AFM: Atomic Force Microscopy II

Transcription

1 AM: Atomic orce Microscopy II Jan Knudsen The MAX IV laboratory & Division of synchrotron radiation research K (Sljus) 4 th of May,

2 Last time: The total force tot chem mag el vdw bonding between tip and sample atoms for magnetically sensitive tips el 1 2 C z V 2 vdw HR 2 6r Chemical sensitivity Atomic resolution Control V bias Use sharp tips

3 Contact mode

4 Different Modes of the AM

5 Jump to Contact - Criterion ts( z) cl( z ) z z k cl z 0 cantilever k cl When (if) the force gradient of the tip-sample interaction exceeds the spring constant of the cantilever, the tip jumps into contact. z force ts orces in AM

6 Jump to contact and the orce Spectroscopy Curve 3 a b c e f (a) As tip approaches the sample, it starts to deflect towards sample (b) Jump to contact-point: When the force gradient exceeds the cantilever spring constant, the tip jumps to contact (c) Pushing the tip further leads to positive deflection, i.e. repulsion. Slope defines relative sample stiffness (S = applied force/distance) (d) The tip retracts to netrual position. (e) Due to adhesion the tip sticks to the surface (f) Jump off: When the cantilever force exceeds the adhesion the cantilever jump out of contact orces in AM

7 Contact Mode Tip is in soft physical contact with the surface. In air, capillary and electrostatic forces add to the tip-sample force. Easy to operate Simple interpretation : topography of constant force Imaging can be destructive Enables riction orce Microscopy Possible in both constant force mode or constant height mode (movie) AM modes

8 Kelvin force microscopy

9 V bias = 0 V The local contact potential V bias = 1.34 V We need a method to continuously modify the contact potential on surfaces where the local contact potential is modulated: Kelvin probe force microscopy! Phys. Rev. Lett. 91, (2003)

10 Electrostatic orces Scanning Probe Microscopy (SPM) Ar + Electrostatic forces between charges trapped in tip and surface. Calculated with Coulombs law from the distribution of charges Surface Oxide Capacitive orces (tip-sample forms a capacitor!): where V CPD is the contact potential difference due to differences in work functions of tip and surface. Compensate with V bias C =? V bias -

11 Kelvin probe orce Microscopy V LCPD V CPD V add ( x, y, z) Non-uniform contact potential difference el DC 2 f 1 C V LCPD VDC VAC sin(2f modt) 2 dz 1 C 2 2 VLCPD VDC VAC 2 dz 1 C 2V LCPD VDC V AC sin(2f 2 dz 1 C 1 2 mod VAC sin(2 2 fmodt) 2 dz 2 f mod 2 mod t) Keep V AC at minimum Used to adjust V DC

12 KPM measurement V bias = 0 V V bias = 1.34 V Phys. Rev. Lett. 91, (2003)

13 Theoretical AM simulations δ + δ - JACS, 3, 517 (2009) Tip model (MgO) Surface model (DT)

14 Comparison with Theory Exp DT Ti O Ti O Ti O Ti Vacancy Exp DT Ti O Ti O Ti O Ti Single OH Double OH Nanotechnology 17, 3436 (2006)

15 Examples

16 Identifying charge state with NC-AM STM constant current STM constant height f f f 0 f0k 2k f0k 2k ts cant ts cant f 2k 0 f 0 cant f d Z Science, 324, 1428 (2009) AM Constant heitght Gross et al., Science, 324, 1428 (2009)

17 Lateral force microscopy (LM) Phys. Rev. Lett. 106, (2011) Rev. Sci. Inst. 82, (2011) PNAS, 99, (2002)

18 The force needed to move a single atom f f f 0 f0k 2k f0k 2k ts cant ts cant f 2k 0 f 0 cant f d Z Science 319, 1066 (2008)

19 How strong is a covalent bond? Science, 283, 1727 (1999)

20 Increasing the atomic resolution: Molecule terminated tips Phys. Rev. Lett. 106, (2011)

21 The AM cantilever concept as a sensor Artificial nose Vibration or static deflection Cantion

22 Membrane proteins Opening Deinococcus radiodurans is an extremophilic bacterium, one of the most radioresistant organisms known. It can survive cold, dehydration, vacuum, and acid, and is therefore known as a polyextremophile and has been listed as the world's toughest bacterium in The Guinness Book Of World Records Scale bars 10nm (upper) Muller et al J. Bacteriology 178, 3025, (1996) 2 successive scans reveal opening and closing of pores

23 Conclusions Jump to contact criteria: Kelvin probe orce Microscopy: Simulations ts( z) cl( z ) z z el 1 2 C dz k cl V V V sin(2 f t 2 LCPD DC AC mod ) Examples