Suspended in Film and Placed Over Microcavities, Quantum Dots Become Brighter

Hank Hogan

According to a group of researchers comprising members from Georgia Institute of Technology in Atlanta, from Iowa State University in Ames and from Agiltron Inc. of Woburn, Mass., suspended quantum dots are brighter because they are removed from the quenching effects of a substrate’s surface. Additionally, if quantum dots are suspended over a reflecting silicon cavity, their light output is enhanced even more.

This schematic shows how researchers suspended quantum dots (QD) within a film composed of layers of polyallylamine hydrochloride (PAH) and polysodium 4-styrenesulfonate (PSS). Keeping the quantum dots away from the surface reduces quenching and makes the particles become brighter. Images reprinted with permission of the American Chemical Society.

With further development, such suspended films could incorporate quantum dots and other types of nanoparticles to create a microfabricated sensing array. “The ultimate goal of our research is the design of a multisensing microarray, which will be sensitive to temperature, pressure, light, sound and some chemicals,” said Vladimir V. Tsukruk, Georgia Tech professor of materials science and engineering. Quantum dots are nanometer-scale semiconductor particles with narrow-bandwidth photoluminescence and an emission peak wavelength dictated solely by size. They have good photostability and are resistant to photobleaching. Because of these characteristics, they are attractive for research and for possible industrial and clinical use. For these applications, it is vital that luminescent materials composed of quantum dots be in a readily usable form. For example, quantum dots placed in a flexible freestanding film could be particularly beneficial for optoelectronic sensing devices, Tsukruk noted.

The researchers investigated quantum dots that were encapsulated in a free-suspended layer-by-layer film. This approach enabled them to place charged nanoparticles such as quantum dots between polymeric layers in a controllable fashion. To do this, they synthesized 5-nm-diameter quantum dots with a CdSe core and a ZnS shell. They fabricated membranes using polyallylamine hydrochloride and polysodium 4-styrenesulfonate on a silicon substrate, spin-coated the film with a quantum dot solution, deposited another polymer membrane and freed the entire construct.

These images show a quantum dot film that is suspended over microscopic silicon cavities. Because of reflection from the silicon at the bottom of the holes, the film is brighter at spots over the holes. Fluorescence images of the film over the array (a) and over a single cavity (c) are shown, with the photoluminescence plotted versus distance across the array (b) and across a single microcavity (d).

Next, they transferred the quantum dot films to a variety of substrates. One of these was a silicon wafer with a 64 × 64 array of holes, dubbed microcavities, that were 80 μm wide and 70 μm deep.

The entire array spanned 0.5 in. By placing the membrane over this array, they formed freestanding quantum dot films over the microcavities.

Tsukruk directed the group at Georgia Tech, with Zhiquin Lin heading the group at Iowa State that, among other tasks, synthesized the quantum dots. Agiltron produced the silicon substrates.

The researchers conducted mechanical and optical measurements of the films both on the substrates and while freestanding. They used a Shimadzu spectrometer to record the ultraviolet and visible spectra and a Leica fluorescence microscope to capture bright-field optical and fluorescence images, attaching a Craic Technologies Inc. point-shot spectrophotometer to the microscope to record luminescence spectra.

They found that the 633-nm peak emission of the quantum dots was the same no matter which substrate they used. However, the intensity of the quantum dots in the freestanding membranes was five times that of those in contact with silicon. The increase, the group concluded, resulted about equally from the removal of quenching caused by the substrate surface and from the highly reflective silicon cavities.

In the next phase of the research, the scientists plan to vary the shape, depth and reflective properties of the cavity with the hope of focusing the light reflected from the bottom of the cavity onto a small spot on the membrane.

Tsukruk said that they expect a very strong increase in the intensity of the emitted light caused by the focusing effect.

Langmuir, Sept. 25, 2007, pp. 10176-10183.