Fluorescent Carbon Nanotube Defects Manifest Substantial Vibrational Reorganization

Abstract

Fluorescent defects have opened up exciting new opportunities to chemically tailor semiconducting carbon nanotubes for imaging, sensing, and photonics needs such as lasing, single photon emission, and photon upconversion. However, experimental measurements on the trap depths of these defects show a puzzling energy mismatch between the optical gap (difference in emission energies between the native exciton and defect trap states) and the thermal detrapping energy determined by application of the van 't Hoff equation. To resolve this fundamentally important problem, here we synthetically incorporated a series of fluorescent aryl defects into semiconducting single-walled carbon nanotubes and experimentally determined their energy levels by temperature-dependent and chemically correlated evolution of exciton population and photoluminescence. We found that depending on the chemical nature and density of defects, the exciton detrapping energy is 14−77% smaller than the optical gap determined from photoluminescence. For the same type of defect, the detrapping energy increases with defect density from 76 to 131 meV for 4nitroaryl defects in (6,5) single-walled carbon nanotubes, whereas the optical gap remains nearly unchanged (<5 meV). These experimental findings are corroborated by quantum-chemical simulations of the chemically functionalized carbon nanotubes. Our results suggest that the energy mismatch arises from vibrational reorganization due to significant deformation of the nanotube geometry upon exciton trapping at the defect site. An unexpectedly large reorganization energy (on the order of 100 meV) is found between ground and excited states of the defect tailored nanostructures. This finding reveals a molecular picture for description of these synthetic defects and suggests significant potential for tailoring the electronic properties of carbon nanostructures through chemical engineering.

Optical Excitation of Carbon Nanotubes Drives Localized Diazonium Reactions

“…Reactions were carried out at bulk solution temperatures ranging from 23 to 50 °C and monitored spectroscopically ( Figure 4 b). It should be noted that although we recognize that PL measurements of SWCNT vary with temperature, 7 we only monitor the change in defect PL intensity for our evaluation here. The temporal evolutions of defect PL at various temperatures fit well per first order reaction kinetics.…”

Semiconducting Single‐Walled Carbon Nanotubes or Very Rigid Conjugated Polymers: A Comparison

“…During their effective lifetime they can move about 100 nm along the nanotube. [169][170][171] With this method a substantial increase in emission efficiency can be achieved. This means that short nanotubes will exhibit much lower photoluminescence yields than longer nanotubes with a saturation at lengths above 1 µm in dispersion.…”

“…This means that short nanotubes will exhibit much lower photoluminescence yields than longer nanotubes with a saturation at lengths above 1 µm in dispersion . This contribution to the overall low photoluminescence of SWCNTs can be circumvented by introducing luminescent sp 3 ‐defects that localize the exciton and enable radiative decay even for very short nanotubes although at lower energies . With this method a substantial increase in emission efficiency can be achieved.…”

A tool box to ascertain the nature of doping and photoresponse in single-walled carbon nanotubes

“…Some studies have reported that several photoluminescence features (such as emission energy and maximum brightness) can be chemically tuned using withdrawing/donating substituents on the aryl functional group. 16,17 It has even been shown that SWCNTs can be photoexcited to induce an acceleration in diazonium functionalization. 18 Even if the intended alteration of the SWCNTs' electronic structure is a fact, by means of diazonium chemistry or any other approach, on many occasions there is a need to unravel the nature of doping (negative, n-or positive, p-) and its extension.…”