Interface-Tailored Secondary Excitation and Ultrafast Charge/Energy Transfer in Ti3C2Tx-MoS2 Heterostructure Films

Abstract

Charge/energy separation across interfaces of plasmonic materials is vital for minimizing plasmonic losses and enhancing their performance in photochemical and optoelectronic applications. While heterostructures combining plasmonic two-dimensional transition metal carbides/nitrides (MXenes) and semiconducting transition metal dichalcogenides (TMDs) hold significant potential, the mechanisms governing plasmon-induced carrier dynamics at these interfaces remain elusive. Here, we uncover a distinctive secondary excitation phenomenon and an ultrafast charge/energy transfer process in heterostructure films composed of macro-scale Ti₃C₂T_x and MoS₂ films. Using Rayleigh-Bénard convection and Marangoni effect-induced self-assembly, we fabricate large-scale (square centimeters) Ti₃C₂T_x and MoS₂ films composed of edge-connected monolayer nanoflakes. These films are flexibly stacked in a controlled sequence to form macroscopic heterostructures, enabling the investigation and manipulation of excited-state dynamics using transient absorption and optical pump-terahertz probe spectroscopy. In the Ti₃C₂T_x-MoS₂ heterostructure, we observe a secondary excitation in MoS₂ driven by the surface plasmon resonance of Ti₃C₂T_x. This phenomenon, with a characteristic rise time constant of ∼70 ps, is likely facilitated by acoustic phonon recycling across the interface. Further interfacial thermal transport engineering─achieved by tailoring the sequence and combination of interfaces in trilayer heterostructures─allows extending the characteristic time to ∼175 ps. Furthermore, we identify a sub-150 fs ultrafast charge/energy transfer process from Ti₃C₂T_x to MoS₂. The transfer efficiency is strongly dependent on the excitation photon energy, resulting in amplified photoconductivity in MoS₂ by up to ∼180% under 3.10 eV excitation. These insights are crucial for developing plasmonic MXene-based heterostructures, paving the way for advancements in photochemical and optoelectronic applications.

Figure 1

Schematics illustrating the preparation of Ti₃C₂T_x-MoS₂ heterostructure films and their excited-state dynamics. (a) Interfacial self-assembly method for fabricating macro-scale Ti₃C₂T_x and MoS₂ films. The heterostructure films are constructed by stacking the MoS₂ film on top of the Ti₃C₂T_x film, followed by an annealing process to completely convert MoS₂ into the semiconducting 2H phase. (b) Schematic representation of three distinct Ti₃C₂T_x-MoS₂ heterostructure configurations: HS-TM, HS-TMT, and HS-TTM. (c) Schematic of time-resolved pump–probe measurements employed to investigate the excited-state dynamics. (d) Schematic showing an ultrafast charge/energy transfer from Ti₃C₂T_x to MoS₂, followed by a secondary excitation in MoS₂ driven by the SP excitation of Ti₃C₂T_x.

Figure 2

Characterizations of Ti₃C₂T_x-MoS₂ heterostructures. Optical and AFM images along with thickness profiles of macro-scale (a) Ti₃C₂T_x film and (b) MoS₂ film (scale bars, 1 μm). (c) Optical microscopy image of HS-TM (scale bar, 20 μm). (d) Raman spectra of the MoS₂ film and HS-TM. (e) Steady-state UV–vis–NIR absorption, (f) PL, and (g) UPS spectra of the Ti₃C₂T_x film, the MoS₂ film, and HS-TM. (h) Schematic showing band alignment in HS-TM.

Figure 3

TA spectra and excited-state dynamics. TDT spectra of (a) the MoS₂ film, (b) the Ti₃C₂T_x film, and (c) HS-TM at 0.1–1000 ps under 1.55 eV excitation with 70 fs-width pulses and 1.2 mJ cm^–2 fluence. (d) TA kinetics pumped at 1.55 eV and probed at 436 nm (C exciton bleaching) of the MoS₂ film, the Ti₃C₂T_x film, HS-TM, and the superposed curve of the MoS₂ film and the Ti₃C₂T_x film (i.e., 1 × MoS₂ + 1 × Ti₃C₂T_x). (e) Pump fluence-dependent PIA-1 (494 nm) kinetics of HS-TM pumped at 1.55 eV. TA kinetics curves of the MoS₂ film, the Ti₃C₂T_x film, and HS-TM at C exciton bleaching pumped at (f) 1.91 eV and (g) 3.10 eV. (h) Schematic illustrating the mechanism governing the secondary excitation in the Ti₃C₂T_x-MoS₂ heterostructure.

Figure 4

Interface-manipulated dynamics. (a) TA kinetics probed at 436 nm (C exciton bleaching) under excitation at 1.55 eV, and (b) the corresponding exponentially fitted time constants. Schematics illustrating the mechanisms governing the interfacial thermal conductivity in (c) HS-TM, (d) HS-TMT, and (e) HS-TTM. The contribution of each Ti₃C₂T_x layer to the secondary excitation of MoS₂ was schematically deconvoluted and illustrated using different colors.

Figure 5

Ultrafast interfacial charge/energy transfer probed by THz spectroscopy. (a–c) THz photoconductivity dynamics normalized by absorbed photon density of the MoS₂ film, the Ti₃C₂T_x film, HS-TM, and the superimposed responses of the MoS₂ film and the Ti₃C₂T_x film under (a) 1.55 eV, (b) 1.91 eV, and (c) 3.10 eV excitations. (d) Comparison of photoconductivity enhancement under different excitations.