A World Beyond the Obvious: Science of the Nanometer Scale

Nanoscience, combining a wide variety of scientific disciplines, is a mature subject today with tremendous technological possibilities. Though it is impossible to do justice to this vast field in a single lecture, or by a single individual, there are some underlying common themes that run through most such studies. I shall discuss some of these at first, with illustrative examples for diverse applications, before taking up one specific aspect, namely photoluminescence, to be dealt in detail.

One of the most spectacular aspects of semiconductor nanocrystals has been their photoluminescence properties, offering wide-ranging tunability of the emitted light and a high degree of quantum efficiency, not usually achievable with corresponding bulk materials; this has led to an intense interest in such materials, both in terms of fundamental science and technological possibilities. There are two distinct classes of light-emitting nanocrystal materials. One class makes use of the band-gap emission, thereby achieving a high degree of tunability as a function of the nanocrystal size via the quantum confinement effect. However, this class of photoluminescence tends to be more easily affected by surface degradation. The other route makes use of de-excitation through atomic-like levels of a dopant ion via energy transfer between the host nanocrystal and the dopant site, thereby achieving a greater stability of the luminescence, but forgoing the tunability with the nanocrystal size. In my presentation, I shall discuss how one may go beyond these expected limitations of each case, making the band-gap emission intrinsically stable and dopant emission tunable, through understanding the fundamental processes involved in each case, that require shifting away from some of the dominant dogmas in the field.