Tag Archives: Bo Li

The brittleness of molybdenum diselenide

With the finding that molybdenum diselenide is not as strong as previously believed, industry may want to reconsider 2D materials before incorporating them in new products according to a Rice University (Texas, US) scientist. From a Nov. 14, 2016 news item on Nanowerk,

Scientists at Rice University have discovered that an atom-thick material being eyed for flexible electronics and next-generation optical devices is more brittle than they expected.

The Rice team led by materials scientist Jun Lou tested the tensile strength of two-dimensional, semiconducting molybdenum diselenide and discovered that flaws as small as one missing atom can initiate catastrophic cracking under strain.

The finding may cause industry to look more carefully at the properties of 2-D materials before incorporating them in new technologies, he said.

 

A Nov. 14, 2016 Rice University news release (also on EurekAlert), which originated the news item, provides more insight into the research,

“It turns out not all 2-D crystals are equal,” said Lou, a Rice professor of materials science and nanoengineering. “Graphene is a lot more robust compared with some of the others we’re dealing with right now, like this molybdenum diselenide. We think it has something to do with defects inherent to these materials.”

The defects could be as small as a single atom that leaves a vacancy in the crystalline structure, he said. “It’s very hard to detect them,” he said. “Even if a cluster of vacancies makes a bigger hole, it’s difficult to find using any technique. It might be possible to see them with a transmission electron microscope, but that would be so labor-intensive that it wouldn’t be useful.”

Molybdenum diselenide is a dichalcogenide, a two-dimensional semiconducting material that appears as a graphene-like hexagonal array from above but is actually a sandwich of metallic atoms between two layers of chalcogen atoms, in this case, selenium. Molybdenum diselenide is being considered for use as transistors and in next-generation solar cells, photodetectors and catalysts as well as electronic and optical devices.

Lou and colleagues measured the material’s elastic modulus, the amount of stretching a material can handle and still return to its initial state, at 177.2 (plus or minus 9.3) gigapascals. Graphene is more than five times as elastic. They attributed the large variation to pre-existing flaws of between 3.6 and 77.5 nanometers.

Its fracture strength, the amount of stretching a material can handle before breaking, was measured at 4.8 (plus or minus 2.9) gigapascals. Graphene is nearly 25 times stronger.

Part of the project led by Rice postdoctoral researcher Yingchao Yang required moving molybdenum diselenide from a growth chamber in a chemical vapor deposition furnace to a microscope without introducing more defects. Yang solved the problem using a dry transfer process in place of a standard acid washing that would have ruined the samples.

To test samples, Yang placed rectangles of molybdenum diselenide onto a sensitive electron microscope platform invented by the Lou group. Natural van der Waals forces held the samples in place on springy cantilever arms that measured the applied stress.

Lou said the group attempted to measure the material’s fracture toughness, an indicator of how likely cracks are to propagate, as they had in an earlier study on graphene. But they found that pre-cutting cracks into molybdenum diselenide resulted in it shattering before stress could be applied, he said.

“The important message of this work is the brittle nature of these materials,” Lou said. “A lot of people are thinking about using 2-D crystals because they’re inherently thin. They’re thinking about flexible electronics because they are semiconductors and their theoretical elastic strength should be very high. According to our calculations, they can be stretched up to 10 percent.

“But in reality, because of the inherent defects, you rarely can achieve that much strength. The samples we have tested so far broke at 2 to 3 percent (of the theoretical maximum) at most,” Lou said. “That should still be fine for most flexible applications, but unless they find a way to quench the defects, it will be very hard to achieve the theoretical limits.”

 

When seen from above, the atoms in two-dimensional molybdenum diselenide resemble a hexagonal grid, like graphene. But in reality, the darker molybdenum atoms are sandwiched between top and bottom layers of selenide atoms. Rice University researchers tested the material for its tensile strength. Courtesy of the Lou Group

When seen from above, the atoms in two-dimensional molybdenum diselenide resemble a hexagonal grid, like graphene. But in reality, the darker molybdenum atoms are sandwiched between top and bottom layers of selenide atoms. Rice University researchers tested the material for its tensile strength. Courtesy of the Lou Group

Here’s a link to and a citation for the paper,

 

Brittle Fracture of 2D MoSe2 by Yingchao Yang, Xing Li, Minru Wen, Emily Hacopian, Weibing Chen, Yongji Gong, Jing Zhang, Bo Li, Wu Zhou, Pulickel M. Ajayan, Qing Chen, Ting Zhu, and Jun Lou. Advanced Materials DOI: 10.1002/adma.201604201 Version of Record online: 3 NOV 2016

© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

This paper is behind a paywall.

2-D melting and surfacing premelting of a single particle

Scientists at the Hong Kong University of Science and Technology (HKUST) and the University of Amsterdam (in the Netherlands) have measured surface premelting with single particle resolution. From a March 15, 2016 HKUST news release on EurekAlert,

The surface of a solid often melts into a thin layer of liquid even below its melting point. Such surface premelting is prevalent in all classes of solids; for instance, two pieces of ice can fuse below 0°C because the premelted surface water becomes embedded inside the bulk at the contact point and thus freeze. Premelting facilitates crystal growth and is critical in metallurgy, geology, and meteorology such as glacier movement, frost heave, snowflake growth and skating. However, the causative factors of various premelting scenarios, and the effect of dimensionality on premelting are poorly understood due to the lack of microscopic measurements.

To this end, researchers from the Hong Kong University of Science and Technology (HKUST) and University of Amsterdam conducted a research where they were able to measure surface premelting with single-particle resolution for the first time by using novel colloidal crystals. They found that dimensionality is crucial to bulk melting and bulk solid-solid transitions, which strongly affect surface melting behaviors. To the surprise of the researchers, they found that a crystal with free surfaces (solid-vapor interface) melted homogenously from both surfaces and within the bulk, in contrast to the commonly assumed heterogeneous melting from surfaces. These observations would provide new challenges on premelting and melting theories.

The research team was led by associate professor of physics Yilong Han and graduate student Bo Li from HKUST. HKUST graduate students Feng Wang, Di Zhou, Yi Peng, and postdoctoral researcher Ran Ni from University of Amsterdam in Netherlands also participated in the research.

Micrometer sized colloidal spheres in liquid suspensions have been used as powerful model systems for the studies of phase transitions because the thermal-motion trajectories of these “big atoms” can be directly visualized under an optical microscope. “Previous studies mainly used repulsive colloids, which cannot form stable solid-vapor interfaces,” said Han. “Here, we made a novel type colloid with temperature-sensitive attractions which can better mimic atoms, since all atoms have attractions, or otherwise they cannot condense into stable solid in air. We assembled these attractive spheres into large well-tunable two-dimensional colloidal crystals with free surfaces for the first time.

“This paves the way to study surface physics using colloidal model systems. Our first project along this direction is about surface premelting, which was poorly understood before. Surprisingly, we found that it is also related to bulk melting and solid-solid transitions,” Han added.

The team found that two-dimensional (2D) monolayer crystals premelted into a thin layer of liquid with a constant thickness, an exotic phenomenon known as incomplete blocked premelting. By contrast, the surface-liquid thickness of the two- or three-layer thin-film crystal increased to infinity as it approaches its melting point, i.e. a conventional complete premelting. Such blocked surface premelting has been occasionally observed, e.g. in ice and germanium, but lacks theoretical explanations.

“Here, we found that the premelting of the 2D crystal was triggered by an abrupt lattice dilation because the crystal can no longer provide enough attractions to surface particles after a drop in density.” Li said. “Before the surface liquid grew thick, the bulk crystal collapsed and melted due to mechanical instability. This provides a new simple mechanism for blocked premelting. The two-layer crystals are mechanically stable because particles have more neighbors. Thus they exhibit a conventional surface melting.”

As an abrupt dilation does not change the lattice symmetry, this is an isostructural solid-solid transition, which usually occurs in metallic and multiferroic materials. The colloidal system provides the first experimental observation of isostructural solid-solid transition at the single-particle level.

The mechanical instability induced a homogenous melting from within the crystal rather than heterogeneous melting from the surface. “We observed that the 2D melting is a first-order transition with a homogeneous proliferation of grain boundaries, which confirmed the grain-boundary-mediated 2D melting theory.” said Han. “First-order 2D melting has been observed in some molecular monolayers, but the theoretically predicted grain-boundary formation has not been observed before.”

Here’s a link to and a citation for the paper,

Imaging the Homogeneous Nucleation During the Melting of Superheated Colloidal Crystals by Ziren Wang, Feng Wang, Yi Peng, Zhongyu Zheng, Yilong Han. Science  05 Oct 2012:
Vol. 338, Issue 6103, pp. 87-90 DOI: 10.1126/science.1224763

This paper is behind a paywall.