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STM study of electronic properties of single CdTe and CdTe/CdSe nanocrystals


A.S. Trifonov, R.B. Vasiliev, I.S. Ezubchenko, M.S. Sokolikova, D.R. Britov, D.E. Presnov, O.V. Snigirev

Size-dependent electronic properties of semiconductor nanocrystals have been intensively studied over the last decades. The methods of colloidal chemistry for high-quality nanocrystals production provide an opportunity to design nanoelectronic devices such as light-emitting diodes and solar cells. Branched nanocrystals, for example tetrapods, represent a new class of nanostructures. A tetrapod-shaped nanocrystal is formed with four arms that branch out at tetrahedral angles from a central region. Due to their shape tetrapods are attractive for electronic and opto-electronic devices, for example solar cell and single-electron transistor. A charge transport phenomenon through a single nanocrystal is of both significant fundamental and practical interest. In present work we studied a monolayer of tetrapod-shaped nanocrystals immobilized on gold surface by scanning tunneling microscopy. We use a CdTe and CdTe/CdSe heterostructured tetrapods with heterojunction formed within single nanocrystal. CdTe/CdSe tetrapods were synthesized by a two-step seed-mediated approach. Monolayers of CdTe and CdTe/CdSe tetrapods were immobilized on gold substrate. According to TEM measurements, the average length of CdTe tetrapod arms was found to be 12 nm and arm width was about 3 nm with size precision of about 10–20%. We investigated electronic properties of single CdTe and CdTe/CdSe heterostructured tetrapods using scanning tunneling microscope (STM). In particular we measured and analyzed a large number of current-voltage characteristics (CVC) recorded under the same conditions in a random place of the CdTe, CdTe/CdSe and pure gold samples. 3 different types of CVC shapes were detected. The first type curve is one typical for gold films and is well fitted by Simmons formula for metal-dielectric-metal (MDM) systems. The second type is a symmetrical curve, which could not be fitted by Simmons formula. Apparently this kind of curve indicates the presence of objects in the tunneling gap, which affects the height of the potential barrier. This shape of CVC is typical for a gold film with CdTe or CdTe/CdSe tetrapods, immobilized on it. The third type is an asymmetrical curve with sharp increase in current at negative voltage. It can be observed in CVCs of a gold film with monolayer of CdTe/CdSe tetrapods. CdTe/CdSe nanocrystals produce multiple channels of current flow, one of which has clear diode dependence with the sharp decline of current on voltage lower than U ~ –1700 mV. The mechanism of current flow through various parts of tetrapod which takes into consideration the deformation of the tetrapod by electrostatic field of a scanning tunneling microscope is proposed. It explains the average shape of the experimental current-voltage characteristics.

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