The perfect atom sandwich requires an extra layer

Aug 05, 2014 by Anne Ju
The perfect atom sandwich requires an extra layer
The left figure demonstrates why the first double layer of strontium oxide is missing when growing a Ruddlesden-Popper oxide thin film. Titanium atoms (yellow) preferentially bond with oxygen atoms (gray) and sit at the center of a complete octahedron, making it energetically more favorable for titanium to switch positions with the topmost strontium oxide layer (red). Because of this, the first double layer of strontium oxide is always missing, and the extra layer rides the surface. By depositing an extra strontium oxide layer first, the desired first double layer is obtained. Credit: Yuefeng Nie

( —Like the perfect sandwich, a perfectly engineered thin film for electronics requires not only the right ingredients, but also just the right thickness of each ingredient in the desired order, down to individual layers of atoms.

Cornell researchers have discovered that sometimes, -by-layer atomic assembly – a powerful technology capable of making new materials for electronics – requires some unconventional "sandwich making" techniques.

The team, led by thin-films expert Darrell Schlom, the Herbert Fisk Johnson Professor of Industrial Chemistry in the Department of Materials Science and Engineering, describes the trick of growing perfect films of oxides called Ruddlesden-Poppers in Nature Communications Aug. 4.  

These oxides are widely studied for their electronically enticing properties, among them superconductivity, magnetoresistance and ferromagnetism. Their layered structure is like a double Big Mac with alternating double and single layers of meat patties – strontium oxide – and bread – titanium oxide – in the case of the Ruddlesden-Poppers studied.

"Our dream is to control these materials with atomic precision," Schlom said. "We think that controlling interfaces between Ruddlesden-Poppers will lead to exotic and potentially useful, emergent properties."

Schlom's lab makes novel thin films with molecular beam epitaxy, a deposition method that controls the order in which atom-thick layers are assembled layer-by-layer, which Schlom likens to precision spray-painting with atoms.

In experiments designed by first author and postdoctoral associate Yuefeng Nie, the researchers found a major difference between assembling atomically precise Ruddlesden-Popper films and the conventional layer-by-layer "sandwich making" of .

This discovery began when co-author Lena Kourkoutis, then a graduate student and now assistant professor of applied and engineering physics, noticed that sample after sample of Ruddlesden-Popper films spray-painted by Schlom's lab were missing a layer of strontium oxide.

"Imagine laying down two meat patties on a bun, followed by a layer of bread, and another two meat patties, only to find that the resulting sandwich consists of just one meat patty below the layer of bread and three above it," Nie explained. "This is the equivalent of what we found to occur with our layers of atoms."

 "After a while we asked, what's going on?" said co-author David A. Muller, professor of applied and engineering physics. "Where did that first layer go?"

It turned out that following a double layer of strontium and oxygen, the next layer of titanium atoms, instead of sitting on top as expected, seeps down between the two strontium oxide layers. That meant the missing first layer of strontium and oxygen ended up on the film's surface – a subtlety overlooked for years.  

To confirm this, researchers used a combination of techniques, including high-energy electron diffraction, X-ray spectroscopy, scanning electron microscopy, and quantum mechanical calculations.

 "This paper is about understanding that this flipping is going on," Schlom said. "The final sandwich structure is not simply the order in which we lay down the layers."

The researchers also designed a modification to their crystal growth to make a Ruddlesden-Popper film – this time truly perfect – by laying down an extra layer of strontium oxide to start. Understanding this growth process has helped them make atomically precise and sharp interfaces between Ruddleseden-Poppers, which paves the way for exploring and harnessing their useful properties for devices.

A competing paper by Argonne National Laboratory researcher June Lee, a graduate of Schlom's group, and published the same week in Nature Materials, arrived at similar conclusions by using different methods.

Explore further: Peeling back the layers of thin film structure and chemistry

More information: "Atomically Precise Interfaces From Non-Stoichiometric Deposition" is available online:

add to favorites email to friend print save as pdf

Related Stories

Interfaces are key in metal oxide superlattices

Sep 05, 2012

(—Materials called transition metal oxides have physicists intrigued by their potentially useful properties—from magnetoresistance (the reason a hard drive can write memory) to superconductivity.

Tunable antenna could end dropped cell phone calls

Oct 16, 2013

( —Why do cell phones drop calls? Like a radio dial tuned to different frequencies (stations), cell phone antennas have tuning circuits that quickly switch frequencies when controlled by a voltage ...

'Exotic' material is like a switch when super thin

Apr 18, 2014

( —Ever-shrinking electronic devices could get down to atomic dimensions with the help of transition metal oxides, a class of materials that seems to have it all: superconductivity, magnetoresistance ...

Recommended for you

Galaxy dust findings confound view of early Universe

Jan 31, 2015

What was the Universe like at the beginning of time? How did the Universe come to be the way it is today?—big questions and huge attention paid when scientists attempt answers. So was the early-universe ...

Evidence mounts for quantum criticality theory

Jan 30, 2015

A new study by a team of physicists at Rice University, Zhejiang University, Los Alamos National Laboratory, Florida State University and the Max Planck Institute adds to the growing body of evidence supporting ...

Scaling up armor systems

Jan 30, 2015

Dermal modification is a significant part of evolution, says Ranajay Ghosh, an associate research scientist in the College of Engineering. Almost every organism has something on its skin that provides important ...

Seeking cracks in the Standard Model

Jan 30, 2015

In particle physics, it's our business to understand structure. I work on the Large Hadron Collider (LHC) and this machine lets us see and study the smallest structure of all; unimaginably tiny fundamental partic ...

The first optically synchronised free-electron laser

Jan 30, 2015

Scientists at DESY have developed and implemented an optical synchronisation system for the soft X-ray free-electron laser FLASH, achieving facility-wide synchronisation with femtosecond precision. The performance ...

User comments : 0

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.