2D Materials: Synthesis, Characterization, and Applications

The isolation of monolayer graphene by Andre Geim and Konstantin Novoselov in 2004 created an explosion of layered materials research in the fields of condensed matter physics, material science, electrical engineering, chemistry, and nanobiology, to name a few. The applications have been broad from enhancing electrode performance in batteries to gas sensing to high-frequency analog flexible electronics. For several years and still to this day, graphene has provided a fertile ground for research due to its superior properties. However, failed efforts to engineer a substantial bandgap, a requirement for digital electronics, led researchers to look elsewhere in the periodic table for other layered materials with rich physics and an even broader application space. Fortunately, the technical expertise developed in the graphene system could, for the most part, be leveraged and modified in these new material systems. This thesis presents a brief history of the field of two-dimensional electronics. The rediscovery - and it can only really be characterized as such since most of these materials were studied in the bulk form going back to the 1960s - of these two-dimensional materials with properties ranging from superconductivity, piezoelectricity, optical and electrical anisotropy, and large magnetoresistivity required the development of new characterization techniques to address the perturbations that accompanied the “thinning” of layers. Several characterization techniques were developed and are presented in this thesis. Moreover, in an effort to push these materials closer towards technological viability, synthesis techniques were developed that enabled the systematic study of a prototypical material system, molybdenum disulfide (MoS₂), in order to address the challenges that accompany scalability and determine the structure-property-function relationship

Options to support sustainable trajectories in a rural landscape : drivers, rural processes, and local perceptions in a colombian coffee-growing region

Altres ajuts: Ean UnivesityUnidad de excelencia María de Maeztu CEX2019-000940-MTrajectories of many rural landscapes in Latin America remain unsustainable. Options to support sustainable rural trajectories should be comprehensive and rooted in the interests of rural actors. We selected a municipality in a coffee-growing region in Colombia with an increasing urban- rural nexus to describe interactions between rural processes and their drivers while identifying and contextualising the perceptions of local actors on major constraints and opportunities for more inclusive and sustainable rural trajectories. We described these interactions by combining secondary data on main drivers, agricultural census data, and interviews with different local actors. Changes in population structure, volatility in coffee prices, in-/out-migration, deagrarianisation, and rurbanisation, among others, are reconfiguring the rural trajectories of the study area. Despite not being a major coffee region, farmers in the study area have developed different strategies, including intensification, diversification, replacement or abandonment of coffee production, and commercialisation. The perceptions of local actors and the multiplicity of agricultural households, food/land use systems, rural processes, and drivers described in this study suggest that more sustainable rural transitions need to be supported by inclusive, integrated, and transformative landscape planning approaches that align with local priorities. However, this transformation needs to be accompanied by changes at a systemic level that address the fundamental bottlenecks to real sustainability

Multi-terminal transport measurements of MoS2 using a van der Waals heterostructure device platform

Atomically thin two-dimensional semiconductors such as MoS2 hold great promise in electrical, optical, and mechanical devices and display novel physical phenomena. However, the electron mobility of mono- and few-layer MoS2 has so far been substantially below theoretically predicted limits, which has hampered efforts to observe its intrinsic quantum transport behaviours. Potential sources of disorder and scattering include both defects such as sulfur vacancies in the MoS2 itself, and extrinsic sources such as charged impurities and remote optical phonons from oxide dielectrics. To reduce extrinsic scattering, here we developed a van der Waals heterostructure device platform where MoS2 layers are fully encapsulated within hexagonal boron nitride, and electrically contacted in a multi-terminal geometry using gate-tunable graphene electrodes. Magneto-transport measurements show dramatic improvements in performance, including a record-high Hall mobility reaching 34,000 cm2/Vs for 6-layer MoS2 at low temperature, confirming that low-temperature performance in previous studies was limited by extrinsic interfacial impurities rather than bulk defects in the MoS2. We also observed Shubnikov-de Haas oscillations for the first time in high-mobility monolayer and few-layer MoS2. Modeling of potential scattering sources and quantum lifetime analysis indicate that a combination of short-ranged and long-ranged interfacial scattering limits low-temperature mobility of MoS2.Physic

Tracking timescales of short-term precursors to large basaltic fissure eruptions through Fe–Mg diffusion in olivine

Petrological constraints on the timescales of pre-eruptive crystal storage and magma degassing provide an important framework for the interpretation of seismic, geodetic and gas monitoring data in volcanically active regions. We have used Fe–Mg diffusion chronometry in 86 olivine macrocrysts from the AD 1783–1784 Laki eruption on Iceland's Eastern Volcanic Zone to characterise timescales of crystal storage and transport in the lead-up to this eruption. The majority of these olivines have core compositions of Fo 81 olivines record Fe–Mg diffusion timescales of ∼124 days; these crystals are likely to have formed in mid-crustal magma chambers, been transferred to storage at shallower levels and then entrained into the Laki melt prior to eruption. Typical Fe–Mg diffusion timescales of 6–10 days are shorter than the average time interval between discrete episodes of the Laki eruption, indicating variable or pulsed disaggregation of stored crystals into the carrier liquid prior to the onset of each episode. The diffusion timescales coincide with historical accounts of strong and frequent earthquakes in southeast Iceland, which we interpret as being associated with mush disaggregation related to melt withdrawal and the initiation of dyke propagation from a crustal magma reservoir at ∼6 ± 3 km depth to the surface. We calculate pre-eruptive CO2 fluxes of 2–6 Mt d−1, assuming a pre-eruptive CO2 outgassing budget of 189.6 Mt for the Laki eruption and a constant rate of CO2 release in the 6–10 days preceding each eruptive episode. Our dataset indicates that petrological constraints on the timescales of magmatic processes occurring in the days leading up to historic eruptions may enhance our ability to forecast the onset of future large eruptions, both in Iceland and further afield