Contact mode Atomic Force Microscopy
In the contact mode, Atomic Force Microscopy relies on scanning a very sharp tip across a surface of interest at a very close proximity or even touching it. The tip “sits” on a thin flexible stripe, usually metal-coated silicon, called a cantilever. Most commonly, the back of the cantilever is illuminated with laser light, and the reflection is observed on a position-sensitive detector (PSD). Due to a large lever of the light path even smallest bends of the cantilever will result in a measurable reflection travel on the detector. Scanning the sample laterally against the cantilever and monitoring the respective displacement of the reflected light on the PSD, one obtains a height map or topography of the sample. The principle of Atomic Force Microscopy is illustrated below, click to expand the image.
Tapping mode Atomic Force Microscopy
In Tapping mode, the cantilever is vibrated, typically by a shaker piezo, so that it oscillates at a certain amplitude, so called free amplitude A0, close to its resonance frequency f0. Once the cantilever starts to interact with the sample’s surface, its free amplitude dampens by a value, defined by a measure of tip-sample interaction, and the resonance frequency shifts. This new engaged amplitude A can be used as a feedback for vertical positioning of the cantilever when it is being scanned across the sample. The ratio of engaged amplitude A to free amplitude A0 is used for approximate quantification of tip-surface interaction: A/A0 = 0.95-0.75 (or 95% .. 75%) is usually called soft tapping, while ratios A/A0 = 0.75-0.5 (or 75% .. 50%) are referred to as hard tapping. Both tapping modes practically eliminate sheer drag force on both sample and the tip, which naturally occur in contact mode, and are essentially less destructive. The majority of AFM studies today is performed in a tapping mode.
Tapping vs Contact modes in AFM
Below the three modes are placed on the Lennard-Jones force curve. Contact mode stays on the steep repulsive branch, at distances shorter than the force minimum. Tapping mode sweeps through the attractive well into the repulsive branch once per oscillation cycle.
See the table below for comparison
| Contact | Tapping (intermittent contact) | Non-contact | |
|---|---|---|---|
| Regime | Repulsive only | Attractive and repulsive, crossed every cycle | Attractive only |
| Tip-sample interaction | Continuous contact | Intermittent contact | No mechanical contact (ideally) |
| Cantilever motion | Static, bent by the load | Oscillates near f₀, amplitude ≈ 10–100 nm | Oscillates near f₀, small amplitude ≈ 0.1–10 nm |
| Feedback | Static deflection (force) | Amplitude at a setpoint below the free amplitude | Usually frequency shift Δf (FM-AFM); in air, sometimes amplitude in the attractive regime |
| Typical cantilever stiffness | Soft, k ≈ 0.01–1 N/m | k ≈ 1–50 N/m | Stiff, k ≈ 20–50 N/m; qPlus ≈ 1800 N/m* |
| Lateral forces | Significant | Largely avoided | None |
| "Extra" data channels | Lateral force (friction) | Phase (dissipation, stiffness) | Dissipation; force spectroscopy |
| Environments | Air, liquid | Air, liquid | UHV**; also works in liquid (FM-AFM) |
| Strengths | Simple, fast, steady contact for contact-based techniques (e.g. contact AFM-IR) | Gentle on soft and loosely bound samples | No tip wear or sample damage; true atomic resolution in UHV |
| Weaknesses | Shear and indentation damage soft samples | Regime switching; harder to quantify | Weak forces, slow feedback; in ambient air, water films and capillary necks make true non-contact hard to achieve and verify |
* qPlus is typically realized in UHV systems, for example, in a combined STM-AFM apparatus
** see “Disadvantages”
Peak Force and Quantitative Nanomechanical Mapping
In order to make the readout from the cantilever more sustainable, a few approaches were developed.
First is a photothermal excitation of the cantilever using a second laser (also know by marketing names blueDrive(TM) or CleanDrive(TM)), as shown in left panel below. The cantilever vibrates at its eigenfrequency due to a bimorph effect in a gold-coated cantilever. This ensures a cleaner resonance curve and allows for mechanical decoupling of the cantilever vibration from those cantilever- or sampleholder eigenfrequencies, which are known as “peak forest” in liquid environment, shown in left panel, bottom.
Peak Force(TM) and WaveMode(TM) belong to a family of off-resonance tapping modes (the tip is driven at 2 .. 4 – 25 kHz) developed by Bruker, Inc., and NanoSurf, respectively where the cantilever vibrates far from its resonance and at every cycle of tapping a force curve is being recorded and analyzed. This mode provides a precise and direct control of interaction between the tip and the surface, instead of an indirect resonance shift in regular tapping. Sampling force curves in every point of the scan allows for simultaneous extraction of such mechanical properties as sample’s stiffness, adhesion, dissipation and indentation.
Infrared imaging and nanospectroscopy with AFM
The illustrations below demonstrate the concept of infrared spectromicroscopy realized with a scanning probe microscope. It is based on a photothermal-photoacoustic response of the sample to excitation with a light source, typically, tunable infrared laser. In contact infrared AFM, cIR AFM shown in (a), an arriving IR pulse causes local sample heating and expansion of a few pm, which “kicks” the cantilever and causes a so-called ringdown shaking, central panel. From the ringdown analysis it is possible then to calculate IR absorption at this wavelength, which is proportional* to thermal expansion and the amplitude of cantilever’s kick. The development of this approach is realized in Resonance-Enhanced contact AFM-IR, or RE-cIR, shown in (b). In this approach, the IR source rate (typically, a tunable IR laser) is synchronized with contact resonance of the cantilever at this particular sample, which improves the signal-to-noise ratio by the Q-factor of the resonance, typically x10-x100 compared to regular cIR. Finally, panel (c) shows the schematics of Tapping IR AFM. There the cantilever vibrates at its second bending mode and the signal pick-up happens at first bending mode**. The laser is then tuned to the difference in frequency between these modes, fL = f2 -f1 so that heterodyne detection occurs. A non-linear tip-sample interaction in this case limits the probing depth to 100-200 nm.
* the mechanical sample parameters (stiffness, expansion) are also a part of a photothermal response, hence quantitative AFM-IR is difficult
** other modes are available, for example when the cantilever is driven at its fundamental bending frequency, f1, and IR readout happens at f2. This still would be a heterodyne detection scheme.
Most recent developments in AFM-IR spectromicroscopy
Tapping AFM-IR
coming soon