- Email: [email protected]
Observation of both Ni and Mo atom images by FIM with imaging plates
surface science ELSEVIER
Applied Surface Science 94/95 (1996) 295-299
Observation of both Ni and Mo atom images by FIM with imaging plates Kouichi Nishikawa a, Takeshi Nishiuchi a, Masahiko Yamamoto a, *, Osamu Nishikawa b a Department of Materials Science and Engineering, Osaka University, Yamadaoka, Suita, Osaka 565, Japan b Department of Electronics, Kanazawa Institute of Technology, Ohgigaoka, Nonoichi, Kanazawa-South, lshikawa 921, Japan Received 7 August 1995; accepted 18 September 1995
Abstract Both Ni and Mo atoms in an ordered Ni4Mo alloy were successfully observed by using field ion microscopy with imaging plates. The details of Ni and Mo atom images are described for both superlattice and fundamental planes. The field evaporation behavior for each case is discussed and can be interpreted in terms of interatomic bond.
1. Introduction A field ion microscope (FIM) image can give an individual atom image. Quantitative analysis of such an individual atom-image can provide tunneling characteristics of electrons from imaging gas atoms to imaged atoms. The visibility of different species of atoms in field ion images of ordered alloys has long been topics and studied for a long time [1,2]. Recently, Yamamoto et al. took photographs in a film and analyzed quantitatively individual atom images in FIM of an ordered Ni4Mo alloy [3]. On the other hand, O. Nishikawa et al. constructed an FIM equipped with imaging plates (IPs) whose dynamic range is 1-10 4 and showed that it can provide higher quality images [4,5].
' Corresponding author. Tel.: +81 6 879 7486; fax: +81 6 876 4729; e-mail: [email protected].
In the present study, we (two groups) have cooperatively studied visibility of Ni and Mo atoms in field ion images of the ordered NiaMo alloy using FIM with IPs. The purpose of this paper is to show that both Ni and Mo atoms can be imaged, to describe field evaporation behavior of Ni and Mo atoms, and to discuss the effect of interatomic bonds on field evaporation behavior.
2. Crystallography of ordered Ni4Mo alloy The crystal structure of an ordered Ni4Mo alloy is body-centered tetragonal (bct) of D1,-type. If the atomic species are neglected, it has a face-centered cubic (fcc) structure. In the present paper, planes are indexed on the basis of bct lattice except the cases noted as fcc. Crystal planes of D1 a-tYpe ordered alloy are classified into two types, fundamental and superlattice
0169-4332/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0 1 6 9 - 4 3 3 2 ( 9 5 ) 0 0 3 9 0 - 8
296
K. Nishikawa et al. / Applied Surjbce Science 9 4 / 9 5 (1996) 295-299
planes. In fundamental planes, each successive layer is crystallographically identical and has the stoichiometric composition of 80 at% Ni and 20 at% Mo. In superlattice planes, each successive layer is either entirely Ni or entirely Mo, with every fifth layer being a Mo layer.
3. Experimental procedure As a specimen, a fully long range ordered Ni4Mo alloy was used. Tips for FIM were made from the alloy wires by electro-polishing. FIM observation was made under a background pressure of better than 10 -8 Pa. Helium was used as an imaging gas. The tip temperature was about 40 K. Exposure time of IPs was 90-180 s. The FIM images of various planes were recorded on the IPs. They were processed by computer and displayed in various forms like a topographic view. Each position of the atom images was determined from the maximum intensity position. As to the resolution of the IPs, the total intensity is classified into 4096 steps, and the size of one pixel is 0.05 mm square which corresponds to about 0.025 nm in a FIM image.
4. Experimental results and discussion 4.1. General features of FIM image of an ordered N i 4 Mo alloy Fig. la is a FIM image of an ordered Ni4Mo alloy and Fig. lb is the schematic representation corresponding to Fig. la. The planes were determined by the method developed by Yamamoto et al. [6,7]. In almost all specimens, the (211) plane of the bct lattice, which correspond to (111) of the fcc lattice, is located at the center of the image. This is because fiber texture occurred after drawing and subsequent annealing. This makes indexing easy. The image shows bct symmetry and thus only Mo atoms are imaged in principle. Although a number of planes were investigated, the following three planes are described here because they clearly give both Ni and Mo atom images as will be described later; (411)
~
(b)
T13
~
1_1
013 111
011
Fig. 1. (a) FIM image of an ordered Ni4Mo alloy and (b) its schematic representation. Planes and zone axes are presented on the basis of bct lattice. The image was taken using channel plates and recorded by a photographic method. Tip voltage is 7 kV.
and (431) as superlattice planes and (842) as fundamental plane.
4.2. Superlattice planes Figs. 2 a - 2 f are the case of a (411) superlattice plane. Fig. 2a is an IP processed FIM image, Fig. 2b shows peak positions obtained from Fig. 2a, and Figs. 2c and 2d show atom positions expected from crystallography of D1 a-type structure.
297
K. Nishikawa et al. / Applied Surfiwe Science 94 / 95 (1996) 295-299
At the rim of a (411) plane, Mo atoms are imaged individually, for example, R~ and R e in Fig. 2b. Moreover, atom images with dumbbell shape have appeared beside the rim of the (411) plane, for example, A I and B t in Fig. 2b. The shape of this dumbbell pair is more clearly demonstrated in a contour map of the intensity, Fig. 2e. When the weight positions of such a dumbbell pair is investigated by comparing between the expected atom positions of Ni and Mo interpolated from the Mo positions at the rim, it is found that one weight of the dumbbell pair is situated at the Mo site, A l in Fig. 2c, and the other weight is at the Ni site, B~ in Fig. 2c. It is also found that the Ni atoms are situated at one atom layer higher than the Mo atom layer (Fig. 2d) and the Ni and Mo atoms of the dumbbell pair is in relation of the first nearest neighbor, lining up along the [120]( II [ll0]f,.~) direction whose projection to the vertical (or sheet) plane is [1 5 ~ ] . This means
(b)
(c) oO
A B~
(e)
A y,,B,
B2 0
o
o oO 'o .o% o
oo
~
o©o
o bT~P~~l"-~~, o o
oq% TM
oo
~
~
(~)(431) normal
(d)
B1
(431) normal
1.257 am
0.251 nm
<110> FCC
Fig. 3. (431) superlattice plane. (a)-(d) are the same as those in Figs. 2a-2d, (e) number of N i - M o interatomic bonds for Ni atoms at sites B~ and B 2 in (c). The meaning of the marks is the same as in Fig. 2.
(o)
0 ,,4lo
.C)o
~,~lJO
(d) o
"~-b"~o [ - ~ 0 [113]
BI /~,' A1 "~,0.0259 , ~..4~ ~E~lv nm (411) J ~ normal
~ L
-=-_ ~'---' II
5T9] [01TI
0.253
1.264
nm
nm
<110>FCC
(f) AI q)
o
Fig. 2. (411) superlattice plane. (a) IP processed FIM image, (b) peak positions obtained from (a), (c) top view, and (d) side view of atom positions expected from crystallography of Dla-type structure, (e) intensity contour map of the area enclosed by rectangle in (b), (f) cross section of intensity distribution along A I-B~. In (b)-(d), closed and open circles represent Mo and Ni atoms, respectively. The size of the circles in (c) shows the depth from the surface. Larger ones are situated in a higher layer.
that one Ni atom layer remains upon the underlying Mo atom layer. In order to compare the intensity of Ni and Mo atom images, a cross-sectional intensity distribution along A ~-B~ was obtained, Fig. 2f. The Ni atom at B a is imaged weaker than the Mo atom at A~ although the Ni atom at B~ is situated 0.0259 nm higher than the Mo atom at A~. Figs. 3a-3e show the case of (431) superlattice plane. Let us see ideal atom arrangement in the (431) plane first. The first atom layer from the top surface is Ni, B 1 in Figs. 3c and 3d, the second atom layer is Ni, B 2, and the third is Mo, A. Ni atoms at B~ are situated beside a Mo atom at A, and Ni atoms at B 2 are situated in the middle region between two rows of Mo atoms. In the FIM image, Fig. 3a, atom rows lining up in the [111] direction can be seen. When the imaged positions of Mo at the rim, A in Fig. 3b, and crystallographic atom arrangement, Figs. 3c and 3d,
298
K. Nishikawa et al. / Applied SurJace Science 94 / 95 (1996) 295-299
are compared, it is found that (i) imaged atom rows, B~ in Fig. 3b, is shifted from the underlying Mo atom row, A, (ii) the situated atom rows are Ni, and (iii) the Ni atoms are arranged in two atomic layers high from the underlying Mo atom layer. On the contrary, there are no atom images in the middle part between two Mo atom rows. Namely Ni atoms in the second atom layer, B 2, do not exist and they have already field evaporated. This field evaporation behavior can be understood by taking into account of the interaction energy of interatomic bond. In field evaporation of an alloy, the ionization energy of solute species and work function of an alloy element contribute in addition to the interaction energy [1,2]. However, work function of each element in an alloy are not known although that in a simple metal is well known. So in the present paper we discuss field evaporation behavior by using only the interaction energy, assuming the other factors do not give large influence. According to the result in theoretical and experimental consideration [8], the interaction energy of N i - M o is larger than of Ni-Ni. For example, UsiNi is about 0.46 eV and UNiMo is about 0.70 eV for the first nearest neighbor when cohesive energy is used. Since the entropy term is small because of low temperature during field evaporation, we can treat the sum of interaction energy as the binding energy. Namely the binding of N i - M o is stronger than of Ni-Ni. In the (431) plane, Ni atoms in the first atom layer, B~, have three N i - M o bonds, as shown in Fig. 3e. On the other hand, Ni atoms in the second atom layer, B 2, have one N i - M o bond as shown in Fig. 3e. The binding of N i - M o is stronger than in Ni-Ni, and the evaporation field of Ni atoms having less N i - M o bonds is lower than that of Ni atoms having many N i - M o bonds. 4.3. Fundamental planes
Figs. 4a-4e show the case of (842) fundamental plane. In order to obtain Fig. 4b, showing peak positions of IP processed FIM image from Fig. 4a, topographic views lightened from various directions were utilized. All the spots are elongated toward the [170] direction and there two or three weak peaks between strong peaks can be seen. Such weak peaks are not symmetrical. Comparing the FIM image, Fig.
[120] FCC
(c)
~
~
[120]
k.~110>
FCC
(d) A1 B1 B 2 B 3 B 4 A 2
>; ~" 0.254 nm
, 0.0603
(842) normal 1272 ,~ nm [120] ~ I <11 O> FOC~¢-' ~i
(e) B1 B2 B~
B4
Fig. 4. (842) fundamental plane. ( a ) - ( d ) are the same as those in Figs. 2 a - 2 d , (e) number of N i - M o interatomic bonds for Ni atoms at sites B 1, B2, B 3 and B 4 in (c). The meaning of the marks is the same as in Fig. 2.
4a, and crystallographic atom arrangement, Fig. 4c, strong peaks correspond to Mo atoms, A I and A 2 in Fig. 4c, and weak peaks correspond to Ni atoms, B~, B2, B 3 and B 4 in Fig. 4c. The weak peaks correspond to two or three of the four Ni atom positions. In the case of two, atom images at B 3 and B 4 have appeared, while in the case of three, atom images at B], B 3 and B 4 have appeared. The above phenomena are also understood by taking into account of the strength of interatomic bonds. Four Ni atoms at B~, B 2, B 3 and B 4 have 1, 1, 2 and 3 nearest neighbor Mo atoms, respectively, as shown in Fig. 4e. When the imaged Ni atoms and the number of the nearest neighbor Mo atoms are compared, it is revealed that the Ni atoms having one N i - M o interatomic bond, field evaporate earlier than the Ni atoms having more than one N i - M o interatomic bond. 4.4. Field evaporation behavior
The present experimental results demonstrate that field evaporation behavior depends on the number of
K. Nishikawa et al. / Applied Surface Science 94 / 95 (1996) 295-299
interatomic bonds. Field evaporation does not always start from the atoms sitting at the rim of the top-most plane. Weakly bound atoms field evaporate earlier than the atoms sitting at the rim of the top-most plane even if weakly bound atoms are situated at a lower atomic level. In the previous atom probe works on this alloy [9], it was found that Ni atoms have the tendency to field evaporate earlier than Mo atoms in one atomic layer. Such field evaporation phenomena in atom probe analysis can be understood by the interpretation from the point of interatomic bond, obtained in the present study.
5. Conclusions Ordered Ni4Mo alloy was observed by FIM with imaging plates. The use of an imaging plate made it possible to observe quantitatively both Ni and Mo atoms. As a result, field evaporation phenomena become clear on an individual atomic basis. Such phenomena were successfully interpreted from the point of interatomic bond. The evaporation field of Ni atoms having less N i - M o bonds is lower than that of Ni atoms having many N i - M o bonds.
299
Acknowledgements The authors are grateful to Dr. Taizo Akimoto and Dr. Tohm Tsuchiya of Fuji Photo Film Co. Ltd. for their help in IP measurements. This work was partially supported by a Grant-in-Aid for Scientific Research on Priority Areas from the Japanese Ministry of Education, Science and Culture.
References [1] M.K. Miller and G.D.W. Smith, in: Atom-Probe Microanalysis (Materials Research Society, Pittsburgh, PA, 1989) ch. 3. [2] T.T. Tsong, in: Atom-Probe Field Ion Microscopy (Cambridge University Press, Cambridge, 1990) chs. 2 and 4. [3] M. Yamamoto, K. Nishikawa and T. Nishiuchi, Appl. Surf. Sci. 87/88 (1995) 291. [4] O. Nishikawa, T. Akimoto, T. Tsuchiya, T. Yoshimura and Y. Ishikawa, Appl. Surf. Sci. 76/77 (1994) 359. [5] O. Nishikawa, M. Kimoto, K. Fukui, H. Yanagisawa, M. Takai, T. Akimoto and T. Tsuchiya, Surf. Sci. 323 (1995) 288. [6] M. Yamamoto, M. Futamoto, S. Nenno and S. Nakamura, J. Phys. Soc. Jpn. 36 (1974) 1330. [7] M. Yamamoto, S. Nenno, M. Futamoto and S. Nakamura, Jpn. J. Appl. Phys. 11 (1972) 437. [8] T. Kingetsu, M. Yamamoto and S. Nenno, Surf. Sci. 144 (1984) 402. [9] M. Yamamoto and D.N. Seidman, Surf. Sci. 129 (1983) 281.
Applied Surface Science 94/95 (1996) 295-299
Observation of both Ni and Mo atom images by FIM with imaging plates Kouichi Nishikawa a, Takeshi Nishiuchi a, Masahiko Yamamoto a, *, Osamu Nishikawa b a Department of Materials Science and Engineering, Osaka University, Yamadaoka, Suita, Osaka 565, Japan b Department of Electronics, Kanazawa Institute of Technology, Ohgigaoka, Nonoichi, Kanazawa-South, lshikawa 921, Japan Received 7 August 1995; accepted 18 September 1995
Abstract Both Ni and Mo atoms in an ordered Ni4Mo alloy were successfully observed by using field ion microscopy with imaging plates. The details of Ni and Mo atom images are described for both superlattice and fundamental planes. The field evaporation behavior for each case is discussed and can be interpreted in terms of interatomic bond.
1. Introduction A field ion microscope (FIM) image can give an individual atom image. Quantitative analysis of such an individual atom-image can provide tunneling characteristics of electrons from imaging gas atoms to imaged atoms. The visibility of different species of atoms in field ion images of ordered alloys has long been topics and studied for a long time [1,2]. Recently, Yamamoto et al. took photographs in a film and analyzed quantitatively individual atom images in FIM of an ordered Ni4Mo alloy [3]. On the other hand, O. Nishikawa et al. constructed an FIM equipped with imaging plates (IPs) whose dynamic range is 1-10 4 and showed that it can provide higher quality images [4,5].
' Corresponding author. Tel.: +81 6 879 7486; fax: +81 6 876 4729; e-mail: [email protected].
In the present study, we (two groups) have cooperatively studied visibility of Ni and Mo atoms in field ion images of the ordered NiaMo alloy using FIM with IPs. The purpose of this paper is to show that both Ni and Mo atoms can be imaged, to describe field evaporation behavior of Ni and Mo atoms, and to discuss the effect of interatomic bonds on field evaporation behavior.
2. Crystallography of ordered Ni4Mo alloy The crystal structure of an ordered Ni4Mo alloy is body-centered tetragonal (bct) of D1,-type. If the atomic species are neglected, it has a face-centered cubic (fcc) structure. In the present paper, planes are indexed on the basis of bct lattice except the cases noted as fcc. Crystal planes of D1 a-tYpe ordered alloy are classified into two types, fundamental and superlattice
0169-4332/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0 1 6 9 - 4 3 3 2 ( 9 5 ) 0 0 3 9 0 - 8
296
K. Nishikawa et al. / Applied Surjbce Science 9 4 / 9 5 (1996) 295-299
planes. In fundamental planes, each successive layer is crystallographically identical and has the stoichiometric composition of 80 at% Ni and 20 at% Mo. In superlattice planes, each successive layer is either entirely Ni or entirely Mo, with every fifth layer being a Mo layer.
3. Experimental procedure As a specimen, a fully long range ordered Ni4Mo alloy was used. Tips for FIM were made from the alloy wires by electro-polishing. FIM observation was made under a background pressure of better than 10 -8 Pa. Helium was used as an imaging gas. The tip temperature was about 40 K. Exposure time of IPs was 90-180 s. The FIM images of various planes were recorded on the IPs. They were processed by computer and displayed in various forms like a topographic view. Each position of the atom images was determined from the maximum intensity position. As to the resolution of the IPs, the total intensity is classified into 4096 steps, and the size of one pixel is 0.05 mm square which corresponds to about 0.025 nm in a FIM image.
4. Experimental results and discussion 4.1. General features of FIM image of an ordered N i 4 Mo alloy Fig. la is a FIM image of an ordered Ni4Mo alloy and Fig. lb is the schematic representation corresponding to Fig. la. The planes were determined by the method developed by Yamamoto et al. [6,7]. In almost all specimens, the (211) plane of the bct lattice, which correspond to (111) of the fcc lattice, is located at the center of the image. This is because fiber texture occurred after drawing and subsequent annealing. This makes indexing easy. The image shows bct symmetry and thus only Mo atoms are imaged in principle. Although a number of planes were investigated, the following three planes are described here because they clearly give both Ni and Mo atom images as will be described later; (411)
~
(b)
T13
~
1_1
013 111
011
Fig. 1. (a) FIM image of an ordered Ni4Mo alloy and (b) its schematic representation. Planes and zone axes are presented on the basis of bct lattice. The image was taken using channel plates and recorded by a photographic method. Tip voltage is 7 kV.
and (431) as superlattice planes and (842) as fundamental plane.
4.2. Superlattice planes Figs. 2 a - 2 f are the case of a (411) superlattice plane. Fig. 2a is an IP processed FIM image, Fig. 2b shows peak positions obtained from Fig. 2a, and Figs. 2c and 2d show atom positions expected from crystallography of D1 a-type structure.
297
K. Nishikawa et al. / Applied Surfiwe Science 94 / 95 (1996) 295-299
At the rim of a (411) plane, Mo atoms are imaged individually, for example, R~ and R e in Fig. 2b. Moreover, atom images with dumbbell shape have appeared beside the rim of the (411) plane, for example, A I and B t in Fig. 2b. The shape of this dumbbell pair is more clearly demonstrated in a contour map of the intensity, Fig. 2e. When the weight positions of such a dumbbell pair is investigated by comparing between the expected atom positions of Ni and Mo interpolated from the Mo positions at the rim, it is found that one weight of the dumbbell pair is situated at the Mo site, A l in Fig. 2c, and the other weight is at the Ni site, B~ in Fig. 2c. It is also found that the Ni atoms are situated at one atom layer higher than the Mo atom layer (Fig. 2d) and the Ni and Mo atoms of the dumbbell pair is in relation of the first nearest neighbor, lining up along the [120]( II [ll0]f,.~) direction whose projection to the vertical (or sheet) plane is [1 5 ~ ] . This means
(b)
(c) oO
A B~
(e)
A y,,B,
B2 0
o
o oO 'o .o% o
oo
~
o©o
o bT~P~~l"-~~, o o
oq% TM
oo
~
~
(~)(431) normal
(d)
B1
(431) normal
1.257 am
0.251 nm
<110> FCC
Fig. 3. (431) superlattice plane. (a)-(d) are the same as those in Figs. 2a-2d, (e) number of N i - M o interatomic bonds for Ni atoms at sites B~ and B 2 in (c). The meaning of the marks is the same as in Fig. 2.
(o)
0 ,,4lo
.C)o
~,~lJO
(d) o
"~-b"~o [ - ~ 0 [113]
BI /~,' A1 "~,0.0259 , ~..4~ ~E~lv nm (411) J ~ normal
~ L
-=-_ ~'---' II
5T9] [01TI
0.253
1.264
nm
nm
<110>FCC
(f) AI q)
o
Fig. 2. (411) superlattice plane. (a) IP processed FIM image, (b) peak positions obtained from (a), (c) top view, and (d) side view of atom positions expected from crystallography of Dla-type structure, (e) intensity contour map of the area enclosed by rectangle in (b), (f) cross section of intensity distribution along A I-B~. In (b)-(d), closed and open circles represent Mo and Ni atoms, respectively. The size of the circles in (c) shows the depth from the surface. Larger ones are situated in a higher layer.
that one Ni atom layer remains upon the underlying Mo atom layer. In order to compare the intensity of Ni and Mo atom images, a cross-sectional intensity distribution along A ~-B~ was obtained, Fig. 2f. The Ni atom at B a is imaged weaker than the Mo atom at A~ although the Ni atom at B~ is situated 0.0259 nm higher than the Mo atom at A~. Figs. 3a-3e show the case of (431) superlattice plane. Let us see ideal atom arrangement in the (431) plane first. The first atom layer from the top surface is Ni, B 1 in Figs. 3c and 3d, the second atom layer is Ni, B 2, and the third is Mo, A. Ni atoms at B~ are situated beside a Mo atom at A, and Ni atoms at B 2 are situated in the middle region between two rows of Mo atoms. In the FIM image, Fig. 3a, atom rows lining up in the [111] direction can be seen. When the imaged positions of Mo at the rim, A in Fig. 3b, and crystallographic atom arrangement, Figs. 3c and 3d,
298
K. Nishikawa et al. / Applied SurJace Science 94 / 95 (1996) 295-299
are compared, it is found that (i) imaged atom rows, B~ in Fig. 3b, is shifted from the underlying Mo atom row, A, (ii) the situated atom rows are Ni, and (iii) the Ni atoms are arranged in two atomic layers high from the underlying Mo atom layer. On the contrary, there are no atom images in the middle part between two Mo atom rows. Namely Ni atoms in the second atom layer, B 2, do not exist and they have already field evaporated. This field evaporation behavior can be understood by taking into account of the interaction energy of interatomic bond. In field evaporation of an alloy, the ionization energy of solute species and work function of an alloy element contribute in addition to the interaction energy [1,2]. However, work function of each element in an alloy are not known although that in a simple metal is well known. So in the present paper we discuss field evaporation behavior by using only the interaction energy, assuming the other factors do not give large influence. According to the result in theoretical and experimental consideration [8], the interaction energy of N i - M o is larger than of Ni-Ni. For example, UsiNi is about 0.46 eV and UNiMo is about 0.70 eV for the first nearest neighbor when cohesive energy is used. Since the entropy term is small because of low temperature during field evaporation, we can treat the sum of interaction energy as the binding energy. Namely the binding of N i - M o is stronger than of Ni-Ni. In the (431) plane, Ni atoms in the first atom layer, B~, have three N i - M o bonds, as shown in Fig. 3e. On the other hand, Ni atoms in the second atom layer, B 2, have one N i - M o bond as shown in Fig. 3e. The binding of N i - M o is stronger than in Ni-Ni, and the evaporation field of Ni atoms having less N i - M o bonds is lower than that of Ni atoms having many N i - M o bonds. 4.3. Fundamental planes
Figs. 4a-4e show the case of (842) fundamental plane. In order to obtain Fig. 4b, showing peak positions of IP processed FIM image from Fig. 4a, topographic views lightened from various directions were utilized. All the spots are elongated toward the [170] direction and there two or three weak peaks between strong peaks can be seen. Such weak peaks are not symmetrical. Comparing the FIM image, Fig.
[120] FCC
(c)
~
~
[120]
k.~110>
FCC
(d) A1 B1 B 2 B 3 B 4 A 2
>; ~" 0.254 nm
, 0.0603
(842) normal 1272 ,~ nm [120] ~ I <11 O> FOC~¢-' ~i
(e) B1 B2 B~
B4
Fig. 4. (842) fundamental plane. ( a ) - ( d ) are the same as those in Figs. 2 a - 2 d , (e) number of N i - M o interatomic bonds for Ni atoms at sites B 1, B2, B 3 and B 4 in (c). The meaning of the marks is the same as in Fig. 2.
4a, and crystallographic atom arrangement, Fig. 4c, strong peaks correspond to Mo atoms, A I and A 2 in Fig. 4c, and weak peaks correspond to Ni atoms, B~, B2, B 3 and B 4 in Fig. 4c. The weak peaks correspond to two or three of the four Ni atom positions. In the case of two, atom images at B 3 and B 4 have appeared, while in the case of three, atom images at B], B 3 and B 4 have appeared. The above phenomena are also understood by taking into account of the strength of interatomic bonds. Four Ni atoms at B~, B 2, B 3 and B 4 have 1, 1, 2 and 3 nearest neighbor Mo atoms, respectively, as shown in Fig. 4e. When the imaged Ni atoms and the number of the nearest neighbor Mo atoms are compared, it is revealed that the Ni atoms having one N i - M o interatomic bond, field evaporate earlier than the Ni atoms having more than one N i - M o interatomic bond. 4.4. Field evaporation behavior
The present experimental results demonstrate that field evaporation behavior depends on the number of
K. Nishikawa et al. / Applied Surface Science 94 / 95 (1996) 295-299
interatomic bonds. Field evaporation does not always start from the atoms sitting at the rim of the top-most plane. Weakly bound atoms field evaporate earlier than the atoms sitting at the rim of the top-most plane even if weakly bound atoms are situated at a lower atomic level. In the previous atom probe works on this alloy [9], it was found that Ni atoms have the tendency to field evaporate earlier than Mo atoms in one atomic layer. Such field evaporation phenomena in atom probe analysis can be understood by the interpretation from the point of interatomic bond, obtained in the present study.
5. Conclusions Ordered Ni4Mo alloy was observed by FIM with imaging plates. The use of an imaging plate made it possible to observe quantitatively both Ni and Mo atoms. As a result, field evaporation phenomena become clear on an individual atomic basis. Such phenomena were successfully interpreted from the point of interatomic bond. The evaporation field of Ni atoms having less N i - M o bonds is lower than that of Ni atoms having many N i - M o bonds.
299
Acknowledgements The authors are grateful to Dr. Taizo Akimoto and Dr. Tohm Tsuchiya of Fuji Photo Film Co. Ltd. for their help in IP measurements. This work was partially supported by a Grant-in-Aid for Scientific Research on Priority Areas from the Japanese Ministry of Education, Science and Culture.
References [1] M.K. Miller and G.D.W. Smith, in: Atom-Probe Microanalysis (Materials Research Society, Pittsburgh, PA, 1989) ch. 3. [2] T.T. Tsong, in: Atom-Probe Field Ion Microscopy (Cambridge University Press, Cambridge, 1990) chs. 2 and 4. [3] M. Yamamoto, K. Nishikawa and T. Nishiuchi, Appl. Surf. Sci. 87/88 (1995) 291. [4] O. Nishikawa, T. Akimoto, T. Tsuchiya, T. Yoshimura and Y. Ishikawa, Appl. Surf. Sci. 76/77 (1994) 359. [5] O. Nishikawa, M. Kimoto, K. Fukui, H. Yanagisawa, M. Takai, T. Akimoto and T. Tsuchiya, Surf. Sci. 323 (1995) 288. [6] M. Yamamoto, M. Futamoto, S. Nenno and S. Nakamura, J. Phys. Soc. Jpn. 36 (1974) 1330. [7] M. Yamamoto, S. Nenno, M. Futamoto and S. Nakamura, Jpn. J. Appl. Phys. 11 (1972) 437. [8] T. Kingetsu, M. Yamamoto and S. Nenno, Surf. Sci. 144 (1984) 402. [9] M. Yamamoto and D.N. Seidman, Surf. Sci. 129 (1983) 281.