Question 1

Will a decrease in the accelerating voltage help increase the resolution of CL spectra and their corresponding images?

Accepted Answer

No, the resolution of a spectrum depends only on the spectrometer’s resolution (slit width and diffraction grating used) and the sample’s alignment to the CL system. The beam current and accelerating voltage do not change the emitted spectral profile, but do influence the number of photons emitted so that it will have an impact on the spectrum’s signal-to-noise ratio.

Question 2

What should be the working distance during CL imaging in an SEM?

Accepted Answer

The exact working distance (WD) depends on the type of detector you have. Many advanced CL detectors include a collection optic in the form of an off-axis parabolic mirror. In this setup, it is critical to locate the specimen at the precise focal point of the parabola. Most systems have this set to be 10 – 13 mm, but there are now versions available that enable CL imaging at <4 mm WD that also allow the use of in-lens detectors.

Question 3

What kind of detector do you need to acquire a color CL image?

Accepted Answer

Dedicated color CL imaging system: The ChromaCL2™ detector allows the simultaneous capture of color CL, BSED, and SE images in a single pass of the electron beam.

Spectrometer-based system: The Monarc^® detector forms color images by sequentially scanning the specimen and filtering the emission by red, green, and blue wavelengths at each image acquisition step. As an added advantage, this detector performs spectral analysis and collects hyperspectral images.

Question 4

What is the spatial resolution of this technique?

Accepted Answer

The spatial resolution is dependent upon the beam spot size and interaction volume of the electron beam. The interaction volume is related to the beam voltage, spot size, and material investigated. Furthermore, it is not binary, meaning the contribution to the CL signal comes from wherever electrons can scatter inside the sample, the population of which decays as a function of depth and distance from the point of injection. For a 2 keV beam with spot size of about 2 nm on GaN, the CL generation is effectively from a range (radius of a few nm with a depth of about 40 nm), with the bulk of the CL coming from less than half of this range (about 14 nm), and lateral extent about the size of the beam spot.

Question 5

What is the mechanism between CL emission and sample charging effects? Could a heavily charging sample completely quench CL emission?

Accepted Answer

Charging of the sample can be a big issue when analyzing many of the wide bandgap semiconductor and insulator materials in the SEM. However, the impact is typically only associated with one’s ability to position the electron beam precisely. The CL results do not degrade further than the secondary electron images.

Question 6

What is the image resolution in a CL system?

Accepted Answer

The image resolution depends on many things, including the SEM settings, sample luminosity, and sensitivity of the detection hardware. In the SEM, CL images can achieve resolutions down to ~10 nm. However, for many practical geological specimens, the best achievable spatial resolution is of the order of 50 – 100 nm.

Question 7

What is a typical interaction volume of CL?

Accepted Answer

The interaction volume is related to the beam voltage, spot size, and material investigated. Furthermore, it is not binary, meaning the contribution to the CL signal comes from wherever electrons can scatter inside the sample, the population of which decays as a function of depth and distance from the point of injection. For a 5 keV beam with spot size of about 5 nm on GaN, the CL generation is effectively from a radius of a few nm with a depth of about 200 nm, known as range. There are numerous publications that estimate the interaction volume of the electron beam for the generation of CL (e.g., Kurniawan, O. et al. Scanning. 29, p. 280-286 (2007))

Question 8

What benefits does the Monarc detector provide for geological samples?

Accepted Answer

Cathodoluminescence detectors are a very effective microanalysis tool for understanding our geological history. They are useful to determine mineral provenance for geochronology and metamorphic alteration studies as well as thermobarometry for petrographic applications.

Many CL detectors only allow the collection of spatial information in the form of unfiltered (black and white) or filtered (colored) images. These techniques provide very limited spectral information that prevent the identification of trace elements, their valence and structural position, or quantitative analysis. In contrast, the Monarc^® detector provides hyperspectral imaging (spectrum imaging) analysis modes to correlate results with spectral information and enable identification of the above attributes.

Question 9

What are the main differences between STEM and SEM CL?

Accepted Answer

	SEM	STEM
Variety of samples	Wide	Limited to efficient emitters
Sample preparation complexity	Low	High
Sample thickness	Thin-bulk	Electron transparent
Field of view	Large	Small
Observe microstructure	No	Yes
Spatial resolution (nm, sample dependent)	10	1
Polarization analysis	Yes	No
Angle-resolved analysis mode	Yes	No

Question 10

What are the main differences between photoluminescence and cathodoluminescence?

Accepted Answer

	SEM	STEM
Variety of samples	Wide	Limited to efficient emitters
Sample preparation complexity	Low	High
Sample thickness	Thin-bulk	Electron transparent
Field of view	Large	Small
Observe microstructure	No	Yes
Spatial resolution (nm, sample dependent)	10	1
Polarization analysis	Yes	No
Angle-resolved analysis mode	Yes	No

Question 11

How do you achieve such a low noise ratio in the band gap measurement?

Accepted Answer

Since CL is an emission spectroscopy technique, the background signal is inherently low in most materials and conditions.

Question 12

Does CL work better on a FEG or tungsten SEM?

Accepted Answer

Similar to EDS analysis in the SEM, the spatial resolution of CL images is typically not limited by the ability of the user to focus the electron probe. Instead, the lateral spread of energetic electrons as they interact with the sample limits the spatial resolution, which is highly dependent on the SEM accelerating voltage used. Using typical analysis conditions for many geological materials (10 or 15 kV), there is little or no difference between a FEG or tungsten SEM since the CL spatial resolution is ~1 µm.

However, there are cases where you can observe sub-100 nm features in CL images when low voltage electrons are used (<5 kV). In tungsten SEMs, it may not be possible to focus the electron beam well at voltages below 5 kV and in this case, FEG-SEMs are superior.

Question 13

Do we need to consider Cherenkov and transition radiation?

Accepted Answer

In an SEM, this radiation tends to be negligible when you compare it to the luminescence signal.

However, in a STEM, it is important to consider both of these radiation mechanisms. Most CL experiments in the STEM are performed in the 60 – 80 kV range, where the scattering cross-section is higher (produce more photons) and the Cherenkov and transition radiation processes are less efficient.

Question 14

Can you use a silicon substrate for CL imaging of metal nanoparticles?

Accepted Answer

Silicon is an excellent choice as a substrate. As an indirect bandgap semiconductor, the background signal contribution from the substrate is very low.

Frequently Asked Questions

Will a decrease in the accelerating voltage help increase the resolution of CL spectra and their corresponding images?

What should be the working distance during CL imaging in an SEM?

What kind of detector do you need to acquire a color CL image?

What is the spatial resolution of this technique?

What is the mechanism between CL emission and sample charging effects? Could a heavily charging sample completely quench CL emission?

What is the image resolution in a CL system?

What is a typical interaction volume of CL?

What benefits does the Monarc detector provide for geological samples?

What are the main differences between STEM and SEM CL?

What are the main differences between photoluminescence and cathodoluminescence?

How do you achieve such a low noise ratio in the band gap measurement?

Does CL work better on a FEG or tungsten SEM?

Do we need to consider Cherenkov and transition radiation?

Can you use a silicon substrate for CL imaging of metal nanoparticles?

Which trace elements can be identified using CL?

When interpreting CL-intensity differences within one mapping, can you differentiate between pure topographic effects and real material-related properties (e.g., a local, higher density of non-radiative centers)?

How do you map elements in a sample? Should you use EDS or CL for mapping?

Is it possible to use CL instead of x-ray diffraction for clay identification?

Is CL useful to analyze luminescent nanocrystals (20 nm in diameter)?

Is CL sensitive enough to reveal trace elements?

How does the thickness of the sample affect CL results?

Crystal defects are probed using CL in material science, are there similar applications in geology?

What is the optical principle to record angular information on the Monarc system?

What is the light collection efficiency?

What is necessary to perform the angle-resolved CL?

What applications benefit from angular-resolved CL? Is it primarily useful for samples with 3D texture or 2D samples, such as flat semiconductor samples?

Is the Monarc performance improvement due to the aberration-corrected diffraction grating?

Is an SEM with 100 – 300 nA of probe current available better than a 200 kV STEM for CL?

How does the Monarc compare to the ChromaCL, MonoCL, and MiniCL?

How does SEM cathodoluminescence compare to other cathodoluminescence techniques like optical cathodoluminescence?

How does CL perform in low vacuum modes (e.g., sensitive geological samples that you cannot coat to minimize charging)?

How do you deal with drift, especially when performing long measurements like spectroscopy mappings?

How do you collect angle-resolved CL in an SEM/STEM?

Does lowering the temperature improve the signal-to-noise ratio (SNR)?

How do I identify each spectral component?

What are suitable substrates for STEM CL?

Frequently Asked Questions

Will a decrease in the accelerating voltage help increase the resolution of CL spectra and their corresponding images?

What should be the working distance during CL imaging in an SEM?

What kind of detector do you need to acquire a color CL image?

What is the spatial resolution of this technique?

What is the mechanism between CL emission and sample charging effects? Could a heavily charging sample completely quench CL emission?

What is the image resolution in a CL system?

What is a typical interaction volume of CL?

What benefits does the Monarc detector provide for geological samples?

What are the main differences between STEM and SEM CL?

What are the main differences between photoluminescence and cathodoluminescence?

How do you achieve such a low noise ratio in the band gap measurement?

Does CL work better on a FEG or tungsten SEM?

Do we need to consider Cherenkov and transition radiation?

Can you use a silicon substrate for CL imaging of metal nanoparticles?

Which trace elements can be identified using CL?

When interpreting CL-intensity differences within one mapping, can you differentiate between pure topographic effects and real material-related properties (e.g., a local, higher density of non-radiative centers)?

How do you map elements in a sample? Should you use EDS or CL for mapping?

Is it possible to use CL instead of x-ray diffraction for clay identification?

Is CL useful to analyze luminescent nanocrystals (20 nm in diameter)?

Is CL sensitive enough to reveal trace elements?

How does the thickness of the sample affect CL results?

Crystal defects are probed using CL in material science, are there similar applications in geology?

What is the optical principle to record angular information on the Monarc system?

What is the light collection efficiency?

What is necessary to perform the angle-resolved CL?

What applications benefit from angular-resolved CL? Is it primarily useful for samples with 3D texture or 2D samples, such as flat semiconductor samples?

Is the Monarc performance improvement due to the aberration-corrected diffraction grating?

Is an SEM with 100 – 300 nA of probe current available better than a 200 kV STEM for CL?

How does the Monarc compare to the ChromaCL, MonoCL, and MiniCL?

How does SEM cathodoluminescence compare to other cathodoluminescence techniques like optical cathodoluminescence?

How does CL perform in low vacuum modes (e.g., sensitive geological samples that you cannot coat to minimize charging)?

How do you deal with drift, especially when performing long measurements like spectroscopy mappings?

How do you collect angle-resolved CL in an SEM/STEM?

Does lowering the temperature improve the signal-to-noise ratio (SNR)?

How do I identify each spectral component?

What are suitable substrates for STEM CL?

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