Mapping of Defects in Individual Silicon Nanocrystals Using Real-Space Spectroscopy

Abstract

The photophysical properties of silicon semiconductor nanocrystals (SiNCs) are extremely sensitive to the presence of surface chemical defects, many of which are easily produced by oxidation under ambient conditions. The diversity of chemical structures of such defects and the lack of tools capable of probing individual defects continue to impede understanding of the roles of these defects in SiNC photophysics. We use scanning tunneling spectroscopy to study the impact of surface defects on the electronic structures of hydrogen-passivated SiNCs supported on the Au(111) surface. Spatial maps of the local electronic density of states (LDOS) produced by our measurements allowed us to identify locally enhanced defect-induced states as well as quantum-confined states delocalized throughout the SiNC volume. We use theoretical calculations to show that the LDOS spectra associated with the observed defects are attributable to Si-O-Si bridged oxygen or Si-OH surface defects.

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

“…For example, close inspection of the d I /d V maps in Figure shows that the intragap states appear to have filament-like structures, contrary to the expected quantum-confined states in crystalline SiNCs, − but in-line with expectations for amorphous silicon. , Another line of evidence for the presence of amorphous silicon is provided by LDOS mapping: the band edges for bistable SiNCs typically show onsets that fluctuate noticeably across the SiNC surface (Figure ), while crystalline SiNCs show band onsets that are independent of location . Consistent with these findings, STM images of bistable SiNCs did not show any obvious crystallographic features, in contrast to SiNCs without bistable states, where evidence of faceting was found in STM imaging . These observations suggest that different SiNCs may have amorphous surface layers of varied thicknesses resulting in varied electronic structures.…”

“…14 Consistent with these findings, STM images of bistable SiNCs did not show any obvious crystallographic features, in contrast to SiNCs without bistable states, where evidence of faceting was found in STM imaging. 14 These observations suggest that different SiNCs may have amorphous surface layers of varied thicknesses resulting in varied electronic structures. Nevertheless, the bistable SiNCs are likely not entirely amorphous because two-dimensional (2-D) LDOS maps (such as the ones shown in Figure 3) show features that appear to have specific orientations (as can be seen, for example, from the boundaries of the bistable areas), which could be explained only by the presence of a substantial amount of crystalline material.…”

“…The spectroscopic data of Figures – demonstrate that the described bistable electronic states are intragap in nature, which, together with their localized character, suggests that these states must be associated with structural defects on 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 .…”

“…11 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), 12 which, in addition to providing the local electronic spectra of quantum-confined states, 13 has produced complete surface maps of the electronic structures of individual SiNCs containing oxidative defects 14 and silicon dangling bonds (DBs). 15 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.…”

Current Rerouting Improves Heat Removal in Few-Layer WSe2 Devices

“…The on-going quest for faster switching, higher density, and better performance, predicted by Moore’s law, has led to persistent downscaling of nanoelectronic devices. Shrinking the device dimensions increases their surface-to-volume ratio and introduces atomic-scale disorder at boundaries and interfaces, , reducing performance and threatening to limit scaling. To avoid these issues, the nanoelectronics community has turned to intrinsically two-dimensional (2D) material platforms.…”