Photovoltaic and photothermal effects induced by visible laser radiation in atomic force microscopy probes

Excitation of electrons into higher energy states in solid state materials can be induced by absorption of visible light, a physical process generally studied by optical absorption spectroscopy. A promising approach for improving the spatial resolution of optical absorption spectroscopy beyond the diffraction limit is the detection of photoinduced forces by an atomic force microscope operating under wavelength-dependent light irradiation. Here, we report on a combined photovoltaic/photothermal effect induced by the absorption of visible light by the microscope probes. By monitoring the photoinduced modifications of the oscillation of the probes, it is found that the oscillation phase-voltage parabolic signals display specific fingerprints which depend on light intensity and the nature of the materials composing the probes. In particular, a localized surface photovoltage (SPV) is evidenced at the tip apex of uncoated Si probes, while none is observed on Au-coated Si probes. The photothermal effects are distinguished from photovoltaic effects by specific shifts of the phase-voltage parabolas. The findings are relevant for the whole range of atomic force microscopy techniques making use of visible light as an additional means of local optical characterization.