Mid-infrared photocurrent nano-spectroscopy exploiting the thermoelectric effect in graphene

We develop a conductive-atomic force microscope coupled to a mid-infrared (IR) quantum cascade laser that allows to measure mid-IR photocurrent maps and spectra with nanometric spatial resolution. Here, we exploit the photo-induced thermoelectric effect of graphene to obtain the mid-IR local absorption spectra of nanoscale devices and materials. By scanning the probe in AFM-contact mode, the thermoelectric photocurrent can be mapped throughout the graphene layer, which acts as a semitransparent transducer. At zero voltage bias applied to the probe tip, we observe very low noise (about 25 pA/ Hz⁠), which makes our technique competitive with others in measuring the local IR absorption in nanomaterials with insufficient thermal expansion coefficient, or to characterize nanoscale devices that significantly absorb IR radiation, such as mid-infrared photodetectors.