Jörg Engstler

Zinc oxide derived from single source precursor chemistry under chimie douce conditions: formation pathway, defect chemistry and possible applications in thin film printing

A series of zinc complexes with oximate ligands is investigated for their suitability as precursors for zinc oxide in inkjet printing. The variation of hydrogen and alkyl groups in the side chains of the oximate framework (R1–ON–C2O2–R2) of the corresponding zinc complexes influences the decomposition temperature, and also important parameters such as solubility and wettability. Detailed investigations of the degradation mechanism reveal their behavior as excellent single source precursors for ZnO under very mild (chimie douce) conditions. Best results for the formation of zinc oxide thin films are obtained with solutions of [2-(methoxyimino)propanato]zinc in methoxyethanol. By calcincation well adherent (tensile strength of 1.95 (±0.95) MPa) nanocrystalline films of zincite are formed. This technique is applied for inkjet printing of ceramic layers on polyethylene-terephthalate thin films. Results of EPR spectroscopy studies on the ZnO nanoparticles are in accord with a core–shell model in which the grain particles of the core consist of vacancy centers which are electronically different from the surrounding shell of the ZnO nanoparticles.

Binary Au/MWCNT and Ternary Au/ZnO/MWCNT Nanocomposites: Synthesis, Characterisation and Catalytic Performance

Gold nanoparticles of 10-24 and 5-8 nm in size were obtained by chemical citrate reduction and UV photoreduction, respectively, on acid-treated multiwalled carbon nanotubes (MWCNTs) and on ZnO/MWCNT composites. The shape and size of the deposited Au nanoparticles were found to be dependent upon the synthetic method used. Single-crystalline, hexagonal gold particles were produced in the case of UV photoreduction on ZnO/MWCNT, whereas spherical Au particles were deposited on MWCNT when the chemical citrate reduction method was used. In the UV photoreduction route, n-doped ZnO serves as the e(-) donor, whereas the solvent is the hole trap. All materials were fully characterised by UV/Vis spectroscopy, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy and BET surface analysis. The catalytic activity of the composites was studied for the selective hydrogenation of alpha,beta-unsaturated carbonyl compound 3,7-dimethyl-2,6-octadienal (citral). The Au/ZnO/MWCNT composite favours the formation of unsaturated alcohols (selectivity=50% at a citral conversion of 20%) due to the presence of single-crystalline, hexagonal gold particles, whereas saturated aldehyde formation is favoured in the case of the Au/MWCNT nanocomposite that contains spherical gold particles.

Catalyst Composition, Morphology and Reaction Pathway in the Growth of “Super‐Long” Carbon Nanotubes

The discovery by Hata et al. of “super-long” carbon nanotubes (CNTs) grown by catalyst-driven, water-assisted chemical vapor deposition represented a major synthetic breakthrough for the integration of CNTs into future device architectures, such as chemical and physical sensors, heterogeneous catalyst arrays, or 3D microand nanoelectromechanical systems (MEMS and NEMS). Their work was preceded by the development of the HiPCO process by Smalley et al. , which allowed, for the first time, a massive formation of unordered singlewalled CNTs (SWCNTs). This discovery was followed shortly thereafter by the first growth process of long (up to 4 mm) vertically aligned SWCNTs by Muruyama et al. Recently, Hata et al. found that CO2, ethers, ketones, and alcohols are, in the same way as water, advantageous for the growth of arranged “super-long” CNTs. All growth enhancers contain oxygen atoms, and act as weak oxidants, which are able to etch away a growing carbon layer, which deactivates the active catalyst, and thus being detrimental to the growth of “superlong” CNTs. 6] The composition of the active catalyst in the “super-long” growth of CNTs needs further investigation, despite experimental efforts in the field of in situ characterization of CNT growth. Herein we report our findings regarding the nature, role, and mechanism of the catalyst composition in the growth of “super-long” CNTs. Our studies allow an understanding of the morphology and structure of the catalyst nanoparticles by high-resolution scanning transmission electron microscopy (STEM) tomograms and of their chemical composition by a combination of spectroscopic and diffraction techniques. Besides catalyst dispersion, the properties of the substrate surface (roughness, active area, and electron-transfer ability to the catalyst surface) are critical for a high-yielding efficient CNT growth. Aluminum is known to work as an efficient buffer layer between catalyst and substrate under “super-long” CNT growth conditions. CNT growth starts with deposition of iron metal particles on a thin aluminum metal film deposited onto a Si wafer having a thin native SiO2 layer. [1] We have chosen two different procedures for catalyst sample preparation, which allow direct observation of the catalyst’s structure and morphology and characterization of its chemical composition. The first set of experiments was carried out on transmission electron microscopy (TEM) grids and the second on a Si wafer substrate. In both cases we deposited a 10 nm-thick aluminum film, followed by a 1 nm-thick layer of iron metal. These deposited films were then heated directly to 750 8C. The resulting catalysts were then tested and found to be active in the water assisted chemical vapor deposition (CVD) growth of CNTs in an independent set of experiments (for details and characterization of the CNTs, see the Supporting Information). The chemical composition of the catalyst system was further studied to clarify whether (a) FexAly intermetallic compound formation or (b) Fe–Al particle formation by cluster intermixing of aluminum and iron elements occurs under growth conditions. The latter are typically nonequilibrium structures, and thermal treatment and surrounding conditions may strongly influence their catalytic behavior. To confirm the shape and particle morphology of the Fe–Al nanocatalyst, firstly high-angle annular dark-field STEM images were recorded (Figure 1 A). The particles exhibit a core–shell contrast with facets. Their size ranges from a few nanometers up to 50 nm and the particles show sharp corners and edges with flat surface structures. Lattice fringes are observed, offering proof of the crystalline nature of the catalyst particles. High-resolution bright-field tomography of these particles was undertaken to clarify their three-dimensional shape. The tomogram data of the catalyst particles reveal a dome-shaped morphology with a void within the core, hence the donut-like contrast in the projection images. A tomogram slice showing the central void and well-defined facets is shown in Figure 1 B. Spatially resolved electron energy-loss spectroscopy (EELS) was performed in a high-resolution STEM to study the element distribution within the catalyst particles (Figure 1 C). Line profiles of 20 EEL spectra were taken across a number of catalyst particles. The evaluation of core-loss signals in the spectra allows plotting of the intensity of Fe and Al concentrations across the particle’s diameter. As can be seen from Figure 1 C, [a] R. Joshi, Dr. J. Engstler, Prof. Dr. J. J. Schneider Technische Universit t Darmstadt, Fachbereich Chemie Eduard-Zintl-Institut f r Anorganische und Physikalische Chemie Petersenstrasse 18, 64287 Darmstadt (Germany) Fax: (+ 49) 6151-163470 E-mail : joerg.schneider@ac.chemie.tu-darmstadt.de [b] Dr. L. Houben, Dr. M. Bar Sadan Ernst-Ruska-Zentrum f r Elektronenmikroskopie (ER-C) 52425 J lich (Germany) [c] Prof. Dr. A. Weidenkaff, Dr. P. Mandaliev Eidgençssische Material-Pr fungsanstalt, EMPA 8600 D bendorf (Switzerland) [d] Dr. A. Issanin Technische Universit t Darmstadt Fachbereich Materialund Geowissenschaften Fachgebiet Oberfl chenforschung Petersenstrasse 23, 64287 Darmstadt (Germany) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cctc.201000037.

A molecular approach to Cu doped ZnO nanorods with tunable dopant content

A novel molecular approach to the synthesis of polycrystalline Cu-doped ZnO rod-like nanostructures with variable concentrations of introduced copper ions in ZnO host matrix is presented. Spectroscopic (PLS, variable temperature XRD, XPS, ELNES, HERFD) and microscopic (HRTEM) analysis methods reveal the +II oxidation state of the lattice incorporated Cu ions. Photoluminescence spectra show a systematic narrowing (tuning) of the band gap depending on the amount of Cu(II) doping. The advantage of the template assembly of doped ZnO nanorods is that it offers general access to doped oxide structures under moderate thermal conditions. The doping content of the host structure can be individually tuned by the stoichiometric ratio of the molecular precursor complex of the host metal oxide and the molecular precursor complex of the dopant, Di-aquo-bis[2-(methoxyimino)-propanoato]zinc(II) 1 and -copper(II) 2. Moreover, these keto-dioximato complexes are accessible for a number of transition metal and lanthanide elements, thus allowing this synthetic approach to be expanded into a variety of doped 1D metal oxide structures.

Electron field emission from carbon nanotubes on porous alumina

We have synthesized carbon nanotubes by chemical vapor deposition using ferrocene as single source organometallic precursor both on commercial (Anodisc®) and electrochemically etched porous alumina templates. Carbon nanotubes of about 20nm diameter and some μm in length appeared apart on the alumina membranes. Integral field emission measurements of these cathodes were performed in a diode configuration with luminescent screen. High emitter number densities of at least 10000∕cm2 and current densities up to 32mA∕cm2 were obtained at an electric field of 7.2V∕μm. Cathode processing at pressures in the range from 10−7to5×10−4mbar resulted in improved current stability measured over 18h. High resolution emitter distributions obtained with the field emission scanning microscope yielded up to 62000emitters∕cm2 at 23V∕μm. Single emitter investigations showed Fowler–Nordheim behavior up to 1μA and current limits up to 12μA in dc operation. Reversible switching between different emission states was also observed. ...

Synthesis, Characterization, Electronic and Gas‐Sensing Properties towards H2 and CO of Transparent, Large‐Area, Low‐Layer Graphene

Low-layered, transparent graphene is accessible by a chemical vapor deposition (CVD) technique on a Ni-catalyst layer, which is deposited on a <100> silicon substrate. The number of graphene layers on the substrate is controlled by the grain boundaries in the Ni-catalyst layer and can be studied by micro Raman analysis. Electrical studies showed a sheet resistance (R(sheet)) of approximately 1435 Ω per □, a contact resistance (R(c)) of about 127 Ω, and a specific contact resistance (R(sc)) of approximately 2.8×10(-4)  Ω cm(2) for the CVD graphene samples. Transistor output characteristics for the graphene sample demonstrated linear current/voltage behavior. A current versus voltage (I(ds)-V(ds)) plot clearly indicates a p-conducting characteristic of the synthesized graphene. Gas-sensor measurements revealed a high sensor activity of the low-layer graphene material towards H(2) and CO. At 300 °C, a sensor response of approximately 29 towards low H(2) concentrations (1 vol %) was observed, which is by a factor of four higher than recently reported.

Imbibition of polystyrene melts in aligned carbon nanotube arrays

We analyze the polymer filling mechanism in composites containing highly ordered and vertically aligned carbon nanotube (CNT) arrays. CNTs are obtained by a template assisted chemical vapor deposition (CVD) method. Different forms of the arrays are studied with one or two carbon layers on top and bottom surface of the array, or freestanding CNTs. Investigation is done by small-angle X-ray scattering (SAXS) in combination with electron microscopy (TEM and SEM) and atomic force microscopy. Tubes are of 40 μm length and 40/90 nm diameter. The original order of the template is only locally preserved in the CNT array. Imbibition of polymer is achieved in the inside of CNTs as well as in between. It modifies the local order of the tubes. We compare structural changes of CNT arrays caused by polymer infiltration. Filling kinetics is followed with time-resolved SAXS. We find two well separated processes that are related to the formation of a precursor film and subsequent partial completion of the imbibition process.

Generation and agglomeration behaviour of size-selected sub-nm iron clusters as catalysts for the growth of carbon nanotubes

Summary Mass-selected, ligand-free FeN clusters with N = 10–30 atoms (cluster diameter: 0.6–0.9 nm) were implanted into [Al@SiOx] surfaces at a low surface coverage corresponding to a few thousandths up to a few hundredths of a monolayer in order to avoid initial cluster agglomeration. These studies are aimed towards gaining an insight into the lower limit of the size regime of carbon nanotube (CNT) growth by employing size-selected sub-nm iron clusters as catalyst or precatalyst precursors for CNT growth. Agglomeration of sub-nm iron clusters to iron nanoparticles with a median size range between three and six nanometres and the CNT formation hence can be observed at CVD growth temperatures of 750 °C. Below 600 °C, no CNT growth is observed.

Charge Transport in Low-Temperature Processed Thin-Film Transistors Based on Indium Oxide/Zinc Oxide Heterostructures

The influence of the composition within multilayered heterostructure oxide semiconductors has a critical impact on the performance of thin-film transistor (TFT) devices. The heterostructures, comprising alternating polycrystalline indium oxide and zinc oxide layers, are fabricated by a facile atomic layer deposition (ALD) process, enabling the tuning of its electrical properties by precisely controlling the thickness of the individual layers. This subsequently results in enhanced TFT performance for the optimized stacked architecture after mild thermal annealing at temperatures as low as 200 °C. Superior transistor characteristics, resulting in an average field-effect mobility (μsat.) of 9.3 cm2 V-1 s-1 ( W/ L = 500), an on/off ratio ( Ion/ Ioff) of 5.3 × 109, and a subthreshold swing of 162 mV dec-1, combined with excellent long-term and bias stress stability are thus demonstrated. Moreover, the inherent semiconducting mechanism in such multilayered heterostructures can be conveniently tuned by controlling the thickness of the individual layers. Herein, devices comprising a higher In2O3/ZnO ratio, based on individual layer thicknesses, are predominantly governed by percolation conduction with temperature-independent charge carrier mobility. Careful adjustment of the individual oxide layer thicknesses in devices composed of stacked layers plays a vital role in the reduction of trap states, both interfacial and bulk, which consequently deteriorates the overall device performance. The findings enable an improved understanding of the correlation between TFT performance and the respective thin-film composition in ALD-based heterostructure oxides.

P2–23: Field emission properties of aligned pure and TiO 2 -coated carbon nanotube block arrays

We report about field emission properties of various arrays of vertically aligned CNT blocks. Multiple emitters per block resulted in nearly aligned and efficient cathode performances. Stable currents up to 300 (100) µA have been achieved for the pure (TiO2 coated) CNT blocks.