Niobium MXene Contacts Unlock High-Performance p-Type 2D Transistors

Two-dimensional semiconductors have long promised a future of atomically thin, flexible, and extraordinarily efficient electronics, but one stubborn problem has held them back: making good electrical contacts, especially to p-type devices. Now a research team writing in Advanced Science reports that a lesser-known member of the MXene family, niobium carbide Nb2CTx, can outperform both the standard titanium-based MXene and traditional high-work-function metals such as platinum, palladium, and gold when paired with the p-type semiconductor molybdenum ditelluride. The finding, demonstrated on wafer-scale transistor arrays, points toward a cheaper, gentler, and more tunable way to wire up the next generation of 2D electronics.

MXenes are two-dimensional metal carbides and nitrides first discovered in 2011, and they have since become one of the most versatile material platforms in nanotechnology. Made by chemically etching away layers from layered ceramic precursors known as MAX phases, they combine metallic electrical conductivity with mechanical flexibility and a chemically tunable surface. Their surfaces are decorated with terminations such as oxygen, fluorine, and hydroxyl groups, and these terminations, together with the choice of metal element, allow researchers to adjust the material’s work function, the energy required to lift an electron out of the solid. That tunability is precisely what makes MXenes attractive as electrode materials, because the quality of a transistor depends critically on how well the energy levels of the electrode and the semiconductor line up.

The contact problem is particularly acute for p-type transition-metal dichalcogenides, the class of 2D semiconductors that conducts current using holes rather than electrons. For n-type devices, low-work-function metals such as indium and bismuth can be deposited gently at low energy, preserving the delicate semiconductor surface. P-type devices, by contrast, demand high-work-function metals, and those metals, including platinum, palladium, and gold, all melt above 1000 degrees Celsius. Depositing them requires energetic vacuum processes that bombard the semiconductor surface, damaging the interface and undermining the very band alignment the metals were chosen to provide. The result has been a persistent bottleneck: the ideal electrode materials are the hardest ones to use without destroying the device.

The new study tackles this dilemma by replacing vacuum-deposited metals with solution-processed MXene films, which can be laminated onto the semiconductor under mild, low-cost conditions that minimize interface damage. The team benchmarked three synthetically mature MXenes: Ti3C2Tx, the most widely studied member of the family, along with Nb2CTx and Mo2CTx, chosen because their work function ranges differ from the titanium compound and thus expand the compositional space for contact engineering. As the semiconductor channel they used 2H-MoTe2, a representative p-type 2D material, grown as thin films by sputtering molybdenum onto silicon dioxide substrates followed by chemical vapor deposition tellurization.

Characterization confirmed that all four materials were synthesized with the intended structure and chemistry. Transmission electron microscopy showed the characteristic flake morphology of the MXenes with hexagonal diffraction patterns indicating high crystallinity, while atomic-resolution scanning transmission electron microscopy revealed the well-defined layered lattice of the 2H-MoTe2 films. Raman spectroscopy, x-ray photoelectron spectroscopy, and x-ray diffraction verified phase purity and surface chemistry, with no significant oxidation detected in the MXene samples and no secondary phases in the semiconductor. These checks matter because even trace impurities or unwanted phases can dominate the behavior of an atomically thin device.

The decisive measurement came from ultraviolet photoelectron spectroscopy, which determined the work functions of Ti3C2Tx, Nb2CTx, and Mo2CTx to be 4.44, 4.72, and 4.23 electron volts respectively, while 2H-MoTe2 showed a work function of 4.40 electron volts with its valence band maximum sitting 0.33 electron volts below the Fermi level. The resulting band alignment diagram revealed that Nb2CTx provides the closest energetic match to the semiconductor’s valence band, meaning the lowest barrier for hole injection. Importantly, the work function of a MXene is not set by the metal element alone; surface terminations modify surface dipoles and charge redistribution, so the measured value reflects the combined contribution of composition and actual surface chemistry. In this case, Nb2CTx’s higher work function translated directly into better contacts.

Because MXenes are hydrophilic while the MoTe2 surface is intrinsically hydrophobic, the researchers could not simply coat one onto the other. Instead they used a polymer-assisted transfer strategy, laminating patterned gold-capped MXene source and drain electrodes onto the semiconductor film. A 20-nanometer gold capping layer improved lateral conductivity and protected the underlying interface, while a consistent spray-coating protocol ensured that differences between devices reflected the MXene composition rather than thickness variation. The choice of transfer polymer proved surprisingly consequential: devices made with cellulose acetate showed significantly degraded characteristics, likely due to unintentional doping, whereas polymethyl methacrylate yielded clean, reproducible interfaces, a sensitivity the authors note is common in TMD devices.

The electrical results were striking. Transistors with Nb2CTx contacts showed clear p-type behavior with hole mobilities reaching about 19.8 square centimeters per volt-second in individual devices and an on/off current ratio of roughly 5.23 times ten to the fourth. Across twenty identically patterned devices measured at random positions on the wafer, the average mobility was 17.4 plus or minus 1.2 square centimeters per volt-second with a narrow performance distribution, indicating excellent uniformity. Output curves were linear and symmetric, confirming low-resistance ohmic-like contacts. Compared head-to-head with platinum-contacted devices, the Nb2CTx devices performed significantly better, and they also surpassed previously reported CVD-grown MoTe2 transistors and nearly all previously reported MXene-based transistors in mobility. According to the authors, this is the first demonstration of wafer-scale MXene contact engineering in p-type systems and the first use of Nb2CTx as an effective contact material. The devices also proved robust, showing negligible degradation after three months in dry air.

Temperature-dependent measurements from 150 to 250 kelvin provided deeper physical insight. As temperature decreased, the on/off ratio rose to about ten to the fifth, because suppressing thermally assisted carrier emission reduces the off-state current. By fitting the current to the thermionic emission model appropriate for 2D semiconductors, the team extracted Schottky barrier heights from Richardson plots as a function of gate voltage. At large negative gate voltages the effective barrier dropped to only a few millielectron volts, signaling the onset of tunneling-assisted carrier injection and explaining the low contact resistance under strong turn-on bias. Together with cross-sectional electron microscopy showing a sharp, well-defined interface, these measurements confirm that the performance gains stem from genuinely clean, low-barrier contacts rather than artifacts.

The broader significance lies in the vast, largely unexplored compositional space of the MXene family. The authors suggest that other compositions such as V2CTx, Ta2CTx, and Ta4C3Tx, as well as the investigated MXenes with different surface terminations, could serve as promising p-type contact candidates if suitable work functions, stable terminations, and clean interfaces can be achieved. Future work with more precisely controlled surface chemistry should help establish a quantitative relationship linking composition, termination, work function, and contact quality. For now, the study offers a clear and practical recipe: pick the MXene whose measured work function best matches the semiconductor’s valence band, deposit it gently from solution, and laminate it without damage. If that approach generalizes, the bottleneck that has slowed p-type 2D electronics may finally be dissolving, opening the door to solution-processed, scalable 2D transistors and optoelectronic devices built without a single high-temperature deposition step.

Subject of Research: MXene electrode contacts for p-type two-dimensional semiconductor transistors

Article Title: MXenes Contacts for p‐type 2D Electronics

Article References: Guo, T., Chen, M., Wang, Y., Liu, C., Xu, X., Luo, L., Zhu, D., Chu, N., Zhang, X., Caraveo, A., Anthopoulos, T. D., Zhang, X., & Alshareef, H. N. (2026). MXenes Contacts for p ‐type 2D Electronics. Advanced Science, 13(56), Article e76591. https://doi.org/10.1002/advs.76591

Image Credits: AI Generated

DOI: 10.1002/advs.76591

Keywords: MXenes, Nb2CTx, 2D semiconductors, MoTe2, p-type transistors, work function, Schottky barrier, contact resistance, solution processing, field-effect transistors, transition-metal dichalcogenides, band alignment