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Experiments with fine-particle beams
Ultramicroscopy 20 (1986) 29-32 North-Holland, Amsterdam
29
EXPERIMENTS WITH FINE-PARTICLE BEAMS
Ryozi UYEDA Research Development Corporation of Japan, c/o Department of Physics, Meijo University, Tenpaku-ku, Nagoya 468, Japan Received 13 June 1986
A new technique for the introduction of fine-particle beams into the electron microscope is described and methods for characterising some of the properties of the beam are briefly summarized.
1. Introduction
2. Experimental details
Experiments with the so-called gas-evaporation technique started in my laboratory in 1962 [1]. Since then, smoke particles of a number of metals and semiconductors, with sizes in the range 10 nm to 1 #m, have been studied by electron diffraction and electron microscopy [2]. Various polyhedra have been seen in these smoke particles and, in some cases, new crystal structures not known for bulk materials were observed. The recent experiments with fine-particle beam summarized here are a continuation of this old work. Smoke particles produced by gas evaporation are introduced into the vacuum after a few stages of differential pumping. This fine-particle beam differs from the usual cluster beam [3] by the size of the particles. Our particles are usually larger than 5 nm whereas clusters are generally much smaller than 5 nm (at most, a few thousand atoms). The initial objective of these studies of fine-particle beams was to introduce smoke particles into the electron microscope without exposure to the atmosphere, and this has been achieved. Various experiments can then be done with the beams produced by this technique. After describing some aspects of the experimental arrangements for producing and characterizing the beams, several experiments which have been carried out at my suggestion by Ichihashi [4] and Kusunoki and Ichihashi [5] are briefly summarized.
Generation of fine-particle beam. The schematic drawn in fig. 1 shows how the beam is generated. Smoke produced by gas evaporation is effused from a nozzle into low vacuum. Most of the gas is pumped out but some of the smoke particles move forward through the aperture, generating a slightly divergent beam of particles. A beam of silver particles with sizes of about 10 nm was generated first. The intensity of this beam was so strong that a silvered disc was visible on the glass plate within several seconds from the opening of the shutter. Estimation of speed by gravity effect. In order to estimate the speed of the particles, two slits with diameters of 0.5 mm were placed in the divergent Nozzle
t
< -~1
I.~[
/Aperture( Irnm~ )
i ll0,Tor r
o'oio'o,. 5ram
/~He or Ar Fig. 1. Schematic diagram showing the experimental setup for the generation of fine-particle beams.
0304-3991/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
R. Uyeda / Experiments withfine-particle beams
30
Glass )late Aperture 51
x1~IIA 5
52
1
o
1
5'0
0
~_~0.5 mm
I()0o/O / I I5 ~°~
-S'
/o o
Fig. 2. Schematic diagram showing the effect of gravity of the fine-particle beam (exaggerated).
beam. Because of gravity, the path of the fine beam passing through the slits was not straight but had a parabolic trajectory, shown exaggerated in fig. 2. For the geometry given in the figure, the distance OB was 0.5 mm, which corresponded to a particle speed of 120 m/s.
Charging of particles by electron bombardment. The beam of silver particles, with an average size of 10 nm, was bombarded by electrons and then deflected by an electric field, as represented in fig. 3. Positively charged, neutral and negatively charged particles were found in the beam. The total beam current was measured by a Faraday cage at various bombardment energies, and fig. 4 shows an example of the results. The maximum found at 150 V presumably corresponds to the maximum efficiency for secondary electron emission in particles of about 10 nm size. It is interesting that the value for an extended flat surface is 800 V and this difference would seem to be worthy of further study.
Measurement of speed by time-of-flight technique. The beam of silver particles was chopped by the deflector (fig. 3) and the particle speed was measured by the time-of-flight technique. This result turned out to be 200 m / s which is substantially higher than the 120 m / s obtained using the gravity method. Various experimental difficulties Bombarder Deflector
l
>
LJ
l
_1_____) J
Detector
I
Pump Fig. 3. Representation of the apparatus used for the electron bombardment, deflection and detection of a fine-particle beam.
/
Fig. 4. Total measured current for a fine-particle silver beam bombarded at various energies.
are clearly still present which prevent accurate results from being obtained.
Direct introduction of smoke particles into the electron microscope. The arrangement for introducing the smoke particles into the microscope is shown in fig. 5. After various technical problems, particularly serious mechanical vibrations, had been overcome, silicon particles were introduced. Once silicon is exposed to air, it becomes covered with amorphous silica layer to about 2 nm in thickness. By direct introduction, the oxide thickness was reduced by about a factor of two. However, complete elimination of the oxide layer remains a difficult task which needs to be overcome at the next stage of development. Heat treatment of fine particles. Annealing of the particles in their isolated state is often required since, when they are heated as an ensemble in a crucible, they usually sinter together. For example, a fine powder of a-A120 a cannot be produced from ~/-A1203 because of the sintering which occurs during the heat treatment. Two Microscope column
Nozzle
I " -$k'mm'rs
I L3
_J'
'
III
#t 7"1-I I ' LIJ
Heor Ar Fig. 5. Schematic diagram showmg the dkect introduction of smoke particles into the transmission electron microscope.
R. Uyeda / Experiments with fine-particle beams
31
Fig. 6. Electron diffraction patterns from fine-particle beams of iron. Left side is from a room temperature sample and all tings correspond to particles in the a-phase (bcc). Right side comes from a beam heated to 1000°C and then quenched to liquid nitrogen temperature. Note the extra fcc (7-phase) rings.
methods are being evaluated for this purpose. The first involves passing the fine-particle beam in vacuum through a heated tube, and the second is to pass the smoke, i.e., the gas containing fine particles, through a heated tube. In the first method, the particles are heated by radiation; in the second, they are heated by radiation and conduction. The first method has been applied to beams of magnesium particles [4]. The effect of heating was apparent by the changes in the particle morphology but no systematic analysis of the effects of the temperature rise has yet been done. The second method has been apphed to smokes of iron which were in the a-phase (bcc) at room temperatures [5]. The smoke was heated up to about 1000°C, where the 7-phase (fcc) was stable, and it was then quenched by being blown against a surface cooled with liquid nitrogen. The deposited iron particles were then examined by electron diffraction and electron microscopy and it was found that the particle size was about 20 nm. Fig. 6 compares electron diffraction patterns from samples which were untreated (left) and heat-
treated (right). This establishes that about 10% of the quenched particles were 7-phase. This result is of considerable importance because particles made in this way are of great interest in the study of magnetism.
3. Conclusion
These experiments with fine-particle beams are only just at a preliminary stage. However, it already appears that the technique will be very productive as a means for the structural and morphological characterization of fine particles. References [1] R. Uyeda, J. Crystal Growth 24/25 (1974) 69. [2] R. Uyeda, in: Morphology of Crystals, Ed. I. Sunagawa (Terra Publ./Reidel, Dordrecht, 1986). [3] K. Satfler, J. Muhlbach and E. Recknagel, Phys. Rev. Letters 45 (1980) 821. [4] T. Ichihashi, Japan. J. Appl. Phys. 25 (1986) 1247. [5] M. Kusunoki and T. Ichihashi, Japan. J. Appl. Phys. 25 (1986) 31.
29
EXPERIMENTS WITH FINE-PARTICLE BEAMS
Ryozi UYEDA Research Development Corporation of Japan, c/o Department of Physics, Meijo University, Tenpaku-ku, Nagoya 468, Japan Received 13 June 1986
A new technique for the introduction of fine-particle beams into the electron microscope is described and methods for characterising some of the properties of the beam are briefly summarized.
1. Introduction
2. Experimental details
Experiments with the so-called gas-evaporation technique started in my laboratory in 1962 [1]. Since then, smoke particles of a number of metals and semiconductors, with sizes in the range 10 nm to 1 #m, have been studied by electron diffraction and electron microscopy [2]. Various polyhedra have been seen in these smoke particles and, in some cases, new crystal structures not known for bulk materials were observed. The recent experiments with fine-particle beam summarized here are a continuation of this old work. Smoke particles produced by gas evaporation are introduced into the vacuum after a few stages of differential pumping. This fine-particle beam differs from the usual cluster beam [3] by the size of the particles. Our particles are usually larger than 5 nm whereas clusters are generally much smaller than 5 nm (at most, a few thousand atoms). The initial objective of these studies of fine-particle beams was to introduce smoke particles into the electron microscope without exposure to the atmosphere, and this has been achieved. Various experiments can then be done with the beams produced by this technique. After describing some aspects of the experimental arrangements for producing and characterizing the beams, several experiments which have been carried out at my suggestion by Ichihashi [4] and Kusunoki and Ichihashi [5] are briefly summarized.
Generation of fine-particle beam. The schematic drawn in fig. 1 shows how the beam is generated. Smoke produced by gas evaporation is effused from a nozzle into low vacuum. Most of the gas is pumped out but some of the smoke particles move forward through the aperture, generating a slightly divergent beam of particles. A beam of silver particles with sizes of about 10 nm was generated first. The intensity of this beam was so strong that a silvered disc was visible on the glass plate within several seconds from the opening of the shutter. Estimation of speed by gravity effect. In order to estimate the speed of the particles, two slits with diameters of 0.5 mm were placed in the divergent Nozzle
t
< -~1
I.~[
/Aperture( Irnm~ )
i ll0,Tor r
o'oio'o,. 5ram
/~He or Ar Fig. 1. Schematic diagram showing the experimental setup for the generation of fine-particle beams.
0304-3991/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
R. Uyeda / Experiments withfine-particle beams
30
Glass )late Aperture 51
x1~IIA 5
52
1
o
1
5'0
0
~_~0.5 mm
I()0o/O / I I5 ~°~
-S'
/o o
Fig. 2. Schematic diagram showing the effect of gravity of the fine-particle beam (exaggerated).
beam. Because of gravity, the path of the fine beam passing through the slits was not straight but had a parabolic trajectory, shown exaggerated in fig. 2. For the geometry given in the figure, the distance OB was 0.5 mm, which corresponded to a particle speed of 120 m/s.
Charging of particles by electron bombardment. The beam of silver particles, with an average size of 10 nm, was bombarded by electrons and then deflected by an electric field, as represented in fig. 3. Positively charged, neutral and negatively charged particles were found in the beam. The total beam current was measured by a Faraday cage at various bombardment energies, and fig. 4 shows an example of the results. The maximum found at 150 V presumably corresponds to the maximum efficiency for secondary electron emission in particles of about 10 nm size. It is interesting that the value for an extended flat surface is 800 V and this difference would seem to be worthy of further study.
Measurement of speed by time-of-flight technique. The beam of silver particles was chopped by the deflector (fig. 3) and the particle speed was measured by the time-of-flight technique. This result turned out to be 200 m / s which is substantially higher than the 120 m / s obtained using the gravity method. Various experimental difficulties Bombarder Deflector
l
>
LJ
l
_1_____) J
Detector
I
Pump Fig. 3. Representation of the apparatus used for the electron bombardment, deflection and detection of a fine-particle beam.
/
Fig. 4. Total measured current for a fine-particle silver beam bombarded at various energies.
are clearly still present which prevent accurate results from being obtained.
Direct introduction of smoke particles into the electron microscope. The arrangement for introducing the smoke particles into the microscope is shown in fig. 5. After various technical problems, particularly serious mechanical vibrations, had been overcome, silicon particles were introduced. Once silicon is exposed to air, it becomes covered with amorphous silica layer to about 2 nm in thickness. By direct introduction, the oxide thickness was reduced by about a factor of two. However, complete elimination of the oxide layer remains a difficult task which needs to be overcome at the next stage of development. Heat treatment of fine particles. Annealing of the particles in their isolated state is often required since, when they are heated as an ensemble in a crucible, they usually sinter together. For example, a fine powder of a-A120 a cannot be produced from ~/-A1203 because of the sintering which occurs during the heat treatment. Two Microscope column
Nozzle
I " -$k'mm'rs
I L3
_J'
'
III
#t 7"1-I I ' LIJ
Heor Ar Fig. 5. Schematic diagram showmg the dkect introduction of smoke particles into the transmission electron microscope.
R. Uyeda / Experiments with fine-particle beams
31
Fig. 6. Electron diffraction patterns from fine-particle beams of iron. Left side is from a room temperature sample and all tings correspond to particles in the a-phase (bcc). Right side comes from a beam heated to 1000°C and then quenched to liquid nitrogen temperature. Note the extra fcc (7-phase) rings.
methods are being evaluated for this purpose. The first involves passing the fine-particle beam in vacuum through a heated tube, and the second is to pass the smoke, i.e., the gas containing fine particles, through a heated tube. In the first method, the particles are heated by radiation; in the second, they are heated by radiation and conduction. The first method has been applied to beams of magnesium particles [4]. The effect of heating was apparent by the changes in the particle morphology but no systematic analysis of the effects of the temperature rise has yet been done. The second method has been apphed to smokes of iron which were in the a-phase (bcc) at room temperatures [5]. The smoke was heated up to about 1000°C, where the 7-phase (fcc) was stable, and it was then quenched by being blown against a surface cooled with liquid nitrogen. The deposited iron particles were then examined by electron diffraction and electron microscopy and it was found that the particle size was about 20 nm. Fig. 6 compares electron diffraction patterns from samples which were untreated (left) and heat-
treated (right). This establishes that about 10% of the quenched particles were 7-phase. This result is of considerable importance because particles made in this way are of great interest in the study of magnetism.
3. Conclusion
These experiments with fine-particle beams are only just at a preliminary stage. However, it already appears that the technique will be very productive as a means for the structural and morphological characterization of fine particles. References [1] R. Uyeda, J. Crystal Growth 24/25 (1974) 69. [2] R. Uyeda, in: Morphology of Crystals, Ed. I. Sunagawa (Terra Publ./Reidel, Dordrecht, 1986). [3] K. Satfler, J. Muhlbach and E. Recknagel, Phys. Rev. Letters 45 (1980) 821. [4] T. Ichihashi, Japan. J. Appl. Phys. 25 (1986) 1247. [5] M. Kusunoki and T. Ichihashi, Japan. J. Appl. Phys. 25 (1986) 31.