NEMS Device for Determination of Shear Stress between ... d… · 3) Contact width: 50% of the tube...

Determination of Shear Stress Between Single-walled Carbon

Nanotubes and Substrates Using NEMS Devices

Student:Huiyan Pan

Advisor: Prof. McGruer Co-advisor: Prof. Adams

Electrical and Computer Engineering

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Outline • Research background Introduction of carbon nanotubes (CNTs) Motivation of this work Shear stress measured by AFM in the literature

• Technique development to study the shear stress between CNTs and substrates Principle of the technique NEMS cantilever based technique Structure design Microfabrication Nanomanipulation tests Testing results Discussion

•Conclusions

2

Graphene

Background

Technique development

Conclusions

What are carbon nanotubes?

Single-walled carbon nanotube

(SWNT)

Multi-walled carbon

nanotube (MWNT) [1]

3 [1] A. R. Hall et al. Reports on Progress in Physics,2012.

Background


Conclusions

Why are the interactions/shear stress between CNTs and substrates so

important?

4

1) Electrophoretic assembly [2] 2) Dielectrophoretic assembly [3] 3) Fluidic assembly [4]

The effect of CNT-substrate interactions on CNT assembly processes

A SWNT film got peeled off from the substrate by nitrogen drying after fluidic assembly [5]

In the fabrication of any CNT-based devices, the mechanical interactions

between tubes and substrates have to be strong enough for the devices to survive the subsequent processes

5 Background


Conclusions

[2] A.R. Boccaccini et al, Carbon ,2006. [3] ] M. R. Islam, et al, Chemical Society Reviews, 2012. [4] S. Somu et al, ACS Nano, 2010. [5] A.R. Boccaccini et al, Carbon ,2006.

The effect of CNT-substrate interactions on CNT-based device performance

CNTs are attached to gold electrodes as probes for a nanotweezer [6]

CNTs contact with electrode during actuation for nanoswitches [7,8]

Background


Conclusions

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[6] P. Kim et al, Science, 2010. [7] S. W. Lee et al, Nano Lett, 2004. [8] C. Ke et al, Small, 2006.

A finite length of SWNT bundle slip into the trench region under electrostatic force during actuation.

AFM of device before testing

AFM of device after testing

The effect of shear stress at CNT-substrate interface on the actuation voltage of nanoswitches

Background


Conclusions

SEM of the SWNT based nanoswitch [9] .

Originally taut SWNT bundle

Axial Slack (the increased length

relative to the originally taut

length)

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[9] P. Ryan et al,Journal of Micromechanics and Microengineering, 2011.

AFM is in the use as lateral force microscopy [10]

Sliding of a MWNT on graphite by AFM (2MPa)[11] A MWNT was pushed on SiO2 surface by AFM (60MPa) [12]

Shear stress measured by AFM in the literature

Background


Conclusions

.

Nanoscale dimension of CNTs

NanoNewton level interaction force

8

[10] P. Reimann et al, New J. Phys. ,2005. [11] M. R. Falvo et al, Nature, 1999. [12] J.-H. et al, Wear, 2009.

AFM manipulation of a SWNT bundle on a mica substrate. A small portion was extracted from the

bundle [14] . A shear stress of 10 MPa is estimated for a bent MWNT on a silicon substrate after AFM

manipulation[13] .

Disadvantages and limitations of the AFM-based technique:

1) Operation difficulty 2) Complicated force calibration 3) Substrate has to be very smooth 4) No shear stress data reported for SWNTs yet based on this technique

Background


Conclusions

9

Shear stress measured by AFM in the literature

[13] T. Hertel, et al, J. Phys. Chem. B, 1998. [14] M. R. Falvo, et al. Microsc. Microanal., 1999

Objective

To develop a new efficient and easily controllable technique to study the shear

stress between SWNTs and substrates

Background


Conclusions

10

Principle of the technique

A SWNT bundle is under tension T

A SWNT bundle gets axial slack S after the gap returns to its original size

Gap elongates by uG

SWNTs stretching SWNTs slipping

Elastic properties Shear stress

Background


Conclusions

Axial slack S

Gap returns to its original size. SWNTs stretching recovers.

The axial slack results from the residual length slipped from the shoulders.

S < uG

Shear stress can be determined by measuring the axial slack

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The testing device has a SWNT bundle contact with two shoulders

A microcantilever is pushed laterally by a nanomanipulator to cause the tubes to slip

Top view of the designed testing device

SWNTs

NEMS cantilever based testing device

Background


Conclusions

Flexible cantilever

Stationary electrode

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SEM of the fabricated cantilever structure (side view)

SEM of the device after SWNT bundle assembly (top view)

Problem: The contact length can not be measured based on the SEM images due to poor contrast.

The original cantilever structure

Background


Conclusions

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Contact length = Cantilever width

Improved structure

Background


Conclusions

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SEM of a fabricated improved structure SEM of the structure with an assembled SWNT bundle

Microfabrication flow

Si

SiO2

W

Au

a) Si/SiO2/ W sputtering

d) Cr/Au evaporation and lift-off

e) SiO2 wet etch for release

b) E-beam lithography and W etch

c) SiO2 wet etch (half of the SiO2 thickness)

Background


Conclusions

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Low yield (~5%) to assemble only one SWNT bundle between the electrodes. Reasons: 1) Average tube length of 0.6 um 2) Assembly can end up with too many

tube bundles or sometimes no tubes assembled

Side electrodes

Side electrodes

NEMS device for SWNT assembly

Dielectrophoretic assembly

Array structure to get more good devices for one assembly process

Background


Conclusions

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Assembly parameters: 1) Concentration of SWNT solution

(310 ng/ml ) ; 2) AC voltage (3.4 Vpp) and frequency

(500 KHz) 3) Electrode gap (~1.8 um) 4) Assembly time (30s)

Expose a spot of the SWNT bundle under electron beam at 500KX magnification for ~2 minutes (Supra 25 SEM).

Before cutting

SWNT cutting by Electron-beam

Background


Conclusions

After cutting

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Nanomanipulation by a Zyvex probe

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After nanomanipulation Before nanomanipulation

Comparison of the same structure before and after nanomanipulation

Background


Conclusions

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Image analysis to obtain the axial slack

-20 degree view

70 degree view

After nanomanipulation Curve fitting in Matlab

Background


Conclusions

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Background


Conclusions

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1) SWNT: hollow cylinder with a radius of 0.65 nm and a wall thickness of 0.34 nm

2) Young’s Modulus: 1TPa Both experimental and theoretical data [15,16] reported in the literature for

SWNTs with the same geometrical details) 3) Contact width: 50% of the tube diameter It is the shear force per unit length instead of shear stress which is

directly determined by the experiments in concert with the theoretical model in this work.

Difficulties in obtaining the contact width: (a)The contact width between tubes and substrates is not well defined or

studied in the literature. (b)For different substrates, the contact width can be different due to

different interaction forces. In this work, we assume 50% of the tube diameter as the contact width

to convert the shear force per unit length to shear stress for all tested substrates.

Modeling assumptions

[15] M. F. Yu et al, Physical Review Letters, 2000. [16] J. P. Lu, Physical Review Letters, 1997.

Modeling assumptions

Background


Conclusions

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4) Nanotubes are assumed to spread out on the contact surface, so the shear stress is independent of the tube number in a bundle.

Fewer tubes contacting would correspond to a larger shear stress. Strong interactions in the bundle, weak interactions at the tube-substrate interface lead to fewer tubes contacting the substrate.

Current assumption: n1=n2 Circular cross section

Anything in between, shear stress τ* = (n1/n2 ) τ, where τ is based on the current assumption, n1: the number of tubes in the bundle, n2 : the number of tubes in

contact with substrate.

For a semicircular cross section with n1 =34 (based on a 5.6 nm bundle height measured by Peter) , n2 =5.6/1.3= 4 , τ* = 8.5 τ

1) Original taut SWNT bundle

Model (by Yu-Chiao Wu) Variables: L: original gap; Lc: contact length;

T: induced tension; f=T/Lc: shear force per unit length; uG: gap elongation; S: axial

slack; : displacement of the free end; EA: extensional rigidity of the tube

(1)

Partial-complete slip Displacement of the completely slipped tube bundle:

2) Partial-partial slip

3) Partial-complete slip

Background


Conclusions

EATL /

)2( EATLc

)2()2( EATLEATLEATL ccuG ++=

1)2( uEATLc +

1)2()2( uu EATLEATLEATL ccG +++=

1u

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Partial-partial slip Suspended tube bundle stretching:

Displacement of the tube bundle from each side:

(2)

4) The SWNT bundle gets some axial slack after the gap recovers to its original size

For partial-complete slip (2) +(3):

With gap elongation D, contact length Lc, original gap L, extensional rigidity EA of the SWNT, and axial slack after testing S all known, the shear stress τ can be

determined.

Background


Conclusions

transition point

)4( EATLc

1)4( uEATLc +

1)4()4( uEATLcEATLcS ++=

)22(2)2( SLSEAf uG −=

)22()( cc LLLSEAf uG +⋅−=

( )( )cG LLLtS uu +== 2,01

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Model (by Yu-Chiao Wu) Gap recovers : Residual displacement of the partially slipped tube bundle: Residual displacement of the completely slipped tube bundle:

(3)

For partial-partial slip (1) +(3):

The transition point:

Shear stress for gold substrates

Background


Conclusions

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Test Original gap L (nm)

Contact length LC

(nm)

Gap elongation

uG (nm)

Axial slack uS

(nm)

Shear stress τ (MPa)

1 338 380 45 32 100 2 338 380 89 80 75 3 270 373 30 20 91 4 275 370 22 13 79 5 301 335 60 53 71 6 361 364 42 28 108

An average shear stress of 87 MPa between SWNTs and gold cantilever surface was obtained.

Gold cantilever surface modification

NH2 SH

2-aminoethanethiol NH2(CH2)2SH

Gold

S

NH 2

S

NH 2

S

NH 2

S

NH 2

S

NH 2

S

NH 2

S

NH 2

S

NH 2

Functionalization (self-assembly) process: 1) Pre-cleaning in piranha for 10 mins 2) Functionalization/Self-assembly in 1

mM ethanolic alkanethiol for 24 hours

3) Rinse with ethanol

SH

2-phenylethanethiol C6H5(CH2)2SH

Background


Conclusions

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Motivation: people modify gold with alkanethiols to change the surface properties in the literature. It is important to understand how the mechanical interactions between tubes and gold surface are changed through the standard chemical functionalization.

Alkanethiols:

Shear stress for 2-aminoethanethiol surfaces

The average shear stress is 142 MPa with more scatter in the results compared to the untreated gold surfaces.

Background


Conclusions

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Contact length LC

(nm)

Gap elongation uG

(nm)

Axial slack uS

(nm)


1 435 281 49 20 275 2 454 271 60 42 172 3 401 301 41 34 73 4 437 247 49 32 196 5 477 249 50 43 74 6 444 273 60 51 89 7 437 257 54 44 113

Results for 2-phenylethanethiol surfaces

Processes of a SWNT bundle slipping on the cantilever surface with a sufficiently low shear stress and the adhesion effect. (a) state before

testing, (b) during gap elongation, and (c) after gap recovery.

Background


Conclusions

The axial slack S is greater than the gap elongation uG

A sufficiently low shear stress τ

(low elastic strain energy)

The work of adhesion Δγ

between tubes and the surface

The SWNT bundle is considered to be an inextensible string by neglecting the strain energy

+

μ = Δγ / τ be determined with an observed axial slack uS, a measured original contact

length LN, two measured curvatures Rw and RN and a given gap elongation uG.

Model (by Yu-Chiao Wu)

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Shear stress for 2-phenylethanethiol surfaces

Background


Conclusions

An average shear stress of 7.2 MPa is determined with an estimated work of adhesion of 0.5 J/m2 (calculated by Yu-Chiao based on a binding energy and a

packing density between a benzene molecule and a CNT). Even with a doubled work of adhesion, the shear stress is 14 MPa, which is still much smaller compared to

untreated gold surfaces.


LR (nm)

LL (nm)

uG (nm)

Axial slack uS

(nm)

RL (nm)

RR (nm)

μ = Δγ/τ (nm)


1 387 315 467 38 52 93 60 102 4.9 2 358 346 456 31 54 74 44 181 2.8 3 378 352 450 40 44 62 45 64 7.8 4 442 276 334 59 85 128 69 113 4.4 5 463 281 334 63 71 92 79 59 8.5 6 437 316 385 32 49 67 75 122 4.1 7 415 301 409 49 63 45 80 111 4.5 8 459 232 340 44 47 61 45 35 14.3 9 448 275 353 54 57 63 65 37 13.6

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Three post-assembly treatments of the devices

Process of the device treatments: (1) Nitrogen annealing at 350 ℃ for 5 minutes (2) E-beam irradiation for the tube-gold contact area with an exposure

dose of 0.7 C/cm2 (3) E-beam induced carbon deposition for different period of time (30

minutes and 60 minutes)

Background


Conclusions

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Motivation: it has been found that electrical contact resistance between metallic pads and SWNTs can be reduced by 3 to 4 orders of magnitude through either nitrogen annealing [17, 18] or e-beam irradiation [18, 19].

Intentional e-beam induced carbon deposition has been used for clamping CNTs onto AFM cantilevers [20, 21].

It is important for us to understand how the above processes affect the shear stress between SWNTs and gold surfaces.

[17] J. O. Lee et al, Journal of Physics D: Applied Physics, 2000. [18]P. J. Ryan, Doctor of Philosophy thesis, 2011. [19]A. Bachtold et al, Applied Physics Letters, 1998. [20]M.-F. Yu et al, Science, 2000. [21]W. Ding et al, Journal of Applied Physics, 2005.

Three post-assembly treatments of the devices

Background


Conclusions

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For the nitrogen annealed devices (2 out of 4 tests) and the e-beam irradiated devices (2 out of 5

tests), some tests resulted in a very nonuniform bundle in the gap, with the axial slack almost

equal to the gap elongation.

Ideas of what is going on: 1) Tubes break at different positions in the

bundle and the outer deposited carbon shell completely breaks

2) The tubes contacting the substrate break, and other tubes not contacting the substrate stay intact. They slip relative to each other in the

bundle 3) Some tubes contact with partial cantilever

width. Tubes contacting the whole cantilever width break and slip in the bundle

All tubes contact with the substrate

Some tubes in the bundle contact with substrate

SEM images for nitrogen annealed devices

Background


Conclusions

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Test 1

Test 4

Test 2

Test 3

Shear stress for nitrogen annealed devices

Background


Conclusions

On average, the shear stress is estimated to be greater than 309 MPa based on a breaking strain of 5.3 % from the literature [22] . (b is the contact width in the

above equation)


Narrow cantilever

width (nm)

Gap elongation uG

(nm)

Axial slack uS

(nm)


1 487 294 42 -- 280*

2 497 271 62 61 301*

3 526 260 68 68 314*

4 571 240 90 76 340*

EALb c

breakτε =

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When a SWNT bundle starts to break, the exact slip length is unknown. However, it is for sure that the slip length Lc is smaller than the narrow cantilever width.

[22] M. F. Yu et al, Physical Review Letters, 2000.

SEM images for e-beam irradiated devices

Background


Conclusions

Test 4 (2 tests show nonuniform bundle after nanomanipulation)

Test 2 (3 tests show uniform bundle after nanomanipulation)

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Shear stress for e-beam irradiated devices

Background


Conclusions

An average shear stress of 202 MPa based on 3 independent tests and an average estimated shear stress larger than 311 MPa based on 2 independent tests with partially broken tubes are obtained.

* Estimated based on breaking strain of 5.3%[22] .


Narrow cantilever

width (nm)

Gap elongation uG

(nm)

Axial slack uS

(nm)


1 500 273 48 30 163

2 532 253 54 30 230

3 526 260 46 23 214

4 519 260 40 38 314*

5 494 266 53 53 307*

35 [22] M. F. Yu et al, Physical Review Letters, 2000.

SEM images for carbon deposited devices

Background


Conclusions

Test 1 (Test 1,2,4 has devices with 30 minute carbon

deposition)

Test 3 (device with 60 minute carbon deposition)

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Shear stress for carbon deposited devices

Background


Conclusions

For the 30 min deposited devices, a shear stress of 124 MPa and 94 MPa are found for test 1,2 respectively. For the 60 min deposited

device in test 3, a shear stress of 200 MPa is obtained. The device in test 4 has tube bundle not covering the whole wide cantilever width,

but the result remains a mystery to us.

* Estimated based on breaking strain of 5.3%[22] .


Contact length LC

(nm)

Gap elongation

uG (nm)

Axial slack uS

(nm)


1 487 279 46 32 124

2 474 279 52 42 94

3 494 274 49 27 200

4 503 284 52 65 --

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Surfaces Average shear stress (MPa)

Number of Tests

Untreated gold surface 87 6

2-aminoethanethiol surface 142 7

2-phenylethanethiol surface 7 9

Discussion of the untreated and functionalized gold results

For untreated gold surfaces, the relatively high shear stress compared to silicon (10MPa) [13] , graphite (2MPa) [11] , and silica (60MPa) [12] surfaces, likely due to charge transfer interactions between the metal and the SWNTs.

Background


Conclusions

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The exposed phenyl groups of the 2-phenylethanethiol surface bind to the SWNT surfaces via relatively weak p-p stacking interactions, which causes a lower shear stress at the interface.

The SWNT-2-aminoethanethiol interface has a shear stress 2 times higher than that for untreated gold, which may result from enhanced bonding between the amino end-groups and the SWNTs.

[11] M. R. Falvo et al, Nature, 1999. [12] J.-H. et al, Wear, 2009. [13] T. Hertel, et al, J. Phys. Chem. B, 1998.

Surfaces Average shear stress (MPa)

Number of Tests

Untreated gold surface 87 6

N2 annealed devices 309* 4

E-beam irradiated devices

202 3 311* 2

Carbon deposited devices

107 2 (30mins deposition) 200 1 (60mins deposition)

Discussion of the post-assembly treatment results

For both N2 annealed and E-beam irradiated devices, the shear stress can be increased to over 300 MPa. It is possible that the greatly increased shear stress is due to the improved contact at the interface during those process. For the carbon deposited devices, the relatively high shear stress compared to the untreated gold surface may result from anchoring of SWNTs to the gold surfaces by the carbon layer coating . Also carbon deposition for longer period of time appears to increase the shear stress more, likely due to a thicker carbon layer coating.

Background


Conclusions

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Discussion of the experimental technique

Background


Conclusions

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1) The effect of incidental e-beam exposure on the shear stress

E-beam irradiation (mainly during SEM imaging before nanomanipulation): the overall exposure dose is estimated to be over 100 times lower than the intentional e-beam exposure dose in the post-assembly treatment tests. Carbon deposition (mainly during SWNT bundle cutting before nanomanipulation): the cutting usually takes ~ 2 mins VS intentional carbon deposition time of 30 mins or 60 mins in the post-assembly treatment tests. Also the SEM chamber is always cleaned by plasma before cutting to minimize the deposition effect.

2) Sensitivity of shear stress relative to experimental parameters

Overall the uncertainty in all experimental parameters contributes to a ~25% scatter in the shear stress for this technique.

Conclusions

1) A new technique based on a NEMS cantilever structure, a nanomanipulator, an SEM along with a theoretical model was developed to study the shear stress at SWNT-substrate interface.

2) For the first time, the interfacial shear stress between dielectrophoretically assembled SWNTs and substrates is determined.

3) It is demonstrated that we can vary the shear stress by a factor of 20 by functionalizing a gold surface with different alkanethiols before SWNT assembly.

4) Some preliminary results also indicate that through post-assembly treatments including N2 annealing, e-beam irradiation and carbon deposition the shear stress can be increased from 87 MPa to over 300 MPa for the case of annealing.

Background

Conclusions


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Recommendations for the future

Background


Conclusions

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1) It will be possible to further tune the shear stresses by varying the nature of the alkanethiol end group. Even finer control may be achieved by systematically varying the length of the carbon spacer between the thiol function and the associated end group.

2) It will be good to get more data points to confirm the consistency of the testing results for post-assembly physical treatments.

3) This technique can also be used for other materials by modifying the cantilever surface through different methods, such as metal layer deposition on the cantilevers or simply making the entire structure out of other materials. Further, instead of using SWNTs, the one-dimensional element can be MWNTs or other types of nanowires.

Publications from this work 1) Pan H Y, Wu Y C, Ryan P J, Adams G G and McGruer N E Measurement of

shear stress between single-wall carbon nanotubes and substrates using NEMS devices. In: Solid-State Sensors, Actuators and Microsystems Conference (TRANSDUCERS), 2011 16th International, pp 629-32

2) Pan H Y, Wu Y C, Adams G G, Miller G P and McGruer N E Interfacial Shear Stress between Single-Walled Carbon Nanotubes and Gold Surfaces with and without an Alkanethiol Monolayer, Journal of colloid and interface science, 2013, 407C, pp. 133-139

3) In preparation of another paper on the results of the post-assembly treatments

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Acknowledgements • Advisors: Prof. Nicol McGruer and Prof. George Adams • Committee: Prof. Carol Livermore and Prof. Ahmed Busnaina • Siva Somu, Peter Ryan, Juan Aceros, Yu-Chiao Wu, and all other

colleagues in CHN; the staff Richard DeVito, Scott McNamara, David McKee, Matthew Botti, and Jessica Viator;

• Others: National Science Foundation and Brewer Science • Work was performed in the George J. Kostas Nanoscale

Technology and Manufacturing Research Center at Northeastern University.

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Thanks for your attention!

Questions?

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NEMS Device for Determination of Shear Stress between ... d… · 3) Contact width: 50% of the tube...

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