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Molecular electronics for Next generation signal processing, data storage, and communication devices

Molecular electronics, also called moletronics, is the branch of nanotechnology where the molecular building blocks are used for the fabrication of electronic components. It is an interdisciplinary area that spans physics, chemistry, and materials science. The smaller size of the electronic components decreases power consumption while increasing the sensitivity (and sometimes performance) of the device. These have wide range of applications in the work areas of chemistry, physics, electronics and nano electronics, technology, artificial intelligence and medical equipment.

As the complexity and volume of global digital data grows, so too does the need for more capable and compact means of processing and storing data. Data storage and processing is central to Department of Defense (DoD) activities across areas including platform design and optimization, sensing, mission planning and logistics, and healthcare. While our current computational architectures remain essential, new complementary approaches are needed to provide advanced capabilities as the complexity and volume of data grows, says DARPA.

DARPA has launched  Molecular Informatics program with aim to discover and define future opportunities for molecules in  information storage and processing. Molecular Informatics program,  seeks a new paradigm for data storage, retrieval, and processing. Instead of relying on the binary digital logic of computers based on the Von Neumann architecture, Molecular Informatics aims to investigate and exploit the wide range of structural characteristics and properties of molecules to encode and manipulate data.

Molecular Electronics

Molecular electronics is defined as the encoding, manipulation, and retrieval of information at a molecular or macromolecular level. These functions are currently performed via lithographic manipulation of bulk materials to generate integrated circuits. Single-molecule devices appear to be ideal candidates for future nano-electronics, as they possess the potential for creating high-density devices with low power consumption in combination with high speed. The motion of the electrons in such devices is inherently governed by quantum mechanics. Molecular electronics provides promising new methodologies for high-speed signal processing and communication, volumetric data storage, novel associative and neural networks, and linear and nonlinear devices and memories.

But so far, no one has actually been able to make complex electrical circuits using molecules. One of the challenges is requirement of sophisticated nanoscopic structures and techniques to contact a single molecule to the outside world, due to the small size of the molecule. The main obstacle hindering progress in this field is the absence of stable contacts between the molecules and metals used that can both operate at room temperature and provide reproducible results.

Another challenge involves the realization of a technology for mass production of single molecule devices. If molecular devices can take advantage of self-assembly processes, then molecular devices may also feature low manufacturing costs. The only known molecules that can be pre-designed to self-assemble into complex miniature circuits, which could in turn be used in computers, are DNA molecules. But no one has yet been able to demonstrate reliably and quantitatively the flow of electrical current through long DNA molecules.

 

Graphene offers new functionalities in molecular electronics

A team of experimentalists from the University of Bern and theoreticians from NPL (UK) and the University of the Basque Country (UPV/EHU, Spain), with the help of collaborators from Chuo University (Japan), have demonstrated the stability of multi-layer graphene-based molecular electronic devices down to the single molecule limit. The findings, reported in the journal Science Advances, represent a major step change in the development of graphene-based molecular electronics, with the reproducible properties of covalent contacts between molecules and graphene (even at room temperature) overcoming the limitations of current state-of-the-art technologies based on coinage metals.

Graphene possesses not only excellent mechanical stability, but also exceptionally high electronic and thermal conductive properties, making the emerging 2-D material very attractive for a range of possible applications in molecular electronics.

An international team of researchers led by the National Physical Laboratory (NPL) and the University of Bern has revealed a new way to tune the functionality of next-generation molecular electronic devices using graphene. The results could be exploited to develop smaller, higher-performance devices for use in a range of applications including molecular sensing, flexible electronics, and energy conversion and storage, as well as robust measurement setups for resistance standards.

Adsorption of specific molecules on graphene-based electronic devices allows device functionality to be tuned, mainly by modifying its electrical resistance. However, it is difficult to relate overall device properties to the properties of the individual molecules adsorbed, since averaged quantities cannot identify possibly large variations across the graphene’s surface. Guided by the theoretical calculations of Dr Ivan Rungger (NPL) and Dr Andrea Droghetti (UPV/EHU), they demonstrated that variations on the graphite surface are very small and that the nature of the chemical contact of a molecule to the top graphene layer dictates the functionality of single-molecule electronic devices.

“We find that by carefully designing the chemical contact of molecules to graphene-based materials, we can tune their functionality,” said Dr Rungger. “Our single-molecule diodes showed that the rectification direction of electric current can be indeed switched by changing the nature of chemical contact of each molecule,” added Dr Rudnev. “We are confident that our findings represent a significant step towards the practical exploitation of molecular electronic devices, and we expect a significant change in the research field direction following our path of room-temperature stable chemical bonding,” summarised Dr Kaliginedi.

 

World’s smallest radio receiver has building blocks the size of two atoms

Researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences have made the world’s smallest radio receiver – built out of an assembly of atomic-scale defects in pink diamonds. This tiny radio—whose building blocks are the size of two atoms—can withstand extremely harsh environments and is biocompatible, meaning it could work anywhere from a probe on Venus to a pacemaker in a human heart. The research was led by Marko Loncar, the Tiantsai Lin Professor of Electrical Engineering at SEAS, and his graduate student Linbo Shao and published in Physical Review Applied.

The radio uses tiny imperfections in diamonds called nitrogen-vacancy (NV) centers. To make NV centers, researchers replace one carbon atom in a diamond crystal with a nitrogen atom and remove a neighboring atom—creating a system that is essentially a nitrogen atom with a hole next to it. NV centers can be used to emit single photons or detect very weak magnetic fields. They have photoluminescent properties, meaning they can convert information into light, making them powerful and promising systems for quantum computing, phontonics and sensing.

Radios have five basic components—a power source, a receiver, a transducer to convert the high-frequency electromagnetic signal in the air to a low-frequency current, speaker or headphones to convert the current to sound and a tuner.

In the Harvard device, electrons in diamond NV centers are powered, or pumped, by green light emitted from a laser. These electrons are sensitive to electromagnetic fields, including the waves used in FM radio, for example. When NV center receives radio waves it converts them and emits the audio signal as red light. A common photodiode converts that light into a current, which is then converted to sound through a simple speaker or headphone.

An electromagnet creates a strong magnetic field around the diamond, which can be used to change the radio station, tuning the receiving frequency of the NV centers. Shao and Loncar used billions of NV centers in order to boost the signal, but the radio works with a single NV center, emitting one photon at a time, rather than a stream of light.

The radio is extremely resilient, thanks to the inherent strength of diamond. The team successfully played music at 350 degrees Celsius—about 660 Fahrenheit. “Diamonds have these unique properties,” said Loncar. “This radio would be able to operate in space, in harsh environments and even the human body, as diamonds are biocompatible.”

Molecular Switch

A molecular switch consists of a molecule which can be shifted from one stable state to another when it is stimulated with chemical, electrical, optical or even a combination of stimuli. These molecules adjust their structural and electronic properties in response to these stimulations. The system returns to the actual state once the input is switched to zero. The two states, between which it transits, can be its isomers, acid and its conjugate base, or oxidised and reduced states.

Dr. Jannic Wolf, chemist at the Univ. of Konstanz, has demonstrated for the first time, switching of a single molecule of diarylethene attached with gold electrode at its ends through light beam. The advantages of this molecule, approximately three nanometers in size, are that it rotates very little when a point in its structure opens and it possesses two nanowires that can be used as contacts.

Researchers in Germany have demonstrated UV light controlled switching between the high conductance “on”-state and a low conductance “off”-state of a molecular switch made of diarylethenes. Switching was achieved in the on-state; the ring-shaped difurylperfluorocyclopentene core of the molecule is closed up, allowing electrical transport across the ring. For the off-state, the opened ring structure leads to a reduced conduction.

Molecular transistor

Transistors are used to amplify or switch the signals. The benzene molecule attached with gold contact performs same as silicon transistor

In order to realize a single-molecule transistor one needs to add a third electrode to a single-molecule junction, which allows applying a gate voltage that regulates the current flowing from source to drain.

International team from Paul-Drude-Institut für Festkörperelektronik (PDI), the Freie Universität Berlin (FUB), the NTT Basic Research Laboratories (NTT-BRL), and the U.S. Naval Research Laboratory (NRL) has built a molecular transistor that can reportedly be controlled precisely, in what could mark an important step toward the advancement of miniaturized electronics.

“Here, we used individual charged atoms, manipulated by a scanning tunnelling microscope, to create the electrical gates for a single-molecule transistor. This degree of control allowed us to tune the molecule into the regime of sequential single-electron tunnelling, albeit with a conductance gap more than one order of magnitude larger than observed previously,” said researchers.

The device was assembled by taking a crystal of indium arsenide and placing 12 indium atoms laid out in a hexagonal shape on top of it, with a phthalocyanine molecule in the middle.

As the researchers explain, the central molecule is only weakly bound to the crystal surface beneath it, and this means that, when the tip of the microscope is very close to the molecule and a voltage is applied, single electrons can tunnel between the surface of the crystal and the tip of the microscope. The positively charged atoms around the molecule act as the gate of the transistor, regulating the electron’s flow and leading to a functioning and reliable molecular transistor.

Kai Sotthewes, Victor Geskin, René Heimbuch, Avijit Kumar, and Harold J. W. Zandvliet, Institute for Nanotechnology, Netherlands University of Mons, Belgium have proposed an alternative scenario, rather than gating the molecule via a conventional gate electrode, they varied the conductance of the molecule by mechanical gating, i.e., compressing or stretching the molecule

Molecular memory

A team from the University of Glasgow’s Schools of Chemistry and Engineering and Rovira Virgili University in Spain have been able to design, synthesise and characterize metal-oxide clusters known as polyoxometalates (POMs) molecules. These molecules can trap charge and act as a flash ram, as well as dope of the inner core of the clusters with selenium to create a new type of memory we call ‘write-once-erase’. The POM clusters provide a balance of structural stability and electronic activity and their electronic functionality is tunable, making them suitable as storage nodes for flash memory.

 

IBM stores one bit of data on a SINGLE atom: Breakthrough may lead to credit card-sized devices capable of holding the entire iTunes library of music

The data storage system uses a single atom of holmium, supported by magnesium oxide to help keep the magnetic poles of the atom stable. By passing an electrical current through the holmium, the scientists are able to reverse these poles at will.
This allows for the switch between a 1 and 0 state – the binary positions used in computing to write and store information.
This information can be read by measuring the current passing through the atom, which will vary depending on its magnetic position.

The researchers showed that two magnetic atoms could be written and read independently, even when they were separated by just one nanometer – a distance one millionth the width of a pin head. This tight spacing could eventually yield magnetic storage that is 1,000 times denser than today’s hard disk drives and solid state memory chips. Hard drives built using this nanostructure would control the position of every atom.

While commercial applications are unlikely to emerge overnight, it does represent a quantum leap in data storage technology and shows great promise for the future. Christopher Lutz is lead nano-science researcher at IBM Research, based at Almaden in San Jose, California. About the project, he said: ‘Magnetic bits lie at the heart of hard-disk drives, tape and next-generation magnetic memory. ‘We conducted this research to understand what happens when you shrink technology down to the most fundamental extreme – the atomic scale.’

 

Molecular Spintronics

Recently, spin transport in molecular devices has started to attract the attention of researchers. The possibility of constructing spin devices utilizing organic molecules was demonstrated in a number of experiments, indicating the emergence of molecular spintronics as a new branch of molecular electronics. Magnetic field and electric potential were considered in the literature as controls for spin flux.

 

References and resources also include:

 

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