Lateral Force: F L = k L * x

Transcription

1 Scanning Force Microscopy (SFM): Conventional SFM Application: Topography measurements Force: F N = k N * k N Ppring constant: Spring deflection: Pieo Scanner Interaction or force dampening field Contact SFM Non-Contact SFM Lateral Force Microscopy (LFM) Application: Tribological studies (friction, adhesion), surface shear mechanical, contrast enhancements, material distinction ("Chemical Force Microscopy") Procedure: The lateral force is typically measured via the torsional bending mode of the cantilever and constant load. Hysteresis analyses of lateral force loops (forces recorded in forward and reverse scan direction) provide information about the reversibility of the measured lateral forces. Irreversible lateral forces are called "friction forces", F F. Load: F N = k N * Lateral Force: F L = k L * F L 0 average F F Pieo Scanner/Feedback F static F dynamic 1

2 4-Quadrant Photodiode Laser Cantilever Topography Friction Scan Input Modulation ω δ Cantilever Response δ = 0 δ > 0 fully elastic viscoelastic ω δ ω δ Spinodal Decomposition of PS/PMMA Blend 50/50 PS/PMMA blend annealed at 180 o C for 1 week PMMA PS PMMA PS SFM Topography comple flow pattern over time 10 µm SFM Lateral Force 2D spinodal decomposition different from bulk Note: The bright spots (PS phase/lateral force image) represent spinodal frustration points of PMMA. 2

3 Scanning Force Microscopy (SFM) continued: Electrostatic Force Microscopy (EFM) Application: Study of the location and lifetime of surface charges on insulating surfaces. Procedure: Long-range electrostatic Coulombic forces are measured with a mechanically modulated conductive or clean silicon cantilever tip. An AC voltage is applied between the tip and the sample with a frequency ω 2 that is smaller than the mechanical modulation frequency ω 1 but larger than the gain of the feedback response. The AC voltage causes a charge and a mirror charge on the tip and the sample, respectively. The mechanically modulating tip is eperiencing a Coulombic force gradient. For an uncharged surface the force grradient will oscillate at 2ω 2, whereas for a charged surface, the force gradient will be modulated at ω 2. A charge signal can be etracted by measuring the f and 2f signal with lock-in technique. The phase of that signal correponds to the sign of the surface charge. Magnetic Force Microscopy (MFM) Application: Measuring of surface magnetic structures Procedure: Using the non-contact mode with magnetically coated cantilever tips. Rheological Force Microscopy Application: An imaging method to determine local moduli (Young's modulus shear modulus) and surface viscous properties (out of phase response). Procedure: The canitlever tip is in contact (at constant load) with the sample and sinusoidally modulated normally or laterally (either directly or indirectly; see below "distance modulation" vs. "force modulation"). For eample, a small and fast (in regards to the feedback response) normal modulation can be superimposed to the pieo normal feedback voltage. Both, the input modulation signal and the response photodiode signal are fed into a dual-phase lock-in amplifier for the determination of the in-phase and out-of-phase response. Load: F N = k N * Lateral Force: F L = k L * Amplitude Response Modulation Signal Input Modulation Signal Pieo sinusoidally modulated either in or Time Delay Time 3

4 =1 Dewetting vs. Grafting Density () Contrast: Friction Elasticity PEA: bright dark PEA-g-PS: dark bright = 1 no dewetting =3 = 3 slow dewetting =5 = 5 fast dewetting Topography Friction Elasticity Topography Imaging: Operational Modes Constant deflection (contact mode) Analog to the constant current STM mode. The deflection of the cantilever probe is used as the feedback signal and kept constant. Constant dampening (AM detection, intermittent contact mode in air or liquid) The response amplitude of sinusoidally modulated cantilevers allow feedback in the pseudo-non-contact regime (intermittent contact) due to fluid dampening. Constant frequency shift (FM detection, non-contact mode in ultrahigh vacuum) Similar to the FM radio, the frequency is measured and frequency shifts are used as feedback system. This approach works only in vacuum where fluid-dampening effects can be neglected. Variable deflection imaging (contact mode) Analog to the variable current STM (constant height) mode. Uses fast scan rates compared to the force deflection feedback (close to ero). Sensitive to local force gradients such as line defects. Improved high resolution capability (atomic resolution). 4

5 Spectroscopy (local probing): Force Spectroscopy (Force-Displacement, F(D), Curves) The normal forces acting on the cantilever are measured as function of the sample-tip displacement. Used for adhesion and force interaction studies. linearly ramped voltage applied to pieo F(D) jump in contact D = D o - vt 0 F(D) forces acting on the tip D jump out of contact 5