Tip Functionalization of Finite Single-Walled Carbon Nanotubes and Its Impact on the Ground and Excited State Electronic Structure

We explore the effect of capping of finite (10,5) carbon nanotube (SWCNT) with various functional groups, including methylene, ether, ester, and carboxylic derivatives, having different electron withdrawing/donating properties. Using density functional theory (DFT) and time-dependent DFT (TDDFT), we found that the sp²-hybridization of the bonding atom of the capping groups and the number of such groups plays the predominant role in eliminating optically inactive edge-localized midgap states, while withdrawing/donating properties of the functional groups are less significant. Our calculations show that two sp²-groups, like methylene derivatives, combined with hydrogens are an optimal capping scheme for (10,5) SWCNT to provide the well-opened energy gap, since sp²-groups can be placed relatively far from each other to minimize the dipole moment at the edges, while preserving conjugation of the edge according to the tube chirality. Absorption spectra demonstrate negligible effects of electron donating/withdrawing groups on the lowest optical E₁₁ band. In contrast, the change in the bond order of the capping groups significantly changes optical spectra, resulting in red-shifted and less intensive E₁₁ band when sp³-capping substitutes two sp²-groups. Our findings can be helpful in choosing the functional groups for tuning the optoelectronic properties of SWCNTs. In addition, a complete understanding of the role of tube’s capping allows for using smaller computational models making computations of a wide range of phenomena in SWNTs practical

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

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

Low-Temperature Single Carbon Nanotube Spectroscopy of sp³ Quantum Defects

Aiming to unravel the relationship between chemical configuration and electronic structure of sp³ defects of aryl-functionalized (6,5) single-walled carbon nanotubes (SWCNTs), we perform low-temperature single nanotube photoluminescence (PL) spectroscopy studies and correlate our observations with quantum chemistry simulations. We observe sharp emission peaks from individual defect sites that are spread over an extremely broad, 1000–1350 nm, spectral range. Our simulations allow us to attribute this spectral diversity to the occurrence of six chemically and energetically distinct defect states resulting from topological variation in the chemical binding configuration of the monovalent aryl groups. Both PL emission efficiency and spectral line width of the defect states are strongly influenced by the local dielectric environment. Wrapping the SWCNT with a polyfluorene polymer provides the best isolation from the environment and yields the brightest emission with near-resolution limited spectral line width of 270 μeV, as well as spectrally resolved emission wings associated with localized acoustic phonons. Pump-dependent studies further revealed that the defect states are capable of emitting single, sharp, isolated PL peaks over 3 orders of magnitude increase in pump power, a key characteristic of two-level systems and an important prerequisite for single-photon emission with high purity. These findings point to the tremendous potential of sp³ defects in development of room temperature quantum light sources capable of operating at telecommunication wavelengths as the emission of the defect states can readily be extended to this range <i>via</i> use of larger diameter SWCNTs