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IOP PUBLISHING ADVANCES IN NATURAL SCIENCES: NANOSCIENCE AND NANOTECHNOLOGY

Adv. Nat. Sci.: Nanosci. Nanotechnol. 1 (2010) 045012 (8pp) doi:10.1088/2043-6262/1/4/045012

Towards highly sensitive strain sensingbased on nanostructured materialsDzung Viet Dao1, Tung Thanh Bui2, Koichi Nakamura1,Van Thanh Dau2,4, Takeo Yamada3, Kenji Hata3 and Susumu Sugiyama1

1 Research Institute for Nanomachine System Technology, Ritsumeikan University, 1-1-1 NojiHigashi,Kusatsu, Shiga, 525-8577, Japan2 Graduate School of Science and Engineering, Ritsumeikan University, 1-1-1 NojiHigashi, Kusatsu,Shiga, 525-8577, Japan3 Nanotube Research Center, National Institute of Advanced Industrial Science and Technology (AIST),1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan

E-mail: dzung@se.ritsumei.ac.jp

Received 30 October 2010Accepted for publication 17 December 2010Published 21 January 2011Online at stacks.iop.org/ANSN/1/045012

AbstractThis paper presents our recent theoretical and experimental study of piezo-effects innanostructured materials for highly sensitive, high resolution mechanical sensors. Thepiezo-effects presented here include the piezoresistive effect in a silicon nanowire (SiNW)and single wall carbon nanotube (SWCNT) thin film, as well as the piezo-optic effect in a Siphotonic crystal (PhC) nanocavity. Firstly, the electronic energy band structure of the siliconnanostructure is discussed and simulated by using the First-Principles Calculations method.The result showed a remarkably different energy band structure compared with that of bulksilicon. This difference in the electronic state will result in different physical, chemical, andtherefore, sensing properties of silicon nanostructures. The piezoresistive effects of SiNW andSWCNT thin film were investigated experimentally. We found that, when the width of 〈110〉

p-type SiNW decreases from 500 to 35 nm, the piezoresistive effect increases by more than60%. The longitudinal piezoresistive coefficient of SWCNT thin film was measured to betwice that of bulk p-type silicon. Finally, theoretical investigations of the piezo-optic effectin a PhC nanocavity based on Finite Difference Time Domain (FDTD) showed extremelyhigh resolution strain sensing. These nanostructures were fabricated based on top-downnanofabrication technology. The achievements of this work are significant for highly sensitive,high resolution and miniaturized mechanical sensors.

Keywords: silicon nanowire, CNT, photonic crystal, piezoresistive effect, piezo-optic effect

Classification numbers: 4.00, 4.08, 5.14, 6.08

1. Introduction

Recently, nanostructured materials have attracted a lot ofattention in Nano ElectroMechanical Systems (NEMS)technology because of their excellent properties for highlysensitive and high resolution physical and chemical sensingapplications [1–3]. Nanostructured materials, such asatomic clusters, nano dot, nano layered films, filamentarystructures and bulk nanostructured materials, are referred to

4 Present address: Research Group (Environmental Health), AgriculturalChemicals Research Laboratory, Sumitomo Chemical Co. Ltd, 2-1Takatsukasa 4 chome, Takarazuka, Hyogo 665-8555, Japan.

those materials with structural elements having nanoscaledimensions, i.e. at least one dimension in the range from1 to 100 nm. In such low-dimensional nanostructures, dueto the quantum confinement effect, the eclectic, magnetic,optical properties of materials were reported to be remarkablydifferent from those of bulk materials, and accordingly,interesting physical and chemical sensing effects innanostructures are also expected. For example, giantpiezoresistive effects were found experimentally in SiNW [1]and SWCNT [2], an extremely high sensitive gas sensingeffect in graphene nanosheet [3] and single-electron resolutionelectric-field sensing based on graphene nanosheets [4]. Such

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Adv. Nat. Sci.: Nanosci. Nanotechnol. 1 (2010) 045012 D V Dao et al

0.2 0.20.10.1 Γ[100],[010] (2π/a0)[110],[1-10]

(2π/a0)

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Figure 1. Band diagrams of the three lowest CB subbands and three highest VB subbands for the hydrogen-terminated Si(001) nanosheetmodels around the 0 point. The energy zero is defined at the valence-band top.

excellent properties are attributed to the facts that whenthe dimensions of materials reduce to the low-dimensionalnanostructure, such as nanodot (0D), nanowire (1D) andnanosheet (2D), most atoms are exposed and the quantumconfinement effect of the carriers plays a major rolein determining their physical properties. Based on thefirst-principles calculations method, we found that low-dimensional structures such as silicon nanosheets (2D) andnanowires (1D) have very different electronic states comparedto microscopic and macroscopic structures [5]. Therefore, thephysical and chemical properties of such nanoscale materialsshould be very different from the macroscopic propertiesof the same substance, offering superior properties for theemerging advanced applications.

In this paper, first, the analysis of the electronic state of Sinanostructure is introduced to illustrate the different electronicstate compared to that of bulk silicon. Next, fabrication andevaluation of piezoresistive effects of SiNW and SWCNTthin films are presented. Finally, a theoretical study of thepiezo-optic effect in a PhC nanocavity is discussed.

2. Electronic state in silicon nanostructures

Electrical characteristics of Si materials are dominated by theelectronic state. In particular, the electronic structure of thevalence band (VB) of Si materials is related to the electricalproperties of p-doped Si semiconductors.

Single-crystal bulk Si has an indirect band gap as is wellknown, where the conduction band (CB) has a multi-valleystructure, and the maximum of the VB top is located at the 0

point. The three highest VB subbands are triply degenerated atthe 0 point by means of non-relativistic regular first-principlecalculation, though one of these subbands should be separatedfrom the other two, the heavy-hole and light-hole bands, in theactual system because of the relativistic spin–orbit coupling.Our novel approach to simulate the piezoresistive effect hassucceeded in a qualitative and quantitative estimation of theprimitive piezoresistive coefficients, π11, π12 and π44, forn-type bulk Si [6], derived from the multi-valley CB structure.

The dimensional reduction to a Si(001) nanosheet bringsa quasi-direct band gap, as shown in figure 1, where theminimum of the CB bottom is located at the 0ïpointdue to the two-dimensional confinement of the electronicstructure [7, 8]. In the VB top of the Si(001) nanosheet

models, the two highest VB subbands are doubly degeneratedat the 0 point by means of regular first-principle calculation,and their characteristics can correspond to the heavy-hole andlight-hole bands with the spin–orbit coupling for bulk Si.However, a different electronic structure from bulk Si can beobtained as the sheet thickness is reduced. The [110] uniaxialtensile stress for the Si(001) nanosheet models causes banddeformation, leading to the redistribution of carriers and adrastic change in conductivity can be observed. The p-typelongitudinal and transverse piezoresistive coefficients aboutthe [110] tensile stress [7] and shear one about (001) shearstrain [8] swell up, respectively, as the nanosheet becomesthinner, and we have obtained high piezoresistive coefficientsof 343 × 10−11 Pa−1 of πl[110], −141 × 10−11 Pa−1 of πt[110]

and 450 × 10−11 Pa−1 of πs for about 1 nm thickness by asimulation on the basis of first-principle calculation.

Through further dimensional reduction to a Si〈001〉

nanowire of a few nanometers radius, the electronic structurehas been completely changed from that of bulk Si. Theuniaxial tensile stress in the longitudinal direction causes anexchange of the order in the band energy of subbands in theVB top, and a sudden change in the hole occupation withthe increase in effective mass bringing a drastic decrease inthe hole conductivity. About 600 × 10−11 Pa−1 of πl〈001〉 isexpected for p-type Si〈001〉 nanowire with a 2 nm diameterby our simulation [9].

3. Piezoresistive effect in silicon nanowire

The piezoresistive effect has been one of the most frequentlyapplied principles in (MEMS) micro electro mechanicalsystems for developing micro mechanical sensors over thepast three decades. For recently emerging applications, suchas camera image stabilization, mobile phone, hard diskprotection, health monitoring and so on, the sensors arerequired to have smaller size, higher sensitivity and lowerpower consumption. However, when the physical size of aninertial sensor decreases, its sensitivity is also drasticallyreduced by the fourth power rule. One of the solutions toovercome this problem is to realize a higher piezoresistiveeffect in SiNW, as briefly discussed in section 2 above andin [9].

In order to evaluate the piezoresistive effect in singlecrystalline SiNW, we used a cantilever model with SiNW

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Adv. Nat. Sci.: Nanosci. Nanotechnol. 1 (2010) 045012 D V Dao et al

Si Nanowires

Force

(100)

Si cantilever

Figure 2. Cantilever model for measurement of the piezoresistiveeffect in SiNW.

1. Ion implantation 2. SiO2 deposition and annealing

3. SiO2 mask patterning

4. SiNW formation 5. Thermal oxidation 6. SiO2 deposition

7. Contact hole opening 8. Interconnection

Si SiO2

AlTiN

SiO2deposition

SiO2thermal

oxidation

Figure 3. Schematic model for measurement of the piezoresistiveeffect in SiNW.

arrays arranged on top, as shown in figure 2. Each arrayconsists of identical SiNWs connected in parallel, and alignedwith longitudinal or transverse axes of the cantilever, as shownschematically in figure 2. To investigate the piezoresistiveeffect in different crystallographic orientations, the SiNWarrays are aligned along different orientations, e.g. 〈100〉,〈010〉 and 〈110〉 directions on the (001) single crystallinesilicon SOI wafer. The width of SiNW in different arrays isdifferent, and it ranges from 35 to 500 nm.

By loading at the free end of the cantilever, thestress/strain will be generated in the SiNW and, therefore,the resistance of the SiNW is changed. Measurement ofthis resistance change and applied stress can provide thepiezoresistive coefficients.

3.1. Fabrication of SiNWs

SiNWs are fabricated by the top-down process described infigure 3 [9]. SIMOX (Separation by Implanted Oxygen) waferwith a 50-nm-thick (100) Si device layer was used. First,boron was implanted uniformly into the 50-nm-thick Si devicelayer to obtain a concentration of 1.2 × 1018 atoms cm−3.Then, a 150-nm-thick SiO2 layer was deposited and annealedto form a hard mask for RIE etching. Next, SiNW patternswere defined by electron beam lithography (EBL) anddilute hydrofluoric (DHF) etching of SiO2. Then, the SiNWstructures were formed by RIE of Si. A 10-nm-thickSiO2 layer was thermally grown to deactivate the sidewalllayers, which were attacked during the RIE process. Next,a 100-nm-thick SiO2 layer was deposited to protect the

SiNWSiO2

500nmContact Pad

Metal ElectrodeSiNW

Contact Hole

(a)(b)

Figure 4. (a) SEM image of SiNW array and (b) FIB cutting imageshowing SiNWs embedded in SiO2 layer.

500nm

35nm

(c)

(d)

500nm(a)

(b)

70nm

Figure 5. SiNW array without SiO2 layer: (a) 〈100〉 SiNW array,(b) close-up view of (a), (c) 〈110〉 SiNW array (d) close-up viewof (c).

SiNW. Then, contact holes were opened by HF wet etching.Finally, 50 nm- TiN/500 nm- Al were deposited, patternedand annealed to form a metal contact. Annealing wasperformed at a temperature of 550 ◦C in N2 for 30 min.

SiNW arrays with a length and thickness of 2 µm and40 nm, respectively, and width ranges from 35–490 nm werefabricated. The resistivity of the SiNW was 0.035 � cm.Figure 4 shows SEM images of a fabricated SiNW array.Figure 4(a) shows a SiNW array covered by a SiO2 layer,and figure 4(b) is a cut-away image showing SiNWsembedded completely in SiO2. Figure 5 shows SEM images ofthe 〈100〉 and 〈110〉 SiNWs after the SiO2 layer was removedto observe the actual size of the nanowires.

3.2. Measurement of piezoresistive coefficients

Identical force is applied on the tip of the cantilever togenerate stress/strain in the SiNWs, as shown in figure 2.The resistance of the SiNWs is measured by using afour-probe resistance measurement technique to eliminate thecontact resistance between the SiNWs and Al interconnectionwires. Figure 6 shows the resistance change in the SiNWsversus applied strain at room temperature. A linear relationbetween the resistance change and the applied strain wasobtained. The gauge factor for the 35-nm-width SiNWs wasdetermined to be G = 91, i.e. 1.75 times larger than that of the490-nm-width SiNWs. Figures 7 and 8 show the dependenceof the longitudinal and transverse piezoresistive coefficientsfor the 〈110〉 and 〈100〉 directions, respectively, on the widthof the SiNWs.

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Adv. Nat. Sci.: Nanosci. Nanotechnol. 1 (2010) 045012 D V Dao et al

Figure 6. Resistance change in different SiNWs versus strain.

Figure 7. With dependence of piezoresistive coefficients in 〈110〉

crystallographic direction. Note: (∗) concentration: 1.2 × 1018 cm−3,(∗∗) concentration of 5 × 1019 cm−3.

The results show a very large increase in the longitudinalpiezoresistive coefficient when the width is smaller than150 nm. The shear piezoresistive coefficients π44 of the 35 nmSiNW was found to be 260 × 10−5 [MPa−1], i.e. around 88%larger than the value reported by Toriyama et al [10], and100% larger than the value reported by Tufte and Steltzer [11].The shear piezoresistive coefficient of the 490 nm SiNWs is164 × 10−5 [MPa−1], which is slightly higher than the valuesreported in [10, 11] for the same impurity concentration of1.2 × 1018 atoms cm−3 (figure 9).

However, there is still a big difference between themeasured and theoretical values due to the difference in thenanowire’s width in the calculation (several nanometers) andin experiment (several tens of nanometers). Therefore, furthertheoretical and experimental studies of the piezoresistiveeffect in SiNW are necessary to eliminate this difference.

4. Piezoresistive effect of CNT thin film deposited ona MEMS structure

Research on carbon nanotubes (CNTs) for sensing applicationis a very promising direction for nanoscale devices due

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Figure 9. Shear piezoresistive coefficient π44.

to CNTs’ excellent electrical and mechanical properties. Inaddition to the characteristics of either a metallic conductoror a semiconductor, which allow CNTs to become promisingcomponents of integrated circuits to continue shrink thetransistor size, CNTs possess a tensile strength larger thanany other known materials [12]. CNTs have been studiedfor various applications, such as tunable electrometricaloscillators, chemical and physical sensors, non-volatilememory and actuators.

The integration of CNTs into a MEMS structure usuallyrelies either on bottom-up (in-situ growth) or top-down(post-growth) methods. The bottom-up technique utilizescatalytic particles directly patterned on a substrate to controlthe position of CNTs, while the top-down method focuseson manipulation of the growth CNTs to specific positions.Although bottom-up methods have particular advantages inthe area of field-effect devices, the poor compatibility withMEMS technology is the main obstacle for MEMS integrationand functional devices. In order to be effectively applied to

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Adv. Nat. Sci.: Nanosci. Nanotechnol. 1 (2010) 045012 D V Dao et al

Figure 10. Integration process of CNT film onto a microstructure. Super-growth synthesized CNT ((a), [16]).

MEMS devices, CNTs should have well-controlled propertiesand orientation, and be easily integrated into the system.An alternative and probably more realistic approach is toassemble a large CNT film at desired locations and patternit by conventional top-down lithography. A recent approachwas reported by Hayamizu et al [13]. This highly efficientprocess is known as ‘super growth’. The ‘super growth’ CNTshave high purity and millimeter-scale length. Therefore,the top-down technique can be used to integrate CNTsonto microstructures. This process allows complex CNTcomponents for integrated devices and, therefore, potentiallyopens a way for low-cost CNT-based MEMS devices.

One of the prominent sensing properties of CNTs isthe piezoresistive effect. This effect was investigated for thefirst time by Tombler et al [14]. A gauge factor (the CNTs’sensitivity to strain) of 1000, i.e. five times larger than thatof single crystal silicon, was reported. Other measurementof Cao et al [15] showed a gauge factor of up to 3000for pre-strained individual SWNTs. This paper reports onthe integration of CNT film into a MEMS structure and thecharacterization of piezoresistive coefficients of integratedCNT elements. The test specimen was made of alignedsingle wall carbon nanotube (SWCNT) forest films, which arepossible for top-down process.

4.1. Integration of CNT film on a microstructure

CNTs have been synthesized by water-assisted chemicalvapor deposition (CVD), a process known as ‘supergrowth’ [16]. Super growth CNTs are catalyst-free SWCNTswith a diameter of 2.8 nm, carbon purity higher than99.9%, vertically aligned, a length in millimeter-scale, andoccupying 3–4% of the total volume of this forest film.Self-assembled SWNTs were vertical aligned in the formof sparse forest films and then zipped into high densitySWNT films by the liquid induced zippering effect. Afterpositioning the film onto the prefabricated silicon device,lithography was performed to pattern the film into the desiredshapes. The integration of CNT film into microstructures

is shown in figure 10. 500 × 500 × 4 µm3 (L × W × T)CNT films were first synthesized by water-assisted CVD(figure 10(a)). 0.3 µm Cr/Au electrodes were created bythe lift-off process on a silicon wafer with a passivationlayer (Si3N4) on top (figures 10(c) and (d)). Thanks to thelarge size of the forest film, the CNT film was manuallyplaced on the electrodes within isopropyl alcohol (IPA)solution and dried naturally. The surface tension of thesolution and strong van der Waals interaction accordinglycompressed the CNT film into a 0.3 µm thick layer andformed a strong contact with the electrodes (figure 10(e)).Once densified, the SWNT film could be considered as acontinuous material layer. A hydrogen silsesquioxane (HSQ)resist was spin-coated and baked at 90 ◦C for 10 min. HSQwas then patterned by EB lithography and developed bytetramethylammoniumhydroxide (TMAH) solution (2.38%,ZTMA—100, Zeon) (figure 10(f)). Then, the CNT film wasetched by reactive ion etching (RIE) (RIE-200L, Samco)with oxygen plasma and argon to define the designed shape(figure 10(g)). The HSQ mask was finally removed bybuffered hydrofluoric acid (figure 10(h)). Figure 11 showsSEM images of the CNT thin film elements as straingauges [17].

To measure the piezoresistive effect, CNT elements werepatterned onto a silicon bar with dimensions of 50 × 5 ×

0.5 mm3, and the bar was bent by the four-point bendingmethod. The 40 × 5 × 0.3 µm3 CNT elements were alignedin either the longitudinal or the transverse direction of thebar and connected by Cr/Au interconnection to facilitatefour-point probe measurement, which eliminates unexpectedcontact resistance. The resistance of the CNT elements wasmeasured to be 3.04 k� and 124.14 k�, corresponding tothe parallel and perpendicular CNT directions, respectively.The I–V characteristic was linear and repeatable during themeasurement. Figure 12 shows the relation between therelative change of resistance 1R/R versus strain applied tothe CNT film [17]. The longitudinal and transverse gaugefactors, measured when the CNT was aligned with thelongitudinal stress direction, were GL = 6.24 and GT = 0.67.

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Adv. Nat. Sci.: Nanosci. Nanotechnol. 1 (2010) 045012 D V Dao et al

CCNT

Electrode CNT

Au

Figure 11. SEM image of integrated CNT elements.

0

25

50

0 5 10 15

Strain (x10-4)

∆R

/R (

x10-4

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Longitudinal

Transversal

Figure 12. Relative change of resistance versus strain.

With Young’s modulus of CNT film 9.7 GPa, as reportedin previous work [13], the piezoresistive coefficients werecalculated to be πL = 81.9 × 10−5 MPa−1 and πT = 7.37 ×

10−5 MPa−1, respectively.Although the piezoresistive coefficient is two times that

of bulk silicon, the current testing showed that SWCNTfilm did not possess giant effects as reported for individualSWCNTs. The main reason could be that the CNT bundlewas embedded in the film. The bundles change theirresistances during the bending test by contact resistance andby elongating CNT bridges, and also due to tunneling contactbetween the nanotubes. Thus, a higher piezoresistive effectwould be expected with smaller, well aligned CNT film.

5. Piezo-optical effect in a photonic crystalnano cavity

A photonic crystal (PhC) is a periodic nanostructure ofmaterials with a high contrast refractive index such as siliconand air. PhCs possess a variety of band dispersions andband gaps, where the propagation of light is prohibited forcertain ranges of wavelength. By choosing different materialsand geometrical parameters, the propagation of light can bemodified in many ways and, therefore, Si PhC-based deviceshave been widely used in light control applications, such aswaveguides, photonic band gap structures and resonators [18].Moreover, because of their sensitivity to small refractive indexchanges, PhC structures have been studied for biochemical

Exciter Detector

r a

t

Figure 13. Schematic of a 2D PhC nanocavity: a = 420 nm,r = 180 nm, t = 220 nm, r/a = 0.38.

sensing by detecting the change in refractive index due tomolecule immobilization in PhC [19, 20]. Recently, a studyof the mechanical sensing effect of PhCs has attracted alot of attention due to its high-resolution strain sensingpossibilities [21–23].

The piezo-optic effect can be defined as the change inoptical properties of the PhC structure upon application of amechanical stress/strain. In order to evaluate the piezo-opticeffect, we will investigate the shift in resonant wavelengthof the PhC nanocavity upon application of mechanicalstress/strain. The detection of the shift in wavelength ischosen due to the lower sensitivity to background noise, whichusually affects the signal level. The PhC nanocavity used inthis study is created by missing a circle of air holes of atriangular lattice PhC, as shown in figure 13. The hole radius,r, is 0.38a (where a = 420 nm is the lattice constant).

The change in PhC geometry upon mechanicalstress/strain is analyzed by using the structural finite elementmethod (FEM), such as ANSYS software. Figures 14(a) and(c) depict the simulated deformation and relocation of anairhole lattice under longitudinal and transverse strains.

Next, the geometric change is then input into a FiniteDifference Time Domain (FDTD) simulation to simulatethe light transmission spectrum of the PhC nanocavity. Theeffective refractive index of air/220 nm-Si/air is calculatedto be 2.81. The analysis performed in this work used TEpolarization light (i.e. the electric field component parallel tothe PhC plane).

Both tensile and compressive strains have been appliedand investigated in this study. Figure 15 shows thetransmission spectra of the PhC cavity correspondingto different applied strains. Figure 16 shows the linear

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Adv. Nat. Sci.: Nanosci. Nanotechnol. 1 (2010) 045012 D V Dao et al

Figure 14. Deformation and relocation of an airhole lattice of PhCstructure under applied strain.

Figure 15. Transmission spectra show resonant wavelength underdifferent strains.

relations of resonant wavelength shift versus longitudinal andtransverse strains. The wavelength shift was 1.9 nm mε−1 forlongitudinal strains, about 2 times larger than that reported byLee and Thillaigovindan [22].

With the minimum detectable wavelength of thepresent commercial optical spectrum analyzer (e.g.Agilent-83453B-HRS) being 0.008 pm, we can obtainthe minimum detectable strain of 5.3 × 10−9, i.e. the smallestdetectable strain known to the authors.

6. Conclusion

In this paper, we have presented a theoretical analysisof the electronic state of silicon nanostructures based onfirst-principles calculation. The results show very differentenergy band structures, i.e. direct band gap compared toindirect band gap of the bulk silicon, due to the quantumconfinement effect. These differences affected the electricaland optical properties and, therefore, would influence thepiezoresistive effects in nanostructures. This assumption wasthen experimentally verified for the case of SiNW, and we

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Figure 16. Resonate wavelength shift versus strain.

found that the smaller the SiNW, the higher the piezoresistivecoefficient. However, the measurement values were stillmuch smaller than the simulation values. This is due to thedifference in width of the SiNW between the simulationand measurement model. Therefore, further experimentalinvestigation for smaller SiNWs, e.g. below 20 nm width, isnecessary. We also reported the integration and patterningmethods of SWCNT film on a MEMS structure, as well asmeasurement of the piezoresistive effect of the SWCNT film.The longitudinal piezoresistive coefficient of the SWCNTfilm was two times higher than that of the bulk silicon.This is promising for CNT-based highly sensitive mechanicalsensors. Finally, theoretical analysis of the piezo-opticaleffect in a PhC nanocavity showed extremely high resolutionstrain sensing, i.e. the smallest detectable strain might bedown to nano strain. These achievements, including thenanofabrication of typical nanostructures and evaluation oftheir piezo-effects, would be significant in developing highlysensitive and high resolution mechanical sensing devicesbased on nanostructured materials.

Acknowledgments

This study was partly supported by the Ministry of Education,Culture, Sports, Science and Technology of Japan under theGrant-in-Aid for Young Scientists (No. 21710141-0001).

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