So what do you do there at the university? Chapter 8: One molecule, that's all it takes - about nanoelectronics

In the nanoelectronics laboratory, we are interested in developing futuristic electrical components in which the size of the smallest element will be of the order of nanometers

I met with Rani Arieli to ask him what is being done there at the university.

Rani is a PhD student in physical chemistry at Tel Aviv University and works in Dr. Yoram Seltzer's nanoelectronics laboratory. He did his previous degrees in the field of physics at Tel Aviv University as well. Rani is a music lover and in his free time enjoys playing the guitar.

Rani, so what are you doing there?

At our nanoelectronics laboratory, we are interested in developing futuristic electrical components in which the size of the smallest element will be of the order of nanometers (nanometer = 10-9 meters, for comparison the diameter of a hair is of the order of several tens of micrometers, micrometer = 10-6 meters).

Can you give an example?

Yes, of course. Imagine for example a gold wire that is used to conduct electricity. The wire can be stretched to the point where in a certain place, just before creating a tear or break, a chain of individual atoms is formed. It is a simple type of electrical device on the nanometer scale. The advantage of such a device is that the electrical conduction in it is 'ballistic', meaning that the electrons pass through it without collisions with atoms and therefore at faster rates. This could lead to more interesting electrical devices. But it really doesn't end here.

We can tie in the middle of the chain, between the gold atoms, an organic molecule which is a molecule made up mainly of carbon and hydrogen atoms. Each such molecule has different electronic properties, for example a different arrangement of the energy values ​​that the electrons in the molecule can receive or a different position of them in space. Therefore, the molecule will affect the conduction characteristic of the electrical component in a different way. For example, the type of molecule affects the chance of an electron of a certain energy passing through it from one gold contact to another. So the molecule affects the relationship between current and electric voltage and essentially determines the nature of the component.

The variety of organic molecules is huge, so we can produce a wide variety of devices with different characteristics. In fact, there is potential here for a modular component that can be 'tailored' according to the user's needs.

How do you make one organic molecule settle between the gold atoms in the chain?

One of the methods, for example, is to start with two gold contacts connected to each other in the form of a bow tie and gently move them away from each other (see photo 2). At some point the narrow junction region is stretched enough that a chain of gold atoms is obtained. Careful continuation of the stretching action will eventually cause the chain to break, so that the detached gold atoms are still close to each other. Now one of the possibilities is to use organic molecules containing a chemical element called thiol. This element is composed of a sulfur atom and a hydrogen atom, and easily bonds to gold. If the molecule contains two thiols, it can bind to two gold atoms and complete the disconnected chain. All we have to do is drip onto the junctions a solution with the appropriate molecules under the right conditions and they do the rest of the work by themselves.

So what do you do with these necklaces?

I will give you an example, in one of the projects in the laboratory we projected laser light on the gold contacts of one of these devices (see photo 3a). What we saw is that the relationship between the voltage and the electric current of the device, i.e. its characteristic, changed following the activation of the laser. We then repeated the experiment with chain-like organic molecules of different lengths and showed that the longer the molecule, the smaller the effect of the laser.

What is the connection between all these things?

There are two types of interaction between light and an electromagnetic field (EMF): a scattering process in which an electron absorbs a photon and rises to a higher energy level, and a process in which the EMF changes the electric potential of the electron system. Both processes lead to an effective increase in the energy of the electrons so that they can now more easily jump over troublesome obstacles, such as an organic molecule, that prevent them from reaching the second gold electrode. In the second electrode there are lots of free energy states for electrons, exactly what electrons like. But that's only part of the story.

One of the important properties of metals is that they are flooded with a sea of ​​electrons that are not bound to their atoms and are just looking for an opportunity to participate in electrical conduction. When light is irradiated on a metal, part of it is absorbed and part is reflected (depending on the wavelength and the dielectric coefficient which is the extent of the effect of an electric field on the material). However, for every metal there is a range of wavelengths where the light is neither exactly absorbed nor exactly reflected, but causes something special.

When we project the laser at this wavelength onto the gold, the many free electrons on the surface begin to move under the influence of the radiation. The movement of the electrons is cyclical and creates fluctuations in their density, thus causing fluctuations in the electric charge (plasmons).

What is the contribution of the electron vibrations to the process?

The fluctuations in the charge spread over the entire electrode like sea waves, and in this way it is possible to more easily manufacture devices in which the area of ​​interest is not exposed to illumination and still be able to be influenced by illumination. Also, when the charge waves propagate towards the tip of the electrode, the electric field induces opposite charges at the tip of The second electrode (see picture 3b). This increases the original optical field and makes it even easier for the electrons to move to the electrode The second is through the organic molecule. All this without applying an additional external voltage, so we gain twice on the laser.

And what about the molecules in this experiment?

We experimented with molecules with identical electronic properties but different lengths. That is, in the experiment the role of the molecules was to determine the distance between the two electrodes. What we saw was that the longer the molecules, the weaker the current we got.

This result can be directly explained by the effect of charge fluctuations. The farther the electrodes were from each other, the less the charge fluctuations at the end of one electrode affected the end of the other electrode. That is, the increase in the electric field was smaller and it was more difficult for the electron to pass the barrier compared to the case where the length of the molecule was shorter.

So what did we have? can you summarize

Today there is a state of uncertainty in the scientific community as to the amount of amplification that occurs due to the formation of plasmons. This situation arose due to the use by each research group of devices with different dimensions and molecular configurations. With the help of the experiment we were able to quantify the phenomenon of the plasmonic amplification due to the illumination and present values ​​for it as a function of the distance between the two electrodes.

In the research, we showed that we are able to compress light into a nanometer structure consisting of a molecular junction, thereby influencing the conduction of the device. Since previous studies have demonstrated the ability to control the progression of plasmons in time, this method can also be used to study the dynamics of electrical conduction in very short times. This dynamic is affected by a number of phenomena that can occur in the device due to the interaction of the electrons with other phenomena in the device, for example vibrations. We marked this topic as our next goal!