14. August 2020

Electron Energy Loss Spectroscopy with a hybrid-pixel detector: pros, no cons!


A 10-minute read

As DECTRIS is a new player on the Elector Microscopy (EM) marketplace, debuting with a collaboration with NION, a world-class developer of advanced electron-optical instruments, was huge. Our work together started a couple of years ago with the tests of a prototype of a hybrid-pixel electron detector, now known as DECTRIS ELA. Today, we are proud to present a summary of this endeavor's results, recently published in Ultramicroscopy and discussed at our latest webinar.

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What started in 2018 as a coffee break chat at M&M in Baltimore turned out to be a success story: a nearly ideal detector for 20-200 keV Elector Energy Loss Spectroscopy (EELS). In a joint publication, NION and DECTRIS tested and characterized DECTRIS ELA, a hybrid-pixel electron detector, and its application potential. In this article, we will summarize some of the core findings.

Zero-loss peak and direct electron counting

One of the main challenges for a detector used in combination with an EEL spectrometer is the significant intensity difference between the Zero-Loss Peak (ZLP) and the low-energy features present in the spectrum. Usually, the microscopist has to make a choice to either expose for the ZLP or to shift the spectrum to avoid saturation and to expose for the features at lower energy. A radiation-hard, high-dynamic range electron-counting detector eliminates this dilemma. The setup of a NION IRIS spectrometer and DECTRIS ELA allowed for the simultaneous acquisition of ZLP and core levels, thus covering the full spectrum. Figure 1 showcases an example of such acquisitions. A hexagonal Boron Nitride (h-BN) spectrum with an intensity range spanning over seven orders of magnitude, a one-pixel point spread function and a negligible background noise shows very accurate features at every energy value.  

Figure 1: h-Boron Nitride spectrum at 60 keV, 105 pA, 100 s exposure.

Detector Sensitivity

Thanks to DECTRIS hybrid-pixel technology, the ELA demonstrates optimal sensitivity for the detection of electron at the typical energies used in commercial TEM/STEM machines. Short dark exposures result in images with zero counts due to the absence of readout noise. However, with longer exposures of the order of several tens of seconds, the detector records the cosmic background radiation at the rate of about four counts per frame per second. Being far below any expected EELS signal, this background noise is irrelevant for practical applications and does not affect the detector’s performance. 

Advantages for multi-pass spectral imaging

Long acquisition times are detrimental for the quality of the experimental data, because of mechanical instabilities and degradation of the sample due to prolonged irradiation. A detector with a fast readout, a high framerate, absent dark current, and high dynamic range presents significant benefits in this regard. The energy-filtered images shown in Figure 2 were obtained as a sequence of shorter acquisitions, aligned and summed. With this approach, the total radiation dose absorbed by the sample can be determined in post processing, any sample drift during the acquisition can be corrected, and even faint high-loss features are effectively visualized.
 

Figure 2: 128 x 128 spectral images (SI) of an STO/BTO/LMSO multilayer, acquired as 32 separate SIs of 8 s each, aligned and summed (4.3 min total).

Detection Quantum Efficiency (DQE)

The data collected by NION during the detector testing have shown that in the 30-100 keV energy range the detection efficiency of a silicon sensor is nearly ideal, and the point-spread function remains below two pixels up to 200 keV. The measured detection quantum efficiency at zero Nyquist is above 0.8 for 60 keV electrons, and the modulation transfer function chases the theoretical limit in a wide interval of frequencies.

Sub-pixel accuracy

DECTRIS ELA is a 2D detector with 1024 x 512 pixels, each of 75 x 75 µm. Since the NION IRIS spectrometer is capable of focusing the ZLP into a single pixel, a sub-pixel averaging technique has been developed, to increase the accuracy with which each feature can be resolved. Figure 3 shows an application of such technique, where several acquisitions with a ¼ pixel shift are combined to obtain richer data and to improve peak-fitting accuracy.

Figure 3: EELS of hexagonal boron nitride with E0 = 60 keV, beam current = 105 pA, 100 s exposure.

Acquisition time

The characterization of the detector was not limited to its performances. NION has used the detector to reproduce the results of several experiments thus illustrating the benefits of the technology. For example, the fast framerate allows revising existing techniques and giving them new life. One significant example is energy-momentum mapping of the phonon dispersion in two-dimensional systems. The NION team replicated the impressive results previously obtained by Senga et al. (Nature 573, 247–250, 2019), acquiring well-defined optical and acoustic phonon branches from an h-BN sample, with a significantly reduced acquisition time  from eight hours to eight minutes.


Your questions about DECTRIS ELA

Virtual pixels between chips and data acquisition?
DECTRIS ELA is made up from eight separate readout chips with 256x256 75 μm pixels each. The assembly with a continuous semiconductor sensor is aided by the presence of three columns of double-sized pixels. From an application point of view, the quality of the final data is not compromised, but the user should pay attention not to locate important features at the corresponding energy channel.

Radiation hardness?
Hybrid Pixel technology is inherently radiation hard. Extended periods of irradiation of the silicon sensor with 80kV electrons did not generate features or undesirable artifacts.

4D-STEM and other applications?
The high dynamic range and high framerate accessible with the DECTRIS ELA do not bring advantages only to EELS experiments. 4D-STEM is the natural candidate for the use of this novel class of fast detectors, and it is already showing impressive preliminary results.

Software integration?
DECTRIS provides for ELA a complete and documented Application Programming Interface, usable with the majority of programming languages. The integration of the detector in any modern data pipeline, or electron microscopy suite, is straightforward. In this work, NION used the powerful Open Source application NionSwift.

Detector compatibility?
Please, contact the manufacturer of your spectrometer for questions regarding DECTRIS ELA compatibility and integration options.

If you have any other questions about ELA detector or its application, do not hesitate to reach out