Communication: Visualization and spectroscopy of defects induced by dehydrogenation in individual silicon nanocrystals

Abstract

We present results of a scanning tunneling spectroscopy (STS) study of the impact of dehydrogenation on the electronic structures of hydrogen-passivated silicon nanocrystals (SiNCs) supported on the Au(111) surface. Gradual dehydrogenation is achieved by injecting high-energy electrons into individual SiNCs, which results, initially, in reduction of the electronic bandgap, and eventually produces midgap electronic states. We use theoretical calculations to show that the STS spectra of midgap states are consistent with the presence of silicon dangling bonds, which are found in different charge states. Our calculations also suggest that the observed initial reduction of the electronic bandgap is attributable to the SiNC surface reconstruction induced by conversion of surface dihydrides to monohydrides due to hydrogen desorption. Our results thus provide the first visualization of the SiNC electronic structure evolution induced by dehydrogenation and provide direct evidence for the existence of diverse dangling bond states on the SiNC surfaces.

Quantum Dot Acceptors in Two-Dimensional Epitaxially Fused PbSe Quantum Dot Superlattices

“…This is in clear contrast with the localization expected for a deep level at the surface of semiconductor materials. 40,41 Our result suggests that the homogeneous contrast in the differential conductance map of Figure 5f reflects the presence of several defects on the unbound facets of a QD, each generating in-gap states. All together, they ensure an efficient transfer of the tunneling electron to the gold substrate via electron hopping at the surface of the defective QD, accounting for the absence of any charging effect.…”

Creation and Annihilation of Charge Traps in Silicon Nanocrystals: Experimental Visualization and Spectroscopy

“…At first glance, this charge-related bistability of DBs may appear to be consistent with the bistable behavior in point F (curves F in Figure d): occupied state H F f indeed disappears after a switching event at a negative voltage possibly leading to extraction of an electron. However, the backward spectrum does not contain any identifiable intragap states, which is inconsistent with LDOS expected for a DB in any charge state . Bistability of states E D f and E E f is also unlikely to be produced by transitions between the different charge states of a DB because one would expect the unoccupied states E D f and E E f to disappear at a positive voltage (upon trapping of an electron), rather than negative voltages, as observed in the forward curves D and E of Figure d.…”

“…In addition to the structure-based interpretation of LDOS bistability, Figure offers the opportunity to clarify the origin of the observed peaks: defect-related states localized on the top surface of the SiNC can be expected to produce LDOS peaks at both bias polarities, a consequence of the existence of two tunneling barriers (tip to SiNC gap, and SiNC itself) separating the state in question from the tunneling contacts . The asymmetric distribution of the applied bias voltage across the two barriers leads to asymmetric onsets of tunneling for the two bias polarities, which manifests itself in Figure c as a noticeable location-dependent onset voltage variation of peak E F f , in contrast to peak H F f which, despite being closely colocalized with peak E F f , does not show any distinguishable onset voltage variation.…”

“…While the impact of specific defects and impurities on the electronic structure and photophysical properties of SiNCs has been addressed in numerous theoretical studies, progress is hampered by the lack of experimental techniques capable of targeting and identifying individual defects, an essential requirement due to the highly varied local structures, and, consequently, properties of different possible defects. This capability was recently demonstrated with a high-stability variant of scanning tunneling spectroscopy (STS), which, in addition to providing the local electronic spectra of quantum-confined states, has produced complete surface maps of the electronic structures of individual SiNCs containing oxidative defects and silicon dangling bonds (DBs) . In addition to oxidative and DB defects, another surprisingly common type of structural irregularity found in SiNCs is a thin (angstrom-scale) amorphous silicon layer found at the SiNC surface .…”

“…Some of the most common defects capable of producing intragap states on the SiNC surface are oxidative defects. − However, elementary defects of this type do not produce deep intragap states such as the ones described above, even though oxidative defects may possibly be contributing to the location-dependent voltage onsets of the band edge states observed, for example, in Figure . A potential source of deep intragap states showing bistable behavior (e.g., “blinking”) are DBs, which could be created, for example, via dehydrogenation induced by the tunneling current . Previous STS work has shown that DBs can be found in different charge states leading to LDOS spectra showing deep intragap peaks .…”