hp 4284a lcr meter at 20 hz and 50 mv swing voltage to include the effects of all potential long lived trap states photocurrent was measured as a function of wavelength for monochromated light and normalized by the measured responsivity at 530 nm the nep at 530 nm was calculated from the responsivity and the photodiode noise taken to be the reverse bias dark current samples for imaging and surface analysis were prepared by ion polishing with a jeol cross section polisher amplitude modulated kpfm was performed with an omicron uhv vt afm base pressure lt 5 u00d7 10 u2212 10 torr equipped with a kevin probe control unit using a nanosensors pt u2014 ir tip nominal resonance u02dc 75 khz kelvin signal was run at the first overtone of cantilever resonance u02dc 475 khz with an applied peak to peak voltage of 200 mv to minimize tip induced band bending the tip work function was calculated by determining the cpd between the tip and a clean polycrystalline gold surface and the sample work function was calculated by the relation u03c6 tip u2212 u03c6 sample cpd samples were cleaned in situ by argon ion sputtering the observed contact potential for both metal electrodes is consistent with previous work function measurements 16 and the measured work function of the selenium semiconductor appears in the middle of its known band gap giving confidence that the surface is reasonably well prepared and representative of the device 39 s electronic structure image analysis was performed with spip and gwyddion scanning confocal raman spectroscopy was.