Colloidal semiconductor nanocrystals have gained interest since their optical and electronic properties can be tuned by varying their shape, size and composition. Recently, 2D square and honeycomb superlattice of lead- and cadmium-chalcogenide quantum dots (QDs) have been prepared. These superstructures are formed by assembling PbSe nanocrystals in a monolayer at the toluene suspension air/interface after which the nanocrystals attach via their four vertical {100} facets [1],[2]. Afterward, cation exchange transforms PbSe into zinc blend CdSe. Theoretical studies show that these 2-D systems have distinct band structures compared to continuous nanosheets, with the appearance of Dirac cones in the case of the honeycomb [3]. Strong electronic coupling via the atomic connections of the QDs in the superstructure may result in a higher mobility compared to the self-assembled lead chalcogenide QDs that are less strongly coupled due to the (in) organic ligands [4].  

In our research, we use electrolyte-gated transistors to study the optoelectronic properties and transport characteristics of 2-D PbSe and CdSe superstructures [5]. The potential of the gate electrode determines the Fermi level with respect to the conduction band (CB) or valence band (VB) of the superstructure. First, to monitor the stability of the superlattice under electron injection we measure the differential capacitance as a function of gate voltage. Second, the conductivity of the network is measured as a function of the Fermi level position. To quantify band occupation into the superlattice, the optical absorption quenching employed. Finally, the mobility of the system is calculated from conductivity and charge density.

We reported the first study of electron transport in a 2-D PbSe system with a square geometry in which band occupation is assured by the electron density of 8 electrons per nanocrystal . The electron mobility between 5 and 18 cm²/Vs is observed for these supersructures [6].­­

In our recent work, we study the electron transport of CdSe superlattices with square and honeycomb geometry. The band occupation is assured by the number of 2 electrons per nanocrystal. The electron mobility of 1 and 10 cm²/Vs is achieved for square and honeycomb geometry respectively.   

1) W.H. Evers et al., Nano Lett., 13, (2013).

2) M.P. Boneschanscher et al., Science, (2014).

3) E. Kalesaki et al., Phys. Rev. B 88, (2013).

4) W.H. Evers et al., Nature Communications 6, (2015).

5) D. Vanmaekelbergh et al., Electrochemica Acta, 53, (2007).

6) M. Alimoradi Jazi et al., Nano Lett., 17, (2017)