Phase transitions of adsorbed oxygen on niobium superfine particles

Surface Science 283 (1993) 387-392 North-Holland

isurface

science

Phase transitions of adsorbed oxygen on niobium superfine particles Kozo Obara a, Masahide Shioga b and Toshikazu Hirose a a Kagoshima University, Korimoto I-21-40, Kagoshima 890, Japan b Recording and Imaging Science Laboratories, Kao Corporation Haga, Tochigi 321-34, Japan Received

21 April

1992; accepted

for publication

30 July 1992

Dynamical characteristics of adsorbed oxygen on niobium superfine particles were investigated by analyzing complex dielectric constants of agglomerations. The partial pressure dependence of oxygen on complex dielectric constants E* changed stepwise at PO2 = 10 and 500 Torr. Below 10 Torr, Re(e *) showed an anomalous dispersion, which became negative at f < 200 Hz and 1 kHz
1. Introduction

Oxygen adsorption on superfine particles is a first step for the formation of surface oxide. In general, the oxidation proceeds passing through various oxygen states: molecular, atomic chemisorbed and incorporated [l]. The oxygen states severely influence the formation processes of oxides, which are metastable non-stoichiometric oxides [2]. To investigate the oxygen states on the superfine particles, we must clarify the dynamics of the surface atoms and the diffusion process at the surface. We obtained information about the diffusion process and defects of the surface oxide on niobium superfine particles (NSFP) from the PO, dependence of the growth rate of oxide by analyzing the DC electrical resistance of agglomerates [3]. The kinetics of oxide growth on NSFP obeyed the parabolic growth law at PO, 2 lO-‘j Torr. The Po, dependence of the growth rates changed near PO, = 10e3 Torr from P&f to P&r. The change in PO, dependence is ascribed to the change of defect clusters from neutral defect clusters [O[(M&lX to monovalent defects clusters [o!‘M&,]‘. In analyzing the DC resistance it is supposed from the assumption of phase bound0039-6028/93/$06.00

0 1993 - Elsevier

Science

Publishers

ary equilibrium that the boundary concentration of diffusing species is proportional to PA<“. In practice, two oxygen states at the surface of NSFP are possible: physisorbed and atomic chemisorbed oxygens. To clarify from the dynamical characteristics the state of the oxygen on the NSFP, we measured complex dielectric constants of agglomerates in the ranges of lop7
2. Preparation

and experimental

methods

NSFPs were formed at PA, = 0.3-0.5 Torr by using a magnetron sputtering system with an orifice to make the particles converge [4] and then deposited under lop4 Torr argon at 300 K on an area of 2 X 4 mm2 of sapphire substrates with four thick evaporated silver electrodes. NSFPs produced by this method wereDcubic crystals with an average size of 260 k 25 A. Figs. la and lb show, respectively, an electron micrograph of NSFPs deposited below the critical density of bond percolation of NSFPs and a typical high resolution lattice image of a NSFP. The NSFP has a lattice constant a = 3.45 A (bee) and rough

B.V. All rights reserved

388

K. Obara et al. / Adsorbed oxygen on niobium superfine particles

(100) surfaces. The small region with a different orientation suggests a crystal growth due to coalescence. The initial resistance of specimens which are agglomerations of NSFPs is adjusted to 300 R. Since each specimen is a three-dimensional random array of NSFPs, almost all junctions between particles statistically consist of a (loo} lateral face of a cubic particle and an apex of another particle. The electrical characteristics of the agglomerations mainly reflect the characteristics of the surface oxide of the NSFPs. The complex dieIectric constants of agglomerations of NSFPs were derived by measuring the capacitance C(w) and conductance a(w) by using

Fig. 2. Frequency dependence of C(w) of agglomerations of superfine particles. PO2dependence of C(w) can be classified into three regions: PO2< 10 Torr, 10 < PO,< 500Torr and 500 < PO2 < 760 Torr.

a precision LCR meter, YHP-4284A, as foliows: E*=Re(e*)+iIm(e*)aC(W)-i

u(w)/w. (I)

~easurcments were done under a constant voltage of 1 V. Each measurement of the frequency dependence was continued for 1.5 h, repeated for each oxygen pressure.

3. Experimental results 3.1. Oxygen pressure dependence of C(w) Fig. 1. (a) Electron micrograph of NSFPs deposited below the critical density of bond percolation of NSFPs, (b) high resolution electron micrograph of a niobium oxjde superfine particle. Lattice intervals of 2.405 and 1.688 A ar,e equal to d,,, and d,,, of bee niobium oxide with a = 3.40 A, respectively.

Fig. 2 shows the frequency dependence of the capacitance C(w) obtained in an oxygen pressure ranging from lo-’ to 760 Torr, at room temperature. PO, dependences of C(w) can be classified

K. Obara et al. /Adsorbed

oxygen on niobium superfine particles

into three types, depending on the oxygen pressure: (1) PO,< 10 Torr. The capacitance became negative at f < 200 Hz and 1 kHz 1 kHz.

389

3.2. Oxygen pressure dependence of a(o) Fig. 3 shows frequency dependences of the conductance a(w) obtained in an oxygen pressure ranging from lo-’ to 760 Torr at room temperature. The ordinate shows “relative conductance” (= AuJu,,, where Au, = u(w) ~(20)). The pressure dependence of the dispersions of the relative conductance was strongly correlated with that of the capacitance. At low oxygen pressures PO,< 10 Torr, the relative conductance decreased largely at f > lo5 Hz. The frequency, at which the relative conductance starts to decrease, increased stepwise at PO,= 10 Torr. At PO,= 500Torr, the decrease of the relative conductance becomes weak in the high frequency region, and at Po,> 500Torr the relative conductance increased in the high frequency region as the frequency increased.

4. Analyses of C(w) and a(w)

Fig. 3. Frequency dependence of relative conductance AU, /u2,, of agglomerations of superfine particles. The pressure dependences of the dispersion of the relative conductance at high frequencies are correlated with C(o).

Dispersions of the capacitance and conductance showed anomalous behaviors during low PO,as is shown in figs. 2 and 3. The most remarkable feature is that the capacitance becomes negative. This feature cannot be interpreted by the dispersion relation of the Debye type relaxation which is usually observed in common dielectric materials. In order to analyze C(w) and a(w), we consider the motion of adsorbed clusters with negative charge elastically bound to an equilibrium position at the surface of NSFP, where a positive charge resides. As the specimens have size distributions of NSFPs and coupling constants of adsorbed clusters, finally we must sum the contributions of many types of adsorbed clusters. Let ni be the density of the ith oxygen cluster with mass mi, negative charge e, natural angular oscillation frequency oi and damping constant yi. By solving the equation of motion of the individual cluster in an alternating field, under the assumption that the displacements are always much smaller than the distance between the clusters, we can get the dielectric constant E:(W) from the

K. Obara et al. /Adsorbed

390

steady-state solution of the displacement ith oxygen cluster, E?(W) =

oxygen on niobium supeqine particles

of the

4n-nie2/mi

wf---‘+iyiw

The complex dielectric constant E*(W) of the total system is expressed as follows: ‘ma,4wnie c*(W)

=A,

+

c i

2

____ mi

+-J (w’

-

co2)2

t- ( yiw)2

where i,, is the maximum index for wj, derived from experimental data, and A, is the contribution from oscillators with i > imax(~i+l > q>. Capacitance and conductance in this model are obtained from eqs. (1) and (3) as follows: imax4vnie2 C(o)

aA,+

C ~ i

(f+&J2)2+(y1&B)2

imax 4rnie2 aC~ i

mi

1

(w’-co’)”

yi and ai. The calculated curves agree well with the experimental data. In order to consider the dynamical characteristics of adsorbed oxygen in the whole PO range, we estimated the spectra of oscillators, IIi”01, which are equal to the distribution of the density of oscillators in frequency i space. Since wi is equal to ~~i/~if where kj is the coupling constant of the ith cluster, we can derive information about the quantities and the coupling constants of adsorbed clusters from the spectrum n(w). Since n(w) stretches logarithmically and the deviations from the proportionality of ni and wi are small, except for f > lo5 Hz, we think in the background spectrum fin(w), wi, yi and yli obey the aforementioned assumptions. The calculated C(w) and a(w) from the background spectrum show flat characteristics. To show clearly the quantities adjusted for fitting, we used the ratio Aa(w)/n,(w> as the spectrum of adsorbed particles, where An(w) = n(w) - n,(w). Fig. 5 shows the PO, dependence of An(w)/ n,(w) in the whole PO, range. The spectra show apparent Po, dependences, especially at f > 100 kHz. At PO, < 10 Torr, relatively large increases of An(o)/n,(w> appear at 1 Hz
1 (4)

mi

w+2

44

Fig. 4. Typical experimental data (dashed line) at POz = 2 X 1O-6 Torr and the results (solid line) calculated by eqs. (4) and (5) under the assumptions (l)-(3).

’

Yio2

* (51 + (Yiikg2

1

When we apply eqs. (4) and (5) to the experimental data of figs. (2) and (3), all data for wi, yi and nj must be given. Since the region where the capacitance becomes negative stretches from 20 Hz to 900 kHz, wi widely distributes in frequency space. After repeated trial and error, we obtained the folIowing assumptions about wj, yi and ni: (1) a group of fwJ forms a power series, (2) yi is proportional to wi, (3) ni is roughly proportional to wi, except for f > lo5 Hz, and finally adjusted so that the calculated C(w) fits the experimental data. Fig. 4 shows a comparison between experimental data (A~lo,/a, and C(w) at PO, = 10M6 Torr) and the results calculated by eqs. (4) and (5) under the aforementioned assumptions about wi,

K. Ubara et al. / Adsorbed oxygen on niobium superfine parrides

391

100 kHz and f < 100 Hz is mainly produced by the decrease of the density of oscillators with wi > 200 kHz and wi = 40 Hz, respectively, and the finestructures of the curve are adjusted by the increases of An(w)/n,(w) at 25 kHz 2.50 kHz becomes larger than that at PO,< 10 Torr. At PO,> 500 Torr, the A~(#)/~a(#) at f > 400 kHz becomes positive and broad spectra appear at 10 Hz
ized by the conductance at PO,= 2 X lop7 Torr, for constant frequencies. C(o), the relative conductance, and AaP/oP, increased stepwise at PO -_10 Torr. Ahhough stepwise increases of C(wf and the relative conductance at PO,= 10and SO0 Torr suggest the increase of adsorbate, the increases do not mean the increase of the number of adsorbed layers. This is because the conductance (AaP/aP,) increases at PO,= 10 and 500 Torr in spite of the increase of the barrier thickness for tunneling electrons. Therefore, the increases of C(w) and Acr,,/+e are ascribed to the increase of small clusters in the layer due to the dissociation of large clusters. This change of adsorbed clusters is induced by the increase of coupling constants due to phase transitions.

5. Discussion Fig. 5. Paz dependence of An(o)/n,(o). A&I) was obtained by using eqs. (4) and (5). The spectra shift to the high frequency region at PO2 = 10 and 500 Torr as PO, increases.

The distribution of geometrical factors of specimens influences the results. The electrical re-

392

K. Obara et al, /Adsorbed

oxygen on niobium superfine particles

sponse of the rough interfaces of specimens has a power-law frequency dependence [6]. The exponent depends on the fractal dimension of the surface. kj and mj of the adsorbed clusters seem to have the same distributions as the roughness of the interface. Therefore we consider that the assumptions (1143) are related to the fractality of the surface. Large dipole moments of physically adsorbed water molecules have been found on de-hydrowlated cu-Fe,O, [71. The change in the dielectric constant, which was of the Debye type, occurred after the adsorption of the first layer. Although the characteristic frequencies of the adsorbed molecules increased from 10 Hz to 10 kHz as the coverage increased, a phase transition was not observed. The stepwise increases of conductance at Po, = 10 and 500 Torr, shown in fig. 6, are related to the large increase of clusters with u)~> 250 kHz, as shown in fig. 5. In order to increase the number of adsorbed clusters without creating a multilayer of them, the two-dimensional structure of the adsorbed oxygen clusters must change at the surface. We consider that the change of the structure is ascribed to the increase of the coupling constant ki of the clusters and the decrease of the mass mi, due to the dissociation of large clusters. This process is induced by the increase of the driving force according to the increase of the chemical potential of gas-phase oxygen, and induces the’shifts of wi to the high frequency side at PO, = 10 and 500 Torr in fig. 5. Since this behavior is reversible with the change of Po2,

these phase transitions are due to the dynamical characteristics of physisorbed oxygen clusters.

6. Conclusion

Dynamical characteristics of adsorbed oxygen in the interface between two NSFPs were interpreted by the ensemble of adsorbed clusters elastically bound to equilibrium positions on NSFPs. In order to analyze the data, we used a model, in which characteristic frequency oi forms a power series and yi and fzi are proportional to wi. From analysis of PO_ dependence of n(o), it is clarified that the step&se changes of C(w) and (T(W) at Po? = 10 and 500 Torr are due to the structural changes of the adsorbed oxygen.

References (11 V.F. Kiselev and O.V. Krylov, Adsorption Processes on Semiconductor and Dielectric Surfaces, Vol. 1 (Springer, Berlin, 1985). [Z] 0. Toft Sgrensen, Nonstoichiometric Oxides (Academic Press, New York, 1981). 131 K. Obara, K. Minami and T. Hirose, J. Cryst. Growth 99 (1990) 192. [4] K. Obara, S. Saito and T. Ogushi, Nucl. Instrum. Methods B 39 (1989) 652. [5] M.W. Roberts and C.S. Mckee, Chemistry of the MetalGas Interface (Oxford University Press, Oxford, 1978) p. 512. [6] T. Kaplan and L.J. Gray, Phys. Rev. B 32 (1985) 7360. [7] E. McCafferty and AC. Zettlemoyer, Discuss. Faraday Sot. 52 (1971) 239.