Osl1 ec

Spectroscopic measurements were carried out with a spectrophotometer (AvaSpec2048, Avantes, Netherlands) coupled to an integrating sphere (2" Integrating Sphere IS200-4, Thorlabs). The transmittance and reflectance were measured in the spectral range from 455 to 705 nm. The entrance port of the integrating sphere is 12.7-mm diameter. It is essential that the samples cover the entire port for accurate measurements. Consequently, we have reduced the entrance port diameter to 10 mm in diameter using a round washer with one face (interior) coated with a highly diffusive paint. The sample is illuminated with a tungsten-halogen light source (Thorlabs OSL1-EC) coupled to a 200-μm diameter fiber collimated with a plug-in SMA collimator.

All electrical measurements were conducted in ambient atmosphere at room temperature. The current-density-voltage (J-V) measurements were performed using a computer-interfaced Keithley 2400 source-meter. As convention, the bias voltage is applied to the silicon substrate and graphene is grounded in all measurements. Laser diodes of wavelength λ = 520, 635 and 980nm as well a monochromator (Spex 500M) with white light source (Thorlabs OSL1EC) were used to record the spectral response of devices under illumination. Neutral density (ND) filters were used to vary the light intensity. The high photoresponsivity of the diodes from near UV to IR allowed performing measurements at DC without need for a lock-in amplifier.