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Graphene Tribotronics for Electronic Skin and Touch Screen Applications

Usman Khan, Tae-Ho Kim, Hanjun Ryu, Wanchul Seung, and Sang-Woo Kim*

Dr. U. Khan, T.-H. Kim, H. Ryu, W. Seung, Prof. S.-W. KimSchool of Advanced Materials Science and EngineeringSungkyunkwan University (SKKU)Suwon 440-746, Republic of KoreaE-mail: [email protected]. S.-W. KimSKKU Advanced Institute of Nanotechnology (SAINT)Sungkyunkwan University (SKKU)Suwon 440-746, Republic of Korea

DOI: 10.1002/adma.201603544

triboelectric nanogenerator (S-TENG) and a graphene FET. When any object (e.g., a human finger) comes into contact with the friction layer of the S-TENG, charges are produced due to the well-known triboelectric effect.[17,20] The triboelectric charges act as a gate bias to the graphene FET and modulate its current transport. The tribotronic transistor, therefore, does not require any external gate voltage, as in traditional metal-oxide semiconductor field-effect transistors.[5] Tribotronic sensors have displayed a sensitivity of ≈2% kPa−1, a limit of detection <1 kPa, and a response time of ≈30 ms. Furthermore, devices can effectively detect touch stimuli from both bare and gloved fingers, which can be a limitation with capacitive touch screens. Tribotronic arrays have also been demonstrated to spatially map the movement of a ball and multiple finger touch stimuli. Besides, tribotronic devices are also transparent, flexible, and structurally simpler. Due to all these features, the graphene tribotronic devices are an ideal candidate for electronic skins (e-skins) and touch screens.

Figure 1a schematically describes the tribotronic design. The poly(dimethylsiloxane) (PDMS) friction layer with an indium tin oxide (ITO)-based back electrode comprises the S-TENG interface of the device. The S-TENG is based on a single-electrode-mode design, as such a design does not require an interconnection between the ITO electrode and the external touching object.[21,22] The graphene channel and ITO electrode are coupled in a coplanar fashion by a 1-ethyl-3-methylimida-zolium bis(trifluoromethylsufonyl)imide ([EMI][TFSI])-based ion-gel gate dielectric. Figure 1b shows an optical image of the fabricated device. The fabrication process is described in detail in the Experimental Section and the design parameters are described in Section S1 (Supporting Information). Figure 1c describes the equivalent electrical circuit diagram of the gra-phene tribotronic device, which shows that the triboelectric potential resulting from external touch stimuli act as a gate bias to the graphene FET. Figure 1d shows the Raman spectra of the CVD graphene where the G-to-2D intensity ratio of ≈0.5 and the symmetric 2D band centered at ≈2660.8 cm−1 indicate that the graphene is monolayer.[19]

When an external object contacts the PDMS friction layer of the TENG interface, charges are produced on the surfaces due to triboelectrification.[17] The polarity of the resulting tribo-electric potential is determined by the relative position of the PDMS friction layer and the external object in the triboelectric series.[23,24] It is negative if the external object is more positive than the PDMS (according to the triboelectric series) and vice versa. Figure 2 schematically describes the working mechanism of the graphene tribotronic device for both the cases of rela-tively more positive and negative touching objects.

For the case of a relatively positive touching object (see Figure 2a), the PDMS friction layer receives electrons from

Mimicry of the touch-sensing characteristic of human skin via electronic devices is of great research interest due to promising applications in touch-screen technologies and artificially intel-ligent systems.[1,2] There are various means for sensing touch stimuli. Capacitive sensors utilize the capacitance change due to a contact from an external object (e.g., a human finger) for touch sensing.[3] Though they have met great commercial suc-cess, capacitive sensors require the touching object to be con-ductive or capacitively grounded[4] and, therefore, can have limitations in the detection of touch stimuli from gloved fingers in comparison to those of bare fingers. Resistive sensors utilize the resistance change for touch sensing when the impact from an external object brings two of its separated electrode layers into contact.[4] However, resistive sensors require a large activa-tion force, suffer from mechanical aging due to cyclic deforma-tion, and display temperature sensitivity.[4] Recently, a new field of tribotronics was introduced, in which the carrier transport in a field-effect transistor (FET) is coupled to the external environ-ment through triboelectrification.[5] Such coupling of the trans-port properties of FETs to the external world can have numerous applications in the fields of security, environmental monitoring, robotics, and human–machine interactive systems.[5–12]

Graphene has excellent electronic transport characteristics and is also transparent.[13,14] Furthermore, graphene under-goes a remarkable ambipolar electric field effect such that both electrons and holes can be induced into a graphene channel depending on the polarity of the gate bias.[15,16] Since the tri-boelectric potential due to contact electrification between two materials can be either positive or negative,[17,18] ambipolar transport characteristics of a tribotronic transistor are highly desirable as the carrier transport can be modulated by either of the triboelectric potential. Therefore, graphene with ambi-polar transport characteristics is an ideal choice for tribotronic devices. Besides, graphene can also be synthesized on large scale using chemical vapor deposition (CVD)[19] which is crucial for industrial-scale applications.

Here, we demonstrate a graphene tribotronic touch sensor that is based on coplanar coupling of a single-electrode-mode

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the external object and, therefore, has net negative charges on its surface. At the moment of contact, the negative charges on the PDMS surface are nullified by the positive charges on the surface of the external object. However, as the external object departs away, the unscreened negative charges on the PDMS surface electrostatically induce positive charges beneath in the ITO electrode and so leaves negative charges on the other end of the ITO electrode. As a consequence, upon contact from a relatively positive touching object, a negative potential is applied to the ion gel dielectric. Application of a negative poten-tial across the ion gel causes ionic migration in it.[25] The ionic migration is such that[25] the cations migrate to the ITO–ion-gel interface in order to screen the negative charges at the inter-face, whereas anions migrate to the graphene–ion-gel interface and accumulate holes in the graphene channel (see Figure 2a). Therefore, “electrical double layers” (EDLs) are formed at each of the interfaces.[25] Figure 2b shows a schematic band diagram that describes the ionic migration and p-type doping of the graphene channel at the advent of a negative potential. On the other hand, for the case of a relatively negative touching object (see Figure 2c), the PDMS friction layer has net positive charges on its surface and, therefore, a positive potential is applied to the ion gel dielectric. Formation of the EDLs at the application of the positive potential is such that electrons are accumulated in the graphene channel (see Figure 2c). Figure 2d shows a schematic band diagram that describes the ionic migration and the n-type doping of the graphene channel at the advent of a positive potential. In all cases, the dependence of the drain-to-source current, Ids, in a graphene channel on gate potential Vg can be described by Equation (1):[26]

µ

µ( )( )

( )=

− + >

− − + <

ds g

e t dsg g,min min g g,min

h t dsg g,min min g g,min

I V

c V Wt

LV V I V V

c V Wt

LV V I V V

(1)

where W, L, and t are the width, length, and thickness of the graphene channel, respectively; μe and μh are the electron and hole mobilities, respectively; ct is the gate capacitance per unit area; Vg,min is the gate bias to achieve the minimum current, Imin, in a graphene channel which is the current at the Dirac point.

We first characterized the electronic transport character-istics of the graphene tribotronic transistor using external gate voltages. Figure 3a shows the transfer characteristics of the graphene FET at a drain-to-source voltage, Vds, of 0.5 V. The coplanar graphene FET undergoes a very strong ambi-polar electric field effect such that very low gate voltages, |Vg| < 3 V, can significantly modulate the current through the gra-phene channel (see Figure 3a). Such low-gate-voltage-based current modulation is due to the ion-gel-based gate dielectric that offers much higher capacitance, typically in the range of 1–12 μF cm−2, than conventional dielectric materials.[25] The ion-gel dielectric also offers the possibility of coupling the TENG and the graphene FET interfaces in a coplanar fashion; the coplanar coupling is effective for realizing FET devices on flexible substrates.[25] Besides, the graphene FET has shown saturation in the current at Vg ≈ −3 V; such current saturation in a graphene FET has already been reported, and is attrib-uted to charge scattering by the interfacial phonons at the

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Figure 1. Graphene tribotronic design. a) Schematic diagram of the graphene tribotronic device. b) An optical image of the device demonstrating its transparency and flexibility. c) Equivalent circuit diagram of the tribotronic touch sensor. d) Raman spectrum of the CVD graphene synthesized for the device.

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interfaces with contacting materials such as substrate.[27] The low-gate-voltage-based current modulation is highly desired for a tribotronic sensor, particularly, in the detection of subtle touch stimuli generating low triboelectric potentials. Though, unlike the ideal case,[16] the charge neutrality point is shifted to the right (see Figure 3a) indicating that the graphene is slightly p-doped due to absorbents from the environment,[28] this does not pose any limitation in the operation of the graphene tribo-tronic sensor. Indeed, both positive and negative triboelectric potentials can effectively modulate the current in the graphene channel. Besides, the graphene FET exhibits a hole mobility of ≈131 cm2 V−1 s−1 and an electron mobility of ≈86 cm2 V−1 s−1. Figure 3b shows the output characteristics of the graphene FET at various gate voltages. It can be seen that the graphene FET has very low operational voltages of ≈0.5 V and consumes very low power, ≈180 μW.

In order to characterize the touch response of the graphene tribotronic sensor, touch stimuli with a force of 2.9 N were applied to the PDMS friction layer from aluminum (Al), nylon, and poly(tetrafluoroethylene) (PTFE). According to the triboe-lectric series (see Section S2, Supporting Information), Al and

nylon are relatively more positive than PDMS, whereas PTFE is relatively more negative than PDMS. The triboelectric potentials due to the touch stimuli from the three materials are shown in Figure S2 (Supporting Information). Figure 3c shows modula-tion in drain-to-source current, ∆Ids, due to the touch stimuli from Al. It can be seen that the tribotronic sensor undergoes a current modulation, ∆Ids, of ≈100 μA due to the Al touch. The current modulation, ∆Ids, due to the touch stimuli from nylon and PTFE are ≈140 and ≈20 μA, respectively, as shown in Figure S3 (Supporting Information). The touch responses in Figure 3c and Figure S3 (Supporting Information) demon-strate that the graphene tribotronic sensor can effectively detect touch stimuli from all kinds of materials, such as electrically conducting and insulting, and more positive and negative with respect to the triboelectric series. Furthermore, we also charac-terized the sensing response against touch stimuli from both bare and gloved fingers (see Section S4, Supporting Informa-tion for details); the sensor undergoes a current modulation, ∆Ids, of ≈110 and ≈90 μA upon bare finger and gloved finger touching, respectively, as shown in Figure S4 (Supporting Information). In summary, the graphene tribotronic sensors

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Figure 2. Working mechanism. a) Schematic description of the triboelectric charges due to the contact from a more positive external object (top), and migration of ions in the ion gel and p-type doping in graphene upon the departure of the external object (bottom). b) Schematic energy-band diagram of the gate/ion-gel/graphene interface describing p-type doping of the graphene channel due to a negative triboelectric potential. c) Schematic descrip-tion of the triboelectric charges due to a contact from a more negative external object (top), and migration of ions in the ion gel and n-type doping of the graphene channel on the departure of the external object (bottom). d) Schematic energy-band diagram of the gate/ion-gel/graphene interface describing n-type doping in graphene due to a positive triboelectric potential.

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can effectively detect touch stimuli from both gloved and bare fingers, which is a limitation with capacitive touch sensors.

In order to characterize the limit of detection, we varied the pressure of a touch signal from 0.816 to 57.16 kPa of an Al object. Figure 3d shows the normalized current modulation, ∆Ids/I0, as a function of the pressure; I0 is the minimum cur-rent through the graphene channel during the touch opera-tion. The normalized current modulation varies linearly until ≈20 kPa and then it starts saturating. The saturation in the cur-rent modulation at pressures in excess of ≈20 kPa is due to the well-known saturation in the triboelectric charges.[29] The touch sensitivity of the tribotronic sensor is ≈2% kPa−1 in a pres-sure range of 10 kPa. Most remarkably, the tribotronic sensor can detect touch stimuli as low as <1 kPa. With a limit of detection in the subtle pressure range (<1 kPa),[1] the graphene

tribotronics are ideal for realizing ultrasensitive e-skins and touch screens. Figure 3e shows the characterization of the response time of the sensor; an Al sheet was utilized as the touching object; the applied force was measured using a com-mercial piezoelectric force sensor (top) and its response was taken as the reference. It can be seen that the tribotronic sensor instantly responds to the touch stimuli with a response time of ≈30 ms (i.e., time taken from the touching moment to the peak value). In order to characterize the stability of the sensor, we applied touch signals from an Al sheet at a force of 39 N for an extended period of ≈4 h, comprising ≈1700 cycles. The cor-responding results in Figure 3f demonstrate that the graphene tribotronic device possesses a very stable response.

A practical touch screen and e-skin require an array of sensing devices in order to spatially map multiple touch

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Figure 3. Characterization of the graphene tribotronic touch sensor. a) Transfer characteristics, Ids–Vgs, at a drain-to-source voltage, Vds, of 0.5 V; Ids and Vgs refer to drain-to-source current and gate-to-source voltage, respectively. b) Output characteristics, Ids–Vds, for different Vgs. c) Touch response of the sensor in terms of current modulation, ∆Ids, due to a contact from Al sheet with a force of 2.9 N; Vds is 0.5 V. d) Normalized current modulation, ∆Ids/I0, as a function of the touch pressure; Vds is 0.5 V; Al is the external touching material. e) Current modulation, ∆Ids, (bottom) due to the touch signals (top); the response time is ≈30 ms; Al is the contacting material and Vds is 0.4 V. f) Stability test in terms of the touch response of the sensor due to a contact from Al with 39 N force; Vds is 0.5 V.

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stimuli. Therefore, we also demonstrated a 3 × 3 array of the graphene tribotronic sensors. Figure 4a schematically describes the tribotronic array; graphene channels are 500 μm wide and 700 μm long, and reside outside the touching area; the touching area comprises a PDMS friction layer with an array of ITO back electrodes of 1 cm × 1 cm size; each ITO electrode is coupled to a distant graphene channel through the ion-gel gate dielectric; the total array size is 6 cm × 5.5 cm. The tribotronic array design proposed here is generic and can be easily adopted to have a higher number of pixels or, in other words, higher resolution.

For characterization, we mounted the graphene tribotronic sensing array on the wrist of an adult man, as shown in Figure 4b, and measured the current modulation, ∆Ids, in the graphene channels due to a double finger touch. Accordingly, we constructed a color map, as shown in Figure 4c, in terms of the current modulation, ∆Ids, due to the double touch signal.

Figure 4b,c demonstrates that the graphene tribotronic array can spatially map multi-touch stimuli. We also utilized the tribotronic array to map the trajectory of a moving ball; 0.9 g metallic ball of 6 mm diameter was utilized for the experiment. Figure 4d–f schematically describes the movement of the ball. Figure 4g–i shows the corresponding reconstructed color maps in terms of the current modulation, ∆Ids, in the graphene chan-nels due to the moving ball. The results in Figure 4d–i show that the graphene tribotronic array accurately mapped the tra-jectory of the moving ball and, therefore, are very suitable for artificial intelligence applications. Additionally, Figure 4d–i also demonstrates that the tribotronic array can potentially detect the touch stimuli from very small objects on the order of a mm or more as the actual contact area of the ball has certainly much less dimensions than its diameter (6 mm). Besides, devices are also wearable and transparent; though Ti–Au contacts are not transparent, they reside outside the touching area.

Figure 4. Graphene tribotronic array. a) Schematic description of a 3 × 3 graphene tribotronic array. b,c) Photograph of the tribotronic array mounted on the wrist with a double finger touch (b) and corresponding spatial mapping in terms of current modulation, ∆Ids (c). d–f) Schematic description of the movement of a ball movement over PDMS friction layer and g–i) corresponding spatial mapping in terms of ∆Ids.

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In summary, we have introduced graphene tribotronics for interfacing electronics to environmental touch stimuli with numerous artificial intelligence applications. The device struc-ture consists of an S-TENG and a graphene FET coupled in a coplanar fashion. The triboelectric charges due to contact from an external object are utilized as a gate bias to tune the cur-rent transport in the graphene FET. The devices exhibit an excellent touch-sensing performance with a limit of detection as low as <1 kPa, a touch sensitivity of ≈2% kPa−1, a response time of ≈30 ms, and stable operation over thousands of cycles. Besides, the sensor requires very low operational voltages ≈0.5 V and consumes very low power ≈180 μW. Furthermore, they are transparent and flexible and can effectively sense the touch stimuli from both gloved and bare fingers. Arrays of the graphene tribotronic sensors can spatially map various kinds of touch stimuli such as multi-touch stimuli and trajectory of a moving ball. Due to all these attributes, graphene tribo-tronics are of great significance for e-skins and touch-screen technologies.

Experimental SectionGraphene Synthesis: Graphene was synthesized on a copper foil using

low-pressure chemical vapor deposition (LPCVD) of methane (CH4) gas at 1000 °C.[19] The Cu foil was first annealed in a CVD chamber at 1000 °C in the presence of H2 flowing at a rate of 30 sccm for 1 h. For graphene synthesis, CH4 was then introduced into the CVD chamber at a rate of 20 sccm for 30 min. Finally, the CVD chamber was cooled in the presence of H2 flow.

Device Fabrication: The graphene tribotronic devices were realized on an ITO-coated poly(ethylene naphthalate) (PEN) substrate. Firstly, the ITO electrode (see Figure 1a) was realized using photolithography and a wet-etching process; for etching, the patterned ITO film was treated with a liquid-crystal-display etchant, LCE-12, for 2 min. Thereafter, the CVD graphene was transferred from the Cu foil onto the substrate using the standard wet-transfer method.[19] The graphene channel was then realized using photolithography and oxygen (O2) plasma etching. Titanium (Ti)–gold (Au)-based drain and source electrodes to the graphene channel (see Figure 1a) were deposited using photolithography, e-beam evaporation, and the lift-off technique; the thicknesses of the Ti and Au layers were 5 and 80 nm, respectively. For coupling (see Figure 1a), the ion-gel dielectric was deposited across the graphene channel and the ITO electrode using the drop-casting method. The ion gel was prepared using the ionic liquid of [EMIM][TFSI], poly(ethylene glycol) diacrylate (PEGDA) monomer, and 2-hydroxy-2-methylpropiophenone (HOMPP) photoinitiator with a weight ratio of 90:7:3. The ion gel, after drop-casting, was cured by exposure to UV (λ = 300 nm) radiation for 1 min. Finally, the friction layer of the tribotronic device was realized by coating a 200 μm-thick PDMS layer using a bar-coater and curing at 80 °C for 40 min. The fabrication of both a single (Figure 1a) tribotronic sensor and arrays (Figure 4a) of sensors was carried out using same steps outlined above.

Characterization and Measurements: The Raman spectra of the CVD graphene were measured using a WiTech confocal Raman microscope with an Nd:YAG laser of 532 nm wavelength. The electronic transport characteristics of the graphene FET were characterized using a probe station (MSTECH, MST5000) using a semiconductor parameter analyzer (Keithley SCS-4200) at room temperature. The touch stimuli to the graphene tribotronic sensor were applied in a programmed manner using a pushing tester (ZPS-100, Z-Tech). During response-time characterization, the force was measured using a commercial piezoelectric force sensor from DYTRAN Instruments. The drain-to-source current, Ids, through the graphene channel, with simultaneously

applied touch stimuli, was measured using an impedance analyzer (IVIUM Technologies).

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsU.K. and T.-H.K. contributed equally to this work. This work was financially supported by Basic Science Research Program (2015R1A2A1A05001851) through the National Research Foundation (NRF) of Korea Grant funded by the Ministry of Science, ICT and Future Planning.

Received: July 6, 2016Revised: August 28, 2016

Published online: October 27, 2016

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