Beams and Scanning Probe Microscopy

Transcription

1 IFN-CNR, Sezione di Trento Istituto Trentino di Cultura of Trento Department of Physics University of Trento Towards the joint use of X-ray Beams and Scanning Probe Microscopy Silvia Larcheri SILS 2005 (Frascati)

2 Nano-scale chemical mapping and surface structural modification by joint use of X-ray X microbeams and tip assisted local detection Specific Targeted Research Project (STRP) Supported by European Commission 6 th Framework Programme

3 Scanning Probe Microscopies (SPM) ZnO thin film on Silicon substrate Red corpuscles 1.0µm Gold island on mica substrate Graphite atoms

4 Scanning Probe Microscopies (SPM)

5 X-ray interaction with matter Sample Incident X-rays Transmitted X-rays hν Visible Light XEOL-PLY e - Total Electron Yield (TEY) hν X-rays Fluorescence (FLY) µx X-ray Absorption Fine Structure (XAFS) These signals are able to be collected by a near field probe! Photon energy h ν

6 Local Probe Microscopy X-Ray Absorption Spectroscopy using Synchrotron Radiation Combined Nano-Scale Spectro-Microscopy

7 X-ray Excited Optical Luminescence Energy - site selection Wavelength [nm] X-rays Visible light Sample PL intensity [a.u.] PLY Absorption [a.u] X-ray energy [ev] Energy [ev]

8 XEOL-XAS XAS setup Grating Mirror C C D Controller X-rays Visible light Sample

9 Scanning Near-Field Optical Microscope (SNOM)

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11 SNOM prototype (Marseille) XEOL-XAS XAS detection set up (Trento) Beamline BM 05 (ESRF Grenoble) September 2005

12 SNOM prototype D. Pailharey, F. Jandard GPEG-CNRS (Marseille) Electronics Z coarse movement of the tip Flange for the vacuum chamber Sample holder and Z-piezo to engage the tip X and Y tip positioning Tip (fixed on the X-ray beam) Piezo scanner

13 Piezo inertial movements

14 XEOL-XAS XAS setup

15 Fabrication techniques Protection-layer etching Heating & Pulling

16 Etched and Pulled Tips

17 Coating equipment V. Sammelselg,, University of Tartu (Estonia)

18 Characterization 1. Scanning Electron Microscopy (SEM) 2. Transmission/collection properties 3. Topographic image of a well-known surface 4. Goniometric characterization

19 Why ZnO thin films? Interesting material for UV photonics, gas sensors, piezoelectric and waveguide devices,, LED, Easy to prepare using many techniques A lot of different nanostructures Two emission bands (visible and UV) Zhong Lin Wang, Materials Today 7, 26 (2004)

20 Optical Luminescence excited by Cu anode 300 dc magnetron sputtering HT2_30s (from met-zn) 341HT_30s ZnOTN_30s TEC1_30s Intensity (a.u) 100 1/2 thick. 338HT_30s 346HT_30s 3%Ta 347HT_30s Intensity (a.u) TEC2_30s TEC3_30s 7%Cd 480HT2_30s %Cd 480LHT_30s Wavelength (nm) 0 TEC4_30s Wavelength (nm)

21 PLY-XANES studies (Beamline BM 08 ESRF Grenoble ) Intensity (a.u./s) Intensity (a.u.) (a) TEC3 TEC5 TEC1 XEOL spectra at 9700 ev (b) Wavelength (nm) TEC5 TEC3 ZnO powder TEC1 PLY-XANES X-ray energy (ev) TEC samples show significant shifts of the XEOL bands in the visible region. PLY-XANES shows significant differences in the local electron density of states between ZnO powder and our nanostructured samples. Changes between different TEC samples might be due to quantum confinement (QC) or to local distortion mainly related to defects or surface states.

22 Comparison between PLY- and FLY-XANES 2.5 ZnO powder: PLY-XANES Intensity (a.u.) TEC5: PLY-XANES TEC5: FLY-XANES X-ray Energy (ev) Changes between PLY- and FLY-XANES spectra of the same sample should be attributed to the peculiar sensitivity of PLY detection mode to the light emitting centers. The origin of these changes is still under study.

23 Future work SNOM PROTOTYPE Absolute alignment Change from grazing to normal incidence Test using laser source and focused and very intense X-ray beamline OPTICAL FIBER PROBES Improve tip shape Full characterization of the optical fiber probes Apertureless probe SAMPLES Detailed study of XAFS spectra from ZnO thin films Selection of other sample suitable both for topography and spectroscopy

24 Scanning Near Field Optical Microscope Current SNOM is directly inspired by the pioneer idea of Synge The tapered and metalized tip has been introduced by E. Betzig et al. Science, 251, 1468 (1991)

25 Generation of Optical Near-Field What happens by shining the light on S? Optical Near Field M.Ohtsu & K.Kobayashi Nuclei and electrons are displaced from their positions and generate electric dipoles p i The electric lines of force connecting the p i are found not only in S but also on its surface; they tend to take the shortest trajectory: this is why the ONF is very thin and localized

26 Detection of Optical Near-Field Introduction of a secondary sphere The ONF cannot be detected like a conventional propagating light because it is localized Detection method: the ONF is disturbed by a secondary sphere P; the disturbed ONF of S is converted to propagating light (scattered light 2) and collected by a photodetector. The two sphere are coupled to each other in response to the incident light but they are mutually isolated material system Optical Near Field M.Ohtsu & K.Kobayashi

27 Electric Field Distribution behind an aperture convolution of the angular spectrum of the incident field and Fourier transform of the aperture B. Hecht et al., J. Chem. Phys., Vol. 12, No. 18, 2000

28 TEC sample and SNOM tip at BM05 (ESRF)

29 TEC sample and SNOM tip at BM05 (ESRF)

30 Al coated tip SEM - 20 KeV, x500

31 Transmission Coefficient of Aperture Probes Attenuation in the taper region depends on the fabrication method reorganization of modes and strong back-reflection B. Hecht et al., J. Chem. Phys. Vol. 12, No. 18, 2000 Attenuation in the sub-λ aperture calculated rigorously by Bethe transmission scales down as a 4 (a denotes aperture diameter) working with a<40-50 nm is often impossible

32 Focused Ion Beam Milling Before... After nm ADVANTAGES clean-cut, circular aperture desired diamete1r (up to 20 nm) higher throughput (up to one order of magnitude) true near-field optical contrast DRAWBACKS extra time added cost failure rate availability of the instrumentation

33 Etching without polymer coating Tube Etching

34 Focus on the body of the optical fiber Focus on the tip Without light coupling With light coupling

35 X-ray Excited Optical Luminescence (XEOL) setup XEOL XEOL X-rays Sample X-rays

36 First Results before applying for Beamtime at Gilda ( ) Photoluminescence (arb.units) ZnO powder RT, Mo anode ZnO:Al 477 ZnO 341 ZnO Wavelength (nm)

37 XANES-XEOL XEOL spectra All our samples 2.0 Intensity (a.u.) ZnO powder (1.3 mg) TEC1 TEC3 TEC5 341HT 346HT (ZnO+W?) 347HT (ZnO:1% Ta+W?) 480HT2 (ZnO:7% Cd) Energy (ev)

38 Formation of ZnO nanostructures by thermal evaporation PL spectra recorded at room temperature. Spectra a, b, and c were recorded from the low temperature site nanowires, the medium temperature site nanoribbons, and the high temperature site needle-like rods, respectively. SEM images showing the three typical morphologies of the asprepared ZnO products: (a) needle-like rods; (b) nanoribbons; (c) nanowires, and (d) their corresponding growing site temperatures. B. D. Yao, Y. F. Chan, and N. Wang, Appl. Phys. Lett (2002).

39 TEC sample characterization Diffracted intensity (a.u.) TEC5 TEC3 TEC1 ZnO powder X-ray diffraction (100) (002) (101) (102) (110) (103) θ (degree) X-ray diffraction reveals an highly (002) oriented crystalline structure. Intensity (photons/sec) 2500 Laser-excited PL after 5 ns after 90 ns 500 after 530 ns Wavelength (nm) TEC samples exhibit a very intense optical luminescence, both XEOL and PL. Time-resolved PL spectra show a very fast excitonic luminescence (not visible after about 200 ns) and a slow defects-related luminescence (visible up to 12 µs). A. Kuzmin,, University of Latvia

40 The excitonic peak appears without collection lens Sample TEC2, bunch of 24 optical fibres E= 9700 ev 1.0 with collection lens - BM08 without collection lens - BM Intensity (a.u./s) Wavelength (nm)

41 Goniometer characterisation Bad tip Intensity (µv) Angle (degree) Good tip Intensity (µv) 76.6 Angle (degree)

42 Radiation damage of the tip Phosphor luminescence spectra 1. directly measured 2. through unirradiated fiber 3. through X-rays irradiated fiber (10 hours)

43 Promising results Luminescence distribution under X-rays for an uniform sample (up) and quantum dots buried under Si layer (down). The scanned surface is 5 µm x 5 µm

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45 Three techniques/setup 1. XAS-STM STM mode photoelectrons emitted under X-ray irradiation collected by the STM tip 2. XAS-AFM AFM mode charge changes induced by X-rays are detected by the AFM tip 3. XAS-SNOM SNOM mode X-ray Excited Optical Luminescence (XEOL) is detected through the optical fiber of a SNOM Two kinds of measurements 1. Conventional scanning for 3D-mapping with element specific contrast 2. Probe fixed at a single point on the sample, X-ray energy tuned across the absorption edge