Solvent- and Wavelength-Dependent Photoluminescence Relaxation Dynamics of Carbon Nanotube sp3 Defect States

Xiaowei He; Kirill A Velizhanin; George Bullard; Yusong Bai; Jean-Hubert Olivier; Nicolai F Hartmann; Brendan J Gifford; Svetlana Kilina; Sergei Tretiak; Han Htoon; Michael J Therien; Stephen K Doorn

doi:10.1021/acsnano.8b02909

Solvent- and Wavelength-Dependent Photoluminescence Relaxation Dynamics of Carbon Nanotube sp³ Defect States

ACS Nano. 2018 Aug 28;12(8):8060-8070. doi: 10.1021/acsnano.8b02909. Epub 2018 Jul 17.

Authors

Affiliations

¹ Center for Integrated Nanotechnologies, Materials Physics and Applications Division , Los Alamos National Laboratory , Los Alamos , New Mexico 87545 , United States.
² Theoretical Division , Los Alamos National Laboratory , Los Alamos , New Mexico 87545 , United States.
³ Department of Chemistry, French Family Science Center , Duke University , Durham , North Carolina 27708 , United States.
⁴ Center for Nonlinear Sciences , Los Alamos National Laboratory , Los Alamos , New Mexico 87545 , United States.
⁵ Department of Chemistry and Biochemistry , North Dakota State University , Fargo , North Dakota 58108 , United States.

PMID: 29995379
DOI: 10.1021/acsnano.8b02909

Abstract

Photoluminescent sp³ defect states introduced to single wall carbon nanotubes (SWCNTs) through low-level covalent functionalization create new photophysical behaviors and functionality as a result of defect sites acting as exciton traps. Evaluation of relaxation dynamics in varying dielectric environments can aid in advancing a more complete description of defect-state relaxation pathways and electronic structure. Here, we exploit helical wrapping polymers as a route to suspending (6,5) SWCNTs covalently functionalized with 4-methoxybenzene in solvent systems including H₂O, D₂O, methanol, dimethylformamide, tetrahydrofuran, and toluene, spanning a range of dielectric constants from 80 to 3. Defect-state photoluminescence decays were measured as a function of emission wavelength and solvent environment. Emission decays are biexponential, with short lifetime components on the order of 65 ps and long components ranging from around 100 to 350 ps. Both short and long decay components increase as emission wavelength increases, while only the long lifetime component shows a solvent dependence. We demonstrate that the wavelength dependence is a consequence of thermal detrapping of defect-state excitons to produce mobile E₁₁ excitons, providing an important mechanism for loss of defect-state population. Deeper trap states (i.e., those emitting at longer wavelengths) result in a decreased rate for thermal loss. The solvent-independent behavior of the short lifetime component is consistent with its assignment as the characteristic time for redistribution of exciton population between bright and dark defect states. The solvent dependence of the long lifetime component is shown to be consistent with relaxation via an electronic to vibrational energy transfer mechanism, in which energy is resonantly lost to solvent vibrations in a complementary mechanism to multiphonon decay processes.

Keywords: electronic-to-vibrational energy transfer; exciton; photoluminescence decay; relaxation dynamics; single wall carbon nanotubes; sp3 defects; thermal detrapping.