microscopical and microanalytical methods...
TRANSCRIPT
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06.11.15 10:15-12:00 Introduction - SPM methods
13.11.15 10:15-12:00 STM
20.11.15 10:15-12:00 STS
27.11.15 10:15-12:00 Novel SPM techniques
04.12.15 10:15-12:00 2-dimensional crystallography, LEED, AES
Erik Zupanič
stm.ijs.si
Microscopical and
Microanalytical Methods
(NANO3)
2k → 20 nm-1
Δd = 0.1 nm
order of magnitude→ difference in the
tunneling probability
Scanning tunneling microscopy
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Scanning Tunneling Spectroscopy (STS)
Different spectroscopy modes:
I-d
V-d
I-V
IETS
(I – tunneling current, V – tunneling voltage, d – tip-sample distance)
Scanning tunneling spectroscopy
resolution limitations:
at 300 K: ΔE ≈ 80 meV
at 4.2 K : ΔE ≈ 1 meV
STS using lock-in amplifier
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STS
STS measurements
STM/STS tips
STS measurements
Best tips for STS: clean, slightly blunt
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Examples
STS mapping – measuring LDOS(VT) in 2D
STM CC image
(1 nA, 40 mV)
LDOS @ 40 mV FFT of LDOS @ 40 mV
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…
Inelastic Electron Tunneling Spectroscopy
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Background-subtracted d2I/dV2 spectra for benzene isotopes and dissociation products with anacetylene spectrum for comparison.
L.J. Lauhon and W. Ho, J. Phys. Chem. A 104 , 2463-2467 (2000)
IETS
Embedded impurities in
Cu(111) surfaces
Co adatom deposition:
a) sample at RT b) in-situ, sample at LT
33 x 33 nm2
60 x 60 nm2450 x 450 nm2
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Embedded impurities in
Cu(111) surfaces
Some Co adatoms stay stable when T ↑
Embedded impurities in
Cu(111) surfaces
Naturally occuring defects
STS mapping @ 100 mV
a) topographic CC image b) tunneling current c) dI/dV value at eVT d) signal phase change
35 x 26 nm2
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Embedded impurities in
Cu(111) surfaces
STS on defects
defect induces broad bound state just below the Cu band edge at 4.2 V←
Embedded impurities in
Cu(111) surfaces
Influence of defects on:
a) Co stability b) the magnetic coupling
Surprising with regard to the high (exponential)dependence of the Kondo temperature on theatomic parameters.
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Embedded impurities in
Cu(111) surfaces
Position of Co relative to the embedded defect
radius ~ 0.26 ± 0.04 nm
Embedded impurities in
Cu(111) surfaces
If number of Co adatoms > number of defects
28 x 28 nm219 x 19 nm2
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Embedded impurities in
Cu(111) surfaces
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
Embedded impurities in
Cu(111) surfaces
DFT results
- EF 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
rapidly with distance from the defect site
Also exhibit a small change in LDOS but areless commonly found as impurities in Cusample
All other either large change of the LDOS atthe defect site of a perturbation with long-ranging tails (suggestive of a standing-wavepattern)
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Embedded impurities in
Cu(111) surfaces
DFT using a 8-by-8 supercell
STM image:
Embedded impurities in
Cu(111) surfaces
DFT binding energies
Co on Cu(111) binding energy on fcc site: 3.222 eV
Co on Cu(111) binding energy on hcp site: 3.212 eV
Co on fcc binding site adjacent to Ag defect: 3.315 eV
Co on hcp binding site adjacent to Ag defect: 3.308 eV
→ defects effectively binds adatom with an additional 90 meV !
Additionally: Zn on binding site adjacent to Zn defect: 2.3 eV
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Embedded impurities in
Cu(111) surfaces
Controlled deposition of Ag on a clean Cu(111) surface , followed by annealing
75 x 75 nm2
Strongly increased concentration of pinning centers!
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