Microscopical and Microanalytical Methods (NANO3)

Transcription

1 Microscopical and Microanalytical Methods (NANO3) :15-12:00 Introduction - SPM methods :15-12:00 STM :15-12:00 STS Erik Zupanič erik.zupanic@ijs.si stm.ijs.si :15-12:00 Novel SPM techniques :15-12:00 2-dimensional crystallography, LEED, AES Scanning tunneling microscopy 2k 20 nm -1 Δd = 0.1 nm order of magnitude difference in the tunneling probability 1

2 Scanning tunneling spectroscopy Scanning Tunneling Spectroscopy (STS) Different spectroscopy modes: I-d V-d I-V IETS (I tunneling current, V tunneling voltage, d tip-sample distance) STS using lock-in amplifier at 300 K: ΔE 80 mev at 4.2 K : ΔE 1 mev resolution limitations: 2

3 STS STS measurements STM/STS tips STS measurements Best tips for STS: clean, slightly blunt 3

4 Examples STS mapping measuring LDOS(V T ) in 2D STM CC image (1 na, 40 mv) 40 mv FFT of 40 mv Inelastic Electron Tunneling Spectroscopy IETS - some of the tunneling electrons lose energy by exciting vibrations of the adsorbate => second tunneling path, which gives an additional current contribution to the tunneling current The inelastic contribution to the current is small compared to the elastic tunneling current (~0.1%) and is more clearly seen as a peak in the second derivative 4

5 IETS Background-subtracted d 2 I/dV 2 spectra for benzene isotopes and dissociation products with an acetylene spectrum for comparison. L.J. Lauhon and W. Ho, J. Phys. Chem. A 104, (2000) Co adatom deposition: a) sample at RT b) in-situ, sample at LT 450 x 450 nm 2 60 x 60 nm 2 33 x 33 nm 2 5

6 Some Co adatoms stay stable when T Naturally occuring defects 35 x 26 nm 2 STS 100 mv a) topographic CC image b) tunneling current c) di/dv value at ev T d) signal phase change 6

7 STS on defects defect induces broad bound state just below the Cu band edge at 4.2 V Influence of defects on: a) Co stability b) the magnetic coupling Surprising with regard to the high (exponential) dependence of the Kondo temperature on the atomic parameters. 7

8 Position of Co relative to the embedded defect radius ~ 0.26 ± 0.04 nm If number of Co adatoms > number of defects 19 x 19 nm 2 28 x 28 nm 2 8

9 Density functional theory (DFT) calculations - very versatile method - able to treat problems with sufficiently high accuracy - computationally relatively simple (compared to q-mc or post-hf theory) Conceivable substitutional defects: - known contaminants in the bulk crystal Ag, Cr, Cd, Fe, Zn - usual surface contaminants on Cu C, O, S - transition metal atoms in the neighborhood of Cu in the periodic table Au, Ni, Pd, Pt - some other elements Al, Bi, Co, Mn, Ti, W DFT results rapidly with distance from the defect site - E F in a plane 0.21 nm above the top-most layer of Cu atoms - three-layer slab geometry - 4-by-4 supercell Induce small change in LDOS which decays Also exhibit a small change in LDOS but are less commonly found as impurities in Cu sample All other either large change of the LDOS at the defect site of a perturbation with longranging tails (suggestive of a standing-wave pattern) 9

10 DFT using a 8-by-8 supercell STM image: DFT binding energies Co on Cu(111) binding energy on fcc site: ev Co on Cu(111) binding energy on hcp site: ev Co on fcc binding site adjacent to Ag defect: ev Co on hcp binding site adjacent to Ag defect: ev defects effectively binds adatom with an additional 90 mev! Additionally: Zn on binding site adjacent to Zn defect: 2.3 ev 10

11 Controlled deposition of Ag on a clean Cu(111) surface, followed by annealing Strongly increased concentration of pinning centers! 75 x 75 nm 2 Conclusions di/dv probing local electronic structure IETS vibrational spectroscopy on adsorbates High spatial resolution (single atoms/molecules) STM possibility of building and probing nanostructures 11