Martin Klaumünzer

Impact of Oxygen Plasma Treatment on the Device Performance of Zinc Oxide Nanoparticle-Based Thin-Film Transistors

Thin-films of zinc oxide nanoparticles were investigated by photoluminescence spectroscopy and a broad defect-related yellow-green emission was observed. Oxygen plasma treatment was applied in order to reduce the number of defects, and the emission intensity was quenched to 4% of the initial value. Thin-film transistors that incorporate the nanoparticles as active semiconducting layers show an improved device performance after oxygen plasma treatment. The maximum drain current and the charge carrier mobility increased more than 1 order of magnitude up to a nominal value of 23 cm(2) V(-1) s(-1) and the threshold voltage was lowered.

Continuous engineering of nano-cocrystals for medical and energetic applications

Cocrystals, solid mixtures of different molecules on molecular scale, are supposed to be tailor made materials with improved employability compared to their pristine individual components in domains such as medicine and explosives. In medicine, cocrystals are obtained by crystallization of active pharmaceutical ingredients with precisely chosen coformers to design medicaments that demonstrate enhanced stability, high solubility, and therefore high bioavailability and optimized drug up-take. Nanoscaling may further advance these characteristica compared to their micronsized counterparts – because of a larger surface to volume ratio of nanoparticles. In the field of energetic materials, cocrystals offer the opportunity to design smart explosives, combining high reactivity with significantly reduced sensitivity, nowadays essential for a safe manipulation and handling. Furthermore, cocrystals are used in ferroelectrics, non-linear material response and electronic organics. However, state of the art batch processes produce low volume of cocrystals of variable quality and only have produced micronsized cocrystals so far, no nano-cocrystals. Here we demonstrate the continuous preparation of pharmaceutical and energetic micro- and nano-cocrystals using the Spray Flash Evaporation process. Our laboratory scale pilot plant continuously prepared up to 8 grams per hour of Caffeine/Oxalic acid 2:1, Caffeine/Glutaric acid 1:1, TNT/CL-20 1:1 and HMX/Cl-20 1:2 nano- and submicronsized cocrystals.

ZnO superstructures via oriented aggregation initiated in a block copolymer melt

A fast and simple one pot synthesis of ZnO nano- and microparticles initiated and driven by an amino block copolymer O,O′-bis(2-aminopropyl)polypropylene glycol-block-polyethylene glycol-block-polypropylene glycol (Jeffamine®) is reported. The specific building mechanism of ZnO mesocrystals is investigated in detail using electron microscopy and diffraction methods. Mesocrystals with a complex superstructure are formed as a result of a consecutive and oriented multiple stage aggregation process: first a 0D → 1D aggregation process is observed, then a 1D → 3D aggregation process occurs in which secondary particles form cones and multiple cone symmetries. Dots, rods, cones, and multiple cones have been isolated within a time resolved study which clearly supports the growth model. To control the morphology of the product particles, the influence of relevant synthesis parameters including stirring and sonication of the intermediate were investigated. Extensive surface characterization of the resulting mesocrystals is presented using infrared and photoluminescence spectroscopies as well as thermogravimetric analysis. Even after multiple washing steps, the particles exhibit a Jeffamine® coated surface that allows for easy dispersion in both polar and nonpolar solvents. The obtained mesocrystals efficiently scatter in the whole range of visible light.

Surface functionalization and electronic interactions of ZnO nanorods with a porphyrin derivative.

To optimize electron transfer and optoelectronic properties in nanoparticulate thin films for electronics we show the surface functionalization of ZnO nanorods by means of replacing surface active 2-[2-(2-methoxyethoxy)ethoxy]acetic acid (TODA) by a redoxactive organic component, that is, 5,10,15,20-(phenoxyacetat)-porphyrin bearing four carboxylic acids as possible ZnO anchors. Microscopy-transmission electron microscopy-and spectroscopy-optical spectroscopy-verifies the successful and homogenous integration of the porphyrin onto the surface of ZnO nanorods, a process that is facilitated by the four anchoring groups. Photophysical investigations based on emission and absorption spectroscopy prompt to distinct electronic interactions between ZnO nanorods and the porphyrins. Consequently, we performed further photophysical studies flanked by pulse radiolysis assays to corroborate the nature of the electronic interactions.

Energetic nanocomposites for detonation initiation in high explosives without primary explosives

The mixing of aluminum nanoparticles with a metal containing oxidizer (here, WO3 or Bi2(SO4)3) gives reactive materials called nanothermites. In this research, nanothermites were combined with high explosive nanoparticles (RDX) to prepare energetic nanocomposites. These smart nanomaterials have higher performances and are much less hazardous than primary explosives. Their flame propagation velocity can be tuned from 0.2 to 3.5 km/s, through their explosive content. They were used to initiate the detonation of a high explosive, the pentaerythritol tetranitrate. The pyrotechnic transduction of combustion into detonation was achieved with short length systems (<2 cm) and small amounts of energetic nanocomposites (∼100 mg) in semi-confined systems.

Transmission Electron Microscopy and Time Resolved Optical Spectroscopy Study of the Electronic and Structural Interactions of ZnO Nanorods with Bovine Serum Albumin

The adsorption behavior and electronic interactions of bovine serum albumin (BSA) with ZnO nanorod surfaces were investigated using high-resolution transmission electron microscopy as well as stationary and time-resolved optical spectroscopy techniques. Transmission electron microscopy shows that ZnO nanorod surfaces are surrounded by a homogeneous amorphous BSA film with thicknesses between ~2.5 and 5.0 nm. The electronic structure and adsorption geometry of BSA were examined using high-angle annular dark field scanning transmission electron microscopy combined with electron energy loss spectroscopy. The adsorption process was observed to result into an unfolded conformation of BSA becoming predominantly bound in the side-on orientation at the ZnO surface. This adsorption mode of the BSA molecules allows for a strong interaction with surface states of the ZnO nanorods. This is obvious from its efficient quenching of the defect-center photoluminescence of ZnO. Complementary information of electronic interactions across the ZnO nanorod interface was obtained from femtosecond transient absorption spectroscopy experiments. The rise dynamics of the measured transients revealed altered hole trapping dynamics and, thus, indicated to heterogeneous charge transfer as emerging from adsorbed BSA molecules to defect centers of the ZnO interface.

Surface Modification of ZnO Nanorods with Hamilton Receptors

A new prototype of a Hamilton receptor suitable for the functionalization of inorganic nanoparticles was synthesized and characterized. The hydrogen bonding receptor was coupled to a catechol moiety, which served as anchor group for the functionalization of metal oxides, in particular zinc oxide. Synthesized zinc oxide nanorods [ZnO] were used for surface functionalization. The wet-chemical functionalization procedure towards monolayer-grafted particles [ZnO-HR] is described and a detailed characterization study is presented. In addition, the detection of specific cyanurate molecules is demonstrated. The hybrid structures [ZnO-HR-CA] were stable towards agglomeration and exhibited enhanced dispersability in apolar solvents. This observation, in combination with several spectroscopic experiments gave evidence of the highly directional supramolecular recognition at the surface of nanoparticles.

Indicating Inconsistency of Desensitizing High Explosives against Impact through Recrystallization at the Nanoscale

ABSTRACT Microscaled 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) is recrystallized by the spray flash evaporation technique to obtain nanoscaled RDX powder. Recrystallization is tracked by diverse methods, including atomic force microscopy and X-ray diffraction. Reduced sensitivity to friction and electrostatic discharge as a consequence of less internal defects within the RDX nanocrystals makes recrystallized material more favorable against unwanted ignition. Increased sensitivity to impact of the nanomaterials is detected and desensitization is discussed in detail with regard to hot-spot theory and potential influencing variables. Surface engineering of nano-RDX by the above-mentioned spray process shows desensitization against stimuli from impact.

Surface Functionalization and Electrical Discharge Sensitivity of Passivated Al Nanoparticles

Passivated aluminum nanoparticles are surface functionalized to elucidate their sensitivity against an electrical discharge. Two size fractions that differ in surface morphology are investigated. Electronic interactions between the partly inert, partly energetic organic molecules used for surface functionalization and the alumina surface are analyzed in detail. The nanoparticle surfaces are modified with the well-established, inert 2-[2-(2-methoxyethoxy)ethoxy]acetic acid, whereas energetic surface modification is achieved using 1,3,5-trinitroperhydro-1,3,5-triazine or the acidic and aromatic 2,4,6-trinitrophenol. A mechanistic model for the chemical surface functionalization of Al nanoparticles is hypothesized and corroborated by comprehensive optical and Fourier transform infrared spectroscopy studies. The surface structures are adjusted by developing a tunable stabilization procedure that prevents sedimentation and hence increases the saturation concentration in the liquid phase and finally affects the sensitivity character in view of electrical discharge ignition of dry powders. Detailed material characterization is conducted using transmission electron microscopy, combined with energy-dispersive X-ray spectroscopy and various absorption spectroscopy techniques (steady state in the infrared and ultraviolet/visible regime). The adjustment of surface structures of the distinct Al nanoparticle samples offers a valuable tool for tuning and tailoring the reactivity, sensitivity, stability, and energetic performances of the nanoparticles, and thereby enables the safe use of these multipurpose nanoparticles.