epitaxial nucleation and growth of organic crystals on inorganic substrates supervisors:willem van...

Epitaxial nucleation and growth of organic crystals on inorganic substrates

Supervisors: Willem van Enckevort

Sander Graswinckel

Mirjam Leunissen

Outline

Introduction What is epitaxy? Why study it?

Systems & experimental methods

Alizarin GeneralOn NaCl {100}‘Hole experiment’On NaCl {111}

Anthraquinone GeneralOn NaCl {100}Molecular mechanics

Paraffins GeneralOn HOPG (0001)

Nucleation theory

Symmetry considerations

Discussion & conclusion What have we learned?Future

Introduction

Subject

Epitaxial three-dimensional nucleation and subsequent growth of organic substances on inorganic substrates from the solution and vapor phase

What is epitaxy?

Present use

Oriented growth of one crystal upon another

Etymology

• Greek derivation

• ‘under order’

• phalanx

Introduction

Epitaxy is the oriented crystal growth of a substance on a crystal surface of the same (‘homo-epitaxy’) or another substance (‘hetero-epitaxy’) in which the structure of the substrate determines the orientation of the guest crystals

Definition

Introduction

• homo -- hetero

• monolayer -- three-dimensional crystal

• inorganic -- organic

• polar -- apolar

• melt / solution / vapor / vacuum evaporation

Types of epitaxy

Introduction

• flat substrate

• 2D unit cell of substrate matches 2D

cell of overlayer (not necessarily 1:1)

• substrate doesn’t dissolve

• similar type of force in substrate and

overlayer (polar/apolar)

Requirements

Substratea1'

a2'

Overlayer

a1

a2

Introduction

Overlayer

Substrate

a1

a2

a1'

a2'

Examples of epitaxy

• ‘natural’: minerals growing together

• crystal growth/seeding

• GaN on sapphire (Al2O3): optical and electronic devices

GaN

Sapphire

[-1010]GaN

(a-axis)

[1-210]sapphire(b-axis)

[1-210]GaN

[10-10]sapphire

Introduction

Goal of present study

To study aspects of epitaxial crystal growth from the solution and vapor phase (e.g. growth mechanism, symmetry aspects)

Why are we interested in epitaxy?

Applications

• grow thick monocrystalline layers of organic compounds on a substrate

• grow ‘on command’ new polymorphs and crystals of substances which

won’t crystallize under ‘normal’ conditions

Approach

Grow oriented three-dimensional nuclei on top of a substrate, which, on continued growth, coalesce and grow together.

Introduction

Outline






Nucleation theory



Systems &

experimental methods

Substrate Guest Solutiondeposition

Vapordeposition

Systemtype

NaCl alizarin yes yes ‘polar’

NaCl anthraquinone yes yes ‘polar’

graphite(HOPG*)

paraffins yes no ‘apolar’

*HOPG: Highly Ordered Pyrolytic Graphite


• substrate cooled to 0 ºC

• (saturated) solution of 45-55 ºC of the guest substance

• 2-10 minutes

• dry with a tissue

Substrate

Solution

Solution deposition


• nitrogen atmosphere

• substrate same temperature as vapor: 165 ºC

• 15-20 hours

N2

Substrate

Guestcompound

Furnace

Vapor deposition


Outline






Nucleation theory



OH

O

OOH

Alizarin

Alizarin: general

General

Crystal structure according to Guilhem (1967): Pa

a = 21.04 Åb = 3.75 Åc = 20.12 Å

β = 104.5º

Alizarin: general

• needle shaped crystals

• long, thin

• hexagonal or rectangular

• sometimes hollow

SEM

Crystallization without substrate

Alizarin: general

Solution deposition: reproduction results of prof. Neuhaus

No great differences between solution and vapor deposition considering general aspects (morphology, orientation)

Alizarin on NaCl {100}

Solution: toluene + 2.5 mass% absolute ethanol

General

Vapor and solution deposition

Alizarin: NaCl {100}

NaCl: cubic Fm-3m

{100} tetragonal symmetry

Orientation

{100}

a

b

c


[010]

[001]

[011][0-11]Alizarin length axis // [011] and [0-11] of NaCl

20 m

20 m

Vapor

Solution

Polarization microscope


Morphology

‘Roof’ like, pointed or topped of

SEM

AFM


Contact face

104.5°

a

b

c

(001)

(-101)(201)

NaCl {100}

(010)

Based on morphology: (001)


Verification with X-ray powder diffraction

Diffraction vector substrate surface for all diffraction anglesEnhanced reflection from (hkl) planes // contact face

Randomly oriented crystallites

(dashed)

Oriented crystallites on NaCl

(solid)(003)


NaCl {100}

Contact face (001):

• alizarin molecules interface

• strong interaction protruding oxygen atoms with ionic substrate


Epitaxial nucleation on faces other than {100}

What is the influence of the substrate orientation on the orientation of the guest crystals?

‘Hole experiment’

Alizarin: hole experiment

100

NaCl

R

ds

12

½ l

110 111

113

112

102

+

100

[100] [110]

Asymmetric unit of point group m3m


• oriented crystallites grow on all faces

• strong dependence substrate orientation and preferred directions

• transition from tetragonal to trigonal symmetry on going from {100} to {111}

• no relationship between size and amount of crystallites and specific substrate orientation


110 111

113

112

102

+

100

[100] [110]

Alizarin on as-grown NaCl {111}

NaCl: {111} trigonal symmetry


{111}

a

b

c

[-101]

[1-10]

[0-11]

10 m

Alizarin length axis // [-101], [1-10] and [0-11] of NaCl


Outline






Nucleation theory



Anthraquinone

O

O

Alizarin structure analogue

Less complicated molecular structure

Monoclinic: P21/c

a = 7.87 Å

b = 3.96 Å

c = 15.78 Å

β = 102.7 º

Vapor and solution deposition

General

Anthraquinone: general

[010]

[001]

[011][0-11]

Orientation

Anthraquinone on NaCl {100}

Anthraquinone length axis // [011] and [0-11] of NaCl

10 m

Optical microscope

Vapor

Solution

20 m

Anthraquinone: NaCl {100}

Morphology

‘Roof’ like, pointed or topped of

AFMSEM


(10-2)

(100)(002)

NaCl {100}

(010)

(002)

(10-2)

c

a(100)

Contact face

Contact face (10-2):

• anthraquinone molecules interface

• strong interaction protruding oxygen

atoms with ionic substrate

a

b

c

102.7°


Dock ‘candidate’ faces onto the NaCl {100} lattice

Size a x b x c : a = # molecules in a row b = # rows per layer c = # layers

Rotate and translate plane until stage of minimal energy is reached

5x2x1

Molecular mechanics: prediction contact face and orientation

‘Candidate’ faces from morphology prediction (Eatt): (100), (002), (10-2)

Anthraquinone: molecular mechanics

[010]

[001]

NaCl {100}

Plot energy as function of orientation angle φ

• 1 row of molecules

• orientation 45 °

Orientation of face (10-2)

• distance between protruding O-atoms within 0.2% identical to distance between Na-ions

Anthraquinone: molecular mechanics

Outline






Nucleation theory



Paraffins

Dotriacontane: C32H66 (orthorhombic)

Tritriacontane: C33H68 (orthorhombic)

Tetracosane: C24H50 (triclinic)

General

Paraffin = n-alkane = CnH2n+2

Apolar

All-trans structure

Diluted n-heptane solutions

Paraffins: general

n-Alkanes adsorb on graphite

Highly Ordered Pyrolytic Graphite (HOPG):

• apolar

• stacking of layers

• (0001) hexagonal

Paraffins: general

Three preferred directions in trigonal pattern

Orientation

Paraffins on HOPG (0001)

Differently oriented domains

Cryo-SEM

Paraffins: HOPG (0001)

Crystal length axis oriented // HOPG periodic bond chain directions

Determination precise orientation by atomic force microscopy

5 x 5 nm


• ‘plate’ like crystals

• steep and high

• flat top faces

• substrate surface

Morphology


Prediction based on morphology crystals without substrate: (100) or (110)

Contact face

a

b

(110)

a

b

(100)

Extinction

directions

Verification: reflection polarization microscopy - different extinction conditions

C32H66 and C33H68: (100)HOPG

(001)(110)

(100)

Molecules (0001)a

b

c


Growth mechanism

Onset of hetero-epitaxial growth: formation monolayer (2 types)


Assumption: same crystal structure with and without substrate

Dock (100) bulk face on HOPG: no existent monolayer structure is obtained

Chain directions in monolayer and bulk crystal

differ ~30º (or equivalent -30º and 90º )

Crystal

1 1 1 1

1' 1' 1' 1'

2 2 2 2

2' 2' 2' 2'

c

a


b

c

Monolayer

30° 90°-30°

Monolayer bulk crystal:

subsequent layers of n-alkane molecules have to be rotated

Gilbert et al. (1994): bilayer

• 1st and 2nd layer mutually rotated by 90 º

• 1st layer consists of rows of parallel n-alkane chains with molecule plane

perpendicular to HOPG surface

Stranski-Krastanov: monolayer followed by three-dimensional nucleation


Outline






Nucleation theory



Nucleation theory

Nucleation theory: general

General

Competition three-dimensional nucleation in bulk phase and on substrate surface

larger number of nucleation sites (bulk) lower activation barrier (substrate)

Assumption: spherical nuclei

Rate of nucleus formation (J)

• surface area critical nucleus

• rate of addition of monomers

• concentration critical nuclei

Homogeneous:

Heterogeneous:

kT

Gf

kTfcfAJ

ochet

hom*

2/121

3/1 )(exp)()(''4

Surface area

substrate

f(α): correction factor for relative volume change critical nucleus

f’’(α): correction factor for reduced surface area critical nucleus

kT

Gc

kTVJ

oco

hom*

21

2/1

hom exp4

Kinetic factor

Activation barrier/ free enthalpy critical

nucleus

Volume fluid Volume

growth unit

Surface energy

Monomer concentration


Contact angle α depends on the surface energy of:

• substrate

• crystal

• interface

γint

γcryst

γsub

Σ Fi,hor = 0


Nucleation competition ratio (NCR)

)

)(ln)(

1

)(ln

)(

3

16exp(

)()(''

213

213

23

2/12/1

3/1

hom

beqb

seqs

c

s

b

o

het

Tcc

kTTcc

kT

f

T

Tff

V

A

J

JNCR

Temperature bulk

Temperature substrate


Conditions for epitaxial nucleation to be favored over bulk nucleation:

1) rate of formation of nuclei on the substrate surface must be higher than in

the bulk: Jhet > Jhomo (NCR>1)

2) rate of formation of nuclei on substrate surface must be reasonable:

Jhet > 105 m-2sec-1 (= 102 nuclei per mm2 in 103 sec)


Nucleation theory: vapor

Application to vapor deposition

Jhet increases dramatically for decreasing γ and α values

2 8 0 3 0 0 3 2 0 3 4 0 3 6 0 3 8 0 4 0 0T e m p e ra tu re (K )

1 E -4

1 E -3

1 E -2

1 E -1

1 E + 0

1 E + 1

1 E + 2

1 E + 3

1 E + 4

1 E + 5

1 E + 6

1 E + 7

1 E + 8

1 E + 9

1 E + 1 0

1 E + 1 1

Het

erog

eneo

us n

ucle

atio

n ra

te

2 8 0 3 0 0 3 2 0 3 4 0 3 6 0 3 8 0 4 0 0T emperature (K)

1 .0 E -4

1 .0 E -3

1 .0 E -2

1 .0 E -1

1 .0 E + 0

1 .0 E + 1

1 .0 E + 2

1 .0 E + 3

1 .0 E + 4

1 .0 E + 5

1 .0 E + 6

1 .0 E + 7

1 .0 E + 8

1 .0 E + 9

1 .0 E + 1 0

1 .0 E + 1 1

1 .0 E + 1 2

Het

erog

eneo

us n

ucle

atio

n ra

te (

1/m

2sec

)

NCR decreases for decreasing surface energy and increasing contact angle

2 8 0 3 0 0 3 2 0 3 4 0 3 6 0 3 8 0 4 0 0 4 2 0 4 4 0T e m p era tu re (K )

1.0E-2

1.0E-1

1.0E+0

1.0E+1

1.0E+2

1.0E+3

1.0E+4

1.0E+5

1.0E+6

Log

(NC

R)

2 8 0 3 0 0 3 2 0 3 4 0 3 6 0 3 8 0 4 0 0 4 2 0 4 4 0T em p era tu re (K )

1 .0 E -2

1 .0 E -1

1 .0 E + 0

1 .0 E + 1

1 .0 E + 2

1 .0 E + 3

1 .0 E + 4

1 .0 E + 5

1 .0 E + 6

Log

(N

CR

)

Nucleation theory: vapor

Outline






Nucleation theory



Prediction number of overlayer domains with different orientation (n): two-dimensional point group symmetry of the two contacting faces

N(S) = number of symmetry operators applying to the two-dimensional point group of the surface

Ssubstrate = {Ss,1; Ss,2; Ss,3; ….; Ss,n}

Scrystal = {Sc,1; Sc,2; Sc,3;…..; Sc,n}

Ss/c,1 = E

)(

)(

crystalsubstrate

substrate

SSN

SNn

Symmetry operators = transformations

(x,y,z) (x,y,z) E

(x’,y’,z’) S2

(xn,yn,zn) Sn



Example: alizarin on NaCl {100}

Alizarin: Pa

Contact face (001): m

N(Salizarin) = 2

NaCl: Fm-3m

Face {100}: 4mm

N(SNaCl) = 8

(x,y,z)

m

(x,y,z)

m

N(SNaCl Salizarin) = 2

42

8

)(

)(

alizarinNaCl

NaCl

SSN

SNn


Single domain monocrystalline layer:

substrate with lowest symmetry possible, i.e. Ssubstrate = {E}

[001]

[010]

NaCl {100}


Outline






Nucleation theory



• expensive ultra high vacuum equipment not necessary

• close relationship between symmetry substrate surface and

preferred orientations

• understanding of processes underlying formation of oriented three-

dimensional nuclei and their subsequent growth

• general conditions for the formation of epitaxial three-dimensional

nuclei to be favored over bulk nucleation

What have we learned?

Discussion & conclusion


• first onset epitaxial growth

• exact role of all factors by precise measurements

• nucleation theory for anisotropic nuclei

• let the separate nuclei grow together into a domain with a single orientation

• general rules to predict suitable combinations guest and substrate compounds

• induction of polymorphism

Future


epitaxial nucleation and growth of organic crystals on inorganic substrates supervisors:willem van...

Documents

anthraquinonegeneralon

paraffinsyesnoapolar

oriented crystal growth

hole experimenton nacl

growth of organic crystals

substance heteroepitaxy

growth mechanism

continued growth