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Characterization of the Photocurrents Generated by the Laser of Atomic Force Microscopes

Yanfeng Ji, Fei Hui, Yuanyuan Shi, Vanessa Iglesias, David Lewis, Jiebin Niu, Shibing Long, Ming Liu, Alexander Hofer, Werner Frammelsberger, Guenther Benstetter, Andrew Scheuermann, Paul C. McIntyre and Mario Lanza

Photoactive materials play a crucial role in the development of energy storage devices, such as solar, electrochemical cells, and others. Conductive atomic force microscopy (CAFM) is the powerful tool for nanoscale electronic characterization of photoactive materials. It is known that environmental light can alter the measurements when scanning photoactive samples. For this reason, measuring in a dark environment has been recognized as the standard CAFM process. However as an optical feedback laser is necessary to acquire topography, the laser used in CAFM can generate a high photocurrent, even without any bias between the conductive tip and the sample. While the laser-induced current signal perturbation is well known within the CAFM community, the observation of abundant currents generated by the optical feedback laser in absence of bias is still not understood and has never been studies in depth.

For the first time, this paper studies and analyses the photocurrent induced in the photoactive materials by the feedback laser. CAFM measurements were carried out on photoactive samples using six standard optical feedback AFMs of different manufacturers and Nanonics tuning-fork based feedback AFM (without using a laser).

The results obtained show that the laser induces abundant parasitic photocurrent even without any bias in the other tested optical feedback AFMs. Only the Nanonics MV4000 system based on TF feedback does not induce parasitic photocurrent and provides a true current map in complete darkness.

a. 3D Collage of topographic and current maps collected on Ni electrode using the Nanonics MV4000 AFM without application of bias. b. 2D Current map. Light Off means the absolute dark, as no laser is used to detect the deflection of the AFM tip and no current is observed. Light On means the light bank is switched on and high current is observed.

An additional solution of the laser induced photocurrent can be the special two pass method, when the feedback laser is switched off in lift mode.