Nanostructuring: A Route for Enhancing Materials Response?

A nanostructured material is nowadays a broad term used to refer to materials that have been either patterned or have structural features in the nanometer (nm) scale. The second approach is the one that allows achieving smaller features (i.e. dimensions below 10 nm) and has been the most widely used in the past for producing nanocrystalline segregation that eventually lead to a 3D network of nanocrystallites with no organization.

More recent and versatile approaches lead to artificial structures such as nm thick multi-layers (i.e. 1D nanometer control), nm sized objects embedded in a host and organised in layers (i.e. 2D nanometer control) or the most complete 3D control in which the organization of the nano-objects within the layer is in addition controlled.

We use the term nanostructuring for the last two approaches in which nano-objects with controlled features smaller than 10 nm are embedded in a host and organised. So far, results from resecrah conducted by Professor Carmen N. Afonso and her colleagues at Instituto de Optica - Consejo Superior de Investigaciones Científicas (CSIC) have demonstrated that the 2 D control is a very promising tool for both understanding fundamental interaction phenomena as well as enhance materials performance. This has been demonstrated for systems having layers whose separation is controlled down to ~ 1 nm, the layers being formed by either metal nanoparticles or rare-earth ions.

The design of these artificially engineering materials in the nanoscale can be tailored to applications in many fields. The main interest of Professor Carmen N. Afonso and her colleagues has been on optical applications and thus we focused on systems formed by metal nanoparticles with dimensions < 10 nm or rare-earth ions embedded in dielectric media with their distribution controlled in-depth within a few nm. The former system has several applications mainly related to its surface plasmon resonance features.

In addition, the nano-objects are large enough as to be imaged by electron microscopy related techniques as shown in the figure and thus prove the concept. The figure shows from left to right: cross-section and plan view images of a specimen containing metal nanoparticles organised in equally spaced layers; cross-section and plan view images of a specimen containing pairs of large and small nanoparticle layers with controlled separation, the two layers being appreciated in the plan view as a bi-modal distribution of large and small nanoparticles; and a cross-section image of a specimen containing layers with different spacings.

This approach has allowed Professor Carmen N. Afonso and her colleagues among others, to reduce the absorption of nanocomposite materials in the neighbourhood of the surface plasmon resonance by choosing an appropriate organisation of the layers¹ or demonstrating the optical activation (in the visible) of magnetic nanoparticles through neighbouring silver nanoparticles for a separation of ~ 4 nm.²

The concept has been extended to rare-earth (RE) ion doping, i.e. the nanostructuring is achieved by organizing the RE ions in layers similarly to the case of metal NPs but the ion layer concentration being two orders of magnitude smaller than that of the metal in the case of nanoparticles.

The nanostructuring approach has been used to optimise key material performance parameters for achieving optical gain at communications wavelength, i.e. lifetime (through the Er-Er separation)³, intensity (through Yb to Er separation)⁴ or bandwidth (through Tm to Er separation)⁵. Additionally, it has been proved an excellent approach to enhance the frequency conversion capability of LiNbO₃ films.⁶

References

1. A. Suarez-Garcia, R. del Coso, R. Serna, J. Solis, and C. N. Afonso, "Controlling the transmission at the surface plasmon resonance of nanocomposite films using photonic structures," Applied Physics Letters 83, 1842-1844 (2003)
2. J. Margueritat, J. Gonzalo, C. N. Afonso, U. Hormann, G. Van Tendeloo, A. Mlayah, D. B. Murray, L. Saviot, Y. Zhou, M. H. Hong, and B. S. Luk'yanchuk, "Surface enhanced Raman scattering of silver sensitized cobalt nanoparticles in metal-dielectric nanocomposites," Nanotechnology 19, 375701 (2008)
3. R. Serna, M. J. de Castro, J. A. Chaos, A. Suarez-Garcia, C. N. Afonso, M. Fernandez, and I. Vickridge, "Photoluminescence performance of pulsed-laser deposited Al2O3 thin films with large erbium concentrations," Journal of Applied Physics 90, 5120-5125 (2001)
4. A. Suarez-Garcia, R. Serna, M. J. de Castro, C. N. Afonso, and I. Vickridge, "Nanostructuring the Er-Yb distribution to improve the photoluminescence response of thin films," Applied Physics Letters 84, 2151-2153 (2004)
5. Z. S. Xiao, R. Serna, and C. N. Afonso, "Broadband emission in Er-Tm codoped Al2O3 films: The role of energy transfer from Er to Tm," Journal of Applied Physics 101, 033112 (2007)
6. J. Gonzalo, J. A. Chaos, A. Suarez-Garcia, C. N. Afonso, and V. Pruneri, "Enhanced second-order nonlinear optical response of LiNbO3 films upon Er doping," Applied Physics Letters 81, 2532-2534 (2002)