Surface Immobilized Heteroleptic Copper Compounds as State Variables that Show Negative Differential Resistance

Surface immobilized bidentate heteroleptic Cu(I) compounds are synthesized using a surface outward sequential synthesis and are characterized using solid-state NMR and atomic force microscopy (AFM). Through use of chemical redox agents, the reversible switching characteristics of SiO₂-immobilized Cu(I) compounds (tetrahedral) to Cu(II) (square planar) are verified via UV−visible absorption spectroscopy and electron paramagnetic resonance. Electrical properties of this system are characterized via preparation of a sandwich-type device using p⁺ silicon and conductive AFM (cAFM). Current−Voltage (I−V) spectroscopy demonstrates that this system reproducibly switches between Cu(I) and Cu(II) states at approximately −0.8 and 2.3 V

Protein Adsorption Alters Hydrophobic Surfaces Used for Suspension Culture of Pluripotent Stem Cells

This Letter examines the physical and chemical changes that occur at the interface of methyl-terminated alkanethiol self-assembled monolayers (SAMs) after exposure to cell culture media used to derive embryoid bodies (EBs) from pluripotent stem cells. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy analysis of the SAMs indicates that protein components within the EB cell culture medium preferentially adsorb at the hydrophobic interface. In addition, we examined the adsorption process using surface plasmon resonance and atomic force microscopy. These studies identify the formation of a porous, mat-like adsorbed protein film with an approximate thickness of 2.5 nm. Captive bubble contact angle analysis reveals a shift toward superhydrophilic wetting behavior at the cell culture interface due to adsorption of these proteins. These results show how EBs are able to remain in suspension when derived on hydrophobic materials, which carries implications for the rational design of suspension culture interfaces for lineage specific stem-cell differentiation

Protein Adsorption Alters Hydrophobic Surfaces Used for Suspension Culture of Pluripotent Stem Cells

This Letter examines the physical and chemical changes that occur at the interface of methyl-terminated alkanethiol self-assembled monolayers (SAMs) after exposure to cell culture media used to derive embryoid bodies (EBs) from pluripotent stem cells. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy analysis of the SAMs indicates that protein components within the EB cell culture medium preferentially adsorb at the hydrophobic interface. In addition, we examined the adsorption process using surface plasmon resonance and atomic force microscopy. These studies identify the formation of a porous, mat-like adsorbed protein film with an approximate thickness of 2.5 nm. Captive bubble contact angle analysis reveals a shift toward superhydrophilic wetting behavior at the cell culture interface due to adsorption of these proteins. These results show how EBs are able to remain in suspension when derived on hydrophobic materials, which carries implications for the rational design of suspension culture interfaces for lineage specific stem-cell differentiation

<i>In Situ</i> STM Investigation of Aromatic Poly(azomethine) Arrays Constructed by “On-Site” Equilibrium Polymerization

Two-dimensional (2D) arrays of π-conjugated aromatic polymers produced by surface-selective Schiff base coupling reactions between an aromatic diamine and an aromatic dialdehyde were investigated in detail using <i>in situ</i> scanning tunneling microscopy. Surface-selective coupling was achieved for almost all diamine/dialdehyde combinations attempted, although several combinations did not proceed even in homogeneous aqueous alkaline solution. Most of the combinations of an aromatic diamine and a dialdehyde, except the combinations of 4,4′-azodianiline with mono/bithiophenedicarboxaldehyde, formed highly ordered π-conjugated polymer arrays on an iodine-modified Au(111) surface in aqueous solution at a suitable pH. The simplest polymer of the various combinations tested, obtained from the combination of 1,4-diaminobenzene with terephthaldicarboxaldehyde, gave a 2D array consisting of linearly connected benzene units. Poly­(azomethine) adlayers caused a positive shift in the electrochemical potential of the butterfly shaped oxidative adsorption and reductive desorption of iodine. The acceleration of the reductive desorption of iodine suggests the existence of a weak interaction between the polymer layer and iodine. Not only the first polymer adlayers but also partially adsorbed secondary adlayers with “on-top” epitaxial behavior were frequently observed for all polymer systems. The alignment of the polymer chains in the adlayers possessed a certain regularity in terms of a regular interval between polymer chains because of repulsive interpolymer interactions

Frequency Response – distributed conductance.

<p>(a) Amplitude spectrum from a Fourier transform of a control device's response to a 2 V, 10 Hz sinusoidal input signal compared to (b) that of a functionalized device which shows enhanced overtones of the input signal with respect to (a). (c) Plot of 2^nd and 3^rd harmonic generation in current response as a function of bias voltage in both functional (black) and control (gray) networks. Harmonic magnitudes are represented as percentage of the fundamental for a 10 Hz sinusoidal input signal.</p

Neuromorphic Atomic Switch Networks

<div><p>Efforts to emulate the formidable information processing capabilities of the brain through neuromorphic engineering have been bolstered by recent progress in the fabrication of nonlinear, nanoscale circuit elements that exhibit synapse-like operational characteristics. However, conventional fabrication techniques are unable to efficiently generate structures with the highly complex interconnectivity found in biological neuronal networks. Here we demonstrate the physical realization of a self-assembled neuromorphic device which implements basic concepts of systems neuroscience through a hardware-based platform comprised of over a billion interconnected atomic-switch inorganic synapses embedded in a complex network of silver nanowires. Observations of network activation and passive harmonic generation demonstrate a collective response to input stimulus in agreement with recent theoretical predictions. Further, emergent behaviors unique to the complex network of atomic switches and akin to brain function are observed, namely spatially distributed memory, recurrent dynamics and the activation of feedforward subnetworks. These devices display the functional characteristics required for implementing unconventional, biologically and neurally inspired computational methodologies in a synthetic experimental system.</p> </div

DC Response – recurrent dynamics.

<p>(a) Time traces of current response to 1 V DC bias show large current increases and decreases at all time scales around a mean of 5.81 µA (172 kΩ); shorter time traces (ii–iii) are subsets of (i). Representative device parameters: R_OFF>10 MΩ, R_ON<20 kΩ, V_T = 3 V during activation (b) Fourier transforms of DC bias response for Ag control (grey) and functionalized Ag-Ag₂S (black) networks. The power spectrum of the functionalized network displays 1/f^β power law scaling (β = 1.34).</p

Network Activation - memristive behavior.

<p>(a) Representative example of initial bias sweeps (0–5 V sweep at 1 V/s) applied to a pristine device which steadily activate higher percentages of atomic switches, resulting in increased current. After 11 sweeps, the device resistance decreases from ∼10 MΩ to ∼500 Ω. Subsequent ±1.5 V bipolar sweeps result in repeatable pinched hysteresis behavior (inset: R_OFF = 25 kΩ, R_ON = 800 Ω), and bistable switching. (b–d) Schematic representation of the mechanism producing the I–V characteristics shown in (a). The network initially consists of weakly memristive junctions and ohmic contacts (b). Continued application of unipolar bias voltage (c) drives the dissolution of silver into silver sulfide, increasing the number of memristive elements, while cation migration across extant memristive junctions leads to filament formation and the onset of hard switching behavior. (d) After the proportion of strong memristors exceeds the percolation threshold (ρ>0.5), the network functions reliably in the hard switching regime.</p

Device Fabrication.

<p>(a) SEM image of complex Ag networks (scale bar = 10 µm) produced by reaction of aqueous AgNO₃ (50 mM) with (inset) lithographically patterned Cu seed posts (scale bar = 1 µm). (b) High resolution image of the functionalized Ag network at the device electrode interface (Pt) showing wire widths ranging from 100 nm to 3 µm (average <1 µm) and lengths extending from a few microns to almost a millimeter (scale bar = 700 nm).</p

Graphene-Assisted Solution Growth of Vertically Oriented Organic Semiconducting Single Crystals

Vertically oriented structures of single crystalline conductors and semiconductors are of great technological importance due to their directional charge carrier transport, high device density, and interesting optical properties. However, creating such architectures for organic electronic materials remains challenging. Here, we report a facile, controllable route for producing oriented vertical arrays of single crystalline conjugated molecules using graphene as the guiding substrate. The arrays exhibit uniform morphological and crystallographic orientations. Using an oligoaniline as an example, we demonstrate this method to be highly versatile in controlling the nucleation densities, crystal sizes, and orientations. Charge carriers are shown to travel most efficiently along the vertical interfacial stacking direction with a conductivity of 12.3 S/cm in individual crystals, the highest reported to date for an aniline oligomer. These crystal arrays can be readily patterned and their current harnessed collectively over large areas, illustrating the promise for both micro- and macroscopic device applications