Scanning Tunneling Microscopy: theory and examples

Transcription

1 Scanning Tunneling Microscopy: theory and examples Jan Knudsen The MAX IV laboratory & Division of synchrotron radiation research K5-53 (Sljus) April 17, 018

2 Last time Scanning Probe Microscopy (STM) Controlling the tip/sample position Vibration isolation Tip preparation

3 Local density of states (LDOS) Scanning Probe Microscopy (STM) Density of state (DOS) means the "number of states at a particular energy level", i.e. the distribution of states over energy. Local Density of state - LDOS(x,y,z,E) - gives the density of electrons of a certain energy at that particular spatial location.

4 Electron tunneling in 1D x m E V x x x E x V x x m 0 0 ) ( ) ( E m V x e x 0 0 Solution: 0 0 ) ( 0 mv a E V m a e e a T W W V 0 E

5 E Bardeen s formalism s s E e - ev t t Pos. bias Scanning Probe Microscopy (STM) Fermi s golden rule: R s t M Fermi Function: 1956: "for their researches on semiconductors and their discovery of the transistor effect 197: "for their jointly developed theory of superconductivity, usually called the BCS-theory 1 kt f / 1 e E F Only elastic tunneling s t

6 1 E Bardeen s formalism I I I s ts st tot s e - ev E t t Pos. bias s s t 1 f M ev f ev t t s 1 I ts 1 and use 3 : I st ev f ev M f 4 E E F F s ev Scanning Probe Microscopy (STM) Fermi s golden rule: R s t M Fermi Function: M ev d t t s M 1956: "for their researches on semiconductors and their discovery of the transistor effect 197: "for their jointly developed theory of superconductivity, usually called the BCS-theory d d 3 1 kt f / 1 e E F Only elastic tunneling Low temp. -> f=1 < E f f=0 > E f * * s t s ds m S t Bardeen, PRL, 6, 57 (1961) s t

7 Tersoff and Hamann s approximations I tot M I ts I st 4 E F E F s ev * * s t s ds m S t M ev d t s-wave function with constant DOS t = const. Expanded in Bloch surface waves Low temperature -> Sharp Fermi functions Tersoff and Hamann: I V0 LDOSs EF, r 0 Tersoff and Hamann, Phys. Rev. B. 31, 805 (1985) Our simple model: T a 0 e a mv 0 M I tot V 0 s E E F t F e a m

8 Measuring empty or filled states by bias switching Scanning Probe Microscopy (STM) Filled state imaging Sample Tip Negative bias Positive bias Empty state imaging Sample Tip Positive bias Negative bias F S e - F T F S e - F T d Potential Barrier d Potential Barrier

9 When is the combined convolution of electronic structure and morphology important? Height [pm] When is this apparent height in STM pictures equal to the real height?

10 Comparing simulated and experimental STM images C 35x35Å Scmid et al. 96, (006) Schnadt et al. 96, (006)

11 Scanning Probe Microscopy (STM) Moiré structures Moiré superstructures depend on: Lattice mismatch Overlayer rotation 6 7

12 FeO(111) 8x79Å HCP FCC TOP Fe-Fe: 3.09 Å Pt-Pt:.77 Å

13 Limitations of the Tersoff-Haman approach Scanning Probe Microscopy (STM) In the simple, approximate model of Tersoff and Hamann, a direct correlation exists between the STM tunnel current and the energyintegrated local density of states (LDOS) projected to the position of the tip apex above the surface. Tersoff and Hamann: I V0 LDOSs EF, r 0 Tip state is not included in the Tersoff-Haman approach! STM topographs of FeO(111) measured with different tip states Merte, Besenbacher et al., Jour. Phys. Chem. C., 115, 089 (011)

14 Tip dependent imaging The contrast observed in the STM images can depend severely on the state of the tip. This is not included in Tersoff-Haman simulations! Fe-mode O-mode I tot V 0 s E E F t F e a m Merte, Knudsen et al., Surface Science, 603, L15 (009)

15 The NiAl(110) surface Scanning Probe Microscopy (STM) K. Hansen, PhD thesis 001, University of Aarhus

16 Tip dependent imaging II Scanning Probe Microscopy (STM) Science, 308, 1440 (005)

17 Organic molecules on surfaces Weigelt, Linderoth, Besenbacher et al., Nature Materials, 5, 11 (006)

18 Following a chemical reaction on a surface with STM Scanning Probe Microscopy (STM) Problems with STM movies Drift due to temp. changes The tip might affect the reaction No chemical sensitivity

19 .7 Å HCP/FCC.46 Å Graphene on Ir(111) TOP 5 Å 50x50 Å

20 Clusters on graphene Cluster nucleation in HCP sites sp 3 sp 0.5 ML Pt on graphene/ir(111) as grown, 380x380Å Ideal model system for studying the adsorption properties with averaging methods.

21 In-situ unpinning of Pt-clusters on supported by graphene/ir(111) Scanning Probe Microscopy (STM) Gerber, Knudsen et al., ACS Nano, 7, 00 (013)

22 CO stability cluster size S av = 7 atoms +1.1 L CO S av = 19 atoms +16 L CO P(CO)< mbar P(CO)= mbar S av = 38 atoms +0 L CO Gerber, Knudsen et al., ACS Nano, 7, 00 (013)

23 CO intercalation: Structure of CO on Ir(111) in the mbar regime 19 (0.7 ML) 8 1mbar CO, RT, 40 min (33 33) structure 0.79 ML Grånäs et al., Jour. Phys. Chem. C, 117, (013)

24 CO intercalation: STM P(CO) = 1 mbar, room temperature, 40 min A B B A A: Islands and stripes Not intercalated B: Islands Intercalated Small islands are not intercalated CO intercalated CO/Ir(111) Grånäs et al., Jour. Phys. Chem. C, 117, (013)

25 CO intercalation: stripes Scanning Probe Microscopy (STM) P(CO) = 1 mbar, room temperature, 40 min TOP: 11 FCC/HCP: 7 Observations: HCP/FCC: 4 Stripes mainly located in FCC/HCP domains When graphene cross Ir(111) step => Stripes at lower step edge Grånäs et al., Jour. Phys. Chem. C, 117, (013)

26 CO intercalation: model for stripes Grånäs et al., Jour. Phys. Chem. C, 117, (013)

27 Etching of ML graphene Scanning Probe Microscopy (STM) 70 nm mbar O 585 K

28 Etching of 0.5 ML graphene Scanning Probe Microscopy (STM) 30 L 80 L 30 L 670 L Observations C-atoms are exclusively removed from the step edges During etching graphene is always intercalated by O- atoms. Free Gr edges are etches faster than Gr-Ir edges Schröder et al., Carbon, 96, 30 (016)

29 Etching of 1 ML graphene Scanning Probe Microscopy (STM) 750 L at 700 K 750 L at 745 K What is the atomic scale structure of these nucleation sites?

30 Etching of 1 ML graphene Etching always start where two additional moiré rows starts The starting point of two additional moiré rows signals 5-7 defects => 5-7 defects are the starting point for graphene etching

31 Conclusions Theoretical expression of the tunnel current: Examples: I tot V 0 s EF t EF E I V0 LDOSs F, r 0 e a m