“In-situ sample rotation as a tool to understand nanotube growth”

Mario Hofmann, Jing Kong EECS Department

MEMS@ MIT open house

“In-situ sample rotation as a tool to understand nanotube growth.”

2

Hypothesis

floating nanotubes can be aligned with the gas flow because this minimizes the drag force induced bending moment on them

Settled Nanotubes will be rotated with the sample.

There will be a difference in angle between nanotubes that were floating and already settled ones after rotation.

“In-situ sample rotation as a tool to understand nanotube growth.”

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Phase 1GrowthGrowth

Phase 2IntermediateIntermediate

Phase 3CoolCool

Hypothesis: Alignment with flow direction

floating nanotubessettled nanotubes

“In-situ sample rotation as a tool to understand nanotube growth.”

4

Observations

Nanotube alignment in three different directions can be observed on SEM pictures

“In-situ sample rotation as a tool to understand nanotube growth.”

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Experimental setup

• common CVD setup with additional outlet to pump• Ethanol CVD @ 925°C (600Ar/440H2 )

mass flow controller

ethanol bubbler

gas supply

SiO2 samplequartz rod

electrical furnace

quartz CVD chamber

micro actuator

“In-situ sample rotation as a tool to understand nanotube growth.”

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Experimental setup (II)

Sample SiO2 piece as pivot pointquartz rod

• quartz rod is used to push sample in different directions while at ~900°C

quartz boat

“In-situ sample rotation as a tool to understand nanotube growth.”

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Straightness of nanotubes

Total floating-mechanism

Even the longest nanotubes are straight and only very few exhibit kinks.This indicates that the whole nanotube is floating and not only segments.

Partial floating-mechanism

“In-situ sample rotation as a tool to understand nanotube growth.”

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Certain nanotubes already fall to the surface during synthesis

Analysis

131m6

-5

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-90 -70 -50 -30 -10 10 30 50 70 90

Alignment angle (°)

# NT

s

Growth

“In-situ sample rotation as a tool to understand nanotube growth.”

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Analysis

131m6

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Alignment angle (°)

# NT

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Intermediate

Growth

These are the nanotubes fall to the surface during the intermediate step

“In-situ sample rotation as a tool to understand nanotube growth.”

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Analysis

131m6

-5

0

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-90 -70 -50 -30 -10 10 30 50 70 90

Alignment angle (°)

# NT

s

Intermediate

Growth

Cool

These are the nanotubes fall to the surface during the cooling step

“In-situ sample rotation as a tool to understand nanotube growth.”

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Term definition

131m6

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-90 -70 -50 -30 -10 10 30 50 70 90

Alignment angle (°)

# N

Ts

22 3

nanotubes settling during phasesettling efficiencyphase phase

=+

1 23

“In-situ sample rotation as a tool to understand nanotube growth.”

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Time dependency

• settling efficiency is affected by time

925°C, 200sccm Ar

0%

10%

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100%

0 2 4 6 8 10 12

duration of phase 2 (min)

settl

ing

effic

ienc

y

“In-situ sample rotation as a tool to understand nanotube growth.”

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200sccm Ar

0%

10%

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0 2 4 6 8 10 12

duration of phase 2 (min)

settl

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effic

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y

200sccm 925°C200sccm cool

Cooling

• settling efficiency is not affected by temperature

750°C

646°C

480°C 380°C

“In-situ sample rotation as a tool to understand nanotube growth.”

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static case

turning off gas flow reduces settling efficiency,but small oxygen leaks limit usage time

no gas flow

0%

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0 2 4 6 8 10 12

duration of phase 2 (min)

settl

ing

effic

ienc

y

200sccm coolno gas flow

“In-situ sample rotation as a tool to understand nanotube growth.”

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Flow rate dependence

925°C, 200sccm Ar

0%

10%

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30%

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70%

80%

90%

100%

0 2 4 6 8 10 12

duration of phase 2 (min)

settl

ing

effic

ienc

y

settling rate [%/min]

“In-situ sample rotation as a tool to understand nanotube growth.”

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Flow rate dependence

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0 500 1000 1500 2000 2500 3000

flow rate [sccm]

settl

ing

rate

high flow rates can stabilize a floating nanotube

“In-situ sample rotation as a tool to understand nanotube growth.”

17-5

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-90 -70 -50 -30 -10 10 30 50 70 90

Alignment (°)

Num

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of N

anot

ubes

Pumping to ~200mbar

• Settling efficiency of pumping is 100% (all nanotubes settle after 30s)

“In-situ sample rotation as a tool to understand nanotube growth.”

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Other Observations

• feature appears frequently

“In-situ sample rotation as a tool to understand nanotube growth.”

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Model

• falling nanotube is caught by another nanotube on the ground

• might be a way to evaluate the necessary force to split a bundle

“bundling force”

drag force?

Flow direction

“bundling force”

drag force?

Flow direction

“In-situ sample rotation as a tool to understand nanotube growth.”

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Which nanotube end touches the ground first?

• Kinks in nanotubes indicate that the upstream end of the nanotube usually touches the substrate first and a zipper- mechanism moves the contact point between substrate and nanotube towards the end

“zipping” of contact point towards the end of the nanotube

“In-situ sample rotation as a tool to understand nanotube growth.”

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Conclusions

• A time resolved investigation of the carbon nanotube behaviors during CVD growth is enabled by in-situ sample rotation

• The mechanism for nanotubes to be floating during the growth is clarified and the settling process is analyzed in detail

• Different settling behaviors are also revealed• These understandings have important

implications regarding the synthesis and integration of nanotubes