Microscopy and Spectroscopy with Tunneling Electrons STM. Sfb Kolloquium 23rd October 2007

Transcription

1 Microscopy and Spectroscopy with Tunneling Electrons STM Sfb Kolloquium 23rd October 2007

2 The Tunnel effect T ( E) exp( S Φ E ) Barrier width s Barrier heigth

3 Development: The Inventors 1981

4 Development: Almost Got it! Stylus Profiler Topografiner Gustev Schmalz, Zeitschrift des Vereines deutscher Ingenieure, Oct 12, 1929, pp R. Young, J. Ward, F. Scire, The Topografiner: An Instrument for Measuring Surface Microtopography, Rev. Sci. Inst., Vol 43, No 7, p 999, 1972.

5 Tricks and Tips The Tips W, Pt/Ir. The Movement Piezoelectrics. The Feedback PID digital or analog. The Surface Conducting/semiconducting. CLEAN!. Single Crystals.

6 Representation of STM data Silicon (111) 7x7 and Silicon Carbide.

7 Advantages of Low Temperature UHV-STM working at 4-5 K High stability Heat shield.. High energy resolution (<2 mev) Window. L. N 2 L. He Low mobility of adsorbates Wobble Stick port. 5 K cold room. 100 K cold room. Extremely high vacuum Control of sample temperature MBE port. Local Spectroscopy Manipulator port. Vacuum pums port.

8 Microscopy on single adsorbates: Atoms and Molecules Single Atoms Single Molecules D. Eigler. IBM, USA. Benzene on Cu(100)

9 Microscopy on molecules: Self-Assembling PVBA / Ag(110) 1-nitronaftaleno Au(111) Basic mechanism for growth of molecular films R. Berndt. Kiel, Alemania. Nucleation centres. Structure and conformation. Chirality.

10 We see topography? Adsorbates D. Eigler. IBM, USA. Atomic structure of surfaces Atomic steps

11 STM is NOT a true Microscopy!. Benzene on Cu(100) Benzene on Ag(110) The adsorbate s shape depends on the chemical interaction with the surface!.

12 STM is NOT a true Microscopy!. Si(111) 7x7 Empty States. E F+eV Tip Sample E F Filled States. I E F + ev E F ρ (E) ρ (E ev) Sample Tip T(eV, z, φ ) p de

13 Modeling the tunneling current 1961: Bardeen solution to the problem of tunneling in one dimension

14 Modeling the tunneling current 1985: Tersoff and Hamann approximation to the problem of imaging plane waves on surfaces Surface: Bloch wave + Tip: S-wave (spherical) (Bias 0 V) Current: Density of States at the tip position

15 From molecular orbitals to resonances I p E a E v +2U +2U 0 SCREENING BROADENING SPLITTING ALIGNMENT & Hybridization LUMO HOMO - E F Free molecule Molecule at the Surface

16 Seeing molecular orbitals? C60 Intramolecular Structure: We do not see the atoms, but the states. C60 on Si(111): Predominantly covalent bond. Predominantly ionic bond. LUMO STM S3 DOS-HOMO * S5 Constant density of States surfaces for the: HOMO S3 S2 C60 sobre Ag(110): S2 DOS-LUMO * Ab Initio calculations with SIESTA by: D. Sánchez-Portal, E. Artacho, P. Ordejón and J.M. Soler. STM

17 Simulation of STM images Tersoff-Hamann simulations may reproduce STM images, but. Ab Initio calculations with VASP by: Manuel Cobian and Nicolas Lorente

18 Principle of scanning tunneling spectroscopy 1961: Bardeen solution to the problem of tunneling in one dimension I E F + ev E F ρ (E) ρ (E ev) T(eV, z, φ ) Sample Tip p de Local Density of states at energy E and position x,y

19 Methods of scanning tunneling spectroscopy Three magnitudes are controlled by the user I E F + ev E F ρ (E) ρ (E ev) T(V, z, φ ) de s p p I - V - Z - The tunnelling current The energy of tunnelling electrons The width of the tunnelling barrier I vs Z - V fixed. I vs. V Z fixed. Z vs. V I fixed. I- Tunneling current (na) F = 5.4 ev = 4.5 ev F = 3.7 ev F Z Tip-sample distance (Å) I - Tunneling current (na) V - Sample voltage (V) Z - Tip-sample distance (Å) F V - Sample voltage (V) 0 d Z / dv (u.a.)

20 STS Fundaments: Energy Resolution. I E F + ev E F ρ (E) ρ (E ev) T(eV, z, φ ) de Sample Tip p Tunnelling current (I) does not provide true Energy Resolution. G(x, y,z, ev) di dv ρ Sample (EF + ev,...) ρtip (EF,...) T(eV, z, φp) +... Differential conductance is proportional to the Surface Density of States (DOS).

21 Spectroscopy of molecular states Filled Empty EF Differential Conductance STS spectrum I (V) GAP LUMO HOMO di/dv (V) Tunnelling Current (na) Sample Bias (V)

22 Single-molecule vibrational spectroscopy. Molecular Vibrations: Chemical fingerprint of a single adsorbate. Less than 10% of electrons interact inelastically with an adsorbate

23 Vibrational Spectroscopy of one molecule. Intramolecular vibrational modes: C 2 H 2 on Cu(100) Adsorbate-substrate bond vibrational modes: CO on Cu(100) Cavity-breathing vibrational mode: C 60 on Ag(110) 30 C 2 H 2 Cu(100) n(c-h) 100 d 2 I/dV 2 (na/v 2 ) mv G = 1.5% G d 2 I/dV 2 (na/v 2 ) 0 n(c-h) Sample Bias (mv) G = 4.5% G Sample Bias (mv) Excitation Mechanisms Dipolar scattering. Long range G < G 1% Resonant scattering. Short range Resonances close to EF G < G 12%

24 Vibrational Spectroscopy of one molecule. W. Ho, UCI, USA. D. Eigler, IBM, USA.

25 Magnetic imaging with atomic resolution.

26 Manipulation of Atoms and Molecules Fabrication of atomic scale devices Electron induced processes

27 Manipulation of Atoms and Molecules 1990 Silver atoms on Ag(111). D. Eigler. IBM Almaden, USA K.-H. Rieder. FU Berlin.

28 Electron induced reactions Electronic excitations Vibrational Tunnelling rate fs ps ns µs ms s 1 na Multiple electrons

29 Electron induced reactions Electronic excitations Vibrational Tunnelling rate fs ps ns µs ms s 1 na I = t e tunn Yα r I n 1 Rate ( = Y I) r α I n Y r α P inel P inel τ t vib tunn P inel τ t vib tunn P inel τ t vib tunn (n)

30 Electron induced reactions Dissociation of O2 on Pt(111) Fix electron energy (voltage bias) Probe statistical dissociation rate vs. Tunnel current Rate ( = Y d I) α I n 300 mev < E dissociation < 400 mev Excitation of O-O stretch Stipe et al. PRL 78, 4410 (97)

31 Summary: STM more than a microscopy that can measure properties. Is a Spectroscopy with high spatial resolution, and a tool to manipulate atoms and molecules.