Contact‐Based and Proximally Thermosensitive Few‐Layer Graphene Ntc Thermistors with Highly Fast Switching Behavior

Patterning graphene onto polymeric materials offers benefits in realizing flexible, stretchable, and wearable multifunctional electrodes. However, the employed integration approaches and use of non‐patternable polymers hinder the patterning of graphene at the sub‐millimeter (mm) scale. Serpentine‐shaped thermally active graphene patterns (thermistors) of 500 × 500 µm2 area are reported by the seamless integration of chemical vapor‐deposited graphene (GCVD) into readily available SU‐8 polymer with MEMS‐compatible cleanroom fabrication processes. The thermistor resistance decreases with an increase in graphene temperature changed by local heat conduction or environmental thermal radiations; hence, exhibits a negative temperature coefficient (NTC) of resistance of 0.0012/°C. Furthermore, very fast resistive switching with 1 s response and 3.2 s recovery time is observed under cyclic heating and cooling. Several application scenarios including, monitoring of surface temperature (e.g., kettle and human body), rapid response (0.25 s) to heat conduction and radiations (0.5 s) from human finger at room temperature for contact and touch‐free proximity switching (e.g., turn ON and OFF an LCD display) are demonstrated. Moreover, owing to its small area less than a ceramic resistor enabled to integrate the fabricated thermistor onto a printed circuit board (PCB) to construct a fully packaged thermometer to monitor ambient temperature.


Introduction
Thermometry is an essential task required to be performed in every field of life to take control and maintain the functionality of operational systems such as industrial machinery, [1,2] home/working environment, [3][4][5] home appliances, [6] food preservation, [7] balanced human life, [8][9][10] biomedical samples, [11] etc. To measure temperature, different sensors including thermocouples, [12,13] resistance temperature detectors (RTDs), [14,15] semiconductor-based temperature sensors, [16] and thermistors have been developed. [17]Thermistors perform thermometry in a fast, efficient, and highly sensitive manner by a rapid change in their resistance upon a slight change in their surface temperature [1,17,18] in comparison to their counterparts.Moreover, several advantages including low power consumption, low cost, and miniature form factor of thermistors, along with their direct electrical readout have facilitated their widespread commercialization.Especially, thermistors with negative temperature coefficient (NTC) characteristics find broad use as temperature sensors due to an increase in their conductivity with an increase in the temperature and vice versa.Likewise, they are also widely utilized as temperature-based current limiters in electronic circuits. [19,20]ommercially available NTC thermistors are transition metal oxide (Mn-Ni-O) based spinels, [21] which can be operated in a diverse temperature range from −50 to 1000 °C with very high thermal index () between 2000-5000 K −1 .However, the brittle and rigid nature of such thermistors limits their application areas especially when they are required to be mounted or conform to a surface with an arbitrary morphology.Therefore, the integration of mechanically stable and thermally active electronic materials onto/into the flexible polymeric carrier materials is necessary to overcome the above-mentioned issues.
Graphene, a 2D material composed of carbon atoms arranged in a hexagonal lattice shows high electron mobility, [22] high thermal conductivity, [23] optical transparency, [24][25][26] high electrical sensitivity to external stimulus (e.g., temperature, humidity, UV light, etc.), [27][28][29][30][31][32] and good mechanical strength. [33,34][50] In addition, graphene shows good thermal conductivity of around 5300 Wm −1 K, −1 [51,52] which is attributed to its electron mobility and primarily due to electron-phonon interactions. [53,54]Another interesting behavior of graphene and its derivatives (graphene oxide─GO and reduced graphene ox-ide─rGO) reported in many studies shows an inverse relation between the conductivity of graphene and temperature stimuli, which confirms the NTC material characteristics of graphene within the range of 298 to 358 K along with high thermal index () 1860 K, respectively. [55]In addition, single and few-layer graphene (FLG) also display a more stable conductivity in humid environments in comparison to its derivatives such as graphene oxides (GO), and reduced graphene oxides (rGO) because of the presence of oxygen groups in their constituent structure. [56]Based on such physical characteristics, graphene is being adopted in the field of thermistors with an operation range of −20 to 250 °C. [57]][60] Therefore, non-conventional patterning techniques such as screen printing, [61] inkjet printing, [55] laser microstructuring, [46,62] and femtosecond laser writing [63] have been used to construct graphene-based thermistors.However, these techniques are not suitable for lending a miniaturized thermistor with dimensions of a few hundred microns that can be integrated into polymeric materials and still display consistent temperature-sensing performance in different environments.In addition, the bulky nature of such thermistors is a fundamental obstacle for bringing them to the micron scale for integration into printed circuit boards (PCBs), or flexible PCBs.
Here, we report a serpentine-shaped, surface mountable, and PCB-integrable graphene thermistor having a thermally active area of less than 0.5 mm 2 and demonstrate its performance in measuring surface (heat conduction) and environmental (thermal radiation) temperature.We utilized transfer-free integration of chemical vapor deposited graphene (G CVD ) into readily available SU-8 polymer by exploiting the nickel (Ni) catalyzer metal at the same time as a sacrificial layer.The prepared thermistor displayed a sheet resistance of 736 Ω/square and optical transparency of 67%.Furthermore, a clear decrease in the resistance of the thermistor was observed by heat conduction within a temperature range of −20 to 100 °C and by thermal radiations from room temperature (RT) to 100 °C; which confirms a negative thermal coefficient (NTC) thermistor with the temperature coefficient of resistance (TCR) value of 0.0012/ °C and maximum temperature sensing resolution of 8 Ω 0.1 °C−1 .Several application scenarios of the fabricated G CVD polymer thermistor have been explored including its use as a thermal switch to control a liquid crystal display (LCD), thermometer to read the surface temperature of continuously heating surfaces such as an electric kettle, and human body with very quick response through a short direct touch or even due to proximal heat transfer through thermal radiation.Furthermore, the highly miniaturized size and free-standing nature of the thermistor facilitates its integration into a printed circuit board (PCB) to construct a room "thermometer" that responds by turning ON and OFF LEDs according to room temperature.

Results and Discussion
A schematic of the thermistor obtained by the integration of serpentine-patterned few-layer chemical vapor-deposited graphene (G CVD ) into a free-standing layer of SU-8 polymer is shown in Figure 1a.Conventional clean room technologies such as e-beam deposition, UV-lithography, and wet/dry etching were utilized to fabricate a temperature-sensitive serpentine-shaped graphene thermistor within an area of 0.5 mm 2 .A thermally active graphene pattern could heat up with the increase in its temperature by heat conduction or thermal radiations and result in a decrease in its resistance and vice versa (Figure 1b).In addition, the highly miniaturized size and free-standing nature of the prepared thermistor have the advantage of being integrated into a PCB as a temperature measurement device as shown in Figure 1b.Since our aim in this work is to show the temperature sensitivity of the patterned few-layered G CVD (shown by camera and optical microscope images of the real sample in Figure 1c,d), we confirm its presence atop the SU-8 film and complete removal of nickel (Ni) sacrificial layer by utilizing Raman spectroscopy shown in Figure 1e.Raman intensity peaks located at 1578 cm −1 are termed as I G , and at 2708 cm −1 as I 2D ; where the intensity ratio (I 2D /I G ) ≈0.6 indicates the multilayer structure of graphene as shown by the lower plot in Figure 1e.This is consistent with the earlier literature which reports that an increase in the number of graphene layers results in an I 2D /I G ratio below unity and a broadened 2D band of graphene, likely due to an increase in the interlayer coupling. [64]Additionally, a slight blue shift in the Raman peaks/intensity (toward lower wavenumbers) was observed before (bottom plot) and after the integration (upper plot) of G CVD onto the SU-8 substrate as shown in Figure 1e, except the emergence of peaks at 1605 and 2918 cm −1 corresponding to the SU-8 substrate.Furthermore, SU-8 integrated graphene showed a sheet resistance of 736 Ω /square and optical transparency of 67% potentially making it suitable for transparent thermal monitoring of photovoltaics. Figure 1f shows the UV-vis spectroscopy analysis of SU-8 and graphene/SU-8 films, where in the inset image the readability of letters located below the SU-8/graphene composite confirms the good optical transparency (≈67%) of the device.Furthermore, capitalizing upon the capabilities of atomic force microscopy enabled us to see a clear view of CVD-grown graphene flakes on the Ni layer (Figure 1g) and thereafter its integration to SU-8 (Figure 1h).Analysis of atomic force microscopy (AFM) images confirms that the change in the root mean square (R q ) value of the surface roughness of graphene flakes before (18.47 nm) and after (19.85 nm) graphene integration was ≈7%, which confirms that the graphene flakes remained stable during the whole fabrication process and were well transferred from nickel (Ni) to SU-8.On the other hand, the AFM image of SU-8 outside the active G CVD thermistor region (Figure 1i) shows a completely different morphology which indicates that graphene and Ni were successfully removed from this area during the etching processes.Moreover, the random surface morphology of the   decreasing behavior of the thermistor and hence, indicating a negative thermal coefficient (NTC) thermistor.Moreover, a nonlinearity in the resistance with an increase in thermistor temperature can be explained by the following expression for semiconductor materials, [65][66][67][68] where, R is the resistance at temperature T and R 0 , E a , k, , are initial resistance, thermal activation energy, Boltzmann constant, and thermal index respectively.The expression in Equation ( 1) can be modified as follows, where a linear trend between 1/T and ln (R) can be obtained as shown by fitted data in Figure 2c.This linear behavior has importance in real-world applications to simplify the postmeasurement process. [58]The thermal index  is ≈100 K of the device was calculated from Equation ( 2), also we found that the activation energy E a = 2k = 0.081 eV.Also, the temperature coefficient of resistance () which reflects the sensitivity of the device toward temperature may explained by the following well-known expression, [65] and sensitivity is defined as, Here from Equation (4), we found the temperature sensitivity from 25 to 100 °C of the integrated graphene thermistor as ≈0.12%/ °C (TCR ≈0.0012/ °C) which is comparable to previously reported TCR values of G CVD. [69,70] Additionally, we confirm the hysteresis behavior in the thermistor's resistance by continuous heating from 23 to 100 °C and cooling down to 23 °C, where the cooling down followed the opposite path to heating with a 0.02% increase in the initial resistance (Supplementary Figure S2, Supporting Information).Moreover, to validate the TCR value of the serpentine-shaped G CVD graphene, we also fabricated ≈200 nm thick gold (Au) film patterns with the same shape and dimensions, where we found that the TCR value of the patterned Au as ≈0.0024/ °C which is similar to the TCR value of Au (see Figure S3, Supporting Information) reported in the literature. [71]A clear trend in resistance decrease (≈740 Ω) is also observed when the temperature of the G CVD thermistor device was gradually increased from 23 to 40 °C with finer steps of 1 °C and 0.1 °C as shown in Figure 2d.Furthermore, based on the available commercial thermometers with the highest resolution, we confirm that one can easily detect meaningful changes in the resistance of the graphene thermistor for temperature differences as low as 0.1 °C, corresponding to a maximum resolution of ≈8 Ω with 0.1 °C change (i.e., 80 Ω/ °C) when the temperature of the thermistor was gradually increased from 28 to 29 °C (inset Figure 2d).Here, unlike existing works, the utilization of conventional microfabrication processes such as UV lithography and plasma etching offers the advantage of repeatable, scalable batch-fabrication of highly sensitive, CVD-graphene integrated thermistors with precise serpentine geometries, which at the same time display comparable performance to others in the literature (see Table S1, Supporting Information).In addition, the low thermal mass (fewer number of layers) of graphene, [72] large serpentine active area, and low thermal conductivity of the carrier substrate are other factors in designing a highly temperaturesensitive graphene thermistor.
Changes in the electrical resistance of graphene (semimetallic material) due to temperature fluctuations is attributed to the combined effect of semiconducting and metallic behavior in graphene, where the former is responsible for thermallygenerated charge carriers to decrease the resistance at elevated temperatures, while the latter dictates charge carrier scattering causing increase in resistance with temperature. [55]Such kind of unusual behavior could originate from the crystallinity, layer number, carrier density, or scatter wave interference. [73]Normally, graphene with high carrier density exhibits metallic behavior while low carrier density graphene with the Fermi level near the Dirac (charge neutrality) point shows semiconducting behavior. [74]Specifically, the presence of oxygen and hydroxyl groups in the chemically derived graphene due to partial reduction of graphene oxide is responsible for leading it to behave as a finite-gap semiconductor. [75]Therefore, the NTC behavior of our CVD-grown graphene thermistor is consistent with previous studies which confirm its semiconducting behavior.
For thermistors, thermal switching is an important characteristic parameter that describes the response and recovery of the device when it is first subjected to a temperature rise due to heating, and then cooling down to steady-state conditions on its own.Figure 3a shows the thermal switching behavior of the integrated thermistor at different temperatures.The thermistor displays a remarkable response to temperature and changes its resistance with a response time as little as 1 s (turn ON) from RT to 100 °C, and has a 3.3 s (turn OFF) recovery time to reach back to normal resistance value at RT.We also show the resistive switching of the thermistor from 30 to 100 °C with smaller increments of 10 °C shown in Figure S4 (Supporting Information), which confirms fast response and recovery in all temperatures from 30 to 100 °C.For comparison, response times of typical metal oxidebased thermistors are over 10 s, which is tenfold longer than the CVD graphene-based thermistor in this work.Since thermal switching behavior is because of the electron transfer between the graphene sheets during heating and cooling cycles, we attribute the highly fast switching response of the thermistor to the very small thermal mass [53] of the few-layer patterned graphene in the active thermistor area.
Moreover, we evaluated the stability of the device at different temperatures (30, 60, and 100 °C) for 10 cycles.Figure 3b shows the cyclic behavior in the resistance of the thermistor when it is placed on a surface with 60 °C temperature and brought back to RT, the percent change in ΔR/R remains almost constant around 3.65% in each cycle.Furthermore, the stability of the thermosensitive device at 30 and 100 °C is shown in Figure S5a (Supporting Information).In addition, we demonstrate the long-term stable operation of the fabricated G CVD thermistor at different temperatures.Figure S5b (Supporting Information) shows a consistent behavior in the ΔR/R of the thermistor for 7 consecutive days even when it was subjected to three different temperatures of 30, 60, and 100 °C for 10 cycles each day.Such thermosensitive devices can be utilized in point-of-care instruments for continuous monitoring of the surface temperature of various objects where we confirm that the resistance of the thermistor remains stable only with minor fluctuations when it was mounted on hot (30, 60, and 90 °C) surfaces for ≈1 h (Figure S6, Supporting Information).We attribute the small fluctuations in thermistor resistance to spontaneous changes in the temperature of the heating surface (i.e., hot plate) occurring due to small variations in ambient conditions which overall prevent a stable equilibrium temperature to be reached at the thermistor's surface.This observation also indicates the very high-temperature sensitivity and fast thermal switching response of the G CVD thermistor as it can even capture the minute thermal fluctuations when mounted on a surface with unstable, time-varying temperatures.Figure 3c shows the response of the G CVD thermometer when mounted onto the surface of a water-boiling kettle (inset Figure 3c).A non-linear change in the resistance (≈6.86%) of the thermometer was read out when the temperature of the kettle surface was increased from RT to 90 °C during the water boiling process.Figure 3d-f shows corresponding thermal camera images of the kettle and mounted thermistor surface during the water boiling process at different temperatures.Therefore, these results suggest that such thermistors can be adopted by replacing mercury-based thermometers or thermal cameras in many processes such as in laboratories where certain chemical reactions require continuous monitoring of the temperature.
Figure 4a shows a stable temperature sensing behavior of the G CVD thermistor at room temperature, where a very quick tap on the sensor surface with a human finger creates a change in the resistance (between 0.2 to 0.3%).This phenomenon is due to the rapid conductive heat transfer from the human finger to the heat-sensitive area of the thermistor formed by G CVD .We also found that when the same tapping was done with other objects at room temperature such as plastic, wood, and glass no resistance change was observed in the thermistor which indicates that tapping with a human finger fundamentally causes thermal conduction as opposed to altering the light-induced generation/recombination mechanism and/or accumulation of electrostatic charges to change the thermistor resistance (see Video S1, Supporting Information).In addition, Figure 4b shows that the heat conduction from a human finger to a thermistor by tapping took 0.25 s (response time) and recovery time of 1.7 s, which are significantly faster than metal-oxide-based NTC thermistors and other reported graphene-based thermistors. [55,76]Again this fast response time is due to the low thermal mass of the fewlayer CVD graphene.Furthermore, when the sensor was gently pressed with the human finger for a longer time (10 s) (shown in Figure 4c) to fully conduct heat from the human finger to the sensor surface, the response time of the sensors increased to 2.4 s (≈0.4% change in resistance) and recovery time reached to 4.1 s shown Figure 4d.Here, prolonged response and recovery could be due to stress and moisture effects from the human finger.
In addition, we demonstrate that heat transfer from the human body to a graphene thermistor is also advantageous for designing thermometers to read human body temperature, as shown in Figure 4e.To show the functionality of the graphene thermistor potentially for wearable temperature monitoring applications, we mount the thermistor onto the human finger, wrist, and forehead to measure the temperature of the human body and also evaluate the thermometer response time.Results of experiments show a reasonable change in the resistances ≈0.53%, ≈1.25%, and ≈1.57% on the finger, wrist, and forehead, respectively.The difference in these resistance values could be because of the slight difference in the temperature of the different human body parts.However, a minimum response time of 1.02 and 5.2 s recovery time was observed on the forehead which is much faster than conventional mercury-based thermometers and comparable to commercially available infrared (IR) thermometers with response times in the range of 0.5 to 1 s.
Proximity (non-touch) sensors and electronic devices are also of great interest in the broad field of electronics and are necessary during pandemics such as COVID-19.For instance, graphenebased proximity sensors were presented previously whose operation relied on modifying the carrier type/density at the interface of a p-type rGO layer and a dielectric PDMS layer, due to charged objects brought above the PDMS surface. [77]This type of sensor operation requires the "proximal object" to have some surface electrostatic charge (including human skin due to friction with air) and in essence, both the object and the dielectric layer should display triboelectrification behavior; therefore, neutral objects like metals fail to be detected with such a proximity sensor.An alternative sensing mechanism that we demonstrate in the present work is based on populating free carriers in graphene by coupling energy from nearby thermal radiation sources which in effect causes resistance to change in the graphene layer.Herein, we exploit this behavior to construct a thermosensitive proximity sensor and thermally driven non-contact switch.
To demonstrate the potential of G CVD thermistors in proximity sensing and non-contact operation, several experiments were performed.Figure 5a shows a 5.6% decrease in the resistance of the graphene thermistor due to convective and radiative heat transfer when a hot rod (from 30 to 100 °C) was placed 5 mm above the thermistor surface.The easily discernible change in the resistance of more than 5% confirms that CVD-grown graphene has good sensitivity to thermal radiation as well as conduction.Moreover, switching in resistance was also confirmed by thermal radiation from a heating rod at 35 °C at steady-state conditions, where the thermistor shows a response in 2.1 s by ≈0.25% decrease in its resistance and recovery time of 6.2 s (Figure 5b).Here, the air gap between the thermistor surface and the heating rod is yet another parameter to determine thermal equilibrium at the thermistor surface and directly affects the response time.We also confirm multiple stable switching behaviors of the thermistor when a heated (at 35 °C) rod was brought close to its surface and pulled back continuously in short time cycles (≈10 s) as shown in Figure 5c.In this case, owing to the gradually declining gap between the "moving" hot rod and the thermistor surface, the response and recovery time of the thermistor reduced to 0.54 and 2.5 s, respectively (Figure 5d).
Aside from the change in the G CVD thermistor resistance by heat conduction from a human finger, similar to the hot rod case described above, we observe that thermal radiation from a human finger close to the thermistor surface can prominently decrease the resistance of the thermistor.Figure 5e shows that the thermistor resistance continuously decreases when a human approaches closer to the thermistor surface, due to increased heat transfer as the distance between the finger and the thermistor narrows.We observe that G CVD thermistor could sense the human finger's thermal radiation from a distance of up to 1.5 cm by a slight change (≈0.018%) in its resistance, which becomes significantly noticeable at closer gaps of 5 and 2.5 mm with sharp changes in thermistor resistance of ≈0.084% and ≈0.16%, respectively.These results show the very high sensitivity of the G CVD thermistor to human body temperature, where it can detect even minute proximal human motions at room temperature easily.Figure 5f shows the response (0.77 s) and recovery (3.42 s) times of the sensor facing a stationary human finger (having 34.2 °C temperature) in a transverse direction at a distance of 5 mm.We attribute the small difference in the ΔR/R values in Figure 5b,f to the slight difference between the rod and the human finger temperature.Moreover, Figure S7 (Supporting Information) shows that the ΔR/R value of the thermistor effectively varies from person to person having different finger temperatures.Figure 5g shows an immediate decrease in the resistance of the thermistor when a human finger was brought 5 mm above its surface and moved back repetitively, with a response of 0.55 s and recovery time of 2.7 s (Figure 5h).In addition, we confirm that a human finger passing by the sensor surface from a random direction at 5 mm above delivers enough heat radiation to decrease the resistance of the thermistor (Figure S8 and Video S2, Supporting Information).°C placed 5 mm above its surface.b) Resistive switching with thermal radiation when a rod radiating heat at 35 °C was brought 5 mm near the thermistor surface for a long time and pulled back.c) Continuous switching of thermistor resistance when a rod radiating heat at 35 °C brought closer to and moved back from the thermistor surface multiple times; and d) demonstration of thermistor response and recovery time of 0.54 and 2.5 s, respectively.Effect of thermal radiations from the human finger on thermistor resistance: e) when a finger was brought closer to the thermistor surface normally from a distance of 5 cm; f) response and recovery time when thermistor faces thermally radiating human finger for a longer time; g) continuous switching of thermistors when a human finger was frequently brought in its proximity at5 mm above; and h) response in recovery time of short switching.A decrease in the thermistor resistance by thermal radiations from a human finger in its proximity can lead to interesting applications such as non-contact, touch-free electronic switches.As a proof-of-concept an electronic circuitry composed of an LCD board connected with a G CVD thermistorbased "switch" was turned ON and OFF successively by utilizing the thermal radiation from a human finger.Figure 6a shows the operation of the thermistor as a non-contact switch where switching between the Sabanci University logo (OFF state) and all other pictures (ON state) indicates the turning ON and OFF the LCD board by bringing a human finger near the surface of the thermistor (for continuous LCD's ON and OFF operation please see Video S3, Supporting Information).
Moreover, a very small size and high sensitivity to thermal radiation enabled us to integrate the G CVD thermistor into a packageable PCB to construct a highly efficient ready-to-use room temperature thermometer.The fabricated miniature G CVD thermistor was integrated into a printed circuit board along with a microcontroller, resistors, and LEDs (Figure 6b); where our thermistor covers a space almost 2 times less than a metal foil resistor (excluding the electrical wiring between thermistor and PCB for which silver paste and lead wire solder were used), and packaged inside a 3D-printed enclosure (Figure 6c insets).Figure 6c shows the almost linear response of the thermistor by decreasing its resistance (≈2.6%) when the ambient-temperature was increased from 22 to 35 °C. Figure 6d shows the operation of the thermometer, where a 1 °C increase in the room temperature successively turns ON an LED in the array.When the temperature goes above 32 °C all LEDs start blinking as shown in Video S4 (Supporting Information).

Conclusion
In conclusion, we report transfer-free integration of CVD-grown graphene in serpentine shape into SU-8 within an area of 500 × 500 μm 2 by using MEMS compatible cleanroom fabrication process and using nickel (Ni) catalyzer metal as a sacrificial layer.The obtained structure showed a decrease in its resistance when its surface temperature was increased by heat conduction (−20 to 100 °C) or thermal radiations (23 to 100 °C), hence, termed a negative thermal coefficient (NTC) thermistor which was used to measure the surface temperature of water boiling electrical kettle.The G CVD thermistor developed in this work showed a very quick response by dropping its resistance in 1 s when mounted on a 100 °C hot surface and 0.25 s when touched by a human finger.Likewise, the G CVD thermistor displays a very fast response time of ≈1 s to measure human body temperature from the forehead, making it useful for wearable thermal monitoring applications.In addition, thermal radiation from a human finger enables switching of the resistance of the thermistor, rendering it a non-contact switch with a response time of 0.55 s to turn ON and OFF an LCD board.Furthermore, a small effective area of the thermistor allows integration into a printed circuit board to construct a room-temperature thermometer.

Experimental Section
Graphene Temperature Sensor Fabrication: A few (6 to 7) layers of chemical vapor deposited (CVD) graphene grown on the nickel (Ni) film with silicon (Si) carrier substrate (Graphene Supermarket) were used without any further processing.Additionally, these samples also contain a SiO 2 layer between Si and Ni which was used as a sacrificial layer in our fabrication process.To fabricate a micron-sized graphene thermistor, first, a gold (Au) layer of ≈300 nm was deposited by e-beam evaporation onto the graphene surface to serve as a protective hard mask for graphene, and to realize the electrical contact pads.Then, a high-resolution photomask prepared by electron-beam lithography which contained serpentine-shaped thermistors each with an effective area of 500 × 500 μm 2 was utilized to pattern the top gold layer by conventional photolithography on a ≈1.4 μmthick layer of spin-coated photoresist.Subsequently, Au was etched by using a wet chemical etching process in potassium iodide (KI) and then graphene by oxygen (O 2 ) plasma-based dry etching.Afterward, the residual photoresist was washed away with acetone, and the protective Au layer on the "patterned graphene" was removed again by wet etching to form the G CVD thermistor structure with only the graphene layer remaining in the active sensing region.Next, to provide mechanical stability to the G CVD thermistor upon release from the substrate surface, a 100 μm-thick SU-8 "handle layer" was spin-coated on the patterned graphene surface followed by UV-lithography which properly structured the SU-8 film into the desired shape and size.Finally, the sample was released from the Si substrate by a sacrificial layer etch process, where the SiO 2 layer was removed first in buffered oxide etchant (BOE), followed by dissolving Ni in iron chloride (FeCl 3 ) solution.After the release step, the samples were cleaned with excessive water and dried under nitrogen flow.
Optical Characterizations: Raman spectroscopy analysis was performed by a Renishaw device, where a 532 nm wavelength of laser light was used to excite the sample.Moreover, for optical transparency, we used a Shimadzu UV-3150 double-beam spectrometer.A sample mounted in the spectrometer was illuminated by a broadband white light source and a transmitted amount of light was collected at the output end.
Electrical Measurements: For electrical measurements, Au pads on the graphene thermometer were connected with 50 μm diameter gold band wires by using silver paste under the microscope.Then, opposite ends of the Au wires were connected with copper (Cu) tape by utilizing silver ink to make efficient contact with measurement instruments.In order to, measure the resistance change with temperature, the sensor was mounted on a highly thermally conductive metal bar and Cu tape was connected to the outer cables of an LCR meter (GW INSTEK LCR-6002).Afterward, the fabricated different thermistors were mounted on a metal bar and placed on a hotplate.Stabilized resistance values were recorded as nominal resistance after making all the connections and completing the measurement setup.Next, the temperature on the hotplate was slowly increased to different levels and monitored with a Fluke Ti9 thermal camera (0.20 °C thermal sensitivity and a temperature measurement range of −20 to 250 °C), while in parallel the thermistor resistance was measured with the LCR meter at desired temperatures.All resistance measurements at specific temperatures were performed after waiting for 3 min to allow stabilization of the resistance value, and experiments were performed at a constant humidity level of the room.The resistance of the sensor was measured at various stages of thermal exposure, including prior to placement on the hotplate, during thermal engagement on the hotplate, and post-removal from the hotplate.
Room Temperature Sensor and LCD Preparation: A full room temperature sensor prepared on a printed circuit board (PCB) was first designed in the Altium Designer software and the microcontroller was programmed by an Arduino simulator.Next, an LCD screen was connected to the programmed microcontroller and the G CVD thermistor was connected as a non-contact switch.For powering these devices, a simple 5 V power supply was used.To electrically interface the thermistor to the PCB, Au bond wires (50 μm in diameter) were connected to the thermistor pads by using conductive silver paste, and the opposite ends of the same Au wires were connected to the PCB by soldering.

Figure 1 .
Figure 1.Schematic showing: a) SU-8 integrated CVD graphene thermistor, and b) its electrical resistance behavior with an increase in local and environmental temperature as well as its integration on a printed circuit board (PCB).c) Photograph (scale bar 1 mm); and d) optical microscope images (scale bar 100 μm) showing the integrated graphene patterns into SU-8.e) Raman spectroscopy analysis indicates few-layer graphene on Ni (bottom) before, and on SU-8 (top) after the whole integration process.f) UV-vis spectroscopy indicating the optical transparency of SU-8 film and CVD graphene onto SU-8.AFM images show the CVD graphene flakes on Ni before g); and, after integration into SU-8 h); as well as (i) only the SU-8 film surface after the fabrication process.The scale bar indicates 2 μm.
Figure 2a shows the curve fitted data (original data shown in supplementary Figure S1, Supporting Information) of non-linear decrease in resistance from 49.82 to 44.39 kΩ of the G CVD thermistor with an increase in temperature from −20 to 100 °C, where error bars indicate the standard deviation from the mean value of measured data collected over 5 trials.Moreover, from I-V data we also confirm an increase in the current value from 17.62 to 19.06 μA at constant open circuit voltage (V oc ) of 1 V when the temperature was increased from room temperature (RT) to 100 °C shown in Figure 2b, which also confirm the resistance

Figure 2 .
Figure 2. Temperature-dependent behavior of the: a) electrical resistance; and b) current at a fixed voltage (I-V measurements), where the inset also shows a non-linear change in resistance.c) Data-fitted curves show the linear relationship between 1000/T and ln (R).(error bars indicate the standard deviation from the mean value of 5 trials) d) Thermistor resistance changes with a small increase of 1 and 0.1 °C in temperature.

Figure 3 .
Figure 3. a)Thermal switching of thermistor showing its response and recovery time from 30, 60, and 100 °C to room temperature.b) Stability in the change in electrical resistance of the thermistor at 60 °C for 10 cycles.c) A thermistor mounted onto the kettle surface informs about the body temperature of the kettle by changing its resistance during the water boiling process.(d-f) Thermal camera images showing the temperature distribution onto and near the thermistor attached to the body of the electrical kettle.

Figure 4 .
Figure 4. a) Switching of the electrical resistance of the thermistor with heat from a human finger under continuous tapping.b) Response (0.25 s) and recovery time (1.7) of the thermistor with a momentary touch with a human finger.c) Cyclic behavior of the thermistor resistance when it was touched by a human finger for a longer duration of 10 s. d) Maximum decrease in the resistance with response and recovery times, when the thermistor was touched by a human finger for a longer time.e) The thermistor shows a very fast switching of its resistance value when placed on different human body parts such as the forehead, wrist, and finger.

Figure 5 .
Figure 5. Proximity sensing behavior of the thermistor.a) Electrical resistance behavior of the thermistor with heat radiating rod from 30 to 100°C placed 5 mm above its surface.b) Resistive switching with thermal radiation when a rod radiating heat at 35 °C was brought 5 mm near the thermistor surface for a long time and pulled back.c) Continuous switching of thermistor resistance when a rod radiating heat at 35 °C brought closer to and moved back from the thermistor surface multiple times; and d) demonstration of thermistor response and recovery time of 0.54 and 2.5 s, respectively.Effect of thermal radiations from the human finger on thermistor resistance: e) when a finger was brought closer to the thermistor surface normally from a distance of 5 cm; f) response and recovery time when thermistor faces thermally radiating human finger for a longer time; g) continuous switching of thermistors when a human finger was frequently brought in its proximity at5 mm above; and h) response in recovery time of short switching.

Figure 6 .
Figure 6.a) Photos showing multiple time switching (ON and OFF) of an LCD due to thermal radiations from a human finger and G CVD thermistor working as a non-contact switch, where the SU-MEMS logo corresponds to OFF and all other images to ON state.b) A printed circuit board (PCB)based room temperature thermometer, where the thermistor acts as heat sensor and is connected to the PCB via lead wire soldering.c) Decreasing electrical resistance of the thermistor with room air temperature (inset shows the size comparison of the thermistor with other PCB compatible discrete components (top) and fully packaged thermometer (bottom)).d) Images showing successive turning ON of LEDs one at a time with a 1 °C increase in room temperature.