Flexible Piezoelectric-Induced Pressure Sensors for Static. Measurements Based on Nanowires/Graphene Heterostructures

Transcription

1 Flexible Piezoelectric-Induced Pressure Sensors for Static Measurements Based on Nanowires/Graphene Heterostructures Zefeng Chen,, Zhao Wang,, Xinming Li,*, Yuxuan Lin, Ningqi Luo, Mingzhu Long, Ni Zhao, and Jian-Bin Xu*, Department of Electronic Engineering, The Chinese University of Hong Kong, Hong Kong SAR, China Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Hubei Key Laboratory of Ferro & Piezoelectric Materials and Devices, Faculty of Physics & Electronic Sciences, Hubei University, 368 Youyi Road, Wuhan , P.R. China Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States Figure and Table S1. XRD patterns and TEM image of PbTiO 3 nanowires. S2. Table of perovskite piezoelectric material and graphene pressure sensor. S3. The band diagram of a pressure/release process in pure PTNWs pressure sensor. S4. Pure PbTiO 3 nanowires pressure sensor. S5. Graphene pressure sensor. S6. Schematic diagram of PTNWs/G pressure transistor. S7. Fitting the transfer curve of PTNWs/G FET. S8. Four additional PTNWs/G FET. S9. Graphene-PTNWs-graphene device. S10. Estimation of the density of charge variations induced by nanowire polarizations. Video S1. The response of PTNWs/graphene FET under pressure. 1

2 S1. XRD patterns and TEM image of PbTiO 3 nanowires (a) (b) Figure S1 (a) XRD pattern of the as-prepared PbTiO 3 nanowires derived from the hydrothermal method, which exhibits sharp diffraction peaks indicating that the samples are well crystallized. All of the diffraction peaks can be indexed as the tetragonal perovskite structure of PbTiO 3 with the lattice parameters: a=3.91 Å and c=4.15 Å, which is corresponding to the high-resolution TEM images (b) of the PbTiO 3 nanowires. The XRD patterns of the PTNWs. All diffraction peaks except those marked by black dots can be indexed into the tetragonal perovskite structure of the PbTiO 3 materials according to the JCPDS Card No The sharp diffraction peaks indicate good crystallinity of the as-synthesized nanowires. In addition, the appearance of the marked diffraction peaks suggested that there are a little amount of PbOx in the samples, which should be due to the precipitated hydrothermal precursor at the initial stage of the hydrothermal treatment. The HRTEM image of the PbTiO 3 nanowire shows clear lattice stripes, which can also prove the good crystallinity of the sample. After converting the image by a fast Fourier transfer method, the interplanar spacing of the nanowire along the axial and radial direction could be calculated as and nm, which belong to the (001) and (100) face of the tetragonal lattice. As a result, the nanowire could be confirmed to be oriented along the [001] direction. 2

3 S2. Table of perovskite piezoelectric material and graphene pressure sensor. Materials Static pressures sensor Dynamic pressure sensor/ nanogenerator PbTiO 3 nanostructure None 1.Pb(Mg 1/3 Nb 2/3 )O 3 xpbtio 3 (PMN PT) nanowires ~7.8 V /2.29 μa [1] Graphene 10-5 kpa -1 [3, 4] None 2. Lead zirconate titanate (PZT) nanofibers ~0.6 V [2] Perovskite nanostructure None 1. NaNbO 3 nanowires under 0.23% compressive strain ~3V/70 na [5] 2. BaTiO 3 nanowires ~ 0.3V/15nA [6] 3. (K, Na)NbO 3 nanowires under 6% strain Changes ~40mV [7] Compound perovskite nanostructure/graphene kpa -1 (This work) Compose of BaTiO 3 nanoparticles and graphene sheet ~ 3V/100nA [8] 3

4 S3. The band diagram of a pressure/release process in pure PTNWs pressure sensor. Once the force loads on, polarization charges of PTNWs will be formed at the surface, as well as a polarization potential, so the voltage rises up quickly; then the free carriers with opposite polarity are attracted to the PTNWs surface for balancing the potential, so the voltage decays with time until potential gets equilibrium and voltage is zero. Once the force releases, the polarization charges disappear and leaving the free carriers, so the voltage quickly falls to a negative value; then the free carriers will move back until equilibrium state, so the voltage rises up to zero again. Press V=V max Hold V=0 No load V=0 Release V=V min Figure. S2 The band diagram of a pressure/release process in pure PTNWs pressure sensor. 4

5 S4. Pure PbTiO 3 nanowires pressure sensor. Figure S3 Short-circuit current under the applied pressure of PbTiO 3 nanowires pressure sensor. PTNWs were dispersed in ethanol by ultrasonic treatment to obtain a suspension of separated PTNWs. After that, a pair of interdigital Au electrodes with 80 nm in thickness is deposited on the commercially available Kapton film. The channel is 3μm. The suspension was then dropped to the electrodes and dried at 80 C for 0.5 h. Consequently, the electrodes were connected with two Ag wires by Ag epoxy-composite to the external circuit. Finally, the system was packaged by PDMS polymer matrix for fixing the PTNWs and preventing physical damages. 5

6 I d (ma) S5. Graphene pressure sensor N 1 N 2 N 3 N 4 N 5 N 0.2 Figure S4. Transfer curve of graphene pressure sensor. It is obvious that the carrier neutral point is consistent under different pressure, as well as the slope of the transfer curve. These mean that the carrier concentration and mobility do not change. Therefore, under a vertical pressure, there should not be any lattice distortion in graphene, resulting in no current change V g (V) 6

7 S6. Schematic diagram of PTNWs/G pressure transistor. Press Sourse Drain V ds V g Gate PTNWs/Graphene Figure S5. Schematic diagram of PTNWs/G pressure transistor. 7

8 Total resistance S7. Fitting the transfer curve of PTNWs/G FET 1000 expriment data fiting R contact =172Ω R contact =322Ω gate voltage (V) Figure S6. The fitting transfer curve of PTNWs/G FET. Fitting result of the transfer curve of PTNWs/G FET (here, we take loading force of 2.5N as an example). The fitting is divided into two parts, holes (red) and electrons (blue) dominated the region, because they have different contact resistance and mobility. In the electron dominated region (blue) the contact resistance is about 320 Ω, much higher than that in holes dominated region. 8

9 I d (ma) I d (ma) I d (ma) I d (ma) S8. Four additional PTNWs/G FET N 5 N N 5 N V g (V) V g (V) N 5 N N 5 N V g (V) V g (V) Figure S7. Transfer curves of four PTNWs/G FET. The four transfer curve show that the voltage of neutral point remains unchanged and the current reduces when applied pressure is loaded on, which is matching with the explanation based scattering model. The response current is always negative (ΔI~0.04mA under pressure of 5N) at zero gate voltage. 9

10 Current (na) Current (na) S9. Graphene-PTNWs-graphene device (a) (b) μm -8 with PTNWs wihtout PTNWs Time (s) (c) 8 Press (d) 4 0 Hold No load (1) Press I = I max (2) Hold I = 0-4 Release Time (s) (4) No load I = 0 (3) Release I = I min Figure S8 (a) Diagram of the graphene-ptnws-graphene structure. (b) and (c) Short current response under pressure/release process. (d) Band structure of graphene-ptnws-graphene. To prove that the polarization charges cannot transfer to graphene to increase current, we design a device of graphene-ptnws-graphene, as shown in Figure S8 (a). The process corresponds to the current signal of the graphene-ptnws-graphene device, shown as figure S8 (b) and (c), which is the same with previous work in nanogenerator. [1, 5, 8] When the PTNWs are pressed or bending, polarization charges will be formed. As known, polarization charges are bound to charge, which means they cannot move but form an electric field on the surface of the PTNWs. Under this polarization-induced electric field, the free carriers on graphene will be attracted and move to the interface Figure S8 (d-1), forming a positive current. But the free carriers of graphene cannot get into PTNWs because of the barrier, the current becomes zero when getting potential equilibrium Figure S8 (d-2). When the pressure is released, the polarization charges of the PTNWs disappear, as well as the polarizationinduced electric field. Then the free carriers of graphene will move back Figure S8 (d-3) until 10

11 equilibrium state Figure S8 (d-4). In a word, there are no carriers exchanging between graphene and PTNWs during the pressing/releasing. Otherwise, there will be a constant current during pressing. S10. Estimation of the density of charge variations induced by nanowire polarizations. For a single nanowire, the electric displacement along the nanowire (D 3 ) as a result of a tensile stress that is applied to the sidewall of the nanowire (T 1 ) can be expressed as: D 3 = d 13 T 1 In our case, d C/N, and the microscopic tensile stress applied onto a single nanowire is proportional to the tensile stress applied on the whole device: T 1 = k ( F A ) For simplicity, we assume k~1. The charge that is generated on a single nanowire is given by Q nw = D 3 πr 2 where r is the radius of the nanowire. And the extra impurity charge density induced on the graphene channel due to the polarization of the nanowires can be expressed as Δn o = Q nw N nw = d 13 kfπr 2 N nw A with N nw the surface density of nanowires. The average value of r and N nw are estimated to be 500 nm, and cm 2, respectively, according to multiple SEM images. Therefore we could estimate Δn o /F to be around cm 2 N 1, which is on the same order of magnitude as the slope as shown in Figure 4(c). 11

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