quantum dot lscRecent quantum dot research at Los Alamos National Laboratory could lead to windows that also act as solar panels. The work shows that the greater light emitting properties of quantum dots are applicable to solar energy, helping more efficiently harvest sunlight.

Quantum dots, extremely small bits of semiconductor material, can be synthesized with almost atomic precision through advanced methods in colloidal chemistry.

The emission color is tunable by varying their dimensions. Color tunability combines with high emission efficiencies approaching 100 percent. These properties have recently become the basis of a new technology, quantum dot displays. These displays are used, for instance, in the newest generation of the Kindle Fire.

“The key accomplishment is the demonstration of large-area luminescent solar concentrators that use a new generation of specially engineered quantum dots,” said Victor Klimov, lead researcher.

Luminescent Solar Concentrators

A luminescent solar concentrator, or LSC, is a photon management device. It is a slab of transparent material holding very efficient emitters like dye molecules or quantum dots.

Sunlight absorbed in the slab is re-radiated at longer wavelengths and guided towards the slab edge equipped with a solar cell.

“The LSC serves as a light-harvesting antenna which concentrates solar radiation collected from a large area onto a much smaller solar cell, and this increases its power output,” said Klimov.

“LSCs are especially attractive because in addition to gains in efficiency, they can enable new interesting concepts such as photovoltaic windows that can transform house facades into large-area energy generation units,” said team member Sergio Brovelli, of University of Milano-Bicocca .

Due to their highly efficient, color-tunable emission and solution processability, quantum dots are ideal materials for use in inexpensive, large-area LSCs.

Engineered Stokes Shift

An overlap between emission and absorption bands in the dots is a challenge, which leads to considerable light losses because the dots re-absorb some of the light they produce.

To overcome this problem the researchers have created their LSCs based on quantum dots with artificially induced large separation between emission and absorption bands, called a large Stokes shift.

The Stokes-shift engineered quantum dots are cadmium selenide/cadmium sulfide (CdSe/CdS) structures in which light absorption is dominated by an ultra-thick outer shell of CdS, while emission occurs from the inner core of a narrower-gap CdSe.

The separation of light-absorption and light-emission functions between the two different parts of the nanostructure results in a large spectral shift of emission with respect to absorption, which greatly reduces losses to re-absorption.

Giant Quantum Dots

To use this concept, researchers fabricated a series of thick-shell, or “giant” CdSe/CdS quantum dots. The dots were incorporated into large slabs, sized in tens of centimeters, of polymethylmethacrylate.

Although giant by quantum dot standards, the active particles are still miniscule, about one hundred angstroms across. (A human hair is about 500,000 angstroms wide.)

Spectroscopic measurements indicated virtually no losses to re-absorption on distances of tens of centimeters. In spite of their high transparency, the fabricated structures showed significant enhancement of solar flux with the concentration factor of more than four.

These results mean that Stokes-shift-engineered quantum dots are a promising materials platform. They could enable the creation of solution processable large-area LSCs with independently tunable emission and absorption spectra.

Reference:

Francesco Meinardi, Annalisa Colombo, Kirill A. Velizhanin, Roberto Simonutti, Monica Lorenzon, Luca Beverina, Ranjani Viswanatha, Victor I. Klimov, Sergio Brovelli.
Large-area luminescent solar concentrators based on ‘Stokes-shift-engineered’ nanocrystals in a mass-polymerized PMMA matrix.
Nature Photonics, 2014; DOI: 10.1038/nphoton.2014.54

Image courtesy of DOE/Los Alamos National Laboratory

red herringResearchers at The University of Texas at Dallas have created a solution that fixes the Heartbleed vulnerability, as well as detects and traps the hackers who may be using it to steal susceptible data.

The sophisticated technique called Red Herring was created by a team of computer scientists to automate the process of creating decoy servers. It makes hackers think they have gained access to confidential, secure information, when actually their actions are being monitored, analyzed and traced back to the source.

“Our automated honeypot creates a fixed Web server that looks and acts exactly like the original — but it’s a trap,” said Dr. Kevin Hamlen, the team’s leader. ”The attackers think they are winning, but Red Herring basically keeps them on the hook longer so the server owner can track them and their activities. This is a way to discover what these nefarious individuals are trying to do, instead of just blocking what they are doing.”

OpenSSL Backdoor

The Heartbleed bug affects around two-thirds of websites previously believed to be secure. They are websites using the computer code library called OpenSSL to encrypt apparently secure Internet connections that are used for sensitive purposes such as online banking and purchasing, sending and receiving emails, and remotely accessing work networks. Heartbleed was announced to the public last week.

In 2012, a new feature named Heartbeat was added to software primarily for slow Internet connections. Heartbeat enabled connections to be held open, even during idle time. But a flaw in its implementation allowed sensitive information to be passed through the connection, thus the name Heartbleed.

Even though Heartbleed is well into the process of being patched, victims have the issue of not knowing who may already be exploiting it to steal the information, and what information they may be going after.

A common fix for this type of problem is to create a trap, a honeypot that lures and exposes attackers. Typically this can involve setting up another Web server somewhere else.

Virtual Fake Servers

“There are all sorts of ad hoc solutions where people try to confuse the attacker by deploying fake servers, but our solution builds the trap into the real server so that attacks against the real server are detected and monitored,” Hamlen said. “Our research idea can build this honeypot really quickly and reliably as new vulnerabilities are disclosed.”

The Red Herring algorithm designed by Hamlen automatically converts a patch, the code widely used to fix new vulnerabilities like Heartbleed, into a honeypot that can catch the attacker at the same time.

“When Heartbleed came out, this was the perfect test of our prototype,” Hamlen said.

Red Herring goes beyond just being a decoy and blocker; it can also lead to catching the attacker.

As the attacker thinks they are stealing data, an analyst is tracking the attack to find out what information the attacker is after, how the malicious code works and who is sending the code.

“In their original disclosure, security firm Codenomicon urged experts to start manually building honeypots for Heartbleed,” Hamlen said. “Since we already had created algorithms to automate this process, we had a solution within hours.”

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