Selective emergence of photoluminescence at telecommunication wavelengths from cyclic perfluoroalkylated carbon nanotubes

Single-walled carbon nanotubes (SWNTs), possessing a tubular graphene structure, exhibit high mechanical strength and unique electronic properties due to their one-dimensionally expanded cylindrical π-electron system. Their properties are dictated by structural parameters like diameter and chiral angle, represented by the chiral index (n,m). SWNTs with mod(n-m, 3) = 0 are metallic; otherwise, they are semiconducting. Since the discovery of near-infrared (NIR) photoluminescence (PL) in semiconducting SWNTs in 2002, their NIR PL properties have attracted significant attention for applications in bioimaging, sensors, and telecommunications. However, challenges remain in enhancing their low quantum yield and controlling the emission wavelength. Chemical functionalization offers a potential solution for tuning the band gap and inducing new PL peaks at longer wavelengths. Previous studies have explored strategies to control SWNT PL wavelength through functionalization, including control of binding configurations, electronic effects of addenda, and local strain. While some success has been achieved in shifting PL wavelengths, generating new peaks >1300 nm for telecommunication applications remains elusive. This study aims to address this gap by investigating the functionalization of SWNTs using 1,4-diiodooctafluorobutane, a reagent incorporating two reactive sites and fluorine atoms, to achieve a significant red-shift in PL emission, thereby enabling the development of advanced materials for optical communication.

Extensive research has focused on modifying the optical properties of SWNTs through chemical functionalization. Studies have demonstrated that the PL wavelength can be controlled by manipulating the binding configurations of the functional groups. Theoretical calculations suggest a strong correlation between binding configurations and local band gap modulation. The electronic effects of the addenda have also been shown to influence the PL wavelength, as evidenced by the control of PL within specific wavelength ranges using different substituents. Furthermore, the introduction of local strain via functionalization has been shown to affect the band gap and hence the PL properties. The use of reagents with multiple reactive sites, such as 1,n-dibromoalkane, has been reported to lead to the selective emergence of new PL peaks at longer wavelengths compared to monofunctionalization. However, achieving selective emergence of new PL peaks beyond 1300 nm from (6,5) SWNTs on a bulk scale remains a challenge.

The study employed 1,4-diiodooctafluorobutane (I(CF2)4I) and sodium naphthalenide for SWNT functionalization, expecting high efficiency due to the six-membered ring structure of the resulting cycloadducts. To study the effects of reactive sites and fluorine atoms, several 1-iodobutane derivatives were also used. The functionalized SWNTs were characterized using absorption and Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL) spectroscopy. The functionalisation degree was evaluated through the D/G band ratio in Raman spectroscopy. PL measurements were conducted in a 1 wt% sodium dodecylbenzenesulfonate (SDBS) D2O solution, with excitation wavelengths optimized for (6,5) SWNT excitation. Gel chromatography with a gradient HPLC system using three surfactant mixed solutions was employed for chiral separation of the functionalized SWNTs, enabling the assignment of the chiral index and analysis of PL properties for SWNTs with different chiral angles. Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD-DFT) calculations were performed to elucidate the mechanism of local band gap modulation and its relationship to the functionalization process, focusing on factors such as spin density of intermediates, relative stability, and transition energy of the SWNT adducts. Specific calculation details, including the basis sets and functionals, were provided in the paper. The Gaussian 09 software suite was used for all computational studies.

Functionalization of SWNTs with 1,4-diiodooctafluorobutane resulted in the selective emergence of a new PL peak at 1318 nm, a wavelength suitable for telecommunication applications. This contrasts with other alkylated SWNTs, which showed shorter PL wavelengths. The D/G ratios from Raman spectroscopy indicated that the functionalisation degree was affected by the number of fluorine atoms close to the reactive site, with more fluorine atoms leading to lower functionalisation degrees. Interestingly, SWNTs functionalized with reagents containing two reactive sites and multiple fluorine atoms exhibited high selectivity, generating single new PL peaks. The influence of fluorine atoms on PL properties was demonstrated by comparing SWNT-(CF2)3CF3 with the corresponding alkylated SWNT-(CH2)3CH3. Regardless of chiral index, SWNT-(CF2)3CF3 consistently produced longer PL wavelengths. Chiral separation of functionalized SWNTs confirmed the effectiveness of the method across different chiral angles, demonstrating the emergence of a new PL peak at 1320 nm for (6,5) SWNTs and other similar peaks in other chiral SWNTs. DFT calculations indicated that the longer PL wavelengths were due to a combination of the electron-withdrawing effect of fluorine and the cyclic structure resulting from the reagent's two reactive sites. Calculations of spin densities of intermediates indicated that the lower spin density of perfluoroalkyl radicals compared to alkyl radicals led to the lower functionalisation degree of perfluoroalkylated SWNTs. Analysis of the transition energy and relative stability of the functionalised SWNTs suggested that the binding configuration was the primary determinant of local band gap, while the number of fluorine atoms played a role in shifting the transition energy to lower values.

The successful generation of a new PL peak at 1318 nm in SWNTs functionalized with 1,4-diiodooctafluorobutane represents a significant advance in the field of NIR light-emitting materials. This achievement directly addresses the challenge of creating materials suitable for telecommunication wavelengths, expanding the potential applications of SWNTs. The high selectivity observed in the PL emission, especially for SWNTs functionalized with reagents containing multiple fluorine atoms and two reactive sites, points toward a regioselective reaction mechanism. The results highlight the importance of both the electronic effect of fluorine atoms and the controlled addition position in achieving a significant red-shift in the PL wavelength. The combined experimental and computational findings provide a comprehensive understanding of the structure-property relationships in functionalized SWNTs, paving the way for more precise control over their optical properties.

This study demonstrates a novel approach for producing NIR-emitting materials with PL at telecommunication wavelengths. The selective emergence of a new PL peak at 1320 nm in (6,5) SWNTs and other chiral SWNTs via cyclic perfluoroalkylation offers a promising pathway for the development of advanced optoelectronic devices. The combined experimental and computational results reveal the crucial role of fluorine atoms and the strategic use of reagents with two reactive sites in achieving this long-wavelength emission. Further research could explore the optimization of functionalization conditions and the investigation of different perfluoroalkylated reagents to achieve even more precise control over PL wavelengths and efficiencies.

The study primarily focused on a limited set of SWNT chiral indices. While the findings suggest broad applicability, further investigation is needed to confirm the generality of the approach for a wider range of SWNT types. The functionalization degree achieved was relatively low, possibly limiting the overall PL intensity. Future research should explore strategies to improve functionalization efficiency while maintaining the selectivity of the long-wavelength PL emission. The computational models utilized for DFT calculations were simplified representations of the actual SWNTs and functionalization, which might introduce some deviations from the real-world system.