Assessing Spatial Resolution of Cathodo-Luminescence Imaging and Spectroscopy at the Nanoscale for Inorganic Phosphor Powders

It is now possible to easily collect CL images (using a photomultiplier tube using total or filtered light) and emission spectra from individual nanometer sized particles using the commercially available Gatan Vulcan attachment for a transmission electron microscope (TEM).  In our case, at Brunel University London, we have fitted this to a JEOL 2100F field emission TEM. This work demonstrates the spatial resolution that has been achieved when imaging various phosphor nano materials and discusses the limitations of the technique.  It also demonstrates the uniformity of excitation/emission from various phosphors, and hence the effectiveness of the synthetic method used to manufacture the phosphor can be seen.   Conventionally, photoluminescence (PL) and cathodoluminescence (CL) spectra are collected from bulk phosphor samples and are the average of all of the particles in the sample.  This is true of most samples made either in the laboratory for research or commercial materials prepared by industrial companies.  Cathodoluminescence spectra have been collected from nanometer-sized crystals of quantum dots (spherical and rods) and various oxide and oxysulfide phosphors.  The latter phosphors have been implemented into CL applications on a vast scale whereas the quantum dot materials are not designed for CL application and therefore great care is required when imaging these materials to minimize the effect of beam damage. 

When observing mixtures of phosphors (Gd₂O₂S:Tb³⁺ and Y₂O₃:Eu³⁺) it is apparent that when the electron beam is positioned onto one of these two materials and the spectrum observed, then the other phosphor emission spectrum can be observed if the particles are nearby.  However when observing quantum dot materials the spatial resolution is only limited by the particle size.  A tentative explanation for this effect is given.

In this work we compare the spectra of nanometer sized phosphor crystals/particles in order to understand any variation in the emission spectra between the phosphor grains and to understand the effect of temperature on their luminescent properties.

Our results will be presented that could only have been found by using CL spectra obtained directly from single nanoparticles.  In conclusion:

1)  The effect of different TEM machine parameters will be discussed

2)  The CL spatial resolution will be demonstrated

Figure 1. Left) HAADF STEM of nanometre-sized single crystals of Y₂O₃Eu³⁺ and Gd₂O₂S:Tb³⁺; The green line spectrum image shows where 10 emission spectra were recorded at equally spaced intervals.  Right) Extracted spectra taken from opposite ends on the line scan show the characteristic CL emission spectrum for both the Eu³⁺ and Tb³⁺ are observed in both spectra.