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Letter
Creation and Annihilation of Charge Traps in Silicon Nanocrystals: Experimental Visualization and Spectroscopy Dmitry A. Kislitsyn, Jon M. Mills, Sheng-Kuei Chiu, Benjamen N. Taber, James D. Barnes, Christian F. Gervasi, Andrea Mitchell Goforth, and George V. Nazin J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b03299 • Publication Date (Web): 24 Jan 2018 Downloaded from http://pubs.acs.org on January 25, 2018
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The Journal of Physical Chemistry Letters
Title: Creation and Annihilation of Charge Traps in Silicon Nanocrystals: Experimental Visualization and Spectroscopy Dmitry A. Kislitsyn†,∥, Jon M. Mills†, Sheng-Kuei Chiu§, Benjamen N. Taber†, James D. Barnes§, Christian F. Gervasi†, Andrea M. Goforth§, and George V. Nazin†,* †
Department of Chemistry and Biochemistry, Materials Science Institute, Oregon Center for
Optical, Molecular and Quantum Science, University of Oregon, 1253 University of Oregon, Eugene, Oregon 97403, United States §
Department of Chemistry, Portland State University, Portland, Oregon 97201, United States
*To whom correspondence should be addressed:
[email protected] 1 ACS Paragon Plus Environment
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ABSTRACT: Recent studies have shown the presence of an amorphous surface layer in nominally crystalline silicon nanocrystals (SiNCs) produced by some of the most common synthetic techniques. The amorphous surface layer can serve as a source of deep charge traps, which can dramatically affect the electronic and photophysical properties of SiNCs. We present results of a scanning tunneling microscopy/scanning tunneling spectroscopy (STM/STS) study of individual intra-gap states observed on the surfaces of hydrogen-passivated SiNCs deposited on the Au(111) surface. STS measurements show that intra-gap states can be formed reversibly when appropriate voltage/current pulses are applied to individual SiNCs. Analysis of STS spectra suggests that the observed intra-gap states are formed via self-trapping of charge carriers injected into SiNCs from the STM tip. Our results provide a direct visualization of the charge trap formation in individual SiNCs, the level of detail which until now had only been achieved in theoretical studies. TOC GRAPHIC
KEYWORDS: sub-bandgap states, trap states, amorphous silicon, quantum dots, scanning tunneling spectroscopy, Staebler-Wronski effect, scanning tunneling microscopy
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The Journal of Physical Chemistry Letters
Silicon nanocrystals (SiNCs) have recently emerged as a promising electronic material with potential applications in photovoltaics,1-4 light-emitting diodes,5-6 sensing7 and thermoelectric devices.8 Important advantages of SiNCs are their efficient light emission and low toxicity, which make SiNCs particularly suitable for biological tagging.9-10 One of the fundamental challenges in realizing these applications lies in achieving precise control of the SiNC properties, in particular control of the SiNC surface chemistry, a difficult task due to the inherent variation in SiNC surface structures and their susceptibility to interaction with environment.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 defects14 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.16 Such amorphous layers have been found even in SiNCs synthesized at high (>1100 °C) temperatures,16 and have been argued to cause trapping of photoexcited holes and nonradiative relaxation of photoexcitation.17 Despite their likely ubiquity, the nature of such charge traps has not been investigated experimentally, and only a limited body of relevant theoretical work is available.18-20
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In this letter, we report STS-based real-space observations of the reversible creation of intragap states in individual hydrogenated SiNCs. Existence of these states is attributable to structural changes expected in amorphous silicon systems.21 Hydrogen-passivated SiNCs were spraydeposited onto a Au(111) substrate in high-vacuum conditions, and investigated using an ultrahigh vacuum cryogenic STM (see Methods for further experimental details). The deposited SiNCs formed a near-monolayer, with individual SiNCs appearing as protrusions with heights in the 2-4 nm range. Characterization of the electronic structures of individual SiNCs was performed by recording the derivative of the tunneling current dI/dV (differential conductance) as a function of the applied bias voltage (V) and the tip location. Obtained dI/dV spectra represent the local density of electronic states (LDOS), with the bias voltage giving the energy scale. Studied SiNCs (~30 in total) typically displayed reproducible current-voltage characteristics and dI/dV spectra when sufficiently low bias voltages and tunneling currents were used in the measurements. Irreversible changes attributable to dehydrogenation of the SiNC surfaces were observed at higher voltages,15 while higher currents often led to motion and/or repositioning of SiNCs on the Au surface. Intriguingly, in five of the studied SiNCs, we found that, in contrast to such irreversible changes, the current-voltage characteristics could be altered reversibly by applying relatively low (
