AFM investigation of bismuth doped silicate glasses

Vacuum/volume

Pergamon PII: SOO42-207X(96)00259-9

AFM investigation

of bismuth

4Unumber

3/4/pages 213 to 21611997 0 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0042-207x/97 $17.00+.00

doped silicate glasses

R Czajka,” K Trzebiatowski,* W Polewska,” 6 KoScielska,* S Kaszczyszyn” and B Susia” a/nstitute of Physics, Poznan University of Technology, ul. Piotrowo 3, 60-965 Poznan, Poland; bDepartment ofApplied Physics, Technical University of Gdarisk, ul. Narutowicza I l/12, 80-233 Gdansk, Poland; ‘Institute of Experimental Physics, University of Wroctaw, Pl. M. Borna 9, 50-204 Wroc!aw, Poland

Bismuth doped glasses were reduced in hydrogen atmosphere at temperatures above 250°C. This process leads to changes in stoichiometry of the glass in the region close to the glass surface. Relatively high surface conductivity of IOm6S allows this material to be applied to channeltron production. AFM investigations of freshly cleaved bismuth dopedglasses were done in constantand friction force modes. The AFM topographical images show the presence of different concentrations of nanocrystals vs distance from the glass surface. Atomic resolution images obtained in both modes of these nanocrystals, together with XRD data, suggest that they are composed of bismuth. 0 7997 Elsevier Science Ltd. All rights reserved

Introduction

Experimental

Silicate glasses with proper amounts of lead. bismuth or antimony oxides’-6 are of great interest because of the possibility of forming surfaces layers with electron surface conductivity of 10-‘2-10 ’ S2 and a large electron secondary emission coefficient (34). Propcrtics of these layers remain unchanged even after exposure to the ambient atmosphere. These features make this type of silicate glass very suitable for the production of channeltrons and microchannel plates. Moreover, the small activation energy of the electric conductivity and negligible changes in other properties within the surface layers mean that they can be applied in a very wide tcmperaturc range. Silicate glasses doped with bismuth are particularly susceptible to (fully controlled) changes in the surface structure by annealing them in a hydrogen atmosphere at increased tcmperaturc. The formation of the type of surface layer mentioned above depends on three processes: (1) diffusion of hydrogen to the bulk; (2) reduction of ions Bi+3 to Bi’; and (3) coalescence increasing the concentration and size of bismuth crystallites. All three processes depend on the chemical composition of the glass. its structure, and reduction time and temperature. Recent developments in scanning tunneling microscopy (STM) and atomic force microscopy (AFM)‘.’ enable detailed investigation of the intentionally modified surface structures even down to the sub-nanometer scale. Glass is an insulator, with a conductive layer resulting from the intentionally modified distribution of dopants toward the surface. The best way to study such a mixed structure is to observe its cross-section. Therefore, the AFL4 method was chosen, suitable for either conductive or insulating materials, to study the influence of technological procedure on bismuth doped silicate glasses used in the channcltron production.

In our studies we used the rod-like glass samples, composed of (%/mol) SiO, 78%1, B&O, 169/o, Sb,O, I?<), KzO 5%. They were synthesized according to standard procedure, at 1250°C in the ceramic and platinum melting pot. The glass samples investigated exhibit extremely good mechanical and applicable properties. such as high transparency, homogeneity and ductility, low tension and lack of visible signs of crystallization during the process of synthesis. The electrical conductivity of samples was measured in a spccially designed chamber.’ Its construction allowed control of the atmosphere as well as the temperature within the range from IOO800 K, which also enabled measurement of surface conductivity variation during annealing? crystallization, oxidation and the reduction process. The influence of time, temperature and the atmosphere on the redistribution of dopants, which takes place during these processes, can be estimated even by eye. because highly reduced samples, with a conductivity of 10 ‘S, exhibit a metallic polish. whereas samples with low reduction degree are much more transparent. The heterogeneous conducting propcrties of glass depend on the position of dopants relative to the surface. and should thus be reflected in the cross-section structure of the sample as non-uniform changes in stoichiometry of the glass in the region close to the surface. Freshly cleaved glass rods were observed in air, with the OMICRON AFM working in the so-called contact mode (at constant force), with loading of the cantilever against the I nN sample. The AFM images were calibrated on mica. The rcsolution, ‘in plane’ as well as in the z-direction, was of the order of 0.01 nm. The OMICRON AFM may also work in so-called friction (lateral) force mode. The AFM was able to monitor sample topography and frictional forces simultaneously. 213

R Czajka et al: AFM investigation

of bismuth

doped silicate glasses

Results and discussion Reasonable values of conductivity. for silicate glasses doped with lead or bismuth, can be obtained at temperatures above 200°C. An increase of surface conductivity with temperature follows the Arrhenius law. and the activation energy, E, estimated from this dependence is much larger than 1.2eV.’ Further annealing of glass, in air at temperatures above 450°C: increases the value of its conductivity and decreases its activation energy, depending on the time of the annealing process. This can be explained in terms of the alkali ions, migration toward the glass surface, causing an increase in the amount of the charge carriers.” The essential structural changes in the glass surface layer take place during annealing in a hydrogen atmosphere. We can observe here a very strong dependence of the rate ofconductivity changes on the annealing temperature. For example, the time to appearance of conductivity, which exceeds the background level, varies from several minutes at 500 C to several days at 160°C. The maximum of conductivity at 5OO’C can bc achieved after 0.5 h. the maximum at 160°C after 20 h. Figure I shows the dependence of the surface conductivity vs annealing time at 35oYz. Such changes in the conductivity should be reflected in the structure of the surface layer and should be visible in the crosssection of a freshly cleaved glass rod. Figure 2 shows a top-view AFM constant force mode image of such a cross-section, with darker colours corresponding to the lower parts of the surface. At the right side of the image we can see the real edge of the cleaved surface. Going from the right to the left side, i.e. to the centre of the cross-section, we can discern two kinds of ‘swellings’; the smaller ones have a diameter of about IOOnm, the bigger ones approximately 200-250 nm. Bigger swellings arc uniformly distributed within the whole cross-section of our sample, whereas smaller swellings are characteristic for the area at the edge of the sample. Their density seems to reach the maximum between 500 and 2000nm relative to the edge of the sample. This could be explained by bismuth precipitation. created during the annealing of the sample in hydrogen atmosphere, creating a kind of dense network responsible for the increased surface conductivity of channeltrons. Figure 3(a) and (b) shows the Loomed part of the cross-section in friction and constant force modes, respectively. WC can explain the different contrast in the map of friction

-12

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100 80 g

60 40 20 0

500

A 1000

1500

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nm Figure 2. Topographical 2000 x 2000 nrn’ image of freshly cleaved bismuth doped silicate glass rod. At the right side of the image the real edge of the cleaved surface is clearly sew. The linear cross-section at the bottom of the image shows the dimensions of swellings.

force; observed in I:igure 3(a), in terms of inhomogeneity of the material. Different friction coefficients of the matrix and dopant may cause the difference in the contrast of the friction force and may reflect the distribution of inhomogeneities within the cleaved surface, indirectly enabling their identification and thus confirming our previous assumption. This friction force mode image corresponds with the AFM constant force image in Figure 3(b), showing the topography of the same spot of the sample crosssection. A closer look into the structure of the single crystallite is presented in Figure 4(a), which shows the 3.4 nm x 3.4 nm AFM top-view of one of the crystallites. An ordered, quasi-hexagonal structure, with interatomic distances of 0.47+0.03 nm, is clearly seen. The same structure, with the same interatomic distances, is observed in the friction force mode in Figure 3(b), and is characteristic of bismuth. Additional data from X-ray diffraction investigations of the glass powders reduced at temperatures above 25O’C, exhibit diffraction spots that characterize the structure of bismuth, whereas the samples reduced at temperatures below 25O’C appear to be amorphic in the X-ray data. Figure 5 shows the influence of the annealing temperature on the bismuth crystallization process in the silicate glasses. The higher the temperature, the better arc the pronounced bismuth crystallitcs found inside the investigated glass sample. Conclusions

1.2

Time [h] Figure 1. The depcndcncc silicate glasses vs annealing temperature of 35O.C.

I20

of surface conductivity of bismuth doped time in hydrogen atmosphere, at constant

According to Hill and Coutts, the character of surface conductivity can be described in terms of bismuth precipitation intcrdistances and their size distribution. Our preliminary AFM results stem to confirm the assumption that changes in surface

R Czajka et al: AFM investigation

of bismuth

doped

silicate

glasses

(4 Length: 2.68 nm Height: 0.014 nm Angle: 83.14

(4 600

0.12 0.10 E 0.08 = 0.06 0.04 0.02 0

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400 run

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@I M-P.-L

-0.034 -0.035 -0.035 -0.036 -0.036 -0.037 -0.037 -0.037 -0.038% -0.038 -0.039 -0.039 -0.039 -0.040 -0.040 -0.041 -0.041

7.891

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7.397 6.904 6.411 5.918 5.425 4.932 4.438 3.945E 3.452= 2.959 2.466 1.973 1.479 0.986 0.493 0 lb0

2bo

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4bo "IIl

5&l

i

0‘

i

4 nm

800

Figure 4. Images of atomically resolved structure of one of the crystallites observed in Figure 3: (a) 3.4 x 3.4 nrn’ in constant force mode; (b) 4 x 4 nm* in friction force mode. Both images show the same spot. From the linear cross-sectIon. presented at the bottom of (a), the value of interatomic distances of the resolved structure may be found.

Figure 3. Topographical 660 x 660 nm’ images showing the yoomcd part of the cross-section presented in Figure 2: (a) friction force mode; (b) constant force mode. Both images were taken simultaneously at the same spot of the sample.

r

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Red. 112 400°C; 20h

(110)28=27.1” 21

23

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Diffraction Figure 5. Influence

of annealing

temperature

in hydrogen

atmosphere

A-

29

angle 28 [deg]

on the Bismuth

crystallization

process in silicate glasses (XRD data). 215

R Czajka et al: AFM investigation

of bismuth doped silicate glasses

electron conductivity of silicate bismuth doped glasses are due to the appearance of metallic bismuth precipitation at the nearsurface layer. A further goal is to show that the surface conductivity saturation as well as the size of the bismuth crystallites depends on time and annealing temperature in hydrogen atmosphere.

Acknowledgements The OMICRON Al-‘M/STM purchase was sponsored by The Foundation for Polish Science under the ‘SEZAM’W Program, Project No. 33/94. This work was supported in part by the Poznan University of Technology Research Program No. BW 62-124, and in part by the University of Wroclaw under Grant 20 16; 1FD:‘96.

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References

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