Piezoelectric coupling in a field-effect transistor with a nanohybrid channel of ZnO nanorods grown vertically on graphene.

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DOI: 10.1039/C4NR04713C

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Piezoelectric Coupling in a Field-Effect Transistor with a Nanohybrid Channel of ZnO Nanorods Grown Vertically on Graphene Cite this: DOI: 10.1039/x0xx00000x

Received Accepted DOI: 10.1039/x0xx00000x www.rsc.org/

V. Q. Dang,a D.-I. Kim,a L. T. Duy,a B.-Y. Kim,b B.-U. Hwang,a M. Jang,b K.-S. Shin,a S.-W. Kim,a,b and N.-E. Leea,b,c,*


Published on 22 October 2014. Downloaded by University of Birmingham on 30/10/2014 17:51:46.

ARTICLE

Piezoelectric coupling phenomena in a graphene field-effect transistor (GFET) with a nano-hybrid channel of chemical-vapor-deposited Gr (CVD Gr) and vertically aligned ZnO nanorods (NRs) under mechanical pressurization were investigated. Transfer characteristics of the hybrid channel GFET clearly indicated that the piezoelectric effect of ZnO NRs under static or dynamic pressure modulated channel conductivity (σ) and caused a positive shift of 0.25% per kPa in the Dirac point. However, the GFET without ZnO NRs showed no change in either σ or Dirac point. Analysis of the Dirac point shifts indicated transfer of electrons from the CVD Gr to ZnO NRs due to modulation of their interfacial barrier height under pressure. High responsiveness of the hybrid channel device with fast response and recovery times was evident in the time-dependent behaviors at a small gate bias. In addition, the hybrid channel FET could be gated by mechanical pressurization only. Therefore, a piezoelectric-coupled hybrid channel GFET can be used as a pressuresensing device with low power consumption and a fast response time. Hybridization of piezoelectric 1D nanomaterials with a 2D semiconducting channel in FETs enables a new design for future nanodevices. Keywords: piezoelectric coupling, ZnO nanorods, graphene, pressure sensor, hybrid channel

1. Introduction Two-dimensional (2D) graphene (Gr), which comprises a one-atom-thick layer of carbon atoms in a honeycomb lattice and that has a unique electronic structure of conduction and valence bands touching each other at a neutral point,1 has attracted great research interest. Its ultrahigh carrier mobility (up to 104 cm2/Vs at room temperature2) combined with excellent conductivity,3,4 high optical transparency,5 thermodynamic stability,6 and mechanical flexibility with elastic modulus7 make Gr a promising material for nanoelectronic devices.8 Interestingly, the electronic properties of Gr are also sensitive to the surroundings due to the low density of states near the Dirac point,1,9 which makes Gr a candidate for sensing devices. The use of Gr in sensing applications such as photodetectors,10 gas sensors,11,12 bio-sensors,13,14 pressure sensors,15,16 strain sensors,17,18 and temperature sensors has been investigated19,20. However, these

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devices often suffer from problems of selectivity, sensitivity, or both. To overcome these problems, hybridization of Gr with other zero-,21– 23 one-,24,25 or two-dimensional26,27 nanostructured materials that have specific stimuli-responsive properties can enhance selectivity as well as sensitivity to a specific stimulus. For example, a hybrid channel of PbS quantum dots (QDs) on Gr in a GFET enhanced sensitivity to infrared (IR) light by many orders of magnitude over the corresponding Gr channel FET, and yielded photoresponsivity to a broad range of wavelengths.10,22,28 A UV sensor based on hybrid ZnO quantum dots (QDs)/Gr showed a rapid response to UV light (325 nm) from tens of minutes to several seconds.29,30 Electron transfer from ZnO QDs to Gr under UV light with the assistance of oxygen molecules in the air made Gr become more n-doped and, therefore, the Dirac point was left-shifted.18 In addition to zero-dimensional nanoparticles (NPs) on the Gr channel in a GFET, 1D nanowires (NW) or nanorods (NR) on 2D

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Gr have the potential to tune or add more functionalities to sensing devices. In a ZnO NR/Gr hybrid structure, for example, the diagonal carbon (C)-atoms of Gr bind only with zinc (Zn)-atoms,29 and the Catoms of Gr bind directly to the Zn-atoms of ZnO in the ZnO NR–Gr interface. Recently, a photo-detector with a ZnO/Gr hybrid structure was shown to be an ultraviolet (UV) sensor; this sensing ability was conferred by the combination of highly conductive Gr and the high UV absorption of ZnO.31 A UV sensor based on a ZnO NR/Gr photoconductor structure was also reported to show a high responsivity of up to 22.7 A/W25, which is 17-fold more than that of a ZnO QD/Gr device (1.3 A/W)30 and over 45,000-fold higher than that of single Gr sheet based on a photodetector (0.5 mA/W).32 Response and relaxation times of milliseconds were much faster than those of the ZnO QD/Gr structure.33 All these observations were attributed to the large surface area of the structures with NRs, resulting in the better UV absorption and more photo-generated electrons that were transferred from ZnO NRs to Gr.25,34 A FET with a hybrid channel of vertically grown ZnO NRs on CVD Gr is an excellent 1D/2D nanohybrid channel FET candidate due to the many responsive properties of ZnO NRs, including their piezoelectric responsiveness, photoresponsivity, and chemoresistivity. However, there have no reports of piezoelectric coupling in GFETs using ZnO NRs on the Gr channel in a FET structure, even though ZnO NR is a piezoelectric material that is highly responsive to pressure or strain.35,36 Herein, we describe a Gr-ZnO NR hybrid channel FET with piezoelectric coupling under mechanical pressure. Understanding piezoelectric coupling of ZnO NRs with CVD Gr channel could enable use of a hybrid channel FET as a pressuresensing device. Gr pressure sensors based on the piezoresistive mechanism of Gr, using either a suspended Gr membrane on substrates or deformable substrates so that the Gr membrane can deform easily under locally applied external pressure or strain, have been reported. However, the low piezoresistive effect in CVD Gr does not allow high sensitivity and it is difficult to obtain pressure sensing under global pressurization, which occurs in many real life applications. Pressurization of the fabricated hybrid channel FET in the present experiments shifted the Dirac point to a positive voltage with a sensitivity of 0.25% per kPa, and modulated the conductivity (σ) even without gate biasing. Analysis of Dirac point shifts indicated a hole doping mechanism involving electron transfer from Gr to ZnO NRs due to lowering of the interfacial barrier height under external compressive pressure. These results indicated that piezoelectric coupling enabled the hybrid channel GFET to function as a pressure-sensing device with low power consumption and a fast response time.

2. Results and discussion 2.1 Characteristic of hybrid channel FET 2.1.1 Surface morphology of gold transferred Gr and ZnO NRs grown on CVD Gr Fabrication of the hybrid channel FET involved two main steps: fabrication of the GFET and then growth of ZnO NRs on the Gr channel of the GFET. A cross-sectional view of the hybrid channel GFET and a description of the process flow are provided in Figure 1. Channel was 80 µm to 110 µm in length and 600 µm in width. Detailed fabrication process is described in the Methods section.

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Figure 1. (a) Cross-sectional view of the hybrid channel GFET and (b) process flow for the fabrication of the hybrid channel GFET.

To evaluate the quality of Gr after transfer onto glass substrate, Raman spectroscopy was carried out. The spectrum in Figure 2a exhibited a G band and 2D band at 1587 cm-1 and 2675 cm-1, respectively. The position of the G band, and the sharpness and intensity of the 2D band, indicated that Gr was single-layer.37 The intensity of the D peak was much less than that of the G peak, and the ratio of peak intensities, ID/IG, was small, which indicated low defect density Gr.38 The FE-SEM image shown in Figure 2b indicates that the Gr film surface was very flat and smooth without any ripples or cracking after transfer. The uniform color contrast of the optical micrograph indicated that the Gr had excellent thickness uniformity and that there was no residual Au on the Gr surface after the transfer process. Therefore, gold transfer was very effective for obtaining high quality Gr without residues on its surface.

After polyethylenimine (PEI) treatment, the Gr surface was enveloped by amine groups. To attract ZnO NPs onto the Gr channel surface, surface treatment of the NPs with oligomeric - acidic esters was necessary. By controlling the concentration of ZnO NPs in butylacetate as well as the spin-casting time, ZnO NPs were uniformly distributed on the Gr surface (Figure 2c). The average size of the ZnO NPs was around 50 nm, similarly to the size of ZnO NPs in solution. A seed layer of highly dense ZnO NPs facilitated a high density of vertically aligned ZnO NRs on the Gr surface. Crosssectional FE-SEM image shown in Figure 2d and the top-view FESEM image (inset in Figure 2d) show ZnO NRs with an average length of nearly 2 µm and a diameter of a few tens nanometers on Gr. Most of the ZnO NRs had a hexagonal morphology.


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Intensity (au)

990

ARTICLE DOI: 10.1039/C4NR04713C

(a)

(b)

660

2D

330

0

G D

1500

2000

2500

3000

3500

4000


Raman shift (cm-1)

(c)

(d)

Figure 2. (a) Raman spectrum of CVD Gr by the gold transfer method, (b) FE-SEM image of CVD Gr by the gold transfer method, (c) FE-SEM image of ZnO NPs seeded on CVD Gr and (d) crosssectional FE-SEM image of ZnO NRs grown on the Gr channel and top-view FE-SEM image (inset). 2.1.2 Transfer characteristic of hybrid channel FET before and after grown ZnO NRs

The transfer characteristics (σ(VG)) after every step of process are shown in Figure 3a. The black curve is the transfer curve of GFET before PEI treatment. The Dirac point voltage (VDirac) of GFET with no PEI treatment was far from zero (nearly 30 V), and the field-effect channel mobility of electrons (µe) was low due to heavy p doping in the Gr layer,39,40 which made the device operate at a high VG. Therefore, GFET was treated by PEI to dope electrons into the Gr film. PEI is an effective n-dopant of Gr because PEI comprises amine-rich and electron-donating macromolecules.41 PEI treatment is a chemical doping method that results in more surface transfer doping than substitutional doping.42 When Gr was doped by PEI treatment, hole conduction was suppressed and electron conduction was preserved.41,42 As a result of PEI treatment, the concentration of electrons surpassed that of holes, which caused a VDirac shift in the negative VG direction and increased the field-effect mobility of electrons (µe) (red curve in Figure 3a). After growth of ZnO NRs, VDirac shifted back in the positive direction and µe decreased (blue curve in Figure 3a), indicating that the Gr was re-doped with holes during the growth of ZnO NRs. The transfer characteristics of the hybrid channel FET shown in Figure 3a indicated that the hybrid channel FET worked very well, even at low VG. Operation of a GFET with a hybrid channel of vertically aligned 1D nanostructured materials on 2D Gr was therefore successful.


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2.2. Pressure sensor based on hybrid channel FET 2.2.1. Transfer characteristics of GFET with and without ZnO NRs under varying pressure

To understand the effect of piezoelectric coupling between ZnO NRs and Gr in the hybrid channel GFET, transfer characteristics of the hybrid channel GFET were investigated at various mechanical pressures. The lower pressure limit was not determined due to the limitation of the measurement equipment (~ 64 kPa), while the upper limit of 160 kPa was due to mechanical damage or delamination of ZnO NRs on the Gr at higher mechanical pressures. For comparison, a GFET with no ZnO NRs was also characterized. For the GFET with no ZnO NRs, the electrical responses to pressure did not change, which indicated no responsivity of GFET to mechanical pressure (Figure 3b). In contrast, the electrical responses of the hybrid channel GFET with ZnO NRs changed significantly with regard to σ(VG) (Figure 3c). Plot of σ(VG) versus applied pressure in Figure 3c indicated changes in the slopes of electron and hole branches, VDirac, and minimum conductivity at the Dirac point (σmin). 2.2.2. Analysis of pressurized hybrid channel FET

Channel conductivity, σ(Vg), in a high carrier density region can be written as 43,44 (1)

Figure 3. (a) Transfer characteristics of the hybrid channel GFET after each process step, (b) transfer characteristics of the GFET with a Gr channel under varying pressures, (c) transfer characteristics of the hybrid channel GFET under varying pressures. The source-drain voltage (VD) was fixed at 1 V. Figures (d), (e), and (f) show Dirac point, sensitivity, and electron concentration transferred from Gr to ZnO NRs in the hybrid channel GFET at varying pressures, respectively.


where σres is residual conductivity and C0 is the capacitance of the gate dielectric per unit area.44 Field-effect channel mobilities of electrons and holes (µe and µh) can be obtained from the slopes of electrons and holes branches, respectively, in linear regimes of σ (VG). For a 150-nm-thick Al2O3 gate dielectric, the measured C0 was about 43.75 nF/cm2. Calculated µh and µe values are described in Table 1 and Figure S3. µh was higher than µe due to the properties of Dirac fermions, which are not observed for massive quasiparticles.43 Table 1. Field-effect channel mobilities of holes and electrons (µh and µe) under varying pressures.

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Journal Name DOI: 10.1039/C4NR04713C

Pressure (kPa)

Initial (0)

64

127

160

Release (0)

µh (cm2V-1s-1)

1984

1140 1000 814

722

1984

µe (cm2V-1s-1)

1820

858

607

1820

96

636

622

As seen in Table 1, both µh and µe values decreased as applied pressure increased, which can be explained by polarization of ZnO NRs causing surface-potential perturbation on Gr channel. When pressure is applied to ZnO NRs, polarization ions appear at two sides of ZnO NRs due to their piezoelectric property.45 Polarization at the interface between ZnO NRs and Gr can cause perturbations of surface potentials on the Gr channel causing scattering of moving electrons and holes. Data shown in Figure 3d indicate that the VDirac value shifted to a positive voltage under compressive pressure due to hole doping into Gr from the ZnO NRs. The VDirac shifts in Figure 3e ∆ versus P, where were quantified as the normalized plot of




,

VDirac and VDirac,0 are the Dirac points with and without pressure, respectively. For practical application, the performance of a pressure sensor can characterized by its sensitivity, which is the slope of a ∆! !

normalized VDirac vs. P plot, . The normalized Dirac point " shifts in Figure 3d had a slope of 1.62 mV/kPa, which translates to 0.25% per kPa (Figure 3e). After the pressure was released, VDirac returned to the initial value, which indicated good recovery. The GFET with no ZnO NRs did not exhibit any changes in either σ or VDirac under pressure (Figure 3b). The positive shift in VDirac in the hybrid channel GFET demonstrated that holes were doped into Gr, which means that some electrons were transferred from Gr to ZnO NRs under pressure. The concentration of transferred electrons were calculated using equation (2) 46,47 ∆#

$ % &,' (

(2)

where e is electronic charge (e = 1.6x10-19 C). Figure 3f shows a concentration plot of transferred electrons with pressure. When a higher pressure was applied, more electrons were transferred across the interface between Gr and ZnO NRs. The number of transferred electrons per unit pressure of 4x108 cm-2 kPa-1 was calculated from the slope in Figure 3f. 2.2.3 Response and recovery times

Figure 4. (a) Comparison of the time-dependent responses of GFETs without (black) and with (blue) ZnO NRs at the applied pressure of 40 kPa, (b) and (c) time-dependent responses of the hybrid channel GFET at different pressures of 40 and 55 kPa at VG = -1 and VG = 0 V, correlatively. (d) and (e) how the response and relaxation time of the hybrid channel GFET, respectively. To appraise the stability of the device as well as its response and recovery times, the time-dependence of responses to applied pressure were measured and the results are shown in Figure 4. In the time-dependent measurements (Figure 4a and b), VD and VG were fixed at 1 and -1 V, respectively. Comparison of σ modulation with applied pressure in GFETs with and without ZnO NRs revealed no responsivity in the GFET without ZnO NRs (black line) and a decrease in σ in the hybrid channel FET under an external pressure of 40 kPa (blue line). A higher signal response of σ was observed with increasing pressurization (Figure 4b). After turning off the pressure, σ returned to its initial value. The hybrid channel FET therefore showed good recovery and excellent stability. The responsivity in the hybrid channel FET without biasing VG is shown in Figure 4c; this indicated operation of the device by piezoelectric gating only. As anticipated, σ increased under external pressure due to hole doping in Gr. This is also in agreement with the data presented in Figure 3c. The response time (rise time) and recovery time (reset time) are presented in Figure 4d and 4e, respectively. Response time was defined as the time required to reach 80% of the final value, )*% 0.8 / (Figure 4d). Recovery time was defined as the time elapsed after release of the pressure to reach 20% of the saturate response, 0 )*% 0.85 / (Figure 4e). We attributed the fast response and recovery times to excellent channel mobility in the Gr channel of the GFET.

2.3. Charge transfer mechanism at interface between Gr and ZnO NRs Charge transfer mechanism as well as the positive shift in Dirac point (Figure 3) was explored using energy diagrams between Gr and ZnO NRs; these are shown in Figure 5. Figure 5a shows the equilibrium

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ARTICLE DOI: 10.1039/C4NR04713C Together, our results indicate that the piezoelectric effects of ZnO NRs on a Gr channel in a GFET structure can be exploited to fabricate a mechanically-coupled FET. As shown in Table 2, the pressure sensitivity of the hybrid channel FET is higher than that of the Gr pressure sensor15 but comparable to that of the sensors with the poly (vinylidenefluoride-co-trifluoroethylene) (P(VDF-TrFE)) thin films.51,52 The low-voltage operating hybrid channel GFET described here has great potential as a practical pressure sensor for global pressurization rather than localized pressurization.

3. Conclusion

Figure 5. Energy band diagram of the hybrid Gr/ZnO NR structure before (a) and after (b) pressurization, P. Due to the piezoelectric attributes of ZnO NRs under mechanical pressure, a deformation in the structure will produce piezo-charges at the interfacial region. It is important to note that polarization charges are distributed within a small depth from the surface, and they are ionic charges, which are non-mobile charges located adjacent to the interface. Positive piezo-charges35,36 may effectively lower the barrier height at the local contact from ФG – χZnO to Ф’ – χ’ so that electrons are transferred directly from Gr Gr

ZnO

to ZnO NRs. To balance Fermi levels at the interface, the Fermi level of ZnO NRs moves closer to the conduction band (Figure 5b) and the Fermi level of Gr is also realigned. Consequently, holes are more dominant in Gr. Hole doping in the Gr shifts VDirac in a positive direction as aforementioned. Piezoelectric effects that generate positive piezo-charges at the interface between ZnO NRs and Gr under pressure was verified in the previous works on nanogenerators.48,49 Nanogenerators were fabricated by growing vertical ZnO NRs on Gr (bottom electrode) and by forming the top electrode on the other side of ZnO NRs. When compressive force was applied to the top of the ZnO NRs, the top surface of the ZnO NRs indicated a negative potential (V-) and the bottom surface reveals a positive potential (V+). Table 2. Comparison of pressure sensitivity of the hybrid channel FET with other sensors Sensing materials

Sensitivity (mV/kPa)

Ref.

P(VDF-TrFE) nanofiber

30

50

P(VDF-TrFE) thin film

0.74

51

PVDF-TrFE (organic FET)

1.094

52

Nanocomposite of P(VDF-TrFE) and BaTiO3 (40 wt%) (organic FET)

3.698

52

CVD graphene

0.085

15

ZnO NRs/Gr hybrid

1.62

Our work


In summary, we successfully fabricated a GFET with a hybrid channel of ZnO NRs/Gr. Positive shift in the Dirac point of 0.25% per kPa, as well as a clear change in the channel conductivity was obtained with the hybrid channel GFET in response to pressurization, while the GFET without ZnO NRs showed no response. Charge transfer at the interface between Gr and ZnO NRs due to the piezoelectric effects of ZnO NRs was responsible for the shifts in the Dirac point and modulation of conductivity at hole and electron branches. The concentration of electrons transferred from Gr to ZnO NRs was calculated to be 4x108 cm-2 kPa-1. The hybrid channel GFET without a biased VG was also responsive to pressurization. Hybrid channel GFET device showed high performance but low power consumption, highlighting the potential of this device in nextgeneration pressure sensors for smart devices.



energy band diagram of the hybrid structure of a Gr/n-type ZnO NR, where the work-function of Gr is ΦGr, and the electron affinity of the semiconductor is χZnO. Eg and EF correspond to the band gap and the Fermi level of the ZnO NRs, respectively. At the equilibrium contacting state, the energy band of ZnO NRs will bend upward at the Gr/ZnO NR interface due to the differences in their work functions, and the Fermi levels at the two sides are brought into coincidence.35,36

4. Experimental Section

Synthesis of Gr. Gr films were grown on 75-µm-thick catalyst-copper foil by thermal CVD following the approach described in another report.53 Cu foil was placed into a hot wall furnace at a temperature of 1000 °C, total pressure of 90 mTorr, and H2 flow rate of 10 sccm. Then, a mixture of H2:CH4=1:2 (10:20 sccm) was introduced into the system to initiate Gr growth. After single-layer Gr was formed on the Cu foil (20 min at a pressure of 350 mTorr), the system was cooled down to room temperature under H2 flow without methane.14

Fabrication of a graphene Field-effect transistor (GFET) device. A graphene channel FET (GFET) was first fabricated, and then ZnO NRs were formed on the Gr channel to fabricate a hybrid channel GFET. Figure 1a shows a cross-section of the device with a ZnO NR/Gr hybrid channel. The process used to fabricate the device is exhibited in Figure 1b. A Ni layer of 60 nm was used as a gate electrode and was deposited onto glass substrate through a shadow mask by e-beam evaporation. Dielectric layer of 150-nm-thick Al2O3 was deposited by atomic-layer deposition (ALD). Single-layer Gr on copper foil was transferred onto the Al2O3 gate dielectric using a gold transfer method.14 Process sequence for the gold transfer of Gr is also shown in Supporting Information (Figure S1). First, gold (30 nm) was deposited onto the Gr/copper foil by thermal evaporation to support the Gr during the transfer process. To etch copper foil, Au/Gr/copper foil was floated on 0.5 M FeCl3 solution for 2 hr. After the copper was completely removed, the Gr sample was transferred to diluted hydrochloric acid solution (15 min) to remove residual ions (e.g. Fe2+, Fe3+, Cu2) beneath the Gr film. Next, Au/Gr was transferred onto Al2O3/Ni/glass substrate after washing three times in de-ionized (DI) water. After that, the sample was placed overnight in a fume hood before gold etching. To eliminate the gold layer on Gr, low concentration KI/I2 etchant solution was used. The gold layer was completely etched by immersion in KI/I2 solution for 30 min.14,54

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Next, the sample was annealed at 200 °C for 2 hr before deposition of the source/drain electrodes. Source/drain electrodes of Cr/Au (5 nm/500 nm) were deposited by thermal evaporation. Due to heavy p-doping in the transferred Gr, polyethyleneimine (PEI) treatment was used to dope electrons into Gr. For this step, the device was immersed in PEI (20% v/v in ethanol) for 3 hr and annealed at 120 °C for 30 min after washing in ethanol and DI water.41,42 Growth of vertically-aligned ZnO NRs on the Gr for a hybrid channel FET. ZnO NRs were grown on the Gr channel of a GFET device using ZnO NPs as a seed material. Before dropping ZnO NPs, tetratetracontance (TTC) was deposited over the source and drain electrodes of the GFET through a shadow mask by thermal evaporation to prevent direct contact between the ZnO NRs and electrodes. Surfaces of ZnO NPs dispersed in butylacetate 40 wt% (Sigma Aldrich) were modified by oligomeric – acidic esters. The average size of nanoparticles was 35 nm and their maximum size was lower than 100 nm. To decrease the concentration of the ZnO NP dispersion, we diluted the initial 40 wt% solution by adding butylacetate solution. A 5 wt% solution of diluted ZnO NP was used to seed ZnO NPs on Gr. Then, the diluted ZnO NPs dispersion was spin-cast on the GFET with a syringe for 5 min. After washing the sample with butylacetate and DI via a water gun for more than 30 s, it was dried at 150 °C for 30 min. The next step was to grow ZnO NRs on the Gr channel in the GFET. Nutrient solution was prepared with equal molar concentrations of 20 mM zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and hexamethylenetetramine (C6H12N4, HMTA) in 200 ml DI water. GFET device with ZnO NPs was placed into this nutrient solution at the growth temperature of 90 °C for 4 hr.55,56 As the last step, polyurethane (PU) was spin-coated on the device for encapsulation and the entire device was annealed at 120 °C for 2 hr. The PU thin layer not only enhanced the adhesion of ZnO NRs and Gr, but also protected the ZnO NRs from contamination or corrosion. More importantly, PU made the ZnO NRs deform uniformly under pressure with minimal mechanical damage within a certain mechanical forcing range. Characterization of the hybrid channel GFET. All measurements were carried out at room temperature under ambient conditions. Quality of Gr after transfer was tested by Raman spectrometry (Alpha 300R (WITec)). Density of ZnO NPs and NRs was investigated by Field-emission scanning electron microscopy (FE-SEM) (JEOL JSM6500F). Pressurization measurements were performed with a pushing tester (ZPS-100 system) (Figure S2). Electric characteristics of the device were measured with a semiconductor parameter analyzer (Agilent, B1500). Source and drain bias (VD) was fixed at 1 V during measurements of transfer characteristics.

Acknowledgements This research was supported by the Basic Science Research Program (Grant No. 2010-0015035 and 2013R1A2A1A01015232) through the National Research Foundation (NRF) funded by the Ministry of Science, ICT& Future Planning

Notes and references a

School of Advanced Materials Science & Engineering, Sungkyunkwan University, Suwon, Kyunggi-do 440-746, Korea. b SKKU Advanced Institute of Nanotechonology (SAINT), Sungkyunkwan University, Suwon, Kyunggi-do 440-746, Korea. c Samsung Advanced Institute for Health Sciences & Technology (SAIHST), Sungkyunkwan University, Suwon, Kyunggi-do 440-746, Korea. * Corresponding author: [email protected]

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Journal Name DOI: 10.1039/C4NR04713C Electronic Supplementary Information (ESI) available: Gr transfer process, Diagram of pressure sensing measurement system, field-effect channel mobilities of electrons and hole at varying pressure. See DOI: 10.1039/b000000x/

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Nanoscale

J. Name., 2014, 00, 1-3 | 7

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