It is known that quantum dots (QDs) can be photoionized . However it was impossible to photoionize selectively individual QDs. Here we study  a method of addressable photo- ionization of individual coated CdSe QDs. The charging is a result of synergetic action of laser light and an atomic force microscope (AFM) probe. The AFM probe squeezes the coating layer of QDs helping the photoelectron to tunnel to either the conductive AFM probe or substrate. The charge can be induced on individual QDs by addressing/locating the QDs with the AFM probe. The charges are stable in ambient conditions, and even recover within a minute after their forced neutralization by airflow of negative ions. These results may be of interest for the development of QDs based sensor, memory, quantum computing, and solar cell applications. For instance, this method could provide recording information at a density of 1Tb/cm2.

An example of the described photoionization is shown below:


Figure. AFM probe-assisted photo-charging of QDs. (a) Experimental setup. QDs dispersed onto graphene film. Electrons in QDs are photo-excited by a green laser. (b) A schematic of the AFM probe -QD interaction. The probe is modeled as a sphere with radius a, and the CdSe nanocrystal as a point charge q. (c) Top: TEM images of dispersed TOPO capped CdSe nanocrystal QDs; middle: a topographic AFM image of the dispersed QDs; bottom: the change of resonance frequency over the same area as shown on the top image. The region inside a dashed rectangle is the area in which the AFM probe scanned the surface during illumination with the laser. Schematics of QDs illuminated by laser without and with touching with the AFM probe are shown on the left.

This method allows measuring accumulations of the charge carrier in various materials, in particular, related to solar cell applications. The error of this method is 0.1e, where e is the charge of electron.

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