Luminescent Defects in Single‐Walled Carbon Nanotubes for Applications

Abstract

most applications, only semiconducting nanotubes and ideally only one (n,m) species (monochiral) are desired. This technological need has pushed the search for more controlled and selective growth of SWCNTs [5,6] and even more importantly fueled the development of post-growth purification and sorting of the different nanotube types and chiralities. Various, quite different techniques, such as gelchromatography, [7][8][9] aqueous two-phase separation (ATPE), [10][11][12] and selective polymer-wrapping, [13][14][15][16][17] now make it possible to produce and work with sufficiently large amounts of not only purely semiconducting nanotubes of a certain diameter range but also monochiral nanotubes and even enantiomerically pure samples. [18,19] This high degree of purification and hence the availability of nanotubes as a material with reproducible properties has been critical for applications in electronics [20,21] and energy conversion, [22,23] as well as imaging [24,25] and sensing [26,27] based on the near-infrared photoluminescence (PL) of SWCNTs and its modulation. However, an important paradigm shift occurred when it became clear that defects in the sp 2 carbon lattice of nanotubes are not always detrimental to the photoluminescence yield, but can indeed lead to new electronic states with highly interesting and useful optical properties. [28][29][30] These specific defects, which are variably named sp 3 defects, luminescent defects, organic color centers or quantum defects [31][32][33][34][35] have been at the heart of many recent studies on carbon nanotubes and promise a wide range of potential applications from single-photon emission to in vivo super-resolution imaging. This review will briefly introduce the underlying photo physics and properties of these defects, peruse recent progress in their controlled creation and discuss the various potential applications in detail.Semiconducting single-walled carbon nanotubes show extraordinary electronic and optical properties, such as high charge carrier mobilities and diameter-dependent near-infrared photoluminescence. The introduction of sp 3 defects in the carbon lattice of these nanotubes creates new electronic states that result in even further red-shifted photoluminescence with longer lifetimes and higher photoluminescence yield. These luminescent defects or organic color centers can be tuned chemically by controlling the precise binding configuration and the electrostatic properties of the attached substituents. This review covers the basic photophysics of luminescent sp 3 defects, synthetic methods for their controlled formation and discusses their application as near-infrared single-photon emitters at room temperature, in electroluminescent devices, as versatile optical sensors, and as fluorophores for bioimaging and potential super-resolution microscopy.

Probing Carrier Dynamics in sp3-Functionalized Single-Walled Carbon Nanotubes with Time-Resolved Terahertz Spectroscopy

“…Monochiral, semiconducting (6,5) SWCNTs (diameter ~0.76 nm, average length ~1.4 µm) were obtained by highly selective dispersion of CoMoCAT nanotubes with a fluorene-bipyridine copolymer (PFO-BPy) in toluene. 30 These polymer-wrapped (6,5) SWCNTs were covalently functionalized with 4-nitrobenzenediazonium tetrafluoroborate 31 to create different densities of sp 3 defects (see Figure 1a) on the nanotubes (for further details, see Methods). The degree of functionalization was quantified with various spectroscopic metrics.…”

“…[23][24][25][26][27] Due to the transient nature of the THz probe with an oscillating electrical field with ~1 ps duration, charge carriers are locally driven over a distance of tens of nm, rendering THz spectroscopy a highly suitable technique to probe charge transport on a microscopic level. 23,28 Previous THz studies revealed significantly longer charge scattering times for (6,5) SWCNTs compared to graphene nanoribbons, 24 as well as very similar carrier dynamics for semiconducting and metallic SWCNT films. 29 Here, we investigate the intrinsic charge transport properties of sp 3 -functionalized, polymersorted (6,5) SWCNTs as a model system by combining (temperature-dependent) OPTP spectroscopy of dispersions and thin films with electrical measurements of nanotube network fieldeffect transistors.…”

“…23,28 Previous THz studies revealed significantly longer charge scattering times for (6,5) SWCNTs compared to graphene nanoribbons, 24 as well as very similar carrier dynamics for semiconducting and metallic SWCNT films. 29 Here, we investigate the intrinsic charge transport properties of sp 3 -functionalized, polymersorted (6,5) SWCNTs as a model system by combining (temperature-dependent) OPTP spectroscopy of dispersions and thin films with electrical measurements of nanotube network fieldeffect transistors. The observed decrease in magnitude and lifetime of the transient photoconductivity with the degree of functionalization corroborates the impact of sp 3 defects on intra-nanotube carrier mobilities and provides a clearer picture of the contributions of intrananotube and inter-nanotube charge transport in pristine and functionalized SWCNT networks.…”

“…Since THz spectroscopy of SWCNT dispersions unambiguously probes only the intrinsic nanotube charge transport, we can assume that the intra-nanotube resistance (increased by the sp 3 defects) in (6,5) SWCNT networks -in contrast to common belief -is not negligible compared to the junction resistance (inter-nanotube hopping). 42 To further corroborate the impact of sp 3 defects on charge transport in individual SWCNTs within networks, we performed temperature-dependent OPTP spectroscopy on dense (6,5) SWCNT films. As shown in Figure S14a (Supporting Information), for pristine nanotubes at 288 K, the THz photoconductivity of thin films was twice as high as that of SWCNTs in dispersion.…”

“…[1][2][3] Recently, the controlled functionalization of SWCNTs with covalently attached aryl or alkyl groups has emerged as a versatile approach to enhance their optical properties. [4][5][6][7] These local sp 3 -hybridized lattice defects, also referred to as quantum defects or organic color centers, form new electronic states that efficiently trap excitons, leading to red-shifted (by ~100-300 meV) nIR photoluminescence (PL) with increased quantum yields. [8][9][10][11][12] Furthermore, such functionalization enables high-purity singlephoton emission in SWCNTs at room temperature.…”