Recent progress in controlling the photoluminescence properties of single-walled carbon nanotubes by oxidation and alkylation

The functionalization of single-walled carbon nanotubes (SWCNTs) has received considerable attention in the last decade since highly efficient near-infrared photoluminescence (PL) has been observed to be red-shifted compared with the intrinsic PL peak of pristine SWCNTs. The PL wavelength has been manipulated using arylation reactions with aryldiazonium salts and aryl halides. Additionally, simple oxidation and alkylation reactions have proven effective in extensively adjusting the PL wavelength, with the resulting PL efficiency varying based on the chosen reaction techniques and molecular structures. This review discusses the latest developments in tailoring the PL attributes of SWCNTs by oxidation and alkylation processes. (6,5) SWCNTs exhibit intrinsic emission at 980 nm, and the PL wavelength can be controlled in the range of 1100-1320 nm by chemical modification. In addition, recent developments in chiral separation techniques have increased our understanding of the control of the PL wavelength, extending to the selection of excitation and emission wavelengths, by chemical modification of SWCNTs with different chiral indices.

Functionalization of Carbon Nanotubes Surface by Aryl Groups: A Review

The review is devoted to the methods of introducing aryl functional groups to the CNT surface. Arylated nanotubes are characterized by extended solubility, and are widely used in photoelectronics, semiconductor technology, and bioelectrocatalysis. The main emphasis is on arylation methods according to the radical mechanism, such as the Gomberg-Bachmann and Billups reactions, and the decomposition of peroxides. At the same time, less common approaches are also considered. For each of the described reactions, a mechanism is presented in the context of the effect on the properties of functionalized nanotubes and their application. As a result, this will allow us to choose the optimal modification method for specific practical tasks.

ortho-Substituted Aryldiazonium Design for the Defect Configuration-Controlled Photoluminescent Functionalization of Chiral Single-Walled Carbon Nanotubes

Defect functionalization of single-walled carbon nanotubes (SWCNTs) by chemical modification is a promising strategy for near-infrared photoluminescence (NIR PL) generation at >1000 nm, which has advanced telecom and bio/medical applications. The covalent attachment of molecular reagents generates sp³-carbon defects in the sp²-carbon lattice of SWCNTs with bright red-shifted PL generation. Although the positional difference between proximal sp³-carbon defects, labeled as the defect binding configuration, can dominate NIR PL properties, the defect arrangement chemistry remains unexplored. Here, aryldiazonium reagents with π-conjugated ortho-substituents (phenyl and acetylene groups) were developed to introduce molecular interactions with nanotube sidewalls into the defect-formation chemical reaction. The functionalized chiral SWCNTs selectively emitted single defect PL in the wavelength range of ∼1230-1270 nm for (6,5) tubes, indicating the formation of an atypical binding configuration, different from those exhibited by typical aryl- or alkyl-functionalized chiral tubes emitting ∼1150 nm PL. Moreover, the acetylene-based substituent design enabled PL brightening and a subsequent molecular modification of the doped sites using click chemistry.

Azide modification forming luminescent sp2 defects on single-walled carbon nanotubes for near-infrared defect photoluminescence

Azide functionalization produced luminescent sp²-type defects on single-walled carbon nanotubes, by which defect photoluminescence appeared in near infrared regions (1116 nm). Changes in exciton properties were induced by localization effects at the defect sites, creating exciton-engineered nanomaterials based on the defect structure design.

Carbon Nanotube Photoluminescence Modulation by Local Chemical and Supramolecular Chemical Functionalization

ConspectusCarbon nanotubes (CNTs) have been central materials in nanoscience and nanotechnologies. Single-walled CNTs (SWCNTs) consisting of a cylindrical graphene show a metallic (met) or semiconducting (sc) property depending on their rolling up manner (chirality). The sc-SWCNTs show characteristic chirality-dependent optical properties of their absorption and photoluminescence (PL) in the near-infrared (NIR) region. These are derived from their highly π-conjugated structures having semiconducting crystalline sp² carbon networks with defined nanoarchitectures that afford a strong quantum confinement and weak dielectric screening. Consequently, photoirradiation of the SWCNTs produces a stable and mobile exciton (excited electron-hole pair) even at room temperature, and the exciton properties dominate such optical phenomena in the SWCNTs. However, the mobile excitons decrease the PL efficiency due to nonradiative relaxation including collision with tube edges and relaxation to lower-lying dark states. A breakthrough regarding the efficient use of the mobile exciton for PL has recently been achieved by local chemical functionalization of the SWCNTs, in which the chemical reactions introduce local defects of oxygen and sp³ carbon atoms in the tube structures. The defect doping creates new emissive doped sites that have narrower band gaps and trap the mobile excitons, which provides locally functionalized SWCNTs (lf-SWCNTs). As a result, the localized exciton produces E₁₁* PL with red-shifted wavelengths and enhanced PL quantum yields compared to the original E₁₁ PL of the nonmodified SWCNTs.In this Account, we describe recently revealed fundamental properties of the lf-SWCNTs based on the analyses by photophysics, theoretical calculations, and electrochemistry combined with in situ PL spectroscopy. The new insight allows us to expand the wavelength regions of the NIR E₁₁* PL derived from the localized exciton, in which upconversion generates a higher energy PL through thermal activation and proximal doped site formation using bis-aryldiazonium modifiers provides a much lower energy PL than typical E₁₁* PL. Moreover, owing to the chemical reaction-dominant doping process, the molecular structure design of modifiers succeeds in producing functionalized lf-SWCNTs; namely, molecular functions are incorporated into the doped sites for their PL modulation. The wavelength changes/switching in the E₁₁* PL selectively occurs by a supramolecular approach using molecular recognition and imine chemistry. Therefore, the local chemical functionalization of the SWCNTs is a key to designing the properties and creating their new functions of the lf-SWCNTs. Fundamental understanding of the doped site properties of the lf-SWCNTs and molecularly driven approaches for exciton and defect engineering would unveil the intrinsic natures of these materials, which is crucial for elevating the SWCNT-based nanotechnologies to the next stage. The resulting materials are of interest in the fields of high performance NIR-II imaging and sensing for bio/medical analyses and single-photon emitters in quantum information technology.