Sorin Lazar

Materials science applications of HREELS in near edge structure analysis and low-energy loss spectroscopy

New experiments made possible with a commercial transmission electron microscope (TEM) equipped with a high-resolution electron energy loss spectrometer (EELS) are presented. With this commercial system, a 100 meV energy resolution using a sub 2 nm probe or 500 meV at a 0.20 nm probe are possible, in combination with other modern techniques available for TEMs. In this paper a number of explorative examples of the first results are shown. The benefit of the increased resolution for detecting more details in near edge structures are shown for the Ti K edge in TiO(2) (brookite) and for the N K edge in cubic and hexagonal GaN. The bandgap of GaN is studied in both crystal structures, as well as the dependency of the low-loss spectrum on the momentum transfer direction in diffraction mode.

A TEM and electron energy loss spectroscopy (EELS) investigation of active and inactive silver particles for surface enhanced resonance raman spectroscopy (SERRS)

A number of silver particles and aggregates of particles were studied using surface enhanced resonance Raman spectroscopy (SERRS), high resolution transmission electron microscopy (HRTEM) and electron energy-loss spectroscopy (EELS). The SERRS mapping/TEM collage method developed previously in our group allows each SERRS active or inactive species to be reliably identified and analysed by each of the techniques in three different instruments. Our aim is to correlate SERRS activity, particle microstructure, chemical composition and electronic properties of each species to gain an insight into the enhancement mechanism. To date, our findings do not reveal any clear link between particle microstructure and SERRS activity. Additionally, the direction of the polarisation of the incident excitation or the presence of interparticle junctions between aggregated particles was not correlated with SERRS activity. However, spectral variations in the EELS data from structurally similar particles and SERRS active and inactive particles suggest that each species is chemically/electronically distinct. Differences in the spectra of single particles, dimers and clusters were also observed. Further analysis of the data, including extraction of the complex dielectric function from the EELS data, will provide an insight into the relationship between these observations and SERRS activity.

Cu2Se and Cu Nanocrystals as Local Sources of Copper in Thermally Activated In Situ Cation Exchange

Among the different synthesis approaches to colloidal nanocrystals, a recently developed toolkit is represented by cation exchange reactions, where the use of template nanocrystals gives access to materials that would be hardly attainable via direct synthesis. Besides, postsynthetic treatments, such as thermally activated solid-state reactions, represent a further flourishing route to promote finely controlled cation exchange. Here, we report that, upon in situ heating in a transmission electron microscope, Cu2Se or Cu nanocrystals deposited on an amorphous solid substrate undergo partial loss of Cu atoms, which are then engaged in local cation exchange reactions with Cu “acceptor” phases represented by rod- and wire-shaped CdSe nanocrystals. This thermal treatment slowly transforms the initial CdSe nanocrystals into Cu2–xSe nanocrystals, through the complete sublimation of Cd and the partial sublimation of Se atoms. Both Cu “donor” and “acceptor” particles were not always in direct contact with each other; hence, the gradual transfer of Cu species from Cu2Se or metallic Cu to CdSe nanocrystals was mediated by the substrate and depended on the distance between the donor and acceptor nanostructures. Differently from what happens in the comparably faster cation exchange reactions performed in liquid solution, this study shows that slow cation exchange reactions can be performed at the solid state and helps to shed light on the intermediate steps involved in such reactions.

Real-space mapping of electronic orbitals

Electronic states are responsible for most material properties, including chemical bonds, electrical and thermal conductivity, as well as optical and magnetic properties. Experimentally, however, they remain mostly elusive. Here, we report the real-space mapping of selected transitions between p and d states on the Ångström scale in bulk rutile (TiO2) using electron energy-loss spectrometry (EELS), revealing information on individual bonds between atoms. On the one hand, this enables the experimental verification of theoretical predictions about electronic states. On the other hand, it paves the way for directly investigating electronic states under conditions that are at the limit of the current capabilities of numerical simulations such as, e.g., the electronic states at defects, interfaces, and quantum dots.

Structural investigation of interface and defects in epitaxial Bi3.25La0.75Ti3O12 film on SrRuO3/SrTiO3 (111) and (100)

The structure of La-doped bismuth titanate (BLT), Bi3.25La0.75Ti3O12, is investigated with atomic resolution high-angle annular dark field (HAADF) scanning transmission electron microscopy. The images reveal evidence of the tilting of TiO6 octahedra within the perovskite-like layers of the BLT unit cell. The tendency of La ions to substitute Bi ions and occupy the top part of the (Bi2O2)2+ layer, previously observed from electron energy loss spectroscopy (EELS) mapping experiments, is explained based on the tolerance factors and stress relief mechanism. The atomic resolution HAADF images also reveal the presence of the out-of-phase boundaries (OPBs). The role of OPBs in BLT is discussed in terms of its fatigue resistance as the OPBs provide extra nucleation sites for ferroelectric domains during polarization reversals. Further, we show evidence that the first deposited atomic layer at the interface also governs the subsequent film growth, resulting in the modulation of the “defect-free” and the “defected”...

Integrated Differential Phase Contrast (iDPC)–Direct Phase Imaging in STEM for Thin Samples

Imaging the phase of the transmission function has always been the ultimate goal of any (S)TEM imaging technique as it is, for thin samples, directly proportional to the projected potential in the sample. Customarily this information is obtained using Holography [1] or by performing focus series reconstruction in TEM (FSR-TEM) [2], recently also in combination with Phase Plates (PP) [3] and/or image Cs correction. Ptychographic reconstruction has also been considered as an alternative [4].

Imaging of gaAs nanowire using combined aberration-corrected tem/stem and exit wave restoration

* Canadian Centre for Electron Microscopy (CCEM), McMaster University, 1280 Main Street West, Hamilton, ON, Canada L8S 4M1 ** FEI Electron Optics, 5600 KA Eindhoven, The Netherlands *** LSME, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland **** LMSC, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland ***** Monash Centre for Electron Microscopy, Monash University, Vic 3800, Australia

Integrated differential phase contrast (iDPC) STEM

In this talk we present Integrated Differential Phase Contrast (iDPC) STEM [1-3], an electron microscopy (EM) technique that directly images the electrostatic potential field produced by charged particles forming the sample. The electrostatic potential field of the sample has clear maxima at the atomic core positions. Therefore it represents an ideal sample map and is the ultimate goal for any EM imaging technique. For thin samples, the phase of the transmission function of the sample is directly proportional to the projected electrostatic potential field. The iDPC-STEM is therefore also a direct phase imaging technique. For non-magnetic samples we know from basic electrostatics that the electric field of the sample (which is a conservative vector field) is the gradient (differential) of the electrostatic potential field of the sample (a scalar field). An electron passing the sample is influenced by this electric field. If the sample is thin, the electric field at the impact point deflects the electron proportionally to its inplane component. This deflection can be measured by detecting the position of the electron at the detector in the far field. Measurement of the electron (beam) deflection is the subject of Differential Phase Contrast (DPC) techniques [4, 5]. In reality, the motion of the electron is described quantum mechanically with its electron wave function. By focusing the electron wave (the probe) we increase the probability that the electron passes at a certain position and is influenced by the electric field at that position. In the far field, at the detector plane, we obtain a corresponding convergent beam electron diffraction (CBED) pattern, a result of many electrons passing the sample. It was indicated [5] and strictly proven [2] that the mathematical expectation of the electron position in the detector plane, in other words, the center of mass (COM) of the CBED pattern preserves a linear relation to the local electric field at the position of the probe. By scanning the probe, the full COM vector field can be obtained as a linear measure of the electric vector field of the sample. An ideal DPC technique should therefore acquire the COM vector field. A straightforward way of performing this is to use a camera. By recording each CBED pattern COM components can be computed directly [2, 6]. Because this requires fast readouts and stable drift-free samples, in practice COM components are measured using detectors with only a few segments [2, 3]. These methods are fast and enable live imaging, as in (A)BFand (HA)ADF-STEM. By integrating the measured COM vector field we obtain the iCOM scalar field, a linear measurement of the electrostatic potential field of the sample [1-3]. iDPC-STEM is a practical method of obtaining iCOM. In this talk iDPC-STEM using a 4 quadrant segmented detector will be explained. Various experimental results and applications will be presented and compared to standard (S)TEM imaging results. It will be demonstrated that iDPC-STEM is capable of imaging light and heavy elements together, has full low frequency transfer and is a low dose technique.

High Performance in Low Voltage HR-STEM Applications Enabled By Fast Automatic Tuning of the Combination of a Monochromator and Probe Cs-Corrector

High tension flexibility in scanning transmission electron microscopy (STEM) enables versatility in the investigation of a broad variety of materials, which formally did not deliver the right contrast or were too beam sensitive at high acceleration voltages. By the introduction of spherical aberration (Cs) correction low voltage STEM has become the major imaging and spectroscopy technique for atomic resolution observation [1,2]. When reducing the acceleration voltage and correcting the Cs of the objective lens, the chromatic aberration (Cc) of the electron source becomes resolution limiting and needs to be addressed. This can be achieved by full correction or minimizing its effect on the image resolution [3]. The use of the monochromator to minimize the effect of Cc is beneficial in STEM application due to the fact that it improves the performance of EELS applications. To obtain reproducible results during daily operation the handling of a monochromized Cs corrected tool at low voltage needs simplification. In this contribution a Wien filter monochromator [4] and a probe Cs corrector [5] combination on a cubed Titan Themis is used to maintain the atomic resolution in low voltage STEM applications. State of the art low voltage images require both the monochromator and the Cs-corrector to be tuned optimally at the same time in an easy and reproducible way. Therefore we developed fast automatic routines to tune the monochromator and the Cs corrector to give easy access to high performance in LV S/TEM. These routines are quick and deliver within minutes a completely tuned tool. We demonstrate with videos the performance of the tuning and level of automation. The automatic tuning of the monochromator requires no sample and uses the flucam of the Titan cubed as a feedback detector. For optimum Cs correction mainly the lower order aberrations vary in operation and need daily retuning. For this reason a correction routine using a series of HRSTEM images is developed to correct the lower order aberration of the Cs-corrector (focus, 2-fold astigmatism, 3-fold astigmatism, coma) This routine can run on a crystalline sample, even on the area of interest in the zone axis of the crystal to ensure best result and fastest time-to-data. An example of the automatic routine is demonstrated in figure 1, where the images of silicon [110] and GaN [211] before and after autotuning of focus and 2-fold astigmatism on the crystalline material are shown. The negative effect of a large energy spread of the non-monochromized source is illustrated in figure 2 by looking at calculations of the transfer function in STEM with different energy resolution The calculated results are compared to images taken with different energy resolution, high tensions and beam currents, set-up via the above described automatic routines. The combination of XFEG gun with monochromator delivers even at low voltages high probe currents for analytical work. We discuss the influence of the energy resolution on the image quality. Therefore HR-STEM analytical and imaging results of various materials are presented to prove the robustness of the alignment procedures and the performance of the Wien filter/Cs corrector combination at low voltages down to 30kV. (Figure 3)

Integrated Differential Phase Contrast (iDPC) STEM: A New Atomic Resolution STEM Technique To Image All Elements Across the Periodic Table

A new, recently introduced Integrated Differential Phase Contrast (iDPC) STEM imaging technique [1] is enabling live imaging of the phase of the transmission function of thin samples. One of the first striking advantages of this new technique is that it is able to image light (C, O, N ...) and heavier elements (Sr, Ti, Ga ...) together in one image whereas a standard (HA)ADF-STEM image shows only the heavier atoms. Figure 1 shows an example of SrTiO3 imaged using the conventional ADF-STEM technique vs. our new iDPC-STEM technique. The oxygen columns and carbon contamination (low frequency information) are clearly visible in the latter and missing in the former.