Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Putting more science into the art of making nanocrystals

11.07.2013
Preparing semiconductor quantum dots is sometimes more of a black art than a science.

That presents an obstacle to further progress in, for example, creating better solar cells or lighting devices, where quantum dots offer unique advantages that would be particularly useful if they could be used as basic building blocks for constructing larger nanoscale architectures.

Andrew GreytakAndrew Greytak, a chemist in the College of Arts and Sciences at the University of South Carolina, is leading a research team that’s making the process of synthesizing quantum dots much more systematic. His group just published a paper in Chemistry of Materials detailing an effective new method for purifying CdSe nanocrystals with well-defined surface properties.

Their process uses gel-permeation chromatography (GPC) to separate quantum dots from small-molecule impurities, and the team went further in characterizing the nanocrystals by a variety of analytical methods. A comparison of their purified quantum dots with those purified by the traditional method of multiple solvation and precipitation cycles underscored the utility of the new method in preparing uniform semiconductor nanocrystals highly amenable to further synthetic manipulation.

Quantum dots

Quantum dots, which are nanocrystals with diameters in the range of 5-10 nanometers, have optical and other physical properties different from those of larger crystals. The reduced size allows them to absorb and emit different colors than bulk quantities of the same compound because of quantum mechanical effects; they also have very large surface-to-volume ratios and can be sensitive to surface treatments.

Greytak’s laboratory typically prepares quantum dots in hydrophobic solvents (such as 1-octadecene), so they come out “capped” with hydrophobic molecules and dissolve readily in nonpolar solvents. “The way the process works, you always have a significant amount of unreacted starting material, high-boiling solvents and extra surfactants in there that are important to the synthesis,” said Greytak. “But once the synthesis is complete, they’re impurities that need to be removed.”

The historic method of quantum dot purification is cycles of solvation, precipitation (such as with alcohol), decanting of impurities and re-solvation. Although the method has been in use for some 20 years, it has a fundamental shortcoming.

“With the precipitation and redissolution process, it’s not actually doing the separation on the basis of the size of the particle, it’s doing it on the basis of the solubility,” said Greytak. “So if you have impurities that have solubility qualities similar to those of the particle, they aren’t removed.”

Gel-permeation chromatography

Greytak groupGreytak directed his team, which included graduate students Yi Shen, Megan Gee and Rui Tan, in developing GPC as a highly effective alternative. A size-exclusion technique, GPC separates chemical species according to molecular weight and is commonly used with macromolecules.

Compared with materials prepared through the precipitation and re-solvation process, the GPC-purified quantum dots had better stability at high temperature. Moreover, a series of NMR measurements assisted by USC research associate professor Perry Pellechia indicated that the GPC method was much more effective in removing weakly adsorbed ligands from the quantum dot surface.

Carrying a synthetic process forward

The team further examined the suitability of the quantum dots for further synthetic manipulation. Again, the GPC-purified products were superior, both in CdS shell growth on CdSe quantum dots as well as ligand exchange of cysteine on CdSe/CdxZn1-xS quantum dots.

Greytak sees the method as a fundamental step forward in being able to further manipulate quantum dots, whether in constructing larger architectures or asserting control over how the nanocrystal colloids behave in solution.

“What we like to say is that we’re developing a sequential, preparative chemistry for semiconductor nanocrystals,” said Greytak. “In most synthetic chemistry, you have a starting material, you do a reaction, and you proceed through a series of intermediates with well-defined structures that can be isolated. For a nanomaterial, it’s much more difficult, because we’re not making molecules, we’re making a population of particles that has, let’s say, a radius of two nanometers. They aren’t all identical, and achieving a consistent product has been challenging, both in terms of how to isolate it and characterize it.

“So we’re really working toward being able to characterize a sample, with, say NMR and thermogravimetric analysis, and being able to really predict with confidence how it’s going to react in a subsequent step.”

Steven Powell | EurekAlert!
Further information:
http://www.sc.edu

More articles from Materials Sciences:

nachricht Graphene origami as a mechanically tunable plasmonic structure for infrared detection
25.04.2018 | University of Illinois College of Engineering

nachricht Scientists create innovative new 'green' concrete using graphene
24.04.2018 | University of Exeter

All articles from Materials Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Why we need erasable MRI scans

New technology could allow an MRI contrast agent to 'blink off,' helping doctors diagnose disease

Magnetic resonance imaging, or MRI, is a widely used medical tool for taking pictures of the insides of our body. One way to make MRI scans easier to read is...

Im Focus: BAM@Hannover Messe: innovative 3D printing method for space flight

At the Hannover Messe 2018, the Bundesanstalt für Materialforschung und-prüfung (BAM) will show how, in the future, astronauts could produce their own tools or spare parts in zero gravity using 3D printing. This will reduce, weight and transport costs for space missions. Visitors can experience the innovative additive manufacturing process live at the fair.

Powder-based additive manufacturing in zero gravity is the name of the project in which a component is produced by applying metallic powder layers and then...

Im Focus: Molecules Brilliantly Illuminated

Physicists at the Laboratory for Attosecond Physics, which is jointly run by Ludwig-Maximilians-Universität and the Max Planck Institute of Quantum Optics, have developed a high-power laser system that generates ultrashort pulses of light covering a large share of the mid-infrared spectrum. The researchers envisage a wide range of applications for the technology – in the early diagnosis of cancer, for instance.

Molecules are the building blocks of life. Like all other organisms, we are made of them. They control our biorhythm, and they can also reflect our state of...

Im Focus: Spider silk key to new bone-fixing composite

University of Connecticut researchers have created a biodegradable composite made of silk fibers that can be used to repair broken load-bearing bones without the complications sometimes presented by other materials.

Repairing major load-bearing bones such as those in the leg can be a long and uncomfortable process.

Im Focus: Writing and deleting magnets with lasers

Study published in the journal ACS Applied Materials & Interfaces is the outcome of an international effort that included teams from Dresden and Berlin in Germany, and the US.

Scientists at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) together with colleagues from the Helmholtz-Zentrum Berlin (HZB) and the University of Virginia...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

Invitation to the upcoming "Current Topics in Bioinformatics: Big Data in Genomics and Medicine"

13.04.2018 | Event News

Unique scope of UV LED technologies and applications presented in Berlin: ICULTA-2018

12.04.2018 | Event News

IWOLIA: A conference bringing together German Industrie 4.0 and French Industrie du Futur

09.04.2018 | Event News

 
Latest News

World's smallest optical implantable biodevice

26.04.2018 | Power and Electrical Engineering

Molecular evolution: How the building blocks of life may form in space

26.04.2018 | Life Sciences

First Li-Fi-product with technology from Fraunhofer HHI launched in Japan

26.04.2018 | Power and Electrical Engineering

VideoLinks
Science & Research
Overview of more VideoLinks >>>