Anisotropic Films

15 Oct 2023

Dielectric Tensor of Microtextured Squaraine Thin Films Obtained by Imaging Mueller Matrix Ellipsometry

  Manuela Schiek, S. Funke, M. Duwe, P. H. Thiesen, K. Hingerl, F. Balzer. di-Iso-Butyl-anilino-Squaraine SQIB transition dipole moment (TDM) along long molecular axis Frenkel excitons incl. Davydov splitting ≥ 2 inequivalent molecules per unit cell orthorhombic polymorph Pbcn CCDC 1567104 (4 inequivalent molecules in primitive unit cell) spincasting + thermal annealing @ 180°C formation of rotational domains with (110) orientation Davydov splitting: 4 inequivalent molecules = 4 Davydov bands (2x UDC & 2x LDC)     linear combination of TDMs within one stack + interstack combination SUM => LDC1 along b-axis & LDC2 along a-axis DIFFERENCE => UDC1 along c-axis & UDC2 = 0 (dark state)   diagonal tensor for orthorhombic crystals: optical indicatrix is congruent with crystallographic axes oscillator model to ensure Kramers-Kronig consistency negative real permittivity in vicinity of the strong resonances explains quasi-metallic shine of SQIB platelets   projected view onto (110) plane Davydov bands are polarized mutually perpendicular projected Davydov splitting ≈ 250 meV spatially resolved polarized absorbance measured (circles) is well reproduced by calculated (solid lines) spectra transfer matrix optical calculations using εa and projection of εB and εy onto (110) plane UDC along c-axis, effective LDC along projected a-b-axes     NanoFilm_EP4 imaging Mueller matrix ellipsometry system, Accurion GmbH, Göttingen   single compensator (PCSA-configuration): 11 normalized MM elements knife-edge illumination to suppress incoherent backside reflections collimated light beam spot diameter 1 mm lateral resolution 2 µm (λ = 400 nm) through microscopy objective, no focusing of incident beam selection of multiple ROIs reflection & transmission scans Theta Stage (TS): azimuthal Φ-rotation Funding & Acknowledgement Linz Institute of Technology (LIT-2019-7-INC-313 SEAMBIOF) PRO RETINA Foundation (especially Franz Badura) We are indebted to Dr. Matthias Schulz and Prof. Dr. Arne Lützen, University of Bonn, for providing SQIB powder.

15 Oct 2023

Optical Characterization of Anisotropic Thiophene-PhenyleneCooligomer Micro Crystals by Spectroscopic Imaging Ellipsometry

Christian Röling, Peter H. Thiesen, Matthias Duwe, Elena Y. Poimanova, and Vladimir V. Bruevich Thiophene-Phenylene Co-oligomer Dhex-TTPTT   semiconducting thiophene-phenylene co-oligomer 1,4-bis(5'-hexyl-[2,2'- bithiophen]-5-yl)benzene (dHexTTPTT) crystals were grown by solvent based self-assembly technique on silicon substrate with 300 nm thermally silicon dioxide. Instrumentation Nanofilm_EP4 + MM-Upgrade Image Stitching - The surface of the complete sample was recorded in a stitched image with an enhanced ellipsometric contrast (AOI = 60°, Lambda = 510 nm). - Even Monolayers of the semi conductive material were observed.   Microscopic Mueller-Matrix Ellipsometry Microscopic Mueller Matrix: 11 elements of normalized (m11) Microscopic MM   Variable Theta, Angle and wavelength spectra: Change of MM-values for anisotropic regions upon rotation of the sample illustrated by MM-element m13 (AOI = 50°, λ = 650 nm) MM-theta scans for several wavelengths Isotropic/anisotropic: Off-diagonal blocks of the MM offer contrast between isotropic and anisotropic regions: isotropic: pixels with MM-value = 0 anisotropic: MM-values non-zero   Multilayer      Monolayer

15 Oct 2023

Biaxial Anisotropic Micro Crystals Investigated by Imaging Ellipsometry

SAMPLE: Organic semiconducting micro crystals show high potential for the applications in microelectronic devices and flexible electronics due to the long-range-ordered molecular packing and absence of grain boundaries. The most organic single crystals indicates a highly anisotropic optical behaviour. The semiconducting thiophene-phenylene cooligomer crystals were grown by solvent based self-assembly technique on silicon substrate with 300 nm thermally silicon dioxide. A ellipsometric high contrast map of the complete sample (right) indicates multi and mono layered crystals on the surfaces.   MEASUREMENT: After rotating the sample into the pseudo-isotropic position we performed spectroscopic measurements (nanofilm_EP4, =400-700 nm) in 5 nm intervals at two different AOI. The optical properties were described by using a Lorentz term in the Ep4-Model. After determining the dispersion of the crystals (left) we converted a recorded Delta and Psi-map into a 2D thickness image (bottom right). Based on a quantitative analysis of the resulting thickness map we have calculated the height of a molecular layer.   RESULTS: - Δ, Ψ values/maps for biaxial anisotropic micro crystals - Fast observation and distinction between mono and multi layered micro crystal in high ellipsometric high contrast images - Optical properties of biaxial semiconducting material - Quantitative analyses of thickness map leads to the height of a molecular layer        

11 Oct 2023

Characterization of black phosphorus as Microstructured Anisotropic 2D Material by Imaging Mueller Matrix Ellipsometry

Thin-film layers of black phosphorus - the most stable allocate of the element phosphorus - is a promising material for future semiconductor electronics based on novel 2D-materials. Ultra-thin films or even monolayers of black phosphorus (b-P) may be fabricated by mechanical exfoliation from bulk material (similar to graphene). 2D-b-P is of particular interest for semiconductor electronics as it features a widely tunable bandgap via the layer thickness (from ~2 eV for monolayers down to 0.3 eV for bulk material). Thus, it closes the energy gap between the other known 2D- materials graphene (zero bandgap) and transition-metal dichalcogenides (bandgap typically 1-2 eV) such as MoS 2