Measurement of Electronic States of PbS Nanocrystal Quantum Dots Using Scanning Tunneling Spectroscopy: The Role of Parity Selection Rules in Optical Absorption

Abstract

We study the structure of electronic states in individual PbS nanocrystal quantum dots by scanning tunneling spectroscopy (STS) using one-to-two monolayer nanocrystal films treated with 1, 2-ethanedithiols (EDT). Up to six individual valence and conduction band states are resolved for a range of quantum dot sizes. The measured states' energies are in good agreement with calculations using the k · p four-band envelope function formalism. A comparison of STS and optical absorption spectra indicates that some of the absorption features can only be explained by asymmetric transitions involving the states of different symmetries (e.g., S and P or P and D), which points towards the relaxation of the parity selection rules in these nanostructures. STS measurements also reveal a midgap feature, which is likely similar to one observed in previous charge transport studies of EDT-treated quantum dot films.

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 …”

Molecular Oxygen Induced in-Gap States in PbS Quantum Dots

“…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.…”

Scanning Probe Characterization of Heterostructured Colloidal Nanomaterials

“…The PbS NCs were spin-cast on a glass substrate coated with indium tin oxide (ITO, 0.5−1 kΩ) and treated with 1,2-ethanedithiol (EDT), which leads to a coupled thin film. 73 Such films of coupled NCs behave as macroscopic semiconductor films in solar cells, field effect transistors, and lightemitting devices, yet they exhibit properties (such as the band gap) resulting from quantum confinement of each NC making up the film. Differential conductance spectra of individual NCs of various sizes within the thin film are shown in Figure 6.…”

“…Therefore, STM/STS can be utilized to gain insight into optical transitions. 73,75 Sample preparation is the crucial first step in being able to make these measurements. Figure 4 summarizes examples of methods that have been commonly used for scanning probe measurements.…”