Colloidal Approaches to Zinc Oxide Nanocrystals.

Zinc oxide is an extensively studied semiconductor with a wide band gap in the near-UV. Its many interesting properties have found use in optics, electronics, catalysis, sensing, as well as biomedicine and microbiology. In the nanoscale regime the functional properties of ZnO can be precisely tuned by manipulating its size, shape, chemical composition (doping), and surface states. In this review, we focus on the colloidal synthesis of ZnO nanocrystals (NCs) and provide a critical analysis of the synthetic methods currently available for preparing ZnO colloids. First, we outline key thermodynamic considerations for the nucleation and growth of colloidal nanoparticles, including an analysis of different reaction methodologies and of the role of dopant ions on nanoparticle formation. We then comprehensively review and discuss the literature on ZnO NC systems, including reactions in polar solvents that traditionally occur at low temperatures upon addition of a base, and high temperature reactions in organic, nonpolar solvents. A specific section is dedicated to doped NCs, highlighting both synthetic aspects and structure-property relationships. The versatility of these methods to achieve morphological and compositional control in ZnO is explicated. We then showcase some of the key applications of ZnO NCs, both as suspended colloids and as deposited coatings on supporting substrates. Finally, a critical analysis of the current state of the art for ZnO colloidal NCs is presented along with existing challenges and future directions for the field.

Photoactive p-Type Spinel CuGa2O4 Nanocrystals.

Herein we report the synthesis and characterization of spinel copper gallate (CuGa2O4) nanocrystals (NCs) with an average size of 3.7 nm via a heat-up colloidal reaction. CuGa2O4 NCs have a band gap of ∼2.5 eV and marked p-type character, in agreement with ab initio simulations. These novel NCs are demonstrated to be photoactive, generating a clear and reproducible photocurrent under blue light irradiation when deposited as thin films. Crucially, the ability to adjust the Cu/Ga ratio within the NCs, and the effect of this on the optical and electronic properties of the NCs, was also demonstrated. These results position CuGa2O4 NCs as a novel material for optoelectronic applications, including hole transport and light harvesting.

Substrate Morphology Directs (001) Sb2 Se3 Thin Film Growth by Crystallographic Orientation Filtering.

Antimony chalcogenide, Sb2 X3 (X = S, Se), applications greatly benefit from efficient charge transport along covalently bonded (001) oriented (Sb4 X6 )n ribbons, making thin film orientation control highly desirable - although particularly hard to achieve experimentally. Here, it is shown for the first time that substrate nanostructure plays a key role in driving the growth of (001) oriented antimony chalcogenide thin films. Vapor Transport Deposition of Sb2 Se3 thin films is conducted on ZnO substrates whose morphology is tuned between highly nanostructured and flat. The extent of Sb2 Se3 (001) orientation is directly correlated to the degree of substrate nanostructure. These data showcase that nanostructuring a substrate is an effective tool to control the orientation and morphology of Sb2 Se3 films. The optimized samples demonstrate high (001) crystallographic orientation. A growth mechanism for these films is proposed, wherein the substrate physically restricts the development of undesirable crystallographic orientations. It is shown that the surface chemistry of the nanostructured substrates can be altered and still drive the growth of (001) Sb2 Se3 thin films - not limiting this phenomenon to a particular substrate type. Insights from this work are expected to guide the rational design of Sb2 X3 thin film devices and other low-dimensional crystal-structured materials wherein performance is intrinsically linked to morphology and orientation.

Solution processed bismuth oxyiodide (BiOI) thin films and solar cells.

Post transition metal chalcohalides are an emerging class of semiconductor materials for optoelectronic applications. Within this class, bismuth oxyiodide (BiOI) is of particular interest due to its high environmental stability, low toxicity, and defect tolerance considered typical of 'ns2' materials. Here we fabricate BiOI thin films using a solution-processed method that affords pin-hole free highly pure films without any residual carbon or other contaminant species. Based on these films, solution processed all-inorganic solar cells with an architecture ITO/NiOx/BiOI/ZnO/Al are fabricated for the first time. Additional device improvements are realised by templating BiOI thin film growth to attain efficiencies that rival some of the best vacuum deposited devices. The BiOI thin films and devices outlined here are an excellent platform for the further development of solution processed bismuth chalcohalide optoelectronic devices.

Cu2ZnSnS(4x)Se(4(1-x)) solar cells from polar nanocrystal inks.

A facile ligand exchange method for dispersing Cu2ZnSnS4 (CZTS) nanocrystals (NCs) in environmentally benign polar solvents, such as ethanol or n-propanol, at high concentrations (up to 200 mg/mL) is demonstrated. This approach has been applied to CZTS nanocrystals synthesized via scalable, noninjection methods to formulate colloidally stable inks that are suitable for the solution processing of solar cell devices. Unlike other inks currently used to fabricate NC solar cells, the CZTS nanocrystal ink developed here circumvents the need for hydrazine, pyridine, or thiol coordinating solvents. By combining our polar CZTS inks with optimized selenization procedures, substrate CZTSSe solar cells have been successfully fabricated with device efficiencies of 7.7%.

Dynamic Structural Evolution and Dual Emission Behavior in Hybrid Organic Lead Bromide Perovskites.

The optoelectronic properties of organic lead halide perovskites (OLHPs) strongly depend on their underlying crystal symmetry and dynamics. Here, we exploit temperature-dependent synchrotron powder X-ray diffraction and temperature-dependent photoluminescence to investigate how the subtle structural changes happening in the pure and mixed A-site cation MA1-xFAxPbBr3 (x = 0, 0.5, and 1) systems influences their optoelectronic properties. Diffraction investigations reveal a cubic structure at high temperatures and tetragonal and orthorhombic structures with octahedral distortion at low temperatures. Steady state photoluminescence and time correlated single photon counting study reveals that the dual emission behavior of these OLHPs is due to the direct-indirect band formation. In the orthorhombic phase of MAPbBr3, the indirect band is dominated by self-trapped exciton (STE) emission due to the higher-order lattice distortions of PbBr6 octahedra. Our findings provide a comprehensive explanation of the dual emission behavior of OLHPs while also providing a rationale for previous experimental observations.

Near-infrared absorbing Cu12Sb4S13 and Cu3SbS4 nanocrystals: synthesis, characterization, and photoelectrochemistry.

Herein, we present the novel synthesis of tetrahedrite copper antimony sulfide (CAS) nanocrystals (Cu12Sb4S13), which display strong absorptions in the visible and NIR. Through ligand tuning, the size of the Cu12Sb4S13 NCs may be increased from 6 to 18 nm. Phase purity is achieved through optimizing the ligand chemistry and maximizing the reactivity of the antimony precursor. We provide a detailed investigation of the optical and photoelectrical properties of both tetrahedrite (Cu12Sb4S13) and famatinite (Cu3SbS4) NCs. These NCs were found to have very high absorption coefficients reaching 10(5) cm(-1) and band gaps of 1.7 and 1 eV for tetrahedrite and famatinite NCs, respectively. Ultraviolet photoelectron spectroscopy was employed to determine the band positions. In each case, the Fermi energies reside close to the valence band, indicative of a p-type semiconductor. Annealing of tetrahedrite CAS NC films in sulfur vapor at 350 °C was found to result in pure famatinite NC films, opening the possibility to tune the crystal structure within thin films of these NCs. Photoelectrochemistry of hydrazine free unannealed films displays a strong p-type photoresponse, with up to 0.1 mA/cm(2) measured under mild illumination. Collectively these optical properties make CAS NCs an excellent new candidate for both thin film and hybrid solar cells and as strong NIR absorbers in general.

Optimal Geometry for Plasmonic Hot-Carrier Extraction in Metal-Semiconductor Nanocrystals.

Plasmon-induced energy and charge transfer from metal nanostructures hold great potential for harvesting solar energy. Presently, the efficiencies of charge-carrier extraction are still low due to the competitive ultrafast mechanisms of plasmon relaxation. Using single-particle electron energy loss spectroscopy, we correlate the geometrical and compositional details of individual nanostructures to their carrier extraction efficiencies. By removing ensemble effects, we are able to show a direct structure-function relationship that permits the rational design of the most efficient metal-semiconductor nanostructures for energy harvesting applications. In particular, by developing a hybrid system comprising Au nanorods with epitaxially grown CdSe tips, we are able to control and enhance charge extraction. We show that optimal structures can have efficiencies as high as 45%. The quality of the Au-CdSe interface and the dimensions of the Au rod and CdSe tip are shown to be critical for achieving these high efficiencies of chemical interface damping.

Highly Conductive and Visibly Transparent p-Type CuCrO2 Films by Ultrasonic Spray Pyrolysis.

The development of high-performing p-type transparent conducting oxides will enable immense progress in the fabrication of optoelectronic devices including invisible electronics and all-oxide power electronics. While n-type transparent electrodes have already reached widespread industrial production, the lack of p-type counterparts with comparable transparency and conductivity has created a bottleneck for the development of next-generation optoelectronic devices. In this work, we present the fabrication of delafossite copper chromium oxide p-type transparent electrodes with outstanding optical and electrical properties. These layers were deposited using ultrasonic spray pyrolysis, a wet chemical method that is fast, simple, and scalable. Through careful screening of the deposition conditions, highly crystalline, dense, and smooth CuCrO2 coatings were obtained. A detailed investigation of the role played by the deposition temperature and the cation ratio enabled the properties of the prepared layers to be reliably tuned, as verified using X-ray diffraction, X-ray photoelectron spectroscopy, optical spectroscopy, Hall effect measurements, and electron and atomic force microscopies. We demonstrate record conductivities for solution-processed CuCrO2, exceeding 100 S cm-1, and we also obtained the highest value for two separate figures of merit for p-type transparent conducting oxides. These performances position solution-deposited CuCrO2 as the leading p-type transparent-conducting oxide currently available.

Fluorine-Doped Tin Oxide Colloidal Nanocrystals.

Fluorine-doped tin oxide (FTO) is one of the most studied and established materials for transparent electrode applications. However, the syntheses for FTO nanocrystals are currently very limited, especially for stable and well-dispersed colloids. Here, we present the synthesis and detailed characterization of FTO nanocrystals using a colloidal heat-up reaction. High-quality SnO2 quantum dots are synthesized with a tuneable fluorine amount up to ~10% atomic, and their structural, morphological and optical properties are fully characterized. These colloids show composition-dependent optical properties, including the rise of a dopant-induced surface plasmon resonance in the near infrared.

Significant Enhancement of Antimicrobial Activity in Oxygen-Deficient Zinc Oxide Nanowires.

The fabrication of antimicrobial surfaces that exhibit enhanced activity toward a large variety of microbial species is one of the major challenges of our time. In fact, the negative effects associated with both bacterial and fungal infections are enormous, especially considering that many microbial species are developing resistance to known antibiotics. In this work, we show how a combination of a specific surface morphology and surface chemistry can create a surface that exhibits nearly 100% antimicrobial activity toward both Gram-negative and Gram-positive bacteria and fungal cells. Arrays of vertically aligned, oxygen-deficient zinc oxide (ZnO) nanowires grown on a substrate exhibit enhanced antimicrobial activity compared with surfaces containing either less defective nanowires or highly oxygen-deficient flat films. This synergistic effect between physical activity (morphology) and chemical activity (surface composition) has been shown to be responsible for the outstanding antimicrobial activity of our surfaces, especially toward notoriously resilient bacterial or fungal species. These findings provide a series of design rules for tuning the activities of antibacterial and antifungal nanomaterials. These rules constitute an excellent platform for the development of next-generation antimicrobial surfaces.

Mapping the optical properties of CdSe/CdS heterostructure nanocrystals: the effects of core size and shell thickness.

Here we present the first comprehensive report on CdSe/CdS heterostructure nanocrystals. The effects of core size and shell thickness on the optical properties of CdSe/CdS heterostructure nanocrystals are investigated. We report a reliable synthetic method to grow thick CdS shells on CdSe cores with sizes ranging from 2.5-4.7 nm. We provide a calibration curve, which enables determination of CdS shell thickness (+/-0.1 nm) over a wide range of core sizes, circumventing the need for time-consuming HRTEM analyses. Epitaxial growth of the shells was verified by HRTEM, XRD, and SAED. In-situ reaction measurements revealed the average per particle (p) deposition rates for cadmium and sulfur to be k(Cd) = 5.38 x 10(-25) mol s(-1) p(-1) and k(S) = 4.83 x 10(-24) mol s(-1) p(-1). Faster sulfur deposition rates are attributed to the absence of strong sulfur binding ligands in the growth medium. Through the rigorous use of high resolution transmission electron microscopy, a direct link between the dimensions of the heterostructures and their band-edge transition energies, quantum yields, and excited state lifetimes is established. The experiments show that the band-edge transition energies of the core samples, which initially span approximately 431 meV, condense to span only 163 meV after the growth of a 6 monolayer-thick CdS shell. Furthermore, shifts in the band-edge transition energies were found to be extremely sensitive to core size. The QY of the as-prepared core/shells ranged from 25 to 60%. The QYs and band-edge lifetimes of the core/shells were found to depend upon the ligands adsorbed to the particle surface. These data prove that one or both of the charge carriers still has access to the particle surface despite the presence of a 2.2 nm thick CdS shell.

Ultrathin Solar Absorber Layers of Silver Bismuth Sulfide from Molecular Precursors.

Here we present a robust molecular precursor-based approach to synthesize high-quality thin films of silver bismuth sulfide (AgBiS2). Pure-phase cubic AgBiS2 thin films are prepared, which are smooth and dense down to thicknesses less than 40 nm. Comprehensive structural and morphological analysis of the as-prepared films as a function of processing temperature and composition is presented, including X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy. The optical properties of the films and their electronic band structure are also presented. The as-prepared films show impressive light absorption properties with absorption coefficients reaching 105 cm-1 for energies above ca. 950 nm. Finally, their photoactivity is demonstrated through photoconductivity measurements on lateral electrodes. The methods outlined herein enable the fabrication of AgBiS2 semiconductor thin films at low processing temperatures (150 °C) with a dense morphology and tunable Ag/Bi composition. Such films provide an excellent platform for the fabrication of AgBiS2-based optoelectronic devices, specifically solar cells.

Accurate control of stoichiometry and doping in barium stannate perovskite oxide nanoparticles.

Barium stannate (BaSnO3) is one of the most promising emerging materials for use as a transparent electrode. However, to date, its synthesis has been proven to be highly irreproducible. In this communication, we present a detailed investigation of the reproducibility issues and provide a robust approach to synthesize BaSnO3 nanomaterials with controlled stoichiometry and doping.

Augmented band gap tunability in indium-doped zinc sulfide nanocrystals.

Doping semiconductor nanocrystals is a powerful tool to impart new and beneficial optical and electrical properties to the host nanocrystals. Doping has been used to improve the performances of nanocrystal-based devices in applications as diverse as optics, magnetism, electronics, catalysis and sensing. In this work we present a low temperature colloidal synthesis of zinc sulfide (ZnS) nanocrystals doped with indium. Through optimization of the reaction parameters and the doping level, quantum confined (∼2 nm in size) crystalline colloids with highly tunable optical properties are achieved. Using a suite of characterization techniques including X-ray diffraction, high-resolution transmission electron microscopy, optical spectroscopies (absorption, emission, and Raman), compositional analyses and first principles simulations, we investigate the structural, morphological and optical properties of the synthesized nanocrystals. Indium dopants are found to heavily influence the band gap of ZnS. This strategy in addition to traditional methods of size control enables the synthesis of nanocrystals with finely tunable band gaps between ∼3.8 eV-4.3 eV. These doped ZnS nanocrystals are fabricated into selective UV thin-film absorbers and discriminatory proof-of-concept UVA-UVB/C photodetectors.

Oxygen-deficient photostable Cu2O for enhanced visible light photocatalytic activity.

Oxygen vacancies in inorganic semiconductors play an important role in reducing electron-hole recombination, which may have important implications in photocatalysis. Cuprous oxide (Cu2O), a visible light active p-type semiconductor, is a promising photocatalyst. However, the synthesis of photostable Cu2O enriched with oxygen defects remains a challenge. We report a simple method for the gram-scale synthesis of highly photostable Cu2O nanoparticles by the hydrolysis of a Cu(i)-triethylamine [Cu(i)-TEA] complex at low temperature. The oxygen vacancies in these Cu2O nanoparticles led to a significant increase in the lifetimes of photogenerated charge carriers upon excitation with visible light. This, in combination with a suitable energy band structure, allowed Cu2O nanoparticles to exhibit outstanding photoactivity in visible light through the generation of electron-mediated hydroxyl (OH˙) radicals. This study highlights the significance of oxygen defects in enhancing the photocatalytic performance of promising semiconductor photocatalysts.

Exfoliation Solvent Dependent Plasmon Resonances in Two-Dimensional Sub-Stoichiometric Molybdenum Oxide Nanoflakes.

Few-layer two-dimensional (2D) molybdenum oxide nanoflakes are exfoliated using a grinding assisted liquid phase sonication exfoliation method. The sonication process is carried out in five different mixtures of water with both aprotic and protic solvents. We found that surface energy and solubility of mixtures play important roles in changing the thickness, lateral dimension, and synthetic yield of the nanoflakes. We demonstrate an increase in proton intercalation in 2D nanoflakes upon simulated solar light exposure. This results in substoichiometric flakes and a subsequent enhancement in free electron concentrations, producing plasmon resonances. Two plasmon resonance peaks associated with the thickness and the lateral dimension axes are observable in the samples, in which the plasmonic peak positions could be tuned by the choice of the solvent in exfoliating 2D molybdenum oxide. The extinction coefficients of the plasmonic absorption bands of 2D molybdenum oxide nanoflakes in all samples are found to be high (ε > 10(9) L mol(-1) cm(-1)). It is expected that the tunable plasmon resonances of 2D molybdenum oxide nanoflakes presented in this work can be used in future electronic, optical, and sensing devices.

Phosphine-free synthesis of CdSe nanocrystals.

High quality CdSe nanocrystals have been prepared using elemental selenium as the chalcogenide precursor dispersed in 1-octadecene (ODE). The conditions used to prepare the Se precursor were found to be critical for successful nanocrystal synthesis. Systematic titration of the Se precursor solution with tri-n-octylphosphine (TOP) allowed the Se reactivity to be tuned and the final particle size to be controlled. Band-edge and surface related emission were observed for samples prepared in the presence and absence of added TOP. In the absence of a selenium passivant, the crystal structure of CdSe nanocrystals could be altered from zinc blende to wurtzite by the addition of bis(2,2,4-trimethylpentyl)phosphinic acid (TMPPA).

Photonic Sintering of Copper through the Controlled Reduction of Printed CuO Nanocrystals.

The ability to control chemical reactions using ultrafast light exposure has the potential to dramatically advance materials and their processing toward device integration. In this study, we show how intense pulsed light (IPL) can be used to trigger and modulate the chemical transformations of printed copper oxide features into metallic copper. By varying the energy of the IPL, CuO films deposited from nanocrystal inks can be reduced to metallic Cu via a Cu2O intermediate using single light flashes of 2 ms duration. Moreover, the morphological transformation from isolated Cu nanoparticles to fully sintered Cu films can also be controlled by selecting the appropriate light intensity. The control over such transformations enables for the fabrication of sintered Cu electrodes that show excellent electrical and mechanical properties, good environmental stability, and applications in a variety of flexible devices.

Plasmonic Ge-doped ZnO nanocrystals.

We present the first colloidal synthesis of Ge-doped ZnO nanocrystals, which are produced by a scalable method that uses only air and moisture stable precursors. The incorporation of tetravalent Ge ions within ZnO nanocrystals generates a surface plasmon resonance in the near-mid infrared, and induces a change in morphology, from isotropic spheroidal nanocrystals to rod-like, elongated structures with a distinctive c-axis orientation.