Charge Percolation Pathways Guided by Defects in Quantum Dot Solids

Abstract

Charge hopping and percolation in quantum dot (QD) solids has been widely studied, but the microscopic nature of the percolation process is not understood or determined. Here we present the first imaging of the charge percolation pathways in two-dimensional PbS QD arrays using Kelvin probe force microscopy (KPFM). We show that under dark conditions electrons percolate via in-gap states (IGS) instead of the conduction band, while holes percolate via valence band states. This novel transport behavior is explained by the electronic structure and energy level alignment of the individual QDs, which was measured by scanning tunneling spectroscopy (STS). Chemical treatments with hydrazine can remove the IGS, resulting in an intrinsic defect-free semiconductor, as revealed by STS and surface potential spectroscopy. The control over IGS can guide the design of novel electronic devices with impurity conduction, and photodiodes with controlled doping.

Molecular Oxygen Induced in-Gap States in PbS Quantum Dots

“…13 A heterogeneous distribution of DOS was observed on measurements over tens of QDs, with about half of them showing IGS. These states are located 0.05À0.1 eV above the Fermi level and 0.15À0.3 eV above the 1S h states (valence band).…”

“…This points to the main difference between the SPS technique and the STS technique: the former is based on electrostatic interactions and only probes the states that can be statically occupied and emptied, while the later is based on the quantum tunneling of electrons and probes a large range of states that electrons can tunnel into/out of. 13,15,33,34 It has been reported that MPA passivates the QD surface more completely than EDT. 7,8,21 Our SPS results on PbS-MPA ( Figure 1B), however, reveal the presence of IGS at nearly the same level as those in the PbS-EDT, indicating that the IGS are not due to incomplete ligand passivation.…”

Revealing Trap States in Lead Sulphide Colloidal Quantum Dots by Photoinduced Absorption Spectroscopy

“…The observation of two distinct distributions may explain why some groups reported states close to the valence level and others associated them with the conduction level. [9][10][11][12][13][14]28] The surface chemistry of PbS CQDs is more complex than simple schemes such as the one in Figure 1a suggest. Even assynthesized CQDs do not only exhibit two differently polarized crystal facets ((001) and (111)), but are also covered with hydroxyl and oleate ions additional to oleic acid.…”

“…Such defect states were observed for PbS before by various techniques. Reported were especially a state 0.2 eV above the valence level of 1,2-ethanedithiol or 1,3-mercaptopropionic acid (EDT, MPA) capped CQDs via Kelvin probe force microscopy (KPFM) and scanning tunneling spectroscopy (STS), [9][10][11][12] Additionally, a quasimetallic midgap band ≈0.4 eV below the conduction …”

Dynamic Charge Carrier Trapping in Quantum Dot Field Effect Transistors

“…The QDs were treated with 1,2-ethanedithiol (EDT), which replaces the native oleate ligands on the surface, reduces the interparticle distance, and activates charge transport. 12,15,16 Previous work by Nagpal et al indicates that in-gap states (IGS) can assist carrier transport in the EDT treated PbS QD films. 7 Our previous results showed that under dark conditions hole transport occurs via valence band (1S h ) states, while electron transport takes place via IGS (Figure 1b), ∼0.2 eV above the 1S h states.…”