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Scanning tunneling microscopy and spectroscopy study of Nd2−xCexCuO4−y
PHYSICA ELSEVIER
Physica C 282-287 (1997) 1503-1504
S c a n n i n g t u n n e l i n g m i c r o s c o p y a n d s p e c t r o s c o p y s t u d y o f Nd2.xCexCuO4.y B. Sustaa, R. Czajkaa, W. Sadowskiu and T. Klimczukb " Institute of Physics, Poznafl University of Technology, ul. Piotrowo 3, 60-965 Poznafl, Poland* u Department of Physics and Mathematics, Technical University of Gdafisk, ul. Majakowskiegoll/12, 80- 952 Gdaflsk, Poland
The results of the tunneling microscopy and spectroscopy (STM/STS as well as atomic force microscopy (AFM) measurements on the Nd2.xCexCuO4.y (NCCO) single crystals in ambient conditions are presented with emphasis on material problems. The detailed spectroscopy and characteristic features of reduced NCCO single crystals in normal state are also discussed.
1.1NTRODUCTION
2. E X P E R I M E N T A L
Tunneling spectroscopy is not a new technique and has been used long before the scanning tunneling microscope (STM) was discovered to examine properties of the tunneling junctions I. The development of the atomic resolution scanning tunneling microscopy (STM) and spectroscopy (STS) by Binnig, Rohrer, and co-workers 2 in 1981 has opened a new era of surface science and engineering. Combined STM/STS studies provide useful information about the relationship between surface-topographic and local electronic properties of Nd2.×CexCuO4.y "214" compounds. Since the invention of the atomic force microscope (AFM) by Binnig et al. 3, the insulating surfaces can be investigated on the atomic scale as well. In addition, local mechanical properties can be probed by force microscopy and spectroscopy. In this paper, we present the experimental results of the STM/STS and AFM studies on the reduced Nd2.xCexCuO4_y single crystal in normal state. We observe reproducible STM and AFM images of the surface in the air. Spectroscopic data showed that the ab-plane of the single crystals NCCO in normal state were semiconductors.
Room-temperature STM/AFM (OMICRON) has been used to study the topography and spectroscopic properties of the crystals. The STM images in constant current mode were taken immediately after cleaving the single crystals in ab-plane at the room temperature. The single crystals of Nd2.×CexCuO4.y used in the present experiments were grown by a flux method in alumina crucibles 4. The as-grown Nd2_xCexCuO4.y crystals are semiconductors and the reduction step is required to transform the materials to the superconducting phase. The superconductivity was achieved by annealed at 900 °C in Ar, and then cooled to room temperature.
3. RESULTS AND D I S C U S S I O N
Figure 1 shows an STM image (atomic image) of the cleaved surface at 300 K , which was taken in the condition of bias voltage V = 800 mV and tunneling current I = 0.2 nA. The atomic image of this figure is in qualitative agreement with that of a square-lattice, which seems to be of the CuO4 plane.
* Acknowledgments - This work was supported partly by Research Program through the Poznafi University o f Technology and by the KBN, under Contract No. 7 T08C 009 11. 0921-4534/97/$17,00 © Elsevier Science B.V. All rights reserved.
PII S0921-4534(97)00859-9
1504
B. Susta et aL IPhysica C 282-287 (1997) 1503-1504
have found out that the local I-V characteristics for reduced NCCO samples are asymmetric the conductance is higher for negative sample polarity (Fig. 3). .........
nA 4 0 . 0 0
' .........
!' . . . . . . . . .
I
' .....
; 7 ~
";
J
o.oo nm
-V2
o.oo nm
Figure 1. STM (3-D) image of Nd2.xCexCuO4.y surface taken at 300 K.
reduced
o.~
I
"
i,/i,e' //i'~ S/'
/ ,:r~, '
This STM image also indicates a modulation structure along the diagonal line of the square (the baxis). The wavelength of the electronic modula-tions is longer than the b-direction lattice constant (a=b=0.395 nm), indicating that these features are not due to individual atoms. This result suggests that the conduction electrons may tunnel into the orbitals of Cu 3dx2. y2 and O2p. on the CuO4 plane 5. The AFM top-view image of 2000x2000 nm ~ surface (not shown here) indicates stepped structures of a few hundred nanometers width and some spiral type structures at step edges. Topography measurements with higher magnification in the vertical direction (Fig. 2) showed the existence of growth spirals. The height of these structures was higher than the c parameter (c=1.207 nm) of the unit cell of NCCO. 15.00 nm 10.00 nrn 5,00 nm 5000.nOOmnm 4043 nrn 300 n m 200 nm 100 nm
v,
-t
--
i
/
//s,
. . . . . . /...,',,i I/ • ....... !. . . . . . . . .
-'=
-;t. ~
-1.oo
o. oo
1, oo
2. oo
V
Figure 3. Experimental I-V characteristics for reduced Nd2.xCexCuO4.ysample. They exhibit a characteristic semiconductor features at room temperature. The energy gap of the semiconductor, Eg in normal state for NCCO single crystals, is expressed in the following equation, Eg = l v , l + l v 2 1 . From Fig. 3 Vl and V2 are about 1.3 V and 0.7 V, respectively, so that Eg is estimated to be (2.0_+0.1) V. The relation between the microstructure of the crystals and the used growth technique have been studied using room-temperature STM/AFM. From scanning tunneling spectroscopy (STS) measurements we could estimate the width of the semiconducting gap of the reduced single crystals. We are firmly convinced that STS is a very useful technique to obtain information about the band gap on nanometer spatial scale. REFERENCES
I
~ _ ~
_~._~____.~__~_~-- ---x----~-a
500 nm
500 nm 0 nm
_~o.~
•.~S,~y
i~" ~
~
0 nm
100 n m
Figure 2. AFM 500x500 nm 2 (3-D) of the reduced NdE.xCexCuO4.y surface taken at 300 K. The individual step region with the structures of the growth spiral type. Investigations of the local surface electronic structure of the NCCO surface were realized by local tunneling spectroscopy measurements. We
1. E.L. Wolf, Principles of electron tunneling spectroscopy, Oxford Univ. Press, NY, 1985. 2. G. Binnig, H. Rohrer, Ch. Gerber, and E. Weibel, Appl. Phys. Lett., 40 (1982) 178. 3. G. Binnig, C. F. Quate, and Ch. Gerber, Phys. Rev. Lett., 56 (1986) 930. 4. W. Sadowski, H. Hegemann, M. Francois, H. Bill, M. Peter, E. Walker, K. Yvon, Physica C170 (1990) 103. 5. B. Susta, R. Czajka, W. Gordon, S. Szuba, and J. Rautuszkiewicz, Material Science and Engineering A217/218 (1996) 419.
Physica C 282-287 (1997) 1503-1504
S c a n n i n g t u n n e l i n g m i c r o s c o p y a n d s p e c t r o s c o p y s t u d y o f Nd2.xCexCuO4.y B. Sustaa, R. Czajkaa, W. Sadowskiu and T. Klimczukb " Institute of Physics, Poznafl University of Technology, ul. Piotrowo 3, 60-965 Poznafl, Poland* u Department of Physics and Mathematics, Technical University of Gdafisk, ul. Majakowskiegoll/12, 80- 952 Gdaflsk, Poland
The results of the tunneling microscopy and spectroscopy (STM/STS as well as atomic force microscopy (AFM) measurements on the Nd2.xCexCuO4.y (NCCO) single crystals in ambient conditions are presented with emphasis on material problems. The detailed spectroscopy and characteristic features of reduced NCCO single crystals in normal state are also discussed.
1.1NTRODUCTION
2. E X P E R I M E N T A L
Tunneling spectroscopy is not a new technique and has been used long before the scanning tunneling microscope (STM) was discovered to examine properties of the tunneling junctions I. The development of the atomic resolution scanning tunneling microscopy (STM) and spectroscopy (STS) by Binnig, Rohrer, and co-workers 2 in 1981 has opened a new era of surface science and engineering. Combined STM/STS studies provide useful information about the relationship between surface-topographic and local electronic properties of Nd2.×CexCuO4.y "214" compounds. Since the invention of the atomic force microscope (AFM) by Binnig et al. 3, the insulating surfaces can be investigated on the atomic scale as well. In addition, local mechanical properties can be probed by force microscopy and spectroscopy. In this paper, we present the experimental results of the STM/STS and AFM studies on the reduced Nd2.xCexCuO4_y single crystal in normal state. We observe reproducible STM and AFM images of the surface in the air. Spectroscopic data showed that the ab-plane of the single crystals NCCO in normal state were semiconductors.
Room-temperature STM/AFM (OMICRON) has been used to study the topography and spectroscopic properties of the crystals. The STM images in constant current mode were taken immediately after cleaving the single crystals in ab-plane at the room temperature. The single crystals of Nd2.×CexCuO4.y used in the present experiments were grown by a flux method in alumina crucibles 4. The as-grown Nd2_xCexCuO4.y crystals are semiconductors and the reduction step is required to transform the materials to the superconducting phase. The superconductivity was achieved by annealed at 900 °C in Ar, and then cooled to room temperature.
3. RESULTS AND D I S C U S S I O N
Figure 1 shows an STM image (atomic image) of the cleaved surface at 300 K , which was taken in the condition of bias voltage V = 800 mV and tunneling current I = 0.2 nA. The atomic image of this figure is in qualitative agreement with that of a square-lattice, which seems to be of the CuO4 plane.
* Acknowledgments - This work was supported partly by Research Program through the Poznafi University o f Technology and by the KBN, under Contract No. 7 T08C 009 11. 0921-4534/97/$17,00 © Elsevier Science B.V. All rights reserved.
PII S0921-4534(97)00859-9
1504
B. Susta et aL IPhysica C 282-287 (1997) 1503-1504
have found out that the local I-V characteristics for reduced NCCO samples are asymmetric the conductance is higher for negative sample polarity (Fig. 3). .........
nA 4 0 . 0 0
' .........
!' . . . . . . . . .
I
' .....
; 7 ~
";
J
o.oo nm
-V2
o.oo nm
Figure 1. STM (3-D) image of Nd2.xCexCuO4.y surface taken at 300 K.
reduced
o.~
I
"
i,/i,e' //i'~ S/'
/ ,:r~, '
This STM image also indicates a modulation structure along the diagonal line of the square (the baxis). The wavelength of the electronic modula-tions is longer than the b-direction lattice constant (a=b=0.395 nm), indicating that these features are not due to individual atoms. This result suggests that the conduction electrons may tunnel into the orbitals of Cu 3dx2. y2 and O2p. on the CuO4 plane 5. The AFM top-view image of 2000x2000 nm ~ surface (not shown here) indicates stepped structures of a few hundred nanometers width and some spiral type structures at step edges. Topography measurements with higher magnification in the vertical direction (Fig. 2) showed the existence of growth spirals. The height of these structures was higher than the c parameter (c=1.207 nm) of the unit cell of NCCO. 15.00 nm 10.00 nrn 5,00 nm 5000.nOOmnm 4043 nrn 300 n m 200 nm 100 nm
v,
-t
--
i
/
//s,
. . . . . . /...,',,i I/ • ....... !. . . . . . . . .
-'=
-;t. ~
-1.oo
o. oo
1, oo
2. oo
V
Figure 3. Experimental I-V characteristics for reduced Nd2.xCexCuO4.ysample. They exhibit a characteristic semiconductor features at room temperature. The energy gap of the semiconductor, Eg in normal state for NCCO single crystals, is expressed in the following equation, Eg = l v , l + l v 2 1 . From Fig. 3 Vl and V2 are about 1.3 V and 0.7 V, respectively, so that Eg is estimated to be (2.0_+0.1) V. The relation between the microstructure of the crystals and the used growth technique have been studied using room-temperature STM/AFM. From scanning tunneling spectroscopy (STS) measurements we could estimate the width of the semiconducting gap of the reduced single crystals. We are firmly convinced that STS is a very useful technique to obtain information about the band gap on nanometer spatial scale. REFERENCES
I
~ _ ~
_~._~____.~__~_~-- ---x----~-a
500 nm
500 nm 0 nm
_~o.~
•.~S,~y
i~" ~
~
0 nm
100 n m
Figure 2. AFM 500x500 nm 2 (3-D) of the reduced NdE.xCexCuO4.y surface taken at 300 K. The individual step region with the structures of the growth spiral type. Investigations of the local surface electronic structure of the NCCO surface were realized by local tunneling spectroscopy measurements. We
1. E.L. Wolf, Principles of electron tunneling spectroscopy, Oxford Univ. Press, NY, 1985. 2. G. Binnig, H. Rohrer, Ch. Gerber, and E. Weibel, Appl. Phys. Lett., 40 (1982) 178. 3. G. Binnig, C. F. Quate, and Ch. Gerber, Phys. Rev. Lett., 56 (1986) 930. 4. W. Sadowski, H. Hegemann, M. Francois, H. Bill, M. Peter, E. Walker, K. Yvon, Physica C170 (1990) 103. 5. B. Susta, R. Czajka, W. Gordon, S. Szuba, and J. Rautuszkiewicz, Material Science and Engineering A217/218 (1996) 419.