Impact of External Electronic Perturbations on Single-Walled Carbon Nanotube Electronic Structure: Scanning Tunneling Spectroscopy and Density Functional Theory

Abstract

Understanding the local impact of environmental electronic perturbations on the local density of states (LDOS) of single-walled carbon nanotubes (CNTs) is critical for developing CNT-based devices. We present scanning tunneling microscopy and spectroscopy (STM/STS) investigations of CNTs adsorbed on both a metal, Au(111), and a dielectric, monolayer RbI on Au(111), serving as models for stronger and weaker electrostatic interactions, respectively. In both cases, STS revealed modulations in the CNT LDOS corresponding to features in the underlying material. We corroborate our STM/STS results with density functional theory calculations of the electronic structure of semiconducting CNTs in the presence and absence of an external dipole. DFT-calculated CNT LDOS qualitatively matched STM/STS results, providing key insight into the local impact external charges have on CNT electronic structure.

Modulation of Carbon Nanotube Electronic Structure by Grain Boundary Defects in RbI on Au(111)

“…Indeed, the downshift observed in the SWCNT LDOS near GBDs is similar in magnitude to the shifts calculated for image potential states in Figure . The localization we observe (approximately 10 nm for unoccupied states in Figure c) is larger than the GBD width (1 nm for narrow and 2 nm for wide structures in Figures and S5), in agreement with previous studies of SWCNT LDOS confinement by external electrostatic interactions, where localization on the order of 4–8 nm was observed for a dipole-like external potential . The exact SWCNT LDOS pattern at each GBD is slightly different, which agrees with the expectation that distinct structures of different GBDs may lead to somewhat different electrostatic potentials.…”

“…The localization we observe (approximately 10 nm for unoccupied states in Figure 1c) is larger than the GBD width (1 nm for narrow and 2 nm for wide structures in Figures 4 and S5), in agreement with previous studies of SWCNT LDOS confinement by external electrostatic interactions, where localization on the order of 4− 8 nm was observed for a dipole-like external potential. 20 The exact SWCNT LDOS pattern at each GBD is slightly different, which agrees with the expectation that distinct structures of different GBDs may lead to somewhat different electrostatic potentials. While there are other various LDOS effects present in the SWCNT studied here, which could be attributed to interactions with point defects or domain edges in RbI, we find that interaction with GBDs leads to significant perturbations of the SWCNT LDOS.…”

“…While a number of thin film dielectrics grown on metal surfaces could serve this purpose, alkali halide films , are particularly appealing primarily due to the relatively well-defined nature − and wide band gaps , of such films. Among alkali halides, NaCl has been the most extensively studied and has been used as a dielectric to investigate adsorbates such as isolated organic molecules, − molecular assemblies, − carbon nanotubes, and individual atoms .…”

“…14 However, previous STM/STS work has primarily focused on SWCNTs on metal substrates, 15−18 while STM/STS studies of interactions between SWCNTs and dielectrics have been less common and focused mainly on SWCNT band bending at the metal/dielectric interface. 19,20 This, in part, is a consequence of the difficulty in preparing SWCNT/dielectric structures with dielectric defects whose locations and properties could be independently ascertained simultaneously with STM/STS measurements of the SWCNT properties. One possible approach that addresses this challenge is based on employing dielectric thin films (grown on metal substrates for compatibility with STM) with spatially extended linear defects with well-defined structures that could be investigated with STM/STS.…”

“…This underperformance can be attributed to the presence of charge trapping defects in the gate dielectric in the vicinity of SWCNTs, , resulting in deteriorated charge transport properties due to scattering and charge trapping, as investigated previously by scanning gate spectroscopy. − Advancing our understanding of these effects is challenging without direct knowledge of the changes induced in the SWCNT local electronic structure by the surrounding environment, which makes scanning tunneling microscopy/spectroscopy (STM/STS) a technique of choice in this endeavor due to its ability to probe electronic structure at the nanoscale . However, previous STM/STS work has primarily focused on SWCNTs on metal substrates, − while STM/STS studies of interactions between SWCNTs and dielectrics have been less common and focused mainly on SWCNT band bending at the metal/dielectric interface. , This, in part, is a consequence of the difficulty in preparing SWCNT/dielectric structures with dielectric defects whose locations and properties could be independently ascertained simultaneously with STM/STS measurements of the SWCNT properties. One possible approach that addresses this challenge is based on employing dielectric thin films (grown on metal substrates for compatibility with STM) with spatially extended linear defects with well-defined structures that could be investigated with STM/STS.…”