High precision characterization instruments are increasingly required to observe, measure, and tailor the properties of nanostructured materials for fundamental analysis and various potential applications [2-3]. Characterization and manipulation of individual nanostructures require not only extreme accuracy but also atomic level resolution. Tremendous efforts have therefore been given to develop and upgrade various microscopic and spectroscopic techniques that will play a central role in characterization and measurement of nanostructured materials. Some of the instruments are surface sensitive, i.e. concerned with what lies within a few Angstroms of the sample surface, while others characterize the bulk of the material. Various techniques that are most widely used in characterizing nanostructured materials include:
(1) Structural characterization (a) X-ray diffraction (XRD) (b) Electron microscopy (EM)
I. Scanning electron microscopy (SEM) II. Transmission electron microscopy (TEM) (c) Scanning probe microscopy (SPM)
I. Scanning tunnelling microscopy (STM) II. Atomic force microscopy (AFM)
(2) Chemical characterization (a) Optical spectroscopy
I. UV-visible spectroscopy
III. Infrared spectroscopy IV. Raman spectroscopy (b) Electron spectroscopy
I. Energy dispersive X-ray spectroscopy (EDS) II. Auger electron spectroscopy (AES)
III. X-ray photoelectron spectroscopy (XPS) (c) Ion spectroscopy
I. Rutherford backscattering spectrometry (RBS) II. Secondary ion-mass spectrometry (SIMS)
Nanomaterials and nanostructures can be analysed and characterised by any combination of the above techniques depending on the specific applications. For example, XRD has been widely used for the determination of crystallinity, crystal structures and lattice constants of nanostructured materials and thin films; SEM and TEM together with electron diffraction pattern have been commonly used for observing the images of materials and structures in nanometer scale.
In this work, the author has extensively employed SEM, TEM and XRD techniques to characterize nanostructured materials and the thin films based on these nanomaterials. The nanostructural characterization of the films has given a deep understanding of their properties, such as the morphology, crystallinity, and orientation of the nanomaterials on the film and film porosity. The characterization results are subsequently linked with the sensor electrical test results to improve performances in Chapter 7.
6.2.1 Scanning Electron Microscopy (SEM)
SEM is one of the most versatile and widely used instruments for the characterization of nanomaterials and nanostructures. It provides morphological and structural information of organic and inorganic materials at nanoscale resolution by scanning an electron probe across a specimen. The popularity of the SEM can be attributed to many factors: the versatility of its various modes of imaging, the excellent spatial resolution, the very modest requirement for sample preparation and conditioning, the relatively straightforward interpretation of the acquired images, and the high levels of automation with user friendliness.
In this PhD thesis, the author has frequently used SEM to examine the nanostructural forms of the sensing materials, film surface topography, lateral homogeneity and chemical composition. Structural characterization of the nanomaterials in terms of length, width, shape, and distribution on the film surface, as well as statistical analysis of these parameters,
were obtained. Throughout the research the author employed a high resolution SEM (Philips XL-30) fitted with an energy dispersive X-ray spectroscopy system (EDS) manufactured by Oxford Instruments. High quality nanoscale images of the sensing materials were obtained by focusing the beam condenser and objective lenses (Fig 6.1). The electron beam accelerating voltage on the instrument was varied from 10 to 30 kV. The SEM figures presented in this thesis specify the operating voltage, spot size, magnification, working distance between the sample and the electron source, the imaging mode and the scale bar. The SEM throughout the study is used in the secondary electron imaging (SEI) mode. In SEI mode, the SEM can produce very high-resolution images of a sample surface, revealing details about 1 to 5 nm in size. Additionally, SEM micrographs in SEI mode have a very large depth of field resulting in a well-defined characteristic three-dimensional appearance useful for understanding the surface structure of a sample.
Figure 6.1: Schematic diagram of an SEM set-up [4].
In SEM, the electron beam is emitted from a heated cathode filament or a field emission tip made of various types of materials, the most common is being tungsten. A voltage is applied to the filament loop causing it to heat up. The application of high voltage in the anode accelerates the electrons from the tungsten filament source towards the anode. This electron beam is focused by a condenser lens and then re-focused by an objective lens onto the specimen. The electron beam scans the specimen in a raster pattern, similar to the way an
Electron source Secondary electron detector Condenser lens Objective lens Limiting aperture Scanning coil Limiting aperture Sample Secondary electron e- Image
electron gun scans the screen in a television set [4]. The beam deflection is done magnetically through magnetic fields generated by electric currents flowing through coils. As the electrons strike and penetrate the surface of the specimen, a number of interactions occur that result in the emission of secondary electrons, back scattered electrons (BSE), characteristic X-rays, and photons from the sample, and SEM images are produced by collecting the emitted electrons or photons on a cathode ray tube (CRT). A schematic representation of the SEM apparatus is shown in Figure 6.1.
The samples placed in the SEM chamber must be either conducting or coated with a thin metal layer in order to avoid electric charging by the electron beam. Additionally, scanning takes place at low pressure so that the electrons are not scattered by the gas molecules [5-6]. The introduction of a field-emission gun as a source of the electron beam in SEM, make it possible to achieve image resolution of about 0.5 nm. The detailed results of the SEM investigation of the nanomaterials are given in the later parts of this chapter.
6.2.2 Transmission Electron Microscopy (TEM)
In TEM, electrons are accelerated from an electron gun (cathode) to focus onto a thin specimen by means of the condenser lens system, and pass through the sample either un- deflected or deflected. The scattered electrons are focused by an objective lens, then amplified by a projector lens, and finally produce the desired image on the screen [7]. The scattering processes experienced by electrons during their passage through the specimen provide the necessary information for imaging. A schematic diagram of a TEM set-up is shown in Figure 6.2.
TEM has the ability to provide both image and selected area diffraction pattern (SADP) from a single sample and one can switch between them by defocusing the condenser lens to produce parallel illumination at the specimen and using a selected area aperture to limit the diffracting volume [7]. To produce a SADP bright field image, the selected area diffraction (SAD) aperture shown in Figure 6.2 only allows the main un-deviated transmitted electrons to pass. In nanotechnology, SADP offers a unique capability to determine the crystal structure of individual nanomaterials, such as nanobelts and nanorods, and the crystal structures of different parts of a sample.
The most recent versions of high-resolution transmission electron microscopes (HRTEM) have the ability to characterize nanostructures with resolution as low as one angstrom (0.1 nm). Using correct operating conditions and well-prepared samples, high-resolution image characteristics are interpretable directly in terms of projections of individual atomic
positions. Thus, the HRTEM has become a powerful and crucial tool for characterising nanostructured materials.
In this PhD work, the TEM images were obtained using a JEOL 2010 and a high resolution JEOL 4000 EX TEM microscope operating at 200 kV and 400 kV, respectively for structural and interfacial analysis as well as crystal structure investigation of the nanomaterials. A detailed TEM investigation of the nanomaterials is given later.
Figure 6.2: Schematic diagram of a transmission electron microscope (TEM) [4].
6.2.3 X-ray Diffraction (XRD)
XRD was employed to determine crystallite phases and the structural properties of the nanostructured sensing materials. In XRD, a collimated beam of X-rays, with a wavelength typically ranging from 0.7 to 2 Å, is directed onto a sample specimen, and the angles at which the beam is diffracted are measured [8]. Generally, the beam is fixed in direction and the crystal is rotated through a broad range of angles to record the X-ray pattern, which is also called diffractogram. Each diffracted X-ray signal corresponds to a coherent reflection, called Bragg reflection, from successive planes of the crystal for which Bragg’s law is satisfied
Electron gun Condenser lens Specimen grid Projector lens Objective lens SAD aperture Image
λ
θ
ndsin =
2 , (6.1)
where d is the spacing between atomic planes in the crystalline phase,
θ
is the incident angle,λ
is the X-ray wavelength, and n=1, 2, 3,… is an integer that usually has the value n=1. The diffraction angles of the X-ray beam depend on the X-ray wavelength, the crystal orientation and the structure of the crystal. The intensity of the diffracted X-rays is measured as a function of the diffraction angle 2θ and the specimen orientation. The exact angle and intensity of a set of peaks is unique to the crystal structure being examined. Figure 6.3 shows a schematic diagram of an XRD set-up. Crystalline phases were examined by a wide angle XRD (PW1820, Philips) with a Cu-Kα
source. The identification of nanostructured materials was achieved by comparing the X-ray diffraction patterns with the JCPDS diffraction database. A detailed XRD investigation of the nanomaterials is given later in this chapter.Figure 6.3: A schematic diagram of an XRD set-up.