Size-Dependent Assembly and Electronic Transport in Epitaxially-Connected Superlattices of Lead Sulfide Quantum Dots

Semiconductor colloidal quantum dots (CQDs) have been widely used in many applications such as optoelectronic and energy-harvesting technology. The intense research on CQDs is triggered by the ease of access to modify both of electrical and optical properties with numerous approaches on pre- and post-synthesis processes. Efforts have been made to optimize the performance of CQDs-based devices, such as controlling the stoichiometry of the QDs as well as the ligand-related surface doping. Recently, it was shown that the performance of charge carrier transport on the CQDs solid assembly is strongly affected by the degrees of dot-to-dot connection and the corresponding structural factors. Nevertheless, obtaining disorder-free CQDs assembly still remains a challenge. Conventionally, ligand-exchange process (long- to short-surface-ligand) must be introduced in order to achieve electronically coupled assembly. Another possible approach is by pursuing the formation of the superstructure of CQDs assembly through the manipulation of the self-assembly process. This process shows an intriguing ability to organize CQD into quasi-2D superstructure with oriented attachments of its facets, leading to a promising way to obtain a structure that is able to support the demonstration of remarkable performance of CQDs-based technology.<br/><br/>Here we demonstrate facile control of lead sulfide (PbS) CQD assemblies to form large-scale epitaxially-connected superlattice with oriented attachment, in which we successfully elucidate their electronic transport. We varied the size of the QDs that become the building block of the assemblies. The superlattice assemblies were prepared by a modified liquid/air interfacial assembly. The attachment and orientational arrangement of QDs were controlled chemically by choosing the proper solvent to support the ligand removal on the [100] facets of the QDs and to further continue the ligand stripping on [111] gently using an amine-based stripping agent. Using combinations of transmission electron microscopy (TEM/HR-TEM), grazing-incidence small-angle x-ray scattering (GISAXS), and grazing-incidence wide-angle x-ray scattering (GIWAXS) measurements, we can observe the orderings in the length scale of the superlattice and determine the orderings in the atomic details to judge the specific orientation and the specific atomic connections on the QD facets. Having large-area strongly ordered 2D superlattices, the electronic transport properties of QDs assemblies were evaluated by using ionic-liquid-gating field-effect-transistors (FETs). Probing by electric-double-layer (EDL)-induced-gating, the size-dependent bandgap of the QDs assemblies can be observed electronically. This electronic transport of the epitaxially-connected superlattices demonstrates high conductivity while the quantum confinement nature of the QDs is still maintained. We will discuss the comparison of the QD size influence on charge carrier transport processes in the epitaxially-connected QD superlattices and in the conventional ligand-bridged QD assemblies. These two different systems showed distinct electronic transport behavior when the size of the QD building blocks are varied, suggesting different key factors in determining the charge carrier transport mechanism. The understanding of charge carrier transport in the epitaxially-connected superlattice of the quantum-confined QDs will reveal the key indicators to significantly enhance the performance of QDs-based optoelectronic and energy harvesting devices for practical applications that rely on efficient charge-carrier transport.<br/>References:<br/>[1] R. D. Septianto, S. Z. Bisri, et al. <i>NPG Asia Materials.</i> 12, 33 (2020)<br/>[2] L. Liu, S. Z. Bisri, et al. <i>Nanoscale</i> 11, 20467 (2019)<br/>[3] L. Liu, S. Z. Bisri, et al. <i>Nanoscale</i> 13, 14001 (2021)<br/>[4] R. D. Septianto, S. Z. Bisri, et al. <i>in preparation</i><br/>[5] N. Yazdani, V. Wood, et al. <i>Nature Comm.</i> 11, 2852 (2020)<br/>[6] T. Chen, B. I. Shklovskii, et al. <i>Nature Materials</i> 15, 299 (2016)