Brightening of the Lowest Exciton in Carbon Nanotubes via Chemical Functionalization Svetlana Kilina, ‡ Jessica Ramirez, † and Sergei Tretiak* ,§ † Quantum Theory Project, University of Florida, Gainesville, Florida 32611, United States ‡ Department of Chemistry and Biochemistry, North Dakota State University, Fargo, North Dakota 58108, United States § Theoretical Division, Center for Nonlinear Studies (CNLS) and Center for Integrated Nanotechnologies (CINT), Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States * S Supporting Information ABSTRACT: Using time-dependent density functional theory, we found that chemical functionalization at low concentrations of single-walled carbon nanotubes (SWNTs) locally alters the π-conjugated network of the nanotube surface and leads to a spatial confinement of the electronically excited wave functions. Depending on the adsorbant positions, the chemisorption significantly modifies the optical selection rules. Our modeling suggests that photoluminescent efficiency of semiconducting SWNT materials can be controlled by selective chemical functionalization. KEYWORDS: Single-walled carbon nanotubes (SWNTs), time-dependent density functional theory (TDDFT), photoluminescent (PL), exciton, chemisorption A variety of low-dimensional materials are highly photo- luminescent (PL). For example, conjugated polymers, 1 semiconductor nanowires, and quantum dots 2 may exhibit near-unity (∼100%) quantum yield (QY). In this respect, semiconducting single-wall carbon nanotubes (SWNTs) are drastically different initially showing only 10 -3 -10 -4 PL efficiency. 3 Such low luminescence of macroscopic SWNT samples is attributed to the presence of tube bundles, metallic tubes, and tubes with multiple defects, which quench emission. 4,5 However, even the high quality SWNT ensem- bles 6,7 and individual SWNTs still have overall low PL efficiency: a few percent or less in water 5,8,9 and up to 20% in organic solvents. 10 Low PL efficiency of semiconducting SWNTs is attributed to the intrinsic low-lying optically forbidden (“dark”) states 11-15 and fast exciton mobility to the quenching sites. 16,17 The lower-energy optical excitations in SWNTs have been ascertained as originating from four exciton subbands 18 with three dark excitons residing below the first optically allowed bright E 11 excitons. 11,12,19,20 The existence of the lowest-energy dark excitons has been explicitly verified via time-resolved spectroscopy, 21 two photon spectroscopy, 22 and PL spectroscopy in the presence of high magnetic fields altering the optical selection rules. 23-25 The excitonic structure and dynamics in SWNTs, however, is even more complicated by the presence of triplet excitons 14,26 and dark or semibright (weakly optically allowed) cross-polarized E 12 excitonic bands in between the bright E 11 and E 22 transitions. 27-29 Despite excitonic state complexity and low PL efficiency, optical properties of semiconducting SWNTs have multiple advantages over other nanomaterials. For example, SWNTs exhibit size-tunable, stable, and nonblinking (at room temper- ature) fluorescence at near-infrared (NIR) wavelength 30 and narrow homogeneous line width. 31 Such robust photophysical features are important for a number of fluorescent-based applications, including bioimaging 32,33 and biosensing. 34 The unique ability of a SWNT to emit a single photon at low temperatures with NIR wavelength is promising for applica- tions in quantum cryptography 35 and optoelectronics. 36 Hence overcoming intrinsic limitations and making highly luminescent SWNTs materials will immediately allow for many new applications and emerging technologies. Chemical functionalization of the nanotube surface offers an attractive route to change the selection rules governing the optical activity in SWNTs. For example, recent experiments on individual SWNTs in water have revealed significant increase in PL efficiency upon addition of reducing agents, such as dithiothreitol and others. 37 Low-energy satellite PL peaks with energies and PL efficiency dependent on the nanotube surroundings have been found in SWNTs ensembles and individual samples. 21,38,39 Other experimental studies have Received: January 13, 2012 Revised: March 9, 2012 Published: April 11, 2012 Letter pubs.acs.org/NanoLett © 2012 American Chemical Society 2306 dx.doi.org/10.1021/nl300165w | Nano Lett. 2012, 12, 2306-2312