Micromanipulator: Here, a glass plate with polymer is placed on a sample holder.  Glass plate with polymer gripper.   Aluminium sample holder.   Microscope.  Illustration: Kirchgassner.dk

New atom building method

Friday 02 Dec 16
by Tom Nervil


Peter Bøggild
DTU Physics
+45 21 36 27 98
DTU researchers have developed a method of constructing atom-thin materials, paving the way for developing materials with new properties.

The invention of the electron microscope 90 years ago enabled scientists for the first time to ‘see’ nature’s microscopic components on the nanoscale. Over time, scientists across many fields wanted not only to study nature’s smallest components, but also to use them as building blocks. Nanotechnology’s ultimate dream is to be able to arrange the individual atoms, thereby creating materials with novel properties.

More unusual materials

Since then, researchers have realized that while unique, graphene is not the only atom-thin material.

“The amazing thing is that graphene is just one of several thousand two-dimensional materials that offer an incredibly wide range of properties covering everything you could possibly want,” says Peter Bøggild, professor at DTU Nanotech.

"The huge potential for these unusual materials is only really triggered when you start to combine them—something we are now able to do with atomic precision here at DTU."
Professor Peter Bøggild, DTU Nanotech

“The huge potential for these unusual materials is only really triggered when you start to combine them—something we are now able to do with atomic precision here at DTU.”

Peter Bøggild and his research group have namely developed a new method to layer atom-thin 2D materials without damaging the layers.

“It’s a bit like laminate flooring. When combined, the different components—foil and various wood layers—provide high tensile strength, easy maintenance, heat insulation, and a high degree of flexibility—all in a single product. We can now do the same thing at the atomic level,” says Peter Bøggild.

When you laminate the atom-thin layer, you can tailor the electrical, optical, and mechanical properties in a new way. However, layering the two-dimensional layers on top of each other without contamination—e.g. from undesirable particles, bubbles, and molecules in between the layers which can easily damage the properties—has proved challenging.

“So we decided to find a solution,” says Peter Bøggild. “Two of our PhD students visited the leading research group in the field at Columbia University in New York in order to learn how the group’s newly developed atom-layering method worked. During their stay, our PhD students not only learned the method, but added a range of crucial improvements—something we call ‘hot pickup,” he explains.

Temperature is crucial

The method consists of precisely controlling the temperature to vary the attraction between the ‘hand’ that picks and places the atom-thin layers and heating under ‘the laminate’ to eliminate impurities between the layers. The improved control means that you can build three-dimensional architectures, where the atom-thin layers are related not only horizontally, but vertically as well. Another advantage is that the method is relatively easy to master and produces predictable results. Peter Bøggild therefore expects more research groups to start building complex structures of atom-thin layers at a high international level.

“Solar cells, sensors, and light-emitting diodes featuring the two-dimensional supermaterials have already seen the light of day, but this is just the beginning. Given the almost infinite combinations in the building of structures with individual atom-thin layers, we are really only limited by our imagination. We are talking about 2,500 different substances that can now be combined freely without restrictions at the atomic level. From a material research perspective, this is an event with far-reaching implications,” concludes Peter Bøggild.

How to build materials in 2D

DTU Nanotech has developed a method whereby atom-thin 2D materials are nano-laminated layer by layer without breaking. Here is the recipe for how graphene is combined with an insulating layer of the 2D material boron nitride that protects the graphene from external influences, vastly speeding up graphene electronics.

Translations to illustrations:

1. Pick-up:
The glass plate with the polymer gripper is brought into contact with a flake of boron nitride on the surface of the sample holder. The PPC polymer changes from solid to sticky when the temperature is increased from 30 to 55 degrees.
PDMS = Polydimethyisiloxane
PPC = Polypropylene carbonate

2. Cooling:
By cooling down to 40 degrees or below, the polymer becomes solid again and boron nitride can now be collected.

Illustration: Kirchgassner.dk
3. Drop-down:
The boron nitride can now be lowered down over a graphene layer at a temperature of 110 degrees. Water vaporizes from the surface, while impurities and dirt are squeezed out (a). The polymer gripper is then carefully peeled away, leaving the layered material on the surface (b).

4. Baking:
By heating the stack of atom-thin film to between 130 and 170 degrees, a firm bond is established between the layers so that they act as a single unit.

Illustration: Kirchgassner.dk

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