Material structure, properties, and dynamics through scanning transmission electron microscopy
Scanning transmission electron microscopy (STEM) has advanced rapidly in the last decade thanks to the ability to correct the major aberrations of the probe-forming lens. Now, atomic-sized beams are routine, even at accelerating voltages as low as 40 kV, allowing knock-on damage to be minimized in beam sensitive materials. The aberration-corrected probes can contain sufficient current for high-quality, simultaneous, imaging and analysis in multiple modes. Atomic positions can be mapped with picometer precision, revealing ferroelectric domain structures, composition can be mapped by energy-dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS), and charge transfer can be tracked unit cell by unit cell using the EELS fine structure. Furthermore, dynamics of point defects can be investigated through rapid acquisition of multiple image scans. Today STEM has become an indispensable tool for analytical science at the atomic level, providing a whole new level of insights into the complex interplays that control material properties.
KeywordsScanning transmission electron microscopy Electron energy loss spectroscopy Energy loss near-edge fine structure Energy-dispersive X-ray spectroscopy Ferroelectric domain structures Lead-free piezoelectrics Point defect dynamics Nanofabrication
With the successful correction of lens aberrations, the STEM has become the dominant microscopy technique used today in material research, due to the availability of simultaneous, multiple, imaging and spectroscopic modes. While these benefits have long been appreciated in principle (Crewe 1966; Crewe et al. 1970; Rose 1974), before aberration correction, it was difficult to get sufficient current into the probe for good quality images, nor could spectroscopic signals be obtained at atomic resolution. Aberration correction, bringing smaller, brighter probes, has overcome the historic disadvantage of STEM, that of poor signal to noise ratio (Pennycook and Nellist 2011). In this review, we highlight some recent achievements and applications to materials. More detailed accounts can be found in a number of recent reviews (Pennycook 2015; Oxley et al. 2016; Varela et al. 2017; Gazquez et al. 2017; Li et al. 2017).
Imaging and spectroscopic modes
Another recent major advance is in higher energy resolution, achieved through monochromation, which is reaching into the meV range (Krivanek et al. 2014), opening the door to phonon spectroscopy (Lagos et al. 2017), and bandgap mapping (Lin et al. 2016). Such energy resolution is comparable to that of a synchrotron, but the microscope provides much better spatial resolution.
However, especially for low losses, the spatial resolution of EELS images may not be as high as for the HAADF or EDX image because electrons only need to pass close enough to the atom to cause an electronic transition, which can occur some distance away, an effect known as delocalization. Egerton (Egerton 2008) has introduced a measure of delocalization as the diameter containing 50% of the excitations, d50. However, it should be noted that this is not the same as image resolution, which is best defined as the full-width-half-maximum of the inelastic image (Oxley et al. 2016). Because of the delocalization effect, EELS images tend to have long tails more resembling a Lorentzian distribution than a Gaussian. The extended tails reduce image contrast more than they reduce resolution. Note also that delocalization is not a simple function of energy loss but depends on the actual electronic transitions. Recently, several examples have been found where low loss images show atomic resolution (Zhou et al. 2012; Zhou et al. 2012; Zhou et al. 2012). Quantum mechanical simulations show that such contrast arises from specific high momentum transfer transitions; hence, there is no violation of the uncertainty principle (Prange et al. 2012; Oxley et al. 2014; Kapetanakis et al. 2015; Kapetanakis et al. 2016).
Imaging, analysis, and nanofabrication in 3D
STEM has developed dramatically in recent years, thanks to the development of aberration correctors which have allowed the advantages of multiple, simultaneous imaging and spectroscopic modes to be exploited with high sensitivity and precision. There are advantages also for in situ and operandi studies since STEM allows good control of dose rate and illumination area (Chang et al. 2011; Jungjohann et al. 2012; Mehdi et al. 2015; Wang et al. 2016). There are also major developments in mathematical image reconstruction techniques, learning from other fields such as computer vision, which are pushing towards lower dose imaging (Stevens et al. 2014; Meyer et al. 2014; Kovarik et al. 2016; Voyles 2017). It is certainly an exciting and rewarding time to be exploring the atomic world.
All authors are grateful to their collaborators on the work cited here. SJP is grateful to the National University of Singapore for funding. CJL is supported by a Lee Kuan Yew Fellowship. Research at Universidad Complutense is sponsored by the European Research Council PoC-2016 POLAR-EM and Spanish MINECO-FEDER MAT2015-66888-C3-3-R. YMK was supported by the Institute for Basic Science (IBS-R011-D1) and Creative Materials Discovery Program through the NRF (National Research Foundation of Korea) grant (NRF-2015M3D1A1070672). J.H. Jang is supported by the Korea Basic Science Institute grant (T37210).
SJP is grateful to the National University of Singapore for funding. CJL is supported by a Lee Kuan Yew Fellowship. Research at Universidad Complutense is sponsored by the European Research Council PoC-2016 POLAR-EM and Spanish MINECO-FEDER MAT2015-66888-C3-3-R. YMK was supported by the Institute for Basic Science (IBS-R011-D1) and Creative Materials Discovery Program through the NRF (National Research Foundation of Korea) grant (NRF-2015M3D1A1070672) in Korea. J.H. Jang is supported by the Korea Basic Science Institute grant (T37210).
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