# Presentation: 903906

## Nanotechnology: Practical Systems 

**M. Meyyappan**
- Chief Scientist for Exploration Technology
- NASA Ames Research Center
- Moffett Field, CA  94035
- email: mmeyyappan@mail.arc.nasa.gov

*Acknowledgements: Jing Li, Y. Lu, Jun Li, Alan Cassell, Hua Chen, Jessica Koehne, Cattien Nguyen *

- Nanotechnology: Practical Systems
- and Integration with MEMS

## Nano - Micro - Macro Integration

- Take-Home Message from this Talk

- •	Emerging of nano will NOT eclipse micro or MEMS

- •	Indeed, in many cases (but not all), nano-based 		products would need MEMS to achieve desirable 	performance.  This means, hierarchical

- •	Nanotechnology, if it succeeds in the market place, 	will breathe new life and renewed vigor into the 		MEMS (research, applications, infrastructure, fabs, 	products, market, companies, profit.....)

## The talk is designed to give some preliminary examples to support the opening statements

- 	*	Introduction
- 	*	CNT Chemical Sensors
- 	*	CNT Based Instrumentation
- 	*	CNT Biosensors
- 	*	Brief Introduction to Nanowires

- Outline

## What Do We Expect from a Well-Designed

- Sensor System?

- •	First, a single device has no value.  We need a system consisting of:
- 			-	Sensor or sensor array
- 			-	Preconcentrator (almost always needed)
- 			-	Micropump?  Microfan?
- 			-	Sample handling, delivery, fluidics
- 			-	Signal processing unit
- 			-	Readout unit (data acquisition, processing, storage)
- 			-	Interface control I/O
- 			-	Integration of the above
- •	Criteria for Selection/Performance
- 			-	Sensitivity (ppm to ppb)
- 			-	Absolute discrimination
- 			-	Small package (size, mass)
- 			-	Low power consumption
- 			-	Rugged, reliable
- 			-	Preferably, a technology that is adaptable to different platforms
- 			-	Amenable for sensor network or sensor web when needed

## Why Nanomaterials/Nanosensors?

- •	Compared to existing systems, potential exists to improve sensitivity limits, 		and certainly size and power needs
- •	Why?  Nanomaterials have a large surface area.  Example: 	SWCNTs have a 		surface area ~1600 m2/gm which translates to the size of a football field for 		only 4 gm.
- •	Large surface area           large adsorption rates for
- 	gases and vapors          	changes some measurable
- 	properties of the nanomaterial           basis for
- 	sensing
- 		-	Dielectric
- 		-	Capacitance
- 		-	Conductance
- 		-	Deflection of a cantilever
- 		-
- 		-

- 4 grams

## Various Carbon Nanostructures

- •	Single-walled carbon nanotubes (SWCNTs)
- 		-	Either a single tube or a film used in the
- 			device
- 		-	Vapor or gas adsorption on the material
- 			leading to a measurable property change
- 		-	Multiwalled carbon nanotubes (MWCNTs) may not be as good
- •	Vertically Oriented Carbon Nanofibers (CNFs)
- 		-	Multiwalled carbon nanofibers
- 		-	Walls are not parallel, instead bamboo-like
- 		-	Individual, free-standing, vertical
- 		-	Function as nanoelectrode, support a probe
- 			molecule at the tip amenable for interaction
- 			with target
- 		-	Nanoelectrode array

## Conductivity Change of CNTs Upon

- Gas/Vapor Adsorption

- •	Early chemical sensors were of the CHEMFET type with SnO2
- 	and other oxide conducting channels
- •	Similar CNT-FETs have been tested in the literature, exposing to 	NH3, NO2, etc.; change in conductivity has been observed
- •	Limitations of CNT-FET
- 		-	Single SWCNT is hard to transfer or grow *in situ*
- 		-	Even a film of SWCNTs by controlled deposition in the 				channel is complex
- 		-	3-terminal device is complex
- 			to fabricate
- 		-	Commercial sensor market is very
- 			cost sensitive

## NASA Ames Alternative to CNT-FET:

- •	Conventional thin film transistor approach is complex and expensive
- •	Two terminal chemiresistor is cheaper, easier to fabricate
- 		1.	Interdigitated electrode device using **simple microfabrication**
- 		2.	Disperse purified SWCNTs in DMF
- 		3.	Solution casting of SWCNTs across the electrodes

- Jing Li et al., Nano Lett., **3**, 929 (2003); Y. Lu et al., Chem. Phys. Lett., Vol. 391, p. 344 (2004).

- Interdigitated Chemiresistor

## Nanosensing Technology

**Using pattern matching algorithms, the data is converted into a unique response pattern**

_**Operation**_**:**
**The relative change of current or resistance is correlated to the concentration of analyte.**
**Array device “learns” the response pattern in the *****training***** mode.**
**Unknowns are then classified in the *****identification***** mode.**

## Scalable Array Approach

**•****	****12 to 36 sensing elements are on a chip (1cm x 1cm) now with heaters and thermistors.**
**•****	****Number of sensing elements can be increased on a chip.**
**•****	****Number of chips can be increased on a 4” wafer.**
**•****	****Wafer size can be increased to 6”, 8”, or 12”.**
**•****	****SWCNT solution-casting by ink jetting or using microarrays**

- (Multi-channel Sensing Chip)

## Sensitivity is the slope of the calibration curves. The average slope is 0.034+0.002 (6%).

## •	Response is larger (10-fold from pure SWCNT sensor). The 		detection limit is 4.6ppb now.

- •	Recovery is better.
- •	Surface response increases.

## SWCNT Sensor Response to 

- SWCNT Sensor Response to
- Nitrotoluene

***Handheld prototype****** ***
- •	Sensor or sensor 		array
- •	< 32 ounces
- •	Low power & cost
- •	BNC, or RS-232, USB

## Nitrotoluene

- Sensing Mechanisms

## Analytes Tested and Selectivity Issues

**•****	****Functionalization of CNTs**
**•****	****CNT + polymer composites**
**•****	****Doping:  catalytic metal clusters**
**•****	****Field effect: gate bias for selective detection        **
**•****	****Programmed temperature control**
**•****	****Sensor array with pattern recognition: **
**	****electronic nose**
**•****	****Combination of the above**

- _Analytes Tested_
- •	NO2
- •	NH3
- •	CH4
- •	SO2
- •	Cl2
- •	HCl
- •	Acetone
- •	Benzene
- •	Toulene
- •	Nitrotoulene
- •	Hydrogen Peroxide
- •	Formaldehyde

## Flight Demo Unit for Satellite

- 32-channel sensor chip

- Ceramic chip carrier

- U.S. Navy Flight in 2006

- A robotic platform for sensor deployment is also under development for submarine and planetary applications

## CNTs in X-ray Imaging

## Carbon Nanotubes in Field Emission

- Single wall nanotube
- (SWCNT)

- Multi wall nanotube
- (MWCNT)

- Carbon nanofibers

- Extremely sharp
- Very good conductors (up to 109 A/cm2 local current density)
- Thermal tolerance (emission reported at 2000K)
- Mechanically robust
- Can be used as single emitter or large film / array

## Flight instrument layout:

- •	volume <1 liter
- •	mass < 1 kilogram
- •	power < 5 watts

- miniature microfocused X-ray tube

- CCD detector optimized for X-ray detection

- Various sample delivery configurations are studied

- CNT emitters

- CheMin XRD/XRF

## Structure of CNT Field Emitters

- •	Nature of nanotubes (SWNTs, MWNTs, CNFs...)
- •	Clean emitting sites vs. adsorbates (water vapor, oxygen...)
- •	Microstructure
- •	Screening effect
- •	Diode vs. triode

- High voltage required and/or gap needs to be adjusted

- Current is controlled by gate voltage, independent of acceleration voltage

## Field Emission of an Individual CNT emitter

- Turn-on field needed for initial field emission decreases as *d* increases
- At large *d *separations, turn-on voltages remain constant as turn-on fields decrease

## Interfacial Resitive Heating 

- Still possible to lose the nanotube due
- to interfacial resistive heating

- Microfabricated Si Structure
- (Ni-coated)

-   Stable emission current > 200 nA per CNT emitter
-   ~105 emitters for 20 mA current (assuming each CNT behaves independently)

- Interfacial Resitive Heating
- from Emission Current

## What is the optimal density?

- 	•	Investigate CNT adjacency effect in a definitive manner
- 		-	as a function of emitter separation
- 		- as a function of CNT length
- 		(Theory: field screening is minimized at separation > 2L)

- •	Robustness and stability of the CNT emitters

## Carbon Nanotube Field Emission

- Silicon and metal substrates
- Film, arrays

- Optimum type of CNT?
- Optimum CNT/substrate attachment?
- Optimum site density?

- CNT emitter fabrication
-  NASA Ames

- Field emission characterization
-  NASA Ames

- Integration in miniature X-ray tube
- (Oxford XTG Inc.)

- Various types of emitters were studied

## Nanoelectrode Array for Biosensors

## Nanoelectrode Array (NEA)

-  Each electrode is well separated with the neighbors (> 6R) so that the NEA behaves similar to a single NE.
-  Can further create an individually addressed multiplex array in an array-in-array format

- Challenges:
- Reliable fabrication techniques with affordable cost, particularly for low-density NEAs.

## Commonly Used Carbon Electrodes

## Macroelectrode vs. Nanoelectrode

- Semi-infinite planar linear diffusion

- Semi-infinite hemispherical diffusion

- Nanoelectrode Array

- * High sensitivity
- * Easily measurable signal
- * Fast detection

- glassy carbon

- I total = Iplanar + Iradial

- When reducing the size (r), orders of magnitude improvements are found in:
- 	(1) Spatial resolution: defined by r
- 	(3)* *Temporal resolution:* **Cell time constant* = RuCd = r Cd0/4
- 	(2) Sensitivity: signal-to-noise ratio: is/in  nFC0D0/r

## CNT DNA Sensor

- Signal from redox bases (Guanine) in the excess DNA single strands
**The signal can be amplified with metal ion mediator oxidation catalyzed by Guanine.**

- Using Electrochemical Detection

## Nanoelectrode Array Fabrication

- 30 dies on a 4” Si wafer

- Embedded CNT Array after planarization

## Carbon Nanotube Electrodes 

- CNT coverage: ~ 20%
- 	(~3.0x109  CNTs/cm2)
- Average nearest-neighbor distance:
- 	~300 nm

- CNT coverage: < 1%
- 	(~1x108 CNTs/cm2)
- Average nearest-neighbor distance:
- 	> 1500 nm

- J. Li, H. T. Ng, A. Cassell, W. Fan, H. Chen, Q. Ye, J. Koehne, J. Han, M. Meyyappan,  *Nano. Lett.*, 2003, **3 **(5), 597.

- CV in 1mM K4Fe(CN)6 in 1M KCl at 20 mV/s

- Carbon Nanotube Electrodes
- at Different Densities

## Electrochemical Detection

- by AC Voltametry

## 30 dies on a 4” Si wafer

- Potential applications:
- Lab-on-a-chip applications
- Early cancer detection
- Infectious disease detection
- Environmental monitoring
- Pathogen detection

- Fabrication of Genechip

##  Conceptual Immunosensor Chip for Bacteria Monitoring

-  Conceptual Immunosensor Chip for Bacteria Monitoring

- Chip size: ~1”x3”

- 100 mL water sample w/ 100 bacteria filtered and condensed by 100 times into ~ 1 mL solution
- 1 mL condensed solution passed by parallel microfluidic channels of ~ 5 to 10 m with multiplex NEAs.
- Bacteria trapped on some NEAs in corresponding zones assisted with dielectrophoresis
- Electrochemical detection done by a computer with multiplex interface cards

**Goal: (1) Detect ~100-1000 bacteria/L in ~10-20 minutes**
**          ****(2) Can detect upto 30 types of bacteria**

- Reagent
- Pumping

## Various Inorganic Nanowires

## Vertically-Aligned Nanowires

- for Device Fabrication

**ZnO Nanowires**

**Germanium Nanowires**

- P. Nguyen et al., Advanced Materials, Vol. 17, p. 549 (2005).

- H.T. Ng et al., Science, Vol. 300, p. 2149 (2003).

## Vertical Surround-Gate

- A process flow outlining the major
- fabrication steps of a VSG-FET.

- Ng *et al.,* Nano Letters, Vol. 4 (7), p. 1247 (2004)

- Field Effect Transistor

## Low Thermal Energy for Programming 
Reduced melting point at 1-D
Reduced programmable element volume 
Reduced activation energy at 1-D

Device Scalability 
Ultra-low current / voltage / power operation 
Reduced thermal interference between neighboring memory cells

- Reduced melting point at 1-D
- Reduced programmable element volume
- Reduced activation energy at 1-D
- Device Scalability
- Ultra-low current / voltage / power operation
- Reduced thermal interference between neighboring memory cells

- Top electrode

- Bottom electrode

- 2-D Thin film PRAM

- 1-D Nanowire PRAM

- Why 1-D Phase-Change Nanowire?

## GeTe Nanowires: TEM, SAED, and EDS

- (a) TEM image of an individual GeTe nanowire with a diameter of about 40 nm. The inset shows an SAED pattern of fcc cubic lattice structure. (b) EDS spectrum of the same GeTe nanowire.

- 40 nm

- <110>

- Ge:Te≈1:1

## GeTe Nanowires: Melting Experiment 

- In-situ Tm measurement of GeTe nanowire under TEM image monitoring (a) The GeTe nanowire is under room temperature. (b) The GeTe nanowire is heated up to 400C when the nanowire is molten and its mass is gradually lost through evaporation. The remaining oxide shell can be seen from the image.

**Liquid GeTe**

- GeTe Nanowires: Melting Experiment
- and In-Situ Monitoring by TEM

## GeTe Nanowires: Melting Point 

- Tm of GeTe nanowires: ~390oC

- 46% reduction!

- The melting temperature of the nanowire is identified as the point at which the electron diffraction pattern _disappears_ and the nanowire starts to be evaporated.
- Lower Tm is translated into potentially much reduced thermal programming energy of data storage device.

- GeTe Nanowires: Melting Point

## Future Outlook for Inorganic Nanowires

**Future Outlook for Inorganic Nanowires**

- Nanowire-based
- Radiation-harden
- Central Processing Unit

- Nanowire-based
- Detector Sensory
- Systems

- Nanowire-based
- Hybrid Energy
- Conversion/Storage
- Unit

- Nanowire-based
- Ultra-high Density
- Data Storage

- Nanowire-based
- Peripheral Optical
- Interconnect/
- Transmitter

## Summary

- •	Emerging of nano will NOT eclipse micro or MEMS
- •	Indeed, in many cases (but not all), nano-based products would need MEMS to 	achieve desirable performance.  This means, hierarchical
- 				Nano - Micro - Macro Integration
- •	Nanotechnology, if it succeeds in the market place, will breathe new life and 		renewed vigor into the MEMS (research, applications, infrastructure, fabs, 		products, market, companies, profit.....)

- *	We have discussed some examples, in terms of chemical, bio 		sensors and other systems, where the “heart and soul” of the system 	is a nanomaterial, nanomechanics or some nano phenomena and the 	system itself needs a seamless integration to micro/macro for a 		technically feasible and commercially viable product.