In situ TEM observation of synergistic electronic-excitation-effects of phase stability in III-V binary compound nanoparticles

Article abstract of DOI:10.1140/epjd/e2007-00094-8

Electronic-excitation-effects of phase stability in III-V binary compound nanoparticles have been studied by TEM. When GaSb particles were excited by 75 keV electrons, the compound transforms to a two-phase consisting of an antimony core and a gallium shell or an amorphous phase, or remains the original crystalline phase, depending on particle size and/or temperature. It is suggested that such nonlinear responses of the phase stability may arise from synergistic effects of bond instability under excited states, formation of high density of excited states, chemical equilibrium under excited states and temperature dependence of defects mobility. EDP Sciences/Societa? Italiana di Fisica/Springer-Verlag 2007.

Full text of DOI:10.1140/epjd/e2007-00094-8

Eur. Phys. J. D 43, 177–180 (2007)
DOI: 10.1140/epjd/e2007-00094-8
THE EUROPEAN
PHYSICAL JOURNAL D
In situ TEM observation of synergistic
electronic-excitation-effects of phase stability
in III-V binary compound nanoparticles
1
,a
1
1
2
2
H. Yasuda , A. Tanaka , H. Usui , H. Mori , and J.G. Lee
1
Department of Mechanical Engineering, Kobe University, Rokkodai, Nada, Kobe 657-8501, Japan
Research Center for Ultra-High Voltage Electron Microscopy, Osaka University, Yamadaoka, Suita, Osaka 565-0871, Japan
2
Received 24 July 2006
Published online 24 May 2007 – ꢀc EDP Sciences, Societ `a Italiana di Fisica, Springer-Verlag 2007
Abstract. Electronic-excitation-effects of phase stability in III-V binary compound nanoparticles have
been studied by TEM. When GaSb particles were excited by 75 keV electrons, the compound transforms
to a two-phase consisting of an antimony core and a gallium shell or an amorphous phase, or remains
the original crystalline phase, depending on particle size and/or temperature. It is suggested that such
nonlinear responses of the phase stability may arise from synergistic effects of bond instability under
excited states, formation of high density of excited states, chemical equilibrium under excited states and
temperature dependence of defects mobility.
PACS. 61.80.-x Physical radiation effects, radiation damage – 81.30.-t Phase diagrams and microstructures
developed by solidification and solid-solid phase transformations – 64.75.+g Solubility, segregation, and
mixing; phase separation
1
Introduction
in the specimen chamber of an electron microscope. The
evaporator consists of two spiral-shaped tungsten fila-
ments. The distance between the filaments and support-
ing film (substrate) for nanoparticles was approximately
In isolated covalent molecules, electronic excitations by
photon induce the dissociation and isomerization of
molecules easily [1,2]. On the other hand, in III-V com-
pounds which have covalent bonding, electronic excita-
tions induce atom displacements resulting from formation
and migration of lattice defects in the interior of bulk
solids and desorption of atoms from the surface, but the
yield of the atom displacements for each phenomenon is
quite low [3–6]. When the size of the III-V compound ma-
terials decreases to nanometer scale, structural changes as
observed in the covalent molecules may be expected under
electronic excitation [7–9].
In the present work, electronic-excitation-effects of
phase stability in III-V binary compound nanoparticles
have been studied by in situ transmission electron mi-
croscopy (TEM) using nanoparticles of GaSb compound,
in order to see the effects of size and/or temperature on
the phase stability in GaSb nanoparticles under electronic
excitation.
1
00 mm. An amorphous carbon film was used as a sup-
porting film and was mounted on a molybdenum grid. Us-
ing the evaporator, gallium was first evaporated from one
filament to produce gallium nanoparticles on the support-
ing film, and then antimony was evaporated from the other
filament onto the same film. The supporting film was kept
at ambient temperature during the deposition. The flux of
17
18
−2 −1
depositing atoms was of the order of 10 ∼10
m
s
.
Vapor-deposited antimony atoms quickly dissolved into
gallium particles to form GaSb (Ga-50at%Sb) compound
particles [10–12]. The particles were then annealed in the
microscope at 573 K for 3.6 ks and were slowly cooled from
the annealing temperature to room temperature in 2.7 ks,
in an attempt to allow high atomic mobility in the par-
ticles which would homogenize the solute concentration.
Electronic excitation experiments and observations were
carried out using the same microscope. The microscope
used was Hitachi H-800 transmission electron microscope
operating at an accelerating voltage of 75 kV. This micro-
scope used was equipped with a turbo-molecular pumping
2
Experimental procedures
system and a liquid nitrogen-cooled anti-contamination
Preparation of size-controlled GaSb particles was carried
out with the use of a double-source evaporator installed
−5
device to achieve a base pressure below 2 × 10
Pa
in the specimen chamber. The electron fluxes used for
20
−2 −1
a
observations and excitations were 1.5 × 10 e m
s
e-mail: yasuda@mech.kobe-u.ac.jp

178
The European Physical Journal D
Fig. 2. An example of the structural change in GaSb parti-
cles kept at 293 K by electronic excitation. (a, a’) A BFI of
particles and the corresponding SAED before excitation, re-
spectively. (b, b’) The same area after excitation for 240 s and
the corresponding SAED, respectively.
Fig. 1. An example of the structural change in GaSb particles
kept at 423 K by electronic excitation. (a, a’) A BFI of particles
and the corresponding SAED before excitation, respectively.
(
b, b’) The same area after excitation for 180 s (i.e., up to
−2
23
the dose of 2.7 × 10 e m ) and the corresponding SAED,
respectively.
after the excitation, there appears a structure consisting of
a core with dark contrast and a shell with bright contrast,
as seen from a comparison of the parts framed squarely
by Ia and IIa in Figure 1a with those framed by Ib and
IIb in Figure 1b, respectively. The SAED taken after the
excitation is shown in Figure 1b’. In the SAED, Debye-
Scherrer rings are recognized, superimposed on a weak
halo ring. The Debye-Scherrer rings can be indexed con-
sistently as those of crystalline antimony, which has the
hexagonal structure with lattice constants of a0 = 0.43 nm
and c0 = 1.13 nm. The values of the scattering vector
2
1
−2 −1
and 1.5 × 10 e m
s
, respectively. The temperature
of particles on the supporting films was kept from 293
to 573 K during the experiments. Structural changes as-
sociated with electronic excitation were observed in situ
by bright-field images (BFIs) and selected-area electron
diffraction patterns (SAEDs).
3
Results
(K = (4πsinθ)/λ) for the halo ring are approximately 31.0
An example of the structural change by electronic exci-
tation in GaSb particles kept at 423 K is shown in Fig-
ure 1. Figures 1a and 1a’ show a BFI of particles with the
diameter of approximately 20 nm before excitation and
the corresponding SAED, respectively. In Figure 1a’, the
Debye-Scherrer rings can be consistently indexed as those
of GaSb which has the zincblende structure with a lattice
constant of a0 = 0.61 nm. The same area after excitation
−
1
and 54.0 nm
which are corresponding to the halos of
liquid gallium. It was confirmed by dark-field electron mi-
croscopy that nanoparticles after the electronic excitation
have the two-phase structure consisting of a crystalline
antimony core and a liquid gallium shell. From these re-
sults, it was evident that two-phase separation took place
in GaSb particles.
21
−2 −1
by the flux of 1.5 × 10 e m
s
for 180 s (i.e., up to the
The structural changes induced by electronic excita-
23
−2
dose of 2.7 × 10 e m ) is shown in Figure 1b. Each of tion in approximately 10nm-sized particles kept at 293 K
observations by BFIs and SAEDs was carried out within have been studied, as shown in Figure 2. In the SAED
20
−2 −1
approximately 5 s by lower flux of 1.5 × 10 e m
s
,
after excitation for 240 s (Fig. 2b’), only haloes are rec-
therefore the excitation effect during BFI and SAED ob- ognized. The values of the scattering vector of the halo
−
1
−1
servations is negligibly small. In the interior of particles rings are approximately 19.0 nm and 29.0 nm which

H. Yasuda et al.: In situ TEM observation of synergistic electronic-excitation-effects of phase stability
179
Fig. 4. Summary of the results on the phase transformations
induced by electronic excitation as functions of particle size and
23
−2
.
temperature. Total electron dose is fixed at 3.6 × 10 e m
of particle size and temperature. Total electron dose is
23
−2
fixed at 3.6 × 10 e m . In this figure, two-phase separa-
tion consisting of a crystalline antimony core and a liquid
gallium shell, formation of an amorphous phase and orig-
inal crystalline phase with the zincblende structure are
schematically illustrated as three kinds of pictures, respec-
tively. From the results, it is evident that the phase trans-
Fig. 3. Typical sequences of excitation process in an approxi-
mately 20 nm-sized GaSb particle. (a –a ) Morphologies in the
same particle kept at 423 K before excitation, after excitation
for 60 s and after excitation for 240 s, respectively. (b –b
Changes in the same particle kept at 293 K before excitation,
1
3
1
3
)
after excitation for 60 s and after excitation for 240 s, respec- formations are suppressed with increasing particle size and
tively.
temperature. The other points noted here are that in ap-
proximately 20 nm-sized particles the phase transforma-
tion is again suppressed with decreasing temperature, but,
correspond to those from amorphous GaSb. This result in approximately 10 nm-sized particles the phase trans-
indicates that amorphization has been induced after the formation varies from phase separation to amorphization
excitation in 10 nm-sized particles kept at 293 K.
with decreasing temperature. Such nonlinear responses of
In order to see the process in the phase separation phase transformations, that is, competitive formations of
and amorphization in the particles, successive stages in three kinds of phases with decreasing particle size and/or
morphological changes have been observed. Figures 3 are temperature were observed by the 75 keV electronic exci-
typical sequences of excitation process in an approxi- tation.
mately 20 nm-sized GaSb particle. Figures 3a1 to 3a3 show
changes in the same particle kept at 423 K during exci-
tation. After excitation for 60 s, phase separation takes 4 Discussion
place to form a crystalline antimony core and a liquid
gallium shell, as shown in Figure 3a2. The shell thick- A candidate for mechanisms of such nonlinear responses of
ness is approximately 3 nm. After excitation for 240 s, the electronic-excitation-induced phase transformations will
shell thickness remains constant because no more compo- be discussed as follows.
sitional changes can be induced after the phase separation,
as shown in Figure 3a3. On the other hand, Figures 3b1– tronic excitations in III-V compounds which have cova-
b3 show changes in the same particle kept at 293 K during lent bonding occur through the process that an excess
It is known that atom displacements induced by elec-
3
excitation. After excitation for 60 s, amorphization takes energy accumulated by local electronic transitions from
place on the surface in the particle, as shown in Figure 3b2. the ground state to excited states, that is the formation of
After excitation for 240 s, the amorphous shell thickness electron-hole pairs or pairs of holes (for example, produced
increases from 3 to 5 nm as seen from a comparison of Fig- by Auger transition), is directly converted into the atomic
ure 3b2 with 3b3. From the results, it was evident that the kinetic energy [13–15]. The introduction of high density
phase separation takes place abruptly after the composi- of excitations is required for the initial step for atom dis-
tional fluctuation in the particle, but the amorphization placements. The interactions of excited states with various
proceeds gradually from the surface of the particle.
kinds of defects (such as surfaces) are effective in the dense
Figure 4 summarizes the results on the phase trans- excitation. High density of excited states tends to intro-
formations induced by electronic excitation as functions duce in nanoparticles which are isolated and have a high

180
The European Physical Journal D
ratio of the surface to the volume. It is considered that the 5 Conclusions
dense excitations and the resulting formation of primary
point defects by atom displacements may act as a trigger Electronic-excitation-effects of phase stability in III-V bi-
for phase separation or amorphization in nanoparticles.
nary compound nanoparticles have been studied by TEM.
From the viewpoint of chemical equilibrium, in the Ga- When GaSb particles were excited by 75 keV electrons, the
Sb binary system in which the heat of formation ∆H is compound transforms to a two-phase consisting of an an-
−
1
negative (–41.9 kJ mol ), the chemical bond between timony core and a gallium shell or an amorphous phase, or
gallium and antimony is stabilized at the ground state. If remains the original crystalline phase, depending on par-
the phase separation from the compound to two pure sub- ticle size and/or temperature. It is suggested that such
stances (i.e., antimony and gallium, in which the cohesive nonlinear responses of the phase stability may arise from
energy is 271 kJ/mol and 265 kJ/mol, respectively) oc- synergistic effects of bond instability under excited states,
curs, the free energy should increase. However, when the formation of high density of excited states, chemical equi-
bonding (valence) electron is excited to the antibonding librium under excited states and temperature dependence
state to form carriers such as electron-hole pairs or pairs of defects mobility.
of holes, the increase of those carriers will bring about
metallic character rather than semiconducting character
in nanoparticles. The metallic character induced under the
excited states forms metallic bonds (between gallium and This work was, in part, supported by the Ministry of Educa-
gallium or between antimony and antimony) as a stable tion, Culture, Sports, Science and Technology (MEXT) Japan
state. It is consequently considered that the free energy under “Grant-in-Aid for Scientific Research” and “Nanotech-
of the reaction for the phase separation GaSb → Ga+Sb nology Support Project”.
becomes negative under the excited states.
The phase stability will be controlled by kinetic process
under such a chemical equilibrium. At elevated tempera-
tures, recombination of the primary defects (i.e., vacancy
and interstitial) suppress phase transformations because
of the high mobility of defects. On the other hand, at
room temperature, the primary defects are frozen in the
compound crystal because of the low mobility of defects
and consequently the amorphization is induced by chemi-
cal disordering and short range atom displacements. How-
ever, in the range of the temperatures, at which vacancies
are mobile but interstitials are immobile, long range diffu-
sion can take place. According to the chemical equilibrium
mentioned above, the compositional fluctuation in the in-
terior of the particles which have the zincblende structure
occurs as a pre-stage of the phase separation. The gallium
atoms diffuse toward the surface and are enriched near the
surface, because the surface energy of solid gallium (i.e.,
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