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Molecular Orientation in Organic Thin Films
Prof. Kathy RowlenDepartment of Chemistry and Biochemistry
University of Colorado, Boulder
Why study molecular orientation in thin films?
interfacial properties
(optical, electronic and mechanical)
molecular interactions
organizational model for complex systems
best means to probe molecular orientation?
does the substrate affect thin film characteristics?
how does molecular structure affect
thin film characteristics?
does molecular orientation and organization vary with time and (or) coverage?
Questions to be addressed:
Organization at Low Surface Coverage?
Photoacoustic Spectroscopy
p
ao
C
KIS
A + hA*
A* A +heat
Angle-Resolved Absorbance withPhotoacoustic Detection (ARAPD)
For a long-axis transition moment:
232 22 sinKsin)/A()(A fZmax
2/1
fZ1*
fZ
*
Zz
Z'z2
Z'zfZ
Kcos
cosKK
Lab Z-axis
Lab Y-axis
Lab X-axis
Molecular z'-axis
Incident Beam
z'Z
Surface Plane
Exhibited second harmonic generation No linear dichroism Apparent orientation angle (by SHG) ~ 45° No change in orientation as layers added
Katz et al. Science (1991) 254, 1485-1487
Angle of Incidence
0 10 20 30 40
Pho
toac
oust
ic S
igna
l (a.
u.)
0
1
5 layers
2 layers
1 layer
0 layers
3 layers
Evolution of Orientation in Multilayer Film
Number of Layers
0 1 2 3 4 5 6 7
Mea
n T
ilt A
ngle
10
15
20
25
30
35
40
Molecular Long Axis Orientation(as a function of number of layers)
“Self-Healing” ?
32° ± 2°One “layer”
Six “layers” 15° ± 1°
Questions:1) How does the angular distribution change?2) What is the effect of surface roughness?
LocalSurfaceNormal, s
sz'
L
sZ
Lab Z-axis
Z
Substrate Roughness
2 2 2
2 1L L L expH
...21Kcos 4382
sZsZ2
Effect of surface roughness on ARAPD(linear dichroism) measurements
in which each value of Kij is equal to cos2ijand the subscripts
indicate the relevant angle, such that z'Z is the angle between the
molecular orientation axis, z', and the macroscopic surface normal, Z.
K K K K Kz Z sZ sz sZ sz 12 1 3
Orientation w.r.t. Local Surface Normal
0 20 40 60 80
App
aren
t Orie
ntat
ion
Ang
le
0
20
40
60
80
L = 5 A
L = 30 A
Error as a function of length scale
Second Harmonic Generation
1064 nm 532 nm
Molecular Orientation by SHG
X-axis
cos
sin
MolecularLong Axiszzz
Z-axis
ZZZ = Nscos3zzz
ZXX = (1/2)Nssin2coszzz
D zzz
zzz zxx
cos
coscos
( )
( ) ( )
3 2
2 22
2
Effect of surface roughness on SHG
D zzz
zzz zxx
cos
coscos
( )
( ) ( )
3 2
2 22
2
Influence of Angular Distribution (on SHG determination of zZ)
Common assumption:
D zzz
zzz zxx
cos
coscos
( )
( ) ( )
3 2
2 22
2
D
,cos
cos
cos exp sin
cos exp sin0
3 3
0 0
2 2
0 0
2 2
2
2
d
d
Influence of Angular Distribution (on SHG determination of zZ)
Common asumption:
Assume Gaussian distribution:
D
,cos
cos
cos exp sin
cos exp sin0
3 3
0 0
2 2
0 0
2 2
2
2
d
d
SHG “Magic Angle”
21
0P cosP1
1cos3P 221
2 cos3cos5P 3
21
3
DP P
P
cos
cos
3 25 3
35 1
1
0
nn dsinPfP
Legendre Polynomial (cos)
DP P
P
cos
cos
3 25 3
35 1
1
SHG Magic Angle
Sequence of events as angular distribution broadens
as <P3> approaches zero, SHG apparent tilt angle converges to 39.2°
as <P2> approaches zero, loss of linear dichroism
as <P1> approaches zero, loss of SHG intensity
1) Heinz, T. F.; Tom, H. W. K.; Shen, Y. R. Phys. Rev. A 1983, 28, 1883. 2) Grubb, S. G.; Kim, M. W.; Rasing, Th.; Shen, Y. R. Langmuir 1988, 4, 452.3) Campbell, D. J.; Higgins, D. A.; Corn, R. M. J. Phys. Chem. 1990, 94, 3681. 4) Shirota, K.; Kajikawa, K.; Takezoe, H.; Fukuda, A. Jpn. J. Appl. Phys. 1990, 29, 750. 5) Li, DeQ.; Ratner, M. A.; Marks, T. J.; Zhang, C. H.; Yang, J.; Wong, G. K. J. Am. Chem. Soc. 1990, 112, 7389. 6) Bubeck, C.; Laschewsky, A.; Lupo, D.; Neher, D.; Ottenbreit, P.; Paulus, W.; Prass, W.; Ringsdorf, H.; Wegner, G. Adv. Mater. 1991, 3, 54. 7) Liu, X.; Liu, L.; Chen, Z.; Lu, X.; Zheng, J.; Wang, W. Thin Solid Films 1992, 219, 221. 8) Bell, A. J.; Frey, J. G.; VanderNoot, T. J. J. Chem. Soc. Faraday Trans. 1992, 88, 2027. 9) Yitzchaik, S.; Roscoe, T. B.; Kakkar, A. K.; Allan, D. S.; Marks, T. J.; Xu, Z.; Zhang, T.; Lin, W.; Wong, G. K. J. Phys. Chem. 1993, 97, 6958. 10) Nalwa, H. S.; Watanabe, T.; Nakajima, K.; Miyata, S. Thin Solid Films 1993, 227, 205. 11) Higgins, D. A.; Naujok, R. R.; Corn, R. M. Chem. Phys. Lett. 1993, 213, 485. 12) Yokoyama, S.; Yamada, T.; Kajikawa, K.; Kakimoto, M.; Imai, Y.; Takezoe, H.; Fukudo, A. Langmuir 1994, 10, 4599. 13) Kezhi, W.; Chunhui, H.; Guangxian, X.; Xinsheng, Z.; Xiaming, X.; Lingge, X.; Tiankai, L. Thin Solid Films 1994, 247, 1. 14) Naujok, R. R.; Higgins, D. A.; Hanken, D. G.; Corn, R. M. J. Chem. Soc. Faraday Trans. 1995, 91, 1411. 15) Lin, W.; Yitzchaik, S.; Lin, W.; Malik, A.; Durbin, M. K.; Richter, A. G.; Wong, G. K.; Dutta, P.; Marks, T. J. Angew. Chem. Int. Ed. Engl. 1995, 34, 1497.16) Marks, T. J.; Ratner, M. A. Angew. Chem. Int. Ed. Engl. 1995, 34, 155.17) Roscoe, S. B.; Kakkar, A. K.; Marks, T. J.; Malik, A.; Durbin, M. K.; Lin, W.; Wong, G. K.; Dutta, P. Langmuir 1996, 12, 4128. 18) Yokoyama, S.; Kakimoto, M.; Imai, Y.; Yamada, T.; Kajikawa, K.; Takezoe, H.; Fukuda, A. Thin Solid Films 1996, 273, 254. 19) Zhang, T.; Feng, Z.; Wong, G. K.; Ketterson, J. B. Langmuir 1996, 12, 2298.
Reported orientation angles (by SHG) within 2 degrees of 39.2°
Nd:YAG
PD QF GLP HWP L
V
IR P IF PMTDM
DM
PiezoInlet Outlet
Nd:YAG
PD QF GLP HWPDM
DM
Combined SHG and ARAPD
SHG
ARAPD
Total Internal Reflection Cell
2
Physisorbed Stilbene Dye (DPB)
Ellipsometry yields an average orientation angle of ~75°
Assuming a 45 Å rod-like molecular length
A a a a K a a a a atot X Y X z Z Z X Y X Z sin sin2 2 2 2
Angle-Resolved Photoacoustic Detection
If a narrow distribution is assumed:
Mean tilt angle ~ 72° ± 3° (monolayer)
SHG for Monolayer DPB
I C s Is XXZ 1
2 22sin
22
54322
5 IsssscossCI ZXXZZZZXXXXZZXXp
Monolayer DPB
If a narrow angular distribution is assumedthe calculated orientation angle is 73° ± 3°
Mean and Angular Distribution
For the DPB monolayer:
ARAPD yields a tilt angle of 72° ± 3°
SHG yields a tilt angle of 73° ± 3°
For DPB, molecular long axis tilted ~70° with respect to surface normal, fairly narrow angular distribution.
DPB Multilayer by ARAPD
If a narrow distribution is assumed:
Mean tilt angle ~ 72° ± 3° (monolayer)
Mean tilt angle ~ 53° ± 0.9° (multilayer)
DPB Multilayer by SHG
If a narrow angular distribution is assumed the calculated orientation angle is 70° ± 3°
Mean and Angular Distribution
For the DPB monolayer:
ARAPD yields a tilt angle of 72° ± 3°
SHG yields a tilt angle of 73° ± 3°
For DPB multilayer:
ARAPD yields a tilt angle of 53° ± 0.9°
SHG yields a tilt angle of 70° ± 3°
Covalent Molecular System: Azo Dye
Water Contact Angle and Ellipsometry
Ellipsometry indicates only 6.5 Å thickness, estimated 0.1 monolayer, 37 Å2 per molecule
Combined SHG and ARAPD Results
Assuming a narrow distribution
for ARAPD: 58° ± 2°
for SHG: 46° ± 2°
Mean and Angular Distribution
K
P
Pz Z z Z
z Z z Z z Z z Z
z Z z Z z Z
cos
cos2
2
0
0
sin d
sin d
D
P d
P dz Z
z Z
z Z
z Z z Z z Z z Z
z Z z Z z Z z Z
cos
cos
cos sin
cos sin
33
0
0
Linear Dichroism
SHG
Assuming a narrow distribution
for ARAPD: 58° ± 2°
for SHG: 46° ± 2°
Mean and Angular Distribution
Assuming a narrow distribution
for ARAPD: 58° ± 2°
for SHG: 46° ± 2°
Mean and Angular Distribution
57 ° ± 30°
Azo Dye with aminopropyl silane linker
SHG for adsorption isotherms andadsorption / desorption kinetics
SHG intensity depends on both the number density and molecular orientation
Experimental geometry can be used to minimize sensitivity to orientation
22
ZZZ4ZXX3XXZ22pp IsssCI
ZZZ = Nscos3zzz
ZXX = (1/2)Nssin2coszzz
Conventional SHG Adsorption Measurements
Single polarization combination (e.g., p-polarized and 2 ), assume molecular orientation does not change with surface coverage
Orientation angle corrected (OAC)
1) Measurement of several polarization combinations at each concentration
2) Calculation of molecular orientation, , at each concentration (assuming narrow angular distribution)
3) Signal normalization
4) Construction of isotherm
Theoretical p-polarized SHG response curves as a function of orientation angle (zzz dominant)
22
54322
52 )()(γcos)γ( IsssssCI ZXXZZZZXXXXZZXXp
RMS deviation for p-polarized 2 zzz dominant)
2
1
3254
51*
3cosγ
ssss
s
*
NO2
N
N
N
OH
Test Case: Disperse Red 1 (in methylene chloride)
Fused Silica
Adsorption isotherm for DR-1 as determined by a variety of polarization conditions
Apparent orientation angle as a function of concentration
Adsorption isotherm for DR-1 after correcting for change in orientation
Ipp Ips Is45 Ip OAC
Keq (M-1) 940 410 470 540 500
± 40 ± 40 ± 50 ± 60 ± 40
Gads (kJ/mol) -16.8 -14.7 -15.1 -15.4 -15.2
± 0.1 ± 0.3 ± 0.3 ± 0.3 ± 0.2
Experimental constants obtained from Langmuir fit to adsorption isotherms
OAC = orientation angle corrected
Adsorption / Desorption Kinetics by SHG
Future Directions
Thin Film Growth Mechanism
Thin Film Growth Mechanism
Thin Film Growth MechanismO
rien
tati
on A
ngle
Time / Coverage
Angular D
istribution
Thin Film Growth Mechanism
Thin Film Growth Mechanism
Thin Film Growth MechanismO
rien
tati
on A
ngle
Time / Coverage
Angular D
istribution
Acknowledgements for Thin Film Work
Susan Doughty (Ph. D. 1996), Patent Lawyer
Garth Simpson (Ph. D. 2000), Post-doc with Prof. Richard Zare at Stanford
Sarah Westerbuhr
Jessica Ekhoff
Funding from Beckman Foundation and theNational Science Foundation
For SHG measurements with an orientation distribution which is isotropic within the surface plane, the three nonzero, independent tensor elements of (2) are given by:62
where is the Euler rotation angle about the molecular z'-axis. Since the X- and Y-axes in the surface plane are equivalent, YYZ = YZY = XXZ and ZYY
= ZXX. The expressions in Eq. 4 are greatly simplified
if only a single tensor element of (2) is dominant, which is often the case experimentally.
zxxxxz22
zzz3
sZZZ 2sinsincoscosN
zxxxxz22
xxzzzz2
s21
ZXX2sinsincos
cossincosN
XXZ XZX s
z z z x x z
z x x x x z
N
12
2
2 2 2
cos sin cos
cos sin sin
Once the apparent orientation angle has been determined, it may be substituted into sin2cos and cos3 in Eq. 5, and the value of Ns calculated by simple rearrangement of the
expressions in Eq. 3:
1**252
12/1psps,s cossinsICN
1**212
12/145s45s,s cossinsICN
1*34
**2322
12/1pppp,s cosscossinssICN
