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SPM Spectroscopy


Traditionally, spectroscopy refers to the study of the emission or absorption of energy. The spectrum of values at which an object absorbs or emits energy provide information about the atomic and molecular composition and structure of the object. In electromagnetic radiation spectroscopy, the energy is proportional to the frequency of the radiation, so that spectroscopy becomes a study of emission or absorption of radiation as a function of the radiation frequency (or wavelength).

One of the main features of an SPM is its ability to make highly localized measurements of the interaction between the sample surface and the probe. In Imaging Modes, this feature is used together with precision raster scanning in X and Y, to generate various types of high resolution images of the sample surface, e.g., topography (or height) image. If raster scanning is disabled, then the interaction between the sample and tip can be studied at a given X,Y coordinate, and this takes us to the subject of Scanning Probe Spectroscopy.

NSOM lends itself very well to spectroscopy in the traditional sense, but with a much enhanced lateral (X,Y) resolution. Highlylocalized optical spectroscopy, including with sub-wavelength resolution, constitute a major application area for NSOM. This is called Near-field Optical Spectroscopy.

Scanning Tunneling
Spectroscopy (STS) was the first spectroscopy technique that used an SPM. Here, the energy levels of an object (e.g., a molecule or an atom) into and out of which a tunneling current flows are studied, as the bias voltage across the tip and the sample changes--in magnitude and in polarity. To the extent that STS probes energy levels directly, it also fits the traditional definition of spectroscopy.

With the invention of the AFM, however, the definition of spectroscopy was expanded to include no only energy, but also force. In AFM Force Spectroscopy, the raster scanning is stopped, allowing for the AFM to measure the tip-sample force at a given X,Y coordinate, while the user changes one or more physical quantities in a controlled manner. Most often, the quantity that is changed, is either ramped or stepped through a range of values at a given time rate of change that the user controls. The ramped parameter is often the length of the Z-actuator, changing the separation between the sample surface and the probe. The most widely the probe. The most widely used AFM spectroscopy mode is Quasi-static AFM Force Spectroscopy (or Contact Mode Force Spectroscopy).

Although STS, Near-field Optical Spectroscopy, and Quasi-static (Contact Mode) AFM Force Spectroscopy are the most widely used SPM spectroscopy modes, others have proven very useful in specialized industrial applications, especially in electrical characterization of semiconductor materials and devices. These involve, for example, applying a bias and measuring a current, for localized I-V curves, or using a resonant capacitance sensor to measure localized C-V curves with the AFM tip.

SPM spectroscopy modes involving Dynamic Mode AFM in which the cantilever is oscillating have been instrumental in understanding the details and the fundamental physics underlying the tip-sample interaction in these modes, namely, Intermittent Contact AFM and FM-AFM.

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