Fully Inkjet-Printed Multilayered Graphene-Based Flexible Electrodes for Repeatable Electrochemical Response

Graphene has proven to be useful in biosensing applications. However, one of the main hurdles with printed graphene-based electrodes is achieving repeatable electrochemical performance from one printed electrode to another. We have developed a consistent fabrication process to control the sheet resistance of inkjet-printed graphene electrodes, thereby accomplishing repeatable electrochemical performance. Herein, we investigated the electrochemical properties of multilayered graphene (MLG) electrodes fully inkjet-printed (IJP) on flexible Kapton substrates. The electrodes were fabricated by inkjet printing three materials – (1) a conductive silver ink for electrical contact, (2) an insulating dielectric ink, and (3) MLG ink as the sensing material. The selected materials and fabrication methods provided great control over the ink rheology and material deposition, which enabled stable and repeatable electrochemical response: bending tests revealed the electrochemical behavior of these sensors remained consistent over 1000 bend cycles. Due to the abundance of structural defects (e.g., edge defects) present in the exfoliated graphene platelets, cyclic voltammetry (CV) of the graphene electrodes showed good electron transfer (k = 1.125 × 10−2 cm s−1) with a detection limit (0.01 mM) for the ferric/ferrocyanide redox couple, [Fe(CN)6]−3/−4, which is comparable or superior to modified graphene or graphene oxide-based sensors. Additionally, the potentiometric response of the electrodes displayed good sensitivity over the pH range of 4–10. Moreover, a fully IJP three-electrode device (MLG, platinum, and Ag/AgCl) also showed quasi-reversibility compared to a single IJP MLG electrode device. These findings demonstrate significant promise for scalable fabrication of a flexible, low cost, and fully-IJP wearable sensor system needed for space, military, and commercial biosensing applications

Additive Manufacturing of Graphene-Based Devices for Flexible Hybrid Electronics

In this work, I investigate and enhance the fundamental sensing properties of printed electronic nanomaterials (e.g., graphene) in real-world environments while decreasing weight, cost, and power consumption. The dissertation addresses this issue with the following foci in mind: (1) developing a straightforward and repeatable process to synthesize graphene ink which is also compatible with Inkjet-printing (IJP) and Aerosol Jet printing (AJP). (2) Tuning additive manufacturing printing (IJP and AJP) parameters to establish a repeatable manufacturing process and print high performing (graphene-based) electrodes and interconnects, compatible with the underlying substrate. (3) Investigate power dissipation and electrical breakdown in AJP printed graphene interconnects. (4) Investigate the IJP printed graphene electrodes\u27 electrochemical sensitivity with pH and selectivity of Na+ ions and K+ ions. (5) Integrate printed electrochemical sensors with flexible silicon integrated circuits (Flex-ICs) for flexible hybrid electronics applications. Herein we demonstrate printed devices using graphene to enhance capabilities relative to sensitivity, conformability, and fast and repeatable responsivity while reducing the monitoring devices\u27 mass. Understanding the structure-property-processing correlations of our graphene-based devices has helped us improve the consistency, repeatability, and uniformity of the printed systems. This marks a significant step forward for designing flexible hybrid sensors as a platform to fabricate sensors for space, military, and commercial applications

Laser-Defined Graphene Strain Sensor Directly Fabricated on 3D-Printed Structure

A direct-write method to fabricate a strain sensor directly on a structure of interest is reported. In this method, a commercial graphene ink is printed as a square patch (6 mm square) on the structure. The patch is dried at 100 °C for 30 min to remove residual solvents but the printed graphene remains in an insulative state. By scanning a focused laser (830 nm, 100 mW), the graphene becomes electrically conductive and exhibits a piezoresistive effect and a low temperature coefficient of resistance of −0.0006 °C−1. Using this approach, the laser defines a strain sensor pattern on the printed graphene patch. To demonstrate the method, a strain sensor was directly fabricated on a 3D-printed test coupon made of ULTEM 9085 thermoplastic. The sensor exhibits a gauge factor of 3.58, which is significantly higher than that of commercial foil strain gauges made of constantan. This method is an attractive alternative when commercial strain sensors are difficult to employ due to the high porosity and surface roughness of the material structure under test

Utilizing a Single Silica Nanospring as an Insulating Support to Characterize the Electrical Transport and Morphology of Nanocrystalline Graphite

A graphitic carbon, referred to as graphite from the University of Idaho thermolyzed asphalt reaction (GUITAR), was coated in silica nanosprings and silicon substrates via the pyrolysis of commercial roofing tar at 800 °C in an inert atmosphere. Scanning electron microscopy and transmission electron microscopy images indicate that GUITAR is an agglomeration of carbon nanospheres formed by the accretion of graphitic flakes into a ~100 nm layer. Raman spectroscopic analyses, in conjunction with scanning electron microscopy and transmission electron microscopy, indicate that GUITAR has a nanocrystalline structure consisting of ~1–5 nm graphitic flakes interconnected by amorphous sp3 bonded carbon. The electrical resistivities of 11 single GUITAR-coated nanospring devices were measured over a temperature range of 10–80 °C. The average resistivity of all 11 devices at 20 °C was 4.3 ± 1.3 × 10−3 Ω m. The GUITAR coated nanospring devices exhibited an average negative temperature coefficient of resistivity at 20 °C of −0.0017 ± 0.00044 °C−1, which is consistent with the properties of nanocrystalline graphite

A Review of Inkjet Printed Graphene and Carbon Nanotubes Based Gas Sensors

Graphene and carbon nanotube (CNT)-based gas/vapor sensors have gained much traction for numerous applications over the last decade due to their excellent sensing performance at ambient conditions. Inkjet printing various forms of graphene (reduced graphene oxide or modified graphene) and CNT (single-wall nanotubes (SWNTs) or multiwall nanotubes (MWNTs)) nanomaterials allows fabrication onto flexible substrates which enable gas sensing applications in flexible electronics. This review focuses on their recent developments and provides an overview of the state-of-the-art in inkjet printing of graphene and CNT based sensors targeting gases, such as NO2, Cl2, CO2, NH3, and organic vapors. Moreover, this review presents the current enhancements and challenges of printing CNT and graphene-based gas/vapor sensors, the role of defects, and advanced printing techniques using these nanomaterials, while highlighting challenges in reliability and reproducibility. The future potential and outlook of this rapidly growing research are analyzed as well

Printed Carbon Nanotube Sensors for Ammonia Gas Detection

Commercially available ammonia gas detectors are expensive and become cost-prohibitive for applications requiring the use of a multitude of these sensors. Additively manufactured flexible electronic sensors can be utilized to adapt to varying environments, with added benefits of low production costs and minimal materials wastage. In terms of a suitable material for Ammonia detection, carbon nanotube (CNTs) have demonstrated the capability of reacting to multiple chemicals with high specificity depending on the formulation of the CNTs. This work seeks to demonstrate the viability of utilizing additive manufacturing technique to deposit metallic CNTs on a silver electrode array as a means to detect ammonia gas. The sensor is fully ink-jet fabricated by first printing a silver reference electrode array and then printing CNTs over the array. Our work also involves optimizing a CNT ink that will continuously jet and react well with ammonia gas, leading to a viable low-cost development of Ammonia sensors that could be widely deployed

Emerging 2D-Nanomaterials for Additive Manufacturing of Space-Grade Hybrid Electronics

The need for robust, sensitive, portable, and inexpensive electronic systems is of significant interest for human space exploration. In general, 2-Dimesional (2D) materials are one to three atoms thick and have modified band structures compared to bulk forms. This quantum confinement gives rise to unique physical and chemical properties. The exemplary electrical and structural properties of 2D materials allow for the design of highly sensitive and selective systems while also limiting the cost, weight and energy consumption of electronic/optoelectronic devices. In this paper, we highlight our recent investigations into the use of 2D material inks for additive manufacturing of electronic and optoelectronic devices. We first report on the electrical transport and power dissipation properties of aerosol-jet printed graphene interconnects, emphasizing the role of device morphology and the substrate on device performance. Secondly, our preliminary data on inkjet printed graphene-based electrodes indicate graphene is a highly sensitive electrode to monitor electrolyte and pH balance in human sweat. Lastly, we also incorporated printed MoS2 (molybdenum disulfide) into a photodetector, highlighting its potential as a semiconducting ink for lightweight optoelectronic devices that can withstand the high radiation exposures in space. We find the photocurrent response of the printed photodetector follows the frequency of the applied light signal as expected. Our results provide new insight into structure-property-processing correlations in printed 2D nanomaterial devices, with broader implications for reliability of printed and flexible electronics for space applications

Additive Manufacturing of Nanomaterial Based Sensors for Extreme Environments

Additive manufacturing, specifically inkjet printing (IJP) and aerosol jet printing (AJP), have shown great potential for rapid prototyping and direct writing of electronic sensors. These additive techniques provide the developer with much flexibility in controlling the sensor response through materials selection and system design. Recently, the research community has started exploring applications of additive manufacturing for extreme environments, such as space and nuclear applications. In this work, we explore both IJP and AJP of sensors for applications in human performance monitoring onboard the International Space Station and field property measurements inside nuclear test reactors. We use IJP of custom graphene inks on flexible substrates to sense pH and electrolyte concentrations, with potential applications in flexible and wearable electronic sensors for real-time analysis of various biological functions. We also explore the utility of AJP in conjunction with commercial nanoparticle inks for temperature melt arrays for in-pile nuclear sensors capable of measuring peak temperatures achieved during long-term irradiation experiments. Our results highlight the importance of structure-property-processing correlations in additively manufactured sensors to their performance in relative extreme environments

The sp\u3csup\u3e2\u3c/sup\u3e-sp\u3csup\u3e3\u3c/sup\u3e Carbon Hybridization Content of Nanocrystalline Graphite from Pyrolyzed Vegetable Oil, Comparison of Electrochemistry and Physical Properties with Other Carbon Forms and Allotropes

Nanocrystalline (nc) graphite produced from pyrolyzed vegetable oil has properties that deviate from typical graphites, but is similar to the previously reported Graphite from the University of Idaho Thermolyzed Asphalt Reaction (GUITAR). These properties include (i) fast heterogeneous electron transfer (HET) at its basal plane and (ii) corrosion resistance beyond graphitic materials. To discover the structural basis for these properties, characterization of this nc-graphite was investigated with Raman and X-ray photoelectron spectroscopies, nano-indentation, density, X-ray diffraction (XRD), thermogravimetric and elemental analyses. The results indicate that this nc-graphite is in Stage-2 of Ferrari’s amorphization trajectory between amorphous carbon (a-C) and graphite with a sp2/sp3 carbon ratio of 85/15. The nano-crystallites size of 1.5 nm from XRD is consistent with fast HET rates as this increases the density of electronic states at the Fermi-level. However, d-spacing from XRD is 0.350 nm vs. 0.335 for graphite. This wider distance does not explain its corrosion resistance. Literature trends suggest that increasing sp2 content in a-C’s increase both HET and corrosion rates. While nc-graphite’s HET rate follows this trend, it exhibits higher than predicted corrosion resistance. In general, this form of nc-graphite matches the best examples of boron-doped diamond in HET and corrosion rates