Humidity-Induced Protein-Based Artificial Synaptic Devices for Neuroprosthetic Applications.

Neuroprosthetics and brain-machine interfaces are immensely beneficial for people with neurological disabilities, and the future generation of neural repair systems will utilize neuromorphic devices for the advantages of energy efficiency and real-time performance abilities. Conventional synaptic devices are not compatible to work in such conditions. The cerebrospinal fluid (CSF) in the central part of the nervous system is composed of 99% water. Therefore, artificial synaptic devices, which are the fundamental component of neuromorphic devices, should resemble biological nerves while being biocompatible, and functional in high-humidity environments with higher functional stability for real-time applications in the human body. In this work, artificial synaptic devices are fabricated based on gelatin-PEDOT: PSS composite as an active material to work more effectively in a highly humid environment (≈90% relative humidity). These devices successfully mimic various synaptic properties by the continuous variation of conductance, like, excitatory/inhibitory post-synaptic current(EPSC/IPSC), paired-pulse facilitation/depression(PPF/PPD), spike-voltage dependent plasticity (SVDP), spike-duration dependent plasticity (SDDP), and spike-rate dependent plasticity (SRDP) in environments at a relative humidity levels of ≈90%.

Precise and rapid point-of-care quantification of albumin levels in unspiked blood using organic field-effect transistors.

Nanowire-based field-effect transistors (FETs) are widely used to detect biomolecules precisely. However, the fabrication of such devices involves complex integration procedures of nanowires into the device and most are not easily scalable. In this work, we report a straightforward fabrication approach that utilizes the grain boundaries of the semiconducting film of organic FETs to fabricate biosensors for the detection of human serum albumin (HSA) with an enhanced sensitivity and detection range. We used trichromophoric pentapeptide (TPyAlaDo-Leu-ArTAA-Leu-TPyAlaDo, TPP) as a receptor molecule to precisely estimate the concentration of HSA protein in human blood. Bi-layer semiconductors (pentacene and TPP) were used to fabricate the OFET, where the pentacene molecule acted as a conducting channel and TPP acted as a receptor molecule. This approach of engineering the diffusion of receptor molecules into the grain boundaries is crucial in developing OFET-based HSA protein sensors, which cover a considerable detection range from 1 pM to 1 mM in a single device. The point-of-care detection in unspiked blood samples was confirmed at 4.2 g dL-1, which is similar to 4.1 g dL-1 measured using a pathological procedure.

Diffusion-Induced Ingress of Angiotensin-Converting Enzyme 2 into the Charge Conducting Path of a Pentacene Channel for Efficient Detection of SARS-CoV-2 in Saliva Samples.

Rapid and accurate identification of a pathogen is crucial for disease control and prevention of the epidemic of emerging infectious like SARS-CoV-2. However, no foolproof gold standard assay exists to date. Nucleic acid-based molecular diagnostic tests have been established for identifying COVID-19. However, viral RNAs are highly unstable in handling with poor laboratory procedures, leading to a false negative that accelerates the spread of the disease. Detection of the spike protein (S1) of the SARS-CoV-2 virus through a proper receptor, commonly used in antigen-based rapid testing kits, also suffers from false-negative predictions due to decreasing viral titers in clinical specimens. Organic field-effect transistor (OFET)-based sensors can be highly sensitive upon properly integrating receptors in the conducting channel. This work demonstrates how angiotensin-converting enzyme 2 (ACE2) molecules can be used as receptor molecules of the SARS-CoV-2 virus in the OFET platform. Integration of ACE2 molecules into pentacene grain boundaries has been studied through the statistical analysis of rough surfaces in terms of lateral correlation length and interface width. The uniform coating of ACE2 molecules has been confirmed through growth studies to achieve better ingress of the receptors into the conducting channel at the semiconductor/dielectric interface of OFETs. We have observed less than a minute detection time with 94% sensitivity, which is the highest reported value. The sensor works with a saliva sample, requiring no sample preparation or virus transfer medium. A prototype module developed for remote monitoring confirms the suitability for point-of-care (POC) application at large-scale testing in more crowded areas like airports, railway stations, shopping malls, etc.

Silver Nanodot Decorated Dendritic Copper Foam As a Hydrophobic and Mechano-Chemo Bactericidal Surface.

The present work investigates the time-dependent antibacterial activity of the silver nanodot decorated dendritic copper foam nanostructures against Escherichia coli (Gram-negative) and Bacillus subtilis (Gram-positive) bacteria. An advanced antibacterial and antifouling surface is fabricated utilizing the collective antibacterial properties of silver nanodots, chitosan, and dendritic copper foam nanostructures. The porous network of the Ag nanodot decorated Cu foam is made up of nanodendrites, which reduce the wettability of the surface. Hence, the surface exhibits hydrophobic nature and inhibits the growth of bacterial flora along with the elimination of dead bacterial cells. The fabricated surface exhibits a water contact angle (WCA) of 158.7 ± 0.17°. Specifically, we tested the fabricated material against both the Gram-positive and Gram-negative bacterial models. The antibacterial activity of the fabricated surface is evident from the growth inhibition percentage of bacterial strains of Escherichia coli (72.30 ± 0.60%) and Bacillus subtilis (48.30 ± 1.71%). The micrographs obtained from scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM) of the treated cells show the damaged cellular structures of the bacteria, which is strong evidence of successful antibacterial action. The antibacterial effect can be attributed to the synergistic mechano-chemo mode of action involving mechanical disruption of the bacterial cell wall by the nanoprotrusions present on the Cu dendrites along with the chemical interaction of the Ag nanodots with vital intracellular components.

Interface engineering of moisture-induced ionic albumen dielectric layers through self-crosslinking of cysteine amino acids for low voltage, high-performance organic field-effect transistors.

The interface roughness between the semiconducting and dielectric layers of organic field-effect transistors (OFETs) plays a crucial role in the charge transport mechanism through the device. Here we report the interface engineering of a moisture induced ionic albumen material through systematic control of the temperature-dependent self-crosslinking of cysteine amino acids in the dielectric layer. The evolution of the surface morphologies of albumen and pentacene semiconducting films has been studied to achieve a smooth interface for enhanced charge transport. A structural transition of pentacene films from crystalline dendrite to amorphous was induced by the higher surface roughness of the albumen film. The devices showed a high transconductance of 11.68 µS at a lower threshold voltage of -0.9 V.

One-Dimensional NiSe-Se Hollow Nanotubular Architecture as a Binder-Free Cathode with Enhanced Redox Reactions for High-Performance Hybrid Supercapacitors.

Selenium-enriched nickel selenide (NiSe-Se) nanotubes supported on highly conductive nickel foam (NiSe-Se@Ni foam) were synthesized using chemical bath deposition with the aid of lithium chloride as a shape-directing agent. The uniformly grown NiSe-Se@Ni foam, with its large number of electroactive sites, facilitated rapid diffusion and charge transport. The NiSe-Se@Ni foam electrode exhibited a superior specific capacitance value of 2447.46 F g-1 at a current density value of 1 A g-1 in 1 M aqueous KOH electrolyte. Furthermore, a high-energy-density pouch-type hybrid supercapacitor (HSC) device was fabricated using the proposed NiSe-Se@Ni foam as the positive electrode, activated carbon on Ni foam as the negative electrode, and a filter paper separator soaked in 1 M KOH electrolyte solution. The HSC delivered a specific capacitance of 84.10 F g-1 at a current density of 4 mA cm-2 with an energy density of 29.90 W h kg-1 at a power density of 594.46 W kg-1 for an extended operating voltage window of 1.6 V. In addition, the HSC exhibited excellent cycling stability with a capacitance retention of 95.09% after 10,000 cycles, highlighting its excellent potential for use in the hands-on applications. The real-life practicality of the HSC was tested by using it to power a red light-emitting diode.

Low Operating Voltage Organic Field-Effect Transistors with Gelatin as a Moisture-Induced Ionic Dielectric Layer: The Issues of High Carrier Mobility.

We have developed low-voltage (<2 V) flexible organic field-effect transistors (OFETs) with high carrier mobility using gelatin as a moisture-induced ionic gate dielectric system. Ionic concentration in the gelatin layer depends on the relative humidity condition during the measurement. The capacitance of the dielectric layer used for the calculation of field-effect carrier mobility for the OFETs crucially depends on the frequency at which the capacitance was measured. The results of frequency-dependent gate capacitance together with the anomalous bias-stress effect have been used to determine the exact frequency at which the carrier mobility should be calculated. The observed carrier mobility of the devices is 0.33 cm2/Vs with the capacitance measured at frequency 20 mHz. It can be overestimated to 14 cm2/Vs with the capacitance measured at 100 kHz. The devices can be used as highly sensitive humidity sensors. About three orders of magnitude variation in device current have been observed on the changes in relative humidity (RH) levels from 10 to 80%. The devices show a fast response with a response and recovery times of ∼100 and ∼110 ms, respectively. The devices are flexible up to a 5 mm bending radius.

Superior optical (λ ∼ 1550 nm) emission and detection characteristics of Ge microdisks grown on virtual Si0.5Ge0.5/Si substrates using molecular beam epitaxy.

We report the optical characteristics of relatively large sized (∼7.0-8.0 µm) but low aspect ratio Ge microdisks grown on a virtual Si0.5Ge0.5 substrate using molecular beam epitaxy following the Stranski-Krastanov growth mechanism. Grown microdisks with very low aspect ratio Ge islands exhibit direct band gap (∼0.8 eV) photoluminescence emission sustainable up to room temperature, enabled by the confinement of carriers into the microdisks. p-i-n diodes with an intrinsic layer containing Ge microdisks have been fabricated to study their emission and photoresponse characteristics at an optical communication wavelength of ∼1550 nm. A strong electroluminescence at 1550 nm has been achieved at low temperatures in the device for a very low threshold current density of 2.56 µA cm-2 due to the strong confinement of injected holes. The emission characteristics of the fabricated device with respect to the injected current density and temperature have been studied. Novel emission and optical modulation characteristics at 1550 nm of the fabricated p-i-n device containing Ge microdisks grown on a virtual SiGe substrate indicate its potential for Si CMOS compatible on-chip optical communications.

Electrodeposited Cu2O Nanopetal Architecture as a Superhydrophobic and Antibacterial Surface.

Bacterial infections being sporadic and uncontrollable demands an urgent paradigm shift in the development of novel antibacterial agents. This work involves the fabrication of Cu2O nanopetals over copper foil that show superlative antibacterial and superhydrophobic properties. A superhydrophobic surface has been fabricated using the electrochemical deposition (ECD) method. Here, it is aimed to establish the superior antibacterial activity as an outcome of the inherent superhydrophobic property of the as-fabricated nanostructures. The present study finds that the elevated value of the water contact angle (154 ± 0.6°) does not allow proper bacterial adhesion, and it is immune from the possibility of biofouling. Specifically, two kinds of bacterial strains have been tested and the time response of the antibacterial activity has been studied over a period of 12 h, taking DH5α Escherichia coli as a Gram-negative model and Bacillus subtilis 168 as a Gram-positive model. Higher antibacterial effects were observed for the Gram-negative model (E. coli) owing to its simplistic cell wall structure which facilitates the easy diffusion of Cu+ ions into the bacterial membrane. The simplicity of the developed method of fabrication along with the superlative superhydrophobic nature and excellent antibacterial property of the material, owing to its synergistic biophysical and biochemical modes of biocidal action, establishes its viability in many applications.

Organic Field-Effect Transistor-Based Ultrafast, Flexible, Physiological-Temperature Sensors with Hexagonal Barium Titanate Nanocrystals in Amorphous Matrix as Sensing Material.

Organic field-effect transistors (OFETs) with hexagonal barium titanate nanocrystals (h-BTNCs) in amorphous matrix as one of the bilayer dielectric systems have been fabricated on a highly flexible 10 µm thick poly(ethylene terephthalate) substrate. The device current and mobility remain constant up to a bending radius of 4 mm, which makes the substrate suitable for wearable e-skin applications. h-BTNC films are found to be highly temperature-sensitive, and the OFETs designed based on this material showed ultraprecision measurement (∼4.3 mK), low power (∼1 µW at 1.2 V operating voltage), and ultrafast response (∼24 ms) in sensing temperature over a range of 20-45 °C continuously. The sensors are highly stable around body temperature and work at various extreme conditions, such as under water and in solutions of different pH values and various salt concentrations. These properties make this sensor unique and highly suitable for various healthcare and other applications, wherein a small variation of temperature around this temperature range is required to be measured at an ultrahigh speed.

P-type ß-MoO2 nanostructures on n-Si by hydrogenation process: synthesis and application towards self-biased UV-visible photodetection.

We report on the synthesis and UV-vis photodetection application of p-type MoO2 nanostructures (NSs) on Si substrate. ß-MoO2 NSs have been synthesized from previously grown α-MoO3 structures/n-type Si via a hydrogenation process at 450 °C. After hydrogenation, the α-MoO3 structures were completely converted into ß-MoO2 NSs without the presence of sub-oxidized phases of molybdenum oxide. The as-grown NSs exhibited very good p-type electrical conductivity of ≈2.02 × 103 S-cm-1 with hole mobility of ≈7.8 ± 1.3 cm2-V-1-Sec-1. To explore optoelectronic properties of p-type ß-MoO2 NSs, we have fabricated a p-MoO2/n-Si heterojunction photodetector device with Au as the top and Al as the bottom contacts. The device exhibits peak photoresponsivity of ≈0.155 A W-1 with maximum detectivity ≈1.28 × 1011 cm-Hz1/2-W-1 and 44% external quantum efficiency around ≈436 nm, following the highest photoresponse (I ph/I d ≈ 6.4 × 102) and good response speed (rise time ∼29 ms and decay time ∼38 ms) at -1.5 V. Importantly, this device also shows good self-powered high-speed (rise time ∼47 ms and decay time ∼70 ms) photodetection performance with peak responsivity and detectivity of ≈45 mA W-1 and ≈4.05 × 1010 cm-Hz1/2-W-1, respectively. This broadband UV-visible light detection feature can be attributed to the coordinated effects of MoO2 band-edge absorption, interfacial defects and self absorption in Si. The photodetection behavior of the device has been understood by proposed energy-band diagrams with the help of an experimentally derived work function, band gap and valence band maximum position of MoO2 NSs.

Effects of Dielectric Material, HMDS Layer, and Channel Length on the Performance of the Perylenediimide-Based Organic Field-Effect Transistors.

It is well-known that the improvement in the performance of organic field-effect transistors (OFETs) relies primarily on growth properties of organic molecules on gate dielectrics, their interface behavior, and on understanding the physical processes occurring during device operation. In this work, the relation of varying the dielectric materials in an n-type OFET device based on 1,7-dibromo-N,N'-dioctadecyl-3,4,9,10-perylenetetracarboxylic diimide (Br2PTCDI-C18) molecule on a low-cost glass substrate at different channel lengths is reported, which is conceptually very important and fundamental in the context of device performance. Anodized alumina (Al2O3) along with dielectric films of polyvinyl alcohol (PVA) or polymethylmethacrylate (PMMA) was used to fabricate the devices and study their influence on various transistor properties. In addition, the effects of a thin hexamethyldisilazane (HMDS) layer on the performance of OFETs including their contact resistances were studied with the channel length variations. The devices with PVA dielectric material exhibited the maximum mobility values of 0.012-0.025 cm2 V-1 s-1 irrespective of varying channel lengths from 25 to 190 µm. The bias-stress measurements were recorded to realize the effects of the channel length and HMDS layer on the stability of the devices. The on/off ratios and electrical stabilities of these devices were enhanced significantly by modifying the surface of the PVA dielectric layer using a thin layer of HMDS. Similarly, in the case of PMMA dielectric layer, a drastic enhancement in the on/off ratio and bias-stress stability was observed. Characterization of all devices at different channel lengths using different dielectric materials permitted us to identify the effects of contact resistance on OFET devices. The stability of the devices in relation to the bias-stress measurements of devices by varying channel lengths and surface modification was systematically investigated. A careful analysis of oxide gate dielectrics modified with polymer-based dielectric materials, contact resistance, influence of thin HMDS layer on the electrical properties, and other parameters on top-contact bottom-gated configured n-type OFET devices is presented herein.

Enhanced environmental stability induced by effective polarization of a polar dielectric layer in a trilayer dielectric system of organic field-effect transistors: a quantitative study.

We report a concept fabrication method that helps to improve the performance and stability of copper phthalocyanine (CuPc) based organic field-effect transistors (OFETs) in ambient. The devices were fabricated using a trilayer dielectric system that contains a bilayer polymer dielectrics consisting of a hydrophobic thin layer of poly(methyl methacrylate) (PMMA) on poly(vinyl alcohol) (PVA) or poly(4-vinylphenol) (PVP) or polystyrene (PS) with Al2O3 as a third layer. We have explored the peculiarities in the device performance (i.e., superior performance under ambient humidity), which are caused due to the polarization of dipoles residing in the polar dielectric material. The anomalous behavior of the bias-stress measured under vacuum has been explained successfully by a stretched exponential function modified by adding a time dependent dipole polarization term. The OFET with a dielectric layer of PVA or PVP containing hydroxyl groups has shown enhanced characteristics and remains highly stable without any degradation even after 300 days in ambient with three times enhancement in carrier mobility (0.015 cm(2)·V(-1)·s(-1)) compared to vacuum. This has been attributed to the enhanced polarization of hydroxyl groups in the presence of absorbed water molecules at the CuPc/PMMA interface. In addition, a model has been proposed based on the polarization of hydroxyl groups to explain the enhanced stability in these devices. We believe that this general method using a trilayer dielectric system can be extended to fabricate other OFETs with materials that are known to show high performances under vacuum but degrade under ambient conditions.

Local diffusion induced roughening in cobalt phthalocyanine thin film growth.

We have studied the kinetic roughening in the growth of cobalt phthalocyanine (CoPc) thin films grown on SiO2/Si(001) surfaces as a function of the deposition time and the growth temperature using atomic force microscopy (AFM). We have observed that the growth exhibits the formation of irregular islands, which grow laterally as well as vertically with coverage of CoPc molecules, resulting rough film formation. Our analysis further disclosed that such formation is due to an instability in the growth induced by local diffusion of the molecules following an anomalous scaling behavior. The instability relates the (ln(t))(1/2), with t as deposition time, dependence of the local surface slope as described in nonequilibrium film growth. The roughening has been characterized by calculating different scaling exponents α, ß, and 1/z determined from the height fluctuations obtained from AFM images. We obtained an average roughness exponent α = 0.78 ± 0.04. The interface width (W) increases following a power law as W ∼ t(ß), with growth exponent ß = 0.37 ± 0.05 and lateral correlation length (ξ) grows as ξ ∼ t(1/z) with dynamic exponent 1/z = 0.23 ± 0.06. The exponents revealed that the growth belongs to a different class of universality.

Organic n-channel transistors based on core-cyanated perylene carboxylic diimide derivatives.

Five core-cyanated perylene carboxylic diimides end-functionalized with fluorine-containing linear and cyclic substituents have been synthesized and employed in the fabrication of air-stable n-channel organic thin-film field-effect transistors with carrier mobilities up to 0.1 cm2/Vs. The relationships between molecular structure, thin-film morphology, substrate temperature during vacuum deposition, transistor performance, and air stability have been investigated. Our experiments led us to conclude that the role of the fluorine functionalization in the air-stable n-channel operation of the transistors is different than previously thought.

Imaging of atomic layer deposited (ALD) tungsten monolayers on alpha-TiO2(110) by X-ray standing wave Fourier inversion.

A single atomic layer of tungsten grown by atomic layer deposition (ALD) on a single-crystal rutile TiO2(110) support is studied by the X-ray standing wave (XSW) technique. The surface structural and chemical properties were also examined using atomic force microscopy, X-ray photoelectron spectroscopy, and low-energy electron diffraction. The XSW measured set of hkl Fourier components for the W atomic distribution function are summed together to produce a model-independent 3D map of the W atoms relative to the rutile lattice. The 3D atomic image shows surface tungsten atoms equally occupying the two nonequivalent Ti sites with a slight outward displacement. This corresponds to the atop and bridge sites with respect to the underlying lattice oxygen atoms. These XSW measurements clearly show that ALD conformal layers can be highly coherent with respect to the substrate lattice.

The controlled evolution of a polymer single crystal.

We present a method for controlling the initiation and kinetics of polymer crystal growth using dip-pen nanolithography and an atomic force microscope tip coated with poly-dl-lysine hydrobromide. Triangular prisms of the polymer epitaxially grow on freshly cleaved mica substrates, and their in-plane and out-of-plane growth rates can be controlled by raster scanning the coated tip across the substrate. Atomic force microscope images were concomitantly recorded, providing a set of photographic images of the process as it spans the nanometer- to micrometer-length scales as a function of environmental conditions.