Precise Control Of Composition Research Articles

Facile synthesis and size dependent visible light photo catalytic properties of bio-compatible silver nanoclusters

Recently tremendous research efforts have been carried out to synthesize metal nanocluster owing to their wide applications in bio-imaging, nonlinear optical materials and catalysis. To explore such applications a precise control of size and composition of the metal nanoclusters are highly desirable. Here we report controlled synthesis and characterization of water soluble silver nanocluster ligated with glutathione. We have developed facile methodologies to prepare nanoclusters with distinct optical and catalytic properties. To achieve tunable size, optical property and chemical composition, here we mainly modulate the nature of metallic salt precursor, reducing agent and reaction conditions. Catalytic properties of as synthesized nanoclusters are explored in visible light induced photo-Fenton like processes for the degradation of model water pollutants such as methyl orange (MO) and Rhodamine B (RhB). The present research work investigates the influence of size of silver nanoclusters (Ag NCs) on photo Fenton like processes under visible light.

Comparing strategies for improving efficiencies in vacuum processed Cu2ZnSnSe4 solar cells

In this study, we detail a Cu2ZnSnSe4 based solar cell fabrication process based on the selenization of metallic precursor stacks with elemental Se. 9.4% efficient devices without antireflection coating have been obtained. First, reproducibility issues of the process are carefully shown and discussed. It is demonstrated that device performances are strongly impacted by the precise control of the precursor composition. Then, starting from this robust process, a review of existing strategies to improve kesterite efficiencies is conducted. A significant increase in efficiency (+1.4% absolute efficiency and +50 mV VOC) is obtained with absorber surface treatment and post-annealing, while no effect of Ge incorporation in the precursor stack is observed. This contradictory result to most of the recent publications raises the question of the universality of this strategy to improve kesterite solar cell performance. Finding a universal activation step to boost kesterite efficiencies and bring it to the market remains a crucial need for the community.

Direct Visualization of the Structural Transformation between the Lyotropic Liquid Crystalline Lamellar and Bicontinuous Cubic Mesophase.

The transition between the lyotropic liquid crystalline lamellar and the bicontinuous cubic mesophase drives multiple fundamental cellular processes involving changes in cell membrane topology, including endocytosis and membrane budding. While several theoretical models have been proposed to explain this dynamic transformation, experimental validation of these models has been challenging because of the short-lived nature of the intermediates present during the phase transition. Herein, we report the direct observation of a lamellar-to-bicontinuous cubic phase transition in nanoscale dispersions using a combination of cryogenic transmission electron microscopy and static small-angle X-ray scattering. The results represent the first experimental confirmation of a theoretical model which proposed that the bicontinuous cubic phase originates from the center of a lamellar vesicle then propagates outward via the formation of interlamellar attachments and stalks. The observation was possible because of the precise control of the lipid composition to place the dispersion systems at the phase boundary of a lamellar and a cubic phase, allowing for the creation of long-lived structural intermediates. By the surveying of the nanoparticles using cryogenic transmission electron microscopy, a complete phase transition sequence was established.

Temperature controlled properties of sub-micron thin SnS films

Tin sulphide (SnS) thin films deposited by thermal evaporation on glass substrates are studied for different substrate temperatures. The increase in substrate temperature results in the increase of the crystallite size and change in orientation of the films. The crystal structure of the films is that of SnS only and for temperatures ≤300 °C the films are of random orientation, whereas for higher temperatures the films become (040) oriented. The variation of Sn/S composition was accompanied by a reduction in optical energy bandgap from 1.47 to 1.31 eV as the substrate temperature increases. The Urbach energy was found stable at 0.169 ± 0.002 eV for temperature up to 350 °C. Photoluminescence emission was observed only for films exhibiting stoichiometric properties and shows that a precise control of the film composition is critical to fabricate devices while an increase in grain size will be essential to achieve high efficiency.

Sp-d Exchange Interactions in Wave Function Engineered Colloidal CdSe/Mn:CdS Hetero-Nanoplatelets.

In two-dimensional (2D) colloidal semiconductor nanoplatelets, which are atomically flat nanocrystals, the precise control of thickness and composition on the atomic scale allows for the synthesis of heterostructures with well-defined electron and hole wave function distributions. Introducing transition metal dopants with a monolayer precision enables tailored magnetic exchange interactions between dopants and band states. Here, we use the absorption based technique of magnetic circular dichroism (MCD) to directly prove the exchange coupling of magnetic dopants with the band charge carriers in hetero-nanoplatelets with CdSe core and manganese-doped CdS shell (CdSe/Mn:CdS). We show that the strength of both the electron as well as the hole exchange interactions with the dopants can be tuned by varying the nanoplatelets architecture with monolayer accuracy. As MCD is highly sensitive for excitonic resonances, excited level spectroscopy allows us to resolve and identify, in combination with wave function calculations, several excited state transitions including spin-orbit split-off excitonic contributions. Thus, our study not only demonstrates the possibility to expand the extraordinary physical properties of colloidal nanoplatelets toward magneto-optical functionality by transition metal doping but also provides an insight into the excited state electronic structure in this novel two-dimensional material.

Preparation of Sn loss-free Cu2SnS3 thin films by an oxide route for solar cell

Ternary Cu2SnS3 (CTS) thin films have attracted lots of attention due to its low cost, nontoxic element for solar cell. However, Tin losses always happen during sulfurization, which will affect the phase purity and morphology of CTS thin films and relevant solar cell. In this work, a low cost oxides route was used to prepare CTS thin films. The influences of sulfurizaton temperature on the crystal structure, morphology and composition were studied carefully. From the XRD, SEM and EDX results, it was found that the stable oxides were conducive to alleviate the Sn loss even at the higher sulfurization temperature (580 °C). Meanwhile, the crystal structure evolution along annealing temperature was also observed from mixed phase (Monoclinic and Tetragonal) to single phase Monoclinic. The Monoclinic CTS thin films sulfurized at 580 °C were used to fabricate CTS solar cell with Mo/CTS/CdS/ZnO/AZO/Ag structure, exhibiting efficiency of 0.16%. The comparable Voc of 300 mV was achieved by benefiting from the precise composition control. Besides, the photoelectric properties of the device were carefully studied as well. This study provides a low-cost and facile strategy for the fabrication of Sn-loss free CTS solar cells.

Real Time Spectroscopic Ellipsometry Analysis of First Stage CuIn1-xGaxSe₂ Growth: Indium-Gallium Selenide Co-Evaporation.

Real time spectroscopic ellipsometry (RTSE) has been applied for in-situ monitoring of the first stage of copper indium-gallium diselenide (CIGS) thin film deposition by the three-stage co-evaporation process used for fabrication of high efficiency thin film photovoltaic (PV) devices. The first stage entails the growth of indium-gallium selenide (In1−xGax)2Se3 (IGS) on a substrate of Mo-coated soda lime glass maintained at a temperature of 400 °C. This is a critical stage of CIGS deposition because a large fraction of the final film thickness is deposited, and as a result precise compositional control is desired in order to achieve the optimum performance of the resulting CIGS solar cell. RTSE is sensitive to monolayer level film growth processes and can provide accurate measurements of bulk and surface roughness layer thicknesses. These in turn enable accurate measurements of the bulk layer optical response in the form of the complex dielectric function ε = ε1 − iε2, spectra. Here, RTSE has been used to obtain the (ε1, ε2) spectra at the measurement temperature of 400 °C for IGS thin films of different Ga contents (x) deduced from different ranges of accumulated bulk layer thickness during the deposition process. Applying an analytical expression in common for each of the (ε1, ε2) spectra of these IGS films, oscillator parameters have been obtained in the best fits and these parameters in turn have been fitted with polynomials in x. From the resulting database of polynomial coefficients, the (ε1, ε2) spectra can be generated for any composition of IGS from the single parameter, x. The results have served as an RTSE fingerprint for IGS composition and have provided further structural information beyond simply thicknesses, for example information related to film density and grain size. The deduced IGS structural evolution and the (ε1, ε2) spectra have been interpreted as well in relation to observations from scanning electron microscopy, X-ray diffractometry and energy-dispersive X-ray spectroscopy profiling analyses. Overall the structural, optical and compositional analysis possible by RTSE has assisted in understanding the growth and properties of three stage CIGS absorbers for solar cells and shows future promise for enhancing cell performance through monitoring and control.

A molecular approach to magnetic metallic nanostructures from metallopolymer precursors

In recent years, metallopolymers have attracted much attention as precursors to generate magnetic metal/metal alloy nanoparticles (NPs) through pyrolysis or photolysis because they offer the advantages of ease of solution processability, atomic level mixing and stoichiometric control over composition. The as-generated NPs usually possess narrow size distributions with precise control of composition and density per unit area. Moreover, patterned NPs can be achieved on various substrates in this way owing to the good film-forming property of metallopolymers and such work is important for many applications based on metal nanostructures. By combining the merits of both the solution processability of metallopolymers and nanoimprint lithography (NIL), a new platform can be created for fabricating bit-patterned media (BPM) and the next-generation of nanoscale ultra-high-density magnetic data storage devices. Furthermore, most of these metallopolymers can be used directly as a negative-tone resist to generate magnetic metallic nanostructures by electron-beam lithography and UV photolithography. Self-assembly and subsequent pyrolysis of metalloblock copolymers can also afford well-patterned magnetic metal or metal alloy NPs in situ with periodicity down to dozens of nanometers. In this review, we highlight the use of metallopolymer precursors for the synthesis of magnetic metal/metal alloy NPs and their nanostructures and the related applications.

Controlled formation of chitosan particles by a clock reaction.

Clock reactions allow precise control of chemical composition in the time domain. Such nonlinear chemical systems have recently been introduced to mimic the self-assembly pathways common in living organisms. Here, we demonstrate the use of a clock reaction to trigger the formation of polymeric nanoparticles. By adjusting the delay of a formaldehyde clock reaction, we controlled the precipitation of chitosan to form particles with sizes tunable in a wide range (from about 200 to 600 nm diameter). The chemical structure of chitosan was not significantly perturbed by the clock reagents.

Live event reconstruction in an optically read out GEM-based TPC

Combining strong signal amplification made possible by Gaseous Electron Multipliers (GEMs) with the high spatial resolution provided by optical readout, highly performing radiation detectors can be realized. An optically read out GEM-based Time Projection Chamber (TPC) is presented. The device permits 3D track reconstruction by combining the 2D projections obtained with a CCD camera with timing information from a photomultiplier tube. Owing to the intuitive 2D representation of the tracks in the images and to automated control, data acquisition and event reconstruction algorithms, the optically read out TPC permits live display of reconstructed tracks in three dimensions. An Ar/CF4 (80/20%) gas mixture was used to maximize scintillation yield in the visible wavelength region matching the quantum efficiency of the camera. The device is integrated in a UHV-grade vessel allowing for precise control of the gas composition and purity. Long term studies in sealed mode operation revealed a minor decrease in the scintillation light intensity.

Encoding Highly Nonequilibrium Boron Concentrations and Abrupt Morphology in p-Type/n-Type Silicon Nanowire Superlattices.

Although silicon (Si) nanowires (NWs) grown by a vapor-liquid-solid (VLS) mechanism have been demonstrated for a range of photonic, electronic, and solar-energy applications, continued progress with these NW-based technologies requires increasingly precise compositional and morphological control of the growth process. However, VLS growth typically encounters problems such as nonselective deposition on sidewalls, inadvertent kinking, unintentional or inhomogeneous doping, and catalyst-induced compositional gradients. Here, we overcome several of these difficulties and report the synthesis of uniform, linear, and degenerately doped Si NW superlattices with abrupt transitions between p-type, intrinsic, and n-type segments. The synthesis of these structures is enabled by in situ chlorination of the NW surface with hydrochloric acid (HCl) at temperatures ranging from 500 to 700 °C, yielding uniform NWs with minimal nonselective growth. Surprisingly, we find the boron (B) doping level in p-type segments to be at least 1 order of magnitude above the solid solubility limit, an effect that we attribute to a high incorporation of B in the liquid catalyst and kinetic trapping of B during crystallization at the liquid-solid interface to yield a highly nonequilibrium concentration. For growth at 510 °C, four-point-probe measurements yield active doping levels of at least 4.5 × 1019 cm-3, which is comparable to the phosphorus (P) doping level of n-type segments. Because the B and P dopants are in sufficiently high concentrations for the Si to be degenerately doped, both segments inhibit the etching of Si in aqueous potassium hydroxide (KOH) solution. Moreover, we find that the dopant transitions are abrupt, facilitating nanoscale morphological control in both B- and P-doped segments through selective KOH etching of the NW with a spatial resolution of ∼10 nm. The results presented herein enable the growth of complex, degenerately doped p-n junction nanostructures that can be explored for a variety of advanced applications, such as Esaki diodes, multijunction solar cells, and tunneling field-effect transistors.

Self-regulation of Bi/(Bi+Fe) ratio in metalorganic chemical vapor deposition of BiFeO3 thin films

Metalorganic chemical vapor deposition (MOCVD) is one of the suitable techniques for practical applications of BiFeO3 films. To develop the potential of MOCVD as a device fabrication process, we investigated the relationship between the film and gas compositions, and the growth under highly oxidizing conditions using O2 and O3 gases. In the growth of epitaxial BiFeO3 thin films on SrRuO3-covered 4° vicinal SrTiO3(001) at 620 °C, the self-regulation of the film composition was achieved for Bi and Fe precursor supply ratios between 62.1 to 78.5% under O2 and 56.1 to 73.2% under 5 wt % O3-mixed O2 atmospheres. The leakage was very sensitive to the precursor supply ratio and oxidizing gas. 150-nm-thick MOCVD-BiFeO3 films grown using O2+O3 gas showed the minimum leakage current density of 2.3 × 10−7 A/cm2 at +1 V. The highly oxidizing growth conditions using O3 can suppress the leakage while precise composition control is required.

Controlled Isotropic and Anisotropic Shell Growth in β-NaLnF4 Nanocrystals Induced by Precursor Injection Rate

Precise morphology and composition control is vital for designing multifunctional lanthanide-doped core/shell nanocrystals. Herein, we report controlled isotropic and anisotropic shell growth techniques in hexagonal sodium rare-earth tetrafluoride (β-NaLnF4) nanocrystals by exploiting the kinetics of the shell growth. A drastic change of the shell morphology was observed by changing the injection rate of the shell precursors while keeping all other reaction conditions constant. We obtained isotropic shell growth for fast sequential injection and a preferred growth of the shell layers along the crystal's c-axis [001] for slow dropwise injection. Using this slow shell growth technique, we have grown rod-like shells around different almost spherical core nanocrystals. Bright and efficient upconversion was measured for both isotropic and rod-like shells around β-NaYF4 nanocrystals doped with Yb3+/Er3+ and Yb3+/Tm3+. Photoluminescence upconversion quantum yield and lifetime measurements reveal the high quality of the core/shell nanocrystal. Furthermore, multishell rod-like nanostructures have been prepared with optically active cores and tips separated by an inert intermediate shell layer. The controlled anisotropic shell growth allows the design of new core/multishell nanostructures and enables independent investigations of the chemistry and physics of different nanocrystal facets.

Copper Metallization of Gold Nanostructure Activated Polypyrrole by Electroless Deposition

We report on the study of novel polymeric electrodes made by the selective, electroless plating, of Copper (Cu) films on gold (Au) nanostructure-modified polypyrrole (Ppy). The main aim of the work is to define the effect of Au nanoparticle seed preparation method on its catalytic properties for Cu electroless deposition. The Au nanostructures were produced by two different techniques, namely, cluster beam deposition from a magnetron sputtering/gas condensation source (with precise size and composition control) and electrochemical deposition. To carry out the research, polypyrrole films were first synthesized by electro-polymerization on Au (200nm)/SiO2/Si substrates, followed by modification of the polymer surface by either electroplating of Au nanoparticles or Au923 clusters deposition from a magnetron cluster source. The gold nanoparticle modified electrodes were subjected to copper electroless deposition for different time intervals. The morphology, growth and nucleation kinetics of the resulting copper film were studied by environmental scanning electron microscopy-Energy dispersive X-ray spectroscopy (ESEM-EDS), atomic force microscopy (AFM) and X-ray fluorescence (XRF). Although both nanoparticle type showed similar incubation time, a faster catalytic response, once deposition had been initiated, was observed when the polypyrrole film was modified with Au923 clusters. The incubation time was independent of cluster size and type. This could be explained by a simple model assuming that the incubation time depends on similar parameters for both nanoparticle types, such as metal-metal (Au-Cu) binding energy, crystallographic misfit (Au- Cu) and lateral growth of the copper film. Further discussion is presented in this paper in attempt to explain the different growth rate of Cu film catalysed by the Au923 clusters and electroplated Au nanoparticles.

Model development for intake gas composition controller design for commercial vehicle diesel engines with HP-EGR and exhaust throttling

Developers of modern diesel engines have to face rigorous restrictions in emission of nitrogen-oxides and particulate matter. Among several methods (DPF, SCR, improved injection etc) which are used to handle these exhaust gas components precise control of the cylinder charge composition seems to be a cost effective solution. The oxygen concentration of the intake gas has a significant impact on the combustion process hence on the pollutant formation. Moreover the precise adjustment of the cylinder charge oxygen concentration can be used to control low temperature combustion processes. The amount of the backflowing exhaust gases is limited by the pressure difference on the EGR duct. To solve this problem the authors suggest the use of novel exhaust brakes with extended functionality: backpressure adjustment for the precise control of the intake gas composition. This way arbitrary EGR rates can be achieved in any engine operation point. For the implementation of such a function the design of an intake manifold oxygen controller is needed, where the control inputs are the EGR valve and the exhaust throttle. This paper demonstrates the development of a control oriented model of the air path system as the first step of this task. The model considers three balance volumes in the air path and has five state variables. Its performance has been evaluated and validated with engine dyno measurements on a medium duty diesel engine.

Plasma Oxidation of Liquid Precursors

Several grand challenges in energy storage and conversion involve the discovery functional materials which many agree will be multi-element solid solution of metal oxides. The conventional wet chemical methods for synthesis of multi-metal oxide often require time-consuming high pressure and temperature processes, and so the challenge is to develop rapid and scalable techniques with precise compositional control. This presentation will discuss the concept of plasma oxidation of liquid precursors for making mixed metal oxides with compositional control and rapid time scales. The concept is demonstrated with binary, ternary and quaternary metal oxide with control over entire composition range using metal precursor solutions.1 For the ternary system, the results show the selective formation of metastable spinel solid solution phases with compositions over the entire range by tuning the metal precursor composition. The concept of plasma oxidation of liquid precursors is investigated to produce a number of materials systems for electrocatalysts, lithium ion batteries and heterogeneous catalysts. For example, in terms of oxygen evolution reaction, ternary alloys involving manganese doped nickel ferrite nanoparticles, NiMnzFe2-zO4 (01), exhibited considerable electrocatalytic activity achieving an overpotential of 0.39V at a benchmarking current density of 10 mAcm-2 for low manganese content at z = 0.2.1 Lithiated NMC compounds exhibit about 220 mAh/g capacity after 50 cycles. Acknowledgements The authors acknowledge support from NSF EPSCoR (1355438) and access to facilities at the Conn Center for Renewable Energy Research. Reference B.P. Ajayi, S. Kumari, D. Jaramillo-Cabanzo, J. Spurgeon, J. Jasinski and M.K. Sunkara, “A rapid and scalable method for making mixed metal oxide alloys for enabling accelerated materials discovery”, J. of Materials Research, DOI: http://dx.doi.org/10.1557/jmr.2016.9231 (11), 1596-1607(2016)

On the study of antimony incorporation in InAs/InAsSb superlattices for infrared sensing

Advanced infrared detector materials utilizing InAs/InAsSb superlattices (SLs) are emerging due to the long minority carrier lifetimes observed in this material system. However, compositional and dimensional changes through Sb segregation alter the detector properties from the original design, and precise compositional control of the Sb in the SL is crucial to advance the state-of-the-art of this novel material system. In this study, epitaxial conditions that can mitigate Sb segregation during growth are explored in order to achieve high-quality SL materials. A nominal SL structure of 77 Å InAs/35 Å InAs0.7Sb0.3 tailored for a midinfrared gap was used to optimize our epitaxial parameters. Since the growth of mixed anion alloys is complicated by the potential reaction of Asx with Sb surfaces, the substrate temperature (Ts), and arsenic cracker temperature (TAs) was varied in order to control the Asx surface kinetics on a Sb surface. Experimental results indicate that the SL sample grown at the lowest investigated Ts of 400 °C produces the highest Sb mole fraction x of ∼0.3 in InAs1-xSbx layers, which is then decreased by 14% as the Ts increases from 400 to 440 °C. This reduction originates from Sb surface segregation during InAsSb growth through the As-Sb exchange process. Although this incorporation was increased with a lower TAs, the crystalline quality of SL layers quickly degraded with the TAs below 850 °C due to the poor adsorption coefficient of As4 at the growth front. Since a change in the designed compositions and effective layer widths related to Sb segregation disrupts strain balance and also significantly impacts the detector performance, further studies to prevent Sb segregation are needed.

Lipid-Dependent Gating of Kv Channels and Excitability Change of Cerebellar Purkinje Neurons in an NPC1 Model Mouse

Recent studies of Kv channels in different membrane systems with altered ratios between phospholipids and nonphospholipids support our lipid-dependent gating hypothesis that the nonphospholipids favor the conformational switch of the voltage sensor domains of a Kv channel to a resting state. To enable more precise control of lipid composition in an artificial membrane and avoid possible solvent contamination or phase separation of lipids in bilayer membranes, we constructed a bead-supported unilamellar membrane (bSUM) and used it to define a more quantitative relationship between nonphospholipid content and change in the gating property of a voltage-gated potassium channel. In the bSUMs, we were able to show for the first time that a small amount of cholesterol (∼10 molar %) in a phospholipid membrane in a fluidic phase exerts a strong inhibitory effect on a KvAP channel, suggesting that the Kv channels might be highly sensitive to change in membrane cholesterol content. To test this prediction in a physiological system where lipid metabolic defects cause significant neurological disorders, we analyzed the neuronal excitability in a mouse model for the Niemann-Pick disease type C, an NPC1-I1061T knockin mice. Systematic comparison of five different groups of cerebellar Purkinje neurons from both knockin mice and control animals revealed a significant decrease in the firing frequency of action potentials in the tonic-burst firing Purkinje neurons of the NPC1 mice, which appears to be consistent with the predicted cholesterol effects on Kv channels. Detailed studies of the gating properties in these Purkinje neurons are being conducted to understand the underlying mechanism. Our data support a potential connection among cholesterol content in cell membranes, cholesterol-dependent gating of voltage-gated K channels and change in neuronal excitability in CNS neurons.

Template-directed synthesis of nitrogen- and sulfur-codoped carbon nanowire aerogels with enhanced electrocatalytic performance for oxygen reduction

Heteroatom doping, precise composition control, and rational morphology design are efficient strategies for producing novel nanocatalysts for the oxygen reduction reaction (ORR) in fuel cells. Herein, a cost-effective approach to synthesize nitrogen- and sulfur-codoped carbon nanowire aerogels using a hard templating method is proposed. The aerogels prepared using a combination of hydrothermal treatment and carbonization exhibit good catalytic activity for the ORR in alkaline solution. At the optimal annealing temperature and mass ratio between the nitrogen and sulfur precursors, the resultant aerogels show comparable electrocatalytic activity to that of a commercial Pt/C catalyst for the ORR. Importantly, the optimized catalyst shows much better long-term stability and satisfactory tolerance for the methanol crossover effect. These codoped aerogels are expected to have potential applications in fuel cells.

Atomic Layer Deposition for Energy and Environmental Applications

Atomic Layer Deposition for Energy and Environmental Applications