On the electronic and transport properties of two-dimensional transition metal carbides and carbonitrides (MXenes)

Kanit Hantanasirisakul

doi:10.17918/00000423

Since their discovery reported in 2011, two-dimensional (2D) transition metal carbides, carbonitrides and nitrides, known collectively as MXenes, have shown promises in applications ranging from energy storage to electromagnetic interference (EMI) shielding, transparent electrodes, optoelectronics, wireless communication, and sensors to name a few. Exceptional performances in the aforementioned applications are enabled by MXenes' high electronic conductivity, that is metallic in nature. In addition, they have a wide range of optical properties, show easy processability and high mechanical strength. MXenes have a general formula of Mn+1XnTx, where M represents early transition metals (Ti, Mo, Cr, etc), X is carbon and/or nitrogen, Tx stands for surface terminations (-O, -OH, -F, -Cl, etc), and n is an integer between 1 and 4, which determines the layer thickness. Because of a large variety of chemical compositions including solid solutions, structural variations, and rich surface chemistry, MXenes are subject to multi-level manipulation of their properties, which is their unique advantage compared to majority other bulk or 2D materials. However, the level of understanding of how the structure and composition of MXenes affect their electronic and transport properties is still insufficient. This dissertation aims to establish relationships between MXenes' electronic properties and the intercalants, transition metals, X elements, and surface terminations with an overarching goal of demonstrating an ability to control MXenes' electronic and transport properties by controlling their structure and chemical composition. To explicitly correlate one of the MXenes' properties to their electronic behavior, the other factors must be precisely controlled and/or monitored. To this end, many in situ measurements were employed in this work such as in situ measurement of MXenes' electrical resistance or work function during thermal annealing under high vacuum, while the evolution of their surface chemistry and structures were monitored by electron energy loss spectroscopy or X-ray photoelectron spectroscopy. Temperature-dependence of resistivity in the range of 2-1050 K and magnetoresistance were primarily used to investigate MXene electronic transport property. My wok shows that large organic intercalants cause expansion of the interlayer spacing and hence higher resistivity of MXenes and alter the low-temperature transport behavior. Furthermore, removal of OH- and F-surface terminations results in lower resistivity of MXenes and more than 1 eV shift of the work function of Ti₃C₂T_x. Moreover, replacing approximately half of the carbon atoms in Ti₃C₂T_x with nitrogen atoms, forming Ti3CNTx, leads to higher electronic resistivity due to lower carrier mobility. Finally, magnetism introduced into a MXene via manipulation of the transition metal layers has been shown to strongly affect magnetotransport properties of Cr2TiC2Tx. This work paves the way toward a better understanding of MXenes' structure, processing, and property, and performance relationships, focusing on electronic and transport properties as well as the realization of MXenes for high-performance optoelectronic devices and 2D heterostructures.

On the electronic and transport properties of two-dimensional transition metal carbides and carbonitrides (MXenes)

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On the electronic and transport properties of two-dimensional transition metal carbides and carbonitrides (MXenes)

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