Organic nanofibers From fundmental optics to devices Horst-Günter Rubahn Fysisk Institut, Syddansk Universitet, Danmark
Topographic background H.C.Andersen Egeskov
Goal New devices on the basis of a new kind of organic nanofibers 5 µm
Quality factors of organic nanofibers Dimensions: height 20-50 nm, width 100-400 nm, length mm Brightness (efficiency 30 percent) Well-defined spectra at room temperature Huge dichroism (polarized emission) Emission into large solid angle Emission color change via modification of basis units Reproduction and alignment via self-assembly Prices
Examples for single color nanoscopic elements p-6p / mica MOP4 / mica p-6p + α-6t / mica, s-polarization p-6p + α-6t / mica, p-polarization α-6t / mica α-4t / mica
Examples for bent single color emitting nanofibers p-6p / mica p-4p / mica funct. p4 / mica α-4t / mica α-6t / mica
What are these elements doing? Intense (blue) light emission, stimulated by UV light or electrons Waveguiding Inrinsic light emission plus waveguiding Light color conversion: nonlinear optical activity Semiconducting
Devices we are heading for New waveguides Ew-coupled self-organized sensor arrays Cheap, high selective markers Polarized, blue LEDs Single crystal OFETs Organic nanolasers
How they are made: DASA Mica a s dipoles b s p-6p γ needle a m Balzer and Rubahn, Appl.Phys..Lett. 2001 Surf.Sci., 2002.
Growth control Intrinsic parameters Substrate and substrate temperature Adsorbate material Coadsorption, templation Extrinsic parameters Deposition rate External fields
Variation of substrate temperature 5 nm p-6p/mica, 0.025 nm/s 340 K 356 K 368 K
Local laser heating 5.7 W/cm 2 0.5 mm Argon Ion laser 4 W/cm 2 Inside focus optimum growth temperature: monodisperse needles 100 µm Balzer and Rubahn, Nanoletters, 2002.
Adsorbate material p-4p p-5p p-6p W = W 2 2 α E 23meV(p 4P);35meV(p 5P);51meV(p 6P) 3 R 28meV thermal
Water modified substrate The magazine of the Optical Society of America, Dec. 2003
Fundamental optics in the sub-wavelength domain
Example 1: nanofiber spectroscopy Epi-fluorescence microscopy Atomic force microscopy
Simplification of emission spectra Luminescence intensity 1.0 0.8 0.6 0.4 0.2 0.0 thick film isolated aggregate 430 440 450 460 470 480 490 500 Wavelength [nm]
Correlation between morphology & spectroscopy morphology 280 Nanofiber width [nm] 240 200 160 120 80 40 0 0 20 40 60 80 Distance from nanofiber end [µm]
Correlation between morphology & spectroscopy morphology spectroscopy 280 18 Nanofiber width [nm] 240 200 160 120 80 40 0 0 20 40 60 80 Distance from nanofiber end [µm] FWHM [nm] 16 14 12 10 8 6 4 2 0 0 10 20 30 40 50 60 70 80 90 Distance from nanofiber end [µm] Optical spectra depend on nanofiber morphology. Simonsen and Rubahn, Nanoletters, 2002.
Example 2 : waveguiding 120 Height [nm] 90 60 30 0 (d) 200 400 600 800 Width [nm]
Waveguiding: far field approach => Direct excitation in the focal point of a fluorescence microscope (a) (b) (c) (d) (e) 0 45 90 µm Balzer, Bordo, Simonsen and Rubahn, Appl.Phys.Lett., 2003.
A note on gains and bills size reduction (packing density) spectroscopic flexibility (new optical properties) self assembly behaviour (bottom-up technology) Fiber diameter 140 µm Reproducible generation and modification of nanoelements Optical analysis of sub-wavelength structures Mobilisation of aggregates/integration into devices
Direct observation of waveguiding Problem: the nanofibers are small! Acceptance angle and diffraction limit problem: Near field approach. H. Sturm BAM, Berlin
Discrimination between near- and farfield Shear mode + 200 nm 45 x 45 µm 2 farfield glowing of junction point Volkov, Bozhevolnyi, Bordo, Rubahn, J.Microscopy, 2004.
Example 3: Nonlinear optical activity 11 nm p-6p, exc.wavelength 786 nm, 80 fs Domain 1 Domain 2 0 o 100 µm 10 µm
Azimuthal anisotropy Variation of NLO activity as a function of polarization angle. pp-orientation Two-photon intensity [arb.units] -0.36 120 o -0.40-0.44-0.48-0.52 0 90 180 270 360 450 540 630 720 Polarization angle [degrees] Orientation of main nonlinear dipole moment along main molecular axis sp-orientation Balzer, Neuendorf, Al-Shamery,Rubahn, Chem.Phys.Lett., 2003.
Why do we see a rotation pattern? 5 µm molecular axes almost perpendicular to long fiber axis 15º
Nonlinear optical microscopy FH sp-orientation FH pp-orientation Fundamental: 25 mw Ti:Sapphire, 790 nm SH 500counts/20ms 14000counts/20ms Scan-size: 10 x 10 micron 2 ss-orientation FH 50counts/20ms 1500counts/20ms ps-orientation FH Beermann, Bozhevolnyi, Bordo, Rubahn, Opt. Communications, 2004.
Local molecular orientations (010) β =72.7 o β = 75.9 o (1-1-1) 60 (a) I pp /I sp (b) I ps /I ss Molecular orientation (deg) 70 80 0 2 4 6 8 10 12 14 16 18 20 22 24 0 2 4 6 8 10 12 14 16 18 20 22 24 72.7 o 75.9 o Position along nanofiber (µm)
Devices
Free floating organic nanowires 1 µm Femtosecond two-photon confocal microscopy (allows to observe the backside of the needles) Markers L. Bagatolli, J. Brewer, H.-G. Rubahn
Combination with other nanoaggregates p6p-needles + polystyrol luminescent 1 µm spheres in water sphere Biosensors
Solution assisted compression -bright - stable (up to 180 C and against chemicals) - huge dichroism - waveguiding s-polarized p-polarized OLEDs
Emission characteristics of individual floating nanofibers after strong excitation side view front view Gain narrowing?
Gain narrowing in p6p nanoneedles FP cavity modes? Sitter et al., Appl.Phys.Lett.(2004)
Term scheme for p-6p p organic light emitters S x S 1 S 0 S 2 ~~> (400 nm) ~~> excited state absorption + (780 nm) excited state luminescence stimulated emission + (460 nm) vibrational progression structural relaxation (100 fs) τs 1 400ps* rapid depletion *Piaggi et al., Opt.Mat.9(1998)489; Ariu et al., CPL 313(1999)405 + Zenz et al., Synth.Met.101(1999)660 ZnO GaN
Nanolasers Elements of the organic nanolaser: High-gain semiconducting medium Efficient pumping via external photons or electrons; low-absorbing substrate, optimum 4-level scheme Strong mode confinement via morphology and feedback via breaks Problems: Photoexcited-state induced oxidation; defect formation (probably via local heating); low resonator quality; small index difference Solution: Use ultrashort laser pulses Use laser cutting and gold plating
Contacting individual nanofibers Ti/Au electrode 1 nanofiber luminescing area Ti/Au electrode 2
IV-curves of individual nanofibers 5e-8 4e-8 Current [A] 3e-8 2e-8 1e-8 0 0.0 0.2 0.4 0.6 0.8 1.0 Voltage [V] High mobility
Acknowledgments A.C. Simonsen, J. Brewer, C. Maibohm, H. Henrichsen, L. Bagatolli Fysisk Institut and MEMPHYS, Syddansk Universitet, Denmark J.Kjelstrup-Hansen Microelectronic Center, DTU, Lyngby, Denmark S.I. Bozhevolnyi, J. Beermann, V.Volkov Fysisk Institut, Aalborg Universitet, Denmark F. Balzer, H. Niehus, L. Kankate Physics Institute, Humboldt University, Germany K. Al-Shamery, R. Neuendorf Chemical Institute, Oldenburg University, Germany V.G. Bordo General Physics Institute, Moscow, Russia FASTNet, PHANTOMSNet, NATO, SNF, Siemensfond