Thickness-dependent Raman spectra, transport properties and infrared photoresponse of few-layer black phosphorus

The crystalline thin layer of black phosphorus (BP) has emerged as a new category of two-dimensional (2D) materials very recently, due to its tunable direct bandgap, promising physical properties, and potential applications in optoelectronics. Herein, the Raman scattering properties of the few layers of BP including the frequency shift and the intensity of the A(g)(1), B-g(2) and A(g)(2) modes have been studied in detail and they show obvious dependence on thickness and light polarization. The optoelectronic performances of few-layer black phosphorus including field-effect properties and photosensitivity to laser light with different wavelengths are also investigated. The optoelectronic parameters including the current modulation, mobility, photoresponsivity and response time vary distinctly with the layer thickness. At room temperature, the obvious bipolar transport properties are obtained (with the hole and electron mobility as high as 240 and 2 cm(2) V-1 s(-1), respectively) in the thicker (15 nm) BP devices, while the thinner (9 nm) BP only shows P-type transportation. The photoresponsivity of BP devices under different laser light illumination reaches several tens of mA W-1, which demonstrates their excellent photo-responsive properties and broadband detection. The thinner (9 nm) BP shows a high photoresponsivity of 64.8 mA W-1 at the communication band of 1550 nm, which is much larger than that of the thicker sample. Our findings reveal that the charge transport and infrared photo-response properties of BP are excellent, and diverse and can be intentionally designed through the thickness control. Such results also suggest BP's great potential in nanoelectronic devices and photodetection from the visible light up to the communication band (infrared light).