X-RAY METHODS FOR ANALYSIS OF MATERIALS Nikolay Petkov, Tyndall, Block B, E-mail: email@example.com
Text Books - Instrumental Methods of Analysis - Atkins’ Physical Chemistry Lecture notes, not really enough but with some background reading should be fine for the exam! Topics • Introduction • X-ray diffraction (XRD) • Crystal structure • X-ray fluorescence analysis (XRF, EDX) • X-ray photoelectron spectroscopy (XPS)
INTRODUCTION Traditional methods of analysis such as: Atomic Adsorption, Mass Spec, Chromatography, Infra-red spectroscopy, UV-Vis spectroscopy etc require dissolution into a fluid phase: destructive Materials or solids analysis has been driven by the requirement to produce non-destructive methods. They can be described in six categories:- Elemental – the atoms present – XRF, EDX Structural – define atomic arrangement - XRD Chemical – define the chemical state - XPS Imaging – what does the morphology look like? – Electron Microscopy Spectroscopic – energy level transitions – IR Thermal – effects of heating (sorption) – TGA, DSC Problems:- Destructive Elemental No understanding of their form in the material
Why to study solid state? • Solid state includes most of the materials in that make modern technology possible • Properties of the solid state differ significantly from the properties of isolated atoms or molecules • The term ‘structure’ takes on a whole new meaning. Example: Nanosized metal particles Consider complete delocalized electrons in the metals, special effect called plasmons (quantized plasma oscillations, collective oscillations of the electron cloud) result is specific colour of metal particles with nanosized dimensions.
Challenges in analysis of solids– compared to molecular analysis • Solids have continuum energy states compared to discrete energy levels in molecular sates. • 2. Very high absorptions so that not much energy gets out! Can lead to adsorption of signal of some elements within the matrix! • 3. Saturation effects can not be simply diluted out. • 4. Structural differences can often be difficult to resolve. • 5. Many of the probes can not be used in simple laboratory environments. • 6. Require complex detection equipment. • Standards can be quire difficult to prepare compared to simple dilution. Phase diagrams and structural change. Matrix effects.
Interaction of the radiation with the matter. Elastic interactions - in which there is no lost in the energy Inelastic interactions – with lost of the exciting energy.
Examples of Methods and Interactions with solids X-ray Diffraction – X-rays in and out but no loss of energy, elastic scattering of x-rays X-ray fluorescence – X-rays in and out but look at x-rays generated by secondary process, inelastic scattering of x-rays X-ray Absorption Spectroscopy - X-rays in and out but look for energy losses X-ray Photoelectron Spectroscopy - X-rays in electrons out (secondary electrons) Electron microscopy – electrons in and out analyse either transmitted (transmission EM) or secondary (secondary electrons, inelastic scattering of electrons) Low Energy Electron Energy Loss Spectroscopy - electrons in and out – energy analyse the electrons to look at energy loss to give vibrational information Ultrasound - Sound in and out – use it to analyse for void formation in solids NMR – radio frequency in and out analyse for energy loss No single techniques is capable of providing a complete characterization of a solid
Bulk vs Surface Analysis Analysing solids is further complicated by the depth sensitivity. X-rays deeply penetrate matter so the analyte depth is high and the analysis is bulk sensitive rather than surface sensitive. However, techniques involving electrons (even if they are excited by high energy techniques) are strongly absorbed and so originate from the surface region. Such are EDX and XPS. This can be an advantage since surface chemistry is of fundamental importance. However, phase separation and segregation can give rise to problems. Quantification An ideal technique should be quantitative e.g. Signal height is proportional to the number of atoms of element or chemical state present. This is rare in a solid. In molecular analysis the intensity is usually directly proportional to the number of molecules in the analyte. This is because the systems are frequently dilute – there is little chance that there are multiple interactions of other interactions with primary or secondary radiation. In a solid exactly the opposite is true. Thus, solids suffer matrix effects.
Analyte Matrix Weak emitter in a strong adsorbing matrix. Analyte emission absorbed by matrix and no signal leaves sample. Weak emitter as a thin layer at the surface on strongly absorbing matrix.
How is heat produced ? Most of the electrons in the incident beam lose energy upon entering a material through inelastic scattering or collisions with other electrons of the material and form heat. In such a collision the momentum transfer from the incident electron to an atomic electron can be expressed as dp = 2e2 / bv, where b is the distance of closest approach between the electrons, and v is the incident electron velocity. The energy transferred by the collision is given by T = (dp)2 / 2m = e4 / Eb2, where m is the electron mass and E is the incident electron energy, given by E = (1 / 2)mv2. By integrating over all values of T between the lowest binding energy, Eo, and the incident energy E, one obtains the result that the total cross section for collision is inversely proportional to the incident energy E.
A second type of interaction in which the incident electron can lose its kinetic energy is an interaction with the nucleus of a target atom. In this type of interaction, the kinetic energy of the projectile electron is converted into electromagnetic energy.
The opposite of a single crystal sample is a polycrystalline sample, which is made up of a number of smaller crystals known as crystallites. Usually those crystallites are connected through a amorphous material to form extended solid. A single crystal, also called monocrystal, is a crystallinesolid in which the crystal lattice of the entire sample is continuous and unbroken to the edges of the sample, with no grain boundaries.