E. Christensen, T. Tumiel, M. Amin, T. Krauss
University of Rochester,
Keywords: atomic force microscopy, electrostatic force microscopy, optical spectroscopy, charges
Summary:Electrostatic force microscopy (EFM) is a modification of atomic force microscopy (AFM) which allows for a quantitative determination of localized charges and dielectric constants at the nanometer scale. In our implementation of EFM, the tip is lifted off the surface and scanned at a constant height under an applied AC and DC voltage, thereby providing sensitivity to extremely small capacitive and Coulombic forces. By modeling the surface charge-cantilever force through a Coulombic interaction, quantitative determinations of surface charge magnitude and location are possible down to less than 0.1 e at room temperature. In this presentation, we will discuss how electrostatic force microscopy EFM was used to characterize the static-charge interactions between an individual single-walled carbon nanotube (SWCNT) and its local environment. As-synthesized, semiconducting SWCNTs are nominally charge neutral. However, ionic surfactants that are commonly used to disperse SWCNTs in solution lead to heterogeneous surface charge buildup along the nanotube. We measured nonuniform spatial charge distributions along the nanotube length with varying magnitudes ranging between ±15 e for dozens of long SWCNTs (> 1.5 microns) solubilized in aqueous suspension using standard ionic surfactants. EFM images acquired after resonant photoexcitation demonstrate charge carrier localization due to electrostatic interactions with charged surfactant aggregates. Charge densities as measured by EFM are used to estimate the depth of this electrostatically induced potential well, calculated to be on the order of hundreds of millielectronvolts, suggesting that surfactant charges heterogeneously covering SWCNTs provide traps for excitons potentially leading to their localization, a result that is most apparent in our correlative AFM and single molecule optical measurements. We have been able to correlate single molecule photoluminescence (PL) spectroscopy and AFM images of the same individual SWCNTs and show that charged surfactant covered areas on the nanotube exhibit brighter, redshifted PL spectra relative to neutral charge regions. Altogether, our experimental results and theoretical work presents a possible explanation for the source of the potential energy minima along the nanotube that localize excitons.