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Showing papers in "Applied Physics Letters in 2012"


Journal ArticleDOI
TL;DR: In this paper, a single-crystal gallium oxide (Ga2O3) metal-semiconductor field effect transistors (MESFETs) with a gate length of 4 μm and a source-drain spacing of 20 μm is presented.
Abstract: We report a demonstration of single-crystal gallium oxide (Ga2O3) metal-semiconductor field-effect transistors (MESFETs). A Sn-doped Ga2O3 layer was grown on a semi-insulating β-Ga2O3 (010) substrate by molecular-beam epitaxy. We fabricated a circular MESFET with a gate length of 4 μm and a source–drain spacing of 20 μm. The device showed an ideal transistor action represented by the drain current modulation due to the gate voltage (VGS) swing. A complete drain current pinch-off characteristic was also obtained for VGS < −20 V, and the three-terminal off-state breakdown voltage was over 250 V. A low drain leakage current of 3 μA at the off-state led to a high on/off drain current ratio of about 10 000. These device characteristics obtained at the early stage indicate the great potential of Ga2O3-based electrical devices for future power device applications.

1,273 citations


Journal ArticleDOI
TL;DR: Using spin torque induced ferromagnetic resonance with a β-W/CoFeB bilayer microstrip, the spin Hall angle was determined to be |θSHβ-W|=0.30±0.02 as mentioned in this paper.
Abstract: We report a giant spin Hall effect in β-W thin films. Using spin torque induced ferromagnetic resonance with a β-W/CoFeB bilayer microstrip, we determine the spin Hall angle to be |θSHβ-W|=0.30±0.02, large enough for an in-plane current to efficiently reverse the orientation of an in-plane magnetized CoFeB free layer of a nanoscale magnetic tunnel junction adjacent to a thin β-W layer. From switching data obtained with such 3-terminal devices, we independently determine |θSHβ-W|=0.33±0.06. We also report variation of the spin Hall switching efficiency with W layers of different resistivities and hence of variable (α and β) phase composition.

1,128 citations


Journal ArticleDOI
Fei Ding, Yanxia Cui, Xiaochen Ge, Yi Jin, Sailing He 
TL;DR: In this article, a microwave ultra-broadband polarization-independent metamaterial absorber is demonstrated, which is composed of a periodic array of metal-dielectric multilayered quadrangular frustum pyramids.
Abstract: A microwave ultra-broadband polarization-independent metamaterial absorber is demonstrated. It is composed of a periodic array of metal-dielectric multilayered quadrangular frustum pyramids. These pyramids possess resonant absorption modes at multi-frequencies, of which the overlapping leads to the total absorption of the incident wave over an ultra-wide spectral band. The experimental absorption at normal incidence is above 90% in the frequency range of 7.8–14.7 GHz, and the absorption is kept large when the incident angle is smaller than 60°. The experimental results agree well with the numerical simulation.

735 citations


Journal ArticleDOI
TL;DR: In this article, the topological insulator (TI) Bi2Te3 is shown to be a very high modulation-depth (up to 95%) saturable absorber.
Abstract: Under strong laser radiation, a Dirac material, the topological insulator (TI) Bi2Te3, exhibits an optical transmittance increase as a result of saturable absorption. Based on an open-aperture Z-scan measurement at 1550 nm, we clearly show that the TI, Bi2Te3 under our investigation, is indeed a very-high-modulation-depth (up to 95%) saturable absorber. Furthermore, a TI based saturable absorber device was fabricated and used as a passive mode locker for ultrafast pulse formation at the telecommunication band. This contribution unambiguously shows that apart from its fantastic electronic property, a TI (Bi2Te3) may also possess attractive optoelectronic property for ultrafast photonics.

556 citations


Journal ArticleDOI
TL;DR: In this article, the performance and environmental effects on back-gated bi-layer MoS2 field effect transistors were investigated and it was shown that vacuum annealing can effectively remove the absorbates and reversibly recover the device performances.
Abstract: Two-dimensional transition-metal dichalcogenides such as MoS2 are promising channel materials for transistor scaling. Here, we report the performance and environmental effects on back-gated bi-layer MoS2field-effect transistors. The devices exhibit Ohmic contacts with titanium at room temperature, on/off ratio higher than 107, and current saturation. Furthermore, we show that the devices are sensitive to oxygen and water in the ambient. Exposure to ambient dramatically reduces the on-state current by up to 2 orders of magnitude likely due to additional scattering centers from chemisorption on the defect sites of MoS2. We demonstrate that vacuum annealing can effectively remove the absorbates and reversibly recover the device performances. This method significantly reduces the large variations in MoS2 device caused by extrinsic factors.

545 citations


Journal ArticleDOI
TL;DR: In this paper, the authors show that perfect absorption can be achieved in a system comprising a single lossy dielectric layer of thickness much smaller than the incident wavelength on an opaque substrate by utilizing the nontrivial phase shifts at interfaces between lossy media.
Abstract: We show that perfect absorption can be achieved in a system comprising a single lossy dielectric layer of thickness much smaller than the incident wavelength on an opaque substrate by utilizing the nontrivial phase shifts at interfaces between lossy media. This design is implemented with an ultra-thin (∼λ/65) vanadium dioxide (VO2) layer on sapphire, temperature tuned in the vicinity of the VO2 insulator-to-metal phase transition, leading to 99.75% absorption at λ = 11.6 μm. The structural simplicity and large tuning range (from ∼80% to 0.25% in reflectivity) are promising for thermal emitters, modulators, and bolometers.

536 citations


Journal ArticleDOI
TL;DR: In this paper, a flat optical device that generates optical vortices with a variety of topological charges is demonstrated, which spatially modulates light beams over a distance much smaller than the wavelength in the direction of propagation by means of an array of V-shaped plasmonic antennas with subwavelength separation.
Abstract: A flat optical device that generates optical vortices with a variety of topological charges is demonstrated. This device spatially modulates light beams over a distance much smaller than the wavelength in the direction of propagation by means of an array of V-shaped plasmonic antennas with sub-wavelength separation. Optical vortices are shown to develop after a sub-wavelength propagation distance from the array, a feature that has major potential implications for integrated optics.

500 citations


Journal ArticleDOI
TL;DR: In this paper, surface roughness-augmented wettability on critical heat flux (CHF) during pool boiling with horizontally oriented surfaces was investigated, and an analytical force-balance model was extended to explain the CHF enhancement.
Abstract: We experimentally investigated surface roughness-augmented wettability on critical heat flux (CHF) during pool boiling with horizontally oriented surfaces. Microstructured surfaces with a wide range of well-defined surface roughness were fabricated, and a maximum CHF of ∼208 W/cm2 was achieved with a surface roughness of ∼6. An analytical force-balance model was extended to explain the CHF enhancement. The excellent agreement found between the model and experimental data supports the idea that roughness-amplified capillary forces are responsible for the CHF enhancement on structured surfaces. The insights gained from this work suggest design guidelines for new surface technologies with high heat removal capability.

477 citations


Journal ArticleDOI
TL;DR: In this paper, the authors report the fabrication of back-gated field effect transistors (FETs) using ultra-thin, mechanically exfoliated MoSe2 flakes.
Abstract: We report the fabrication of back-gated field-effect transistors (FETs) using ultra-thin, mechanically exfoliated MoSe2 flakes. The MoSe2 FETs are n-type and possess a high gate modulation, with On/Off ratios larger than 106. The devices show asymmetric characteristics upon swapping the source and drain, a finding explained by the presence of Schottky barriers at the metal contact/MoSe2 interface. Using four-point, back-gated devices, we measure the intrinsic conductivity and mobility of MoSe2 as a function of gate bias, and temperature. Samples with a room temperature mobility of ∼ 50 cm2/V·s show a strong temperature dependence, suggesting phonons are a dominant scattering mechanism.

470 citations


Journal ArticleDOI
TL;DR: In this article, the authors show that metamaterials with hyperbolic dispersion support a large number of electromagnetic states that can couple to quantum emitters leading to a broadband Purcell effect.
Abstract: We show that metamaterials with hyperbolic dispersion support a large number of electromagnetic states that can couple to quantum emitters leading to a broadband Purcell effect. The proposed approach of radiative decay engineering, useful for applications such as single photon sources, fluorescence imaging, biosensing, and single molecule detection, also opens up the possibility of using hyperbolic metamaterials to probe the spontaneous emission properties of atoms and artificial atoms such as quantum dots.

442 citations


Journal ArticleDOI
TL;DR: In this paper, the authors employ both theory and experiment to assess the PV relevant properties of SnS and clarify on whether SnS has an indirect or direct band gap and what is the minority carrier effective mass as a function of the film orientation.
Abstract: SnS is a potential earth-abundant photovoltaic (PV) material. Employing both theory and experiment to assess the PV relevant properties of SnS, we clarify on whether SnS has an indirect or direct band gap and what is the minority carrier effective mass as a function of the film orientation. SnS has a 1.07 eV indirect band gap with an effective absorption onset located 0.4 eV higher. The effective mass of minority carrier ranges from 0.5 m0 perpendicular to the van der Waals layers to 0.2 m0 into the van der Waals layers. The positive characteristics of SnS feature a desirable p-type carrier concentration due to the easy formation of acceptor-like intrinsic Sn vacancy defects. Potentially detrimental deep levels due to SnS antisite or S vacancy defects can be suppressed by suitable adjustment of the growth condition towards S-rich.

Journal ArticleDOI
TL;DR: In this article, the authors demonstrate the design, characterization, and interference-theory interpretation of a terahertz triple-band metamaterial absorber (MA) with three distinctive absorption peaks at 0.5, 1.03, and 1.71 THz with absorption rates of 96.4, 96.3, and 96.7%, respectively.
Abstract: We demonstrate the design, characterization, and interference-theory interpretation of a terahertz triple-band metamaterial absorber (MA). The experiments show that the fabricated MA has three distinctive absorption peaks at 0.5, 1.03, and 1.71 THz with absorption rates of 96.4%, 96.3%, and 96.7%, respectively. We use the multi-reflection interference theory to investigate the physical insight of the proposed triple-band terahertz MA, which provides a design guideline for MA of such type. The theoretical predictions of the interference model have excellent agreements with experimental results. The designed multiband absorber is easy to manufacture and insensitive to incident polarizations with high absorption, which is favorable for various applications.

Journal ArticleDOI
TL;DR: In this article, the authors demonstrate that elastocaloric cooling (EC), a type of solid-state cooling mechanism based on the latent heat of reversible martensitic transformation, can have the coefficient of performance as high as ≈11, with a directly measured ΔT of 17 C.
Abstract: Vapor compression (VC) is by far the most dominant technology for meeting all cooling and refrigeration needs around the world. It is a mature technology with the efficiency of modern compressors approaching the theoretical limit, but its environmental footprint remains a global problem. VC refrigerants such as hydrochloroflurocarbons (HCFCs) and hydrofluorocarbons (HFCs) are a significant source of green house gas emissions, and their global warming potential (GWP) is as high as 1000 times that of CO2 [Buildings Energy Data Book (Building Technologies Program, Department of Energy, 2009)]. There is an urgent need to develop an alternative high-efficiency cooling technology that is affordable and environmentally friendly [A. D. Little, Report For Office of Building Technology State and Community Programs, Department of Energy, 2001]. Here, we demonstrate that elastocaloric cooling (EC), a type of solid-state cooling mechanism based on the latent heat of reversible martensitic transformation, can have the coefficient of performance as high as ≈11, with a directly measured ΔT of 17 °C. The solid-state refrigerant of EC completely eliminates the use of any GWP refrigerants including HCFCs/HFCs.

Journal ArticleDOI
TL;DR: In this paper, a sputtered CoFeB/MgO-based magnetic tunnel junction with a perpendicular magnetic easy axis in a static external magnetic field is realized for a ∼180° magnetization reversal, where the bias voltage pulse duration is adjusted to a half period of the precession.
Abstract: The electric field-induced ∼180° magnetization reversal is realized for a sputtered CoFeB/MgO-based magnetic tunnel junction with perpendicular magnetic easy axis in a static external magnetic field. Application of bias voltage with nanoseconds duration results in a temporal change of magnetic easy axis in the free layer CoFeB to in-plane, which induces precessional motion of magnetization in the free layer. The magnetization reversal takes place when the bias voltage pulse duration is adjusted to a half period of the precession. We show that the back and forth magnetization reversal can be observed by using successive application of half-period voltage pulses.

Journal ArticleDOI
TL;DR: In this paper, the authors describe the fabrication and measurement of microwave coplanar waveguide resonators with internal quality factors above 107 at high microwave powers and over 106 at low powers, with the best low power results approaching 2×106.
Abstract: We describe the fabrication and measurement of microwave coplanar waveguide resonators with internal quality factors above 107 at high microwave powers and over 106 at low powers, with the best low power results approaching 2×106, corresponding to ∼1 photon in the resonator. These quality factors are achieved by controllably producing very smooth and clean interfaces between the resonators’ aluminum metallization and the underlying single crystal sapphire substrate. Additionally, we describe a method for analyzing the resonator microwave response, with which we can directly determine the internal quality factor and frequency of a resonator embedded in an imperfect measurement circuit.

Journal ArticleDOI
TL;DR: In this article, the pentamode theoretical ideal suggested by Milton and Cherkaev in 1995 can be approximated by a metamaterial with current state-of-the-art lithography.
Abstract: Conceptually, all conceivable three-dimensional mechanical materials can be built from pentamode materials. Pentamodes also enable to implement three-dimensional transformation elastodynamics—the analogue of transformation optics. However, pentamodes have not been realized experimentally. Here, we investigate inasmuch the pentamode theoretical ideal suggested by Milton and Cherkaev in 1995 can be approximated by a metamaterial with current state-of-the-art lithography. Using numerical calculations calibrated by our fabricated three-dimensional microstructures, we find that the figure of merit, i.e., the ratio of bulk modulus to shear modulus, can realistically be made as large as about 1000.

Journal ArticleDOI
TL;DR: In this paper, the authors report on synthesis and thermal properties of the electrically conductive thermal interface materials with the hybrid graphene-metal particle fillers and show that the thermal conductivity of resulting composites was increased by ∼500% in a temperature range from 300 to 400 k at a small graphene loading fraction of 5-vol.-%.
Abstract: The authors report on synthesis and thermal properties of the electrically conductive thermal interface materials with the hybrid graphene-metal particle fillers. The thermal conductivity of resulting composites was increased by ∼500% in a temperature range from 300 K to 400 K at a small graphene loading fraction of 5-vol.-%. The unusually strong enhancement of thermal properties was attributed to the high intrinsic thermal conductivity of graphene, strong graphene coupling to matrix materials, and the large range of the length-scale—from nanometers to micrometers—of the graphene and silver particle fillers. The obtained results are important for the thermal management of advanced electronics and optoelectronics.

Journal ArticleDOI
TL;DR: The proposed energy harvesters are robust to variation of heart rate and can meet the power requirement of pacemakers.
Abstract: Linear and nonlinear piezoelectric devices are introduced to continuously recharge the batteries of the pacemakers by converting the vibrations from the heartbeats to electrical energy. The power requirement of a pacemaker is very low. However, after few years, patients require another surgical operation just to replace their pacemaker battery. Linear low frequency and nonlinear mono-stable and bi-stable energy harvesters are designed according to the especial signature of heart vibrations. The proposed energy harvesters are robust to variation of heart rate and can meet the power requirement of pacemakers.

Journal ArticleDOI
TL;DR: In this paper, a terahertz (THz) frequency-reconfigurable antenna using graphene is presented, which exploits dipole-like plasmonic resonances that can be frequency-tuned on large range via the electric field effect in a graphene stack.
Abstract: The concept and analysis of a terahertz (THz) frequency-reconfigurable antenna using graphene are presented. The antenna exploits dipole-like plasmonic resonances that can be frequency-tuned on large range via the electric field effect in a graphene stack. In addition to efficient dynamic control, the proposed approach allows high miniaturization and good direct matching with continuous wave THz sources. A qualitative model is used to explain the excellent impedance stability under reconfiguration. These initial results are very promising for future all-graphene THz transceivers and sensors.

Journal ArticleDOI
TL;DR: In this paper, the growth of vertically standing zincblende InP nanowire arrays on InP (100) substrates in the vapor-liquid-solid growth mode using low-pressure metal-organic vapor-phase epitaxy was investigated by electron beam lithography.
Abstract: We investigate the growth of vertically standing [100] zincblende InP nanowire (NW) arrays on InP (100) substrates in the vapor-liquid-solid growth mode using low-pressure metal-organic vapor-phase epitaxy. Precise positioning of these NWs is demonstrated by electron beam lithography. The vertical NW yield can be controlled by different parameters. A maximum yield of 56% is obtained and the tapering caused by lateral growth can be prevented by in situ HCl etching. Scanning electron microscopy, high-resolution transmission electron microscopy, and micro-photoluminescence have been used to investigate the NW properties.

Journal ArticleDOI
TL;DR: In this paper, the authors developed the fluctuational electrodynamics of metamaterials with hyperbolic dispersion and showed the existence of broadband thermal emission beyond the black body limit in the near field.
Abstract: We develop the fluctuational electrodynamics of metamaterials with hyperbolic dispersion and show the existence of broadband thermal emission beyond the black body limit in the near field. This arises due to the thermal excitation of unique bulk metamaterial modes, which do not occur in conventional media. We consider a practical realization of the hyperbolic metamaterial and estimate that the effect will be observable using the characteristic dispersion (topological transitions) of the metamaterial states. Our work paves the way for engineering the near-field thermal emission using metamaterials.

Journal ArticleDOI
TL;DR: In this article, the coupling of surface plasmon polaritons (SPPs) between spatially separated graphene sheets was theoretically investigated, and it was employed to design zero insertion loss optical splitters, 1'×'2 digital optical spatial switches, and ultra-compact Mach-Zehnder interferometers with the arm length far below the diffraction limit.
Abstract: In this letter, we theoretically investigate the coupling of far-infrared surface plasmon polaritons (SPPs) between spatially separated graphene sheets By using the finite-difference frequency-domain method, we numerically illustrate the SPP propagation and modulation in the graphene sheets The coupling of SPPs is employed to design zero insertion loss optical splitters, 1 × 2 digital optical spatial switches, and ultra-compact Mach-Zehnder interferometers with the arm length far below the diffraction limit The study provides an effective way in designing graphene based high-speed and ultra-compact optoelectronic devices

Journal ArticleDOI
TL;DR: In this paper, a high-efficiency purely organic luminescent material, 2,4-bis{3-(H-carbazol-9-yl)-9, 9, carbazol 9-yl}-6-phenyl-1,3,5-triazine (CC2TA) comprising the bicarbazole donor and phenyltriazine acceptor units, which is capable of emitting thermally activated delayed fluorescence.
Abstract: We have designed and synthesized a high-efficiency purely organic luminescent material, 2,4-bis{3-(9 H-carbazol-9-yl)-9 H-carbazol-9-yl}-6-phenyl-1,3,5-triazine (CC2TA) comprising the bicarbazole donor and phenyltriazine acceptor units, which is capable of emitting thermally activated delayed fluorescence. The molecular design of CC2TA allows spatial separation of HOMO and LUMO on the donor and acceptor fragments, respectively, leading to an exceptionally small singlet–triplet exchange energy (ΔEST = 0.06 eV) together with a high triplet energy. Furthermore, a high external electroluminescence quantum efficiency as high as 11% ± 1% has been achieved in the sky-blue organic light-emitting diodes employing CC2TA as an emitter.

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the mechanism of ice adhesion on surfaces and found that the super-hydrophilic surface cannot reduce the ice-adhesion, and the strength of the superhydrophobic surface is almost the same.
Abstract: Understanding the mechanism of ice adhesion on surfaces is crucial for anti-icing surfaces, and it is not clear if superhydrophobic surfaces could reduce ice adhesion Here, we investigate ice adhesion on model surfaces with different wettabilities The results show that the superhydrophobic surface cannot reduce the ice adhesion, and the ice adhesion strength on the superhydrophilic surface and the superhydrophobic one is almost the same This can be rationalized by the mechanical interlocking between the ice and the surface texture Moreover, we find that the ice adhesion strength increases linearly with the area fraction of air in contact with liquid

Journal ArticleDOI
TL;DR: P pseudo-thermal light ghost imaging is extended to the area of remote imaging and a ghost imaging lidar system is proposed and the results demonstrate that the real-space image of a target at about 1.0 km range with 20 mm resolution is achieved by ghost imaging via sparsity constraints (GISC) technique.
Abstract: For remote sensing, high-resolution imaging techniques are helpful to catch more characteristic information of the target. We extend pseudo-thermal light ghost imaging to the area of remote imaging and propose a ghost imaging lidar system. The experimental results demonstrate that the real-space image of a target at about 1.0 km range with 20 mm resolution is achieved by ghost imaging via sparsity constraints (GISC) technique. The characters of GISC technique compared to the existing lidar systems are also discussed.

Journal ArticleDOI
TL;DR: In this paper, the authors showed that the control of an interfacial MoSe2 layer thickness and the introduction of an adequate Se partial pressure (PSe) during annealing are essential to achieve high efficiency CZTSe solar cells.
Abstract: We have examined Cu2ZnSnSe4 (CZTSe) solar cells prepared by thermal co-evaporation on Mo-coated glass substrates followed by post-deposition annealing under Se ambient. We show that the control of an interfacial MoSe2 layer thickness and the introduction of an adequate Se partial pressure (PSe) during annealing are essential to achieve high efficiency CZTSe solar cells—a reverse correlation between device performance and MoSe2 thickness is observed, and insufficient PSe leads to the formation of defects within the bandgap as revealed by photoluminescence measurements. Using a TiN diffusion barrier, we demonstrate 8.9% efficiency CZTSe devices with a long lifetime of photo-generated carriers.

Journal ArticleDOI
TL;DR: In this article, the design of an optomechanical crystal nanobeam cavity that combines finite-element simulation with numerical optimization is presented, and considers the optomchanical coupling arising from both moving dielectric boundaries and the photo-elastic effect.
Abstract: We present the design of an optomechanical crystal nanobeam cavity that combines finite-element simulation with numerical optimization, and considers the optomechanical coupling arising from both moving dielectric boundaries and the photo-elastic effect. Applying this methodology results in a nanobeam with an experimentally realized intrinsic optical Q-factor of 1.2×10^6, a mechanical frequency of 5.1 GHz, a mechanical Q-factor of 6.8×10^5 (at T = 10 K), and a zero-point-motion optomechanical coupling rate of g = 1.1 MHz.

Journal ArticleDOI
TL;DR: In this article, the authors investigated a recording structure consisting of two CoFeB-MgO interfaces, MgO/CoFeB (1.6 nm)/Ta (0.4 n) with a recording size of 70 nm.
Abstract: We investigated perpendicular CoFeB-MgO magnetic tunnel junctions (MTJs) with a recording structure consisting of two CoFeB-MgO interfaces, MgO/CoFeB (1.6 nm)/Ta (0.4 nm)/CoFeB (1.0 nm)/MgO. Thermal stability factor of MTJ with the structure having junction size of 70 nmφ was increased by a factor of 1.9 from the highest value of perpendicular MTJs with single CoFeB-MgO interface having the same device structure. On the other hand, intrinsic critical current for spin transfer torque switching of the double- and single-interface MTJs was comparable.

Journal ArticleDOI
TL;DR: In this article, it was shown that barium titanate glass microspheres with diameters in the range 2-220μm and with high refractive index (n∼∆ 1.9-2.1) can be used for super-resolution imaging of liquid-immersed nanostructures.
Abstract: It is experimentally shown that barium titanate glass microspheres with diameters (D) in the range 2–220 μm and with high refractive index (n ∼ 1.9–2.1) can be used for super-resolution imaging of liquid-immersed nanostructures. Using micron-scale microspheres, we demonstrate an ability to discern the shape of a pattern with a minimum feature size of ∼λ/7, where λ is the illumination wavelength. For spheres with D > 50 μm, the discernible feature sizes were found to increase to ∼λ/4. Detailed data on the resolution, magnification, and field-of-view are presented. This imaging technique can be used in biomedical microscopy, microfluidics, and nanophotonics applications.

Journal ArticleDOI
TL;DR: In this paper, a charge tunneling model was used to explain the piezoresistive characteristics of nanographene films, which indicates their results provide a different rout toward ultra-sensitive strain sensors.
Abstract: Graphene shows promise on strain sensor applications, but the piezoresistive sensitivity of perfect graphene is low due to its weak electrical conductivity response upon structural deformation. In this paper, we used nanographene films for ultra-sensitive strain sensors. The piezoresistive sensitivity of nanographene films with different thicknesses and conductivities was systematically investigated and a nearly inverse proportional correlation was found. A gauge factor over 300, the highest so far for graphene-based strain sensors, was achieved. A charge tunneling model was used to explain the piezoresistive characteristics of nanographene films, which indicates our results provide a different rout toward ultra-sensitive strain sensors.