Atomic Force Microscopy (AFM) is a type of scanning probe microscopy commonly used in nanoscience. It acquires images by rastering a sharp tip attached to a flexible cantilever across the surface of a specimen, which is then deflected by the forces between the surface and the sharp tip. The amount of deflection is measured by using a laser spot focused on the top surface of the cantilever that is reflected onto an array of photodiodes. AFM is used
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in this thesis in phase (tapping) mode. Phase mode monitors the difference in phase before and after the cantilever interacts with the sample.It can be used to highlight changes in the properties of a material across its surface, including adhesion and friction. Phase images can be measured simultaneously with topographic images, allowing direct comparisions to be made between the two measurement types. Phase imaging typically shows high lateral resolution in samples of different material properties, while topographic imaging shows variation in height across a sample. Used together, these measurements can give a wealth of information regarding 2D materials, and can serve as confirmation of layer number of 2D TMDs measured by Raman and PL. AFM measurements presented in this work were carried out using a Veeco Dimension 3100 in tapping mode, with 40 N/m probes from Budget Sensors with a tip radius of < 10 nm and a resonant frequency of 300 kHz. Measurements in this work were taken with the assistance of Dr. Toby Hallam.
2.3.2 Transmission Electron Microscopy
Transmission Electron Microscopy (TEM) is a microscopic technique whereby a beam of electrons interacts with and passes through a specimen, and is elastically or inelastically scattered in the process. Apertures and detectors are used in order to collect a certain fraction of scattered electrons, allowing an image to be formed of the specimen being examined.
TEM is commonly used as a characterisation tool for 2D materials,2, 29, 88 due to its ability to image nanomaterials down to atomic resolution, and provide information about long-range crystallinity through the analysis of electron diffraction patterns. TEM can give other useful information on nanomaterials, including shape, composition, crystal structure and level of defects. Some difficulties in TEM can include the small sampling size possible at a given time, electron beam damage to the sample, the possible misinterpretation of
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images, and image distortion due to aberrations. Aberrations in TEM can limit the resolution that is obtainable, and can cause points to appear blurry on an imaged object. Aberrations can be spherical, meaning they arise due to the lens geometry, or chromatic, meaning they arise due to the inability of the lens to focus electrons of different energies to the same focal plane. Recent advances in microscopy have led to the development of aberration correctors, which can be incorporated into TEM hardware, allowing higher resolution limits to be achieved.89 Aberration effects are negated by using correctors that
produce negative spherical aberration, reversing the spherical aberrations caused by the lens, and by using electron sources with minimal energy spread, in order to refract all beams and focus them to the same focal plane, reducing chromatic aberration effects. A microscope is generally operated in two principal modes - parallel beam or convergent beam, as illustrated in Figure 12. Parallel beam mode allows the acquisition of High Resolution TEM (HRTEM) images. HRTEM analysis was performed in an FEI Titan transmission electron microscope at an acceleration voltage of 300 kV. Convergent beam mode allows Scanning TEM (STEM) images to be recorded, where a focused beam of electrons is rastered across a sample to generate an image of it. Atomic-resolution images in Chapter 4 on PtSe2 were obtained by collaborators in the University of Vienna, with a
Nion UltraSTEM 100, using a high angle annular dark field (HAADF) detector, operated at 60 kV. Where images were taken by collaborators, this will be clearly stated.
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Figure 12
Schematic of a TEM, outlining the principles of parallel and convergent operating modes.
2.3.3 Scanning Electron Microscopy
Scanning electron microscopy (SEM) is a microscopic technique whereby a beam of electrons rasters over a sample, causing multiple interactions to occur. A schematic of an SEM is shown in Figure 13. A schematic is also shown in Figure 13 of the various interactions that arise due to electron beam interaction with the sample, including backscattered electrons, secondary electron and Auger electron emission, and X-Ray emission. The different interactions are then analysed by detectors in the SEM. Backscattered electrons are those which are produced by the scattering of the incident electrons in the sample, and can be used to provide information on the chemical composition of a material by the contrast in the SEM image. Secondary electrons originate from the surface or a few nm into the sample. This means that secondary electron detection, using an InLens detector, can highlight topographical features. When analysing topographical features, low voltages are used in order to only image the surface features. In the results presented in this work, SEM was used to examine the uniformity and topography of synthesised materials. SEM images were acquired using a Zeiss Ultra Plus
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SEM and an InLens detector at a low accelerating voltage of 1 kV, in order to increase sensitivity to surface features.
Figure 13
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3 Vapour Phase Synthesis of Transition
Metal Dichalcogenides
This chapter serves to provide specific experimental details on how the materials studied in the following chapters were produced. Some of the results presented in this chapter were published in the journals “Scientific Reports”90 and “Chemical Physics Letters”.91