Graphene-based films in sensor applications

#The present invention relates to the graphene thin films fabricated by inkjet printing of graphene oxide with subsequent reduction to graphene, and, more specifically to the use of such graphene thin films as electrical sensors for sensing temperature, humidity, mechanical stresses or strains, or chemical, biological species.

#Temperature sensors such as thermistors are used in various applications, such as in gas sensors or in battery packs for optimization of battery life. Temperature sensors have elements with temperature-dependent electrical resistances. The temperature-resistance coefficient (i.e., the change in resistance with change in temperature) is positive for platinum-based materials and negative for metal oxides and semiconductors (i.e., metal oxides and semiconductors exhibit negative temperature coefficients (“NTC”)). Such sensor elements are typically fabricated in the form of discs and pellets. For advanced applications, NTC elements should maintain their properties over time (i.e., they should not age too quickly for long-term use), and should remain mechanically flexible. The mechanism of tracking the electrical resistance as response to environmental conditions can also be used in other sensing applications such as sensing mechanical strain or stress, pressure, humidity, or the chemical and biological species. Respectively, the corresponding changes in electrical resistance can be directly correlated to the mechanical strain or stress, relative humidity and concentrations of chemical or biological species in gas or liquid media.

#In embodiments of the present invention, graphene thin films (also referred to herein as “graphene films”) are used as NTC temperature sensing elements. In some embodiments of the invention, inks prepared by dispersing graphene oxide sheets in water are used to print micropatterns of graphene oxide on a variety of flexible substrates (e.g., polyethylene terephthalate (PET), or Kapton® polyimide membrane) using a commercial inkjet printer. In some such embodiments of the invention, the printed micropatterns are subsequently reduced to graphene in minutes using an infrared heat lamp at a temperature of about 220° C. The spacing between adjacent ink droplets and the number of printing layers deposited may be adjusted to tailor the graphene film's electrical resistance. The graphene film behaves as a NTC sensing element, wherein electrical resistance decreases rapidly as temperature increases, providing a faster response time than any other NTC material reported in the literature to date. The electrical properties of the graphene film are also stable under mechanical bending of the graphene film. Embodiments of the present invention may be prepared to provide selected mechanical properties useful for mechanical sensors.

#For a more complete understanding of the present invention, reference is made to the following detailed description of exemplary embodiments considered in conjunction with the accompanying drawings, in which:

#FIG. 1 is a schematic illustration of a process of forming flexible graphene micropatterns, according to a method of the present invention;

#FIG. 2 is a plot of the changes in resistance of the micropattern as graphene oxide in the micropattern is converted to graphene using a heat lamp, according to an embodiment of the present invention;

#Graphene has received significant attention in a variety of technical fields because of its potential use in highly-flexible electrically-conductive films for applications such as optoelectronics, energy storage, environmental sensors, and biomedical devices. Inkjet printing of aqueous dispersions of graphene oxide sheets and their subsequent reduction to graphene for use as supercapacitor electrodes is described in U.S. Patent Application Publication No. 2012/0170171, published on Jul. 5, 2012, the disclosure of which is incorporated by reference herein in its entirety. A similar approach is used in embodiments of the present invention to fabricate graphene thin films which exhibit NTC behavior with high sensitivity to temperature changes and fast response time, enabling their use as writable, very thin, flexible, and transparent sensing elements.

#FIG. 1 is a schematic illustration of a process of forming flexible graphene micropatterns according to an embodiment of the present invention. The micropattern 10 is produced by depositing droplets 12 of an ink comprising graphene oxide dispersed in water from an inkjet printer head 14 onto current collectors (not shown) formed on a flexible substrate 16. Dot-like structures 18 are formed from the droplets 12 upon impact of the droplets 12 on the substrate 16. A commercial inkjet printer (e.g., a FUJIFILM Dimatix Materials Printer, DMP2800) may be used. In embodiments of the inventions, inks having a concentration of about 0.2% graphene oxide by weight may be used. The average lateral dimension of the graphene oxide sheets in an exemplary ink may be about 530 nm with about 35% of the graphene oxide sheets being smaller than 300 nm and about 30% of the graphene oxide sheets being larger than about 1000 nm. Sheets of graphene or graphene oxide are also referred to herein as “flakes”. FIG. 1 also indicates the printing parameters of droplet spacing (D) and the number of layers printed (N) which affect the electrical properties of the graphene film.

#In an exemplary embodiment of the invention, exposure to radiation from an infrared (IR) lamp reduced the graphene oxide in the printed micropatterns to graphene in about 10 minutes in an ambient room environment. As shown in FIG. 2, the substrate temperature rose to about 220° C. during a 12 minute exposure. The electrical resistance of the graphene film became measurable at about 5 minutes into the exposure period, and continuously decreased until it reached a steady state value after an exposure of about 10 minutes.