surface diffraction studies of organic thin films mehmet fatih danışman middle east technical...
TRANSCRIPT
Surface Diffraction Studies
of Organic Thin Films
Mehmet Fatih Danışman
Middle East Technical UniversityDepartment of Chemistry
Methanethiol Self-Assembled Monolayers (SAMs) on Au(111)
Pentacene (C22H14) Thin Films on Ag(111)
Side view
Top view
Octadecyltrichlorosilane (OTS) SAMs on Silica
Advantages of helium atom diffraction
•Low-energy (~14meV) He-atoms produced by supersonic expansion
• λ≈1 Å comparable to unit cell dimensions • Sensitive only to topmost layer • Non-perturbing• Very sensitive to surface corrugation • Very sensitive to adsorbate coverage due to large
cross-sections Real time monitoring of film growth
Rota table Preamplifier
r
Helium Beam
Crsytal
Bolometer
Dose line Thiol feed
L He P=1 Torr T=1.6 K
T=40 K
T=77 K
L N2 77 K
L N2 77 K
To rotary pump
Motor
Radiation Shield
20 10
Helium Beam Source Chamber
Diffraction Chamber
PumpingChamber
40
30
Seeded Supersonic Beam
Beam Source
20 10
Helium Beam Source Chamber
Diffraction Chamber
PumpingChamber
40
30
Seeded Supersonic Beam
Beam Source
20 10
Helium Beam Source Chamber
Diffraction Chamber
PumpingChamber
40
30
Seeded Supersonic Beam
Beam Source
20 10
Helium Beam Source Chamber
Diffraction Chamber
PumpingChamber
40
30
Seeded Supersonic Beam
Beam Source
• Main Beam Source kept at 70 K (Δv/v < 2%)
• Diffraction data obtained at low surface temperature (40 K)
Low level of inelastic scattering
Experimental highlights and the diffraction chamber
Tnozzle=70Kdnozzle=20 μmP: 100 psi (~7 atm)
fi
a
For constructive interference path length difference should be equal to a multiple of wavelength.
Bragg condition
nλ)i
θf
θa(i
θaf
θa sinsinsinsin
Diffraction from a surface
an)
iθ
fθ(
2
sinsin2
ki = 5.1 Å-1
ΔK// = ki (sin θf – sin θi)
Diffraction Geometry and the Ewald Sphere Construction
a*
b*
θiθf
kiz
kfz
Ki
ki
Kf
kf
00-10-20-30
ΔK
EwaldSphere
-31
k00
-40-50
Laue Condition:
Ki+G=Kf
where G=ma*+nb* m,n N
denotes a reciprocal lattice vector
imE
h
2
ki = 2/
“Self-assembled monolayers (SAMs) are ordered molecular assemblies that are formed spontaneously by the adsorption of a surfactant with a specific affinity of its head group to a substrate.”
Self-Assembled Monolayers
J. C. Love et al., Chem. Rev. 105, 1103 (2005)
F. Schreiber, Prog. Surf. Sci. 65, 151 (2000)
• What is the effect of intermolecular interactions on the adsorption geometry of
alkanethiol self assembled monolayers (SAMs)?
•Will the (3x23) will be observed for the shortest alkanethiol (CH3S) too?
• What is adsorption site of the sulfur atom?
•The bridge site as the DFT calculations predict
•Or the atop site found experimentally by X-ray photoelectron diffraction and X-ray
standing waves
Why Methanethiol Monolayers on Au(111)?
We tackled the problem by using three different
complementary probes to have a complete picture
• Methanethiol forms a (3x4) superlattice (though after a complex deposition/annealing
procedure)
• A superlattice, although different from the (3x23) phase, exists even for the shortest
thiol monolayer Interchain interactions are not essential for superlattice formation
X-ray photoelectron diffraction, MD simulations
• A dynamic equilibrium exists between bridge site adorption and a novel
quasi-ontop site adsorption.
Helium and X-ray diffraction
b) Kx
Ky
b) Kx
Ky
-7 -6 -5 -4 -3 -2 -1 0 10
2
4
6
8
10
12
14
16
18 = 30o
= 20o
= 10o
= 0o
Inte
nsit
y (a
rb.u
nits
)
k// (Å-1)
Diffraction scans taken along four different azimuthal angles. (=0°) corresponds to the <1-10> direction, and (=30°) corresponds to the <1-10> direction. Expected positions of (3x3) lattice points in the <1-10> (<1-10>) directions are indicated by dashed (dotted) lines.
Expected diffraction peak positions for (3x4) () and (3x3) (■) lattices overlaid on the experimental data before misalignment correction (a) and after the correction (b). Au(111) diffraction peak positions are indicated by solid squares.
<1-10>
<11-2>
X-ray penetration depth and specular reflection intensity as a function of incidence angle
incidence angle (degr.)
Grazing incidence X-ray diffraction geometry
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
-0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
(0,0)
(1,2)
(3/4,4/3)
(1/3,2/3)
(2/3,4/3)/16
(1,1)
(1,4/3)
(7/4,2)
(2/4,5/3)
(3/4,5/3)
(2/4,4/3)
(1/4,1)
(0,1)
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
-0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
(0,0)
(1,2)
(3/4,4/3)
(1/3,2/3)
(2/3,4/3)/16
(1,1)
(1,4/3)
(7/4,2)
(2/4,5/3)
(3/4,5/3)
(2/4,4/3)
(1/4,1)
(0,1)
Observed reciprocal space map at ki=1.306 Å-1 and kz=0.13 Å-1. The
radii of the circles correspond to the relative intensities of the Bragg peaks. Yellow, red and blue circles correspond to Au, (3x3) and (3x4) lattices. The black dots indicate the (3x23) reciprocal space lattice points for which no diffraction intensity could be measured.
Measured Diffraction Intensity (arbitrary units) as a function of perpendicular momentum transfer L; circles experimental data, solid line best fit, (h,k) indexes refer to Au(111) unit cell
(2/3,1/3) (1/3,2/3)
(2/3,4/3)
(3/3,6/3)
(4/3,2/3) (4/3,5/3)
(7/3,5/3)
(5/3,4/3)
(0,-3/3)
Grazing Incidince X-Ray Diffraction:
Theory and Results
Electron elastic scattering factor for Ni.
X-Ray Photoelectron Diffraction: Theory
j
jjjjj krifaA cos1exp1k
f : Scattering factor : Scattering angle, r : Distance between the emitter and the scatterer, : scattering phase shift
a : Amplitude of the photoemitted wavefield at the scatterer. Decreases exponentially due to inelastic scattering, thermal
motion of the atoms and 1/r dependence of the wavefield.
Hence the main contribution to the diffraction intensities, A(k)2, is made by the nearest neighbors of the emitter atom
which makes X-ray photoelectron diffraction a local structural probe.
),(
),(),(),(
0
0
k
kkk
kI
kIkIk
Reliabilty factor, rf
rf = 0 for perfect fit
Theoretical fit
rf= 0.49 rf= 0.81
rf= 0.52 rf= 0.48
X-Ray Photoelectron Diffraction: Results
PED fits (lines) to experimental data (circles) collected at S 2p3/2 peak. Energy scan is performed in normal emission in the range 250 – 630 eV. Polar scans are performed at 250 eV photon energy.
Why pentacene thin films ?
Device characteristics Mobility, On/off ratio, Turn on voltage
Structural and Morphological properties of the film
Molecular orientation, Molecular packing, Grain boundaries, Defect
concentration, Domain size, Contacts
Substrate and Growth conditions
Adhesion energy, Substrate temperature, Flux, Surface steps
Pentacene film morphology and mobility
aabb
cc
C.D. Dimitrakopuolos and D.J. Mascaro, IBM J. Res. & Dev. 45, 11 (2001)
Triclinic unit cell, with single cleavage plane; molecules in each ac plane have tilted herringbone structure
Dimitrakopoulos C.D., Adv. Mat., 2002, 14, 99
Pentacene film morphology at the
gold electrode – SiO2 interface of a Thin Film Transistor
Small domains Domain size increases Big domains
Typical pentacene film morphology on SiO2
Mobility is limited by the charge carrier
injection at the electrodes
Ruiz R. et al., Phys. Rev. B, 2003, 67, 125406
Effect of substrate properties and the growth parameters on the film morphology
Pratontepa S. et al., Synthetic Metals 2004, 146, 387
Low deposition rates and high substrate
temperatures result in larger domain sizes
Reduced surfaceReduced surface Oxidized surfaceOxidized surface
Smaller domain size
Layer by layer growth
Larger domain size
Dewetting
Seeded supersonic molecular beam source vs. conventional vapor phase deposition
Organic source material is evaporated at sublimation
temperature either in UHV or in flux of carrier gas.
kT≈0.05 eVkT≈0.05 eV
Ekin5 mpentacene R TNozzle
2 maverage
P0≈400 Torr
PPen≈10-3 Torr
Pb≈10-5 Torr
Ekin≈5 eV for He
Ekin≈0.4 eV for Kr
Heavy species is accelerated by seeding into a lighter carrier gas
Carrier gas P0, T0Carrier gasCarrier gas P0, T0
• Using high kinetic energy molecules during the deposition results in sharper
photoluminescence bands than those of the thicker films grown by conventional techniques.
• As the kinetic energy of the molecules increase the bands get narrower
Low defect density or different film structures?
Iannotta S. et al., Appl. Phys. Lett. 76, 1845 (2000)
Indirect evidence obtained from photoluminescence measurements
about the high quality of quaterthiophene films
prepared by supersonic molecular beam deposition
quaterthiophene
0 200 400 600 800 1000
0.010
0.012
0.014
0.016
0.018
0.20.30.40.50.60.70.80.91.0
0 5 10 15
0.8
0.9
1.0
Completion of second layer?
Completion of monolayer
Inte
nsi
ty (
a.u
.)
Time (s)
Saturationof steps
Pentacene growth on the “stepped” Ag(111)
Specularity of the clean Ag(111) surface 30%
(surface miscut 0.56, av. terrace width 380 Å)
Specular Intensity vs. Exposure
TS=200K Ekin≈5 eV
-7 -6 -5 -4 -3 -2 -1 0 110-4
10-3
Multilayer
Monolayer
Inte
nsity
(ar
b.un
its)
K// (Å-1)
0 200 400 600 800 1000
0.010
0.012
0.014
0.016
0.018
0.20.30.40.50.60.70.80.91.0
0 5 10 15
0.8
0.9
1.0
Completion of second layer?
Completion of monolayer
Inte
nsi
ty (
arb
.un
its)
Time (s)
Saturationof steps
Pentacene growth on the “stepped” Ag(111)
TS=200K Ekin ≈5 eV
Monolayer and the multilayer have different structures
-8 -6 -4 -2 010-4
10-3
Ts = 250 K
Ts = 200 K
Ts = 150 K
Inte
nsity
(ar
b.un
its)
K// (Å-1)
-6 -4 -2 010-4
10-3
10-2
Ts = 150 K
Ts = 200 K
Ts = 250 K
K// (Å-1)
Inte
nsi
ty (
arb
.un
its)
Effect of substrate temperature on film growth
Competition between local and global annealing
Optimum substrate temperature for multilayer growth is 200 K
Poor structure at higher temperatures may be caused by dewetting
Monolayer, Ekin ≈ 5 eV Multilayer, Ekin ≈ 5 eV
Effect of kinetic energy on the film growth
Surface diffusion is activated by the extra kinetic energy.
Improvement in multi layer structure.
Monolayer, TS=200 K Multilayer, TS=200 K
-8 -6 -4 -2 010-4
10-3
In
tens
ity (
arb.
units
)
Ekin
4 eV
Ekin
0.4 eV
K// (Å-1)
Ekin≈5 eV
Ekin≈0.4 eV
-6 -4 -2 010-4
10-3
10-2
Ekin
0.4 eV
Ekin
4 eV
Inte
nsity
(a
rb.u
nits
)
K// (Å-1)
Ekin≈5 eV
Ekin≈0.4 eV
Multilayer film structure
• Full lines indicate a periodicity of 6.1 Å
along <11-2> direction
• Dashed lines indicate a periodicity of
15.3 Å along <1-10> direction
Multilayer, Ts=200K, Ekin ≈ 5 eV Ex-situ X-Ray Reflectivity
• X-ray peak indicates a periodicity of 3.72 Å
along the z direction
• The asymmetry indicated a flat lying
monolayer structures with a thickness of
7.8 Å
a = 7.90 Åa = 7.90 ÅBulk : b = 6.06 ÅBulk : b = 6.06 Å c = 16.01 Åc = 16.01 Å
a = 7.44 Åa = 7.44 Å: b = 6.1 Å: b = 6.1 Å c = 16.5 Åc = 16.5 Å
Thin film Thin film
on Ag(111)on Ag(111)
Proposed Model for the Thin Film Structure
Top view
7.8 Å7.8 Å
Side view
Side view
Top view
a
b
Molecules in the film rest Molecules in the film rest tilted on their long side and form tilted on their long side and form a 2-D lattice which is very similar a 2-D lattice which is very similar to the b-c face of the bulk latticeto the b-c face of the bulk lattice
We had to change the crystal and by chance end up with an almost flat surface that led us to study the
effect of step density on the film growth
5 10 15 20 25 30 35 40 45 50 55 60 6510-5
10-4
10-3
10-2
10-1
100
Polar angle / degrees
I/I
0
() from Ag(111) surface with relatively high step density
(, ) from Ag(111) surface with very low step density
along different azimuthal directions
Helium scattering intensity, relative to the main beam intensity
0 100 200 300 400 500 600 700
10-3
10-2
10-1
0.00 0.02 0.04 0.06 0.080.7
0.8
0.9
1.0
0 1 2 3 410-3
10-2
Coverage (ML)
4
A
Inte
nsity
(ar
b. u
nits
)
Time (s)
B
1
2 3
5 6
I/I0
I/I0
Coverage (ML)
Effect of step density and substrate quality on the film growth
TS= 200K Ekin ≈ 5 eV
-4 -3 -2 -1 0
10-3
10-2
10-1
6
A
B
Diffraction scans along <11-2> direction
5
4
3
2
1
K// (Å-1)
Inte
nsity
(ar
b. u
nits
)
•Specularity of the “stepped” Ag(111) surface 30%
(surface miscut 0.56, av. terrace width 380 Å)
•Specularity of the “flat” Ag(111) surface 90%
(surface miscut < 0.1, av. terrace width > 2000 Å)
Effect of step density and substrate quality on the film growth
-4 -3 -2 -1 0
10-3
10-2
10-1
Inte
nsity
(ar
b. u
nits
)5
B
4
6
1
2
3
A
Diffraction scans along <1-10> direction
K// (Å-1)
0 100 200 300 400 500 600 700
10-3
10-2
10-1
0.00 0.02 0.04 0.06 0.080.7
0.8
0.9
1.0
0 1 2 3 410-3
10-2
Coverage (ML)
4
A
Inte
nsity
(ar
b. u
nits
)
Time (s)
B
1
2 3
5 6
I/I0
I/I0
Coverage (ML)
TS= 200K Ekin ≈ 5 eV
•Specularity of the “stepped” Ag(111) surface 30%
(surface miscut 0.56, av. terrace width 380 Å)
•Specularity of the “flat” Ag(111) surface 90%
(surface miscut < 0.1, av. terrace width > 2000 Å)
France et. al, Langmuir, 2003, 19, 1274
Comparison with previous STM studies
Increasing pentacene film coverage on Au(111)
Unit cell size decreases as the coverage increases
-6.56 -5.74 -4.92 -4.10 -3.28 -2.46 -1.64 -0.82 -0.41 0.00 0.41
10-4
10-3
10-2
= 20o
= 10o
<1-10> = 30o
<11-2> = 0o
K// (Å-1)
Inte
nsity
(ar
b.un
its)
Diffraction scans are taken in a 180o azimuthal, , range with 5o increments (36
scans). The results are combined to obtain a contour plot of the reciprocal space
shown in the right figure. Grid lines in the left plot and the dots in the contour map
indicate the expected positions for 6.1x3 unit cell.
Reciprocal space map of structure “4”
<1-10>
<11-2>17.6 Å
8.67 Å
Structural model of “4” and comparison with theory
• Experimental data suggests a (6.1x3) unit cell
• Close coupling calculations reproduce the data quite well for small K// values
however the code should be refined in order to obtain a better fit for large K//
values (convergence problem).
-4 -3 -2 -1 0
1.0x10-4
1.5x10-4
2.0x10-4
2.5x10-4
3.0x10-4
3.5x10-4
4.0x10-4
Ekin 0.4eV
Ekin 4eV
Inte
nsity
(arb
. uni
ts)
K// (Å-1)
-7 -6 -5 -4 -3 -2 -1 010-4
10-3
10-2
Inte
nsity
(arb
.uni
ts)
K// (Å-1)
Ekin0.4eV
Ekin 5eV
Monolayer <11-2> direction, Ts=200 K Multilayer <1-10> direction, Ts=200 K
Effect of kinetic energy
-5 -4 -3 -2 -1 00.0001
0.0002
0.0003
0.0004
0.0005
0.0006
0.0007
0.0008
TS=300 K
TS=250 K
TS=200 K
TS=150 K
Inte
nsity
(a.
u.)
K// (Å-1)
Effect of substrate temperature and annealing on the film structure
-4 -3 -2 -1 010-4
10-3
10-2
10-1
Diffraction scans of "4" as a function of annealing temperature
along <11-2> direction
Inte
nsity
(ar
b.un
its)
K// (Å-1)
after deposition at 200K substrate temperature
After 250K anneal After 300K anneal After 350K anneal After 400K anneal
-6 -5 -4 -3 -2 -1 010-4
10-3
10-2
10-1
TS=300 K
Diffraction scans of "4" as a function of substrate temperature
along <11-2> direction
TS=250 K
TS=200 K
TS=150 K
Inte
nsity
(ar
b.un
its)
K// (Å-1)
Diffraction scans of the multilayer as a function of substrate temperature
along <11-2> direction
100 150 200 250 300 350 400
1.2x10-4
1.5x10-4
1.8x10-4
2.1x10-4
2.4x10-4
He specular reflection intensity
Inte
nsity
(a.
u.)
Temperature (K)
-1.0x10-4
-5.0x10-5
0.0
5.0x10-5
1.0x10-4
1.5x10-4
Derivative of the specular intensity
dI/d
T
The low temperature at 328 K corresponds to a desorption energy of 94 kJ/mol
The higher temperature rise at 382 K corresponds to 109 kJ/mol
Temperature programmed desorption measurement
performed by monitoring He specular reflection intensity
Conclusions
For Ag(111) surface with relatively high step density
• Optimum growth is achieved by using high kinetic energy molecules, at low substrate temperatures
• Local annealing induced by the impact of high energy pentacene molecules has a decisive role in improving the growth: keeping the substrate temperature low, in fact, processes like de-wetting or disorder induced by the growth of different polymorphs are hindered
• The monolayer and the multilayer have different structures, monolayer having a (6.1x3) lattice and the multilayer having a unit cell very similar to that of bulk crystal.
Conclusions
For the extremely flat Ag(111) surface
• While the film characteristics follow the same trend, as a function of substrate temperature, as the films grown on the stepped surface, increasing kinetic energy does not improve the film quality considerably.
• The multilayer has a different structure and worse quality than that of the films grown on the stepped surface.
• This is probably due to the missing of steps. On the high step density surface, step edges provide extra dimensionality and act as nucleation centers for the tilted multilayer molecules which result in a step flow growth.
What next ?
• Integration of a mass spectrometer to the He Atom Diffraction system, to detect the speed of the organic molecules, in order to have a more precise measure of the kinetic energy.
• Use vicinal surfaces in order to study the effect of step density on the film growth more systematically.
• Integrate a commercial Quartz Crystal Microbalance to the He Atom Diffraction system in order to measure the flux of organic molecules independently.
• Grow organic films on gold surfaces coated on Quartz crystals in order to measure the film coverage simultaneously by both Quartz Crystal Microbalance technique and He scattering.
Acknowledgements
Middle East Technical UniversityProf. İlker Özkan, Prof. Metin Zora, Prof. Erdal Bayramlı
Prof. Hüseyin İşçi, Sevil Güçlü
Higher Education Board of Turkey (YÖK)
Princeton UniversityProf. Giacinto Scoles
Dr. Loredana Casalis, Dr. Bert Nickel
Prof. Kevin Lehmann, Scoles and Lehmann Research Groups
Brookhaven National Laboratory, National Syncrotron Light Source, X10B beamline staff
Sincrotrone TriesteALOISA beamline staff
Penn State UniversityProf. David L. Allara, Prof. John V. Badding, Jacob Calkins