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Submitted on 1 Jan 1987
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FIM AND ATOM-PROBE STUDY OF POLYMERS
T. Maruyama, Y. Hasegawa, T. Nishi, T. Sakurai
To cite this version:
T. Maruyama, Y. Hasegawa, T. Nishi, T. Sakurai. FIM AND ATOM-PROBE STUDY OF POLY- MERS. Journal de Physique Colloques, 1987, 48 (C6), pp.C6-269-C6-274. �10.1051/jphyscol:1987644�.
�jpa-00226849�
FIM A N D ATOM-PROBE STUDY O F POLYMERS
T. ~aru~arna*, Y. Hasegawa, T. ~ishi*and T. Sakurai
The Institute for Solid State Physics (ISSP), The University of Tokyo, Minato-ku, Tokyo, Japan
*Department of Applied Physics, The University of Tokyo, Bunkyo-ku, Tokyo, Japan
Abstract - A field ion microscope makes it possible to observe the surface structures of metals and semiconductors at the atomic level and an atom-probe (FIM), which is a FIM with a mass spectrometer, has been widely used to study the chemical compositions of specimens.
Some attempts were made previously to visualize polymers such as biological molecules. They are usually insulators and, thus, are difficult to be imaged by the FIM. Furthermore, molecules are known to decompose or desorb before a needed high field is reached.
In the present study, we have used conductive plolymers (polypyrrole) as a specimen and have explored the possibility of visualizing a polymer with atomic resolution using an imaging atom-probe. The atom-probe analysis has also been carried out to detect monomer, dimer and trimer species.
An atom probe FIM is a very effective instrument to obtain information on the surface structures of metals or semiconductors.
However, its applications for polymer samples have been limited.
One of the reasons for this situation is that it is difficult to prepare a needle shape sample of polymers. Besides, most polymers are insulators and, thus, an application of high voltage-nanosecond pulses needed to trigger field evaporation events is difficult for these samples due to the charging effect.
In recent years, conducting polymers have attracted much
attention because of many unique physical phenomena, such as soliton or bipolaron type excitations./l/ Even though these polymers
themselves are perfect insulators and their electric conductivities are below 10-lo ( a cm)-I, doping procedures improve their
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987644
C6-270 JOURNAL DE PHYSIQUE
conductivities by many orders of magnitude, even up to 100 ( ~cm)-l.
This conductivity is comparable to that of a doped semiconductor and, therefore, the atom probe is expected to be amanable to the microscopic analysis of these conducting polymers.
In the present study we used polypyrrole as a sample.
Polypyrrole is one of the most popular heterocyclic conducting polymers. Our attempt of obtainging a high quality FI image or desorption image has not been successful so far. However, the atom-probe analysis has met with some success. We have succeeded in detecting polymer specimens deposited on the tungsten tip surface by the monomer unit for the first time./2/
Polypyrrole used in the present study, is a pyrrole based conducting polymer. Fig. 1 (a) shows the pyrrole monomer. One nitrogen and four carbon atoms form a hetrocycle. When an adequate voltage is applied between the electrodes in the pyrrole solution, monomers are electro-chemically oxidized to become radical ions.
Sequential polymerization follows.(Fig. 1 b) During these processes oxidation of the polymer by dopants ions takes place
simultaneously. Therefore, when the dopant is a perchlorate, the synthesized polypyrrole results in a highly doped and high
conductivity polymer.(Fig. 1 c)
(a) Pyrrole monomer H
(b) polymerization
n (CIHIN) + (CaliiN) ,+On c - + 2 n H o
( c ) doping reaction
(CdHaN) . + y n C 1 0 4 - 4 ( (CIHSN) ** ( C I 01.) .) .+y n s-
FIG. 1. Characteristics of Polypyrrole.
FIM) as the anode and a platinum plate as the cathode. The solution consists of pyrrole, lithium perchrolate and nitrobenzen. Applying 4 V for 90 seconds, the surface of the tungsten F I tip is covered with polypyrrole amorphous film. This tip is used as the polymer specimen in the atom-probe study. The tungsten tip has been observed by the FIM before depositing polymers in order to assure that this tip had an adequate shape and low evapolation field.
Fig. 2 shows the atom probe FIM used for the present analysis.
The details of the system can be found in previous publications./3/
One unique feature worth noting in the polymer study is that both the straight-type time-of-flight and focused-type time-of-flight modes can be used almost simultaneously so that umambiguous analysis of the atom-probe mass spectra can be carried out, even though the specimens are not ideally conductive.
CRVO PUMP
bNNEL R A T E . SCREEN.
cnmNEL RATE AN0 SCREEN I ROTATABLE I
~ ~ c m o s ~ a n c LENS
TOP VIEW ! I TIP HOLDER i
Fig. 2. Schematic of the focusing-type ToF atom-probe FIM.
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Fig. 3 is the atom-probe data of the polypyrrole. An
interesting observation is that there are three peaks at m/n = 65, 140 and 210, equally spaced by approximetely 70. Taking into
account that the polypyrrole chain structure consists of pyrrole monomers of mass 67 amu, it is reasonable to assume that these three peaks are pyrrole monomer, dimer and trimer, respectively. There are few signals after the trimer peak. The observations that there are many small peaks and that each peak, especially at the higher mass range, has a broad mass distribution are commom in the case of high purity semiconductor specimens, and may be explained by charging effects of the surface polymers.
M A S S TO C H A R G E R A T I O ( r n / n >
Fig. 3. Mass histogram of polypyrrole deposited on a W emitter. Pyrrole monomer, dimer and trimer are detected.
When the applied voltage increases during the analysis, all the polymers are field desorbed and the bare tungsten surface appears at the field much lower than the evaporation field of tungsten.. Thus, the field ranges for detecting polypyrrole related ion signals and tungsten ions are clearly different and no mistaking of W and pyrrole signals takes place in the present case, even though m/nls are similar for pyrrole monomer and
w3+
ions. An example is shown in Fig. 4. Fig. 4(a) is the data for the polypyrrole and Fig. 4(b) .is the data for the tungsten surface after the polypyrrole film is completely evaporated. In Fig. 4(b), the peaks around 45 are ofw4+
ions and those around 60 are ofw3+
ions. Fig. 4(a) does show high noise levels, while Fig. 4(b) has no noise at all other than H+, H2+, and He+. We also no e that the abundance and positions of individual peaks at m4n = 45 and 60 coinclde perfectly with tungsten isotopes.F i g . 4 . mass s p e c t r a of p o l y p y r r o l e ( a ) and s u b s t r a t e t u n g s t e n (b).
Pyrrole detection range in the atom-probe experiments can also be divided into two region. Fig. 5 shows the relationship between the number of trigger pulse and the number of ions detected during the sequential evaporation scheme. A broken line in the figure is for Hz+ + Hj+, and a broken line is for H + . In a low
voltage region, detected hydrogen ions are only H+ and in a high voltage region only Hz+ and H3+ are detected: a striking
difference between these two regions. Furthermore, the amounts of detected ions are very different. The number of H+ ions is
comparable to those of other ion signals, while the intensities of Hz+ and H3+ ions are extraordinally large. It is thought
that H+ is produced by decomposition of polymer and that Hz+
or H3+ ions are coming from the residual gas.
Similar differences in the signals are found even in the pyrrole related ions at the low and high field ranges. There is a large C+ peak at the lower field. Dimer and trimer peaks are observed only in the higher fiels range. The positions of these peaks are a slightly higher. The existence of C+ ions in the low voltage region supports our assumption that H+ ions are by the decomposition of polymer, since C and H co-exist and are bound
C6-274 JOURNAL DE PHYSIQUE
tightly to form a pyrrole ring. The fact that the mass of the dimers and trimers are silghtly larger than should be may be
explained by the adsorption of extra hydrogen atoms in the presence of a high field.
We note that the present result is strikingly different from that reported by Kato et a1./2/. Kato et al. did not detect any pyrrole species and almost all ion species detected by them contained oxygen, which should not be in polypyrrole polymer. A
further study is currently being carried out.
Fig. 5 . H and (Hz
+
H3) s i g n a l s are p l o t t e d against sequential t r i g g e r pulsed together with polypyrrole s i g n a l s . Note large (Hz+
H3)s i g n a l i n t e n s i t y .
REFERENCES
/ I / Yakushi, K., et al., J. Chem. Phys. 79 (1983) 4774.
/2/ Kato, H. et al., J. de Physiq. C7 ( 1 B 6 ) 434.
/3/ T. Sakurai & T. Hashizume, Rev. Sci. Instrum. 57 (1986) 236.
