Alpana Nayak

Atomically controlled electrochemical nucleation at superionic solid electrolyte surfaces

Electrochemical equilibrium and the transfer of mass and charge through interfaces at the atomic scale are of fundamental importance for the microscopic understanding of elementary physicochemical processes. Approaching atomic dimensions, phase instabilities and instrumentation limits restrict the resolution. Here we show an ultimate lateral, mass and charge resolution during electrochemical Ag phase formation at the surface of RbAg(4)I(5) superionic conductor thin films. We found that a small amount of electron donors in the solid electrolyte enables scanning tunnelling microscope measurements and atomically resolved imaging. We demonstrate that Ag critical nucleus formation is rate limiting. The Gibbs energy of this process takes discrete values and the number of atoms of the critical nucleus remains constant over a large range of applied potentials. Our approach is crucial to elucidate the mechanism of atomic switches and highlights the possibility of extending this method to a variety of other electrochemical systems.

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

The switching time of a Cu(2)S-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 Cu(2)S 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 Cu(2)S 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.

Sensory and short-term memory formations observed in a Ag2S gap-type atomic switch

Memorization caused by the change in conductance in a Ag2S gap-type atomic switch was investigated as a function of the amplitude and width of input voltage pulses (Vin). The conductance changed little for the first few Vin, but the information of the input was stored as a redistribution of Ag-ions in the Ag2S, indicating the formation of sensory memory. After a certain number of Vin, the conductance increased abruptly followed by a gradual decrease, indicating the formation of short-term memory (STM). We found that the probability of STM formation depends strongly on the amplitude and width of Vin, which resembles the learning behavior of the human brain.

Atomic switches: atomic-movement-controlled nanodevices for new types of computing

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.

Theoretical investigation of kinetics of a Cu2S-based gap-type atomic switch

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 Cu2S-based atomic switch as a function of bias, temperature and off-resistance, providing a significant physical insight into the mechanism.

Discogen-DNA Complex Films at Air-Water and Air-Solid Interfaces

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.

Nanoarchitectonics for Controlling the Number of Dopant Atoms in Solid Electrolyte Nanodots

Controlling movements of electrons and holes is the key task in developing todays highly sophisticated information society. As transistors reach their physical limits, the semiconductor industry is seeking the next alternative to sustain its economy and to unfold a new era of human civilization. In this context, a completely new information token, i.e., ions instead of electrons, is promising. The current trend in solid-state nanoionics for applications in energy storage, sensing, and brain-type information processing, requires the ability to control the properties of matter at the ultimate atomic scale. Here, a conceptually novel nanoarchitectonic strategy is proposed for controlling the number of dopant atoms in a solid electrolyte to obtain discrete electrical properties. Using α-Ag2+δ S nanodots with a finite number of nonstoichiometry excess dopants as a model system, a theory matched with experiments is presented that reveals the role of physical parameters, namely, the separation between electrochemical energy levels and the cohesive energy, underlying atomic-scale manipulation of dopants in nanodots. This strategy can be applied to different nanoscale materials as their properties strongly depend on the number of doping atoms/ions, and has the potential to create a new paradigm based on controlled single atom/ion transfer.

Discotic Mesogen – DNA Complex Films at Interfaces

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.

Evaluation of silicon- and carbon-face SiO2/SiC MOS interface quality based on scanning nonlinear dielectric microscopy

A strong positive correlation was found between the trap density (Dit) at the SiO2/SiC interface and signal variation in a scanning nonlinear dielectric microscopy (SNDM) image. Si-face and C-face SiC wafers with a 45-nm-thick oxide layer were examined by the conventional high-low method and SNDM, which is a type of scanning probe microscopy. The Dit value measured by the high-low method and the standard deviation of normalized SNDM images exhibited a strong positive correlation, which means that the standard deviation of the normalized SNDM image can be used as a universal measure of the SiO2/SiC interface quality. Using this measure, a quick evaluation of Dit using SNDM is possible.

Universal parameter evaluating SiO 2 /SiC interface quality based on scanning nonlinear dielectric microscopy

Oxidized both silicon-face (Si-face) and carbon-face (C-face) wafers with various post-oxidation-annealing conditions were measured by scanning nonlinear dielectric microscopy (SNDM) and method for evaluating SiO2/SiC interface quality using SNDM was investigated. We found that the normalized standard deviation of SNDM image was good parameter to evaluate SiO2/SiC interface of Si and C-face. SNDM measurement does not need electrode fabrication to measure C-V curve, but needs just scan on the oxidized wafer with conductive tip, which is easier and quicker. This technique enables us to quickly examine the effect of variation of process parameters in MOS fabrication and to effectively reduce the time needed for R&D cycle.