Abstract:
An environmental sensor comprises a graphene thin-film as an environmentally responsive material. Such graphene films exhibit negative temperature coefficients (NTC), resulting in rapid decreases in electrical resistance as temperature increases, as well as a much faster response time than any other NTC material reported in the literature. The graphene film is also mechanically stable under bending, and, therefore, can be adapted for use in a mechanical sensor or pressure sensor, because the electrical resistance of the graphene film changes upon deflection and/or changes in pressure. The electrical resistance of the graphene film also increases in response to increases in environmental humidity. The electrical resistance changes of the graphene film can also be used as a sensing mechanism for changes in chemical and biological parameters in the environment of the sensor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims the benefit of U.S. Provisional Patent Application No. 61/713,872, filed on Oct. 15, 2012, the disclosure of which is incorporated by reference herein in its entirety. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
       [0002]    The technology disclosed herein was in part derived from research supported by the U.S. Government as part of Task Order #76 of Contract W15QKN-05-D-0011-0076 “Integrated Flexible Electronics and Energetics”. The U.S. Government may have certain rights to the invention disclosed herein. 
     
    
     FIELD OF THE INVENTION 
       [0003]    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. 
       BACKGROUND OF THE INVENTION 
       [0004]    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. 
       SUMMARY OF THE INVENTION 
       [0005]    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&#39;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. 
     
    
     
       BRIEF DESCRIPTION OF FIGURES 
         [0006]    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: 
           [0007]      FIG. 1  is a schematic illustration of a process of forming flexible graphene micropatterns, according to a method of the present invention; 
           [0008]      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; 
           [0009]      FIG. 3  presents plots of the Fourier transform infrared (FTIR) characteristics of graphene oxide sheets before (upper plot) and after (lower plot) reduction to graphene using an infrared heat lamp, according to an embodiment of the present invention; 
           [0010]      FIG. 4  presents Raman spectra of graphene oxide sheets before (upper spectrum) and after (lower spectrum) reduction to graphene using an infrared heat lamp, according to an embodiment of the present invention; 
           [0011]      FIG. 5  is a plot of showing the temperature dependence of the electrical resistance of a graphene film prepared according to an embodiment of the present invention; 
           [0012]      FIG. 6  is a plot showing a linear fit between the log of the electrical resistance of a graphene film as a function of the inverse of the temperature of the graphene film, the graphene film being an embodiment of the present invention; 
           [0013]      FIG. 7  is a plot showing the changes in relative electrical resistance of a graphene film prepared according to an embodiment of the present invention in response to repeated contact with a human fingertip; and 
           [0014]      FIG. 8  is a plot showing the change of electrical resistance with respect to mechanical bending of a graphene film prepared according to an embodiment of the present invention; and 
           [0015]      FIG. 9  is a plot showing the change of electrical resistance with respect to relative humidity in the environment of a graphene film prepared according to an embodiment of the present invention; and 
           [0016]      FIG. 10  is a schematic drawing of a design for a graphene film sensor according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    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. 
         [0018]      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. 
         [0019]    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. 
         [0020]    The aforesaid changes in electrical properties of the printed film suggest that there is a significant removal of O—H functional groups from the exposed graphene oxide sheets during thermal reduction.  FIG. 3  presents plots of the Fourier transform infrared (FTIR) characteristics of graphene oxide sheets before (upper plot) and after (lower plot) reduction to graphene using an infrared heat lamp, according to an embodiment of the present invention. Prior to thermal reduction, infrared absorption peaks characteristic of graphene oxide were present in the FTIR spectrum of the material including: (1) C═O stretching vibration at 1735 cm −1 , (2) O—H stretching vibration at 3428 cm −1 , (3) O—H deformation vibration at 1411 cm −1 , (4) aromatic C═C stretching vibration at 1610 cm −1 , and (5) alkoxy C—O stretching vibration at 1041 cm −1 . After thermal reduction, the 1411 cm −1  and 1041 cm −1  peaks disappeared, while the 3428 cm −1  peak significantly decreased. The small 1735 cm −1  peak still remained. However, the 1735 cm −1  peak did not disappear, suggesting that the C═O stretching vibration of lactones was still present. The 1610 cm −1  C═C peak was present, indicating that the sp 2  structure of carbon atoms was retained. 
         [0021]      FIG. 4  presents Raman spectra of graphene oxide sheets before (upper spectrum) and after (lower spectrum) reduction to graphene using an infrared heat lamp, according to an embodiment of the present invention. Two prominent Raman peaks were observed before and after the thermal reduction step: (1) a G band (at about 1602 to 1607 cm −1 ) corresponding to the first-order scattering of photons by sp 2  carbon atoms, and (2) a D band (at about 1354 cm −1 ) arising from small domain-sized graphitic regions. The intensity ratio of the D to G bands (I D /I G ) increased from 0.79 to 0.94 upon reduction. This change of ratio suggested that: (1) most of the oxygenated functional groups were removed from graphene oxide sheets by the reduction step, and (2) a sp 2  network was established. Upon reduction, the G band was slightly shifted to 1602 cm −1  from 1607 cm −1 . However, the G and D bands of the reduced graphene oxide sheets present at 1602 cm −1  and 1354 cm −1  were considerably higher than those typically observed at 1575 cm −1  and 1350 cm −1  for chemically vapor deposited (CVD) graphene. These peak shifts indicate that a relative lack of sp 2  character and a remaining presence of some oxygenated functional groups, consistent with the FTIR results. 
         [0022]    The FTIR and Raman results discussed with respect to  FIGS. 3 and 4  suggest that thermal reduction using an infrared lamp is effective in reducing printed graphene oxide films to graphene films to a significant extent, but not completely. Reduction of graphene oxide by this method is particularly useful for films printed on thermally- and chemically-sensitive materials and devices. Also, the use of an infrared lamp is advantageous for easy integration with roll-to-roll, additive manufacturing since it only takes minutes to effect a suitable reduction, as opposed to the hours required for the thermal and chemical methods, without the need for controlled reduction environments and sophisticated equipment. 
         [0023]    By varying printing parameters such as droplet spacing (D) and the number of layers printed (N), the electrical resistance and optical transparency of the graphene film can be tailored for specific applications. Variations in D and N are highly related to the morphology of the electrode patterns, and affect the electrical conductivity of the micropatterns by altering the number of available electron transport paths, as well as the availability of light-transmitting paths. In an exemplary embodiment of the invention, at D=20 μm, sheet resistance (R s ) decreased from about 12 MΩ/□ to about 0.3MΩ/□ upon increasing N from 2 to 5, and transparency of the film rapidly decreased from about 76% to about 45%. Optimization and control of the electrical resistance and optical properties can be used to achieve a desired purpose, whether for a temperature-sensing application or for an electrochemical sensing application, or even for other sensing applications. 
         [0024]    In a demonstration of the performance of the printed graphene temperature-dependent film of an embodiment of the present invention, D=20 μm and N=2 were determined to be optimum printing parameters for producing a continuous morphology of a graphene printed on Kapton, with the film having R s =12 MΩ/□ and a 76% transparency. This optoelectrical performance compares well with a printed graphene electrode that was produced by ultra-sonic exfoliation of a graphite powder and dispersal in organic solvent (see, e.g., Torrisi et al., Inkjet-Printed Graphene Electronics,  ACS Nano,  2012, 6 (4), pp 2992-3006, which is incorporated by reference herein). 
         [0025]    The temperature-dependence performance of a printed graphene temperature sensor according to an embodiment of the present invention is shown in  FIG. 5 . The resistance of the graphene film decreases significantly with increased temperature. Referring to  FIG. 6 , the following equation was used to model the observed temperature-dependence as a negative temperature coefficient (NTC) behavior: 
         [0000]    
       
         
           
             
               R 
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         [0000]    where R T  is the electrical resistance as a function of temperature (T), B is the material constant and a measure of temperature sensitivity, and R 0  is the resistance at the reference temperature (T 0 =298 K). By data fitting, B was determined to have a value of 1860 K in the temperature range of 298 to 358 K with the respective resistance changes from 4.4×10 6  to 2.4×10 6 Ω. This B value is close to that of the conventional metal oxide ceramic NTC materials. The temperature coefficient of resistance (α) was also used as another measure of temperature sensitivity where 
         [0000]    
       
         
           
             α 
             = 
             
               
                 R 
                 
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         [0000]    The value of α for exemplary graphene films of the present invention was determined to be −0.0148 K −1  at 298 K, which is comparable with α-values reported for other graphene films, such as chemically-reduced graphene oxide sheets and metal-diffused graphene oxide sheets. Also, the α-value of exemplary graphene films of the present invention is about three orders of magnitude higher than that of carbon nanotubes. 
         [0026]    The temperature-sensing function of the graphene film of the present invention was evaluated by lightly tapping the film with a human finger (at the temperature of the human body) in the ambient room environment. The repeated taps resulted in the resistance decreases shown in the  FIG. 7 . In contrast, no change in resistance was observed when the film was tapped with other objects that were in thermal equilibrium with the room environment (not shown). 
         [0027]    The response time of the graphene film to temperature changes resulting from touching the film with a finger tip was about 0.5 seconds, and the recovery time to its initial resistance value upon removing the finger tip was about 10 seconds. In comparison, a typical response time for warming a conventional NTC metal oxide material is more than 10 seconds, suggesting an order-of-magnitude faster temperature-sensing function of the graphene film. 
         [0028]    The mechanical stability of the graphene film of the present invention upon bending is shown in  FIG. 8 , where an overall decrease of R of 5.6% was not achieved until the film was bent to 2θ=27.4°. These observations indicated that the effect of slight substrate flexing during tapping on the resistance of the graphene film was much smaller than that of warming the graphene with the finger tip. Graphene film was constructed on the flexible substrate, therefore any change in the degree of bending of the substrate would lead to changes in the electrical resistance and one can use the correlation to develop the mechanical sensor. 
         [0029]    Turning to  FIG. 9 , the humidity sensing function of a graphene film made according to the present invention was evaluated by constructing a sensor as later described with respect to  FIG. 10 , and measuring the changes in the electrical resistance of the sensor during exposure to different relative humidities in the environment of the sensor. The sensor was first dried overnight in an oven at 70° C. to remove excess water inside the sensor before the measurements of the electrical resistances of two different sensors (i.e., sensors A and B) were recorded at three relative humidity levels (i.e., 1%, 60% and 100% relative humidity) in the environment if the sensor. The sensitivity of the sensor to changes in humidity was calculated based on the changes of electrical resistance after normalizing to a relative humidity level at 1% (R 0 ). It was found that the resistances of both sensors increased with respect to higher relative humidities. Sensor A showed increases in resistance of 36% and 107% at relative humidities of 60% and 100%, respectively. Sensor B showed increases in resistance of 15% and 39% at relative humidities of 60% and 100%, respectively. It was also observed that the resistance sensor took no more than two minutes to stabilize when the sensor was exposed to the humid environment. 
         [0030]    As demonstrated by the foregoing exemplary embodiments of the invention, inkjet-printed graphene has properties useful in fabricating temperature sensors having fast response times.  FIG. 10  is a schematic drawing of a design for a graphene film temperature sensor  20  according to an embodiment of the present invention. Referring to  FIG. 10 , a graphene film  22  is inkjet-printed on the surface of an electrically insulating substrate  24  (e.g., Kapton® polyimide membrane) over a cross-finger, inter-digitized configured current collector  26 . The current collector  26  may be formed from an electrically-conductive material (e.g., silver) and may be applied to the substrate  26  as a suspension of nanoparticles. Polymeric materials, including those suitable for use as the substrate  26 , are often hydrophobic, and may resist adhesion of a water-based graphene oxide ink, such as those which may be used in embodiments of the present invention. In such cases, the substrate material may be pretreated to improve the adhesion and wetting behavior of the ink droplets. For example, a Kapton® polyimide substrate may be pretreated with O 2  plasma for 30 seconds prior to printing (e.g., using a plasma cleaner) to improve adhesion and wetting behavior of the ink. A suitable seal may be formed from a polymeric membrane (e.g., Kapton® FN polyimide film) that is heat-sealed to the electrically-insulating material over the graphene film  22 . Adhesive bonding (e.g., by soft-lithographic techniques) may also be used to form a suitable seal. Routine experimentation based on ordinary knowledge of the materials&#39; properties will be sufficient for most cases, but this area is also open to invention of novel materials and novel combinations of known materials. 
         [0031]    It should be understood that the embodiments of the invention described herein are merely exemplary and that a person skilled in the art may make many variations and modifications without departing from the spirit and scope of the invention. All such variations and modifications are intended to be included within the scope of the invention as defined in the appended claims.