Noncontact-AFM (nc-afm)

Transcription

1 Noncontact-AFM (nc-afm)

2 Quantitative understanding of nc-afm A attractive interaction Δf Resonance frequency: f 0 Width of resonance curve (FWHM): Γ Γ+ΔΓ Γ Q-factor: Q π f Γ = 0 f 0 f Conservative forces shift of resonance curve Δf Dissipative forces broadening of curve ΔΓ

3 Forces in nc-afm Frequency modulation: f 0 = 1 π k m * Δf = f0 k F z tot measured topography = surface of constant F z F = F + F + F + tot chem mag el F vdw bonding between tip and sample atoms (only for d < 5 Å) only for magnetically sensitive tips F el = 1 C z V F vdw = HR 6d

4 Dynamic Mode, non-contact 100 III II I Standardwerte: z min = nm A = 0-60 nm region I: attractive forces non-contact mode Δf (-Hz) 10 region II: attractive forces atomic resolution z min (nm) region III: repulsive forces tapping mode

5 Molecular nanowires on KBr Topography Damping Asymmetric porphyrins on KBr with pits 100 nm Straight edges are decorated Image height approx 1nm, Single molecules? 4 nm

6 Cutting a molecular wire topo df damping 1 50nm 50nm 50nm 50nm 50nm 50nm df -6 Hz 3 df -30 Hz df -6 Hz 50nm 50nm 50nm df -14 Hz 4 50nm 50nm 50nm

7 Wieviel Kraft braucht man für einen molekularen Schalter?

8 Force spectroscopy of Cu-TBPP molecules on Cu(100) Copper 0 Curves on 4x5 Cu-TBPP island; thermal drift 5nm/h Ch. Loppacher et al., PRL 90, (003)

9 Force spectroscopy above a leg of Cu-TBPP Force derived from Δf (Algorithm from F. Giessibl) J P<7fW Difference (Δf leg - Δf Cu ) 10-4 switching energy of a state-of-the-art transistor

10 inhomogeneous sample: HOPG + ½ monolayer C60 V bias = 0 V Topography V bias = 1.34 V z (nm) 500 nm 4 0 HOPG C 60 HOPG x (nm) z (nm) 500 nm 4 0 HOPG C 60 HOPG x (nm) S. Sadewasser et al., PRL (003) contrast inversal: HOPG C 60

11 inhomogeneous sample: HOPG + ½ monolayer C60 topography contact potential z (nm) HOPG C 60 HOPG x (nm) 500 nm 500 nm CP (V) HOPG HOPG C x (nm) HOPG: V CP 0.61 V C 60 : V CP 0.66 V NC-AFM: residual electrostatic force for fixed V bias

12 Makroskopische Kelvin-Sonde Lord Kelvin 1861 Verschiebestrom I(t) = (U dc U CPD ) f ΔC cos ωt.

13 Kelvin Principle tip Φ sample Φ tip E vac sample E F electrostatic force CP=ΔΦ e ΔΦ = eu dc U = U dc +U ac sinωt U dc

14 Electrostatic Forces in nc-afm Principle of Kelvin Probe Force Microscopy F el = 1 C z V eff F el = 1 C z ( V V ) bias CP V CP = 1 e ( Φ Φ ) tip sample contact potential Φ - work function apply bias: V bias = V dc + V ac sin( ω t)

15 Kelvin Probe Force Microscopy F el = F F F 1 dc ω ω C z V eff = F dc + F ω C 1 = dc z C = ( Vdc VCP ) V z C Vac = cos(ω t) z 4 + ( V V ) CP ac F ω V + 4 ac sin( ω t) AM-KPFM Amplitude Modulation FM-KPFM Frequency Modulation

16 FM KPFM Frequency Modulation Detection Fel C Δf( ω) z z ( V V ) V sin( ωt) dc CP ac frequency ω of V ac between 1-3 khz detection of the oscillation of A(Δf 1 ) with a lock-in limiting factor: bandwidth of the FM-demodulator / PLL A(Δf 1 ) F el / z

17 F AM KPFM Amplitude Modulation Detection ω C = z ( V V ) V sin( ω t) dc CP ac tune ω to the second resonance f detection of the oscillation amplitude A ω with a lock-in limiting factor: bandwidth of the photodiode A ω F ω

18 Experimental Setup nc-afm & AM-KPFM

19 KPFM calibration and absolute work function Contact Potential vs. Si [ev] Work Function (literature) [ev] Φ-Si-Cantilever = 4.70 (±0.1) ev absolute and quantitative work function determination U ac = 100 mv

20 Polished Cross Section of a CuGaSe Solar Cell CuGaSe solar cell device: V oc = 80 mv, η = 4.6 % polished and Ar-ion sputtered cross section topography work function Mo, Φ=4.0eV MoSe, Φ=4.40eV CuGaSe, Φ=4.80eV 500 nm Δz = 65 nm Glatzel et al., APL 81, 017 (00) n-zno:ga, Φ=4.0eV

21 AM-KPFM measurement on GaP pn-junction n-type GaP wafer with p-type GaP layer, cm -3, UHV cleavage along (110) surface topography work function Δz = 4.5 nm.5 5 μm Φ = ev most III-V semiconductors: no surface states on the (110) surface GaP does show surface states discrepancy of Φ exp to Φ theo due to surface states! Φ (ev) expected curve experiment x (nm)

22 surface effects no surface states E Vac Φ E CB E F E VB p-type semiconductor surface states SPV E Vac Φ E CB E F E VB p-type semiconductor

23 Surface Photovoltage n- GaP p- GaP GaP pn-interface n- GaP p- GaP 5.6 Δz = 4.5 nm (5000x500 nm) Φ (ev) ohne Beleuchtung, ΔΦ = 100 mev 0. mw,.81 ev, ΔΦ = 500 mev 90 mw,.81 ev, ΔΦ = 300meV x (nm) Φ = ( )eV

24 Surface Photovoltage MDMO-PPV/PCBM 675nm 1000 nm 1000 nm 0 nm 105.6nm 4.19eV 4.6eV -50 mv 0mV

25 Die Nano-Schweiz 9.3 x 7.1 nm NaCl-Insel mit AFM abgebildet