Alpana Nayak K. A. Suresh Santanu Kumar Pal Sandeep Kumar

Cite this: J. Phys. Chem. B 2007, 111, 38, 11157–11161Publication Date:August 31, 2007 https://doi.org/10.1021/jp073196z

Abstract

Discotic molecules are known to form highly anisotropic structures at the air−water (A−W) interface. We have studied two novel ionic discotic mesogenic molecules, viz., pyridinium tethered with hexaalkoxytriphenylene with bromide counterion (Py-Tp) and imidazolium tethered with hexaalkoxytriphenylene with bromide counterion (Im-Tp) at A−W and air−solid interfaces. The monolayer phases were investigated at the A−W interface employing surface manometry and Brewster angle microscopy techniques. They indicate a uniform monolayer phase which shows negligible hysteresis on expanding and compressing. Also, in both the systems the collapsed state completely reverts to the monolayer state. These monolayer films transferred at different surface pressures by Langmuir−Blodgett technique were studied by employing atomic force microscopy. The topographies of these films transferred at the low and high surface pressure region of the isotherm indicate a transformation of the monolayer from face-on to edge-on structure.

The post Films of Novel Mesogenic Molecules at Air−Water and Air−Solid Interfaces appeared first on Nano Systems Laboratory.

Alpana Nayak and K. A. Suresh

Phys. Rev. E 78, 021606 – Published 27 August 2008

ABSTRACT

We have studied the electrical conductivity in monolayer films of an ionic disk-shaped liquid-crystal molecule, pyridinium tethered with hexaalkoxytriphenylene (PyTp), and its complex with DNA by current-sensing atomic force microscopy (CS-AFM). The pure PyTp and PyTp-DNA complex monolayer films were first formed at the air-water interface and then transferred onto conducting substrates by the Langmuir-Blodgett (LB) technique to study the nanoscale electron transport through these films. The conductive tip of CS-AFM, the LB film, and the metal substrate form a nanoscopic metal-LB film-metal (M−LB−M) junction. We have measured the current-voltage (I−V) characteristics for the M−LB−M junction using CS-AFM and have analyzed the data quantitatively. We find that the I−V curves fit well to the Fowler-Nordheim (FN) model, suggesting electron tunneling to be a possible mechanism for electron transport in our system. Further, analysis of the I−V curves based on the FN model yields the barrier heights of PyTp-DNA complex and pure PyTp films. Electron transport studies of films of ionic disk-shaped liquid-crystal molecules and their complex with DNA are important from the point of view of their applications in organic electronics.

The post Conductivity of Langmuir-Blodgett films of a disk-shaped liquid-crystalline molecule–DNA complex studied by current-sensing atomic force microscopy appeared first on Nano Systems Laboratory.

• Alpana Nayak^‡
• K. A. Suresh^*§

View Author Information Cite this: J. Phys. Chem. B 2009, 113, 12, 3669–3675Publication Date:February 16, 2009https://doi.org/10.1021/jp806600g

Abstract

We have studied the mechanical properties of films of a novel ionic discogen, pyridinium tethered with hexaalkoxytriphenylene (PyTp) and its complex with DNA (PyTp−DNA) using atomic force microscope (AFM). The PyTp and PyTp−DNA complex monolayer films were first formed at air−water interface and then transferred onto silicon substrates by Langmuir−Blodgett (LB) technique. For the mechanical properties, particularly to obtain elastic modulus, we have carried out nanoindentation measurements on the LB films of PyTp and also PyTp−DNA complex. The load versus indentation curves from the nanoindentation measurements were analyzed quantitatively using Hertz model. Our analysis yields Young’s modulus values of 54 and 160 MPa for the PyTp and PyTp−DNA complex films, respectively. In addition, the LB films were imaged in the tapping mode AFM to obtain topography and phase images simultaneously. The energy dissipation maps were constructed from the phase images to determine qualitatively the variation in stiffness on the film surfaces. We find that the complex film exhibits a nonuniform surface with varying stiffness while the pure film exhibits a uniform surface.

The post Mechanical Properties of Langmuir−Blodgett Films of a Discogen−DNA Complex by Atomic Force Microscopy appeared first on Nano Systems Laboratory.

• Alpana Nayak
• K. A. Suresh

View Author Information Cite this: J. Phys. Chem. B 2008, 112, 10, 2930–2936Publication Date:February 19, 2008 https://doi.org/10.1021/jp710084q

Abstract

We have studied films of an ionic discogenic (discotic mesogenic) molecule (pyridinium salt tethered with hexaalkoxytriphenylene (PyTp)) and DNA complex at air−water (A−W) and air−solid interfaces. We have formed an PyTp monolayer on an aqueous subphase containing a small amount of DNA to obtain a PyTp−DNA complex at the A−W interface. Compared to the pure PyTp monolayer, the PyTp−DNA complex monolayer exhibits a higher collapse pressure and lower limiting area, indicating condensation and better stability. A Brewster angle microscope was used for in situ observation of the morphology of the film at the A−W interface. The PyTp−DNA complex films on silicon wafers were prepared using the Langmuir−Blodgett (LB) technique. We find that several tens of layers of the PyTp−DNA complex monolayer can be transferred with good efficiency. Fourier transform infrared spectroscopy studies confirm the presence of DNA in the LB films of the PyTp−DNA complex. Nanoindentation measurements using atomic force microscope reveal that the PyTp−DNA complex films are about two times harder as compared to the pure PyTp films.

The post Discogen−DNA Complex Films at Air−Water and Air−Solid Interfaces appeared first on Nano Systems Laboratory.

K. A. Suresh  &Alpana Nayak Pages 57/[1903]-80/[1926] | Published online: 05 Oct 2009

• https://doi.org/10.1080/15421400903050459

Abstract

In this article, we review our studies on discotic mesogen – DNA complex films at interfaces. Discotic molecules are known to form highly anisotropic structures at the air-water (A-W) interface. We have studied films of a novel ionic discotic mesogenic molecule, pyridinium salt tethered with hexaalkoxytriphenylene (PyTp) and its complex with DNA (PyTp-DNA) at A-W and air-solid interfaces. The PyTp monolayer was formed on the aqueous subphase containing small amount of DNA. The electrostatic interaction between PyTp and DNA molecules results in a PyTp-DNA complex monolayer. Compared to the pure PyTp monolayer, the PyTp-DNA complex monolayer exhibits higher collapse pressure and lower limiting area. Surface manometry and Brewster angle microscope studies of the PyTp-DNA complex monolayer film indicate the molecules to be in the edge-on configuration. With increase in surface pressure, the complex monolayer undergoes a transition from a loosely packed monolayer phase to a compactly packed monolayer phase. The PyTp-DNA complex films on silicon wafers were prepared using Langmuir-Blodgett (LB) technique. We find that several tens of layers of PyTp-DNA complex monolayer can be transferred with good efficiency. We have carried out nanoscale electrical conductivity measurements for the pure PyTp and PyTp-DNA complex LB films using current sensing atomic force microscope. We have studied the current-voltage (I-V) characteristics for the metal-LB film-metal junction and our analysis shows that the I-V curves followed the Fowler-Nordheim tunneling model.

The post Discotic Mesogen – DNA Complex Films at Interfaces appeared first on Nano Systems Laboratory.

• Alpana Nayak^*†
• Takuro Tamura^‡
• Tohru Tsuruoka^†
• Kazuya Terabe^†
• Sumio Hosaka^‡
• Tsuyoshi Hasegawa^†
• Masakazu Aono^†

View Author Information Cite this: J. Phys. Chem. Lett. 2010, 1, 3, 604–608Publication Date:January 11, 2010 https://doi.org/10.1021/jz900375a

Abstract

The switching time of a Ag₂S atomic switch, in which formation and annihilation of a Ag atomic bridge is controlled by a solid-electrochemical reaction in a nanogap between two electrodes, is investigated as a function of bias voltage and temperature. Increasing the bias voltage decreases the switching time exponentially, with a greater exponent for the lower range of bias than that for the higher range. Furthermore, the switching time shortens exponentially with raising temperature, following the Arrhenius relation with activation energy values of 0.58 and 1.32 eV for lower and higher bias ranges, respectively. These results indicate that there are two main processes which govern the rate of switching, first, the electrochemical reduction Ag⁺ + e⁻→Ag and, second, the diffusion of Ag⁺ ions. This investigation advances the fundamental understanding of the switching mechanism of the atomic switch, which is essential for its successful device application.

The post Rate-Limiting Processes Determining the Switching Time in a Ag2S Atomic Switch appeared first on Nano Systems Laboratory.

• Tsuyoshi Hasegawa, 
• Alpana Nayak, 
• Takeo Ohno, 
• Kazuya Terabe, 
• Tohru Tsuruoka, 
• James K. Gimzewski & 
• Masakazu Aono 

Applied Physics A volume 102, pages811–815 (2011)Cite this article

• 379 Accesses
• 38 Citations
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Abstract

We demonstrate memristive operations using gap-type Ag₂S atomic switches, in which the growth and shrinkage of an Ag protrusion are controlled by using solid-electrochemical reactions. In addition to conventional memristive operations such as those proposed and demonstrated by resistive random-access memories (ReRAMs) using metal oxide compounds, gap-type Ag₂S atomic switches also show new types of memristive operations by storing information from input signals without changing their output until a sufficient number of signals are inputted. The new types of memristive operations resemble the learning process seen in neuroplasticity, where changes occur in the organization of the human brain as a result of experience.

The post Memristive operations demonstrated by gap-type atomic switches appeared first on Nano Systems Laboratory.

Takami Hino ¹, Tsuyoshi Hasegawa ², Kazuya Terabe ¹, Tohru Tsuruoka ², Alpana Nayak ¹, Takeo Ohno ², Masakazu Aono ¹

Affiliations

• ¹WPI Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan.
• ²WPI Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan; Japan Science and Technology Agency, CREST, 5 Sanbancho, Chiyoda-ku, Tokyo 102-0075, Japan.

• PMID: 27877376
• PMCID: PMC5090398
• DOI: 10.1088/1468-6996/12/1/11660945

Abstract

Atomic switches are nanoionic devices that control the diffusion of metal cations and their reduction/oxidation processes in the switching operation to form/annihilate a metal atomic bridge, which is a conductive path between two electrodes in the on-state. In contrast to conventional semiconductor devices, atomic switches can provide a highly conductive channel even if their size is of nanometer order. In addition to their small size and low on-resistance, their nonvolatility has enabled the development of new types of programmable devices, which may achieve all the required functions on a single chip. Three-terminal atomic switches have also been developed, in which the formation and annihilation of a metal atomic bridge between a source electrode and a drain electrode are controlled by a third (gate) electrode. Three-terminal atomic switches are expected to enhance the development of new types of logic circuits, such as nonvolatile logic. The recent development of atomic switches that use a metal oxide as the ionic conductive material has enabled the integration of atomic switches with complementary metal-oxide-semiconductor (CMOS) devices, which will facilitate the commercialization of atomic switches. The novel characteristics of atomic switches, such as their learning and photosensing abilities, are also introduced in the latter part of this review.

Keywords: atomic switches; learning ability; nanoionic devices; nonvolatility; photosensing ability; three-terminal switch.

The post Atomic switches: atomic-movement-controlled nanodevices for new types of computing appeared first on Nano Systems Laboratory.

Nanotechnology

Switching kinetics of a Cu₂S-based gap-type atomic switch

Alpana Nayak¹, Tohru Tsuruoka¹, Kazuya Terabe¹, Tsuyoshi Hasegawa¹ and Masakazu Aono¹

Published 11 April 2011 • IOP Publishing Ltd
Nanotechnology, Volume 22, Number 23 Citation Alpana Nayak et al 2011 Nanotechnology 22 235201

Abstract

The switching time of a Cu₂S-based gap-type atomic switch is investigated as a function of temperature, bias voltage, and initial off-resistance. The gap-type atomic switch is realized using a scanning tunneling microscope (STM), in which the formation and annihilation of a Cu-atom bridge in the vacuum gap between the Cu₂S electrode and the Pt tip of the STM are controlled by a solid-electrochemical reaction. Increasing the temperature decreases the switching time exponentially with an activation energy of about 1.38 eV. Increasing the bias voltage also shortens the switching time exponentially, exhibiting a greater exponent for the lower bias than for the higher bias. Furthermore, faster switching has been achieved by decreasing the initial off-resistance between the Cu₂S electrode and STM tip. On the basis of these results, we suggest that, in addition to the chemical reaction, the electric field in the vacuum gap plays a significant role in the operation of a gap-type atomic switch. This investigation advances our understanding of the operating mechanism of an atomic switch, which is a new concept for future electronic devices.

The post Switching kinetics of a Cu2S-based gap-type atomic switch appeared first on Nano Systems Laboratory.

Appl. Phys. Lett. 98, 233501 (2011); https://doi.org/10.1063/1.3597154 Alpana Nayak^a), Tohru Tsuruoka, Kazuya Terabe, Tsuyoshi Hasegawa, and Masakazu Aono

ABSTRACT

Atomic switch, operating by forming and dissolving a metal-protrusion in a nanogap, shows an exponentially large bias dependence and a faster switching with increasing temperature and decreasing off-resistance. These major characteristics are explained with a simple model where the electrochemical potential at the subsurface of solid-electrolyte electrode determines the precipitation rate of metal atoms and the electric-field in the nanogap strongly affects the formation of metal-protrusion. Theoretically calculated switching time, based on this model, well reproduced the measured properties of a Cu2SCu2S-based atomic switch as a function of bias, temperature and off-resistance, providing a significant physical insight into the mechanism.Part of this work was conducted under the Key-Technology Research Project, “Atomic Switch Programmed Device,” supported by the MEXT, and the Strategic Japanese-German Cooperative Program, “Faradaic currents and ion transfer numbers in electrochemical atomic switches,” supported by JST. We would like to thank Prof. J. M. Ruitenbeek and Dr. M. Morales-Masis for fruitful discussions on the growth measurement.

The post Theoretical investigation of kinetics of a Cu2S-based gap-type atomic switch appeared first on Nano Systems Laboratory.