Kristina Buchholt

Ohmic contact properties of magnetron sputtered Ti3SiC2 on n- and p-type 4H-silicon carbide

Epitaxial Ti3SiC2 (0001) thin film contacts were grown on doped 4H-SiC (0001) using magnetron sputtering in an ultra high vacuum system. The specific contact resistance was investigated using linea ...

Phase-stabilization and substrate effects on nucleation and growth of (Ti,V)(n+1)GeC(n) thin films

Phase-pure epitaxial thin films of (Ti,V)(2)GeC have been grown onto Al(2)O(3)(0001) substrates via magnetron sputtering. The c lattice parameter is determined to be 12.59 A, corresponding to a 50/50 Ti/V solid solution according to Vegards law, and the overall (Ti,V): Ge: C composition is 2:1:1 as determined by elastic recoil detection analysis. The minimum temperature for the growth of (Ti,V)(2)GeC is 700 degrees C, which is the same as for Ti(2)GeC but higher than that required for V(2)GeC (450 degrees C). Reduced Ge content yields films containing (Ti,V)(3)GeC(2) and (Ti,V)(4)GeC(3). These results show that the previously unknown phases V(3)GeC(2) and V(4)GeC(3) can be stabilized through alloying with Ti. For films grown on 4H-SiC(0001), (Ti,V)(3)GeC(2) was observed as the dominant phase, showing that the nucleation and growth of (Ti,V)(n+1)GeC(n) is affected by the choice of substrate; the proposed underlying physical mechanism is that differences in the local substrate temperature enhance surface diffusion and facilitate the growth of the higher-order phase (Ti,V)(3)GeC(2) compared to (Ti,V)(2)GeC.

Electrochemically Synthesised Pd- and Au-Nanoparticles as Sensing Layers in NOx-Sensitive Field Effect Devices

An environmental pollutant of great concern is NO_x (nitrogen monoxide and nitrogen dioxide). Here we report the utilisation of electrochemically synthesised gold and palladium nanoparticles as catalytically active gate material on gas sensitive field effect sensor devices. The synthesised nanoparticles have been characterised by TEM and XPS, and the morphology of the thermally treated nanostructured sensing layers has been investigated using SEM and XPS. Measurements on the gas response of the palladium as well as the gold nanoparticle sensors towards a number of analytes found in automotive gas exhausts were performed and their response patterns were compared. The initial gas response measurements show interesting sensing properties for both the gold and the palladium nanoparticle sensors towards NO_x detection.

A Comparison of the Gas Sensing Properties of Purified and Platinum Decorated Chemical Vapour Deposition Grown Multi Walled Carbon Nanotubes

Multi walled carbon nanotubes (MWCNTs) are known to respond well to a range of gases and vapours. Metallic additives are often introduced to improve the sensitivity and selectivity to some gases. Here the difference in response between MWCNT sensors with and without a metal additive is discussed for a range of gases. Resistive sensors fabricated from chemical vapour deposition (CVD) grown multi walled carbon nanotubes (MWCNTs) collected between gold microelectrodes by di-electrophoresis are presented as a cheap, scaleable and facile method of producing carbon nanotube gas sensors. The MWCNTs sensors were exposed to a series of test gases including NO2, NH3, CO and H2 and exhibited low ppm. detection at room temperature. Increasing the temperature not only reduced the recovery time of the sensors, but also increased the sensitivity to some gases whilst a decrease in the sensitivity was seen for other gases. A method for decorating MWCNTs with nanometer sized Pt particles is presented and compared as a sensing material to purified MWCNTs which have undergone acid reflux and base washing. The role of defects and the sensing mechanisms are discussed.

FET Transduction of Electric Dipole Changes in Organic Layers

Modern chemistry makes possibile the production of artificial receptors endowed with interaction sites specific towards selected analytes. To fabricate chemical sensors a transducer able to preserve the selectivity of such sensing layers is requested. We demonstrate the Field Effect Transistors (FET) coated by metalloporphyrins emphasizes the coordination of analytes with the sensing molecules with respect to the weakest forces that tend to cancel the selectivity. Molecular simulations show that coordination of gases such as carbon monoxide and nitric oxide greatly change the porphyrin electric dipole both in magnitude and direction inducing a large FET signal shift.

Gold nanoparticle sensors for environmental pollutant monitoring

Gold nanoparticles (Au-NPs) have been synthesised using a sacrificial anode electrolysis in the presence of tetra-alkyl-ammonium halides, employed as cationic stabilizers. Catalytic NPs have been then deposited on top of Field Effect (FE) gas sensing devices and subjected to mild annealing procedures. Transmission Electron Microscopy (TEM) shows that the NP average core diameter is around 5 nm. X-Ray Photoelectron Spectroscopy (XPS) and Scanning Electron Microscopy (SEM) have been applied to the surface characterization of the annealed NP films used as active sensing layers. Morphological and spectroscopic results demonstrate that the annealed inorganic nano-clusters are finely dispersed and maintain a metallic oxidation state. Au-NPs can be proficiently employed as gate material in Si-Field Effect Gas Sensors. Preliminary results show interesting selectivity and sensitivity sensing features towards NOx detection.