Spectroscopy of Hybrid Inorganic/Organic Interfaces Vibrational Spectroscopy Dietrich RT Zahn.

Spectroscopy of Hybrid Inorganic/Organic Interfaces

Vibrational Spectroscopy

Dietrich RT Zahn

Dietrich RT Zahn, TU Chemnitz

The Overall Device Performance

GaAs(100)

Organic Interlayer

Metal

V

I

(iv) The Interface between the Organic Molecules and the Metal

(iii) The Organic Molecular Film

(ii) The Interface between GaAs Substrate and Organic Molecules

(i) The GaAs Substrate Surface

The Application of Raman Spectroscopy in the DIODE Project


PTCDA: 3,4,9,10- Perylenetetracarboxylic diAnhydride DiMe-PTCDI: 3,4,9,10- Perylenetetracarboxylic diImide

Symmetry D2h Raman active: 19Ag+18B1g+10B2g+7B3g

IR active: +10B1u+18B2u+18B3u

Silent: + 8Au

108 internal vibrations

Molecular Vibrational Properties

CC2424HH88OO66

Monoclinic crystallographic system in thin films:

• PTCDA: - and -phases: S. R. Forrest, Chem. Rev. 97 (1997), 1793.

• DiMe-PTCDI: Cambridge Structural Database.

CC2626HH1414OO44NN22

C2h

44Ag+22Bg

+23Au+43Bu

+ 8Au

132 internal vibrations


2-fold

Davydov Splitting

internal molecular modes: external molecular modes (phonons):

CC- - CC- - OOBBgg

CC--HH CC--CC

CC--CC

SymmetrySymmetry: : DD2h2h CC2h2h (monoclinic) (monoclinic)

6 rotational vibrations: 3Ag+3Bg

19Ag+18B1g+

10B2g+7B3g

BBgg

AAgg

AAgg

BBgg

AAgg

Raman-active vibrations of PTCDA (C24H8O6):Effect of crystal formation


19 Ag breathing modes

very good agreement

between experimental and

calculated frequencies !

Vibration modes of Vibration modes of PTCDAPTCDA molecule molecule

Exp DFTB

freq / cm-1

freq / cm-1

freq / cm-1

Raman Activity

12 233 233 233 0,99

19 389 383 389 6,43

25 476 474 476 0,88

29 537 550 539 122,86

33 624 639 627 72,23

41 727 728 732 12,01

49 858 863 853 2,63

64 1054 1070 1059 346,71

67 1150 1140 1151 161,30

75 1305 1285 1269 1030,13

76 1340 1304 1303 0,00

79 1381 1347 1346 5362,37

83 1389 1393 1389 551,12

86 1544 1527 1472 1083,97

92 1572 1616 1611 0,01

94 1590 1623 1620 1395,17

100 1774 1723 1800 1284,66

103 3173 3227 173,49

107 3190 3253 236,78

B3LYP / 3-21GMode

CC--OO BBgg

CC--HH CC--CC

CC--CC


200 400 600 1200 1350 1500 1650

Inte

nsi

ty /

arb

. un

its

Raman shift / cm-1

Raman Spectra of a PTCDA Crystal

• assignment of modes and their relative atomic contribution using Gaussian `98 (B3LYP, 3-21G)

x0.1


200 400 600 1200 1350 1500 1650

Inte

nsi

ty /

arb

. un

its

Raman shift / cm-1

x0.1

Ag Raman Modes of PTCDAwith In


x0.1

200 400 600 1200 1350 1500 1650

Inte

nsi

ty /

arb

. un

its

Raman shift / cm-1

• assignment of modes and their relative atomic contribution using Gaussian `98 (B3LYP:3-21G).

x0.5

C=O

ring

C-H

and a and a DiMe-PTCDIDiMe-PTCDIRaman Spectra of a Raman Spectra of a PTCDAPTCDA Crystal Crystal


200 400 600 1200 1350 1500 1650

Inte

nsi

ty /

arb

. un

its

Raman shift / cm-1

Raman Spectra of a Raman Spectra of a PTCDAPTCDA Crystal Crystal

• assignment of modes and their relative atomic contribution using Gaussian `98 (B3LYP:3-21G).

Raman shift /cm-1

and a and a DiMe-PTCDIDiMe-PTCDI

DiMe-PTCDI PTCDA

PTCDA DiMe-PTCDI

DiMe-PTCDI

PTCDA experimental

ω m= =0.97

ω m

ω 221= =0.95

ω 233


external molecular modes (phonons): 6 rotational vibrations: 3Ag+3Bg

SymmetrySymmetry: : CC2h2h (monoclinic) (monoclinic)

Raman-active vibrations of PTCDA:Effect of crystal formation

BBgg

AAgg

BBgg


750 1000 1250 1500 1750

1.0

1.1

1.2

1.3

1.4

Rsa

mp

le/R

sub

stra

te

Wavenumber / cm-1

Infrared Modes in Films on S-GaAs

•Assignment of modes using Gaussian `98 (B3LYP, 3-21G).

Reflection, s-polarized light.

C=O

ring

C-O-C

C-O+

C-C

C-H

(oop)

C-H+C-N-C


Sample PreparationEpi-ready GaAs (100)

DegreasingAcetone, Ethanol, Di-Water

Wet Chemical TreatmentS2Cl2:CCl4=1:3 (10 sec)

Rinsing(CCl4, Acetone, Ethanol, Di-Water)

Annealing at 620 K, 30 min

OMBD deposition:PTCDA, DiMe-PTCDI

Thickness: 0.1 nm ÷15 nm

S-GaAs(100):2x1

Metal deposition:Ag, In

Thickness: 0.1 nm ÷260 nm


Ex Situ (Micro-) and In Situ (Macro- Configuration)

Raman Spectroscopy

Dilor XY 800 SpectrometerMonochromatic light source: Ar+ Laser (2.54eV), Detector: CCD • resonance condition with the absorption band of the organic crystalline material.• resolution: 1.2 cm-1 to 3.5 cm-1.

1.5 2.0 2.5 3.0 3.5 4.0

0

2

4

6

Abs

orbt

ion

coef

ficie

nt *

105

Abs

orbt

ion

coef

ficie

nt *

105

S0-S

2 transition

S0-S

1 transition

DiMe-PTCDI

PTCDA

Energy / eV

800 700 600 500 400

0

2

4

Wavelength / nm

Ar+ line


Monitoring of PTCDA Film Growth on S-GaAs

• Phonons are well resolved as soon as 20 nm of PTCDA are deposited.

• The relative intensity of internal modes does not change upon deposition.

E = 2.54 eV

M. Ramsteiner et al., Appl. Opt. 28 (18) (1989), 4017.

weak interaction of the molecules with the S-passivated substrate.

0 20 40 60 80 100 120

0.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

Inte

nsity

Thickness / nm

431 cm-1

386 cm-1

233 cm-1

simulation, k= 0.99


1300 1400 1500 1600

Inte

nsity

/ ct

s m

W-1 s

-1

Raman shift / cm-1

0.0

02

Annealing at 623 K for 30 min:• Molecules remaining at the surface:NPTCDA(0.04nm)~1013 cm-2

NdSi ~ 1012 cm-2

• Spectrum of annealed film similar to that of an annealed PTCDA film on Si(100).

The strongest interaction: between the PTCDA molecules and defects due to Si at the GaAs surface.

Chemistry at Organic/S-GaAs(100):2x1Vibrational Properties:

PTCDA

40 nmx 0.01

0.45 nm(x 0.6)

0.18 nm

ann.x 4.4


300 600 900

0

10

20

30

1200 1400 1600

0

500

1000

1500

Inte

nsity

/ A

4 am

u-1

Raman shift / cm-1

Calculated Vibrational Properties: PTCDA

1340 1350

2.7 cm-1


1300 1400 1500 1600 1700

0

500

1000

1500

Inte

nsity

/A4 a

mu-1

Raman shift / cm-1

neutral

negative

Calculated Vibrational Properties: PTCDA

Molecular charging with one elementary charge:

positive fractional charge transfer between the PTCDA and the defects at the GaAs surface.

• significant spectral changes predicted for the C=C modes around 1600 cm-1


In Situ Raman: Monitoring of Indium Deposition onto PTCDA (15 nm)

1200 1400 1600

0.05

Raman shift / cm-1200 400 600

Inte

nsity

/ c

ts m

W-1s-1

0.005

43/5

In thickness / nm

00.4/0.71.1/1.52.8/135.0

/288.0/3315.0

/5826.0/10


Influence of Indium on Vibrational Spectra of PTCDA

1200 1400 1600

0.0025

+ InB

3g

B1u

Ag

Ag

B3g

B2u

Ag

B3g

(B3g

)B

1uB

3gA

g

Raman shift / cm-1

PTCDA

200 400 600

B2uA

g

B3g

Ag

Ag

In15 nm

Inte

nsi

ty /c

ts m

W-1s-1

0.0025Ag

B2g

GaAs


800 1000 1200 1400 1600 1800 2000

1.0

1.4

1.6

C-H,

C-O

C-H

R/R

subs

trat

e

Wavenumber/ cm-1

In(15nm)/PTCDA(15 nm)

PTCDA(130 nm)

C=C

C=OC-H

C-C,

C-O

C-H

(z)

Influence of Indium on Vibrational Spectra of PTCDA

• all PTCDA modes are preserved in the spectrum of In/PTCDA.• observation of C=O modes (around 1730-1770cm-1)

In does not react with the O of PTCDA !

• organic films grown on S-GaAs(100):2x1

• reflection measurements at 20° incidence.


Indium/PTCDA: Separation of Chemical

and Structural Properties

1200 1350 1500 1650

Inte

nsity

/ ct

s m

W-1s-1 0.03

PTCDA

PTCDA(15 nm)

Raman shift / cm-11350 1500 1650

PTCDA(0.4 nm)

0.001

x 0.017+ Inx0.045

In: 0 100 nm

In: 1 nm/min

PTCDA~0.4 nm(~1 ML)

S-GaAs(100)

~15 nm(~50ML)

PTCDA

S-GaAs(100)


S-GaAs(100)

In: 1 nm/min Ag:1.6 ÷ 5.5 nm/min

Comparison of Indium and Silver Deposition on PTCDA and DiMe-PTCDI

1200 1400 1600

0.03

In

tens

ity /

cts

mW

-1s-1

Raman shift / cm-11200 1400 1600

0.01

DiMe-PTCDI (15 nm)

PTCDA(15 nm)

S-GaAs(100)

+ In+ Ag


Comparison of Indium and Silver Deposition on PTCDA and DiMe-PTCDI

• the PTCDA external modes: are preserved broadened after 0.3 nm Ag deposition. disappear after 0.4 nm In.

• the DiMe-PTCDI external modes: less affected compared to PTCDA. probably due to less compact crystalline structure.

100 200 100 200

0.4 nm In

PTCDA

x 4

0.4 nm In

DiMe-PTCDI

/ 2

0.3 nm Ag

Inte

nsity

/a.u

.

Raman shift / cm-1

/ 2

0.4 nm Ag


1300 1400 1500 1600

/5

/10

+ Mg

PTCDA

+ In

+ Ag

Inte

nsi

ty /

cts.

mW

-1s-1

Raman shift / cm-1

200 300 400 500 600

0.03

0.01

PTCDA (15 nm)Mg, In, Ag on PTCDA


200 300 400 500 600

1300 1400 1500 1600

Inte

nsi

ty

Raman shift / cm-1

5x10-2

cts.mW-1s-1

5x10-3

cts mW-1

s-1

DiMe-PTCDI (15 nm)

+ In

DiMe-PTCDI

+ In

+ Ag

Mg, In, Ag on DiMe-PTCDI

+Mg


PTCDA (15 mn) DiMe-PTCDI (15 nm)

Indium and Silver Deposition:Enhancement Factors

0 10 20 30 401

10

100

Rel

ativ

e A

rea

In thickness / nm

0 10 20 30 40

1

10

100

1236 cm-1

1568 cm-1

1610 cm-1

Ag Thickness / nm

0 10 20 30 401

10

100

1000

In thickness / nm

Rel

ativ

e A

rea

0 10 20 30 40

1

10

100

1244 cm-1

1570 cm-1

1615 cm-1

Ag Thickness / nm


Determination of Molecular Orientation:Determination of Molecular Orientation:DiMe-PTCDIDiMe-PTCDI

Azimuthal rotation of a 120 nm thick film; normal incidence. Periodic variation of signal in crossed and parallel polarization.

M. Friedrich, G. Salvan, D. Zahn et al., J. Phys. Cond. Mater. submitted.

=0°: x II [011]GaAs

=90°:x II [0-11]

phononsphonons phononsphonons


Determination of Molecular Orientation:Determination of Molecular Orientation:DiMe-PTCDIDiMe-PTCDI

yx

xx

IDep =

I

56 4 ;

,

g

-1g

m= R , ,A ,A, R , , Good agreement with IR and NEXAFS results

s igAI = e e

0 60 120 180 240 300 3600.0

0.5

1.0

1.5

2.0

2.5

De

po

lari

zatio

n R

atio

/ a.u

.

Experimental angle ()/°

BreathingBreathing mode at 221 cm mode at 221 cm-1-1


Molecular Orientation with respect to GaAs substrate:

PTCDA: ~ 9°


DiMe-PTCDI: ~ 6° ~ 60°

[-110][-110]


Interface reactions Internal Modes:

Shifts, Intensities

Film thickness Intensity modulations

Crystalline

Order

Growth Mode Intensity modulations

Crystallinity Occurrence of Phonon-like Modes, FWHM

Crystal modifications Phonons,

Davydov Splitting of Internal Modes

Orientation Further investigations

Raman Characterization of Organic Thin Films:

Achievements and Outlook


Raman Spectroscopy Team:

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Spectroscopy of Hybrid Inorganic/Organic Interfaces Vibrational Spectroscopy Dietrich RT Zahn.

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