2024 Nanotech L5

We apply a bias voltage between the STM tip and sample, because

• a voltage is needed to emit electrons from the STM tip
• tunneling only occurs at finite bias voltages
• to reduce the tunnel barrier and allow higher currents
• otherwise tunneling in both directions cancel the current

What information does an STM image contain?

• topography (atomic structure)
• local electronic structure (density of states)
• a combination of topography and electronic structure effects
• local mechanical properties of samples

Tunneling spectroscopy provides infomation about

• local chemical bond configuration
• local electronic desnity of states of the sample
• overall electrical conductivity of the smaple
• local mechanical properties of samples

The main advantage of AFM over STM is

• faster measuremetns
• higher resolution
• the ability to investigate electrically insulating samples
• smaller tip-sample interactions, less destructive

The interaction between the AFM tip and sample surface is

• always repulsive
• always attractive
• repulsive at large (tip-sample) distances, attractive at small
• attractive at large (tip-sample) distances, repulsive at small

The main difference between contact, tapping and non-contact AFM measurement modes is

• the strenght of the tip-sample interaction
• the driving frequency of the tip
• the detection mode of the signal
• the oscillation amplitude of the tip

In tapping or non-contact mode AFM measurements we drive the tip oscillations at

• cantilever resonance frequency
• away from resonance, to avoid strong vibrations
• slightly off-resonance, to achieve maximum sensitivity
• does not matter, works well for a wide driving frequency range