Scanning Probe Microscopy (SPM) Real-Space Surface Microscopic Methods

SPM Principle Probes that are nanosized (accomplished microlithographically), scanning and feedback mechanisms that are accurate to the subnanometer level (achieved with piezoelectric material), and highly sophisticated computer controls (obtained with fast DACs (digital analog converters, etc.). Consists of

Schematic of SPM Principle

Resolution Comparison

3 Axis Cylindrical Piezo

SPM Tree

The Three Basic SPM Systems Scanning Tunneling Microscope (STM) Scanning Force Microscope (SFM) Scanning Nearfield Optical Microscope (SNOM)

Scanning Tunneling Microscopy (STM) Signal: Tunnel Current The tunnel current depends on the tip-sample distance, the barrier height, and the bias voltage. Studying the bias dependence provides important spectroscopic information on the occupied and unoccupied electronic states (-> local LDOS studies). TT SS SS TT Positive sample bias: Net tunneling current arises from electrons that tunnel from occupied states of the tip into unoccupied states of the sample Negative sample bias: Net tunneling current arises from electrons that tunnel from occupied states of the sample into unoccupied states of the tip. The tunnel current is strongly distance, Dz, dependent A = const.

Conventional STM Tunneling Current, I Bias Voltage, V Conductive Sample STM Tip Piezo Scanner

STM Modes of Operations Examples: Constant height imaging or variable current mode (fast scan mode) The scan frequency is fast compared to the feedback response, which keeps the tip in an average (constant) distance from the sample surface. Scanning is possible in real-time video rates that allow, for instance, the study of surface diffusion processes. Differential tunneling microscopy Tip is vibrated parallel to the surface, and the modulated current signal is recorded with lock-in technology. Tracking tunneling microscopy Scanning direction is guided by modulated current signal (e.g., steepest slope). Scanning noise microscopy Use current noise as feedback signal at zero bias. Nonlinear alternating-current tunneling microscopy Conventionally, STM is restricted to non-conducting surfaces. A high frequency AC driving force causes a small number of electrons to tunnel onto and off the surface that can be measured during alternative half-cycles (third harmonics).

Scanning Force Microscopy (SFM) Sample SFM Tip Piezo Scanner z Force: F N = k N *  z Ppring constant: k N Spring deflection:  z Interaction or force dampening field Contact Method:“Non-Contact” Method:

Rheological SFM Sample SFM Tip Piezo sinusoidally modulated either in x or z z Load: F N = k N *  z Lateral Force: F L = k L *  x x Input Modulation Signal Response Modulation Signal Amplitude Time Time Delay

Topography Modes of SPM 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 zero). Sensitive to local force gradients such as line defects. Improved high resolution capability (atomic resolution).

SFM Force Spectroscopy Sample F(D) forces acting on the tip linearly ramped voltage applied to piezo D = D o - vt F(D) 0 D jump in contact jump out of contact

Cantilevers Probes for SFM

Scanning Near-field Optical Microscopy (SNOM) SNOM Principle (Pohl et al. 1984): A tiny aperture, illuminated by a laser beam from the rear side, is scanned across a samle surface, and the intensity of the light transmitted through the sample is recorded. To achieve high lateral resolution (first experiments provided already tens of nanometer resolution), the aperture had to be nanometer sized, and maintained at a scanning distance of less than 10 nm from the sample surface (i.e., within the evanescent field).

SNOM Schematic Examples Small aperture Evanescent Field Regime Illumination Objective Detector Sample Illumination Objective Detector Sample Illumination Mode Reflection Mode

SNOM