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[Daichi Kozawa](https://orcid.org/0000-0002-0629-5589), Yuto Shiota, Mengyue Wang, [Yuichiro K. Kato](https://orcid.org/0000-0002-9942-1459)

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[Deterministic Formation of Single Organic Color Centers in Single-Walled Carbon Nanotubes](https://mdr.nims.go.jp/datasets/adad6488-9696-41af-912d-6600e7dbd4fa)

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Deterministic Formation of Single Organic Color Centers in Single-Walled Carbon NanotubesDeterministic Formation of Single Organic Color Centers in Single-Walled Carbon NanotubesDaichi Kozawa,* Yuto Shiota, Mengyue Wang, and Yuichiro K. Kato*Cite This: Nano Lett. 2025, 25, 13103−13109 Read OnlineACCESS Metrics & More Article Recommendations *sı Supporting InformationABSTRACT: Quantum light sources using single-walled carbon nanotubes showpromise for quantum technologies but face challenges in achieving precise controlover color center formation. Here, we present a novel technique for deterministiccreation of single organic color centers in carbon nanotubes using in situphotochemical reaction. By monitoring discrete intensity changes in photo-luminescence spectra, we achieve precise control over the formation of individualcolor centers. Furthermore, our method allows for position-controlled formationof color centers as validated through photoluminescence imaging. We alsodemonstrate photon antibunching from a color center, confirming the quantumnature of the defects formed. This technique represents a significant step forward in the precise engineering of atomically definedquantum emitters in carbon nanotubes, facilitating their integration into advanced quantum photonic devices and systems.KEYWORDS: single-walled carbon nanotubes, color centers, quantum emitters, photon antibunching, photoluminescenceQ uantum light sources capable of emitting single photonson demand are essential for emerging quantumtechnologies,1 enabling advances in communication,2 comput-ing,3 and sensing applications.4 Notable developments towardthese applications include single-photon emission demonstra-ted in various solid-state systems such as nitrogen-vacancycenters in diamond,5 semiconductor quantum dots,6 anddefects in two-dimensional materials.7−9 Particularly desirableare quantum light sources that operate at room temperatureand within the telecom wavelength range,10,11 while materiallimitations and emission efficiency constraints need to beconsidered.Single-walled carbon nanotubes (SWNTs) have emerged asa promising platform for quantum light sources,11 owing totheir unique one-dimensional structure,12 exceptional opticalproperties,13,14 and compatibility with telecom wave-lengths.10,15−17 The introduction of organic color centers inSWNTs18 has further enabled single-photon emission at roomtemperature10 with large tunability of the emission energy.19Nevertheless, one of the critical challenges is the deterministiccontrol over the formation of these quantum defects,particularly the number of defects introduced and their spatialpositioning within single carbon nanotubes.20,21 Existingmethods often lack the capability or the precision required,limiting their potential for scalable quantum photonicapplications.22In this work, we develop a technique for the deterministiccreation of single organic color centers using in situphotochemical reaction. As nanotubes are functionalized,discrete photoluminescence (PL) intensity changes corre-sponding to the formation of individual color centers areobserved. By stopping the reaction upon detecting the discreteincrease, we are able to deterministically create single colorcenters. Statistical analysis of PL spectra from individual colorcenters is conducted to obtain quantitative insight into thedistribution of color center types. In addition, this techniqueallows for position-controlled formation of color centers whichis validated by excitation PL imaging. Furthermore, we observephoton antibunching from a color center by performing aphoton correlation measurement, showing that single quantumdefects can be formed using this technique.Air-suspended SWNTs are grown across trenches on Sisubstrates by chemical vapor deposition (CVD),11,17,23 withthe nanotube density carefully controlled via growthparameters to allow for single tube measurements. Thesubstrate is then placed inside a sealed reaction cell, where adroplet of iodobenzene is deposited next to the substrate andleft for 10 min to saturate the chamber with vapor. The cell issubsequently mounted on a motorized three-dimensionalfeedback stage, which enables precise spatial targeting ofindividual SWNTs. Local functionalization is performed via avapor-phase photochemical reaction24 by focusing an ultra-violet (UV) laser through a quartz window onto a selectedSWNT (Figure 1a; see Methods and Supporting Information,Section 1, for details). Initially, the UV laser is blocked by ashutter, and PL spectra are acquired over a time period of 5 sReceived: April 28, 2025Revised: August 17, 2025Accepted: August 18, 2025Published: August 21, 2025Letterpubs.acs.org/NanoLett© 2025 The Authors. Published byAmerican Chemical Society13103https://doi.org/10.1021/acs.nanolett.5c02378Nano Lett. 2025, 25, 13103−13109This article is licensed under CC-BY 4.0Downloaded via NATL INST FOR MATLS SCIENCE (NIMS) on September 9, 2025 at 00:56:46 (UTC).See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Daichi+Kozawa"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Yuto+Shiota"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Mengyue+Wang"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Yuichiro+K.+Kato"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/showCitFormats?doi=10.1021/acs.nanolett.5c02378&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c02378?ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c02378?goto=articleMetrics&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c02378?goto=recommendations&?ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c02378?goto=supporting-info&ref=pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.5c02378/suppl_file/nl5c02378_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.5c02378/suppl_file/nl5c02378_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c02378?fig=tgr1&ref=pdfhttps://pubs.acs.org/toc/nalefd/25/35?ref=pdfhttps://pubs.acs.org/toc/nalefd/25/35?ref=pdfhttps://pubs.acs.org/toc/nalefd/25/35?ref=pdfhttps://pubs.acs.org/toc/nalefd/25/35?ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://doi.org/10.1021/acs.nanolett.5c02378?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://pubs.acs.org/NanoLett?ref=pdfhttps://pubs.acs.org/NanoLett?ref=pdfhttps://acsopenscience.org/researchers/open-access/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/using Ti:sapphire laser excitation. At t = 0 s, the shutter isopened and UV irradiation begins while PL spectra continue tobe recorded.During the photochemical reaction of a (9,7) SWNT, PLspectra show temporal changes (Figure 1b). The pristinenanotube initially exhibits only E11 emission, while anadditional peak labeled E11 emerges at t = 190 s, indicatingthat functionalization has occurred. Spectral diffusion orbroadening of the emission peaks is insignificant in mostnanotubes during the reaction (Figure S1), enabling us tofocus on the emission intensity for further analysis. We extracttemporal profiles of the emission intensity by spectrallyintegrating the PL spectra of each relevant peak in the timetrace map (Figure 1c; see Methods for details). Notably, weobserve discrete intensity steps in both E11 and E11 emission,which can be attributed to the formation of individual organiccolor centers. For the E11 emission, most of these discrete stepscorrespond to intensity reductions, likely caused by theintroduction of color centers or quenching sites. While manyof the intensity steps occur synchronously in both E11 and E11,Figure 1. (a) A schematic of an experimental setup for the in situ functionalization. (b) A time-trace map of PL spectra for a (9,7) SWNT excitedwith 1.59 eV and 100 μW. (c) Temporal profiles of emission peaks E11 and E11 extracted from the time trace (b) by spectrally integrating theintensity with a bin width of 20 meV at each time point. The solid lines are fits by the Gaussian mixture model. The shutter for the UV laser is openafter t = 0 s in panels (b, c).Figure 2. (a) A flowchart of the algorithm for a single color center formation. (b) Spectral difference before and after reaction of a (9,7) SWNT.(c) Peak probability densities of the E11 intensity before and after the reaction. The black dotted lines are sum of the Gaussian fit with eq 1, theorange lines are individual Gaussian components, and the vertical gray broken lines indicates 1σ of the intensity distribution before the reaction.The functionalization is conducted with a UV laser power of 5 nW, while PL spectra are taken with an excitation energy of 1.59 eV and a power of100 μW. The PL intensity is obtained by computing the mean intensity within a 12 meV window centered at each emission peak.Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.5c02378Nano Lett. 2025, 25, 13103−1310913104https://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.5c02378/suppl_file/nl5c02378_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c02378?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c02378?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c02378?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c02378?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c02378?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c02378?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c02378?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c02378?fig=fig2&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.5c02378?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-assome steps in the E11 intensity occur without changes in theE11 intensity, suggesting the formation of quenching sitesindependent of organic color center formation.The time traces can be well reproduced by the Gaussianmixture model (GMM),25 which provides a statistical frame-work for distinguishing discrete emissive states. We combineGMM with the Akaike information criterion to minimizeoverfitting while ensuring an accurate representation of thedata, allowing for an objective determination of the optimalnumber of formed color centers. The GMM validates thatthese steps are not random intensity fluctuations but discrete,quantized transitions, suggesting the individual creation ofcolor centers.Using the discrete intensity steps observed in the PL spectra,we can control the photochemical reaction26 to reliably formsingle color centers. Figure 2a summarizes the algorithmemployed for this process, in which we first acquire a PLspectrum before initiating the reaction. After we start UVirradiation, the reaction is closely monitored in real time byrepeatedly acquiring PL spectra every 0.5 s. In order to detectthe emergence of color center emission, we compute thedifference of the real time spectrum and the one taken beforeUV irradiation. The mean of the difference intensity iscomputed within an energy window of 12 meV below E11,and a predefined threshold given by the root-mean-squareintensity σ of all the spectra before the reaction is used as acriterion to stop UV irradiation. This procedure is adopted forall subsequent preparation of color centers. In Figure 2b, atypical spectral difference before and after the reactionhighlights the appearance of an emission peak, which clearlyindicates color center formation. The calculation of thedifference can cancel out the contribution of a phononsideband around E11 which is assigned to the out-of-planetransverse optical/out-of-plane transverse acoustic phononmodes at the K point.14To characterize the effectiveness in creating color centers,we analyze the statistical significance of the intensity change in274 PL spectra of individual functionalized (9,7) SWNTs. Asshown in Figure 2c, the peak probability density of the E11emission intensity demonstrates a significant shift beyond 1.5σbefore the reaction. The histogram after the reaction shows along tail toward high intensities, which may indicate thepresence of multiple color centers. We therefore fit the PLintensity distribution using a multiple Gaussian function togain insight into the formation of color centers. Thedistribution of the PL intensity I is well described by aprobability density functionP I aI j( ) exp( )2jnj022ikjjjjjy{zzzzz== (1)where n is the maximum number of color centers considered,aj represents the peak probability density for j color centerscreated, and μ is the mean intensity of a single color center.The prereaction distribution is well described by n = 0, whilethe postreaction distribution requires n = 4 for optimal fitting.When fitting the postreaction PL intensity distribution, we usethe σ value obtained from the prereaction fit, assumingdetector noise is the dominant source of the broadening. Thefitted distribution reveals evenly spaced intensity clusters,which can be explained by the formation of j = 0 through 4color centers.The distribution obtained from the experiments shows noevidence of higher probabilities for multiple defect forma-tion.27−29 This suggests that color center formation is notinfluenced by the presence of other color centers. If the initialradical attachment to the nanotube wall alters the localelectronic and chemical environment, the reactivity of adjacentcarbon atoms may be enhanced to favor successive formation.Direct structural characterization using scanning transmissionelectron microscopy or scanning tunneling microscopy wouldprovide deeper understanding of the color center formationprocess.The fraction of spectra with j > 1 is relatively small, beingless than 23% as observed in the intensity histogram (Figure2c). The formation of multiple color centers can be furtherminimized by lowering the reaction rate, which will providesufficient time to stop the reaction before multiple colorcenters are formed. Additionally, the probability for j = 0 canbe suppressed by increasing the integration time of the PLspectra to reduce σ. Our in situ reaction and monitoringtechnique therefore allows for deterministic formation of singlecolor centers on demand.We now proceed to analyze the spectra acquired after theformation of the color centers and examine the statisticaldistribution of the emission energies to obtain insight into thedefect types. Typical PL spectra shown in Figure 3a highlightthe emission features from two distinct defect types E11 andE11*. The difference in the emission energy has beeninterpreted to arise from the binding configurations ofFigure 3. Statistical distribution of the emission energies after thefunctionalization of (9,7) SWNTs. (a) PL spectra displaying E11 andE11* defect emission, where the spectrum with E11 is verticallydisplaced. (b) Histograms of the peak center obtained with a fit by theLorentz function. The functionalization is conducted with a UV laserpower of 5 nW, while PL spectra are taken with an excitation energyof 1.59 eV and a power of 100 μW. The bin width of the histogramsare determined by Freedman Diaconis estimator. For analysis ofreacted nanotubes, only spectra with an E11 peak area of at least 60eV·counts/s and a line width between 10 and 80 meV are used.Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.5c02378Nano Lett. 2025, 25, 13103−1310913105https://pubs.acs.org/doi/10.1021/acs.nanolett.5c02378?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c02378?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c02378?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c02378?fig=fig3&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.5c02378?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asfunctional groups,30−32 where monovalent aryl functionaliza-tion can yield six distinct aryl-H binding configurations relativeto the SWNT lattice. Due to the chiral structure of carbonnanotubes, each binding configuration possesses a differentelectronic structure and corresponding emission energy. We fitall collected PL spectra by a double Lorentzian function wherethe emission peak with an energy lower than E11 is assigned tocolor center emission. Histograms of the emission energiesshow a distribution corresponding to the different defect types(Figure 3b). The main peak in the distribution underscores thereproducibility of defect formation and indicates preferentialpathways in the photochemical reaction to form E11 colorcenters. Furthermore, selecting the emission energy of colorcenters can be extended by using different chiralities, coveringthe telecom C and O bands (Figure S3).In addition to controlling the number of color centers, wedemonstrate position-controlled formation in individualSWNTs. By targeting specific regions of the nanotubes, weperform localized functionalization as depicted schematically inFigure 4a−c where the UV laser is focused at the top, middle,and bottom regions of the tubes. Locations of color centers arecharacterized by excitation imaging measurements where wescan over the nanotubes to excite and collect PL spectra.Intensity maps for the color center peaks (Figure 4d−f) arethen constructed by spectrally integrating the intensity withinenergy windows at the color center emission peaks (Figure4g−i). The images provide visual evidence of the position-controlled color center formation. The resolution of this spatialcontrol is determined by the focused UV beam diameter of 1.6μm. While the microscope stage offers 50 nm positioningaccuracy, the practical spatial resolution is limited by the beamdiameter. This resolution nevertheless enables the positioningof multiple defects along individual nanotubes with micron-scale separation, which is sufficient for integration intoquantum photonic devices.We note that the size of the bright regions in the PL imagesdiffers from defect to defect, which does not necessarily implydifferences in defect numbers or distributions since theintensity maps are not emission images but are obtained byscanning the excitation laser. The bright region is mostlocalized in Figure 4d, which is attributed to formation ofquenching defects in proximity to the bright color center. Incomparison, a noticeable blur around the defect sites, visible inFigure 4e and f, suggests the influence of excitondiffusion.17,23,33 The difference in sharpness between theimages supports the interpretation that quenching defects andcolor centers are generated independently during the reaction.The observed spatial extent reflects exciton diffusion in thenanotube prior to recombination at color center sites, whereexcitons created by the laser can diffuse along the nanotubewith typical diffusion lengths of several hundred micrometersbefore being trapped and emitting at a color center.The microscopic chemical environment remains uniformacross air-suspended nanotube segments, ensuring consistentreaction probability regardless of position along the suspendedportion. While reaction times vary between individualnanotubes due to the stochastic nature of the photochemicalprocess, the probability of color center formation should notdiffer depending on position along a suspended segment, asthis uniform environment is maintained throughout the air-suspended portion. However, we expect that reactionprobability would depend on position when the UV spotoverlaps with trench edges where nanotubes contact thesubstrate, as UV light reflection and scattering at these edgesFigure 4. Spatial control over color center formation in SWNTs. (a−c) Schematics of air-suspended SWNTs where target positions are indicatedby red dots and trench edges are indicated by broken lines. (d−f) Excitation PL images of nanotubes functionalized at different positions where theimages (d−f) are obtained with the spectrally integrated intensity within 4-meV windows at the color center emission peaks indicated by arrows inPL spectra (g−i), respectively. The functionalization is conducted with a UV laser power of 5 nW for (g) 709.2, (h) 167.6, and (i) 89.0 s, while PLspectra are taken with an excitation energy of 1.59 eV and a power of 10 μW. The scale bars in the panels d−f are 1 μm.Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.5c02378Nano Lett. 2025, 25, 13103−1310913106https://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.5c02378/suppl_file/nl5c02378_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c02378?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c02378?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c02378?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c02378?fig=fig4&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.5c02378?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ascould modify the reaction rate. Our experiments avoid thisregime to ensure consistent reaction conditions.The capabilities for forming single color centers at desiredlocations provide an important step toward applications inquantum light sources.34 To this end, we perform a photoncorrelation measurement using a Hanbury-Brown-Twiss setupunder pulsed excitation which is a definitive test for single-photon emission, as it allows us to evaluate the photonantibunching behavior. Figure 5a shows PL spectra of a (11,3)SWNT with and without a long-pass filter, where the filterisolates the E11 color center emission from the E11 emission.The photon correlation data presented in Figure 5b reveal aclear antibunching behavior characterized by a second-ordercorrelation function value of g(2)(0) = 0.45, indicating asignificant suppression of multiphoton events compared torandom photon emission and confirming the single-photonnature of the emission.In summary, we have developed in situ photochemicalreaction technique that enables the deterministic formation ofsingle organic color centers in air-suspended SWNTs. Bymonitoring PL spectra in real time, we have observed discreteintensity changes that correspond to the formation ofindividual color centers. The introduction of the color centerscan then be precisely controlled by blocking UV irradiationupon the first detection of color center emission. Statisticalanalysis of PL spectra has revealed a preference for theformation of E11 emitters, and spatial control over defectplacement has been demonstrated via excitation PL imaging.Furthermore, the photon correlation measurement hasconfirmed single-photon emission, establishing the quantumnature of the defect. Our deterministic functionalizationtechnique offers the potential for on-demand use of quantumdefects with desired emission energies, while providingcapability for broader spectral coverage by the choice ofchirality24 and molecular precursors.19,35 This level of controlpaves the way for the development of atomically definedtechnology for scalable quantum photonic circuits, operating atroom temperature within the telecom band.■ METHODSFabrication of Air-Suspended Carbon Nanotubes.Our process for fabricating air-suspended single-walled carbonnanotubes (SWNTs) utilizes a combination of electron-beamlithography and dry etching techniques.17 We begin byemploying these methods to create trenches on siliconsubstrates. The trenches are approximately 1.0-μm deep andcan be up to 4.0-μm wide. Following this step, a secondaryelectron-beam lithography process is performed to define thecatalyst areas in the vicinity of these trenches. We then spin-coat these areas with a Fe-silica catalyst that has been dispersedin ethanol. Excess catalyst is removed through a lift-off process,ensuring that it remains only within the predefined areas.Synthesis of the SWNTs occurs over these trenches withalcohol CVD.11,23 The synthesis process is conducted under aflow of ethanol with a carrier gas mixture of argon andhydrogen at 800 °C for 1 min. The result is air-suspendedSWNTs positioned over the trenches, ready for furtherexperiments.Micro-PL Measurements. We conduct PL character-ization using a home-built scanning confocal microscope.17,23For these experiments, we use a continuous-wave Ti:sapphirelaser for excitation and a liquid-N2-cooled InGaAs photodiodearray attached to a 30 cm spectrometer for detection. The laserpolarization is maintained perpendicular to the trenches, andthe beam is focused using a reflective objective lens with anumerical aperture of 0.5 and a working distance of 7.8 mm.The 1/e2 diameter of the focused beam is 1.32 μm for anexcitation energy of 1.59 eV. This diameter is characterized byperforming PL line scans perpendicular to a suspended tube.The confocal pinhole defines the collection spot size, which isapproximately 5.5 μm in diameter. For excitation PL imaging,we scan over a SWNT to collect PL spectra and constructintensity maps for color center emission by spectrallyintegrating the intensities of each peak. All PL spectra aretaken at the center of the nanotubes except for thehyperspectral PL images. All measurements are carried out atroom temperature.Formation of Organic Color Centers. To functionalizethe air-suspended nanotubes with iodobenzene as a precursor,we use vapor-phase reaction.24 As-grown SWNTs on Sisubstrates are placed in a reaction cell with an inner volumeof 7.4 mL (Figure S2). We introduce 20 μL of iodobenzene(Fujifilm Wako Pure Chemical Corporation, ≥97% purity,used without further purification) to the bottom of the cellusing a micropipette, and the cell is sealed in air. Controlexperiments confirm that oxygen and water in air do not play amajor role in the reaction (Supporting Information, Section 2).After 10 min to allow the cell to fill with iodobenzene vapor,we perform the reaction by irradiating the SWNTs with a 4.09eV UV laser through the quartz window of the cell, which has athickness of 0.5 mm. The focused UV laser beam has a 1/e2diameter of 1.6 μm, as determined by the knife-edge method.This spot size represents a deviation from the diffraction limit,which can be attributed to the nonideal spatial beam profile ofour UV laser diode source. We control the UV laser exposureusing a motorized shutter with a closing time of 8.0 ms,allowing precise on−off switching during the photochemicalreaction. The UV laser beam is colinearly aligned with theFigure 5. (a) PL spectra of a functionalized (11,3) SWNT showing anE11 emission peak. The orange and blue spectra are taken with andwithout a long-pass filter having a cutoff energy of 0.992 eV,respectively. (b) Photon correlation showing g(2)(0) = 0.45 taken with200 nW, 1.59 eV excitation and a 2-h accumulation time at roomtemperature.Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.5c02378Nano Lett. 2025, 25, 13103−1310913107https://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.5c02378/suppl_file/nl5c02378_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.5c02378/suppl_file/nl5c02378_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c02378?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c02378?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c02378?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c02378?fig=fig5&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.5c02378?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asTi:sapphire laser beam, where the two beams are overlapped ata long-pass dichroic mirror just before the reflective objectivelens. The polarization of the UV laser is kept parallel to thetrenches. Following the reaction, we remove the samples fromthe cell and store them in dark for subsequent spectroscopiccharacterization.Analysis of PL Time Traces. In the analysis of PL spectraltime traces, the first step is to identify emission peaks byapplying a peak-finding algorithm to a variance spectrumderived from a time trace of PL spectra. Once identified, theintensity of each emission peak is spectrally integrated using abin width of 20 meV, and the resulting intensity is plotted as afunction of time to construct temporal profiles. The PLintensity profiles are then fitted using the Gaussian mixturemodel, with the number of intensity levels determined basedon the Akaike information criterion.Our intensity-based approach for counting individual defectformation is validated by both established methodology andthe unique advantages of air-suspended nanotubes thatminimize environmental perturbations. The interpretation ofdiscrete, stepwise intensity changes as individual defectformation events has been widely established in carbonnanotube studies,36,37 where the stepwise increase of E11emission intensity directly corresponds to the formation ofindividual color centers. Crucially, our air-suspended nanotubeconfiguration significantly minimizes environmental interac-tions compared to substrate-supported systems,23 reducingdielectric screening effects and substrate-induced perturbationsthat could complicate intensity interpretation. The high qualityof this system is evidenced by minimal spectral broadening andnegligible spectral diffusion during the reaction process (FigureS1), confirming that the discrete, quantized intensity steps inFigure 1c arise from individual defect formation rather thanenvironmental effects.Photon Correlation Measurement. The photon corre-lation measurement is conducted using a Hanbury-Brown-Twiss setup with a 50:50 fiber coupler under laser excitation of100 fs pulses from a Ti:sapphire laser operating at a repetitionrate of 76 MHz.38,39 The excitation laser beam is focused ontothe sample through a transmissive objective lens with anumerical aperture of 0.85. PL from the center of the nanotubeis coupled via single-mode fibers to superconducting single-photon detectors, and data collection is performed using atime-correlated single-photon counting module.■ ASSOCIATED CONTENT*sı Supporting InformationThe Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c02378.Time-resolved spectral analysis, gas-reaction cell design,chirality-dependent photoluminescence spectra, reactionscheme, and control experiments under inert atmos-phere (PDF)■ AUTHOR INFORMATIONCorresponding AuthorsDaichi Kozawa − Quantum Optoelectronics Research Team,RIKEN Center for Advanced Photonics, Wako, Saitama 351-0198, Japan; Nanoscale Quantum Photonics Laboratory,RIKEN Pioneering Research Institute, Wako, Saitama 351-0198, Japan; Research Center for MaterialsNanoarchitectonics, National Institute for Materials Science,Ibaraki 305-0044, Japan; orcid.org/0000-0002-0629-5589; Email: kozawa.daichi@nims.go.jpYuichiro K. Kato − Quantum Optoelectronics Research Team,RIKEN Center for Advanced Photonics, Wako, Saitama 351-0198, Japan; Nanoscale Quantum Photonics Laboratory,RIKEN Pioneering Research Institute, Wako, Saitama 351-0198, Japan; orcid.org/0000-0002-9942-1459;Email: yuichiro.kato@riken.jpAuthorsYuto Shiota − Nanoscale Quantum Photonics Laboratory,RIKEN Pioneering Research Institute, Wako, Saitama 351-0198, Japan; Department of Applied Physics and Physico-Informatics, Keio University, Yokohama, Kanagawa 223-8522, JapanMengyue Wang − Quantum Optoelectronics Research Team,RIKEN Center for Advanced Photonics, Wako, Saitama 351-0198, Japan; Nanoscale Quantum Photonics Laboratory,RIKEN Pioneering Research Institute, Wako, Saitama 351-0198, JapanComplete contact information is available at:https://pubs.acs.org/10.1021/acs.nanolett.5c02378NotesThe authors declare no competing financial interest.■ ACKNOWLEDGMENTSThis work was supported in part by JSPS KAKENHI GrantsJP24H01210, JP23K23161, JP25K21704, and JP23H00262;JST ASPIRE Grant JPMJAP2310; Canon Foundation;Mitsubishi Foundation; Murata Science and EducationFoundation; and MEXT ARIM Grant JPMXP1222UT1136.We thank the Advanced Manufacturing Support Team atRIKEN for their technical assistance.■ REFERENCES(1) Aharonovich, I.; Englund, D.; Toth, M. 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