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Mapping Structure-Property Relationships of Organic Color Centers
Article
Mapping Structure-Property Relationships of Organic Color Centers Mijin Kim, Xiaojian Wu, Geyou Ao, ..., Ming Zheng, Stephen K. Doorn, YuHuang Wang [email protected]
HIGHLIGHTS Bonding functional groups to carbon semiconductors creates organic color centers Organic color centers produce tunable near-infrared emission by as much as 400 meV The electronic coupling between a color center and its host can be chemically tailored
Organic color centers are an emergent class of quantum emitters that hold vast potential for photonic, sensing, and optoelectronic applications. Covalently bonding functional groups to semiconducting single-walled carbon nanotubes creates molecularly tunable color centers where mobile excitons can be trapped and fluoresce brightly. Comparative studies of a series of 14 high-purity nanotube hosts and 30 organic color centers reveal new insights on the structure-property relationships that may be used to guide the synthesis of these centers featuring tailored light-emitting properties.
Kim et al., Chem 4, 1–12 September 13, 2018 ª 2018 Elsevier Inc. https://doi.org/10.1016/j.chempr.2018.06.013
Please cite this article in press as: Kim et al., Mapping Structure-Property Relationships of Organic Color Centers, Chem (2018), https://doi.org/ 10.1016/j.chempr.2018.06.013
Article
Mapping Structure-Property Relationships of Organic Color Centers Mijin Kim,1 Xiaojian Wu,1 Geyou Ao,2 Xiaowei He,3 Hyejin Kwon,1 Nicolai F. Hartmann,3 Ming Zheng,2 Stephen K. Doorn,3 and YuHuang Wang1,4,5,*
SUMMARY
The Bigger Picture
Organic color centers are an emergent class of quantum emitters that hold vast potential for applications in bioimaging, chemical sensing, and quantum information processing. Here, we show that these synthetic color centers follow interesting structure-property relationships through comparative spectral studies of 14 purified single-walled carbon nanotube chiralities and 30 different functional groups that vary in electron-withdrawing capability and bonding configurations. The defect emission is tunable by as much as 400 meV in the near-infrared as a function of host structure and the chemical nature of the color centers. However, the emission energy is nearly free from chiral angle and family patterns of the nanotube host (although this strongly depends on the nanotube diameter), suggesting that a trapped exciton at the organic color centers to some degree electronically decouples from the one-dimensional semiconductor host. Our findings provide important insights for designing and controlling this new family of synthetic color centers.
Color centers, such as nitrogen vacancies in diamond, possess intriguing optical properties that have been intensively studied for imaging, sensing, lasing, and quantum information. However, most color centers occur as native defects, making tailored synthesis difficult. By covalently bonding functional groups to carbon nanotube semiconductors, we can synthetically create a whole new class of color centers that are organic and can be chemically tailored in highly predictable manners. These synthetic quantum emitters allow systematic tuning of the emission color in the near-infrared, a spectral regime that is important to biochemical imaging and quantum communications but had been largely unexplored. Unlike nitrogen vacancy centers in diamond, which is an electrical insulator, organic color centers are synthetically created in a semiconductor and electronically coupled to the host, opening the possibility for electrical addressing and chemical tailoring with molecular precision.
INTRODUCTION Organic color centers are an emerging class of synthetic defect emitters that can be chemically incorporated into the sidewall of a single-walled carbon nanotube (SWCNT) through covalently bonding functional groups to the semiconductor host.1–4 The introduced chemical defect locally modulates the electronic structure of the SWCNT host to enable trapping of excitons at the defect site, where they burst brightly as single photons.1,5 The defect-induced emission (E11) can be significantly brighter than the native photoluminescence (PL) (E11) of the nanotube and emits at the near-infrared (NIR),1 suggesting vast potential for applications in imaging, sensing, and creating single photon sources at telecom bands.3,6–8 The electronic and optical properties of organic color centers vary substantially with the structure of the SWCNT host, each of which is assigned a specific (n,m) chirality that describes its construction as a rolled-up graphene sheet. On the other hand, there are increasingly large numbers of functional groups that are being identified capable of creating organic color centers, suggesting virtually unlimited opportunities in broadly applying organic chemistry in this emergent field.1,2,4,9–13 The unfunctionalized SWCNT hosts are quasi-one-dimensional nanostructures with highly predictable electronic structure and optical properties.14 Particularly, the excitonic transition energies of semiconducting SWCNTs show a strong dependence on both diameter and chiral angle, as summarized in correlation plots known as the Kataura plots.15,16 In contrast, systematic analysis of the structure-dependence of organic color centers has yet to be established. It is important to understand how the defect state is related to the nanotube family pattern and chirality, and how
Chem 4, 1–12, September 13, 2018 ª 2018 Elsevier Inc.
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the one-dimensional SWCNT couples to the zero-dimensional quantum state of the organic color centers, in order to provide a predicative understanding that will enable the design and synthesis of this family of quantum emitters for bioimaging, biosensing, quantum computing, and nanophotonics, as well as probing the rapidly unraveled fundamentally new phenomena of defect chemistry and physics. Even simply knowing the energies of these defect states for specific (n,m) structures and organic color centers would be important not only for fundamental photophysics of defect-trapped excitons but also for applications in infrared imaging, chemical sensing, and tailored design and synthesis of quantum materials. Herein, we aim at establishing this structure-property relationship for organic color centers through controlled synthesis and comparative spectral studies of 30 chemically distinct organic color centers and 14 purified nanotube hosts. We observed distinct defect-induced PL features for 14 semiconducting SWCNT species (ranging from 0.62 to 0.94 nm in diameter and from 0 to 27.5 in chiral angle) using 30 different functional groups. Based on the spectrofluorometric measurements of perfluorohexyl defect-tailored (n,m)-SWCNTs, we analyzed the E11 as empirically fitted functions of nanotube diameter. Our results show that the measured E11 PL energy is tunable by 400 meV in the NIR as a function of SWCNT diameter. However, the emission energy of defect-trapped excitons is nearly free from chiral angle and family patterns, suggesting that an exciton at an organic color center to some degree behaves independently from the nanotube host but this color center-host coupling is chemically tunable depending on the chemical nature of the defect in terms of the group’s electron-withdrawing ability and bonding configuration. These findings provide a comprehensive picture of the structure-property relationships of organic color centers that is required to guide controlled and tailored synthesis of this new family of quantum emitters.
RESULTS AND DISCUSSION Incorporation of Organic Color Centers in SWCNTs We covalently attached functional groups into a series of (n,m)-SWCNTs to study the correlation of the photon energy emitted from the resulting organic color centers with the structure of the SWCNT host (Figure 1). In this work, we sorted 14 types of semiconducting SWCNT species into samples highly enriched in single chiralities using polymer aqueous two-phase (ATP) extraction17 or gel chromatography18 (Figure S1). The sorted SWCNTs were stabilized in 1 wt/v % sodium dodecyl sulfate (SDS) in D2O for subsequent functionalization (see Experimental Procedures for detailed protocols). The semiconducting SWCNT structures studied in this work ranged from 0.62 to 0.94 nm in diameter and from 0 to 27.5 in chiral angle (Table S1). We then used diazonium1 or alkyl/aryl halide2,4 chemistry to covalently attach 30 different functional groups to the selected SWCNT chiralities (Table S2). The attached functional groups vary in electron-withdrawing capability and bonding configurations, enabling us to systematically modify the energy level of the defect state relative to the native electronic structure of the nanotube. The functionalized SWCNTs were characterized by UV-visible (vis)-NIR absorption and PL spectroscopy. The E11 and E11 wavelengths were determined by fitting the PL spectrum at E22 excitation using Voigt profiles. Figure 1 displays the absorption spectra and emission-excitation PL maps of three exemplary nanotube species: (5,4), (6,5), and (10,3). Incorporation of perfluorohexyl organic color centers produces a new E11 PL peak at a redshifted wavelength from the native E11 emission of the nanotube host. The E11 peak originates from the
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1Department
of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742, USA
2Materials
Science and Engineering Division, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD 20899, USA
3Center
for Integrated Nanotechnologies, Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
4Maryland
NanoCenter, University of Maryland, College Park, MD 20742, USA
5Lead
Contact
*Correspondence: [email protected] https://doi.org/10.1016/j.chempr.2018.06.013
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Figure 1. Organic Color Centers in Semiconducting SWCNT Hosts (A) Schematic representation of organic color centers incorporated as perfluorohexyl defects into a series of (n,m)-SWCNTs. An individual color center creates a host-structure-dependent potential well where a mobile exciton can be trapped and fluoresce brightly. (B) UV-vis-NIR absorption spectra of purified (5,4), (6,5), and (10,3) SWCNT solutions dispersed in 1 wt/v % SDS in D 2 O. (C) Excitation-emission PL maps of perfluorohexyl functionalized (5,4), (6,5), and (10,3) SWCNTs, from top to bottom.
radiative recombination of trapped excitons from fluorescent defect sites,1 as shown by the fact that both E11 and E11 PL peaks resonate with the E22 excitation of the SWCNT in excitation-emission maps of the functionalized samples (Figure 1). Although the attached functional groups (-C6F13) are the same, the emission wavelength of E11 was found to vary dramatically with the nanotube species, which implies a correlation with the host structures. We tabulated the emission wavelengths of E11 and E11 PL for the different nanotube chiralities studied and the energy difference between these two PL peaks (optical energy gap, DE = E11 E11) in Tables 1 and S3. The results provided the basis for an empirically determined energy plot of defect PL versus (n,m) chirality for semiconducting SWCNTs (Figures 2 and 3). Among the species investigated here, the longest wavelength of defect PL appears at 1,487 nm for perfluorohexyl functionalized (11,1) nanotube, (11,1)-SWCNT-C6F13. The shortest wavelength of defect PL occurs at 1,000 nm for (5,4)-SWCNT-3,5-C6H3(NO2)2. The largest DE value was 282 meV for (5,4)-SWCNT-C6H4NO2, while the smallest was 92 meV for (7,6)-SWCNT-C6H13. This wide emission wavelength range covers the biological transparency window19 and most of the telecommunication regime,3,8 and can be exploited for chemical sensing, bioimaging, quantum information, and a host of optoelectronic applications that require bright, high-quality light sources in the NIR wavelengths.3,6–8
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Table 1. Optical Characteristics of Selected Combinations of Nanotube Chirality and Organic Color Centers
>CF2 -3,5-C6H3(NO2)2
-4-C6H4NO2
(5,4)
(6,4)
(7,3)
(9,1)
(6,5)
(8,3)
-4-C6H4N(C2H5)2
-C6F13
-C6H13
1,052 nm
1,027 nm
–
282 meV
265 meV
211 meV
199 meV
1,066 nm
1,082 nm
1,049 nm
1,084 nm
1,053 nm
1,097 nm
247 meV
264 meV
228 meV
265 meV
232 meV
277 meV
185 meV
198 meV
171 meV
225 meV*
174 meV
208 meV
1,160 nm
1,190 nm
1,150 nm
1,177 nm
–
1,211 nm
172 meV
198 meV
156 meV
184 meV
206 meV
129 meV
148 meV
117 meV
138 meV
154 meV
1,115 nm
1,128 nm
1,086 nm
1,169 nm
240 meV
241 meV
202 meV
287 meV
180 meV
181 meV
151 meV
215 meV
1,145 nm
1,152 nm
1,105 nm
178 meV
190 meV
133 meV
142 meV
1,159 nm
1,169 nm
1,124 nm
229 meV
238 meV
195 meV
171 meV
178 meV
146 meV
160 meV*
150 meV
1,310 nm
–
–
–
–
–
–
1,000 nm
–
238 meV
1,025 nm 275 meV
178 meV
206 meV
–
–
1,160 nm
1,132 nm
1,168 nm
137 meV
190 meV
161 meV
200 meV
103 meV
143 meV*
121 meV
150 meV
1,171 nm
1,127 nm
–
237 meV
200 meV
137 meV (9,2)
123 meV
(7,5)
(8,4)
1,189 nm
1,206 nm
1,174 nm
1,192 nm
1,164 nm
167 meV
173 meV
151 meV
162 meV
136 meV
152 meV
156 meV
130 meV
146 meV
122 meV
1,276 nm
1,284 nm
1,228 nm
1,282 nm
1,242 nm
141 meV
149 meV
102 meV
146 meV
114 meV
127 meV
134 meV
92 meV
132 meV*
103 meV
–
–
–
1,225 nm
–
–
–
–
160 meV 144 meV
(11,0)
(7,6)
1,281 nm
1,291 nm
1,228 nm
1,281 nm
1,227 nm
135 meV
134 meV
92 meV
124 meV
92 meV
121 meV
120 meV
83 meV
112 meV
83 meV (Continued on next page)
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Table 1. Continued
>CF2 -3,5-C6H3(NO2)2
-4-C6H4NO2 –
-4-C6H4N(C2H5)2
-C6F13
-C6H13
1,270 nm
–
–
–
–
–
–
–
–
–
–
–
–
137 meV 123 meV
(9,4) 1,486 nm
(11,1)
(10,3)
137 meV
137 meV
123 meV
123 meV
1,455 nm
1,445 nm
127 meV
126 meV
114 meV
113 meV
Listed are the emitting wavelength (E11, top), the optical shift from that of the native exciton (DE, middle), and the thermal trapping potential of the E11 exciton (Etrap, bottom). Note that the PL spectra were obtained from SWCNTs in 1% SDS-D2O. Changes in the dispersion media will cause deviations in the emission wavelengths from the reported data due to dielectric screening effects.20 Also note that some Etrap values are derived from linear extrapolation of experimentally measured series (asterisks).
Structure-Property Relationships of the Defect PL Emission Each SWCNT structure can be uniquely indexed by a pair of (n,m) integers to indicate the chirality of the nanotube based on the roll-up vector of a graphene sheet (Figure 2). The direction of rolling determines not only the physical structure, such as diameter and chiral angle,21 but also the electronic band structure of the nanotube.14,15,22 Specifically, since an SWCNT imposes boundary conditions on the electron wave function in the direction of rolling, for semiconducting (n,m) SWCNTs, dividing (n m) by 3 leaves a remainder of 1 or 2, which is classified as mod (n m, 3) = 1 or mod (n m, 3) = 2, respectively. If (n m) is evenly divisible by 3, the SWCNT is metallic and therefore does not emit PL. Covalent functionalization may break the intrinsic symmetry of the SWCNT by converting sp2 carbon to sp3, which modifies the energy level of the functionalized nanotube locally at the site of the defect.1,2 At this local defect state, the optical properties, including the emission energy and the size of the exciton,23 are different from those of the intrinsic E11 excitons. Hence, we analyzed the structure-dependent properties of defect-trapped excitons in (n,m)-SWCNTs with perfluorohexyl, 4-nitroaryl, and 3,5-dinitroaryl defects, and mapped out the structure dependence of the E11 emission wavelength onto the graphene lattice (Figures 2 and S2). The results show in general that the E11 for larger SWCNT diameters emits at longer wavelength, but it also depends on the chiral angle of the nanotube. We observed a positive correlation between E11 and E11 emission energies for the studied chiralities with the same perfluorohexyl functional groups (Figure 3A), indicating that the defect state is closely related to the E11 excitonic state. The energy difference between E11 and E11 (DE values) were examined for dependence on diameter (d),
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Figure 2. Tunable Defect Photoluminescence as a Function of Host Chirality Experimentally determined E 11 wavelength of (A) (n,m)-SWCNT-C 6 H4 NO2 , (B) (n,m)-SWCNT-C 6 F 13 , and (C) (n,m)-SWCNT-C 6 H3 (NO2 ) 2 on a graphene sheet showing (n,m) lattice points. The color bar shows the emission wavelength of the defect PL. A lattice with diagonal stripes represents non-emitting metallic SWCNTs, in which (n – m)/3 is equal to an integer. A filled lattice indicates a semiconducting SWCNT chirality studied in this work. The blue arrow and q in (A) represent the chiral vector and chiral angle of (6,2)-SWCNTs as a demonstration of SWCNT chirality. The molecular structure of the organic color center is specified in each part of the figure.
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Figure 3. Semiconductor Host-Color Center Relationship Closed and open circles indicate mod 1 and mod 2, respectively. (A) Positive correlation between native (E 11 ) and defect-induced PL (E 11 ) wavelengths. The solid line is a quadratic function drawn to guide the eye. (B) Diameter dependence of DE. The solid line is an empirical fitting to the inverse second order of the diameter (Equation 1). (C) Curvature chirality effect on DE. The dashed line indicates metallic armchair SWCNTs with q = 30 . Each point represents different (n,m) species that are covalently functionalized with perfluorohexyl defects.
chiral angle (q), and mod (n – m, 3). We explored the diameter dependence by fitting DE with the inverse second-order equation of diameter (Equation 1): DE=meV = 11:3 d2 + 253d1 158;
(Equation 1)
in which d is the SWCNT diameter in nanometers. Figure 3B shows in general an inverse correlation between DE and diameter. However, we also observe 28% deviation of DE on average from Equation 1 for the range of diameters studied, suggesting diameter alone cannot account for DE variation, thus the chirality effect should be considered. Although the DE versus chirality plot displays no obvious pattern (Figure 3C), we found that the deviation of DE is mod dependent with higher DE values for structures of mod (n m, 3) = 2 than for mod (n m, 3) = 1. This mod-dependent deviation of DE is reminiscent of what was reported for the E11 energy and diameter correlation.14,16 The mod-dependent deviation represents the degree of electronic decoupling and is discussed in the next section. Even though the empirical fitting provides an estimate of the defect PL energy on the ensemble level, the PL energy of an individual quantum defect can vary due to the bonding configuration, as predicted by Kwon et al.2 and Gifford et al.24 Exciton Trapping Potential at Organic Color Centers The exciton trapping potential (Etrap) is the minimum energy required to detrap the E11 exciton from the defect trap to restore as a free E11 exciton. It is an important parameter to understand the structure-related properties of excitons, as well as to predict the PL stability. As established in our previous work,25 the trapping potential of a defect state can be experimentally determined by monitoring the E11 and E11 PL as a function of temperature or calculated from the difference between the optical energy gap (DE) and the reorganization energy (l). Reorganization occurs due to deformation of the nanotube geometry upon exciton trapping at the defect site25 and is related to the exciton localization at the defect site,26,27 with greater localization presumably leading to a larger reorganization energy. When the density of defects increases, the exciton wave function may be delocalized across multiple defects, leading to weaker localization of excitons at the defect site and smaller reorganization energy. For the series of perfluorohexyl defect-tailored SWCNTs, we also experimentally derived a larger l in smaller diameter SWCNTs (d < 0.84 nm, Figure S3). Such greater spatial localization of the wave
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Figure 4. Trapping Potential of E11 Exciton at Organic Color Centers (A) Diameter dependence of the trapping potential of E 11 excitons in (n,m)-SWCNT-C6 F 13 . (B) Diameter dependence of E 11 (gray) and E 11 (blue) emission energies. The labels categorize the (2n + m) families. Filled and open circles mark mod 1 and 2 SWCNT structures, respectively.
function effectively increases the amount of exciton-phonon coupling and thus increases reorganization energy in small-diameter SWCNTs. By linear extrapolation from the experimentally determined series (Table 1), we derived that the trapping potential of perfluorohexyl defect-trapped excitons ranges from 113 to 200 meV, as shown in Figure 4A. Our results suggest that the trapping potential is related to the size of the trapped exciton (electron-hole separation). For a larger trapping potential, the wave function of the exciton, which can be computed using density functional theory,25 is more spatially localized at the defect site.23,25,28 This is congruent with our previous theoretical prediction that an E11 exciton of a (6,5)-SWCNT is squeezed by 17% in size when trapped at an aryl defect.25 The inverse correlation between Etrap and diameter (regardless of l values) implies that, in smaller diameter SWCNTs, a defect trap can effectively reduce the exciton size and increase the oscillator strength of the defect state.28 Strong localization at a deep trap may enhance the binding energy of the E11 exciton, and improves the stability of the defect-trapped excitons. This may be related to the diameter-dependent quantum yield enhancement1 and PL stability of E11 excitons in single photon emission.8,29 Although the photon conversion efficiency is an important parameter for many potential applications of organic color centers, it remains a challenging and labor-intensive task,1 which warrants the development of more efficient techniques to quantify this value. Extrapolating beyond d > 0.94 nm (the largest diameter studied here), our empirical fitting of Etrap (solid line in Figure 4A) predicts that sp3 defects created by covalent functionalization create shallow traps in large-diameter SWCNTs (e.g., few millielectronvolts for d > 1.3 nm), making trapping and radiative recombination of E11 excitons less efficient. Another interesting finding is that only a weak chirality dependence was observed for the E11 fitting. Figure 4B shows the empirically fitted emission energy of the E11 and E11 exciton as a function of diameter for 12 SWCNT chiralities that are tailored with perfluorohexyl organic color centers. There is a clear inverse correlation between nanotube diameter and exciton emission energy. The deviation from the diameter fitting is due to the chiral angle dependence. This dependence becomes more apparent if we group the same nanotube families (2n + m) with a solid line, indicating the set of SWCNTs with similar diameters but different chiral
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Figure 5. Trapping Potential of E11 Excitons at Organic Color Centers in (6,5)-SWCNT Host (A) The inductive and resonance effects of terminating aryl moieties on E trap . (B) Inductive effects of alkyl chains on E trap . The solid lines in (A) and (B) are linear fitting of the correlations. (C) Divalent methyl (square), aminoaryl (circle), aryl (triangle), and perfluoromethyl (diamond) defects have larger E trap compared with their monovalent counterparts. The dashed line is E trap (monovalent) = E trap (divalent) drawn to guide the eye.
angles.15,16,22 It is also clear that the curvature effects in E11 are less significant than those in E11. These two trends may be understood by examining the electronic structure of the nanotube host and the molecular nature of the defects. The E11 energy levels influence the energy levels of the defect states because the defect state originates from the splitting of the doubly degenerate frontier orbitals of the SWCNT host1,23 and thus exhibits some degree of chiral angle dependence that arises from the trigonal wrapping effect inherent in the host.22 Meanwhile, once a mobile exciton is trapped at an sp3 defect, the trapped exciton manifests photophysics that is different from the E11 exciton, which is governed by the sp2 symmetry of the SWCNT lattice. Thus, our data suggest that, as the defect trap becomes deeper, the exciton localization is stronger, and the optical and electronic properties of the trapped excitons become closer to an isolated zerodimensional system with weaker dependence on the chiral angle of the nanotube host. Lastly, Figure 5 demonstrates that the trapping potential and emission energies of defect-trapped excitons are highly tunable depending on the chemical nature of the defects. Here we specifically studied (6,5)-SWCNT because it is easier to prepare high-purity samples compared with other chiralities. The spectral characterization of all defect-tailored (6,5)-SWCNTs studied in this work is available in Figure S4. We note that this correlation between the chemical natures of the defect and DE is observed for other chiralities as well (Table 1). Due to the wide choice of defects (30 different functional groups), the defect PL of (6,5)-SWCNTs can be broadly tuned over the emission wavelength of 1,094–1,164 nm. We found that, in general, Etrap, as well as the E11 wavelength and DE, increased as the electron-withdrawing ability of the defects became stronger. The electronwithdrawing ability can be quantified using Hammett constants for aryl defects30 and Taft constants for alkyl defects.31 From the study presented here, we show that the chemical nature of defects has a significant impact on the organic color centers, as manifested in Etrap, which follows a linear correlation with both Hammett (s) (Figure 5A) and Taft constants (s*) (Figure 5B). Combining the results of Figures 4 and 5, we derived empirical prediction models of DE for monovalent aryland alkyl-defect-tailored SWCNTs. For aryl-defect functionalized SWCNTs, DE 17:3 235:5 559 = 146:3 + Ds 613:2 : (Equation 2) + 2 meV ðd=nmÞ d=nm d=nm
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For alkyl-defect functionalized SWCNTs, DE 11:3 253 248:6 = 158 + Ds ; 205:1 + meV ðd=nmÞ2 d=nm d=nm
(Equation 3)
in which Ds is s 0.788 and Ds* is s* 4.87. Due to the chiral angle dependence of the E11 and E11 PL, the chiral angle dependence generates, on average, 9% deviation from the experimentally determined values from the diameter fitting. Lastly, we note that bonding configuration also influences Etrap and E11 PL (Figure 5C). Divalent defects, including >CF2, >CH2, >C6H4, and >C6H3NH2, tend to create deeper defect potentials and increase the exciton trapping potentials compared with their monovalent counterparts (-CF3, -CH3, -C6H5, and -C6H4NH2). In conclusion, we have synthesized a series of organic color centers in semiconducting SWCNTs and determined the diameter and chirality dependence of PL emission wavelengths and the thermal trapping potential for defect-trapped excitons. Similar to the native exciton (E11) in unfunctionalized SWCNTs, the E11 PL is strongly correlated with nanotube diameter. The emission energy of defect-trapped excitons, however, is largely free from chiral angle and family patterns of the semiconductor host, suggesting that an exciton at an organic color center to some degree decouples from the one-dimensional nanotube host. Our work establishes the structure-property relationships for organic color centers, which may help guide the controlled and tailored synthesis of this new family of quantum emitters for applications in bioimaging, quantum information, and optoelectronics in general.
EXPERIMENTAL PROCEDURES A detailed description of the synthesis protocols can be found in the Supplemental Experimental Procedures. High-Purity SWCNT Sorting We sorted single-chirality SWCNTs, including (11,1), (10,3), (11,0), (7,5), (6,5), (9,1), (6,4), and (5,4), from CoMoCAT SG65i or SG76 SWCNTs (Southwest Nanotechnologies) using polymer ATP separation.5,17 For the ATP separation process using single-stranded DNA,17 the sorted SWCNTs were precipitated from DNA solution by sodium thiocyanate (Sigma-Aldrich, 98%) and re-dispersed in 1 wt/v % sodium deoxycholate (DOC, Sigma-Aldrich, R97%). In other ATP experiments,5 we pressure filtrated the SWCNT-polymer mixture (Amicon, number 5123, using 100 kDa ULTRAcel regenerated cellulose filter membranes) with 1 wt/v % DOC to dilute the polymer concentration by a factor of 103. Alternatively, gel chromatography was used to sort (7,6) + (8,4), (8,3) + (8,4), (8,3) + (7,3), and (6,5)-SWCNT enriched samples from HiPco SWCNTs (Rice University, batch #194.3) as described previously by Liu et al.18 The purified SWCNTs were then suspended in 1 wt/v % SDS (Sigma-Aldrich, R98.5%) in D2O (Cambridge Isotope Laboratories, 99.8%) for subsequent functionalization. Covalent Functionalization of Organic Color Centers The optical densities of the SWCNT solutions were adjusted to 0.03–0.12 at the E11 transition for subsequent optical studies. Monovalent aryl defects were introduced by diazonium chemistry.1,11 For some aryl defects that cannot be incorporated using diazonium reactions, such as divalent aryl and aminoaryl defects, light activated arylation was used.2,4 A series of alkyl defects were introduced using alkyl halides
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as reported by Kwon et al.2 Covalent functionalization was monitored by defect PL evolution at E22 excitation light, as described in the following section. Spectroscopic Characterization The SWCNT PL was characterized with a NanoLog spectrofluorometer (Horiba Jobin Yvon) using a liquid-N2-cooled InGaAs array. The SWCNTs were excited with monochromator-selected light (10 nm slit width) from a 450 W xenon arc lamp. The excitation power was lower than 10 mW with an integration time of 1–10 s. The spectral resolution was 10 nm for the emission detection channel. UV-vis-NIR absorption spectra were obtained with a spectrophotometer equipped with a broadband InGaAs detector (Lambda 1050, PerkinElmer). The path length of absorption measurements was 10 mm.
SUPPLEMENTAL INFORMATION Supplemental Information includes Supplemental Experimental Procedures, four figures, and three tables and can be found with this article online at https://doi. org/10.1016/j.chempr.2018.06.013.
ACKNOWLEDGMENTS This work was supported by the NSF (grant number CHE-1507974) and also partially by the NIH NIGMS (grant number R01GM114167). Part of this work was performed at the Center for Integrated Nanotechnologies, US Department of Energy, Office of Science user facility through user project 2017AC0164. S.K.D., X.H., and N.F.H. acknowledge support from the LANL Laboratory Directed Research and Development program.
AUTHOR CONTRIBUTIONS Conceptualization, Y.W. and M.K.; Investigation, M.K. and H.K.; Resources: G.A., X.H., and N.F.H.; Writing, M.K., X.W., and Y.W.; Funding Acquisition, Y.W.; Supervision, M.Z., S.K.D., and Y.W.
DECLARATION OF INTERESTS The authors declare no competing interests. Received: March 8, 2018 Revised: June 18, 2018 Accepted: June 25, 2018 Published: July 19, 2018
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Mapping Structure-Property Relationships of Organic Color Centers Mijin Kim, Xiaojian Wu, Geyou Ao, ..., Ming Zheng, Stephen K. Doorn, YuHuang Wang [email protected]
HIGHLIGHTS Bonding functional groups to carbon semiconductors creates organic color centers Organic color centers produce tunable near-infrared emission by as much as 400 meV The electronic coupling between a color center and its host can be chemically tailored
Organic color centers are an emergent class of quantum emitters that hold vast potential for photonic, sensing, and optoelectronic applications. Covalently bonding functional groups to semiconducting single-walled carbon nanotubes creates molecularly tunable color centers where mobile excitons can be trapped and fluoresce brightly. Comparative studies of a series of 14 high-purity nanotube hosts and 30 organic color centers reveal new insights on the structure-property relationships that may be used to guide the synthesis of these centers featuring tailored light-emitting properties.
Kim et al., Chem 4, 1–12 September 13, 2018 ª 2018 Elsevier Inc. https://doi.org/10.1016/j.chempr.2018.06.013
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Article
Mapping Structure-Property Relationships of Organic Color Centers Mijin Kim,1 Xiaojian Wu,1 Geyou Ao,2 Xiaowei He,3 Hyejin Kwon,1 Nicolai F. Hartmann,3 Ming Zheng,2 Stephen K. Doorn,3 and YuHuang Wang1,4,5,*
SUMMARY
The Bigger Picture
Organic color centers are an emergent class of quantum emitters that hold vast potential for applications in bioimaging, chemical sensing, and quantum information processing. Here, we show that these synthetic color centers follow interesting structure-property relationships through comparative spectral studies of 14 purified single-walled carbon nanotube chiralities and 30 different functional groups that vary in electron-withdrawing capability and bonding configurations. The defect emission is tunable by as much as 400 meV in the near-infrared as a function of host structure and the chemical nature of the color centers. However, the emission energy is nearly free from chiral angle and family patterns of the nanotube host (although this strongly depends on the nanotube diameter), suggesting that a trapped exciton at the organic color centers to some degree electronically decouples from the one-dimensional semiconductor host. Our findings provide important insights for designing and controlling this new family of synthetic color centers.
Color centers, such as nitrogen vacancies in diamond, possess intriguing optical properties that have been intensively studied for imaging, sensing, lasing, and quantum information. However, most color centers occur as native defects, making tailored synthesis difficult. By covalently bonding functional groups to carbon nanotube semiconductors, we can synthetically create a whole new class of color centers that are organic and can be chemically tailored in highly predictable manners. These synthetic quantum emitters allow systematic tuning of the emission color in the near-infrared, a spectral regime that is important to biochemical imaging and quantum communications but had been largely unexplored. Unlike nitrogen vacancy centers in diamond, which is an electrical insulator, organic color centers are synthetically created in a semiconductor and electronically coupled to the host, opening the possibility for electrical addressing and chemical tailoring with molecular precision.
INTRODUCTION Organic color centers are an emerging class of synthetic defect emitters that can be chemically incorporated into the sidewall of a single-walled carbon nanotube (SWCNT) through covalently bonding functional groups to the semiconductor host.1–4 The introduced chemical defect locally modulates the electronic structure of the SWCNT host to enable trapping of excitons at the defect site, where they burst brightly as single photons.1,5 The defect-induced emission (E11) can be significantly brighter than the native photoluminescence (PL) (E11) of the nanotube and emits at the near-infrared (NIR),1 suggesting vast potential for applications in imaging, sensing, and creating single photon sources at telecom bands.3,6–8 The electronic and optical properties of organic color centers vary substantially with the structure of the SWCNT host, each of which is assigned a specific (n,m) chirality that describes its construction as a rolled-up graphene sheet. On the other hand, there are increasingly large numbers of functional groups that are being identified capable of creating organic color centers, suggesting virtually unlimited opportunities in broadly applying organic chemistry in this emergent field.1,2,4,9–13 The unfunctionalized SWCNT hosts are quasi-one-dimensional nanostructures with highly predictable electronic structure and optical properties.14 Particularly, the excitonic transition energies of semiconducting SWCNTs show a strong dependence on both diameter and chiral angle, as summarized in correlation plots known as the Kataura plots.15,16 In contrast, systematic analysis of the structure-dependence of organic color centers has yet to be established. It is important to understand how the defect state is related to the nanotube family pattern and chirality, and how
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1
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the one-dimensional SWCNT couples to the zero-dimensional quantum state of the organic color centers, in order to provide a predicative understanding that will enable the design and synthesis of this family of quantum emitters for bioimaging, biosensing, quantum computing, and nanophotonics, as well as probing the rapidly unraveled fundamentally new phenomena of defect chemistry and physics. Even simply knowing the energies of these defect states for specific (n,m) structures and organic color centers would be important not only for fundamental photophysics of defect-trapped excitons but also for applications in infrared imaging, chemical sensing, and tailored design and synthesis of quantum materials. Herein, we aim at establishing this structure-property relationship for organic color centers through controlled synthesis and comparative spectral studies of 30 chemically distinct organic color centers and 14 purified nanotube hosts. We observed distinct defect-induced PL features for 14 semiconducting SWCNT species (ranging from 0.62 to 0.94 nm in diameter and from 0 to 27.5 in chiral angle) using 30 different functional groups. Based on the spectrofluorometric measurements of perfluorohexyl defect-tailored (n,m)-SWCNTs, we analyzed the E11 as empirically fitted functions of nanotube diameter. Our results show that the measured E11 PL energy is tunable by 400 meV in the NIR as a function of SWCNT diameter. However, the emission energy of defect-trapped excitons is nearly free from chiral angle and family patterns, suggesting that an exciton at an organic color center to some degree behaves independently from the nanotube host but this color center-host coupling is chemically tunable depending on the chemical nature of the defect in terms of the group’s electron-withdrawing ability and bonding configuration. These findings provide a comprehensive picture of the structure-property relationships of organic color centers that is required to guide controlled and tailored synthesis of this new family of quantum emitters.
RESULTS AND DISCUSSION Incorporation of Organic Color Centers in SWCNTs We covalently attached functional groups into a series of (n,m)-SWCNTs to study the correlation of the photon energy emitted from the resulting organic color centers with the structure of the SWCNT host (Figure 1). In this work, we sorted 14 types of semiconducting SWCNT species into samples highly enriched in single chiralities using polymer aqueous two-phase (ATP) extraction17 or gel chromatography18 (Figure S1). The sorted SWCNTs were stabilized in 1 wt/v % sodium dodecyl sulfate (SDS) in D2O for subsequent functionalization (see Experimental Procedures for detailed protocols). The semiconducting SWCNT structures studied in this work ranged from 0.62 to 0.94 nm in diameter and from 0 to 27.5 in chiral angle (Table S1). We then used diazonium1 or alkyl/aryl halide2,4 chemistry to covalently attach 30 different functional groups to the selected SWCNT chiralities (Table S2). The attached functional groups vary in electron-withdrawing capability and bonding configurations, enabling us to systematically modify the energy level of the defect state relative to the native electronic structure of the nanotube. The functionalized SWCNTs were characterized by UV-visible (vis)-NIR absorption and PL spectroscopy. The E11 and E11 wavelengths were determined by fitting the PL spectrum at E22 excitation using Voigt profiles. Figure 1 displays the absorption spectra and emission-excitation PL maps of three exemplary nanotube species: (5,4), (6,5), and (10,3). Incorporation of perfluorohexyl organic color centers produces a new E11 PL peak at a redshifted wavelength from the native E11 emission of the nanotube host. The E11 peak originates from the
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1Department
of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742, USA
2Materials
Science and Engineering Division, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD 20899, USA
3Center
for Integrated Nanotechnologies, Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
4Maryland
NanoCenter, University of Maryland, College Park, MD 20742, USA
5Lead
Contact
*Correspondence: [email protected] https://doi.org/10.1016/j.chempr.2018.06.013
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Figure 1. Organic Color Centers in Semiconducting SWCNT Hosts (A) Schematic representation of organic color centers incorporated as perfluorohexyl defects into a series of (n,m)-SWCNTs. An individual color center creates a host-structure-dependent potential well where a mobile exciton can be trapped and fluoresce brightly. (B) UV-vis-NIR absorption spectra of purified (5,4), (6,5), and (10,3) SWCNT solutions dispersed in 1 wt/v % SDS in D 2 O. (C) Excitation-emission PL maps of perfluorohexyl functionalized (5,4), (6,5), and (10,3) SWCNTs, from top to bottom.
radiative recombination of trapped excitons from fluorescent defect sites,1 as shown by the fact that both E11 and E11 PL peaks resonate with the E22 excitation of the SWCNT in excitation-emission maps of the functionalized samples (Figure 1). Although the attached functional groups (-C6F13) are the same, the emission wavelength of E11 was found to vary dramatically with the nanotube species, which implies a correlation with the host structures. We tabulated the emission wavelengths of E11 and E11 PL for the different nanotube chiralities studied and the energy difference between these two PL peaks (optical energy gap, DE = E11 E11) in Tables 1 and S3. The results provided the basis for an empirically determined energy plot of defect PL versus (n,m) chirality for semiconducting SWCNTs (Figures 2 and 3). Among the species investigated here, the longest wavelength of defect PL appears at 1,487 nm for perfluorohexyl functionalized (11,1) nanotube, (11,1)-SWCNT-C6F13. The shortest wavelength of defect PL occurs at 1,000 nm for (5,4)-SWCNT-3,5-C6H3(NO2)2. The largest DE value was 282 meV for (5,4)-SWCNT-C6H4NO2, while the smallest was 92 meV for (7,6)-SWCNT-C6H13. This wide emission wavelength range covers the biological transparency window19 and most of the telecommunication regime,3,8 and can be exploited for chemical sensing, bioimaging, quantum information, and a host of optoelectronic applications that require bright, high-quality light sources in the NIR wavelengths.3,6–8
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Table 1. Optical Characteristics of Selected Combinations of Nanotube Chirality and Organic Color Centers
>CF2 -3,5-C6H3(NO2)2
-4-C6H4NO2
(5,4)
(6,4)
(7,3)
(9,1)
(6,5)
(8,3)
-4-C6H4N(C2H5)2
-C6F13
-C6H13
1,052 nm
1,027 nm
–
282 meV
265 meV
211 meV
199 meV
1,066 nm
1,082 nm
1,049 nm
1,084 nm
1,053 nm
1,097 nm
247 meV
264 meV
228 meV
265 meV
232 meV
277 meV
185 meV
198 meV
171 meV
225 meV*
174 meV
208 meV
1,160 nm
1,190 nm
1,150 nm
1,177 nm
–
1,211 nm
172 meV
198 meV
156 meV
184 meV
206 meV
129 meV
148 meV
117 meV
138 meV
154 meV
1,115 nm
1,128 nm
1,086 nm
1,169 nm
240 meV
241 meV
202 meV
287 meV
180 meV
181 meV
151 meV
215 meV
1,145 nm
1,152 nm
1,105 nm
178 meV
190 meV
133 meV
142 meV
1,159 nm
1,169 nm
1,124 nm
229 meV
238 meV
195 meV
171 meV
178 meV
146 meV
160 meV*
150 meV
1,310 nm
–
–
–
–
–
–
1,000 nm
–
238 meV
1,025 nm 275 meV
178 meV
206 meV
–
–
1,160 nm
1,132 nm
1,168 nm
137 meV
190 meV
161 meV
200 meV
103 meV
143 meV*
121 meV
150 meV
1,171 nm
1,127 nm
–
237 meV
200 meV
137 meV (9,2)
123 meV
(7,5)
(8,4)
1,189 nm
1,206 nm
1,174 nm
1,192 nm
1,164 nm
167 meV
173 meV
151 meV
162 meV
136 meV
152 meV
156 meV
130 meV
146 meV
122 meV
1,276 nm
1,284 nm
1,228 nm
1,282 nm
1,242 nm
141 meV
149 meV
102 meV
146 meV
114 meV
127 meV
134 meV
92 meV
132 meV*
103 meV
–
–
–
1,225 nm
–
–
–
–
160 meV 144 meV
(11,0)
(7,6)
1,281 nm
1,291 nm
1,228 nm
1,281 nm
1,227 nm
135 meV
134 meV
92 meV
124 meV
92 meV
121 meV
120 meV
83 meV
112 meV
83 meV (Continued on next page)
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Table 1. Continued
>CF2 -3,5-C6H3(NO2)2
-4-C6H4NO2 –
-4-C6H4N(C2H5)2
-C6F13
-C6H13
1,270 nm
–
–
–
–
–
–
–
–
–
–
–
–
137 meV 123 meV
(9,4) 1,486 nm
(11,1)
(10,3)
137 meV
137 meV
123 meV
123 meV
1,455 nm
1,445 nm
127 meV
126 meV
114 meV
113 meV
Listed are the emitting wavelength (E11, top), the optical shift from that of the native exciton (DE, middle), and the thermal trapping potential of the E11 exciton (Etrap, bottom). Note that the PL spectra were obtained from SWCNTs in 1% SDS-D2O. Changes in the dispersion media will cause deviations in the emission wavelengths from the reported data due to dielectric screening effects.20 Also note that some Etrap values are derived from linear extrapolation of experimentally measured series (asterisks).
Structure-Property Relationships of the Defect PL Emission Each SWCNT structure can be uniquely indexed by a pair of (n,m) integers to indicate the chirality of the nanotube based on the roll-up vector of a graphene sheet (Figure 2). The direction of rolling determines not only the physical structure, such as diameter and chiral angle,21 but also the electronic band structure of the nanotube.14,15,22 Specifically, since an SWCNT imposes boundary conditions on the electron wave function in the direction of rolling, for semiconducting (n,m) SWCNTs, dividing (n m) by 3 leaves a remainder of 1 or 2, which is classified as mod (n m, 3) = 1 or mod (n m, 3) = 2, respectively. If (n m) is evenly divisible by 3, the SWCNT is metallic and therefore does not emit PL. Covalent functionalization may break the intrinsic symmetry of the SWCNT by converting sp2 carbon to sp3, which modifies the energy level of the functionalized nanotube locally at the site of the defect.1,2 At this local defect state, the optical properties, including the emission energy and the size of the exciton,23 are different from those of the intrinsic E11 excitons. Hence, we analyzed the structure-dependent properties of defect-trapped excitons in (n,m)-SWCNTs with perfluorohexyl, 4-nitroaryl, and 3,5-dinitroaryl defects, and mapped out the structure dependence of the E11 emission wavelength onto the graphene lattice (Figures 2 and S2). The results show in general that the E11 for larger SWCNT diameters emits at longer wavelength, but it also depends on the chiral angle of the nanotube. We observed a positive correlation between E11 and E11 emission energies for the studied chiralities with the same perfluorohexyl functional groups (Figure 3A), indicating that the defect state is closely related to the E11 excitonic state. The energy difference between E11 and E11 (DE values) were examined for dependence on diameter (d),
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Figure 2. Tunable Defect Photoluminescence as a Function of Host Chirality Experimentally determined E 11 wavelength of (A) (n,m)-SWCNT-C 6 H4 NO2 , (B) (n,m)-SWCNT-C 6 F 13 , and (C) (n,m)-SWCNT-C 6 H3 (NO2 ) 2 on a graphene sheet showing (n,m) lattice points. The color bar shows the emission wavelength of the defect PL. A lattice with diagonal stripes represents non-emitting metallic SWCNTs, in which (n – m)/3 is equal to an integer. A filled lattice indicates a semiconducting SWCNT chirality studied in this work. The blue arrow and q in (A) represent the chiral vector and chiral angle of (6,2)-SWCNTs as a demonstration of SWCNT chirality. The molecular structure of the organic color center is specified in each part of the figure.
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Figure 3. Semiconductor Host-Color Center Relationship Closed and open circles indicate mod 1 and mod 2, respectively. (A) Positive correlation between native (E 11 ) and defect-induced PL (E 11 ) wavelengths. The solid line is a quadratic function drawn to guide the eye. (B) Diameter dependence of DE. The solid line is an empirical fitting to the inverse second order of the diameter (Equation 1). (C) Curvature chirality effect on DE. The dashed line indicates metallic armchair SWCNTs with q = 30 . Each point represents different (n,m) species that are covalently functionalized with perfluorohexyl defects.
chiral angle (q), and mod (n – m, 3). We explored the diameter dependence by fitting DE with the inverse second-order equation of diameter (Equation 1): DE=meV = 11:3 d2 + 253d1 158;
(Equation 1)
in which d is the SWCNT diameter in nanometers. Figure 3B shows in general an inverse correlation between DE and diameter. However, we also observe 28% deviation of DE on average from Equation 1 for the range of diameters studied, suggesting diameter alone cannot account for DE variation, thus the chirality effect should be considered. Although the DE versus chirality plot displays no obvious pattern (Figure 3C), we found that the deviation of DE is mod dependent with higher DE values for structures of mod (n m, 3) = 2 than for mod (n m, 3) = 1. This mod-dependent deviation of DE is reminiscent of what was reported for the E11 energy and diameter correlation.14,16 The mod-dependent deviation represents the degree of electronic decoupling and is discussed in the next section. Even though the empirical fitting provides an estimate of the defect PL energy on the ensemble level, the PL energy of an individual quantum defect can vary due to the bonding configuration, as predicted by Kwon et al.2 and Gifford et al.24 Exciton Trapping Potential at Organic Color Centers The exciton trapping potential (Etrap) is the minimum energy required to detrap the E11 exciton from the defect trap to restore as a free E11 exciton. It is an important parameter to understand the structure-related properties of excitons, as well as to predict the PL stability. As established in our previous work,25 the trapping potential of a defect state can be experimentally determined by monitoring the E11 and E11 PL as a function of temperature or calculated from the difference between the optical energy gap (DE) and the reorganization energy (l). Reorganization occurs due to deformation of the nanotube geometry upon exciton trapping at the defect site25 and is related to the exciton localization at the defect site,26,27 with greater localization presumably leading to a larger reorganization energy. When the density of defects increases, the exciton wave function may be delocalized across multiple defects, leading to weaker localization of excitons at the defect site and smaller reorganization energy. For the series of perfluorohexyl defect-tailored SWCNTs, we also experimentally derived a larger l in smaller diameter SWCNTs (d < 0.84 nm, Figure S3). Such greater spatial localization of the wave
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Figure 4. Trapping Potential of E11 Exciton at Organic Color Centers (A) Diameter dependence of the trapping potential of E 11 excitons in (n,m)-SWCNT-C6 F 13 . (B) Diameter dependence of E 11 (gray) and E 11 (blue) emission energies. The labels categorize the (2n + m) families. Filled and open circles mark mod 1 and 2 SWCNT structures, respectively.
function effectively increases the amount of exciton-phonon coupling and thus increases reorganization energy in small-diameter SWCNTs. By linear extrapolation from the experimentally determined series (Table 1), we derived that the trapping potential of perfluorohexyl defect-trapped excitons ranges from 113 to 200 meV, as shown in Figure 4A. Our results suggest that the trapping potential is related to the size of the trapped exciton (electron-hole separation). For a larger trapping potential, the wave function of the exciton, which can be computed using density functional theory,25 is more spatially localized at the defect site.23,25,28 This is congruent with our previous theoretical prediction that an E11 exciton of a (6,5)-SWCNT is squeezed by 17% in size when trapped at an aryl defect.25 The inverse correlation between Etrap and diameter (regardless of l values) implies that, in smaller diameter SWCNTs, a defect trap can effectively reduce the exciton size and increase the oscillator strength of the defect state.28 Strong localization at a deep trap may enhance the binding energy of the E11 exciton, and improves the stability of the defect-trapped excitons. This may be related to the diameter-dependent quantum yield enhancement1 and PL stability of E11 excitons in single photon emission.8,29 Although the photon conversion efficiency is an important parameter for many potential applications of organic color centers, it remains a challenging and labor-intensive task,1 which warrants the development of more efficient techniques to quantify this value. Extrapolating beyond d > 0.94 nm (the largest diameter studied here), our empirical fitting of Etrap (solid line in Figure 4A) predicts that sp3 defects created by covalent functionalization create shallow traps in large-diameter SWCNTs (e.g., few millielectronvolts for d > 1.3 nm), making trapping and radiative recombination of E11 excitons less efficient. Another interesting finding is that only a weak chirality dependence was observed for the E11 fitting. Figure 4B shows the empirically fitted emission energy of the E11 and E11 exciton as a function of diameter for 12 SWCNT chiralities that are tailored with perfluorohexyl organic color centers. There is a clear inverse correlation between nanotube diameter and exciton emission energy. The deviation from the diameter fitting is due to the chiral angle dependence. This dependence becomes more apparent if we group the same nanotube families (2n + m) with a solid line, indicating the set of SWCNTs with similar diameters but different chiral
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Figure 5. Trapping Potential of E11 Excitons at Organic Color Centers in (6,5)-SWCNT Host (A) The inductive and resonance effects of terminating aryl moieties on E trap . (B) Inductive effects of alkyl chains on E trap . The solid lines in (A) and (B) are linear fitting of the correlations. (C) Divalent methyl (square), aminoaryl (circle), aryl (triangle), and perfluoromethyl (diamond) defects have larger E trap compared with their monovalent counterparts. The dashed line is E trap (monovalent) = E trap (divalent) drawn to guide the eye.
angles.15,16,22 It is also clear that the curvature effects in E11 are less significant than those in E11. These two trends may be understood by examining the electronic structure of the nanotube host and the molecular nature of the defects. The E11 energy levels influence the energy levels of the defect states because the defect state originates from the splitting of the doubly degenerate frontier orbitals of the SWCNT host1,23 and thus exhibits some degree of chiral angle dependence that arises from the trigonal wrapping effect inherent in the host.22 Meanwhile, once a mobile exciton is trapped at an sp3 defect, the trapped exciton manifests photophysics that is different from the E11 exciton, which is governed by the sp2 symmetry of the SWCNT lattice. Thus, our data suggest that, as the defect trap becomes deeper, the exciton localization is stronger, and the optical and electronic properties of the trapped excitons become closer to an isolated zerodimensional system with weaker dependence on the chiral angle of the nanotube host. Lastly, Figure 5 demonstrates that the trapping potential and emission energies of defect-trapped excitons are highly tunable depending on the chemical nature of the defects. Here we specifically studied (6,5)-SWCNT because it is easier to prepare high-purity samples compared with other chiralities. The spectral characterization of all defect-tailored (6,5)-SWCNTs studied in this work is available in Figure S4. We note that this correlation between the chemical natures of the defect and DE is observed for other chiralities as well (Table 1). Due to the wide choice of defects (30 different functional groups), the defect PL of (6,5)-SWCNTs can be broadly tuned over the emission wavelength of 1,094–1,164 nm. We found that, in general, Etrap, as well as the E11 wavelength and DE, increased as the electron-withdrawing ability of the defects became stronger. The electronwithdrawing ability can be quantified using Hammett constants for aryl defects30 and Taft constants for alkyl defects.31 From the study presented here, we show that the chemical nature of defects has a significant impact on the organic color centers, as manifested in Etrap, which follows a linear correlation with both Hammett (s) (Figure 5A) and Taft constants (s*) (Figure 5B). Combining the results of Figures 4 and 5, we derived empirical prediction models of DE for monovalent aryland alkyl-defect-tailored SWCNTs. For aryl-defect functionalized SWCNTs, DE 17:3 235:5 559 = 146:3 + Ds 613:2 : (Equation 2) + 2 meV ðd=nmÞ d=nm d=nm
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For alkyl-defect functionalized SWCNTs, DE 11:3 253 248:6 = 158 + Ds ; 205:1 + meV ðd=nmÞ2 d=nm d=nm
(Equation 3)
in which Ds is s 0.788 and Ds* is s* 4.87. Due to the chiral angle dependence of the E11 and E11 PL, the chiral angle dependence generates, on average, 9% deviation from the experimentally determined values from the diameter fitting. Lastly, we note that bonding configuration also influences Etrap and E11 PL (Figure 5C). Divalent defects, including >CF2, >CH2, >C6H4, and >C6H3NH2, tend to create deeper defect potentials and increase the exciton trapping potentials compared with their monovalent counterparts (-CF3, -CH3, -C6H5, and -C6H4NH2). In conclusion, we have synthesized a series of organic color centers in semiconducting SWCNTs and determined the diameter and chirality dependence of PL emission wavelengths and the thermal trapping potential for defect-trapped excitons. Similar to the native exciton (E11) in unfunctionalized SWCNTs, the E11 PL is strongly correlated with nanotube diameter. The emission energy of defect-trapped excitons, however, is largely free from chiral angle and family patterns of the semiconductor host, suggesting that an exciton at an organic color center to some degree decouples from the one-dimensional nanotube host. Our work establishes the structure-property relationships for organic color centers, which may help guide the controlled and tailored synthesis of this new family of quantum emitters for applications in bioimaging, quantum information, and optoelectronics in general.
EXPERIMENTAL PROCEDURES A detailed description of the synthesis protocols can be found in the Supplemental Experimental Procedures. High-Purity SWCNT Sorting We sorted single-chirality SWCNTs, including (11,1), (10,3), (11,0), (7,5), (6,5), (9,1), (6,4), and (5,4), from CoMoCAT SG65i or SG76 SWCNTs (Southwest Nanotechnologies) using polymer ATP separation.5,17 For the ATP separation process using single-stranded DNA,17 the sorted SWCNTs were precipitated from DNA solution by sodium thiocyanate (Sigma-Aldrich, 98%) and re-dispersed in 1 wt/v % sodium deoxycholate (DOC, Sigma-Aldrich, R97%). In other ATP experiments,5 we pressure filtrated the SWCNT-polymer mixture (Amicon, number 5123, using 100 kDa ULTRAcel regenerated cellulose filter membranes) with 1 wt/v % DOC to dilute the polymer concentration by a factor of 103. Alternatively, gel chromatography was used to sort (7,6) + (8,4), (8,3) + (8,4), (8,3) + (7,3), and (6,5)-SWCNT enriched samples from HiPco SWCNTs (Rice University, batch #194.3) as described previously by Liu et al.18 The purified SWCNTs were then suspended in 1 wt/v % SDS (Sigma-Aldrich, R98.5%) in D2O (Cambridge Isotope Laboratories, 99.8%) for subsequent functionalization. Covalent Functionalization of Organic Color Centers The optical densities of the SWCNT solutions were adjusted to 0.03–0.12 at the E11 transition for subsequent optical studies. Monovalent aryl defects were introduced by diazonium chemistry.1,11 For some aryl defects that cannot be incorporated using diazonium reactions, such as divalent aryl and aminoaryl defects, light activated arylation was used.2,4 A series of alkyl defects were introduced using alkyl halides
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as reported by Kwon et al.2 Covalent functionalization was monitored by defect PL evolution at E22 excitation light, as described in the following section. Spectroscopic Characterization The SWCNT PL was characterized with a NanoLog spectrofluorometer (Horiba Jobin Yvon) using a liquid-N2-cooled InGaAs array. The SWCNTs were excited with monochromator-selected light (10 nm slit width) from a 450 W xenon arc lamp. The excitation power was lower than 10 mW with an integration time of 1–10 s. The spectral resolution was 10 nm for the emission detection channel. UV-vis-NIR absorption spectra were obtained with a spectrophotometer equipped with a broadband InGaAs detector (Lambda 1050, PerkinElmer). The path length of absorption measurements was 10 mm.
SUPPLEMENTAL INFORMATION Supplemental Information includes Supplemental Experimental Procedures, four figures, and three tables and can be found with this article online at https://doi. org/10.1016/j.chempr.2018.06.013.
ACKNOWLEDGMENTS This work was supported by the NSF (grant number CHE-1507974) and also partially by the NIH NIGMS (grant number R01GM114167). Part of this work was performed at the Center for Integrated Nanotechnologies, US Department of Energy, Office of Science user facility through user project 2017AC0164. S.K.D., X.H., and N.F.H. acknowledge support from the LANL Laboratory Directed Research and Development program.
AUTHOR CONTRIBUTIONS Conceptualization, Y.W. and M.K.; Investigation, M.K. and H.K.; Resources: G.A., X.H., and N.F.H.; Writing, M.K., X.W., and Y.W.; Funding Acquisition, Y.W.; Supervision, M.Z., S.K.D., and Y.W.
DECLARATION OF INTERESTS The authors declare no competing interests. Received: March 8, 2018 Revised: June 18, 2018 Accepted: June 25, 2018 Published: July 19, 2018
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