Epitaxial nucleation and growth of organic crystals on inorganic substrates Supervisors:Willem van...
-
date post
15-Jan-2016 -
Category
Documents
-
view
230 -
download
1
Transcript of 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
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
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
Systems & experimental methods
• 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
Systems & experimental methods
• nitrogen atmosphere
• substrate same temperature as vapor: 165 ºC
• 15-20 hours
N2
Substrate
Guestcompound
Furnace
Vapor deposition
Systems & experimental methods
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
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
Alizarin: NaCl {100}
[010]
[001]
[011][0-11]Alizarin length axis // [011] and [0-11] of NaCl
20 m
20 m
Vapor
Solution
Polarization microscope
Alizarin: NaCl {100}
Morphology
‘Roof’ like, pointed or topped of
SEM
AFM
Alizarin: NaCl {100}
Contact face
104.5°
a
b
c
(001)
(-101)(201)
NaCl {100}
(010)
Based on morphology: (001)
Alizarin: NaCl {100}
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)
Alizarin: NaCl {100}
NaCl {100}
Contact face (001):
• alizarin molecules interface
• strong interaction protruding oxygen atoms with ionic substrate
Alizarin: NaCl {100}
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
Alizarin: hole experiment
• 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
Alizarin: hole experiment
110 111
113
112
102
+
100
[100] [110]
Alizarin on as-grown NaCl {111}
NaCl: {111} trigonal symmetry
Alizarin: NaCl {111}
{111}
a
b
c
[-101]
[1-10]
[0-11]
10 m
Alizarin length axis // [-101], [1-10] and [0-11] of NaCl
Alizarin: NaCl {111}
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
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
Anthraquinone: NaCl {100}
(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°
Anthraquinone: NaCl {100}
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
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
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
Paraffins: HOPG (0001)
• ‘plate’ like crystals
• steep and high
• flat top faces
• substrate surface
Morphology
Paraffins: HOPG (0001)
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
Paraffins: HOPG (0001)
Growth mechanism
Onset of hetero-epitaxial growth: formation monolayer (2 types)
Paraffins: HOPG (0001)
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
Paraffins: HOPG (0001)
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
Paraffins: HOPG (0001)
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
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
Nucleation theory: general
Contact angle α depends on the surface energy of:
• substrate
• crystal
• interface
γint
γcryst
γsub
Σ Fi,hor = 0
Nucleation theory: general
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
Nucleation theory: general
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: general
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
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
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
Symmetry considerations
Symmetry considerations
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
Symmetry considerations
Single domain monocrystalline layer:
substrate with lowest symmetry possible, i.e. Ssubstrate = {E}
[001]
[010]
NaCl {100}
Symmetry considerations
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
• 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
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
Discussion & conclusion