Enabling metallic behaviour in two-dimensional superlattice of semiconductor colloidal quantum dots

Abstract

Semiconducting colloidal quantum dots and their assemblies exhibit superior optical properties owing to the quantum confinement effect. Thus, they are attracting tremendous interest from fundamental research to commercial applications. However, the electrical conducting properties remain detrimental predominantly due to the orientational disorder of quantum dots in the assembly. Here we report high conductivity and the consequent metallic behaviour of semiconducting colloidal quantum dots of lead sulphide. Precise facet orientation control to forming highly-ordered quasi-2-dimensional epitaxially-connected quantum dot superlattices is vital for high conductivity. The intrinsically high mobility over 10 cm² V^-1 s^-1 and temperature-independent behaviour proved the high potential of semiconductor quantum dots for electrical conducting properties. Furthermore, the continuously tunable subband filling will enable quantum dot superlattices to be a future platform for emerging physical properties investigations, such as strongly correlated and topological states, as demonstrated in the moiré superlattices of twisted bilayer graphene.

Fig. 1. Epitaxially-connected PbS quantum dot superlattices (QD-SL).

a Transmission electron micrograph (TEM) of the QD-SL containing 8.1 nm diameter PbS QD. (inset) Fast-Fourier transform (FFT) of the micrograph. b The grazing incident small-angle X-ray scattering (GISAXS) pattern of the corresponding sample indicates the two-dimensional ordering of the QD monolayer. c The plot compares the extracted average diameter of the QDs versus the centre-to-centre (inter-QD) distance from the TEM images (black squares), and GISAXS measurement (red dots) of QD-SLs fabricated from various diameters of QD building blocks. Error bars represent standard deviation. d High-resolution TEM shows the epitaxial connection of the QDs and e selected area electron diffraction (SAED), demonstrating the PbS atomic orientation in the QD-SLs. It is a conclusion supported by the f grazing incident wide-angle scattering (GIWAXS) pattern taken from a much larger sample cross-section. g Azimuthal line cut-off (χ) on scattered X-ray wavevector (q) for {111}_AL and h {200}_AL of the GIWAXS patterns show the consistency of the ordering for all QD diameters. i An illustration reconstructing the PbS QD-SL alignment on the substrate showing the atomic lattice orientation relative to the normal surface.

Fig. 2. In-plane arrangement of the PbS QD-SLs.

a TEM image of oleic-acid (OA)-capped PbS QDs showing the 2-dimensional hexagonal structure, and b the TEM image of the epitaxially-connected PbS QD-SL. Upon assembly, selective ligand stripping and re-orientation, the QD-SLs transform from hexagonal lattices to quasi-square lattices or rhombic lattices with superlattice angle α. c Plot comparing the superlattice angle α as the function of QD diameters when prepared with different interparticle spacing: epitaxially connected by DMSO + EDA treatment (blue dots); EDT-bridged by ligand exchange (red triangles); or OA-capped (black square). Error bars represent the standard deviation.

Fig. 3. Size-independent mobility in electric-double-layer transistors (EDLT) of PbS epitaxially-connected QD-SLs.

a A schematic of the two-terminal (2T) EDLT device structure of the PbS QD-SL using EMIM-TFSI ionic liquid as electrolyte gate. b I_D–V_G transfer characteristics of the QD-SLs prepared from PbS QD with different diameters show ambipolar transport with a well-defined intrinsic region and a high on/off ratio for holes and electrons. c The gate-voltage-dependent capacitance value of the EDLT (red circle) and the corresponding accumulated sheet electron density (blue square) were extracted from potentiostatic electrochemical-impedance-spectroscopy (EIS) measurements. d Plots of the characteristic electron mobility values of the epitaxially-connected QD-SLs EDLT versus the diameter of the QDs (red diamond) showing weakly size-dependency, in contrast to the conventional (decreasing) size-dependent electron mobility in the ligand-capped QD assemblies (i.e. EDT-bridged PbS QD, grey circle). The four-terminal (4T)-EDLT mobility values of the epitaxially-connected PbS QD-SLs (blue triangle) show higher values than their two-terminal (2T)-EDLT mobility values, with a similar size-independent trend. Error bars represent the standard deviation.

Fig. 4. Electron delocalisation in the epitaxially-connected PbS QD-SLs.

a Temperature-dependent Arrhenius plot of the two-terminal (2T) electron sheet conductance (G_sheet) in EDT-bridged PbS QDs assembly under application of different charge carrier density values accumulated by field-induced doping of EDLT. b A similar plot of the electron transport in the epitaxially-connected PbS QD-SL shows higher electron conductivity and stronger deviation from Arrhenius-type hopping transport behaviour. c The corresponding four-terminal (4T) G_sheet of the EDLT demonstrates even higher intrinsic electron conductance, free from the influence of the significant contact resistance. Upon application of much higher carrier density (V_G > 1.4 V), the electron transport process transformed on the verge of delocalisation (dG_sheet/dT < 0). (inset) Illustration of the 4T-EDLT device.