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In an article recently published in the journal ACS Applied Electronic Materials, researchers synthesized two-dimensional (2D) transition metal carbides, nitrides, and/or carbonitrides (Ti3C2TX MXene) nanosheets and studied the electron emission of both Ti3C2TX MXene and Ti3SiC2 MAX to inform the design of vacuum electronic devices.

Comparative Study of Cold Electron Emission from 2D Ti3C2TX MXene Nanosheets with Respect to Its Precursor Ti3SiC2 MAX Phase. ​​​​​​​Image Credit: sKjust/Shutterstock.com

The nanosheets were prepared by etching titanium silicon carbide (Ti3SiC2) MAX phase, which was synthesized by heating the mixture of elemental titanium (Ti), silicon (Si) and carbon (C) at a high temperature. Density functional theory (DFT) simulations determined the electronic and structural properties of Ti3C2TX MXene and Ti3SiC2 MAX.

A deformation in the surface potential barrier that occurs in a metal nearly becomes triangle-shaped in the presence of a strong electric field. If the deformed potential barrier width at the Fermi energy level becomes equal to the De Broglie wavelength of an electron, the probability of finding an electron outside the barrier becomes nonzero; this is termed field emission (FE). Since the tunneling probability is explained at 0 kelvin, it is termed cold emission and electro emitter is called field emitter.

The mechanism of field electron emission takes work function (Φ) and geometry of the emitter material, giving a special focus to the field emission (FE) behavior of one-dimensional nanostructured materials. To this end, the promising one-dimensional (1D) field emitting materials include molybdenum (Mo), tungsten (W), carbon nanotubes, and semiconductors such as zinc oxide (ZnO), titanium oxide (TiO2), tine oxide (SnO2), tungsten oxide (WO3), and more.

The efficiency and applicability of field emitters are controlled by factors such as low work function, morphology, ease of synthesis, and stable emission current. The 1D-field emitter’s temporal stability suffers from the drawback of tip “burnout”. To this end, carbon and non-carbon thin-film-based field emitters can overcome these limitations.

In addition to 1D planar emitters, 2D material like graphene was considered to study the FE characteristics. Extensive research on graphene and graphene-based hybrids/ composites showed their efficiency as vacuum field emission devices, yet their fabrication methods restrict their practical applicability.

MXenes are 2D materials with unique layered structures with attractive properties. The morphology of MXenes is tailored into a single/multilayer based on the etching methods. Due to their huge specific area, Mxenes have a wide range of applications in sensing, energy conversion and storage, photocatalysis, and adsorption. Thus, there is a demand for further investigations on Mxenes.

In the present work, the authors applied 1 microampere per square centimeter of current density and achieved a turn-on voltage of 4.7 volts per micrometers for pristine Ti3C2TX Mxene, without any morphology reconstruction or surface treatment. This reduced turn-on field is due to ultrathin edges of 2D Mxene nanosheets with the oxygen (O) and hydroxyl (OH)-terminated surfaces, imparting a negative surface charge to reduce the potential energy barrier. This surface type allows the tunneling of electrons into the vacuum.

The researchers envisaged the great potential of MXene nanosheets in creating a high-performance electron emitter. DFT calculations revealed that the interaction of Ti3C2 MXene with the -OH functional group is the result of charge transfer from former to latter.

X-ray diffraction (XRD) studies on the as-prepared Ti3SiC2 MAX phase after etching and delamination showed the presence of two diffraction peaks at 9.9 and 39.5 degrees that indexed the planes (002) and (104) and confirmed the formation of Ti3SiC2 MAX phase. 

The XRD pattern of Ti3C2TX MXene showed a blue shift with the characteristic (002) plane from 9.9 and 8.9 degrees, confirming the etching of the "Si" layer in the Ti3SiC2 MAX phase. The (002) plane shifted from 8.9 to 6.1 degrees, which was attributed to tetrabutylammonium (TBA+) ions which intercalated between Ti3C2TX layers and increased d spacing.

The field emission scanning electron microscope (FESEM) image of the Ti3SiC2 MAX phase revealed the presence of compact, plate-like, and stacked morphology of Ti3SiC2 MAX powder with a smooth surface. The well-defined structure of the Ti3SiC2 MAX phase was confirmed by Ti3C2TX MXene, with each layer having a thickness of 1 to 2 nanometers and interlayer distance of about 1.5 nanometers.

Energy-dispersive X-ray (EDX) spectra revealed a decrease in Si atomic weight in Ti3C2TX MXene, indicating successful etching of Si in the Ti3SiC2 MAX phase. The presence of fluorine (-F) and -O in Ti3C2TX MXene indicated the etching of HF. Transmission electron microscope (TEM) images of Ti3SiC2 showed a particle size of 20 nanometers.

In conclusion, to construct a vacuum electronic devise, the team explored the field emission properties of Ti3C2TX MXene nanosheets and their precursor Ti3SiC2 MAX phase. The measured turn-on and threshold field of Ti3C2TX MXene were 4.7 and 5 volts per micrometer, respectively, and for the Ti3SiC2 MAX phase, the same were 6.5 and 7.5 volts per micrometer, respectively.

Ti3C2TX MXene exhibited superior field emission properties due to −OH, −O, −F terminal groups, which contributed to the reduction in electron tunneling potential barrier and the consequent emission.

Kiran, N U., Deore, Amol B., More, M.A. et.al. (2022) Comparative Study of Cold Electron Emission from 2D Ti3C2TX MXene Nanosheets with Respect to Its Precursor Ti3SiC2 MAX Phase. ACS Applied Electronic Materials. https://pubs.acs.org/doi/10.1021/acsaelm.2c00128

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Bhavna Kaveti is a science writer based in Hyderabad, India. She has a Masters in Pharmaceutical Chemistry from Vellore Institute of Technology, India, and a Ph.D. in Organic and Medicinal Chemistry from Universidad de Guanajuato, Mexico. Her research work involved designing and synthesizing heterocycle-based bioactive molecules, where she had exposure to both multistep and multicomponent synthesis. During her doctoral studies, she worked on synthesizing various linked and fused heterocycle-based peptidomimetic molecules that are anticipated to have a bioactive potential for further functionalization. While working on her thesis and research papers, she explored her passion for scientific writing and communications.

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