In a recent article in the journal ACS Applied Electronic Materials, researchers synthesized nanosheets of two-dimensional (23) transition metal (2D) carbide, nitride, and / or carbonitride nanosheets (Ti3C2TX MXene) and studied the emission of both Ti3C2TX electrons. MXene as Ti3SiC2 MAX to inform the design of vacuum electronic devices.
Comparative study of the emission of cold electrons from 2D Ti3C2TX MXene nanosheets with respect to their precursor phase Ti3SiC2 MAX. Image credit: sKjust / Shutterstock.com
The nanosheets were prepared by etching the MAX phase of titanium silicon carbide (Ti3SiC2), which was synthesized by heating the mixture of elemental titanium (Ti), silicon (Si), and carbon (C) to high temperature. Functional density theory (DFT) simulations determined the electronic and structural properties of Ti3C2TX MXene and Ti3SiC2 MAX.
Field emitting materials
A deformation of the surface potential barrier that occurs in a metal almost becomes triangular in the presence of a strong electric field. If the width of the deformed potential barrier at the Fermi energy level is equal to the De Broglie wavelength of an electron, the probability of finding an electron outside the barrier becomes nonzero; this is called field emission (FE). Since the probability of tunneling is explained at 0 kelvin, it is called cold emission and the electro emitter is called field emitter.
The field electron emission mechanism takes the working function (Φ) and geometry of the emitting material, giving special focus to the field emission behavior (FE) of one-dimensional nanostructured materials. To this end, promising one-dimensional (1D) field emitting materials include molybdenum (Mo), tungsten (W), carbon nanotubes and semiconductors such as zinc oxide (ZnO), titanium oxide (TiO2), tooth oxide (SnO2), tungsten oxide (WO3) and more.
The efficiency and applicability of field emitters are controlled by factors such as low working function, morphology, ease of synthesis, and stable emission current. The temporary stability of the 1D field emitter suffers from the disadvantage of the “burnout” of the tip. To this end, carbon and non-carbon thin film field emitters can overcome these limitations.
In addition to 1D planar emitters, 2D material such as graphene was considered to study FE characteristics. Extensive research into graphene- and graphene-based hybrids / composites showed their efficiency as vacuum field emitting devices, but their manufacturing methods restrict their practical applicability.
MXenes are 2D materials with unique layered structures with attractive properties. MXenes morphology adapts to single / multilayer depending on the engraving methods. Due to its huge specific area, Mxenes has a wide range of applications in energy detection, conversion and storage, photocatalysis and adsorption. Thus, there is a demand for new research on Mxenes.
2D Ti3C2TX Mxene cold electron emission
In the present work, the authors applied 1 microamperes per square centimeter of current density and achieved an ignition voltage of 4.7 volts per micrometer for pristine Ti3C2TX Mxene, without any reconstruction of morphology or surface treatment. This reduced ignition field is due to the ultra-thin edges of Mxene 2D nanosheets with oxygen (O) and hydroxyl (OH) finished surfaces, which impart a negative surface charge to reduce the potential energy barrier. This type of surface allows the electron tunnel to be vacuumed.
Researchers predicted the great potential of MXene nanosheets to create a high-performance electron emitter. DFT calculations revealed that the interaction of Ti3C2 MXene with the -OH functional group is the result of the charge transfer from the first to the second.
Research results
X-ray diffraction (XRD) studies on the Ti3SiC2 MAX phase as prepared after etching and delamination showed the presence of two diffraction peaks at 9.9 and 39.5 degrees which indexed the planes ( 002) and (104) and confirmed the formation of Ti3SiC2 MAX. phase.
The XRD pattern of Ti3C2TX MXene showed a blue offset with the characteristic plane (002) of 9.9 and 8.9 degrees, confirming the engraving of the “Si” layer in the Ti3SiC2 MAX phase. The plane (002) went from 8.9 to 6.1 degrees, which was attributed to the tetrabutylammonium ions (TBA +) that interspersed between the Ti3C2TX layers and increased the space d.
The image of the Ti3SiC2 MAX phase field emission scanning electron microscope (FESEM) revealed the presence of a 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 1 to 2 nanometers thick and a layer spacing of about 1.5 nanometers.
Energy dispersion X-ray (EDX) spectra revealed a decrease in the atomic weight of Si in Ti3C2TX MXene, indicating a successful etching of Si in the Ti3SiC2 MAX phase. The presence of fluorine (-F) and -O in Ti3C2TX MXene indicated HF etching. Ti3SiC2 transmission electron microscope (TEM) images showed a particle size of 20 nanometers.
Conclusion
In conclusion, to build an electronic vacuum device, the team explored the field emission properties of the Ti3C2TX MXene nanosheets and their precursor phase Ti3SiC2 MAX. The measured ignition and threshold field of Ti3C2TX MXene were 4.7 and 5 volts per micrometer, respectively, and for the Ti3SiC2 MAX phase, they were 6.5 and 7.5 volts per micrometer, respectively.
Ti3C2TX MXene showed superior field emission properties due to the -OH, -O, -F terminal groups, which contributed to the reduction of the electron tunnel potential barrier and the consequent emission.
Reference
Kiran, N U., Deore, Amol B., More, MA et al. (2022) Comparative study of the emission of cold electrons from 2D Ti3C2TX MXene nanosheets with respect to their precursor phase Ti3SiC2 MAX. Applied electronic materials ACS. https://pubs.acs.org/doi/10.1021/acsaelm.2c00128
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