Ion Beam-Induced Changes in Optical Properties of MgO

The implantation of Ag into MgO (100) single crystals, followed by thermal annealing at 1100°C. leads to dramatic changes in their optical properties. The changes in the optical properties are due to the presence of small Ag clusters which are formed in the annealed samples. The small Ag cIusters are obtained by thermd annealing of the implanted MgO crystals between 600°C and 1100°C to investigate the changes in cluster sizes and to correlate with changes in their optical properties. Sample characterization is carried out using optical spectrophotometry to confirm the effective presence of Ag clusters and Rutherford Backscattering Spectrometry (RBS) to study the profile of Ag clusters.


INTRODUCTION
IOR impiantation is the most successfiil and widespread surface modification technique in insulators and ceramic materials. An area which has drawn considerable attention is the use of implzntation to cause changes in the optical properties of these materials [1] [2]. One way to cause dramatic changes in the optical properties is through the formation of small metailic clusters which absorb light at the surface plasmon resonance fiequency [3-51.
With regard to the phase formation in MgO implanted with a high dose (more than 10l6 ionsicrn') of metallic ions, three cases have been distinguished [6] a) the alkaline ions [7] (Li, Nay K. Rb), which form metallic clusters, b) Ag and Au [SI, which form metallic precipitates and bina? alloys with magnesium, and e) Fe, which forms oxides and spinel femtes [9].
In the present study, we have found that the implantation of Ag into MgO (100) single crystais, followed by thermal annealing at 1 IOO' C, results in the formation of Ag clusters. These small metailic clusters are identified by optical absorption and Rutherford Backscattering Spectrometry (RBS). In addition, we have investigated the effects of thermal annealing on smaII metallic cluster sizes and correlated with changes in optical properties. The positions of the maximum and the &I1 width at half maximum (FWHM) of the optical absorption band are d a t e d IO the sizes of the sinail nietalIic ctisiers. Toward this objective, MgO single ciystak were implanted with 1.5 MeV Ag and then subjected to thermal annealing at 600"C, 8OO0C, 1000°C and 1 1 OOOC, each sample for 30 minutes.

SamDie Preuaration
Samples of approximately 10 x 10 x 0.5 mm3 of MgO (100) single crystais were implamed with Ag' ions using a 1.7 MV General Ionex Tandem ion accelerator at Oak Ridge Nationai Laboratory. The beam energy was 1.5 MeV and the current density was maintained rather low (-1 pA cm-' ) in order to restrict thermal effects during implantation. The theoretical 'range from the TRIM calculation [lo] for 1.5 MeV silver ions implanted in MgO gives a projected depth A x = O . 4 5 p . The ion beam scan area was 10 x 10 mm'. To produce small clusters of silver in MgO crystals, each implanted sample was heated in air to 1100°C in increments of 100°C from 6OO0C, with a 30 minute dwelI time at each annealing.

Rutherford Backscattering Spectrometrv (RBS)
. Rutherford Backscattering analysis performed using 3.5 MeV particles at RT gives information on the depth concentration profile of implanted particles in crystals. The analyzed surface is about 2 mm' . Backscattered particles are detected with a surface barrier detector located at an angle of 170".

Optical Absomtion Measurements
Optical absorption measurements were performed at room temperature using a Cary 13E spectrophotometer capable of measuring absorption in U V and visible portions of the spectrum (Le., fiom 190 to 900 nm). For all these measurements, the unimplanted part of the sample was used as a reference.

Theoretical Considerations
It has long been known that small metallic particles or colloids embedded in dielectrics produce beautifid colors associated with optical absorption at the surface plasmon resonance frequency [ll-131. For clusters with diameters much smaller than the wavelength of light (A), the theories of M e [3] can be used to calculate the absorption coefficient (cm-') of the composite: where Q is the volume fraction occupied by the metallic particles, no is the refractive index of the host medium, and eland € 2 are the real and imaginary parts of the fiequency-dependent dielectric constant of the bulk metal. Equation (1) is a Lorentzian fhction with a maximum value at the surface plasmon resonance frequency (ap), where E1(olp) + 2n2, = 0 In the above, & i = n 2 -IC' where n and k are the optical constants of the bulk metal. Using the tabulated [14] optical constants for Ag, the value of c1 for silver has been plotted as a function of the photon wavelength (Fig. 1). Since we know q, = 1.73 for MgO, Equation (2) predicts a photon waveiength of 430 nm for the surface plasmon resonance frequency for colloidal Ag in MgO. On the other hand, the average radius of the metallic clusters, r, will be estimated from the absorption spectrum and using Doyle theory 1141, according to the equation I where vp is the Fermi velocity of metal ( v,=l .39x10s c d s e c for silver [SI ) and Aa 112 is the full width at half maximum (FWHM) of the absorption band due to the plasmon resonance of silver particles. The value of the FWHM is derived from the absorption band (Aa 1/2 = 2 x c 3 where Ah is the full width at half maximum wavelength of the plasmon band and h, represents the peak waveiength of the plasmon band). The implantation at room temperature of silver ions into l l g 0 for 6 0 K IO'" and 1.2 x l 0'' Ag'/cm' doses produced defects in the oxygen and magnesium sublattices These defects are identified by optical absorption measurement. Figure 2 shows the spcctnini ofthe as implanted MgO. Two main absorption bands are observed at 250 nm in relation to F' and F-centers (oxygen vacancies with one or two trapped electrons) and at 575 nni corresponding respectively to absorption from V-type centers ( magnesium vacancies) generated t hroush nuclear elastic collisions.

Effect of heat-treatment on the Ag colloid particles implanted MsO
Absorption spectra have been studied following 30 min nnneais between 600°C and 1100°C in order to determine the modifications induced in implanted species. For a sample implanted with l.2~10'' Ag' ions/cm' and subsequently isochronally annealed for 30 min, Fig. 3 shows an absorption at 410 nm after 60OoC. The position of this band does not change or changes only a little for annealing temperature between 600°C and S O O T , however, the band  For annealing temperatures higher than 8OO0C, the band maximum shifts to longer wavelengths and reaches 420 nm at 1100°C. The sample is brown in this case. This measured vahe of 420 nm is in good agreement with a photon wavelength of 430 nm deduced from Equation (2) for the surface plasmon resonance frequency for colloidal Ag in MgO. Fig.3 shows that the spectrum also has an F-center band and a V-type center band after annealing. Only the optical density decreases. Additionally, a small sharp band located at 360 nm appears after annealing at 800°C. The 360 nm bands are F-aggregate centers 1151. The shift of the absorption maximum to longer wavelengths accompanied by the increase in absorption and a narrowing of the absorption band may be related to the growth of Ag coiloids during annealing. The particle sizes deduced fiom Equation (3) and Fig.3 show that for annealing up to 1 100°C the radii of the silver precipitates vary between 0.8 and 3 nm. However, positive identification of the precipitates and their size distribution can only be obtained by TEM measurements. These are in progress and will be reported in the future. Rutherford backscattering spectroscopy-channeling results indicate that the precipitates form without the recovery of implantation-produced extended defects in the MgO lattice. Additionally, no long-range diffusion of silver is observed after annealing at 1 100°C (Fig.4).

CONCLUSION
Ion implantation of Ag into MgO followed by thermal annealing at 1100°C forms crystalline Ag clusters (average size -3nm). These clusters produce a strong optical absorption band at 420 nm due to surface plasmon resonance absorption, which leads to a brown color in the implanted region. Absorption spectrophotometer and RBS results indicate that the Ag clusters form without the recovery of implantation produced extended defects in the MgO lattice. MgO. Arrow with labels indicates the channel number for surface scattering from Ag, Mg and 0 .