Quantum Jump of Electrons in Atomic Layers

Illustration of Laser shown onto multilayers. Image source:The University of Kansas

Electrons are tiny little negative charges that orbit around the nucleus of its atom. Electrons can behave in a peculiar manner – they don’t move around the way you’d expect. They often do something called a Quantum jump.

The way that electrons orbit around its atom’s nucleus depends on what valence shell (how far from the nucleus) they are on. These shells are at finite distances depending on the specific atom. This means that a group of electrons on the same shell will be the same distance away from the nucleus as one another.

However, an electrons can perform quantum jump and easily switch places amongst these shells.

QUANTUM JUMP IN A SINGLE ATOM
Quantum Jump : Photon in a Bohr atom
Image shows electronic jumps of an electron within shells in an atom by absorbing/emitting energy. Credits: Kurzon, CC BY-SA 3.0, via Wikimedia Commons

This classical picture of the electronic jumps can be a bit misleading. What makes electrons interesting is that they don’t need to move step by step up the orbitals. They can jump multiple places, leaving what’s known as a “hole” in its previously occupied shell.

Once a hole is created, an electron from a higher layer drops down into the vacant hole. The process of the electron moving down the layers emits a wavelength of energy.

Since the (layers) shells are finite, there are only a set distances that each electron can fall to. This explains why each atom has its own specific discrete wavelengths that it can emit.

This is a standard process for the excitation of electrons within an atom. But what happens when it’s a more complicated process with multiple layers of different compounds made up of different atoms?

QUANTUM JUMP IN TRILAYERS

Research carried out by The University of Kansas aimed to examine the motion of electrons in a trilayer MoS2–WS2–MoSe2 compound. They wanted to see specifically if the transfer of electrons from MoSe2 to MoS2 interacted with the WS2 layer at all.

The trilayer was made by using the scotch tape method to lift single molecular layers from their crystals. Each trilayer was optically examined under a microscope to make sure the structure was correct and consistent.

These three compounds are light sensitive semiconductor materials. This means that shining a laser pulse helps to stimulate the electrons. In this case, a laser was pulsed onto the layers for 100 femtoseconds to stimulate the electrons on the MoSe2 layer.

“The color of the laser pulse was chosen so that only electrons in the top layer can be liberated,” says Hiu Zhao, a researcher in this study. “If electrons were stimulated and emitted by any of the other 2 layers, it could give erroneous results. We then used another laser pulse with the ‘right’ color for the bottom MoS2 layer to detect the appearance of these electrons in that layer. The second pulse was purposely arranged to arrive at the sample after the first pulse by about 1 picosecond, by letting it travel a distance 0.3 mm longer than the first.”

As it turned out, to the surprise of the researchers, the electrons didn’t interact with the second layer whatsoever. Instead, in the one picosecond it took for the transfer to happen, the electron traveled from the third layer down to the first layer, completely bypassing the second layer.

A third and final pulse was used to examine the middle layer, which conclusively found that no electrons were present.

“If electrons were things that followed ‘common sense,’ like so-called classical particles, they’d be in the middle layer at some point during this one picosecond,” explained Hiu Zhao.

Quantum Jump
An image showing the movement of the excited electron moving from MoSe2 to MoS2. The WS2 layer is completely bypassed by the electron. Image source: American Chemical Society

 “This study showed electrons can transfer between these layers in a quantum fashion, just like in other conductors and semiconductors,” Zhao concluded.

HOW THESE RULES APPLY TO NANOTECHNOLOGY

These findings have interesting results when it comes to nanotechnology. Nanotechnology involves compounds that are potentially only one atom thick, sometimes stacked over each other with Van der Waals forces – like graphene.

By understanding the characteristics of electrons in this study, it can be more broadly applied to optoelectronics which employ a similar multi layer structure.

Source American Chemical Society The University of Kansas

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