Diversity of sub-bandgap states in lead-sulfide nanocrystals: real-space spectroscopy and mapping at the atomic-scale

Abstract

Colloidal semiconductor nanocrystals have emerged as a promising class of technological materials with optoelectronic properties controllable through quantum-confinement effects. Despite recent successes in this field, an important factor that remains difficult to control is the impact of the nanocrystal surface structure on the photophysics and electron transport in nanocrystal-based materials. In particular, the presence of surface defects and irregularities can result in the formation of localized sub-bandgap states that can dramatically affect the dynamics of charge carriers and electronic excitations. Here we use Scanning Tunneling Spectroscopy (STS) to investigate, in real space, sub-bandgap states in individual ligand-free PbS nanocrystals. In the majority of studied PbS nanocrystals, spatial mapping of electronic density of states with STS shows atomic-scale variations attributable to the presence of surface reconstructions. STS spectra show that the presence of surface reconstructions results in formation of surface-bound sub-bandgap electronic states. The nature of the surface reconstruction varies depending on the surface stoichiometry, with lead-rich surfaces producing unoccupied sub-bandgap states, and sulfur-rich areas producing occupied sub-bandgap states. Highly off-stoichiometric areas produce both occupied and unoccupied states showing dramatically reduced bandgaps. Different reconstruction patterns associated with specific crystallographic directions are also found for different nanocrystals. This study provides insight into the mechanisms of sub-bandgap state formation that, in a modified form, are likely to be applicable to ligand-passivated nanocrystal surfaces, where steric hindrance between ligands can result in under-coordination of surface atoms.

Energetics at the Surface: Direct Optical Mapping of Core and Surface Electronic Structure in CdSe Quantum Dots Using Broadband Electronic Sum Frequency Generation Microspectroscopy

“…Understanding and controlling nanomaterial surfaces is a well-known barrier to developing the next generation of these materials for practical applications. − Such applications are wide ranging and include catalytic transformations, − chemical sensors, − and biological imaging. , Of relevance to the work presented here are semiconducting quantum dots (QDs) that through a size-tunable electronic structure have proven to be advantageous in both light absorbing and light emitting applications. ,,− The tunable optical properties are generally considered to be a bulk characteristic arising from confinement in the core of the nanoparticle; however, uncoordinated surface atoms and related point defects can result in states that are optically dark and serve as charge trapping sites. ,− The presence and characteristics of these defects are typically inferred by measuring the photoluminescence (PL) spectrum/quantum yield or excited state dynamics, which are influenced by surface functionalization and termination due to the aforementioned surface trapping. ,,− In fact, the presence of only a handful of defects at the surfaces of QDs can strongly suppress quantum yields by opening up new nonradiative relaxation pathways. ,− An atomistic understanding of both the characteristics and origins of these surface trap states is an active field of research, where a large diversity of types and sources have been reported that strongly depend on the nanoparticle’s composition, crystal structure, surface passivation, and growth conditions. ,,,− …”

Quantum Dots Microstructural Metrology: From Time‐Resolved Spectroscopy to Spatially Resolved Electron Microscopy

“…Surface-bound sub-bandgap electronic states with spectral and spatial properties sensitive to the local stoichiometry of nanocrystal surfaces are observed in the reconstructed surface images. [127]…”

“…Variation of chemical composition in the sample can be obtained from EDX point analysis, and Reproduced with permission. [127] Copyright 2015, The Royal Society of Chemistry.…”

Hydrogen Treatment as a Detergent of Electronic Trap States in Lead Chalcogenide Nanoparticles

“…Despite several reports confirming the presence of trap states in films of lead chalcogenide nanoparticles, − , their atomistic origin has remained elusive but for a few exceptions. − Ab initio calculations have suggested the presence of defect states in off-stoichiometric NPs and pointed out the possible use of ligands to achieve charge-orbital balance to heal trap states. ,, Charge-orbital balance is defined by assuming that the formal charge state of all NP constituents, including the ligands, add up to zero, thus leading in principle to a gap without any defect states (or clean gap). , However, recent calculations pointed out that even perfectly charge-balanced NPs may have states in their gap . For example, in the presence of charge balanced Se vacancies, deep unoccupied states may be present in the electronic gap of PbS NPs.…”

“…35 Although strategies to turn the presence of deep gap states into an advantage have been suggested, 36−38 it is believed that deep states are overall harmful; for example, they are the cause for the observed low open circuit voltages of NP solar cells. 39 Despite several reports confirming the presence of trap states in films of lead chalcogenide nanoparticles, [23][24][25][26]40 their atomistic origin has remained elusive but for a few exceptions. 26−30 Ab initio calculations have suggested the presence of defect states in off-stoichiometric NPs and pointed out the possible use of ligands to achieve charge-orbital balance to heal trap states.…”