As the complexity and volume of global digital data grow, so too does the need for more capable and compact means of processing and storing data. At present, traditional electronic devices based on semiconductor materials face severe challenges, not only technical and technological limitations but also key theoretical limitations.
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 a wide range of applications in the work areas of chemistry, physics, electronics and nanoelectronics, technology, artificial intelligence, and medical equipment. It seems very unlikely that molecular electronics will ever replace silicon-based electronics, but there are good reasons to believe that it can complement it by providing, for instance, novel functionalities out of the scope of traditional solid state devices.
With the rapid development of nanotechnology and in-depth research, researchers have made progress in the theory and practice of molecular electronic devices in recent years.
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 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 electronic devices are devices that use molecules (including biomolecules) with certain structures and functions to build an ordered system in a molecular scale or supramolecular scale. They make use of the quantum effect of electrons to work, control the behavior of single electrons, and realize the functions of information detection, processing, transmission and storage, such as molecular diodes, molecular memories, molecular wires, molecular field-effect transistors, and molecular switches.
As a stable quantum system with abundant photoelectric properties, molecules have many electronic transport properties different from semiconductor devices. Molecular electronic devices have the following advantages:
(1) small molecular volume, which can improve the integration and operation speed. Although most molecules are poorly conductive, good molecular wires could reduce the transit time of typical transistors (∼ 10−14s), reducing so the time needed for an operation.
(2) selecting appropriate components and structures can widely change the electrical properties of molecules. By choice of composition and geometry, one can extensively vary a molecule’s transport, binding, optical, and structural properties. The tools of molecular synthesis are highly developed;
(3) molecules are easy to synthesize, and the required structure can be formed by a self-assembly method; and
(4) the molecular scale is on the nanometer scale and has advantages in cost, efficiency, and power consumption.The reduce size of small molecules (between 1 and 10 nm) could lead to a higher packing density of devices with the subse-quent advantages in cost, eﬃciency, and power dissipation;
(5) New functionalities. Special properties of molecules, like the exis-tence of distinct stable geometric structures or isomers, could leadto new electronic functions that are not possible to implement inconventional solid state devices . 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.
Molecules have also obvious disadvantages such as instabilities at high temperatures. Moreover, the fabrication of reliable molecular junctions requires sometimes to control matter at an unprecedented level, which can be not only diﬃcult but also slow and costly.
But so far, no one has actually been able to make complex electrical circuits using molecules. One of the challenges is a 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.
With traditional silicon-based electronic devices becoming ever smaller, the impact of quantum effects is starting to emerge. Research on molecular electronics has made significant breakthroughs. Researchers are increasing their understanding of characteristics such as potential thermoelectric effects, new thermally induced spin transport phenomena, and negative differential resistance, and believe that smaller, faster, and “cooler” high-tech products will eventually be realized in the future.
Researchers from the University of Basel have reported a new method that allows the physical state of just a few atoms or molecules within a network to be controlled. It is based on the spontaneous self-organization of molecules into extensive networks with pores about one nanometer in size. In the journal Small, the physicists reported on their investigations, which could be of particular importance for the development of new storage devices.
Switches are one of the cornerstones of electrical circuitry, but it has been notoriously difficult to develop molecular switches that perform well injunctions.
A molecular switch consists of a molecule that 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.
Electronic switches fabricated with single molecules are expected to break through the bottleneck in the development of semiconductor device miniaturization. However, at present time, most development on single molecule switches only provides one-way switching functions. Stable bidirectional reversible single-molecule switches are rarely reported.
Dr. Jannic Wolf, chemist at the Univ. of Konstanz, has demonstrated for the first time, switching of a single molecule of diarylethene attached with a gold electrode at its ends through the 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 reduced conduction.
In upcoming technologies, such as, neuromorphic computing, it is important to development new types of molecular switches that can reduce the footprint of devices, decrease power consumption, and enable complementary functionalities to existing solutions. UNIVERSITY OF TWENTE developing new types of molecular switches that can toggle between multiple functionalities For instance, we created a dual-functional molecular switch that works as both a diode and a memory element. The device is only 2 nm thick and only requires a low drive voltage of less than 1 V, a significant improvement over other 1D-1R memory devices.
Single Molecule Switch, reported in June 2020
According to an international team of scientists, we could be one step closer to molecular electronics with a single molecule ‘switch’ that acts like a transistor and could potentially store binary information. The team behind the switch believes that molecules like the ones discovered could hold 250 terabits of information per square inch. This is according to their study which shows that organic salt molecules can be switched to appear either bright or dark by using a small electrical current. This switching between bright or dark provides binary information that can be written, read, and erased at room temperature and in normal air pressures. In contrast, much of the previous research that has gone into molecular electronics had to be conducted at very low temperatures in a vacuum.
In a statement, Dr. Stijn Mertens, Senior Lecturer in Electrochemical Surface Science at Lancaster University, who led the research study, said: “There is an entire list of properties that a molecule has to possess to be useful as a molecular memory…. Ours is the first example that combines all these [useful] features in the same molecule.” These useful features are that aside from being switchable in both directions under ambient conditions, the molecule must be stable in the long-term in both the bright and dark state, and also spontaneously form highly ordered layers that are one molecule thick, a process known as self-assembly.
To prove their discovery’s worthiness, the research team used small electric pulses in a scanning tunneling microscope to switch individual molecules from bright to dark. During switching, the electric pulse changes the way the organic salt’s cations and anions are stacked together, and this causes the molecule to appear either bright or dark. According to the Lancaster research team, the spontaneous ordering of these molecules is crucial. Via self-assembly, the molecules find their way into a two-dimensional crystal without the need for expensive manufacturing tools or processes that are currently used for electronics. “Because chemistry allows us to make molecules with sophisticated functions in enormous numbers and with atomic precision, molecular electronics may have a very bright future,” Dr. Mertens said.
Data storage using individual molecules
Around the world, researchers are attempting to shrink data storage devices to achieve as large a storage capacity in as small a space as possible. In almost all forms of media, phase transition is used for storage. For the creation of CD, for example, a very thin sheet of metal within the plastic is used that melts within microseconds and then solidifies again. Enabling this on the level of atoms or molecules is the subject of a research project led by researchers at the University of Basel.
In principle, a phase change on the level of individual atoms or molecules can be used to store data; storage devices of this kind already exist in research. However, they are very labor-intensive and expensive to manufacture. In Dec 2018, it was reported that group led by Professor Thomas Jung at the University of Basel is working to produce such tiny storage units consisting of only a few atoms using the process of self-organization, thereby enormously simplifying the production process. To this end, the group first produced an organometallic network that looks like a sieve with precisely defined holes. When the right connections and conditions are chosen, the molecules arrange themselves independently into a regular supramolecular structure.
The physicist Aisha Ahsan, lead author of the current study, has now added individual Xenon gas atoms to the holes, which are only a bit more than one nanometer in size. By using temperature changes and locally applied electrical pulses, she succeeded in purposefully switching the physical state of the Xenon atoms between solid and liquid. She was able to cause this phase change in all holes at the same time by temperature. The temperatures for the phase transition depend on the stability of the Xenon clusters, which varies based on the number of Xenon atoms. With the microscope sensor she has induced the phase change also locally ,for an individual Xenon containing pore.
As these experiments have to be conducted at extremely low temperatures of just a few Kelvin (below -260°C), Xenon atoms themselves cannot be used to create new data storage devices. The experiments have proven, however, that supramolecular networks are suited in principle for the production of tiny structures, in which phase changes can be induced with just a few atoms or molecules.
“We will now test larger molecules as well as short-chain alcohols. These change state at higher temperatures, which means that it may be possible to make use of them,” said Professor Thomas Jung, who supervised the work.
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, reported in Mar 2017
BM…announced…that it…successfully managed to store data on a single atom,…an achievement that could potentially change the way storage devices are developed in the future…modern hard drives utilize roughly 100,000 atoms to store a single bit, so shrinking things down to the size of just one atom is obviously a massive achievement.
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.’
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
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.”
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.
Researchers have developed a step-by-step recipe to produce single-atom transistors in reported inMay 2020
Now, researchers at the National Institute of Standards and Technology (NIST) and their colleagues at the University of Maryland have developed a step-by-step recipe to produce the atomic-scale devices. Using these instructions, the NIST-led team has become only the second in the world to construct a single-atom transistor and the first to fabricate a series of single electron transistors with atom-scale control over the devices’ geometry.
The scientists demonstrated that they could precisely adjust the rate at which individual electrons flow through a physical gap or electrical barrier in their transistor — even though classical physics would forbid the electrons from doing so because they lack enough energy. That strictly quantum phenomenon, known as quantum tunneling, only becomes important when gaps are extremely tiny, such as in the miniature transistors. Precise control over quantum tunneling is key because it enables the transistors to become “entangled” or interlinked in a way only possible through quantum mechanics and opens new possibilities for creating quantum bits (qubits) that could be used in quantum computing.
To fabricate single-atom and few-atom transistors, the team relied on a known technique in which a silicon chip is covered with a layer of hydrogen atoms, which readily bind to silicon. The fine tip of a scanning tunneling microscope then removed hydrogen atoms at selected sites. The remaining hydrogen acted as a barrier so that when the team directed phosphine gas (PH3) at the silicon surface, individual PH3 molecules attached only to the locations where the hydrogen had been removed (see animation). The researchers then heated the silicon surface. The heat ejected hydrogen atoms from the PH3 and caused the phosphorus atom that was left behind to embed itself in the surface. With additional processing, bound phosphorus atoms created the foundation of a series of highly stable single- or few-atom devices that have the potential to serve as qubits.
Two of the steps in the method devised by the NIST teams — sealing the phosphorus atoms with protective layers of silicon and then making electrical contact with the embedded atoms — appear to have been essential to reliably fabricate many copies of atomically precise devices, NIST researcher Richard Silver said.
In the past, researchers have typically applied heat as all the silicon layers are grown, in order to remove defects and ensure that the silicon has the pure crystalline structure required to integrate the single-atom devices with conventional silicon-chip electrical components. But the NIST scientists found that such heating could dislodge the bound phosphorus atoms and potentially disrupt the structure of the atomic-scale devices. Instead, the team deposited the first several silicon layers at room temperature, allowing the phosphorus atoms to stay put. Only when subsequent layers were deposited did the team apply heat.
“We believe our method of applying the layers provides more stable and precise atomic-scale devices,” said Silver. Having even a single atom out of place can alter the conductivity and other properties of electrical components that feature single or small clusters of atoms.
The team also developed a novel technique for the crucial step of making electrical contact with the buried atoms so that they can operate as part of a circuit. The NIST scientists gently heated a layer of palladium metal applied to specific regions on the silicon surface that resided directly above selected components of the silicon-embedded device. The heated palladium reacted with the silicon to form an electrically conducting alloy called palladium silicide, which naturally penetrated through the silicon and made contact with the phosphorus atoms.
In a recent edition of Advanced Functional Materials, Silver and his colleagues, who include Xiqiao Wang, Jonathan Wyrick, Michael Stewart Jr. and Curt Richter, emphasized that their contact method has a nearly 100% success rate. That’s a key achievement, noted Wyrick. “You can have the best single-atom-transistor device in the world, but if you can’t make contact with it, it’s useless,” he said.
Fabricating single-atom transistors “is a difficult and complicated process that maybe everyone has to cut their teeth on, but we’ve laid out the steps so that other teams don’t have to proceed by trial and error,” said Richter.
“Because quantum tunneling is so fundamental to any quantum device, including the construction of qubits, the ability to control the flow of one electron at a time is a significant achievement,” Wyrick said. In addition, as engineers pack more and more circuitry on a tiny computer chip and the gap between components continues to shrink, understanding and controlling the effects of quantum tunneling will become even more critical, Richter said.
First Molecular Electronics Chip Developed, reported in Feb 2022
The first molecular electronics chip has been developed, realizing a 50-year-old goal of integrating single molecules into circuits to achieve the ultimate scaling limits of Moore’s Law. Developed by Roswell Biotechnologies and a multi-disciplinary team of leading academic scientists, the chip uses single molecules as universal sensor elements in a circuit to create a programmable biosensor with real-time, single-molecule sensitivity and unlimited scalability in sensor pixel density. This innovation, appearing this week in a peer-reviewed article in the Proceedings of the National Academy of Sciences (PNAS), will power advances in diverse fields that are fundamentally based on observing molecular interactions, including drug discovery, diagnostics, DNA sequencing, and proteomics.
“Biology works by single molecules talking to each other, but our existing measurement methods cannot detect this,” said co-author Jim Tour, PhD, a Rice University chemistry professor and a pioneer in the field of molecular electronics. “The sensors demonstrated in this paper for the first time let us listen in on these molecular communications, enabling a new and powerful view of biological information.”
The molecular electronics platform consists of a programmable semiconductor chip with a scalable sensor array architecture. Each array element consists of an electrical current meter that monitors the current flowing through a precision-engineered molecular wire, assembled to span nanoelectrodes that couple it directly into the circuit. The sensor is programmed by attaching the desired probe molecule to the molecular wire, via a central, engineered conjugation site. The observed current provides a direct, real-time electronic readout of molecular interactions of the probe. These picoamp-scale current-versus-time measurements are read out from the sensor array in digital form, at a rate of 1000 frames per second, to capture molecular interactions data with high resolution, precision and throughput.
The paper also presents a molecular electronics sensor capable of reading DNA sequence. In this sensor, a DNA polymerase, the enzyme that copies DNA, is integrated into the circuit, and the result is direct electrical observation of the action of this enzyme as it copies a piece of DNA, letter by letter. Unlike other sequencing technologies that rely on indirect measures of polymerase activity, this approach achieves direct, real-time observation of a DNA polymerase enzyme incorporating nucleotides. The paper illustrates how these activity signals can be analyzed with machine learning algorithms to allow reading of the sequence.
“The Roswell sequencing sensor provides a new, direct view of polymerase activity, with the potential to advance sequencing technology by additional orders of magnitude in speed and cost,” said Professor George Church, a co-author of the paper, member of the National Academy of Sciences, and a Roswell Scientific Advisory Board member. “This ultra scalable chip opens up the possibility for highly distributed sequencing for personal health or environmental monitoring, and for future ultra-high throughput applications such as Exabyte-scale DNA data storage.”
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