Self Assembled Growth Self Assembled Growth of Nanostructuresof Nanostructures

ArshadArshad SaleemSaleem BhattiBhattiDepartment of PhysicsDepartment of Physics

COMSATS Institute of Information Technology, Islamabad.COMSATS Institute of Information Technology, Islamabad.

Collaborator: U. Collaborator: U. ManzoorManzoor & Co& CoStudents: M. Students: M. HafeezHafeez, Muhammad , Muhammad FakharFakhar Zia, Zia, JunaidJunaid Ali, Ali, AsifAsif

IqbalIqbal ZiaZia

OverviewOverview

Concept of self assemblyConcept of self assemblyThin film growth mechanismsThin film growth mechanismsFormalism of VLS growth mode for Formalism of VLS growth mode for nanowiresnanowiresExperiments (Experiments (ZnSZnS--NWs, NWs, ZnOZnO--NWs, SiNWs, Si--NWs, NWs, CNTsCNTs))Sensor applications of nanowiresSensor applications of nanowiresConclusionsConclusions

One dimensional structures in NatureOne dimensional structures in Nature

Spider webSpider webSilk fiberSilk fiber…………

Spider webSpider webThe average diameter of a thread in a orb web is around 0.15 mm (150 μm). The smallest measured thread was only 0.02 mm (20 μm) thick. We see the web only because of the reflection of sunlight These thin wires are capable of stopping a bee flying at full speed. This thread is not only strong but also very elastic.

• What is the thread made of? • It is a protein of a molecular mass of 30,000 Dalton in the gland.

Outside the gland it polymerizes to a molecule named fibroin with a molecular mass of around 300,000 Dalton.

• Activation of polymerization process is not clear.

E. Kullmann, H. Stern, Leben am seidenen Faden, Die rätselvolle welt der Spinnen, 1975, VerlagsgruppeBertelsmann Verlag, Munchen, Germany, ISBN 90 222 0239 9

Spider web: propertiesSpider web: properties

• Structure of spider silk. Inside a typical fiber, one finds crystalline regions separated by amorphous linkages. The crystals are beta-sheets that have assembled together.

• Remarkably strong material, tensile strengthis superior to that of high-grade steel, and as strong as aramid filaments, such as Twaron or Kevlar.

• Extremely lightweight: a strand of spider silk long enough to circle the Earth would weigh less than 500 grams (1 lb.)

• Ductile, able to stretch up to 140% of its length without breaking.

• Can hold its strength below -40 degrees Celsius. This gives it a very high toughness (or work to fracture), which themselves are benchmarks of modern polymer fiber technology."

Stretching of silk, 1, 5 and 20 times.

ManMan’’s way: Approach to Matter at Nanoscales way: Approach to Matter at Nanoscale

Top Top –– DownDownBottom Bottom –– upup (this is how Nature Works?)(this is how Nature Works?)

There are two routs for the synthesis of 1There are two routs for the synthesis of 1--D nanostructuresD nanostructures

Top-down technique

– Growth/over growth

– Pattern transfer

– Etching

Bottom-up technique

– Vapor-Liquid-Solid (VLS)

– Vapor-Solid (VS)

– Solution-Liquid-Solid

Substrate

Thermal SpeciesThermal Species

• Adsorb• Diffuse• Desorb• Coalesce into

clusters• Cluster growth

Atomic-scale phenomena affecting N&G

Strain Assisted/Lattice MismatchStrain Assisted/Lattice MismatchImportant for Semiconductor IndustryImportant for Semiconductor Industry

Layered growth of MonolayersLayered growth of MonolayersThree Growth Modes in Thin FilmsThree Growth Modes in Thin Films

FM (no or very small lattice mismatch < 4%)FM (no or very small lattice mismatch < 4%)SK (moderate Lattice mismatch < 10%)SK (moderate Lattice mismatch < 10%)VW (Large mismatch > 10%)VW (Large mismatch > 10%)

Fundamental Physical ProcessFundamental Physical ProcessRole of Adsorbed atoms and substrateRole of Adsorbed atoms and substrateSurface energies of adsorbed specie, substrate and Surface energies of adsorbed specie, substrate and

interface energyinterface energy

transfer among nuclei; ripening

deposition flux [cm-2s-1]

a step edge acts as an infinite size nucleus.

metastableclusters

surface vacancy

adatom diffusion

Nucleation and Growth

stableclusters

Cluster Size (atoms)

The critical radius r*

The nucleation energy barrier, E*

-2.0-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

Clu

ster

Ene

rgy

(Arb

itrar

y U

nits

)

1 3 5 7 9 11

volume volume energyenergy

surface surface energyenergy

Vapor liquid solid (VLS) mechanismVapor liquid solid (VLS) mechanism

Schematic representation of the VLS mechanism

Firstly reported by Wagner et al. (1964).

A metal thin film (or nanodroplets) used as a catalyst for the growth of 1-D nanostructures.

At elevated temperatures, this thin film on substrate to make the alloy droplets which behaves as nucleation sites for the growth of 1-D nanostructures.

Eutectic point of Au-Si alloy is 363 oCwhile for the Zn-Si it is 420 oC.

Vapors of the source material are transported to the droplet by using a carrier gas flow through the growth chamber.

The metal used as catalyst must have high values of surface tension and accommodation coefficient.

Time

R.S.Wagner and W.C.Ellis: Appl. Phys. Lett. 4, 89 (1964).

Onset and formation of nucleiOnset and formation of nuclei

Formed on the substrate Initial growth of the nanowire.

The hillock shape of the nanowire root

Modified young’s equation

Theoretical study of the VLS growthTheoretical study of the VLS growth

VSoB

o rlTKlrG σπηπδ 2ln2 +Ω

−=

The change in Gibbs free energy is given by

eqZnS

ZnS

PP

=η

Volume of the unit cell.

The reduction in the Gibbs free energy due to crystal formation.

The increase in the surface energy due to increase in surface area.

is the vapor pressure super saturation ratio.

• T.Y. Tan: Mat. Sci. Eng. B 10, 227 (1991).• T.Y. Tan, N. Li, U. Gösele, Appl. Phys. A, 78, 519 (2004). • N. Li , T.Y. Tan, U. Gösele, Appl. Phys. A, 86, 433 (2007).

cLSc rrG τπσπδ 22 +=

ησ lnΩ

−=TKl B

oLSc

VSoc l στ =

Now assume the existence of effective surface tension, and

Effective line tension, which swept out on the nanowire growth front the area and the circumference length respectively.

Effective surface tension component of the chemical tension

Effective line tension component of the chemical tension

Theoretical study of the VLS growth (cont.)Theoretical study of the VLS growth (cont.)

The effective surface tension component acts along the liquid-solid interface as an in plane vector quantity.

Effective line tension component acts along the circumference of the liquid-solid interface as in plane vector quantity

Effective chemical TensionEffective chemical Tension

= effective surface tension component of the chemical tension,

= effective line tension

= volume of the unit cell

= ZnS super saturation

= Radius of the growing droplet

rlTK

lr

VSoBo

C

LSCC σ

ητσσ +Ω

−=+= ln

0<Cσ0>Cσ

1D growth will occur

0D growth will occur

Effective Chemical Tension (σc) = Surface Tension + Line Tension

Theoretically Calculated Growth rateTheoretically Calculated Growth rate

db

TKb

TKV n

B

VSn

B

on 14 111 σμ Ω−

Δ=

For a wire with l = 30 μm: η > 4.5

For a wire with l = 20 μm: 3 < η < 4.5

Theoretically calculated Growth rate

In our case we have obtained 50 µm or long wires so the super saturation ratio is b/w 15 and 20.

Dots are grown due to absence of supersaturation condition at extremely low flow rates

At high flow rate and temperature the wire with small diameter grows faster than at low flow rate .

0 20 40 60 80 100 120 140 160 180 200 220

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0η = 17

Length of wire = 50 μm

V1/2 (n

m/S

)Diameter (nm)

Length of wire= 20 μm

Length of wire = 30 μm

η = 1.5

η = 3

η = 4.5

ZnSZnS

IIII––VI polar compound semiconductor.VI polar compound semiconductor.

EEgg = = 3.56 3.56 –– 3.91 3.91 eVeV at 300 K.at 300 K.

Applications: Applications: OptoelectronicsOptoelectronicsFlatFlat--panel displayspanel displaysElectroluminescent devices Electroluminescent devices Infrared windows Infrared windows Sensors and lasersSensors and lasersbiologybiology

Excellent mechanical properties:Excellent mechanical properties:ZnSZnS doped with certain metal doped with certain metal cationscations emit intense light upon stress.emit intense light upon stress.

Polar planes in wurtziteStructures

Crystal Structures of Crystal Structures of ZnSZnS

(b) Wurtzite

ZnS has two crystal structure “zinc blend (cubic) structure and Wurtzite (hexagonal) structure.

ZnS has a stable Zinc blend structure at room temperature

At 1020 oC, it undergoes a transformation from zinc blend to Wurtzite crystal structure

the repeating pattern of the zinc blend (111) planes is ABCABCABCABC is very similar to the ABABABAB of the Wurtzite (0001) planes

ZnS in Wurtzite form exhibits interesting morphology due to different planes have different energies and surface polarity and chemical activities.

(a) Zinc blend

Formation of nanowires, Formation of nanowires, nanobeltsnanobelts, , nanosheetsnanosheets

Growth set upGrowth set up

-50 -40 -30 -20 -10 0 100

200

400

600

800

10001100 oC

675 oC

460 oC

Tem

pera

ture

(o C)

Distance (cm)

ZnS

S1

S2

S3

850 oC

Schematic of tube furnace employed for nanowire growth

Temperature profile of reaction chamber as a function of distance

Furnace temperature 800 0C for 30 mins1100 0C for 3 hours

0 50 100 150 200 250 300 350

0

200

400

600

800

1000

1200

Tem

pera

ture

(o C)

Time (min)

30 min

Growth at 20 Growth at 20 SccmSccm Flow rateFlow rate

T = 850 oC

Dav = 120 ± 29 nm

T = 675 oC

Dav = 200 ± 83 nm

T = 460 oC

Dav = 38 ± 7 nm

ρ = 8 x 1010 cm-2

21

Growth at 2 Growth at 2 sccmsccm Flow RateFlow Rate

T = 460 oCDav = 13 ± 2 nm ρ = 5 x 1010 cm-2

T = 850 oCDav = 275 ± 33 nm ρ = 8 x 108 cm-2

400 500 600 700 800 900

0

50

100

150

200

250

300

Substrate Temp oC

Dav

(nm

)

0

100

200

300

400

500

Density (10

8 cm-2)

T = 675 oCDav = 143 ± 46 nm ρ = 4.4 x 109 cm-2

Variation of density with substrate temperature

Growth at 60 Growth at 60 SccmSccm Flow rateFlow rate

T = 460 oCDav = 95 ± 33 nmρ = 1.9 x 109 cm-2

T = 675 oCT = 850 oC

Various nanostructures: summaryVarious nanostructures: summary

Growth of nanoleaves at 880 oC

Growth at 20 Sccm at (a) 880 oC, (b) 810 oC,

Growth at 20 Sccm at (a) 765 oC, (b) 630 oC,

Some more structuresSome more structures

2520 30 40 50 60 70 80

0

50

100

150

200

250

Nanoleaves

Nanowires

2θ

(011)

(019)(0 0 66)

Nanodots(a)

0

100

200

300

400

500(b)

Inte

nsity (100)

(002)

(101)

(102)

(110)

(103)(112)

(203)

Wurtzite 2H

0

200

400

600

800

1000

1200(c)

(100)

(002)

(101)

(102)

(110)

(103)(112)

(203)(210)

Wurtzite 2H

XRD Analysis At 850 oC

Nanoleaves and Nanowires, the growth is under high supersaturation conditions and thus the value of Gibbs free energy is very high, responsible for the crystallization at the L-S interface., shown middle and upper spectrum.

At the 2 Sccm flow rate, nanodots show low order of oxidized crystalline structures (ZnO) shown in the bottom spectrum.

These are formed as the carrier gas flow rate is too low to obtain supersaturation condition around the substrate surface, which inhibits the growth of crystal at the L-S interface.

Oxidation took place due to the presence of native oxide of Si, which could not be flushed out due to very low flow rate.

26

Absorption Spectroscopy At 850 oC

2.5 3.0 3.5 4.0 4.50.06

0.08

0.10

0.12

0.14 Nanodots

Energy (eV)

3.12(ZB-ZnO)

3.60(ZB-ZnS)

0.0

0.2

0.4

0.6

0.8

1.0 Nanowires

Abs

orpt

ion

3.56(zB-ZnS)

3.89(W-ZnS)

0.0

0.2

0.4

0.6

0.8

1.0

3.60(ZB-ZnS)

3.90(W-ZnS)

Nanoleaves

The spectrum shows the mixed growth of the ZnS/ZnO for nanodots.

The peak centered at 3.12 eV is due to Zinc blend ZnO and at 3.60 eV is due to Zinc blend ZnS.

The absorption spectroscopy spectra of nanowires and nanoleaves show the mixed growth of Zinc blend and Wurtzite ZnS having band gaps 3.60 eV and 3.90 eV .

Hafeez, Manzoor and Bhatti, submitted to Appl. Phys A (20

The absorption spectroscopy graphs for samples grown at T= 395oC

Absorption spectra from samples grown at T = 880oC, 810oC and 765oC

• Nanostructures grown at high temperature show sharp absorption

• Nanostructures grown at moderate and low temperatures show micture of phases and signatures of ZnO

Hafeez, Manzoor and Bhatti, submitted to Appl. Phys A (20

0 100 200 300 400 5000.0

2.0x107

4.0x107

6.0x107

8.0x107

1.0x108

1.2x108

Gas on Gas on Gas on

Gas off Gas off

Res

ista

nce

(Ohm

s)

Time ( Seconds)

Gas off

Gas on

(a)

-50 0 50 100 150 200 250 300 350 400 450 500

20700000

21000000

21300000

21600000

21900000

22200000

22500000Gas on

Res

ista

nce

(Ohm

s)Time (Seconds)

Gas on

Gas off

Gas on

Gas off

Gas on

Gas off

(b)

0 50 100 150 200 250 300 3505420000

5440000

5460000

5480000

5500000

5520000

5540000

5560000

Gas off

Gas off

Gas off

Gas on

Gas on

Gas on

Res

istn

ace

(Ohm

s)

Time (Seconds)

(c)

Hydrogen SensingHydrogen Sensing

Change in electrical conductivity due to the interaction between gas molecules to be detected and the surface complexes such as O−, O2− reactive chemical species (S2−)

ZnS nanowires exhibited better response to the hydrogen because the nanowires have high aspect ratio

( ) ( ) ( )

( ) ( ) ( )−−

−−

+→+

+→+

eSHSH

eOHOH

gsg

gsg

22

2

22

2 2

60 80 100 120 140 160 180

0.0

0.2

0.4

0.6

0.8

1.0

Nano leaves

DotsNor

mal

ized

resi

stan

ce

Time (seconds)

NW

Hafeez, Manzoor and Bhatti, submitted to Appl. Phys A (20

ZnO NanostructuresZnO Nanostructures--Effect of Ar Flow RateEffect of Ar Flow Rate

Diameter of the nanostructures increase with Ar flow rate

Umair Manzoor & Do Kyung Kim, Physica E 2009, 41, 500-505.

Umair Manzoor & Do Kyung Kim, Scripta Materialia 2006, 54, 807-811.

Unique ZnO NanocombsUnique ZnO Nanocombs

Only Nanocombs were presentMost nanocombs don’t have a clear primary armA thin film is present between 1 side of the secondary arms

Experimental Conditions900°C, 15 min, Ar&O2 Flow: 150&2sccm

Ultra Long ZnO Ultra Long ZnO NanobeltsNanobelts

-2 0 0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 00 .0 0 E + 0 0 0

5 .0 0 E + 0 0 8

1 .0 0 E + 0 0 9

1 .5 0 E + 0 0 9

2 .0 0 E + 0 0 9

C O

H 2 S e n s itiv ity 1 5 .5 )

N H 3 S en sitivity 22 .1

Res

ista

nce

(ohm

s)

T im e (m in )

1000 ppm of gas

Umair Manzoor et al, unpublished data

ZnO nanobelts are highly selective

NH3 gas is detected with good sensitivity

Intermediate sensitivity and selectivity for H2

GaGa doped Si Nanowiresdoped Si Nanowires=

ZnSZnS nanostructures of different morphology and size were successfulnanostructures of different morphology and size were successfully ly synthesized by changing the substrate temperature and carrier gasynthesized by changing the substrate temperature and carrier gas s flow rate. flow rate.

This enabled us to control the supersaturation regime for growthThis enabled us to control the supersaturation regime for growth of of different nanostructures. different nanostructures.

Highly crystalline nanowires were obtained at the flow rate of 2Highly crystalline nanowires were obtained at the flow rate of 20 0 sccmsccmat high temperatures (875at high temperatures (875ooC). C).

These showed excellent phase purity with sharp band edge and These showed excellent phase purity with sharp band edge and excellent response to hydrogen gas sensing characteristics with excellent response to hydrogen gas sensing characteristics with a a response time is less than 1 second for the 1D response time is less than 1 second for the 1D ZnSZnS nanostructures and nanostructures and response sensitivity of 8, much higher than leaves and dots.response sensitivity of 8, much higher than leaves and dots.

ConclusionsConclusions

AcknowledgmentsAcknowledgments

HEC (074HEC (074--17451745--ps4ps4--388)& for funding of MH, 388)& for funding of MH, Research Grant # 261 and HEC development grant Research Grant # 261 and HEC development grant for micro devices. for micro devices.

COMSATS IIT COMSATS IIT

UIUC and GCU UIUC and GCU

Comparison: Optical propertiesComparison: Optical properties

2.0 2.5 3.0 3.5 4.0 4.5 5.0

0.20.30.40.50.60.70.80.91.01.1

T=850 oC

T=675 oC

T=460 oC

Energy (eV)

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Abs

orpt

ion

0.0

0.2

0.4

0.6

0.8

1.0

3.10 eV

3.63 eV

3.64 eV

3.58 eV

3.88 eV(b)

2.5 3.0 3.5 4.0 4.5 5.0

0.5

0.6

0.7

0.8

0.9

1.0

3.57 eV

3.58 eV

3.31 eV

3.81 eV

3.88 eV

3.89 eV

Energy (eV)

3.12 eV

0.2

0.4

0.6

0.8

1.0

Abso

rptio

n

0.0

0.2

0.4

0.6

0.8

1.0

T=460 oC

T=675 oC

T=850 oC(a)

2.5 3.0 3.5 4.0 4.5 5.00.70

0.75

0.80

0.85

0.90

0.95

1.00

3.07 eV

3.29 eV

3.31 eV

Energy (eV)

3.34 eV

3.07 eV

0.6

0.7

0.8

0.9

1.0

1.1

Abs

orpt

ion

0.06

0.08

0.10

0.12

0.14

T=460 oC

T=675 oC

T=850 oC

2 sccm 20 sccm 60 sccm

leaves

pallets

dots

dots

dots

dots

wires

dots

wires/rods

Absorption spectroscopy spectra of the nanodots, nanowires and nanoleaves grown under different conditions. Absorption due to ZnS (Wurtzite and zinc blend) and ZnO is observed.

Silk FibersSilk Fibers

Physical propertiesPhysical propertiesSilk fibers from the Silk fibers from the BombyxBombyx morimori silkworm have a silkworm have a triangulartriangular cross sectioncross section with rounded corners, 5with rounded corners, 5--10 10 μμmm wide. The wide. The fibroinfibroin--heavy chain is composed mostly of betaheavy chain is composed mostly of beta--sheets, due to a 59sheets, due to a 59--mer mer aminoacidaminoacid repeat sequence repeat sequence GAGAGSGAAG[SGAGAG]8Y with some variations.GAGAGSGAAG[SGAGAG]8Y with some variations.[14][14] The flat surfaces of the fibrils reflect The flat surfaces of the fibrils reflect lightlight at many angles, at many angles, giving silk a natural shine. The crossgiving silk a natural shine. The cross--section from other silkworms can vary in shape and diameter: cresection from other silkworms can vary in shape and diameter: crescentscent--like for like for AnapheAnaphe and elongated wedge for and elongated wedge for tussahtussah. Silkworm fibers are naturally extruded from two silkworm gland. Silkworm fibers are naturally extruded from two silkworm glands as a pair of s as a pair of primary filaments (primary filaments (brinbrin) which are stuck together, with ) which are stuck together, with sericinsericin proteins acting like proteins acting like glueglue, to form a . , to form a . BaveBave diameters for diameters for tussah silk can reach 65 tussah silk can reach 65 μμmm. See cited reference for cross. See cited reference for cross--sectional SEM photographs.sectional SEM photographs.[15][15]Silk has a smooth, soft Silk has a smooth, soft texturetexture that is not slippery, unlike many that is not slippery, unlike many synthetic fiberssynthetic fibers. Its . Its denierdenier is 4.5 is 4.5 g/dg/d when dry and 2.8when dry and 2.8--4.0 4.0 g/dg/d when moist.when moist.Silk is one of the strongest natural fibers but loses up to 20% Silk is one of the strongest natural fibers but loses up to 20% of its strength when wet. It has a good of 11%. Its of its strength when wet. It has a good of 11%. Its elasticityelasticityis moderate to poor: if elongated even a small amount, it remainis moderate to poor: if elongated even a small amount, it remains stretched. It can be weakened if exposed to too much s stretched. It can be weakened if exposed to too much sunlight. It may also be attacked by insects, especially if leftsunlight. It may also be attacked by insects, especially if left dirty.dirty.Silk is a poor conductor of Silk is a poor conductor of electricityelectricity and thus susceptible to and thus susceptible to static clingstatic cling..Unwashed silk chiffon may shrink up to 8% due to a relaxation ofUnwashed silk chiffon may shrink up to 8% due to a relaxation of the fiber macrostructure. So silk should either be prethe fiber macrostructure. So silk should either be pre--washed prior to garment construction, or washed prior to garment construction, or dry cleaneddry cleaned. . Dry cleaningDry cleaning may still shrink the chiffon up to 4%. Occasionally, may still shrink the chiffon up to 4%. Occasionally, this shrinkage can be reversed by a gentle steaming with a pressthis shrinkage can be reversed by a gentle steaming with a press cloth. There is almost no gradual shrinkage nor shrinkage cloth. There is almost no gradual shrinkage nor shrinkage due to moleculardue to molecular--level deformation.level deformation.[[editedit] Chemical properties] Chemical propertiesSilk is made up of the Silk is made up of the amino acidsamino acids GlyGly--SerSer--GlyGly--AlaAla and forms and forms Beta pleated sheetsBeta pleated sheets. form between chains, and side chains . form between chains, and side chains form above and below the plane of the Hform above and below the plane of the H--bond network.bond network.The high proportion (50%) of The high proportion (50%) of glycineglycine, which is a small , which is a small amino acidamino acid, allows tight packing and the fibers are strong and , allows tight packing and the fibers are strong and resistant to stretching. The tensile strength is due to the manyresistant to stretching. The tensile strength is due to the many interseededinterseeded hydrogen bonds. Since the protein forms a Beta hydrogen bonds. Since the protein forms a Beta sheet, when stretched the force is applied to these strong bondssheet, when stretched the force is applied to these strong bonds and they do not break.and they do not break.Silk is resistant to most Silk is resistant to most mineral acidsmineral acids, except for , except for sulfuric acidsulfuric acid which dissolves it. It is yellowed by perspiration.which dissolves it. It is yellowed by perspiration.