Fig. 4. Electrical and infrared optical properties of the as-grown BP films.

a Output and transfer curves of the FETs utilizing the synthesized BP film as channel materials. Transfer characteristics was measured under gate voltage V_g swept from −30 to 50 V with fixed source-drain voltage V_ds = 100 mV. The output result is demonstrated in the insert, measured under V_ds swept from −50 to 50 mV, and different gate voltages from −40 to 20 V with 20 V as step. Electrical characterizations demonstrate a typical p-type ambipolar transport behavior of the BP sample. b Carrier mobility and current on/off ratio as a function of temperature in a synthesized BP film. Field-effect mobility of over 1200 cm² V⁻¹ s⁻¹ and excellent current on/off ratio as high as 10⁶ for gate voltages in the range of −40 to 40 V can be achieved. c Hall mobility (red line) and carrier concentration (blue line) of the BP film as a function of temperature. Insert is the AFM topography image of a Hall-bar device employed for the characterization, where the line profile indicates the thickness of the BP is ~ 8 nm. d Infrared absorption spectra of the as-grown lamellar structure BP film (red line) and the BP derived from conventional bulk crystal (blue line). e PL spectra of the as-grown lamellar structure BP film (red line) compared with BP derived from conventional bulk crystal (blue line). f The thickness dependence of the full width at half maximum of the PL spectra for the as-grown lamellar structure BP film (red line) and those derived from conventional BP bulk crystal (blue line). All the spectra measurements were carried out at ambient conditions, and employing two types of BP with the same thicknesses.