White Photoluminescent Ti3C2 MXene Quantum Dots with Two‐Photon Fluorescence

Abstract A recently created class of inorganic 2D materials, MXenes, has become a subject of intensive research. Reducing their dimensionality from 2D to 0D quantum dots (QDs) could result in extremely useful properties and functions. However, this type of research is scarce, and the reported Ti3C2 MXene QDs (MQDs) have only shown blue fluorescence emission. This work demonstrates a facile, high‐output method for preparing bright white emitting Ti3C2 MQDs. The resulting product is two layers thick with a lateral dimension of 13.1 nm. Importantly, the as prepared Ti3C2 MQDs present strong two‐photon white fluorescence. Their fluorescence under high pressure is also investigated and it is found that the white emission is very stable and the pressure makes it possible to change from cool white emission to warm white emission. Hybrid nanocomposites are then fabricated by polymerizing Ti3C2 MQDs in polydimethylsiloxane (PDMS) solution, and the bright white emitting hybrid materials in white light‐emitting diodes are used. This work provides a facile and general approach to modulate various nanoscale MXene materials, and could further aid the wide development of applications for MXene materials in various optical‐related fields.


Femtosecond transient absorption setup:
A regeneratively amplified Ti:sapphire laser system (Coherent Libra,50fs,1kHz) provides the fundamental light source. The pump pulse (400 nm) is generated by focusing a portion of fundamental light into BBO crystal. In order to avoid the influence of rotational relaxation effects on dynamics, the polarization of pump pulse is randomized by depolarizing plate. The other fundamental pulse provides broadband probe pulse (white light continuum) that is produced by focusing 800 nm fundamental light into sapphire plate (3mm). The pump and probe beams are overlapped in the sample with crossing areas of 600 m and 150 m. After passing through the sample, the probe pulse is focused into optical fiber that is coupled to spectrometer(AvaSpec-1650F). The energy of 400 nm excitation pulse is adjusted to about 1.5 J/pulse by a neutral density optical filter. The pump pulse is chopped at 500 Hz to acquire pumped (signal) and un-pumped (reference) probe spectra, and the OD spectrum can be obtained by processing them. The solutions are placed in 2 mm optical path length quartz cuvette. Both the instrument response function (100 fs) and temporal chirp in the probe light are determined by measuring the cross modulation of ethanol. The group velocity dispersion effect on the experiment data is corrected by home-made chirp program. For each measurement, the pump-probe delay scan is repeated three times to give the averaged experiment data.

Two -photon fluorescence (TPF) setup:
The excited source consists of a regeneratively amplified Ti:sapphire laser system (Coherent Libra, 50fs, 1kHz, 800 nm). The TPF signal is collected at the right angles from excited solution placed in the 1cm fluorescence cuvette. The signal is coupled into the spectrometer (Spectra Pro 500i,PI Acton) through a fiber. Finally, the TPF signal is detected by an intensified charge coupled device (ICCD, PIMAX4, Princeton Instruments) triggered by laser.

Ti 3 C 2 Tx suspension:
The MAX phase was prepared by mixing with TiH 2 , Al and graphite in a molar ratio of 3:1.1:2, and followed by ball milling for 12 h. The mixture was then heated under a protection of argon (Ar) gas in a tube furnace for 2 h at 1400 °C. The resultant bulk was powdered and sieved by a 400 mesh sieve. The Ti 3 AlC 2 powder was soaked into 49% HF solution at 60°C, and then placed in a 70°C vacuum oven overnight to obtain Ti 3 C 2 Tx powder.

Synthesis of Ti 3 C 2 nanosheet:
Ti 3 C 2 nanosheets were synthesized in the following method. Briefly, 20 mL oleylamine was added to a 50 mL three-neck flask and heated to 100°C to eliminate air by bubbling nitrogen. 0.01g Ti 3 C 2 was added to the solution and the stirring speed adjusted to 500 rpm, and then ventilated with inert gas for 30 min to purge any air.
The resulting solution was heated to 250°C and kept at this temperature for 3h.

Synthesis of Ti 3 C 2 MQDs:
The Ti 3 C 2 MQDs were synthesized by hydrolyzing the as prepared Ti 3 C 2 nanosheets.
The Ti 3 C 2 nanosheets in oleylamine was strongly sonicated for 6 h ( 600 w), then the uniformly-dispersed solution was added to a Teflon-lined autoclave and heated at Figure S1. Energy-dispersive X-ray spectroscopy (EDX) of Ti 3 C 2 MQDs displaying that the intrinsic chemical structure of Ti 3 C 2 was well maintained Figure S2. High-resolution XPS spectra of Ti 2p of Ti 3 C 2 MQDs displaying that the intrinsic chemical structure of Ti 3 C 2 was well maintained.