Controlling the optical properties of carbon nanotubes with organic colour-centre quantum defects

Abstract

Previously unwelcome, defects are emerging as a new frontier of research, providing a molecular focal point to study the coupling of electrons, excitons, phonons and spin in low-dimensional materials. This opportunity is particularly intriguing in semiconducting single-walled carbon nanotubes, in which covalently bonding organic functional groups to the sp ² carbon lattice can produce tunable sp ³ quantum defects that fluoresce brightly in the shortwave IR, emitting pure single photons at room temperature. These novel physical properties have made such synthetic defects, or 'organic colour centres', exciting new systems for chemistry, physics, materials science, engineering and quantum technologies. This Review examines progress in this emerging field and presents a unified description of this new family of quantum emitters, as well as providing an outlook of the rapidly expanding research and applications of synthetic defects.

Fig. 1 |. Organic colour centres in a semiconductor host.

a | Organic colour centres are quantum emitters that can be synthesized by covalently attaching organic functional groups to a semiconducting host, in this case, a single-walled carbon nanotube (SWCNT). b | The organic functional group acts as a defect on the SWCNT and produces a localized quantum well in which mobile excitons from the host semiconductor are trapped and efficiently recombine to emit single photons. Part a is adapted with permission from REF., ACS.

Fig. 2 |. Optical properties of organic colour centres.

An organic colour centre creates a localized two-level state in the single-walled carbon nanotube (SWCNT) host, introducing new chemical functionality and optical properties. a | Covalent bonding of an aryl functional group to the sidewall of a (6,5)-SWCNT creates a two-level state that enables dark excitons from the host to be harnessed. The band structure and theoretically predicted excitonic states for the functionalized SWCNT feature bright singlet and dark E₁₁ excitons of the SWCNT host (in blue and black, respectively) and the E₁₁⁻ state (red) generated from the sp molecular defect. b | Defect-induced photoluminescence (E₁₁⁻) arises as more 4-nitrobenzene organic colour centres are incorporated into (6,5)-SWCNTs through a reaction using diazonium salts. Part a and data for part b are adapted from REF., Springer Nature Limited.

Fig. 3 |. Key evidence of exciton trapping by organic colour centres.

a | Organic colour centre (OCC) photoluminescence can be tuned through the incorporation of various functionalities to the surface of single-walled carbon nanotubes (SWCNTs). b | Photoluminescence arising from the E₁₁ transition (green) and E₁₁⁻ transition (red) along an individual (6,5)-SWCNT-C₆H₄OCH₃ (position indicated by the dashed line). The OCC photoluminescence is spatially isolated and spectrally distinct from the native emission of the nanotube. c | Diffraction-limited E₁₁⁻ photoluminescence image of an ultrashort (6,5)-SWCNT-C₆F₁₃, in which the individual location of the defect(s) cannot be resolved. d | Super-localization of fluorescent OCCs on (6,5)-SWCNT-C₆F₁₃ displays two super-resolved perfluorohexyl chain (−C₆F₁₃) defects at the SWCNT ends. Part a is adapted with permission from REF., ACS. Part b is adapted with permission from REF., RSC. Parts c and d are adapted with permission from REF., ACS.

Fig. 4 |. Dark exciton harvesting at fluorescent organic colour centres.

a | The energy shift between the E₁₁ and E₁₁⁻ emissions as a function of the nanotube chirality diameter approximately corresponds to the energy of the D-phonon mode associated with defect sites in the nanotube structure. This suggests that exciton–phonon coupling enhances dark exciton brightening by the organic colour centre. b | The redshift of E₁₁⁻ is enhanced by the use of electron-withdrawing substituents of aryl-functionalized (6,5)-single-walled carbon nanotubes. Reproduced with permission from REF., Springer Nature Limited.

Fig. 5 |. Chemistry of organic colour centres.

Various reactions have been developed to add aryl^,,, and alkyl^, organic colour centres to the surface of single-walled carbon nanotubes at controlled densities, converting sp²-hybridized carbon atoms of the host substrate to sp³. a | Arylation by reaction with a diazonium salt. b | Arylation by reaction with diazoethers. c | Arylation by reaction with aryl halides. d | Arylation by reaction with bisdiazonium salt. e | Alkylation by Billups–Birch reduction. f | Alkylation by reaction with an alkyl halide. The choice of the substituent alkyl or aryl groups employed enables the nanotube electronic structure and resulting organic colour centre emission to be precisely tuned.

Fig. 6 |. Emergent applications of fluorescent organic colour centres.

a | Optical pH sensing in complex solutions within the biological window. A 4-N,N-diethylaminoaryl organic colour centre (OCC) on a (6,5)-single-walled carbon nanotube (SWCNT), which is protonated at low pH (pK_a = 6.28). b | The OCC photoluminescence shifts after changing the pH of a suspension of (6,5)-SWCNT-C ₆H₄N(C₂H₅)₂. The asterisk denotes other nanotube chiralities present in the suspension. c | Radiative decay of OCC-trapped excitons produces single photons. The dashed lines represent the measured data, and solid lines represent isolated E₁₁ and E₁₁^– peaks from spectral fitting. d,e | The photoluminescence spectrum (part d) and second-order time correlation (part e) of OCC photoluminescence in (7,5)-SWCNT-C ₆H₄OCH₃ showing high-purity single-photon emission in the shortwave IR at room temperature. a.u., atomic units. Parts a and b are adapted with permission from REF., ACS. Parts c–e are reproduced from REF., Springer Nature Limited.

Fig. 7 |. Opportunities and challenges for organic colour centres.

a | Organic colour centres (OCCs) provide a molecular site for studying the coupling of electrons, excitons, phonons and spin in low-dimensional materials, which are largely unexplored. b | Two-dimensional materials such as MoS₂ may function as interesting substrates for OCCs, in which the attachment of various organic groups to the crystal substrate could potentially produce new fluorescent emissions with controllable intensity and wavelength. c | OCCs break the symmetry of extended solids, posing significant challenges to theory in predicting the relative bonding sites, as highlighted by the many possible atomic configurations, including 1,2 sites (blue) and 1,4 sites (green), and describing the excited states. Part b is adapted with permission from REF., ACS.