The versatile property suite of two-dimensional MXenes is driving interest in various applications, including energy storage, electromagnetic shielding, and conductive coatings. Conventionally, MXenes are synthesized by a wet-chemical etching of the parent MAX-phase in HF-containing media. The acute toxicity of HF hinders scale-up, and competing surface hydrolysis challenges control of surface composition and grafting methods. Herein, we present an efficient, room-temperature etching method that utilizes halogens (Br 2 , I 2 , ICl, IBr) in anhydrous media to synthesize MXenes from Ti 3 AlC 2 . A radicalmediated process depends strongly on the molar ratio of the halogen to MAX phase, absolute concentration of the halogen, the solvent, and temperature. This etching method provides opportunities for controlled surface chemistries to modulate MXene properties.

MXenes comprise a class of 2D materials with attractive properties for use in electronically functional composites. However, a fundamental understanding of these properties at the scale of individual flakes is necessary. Here, we use resonance Raman scattering of Ti3C2T x MXenes to study their vibrational modes as a function of the flake thickness and surface terminations. Enhancement enables a precise comparison of the peak shifts in MILD/HF-method etching environments that produce different functional group densities on the MXene layer surfaces. We find that the enhanced Raman modes are sensitive to the different electron-withdrawing character of the surface functional groups.

We present continuous wave photoinduced absorption spectroscopy of single-walled carbon nanotubes dispersed in a polymer matrix. The spectrum is dominated by a modulation of the absorption line shape, predominantly of large diameter tubes, that we assigned to electroabsorption caused by local electric fields arising from trapped photoinduced charges. The lack of selectivity in the excitation points to an efficient migration of the photoexcited states, either the singlet excitons or the charges resulting from their dissociation.

Unambiguous determination of the optical dynamics in MXenes is necessary for their reliable development into applications such as EMI shielding, energy storage, and laser systems. Here, simultaneous ultrafast transmission and reflection (SUTR) is used to determine the temporal change in refractory index (n) and extinction coefficient (k) of Ti 3 C 2 T x from 450 to 1300 nm which is dominated by intraflake mechanisms. We assign the dynamics for wavelengths below 600 nm with interband transitions, while those at 800 nm are assigned to a plasmon resonance. The response from 1000 to 1300 nm shows changes in index of refraction with no changes in extinction coefficient, consistent with free carriers.