Browsing by Subject "Two-dimensional materials"
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Item Device physics and device mechanics for flexible MoS2 thin film transistors(2015-08) Chang, Ph. D., Hsiao-Yu; Akinwande, Deji; Lu, Nanshu; Lee, Jack; Lai, Keji; Dodabalapur, AnanthWhile there has been increasing studies of MoS2 and other two-dimensional (2D) semiconducting dichalcogenides on hard conventional substrates, experimental or analytical studies on flexible substrates has been very limited so far, even though these 2D crystals are understood to have greater prospects for flexible smart systems. In the first part, we report detailed studies of MoS2 transistors on industrial plastic sheets. Failure mechanisms under strain are studied with bending test and stretching test. Experimental investigation identifies that crack formation in the dielectric and the buckling delamination in MoS2 are responsible for the degradation of the device performance. Several approaches to improve device flexibility were discussed. In the second part, electronic transport properties in multilayer MoS2 are investigated with Y-function method. By combining experiments and analysis, we show that the Y-function method offers a robust route for evaluating the low-field mobility, threshold voltage and contact resistance even when the contact is a Schottky-barrier as is common in two-dimensional transistors. In addition, an independent transfer length method (TLM) evaluation corroborates the modified Y-function analysis. The last part, we demonstrate the first RF performance for transferred CVD-grown MoS2 FETs on the flexible substrate. Our result suggests that the large-area CVD-grown MoS2 provides a practical route to realize low-power, high speed electronics circuit applications in the future.Item Four-probe measurements of anisotropic in-plane thermal conductivities of thin black phosphorus(2016-08) Smith, Brandon Paul; Shi, Li, Ph. D.; Akinwande, DejiPhosphorene, a two-dimensional material exfoliated from black phosphorus (BP), is a promising p-type, high-mobility semiconductor. Phosphorene and BP display intrinsic in-plane anisotropic transport properties due to its puckered honeycomb lattice with distinct armchair and zigzag crystallographic orientations. The anisotropic thermal transport properties of BP and phosphorene influence the performance and reliability of functional devices made from these materials, and remain to be better understood. Here, we report the anisotropic in-plane thermal conductivities of suspended multi-layer BP samples, which are measured by a four-probe thermal transport measurement method. The measurement device consists of four microfabricated, suspended Pd/SiNx lines that act as resistive heaters and thermometers. The BP flake is suspended across the microstructure in contact with all four lines. This four-probe thermal transport measurement is equipped with the unique ability to isolate the intrinsic thermal resistance from the contact thermal resistance, which can be a major source of error in thermal conductivity measurements of nanostructures. Four BP samples were measured with thicknesses ranging from 39.2 nm to 274 nm and a peak thermal conductivity of 142 W m-1 K-1 at 80 K for a 55.6 nm thick zigzag oriented flake. The measurement results exhibit more pronounced temperature dependence with a higher peak thermal conductivity together with a weaker thickness dependence than prior reports. The results suggest the important role of defects in thermal transport in thin BP flakes, which can degrade upon exposure to air and water.Item Two-dimensional coherent spectroscopy of monolayer transition metal dichalcogenides(2015-08) Dass, Chandriker Kavir; Li, Elaine; Downer, Michael; Lai, Keji; Shih, Chih-Kang; Tutuc, EmanuelTwo-dimensional semiconductors have long been studied for their unique optical and electronic properties, but with the work of Novoselov and Geim on van der Waals materials, two-dimensional semiconductors have seen a surge of renewed interest. This dissertation focuses on monolayer transition metal dichalcogenides (TMDCs), a class of two-dimensional materials that can easily be fabricated by mechanical exfoliation, much like graphene. In their bulk form, these materials have indirect band gaps, but transition to direct gap semiconductors in the monolayer limit. The band-edge optical response of TMDCs, like WSe₂ and MoS₂, is dominated by exciton absorption occurring at the ±K-points of the Brillouin zone. Because of the unique electronic structure of these materials, these two points form distinct valleys in the band structure which can be exploited to produce valley polarization. Exciton quantum dynamics are characterized by two fundamental parameters, one of which is the dephasing rate, γ, which describes quantum dissipation arising from the interaction of the excitons with their environment (i.e. other excitons, impurities, etc…). This dissertation focuses on measuring the fundamental property of dephasing time (which is inversely proportional to the dephasing rate and homogeneous linewidth) in monolayer WSe₂ through the use of two-dimensional coherent spectroscopy. Our measurements have revealed a homogeneous linewidth consistent with dephasing times in the sub-picosecond regime. We also characterize the role of exciton-exciton and exciton-phonon interactions, on the homogeneous linewidth, through excitation density and temperature dependent studies. These studies have revealed strong many-body effects and nonradiative population relaxation as the primary dephasing mechanisms. Microscopic calculations show that in perfect crystalline samples of monolayer TMDCs, the radiative lifetimes are also in the sub-picosecond regime due to the large oscillator strengths inherent in these materials. This result is consistent with the short dephasing times found experimentally.