Two-dimensional (2D) atomic sheets yield collective properties of mechanical flexibility, electrical control, optical transparency and high surface-to-volume ratio, which hold promise for advanced flexible nanoelectronics and sensors. This work explores two newly emerging 2D materials, silicene and phosphorene (the Si and P equivalent to graphene) in terms of their air-stability and device properties. Silicene, IVA family cousin of graphene, is predicted to offer a host of exotic electrical properties depending on its material phase, interface and external fields. Recent effort addressed long-lasting air-stability and portability issues, allowing silicene transistor to make its debut, corroborating theoretically predicted ambipolar transport indicating Dirac band structure. Electrostatic characterization on prototype silicene transistors observed carrier mobility ~100 cm2/V-s and 10× gate modulation at RT. In theory, pristine free-standing silicene may offer intrinsic mobility over 1000 cm2/V-s without non-ideal limiting factors, e.g. phase boundary scattering and electron-phonon coupling. The debut of silicene transistor confirms ambipolar transport behavior in atomically thin Si with greater gate modulation than graphene, indicating potential device reach beyond graphene. On the other hand, phosphorene exhibits high mobility and tunable direct bandgap even on plastic substrates, making it the most suitable contemporary 2D semiconductor that combines the merits of graphene and transitional metal dichalcogenides. Phosphorene, phosphorus analog to graphene, is a contemporary semiconductor promising for 2D nanoelectronics due to its direct bandgap bridging between graphene and transitional metal dichalcogenides. Recent few-layer black phosphorus has demonstrated high carrier mobility 310-1500 cm2/Vs with gate modulation 103-105 on flexible polyimide substrate, sustaining under ex-situ bending test with tensile strain up to 1.5%. This recent progress on silicene and phosphorene represent a renewed opportunity for future nanoscale and flexible devices.