Anirban Bandyopadhyay

Large conductance switching and memory effects in organic molecules for data-storage applications

We have observed a large electrical conductance switching (ON:OFF ratio=105) in single-layer sandwich structures based on organic molecules at room temperature. The switching devices showed an associated memory effect for data-storage applications. We could write or erase a state and read it for many cycles. In switching devices, the active semiconductor retained its high conducting state until a reverse voltage erased it. A high conducting state arose due to restoration of conjugation in the molecule via electroreduction. Such a high ON–OFF ratio in a single layer sandwich structure, as compared to contemporary switching devices, is due to low off-state leakage current. The concept of conjugation restoration has been verified in supramolecular structures by adding donor groups to the molecule, which resulted in increased off-state current and hence lower ON–OFF ratio. Our work set a generalized example of selecting organic molecules to obtain higher ON–OFF ratio in molecular switching devices.

Multi-level memory-switching properties of a single brain microtubule

We demonstrate that a single brain-neuron-extracted microtubule is a memory-switching element, whose hysteresis loss is nearly zero. Our study shows how a memory-state forms in the nanowire and how its protein arrangement symmetry is related to the conducting-state written in the device, thus, enabling it to store and process ∼500 distinct bits, with 2 pA resolution between 1 nA and 1 pA. Its random access memory is an analogue of flash memory switch used in a computer chip. Using scanning tunneling microscope imaging, we demonstrate how single proteins behave inside the nanowire when this 3.5 billion years old nanowire processes memory-bits.

Multilevel conductivity and conductance switching in supramolecular structures of an organic molecule

We have demonstrated conductance switching between multilevel states in devices based on Rose Bengal molecules embedded in supramolecular matrices. Two mechanisms, namely electroreduction and conformational change of the molecules, resulting in conjugation modification have been proposed to be applicable in these devices. In a low voltage region, reverse-bias induced electroreduction of Rose Bengal facilitated conjugation restoration in the backbone of the molecule and, hence, switching to a high-conducting state. At high biases, the two perpendicular planes present in Rose Bengal, which have permanent dipole moments, allowed forward-bias induced conformation change to occur, and results in conductance switching. We have demonstrated how the devices can switch between two pair of conducting states for random-access memory and read-only memory applications for several hours.

Memory device applications of a conjugated polymer: role of space charges

Conjugated polymers have been used in data-storage devices. A “state” has been written by applying a voltage pulse. The state of the device has been “read” from the current under a small probe voltage (0.2 V). The polymer retained the state for more than 1 h which can be refreshed or erased at will. The stored space charges under a voltage pulse have been found to control the charge injection and hence the device current. Their slow relaxation process has resulted in the use of conjugated polymers in memory device applications. Hysteresis-type behavior has been observed in the current–voltage characteristics. The density of stored charges at the polymer layer near the metal/polymer interface has been found to depend on the voltage amplitude. The relaxation of the stored charges has been studied by applying two voltage pulses. By varying the delay between the two pulses, during which the space charges relax or redistribute, the time constant for charge relaxation has been calculated. The time constant was found to be independent of the density of the space charges or of the pace at which they were stored.Conjugated polymers have been used in data-storage devices. A “state” has been written by applying a voltage pulse. The state of the device has been “read” from the current under a small probe voltage (0.2 V). The polymer retained the state for more than 1 h which can be refreshed or erased at will. The stored space charges under a voltage pulse have been found to control the charge injection and hence the device current. Their slow relaxation process has resulted in the use of conjugated polymers in memory device applications. Hysteresis-type behavior has been observed in the current–voltage characteristics. The density of stored charges at the polymer layer near the metal/polymer interface has been found to depend on the voltage amplitude. The relaxation of the stored charges has been studied by applying two voltage pulses. By varying the delay between the two pulses, during which the space charges relax or redistribute, the time constant for charge relaxation has been calculated. The time constant was ...

Atomic water channel controlling remarkable properties of a single brain microtubule: Correlating single protein to its supramolecular assembly

Microtubule nanotubes are found in every living eukaryotic cells; these are formed by reversible polymerization of the tubulin protein, and their hollow fibers are filled with uniquely arranged water molecules. Here we measure single tubulin molecule and single brain-neuron extracted microtubule nanowire with and without water channel inside to unravel their unique electronic and optical properties for the first time. We demonstrate that the energy levels of a single tubulin protein and single microtubule made of 40,000 tubulin dimers are identical unlike conventional materials. Moreover, the transmitted ac power and the transient fluorescence decay (single photon count) are independent of the microtubule length. Even more remarkable is the fact that the microtubule nanowire is more conducting than a single protein molecule that constitutes the nanowire. Microtubules vibrational peaks condense to a single mode that controls the emergence of size independent electronic/optical properties, and automated noise alleviation, which disappear when the atomic water core is released from the inner cylinder. We have carried out several tricky state-of-the-art experiments and identified the electromagnetic resonance peaks of single microtubule reliably. The resonant vibrations established that the condensation of energy levels and periodic oscillation of unique energy fringes on the microtubule surface, emerge as the atomic water core resonantly integrates all proteins around it such that the nanotube irrespective of its size functions like a single protein molecule. Thus, a monomolecular water channel residing inside the protein-cylinder displays an unprecedented control in governing the tantalizing electronic and optical properties of microtubule.

Massively parallel computing on an organic molecular layer

Modern computers operate at enormous speeds—capable of executing in excess of 1013 instructions per second—but their sequential approach to processing, by which logical operations are performed one after another, has remained unchanged since the 1950s. In contrast, although individual neurons of the human brain fire at around just 103 times per second, the simultaneous collective action of millions of neurons enables them to complete certain tasks more efficiently than even the fastest supercomputer. Here we demonstrate an assembly of molecular switches that simultaneously interact to perform a variety of computational tasks including conventional digital logic, calculating Voronoi diagrams, and simulating natural phenomena such as heat diffusion and cancer growth. As well as representing a conceptual shift from serial-processing with static architectures, our parallel, dynamically reconfigurable approach could provide a means to solve otherwise intractable computational problems. The processors of most computers work in series, performing one instruction at a time. This limits their ability to perform certain types of tasks in a reasonable period. An approach based on arrays of simultaneously interacting molecular switches could enable previously intractable computational problems to be solved.

Origin of Negative Differential Resistance in a Strongly Coupled Single Molecule-Metal Junction Device

A new mechanism is proposed to explain the origin of negative differential resistance (NDR) in a strongly coupled single molecule-metal junction. A first-principles quantum transport calculation in a Fe-terpyridine linker molecule sandwiched between a pair of gold electrodes is presented. Upon increasing the applied bias, it is found that a new phase in the broken symmetry wave function of the molecule emerges from the mixing of occupied and unoccupied molecular orbitals. As a consequence, a nonlinear change in the coupling between the molecule and the lead is evolved resulting in NDR. This model can be used to explain NDR in other classes of metal-molecule junction devices.

Data-storage devices based on layer-by-layer self-assembled films of a phthalocyanine derivative

Abstract An organic dye, namely nickel phthalocyanine, has been used in data-storage devices. A “high state” has been written by applying a voltage pulse. The state of the device has been “read” by applying a small probe voltage. The dye embedded in an inert polymer matrix retained the high state for more than an hour, which can be refreshed or erased at will. Hysteresis-type behaviour has been observed in the current–voltage characteristics. The space charges at the metal/semiconductor interfaces, stored under the voltage pulse, have been found to control the charge injection and hence the current in these devices. The formation of space charges near the interfaces, and relaxation have been studied in the data-storage devices. The space charges’ slow relaxation process has been shown to result in the memory device applications of the semiconducting dyes.

A study of intermetallic compound formation in a copper–tin bimetallic couple

The formation of intermetallics in copper–tin bimetallic couples has been studied from room temperature to 183 °C by measuring the evolution of contact resistance and composite electrical resistance with time and temperature in order to assess the kinetic behavior of the system. X‐ray diffractogram (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) studies have also been performed on the samples. As regards bulk diffusion, copper diffuses interstitially into tin rapidly at room temperature with the formation of η’‐Cu6Sn5 intermetallic compound. Further diffusion through this phase as evaluated by composite electrical resistivity measurements is given by 0.40 eV, assuming a model of defect‐assisted diffusion into the grains. The grain‐boundary diffusion is found to occur with an activation energy of 0.78 eV as estimated from contact resistivity measurements. SEM confirms the presence of grain‐boundary diffusion of tin in copper, whereas XRD and TEM measurements indicate t...

Live visualizations of single isolated tubulin protein self-assembly via tunneling current: effect of electromagnetic pumping during spontaneous growth of microtubule

As we bring tubulin protein molecules one by one into the vicinity, they self-assemble and entire event we capture live via quantum tunneling. We observe how these molecules form a linear chain and then chains self-assemble into 2D sheet, an essential for microtubule, —fundamental nano-tube in a cellular life form. Even without using GTP, or any chemical reaction, but applying particular ac signal using specially designed antenna around atomic sharp tip we could carry out the self-assembly, however, if there is no electromagnetic pumping, no self-assembly is observed. In order to verify this atomic scale observation, we have built an artificial cell-like environment with nano-scale engineering and repeated spontaneous growth of tubulin protein to its complex with and without electromagnetic signal. We used 64 combinations of plant, animal and fungi tubulins and several doping molecules used as drug, and repeatedly observed that the long reported common frequency region where protein folds mechanically and its structures vibrate electromagnetically. Under pumping, the growth process exhibits a unique organized behavior unprecedented otherwise. Thus, “common frequency point” is proposed as a tool to regulate protein complex related diseases in the future.