Tag Archives: van der Waals interactions

Psst: secret marriage … Buckyballs and Graphene get together!

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A March 1, 2018 news item on Nanowerk announces  a new coupling,

Scientists combined buckyballs, [also known as buckminsterfullerenes, fullerenes, or C60] which resemble tiny soccer balls made from 60 carbon atoms, with graphene, a single layer of carbon, on an underlying surface. Positive and negative charges can transfer between the balls and graphene depending on the nature of the surface as well as the structural order and local orientation of the carbon ball. Scientists can use this architecture to develop tunable junctions for lightweight electronic devices.

The researchers have made this illustration of their work available,

Researchers are developing new, lightweight electronics that rapidly conduct electricity by covering a sheet of carbon (graphene) with buckyballs. Electricity is the flow of electrons. On these lightweight structures, electrons as well as positive holes (missing electrons) transfer between the balls and graphene. The team showed that the crystallinity and orientation of the balls, as well as the underlying layer, affected this charge transfer. The top image shows a calculation of the charge density for a specific orientation of the balls on graphene. The blue represents positive charges, while the red is negative. The bottom image shows that the balls are in a close-packed structure. The bright dots correspond to the projected images of columns of buckyball molecules. Courtesy: US Department of Energy Office of Science

A February 28, 2018 US Department of Energy (DoE) Office of Science news release, which originated the news item, provides more detail,

The Impact

Fast-moving electrons and their counterpart, holes, were preserved in graphene with crystalline buckyball overlayers. Significantly, the carbon ball provides charge transfer to the graphene. Scientists expect the transfer to be highly tunable with external voltages. This marriage has ramifications for smart electronics that run longer and do not break as easily, bringing us closer to sensor-embedded smart clothing and robotic skin.

Summary

Charge transfer at the interface between dissimilar materials is at the heart of almost all electronic technologies such as transistors and photovoltaic devices. In this study, scientists studied charge transfer at the interface region of buckyball molecules deposited on graphene, with and without a supporting substrate, such as hexagonal boron nitride. They employed ab initio density functional theory with van der Waals interactions to model the structure theoretically. Van der Waals interactions are weak connections between neutral molecules. The team used high-resolution transmission electron microscopy and electronic transport measurements to characterize experimentally the properties of the interface. The researchers observed that charge transfer between buckyballs and the graphene was sensitive to the nature of the underlying substrate, in addition, to the crystallinity and local orientation of the buckyballs. These studies open an avenue to devices where buckyball layers on top of graphene can serve as electron acceptors and other buckyball layers as electron donors. Even at room temperature, buckyball molecules were orientationally locked into position. This is in sharp contrast to buckyball molecules in un-doped bulk crystalline configurations, where locking occurs only at low temperature. High electron and hole mobilities are preserved in graphene with crystalline buckyball overlayers. This finding has ramifications for the development of organic high-mobility field-effect devices and other high mobility applications.

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

Molecular Arrangement and Charge Transfer in C60 /Graphene Heterostructures by Claudia Ojeda-Aristizabal, Elton J. G. Santos, Seita Onishi, Aiming Yan, Haider I. Rasool, Salman Kahn, Yinchuan Lv, Drew W. Latzke, Jairo Velasco Jr., Michael F. Crommie, Matthew Sorensen, Kenneth Gotlieb, Chiu-Yun Lin, Kenji Watanabe, Takashi Taniguchi, Alessandra Lanzara, and Alex Zettl. ACS Nano, 2017, 11 (5), pp 4686–4693 DOI: 10.1021/acsnano.7b00551 Publication Date (Web): April 24, 2017

Copyright © 2017 American Chemical Society

This paper is behind a paywall.

Upending the textbooks on Van Der Waals interactions

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It’s always exciting when textbook information has to be changed due to a discovery; it’s also a reminder that science is constantly changing. From a June 29, 2017 news item on Nanowerk (Note: A link has been removed),

Van der Waals interactions between molecules are among the most important forces in biology, physics, and chemistry, as they determine the properties and physical behavior of many materials. For a long time, it was considered that these interactions between molecules are always attractive. Now, for the first time, Mainak Sadhukhan and Alexandre Tkatchenko from the Physics and Materials Science Research Unit at the University of Luxembourg found that in many rather common situations in nature the van der Waals force between two molecules becomes repulsive. This might lead to a paradigm shift in molecular interactions.

“The textbooks so far assumed that the forces are solely attractive. For us, the interesting question is whether you can also make them repulsive,” Prof Tkatchenko explains. “Until recently, there was no evidence in scientific literature that van der Waals forces could also be repelling.”

Now, the researchers have shown in their paper, published in Physical Review Letters (“Long-Range Repulsion Between Spatially Confined van der Waals Dimers”), that the forces are, in fact, repulsive when they take place under confinement.

A June 30, 2017 Université de Luxembourg press release, which originated the news item, expands on the theme,

The ubiquitous van der Waals force was first explained by the German-American physicist Fritz London in 1930. Using quantum mechanics, he proved the purely attractive nature of the van der Waals force for any two molecules interacting in free space. “However, in nature molecules in most cases interact in confined spaces, such as cells, membranes, nanotubes, etc. In is this particular situation, van der Waals forces become repulsive at large distances between molecules,” says Prof. Tkatchenko.

Mainak Sadhukhan, the co-author of the study, developed a novel quantum-mechanical method that enabled them to model van der Waals forces in confinement. “We could rationalise many previous experimental results that remained unexplained until now. Our new theory allows, for the first time, for an interpretation of many interesting phenomena observed for molecules under confinement,” Mainak Sadhukhan says.

The discovery could have many potential implications for the delivery of pharmaceutical molecules in cells, water desalination and transport, and self-assembly of molecular layers in photovoltaic devices.

Prof. Tkatchenko’s research group is working on methods that model the properties of a wide range of intermolecular interactions. Only in 2016, they found that the true nature of these van der Waals forces differs from conventional wisdom in chemistry and biology, as they have to be treated as coupling between waves rather than as mutual attraction (or repulsion) between particles.

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

Long-Range Repulsion Between Spatially Confined van der Waals Dimers by Mainak Sadhukhan and Alexandre Tkatchenko. Phys. Rev. Lett. 118, 210402 (Vol. 118, Iss. 21 — 26 May 2017) DOI:https://doi.org/10.1103/PhysRevLett.118.210402 Published 25 May 2017

This paper is behind a paywall.

The birth of a molecule

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This research comes from Korea’s Institute of Basic Science in a Feb. 27, 2015 news item on Azonano,

The research team of the Center for Nanomaterials and Chemical Reactions at the Institute for Basic Science (IBS) has successfully visualized the entire process of bond formation in solution by using femtosecond time-resolved X-ray liquidography (femtosecond TRXL) for the first time in the world.

A Feb. 18, 2015 IBS press release, which originated the news item, provides more details,

Every researcher’s longstanding dream to observe real-time bond formation in chemical reactions has come true. Since this formation takes less than one picosecond, researchers have not been able to visualize the birth of molecules.

The research team has used femtosecond TRXL in order to visualize the formation of a gold trimer complex in real time without being limited by slow diffusion.

They have focused on the process of photoinduced bond formation between gold (Au) atoms dissolved in water. In the ground (S0) state, Au atoms are weakly bound to each other in a bent geometry by van der Waals interactions. On photoexcitation, the S0 state rapidly converts into an excited (S1) state, leading to the formation of covalent Au-Au bonds and bent-to-linear transition. Then, the S1 state changes to a triplet (T1) state with a time constant of 1.6 picosecond, accompanying further bond contraction by 0.1 Å. Later, the T1 state of the trimer transforms to a tetramer on nanosecond time scale, and Au atoms return to their original bent structure.

“By using femtosecond TRXL, we will be able to observe molecular vibration and rotation in the solution phase in real time,” says Hyotcherl Ihee, the group leader of the Center for Nanomaterials at IBS, as well as the professor of the Department of Chemistry at Korea Advanced Institute of Science and Technology.

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

Direct observation of bond formation in solution with femtosecond X-ray scattering by Kyung Hwan Kim, Jong Goo Kim, Shunsuke Nozawa, Tokushi Sato, Key Young Oang, Tae Wu Kim, Hosung Ki, Junbeom Jo, Sungjun Park, Changyong Song, Takahiro Sato, Kanade Ogawa, Tadashi Togashi, Kensuke Tono, Makina Yabashi, Tetsuya Ishikawa, Joonghan Kim, Ryong Ryoo, Jeongho Kim, Hyotcherl Ihee & Shin-ichi Adachi. Nature 518, 385–389 (19 February 2015) doi:10.1038/nature14163 Published online 18 February 2015

This paper is behind a paywall although there is a free preview via ReadCube access.