Ultramicroscopy 36 (1991) 1733185

North-Holland

173

Photoemission electron microscopy in metallurgical research: applications using the Balzers KE3 Metioscope

Christopher Hammond

School of Materials. University of Leeds, Leeds LS2 9JT, UK

and

M. Ashraf Imam

Materrals Sctence and Technology Division, Naval Research Laboratory, Washtngton. DC 20375.5000, USA

Received 5 September 1990; at Editorial Office 1 December 1990

The sources of contrast in metals and alloys, which are exploited in photoemission electron microscopy. and the problems of image interpretation which may arise, are summarised and discussed in relation to the design of the Bakers KE3 Metioscope.

The preparation procedures for the different types of specimen which may be used are briefly described. The relative

advantages and disadvantages of photoemission electron microscopy in comparison with other “in situ” high-temperature

dynamical microscopical techniques are discussed in relation to the field of metallurgical research, and some examples of

recent results obtained with the Bakers KE3 Metioscope are given which illustrate the potential of the technique.

1. Introduction: sources of contrast in metals and alloys

Photoemission electron microscopy in metal- lurgy has had a somewhat chequered history. De-

spite the fact that it can be and has been used to observe directly changing microstructures such as,

for example, the processes of recrystallisation and grain growth or phase transformations either iso- thermally or during heating or cooling, its great potential in this respect has not been fully realised and it has not established itself as a standard microscopical technique. Indeed, the Balzers KE3 Metioscope (in German “Metioskop”), which was specifically named and designed for metallurgical applications, is no longer in commercial produc- tion [1,2].

This situation is unfortunate and unsatisfactory since photoemission electron microscopy (PEEM) has a number of advantages over the rival tech- nique of hot- and straining-stage scanning electron

0304-3991/91/$03.50 0 1991 - Elsevier Science Publishers B.V

microscopy, which tend to be overlooked. It is by no means obvious that the development of in situ heating and straining stages and of signal collec- tion and processing techniques in the SEM have made, or will make, PEEM redundant.

The basis of photoemission electron mi- croscopy is the photoelectric effect, discovered by

Hertz in 1887 and subsequently investigated by Lenard and Millikan [3]. Its explanation by Ein- stein in 1905 in terms of Planck’s quantum theory of radiation constitutes one of the most striking successes of the theory. When light falls on a metal surface, electrons are emitted, their number increasing (up to a saturation level) with both the intensity and the frequency of the incident light. However, below a certain frequency v,, (or above a certain wavelength h,) no electrons are emitted, irrespective of the intensity of the light. The en- ergy of the photons E, at this “threshold” frequency v0 (which are related by Planck’s equa- tion E, = hv, = he/X,, where h is Planck’s con-

(North-Holland)

stant and c the velocity of light) is equal to the minimum energy or “work function” required for the emission of electrons from the metal surface.

For the alkali metals this threshold value of vC, or A,, lies in the visible range, but for heavier metals

it lies in the higher frequencies or shorter wave- lengths of the ultraviolet range.

For polycrystalline metals or alloys the work

function of the surface is not constant but varies from grain to grain depending upon crystallo-

graphic orientation. bulk composition, tempera- ture and the extent of adsorption of impurities or

oxidation of the surface. Herein lies the basis of the microscopical application of the effect. for when such a surface is irradiated with ultraviolet light the intensity of electrons emitted also varies, and these electrons can be used to form an image of the surface.

Furthermore. the overall intensity of emitted electrons increases with increase in temperature,

resulting in a greater brightness of the image: and above about 0.77;,, (where T,, is the melting tem- perature of the material), thermionic electron emission becomes dominant. Thus, strictly speak- ing, emission electron microscopy over a range of temperatures utilizes the processes of both photo- emission and thermionic electron emission.

In addition to the image contrast which arises from crystallographic orientation and composi- tion. the intensity of the emitted electrons is de- pendent upon surface topography. Protrusions or recesses in the surface experience different poten- tial gradients and hence emit electrons more or less strongly (see section 2 below). This is an important additional source of contrast in that grain boundaries can be revealed by etching or thermal grooving, but in practice too much devia- tion from planarity in the specimen surface gives too great a variation in emission; in particular, the “bright spots” tend to obscure the contrast arising from the other effects and give rise to instability in the image. Hence, correct specimen preparation, as described in section 3 below, is critical.

In order to achieve the greatest contrast in the photoemission electron microscope, it is desirable that the energy of the incident UV photons should be about equal to the maximum value of the work functions across the surface: in this case the grains

with the highest work functions will emit weakly and appear dark, and those with the lowest work functions will emit strongly and appear bright. The situation is illustrated diagrammatically in fig.

1 which shows the variation in intensity of photo- electron emission with increasing energy E of the

incident photons for three grains with work func-

tions E,,1, E,f and E‘,:. Clearly. UV light with an energy represented by the dotted line will intersect the linear regions of the emission curves and give rise to the greatest variations of emission and

therefore contrilst. whereas UV light with a higher energy (higher frequency or lower wavelength) will intersect the flatter saturation regions of the emis- aion curves giving much lower contrast. Ideally, for any given specimen, it would be advantageous to be able to “tune” the wavelength of the inci- dent UV light to the position of greatest contrast, but unfortunately such a facility ia not available with the mercury arc vapor lamps (A = 280 nm) of the Balzers KE3 Metioscope. In practice good orientation and composition contrast is obtained in steel. copper and titanium alloys and many ceramic materials but not (for example) in aluminum alloys. It should be stressed again that the work functions of a metal or ceramic surface

Fig. I. A shematic diagram showng the variation in photo-

electron cmlssion I wth energy E of inudent UV photons l’or

three surfaces with work functions E,j. E,f. E,:. The dotted line

represents the photon energy w,hlch provides the greatest con-

trii\t. From ref. [4].

C. Hammond. M.A. Imam / PEEM in metallurgical research 175

are

[41.

2. The Balzers KE3 Metioscope

Fig. 2 is a simplified schematic diagram show- ing the major components of the Balzers KE3

Metioscope. The specimen is situated above the anode of a three-stage electromagnetic imaging system. Ultraviolet light from four mercury arc

vapor lamps is reflected from the polished top surface of the anode plate and focused up onto the specimen surface. The photoelectrons emitted from the surface are then accelerated towards the anode

UV lamps

solld spec,mcn

pwccd anode

ObJectwe lens

fluoresccn~ screen. camcra-

Fig. 2. A schematic diagram illustrating the major components of the Bakers KE3 Metioscope. From ref. [2].

through a potential difference of 30-50 kV, pass through an aperture in the anode to the objective lens and are finally imaged onto the fluorescent

screen in the usual way. Fig. 3 is a cross-sectional view showing the

construction of the Metioscope and its ancillary apparatus. The instrument as supplied incorpo- rated only a 5-plate camera (fig. 3, No. 13) - quite inadequate for recording “real time” changes in

microstructures. The camera has been augmented on the Leeds instrument by Mr. A. Nichells, who has incorporated a video camera operating in con- junction with a high-resolution transmission screen

made from a thin single crystal of YAG (yttrium aluminum garnet).

The vacuum system of the KE3 Metioscope consists of a rotary pump plus an oil diffusion

pump, and pressures of 10m6 Torr are normally achieved. Hence it is not an ultra-high or an ultra-clean vacuum system design, and contamina- tion of the specimen surface occurs by adsorption or absorption of oxygen, nitrogen, hydrocarbons, etc. It is very difficult to assess the effects of such contaminants on the contrast and/or brightness of the image since their effects upon the work function of the surface are unknown. There is no reason to suppose, however, that their effects are wholly deleterious - sometimes contrast appears to “improve” during observation, only to be re- duced again after surface cleaning [5].

Specimen cleaning may be carried out in situ in

two ways. Firstly, the specimen may be cleaned during observation by a stream of argon atoms incident at a grazing angle to the surface, the argon “neutral particle” gun (fig. 3, No. 10) being directed in the gap between the specimen surface and anode plate. Secondly, the whole specimen /holder assembly may be rotated into the promi- nent side-chamber (as indicated in fig. 3, No. 2) into which are fitted normal and inclined inci- dence argon ion beam guns, one of which is shown in fig. 3, No. 3. These enable surface films or particles to be removed, following which the speci- men/holder assembly is rotated back into the vertical observation position (fig. 3, No. 9).

Fig. 3, No. 1, shows the specimen holder or stage assembly in which the specimen must be maintained at the high negative potential of 30-50

176 Hummond. M.A. lmam / PEEM rn metullurgicul rrsrurch

I2

13

Fig. 3. A cross-sectional view of the Balzers KE3 Metioscope showing the construction and ancillary apparatus. Number legends: (I) specimen holder; (2) preparation position; (3) ion source for ion cleaning; (4) specimen vacuum lock; (5) cold trap; (6) vacuum-tight

Joint; (7) UV illumination; (8) cross-table; (9) specimen; (10) neutral particle source; (11) electromagnetic lenses; (12) fluorescence screen; (13) photographic or film camera. From ref. [2].

kV. Two types of holders are provided, one which which allows a specimen to be simultaneously

incorporates a cylindrical furnace allowing a heated and strained. Such a straining stage assem-

specimen to be heated, and one which incorpo- bly is illustrated in fig. 4. The Balzers KE3 at

rates an electron beam heater and a drive rod Leeds is illustrated in fig. 5. When the straining

C. Hammond, M.A. Imam / PEEM in metaNurgrcal research 171

Fig. 4. The Balzers KE3 straining-stage assembly showing the

drive unit controller (top) and the guiding tube and (side]

connector to the high-voltage supply.

stage assembly is in use, the multiplicity of man- ual controls requires the collaboration of several additional operators.

3. Types of specimen and preparation techniques

In order to achieve satisfactory images - or to achieve images at all - specimen preparation is critical. The surface must be flat, exactly parallel with the anode plate and polished to a bright metallographic finish with the finest grade (< 1 pm) diamond or alumina powder. Any bevelling or surface discontinuities such as oxide particles or inclusions will lead to severe non-uniformity of the anode-specimen field, instability in the image, and “ flashover”, i.e., arcing between the specimen

and anode plate [5]. The specimen may take a number of forms:

(a) A cylindrical specimen 12 mm in diameter and 32 mm long machined with three clamping lugs. This fits into a wire-wound furnace. A hole is drilled from the back to within 2 mm of the

specimen surface to receive a thermocouple. The specimen and furnace assembly is illustrated in fig. 6.

Preparation of the surface is best carried out by mounting the specimen in a cylindrical jig which will eliminate bevelling during polishing and en-

sure the necessary conditions of flatness and uni- formity.

(b) A cylindrical “insert” specimen 8 mm in diameter and 4 mm deep which is fitted into a

specimen holder fabricated in the form shown in fig. 6, but made with a cylindrical recess to receive

Fig. 5. The Bakers KE3 Metioscope in use in the School of Materials, University of Leeds.

Fig. 6. The cylindrical specimen (A) showing the three clamp-

~ng lugs. and the Balzers wire-wound furnace (B) with the

thermocouple (C) at the centre.

the insert specimen. In addition, three threaded holes are made around the perimeter of the holder to receive grub screws which help to fix the insert specimen in position. The whole specimen/ holder assembly is then polished as described in (a) above. However, the use of an “insert” specimen should only be resorted to if insufficient material is avail- able to fabricate a cylindrical specimen as de-

scribed in (a) above. It is much more difficult to achieve a uniformly flat surface, and specimen movement in the holder and outgassing tend to occur, which lead to instability in the field be- tween the specimen and anode plate. In addition, the materials of the specimen and holder must be carefully matched to avoid chemical interactions at high temperature. For example, the juxtaposi- tion of a titanium specimen and stainless steel holder at 950°C leads to such rapid and extensive interdiffusion that the specimen appears to “melt” into the holder and along the thermocouple hole

[51. (c) A flat sheet “tensile” specimen 6 mm in

width, 32 mm long, the ends of which are bent over and drilled with holes in order to secure it to the jaws of a straining stage holder which can move apart, hence allowing the direct observation

of high-temperature deformation (figs. 4 and 7). Heating is localised in the center of the “gauge length” of the specimen by means of an electron beam heater, and a thermocouple is spot-welded to the rear side edge. Separation of the jaws about a fulcrum is controlled by a push-rod sliding in a guide tube and a load cell assembly (as described above). In practice it has proved impossible to

make any other than qualitative estimates of the stress, strain or even temperature conditions of

high-temperature deformation processes because of the problems of estimating the effective gauge length, the temperature gradient from the centre of the specimen to the jaws, etc. In addition. the movement of the jaws about a fulcrum tends to “bow” the specimen surface, which leads to image instability. These defects in design have been partly overcome by redesigning the specimen/jaw clamp

ing assembly; the gauge length has been reduced in length to correspond more closely with the surface area of the electron beam heater, and the clamping arrangement has been modified to allow the ends of the specimen to be bent to a much lower angle ~ 30 o instead of 90 “; they are sep- arated from the steel jaws by the use of shaped ceramic washers [5]. Hence the problems of crack-

Fig. 7. An “exploded” view of the Bakers tensile speumen and

btrnining stage. The specimen (bent strip of metal) is screwed

into the jaws of the stage and is heated hy the wirc-wound

electron henm heater. The thermocouple is spot-welded to the

near rear side edge ol’ the specimen.

C. Hammond, M.A. Imam / PEEM in metallurgical research 179

ing during the bending of specimens of limited ductility and heat losses to the jaws have been partially (but not wholly) obviated.

4. Image interpretation in the PEEM: surface ef- fects (comparisons between PEEM, hot-stage SEM

and TEM)

In interpreting the results of any dynamic in-situ experiments, whether in PEEM or hot-stage SEM which involve observations of a single surface, or hot-stage TEM, which involves observations in a

thin region between two surfaces, the question always arises as to what extent do the changing microstructures observed at the surface or in the

thin foil correspond with those characteristic of bulk material? This is indeed the most serious

problem of any high-temperature or dynamic mi- croscopical technique and has probably inhibited their development more than any other factor. The arguments are not dissimilar to those made about the interpretation of the microstructures in thin foils at ambient temperatures in transmission electron microscopy - the problems of sponta- neous transformations, annealing out of disloca- tions and so on. However, such problems have not inhibited the applications of transmission electron

microscopy ~ not because of any general counter- argument but because cumulative experience has enabled those situations in which the observed structures do deviate from those of the bulk

material to be recognised and accounted for. Simi- lar considerations apply in the case of dynamic experiments in high-voltage transmission electron microscopy (which extends the thickness range of thin foils which may be imaged). The review by Butler and Hale [6] summarises the data and pro- vides useful guidelines by which “ thin foil” effects may be distinguished from effects characteristic of the bulk material.

It would be expected that the problem of single surface artifacts would be the same in hot stage SEM and PEEM. What, then, are the advantages and disadvantages of PEEM as compared with hot- and straining-stage SEM? One obvious prac- tical disadvantage of PEEM is that the specimen assembly is the cathode of the electron microscope

column and is at a negative potential of 30-50 kV. All the control circuits to the specimen must therefore be isolated, implying that even slight

modifications ~ the provision of additional ther- mocouples, for example - necessitate a substantial redesign of the specimen stage. This is particularly the case with the straining stage (figs. 4 and 7) where the temperature gradients across the speci- men need to be recorded. The advantages of PEEM

are that the contrast is determined primarily by the variations in crystallographic orientation and composition (apart from the topographical con- trast mentioned above), and although these varia-

tions may also provide a source of contrast in the scanning electron microscope when operated in the primary back-scattered electron or electron channelling modes, these modes are in general less sensitive to such variations. Furthermore, the reso- lution obtainable in PEEM is greater than that obtainable in SEM when the primary back- scattered electrons are used to form the image. In both cases the resolution is approximately equal to the depth from which the detected electrons are emitted; photoelectrons escape from a depth of the order of 5-20 nm, whereas primary back- scattered electrons of much higher energy may arise from a depth of 100-200 nm. The resolving power of the photoemission electron microscope is therefore about an order of magnitude greater than that of the scanning electron microscope when the latter is operated in the back-scattered electron mode. However, it must be pointed out that rapid developments of signal collection and

image processing techniques and the increasing use of lower operating potentials in the scanning electron microscope are likely to erode this ad- vantage.

5. Image interpretation in the Bakers ICE3 PEEM and some recent metallurgical applications

As pointed out in section 4 above, it is essential to determine whether the structures and transfor- mations observed in the PEEM are characteristic of the bulk material. In general, it has been found that large-scale phase transformations and grain- growth processes are characteristic of the bulk

material. but that the nucleation and growth processes of fine-scale precipitates tnuy be af- fected by the presence of the free surface, in which

case both the morphology and composition of the surface-nucleated precipitates will be different

from those characteristic of the bulk material. The

problem may be complicated by the possibility that in a given microstructure some of the oh- served precipitates may be surface nucleated and others may not.

Examples of all these cases, taken largely from work at Leeds, are given below. For results of other groups see ref. [7], and the reviews [2,8,9]. Fig. 8a shows a triple point, where three grain boundaries of the /3 phase are in equilibrium, of a titanium alloy IMI 685 (Ti-6%Al-5%Zr- OS%Mo-0.25%Si) [lo] imaged at 1070°C. The rip- ple-like variations in contrast across the grain

surfaces arise as a result of “thermal faceting” ~ the initial flat polished surface tends to develop facets corresponding to surfaces of lower surface energy. They may be removed. or partly removed, by argon ion cleaning which indicates that adsorp- tion of gases plays a part in their formation. The light-dark contrast is topographic. Fig. 8a also shows the presence of grain boundary LY (at A and

c’) and a thermal groove indicating migration of the grain boundary B ~B. Fig. Xb shows the same area after cooling to 850°C and shows the nuclea- tion (from the prior j3 grain boundaries) and growth of platelets of the N phase into the ,/I

phase. The resulting “basket weave” structure of N platelets is entirely characteristic of the bulk

material. The (Y - j3 transformation involves shape changes. and the contrast is again topographic. Similarly with the reverse transformation on heat- ing. Fig. 9 shows a sequence of micrographs taken during heating the alloy IMI 685 in the as-forged condition. Fig. 9a (600°C) shows several colonies of cy platelets, some of which are curved and distorted as a result of the prior deformation. and the presence at the platelet boundaries of thin films of the high-temperature j!! phase. The con-

trast arises from compositional differences be- tween the two phases. On further heating the /3 layers thicken as shown in fig. 9b (700°C) and fig. 9c (95O”C), and eventually the LY is wholly trans- formed to the /3 phase. These results are of par- ticular interest because they show that the inter- platelet /3 phase within an LY colony is of the same crystallographic orientation and therefore must derive from a prior single j3 grain. In addition.

Fig. 8. (a) Titanium alloy IMI 685 at 1070°C showing a triple point between fl grains. grain boundary LY particles (at A and C) and thermal grooving corresponding to the prior position of a /3 grain boundary (at B-B). (h) The same area after cooling to X5O”C

showing the transformation of /3 to a “hasketweave” structure (BW) of n platelets.

C. Hammond, M.A. Imum / PEEM in metullurgid research 181

Fig. 9. (a) Titanium alloy IMI 685 at 600°C showing colonies

of deformed a platelets and interplatelet layers of the /3 phase.

(b) The same area at 700°C showing thickening of the inter-

platelet p phase. (c) The same area at 950°C showing further growth of the interplatelet /3 phase.

nucleation of new grains of the p phase within the

a platelets is not observed to occur.

The isothermal transformation occurs by the nucleation of ferrite grains (the low-temperature form of iron) at the austenite grain boundaries, the growth of these ferrite grains into the austenite and at the same time the precipitation of fine plate-like or needle-like particles of molybdenum carbide at the migrating austenite-ferrite inter- faces. The progress of this transformation can be seen in fig. 11, which shows several ferrite grains, with fine internal substructures of molybdenum carbide particles, growing inward from the grain boundaries of an austenite grain. The bright re- gion in the center of the field of view is the residual austenite grain on the surface of which

are clear surface-nucleated dendrites of molybde- num carbide. In this example, the surface- nucleated phase can easily be identified by subse-

quent X-ray analysis. This is not always the case. In work currently in progress on a low-carbon steel A710, unidentified surface and grain boundary precipitates occur. Fig. 12a shows the structure of the steel cooled from 950” C to - 745°C. The austenite grain boundaries are de- corated with a “necklace” of LY grains entirely characteristic of the bulk transformation to ferrite. However, at lower temperatures an unidentified precipitate occurs both at the grain boundaries and at the surface (fig. 12b).

A clear example of a surface-nucleated phase is In the above examples the transformations pro- given in fig. 10 which shows the structure of a ceed sufficiently slowly, or can be arrested, by high-carbon steel (Fe-0.9%) which has been an- holding the temperature constant, such that the nealed at 1000°C and cooled to 820°C [ll]. The structures can be recorded using the plate camera.

expected structure is austenite (the high-tempera- ture form of iron with the carbon in solid solu- tion) together with a precipitation of cementite (iron carbide) at the grain boundaries. The austenite grains are clearly visible in fig. 10, the strong contrast between the grains and from the twins within the grains arising from differences in

crystallographic orientation (orientation contrast). In addition, surface-nucleated “rosettes” of gra- phite occur, clearly visible at A.

An example of the co-occurrence of a precipita- tion reaction which is partially surface nucleated is given in fig. 11, which shows the structure of an alloy steel (Fe-4%Mo-0.2%) which has been

annealed at lOOO”C, cooled to 65O”C, and held at this temperature [12].

1x2

In other cases it is essential to use video record- morphology of the transformed phases during the ings. This is the case with respect to work cur- early period of their evolution. The characteristics rentlq’ in progress where the KE3 Metioscope is of the grain boundary cy allotriomorphs and the being used to understand the loss in ductility WidmanstStten a. as well as the sequence of their which occurs in titanium alloys in the temperature formation during continuous cooling in an experi- range 1000-1150 K by critically examining the mental alloy Ti-6%A1-2%V-6’%Zr, are shown in transformation process in-situ and evaluating the fig. 13 which shows the untransformed /3 grains at

I-?- 11. Iwrhermal transformation at 650°C in an Fc-4TMo-O.ZSiC \trel, cooled from the au\tenite range. showing the growth of

feryite grains into the (central) austenite grain. fine needle- or plate-llke particles of molybdenum carhide in the ferrite which nuclealc

on tile migrating austenitr/ferrite boundaries and the wrfacr nucleation of dendritic molvhdenum carbide particles.

Fig. 12. (a) Ferrite grains nucleated at austenite grain

boundaries with carbide precipitate in low-carbon steel A 710

at 745’C. (b) The same area at 690°C showing ferrite grain

boundary and copper precipitates along with carbide precipi-

tates.

PEEM WI metallurgical research 183

1030°C (fig. 13a), (Y allotriomorphs and the onset of Widmanstatten (platelet) (Y at 820°C (fig. 13b), and the end microstructural condition after trans- formation is nearly complete at 805°C (fig. 13~). The micrographs in fig. 13 are single frames ex- tracted from a continuous video record of the transformation events occurring at a cooling rate

of about lO”C/min from 1050°C. The (Y al- lotriomorph nucleations appeared at about 900 o C,

nearly 80°C above the onset of the fast-growing Widmanstatten (Y platelets. Lattice incompatibility of the interface between the (Y allotriomorphs and the /3 grain at the temperature of transformation is suggested [13] to contribute an abrupt loss of

ductility at a narrow temperature range in these

alloys when subjected to an externally imposed

stress. Finally, figs. 14 and 15 show some results ob-

tained with the straining stage which has been used to simulate the temperature-strain-rate con- ditions characteristic of superplasticity [14]. Fig. 14 shows a fine-grained aluminum alloy 7475 (Al-

0.1%Si-0.12%Fe-1.2/1.9%Cu-0.06%Mn-1.2/ 2.6%Mg-5.2/6.2%Zn-0.18/0.25%Cr-O.O6%Ti) which has been deformed at 500 o C to 1% strain (i.e. stretched in the central heated region by roughly 1%) (fig. 14a) and - 10% strain (fig. 14b). The tensile axis is horizontal as indicated. The grain offsets arising from grain boundary sliding

processes are clearly visible, primarily as a result

Fig. 13. Transformation sequence in an experimental titanium alloy (Ti-6%Al-2RV-6RZr) from a continuous video record: (a) at 1030°C showing untransformed p grains; (b) at 82O”C, showing partial transformation to Widmanstltten a; and (c) at 805” C

showing the near-completion of the p + a transformation. Marker indicates 50 pm.

Two-phase titanium alloys present a slightI>

more satisfactory case in that strong exoelectron

emission does not obscure the image, and the differential contribution to the overall strain made by the two phases can be observed. Fig. 15a shows the titanium alloy IMI 318 (Ti-6%Al-4SBV) de-

formed at 900°C‘ to 12 strain and subsequently to

the frahly exposed grain boundary surfaces. From ref. [14]. are a constant source of inspiration to all those

C. Hammond, M.A. lnmm / PEEM in metallurgical research 185

working in the field of photoemission electron microscopy.

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