2. EXPERIMENTAL TECHNIQUES
voltage/high resolution TEMs, utilizing 200 keV to 1 MeV, have permitted the routine imaging of crystal structures with atomic resolution, allowing materials researchers to monitor and design materials with custom-tailored properties. With the addition of energy dispersive X-ray analysis (EDX) or electron energy loss spectroscopy (EELS), the TEM can also be used as an elemental analysis tool, capable of identifying the elements at the nanoscale. The parts of a TEM are depicted in figure 2.6 and the most important are: (1) two or three condenser lenses to form the electron wave that illu-minates the specimen, (2) an objective lens to form the di↵raction pattern in the back focal plane and the image of the sample in the image plane, (3) several post-specimen lenses to magnify the image or the di↵raction pattern on the screen. If the sample is thin ( 200 nm) and constituted of light chemical elements, the image presents a very low contrast when it is focused. To obtain images in di↵raction contrast, a small
3. Transmission Electron Microscopy
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focused. To obtain an amplitude contrasted image, an objective diaphragm is inserted in the back focal plane to select the transmitted beam (and possibly few diffracted beam): the crystalline parts in Bragg orientation appear dark and the amorphous or not Bragg oriented parts appear bright. This imaging mode is called bright field mode BF (Fig. 3.7). If the diffraction is constituted by many diffracting phases, each of them can be differentiated by selecting one of its diffracted beams with the objective diaphragm. To do that, the incident beam must be tilted so that the diffracted beam is put on the objective lens axis to avoid off-axis aberrations (Fig. 3.8). This mode is called dark field mode DF. The BF and DF modes are used for imaging materials to nanometer scale.
Fig. 3.7 Ray diagram for a transmission electron microscope in image mode. In diffraction mode, an other intermediate lens is inserted to image on the screen the diffraction pattern of the back focal plane.
crystalline or oriented
amorphous or not oriented
anode
condenser lens 1 condenser lens 2 condenser diaphragm
sample
objective diaphragm (back focal plane)
selected area diaphragm objective lens
intermediate lens
projective lens
final image screen
Figure 2.6: - Schematic diagram for a transmission electron microscope in image mode.
In di↵raction mode, the back focal plane (di↵raction pattern) is projected on the screen.
objective aperture is inserted into the back focal plane of the objective lens allowing
64
2.3 Morphological analysis
either the transmitted beam or a di↵racted beam to contribute to the image formation.
These imaging modes are called BF and DF, respectively, and correspond to magnified maps of the intensity distribution of the transmitted selected beam on the exit surface of the thin crystalline sample. The complementary information obtained by BF and DF modes allows compositional analysis, crystal defect investigation and is also used to map di↵erent crystal phases at the nanoscale.
Scanning Transmission Electron Microscopy (STEM).
The basic principle of image formation in the STEM is fundamentally di↵erent from static beam TEM. An STEM machine is equipped with a scanning coil that moves the beam on the sample and a series of detector at di↵erent positions. For each beam posi-tion the signal of each detector is collected, and the final image is obtained assigning to each position the corresponding intensity.The standard detectors attached to an STEM are mainly distinguished in the BF and DF detectors with a terminology that reminds single beam di↵raction contrast. Indeed a BF detector includes the transmitted beam while DF (usually anular shaped) does not. The best resolution in STEM is obtained using an annular DF detector with a large inner aperture angle being the state-of-the-art resolution 0.7 ˚A. This technique can be referred to as HAADF (high-angle annular dark field) or simply Z contrast since it shows a high sensitivity to the atomic number Z.
High Resolution Transmission Microscopy (HRTEM).
Conventional TEM uses only the transmitted or one di↵racted beam to form a di↵rac-tion contrast image. HRTEM uses the transmitted and several di↵racted beams to create an interference image. The understanding of the image formation must take into account the two following steps: (1) the propagation of the incident wave through the object, which depends on the specimen, (2) the (degrading) influence of the optical system in the scattered wave (which depends on the coherence of the electron source, the stability of the microscope and the aberrations introduced by the objective lens).
Therefore, HRTEM imaging requires a high performance microscope (low spherical aberration, high stability of the accelerating high tension and of the lens currents, and high mechanical stability of the column).
2. EXPERIMENTAL TECHNIQUES
2.3.2 Scanning probe microscopes (SPM)
Scanning probe microscopes (SPM) define a broad group of instruments used to image and measure properties of material, chemical, and biological surfaces. SPM images are obtained by scanning a sharp probe across a surface while monitoring and compiling the tip-sample interactions to provide an image. The two primary forms of SPM are scanning tunneling microscopy (STM) and atomic force microscopy (AFM). STM was first developed in 1982 at IBM in Zurich by Binnig (Nobel Prize in Physics in 1986) [92].
Although the ability of the STM to image and measure material surface morphology with atomic resolution has been well documented, only good electrical conductors are candidates for this technique. This significantly limits the materials that can be studied using STM. In 1986, the Atomic Force Microscope was developed to overcome the basic drawback with STM [93]. The AFM, however, has the advantage of imaging almost any type of surface, including polymers, ceramics, composites, glass, and biological samples.
AFM provides a number of advantages over conventional microscopy techniques. AFMs probe the sample and make measurements in three dimensions, x, y, and z (normal to the sample surface), thus enabling the presentation of three-dimensional images of a sample surface. This provides a great advantage over any microscope available previously. With good samples (clean, with no excessively large surface features), resolution in the x-y plane ranges from 0.1 to 1.0 nm and in the z direction is 0.01 nm (atomic resolution). AFMs require neither a vacuum environment nor any special sample preparation, and they can be used in either an ambient or liquid environment.
With these advantages AFM has significantly impacted the fields of materials science, chemistry, biology, physics, and the specialized field of semiconductors. Today, most AFMs use a laser beam deflection system (Fig. 2.7), introduced by Meyer and Amer, where a laser is reflected from the back of the reflective AFM lever and onto a position-sensitive detector. AFM tips and cantilevers are microfabricated from Si or Si3N4. Typical tip radius is a few of nm. Because the atomic force microscope relies on the forces between the tip and sample, knowing these forces is important for proper imaging.
The force is not measured directly, but calculated by measuring the deflection of the lever, and knowing the sti↵ness of the cantilever. Hooks law gives F = kz, where F is the force, k is the sti↵ness of the lever, and z is the distance the lever is bent.
2.3 Morphological analysis
Because of AFMs versatility, it has been applied to a large number of research topics.
Figure 2.7: - Beam deflection system, using a laser and photodetector to measure the beam position.
The Atomic Force Microscope has also gone through many modifications for specific application requirements. Contact mode AFM is one of the more widely used scanning probe modes, and operates by rastering a sharp tip, attached to a low spring constant cantilever, across the sample. An extremely low force (⇡10 9 N) is maintained on the cantilever, thereby pushing the tip against the sample as it rasters. Either the repulsive force between the tip and sample or the actual tip deflection is recorded relative to spatial variation and then converted into an analogue image of the sample surface. Although operating in the contact mode has proven successful, it su↵ers from a number of drawbacks that limit its use on a number of sample types. First, the constant downward force on the tip often damages (and thus changes) many softer surfaces (polymers and biological samples) and even some hard surfaces such as silicon.
Also, many samples, such as small particles or biological samples like DNA and cells, must be placed on a substrate for imaging purposes. In contact mode, the sample is often destroyed or even pushed out of the field of view by the rastering tip. These complications have been addressed through the development of Tapping Mode AFM.
In the Tapping Mode, the AFM tip-cantilever assembly oscillates at the sample surface while the tip is scanned; thus, the tip lightly taps the sample surface while rastering and only touches the sample at the bottom of each oscillation. This prevents damage to soft specimens and avoids the pushing of specimens around on the substrate. By using a constant oscillation amplitude, a constant tipsample distance is maintained until the scan is complete. Tapping Mode AFM can be performed on both wet and dry sample surfaces. Lift Mode AFM provides the operator with a tool to record dual information about a sample surface at one location, such as topography and magnetic gradients (obtained in the magnetic force microscopy (MFM), thereby allowing the
2. EXPERIMENTAL TECHNIQUES
useful association of the two images. Lift Mode AFM operates by first scanning a line on the sample surface in Tapping Mode to obtain the topographical information. Then, the tip is lifted to a distance above the sample set by the operator and the same line retraced in a non-contact mode to obtain (for example) near surface magnetic field information. The process is repeated until the scan is complete and both images are saved. To perform MFM, a ferromagnetic tip and a ferromagnetic or paramagnetic sample are required. In the standard MFM experiment, the phase shift ( ), of the cantilever oscillation is related to the force (F) experienced by the tip due to the magnetic stray field (H) generated by the sample according to the expression:
= Q
K
@Fz
@z = Q
K
@2Hz
@z2 (2.3)
where Q is the quality factor of the oscillation, K is the force constant of the cantilever and z the distance between the tip and the sample. Bright and dark regions in the MFM images correspond to the opposite out-of-plane components of the magnetization vector.
Lift Mode AFM can also be used to record topography and electric fields or phase imaging data.
In this work sample surface morphology has been investigated by a Dimension 3100 Atomic Force Microscope (AFM) equipped with a Nanoscope IVa controller (Veeco Instruments) and to obtain magnetic information MFM mode has been used. The AFM technology provides scientists with a powerful tool to characterize a variety of sample surfaces. Minimal sample preparation, use in ambient conditions, and the ability to image nonconducting specimens at the atomic scale (in some cases) makes AFM an extremely versatile and useful form of microscopy.