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Ultrathin materials have opened a new chapter in condensed matter physics that is still being extensively written as new layered materials are synthesized or extracted from bulk. Fabrication of field-effect transistors (FETs) with two-dimensional materials allows precise control of their properties. New exciting physics is observed when such materials are combined in heterostructures, with or without mechanical strain or in-plane twisting. This dissertation is focused on studies on two materials that recently became available for 2D materials research: indium selenide (InSe) and chromium iodide (CrI3).The first part of the dissertation focuses on CrI3. As a 2D ferromagnetic semiconductor with magnetic ordering, this material is one of the latest additions to the family of 2D materials. However, realistic exploration of CrI3-based devices and heterostructures is challenging, due to its extreme instability under ambient conditions. Here we present Raman characterization of CrI3 and demonstrate that the main degradation pathway of CrI3 is the photocatalytic substitution of iodine by water. While simple encapsulation by Al2O3, PMMA and hexagonal BN (hBN) only leads to modest reduction in degradation rate, minimizing exposure of light markedly improves stability, and CrI3 sheets sandwiched between hBN layers are air-stable for >10 days. By monitoring the transfer characteristics of CrI3/graphene heterostructure over the course of degradation, we show that the aquachromium solution hole-dopes graphene.In the second part of this thesis, we focus on charge transport studies of atomically thin InSe and demonstrate SOC and intrinsic spin-splitting therein can be modified over an unprecedently large range. From beating patterns in quantum oscillations, we establish that the SOC parameter α is thickness-dependent; it can be continuously modulated over a wide range by an out-of-plane electric field, achieving intrinsic spin splitting tunable between 0 and 20 meV. Surprisingly, α could be enhanced by an order of magnitude in some devices, suggesting that SOC can be further manipulated by variations in interlayer spacing induced by stacking and/or electrostatic compression. Our work highlights the extraordinary tunability of SOC in 2D materials, which can be harnessed for in operando spintronic and topological devices and applications.Additionally, we examine magnetotransport in ultrathin InSe in quantum Hall regime. By controlling the sample temperature, we extract energy gaps between Landau levels and study their dependence on perpendicular magnetic field. We demonstrate that the values of energy gaps can be described by a model of a semiconductor with strong Rashba SOC, and obtain the values of and g-factors and effective masses at different filling factors.Furthermore, we demonstrate the population of the 2nd electronic subband at high charge densities, which manifest as the appearance of an additional Landau fan in magnetotransport data. We obtain the effective mass of electrons and show that the onset charge density of the second subband can be increased by application of perpendicular electric field. Transport measurements at high magnetic fields reveal intriguing circular shape of crossings of Landau levels, which we explain to result from a transition from paramagnetic to ferromagnetic states.