Optical properties of metallic nanoparticles in Ni-ion-

implanted α-Al2O3 single crystals

X. Xiang and X. T. ZuS. Zhu and L. M. Wang

Younes Sina

Potential promise as a means of modifying the near surface mechanical, electrical, optical, and magnetic properties in insulator materials due to quantum confinement effects

Ion implantation

A powerful technique for forming nanoparticles

α-Al2O3 has been often used in microelectronic devices or as window materials

Single crystal Sapphire has a high thermal conductivity, volume resistivity, hardness and wear resistance, as well as a high upper use temperature.

The energy band gap is large (≈9 eV).

Sapphire is a transparent material having useful transmission of wavelengths in the range 0.2 to 5.5 microns. It is much stronger than glass and can be used as viewing windows in high pressure and vacuum applications.

Vacuum

<0.5 μA/Cm2

1.8x10-3

Pa

64 keV

5x1016 Ni+/cm2

To avoid heating the sample

1 mm

15 m

m

1 0 mm

Charge stateXPS (X-ray photoelectron spectroscopy)

Structure of metallic Ni nanoparticles in Ni- implanted Al2O3 crystalsTEM ( Transmission electron microscopy)

Optical propertiesOptical spectroscopy

XRP (X-ray photoelectron spectroscopy)at room temperature with KRATOS X SAM 800 with monochromatic Al Kα (hν =1486 eV)

Ar+ ion beam etching of sample at energy of 3 keV andcurrent density of 10 μA/Cm2

Etching rate = 1 nm/min

2 nm etched Ni-implanted samples with Ar+ in order to remove surface contamination

X-ray photoelectron spectra of Ni 2p3/2 energy level of Ni+-implanted Al2O3 crystals showing implanted Ni ions mainly in charge state of metallic Ni0. The charge states of Ni have no distinct change with the increasing etching depth. Real line-experimental curve, point line Gaussian simulated curve.

Etching depth 2 nm Etching depth 12 nm

852.3

Ni0 Ni0

Ni2+(NiAl2O4)Ni3+ (Ni2O3)

852.3856.6 856.6

Ni2+(NiAl2O4)Ni3+ (Ni2O3)

The charge states of Ni have no distinct change with the increasing etching depth.

Etching depth 2 nm Etching depth 12 nm

Ni concentration= 9.55% Ni concentration= 9.84%

JEOL 2010 F (STEM & TEM) at 200 keV bright-fieldHAADF ( high angle annular dark-field)Probe size 0.2 nm Collection angle of 50 mradHREM ( high-resolution electron microscopy)

A bright-field (a) and a HAADF STEM (b) cross-sectional image in the near surface of as-implanted Al2O3 matrix. Nearly spherical embedded nano-particles, 1–5 nm in diameter, are distributed from the surface to 30 nm below the surface.

bright-field HAADF STEM

Al2O3

Φ = 1 – 5 nm

A HREM image showing structure of Ni nanoparticles in the near surface of Al2O3 matrix

HREM

Ni-ion implanted area is amorphized entirely

Optical SpectroscopyWith SHIMATZU UV-2100 spectrophotometerWavelength from 200-800 nmLuminescence testWith HITACHI 850 photoluminescence spectrophotometerXe lamp excitation source

Optical absorption spectra of as-grown, as-implanted and electron irradiated Al2O3 crystals. A broadband peaked at 400 nm appeared in the Ni-implanted sample. (1)as-grown (2) electron-irradiated, and (3) as-implanted

as-grown

electron-irradiated

as-implanted

225 nm 250 nm 400 nm

No detection of emission band in the as-implanted sample under a Xe lamp excitation wavelength of 250-430 nm

F+ color center form by O2- vacancy captured single e

F color center: F++e→ FBroadband peaked at 400 nm due to SPR of Ni0 Crystal turning gray

Surface Plasmon resonance

Single crystal of α-Al2O3 (rhombohedral) has a wide band gap (Eg≈9 eV). Therefore, its intrinsic absorption is in the ultraviolet wave band. Pure single crystals of α-Al2O3 look colorless and transparent in the visible region. Then from 380-740 nm (visible wave band) its absorption spectrum is a smooth line.

intrinsic absorption

In general, ion implantation techniques used to form nanocrystals may be categorized as follows:

Room temperature implantation followed by high temperature annealingRoom temperature implantation at dosage above the threshold dose for spontaneous nanocrystals formation Ion implantation at elevated temperatures

The degree of disorder during implantation depends upon:

Ion doseIon speciesTarget temperatureOrientation of the ion beam relative to crystallographic axes

Examples:

Disorder increases faster with increasing dose for the c-axis orientation than the a-axis orientation.

(0001) Surface implanted sapphire with Fe ion become amorphous at a dose between 1x1017 and 2x1017

Metallic nanocrystals has a small size and high specific surface area.

Optical properties of metallic nanoparticles embedded in a dielectric host are dominated by Surface Plasmon Resonance (SPR).

For the smaller particle size, the plasmon band are relatively weak, broadened, and is shifted to higher energy due to quantum confinement.

target Refractive Index

ion SPR absorption (nm)

SiO2 1.52 Au 530

SiO2 1.52 Ag 400-410

CuS - Cu 560

SiO2 1.52 Ni 350

Al2O3 1.76 Ni 400

Based on Xu and Käll model using Mie theory, SPR peak shift towards low energy (higher wavelength) with increasing refractive index of surrounding medium.

Kurdistan