Higher TERS enhancement can be seen when the substrate is also a noble metal (e.g., gold); commonly referred to as āgap-mode TERSā.
TERS enhancement depends on many factors: tip size, tip shape and tip material composition. Higher enhancement can be seen when the substrate is also a noble metal (e.g., gold); commonly referred to as āgap-mode TERSā. This requires a sample preparation step where the nano-objects of interest are transferred (by spin- or dip-coating methods) to the gold surface before being probed. The typical gold substrate consists of a gold thin-film of a few hundred nanometers deposited on a glass substrate right after an adhesion layer of chromium. A step of flame annealing can be used in order to reconstruct the gold surface and obtain atomically flat gold grains with micrometric areas.
Where the sample is suitable for such preparation, increased enhancement can be achieved by the use of gap-mode TERS. In other cases, however, such transfer may not be possible or there may be specific reasons for analysis in-situ on another substrate and glass, mica, silicon, silicon dioxide, or other commonly used flat materials can all be used as substrates in the TERS system based on AFM or Shear/Normal force Microscopy. This referred to as ānon-gap-mode TERSā. The choice of the substrate is also imposed by the TERS instrumentation. In the STM-based TERS system, because of the detection of a tunneling current between the tip and the sample surface, ideally a metal single crystal or a metal (coated) surface is used. Also, if bottom illumination and bottom detection are used, the substrate must be transparent, but since gold and silver single crystals can be made so thin that they are still transparent, this is not a serious restriction.
