au. Such SiO2-Au-Pt nanoparticles are attractive for hot-electron production due to the tunability of their plasmonic resonance and enhanced catalytic activity.

Confinement Effects and Charge Dynamics in Zn3N2 Colloidal Quantum Dots: Implications for QD-LED Displays.

Zinc nitride (Zn3N2) colloidal quantum dots are composed of nontoxic, low-cost, and earth-abundant elements. The effects of quantum confinement on the optical properties and charge dynamics of these dots are studied using steady-state optical characterization and ultrafast fluence-dependent transient absorption. The absorption and emission energies are observed to be size-tunable, with the optical band gap increasing from 1.5 to 3.2 eV as the dot diameter decreased from 8.9 to 2.7 nm. Size-dependent absorption cross sections (σ = 1.22 ± 0.02 × 10-15 to 2.04 ± 0.03 × 10-15 cm2), single exciton lifetimes (0.36 ± 0.02 to 0.65 ± 0.03 ns), as well as Auger recombination lifetimes of biexcitons (3.2 ± 0.4 to 5.0 ± 0.1 ps) and trions (20.8 ± 1.8 to 46.3 ± 1.3 ps) are also measured. The degeneracy of the conduction band minimum (g = 2) is determined from the analysis of the transient absorption spectra at different excitation fluences. The performance of Zn3N2 colloidal quantum dots thus broadly matches that of established visible light emitting quantum dots based on toxic or rare elements, making them a viable alternative for QD-LED displays.

The passivating effect of cadmium in PbS/CdS colloidal quantum dots probed by nm-scale depth profiling.

Achieving control of the surface chemistry of colloidal quantum dots (CQDs) is essential to fully exploit their properties in solar cells, but direct measurement of the chemistry and electronic structure in the outermost atomic layers is challenging. Here we probe the surface oxidation and passivation of cation-exchanged PbS/CdS core/shell CQDs with sub nm-scale precision using synchrotron-radiation-excited depth-profiling photoemission. We investigate the surface composition of the topmost 1-2.5 nm of the CQDs as a function of depth, for CQDs of varying CdS shell thickness, and examine how the surface changes after prolonged air exposure. We demonstrate that the Cd is localized at the surface of the CQDs. The surface-localized products of oxidation are identified, and the extent of oxidation quantified. We show that oxidised sulfur species are progressively eliminated as Cd replaces Pb at the surface. A sub-monolayer surface 'decoration' of Cd is found to be effective in passivating the CQDs. We show that the measured energy-level alignments at PbS/CdS colloidal quantum dot surfaces differ from those expected on the basis of bulk band offsets, and are strongly affected by the oxidation products. We develop a model for the passivating action of Cd. The optimum shell thickness (of around 0.1 nm, previously found to give maximised power conversion efficiency in PbS/CdS solar cells) is found to correspond to a trade-off between the rate of oxidation and the introduction of a surface barrier to charge transport.

Importance of the structural integrity of a carbon conjugated mediator for photocatalytic hydrogen generation from water over a CdS-carbon nanotube-MoS2 composite.

Incorporation of CdS quantum dots is shown to significantly promote photocatalytic hydrogen production from water over single-layer MoS2 in a remote manner via their dispersions on a carbon nanotube as a nanocomposite: the hydrogen evolution rate is found to be critically dependent on the content and structural integrity of the carbon nanotube such that the double-walled carbon nanotube shows superior H2 production to a single-walled carbon nanotube because the inner carbon tubules survive from the structural damage during functionalization.

Hydrophobin-Encapsulated Quantum Dots.

The phase transfer of quantum dots to water is an important aspect of preparing nanomaterials that are suitable for biological applications, and although numerous reports describe ligand exchange, very few describe efficient ligand encapsulation techniques. In this report, we not only report a new method of phase transferring quantum dots (QDs) using an amphiphilic protein (hydrophobin) but also describe the advantages of using a biological molecule with available functional groups and their use in imaging cancer cells in vivo and other imaging applications.

Dynamics in next-generation solar cells: time-resolved surface photovoltage measurements of quantum dots chemically linked to ZnO (101[combining macron]0).

The charge dynamics at the surface of the transparent conducting oxide and photoanode material ZnO are investigated in the presence and absence of light-harvesting colloidal quantum dots (QDs). The time-resolved change in surface potential upon photoexcitation has been measured in the m-plane ZnO (101[combining macron]0) using a laser pump-synchrotron X-ray probe methodology. By varying the oxygen annealing conditions, and hence the oxygen vacancy concentration of the sample, we find that dark carrier lifetimes at the ZnO surface vary from hundreds of µs to ms timescales, i.e. a persistent photoconductivity (PPC) is observed. The highly-controlled nature of our experiments under ultra-high vacuum (UHV), and the use of band-gap and sub-band-gap photoexcitation, allow us to demonstrate that defect states ca. 340 meV above the valence band edge are directly associated with the PPC, and that the PPC mediated by these defects dominates over the oxygen photodesorption mechanism. These observations are consistent with the hypothesis that ionized oxygen vacancy states are responsible for the PPC in ZnO. The effect of chemically linking two colloidal QD systems (type I PbS and type II CdS-ZnSe) to the surface has also been investigated. Upon deposition of the QDs onto the surface, the dark carrier lifetime and the surface photovoltage are reduced, suggesting a direct injection of charge carriers into the ZnO conduction band. The results are discussed in the context of the development of next-generation solar cells.

Influence of luminescence quantum yield, surface coating, and functionalization of quantum dots on the sensitivity of time-resolved FRET bioassays.

In clinical diagnostics, homogeneous time-resolved (TR) FRET immunoassays are used for fast and highly sensitive detection of biomarkers in serum samples. The most common immunoassay format is based on europium chelate or cryptate donors and allophycocyanin acceptors. Replacing europium donors with terbium complexes and the acceptors with QDs offers large photophysical advantages for multiplexed diagnostics, because the Tb-complex can be used as FRET donor for QD acceptors of different colors. Water-soluble and biocompatible QDs are commercially available or can be synthesized in the laboratory using many available recipes from the literature. Apart from the semiconductor material composition, an important aspect of choosing the right QD for TR-FRET assays is the thickness of the QD coating, which will influence the photophysical properties and long-term stability as well as the donor-acceptor distance and FRET efficiency. Here we present a detailed time-resolved spectroscopic study of three different QDs with an emission maximum around 605 nm for their application as FRET acceptors (using a common Tb donor) in TR-bioassays: (i) Invitrogen/Life Technologies Qdot605, (ii) eBioscience eFluorNC605 and iii) ter-polymer stabilized CdSe/CdS/ZnS QDs synthesized in our laboratories. All FRET systems are very stable and possess large Förster distances (7.4-9.1 nm), high FRET efficiencies (0.63-0.80) and low detection limits (0.06-2.0 pM) within the FRET-bioassays. Shapes, sizes and the biotin/QD ratio of the biocompatible QDs could be determined directly in the solution phase bioassays at subnanomolar concentrations. Both commercial amphiphilic polymer/lipid encapsulated QDs and self-made ligand-exchanged QDs provide extremely low detection limits for highly sensitive TR-FRET bioassays.

Low temperature phase selective synthesis of Cu(2)ZnSnS(4) quantum dots.

The application of indium-free quaternary chalcogenides, such as Cu(2)ZnSnS(4) (CZTS), in photovoltaics has created tremendous interest in recent years. In this paper we develop a method to synthesize high quality CZTS nanoparticles with thermodynamically stable kesterite and wurtzite phases via a simple, one-pot, low-cost solution method.