SPM Users Basic Training August 2010 Lecture VIII – AC Imaging Modes: ACAFM and MAC Imaging methods using oscillating cantilevers.

SPM Users Basic Training August 2010 Acoustic AC (AAC) Mode Cantilever oscillated near resonance frequency. Amplitude rather than deflection is feedback loop input. Uses stiffer cantilevers than contact mode.

SPM Users Basic Training August 2010 Acoustic AC (AAC) Mode: Why Use It? Minimal lateral forces to distort sample and degrade resolution. Generally less damage to tip and/or sample and higher resolution than contact mode. Different information available due to dynamic nature of mode. Phase lag of cantilever is sensitive to changes in sample physical properties. Higher order responses Contact ACAFM Contact Mode ACAFM

SPM Users Basic Training August 2010 AAC Mode: Typical Image Types Topography: Same idea as in contact (calibrated voltage sent to piezo) but oscillation amplitude is feedback loop input, not deflection. Amplitude: Error signal, difference between setpoint amplitude and actual amplitude. Phase: Lag between phase of driving signal and response of cantilever. Very sensitive to material differences in sample and can show details not readily seen in Topography image. 9 degrees/V conversion factor.

SPM Users Basic Training August 2010 A piezoelectric transducer shakes the cantilever holder at or near the resonant frequency of the cantilever ( kHz typically). Interaction with the sample reduces the oscillation amplitude. This reduced amplitude is used as the feedback signal. AC Methods - Acoustic Drive (AAC)

SPM Users Basic Training August 2010 The cantilever is coated on the top side with a proprietary magnetic film. A solenoid applies an oscillating magnetic field which is used to vibrate the cantilever. Since only the cantilever is oscillating, fewer resonances are excited. AC Methods – Magnetic AC (MAC) Mode

SPM Users Basic Training August 2010 AAC vs. MAC Mode Drive AAC Mode: Drive “transmitted” through nose holder to tip. Cantilever chip vibrates along with tip. Vibration of nose causes turbulence in solution which may interfere with amplitude vs. frequency response. Higher frequencies than MAC Mode. Best in air but works in solution. MAC Mode: Drives the cantilever “directly”. Cantilever chip does not vibrate. No vibration of nose or cantilever chip so no problems with interference in solution. Limited frequency ranges due to LRC circuit limitations. Best in solution but will also work in air.

SPM Users Basic Training August 2010 Magnetic AC (MAC) Mode: Why Use It? Comparison of MAC and AAC frequency response curves in liquid. Lower Q of AAC cantilever in fluid cannot filter out piezo resonances.

SPM Users Basic Training August 2010 AAC Mode: Two Regimes

SPM Users Basic Training August 2010 AAC Mode: Resonant Frequency Shift During Approach

SPM Users Basic Training August 2010 AAC Mode: Resonant Frequency Shift During Approach In net attractive regime amplitude increases and phase decreases if cantilever drive frequency is less than resonant frequency (vertical blue line). If cantilever drive frequency is above resonant frequency (vertical green line), both amplitude and phase decrease in net attractive regime. As cantilever enters net repulsive regime the resonant frequency increases and the opposite amplitude and phase effects are seen.

SPM Users Basic Training August 2010 Dynamics of AC Mode Imaging The markers indicate the transitions from net- attractive to net-repulsive. Note that transitions occur at different distances during approach (red lines) and withdrawal (blue lines). Amplitude and Phase vs. Distance plots with drive frequency 100Hz above resonant frequency of cantilever. Amplitude and Phase vs. Distance plots with drive frequency 300Hz below resonant frequency of cantilever.

SPM Users Basic Training August 2010 AAC Mode: Why Monitor Phase Signal? Topography (left) and Phase (right) images of rubber sample. Bright spots in Phase image show areas of different composition. No indication of these differences in Topography image.

SPM Users Basic Training August 2010 Setting up AC Mode PicoView Controls > Setup > Options: Select the Serial Port AC Mode Controller check box. Some models (e.g. 5400) have internal AC Drive so this step is not applicable.

SPM Users Basic Training August 2010 AC Mode Controls Available AC Mode Controls: Drive (%) – Strength of signal going to nose cone. Frequency (kHz) – Value used for oscillating cantilever during imaging. Gain factor for signal from photodetector. Select drive type (AAC, MAC, Top MAC). Define phase to be zero at driving frequency.

SPM Users Basic Training August 2010 Advanced AC Mode Controls Bandwidth of lock-in. Input signal: Deflection or Friction. Can choose higher harmonics of signal.

SPM Users Basic Training August 2010 AC Mode Frequency Tune Controls Available AC Mode Tune Controls: Auto Tune: Start (kHz) End (kHz) Peak Amplitude (V) Off Peak Manual Tune (does one sweep): Start (kHz) End (kHz) Data Points Plot types (Cantilever specific)

SPM Users Basic Training August 2010 AC AFM Mode Imaging Pre-Engage Check-list Servo Gain Settings – I and P Gain (%) settings should both be initially set close to These will be adjusted while imaging. Setpoint (V) – Automatically set by the system based on the Stop At value. Range (um) – Should be the full Z range of the scanner. Sample Bias – This is typically not used for standard ACAFM imaging. Set to

SPM Users Basic Training August 2010 AC AFM Mode Imaging Pre-Engage Check-list Stop At (%) – Fraction of free air oscillation amplitude at which approach will stop. Typically this is set in the 80% to 95% range. Speed (um/s) – This is the rate at which the tip approaches the surface. The system is now ready to approach. Follow the same approach and optimization procedures as described in the Contact AFM section.

SPM Users Basic Training August 2010 AC AFM Mode Imaging Pre-Engage Check-list Auto Tune: Set frequency range Set Peak Amplitude Set Off Peak location Manual Tune: Start frequency End frequency # of Data Points Plots: Amplitude Phase X and Y components AC Mode Tune

SPM Users Basic Training August 2010 AC AFM Mode Imaging Pre-Engage Check-list 1.Set the Mode to AC AFM (Mode  ACAFM). 2.Choose correct scanner calibration file (Scanner  select correct scanner calibration file). 3.Re-check photo-diode alignment (Laser Alignment window). 4.Find cantilever resonance and set Drive Frequency and Drive %. 5.Verify servo settings (Servo window). 6.Check approach speed and Stop At value (Scan and Motor window, Motor Tab). 7.Select Scan Size. 8.Approach.

SPM Users Basic Training August 2010 Preparing Software for AC Mode Imaging