Page 52 - Nanotechnology an introduction

conducting. Continuous incremental improvements in the technology of scanning electron microscopy now make it possible to obtain images in the
presence of air at a pressure of a few thousandths of an atmosphere. This is called environmental scanning electron microscopy (ESEM). Some
resolution is thereby sacrificed, but on the other hand it is not necessary to dehydrate the sample, nor is it necessary to coat it with a metal if it is
nonconducting—the remaining air suffices to conduct excess electrons away. A combination of transmission and scanning microscopies (STEM,
scanning transmission electron microscopy) was first realized by Manfred von Ardenne but again only commercialized several decades later. It has
recently been demonstrated that a commercial instrument, the Nion UltraSTEM, has been practically able to achieve the nanometrologist's goal of
imaging and chemically identifying every atom in an arbitrary three-dimensional structure.
Instead of electrons, ions can also be used as imaging particles (FIB, fast ion bombardment). Originally developed as a way of controllably etching
parts of samples in order to be able to more completely investigate their inner structures using electron microscopy, the instrumentation is now
sufficiently advanced to enable it to be used as an ultramicroscopy technique in its own right.
5.2. Chemical Surface Structure (Chemography)
Many of the available techniques are essentially nonimaging approaches because they lack lateral resolution in the nanoscale. Often nanoscale
resolution is only achievable for regular structures. The best-known approach of this type is probably X-ray diffraction (which could equally well be
considered as a technique for determining structure). A beam of X-rays is made to impinge on the sample making an angle θ with a plane of atoms
within it, and the spacial distribution of the scattered X-rays is measured. Because the wavelength λ of X-rays is of the order of interatomic-plane
distance d (tenths of a nanometer), crystalline material, or at least material with some order in its atomic arrangement, diffracts the beam. The key
condition for constructive interference of the reflected beam is Bragg's law:

(5.4)
This metrology technique was developed soon after the discovery of X-rays by Röntgen in 1895, in other words long before the era of
nanotechnology.
The main family of classical surface chemical analytical methods involves firing one kind of photon (or electron) at the sample and observing the
energy of the photons (or electrons) whose emission is thereby triggered.
In Auger electron spectroscopy (AES), an incident electron beam ejects an electron from a core level; the resulting vacancy is unstable and is filled
by an electron from a high level, releasing energy that is either transferred to another (Auger) electron (from a yet higher level), or emitted as an X-
ray photon. The measurement of the spectrum of the Auger electrons is called AES. In energy dispersive X-ray spectroscopy (EDS, EDX) it is the
X-ray photons whose spectrum is measured. Both these techniques are capable of good lateral resolution (within the nanoscale), because the
incident electron beam can be finely focused. EDS is typically carried out within a scanning electron microscope equipped with a suitable X-ray
detector. It yields quantitative elemental abundances with an accuracy of around 1 atom%. AES, on the other hand, can additionally identify the
chemical state of the element. All these techniques yield an average composition within a certain depth from the surface of the sample, which is a
complicated function of the scattering of the incident and emergent radiations. Typically AES samples only the first few nanometers from the
surface, whereas EDS averages over a somewhat greater depth, which might be as much as 1 μm.

Techniques such as X-ray fluorescence and X-ray photoelectron spectroscopy, in which the incident photon is an X-ray, have insufficient lateral
resolution to be useful for mapping nanotexture.
A different kind of technique is secondary ion mass spectrometry (SIMS), in which a beam of energetic ions (typically gallium or oxygen) focused on
the sample knocks out ions from it, which are detected in a mass spectrometer. Recent advances in ion beam technology have resulted in the
introduction of nanoSIMS, with a lateral resolution of a few tens of nanometers. The relationship between the detected ion abundances and the
original sample composition strongly depends on the overall constitution of the sample, and hence quantification is usually a difficult challenge. One
advantage of SIMS is that the incident ion beam can be used to systematically etch away the sample, allowing the depth profile of the chemical
composition to be obtained.

Atom probe field ion microscopy (APFIM) requires an acicular sample (hence limiting its practical applications); upon applying a high electric field
between the counterelectrode and the sample, some atoms evaporate from the sample as ions and move towards a detector, their position on
which is directly related to their original position in the sample.
Continuing advances in instrumentation now make it feasible to nondestructively map the inner sections of three-dimensional objects from which
they can be completely reconstructed (tomography). Although initially demonstrated by Hounsfield with X-rays, it is now available for a variety of the
techniques discussed above, including APFIM, TEM, STEM, AFM, etc., with the capability of nanoscale resolution in all three coordinates.
Atomic force microscopy (Figure 5.1) can also be used to determine the chemical structure since the force–distance relationships as the tip is
made to approach and then retracted from a particular patch on the surface are characteristic of the chemistry of that patch (Chapter 3). Careful
measurement of those relationships at each patch (Figure 5.4), although laborious, allows a complete map of the chemical variegation of the
surface to be obtained. Advances in nanoscale tip fabrication already offer considerable flexibility in choosing tips made from different materials in
order to maximize the contrast between different patches. If a rather stiff cantilever is used, its motion will tend to follow the topography of the Born
repulsion. A more flexible cantilever will be sensitive to the longer-range, but weaker, electron donor–acceptor and electrostatic interactions
(Section 3.2), which depend upon the chemical composition of the sample at the point being measured.