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Scanning tunneling microscopy of metal surfaces in air
Surface Science 181 (1987) 119-125 North-Holland, Amsterdam
119
SCANNING TUNNELING MICROSCOPY OF METAL SURFACES IN AIR
S. MORITA Research institute
of EIeetrical C~mmunica~i#n,
Tohoku university,
Sendai 980, Japan
T. OKADA Oiympur Optical Co., Ltd., fshikawa-cho
Y. ISHIGAME
2951, Hachioji 192, Japan
and N. MIKOSHIBA
Research Institute of Elecirical Communication,
Received
14 July 1986, accepted
for publication
Tohoku Unillersity, Sendai 980, Japan
30 July 1986
We have constructed a scanning tunneling microscope (STM) for operation in air. We obtained surface images of a commercial titanium (Ti) plate, a barrier layer of anodic aluminum oxide (Also,) and platinum (Pt) fine particles supported on a TiO,/Ti plate. We found the following results: (1) The resolution of our STM is better than 10 A in air. (2) The surface image of the barrier layer has a somewhat complicated structure compared with hexagonal cells observed by SEM. (3) The surface of Pt fine particles is flat and shows a sharp and deep cliff down to the TiOZ/Ti plate.
1. In~~ucti~n
A scanning tunneling microscope (STM) can provide surface topographic images down to the atomic scale [l] and spectroscopic information on the surface density of electronic states [2]. In contrast to electron microscopes which can be operated only in vacuum, the STM has been operated even in air, in gaseous environment and in liquid as well as in vacuum [3]. These unique features make STM suitable to investigate chemical processes such as catalysis and electrochemistry. In the present experiments, we investigated, in air, surfaces of a commercial titanium (Ti) plate, a barrier layer of electrochemically treated, i.e., anodic aluminum oxide (A1203) and platinum (Pt) fine particles as a catalytic agent supported on a TiO,/Ti plate. ~39-6028/87/$03.50 Q Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
120
2. Construction
S. Moritu et al. / STM of metal surfaces in air
of the STM
As a rough positioning mechanism in our STM, we used a simple differential micrometer. We added a three-dimensional (3D) positioning mechanism in addition to a 3D-scanning mechanism as shown in fig. 1. We made these two sets of 3D-translators using small piezoelectric cubes interconnected by stainless-steel pieces [4]. We calibrated piezoelectric constants of these cubes using a stylus profilometer and obtained piezoelectric constants of 3 A/V for 3D-scanner elements and 8 A/V for 3D-positioner elements, respectively. These calibrated values agreed with nominal values of the piezoelec$c conelements and 6.4 A/V for stants d,, which were 3 A/V for 3D-scanner 3D-positioner elements. For damping of vibrations, we used two stages of commercial air suspension vibration isolators in addition to a stack of steel plates with pieces of O-rings [5]. For shieldings of acoustic and electrical noises, we used glass wool and magnetically shielded boxes, respectively. Here, glass wool also works as a heat insulator. We placed a voltage follower for a tunneling bias, a current-to-voltage converter and a preamplifier near the STM to reduce the noise. We used a logarithmic amplifier to make the feedback-loop gain constant. Using this STM, we measured STM images of metal surfaces in air by applying a tunneling voltage from 30 mV up to 6 V.
Stainless
3D-SCANNER
Ypz
Steel
3D-POSITIONER
Fig. 1. Structure of 3D-scanner and 3D-positioner
in our STM.
S. Morita et al. / STM of metal surfaces in air
121
3. Experimental results and discussion 3.1. STM images of a commercial Ti plate As shown in fig. 2a, the surface of a commercial Ti plate was smooth, though we found several hills and valleys. From these images, we estimated the resolution of our STM to be better than 10 A in air. After rather strong contacts using the differential micrometer, we found a larger structure of - 80 A as shown in fig. 2b, where a valley was formed by the contacts and where the image of the valley seemed to be unstable. We conjectured that an oxidation process at this valley on the Ti surface was in rapid progress which caused a fluctuation of the tunneling current [6]. We also investigated current-voltage characteristics (I- I/ curves) [7]. We found a strong nonlinearity and a clear hysteresis of I-V curves. 3.2. STM images of a barrier layer of anodic aluminum oxide It is well-known that porous anodic aluminum oxide films can be fabricated by anodizing in acid electrolytes such as sulfuric acid, oxalic acid or phosphoric acid [8]. As shown in fig. 3, the anodic film on an aluminum substrate consists of two regions, the so-called porous layer and the barrier layer. The porous layer consists of close-packed cells of oxide, predominantly hexagonal in shape, each containing a single pore formed perpendicularly to the surface of the aluminum substrate. The barrier layer is a thin compact inner region lying adjacent to the aluminum and consists of predominantly hexagonal cells. Each is a semi-sphere in the shape as shown in fig. 3.
Fig. 2. STM images of a commercial Ti plate. From (a), we estimated the resolution of the STM to be better than 10 A in air; (b) was obtained after rather strong contacts using the differential micrometer.
122 aluminum
Fig. 3. Structure
Fig. 4 STM images of a barrier
of anodic
layer of anodic
aluminum
aluminum
oxide (AlzO,)
oxide prepared
with a 20 V cell voltage.
S. Morita et al. / STM of metal surfaces in air
123
We measured STM images of the surface of the barrier layer by evaporating 300 A Pt-Pd thin films after the removal of the Al substrate using a 20 wt% HCl, O.lM CuCl, solution. For the anodization of Al, we used sulfuric acid (20 wtW) as the acid electrolytes. The cell voltage controlled by a DC power supplier was 20 V. From these conditions, . we expected a pore diameter region, D z.500 A [9]. 2r0 = 100 A and a total diameter, of semi-spherical These preparation methods were reported in a previous paper [9]. Fig. 4 shows a series of STM images measured by changing the positioning voltage VY,in the y-direction. In this figure, we can see several domains which have a nearly circular shape with a diameter D = 500 A consistent with the expected value. However, the surface of domains is flat and the depth of domain walls is shallow compared with the expected value of D/2 - 250 A. We attributed these results to the thick Pt-Pd film. In fact, by evaporating a thinner Pt-Pd film, we obtained a more spherical surface and a deeper boundary between domains [lo]. As shown in fig. 4, we also found several additional structures which could not be correlated with the domain boundaries. As we found such a structure even in a sample with a thinner Pt-Pd film, we conjectured that a part of such a structure corresponds to a grain boundary of the evaporated Pt-Pd thin film. Using fig. 4, we evaluated the performance of 3D-positioner by measuring the displacement of STM images. As shown in fig. 5, we found that the displacement of STM images shows a hysteresis as a function of the positioning voltage vY, though an averaged piezoelectric constant of - 7 A/V agreed with the nominal and calibrated values of d,,. In fig. 4, we also found that the STM image drifted by about 200 A in the x-direction in 16 min. At the best condition, the drift both in the x-direction and in the z-direction was less than a few angstroms in a minute even in air.
Fig. 5. Displacement of STM images as a function of a positioning voltage V,, in the y-direction.
W
fd
4OA x Fig. 6. STM
images
of Pt fine particles supported on a TiO,/Ti location different from (a).
plate:
(b) shows
an image
at
3.3. STM images of Pt fine particles supporter on a TiO, / Ti plate
Pt fine particles on a thin TiO, layer on a Ti plate were prepared by depositing a mist of an aqueous solution of hexachloroplatinic acid produced by an ultrasonic humidifier and reducing it by hydrogen at 300°C. Fig. 6 shows STM images of this sample. In fig. 6a, we found a very flat region and a very deep cliff of - 400 A which is much deeper than hills and valleys of fig. 2b in the TiO,/Ti plate without Pt fine particles. Therefore, we concluded that the surface of Pt fine particles is flat and have a sharp and deep cliff down to the TiO,/Ti plate. We also found several steps at the flat surfaces of Pt fine particles as shown in fig. 6b. We conjectured that these steps correspond to boundaries between adherent Pt fine particles.
4. Summary
We have constructed a scanning tunneling microscope with a 3D-positioning mechanism in addition to a 3D-scanning mechamism for operation in air. We investigated surfaces of a commercial Ti plate, a barrier layer of anodic aluminum oxide and Pt fine particles supported on the TiOJTi plate. Our results present experimental support for the possibility that we can investigate the metal surface with STM even at atmospheric pressure.
S. Morita et al. / STM of metal surfaces in air
125
Acknowledgments The authors are grateful to Professor K. Itaya for the kind supply of the samples of anodic aluminum oxide. We are also grateful to Dr. M. Komiyama for the kind supply of the samples of Pt fine particles supported on the TiO,/Ti plate.
References [l] [2] [3] [4] [5] [6] [7] [8] [9] [lo]
G. Binmg, H. Rohrer, Ch. Gerber and E. Weibel, Phys. Rev. Letters 50 (1983) 120. R. Garcia, J.J. Saenz and N. Garcia, Phys. Rev. B33 (1986) 4439. R. Sonnenfeld and P.K. Hansma, Science 232 (1986) 211. G.F.A. van de Walle, J.W. Gerritsen, H. van Kempen and P. Wyder, Rev. Sci. Instr. 56 (1985) 1573. Ch. Gerber, G. Binnig, H. Fuchs, 0. Marti and H. Rohrer, Rev. Sci. Instr. 57 (1986) 221. A. Bryant, D.P.E. Smith and C.F. Quate, Appl. Phys. Letters 48 (1986) 832. S. Morita, T. Okada, Y. Ishigame, C. Sato and N. Mikoshiba, Japan. J. Appl. Phys. 25 (1986) L516. J.W. Diggle, T.C. Downie and C.W. Goulding, Chem. Rev. 69 (1969) 365. K. Itaya, S. Sugawara, K. Arai and S. Saito, J. Chem. Eng. Japan 17 (1984) 514. S. Morita, K. Itaya and N. Mikoshiba, Japan. J. Appl. Phys. 25 (1986) L743.
119
SCANNING TUNNELING MICROSCOPY OF METAL SURFACES IN AIR
S. MORITA Research institute
of EIeetrical C~mmunica~i#n,
Tohoku university,
Sendai 980, Japan
T. OKADA Oiympur Optical Co., Ltd., fshikawa-cho
Y. ISHIGAME
2951, Hachioji 192, Japan
and N. MIKOSHIBA
Research Institute of Elecirical Communication,
Received
14 July 1986, accepted
for publication
Tohoku Unillersity, Sendai 980, Japan
30 July 1986
We have constructed a scanning tunneling microscope (STM) for operation in air. We obtained surface images of a commercial titanium (Ti) plate, a barrier layer of anodic aluminum oxide (Also,) and platinum (Pt) fine particles supported on a TiO,/Ti plate. We found the following results: (1) The resolution of our STM is better than 10 A in air. (2) The surface image of the barrier layer has a somewhat complicated structure compared with hexagonal cells observed by SEM. (3) The surface of Pt fine particles is flat and shows a sharp and deep cliff down to the TiOZ/Ti plate.
1. In~~ucti~n
A scanning tunneling microscope (STM) can provide surface topographic images down to the atomic scale [l] and spectroscopic information on the surface density of electronic states [2]. In contrast to electron microscopes which can be operated only in vacuum, the STM has been operated even in air, in gaseous environment and in liquid as well as in vacuum [3]. These unique features make STM suitable to investigate chemical processes such as catalysis and electrochemistry. In the present experiments, we investigated, in air, surfaces of a commercial titanium (Ti) plate, a barrier layer of electrochemically treated, i.e., anodic aluminum oxide (A1203) and platinum (Pt) fine particles as a catalytic agent supported on a TiO,/Ti plate. ~39-6028/87/$03.50 Q Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
120
2. Construction
S. Moritu et al. / STM of metal surfaces in air
of the STM
As a rough positioning mechanism in our STM, we used a simple differential micrometer. We added a three-dimensional (3D) positioning mechanism in addition to a 3D-scanning mechanism as shown in fig. 1. We made these two sets of 3D-translators using small piezoelectric cubes interconnected by stainless-steel pieces [4]. We calibrated piezoelectric constants of these cubes using a stylus profilometer and obtained piezoelectric constants of 3 A/V for 3D-scanner elements and 8 A/V for 3D-positioner elements, respectively. These calibrated values agreed with nominal values of the piezoelec$c conelements and 6.4 A/V for stants d,, which were 3 A/V for 3D-scanner 3D-positioner elements. For damping of vibrations, we used two stages of commercial air suspension vibration isolators in addition to a stack of steel plates with pieces of O-rings [5]. For shieldings of acoustic and electrical noises, we used glass wool and magnetically shielded boxes, respectively. Here, glass wool also works as a heat insulator. We placed a voltage follower for a tunneling bias, a current-to-voltage converter and a preamplifier near the STM to reduce the noise. We used a logarithmic amplifier to make the feedback-loop gain constant. Using this STM, we measured STM images of metal surfaces in air by applying a tunneling voltage from 30 mV up to 6 V.
Stainless
3D-SCANNER
Ypz
Steel
3D-POSITIONER
Fig. 1. Structure of 3D-scanner and 3D-positioner
in our STM.
S. Morita et al. / STM of metal surfaces in air
121
3. Experimental results and discussion 3.1. STM images of a commercial Ti plate As shown in fig. 2a, the surface of a commercial Ti plate was smooth, though we found several hills and valleys. From these images, we estimated the resolution of our STM to be better than 10 A in air. After rather strong contacts using the differential micrometer, we found a larger structure of - 80 A as shown in fig. 2b, where a valley was formed by the contacts and where the image of the valley seemed to be unstable. We conjectured that an oxidation process at this valley on the Ti surface was in rapid progress which caused a fluctuation of the tunneling current [6]. We also investigated current-voltage characteristics (I- I/ curves) [7]. We found a strong nonlinearity and a clear hysteresis of I-V curves. 3.2. STM images of a barrier layer of anodic aluminum oxide It is well-known that porous anodic aluminum oxide films can be fabricated by anodizing in acid electrolytes such as sulfuric acid, oxalic acid or phosphoric acid [8]. As shown in fig. 3, the anodic film on an aluminum substrate consists of two regions, the so-called porous layer and the barrier layer. The porous layer consists of close-packed cells of oxide, predominantly hexagonal in shape, each containing a single pore formed perpendicularly to the surface of the aluminum substrate. The barrier layer is a thin compact inner region lying adjacent to the aluminum and consists of predominantly hexagonal cells. Each is a semi-sphere in the shape as shown in fig. 3.
Fig. 2. STM images of a commercial Ti plate. From (a), we estimated the resolution of the STM to be better than 10 A in air; (b) was obtained after rather strong contacts using the differential micrometer.
122 aluminum
Fig. 3. Structure
Fig. 4 STM images of a barrier
of anodic
layer of anodic
aluminum
aluminum
oxide (AlzO,)
oxide prepared
with a 20 V cell voltage.
S. Morita et al. / STM of metal surfaces in air
123
We measured STM images of the surface of the barrier layer by evaporating 300 A Pt-Pd thin films after the removal of the Al substrate using a 20 wt% HCl, O.lM CuCl, solution. For the anodization of Al, we used sulfuric acid (20 wtW) as the acid electrolytes. The cell voltage controlled by a DC power supplier was 20 V. From these conditions, . we expected a pore diameter region, D z.500 A [9]. 2r0 = 100 A and a total diameter, of semi-spherical These preparation methods were reported in a previous paper [9]. Fig. 4 shows a series of STM images measured by changing the positioning voltage VY,in the y-direction. In this figure, we can see several domains which have a nearly circular shape with a diameter D = 500 A consistent with the expected value. However, the surface of domains is flat and the depth of domain walls is shallow compared with the expected value of D/2 - 250 A. We attributed these results to the thick Pt-Pd film. In fact, by evaporating a thinner Pt-Pd film, we obtained a more spherical surface and a deeper boundary between domains [lo]. As shown in fig. 4, we also found several additional structures which could not be correlated with the domain boundaries. As we found such a structure even in a sample with a thinner Pt-Pd film, we conjectured that a part of such a structure corresponds to a grain boundary of the evaporated Pt-Pd thin film. Using fig. 4, we evaluated the performance of 3D-positioner by measuring the displacement of STM images. As shown in fig. 5, we found that the displacement of STM images shows a hysteresis as a function of the positioning voltage vY, though an averaged piezoelectric constant of - 7 A/V agreed with the nominal and calibrated values of d,,. In fig. 4, we also found that the STM image drifted by about 200 A in the x-direction in 16 min. At the best condition, the drift both in the x-direction and in the z-direction was less than a few angstroms in a minute even in air.
Fig. 5. Displacement of STM images as a function of a positioning voltage V,, in the y-direction.
W
fd
4OA x Fig. 6. STM
images
of Pt fine particles supported on a TiO,/Ti location different from (a).
plate:
(b) shows
an image
at
3.3. STM images of Pt fine particles supporter on a TiO, / Ti plate
Pt fine particles on a thin TiO, layer on a Ti plate were prepared by depositing a mist of an aqueous solution of hexachloroplatinic acid produced by an ultrasonic humidifier and reducing it by hydrogen at 300°C. Fig. 6 shows STM images of this sample. In fig. 6a, we found a very flat region and a very deep cliff of - 400 A which is much deeper than hills and valleys of fig. 2b in the TiO,/Ti plate without Pt fine particles. Therefore, we concluded that the surface of Pt fine particles is flat and have a sharp and deep cliff down to the TiO,/Ti plate. We also found several steps at the flat surfaces of Pt fine particles as shown in fig. 6b. We conjectured that these steps correspond to boundaries between adherent Pt fine particles.
4. Summary
We have constructed a scanning tunneling microscope with a 3D-positioning mechanism in addition to a 3D-scanning mechamism for operation in air. We investigated surfaces of a commercial Ti plate, a barrier layer of anodic aluminum oxide and Pt fine particles supported on the TiOJTi plate. Our results present experimental support for the possibility that we can investigate the metal surface with STM even at atmospheric pressure.
S. Morita et al. / STM of metal surfaces in air
125
Acknowledgments The authors are grateful to Professor K. Itaya for the kind supply of the samples of anodic aluminum oxide. We are also grateful to Dr. M. Komiyama for the kind supply of the samples of Pt fine particles supported on the TiO,/Ti plate.
References [l] [2] [3] [4] [5] [6] [7] [8] [9] [lo]
G. Binmg, H. Rohrer, Ch. Gerber and E. Weibel, Phys. Rev. Letters 50 (1983) 120. R. Garcia, J.J. Saenz and N. Garcia, Phys. Rev. B33 (1986) 4439. R. Sonnenfeld and P.K. Hansma, Science 232 (1986) 211. G.F.A. van de Walle, J.W. Gerritsen, H. van Kempen and P. Wyder, Rev. Sci. Instr. 56 (1985) 1573. Ch. Gerber, G. Binnig, H. Fuchs, 0. Marti and H. Rohrer, Rev. Sci. Instr. 57 (1986) 221. A. Bryant, D.P.E. Smith and C.F. Quate, Appl. Phys. Letters 48 (1986) 832. S. Morita, T. Okada, Y. Ishigame, C. Sato and N. Mikoshiba, Japan. J. Appl. Phys. 25 (1986) L516. J.W. Diggle, T.C. Downie and C.W. Goulding, Chem. Rev. 69 (1969) 365. K. Itaya, S. Sugawara, K. Arai and S. Saito, J. Chem. Eng. Japan 17 (1984) 514. S. Morita, K. Itaya and N. Mikoshiba, Japan. J. Appl. Phys. 25 (1986) L743.