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Novel Layered and 2D Materials for Functionality Enhancement of Contacts and Gas Sensors
Linköping University, Department of Physics, Chemistry and Biology, Applied Sensor Science. Linköping University, Faculty of Science & Engineering.
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Chemical gas sensors are widely-used electronic devices for detecting or measuring the density levels of desired gas species. In this study, materials with established or potential applications for gas sensors are treated. For the case of high-temperature applications (≈ 600 °C), semiconductor-based gas sensors suffer from rapid oxidation of the metallic ohmic contacts, the same cause-of-failure as for the general case of high-temperature semiconductor electronics. 4H-SiC is an ideal semiconductor for high-temperature applications. Ti3SiC2 is a known ohmic contact to 4H-SiC with the known two-step synthesis process of post-annealing of pre-deposited Ti/Al multilayers or sputter-deposition of Ti3SiC2 films at > 900 °C. Here, sputter-deposition of Ti on 4H-SiC at > 900 °C is presented as a novel single-step method for the synthesis of Ti3SiC2 ohmic contacts, based on a concurrent reaction between sputter-deposited Ti and 4HSiC. Ti3SiC2, similar to any other known ohmic contact, degrade rapidly in high-temperature oxidizing ambient. To try to overcome this obstacle, noble metal diffusion into Ti3SiC2 has been s studied with the goal to retain ohmic properties of Ti3SiC2 and harnessing oxidation resistivity of noble metals. A novel exchange intercalation between Ti3SiC2 and Au is discovered which results in the almost complete exchange of Si with Au giving rise to novel Ti3AuC2 and Ti3Au2C2. Ti3IrC2 is also synthesized through exchange intercalation of Ir into Ti3Au2C2. All the aforementioned phases showed ohmic properties to 4H-SiC. This technique is also studied based on Ti2AlC and Ti3AlC2 resulting in the synthesis of novel Ti2Au2C and Ti3Au2C2, respectively. Using Ti3AuC2 and an Au/IrOx capping layer, an ohmic contact was manufactured, which maintained ohmic properties and showed no structural defects after 1000 h of aging at 600 °C air.

Ti3SiC2 is a member of a large family of materials known as Mn+1AXn phases. While exchange reactions of Si (or Al) planes in Ti3SiC2 (Ti2AlC and Ti3AlC2) is presented here, a world-wide research already exists on chemical removal of the same atomic planes from different Mn+1AXn phases and the synthesis of Mn+1Xn sheets known as MXenes. I performed a theoretical study regarding simulation of electronic and structural properties of more than120 different possible MXene phases. The results show that some MXene phases, when terminated by particular gas species, turn into Dirac materials. That is, they possess massless Dirac fermions with different properties compared to graphene such as higher number of Dirac points at the Fermi level, giant spin orbit splitting, and preserved 2D-type electronic properties by extending the dimensionality. The general substantial change of the electronic properties of MXenes under different gas adsorption configurations stands out and can thus be harnessed for sensing applications.

Growth of monolayer iron oxide on porous Pt sensing layers is another novel approach used in this study for applying the unique properties of 2D materials for gas sensors. A low temperature shift in CO oxidation characteristics is presented. The approach is similar to that previously reported using bulk single crystal Pt substrate, the latter being an unrealistic model for sensors and catalysts. Monolayer-coated Pt sensing layers were fabricated as the metal component of a metal oxide semiconductor (MOS) capacitor device, whereby the electrical response of the MOS device could be used to map out the catalytic properties of the sensing layer. The monolayer-coated Pt surface showed to be stable with retained improved catalytic properties for > 200 h. The MOS device measurements are here utilized as a handy method for in-situ monitoring of the surface chemical properties of the monolayer-coated Pt and the approach is highly functional for use and characterization of monolayer coatings of widely used sensingor catalytic layers.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2016. , 55 p.
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1784
National Category
Physical Sciences Materials Chemistry Condensed Matter Physics Inorganic Chemistry Other Chemical Engineering Other Materials Engineering
URN: urn:nbn:se:liu:diva-131474DOI: 10.3384/diss.diva-131474ISBN: 9789176856994 (Print)OAI: diva2:972762
Public defence
2016-10-21, Plank, Fysikhuset, Campus Valla, Linköping, 10:15 (English)
Available from: 2016-09-22 Created: 2016-09-22 Last updated: 2016-09-22Bibliographically approved
List of papers
1. Single-step synthesis process of Ti3SiC2 ohmic contacts on 4H-SiC by sputter-deposition of Ti
Open this publication in new window or tab >>Single-step synthesis process of Ti3SiC2 ohmic contacts on 4H-SiC by sputter-deposition of Ti
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2015 (English)In: Scripta Materialia, ISSN 1359-6462, E-ISSN 1872-8456, Vol. 99, 53-56 p.Article in journal (Refereed) Published
Abstract [en]

We report a single-step procedure for growth of ohmic Ti3SiC2 on 4H-SiC by sputter-deposition of Ti at 960 °C, based on the Ti–SiC solid-state reaction during deposition. X-ray diffraction and electron microscopy show the growth of interfacial Ti3SiC2. The as-deposited contacts are ohmic, in contrast to multistep processes with deposition followed by rapid thermal annealing. This procedure also offers the possibility of direct synthesis of oxygen-barrier capping layers before exposure to air, potentially improving contact stability in high-temperature and high-power devices.

Place, publisher, year, edition, pages
Elsevier, 2015
Silicon carbide, MAX phase, Physical vapor deposition, High temperature
National Category
Physical Sciences
urn:nbn:se:liu:diva-113760 (URN)10.1016/j.scriptamat.2014.11.025 (DOI)000348881100014 ()

We acknowledge the support from the VINN Excellence Center in research and innovation on Functional Nanoscale Materials (FunMat) by the Swedish Governmental Agency for Innovation Systems. P.E and J.L. also acknowledge support from the Swedish Foundation for Strategic Research through the Future Research Leaders 5 program and the Synergy Grant FUNCASE, Functional Carbides and Advanced Surface Engineering. In addition, we thank Dr. Hans Hogberg, Dr. Arni Sigurdur Ingason and Dr. Fredrik Eriksson for discussions and help with experiments.

Available from: 2015-01-30 Created: 2015-01-30 Last updated: 2016-09-22Bibliographically approved
2. Dirac points with giant spin-orbit splitting in the electronic structure of two-dimensional transition-metal carbides
Open this publication in new window or tab >>Dirac points with giant spin-orbit splitting in the electronic structure of two-dimensional transition-metal carbides
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2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 15Article in journal (Refereed) Published
Abstract [en]

We investigated the structural and electrical properties of 2D MXene sheets by means of firstprinciples density functional theory (DFT) calculations. To describe the Kohn-Sham states, plane wave basis set and projector augmented wave method (PAW) were used as implemented in the Vienna ab initio Simulation Package (VASP). We applied PBE parameterization of the generalized gradient approximation of the exchange and correlation energy functional to account for many-body effects of the interacting electron system. Convergent sampling of the Brillouin-zone was achieved by a Γ-centered 15×15×1 grid. In order to model a single sheet of MXene we ensured at least 30 Å vacuum between the periodically repeated sheets. For the structural optimization 1×10−3 eV/Å force criteria was used. The relativistic spin-orbit coupling effects were also included in our simulations regarding band structure and density of states.

Cone-point, MAX phase, MXene, Dirac fermion, Spin-orbit coupling
National Category
Physical Sciences
urn:nbn:se:liu:diva-113761 (URN)10.1103/PhysRevB.92.155142 (DOI)000363512700002 ()
Available from: 2015-05-01 Created: 2015-01-30 Last updated: 2016-10-11Bibliographically approved

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