Dogan Sinar

Printed graphene interdigitated capacitive sensors on flexible polyimide substrates

Interdigitated capacitors (IDC) are extensively used for a variety of chemical and biological sensing applications. Printing and functionalizing these IDC sensors on bendable substrates will lead to new innovations in healthcare and medicine, food safety inspection, environmental monitoring, and public security. The synthesis of an electrically conductive aqueous graphene ink stabilized in deionized water using the polymer Carboxymethyl Cellulose (CMC) is introduced in this paper. CMC is a nontoxic hydrophilic cellulose derivative used in food industry. The water-based graphene ink is then used to fabricate IDC sensors on mechanically flexible polyimide substrates. The capacitance and frequency response of the sensors are analyzed, and the effect of mechanical stress on the electrical properties is examined. Experimental results confirm low thin film resistivity (~6;.6×10-3 Ω-cm) and high capacitance (>100 pF). The printed sensors are then used to measure water content of ethanol solutions to demonstrate the proposed conductive ink and fabrication methodology for creating chemical sensors on thin membranes.

Graphene-based inkjet printing of flexible bioelectronic circuits and sensors

Bioelectronics involves interfacing functional biomolecules or living cells with electronic circuitry. Recent advances in electrically conductive inks and inkjet printing technologies have enabled bioelectronic devices to be fabricated on mechanically flexible polymers, paper and silk. In this research, non-conductive graphene-oxide (GO) inks are synthesized from inexpensive graphite powders. Once printed on the flexible substrate the electrical conductivity of the micro-circuitry can be restored through thermal reduction. Laser irradiation is one method being investigated for transforming the high resistance printed GO film into conductive oxygen reduced graphene-oxide (rGO). Direct laser writing is a precision fabrication process that enables the imprinting of conductive and resistive micro-features on the GO film. The mechanically flexible rGO microcircuits can be further biofunctionalized using molecular self-assembly techniques. Opportunities and challenges in exploiting these emerging technologies for developing biosensors and bioelectronic cicruits are briefly discussed.

Laser assisted reduction of printed GO films and traces

Non-conductive graphene oxide (GO) particles (70% carbon, 30% oxygen) synthesized directly from inexpensive graphite powders are hydrophilic and can be dispersed homogeneously in water based solutions. In this study, thin film electrode traces are produced from a GO aqueous suspension using a commercially available inkjet printer. Once deposited on the functionalized substrate the printed film is thermally annealed to remove excess water and harden the material. The electrical conductivity of the high resistance (>10MΩ) annealed film is increased through the removal of oxygen molecules using a focused 775nm, 120fs pulsed laser. The electrical properties of select target areas on the thermally reduced graphene oxide (rGO) film can be tuned by adjusting the laser power, material feed rate, and the number of passes that the beam makes over the target surface. Experiments demonstrate that the rGO trace can be modified over a wide range of sheet resistance values (1MΩ to 2kΩ) for a ~1μm thick film. The proposed fabrication method can also be used to create a variety of resistive and semiconductor components on printed thin films, conductive electrodes and micro-circuit traces.

Laser micromachining of oxygen reduced graphene-oxide films

Non-conductive graphene-oxide (GO) inks can be synthesized from inexpensive graphite powders and deposited on functionalized flexible substrates using inkjet printing technology. Once deposited, the electrical conductivity of the GO film can be restored through laser assisted thermal reduction. Unfortunately, the inkjet nozzle diameter (~40μm) places a limit on the printed feature size. In contrast, a tightly focused femtosecond pulsed laser can create precise micro features with dimensions in the order of 2 to 3 μm. The smallest feature size produced by laser microfabrication is a function of the laser beam diameter, power level, feed rate, material characteristics and spatial resolution of the micropositioning system. Laser micromachining can also remove excess GO film material adjacent to the electrode traces and passive electronic components. Excess material removal is essential for creating stable oxygen-reduced graphene-oxide (rGO) printed circuits because electron buildup along the feature edges will alter the conductivity of the non-functional film. A study on the impact of laser ablation on the GO film and the substrate are performed using a 775nm, 120fs pulsed laser. The average laser power was 25mW at a spot size of ~ 5μm, and the feed rate was 1000-1500mm/min. Several simple microtraces were fabricated and characterized in terms of electrical resistance and surface topology.

Microfabrication of passive electronic components with printed graphene-oxide deposition

Flexible electronic circuitry is an emerging technology that will significantly impact the future of healthcare and medicine, food safety inspection, environmental monitoring, and public security. Recent advances in drop-on-demand printing technology and electrically conductive inks have enabled simple electronic circuits to be fabricated on mechanically flexible polymers, paper, and bioresorbable silk. Research has shown that graphene, and its derivative formulations, can be used to create low-cost electrically conductive inks. Graphene is a one atom thick two-dimensional layer composed of carbon atoms arranged in a hexagonal lattice forming a material with very high fracture strength, high Young’s Modulus, and low electrical resistance. Non-conductive graphene-oxide (GO) inks can also be synthesized from inexpensive graphite powders. Once deposited on the flexible substrate the electrical conductivity of the printed GO microcircuit traces can be restored through thermal reduction. In this paper, a femtosecond laser with a wavelength of 775nm and pulse width of 120fs is used to transform the non-conductive printed GO film into electrically conductive oxygen reduced graphene-oxide (rGO) passive electronic components by the process of laser assisted thermal reduction. The heat affected zone produced during the process was minimized because of the femtosecond pulsed laser. The degree of conductivity exhibited by the microstructure is directly related to the laser power level and exposure time. Although rGO films have higher resistances than pristine graphene, the ability to inkjet print capacitive elements and modify local resistive properties provides for a new method of fabricating sensor microcircuits on a variety of substrate surfaces.

Printed Graphene Derivative Circuits as Passive Electrical Filters

The objective of this study is to inkjet print resistor-capacitor (RC) low pass electrical filters, using a novel water-based cellulose graphene ink, and compare the voltage-frequency and transient behavior to equivalent circuits constructed from discrete passive components. The synthesized non-toxic graphene-carboxymethyl cellulose (G-CMC) ink is deposited on mechanically flexible polyimide substrates using a customized printer that dispenses functionalized aqueous solutions. The design of the printed first-order and second-order low-pass RC filters incorporate resistive traces and interdigitated capacitors. Low pass filter characteristics, such as time constant, cut-off frequency and roll-off rate, are determined for comparative analysis. Experiments demonstrate that for low frequency applications (<100 kHz) the printed graphene derivative circuits performed as well as the circuits constructed from discrete resistors and capacitors for both low pass filter and RC integrator applications. The impact of mechanical stress due to bending on the electrical performance of the flexible printed circuits is also investigated.

Flexible hydrogel actuated graphene-cellulose biosensor for monitoring pH

The level of pH in body fluids can indicate the onset of infection or a chronic condition. A mechanically flexible pH sensitive graphene-cellulose interdigitated capacitive (IDC) biosensor for monitoring acidity in various types of body fluids is introduced in this paper. The planar IDC is printed on a nonrigid polymer substrate material and coated with a very thin passivation layer to prevent an electrical short with the material under test. The electrically conductive ink is synthesized by using carboxymethyl cellulose (CMC) to suspend hydrophobic graphene (G) sheets in a water-based solvent. Once deposited on the substrate the conductivity of the printed G-CMC IDC electrodes is increased using thermal reduction. To enable the biosensor to respond to changes in fluid acidity, a biocompatible pH-sensitive chitosan hydrogel is immobilized on the IDC electrodes. As the hydrogel responds to changes in pH, the gel swells or de-swells over the interdigitated electrodes causing a measureable change in circuit capacitance. Issues associated with mechanical and chemical stability are discussed.

Graphene and silver-nanoprism dispersion for printing optically-transparent electrodes

Optically transparent electrodes (OTEs) are used for bioelectronics, touch screens, visual displays, and photovoltaic cells. Although the conductive coating for these electrodes is often composed of indium tin oxide (ITO), indium is a very expensive material and thin ITO films are relatively brittle compared to conductive polymer or graphene thin films. An alternative highly conductive optically transparent thin film based on a graphene (G) and silver-nanoprism (AgNP) dispersion is introduced in this paper. The aqueous G ink is first synthesized using carboxymethyl cellulose (CMC) as a stabilizing agent. Silver (Ag) nanoprisms are then prepared separately by a simple thermal process which involves the reduction of silver nitrate by sodium borohydride. These Ag nanoprisms are only a few nanometers thick but have relatively large surface areas (>1000 nm2). As a consequence, the nanoprisms provide more efficient injection of free carriers to the G layer. The concentrated G-AgNP dispersions are then deposited on optically transparent glass and polyimide substrates using an inkjet printer with a HP6602A print head. After printing, these optically thin films can be thermally treated to further increase electrical conductivity. Thermal treatment decomposes CMC which frees elemental carbon from polymer chain and, simultaneously, causes the film to become hydrophobic. Preliminary experiments demonstrate that the G-AgNP films on glass substrates exhibit high conductivity at 70% transparency (550 nm). Additional tests on the Gr-AgNP thin films printed on polymide substrates show mechanical stability under bending with minimal reduction in electrical conductivity or optical transparency.

Flexible electrical circuits printed on polymers using graphene-cellulose inks

A graphene based ink for printing passive electrical components (conductive traces, resistors, capacitors, inductors) and circuitry on mechanically flexible polymer substrates is described in this paper. The ink is synthesized by using carboxymethyl cellulose (CMC), a hydrophilic cellulose derivative, to suspend the naturally hydrophobic graphene (G) sheets in an aqueous solvent composed of 70% DI water and 30% 2-butoxyethanol. Once deposited on the functionalized substrate the conductivity of the printed electrical components can be optimized by decomposing the cellulose stabilizer using thermal reduction. Several printed passive electrical circuits are fabricated and tested including an interdigitated capacitive chemical sensor and a low-pass electrical filter. The measured characteristics of the printed circuits are compared with theoretical values for performance validation. Mechanical bending tests were also performed to demonstrate that the thermally modified G-CMC films can absorb large levels of strain without fracturing or degrading the electrical properties.