Papers by Author: L. Hultman

Abstract: Epitaxially grown single layer graphene on silicon carbide (SiC) resistive sensors were characterised for NO2 response at room and elevated temperatures, with an n-p type transition observed with increasing NO2 concentrations for all sensors. The concentration of NO2 required to cause this transition varied with different graphene samples and is attributed to varying degrees of substrate induced Fermi-level pinning above the Dirac point. The work function of a single layer device demonstrated a steady increase in work function with increasing NO2 concentration indicating no change in reaction mechanism in the concentration range measured despite a change in sensor response direction. Epitaxially grown graphene device preparation is challenging due to poor adhesion of the graphene layer to the substrate. A field effect transistor (FET) device is presented which does not require wire bonding to contacts on graphene.

Abstract: The inherent resiliency, hardness and relatively low friction coefficient of the fullerene-like (FL) allotrope of carbon nitride (CN_x) thin solid films give them potential in numerous tribological applications. In this work, we study the substitution of N with P to grow FL-CP_x to achieve better cross- and inter-linking of the graphene planes, improving thus the material’s mechanical and tribological properties. The CN_x and CP_x films have been synthesized by DC magnetron sputtering. HRTEM have shown the CP_x films exhibit a short range ordered structure with FL characteristics for substrate temperature of 300 °C and for a phosphorus content of 10-15 at.%. These films show better mechanical properties in terms of hardness and resiliency compared to those of the FL-CN_x films. The low water adsorption of the films is correlated to the theoretical prediction for low density of dangling bonds in both, CN_x and CP_x. First-principles calculations based on Density Functional Theory (DFT) were performed to provide additional insight on the structure and bonding in CN_x, CP_x, and a-C compounds.

Abstract: Epitaxial Mn+1AXn phase (n=1, 2 or 3) thin films from the chemically related Ti-Si-C, Ti-Ge-C, and Ti-Sn-C systems were grown on Al2O3(0001) substrates at temperatures in the region of 700-1000 oC, using d.c. magnetron sputtering from individual sources. In addition to growth of the known phases Ti3SiC2, Ti3GeC2, Ti2GeC, and Ti2SnC the method allows synthesis of the new phases Ti4SiC3, Ti4GeC3, and Ti3SnC2 as well as the intergrown structures Ti5A2C3 and Ti7A2C5 in the Si and Ge systems. Characterization by XRD, TEM and nanoindentation show similarities with respect to phase distribution, mechanical, and electrical properties, particularly pronounced when comparing Si and Ge. The Ti-Sn-C system is, however, the most liable system with respect to surface segregation of the A-element. This causes less favorable growth of MAX phases as seen by a preferential growth of the binary carbide TiC and metallic Sn. Nanoindentation on films from the Ti-Si-C and Ti-Ge-C systems shows large plastic deformation with extensive pile up. The typical thin film hardness is
20 GPa, and the Young’s modulus in the region of 320 GPa. The four-point probe resistivity is low for all systems, but differs depending on materials system and phase, with values of
25 μcm for Ti3SiC2, and
17 μcm for Ti2GeC.