A Japanese research group successfully controlled the diameter of a semiconductor nanoparticle having a novel composition of (AgIn)xZn(1-2x)S (silver, indium, zinc, sulfur) by photoetching and emitted fluorescent light in various colors.
The group is led by Susumu Kuwabata, a professor at the Graduate School of Engineering of the Osaka University, Tsukasa Torimoto, a professor at the Graduate School of Engineering of the Nagoya University, and IDEC Corp.
When a solution in which AgInZnS semiconductor nanoparticles with different diameters (particle diameter: 2-3nm) are dispersed is irradiated with UV light having a wavelength of 350nm, the solution emits fluorescent light in various colors ranging from green to red.
The group has been involved in research and development based on photoetching using a weak acid solution containing oxygen in which semiconductor nanoparticles with photocatalytic reactivity are dispersed. When the solution is irradiated with UV light, an oxidation reaction resulting from a photocatalytic reaction occurs at the nanoparticle surface. As a result, the nanoparticles oxidize themselves and are dissolved in the solution as oxide ions so that the particle diameters will be reduced.
As part of the research achievement, the group discovered that the diameter of a CdTe (cadmium, tellurium) semiconductor nanoparticle can be precisely controlled by irradiating it with UV light. Based on this finding, the group developed "size-selective photoetching."
In this technique, when the wavelength of approximately 350nm UV light is gradually varied in the order of nanometer, the diameter of a nanoparticle is determined in accordance with a width of the forbidden band (width of the bandgap between the valence and conduction bands) corresponding to the wavelength.
For example, when the wavelength of approximately 350nm UV light is varied in increments of 2nm, nanoparticles having large diameters will be oxidized and dissolved, and their diameters will be gradually decreased. Then, the dissolution stops when the diameter becomes equal to the width of the forbidden band, making it possible to obtain nanoparticles with a fixed diameter.
When the wavelength was changed in increments of 2nm, "the diameters differed in increments of about 0.02nm," Kuwabata said. When CdTe nanoparticles having slightly different diameters were irradiated with UV light, fluorescent light in 12 colors ranging from green to red could be emitted. For example, green light glows in six different tonal levels.
The CdTe semiconductor nanoparticle that enabled size-selective photoetching includes cadmium (Cd) as one of the constituent elements. Cd is regulated by RoHS, a directive that prohibits the use of specified hazardous substances, which was put into effect in July 2006 in Europe.
Therefore, Kuwabata's group searched for a semiconductor nanoparticle that does not contain Cd and succeeded in emitting fluorescent light by the use of the AgInZnS semiconductor nanoparticles. Like CdTe, the AgInZnS semiconductor nanoparticle features a high quantum efficiency (ratio of the product to one photon) of 0.4 or higher.
With the newly developed AgInZnS semiconductor nanoparticle, it is possible to emit fluorescent light in a variety of colors by gradually changing the factor "x," which determines the composition of (AgIn)xZn(1-2x)S. When the particles are irradiated with 350nm UV light, the emission color changes, in a non-consecutive manner, to green when x = 0.2, yellow when x = 0.35, orange when x = 0.4 and red when x = 0.5.
The novelty of the latest achievement is that the group established the semiconductor nanoparticle synthesis method that can precisely control the particle diameters.
The group also confirmed that when a transparent resin into which semiconductor nanoparticles having different diameters are kneaded is irradiated with UV light, the resin emits fluorescent light in green, yellow, red, etc. For example, the group confirmed the light emission by using semiconductor nanoparticles embedded in a resin sheet made of polymethylmethacrylate (PMMA), a typical transparent resin.
The research is being promoted to develop applied products such as light emitting devices in the future. The research group is "considering various applications including an anti-counterfeiting sheet," according to IDEC.