Aldo Brillante Alma Mater Studiorum, University of Bologna, Department of Industrial Chemistry "Toso Montanari" Polymorphism and phase mixing in organic semiconductors probed by Lattice Phonon Raman Microscopy [email protected]http://www.unibo.it/docenti/aldo.brillante
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Polymorphism and phase mixing in organic …Polymorphism is intrinsic to molecular materials selected as best bet for organic electronics Large molecules, with extended πconjugation
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Aldo BrillanteAlma Mater Studiorum, University of Bologna,
Department of Industrial Chemistry "Toso Montanari"
Polymorphism and phase mixing in organic semiconductors probed by Lattice Phonon Raman Microscopy
1) Polymorphism and OSC: problems and solutions2) Raman Mapping in bulk crystals3) Extension to thin films down to sub-ML4) Applications to actual devices
MATERIALS FOR ORGANIC ELECTRONICSMATERIALS FOR ORGANIC ELECTRONICS
What are we dealing with…………
Classical building blocks
S
S
S
S
Polymorphism is intrinsic to molecular materialsselected as best bet for organic electronics
Large molecules, with extended π conjugation kept together by weak forces.
A variety of different molecular geometry into the lattice may easily lead toconformational or packing polymorphism
This situation is ideal for the existence of many alternative crystal structures, withsmall differences in energy.
How polymorphism can be probed
- Not only XRD, GIXRD- Multidisciplinary approach
where Raman gets in
Lattice phonons probe the inter-molecular interactions and thus are very sensitive to differences in molecular packing
WHY LATTICE PHONONS?
Different crystalline polymorphs can be identified by their Ramanspectra in the region of lattice phonons (10-150 cm–1)
CONFOCAL RAMAN MICROSCOPY AND CRYSTAL STRUCTURECONFOCAL RAMAN MICROSCOPY AND CRYSTAL STRUCTURE
Lattice phonon Raman
spectroscopy
X-ray diffraction
Lattice dynamics
Lattice geometry
Lattice phonon Raman
spectroscopy
X-ray diffraction
Lattice dynamics
Lattice geometry
ExperimentalExperimental procedureprocedure
………some typical examples for organic materials:
In molecular crystals lattice phonons are the fingerprints of the crystal structure.
XX
XX
H
C
X-RAY DIFFRACTION LATTICE PHONON SPECTRA
Lattice phonon Raman spectra of the two polymorphic forms of pentacene, tetracene, α-sexithiophene and α-quaterthiophene and their corresponding crystal structures.
POLYMORPHISM in
ORGANIC ELECTRONICSis
AN ISSUE
1) Organic molecular materials commonly show polymorphism2) Carrier mobility in organic electronics depends on crystal structure3) Polymorphism may occur as phase mixing on a micro-scale4) Phase in-homogeneity acts as an intrinsic source of disorder with
detrimental effects on the charge transport properties of organic semiconductors.
Lattice phonon Raman spectra show that polymorphism may occur as phase mixing on a microscale
PENTACENE TETRACENE SEXITHIOPHENE
LT
HT
LT+HT
HT+LT
LT
HT
LT+HT
S
S
S
S
?
Phase mixing in DB-TTF
20 µm
20
40
60
80
100
120
1400 50 100 150
CrystEngComm. 10, 1899 (2008)
Phase mixing in DT-TTF(more subtle…)
Phase homogeneity cannot be taken for granted, even for well formed large crystal domains.
PHASE MIXING AND ORGANIC ELECTRONICSPHASE MIXING AND ORGANIC ELECTRONICS
1)1) Helps in discriminating crystal structures of domains of ~1Helps in discriminating crystal structures of domains of ~1m (m (and belowand below))2)2) Can be used as control technique of phase homogeneityCan be used as control technique of phase homogeneity
CONFOCAL RAMAN MAPPING
Sketch of a confocal Raman mapping experiment.
with a false color palette different phases
can be discriminated
HOW
1. Assign spectra to structures2. Scan selected crystal regions3. Identify phase mixing4. Map of structural homogeneity
WHAT
Get a visual displayof phase purity
CONFOCAL RAMAN MAPPINGCONFOCAL RAMAN MAPPING
CrystEngComm. 10, 937 (2008)
Optical Images and Raman Maps of PolymorphsOptical Images and Raman Maps of Polymorphs RESULTS
No relationship between morphology and the actual crystal phase:phase homogeneity can be efficiently monitored only from the lattice phonon confocal Raman maps.
The effect of penetration depthThe effect of penetration depth:Use confocal properties of micro-Raman to
investigate phase coexistence inside the bulk
The less stable form grows preferentially on surface/edges of the otherwise “perfect” crystal.Related examples are T6 etc.
0.15
0.10
0.05
0.00
%HT
LT Phase
HT Phase
LT+HT
getting into the bulk
450 m
25 m
7.5 m
theoreticalfield depth
1) Phase recognition in thin films (lattice phonon spectra)2) Crystal growth by ML deposition and phase homogeneity3) Control of semiconductor layer (Raman mapping)
Fast, visual and efficient test: - quality “test” for deposition- quality test for the operating device
fromBULK CRYSTALS
toTHIN FILMS
andTEST PATTERNS FOR ELECTRONIC DEVICES
1) Phase recognition in thin films (lattice phonon spectra)2) Crystal growth by ML deposition and phase homogeneity3) Control of semiconductor layer (confocal Raman)
Fast, visual and efficient test (quality “test” for deposition)
fromBULK CRYSTALS
toTHIN FILMS
andTEST PATTERNS FOR ELECTRONIC DEVICES
BULK
thin film(60 nm)
HT
LT
LT
LT PHASE IS CLEARLY IDENTIFIED
The presence of a lattice phonon spectrum is a proofof crystalline order.Polarized spectra can reveal orientation, if any.
furtherhints
PHASE RECOGNITION OF T6 DEPOSITED ON MICA
PHASE RECOGNITION OF T6 DEPOSITED ON Si/SiOx
aa
A crystal structure (LT phase) starts between 8 and 16 ML(20-40 nm).The initial deposition may bedisordered.
J.-F. Moulin, F. Dinelli, M. Massi, C. Albonetti, R. Kshirsagar, F. Biscarini,Nucl. Instrum. Methods Phys. Res. Sect. B 2006, 246, 122.
> 6 ML (15 nm)
“……..Upon further growth of the films, the bulk structure appearsaround 6 ML…………”
THE EXTRAORDINARY CASE OF PENTACENE
1ML (15Å) ofPentaceneon Si/SiOx
Ideal case to investigate
growth dynamics
ML growth is along c
High rate growthΦ1= 0.36 ± 0.02 Å/s
2D regime
PHASE RECOGNITION OF PENTACENE DEPOSITED ON Si/SiOx
The presence of low wavenumberphonons, indicates that we do have a lattice structure startingfrom 1ML, in agreement withGIXRD experiments.
The structure formed is rate-dependent
Relationship between morphology and lattice structure
3D 2Dintra-layerinter-layer
surface depositiongrowth
direction
Topography StructureΦ1= 0.36 Å/s Φ2= 0.033 Å/s
Phys. Rev. B 85 195308 (2012)
The bulk crystal structureinitially formed is the lessstable HT phase!
1) There is a loss of periodicity along the direction normal to the substrate (ab plane), in a 2D-like structure
2) The more a mode depends on k (i.e. is dispersed), the higher its dependence on order and therefore dimensionality
3) The high frequency lattice phonons undergo no dispersion: they show at 1ML and in a 2D-like thin film structure.
4) The low frequency lattice phonons have high dispersion and will be washed out when correlation along c* is lost.
5) At high growth rate the spatial correlation along c* is lost
Thin film structure (TF) vs bulk structure
“Surface selected crystal structure”
“Surface induced crystal structure”
Two examples
Non-eq
eq
….a new grammar….
Pentacenequinone
six allowed lattice phonons
Z = 2 monoclinic
three allowed lattice phonons
Z = 1 surface selected structure
Perfluoropentacene
“Surface selected crystal structure”
ACS Nano 12, 10874 (2012)
1) Phase recognition in thin films (lattice phonon spectra)2) Crystal growth by ML deposition and phase homogeneity3) Control of semiconductor layer (confocal Raman)
Fast, visual and efficient test (quality “test” for deposition)
fromBULK CRYSTALS
toTHIN FILMS
andTEST PATTERNS FOR ELECTRONIC DEVICES
x2
Raman signal is detected down to 2 monolayers
(nominal thickness ~5 nm)
T6
Topography of layer by layer growth
Unlikepentacene, no
lattice structure in the
first few ML
T6 on Si/SiOx
aa
T6 on gold
Phonon spectra vs coverageDifferent spectralevolution for differentmorphology
60 °C 90 °C
120 °C 150 °C
T6 on Au at different deposition temperatures
Relationship morphology-crystallinity: 3D growth on increasing temperature
PENTACENE on Si/SiOx
layer by layergrowth
bulk growth
1ML=15Å
1) Phase recognition in thin films (lattice phonon spectra)2) Crystal growth by ML deposition and phase homogeneity3) Control of semiconductor layer (Raman mapping) of
actual devices
Fast, visual and efficient test (quality “test” for deposition)
fromBULK CRYSTALS
toTHIN FILMS
andTEST PATTERNS FOR ELECTRONIC DEVICES
Confocal Raman mapping of devices based on T6 thin films grown on micro-fabricated test patterns
10 nm(4ML)
25 nm(10ML)
60 nm(25 ML)
The electrode-channel steps actas a template for deposition of
molecules on the substrate
Above ~20 ML the template effect given by the patterned electrodes is lost and T6 growth proceeds in a homo-epitaxial mode
“Quality test”for deposition
Adv. Funct. Mater. 17, 3119 (2007)
H4T6 (tetrahexyl-sexithiophene)/PS composite as active layer in a FET device
showspolymorphism and
phase mixing
The relative amount of the twopolymorphs for a series of devices wasrelated to the transistor efficiency.
Yellow-phase
Red-phase
050504-1Y+R
1
In the transistors where H4T6 metastable “red phase” is the largest, the device response and the charge mobility are comparable to those of sexithienyl thin films grown by high vacuum sublimation.
red
yellow
working transistor
YES NO
Langmuir 23, 2030 (2007)
Dithiophene-Tetrathiafulvalene (DT-TTF)
High-Performance Single Crystal OrganicFET based on two DT-TTF polymorphs
Device with graphite source (S) and drain (D) electrodes and SiO2 as dielectric.
Device with gold source (S) and drain (D) electrodes and Parylene C as dielectric.
2
Adv. Mater. 22, 4198 (2010)
Blend of organic source and drain electrodes withsemiconducting organic single crystal attive layer(DT-TTF) in field-effect transistors
from toluene from PhCl
3
J. Mater. Chem. 22, 16011 (2012)
From tolueneLower blending
High contact resistence
From PhClHigher blending
Low contact resistence
CONCLUSIONSCONCLUSIONSChemical purity and physical purity (phase homogeneity) are both required for optimal device response, effectively ensuring that performance parameters are due to the intrinsic properties of the material.
1. Lattice phonon confocal Raman mapping as ideal tool (fast, versatile, in situ, non-destructive) to monitor crystal phases and phase mixing in organic semiconductors.
2. Especially useful when crystal morphology cannot assist phase recognition.
3. Application to thin film yields information on crystallinity, phaserecognition and homogeneity of the deposited material.
4. Control of semiconducting layer in organic electronic devices: “quality test” of deposition.
Tommaso SalzilloElisabetta VenutiRaffaele Guido Della Valle
Former postdoc: Ivano Bilotti
Acknowledgements
Major funding: EU Integrated Project NAIMO (No NMP4-CT-2004-500355) 2004-2008EU Large Project One-P (FP7-NMP-2007-212311) 2009-2012
Alberto Girlando
UNIV. PARMA
Matteo Masino
Acknowledgements
Major funding: EU Integrated Project NAIMO (No NMP4-CT-2004-500355) 2004-2008EU Large Project One-P (FP7-NMP-2007-212311) 2009-2012
Tommaso SalzilloElisabetta VenutiRaffaele Guido Della Valle