Surface Diffraction Studies of Organic Thin Films

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