Investigating laser induced phase engineering in MoS2 transistors

Phase engineering of MoS2 transistors has recently been demonstrated and has led to record low contact resistances. The phase patterning of MoS2 flakes with laser radiation has also been realized via spectroscopic methods, which invites the potential of controlling the metallic and semiconducting phases of MoS2 transistors by simple light exposure. Nevertheless, the fabrication and demonstration of laser patterned MoS2 devices starting from the metallic polymorph has not been demonstrated yet. Here, we study the effects of laser radiation on 1T/1T'-MoS2 transistors with the prospect of driving an in-situ phase transition to the 2H-polymorph through light exposure. We find that although the Raman peaks of 2H-MoS2 become more prominent and the ones from the 1T/1T' phase fade after the laser exposure, the semiconducting properties of the laser patterned devices are not fully restored and the laser treatment ultimately leads to degradation of the transport channel

On the origin of critical temperature enhancement in atomically-thin superconductors

Recent experiments showed that thinning gallium, iron selenide and 2H tantalum disulfide to single/several monoatomic layer(s) enhances their superconducting critical temperatures. Here, we characterize these superconductors by extracting the absolute values of the London penetration depth, the superconducting energy gap, and the relative jump in specific heat at the transition temperature from their self-field critical currents. Our central finding is that the enhancement in transition temperature for these materials arises from the opening of an additional superconducting gap, while retaining a largely unchanged bulk superconducting gap. Literature data reveals that ultrathin niobium films similarly develop a second superconducting gap. Based on the available data, it seems that, for type-II superconductors, a new superconducting band appears when the film thickness becomes smaller than the out-of-plane coherence length. The same mechanism may also be the cause of enhanced interface superconductivity.Comment: 43 pages, 12 figure

Thickness dependent interlayer transport in vertical MoS2 Josephson junctions

We report on observations of thickness dependent Josephson coupling and multiple Andreev reflections (MAR) in vertically stacked molybdenum disulfide (MoS2) - molybdenum rhenium (MoRe) Josephson junctions. MoRe, a chemically inert superconductor, allows for oxide free fabrication of high transparency vertical MoS2 devices. Single and bilayer MoS2 junctions display relatively large critical currents (up to 2.5 uA) and the appearance of sub-gap structure given by MAR. In three and four layer thick devices we observe orders of magnitude lower critical currents (sub-nA) and reduced quasiparticle gaps due to proximitized MoS2 layers in contact with MoRe. We anticipate that this device architecture could be easily extended to other 2D materials.Comment: 18 pages, 6 figures including Supporting Informatio

Utilizing Muscovite to Create High Mobility Molybdenum Disulfide Transistors

Molybdenum disulfide transistor devices were fabricated utilizing muscovite mica as dielectrics in order to test the hydrophilic behavior of mica. This was done by probing the device for its transconductance plot to show hysteretic patterns. Devices were fabricated using a clean van der Waals technique to stack two-dimensional materials into heterostructures. The devices showed a hysteretic trend in the transconductance curve. We compared the hysteretic behavior from mica with that of another well-known dielectric, silicon dioxide. The devices with mica dielectrics showed larger hysteresis in the gate sweeps than silicon dioxide. Devices utilizing mica as dielectrics are expected to have hysteretic behaviors due to the interfacial water on the mica surface. It is also speculated that water accumulation will continue to grow on the surface as long as the device is in ambient conditions, so the hysteresis may worsen over time. We aim to mitigate water absorption at the surface of mica and suggest future work to accomplish this goal.https://digitalscholarship.unlv.edu/durep_podium/1020/thumbnail.jp

On-Chip Sub-Diffraction THz Spectroscopy of Materials and Liquids

This chapter summarizes the trends in terahertz measurements on the surface of rigid and flexible substrates. It focuses on research incorporating fast photoconductive switches to generate and detect on-chip THz pulses using a femtosecond laser. The chapter aims to review progress toward the study of picosecond dynamics and THz spectroscopy of materials and liquids. We emphasize general sub-diffraction techniques for THz spectroscopy, transmission line and waveguide design considerations, time-domain measurements for studies of material dynamics, and provide a survey of recent research on the THz spectroscopy of materials and liquids on-chip. We conclude with an outlook on the field and highlight promising new directions. This chapter is meant to be an introduction and a general guide to this emerging field for new researchers interested in on-chip THz studies

Van Der Waals Heterostructure Engineered Quantum Anomalous Hall Effect

The quantum anomalous hall effect (QAHE) is a phase of matter in which a dissipationless current is made to flow around the edge of a two dimensional (2D) material. Making use of this effect for next generation electronics could lead to faster processors and low power devices. There are very few materials that exist in nature that intrinsically possess the QAHE, however by sandwiching target 2D materials together we can establish this highly sought after phase. By using three 2D materials: graphene, molybdenum disulfide (MoS2) and chromium tri-iodide (CrI3) forming a van der Waals heterostructure we can create a proximity induced magnetism effect. Here, we took highly sensitive capacitance measurements of graphene on MoS2 devices at low temperatures and high magnetic fields. By taking measurements of the penetration field capacitance vs charge density and polarization of a graphene and MoS2 device at 2 Kelvin and zero external magnetic field, we are able to see the charge neutrality point in graphene and the conduction band of MoS2. Using this method of capacitance measurements we plan to integrate thin CrI3 flakes into our graphene and MoS2 devices to develop a full device to study the proximity induced QAHE.https://digitalscholarship.unlv.edu/durep_podium/1019/thumbnail.jp