Optically Active MXenes in Van der Waals Heterostructures

Nanoscale Surface and Bulk Electronic Properties of Ti3C2Tx MXene Unraveled by Multimodal X-Ray Spectromicroscopy

2D layered materials, such as transition metal carbides or nitrides, known as MXenes, offer an ideal platform to investigate charge transfer processes in confined environment, relevant for energy conversion and storage applications. Their rich surface chemistry plays an essential role in the pseudocapacitive behavior of MXenes. However, the local distribution of surface functional groups over single flakes and within few- or multilayered flakes remains unclear. In this work, scanning X-ray microscopy (SXM) is introduced with simultaneous transmission and electron yield detection, enabling multimodal nanoscale chemical imaging with bulk and surface sensitivity, respectively, of individual MXene flakes. The Ti chemical bonding environment is found to significantly vary between few-layered hydrofluoric acid-etched Ti3C2Tx MXenes and multilayered molten salt (MS)-etched Ti3C2Tx MXenes. Postmortem analysis of MS-etched Ti3C2Tx electrodes cycled in a Li-ion battery further illustrates that simultaneous bulk and surface chemical imaging using SXM offers a method well adapted to the characterization of the electrode-electrolyte interactions at the nanoscale.

A Molecular Coordination Strategy for Regulating the Interface of MoS2 Field Effect Transistors

Chemically modifying monolayer two-dimensional transition metal dichalcogenides (TMDs) with organic molecules provides a wide range of possibilities to regulate the electronic and optoelectronic performance of both materials and devices. However, it remains challenging to chemically attach organic molecules to monolayer TMDs without damaging their crystal structures. Herein, we show that the Mo atoms of monolayer MoS2 (1L-MoS2) in defect states can coordinate with both catechol and 1,10-phenanthroline (Phen) groups, affording a facile route to chemically modifying 1L-MoS2. Through the design of two isomeric molecules (LA2 and LA5) comprising catechol and Phen groups, we show that attaching organic molecules to Mo atoms via coordinative bonds has no negative effect on the crystal structure of 1L-MoS2. Both theoretical calculation and experiment results indicate that the coordinative strategy is beneficial for (i) repairing sulfur vacancies and passivating defects; (ii) achieving a long-term and stable n-doping effect; and (iii) facilitating the electron transfer. Field effect transistors (FETs) based on the coordinatively modified 1L-MoS2 show high electron mobilities up to 120.3 cm2 V-1 s-1 with impressive current on/off ratios over 109. Our results indicate that coordinatively attaching catechol- or Phen-bearing molecules may be a general method for the nondestructive modification of TMDs.

Interfacial interactions of doped-Ti3C2 MXene/MAPbI3 heterostructures: surfaces and the theoretical approach

The work function (WF) of perovskite materials is essential for developing optoelectronic devices enabling efficient charge transfer at their interfaces. Perovskite's WF can be tuned by MXenes, a new class of two-dimensional (2D) early transition metal carbides, nitrides, and carbonitrides. Their variable surface terminations or the possibility of introducing elemental dopants could advance perovskites. However, the influence of doped-MXenes on perovskite materials is still not fully understood and elaborated. This study provides mechanistic insight into verifying the tunability of MAPbI3 WF by hybridizing with fluorine-terminated Ti3C2Tx (F-MXene) and nitrogen-doped Ti3C2Tx (N-MXene). We first reveal the interfacial interaction between MAPbI3 and MXenes via X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and photoluminescence spectroscopy (PL). UPS supported by density functional theory (DFT) calculations allowed the description of the influence of F and N on MXene's WF. Furthermore, we developed MAPbI3/MXene heterostructures using F- and N-MXenes. The F-MXenes extended the most WF of MAPbI3 from 4.50 eV up to 3.00 eV, compared to only a small shift for N-MXene. The underlying mechanism was charge transfer from low WF F-MXene to MAPbI3, as demonstrated by PL quenching in MAPbI3/F-MXene heterostructures. Altogether, this work showcases the potential of fluorine-doped MXenes over nitrogen-doped MXenes in advancing perovskite heterostructures, thus opening a door for efficient optoelectronic devices.

Ambient Processed rGO/Ti3CNTx MXene Thin Film with High Oxidation Stability, Photosensitivity, and Self-Cleaning Potential

Solution-based processing offers advantages for producing thin films due to scalability, low cost, simplicity, and benignity to the environment. Here, we develop conductive and photoactivated self-cleaning reduced graphene oxide (rGO)/Ti3CNTx MXene thin films via spin coating under ambient conditions. The addition of a thin rGO layer on top of Ti3CNTx resulted in up to 45-fold improvement in the environmental stability of the film compared to the bare Ti3CNTx film. The optimized rGO/Ti3CNTx thin film exhibits an optical transmittance of 74% in the visible region of the spectrum and a sheet resistance of 19 kΩ/sq. The rGO/Ti3CNTx films show high rhodamine B discoloration activity upon light irradiation. Under UV irradiation, the electrically conductive MXene in combination with in situ formed semiconducting titanium oxide induces photogenerated charge carriers, which could potentially be used in photocatalysis. On the other hand, due to film transparency, white light irradiation can bleach the adsorbed dye via photolysis. This study opens the door for using MXene thin films as multifunctional coatings with conductive and potentially self-cleaning properties.