Two-Dimensional MXenes Mo₂Ti₂C₃T_z and Mo₂TiC₂T_z: Microscopic Conductivity and Dynamics of Photoexcited Carriers

Abstract

MXenes are a recently discovered family of two-dimensional transition metal carbides, nitrides, and carbonitrides with electronic properties that can be tuned by their chemistry and structure. Herein THz spectroscopy was used to investigate the microscopic conductivity and photoexcited charge carrier dynamics in two Mo-based MXenes: Mo2Ti2C3T z and Mo2TiC2T z . We find that both have high intrinsic carrier densities (∼1020 cm–3 in Mo2Ti2C3T z and ∼1019 cm–3 in Mo2TiC2T z ) and mobilities and exhibit high conductivities within individual nanosheets. Optical excitations result in a transient conductivity increase in both compositions, in stark contrast with the most studied member of the MXene family, Ti3C2T z , where photoexcitation suppresses the conductivity for nanoseconds. Deintercalation of water, and other species, from between the nanosheets by mild vacuum annealing at 200 °C further improves the long-range, internanosheet transport of the photoexcited carriers and increases their lifetime. High, and long-lived, photoinduced conductivity that can be engineered by substituting Mo for Ti renders these Mo-based MXenes attractive for a variety of optoelectronic, sensing, and photoelectrochemical applications.

Terahertz Polarizers Based on 2D Ti3C2Tz MXene: Spin Cast from Aqueous Suspensions

“…The properties of in the THz range were parameterized using a Drude–Smith model, [ 37,38 ] following previous reports. [ 31,32,39 ] To model the polarizers, we assumed a carrier density of 4 × 10 20 cm −3 , a scattering time of 40 fs, and a Drude–Smith c‐parameter of –0.75, representing the effect of nanoflake boundaries and disorder on the long‐range electron transport. Details of the Drude–Smith analysis are provided in Figure S5, Supporting Information.…”

Double transition-metal MXenes: Atomistic design of two-dimensional carbides and nitrides

“…55,64,69 However, DTM MXenes with uniform surface functional groups (for example, fully F-terminated) have not yet been synthesized, and all synthesized out-of-plane DTM MXenes to date have a mixture of surface terminations and have been electrically conductive. 15,62,66,67,73 Out-of-plane ordered DTM MXenes are less conductive than Ti3C2Tx, with the highest film conductivity of 1490 S•cm -1 observed for Mo2Ti2C3Tx (annealed at ~ 500°C) compared to 15,100 S•cm -1 for Ti3C2Tx (annealed at 200°C). 28,66 Additionally, the conductivity of Mo-containing out-of-plane DTM MXene films is shown to be more dependent on interlayer spacing between MXene flakes due to intercalated molecules used in delamination of out-of-plane DTM MXenes as compared to Ti3C2Tx.…”

“…58 Once ordered MAX phases are selectively etched, their derivative MXenes retain the structural ordering of the corresponding MAX phases. 39,57,62 From out-of-plane ordered MAX phases, the only synthesized DTM out-of-plane MXenes thus far include Mo2ScC2Tx, 57 Mo2TiC2Tx, 39 Cr2TiC2Tx, 39 and Mo2Ti2C3Tx. 39 Figure 4a illustrates a high-resolution scanning transmission electron microscope (STEM) image of Mo2TiAlC2 MAX phase, 56 which is then selectively etched in 48 wt.% HF at 55ºC for 48 h to yield the Mo2TiC2Tx MXene (Figure 4b).…”

Strategies for Fabricating High‐Performance Electrochemical Energy‐Storage Devices by MXenes

“…[102] Modifying the electrical properties of double-transitionmetal MXenes has been an interesting topic since their discovery in 2015. [95,103,104] In addition to MXenes with out-ofplane ordered structure (such as the above-mentioned Mo 2 TiC 2 O 2 ), solid-solution species (molybdenum vanadium carbides, Mo x V 4-x C 3 ) were recently synthesised by removing Al layers from Mo x V 4-x AlC 3 (x = 1, 1.5, 2 and 2.7) by HF (~50 %). [105] Due to the low affinity of À F terminations on Mo surface, the fluorine content increased from 0.72 � 0.08 to 1.88 � 0.15 moles by decreasing x from 2.7 to 1.…”