Pyramid-Shaped Single-Crystalline Nanostructure of Molybdenum with Excellent Mechanical, Electrical, and Optical Properties

Abstract

Specific geometric morphology and improved crystalline properties are of great significance for the development of materials in micro–nano scale. However, for high-melting molybdenum (Mo), it is difficult to get high-quality structures exhibiting a single-crystalline nature and preconceived morphology simultaneously. In this paper, a pyramid-shaped single-crystalline Mo nanostructure was prepared through a thermal evaporation technique, as well as a series of experimental controls. Based on detailed characterizations, the growth mechanism was demonstrated to follow a sequential process that includes MoO2 decomposition and Mo deposition, single-crystalline islands formation, layered nucleation, and competitive growth. Furthermore, the product was measured to show excellent physical properties. The prepared nanostructures exhibited strong nano–indentation hardness, elastic modulus, and tensile strength in mechanical measurements, which are much higher than those of the Mo bulks. In the measurement of electronic characteristics, the individual structures indicated very good electrical transport properties, with a conductance of ∼0.16 S. The prepared film with an area of 0.02 cm2 showed large-current electron emission properties with a maximum current of 33.6 mA and a current density of 1.68 A cm–2. Optical properties of the structures were measured to show obvious electromagnetic field localization and enhancement, which enabled it to have good surface enhanced Raman scattering (SERS) activity as a substrate material. The corresponding structure–response relationships were further discussed. The reported physical properties profit from the basic features of the Mo nanostructures, including the micro–nano scale, the single-crystalline nature in each grain, as well as the pyramid-shaped top morphology. The findings may provide a potential material for the research and application of micro–nano electrons and photons.

Study on Pyramidal Molybdenum Nanostructures Cold Cathode with Large-Current Properties Based on Self-Assembly Growth Method

“…Under an operating pulsed electric field of 28 V μm −1 , the nanostructures after optimization present the largest current of 47.62 mA, and the highest current density of 2.38 A cm −2 , which are about 5 times better than the pristine structures with intermediate layer of 31.4 nm (∼9.84 mA and ∼0.49 A cm −2 ). Such extracted values show a big advantage compared with all of the other self-assembly grown Mo nanostructures reported so far, [22][23][24][25]27,36 and are much higher than most of other cathode materials (see Table 4). 9,38−58 It indicates the attractive perspective of the prepared pyramidal nanostructures for the power vacuum electron devices.…”

“…The pyramidal shape at the top was reported to form due to the single crystalline layered nucleation of body-centered cubic (bcc) Mo. 36 Here, it can be further sharpened by gradually widening the distance between the evaporating source and the substrate during the modified preparation. This way, less and less Mo atoms are deposited, and the final morphology of the nanostructure is trimmed.…”

“…This is mainly due to the competitive growth behavior of different individuals in the growth process. [22][23][24][25]36 The upper pyramid is about 1 μm in height. In our modified preparation, the pyramid was further sharpened artificially by depositing less and less Mo atoms during the growth.…”

“…In our recent work, a pyramidal nanostructure of pure Mo has been prepared by means of bottom-up self-assembled growth. 36 Such a nanostructure has been reported to show excellent single crystalline nature, mechanical strength, and conductivity. In this paper, we investigated the performance of this nanostructure as a large-current field emission cold cathode for possible power vacuum electronic device applications.…”

“…In our recent work, a pyramidal nanostructure of pure Mo has been prepared by means of bottom-up self-assembled growth . Such a nanostructure has been reported to show excellent single crystalline nature, mechanical strength, and conductivity.…”