Inter-flake transport and humidity response of Ti3C2Tx MXene at the nanoscale

TL;DR

Ti_3C_2T_x MXene networks exhibit transport dominated by inter-flake junctions, with junction resistances $R_J$ often exceeding intra-flake resistances $R_{NS}$; the authors quantify $R_J$ via 4-probe STM, STP, and C-AFM across single flakes, conductive paths, and lithographically defined devices, in both UHV and humid environments, and link humidity response to junction dynamics with network configurations showing the fastest sensing. The data support a model where charge transport is the sum of isopotential flakes separated by discrete junctions, i.e., $R_{total} \,\approx\ \\sum R_{NS} + \\sum R_J$, with $R_J$ strongly morphology-dependent. Humidity sensing kinetics are dominated by junctions, with network-scale responses faster than individual flakes due to multiple junctions. These findings highlight the crucial role of junction engineering and morphology control for reproducible MXene-based electronics and chemiresistive/humidity-sensing applications.

Abstract

Understanding charge transport in networks of two-dimensional crystals is essential for developing reliable applications such as chemiresistors or electromagnetic shields. For this purpose, intra- and inter-flake contributions to the network resistance must be disentangled. MXenes, such as Ti3C2Tx, are prime examples of 2D crystals often employed as thin networks of interconnected flakes deposited on substrates to realize functional devices. While a significant number of studies focused on transport in individual MXene flakes, inter-flake transport remains scarcely explored. Here, we demonstrate that charge transport in multi-flake conductive paths of Ti3C2Tx is dominated by interflake junctions and provide quantitative estimates of junction resistances. Scanning probe measurements reveal that in a MXene multi-flake conductive path, individual flakes behave as isopotential domains, since the voltage drop is localized precisely at the inter-flake junctions. We further investigate the chemiresistive response to humidity at the single flake, multi-flake and flake network scale, evidencing the leading impact of junctions on sensing kinetics. These findings underline the crucial role of junctions in charge transport and sensing capabilities of MXenes.

Figures (6)

• Figure 1: (a) 3D view of the AFM topography of a Ti_3C_2T_x flake deposited on a Si/SiO_2 substrate, obtained in tapping mode. (b) I-V characteristic obtained by 4-probe measurement on the flake shown in (a). (c) Height profile along the white dotted line in (a). (d) SEM image of the Ti_3C_2T_x with the tungsten probes placed in the 4-contact measurement configuration.
• Figure 2: (a) 3D view of the AFM topography of a conductive path formed by Ti_3C_2T_x flakes deposited on a Si/SiO_2 substrate, obtained in tapping mode. (b) Height profile along the white dotted line in (a). (c-d) SEM images of the 4-probe measurement along the conductive path, for two different measurement points, with the distance between the measuring probes being 4.5 and 11.7 µm, respectively. (e) I-V characteristics for both measurements points shown in (c) and (d). Note that the vertical (voltage) scale for the light blue curve (left axis) is orders of magnitude larger than for the dark blue curve (right axis). (f) Electrical resistance along the conducting path as a function of the distance between the measuring probes.
• Figure 3: (a) AFM topography of the junction resistance, obtained in tapping mode. (b) STM topography and potentiometry inside the rectangle highlighted in (a). (c) SEM image of the probe configuration of the potentiometry measurement obtained with a 100 nA current between the two outer probes. (d) Profile of the potential along the white dashed line in (b).
• Figure 4: (a) 3D view of the AFM topography of a device with metallic contacts on Ti_3C_2T_x flakes, obtained in tapping mode. (b) Electrical resistance along the conducting path as a function of the distance between the measuring contacts. The inset shows I-V curves acquired on one junction and on the individual flake. (c-d) Topographical details of the junctions between the flakes of the device shown in (a). The insets show height profiles extracted at the locations indicated by the dashed lines on the AFM scans. (e) Junction resistance values, along with flake thickness, for this work and the value reported in Loes et al. loes2024layer. (f) Boxplot representing all the values obtained from fabricated devices and from 4-probe STM measurement for flake resistance ($R_{NS})$ and junction resistance ($R_J$).
• Figure 5: (a) 3D view of the AFM topography and (b) current map of Ti_3C_2T_x flakes overlapping a metallic electrode and an insulating substrate, obtained with Conductive AFM with a 3.5 mV bias between tip and sample. (c) Detail in the current map shown in (b). (d) C-AFM current profile along the white dotted line in (c), taken with a 5-pixel thickness (34 nm).