Quantum Dots Are Finding Their Place in the World

Lynn Savage, Features Editor, lynn.savage@photonics.com

There has been a panoply of research into the next big thing in quantum dots – those semiconducting artificial atoms that are ubiquitous in fluorescence imaging, biological and chemical sensing, and display applications. Quantum dots of more (or less) exotic materials and with more (or less) interesting shapes are demonstrated on almost a weekly basis. It is the heyday for the field. But getting less attention are the practical issues of handling quantum dots in such a way that their functionality can be maximized and their use more broadly commercialized.

What follows is a look at some important steps in that direction.

Dropping through the grate

Negative-refractive-index materials, superlenses and Raman scattering-based spectroscopy are just a few applications that have come about – or that have been improved – through research developments involving surface plasmons. Quasiparticles analogous to photons, surface plasmons are the faint residue of electronic oscillations that occur naturally at the interface between metal and dielectric materials, such as silver and air, respectively. Integrate photons into the situation, and you obtain surface plasmon polaritons, another form of energetic quasiparticle that moves across the metal surface until reabsorbed or radiated away.

One way to improve the signal quality in a sensor is to use surface plasmon polaritons to enhance the fluorescence signal of nearby dye particles. For example, using a metallic surface rather than glass can improve the emission intensity of a fluorescent dye by as much as 20 times, perhaps more. Furthermore, if the metal surface is grated, more enhancement can be coaxed from the system because the coupling of photons to surface plasmons is boosted.

To further increase the applicability of fluorescence enhancement to sensor technology is to use quantum dots instead of dyes. Quantum dots have several advantages over fluorescent dyes, including size-dependent emission spectra, wide absorption bands, insusceptibility to photobleaching and higher quantum yields.

“In our work, we were able to take advantage of several of the special properties of quantum dots relative to traditional fluorescent dyes,” said Ehren Hwang of the University of Maryland, College Park. “The broadband absorption allowed us to use a pump wavelength widely separated from the quantum dot emission wavelength during the earlier phases of [our] experiment. We also were able to take advantage of the intrinsic resistance of quantum dots to photobleaching, [which] allowed us to interrogate our samples for extended periods of time – several hours – with repeated use over the course of months.”

Shown, are micrographs of (A) PMMA/quantum dot composite film on gold, with an integration time of 240 s; (B) pre-spun quantum dots over PMMA on gold, 30 s; and (C) pre-spun quantum dots over PMMA on chromium, 30 s. Courtesy of Ehren Hwang, University of Maryland, College Park.

Hwang, his adviser, professor Christopher C. Davis, and Igor I. Smolyaninov, now with BAE Systems in Washington, recently reported the effects of using quantum dots instead of fluorescent dyes in these systems. They used the polymer PMMA to create gratings over a 50-nm-thick layer of gold on glass. PMMA, the researchers noted, is inexpensive and easy to scribe with electron beam lithography. The polymer tends to absorb dye particles, making measurement of their positions and emissions less than certain, but quantum dots are much bigger, don’t get absorbed into the PMMA and, thus, are more certain when it comes to measurements.

“The physical positioning of the quantum dots was of paramount importance,” Hwang said. “The propensity of quantum dots to remain at the surface of the film was instrumental in enabling us to study closely the relationship between the grating and fluorescence enhancement.” He cautioned, however, that the size of quantum dots may be problematic for biosensing, especially if the particles are expected to be taken up by cells. In addition, the particles also tend to be made of cadmium or other materials that can be toxic to cells.

For some tests, the investigators directly deposited CdSe/ZnS quantum dots onto PMMA; for others, they mixed the PMMA with the quantum dots prior to coating the glass/gold base. The particles had an emission peak of 640 to 660 nm and quantum efficiency of 40 to 50 percent. They also tested grated and ungrated PMMA samples and, to determine whether the surface plasmon polaritons were the sole source of fluorescence enhancement, they made still other samples using chromium or indium tin oxide instead of gold.

After irradiating each sample with a filtered mercury discharge lamp, the investigators found that the gold layer clearly enhanced fluorescence compared with the alternate materials. Interestingly, quantum dots deposited prior to carving out the grating via electron beam lithography remained on the surface afterward.

Coating a PMMA-gold-glass system with quantum dots before etching a grating pattern into the PMMA material makes possible novel ways to increase the sensitivity of biosensors based on the interactions of surface plasmon and light. Reprinted with permission of the American Chemical Society.

Quantum dots that were mixed in with the PMMA and that drifted close to the gold thin-film layer were quenched, which was expected.

Overall, Hwang and his colleagues reported in the Dec. 30, 2009, issue of NanoLetters, the thickness of the PMMA layer, the periodicity of the grating and the grating geometry all had an effect on quantum dot-based outcomes.

According to Hwang, he and Davis now are performing experiments to investigate the influence of the substrate material thickness on the quantum dot enhancement effect. They also are drilling down into the nature of fluorescence enhancement in other ways.

“If possible, we would also like to probe our substrates using a more standard Kretschmann prism-coupling setup and to study the dependence of the system on azimuthal rotation of the gratings relative to an off-axis illumination source – that is, spinning the gratings around the Z-axis,” Hwang said. Additionally, “we are interested in probing our structures using a near-field scanning optical microscope in order to examine the near-field response of the interaction.”

Laying out an array of quantum dots with photo-oxidizable bonds, then breaking those bonds selectively with a scanning laser, enables changes in either emission intensity (top) or wavelength (bottom). Courtesy of James D. Batteas, Texas A&M University.

Taking a firm position

The quantum dot deposition method used by Hwang and his colleagues is typical but, ultimately, random. You can get a layer of fairly uniform thickness using spin-coating techniques, for example, but it is no way to get a finely arranged array of particles with submicron resolution between particles.

The ability to precisely lay down individual quantum dots could be the next step toward improved biosensors, but also to better LEDs, organic LEDs, solar panels and other optoelectronics.

But it’s not easy, by any stretch of the imagination. First, one must create the particles, then move them around individually to the desired location. Doing this with optical tweezers would be direct, but besides other challenges, would be dreadfully slow when spacing out an array of particles on a commercial scale.

Instead, placing arrangements of quantum dots has typically been done through photochemical means. One such method is to deposit quantum dots with molecular tags, or ligands, that adhere the particles onto a substrate, then, with lasers, chemicals or both, strip the bonds of select adherents, leaving only the quantum dots required.

To researchers at Texas A&M University’s chemistry department in College Station, however, this and other methods are too labor-intensive. To address the issues, they developed a method, dubbed “lithosynthesis,” that takes advantage of the photo-oxidation process.

According to James D. Batteas, associate professor of chemistry and materials science and engineering, lithosynthesis is a process wherein CdSe quantum dots are capped with a photo-oxidizable molecule, then spread onto a positively charged glass, silicon or other substrate. When a laser scans the layered surface, a portion of the caps are broken, leaving behind patterned arrays of quantum dots that have various emission intensities and wavelengths.

Batteas and his colleagues report in the March 10, 2010, Journal of the American Chemical Society that they used 16-mercaptohexadecanoic acid (16-MHA) to cap 4-nm-diameter CdSe quantum dots and positively charged poly(diallyldimethylammonium chloride) to enable the photo-oxidation effect in their experiments.

Using a 488-nm laser scanning at 0.1 to 4 μm/s, researchers changed the emission intensity (B) and peak positions (C) of the arrayed quantum dots. Reprinted with permission of the American Chemical Society.

Using a 488-nm argon-ion laser, they raster-scanned the landscape of capped particles, photo-oxidizing the tandems, breaking their bonds and leaving behind a composition of quantum dots with various intensities and wavelengths.

“This approach allows for the high spatial resolution patterning of quantum dots in which both emission intensity and wavelength can be spatially modified on a single layer of quantum dots,” Batteas said. “With regards to chemical sensing, this has many advantages over carrying out these measurements in solution.”

In particular, he noted, using quantum dots in solution often results in their untimely aggregation and, ultimately, their precipitation. “Since our platform is on a surface, this cannot occur.”

The group also considered ligands besides 16-MHA as caps, but despite the prospect of lower quantum yields, found that quantum dots capped with 16-MHA exhibited a dramatic increase in luminescence after laser exposure. The researchers also noted that the increased luminescence following photo-oxidation is reversible by dipping the quantum dot array into a 16-MHA bath, where reconnections can occur.

Luminescence intensity changes also are affected by materials used. (A) shows a control sample patterned in air and soaked in CH₂Cl₂; (B) shows a sample that has been immersed in 16-MHA; and (C) shows a sample that has been soaked in porphyrin thiol solution. Reprinted with permission of the American Chemical Society.

An additional advantage to the technique is that the photo-oxidized quantum dots become more capable of binding to newly introduced molecules than the other, unoxidized material.

“The ability to bind new molecules to the photo-oxidized quantum dots allows us to turn their emission on and off, allowing for data storage capabilities,” Batteas said. “They also show the propensity to selectively bind new molecules, allowing the modified quantum dots to be readily adapted for chemical sensing applications.”