64	3 Electron-based techniques
	Table 3.1. Particle scattering techniques
		Electrons (E0, k0, . . . spin s)		{	Electrons (E, k, . . . spin s)
	Probe	Radiation (v0, k0, . . . polarization)	Response		Radiation (v, k, . . . polarization)
		Atoms (E0, k0, atomic number Z)			Atoms (E, k, atomic number Z9)
		{Ions (E0, k0, Z, charge state 6n)			Ions (E, k, Z9, charge state 6n9)

The probe will be formed from a particular type of particle, and typically will have a well de®ned energy E0, and often a well de®ned wave vector k0, or equivalently momentum p05"k0. The response can either be the same or a diVerent particle, and, depending on the detection system, its energy E and/or its wave vector (momentum) k(p), and maybe other attributes, can be measured. If we understand the nature of the scattering process, then we can interpret the experiment and deduce the corresponding characteristics of the sample. It is easy to see from table 3.1 how the number of techniques, and the corresponding acronyms, can be very large, especially once one realizes that any probe particle can give rise to several responses, and that we may have diVerent names for essentially the same technique used at diVerent energy, and diVerent wavevector (momentum or angular) regimes.

3.1.2Reasons for surface sensitivity

The next question is `which techniques are actually useful for studying surfaces?'. There are two cases. In the ®rst case, the emergent (i.e. response) particle or the probe particle has a short mean free path, l. This leads to a useful `single surface' technique.

Examples are Auger electron spectroscopy (AES), where the emerging electrons in the energy range 100±2000 eV have l for inelastic scattering in solids typically in the range 0.5±2.5nm. Using an energy analyzer to measure only those Auger electrons which have not lost energy, attenuates the signal from subsurface layers strongly. A cruder form of energy discrimination is used in observing LEED patterns, where both the incident and the emergent electrons have short mean free paths for energy loss processes. In SIMS (secondary ion mass spectrometry), the emergent ions have a very high probability of being neutralized if they do not originate very near the surface. ICISS (impact collision ion scattering spectroscopy) is surface sensitive because the incident ion will be neutralized, and thereby not detected, if the probe particle penetrates the solid. An introduction to ion based techniques can be found in Feldman & Mayer (1986), in Rivière (1990), and in several chapters contained in Walls (1990); many other books can be unearthed via the web.

In the second case, the sample has a large surface to volume ratio. This condition allows us to extract surface information from techniques which are not particularly surface sensitive. We can perform heat capacity or other thermodynamic measurements, or study structures and dynamics by X-ray or neutron scattering. Here we need to know the signal from the bulk, and maybe subtract it in a diVerential measurement. Much of physical chemistry work on surfaces has been done this way, on powdered, or exfoliated, samples.

3.1 Classi®cation of surface and microscopy techniques	65

Figure 3.1. Schematic geometries for TEM, STEM, SEM and REM (after Venables et al. 1987, redrawn with permission).

3.1.3Microscopic examination of surfaces

Microscopy can be categorized into ®xed beam, scanned beam and scanned probe techniques. A typical ®xed beam technique is transmission electron microscopy (TEM); the same instrument can often be used for re¯ection electron microscopy (REM). These techniques are illustrated schematically in ®gures 3.1(a) and (b). Examples will be given later which show that it is not essential to have these instruments operating at UHV in order to produce useful surface related information: UHV experiments followed by ex situ examination can be very informative, provided the ®nal samples are not too reactive in air.

The central element of TEM or REM is the objective lens, a cylindrically symmetric strong magnetic ®eld positioned just after the sample. As the equivalent of the ®rst stage of an ampli®er in electronics, this element is critical to the performance of the microscope, and the aberrations and phase transfer characteristics of this lens determine both the resolution and contrast that are seen in the image. A particularly useful feature is the use of the aperture situated in the back focal plane of the objective lens

663 Electron-based techniques

to select individual diVraction features and to relate these features to the image. In the case of REM, the image is strongly foreshortened, but this does not mean that the images are particularly diYcult to interpret, as we experience the same sort of image foreshortening when we look ahead driving a car along the road. Both TEM and REM have been able to image surface steps, particularly on high atomic number relatively inert materials such as Au(111) and Pt(111), without requiring that the surfaces were truly clean.

There are many books on electron microscopy, and TEM in particular has the reputation for being diYcult to understand, primarily due to the need for a dynamical theory of electron diVraction to interpret the images of crystalline samples. For an overview of the ®eld, see Buseck et al. (1988), which includes a chapter on surfaces (Yagi 1988); recent surveys of high resolution (HR)-TEM, describing the approach to atomic resolution at surfaces and interfaces, are given by Smith (1997) and Spence (1999), both with extensive references. I have attempted a ten-minute sketch of the various techniques in Venables et al. (1987).

A few groups have converted their instruments to, or constructed instruments for, UHV operation, and in situ experiments. These instruments, which can also be used for transmission high energy electron diVraction (THEED) and re¯ection high energy electron diVraction (RHEED), have produced highly valuable information on surface studies, as reviewed, for example, by Yagi (1988, 1989, 1993). More recently low energy electron microscopy (LEEM) has been developed, which can be combined with LEED, and is making a major contribution (Bauer, 1994). This instrument can also be used for photoemission microscopy (PEEM), which has been developed in several diVerent versions. A specialist form of microscopy with a venerable history is ®eld ion microscopy (FIM), which is especially useful for studying individual atomic events such as diVusion and cluster formation, as discussed by Bassett (1983), Kellogg (1994), Ehrlich (1991, 1994, 1995, 1997) and Tsong & Chen (1997).

The great virtue of ®xed beam techniques is that the information from each picture element (pixel) is recorded at the same time, in parallel. This leads to relatively rapid data acquisition, and the ability to study dynamic events, often in real time, e.g. via video recording. In contrast, data in a scanned beam technique, such as scanning electron microscopy (SEM) or scanning transmission electron microscopy (STEM), is collected serially, point by point, with the sample placed after the objective lens as illustrated in ®gures 3.1(c) and (d).

This con®guration means that multiple signals (not just electrons at the probe energy as in TEM or REM) can be used, which makes the instruments very versatile. It also makes them ideally adapted for computer control and computer-based data collection, but can have a corresponding disadvantage: the need to concentrate a very high current density into a small spot means that not all forms of information can be obtained rapidly, that there will be substantial signal to noise ratio (SNR) problems, and that the beam can cause damage to sensitive specimens. Nonetheless SEM and STEM form the basis of a very useful class of techniques; UHV-SEM has been developed in several laboratories, including the University of Sussex, and UHV-STEM especially at Arizona State University. We examine particular developments in section 3.5.

The above techniques have been available for several decades, and have been

3.1 Classi®cation of surface and microscopy techniques	67

substantially developed in an evolutionary sense, year by year. By contrast, the scanned probe techniques burst upon the scene in the early 1980s, in the revolutionary development of ®rst scanning tunneling microscopy (STM) (Binnig et al. 1982), followed in quick succession by atomic force microscopy (AFM), scanning near-®eld optical microscopy (SNOM), and related spectroscopies. The ®rst two techniques as illustrated in ®gure 3.2.

The central feature of STM operation is the tip, which has to be brought extremely close to the sample in a controlled manner to eVect tunneling, as shown schematically in ®gure 3.2(a). In the simplest mode of operation, the z-motion is used to keep the tunneling current IT constant. In addition, one has to be able to move the tip and sample relative to each other, both as a shift to ®nd out where you are, and as a scan in x and y to produce the image. There are many diVerent STM designs, but all successful designs have been based on piezo-electric elements and have paid due regard for design symmetry, which is necessary to minimize thermal drift.

A particularly appealing design is the `beetle' STM developed by Besocke (1987) and Frohn et al. (1989) in which the sample rests on three piezo-tube `legs' and probes the sample with the tip mounted on a piezo-tube `feeler'. This design is shown in ®gure 3.2(b) in the version developed by Voigtländer & Zinner (1993) and Voigtländer (1999) for in situ deposition experiments. Coarse approach of the sample is eVected by a special holder, in which a rotational motion is translated via a shallow ramp into z- motion; once the sample and tip can `feel' each other, the feeler piezo takes over and STM proper can begin. Coarse movement in x and y uses the leg piezo drives in stickslip motion; a fast jerk on the legs causes them to slip and the stage to move relative to the leg and tip, but a slow movement translates the stage and legs together. By repeated alternating stick and slip motions, stage translation can be made remarkably reproducible; the design will work either way up, though not on its side; it uses gravity. Either the sample holder or the tip holder assembly can be readily withdrawn for sample preparation.

The AFM also comes in many forms, and has the great advantage that it can be used on insulators as well as conductors. A key element here was the development of sensitive cantilever arms, whose de¯ection is typically monitored by a low powered He±Ne laser re¯ected onto a position sensitive diode array detector, as shown in ®gure 3.2(c) (Meyer & Amer 1988). These arms are usually made of lithographically etched silicon, with silicon nitride (Si3N4) as the tip material. Such an arm will have a characteristic resonant frequency, so that, in addition to steady (d.c.) measurements of tip displacement, many a.c. and phase sensitive measurement schemes are possible. Figure 3.2(d) shows a close up SEM view of such a Si3N4 tip (Albrecht et al. 1990).

Of the many recent books on scanned probe microscopy, arguably the best to start from are Chen (1993) and Wiesendanger (1994). There are several multi-author texts, including Stroscio & Kaiser (1993) and many review and specialist articles. Indeed, there are now a large number of techniques for studying surfaces on a microscopic scale: a description of these techniques and their applications would take a very long time. It is not possible to do justice to the full range of extraordinary possibilities oVered by these techniques here, but several examples are given throughout the book which show how valuable they are in particular cases.

68 3 Electron-based techniques

Pz

Px

	Control	z	I
	unit			T

(a) (a)

Py

Contrast due to

steps and electronic patches

scan line

(b)

(c)

Figure 3.2. Scanned probe techniques: (a) principles of STM operation, indicating x, y and z piezo-elements Px, Py and Pz and contrast due to steps and electronic eVects (after Binnig

et al. 1982); (b) the `beetle' STM design of Besocke (1987) and Frohn et al. (1989), as used for in situ deposition experiments by Voigtländer & Zinner (1993) and Voigtländer (1999);