Nanocrystals through De-Wetting and De-mixingAlokmay DattaSurface Physics and Materials Science DivisionSaha Institute of Nuclear Physics

Self-organization and Nanocrystals

• Self-organization is essentially a non-equilibrium phenomenon

• It requires two or more competing forces that are close in strength

• Two common areas where such forces are present are • De-wetting and• De-mixing

• These maybe utilized to build spontaneously ordered supramolecular patterns and structures such as Nanocrystals

De-Wetting

Wetting & dewetting

Substrate

LiquidLiquid

Substrate

SG = SL + LG cosc

Substrate

Wetting S 0

DewettingS 0

Hydrophobic surface

Liquid

Spreading coefficient,S = SG – (SL + LG )

S = LG (cosc – 1)

Young’s equation

Young-Dupre equation

Fatty Acid

Head

Tail

Hydrophobic tail Hydrophilic head

Simpler Picture

Langmuir Trough Langmuir Monolayer

Dissolve in solvent 1.Lighter than water2.Insoluble in water3.VolatileSpread on waterWait for solvent to

EvaporateCompress to required

Surface Density

Amphiphiles and Monolayers

Water

Tail

Hydrogen

Oxygen Carbon

Metal2+ 2+

2++ + +

Head

-

Langmuir Monolayer

Langmuir monolayers with divalent Metal ions in water

Langmuir-Blodgett film deposition with one-tailed amphiphiles

Langmuir-Blodgett Films

a)Front view

(b) Closer view of the dipper

Found from 1.X-ray diffraction2.X-ray and Neutron Reflectivity using deuterated and normal hydrocarbons in tailsMalik et al, Phys. Rev. B 52, R11654 (1995)Phys. Rev. B 65, 033409 (2002)

What Happens with Two-tailed Amphiphiles?

200nm

1.0µm

3.0µm

1 ML

3 ML

9 ML

Wetting on High Energy Surface: ZnSt LB Films on Si

Complete wetting

Partial Wetting

EDP shows full coverage in ZnSt monolayerpartial coverage in ZnSt multilayers

XRR results

400nm

400nm

Cd-T

Co-T

Defect-free morphology

`Pinhole’ defects present

AFM image of LB Templates

Template Formation: CdSt and CoSt trilayers (AML + SML) deposited on hydrophilic silicon (100) substrate by three subsequent vertical passages of substrate through air/water interface with Stearic acid Langmuir Monolayer (LM), containing Cd2+ and Co2+ in subphase, starting from water.

AML

SML

De-wetting on Low Energy Surface: ZnSt LB Films on Organic Films

CdSt and CoSt LB films are used as organic templatesCdSt is `solid-like’ but CoSt is `liquid-like’

C O M

Cd-T (unidentate co-ordination)

First Layer(asymmetric monolayer)

Next Layer(symmetric monolayer)

The Templates

Headgroup Bondingsand Co-ordinations (from FTIR Spectroscopy)

Surface Morphology (from AFM)

Co-T (bidentate co-ordination)

Nanocrystal Formation: The Cd-Stearate (Cd-T) and Co-Stearate (Co-T) Langmuir-Blodgett templates were vertically passed through the air/water interface with Stearic acid LM containing Zn2+ in subphase , from air to water and back to air.

Sample 1: ZnSt nanocrystal on Cadmium bearing template (Cd-T)Sample 2: ZnSt nanocrystal on Cobalt bearing template (Co-T)

AML

SML

Zinc Stearate (ZnSt) nanocrystals

Sample 1 Sample 2

Growth of ZnSt Nanocrystals

LB Deposition Conditions

Surface pressure: 30mN/mTemperature: 19°CpH ~ 6 adjusted by NaHCO3

Nanodroplet by de-wetting A. Checco et al Phys. Rev. Lett. 91, 186101,

(2003)

1.6µm 1.6µm

Self-assembled ZnSt nanocrystals show difference in structure on Cd-T and Co-T templates

ZnSt on Cd-T ZnSt on Co-T

(Height of nanocrystal ~ 25 nm)0.12 0.18 0.24

0.0

1.0x10-9

2.0x10-9

3.0x10-9

4.0x10-9

Inte

nsity

qz ()

Zinc Stearate on Cadmium Stearate template (E=260 eV)

0.14 0.21

0.0

7.0x10-10

1.4x10-9

Zinc Stearate on Cobalt Stearate template (E=260 eV)

Inte

nsity

qz ()

Diffraction measurements in the vicinity of C K-edge (BEAR beamline, Elettra Synchrotron, Italy)

Peak Zn on Cd-T Zn on Co-T Reflection Plane

1 51.93 (M) x (1/2,1/2,1/2)

2 43.33 (S) 43.94 (S) (001)

3 30.50 (S) X (101)+(011)

4 25.65 (W) 25.96 (S) (111)

Assigned diffraction peaks

1

2 3

4

2 4

1. Peak 2 (d ~ 43 Å) correspond to headgroup separation in multilayers of untilted ZnSt molecules.

2. Peak 2 correspond to reflection from (001) planes (assuming d to be lattice spacing c).

3. Peak 3 (Cd-T only) correspond to doubly degenerate (101) + (011) reflection.

4. Peak 4 correspond to reflection from (111) plane.

Structure of ZnSt nanocrystals

Diffraction Results:

Molecule

Non Close Packed

Structure

Unit Cell

Close Packed

Structure

Head

Structure

Bidentate Unidentate

OxygenZincCarbon

Tail Structure

Carbon

Hydrogen

Molecule

Molecular Structure

• Intensity of (111) peak is considerable for ZnSt on Co-T but very weak in case of Cd-T, consistent with a close packed structure for nanocrystals on Co-T and a non-close packed structure of the same on Cd-T.

• These structures for the nanocrystals are not observed in bulk ZnSt crystals.• Peak 1 (CdSt) corresponds to reflection from a (½½½) plane, i.e., some superlattice.• Exact nature of this superlattice has not been ascertained but most probably it is coming from the

fact that Zn-bearing carboxylate group has two structures – unidentate and bidentate bridged. Absence of this superlattice in Co-T suggests a mixture of the two structures in each unit cell, as seen in ZnSt LB multilayers.

Dipole moment mismatch at interface

ZnSt on Co-T

CdSt on Co-T

CoSt on Cd-T

ZnSt on Cd-T

Mater. Res. Express 1 (2014) 025006

The De-wetting 'Force'

De-Mixing

Our Main Actor – The Liquid Crystal MBBA(N-(4-Methoxybenzylidene)-4-butylaniline)

•The Imine (-C=N) has an electron lone-pair located on nitrogen.

•MBBA has a well-defined Nematic-Isotropic (N-I) Phase Transition and no well-defined Nematic-Smectic Transition

The Nematic-Isotropic Transition

• The Nematic phase is unique since it requires no extra interaction over a fluid phase, it is a ‘higher density liquid phase’ just as a simple liquid is a ‘high density gas phase’

• However, unlike the simple liquid, it breaks the rotational symmetry simply due to the anisotropy in the molecular shape, which is decided by the molecular conformation. This has made the categorization of N-I transition ambiguous.

• We shall see in our results the strong role played by this idea

Growth of Au Nano-prisms

Basic Ideas: 1.The electron lone-pair on the imine group may be used to reduce HAuCl4 to Au in a slow and regulated way 2.The Liquid Crystal, in this reactive and charged environment, may develop positional order, i.e. go over to a ‘Smectic-like’ phase, and this phase, in turn would provide an ordered matrix for nanoparticle growth

The growth technique consisted of prolonged slow heating of a solution of HAuCl4 and MBBA in alcohol. It was seen that alcohol was necessary but other than a slowing down of the reaction rate with hydrocarbon chain length no change was observed from methanol to propanol

Characterization of Gold Nano Particle

Transmission Electron Microscopy (TEM)

Existence of Highly Faceted Nano Particle

SAED Pattern showing different crystal planes of Au that matched well with grazing incidence x-ray diffraction

EDAX Spectrum

Faceted crystals

Spheres

20 nm

20 nm 20 nm

20 nm

20 nm

0˚ 10˚

20˚ 30˚

40˚6.8 nm

Electron Tomography Pattern showing the 3-Dstructure

Highly Symmetric Au Nano prism with each side 12 nm

5 nm

-111 -11-1

002

00-2

[110]

{111}

Fast Fourier Transform

Grazing Incidence X-Ray Diffraction (GIXRD)

Interplanar spacing ≈ 0.207 nm Lattice Spacing ≈ 0.413 nm

Good Agreement with TEM and standard literature

Average Crystallite Size ≈ 30 nm

UV-Visible Absorption Spectroscopy

Characteristic Plasmon resonance bandClearly visible at 513 nm

Size of the Au Nano particle increases with Increasing Nano particle precursor

Effect of Solvent in Nano Particle Production

Role of Methanol is very important in nano particle production. Methanol itself cannot reduce HAuCl4 to produce nano particle in the matrix.

Fourier Transform Infrared Spectroscopy

Red Shift in imine stretch by 26 cm-1 signifies co-ordination between MBBA–AuNP System.

A new peak is appearing at 1655 cm-1 due to oxidationof methanol

Probable Reaction Mechanism

Nematic to Smectic Texture Transformation of MBBA during synthesis

Differential Scanning Calorimetry Study

The MBBA-AuNP conjugate system clearly indicates much higher phase transitionTemperature compared to pristine MBBA which indicates system trying to stabilize atmuch higher TC due to induced smectic ordering

Ag-60-Au-40AgNO3 + HAuCl4

High ResolutionTransmissionElectronMicroscopy

The Star Structure

Au Ag

Au AgAu Core Ag Shell (Star)

Total Core-Shell Composite

The Star Core-Shell Structure from Element Mapping

People Involved

1. Smita Mukherjee, UPMC, Paris

2. Nupur Biswas, IISc, Bangalore

3. Kaustabh Dan, SINP

4. Biswarup Satpathi, SINP

5. Madhusudan Roy, SINP

6. Stefano Nannarone, Elettra Sincotrone, Trieste

7. Angelo Giglia, Elettra Sincotrone, Trieste

8. Sarah Hidki, UPMC, Paris

Thank You!