Transmission Electron Microscopy

TEM can provide image resolution down to the sub-angstrom regime and additional analytical measurements can be performed making an impressive amount of information accessible. Besides TEMs exceptional image resolution, it is possible to characterize crystallographic phases and their orientation with diffraction experiments, generate elemental maps by using EDX or electron energy loss spectroscopy, and to acquire images highlighting elemental contrast.12 _{Ultrathin samples along with a certain electron}

beam stability of the samples are a prerequisite for TEM investigations that are besides costs usually the bottleneck of the method. Samples investigated in this thesis were prepared in three different ways: As standard grid samples for bulk materials and nanosheets were used, while a lift-out technique or conventional cross-section sample preparation methods was applied for LBL deposited material.

In the first case, the solid sample is suspended in a solvent, ultrasonicated to reduce its size to electron transparency and the resulting suspension dropped onto a TEM grid (Cu, Au, etc.) that is covered by an amorphous lacey-carbon film. In the case of 2D nanosheets only dilution of the suspension is necessary.

The lift-out technique is performed in a focused ion beam (FIB) microscope.19 _{At high}

beam currents a focused beam of ions, e.g. Ga, can be used for site specific sputtering or milling to prepare a TEM lamella. The region of interest is coated with a protective layer, e.g. Pt, before two trenches are milled next to this region. The central membrane between the two trenches is thinned and release cuts are introduced below the formed lamella. The lamella is then fixed on one side to a probe, cut off from the remaining connections, transferred to a carrier holder and thinned to electron transparency using a low current Ga beam.

Cross-sectional TEM samples were prepared as illustrated in Figure 2.3.5.20 _{In the first}

step, two substrates are glued together with a two-component glue with the coated surfaces facing each other. The achieved “sandwich” structure is than cut down with a diamond wire saw so that it fits into a brass tube with a diameter of ~2 mm, where it is immobilized with a two-component glue. From this brass tube discs of ~200 µm are cut off with the diamond wire saw, that are further thinned down to ~50 µm by grinding. The thickness in the center of the disc is then reduced to 15 µm with a dimpling wheel in combination with a diamond paste. In the final step, a precision ion polisher operated with two Ar ion beams removes wedge shape like remaining material until a hole forms in the middle of the sample. Right next to this hole, the sample is thin enough for TEM investigations, typically below 100 nm.

Figure 2.3.5: Illustration of different steps in TEM cross-sectional preparation: a)+b) gluing of two coated wafers into a “sandwich” structure; c) cutting of the “sandwich” into pieces that fit into d) a brass tube, where they are embedded in glue and further cutted into discs tha t are grinded down to ~50 μm; e) the inner part is further thinned with a dimple grinder and e) finalized with help of an ion polishing system.

In the TEM, high energetic electrons are transmitted through the specimen and the various interactions are used to form an image or diffraction pattern and to gain analytical information. The instrument itself can be divided into three components: the illumination system, the objective lens/stage, and the imaging system.12 _{The role of the}

illumination system is to extract electrons from the gun and to transfer them through condensor lenses to the specimen. Depending on the mode the illumination can be either with a parallel beam or a convergent beam. While the former is used in conventional TEM imaging and selected area electron diffraction (SAED), the second is performed in scanning TEM (STEM) imaging and analytical experiments. In the center of the TEM the objective lens and the specimen stage are located and here the beam - specimen interactions take place. In the last part of the TEM, the imaging system magnifies and focuses the produced image or diffraction pattern on the viewing screen or detector, respectively.

Figure 2.3.6 describes basically the two operation modes of the conventional TEM: the diffraction mode and the imaging mode.12 _{As one can see, the diffraction pattern and the}

image are simultaneously present. The objective lens forms a diffraction pattern in the back focal plane (BFP) with electrons scattered by the sample and combines them to a first image.21 _{Controlled by the strength of the intermediate lens either the diffraction}

Figure 2.3.6: Schematic drawing of the two basic modes of conventional TEM and corresponding beam pathways: diffraction mode (left) and imaging mode (right).

For the diffraction mode the diffraction pattern can be confined to a selected area of the specimen by insertion of a SAD aperture into the image plane. In the imaging mode, positioning of an objective aperture at a specific location in the BFP can be used to select electrons that have been diffracted by a specific angle.12 _{When the aperture is}

positioned to pass only the direct electron beam, a bright-field (BF) image is formed. As the contrast in conventional TEM is mainly due to diffraction contrast and mass- thickness contrast, strongly scattering regions of the specimen (heavy elements, large thickness) show a lower intensity in the BF images than weakly scattering regions (light elements, small thickness). When the aperture is positioned to pass only diffracted electrons at a certain angle, a dark-field (DF) image is formed, which can give useful information, e.g. about planar defects, stacking faults or particle size. Hence, combination of both imaging modes can be used to obtain complementary information on the sample. SAED diffraction experiments are applied to determine the lattice plane distances and the crystal structure.

The switchover from conventional TEM with a parallel beam to STEM with a convergent electron beam can be performed by a change in the illumination system.12 _{The quality of}

on the size of the formed probe and hence is related directly to the quality in the illumination system. For each scanned position the scattered signal is measured and an image is formed point by point. Three different detectors are placed in dependence on the scattering angle θ of the transmitted electrons with respect to the optical axis, so that complementary information of the specimen can be obtained (Figure 2.3.7): A BF detector with θ < 10-25 mrad, an annular dark-field (ADF) detector with 25 < α < 50 mrad and a high angle annular dark-field (HAADF) detector with β > 75 mrad. In STEM it is possible to achieve spatial resolutions of < 0.1 nm. In addition analytical signals, like EDX and EELS, can be acquired with a high spatial resolution.

Figure 2.3.7: Schematic drawing of the various electron detectors in a STEM: BF - (θ < 10-

25 mrad), ADF- (25 < α < 50 mrad) and HAADF (β > 75 mrad) detector.

In this thesis, TEM in combination with related spectroscopies was the main analytical tool for various purposes. First of all, TEM in addition to XRD and Raman spectroscopy was used for structure determination of various “new” layered bulk materials. Second, TEM was critical for the evaluation of exfoliation products and further the extraction of 2D specific properties. Third, TEM allowed for in-depth characterization of heterostructures consisting of 2D nanosheets building blocks. Thus, several instruments were used in dependence of the purpose. An overview of all instruments is given in Table 2.3.1.

Table 2.3.1: Different TEMs used in this work, their specifications and their purposes .

Microscope Specifications Purpose

JEOL 2011 (JEOL Ltd., Tokyo) LaB6 cathode, 200 keV Pre-Investigations

Philips CM30 ST (Royal Philips Electronics, Amsterdam)

LaB6 cathode, 300 keV Basic nanosheet and

LBL characterization, SAED of bulk materials

FEI Titan 80-300 (S)TEM (FEI, Hillsboro)

Field emission gun, 80-300 keV EDAX Sapphire Si(Li) detector (EDAX, Mahwah)

Model 3000 HAADF detector (Fischione Instruments, Export)

(HR)TEM, EDX and EELS of nanosheets and hybrid structures

FEI Titan 80-300 Cubed (STEM) (FEI, Hillsboro)

High brightness X-FEG, 80-300 keV Two Cs correctors

Gatan GIF (model 866) spectrometer

STEM, EDX and EELS of nanosheets and hybrid structures FEI Titan 80-300 (S)TEM

(FEI, Hillsboro)

Field emission gun, 80-300 keV Cs corrector (imaging lens)

Gatan UltraScan 1000 (2k × 2k) slow scan CCD

HRTEM of bulk materials

FEI Titan 80-300 (S)TEM (FEI, Hillsboro)

Field emission gun, 80-300 keV Cs corrector (probe lens)

Gatan UltraScan 1000 (2k × 2k) slow scan CCD

Wien-type monochromator

STEM, EELS and EDX of bulk materials

FEI Titan 80-300 (S)TEM (FEI, Hillsboro)

Field emission gun, 80-300 keV 2k x 2k CCD

Gatan Tridiem 866 energy filter Wien-type monochromator

STEM and VEELS on bulk materials and nanosheets

TEM measurements were performed by Viola Duppel (Chapter 3.1-4.3); Teresa Dennenwaldt, Marc Heggen, Juri Barthel, Anna Frank and Christina Scheu (Chapter 3.1); Matthieu Bugnet (Chapter 3.2/4.1); Kulpreet Virdi and Yaron Kauffmann (Chapter 5.1-5.2). Additional cross-section preparation were performed by Tanja Holzmann as well as Arne Schwarze (Chapter 4.1) and Katarina Markovic (Chapter 4.2- 4.3). FIB samples were prepared by Bernhard Fenk (Chapter 4.3).