Resolution and Performance of 3D Confocal Raman Imaging Systems

Abstract

Confocal Raman microscopes are the instruments of choice for chemical characterization in a wide variety of applications including geosciences (Jenniskens et al., in Nature 458(7237): 485, 2009 [1]), (Rotundi et al., in Planet Sci 43(1–2):367, 2008, [2]), (Heim et al., Geobiology in 10(4):280, 2012 [3]), biology (Hild et al., in J Struct Biol 168(3):426, 2009 [4]), (Gierlinger and Schwanninger, in J Spectrosc 21(2):69, 2007 [5]), (Hermelink et al., in Analyst 134(6):1149, 2009 [6]), nano-carbon materials (Xu et al., in ACS nano 5(1):147, 2010 [7]), (You et al., in Appl Phys Lett 93(10):103111, 2008 [8]), (Yu et al., in J Phys Chem C 112(33):12602, 2008 [9]) and pharmaceutics (Matthäus et al., in Mol Pharm 5(2):287, 2008 [10]), (Chernenko et al., in ACS nano 3(11):3552, 2009 [11]) to name but a few. Many pertinent examples can be found within this book. This chapter intends to shed light on the possibilities of confocal Raman systems in general, while briefly reviewing their origins and describing considerations such as spectral and spatial resolution as well as throughput. 3D confocal Raman imaging as well as compensation for surface topography during a confocal Raman image scan will also be outlined.

A. Appendix

1.1 A.1 Ideal Performance Test Samples

Based on the methods and theories described in this chapter, ideal samples for performance determination are outlined in the following. An ideal system can achieve optimal results for each of the categories in the same configuration.

1.1.1 A.1.1 Lateral Resolution

Ideal sample: Isolated Single- or Multi-walled Carbon Nano-Tubes (CNTs)

• Recommended maximum diameter: 75 nm

• Recommended minimum length: 1 $\upmu$m

• Recommended substrate: Si or glass (required for resolution determination using immersion objectives)

1.1.2 A.1.2 Depth Resolution and Confocality

Ideal sample: Suspended Graphene or Ultrathin Graphite or Si when using only the rising flank of the depth profile

• Recommended maximum sample thickness (for suspended samples): 50 nm

• Recommended minimum hole diameter (for the suspended part): 5–10 $\upmu$m

• Recommended substrate: Si

1.1.3 A.1.3 Spectral Resolution

Ideal sample: Atomic Emission Lines

The emission should be guided along exactly the same beam path as the scattered Raman light.

• Recommended emission lines: Within the Raman detection range.

  532 nm: Hg @ 546.0735 nm [34]

  785 nm: Ar @ 912.2967 nm [35]

1.1.4 A.1.4 Throughput

Ideal sample: Si

Crucial parameters for possible comparison:

• Laser power

• Laser wavelength

• Objective

• Spectral dispersion @ $\approx$1950 rel. cm$^{-1}$

• Integration time

• Confocality (measured by comparison of the peak height of the 4th order Si peak relative to the intensity of the O$_\text {2}$ and N$_\text {2}$ emission lines - see Fig. 6.15).