Paving the way for the transfer of information in small circuits

STEM (scanning transmission electron microscopy) Image of a one-dimensional matrix of F4TCNQ molecules (yellow-orange) in a graphene device for tuning the door. Credit: Berkeley Lab

Removing a charged molecule from a one-dimensional matrix causes others to “turn on” or “turn off,” paving the way for the transfer of information in tiny circuits.

Small electronic circuits feed our daily lives, from small cameras on our phones to microprocessors on computers. To make these devices even smaller, scientists and engineers design circuit components with a single molecule. In addition to providing advantages in increasing the density, speed, and energy efficiency of the device in miniaturized circuits, for example, in flexible electronics or data storage, devices with unique functionality can be created using the physical properties of specific molecules. However, the development of practical nanoelectronic devices from single molecules requires precise control over the electronic behavior of these molecules, and a reliable method of manufacturing them.

Now, the magazine reported Nature Electronics, researchers have developed a method for fabricating a set of individual molecules and precisely controlling their electronic structure. Carefully tuned the voltage applied to a chain of molecules embedded in a one-dimensional carbon (graphene) layer, a team led by researchers at the Lawrence Berkeley National Laboratory (Berkeley Lab) found that all molecules, none or some or some of them, were capable of controlling whether they had an electric charge. The resulting charge pattern would change across the chain by manipulating the individual molecules at the end of the chain.

“If you are going to build electrical devices from individual molecules, you need molecules with useful functionality and you have to figure out how to organize them into a usable model. We did both of these things in this work, ”said Michael Crommie, senior professor in the Materials Science Division at Berkeley Lab, who led the project. The research is located within the U.S. Department of Energy (DOE) Science Program for the Characterization of Functional Nanomachines. Its main goal is to understand the electrical and mechanical properties of molecular nanostructures and to create new nanomachines based on molecules, converting energy from one form to another at the nanoscale.

The main feature of a molecule rich in fluoride selected by the Berkeley Lab is a high tendency to accept electrons. To control the electronic properties of a specific aligned chain of 15 molecules placed on a graphene substrate, Crommie, also a professor of physics at UC Berkeley, and his colleagues placed a metal electrode under graphene. thin insulating layer. Applying the voltage between the molecules and the electrode inserts or removes electrons from the molecules. In this way, the graphene-assisted molecules act like a capacitor, an electrical component used in a charge storage and release circuit. But, unlike a “normal” macroscopic capacitor, by synthesizing the voltage of the bottom electrode, researchers can control which molecules were charged and which were kept neutral.

Molecular Chain

A set of one-dimensional molecules charged with electricity (blue dot) changes to neutral (empty dot) when the odd number of molecules is removed from the end of the model. This forces an electron to be a molecule from end to end, and thus the other molecules will change their charge state, thus changing the alternating charge pattern. Credit: Berkeley Lab

In previous studies of molecular assemblies, the electronic properties of molecules could not have been both tuned and represented on scales of atomic length. Without additional image capabilities the relationship between structure and function cannot be fully understood in the context of electrical devices. The molecules were placed on a specially designed graphene substrate developed at the nanoscale Molecular Foundry nanoscale at Berkeley Lab, where Crommie and his colleagues ensured that the molecules were fully accessible for microscopic observation and electrical manipulation.

As expected, they were filled with electrons by applying a strong positive voltage to the metal electrode under the graphene that supports the molecules, leaving the entire molecular set in a negative charge. Removing or reversing this stress caused all the attached electrons to leave the molecules, returning the entire matrix to neutral charge. At an intermediate voltage, however, the electrons fill only all the other molecules in the matrix, thus creating a “lady” charge pattern. Crommie and his team explain this through the new behavior that electrons repel each other. If the two charged molecules were to occupy adjacent sites for the time being, their repulsion would force one of the electrons to move away and place one site further away from the molecular row.

“We can do all the molecules without charge, all full or alternating. We call it the collective charge model because it is determined by the electron-to-electron repulsion throughout the structure, ”Crommie said.

It is estimated that in a set of molecules with alternating charges the terminal molecule of the matrix should always have an additional electron, as this molecule has no second neighbors to cause repulsion. To investigate this type of behavior experimentally, the Berkeley Lab team removed the last molecule from a set of molecules with alternating charges. They found that the original charge pattern was changed by one molecule: the charged sites were neutral and vice versa. The researchers concluded that before removing the charged terminal molecule, the molecule next to it must be neutral. In its new position at the end of the matrix, the first second molecule was charged. To maintain the alternating pattern between charged and uncharged molecules, the entire charge pattern had to be changed for one molecule.

If the charge of each molecule is thought to be a bit of information, the removal of the last molecule causes the entire information pattern to shift from one position to another. This behavior mimics the electronic displacement record of a digital circuit and offers new possibilities for transmitting information from one region of a molecular device to another. Moving a molecule to one end of an array can serve to turn the switch on or off elsewhere in the device, providing useful functionality for a future logic circuit.

“One thing we found really interesting about this result is that the electronic charge and so we’ve been able to change the properties of the molecules from a great distance. That level of control is something new,” Crommie said.

With their molecular assemblies, the researchers were able to create a structure with very specific functionality; that is, by applying a strain the molecular charges can be tuned between different possible states. Changing the charge on molecules causes a change in electronic behavior and, consequently, in the functionality of the entire device. This work is the result of a DOE effort to construct specific molecular nanostructures with well-defined electromechanical functionality.

The Berkeley Lab Group’s molecular charge control techniques can lead to new designs including nanoscale electronic component transistors and logic gates. The technique could also be generalized to other materials and incorporated into more complex molecular networks. One option is to tune the molecules to create more complex charge patterns. For example, replacing one atom with another it can change the properties of the molecule in one molecule. Placing these modified molecules in the matrix can create new functionality. Based on these results, the researchers aim to study the functionality that arises as a result of new variations in molecular enrollment, as well as whether they can be used as small circuit components. Ultimately, they plan to incorporate these structures into more practical nanoscale devices.

References: Hsin-Zon Tsai, Johannes Lischner, Arash A. Omrani, Franklin Liou, Andrew S. Aikawa, Christoph Karrasch, Sebastian Wickenburg, Alexander Riss, Kyler C. Natividad, Jin Chen, Won-Woo Choi, Kenji Watanabe, Takashi Taniguchi , Chenliang Su, Steven G. Louie, Alex Zettl, Jiong Lu and Michael F. Crommie, September 28, 2020, Nature Electronics.
DOI: 10.1038 / s41928-020-00479-4

Molecular Foundry is a facility for use by the DOE Office of Science located at Berkeley Lab.

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