Pawan Tyagi

Patternable nanowire sensors for electrochemical recording of dopamine

Spatially resolved electrochemical recording of neurochemicals is difficult due to the challenges associated with producing nanometer-scale patternable and integrated sensors. We describe the lithographic fabrication and characterization of patternable gold (Au) nanowire (NW) based sensors for the electrochemical recording of dopamine (DA). We demonstrate a straightforward NW-size-independent approach to align contact pads to NWs. Sensors, with NW widths as small as 30 nm, exhibited considerable insensitivity to scan rates during cyclic voltammetry, a nonlinear increase in oxidation current with increasing NW width, and the selectivity to measure submaximal synaptic concentrations of DA in the presence of interfering ascorbic acid. The electrochemical sensitivity of Au NW electrode sensors was much larger than that of Au thin-film electrodes. In chronoamperometric measurements, the NW sensors were found to be sensitive for submicromolar concentration of DA. Hence, the patternable NW sensors represent an attractive platform for electrochemical sensing and recording.

Reversible Actuation of Microstructures by Surface‐Chemical Modification of Thin‐Film Bilayers

Adv. Mater. 2010, 22, 407–41

Self-Assembly Based on Chromium/Copper Bilayers

In this paper, we detail a strategy to self-assemble microstructures using chromium/copper (Cr/Cu) bilayers. Self-assembly was primarily driven by the intrinsic residual stresses of Cr within these films; in addition, the degree of bending could be controlled by changing the Cu film thickness and by introducing a third layer with either a flexible polymer or a rigid metal. We correlate the observed curvature of patterned self-assembled microstructures with those predicted by a published multilayer model. In the model, measured stress values (measured on the unpatterned films using a substrate curvature method) were utilized. We also investigated the role of two different sacrificial layers: 1) silicon and 2) water-soluble polyvinyl alcohol. Finally, a Taguchi design of experiments was performed to investigate the importance of the different layers in contributing to the stress-thickness product (the critical parameter that controls the curvature of the self-assembled microstructures) of the multilayers. This paper facilitates a deeper understanding of multilayer thin-film-based self-assembly and provides a framework to assemble complex microstructures, including tetherless self-actuating devices.

MOLECULAR SPIN DEVICES: CURRENT UNDERSTANDING AND NEW TERRITORIES

Molecular spin devices (MSDs) are the most promising candidate for futuristic quantum computation, having potential to resolve spin scattering issue which compromise the utility of conventional spin devices. The MSDs have been extensively reviewed from the view points of device physics and the application of target molecules, such as single molecular magnets. Fabrication of a competent MSD still remains an intractable task. In this review, we first describe the experimental studies where spin state of molecule and/or electrode affected the device transport, especially under magnetic field. Then, we correlated the number of theoretical and experimental results from various domains of nanomagnetism to highlight the scope and future directions panoramically. Finally, the key designs of various MSDs, including our recently developed multilayer edge molecular electrode, have been discussed. A multilayer edge molecular electrode, prepared by bridging the molecular clusters on the exposed edges of a customized ferromagnet–insulator–ferromagnet junction, can be a promising platform for testing the variety of molecular magnets.

Monte Carlo and Experimental Magnetic Studies of Molecular Spintronics Devices

Molecular spintronics devices (MSDs) are highly promising candidates for enabling quantum computation and revolutionizing computer logic and memory. An advanced MSD will require the placement of magnetic molecules between the two ferromagnetic (FM) electrodes. Recent experimental studies showed that some magnetic molecules produced unprecedented strong exchange couplings between the two FM electrodes leading to intriguing magnetic and transport properties in a MSD. Future development of MSDs will critically depend on obtaining an in-depth understanding of the molecule induced exchange coupling, and its impact on switchability, functional temperature range, and stability. However, the large size of MSD systems and fragile device fabrication scheme continue to limit the theoretical and experimental studies of magnetic attributes produced by molecules in a MSD. This paper theoretically studies the MSD by performing Monte Carlo simulations (MCS). Our MCS encompasses the full range of MSDs that can be realized by establishing different kinds of magnetic interaction between magnetic molecules and FM electrodes. Our MSDs are represented by a 2D Ising model. We studied the effect of a wide range of molecule-FM electrode couplings on the basic properties of MSDs. This wide range covered (i) molecule possessing ferromagnetic coupling with both FM electrodes, (ii) molecule possessing antiferromagnetic coupling with both FM electrodes, and (iii) molecule possessing ferromagnetic coupling with one electrode and antiferromagnetic coupling with another FM electrode. Our MCS will enable the fundamental understanding and designing of a wide range of novel MSDs utilizing a variety of molecules and FM electrodes; these studies will also benefits nanomaterials based spintronics devices employing nanoclusters and quantum dots as the device elements.Molecule-based spintronics devices (MSDs) are highly promising candidates for discovering advanced logic and memory computer units. An advanced MSD will require the placement of paramagnetic molecules between the two ferromagnetic (FM) electrodes. Due to extreme fabrication challenges, only a couple of experimental studies could be performed to understand the effect of magnetic molecules on the overall magnetic and transport properties of MSDs. To date, theoretical studies mainly focused on charge and spin transport aspects of MSDs; there is a dearth of knowledge about the effect of magnetic molecules on the magnetic properties of MSDs. This paper investigates the effect of magnetic molecules, with a net spin, on the magnetic properties of 2D MSDs via Monte Carlo (MC) simulations. Our MC simulations encompass a wide range of MSDs that can be realized by establishing different kinds of magnetic interactions between molecules and FM electrodes at different temperatures. The MC simulations show that ambient thermal energy strongly influenced the molecular coupling effect on the MSD. We studied the nature and strength of molecule couplings (FM and antiferromagnetic) with the two electrodes on the magnetization, specific heat and magnetic susceptibility of MSDs. For the case when the nature of molecule interaction was FM with one electrode and antiferromagnetic with another electrode the overall magnetization shifted toward zero. In this case, the effect of molecules was also a strong function of the nature and strength of direct coupling between FM electrodes. In the case when molecules make opposite magnetic couplings with the two FM electrodes, the MSD model used for MC studies resembled with the magnetic tunnel junction based MSD. The experimental magnetic studies on these devices are in agreement with our theoretical MC simulations results. Our MC simulations will enable the fundamental understanding and designing of a wide range of novel spintronics devices utilizing a variety of molecules, nanoclusters and quantum dots as the device elements.

Scanning surface-enhanced Raman spectroscopy (SERS) of chemical agent simulants on templated Au-Ag nanowire substrates

We report the results of scanning micro-Raman spectroscopy obtained on Au-Ag nanowires for a variety of chemical warfare agent simulants. Rough silver segments embedded in gold nanowires showed enhancement of 105 - 107 and allowed unique identification of 3 of 4 chemical agent simulants tested. These results suggest a promising method for detection of compounds significant for security applications, leading to sensors that are compact and selective.

Nanowire-based surface-enhanced Raman spectroscopy (SERS) for chemical warfare simulants

Hand-held instruments capable of spectroscopic identification of chemical warfare agents (CWA) would find extensive use in the field. Because CWA can be toxic at very low concentrations compared to typical background levels of commonly-used compounds (flame retardants, pesticides) that are chemically similar, spectroscopic measurements have the potential to reduce false alarms by distinguishing between dangerous and benign compounds. Unfortunately, most true spectroscopic instruments (infrared spectrometers, mass spectrometers, and gas chromatograph-mass spectrometers) are bench-top instruments. Surface-acoustic wave (SAW) sensors are commercially available in hand-held form, but rely on a handful of functionalized surfaces to achieve specificity. Here, we consider the potential for a hand-held device based on surface enhanced Raman scattering (SERS) using templated nanowires as enhancing substrates. We examine the magnitude of enhancement generated by the nanowires and the specificity achieved in measurements of a range of CWA simulants. We predict the ultimate sensitivity of a device based on a nanowire-based SERS core to be 1-2 orders of magnitude greater than a comparable SAW system, with a detection limit of approximately 0.01 mg m-3.

Dielectrophoretic assembly of ordered nanostructures: Harnessing thermal randomness and inter-particle interactions

If sufficiently developed, directed assembly of engineered nanostructures could revolutionize the manufacture of integrated devices. We describe the assembly of metallic and heterostructured nanowires into ordered arrays, and show that such assembly can be controlled by balancing the various relevant energy scales in the process in a manner analogous to crystal growth. These assemblies offer examples of structures that could be applicable to next-generation sensors and electronics.

Taguchi Design of Experiment Enabling the Reduction of Spikes on the Sides of Patterned Thin Films for Tunnel Junction Fabrication

Sputter thin film deposition after photolithography often produces unwanted spikes along the side edges. These spikes are a significant issue for the development of magnetic tunnel junction (MTJ)-based memory and molecular spintronics devices, microelectronics, and microelectromechanical systems because they influence the properties of the other films deposited on the top. Our molecular spintronics devices that utilizes MTJ as the testbed are almost short-lived and encountered high background current that masked the effect of molecular transport channels placed along the sides of MTJs. Therefore, tapered thin film edges are critically needed in devices. Here, we report a very cost efficient and fast way of creating an optimum photoresist profile for the production of ‘spike-free’ patterned films. This approach is based on performing a soaking in the photoresist developer after baking and before the UV exposure. However, the success of this method depends on multiple factors accounted for during photolithography photoresist thickness (spin speed), baking temperature, soaking time and exposure time. Our recent experiments systematically studied the effect of these factors by following the L9 experimental scheme of the Taguchi Design of experiment (TDOE). The L9 experimental scheme effectively accommodated the study of four photolithography factors, each with three levels. After performing photolithography as per L9 TDOE, we conducted sputtering thin film deposition of 20 nm Tantalum. Then we conducted an atomic force microscope (AFM) study of thin film patterns and measured the spikes along the edges of the deposited Tantalum. We utilized spike height as the desired property and chose “smaller the better” criteria for TDOE analysis. TDOE enabled us to understand the relative importance of the parameters, relationship amongst the parameters, and impact of the various levels of the parameters on the edge profile of the thin film patterns. We discovered that baking temperature was the most influential parameter; presoak time and photoresist thickness were two other influential factors; exposure time was the least effective factor. We also found that 4000 rpm, 100 C soft baking, 60 s soaking and 15 s UV exposure yielded the best results. Finally, the paper also discusses the interdependence of selected factors, and impact of the individual levels of each factor. This study is expected to benefit MEMS and micro/nanoelectronics device researchers because it attempts at finding a cheaper and faster alternative to creating an optimum photoresist profile.