Quantum theory suggested that nanotube photoluminescence was intrinsically inefficient because of low-lying so-called dark exciton states. Here a significant brightening of nanotube photoluminescence (up to 28-fold) was demonstrated through the creation of an optically allowed defect state that resided below the predicted energy level of the dark excitons. Emission from this new state generated a photoluminescence peak that was red-shifted by as much as 254meV from the nanotube’s original excitonic transition. It was also found that the attachment of electron-withdrawing substituents to carbon nanotubes systematically drove this defect state further down the energy scale. The experiments showed that the material’s photoluminescence quantum yield increased exponentially as a function of the shifted emission energy. This laid the foundation for chemical control of defect quantum states in low-dimensional carbon materials.

Brightening of Carbon Nanotube Photoluminescence through the Incorporation of sp3 Defects. Y.Piao, B.Meany, L.R.Powell, N.Valley, H.Kwon, G.C.Schatz, Y.Wang: Nature Chemistry, 2013, 5[10], 840-5