Organic nanofibers. From fundmental optics to devices

Transcription

1 Organic nanofibers From fundmental optics to devices Horst-Günter Rubahn Fysisk Institut, Syddansk Universitet, Danmark

2 Topographic background H.C.Andersen Egeskov

3 Goal New devices on the basis of a new kind of organic nanofibers 5 µm

4 Quality factors of organic nanofibers Dimensions: height nm, width 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

5 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

6 Examples for bent single color emitting nanofibers p-6p / mica p-4p / mica funct. p4 / mica α-4t / mica α-6t / mica

7 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

8 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

9 How they are made: DASA Mica a s dipoles b s p-6p γ needle a m Balzer and Rubahn, Appl.Phys..Lett Surf.Sci., 2002.

10 Growth control Intrinsic parameters Substrate and substrate temperature Adsorbate material Coadsorption, templation Extrinsic parameters Deposition rate External fields

11 Variation of substrate temperature 5 nm p-6p/mica, nm/s 340 K 356 K 368 K

12 Local laser heating 5.7 W/cm mm Argon Ion laser 4 W/cm 2 Inside focus optimum growth temperature: monodisperse needles 100 µm Balzer and Rubahn, Nanoletters, 2002.

13 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

14 Water modified substrate The magazine of the Optical Society of America, Dec. 2003

15 Fundamental optics in the sub-wavelength domain

16 Example 1: nanofiber spectroscopy Epi-fluorescence microscopy Atomic force microscopy

17 Simplification of emission spectra Luminescence intensity thick film isolated aggregate Wavelength [nm]

18 Correlation between morphology & spectroscopy morphology 280 Nanofiber width [nm] Distance from nanofiber end [µm]

19 Correlation between morphology & spectroscopy morphology spectroscopy Nanofiber width [nm] Distance from nanofiber end [µm] FWHM [nm] Distance from nanofiber end [µm] Optical spectra depend on nanofiber morphology. Simonsen and Rubahn, Nanoletters, 2002.

20 Example 2 : waveguiding 120 Height [nm] (d) Width [nm]

21 Waveguiding: far field approach => Direct excitation in the focal point of a fluorescence microscope (a) (b) (c) (d) (e) µm Balzer, Bordo, Simonsen and Rubahn, Appl.Phys.Lett., 2003.

22 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

23 Direct observation of waveguiding Problem: the nanofibers are small! Acceptance angle and diffraction limit problem: Near field approach. H. Sturm BAM, Berlin

24 Discrimination between near- and farfield Shear mode nm 45 x 45 µm 2 farfield glowing of junction point Volkov, Bozhevolnyi, Bordo, Rubahn, J.Microscopy, 2004.

25 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

26 Azimuthal anisotropy Variation of NLO activity as a function of polarization angle. pp-orientation Two-photon intensity [arb.units] o Polarization angle [degrees] Orientation of main nonlinear dipole moment along main molecular axis sp-orientation Balzer, Neuendorf, Al-Shamery,Rubahn, Chem.Phys.Lett., 2003.

27 Why do we see a rotation pattern? 5 µm molecular axes almost perpendicular to long fiber axis 15º

28 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.

29 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) o 75.9 o Position along nanofiber (µm)

30 Devices

31 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

32 Combination with other nanoaggregates p6p-needles + polystyrol luminescent 1 µm spheres in water sphere Biosensors

33 Solution assisted compression -bright - stable (up to 180 C and against chemicals) - huge dichroism - waveguiding s-polarized p-polarized OLEDs

34 Emission characteristics of individual floating nanofibers after strong excitation side view front view Gain narrowing?

35 Gain narrowing in p6p nanoneedles FP cavity modes? Sitter et al., Appl.Phys.Lett.(2004)

36 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

37 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

38 Contacting individual nanofibers Ti/Au electrode 1 nanofiber luminescing area Ti/Au electrode 2

39 IV-curves of individual nanofibers 5e-8 4e-8 Current [A] 3e-8 2e-8 1e Voltage [V] High mobility

40 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