The properties of nanoribbon edges play a crucial role in their potential applications in electronic devices, sensors, and catalysts.
A team of researchers from Japan and China recently conducted a study on the mechanical behavior of single-layer molybdenum disulfide nanoribbons with armchair edges, using in situ transmission electron microscopy. Their findings revealed that Young's modulus of the nanoribbons decreased as their width decreased below 3 nm.
In the modern world, sensors have turned out to be ubiquitous, with applications varying from detecting explosives and quantifying physiological spikes of glucose or cortisol non-invasively to evaluating greenhouse gas levels present in the air.
The main technology needed for sensors is a mechanical resonator. Conventionally, quartz crystals have been utilized for this as a result of their availability and high rigidity. Recently, this technology has set the stage for advanced nanomaterials. One such material is the single-walled molybdenum disulfide (MoS2) nanoribbon.
Determining the chemical and physical properties of nanoribbon edges is vital for their applications in sensors, electronic devices, and catalysts. However, the mechanical response of MoS2 nanoribbons—anticipated to be reliant on their edge structure—has remained undiscovered, thereby impeding their practical implementation in thin resonators.
Against this background, a research group from Japan and China, headed by Professor Yoshifumi Oshima from Japan Advanced Institute of Science and Technology (JAIST), has analyzed the mechanical properties in recent times—namely the Young’s modulus—of single-layer MoS2 nanoribbons along with armchair edges as a function of their width utilizing a micromechanical measurement technique.
Their study, reported in the Advanced Science journal, was co-authored by Associate Professor Kenta Hongo and Professor Ryo Maezono from JAIST, Lecturer Chunmeng Liu, and Lecturer Jiaqi Zhang from Zhengzhou University, China.
We have developed the world’s first micromechanical measurement method to clarify the relationship between the atomic arrangement of atomic-scale materials and their mechanical strength by incorporating a quartz-based length extension resonator (LER) in an in situ transmission electron microscopy (TEM) holder.
Yoshifumi Oshima, Professor, Japan Advanced Institute of Science and Technology
As the resonance frequency of a quartz resonator alters when it senses contact with a material, the equivalent spring constant of the material could be evaluated with high accuracy by the change in this resonance frequency.
Furthermore, it is feasible to capture high-resolution TEM images as the LER vibration amplitude essential for the measurement is as small as 27 pm. The novel technique developed by the scientists controlled to conquer the flaws of traditional methods, thereby obtaining high-precision measurements.
Initially, the scientists synthesized a single-layer MoS2 nanoribbon by peeling off the outermost layer of the folded edge of a MoS2 multilayer utilizing a tungsten tip. Consequently, the single-layer nanoribbon was supported between the multilayer and the tip. The TEM image of this MoS2 nanoribbon disclosed that its edge consisted of an armchair structure.
The width and length of the nanoribbon were also measured from the image, and the corresponding equivalent spring constant was determined from the frequency shift of the LER to obtain the Young's modulus of this nanoribbon.
Chunmeng Liu, Lecturer, Zhengzhou University
The scientists discovered that the Young’ modulus of the single-layer MoS2 nanoribbons with armchair edges was reliant on their width.
While it stayed constant at around 166 GPa for broader ribbons, it displayed an inverse relation to the width for ribbons less than 3 nm in width, increasing from 179 GPa to 215 GPa as the nanoribbon width reduced from 2.4 nm to 1.1 nm. The scientists attributed this to a higher bond stiffness for the edges in comparison to that of the interior.
Furthermore, density functional theory calculations executed by the scientists for describing their observation disclosed that the Mo atoms buckled at the armchair edge, which led to an electron transfer to the S atoms on both sides. This, in return, increased the Coulombic attraction between the two atoms, thereby improving the edge strength.
The current study provides significant knowledge on the mechanical properties of MoS2 nanoribbons, which could help in streamlining the design of nanoscale, ultra-thin mechanical resonators.
Nanosensors based on such resonators can be integrated into smartphones and watches, which will enable people to monitor their environment as well as communicate the sense of taste and smell in the form of numerical values.
Jiaqi Zhang, Lecturer, Zhengzhou University
Liu, C., et al. (2023) Stiffer Bonding of Armchair Edge in Single-Layer Molybdenum Disulfide Nanoribbons. Advanced Science. doi.org/10.1002/advs.202303477.