Low-dimensional semiconductorsSynthesis, properties and devices

Resumen

The discovery of graphene, an atomically thin layer of carbon atoms, in 2004 by Andre Geim and Konstantin Novoselov [Novoselov, 2004] stirred up the scientific community due to the novel and interesting properties found in the material. High electron mobility, strong Young modulus, very high thermal conductivity, complete impermeability to any gases or high cur-rent density are some of the main properties of this material [Geim, 2007; Novoselov, 2012]. Although it might look like graphene is the wonder mate-rial for several applications, its use in electronics has become highly chal-lenging due to the absence of a bandgap in its electronic structure. There have been several attempts to introduce a bandgap in the electronic structure of graphene [Zhang, 2009] and the further fabrication of graphene-based transistors [Schwierz, 2010], but it always results in a reduction of the elec-tron mobility or the requirement of high voltages [Xia, 2010]. Nevertheless, even if graphene is not such a great candidate for electron-ic applications, it opened the door to the fabrication and synthesis of low-dimensional electronic structures. In this way, the first semiconductor isolat-ed in a two-dimensional configuration was molybdenum disulfide (MoS2), in 2011, when the first field-effect transistor (FET) employing a two-dimensional semiconductor was developed [Radisavljevic, 2011]. From that moment, more than ten two-dimensional semiconductors have emerged: transition metal dichalcogenides (MoS2, MoSe2, WS2 or WSe2), black phos-phorus, Re-based chalcogenides or transition metal trichalcogenides [Castellanos-Gomez, 2016]. Together with the two-dimensional materials, the one-dimensional materials have also gained attention, finding interesting applications with nanowires of different semiconductors such as SnO2 [Lin, 2008], TiO2 [Tsai, 2012], carbon nanotubes [Sapmaz, 2003] or ZnO [Bai, 2011; Ates, 2012]. These materials present different bandgaps, meaning that each one can be suitable from different applications like FETs, photodetec-tors, solar cells, etc [Wang, 2012; Lopez-Sanchez, 2013; Buscema, 2014; Island, 2014]. Therefore, the quest for finding the best low-dimensional ma-terial for each application is opened. From a synthesis point of view, the most used method to obtain two-dimensional crystals is the so-called Scotch tape method, which basically consist of mechanically exfoliating layered materials with an adhesive tape to reduce the material thickness, obtaining micrometer-size (in lateral dimen-sions) crystals. By this method, the exfoliated materials can be transferred to arbitrary substrates by just placing the adhesive tape holding the crystals on the substrate and peeling it off really slow. Another approach consist of di-rectly synthesizing the two-dimensional crystals on a substrate by molecular beam epitaxy (MBE) or chemical vapor deposition (CVD), but these meth-ods involve complex and time-consuming procedures which are difficult to scale up. In this thesis we explore new semiconductors in low-dimensional sys-tems such as field-effect devices or photodetectors with two-dimensional crystals or one-dimensional nanoribbons or nanowires as semiconducting channel. We employ different methods to synthesize the materials (mechan-ical exfoliation, air-pressure CVD) and fabricate functional devices (deter-ministic transfer, e-beam lithography), explained in Chapter 2. We use dif-ferent experimental techniques to study the properties of the materials (scanning electron microscopy, STM), as well as the characterization of the synthetized material (X-ray photoelectron spectroscopy, XPS) and the fabri-cated devices (electronic transport and photoresponse in field-effect devic-es), which are explained in Chapter 3. We study four different materials: TiS3, TiO2, MoO3 and franckeite. We first explore one-dimensional systems based on TiS3 nanoribbons and TiO2 nanofibers by studying the bandgap and the exciton binding energy of TiS3 nanoribbons, as well as its thermal stability in field-effect transistors. We also characterize the photoresponse of TiO2 single nanofiber-based UV-photodetectors (Chapter 4). We also study two-dimensional systems based on centimeter-scale ul-trathin MoO3, used in field-effect devices, UV-photodetectors and self-driven voltage devices, and in franckeite, a two-dimensional van der Waals heterostructure that has been employed in field-effect devices, NIR-photodetectors and p-n junctions (Chapter 5). References [Ates, 2012] Ates, E. S., S. Kucukyildiz and H. E. Unalan. "Zinc Oxide Nanowire Photodetectors with Single-Walled Carbon Nanotube Thin-Film Electrodes." ACS Applied Materials & Interfaces 4(10): 5142-5146 (2012). [Bai, 2011] Bai, S., W. Wu, Y. Qin, N. Cui, D. J. Bayerl and X. Wang. "High-Performance Integrated ZnO Nanowire UV Sensors on Rigid and Flexible Substrates." Advanced Functional Materials 21(23): 4464-4469 (2011). [Buscema, 2014] Buscema, M., D. J. Groenendijk, G. A. Steele, H. S. J. van der Zant and A. Castellanos-Gomez. "Photovoltaic effect in few-layer black phosphorus PN junctions defined by local electrostatic gating." Nat Commun 5 (2014). 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