NIST Studies Why Quantum Dots Suffer From 'Fluorescence Intermittency'

Don't Blink! NIST Studies Why Quantum Dots Suffer From 'Fluorescence Intermittency'

Researchers at the National Institute of Standards and Technology (NIST), working in collaboration with the Naval Research Laboratory, have found that a particular species of quantum dots that weren't commonly thought to blink, do.

NIST Studies Why Quantum Dots Suffer From 'Fluorescence Intermittency'

So what? Well, although the blinks are short—on the order of nanoseconds to milliseconds—even brief fluctuations can result in efficiency losses that could cause trouble for using quantum dots to generate photons that move information around inside a quantum computer or between nodes of a future high-security internet based on quantum telecommunications.

Beyond demonstrating that the dots are blinking, the team also suggests a possible culprit.*

Scientists have regarded indium arsenide and gallium arsenide (InAs/GaAs) quantum dots to be promising as single photon sources foruse in different future computing and communication systems based on quantum technologies. Compared to other systems, researchers have preferred these quantum dots because they appeared to not blink and because they can be fabricated directly into the types of semiconductor optoelectronics that have been developing over the past few decades.

The NIST research team also thought these quantum dots were emitting steady light perfectly, until they came upon one that was obviously blinking (or was "fluorescently intermittent," in technical terms). They decided to see if they could find others that were blinking in a less obvious way.

While most previous experiments surveyed the dots in bulk, the team tested these dots as they would be used in an actual device. Using an extremely sensitive photon autocorrelation technique to uncover subtle signatures of blinking, they found that the dots blink over timescales rangingfrom tens of nanoseconds to hundreds of milliseconds. Their results suggest that building photonic structures around the quantum dots—something you'd have to do to make many applications viable—may make them significantly less stable as a light source.

"Most of the previous experimental studies of blinking inInAs/GaAs quantum dots looked at their behavior after the dots have been grown but before the surrounding devices have been fabricated," says Kartik Srinivasan, one of the authors of the study. "However, there is no guarantee that a quantum dot will remain non-blinking after the nanofabrication of a surrounding structure, which introduces surfaces and potential defects within 100 nanometers of the quantum dot. We estimate the radiative efficiency of the quantum dots to be between about 50 and 80 percent after the photonic structures are fabricated, significantly less than the 100 percent efficiency that future applications will require."

According to Marcelo Davanço, another author of the study, future work will focus on measuring dots both before and after device fabrication to better assess whether the fabrication is indeed a source of the defects thought to cause the blinking. Ultimately, the authors hope to understand what types of device geometries will avoid blinking while still efficiently funneling the emitted photons into a useful transmission channel, such as an optical fiber.

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The NIST Center for Nanoscale Science and Technology (CNST) is a national nanotechnology user facility that enables innovation by providing rapid access to the tools needed to make and measure nanostructures. Researchers interested in accessing the techniques described here or in collaborating on their future development should contact Kartik Srinivasan.

*M. Davanço, C. Stephen Hellberg, S. Ates, A. Badolato and K. Srinivasan. Multiple time scale blinking in InAs quantum dot single-photon sources. Phys. Rev. B 89, 161303(R) – Published 16 April 2014.

News Release Source : Don't Blink! NIST Studies Why Quantum Dots Suffer From 'Fluorescence Intermittency'

New quantum dots herald a new era of electronics operating on asingle-atom level

New quantum dots herald a new era of electronics operating on a single-atom level

New types of solotronic structures, including the world's first quantum dots containing single cobalt ions, have been created and studied at the Faculty of Physics at the University of Warsaw. The materials and elements used to form these structures allow us forecast new trends in solotronics – a field of experimental electronics and spintronics of the future, based on operations occurring on a single-atom level.

New quantum dots herald a new era of electronics operating on a single-atom level

Electronic systems operating on the level of individual atoms would seem to be the natural consequence of efforts to achieve ever-greater miniaturization. Already now, we are able to control the behavior of individual atoms by situating them within special semiconductor structures – this is the method used to form quantum dots that contain single magnetic ions. Until recently, only two variants of such structures were known. However, physicists from the Institute of Experimental Physics at the Faculty of Physics at the University of Warsaw (FUW) have successfully created and studied two completely new types of the structures. The materials and elements used in the process make it wholly likely that solotronic devices may come into widespread use in the future.

The results, the Warsaw physicists have just published in Nature Communications, pave the way for developing the field of solotronics.

"Quantum dots are semiconductor crystals on a nanometer scale. They are so tiny that the electrons within them exist only in states with specific energies. As such, quantum dots exhibit similar characteristics to atoms, and – just like atoms – they can be stimulated with light to reach higher energy levels. Conversely, this means they emit light as they return to states with lower energy levels," says Prof. Piotr Kossacki (FUW).

The University laboratory creates quantum dots using molecular beam epitaxy. The process involves precision-heating crucibles containing elements placed in a vacuum chamber. Beams of elements are deposited on the sample. By carefully selecting materials and experimental conditions, the atoms assemble into tiny islands, known as quantum dots. The process is similar to how water vapor condenses on a hydrophobic surface.

While the dots settle, a small quantity of other atoms (for example magnetic ones) can be introduced into the vacuum chamber, with some becoming a part of the emerging dots. Once the sample is removed, it can be examined under a microscope to detect quantum dots containing a single magnetic atom at the center.

"Atoms with magnetic properties disrupt the energy levels of electrons in a quantum dot, which affects how they interact with light. As a result, the quantum dot becomes a detector of such an atom's state. The relationship also works the other way: by changing energy states of electrons in quantum dots, we can affect the respective magnetic atoms," explains Michał Papaj, a student at the UW Faculty of Physics, awarded the Gold Medal in Chemistry during last year's national competition for the best B.Sc. thesis held by the Institute of Physical Chemistry of the Polish Academy of Sciences for his work on quantum dots containing single cobalt ions.

The most powerful magnetic properties are observed in manganese atoms stripped of two electrons (Mn2+). In experiments conducted thus far, the ions have been mounted in quantum dots made of cadmium telluride (CdTe) or indium arsenide (InAs). Using CdTe dots prepared by Dr. Piotr Wojnar at the PAS Institute of Physics, in 2009 Mateusz Goryca from the University of Warsaw demonstrated the first magnetic memory operating on a single magnetic ion.

"It was commonly believed that other magnetic ions, such as cobalt (Co2+), cannot be used in quantum dots. We decided to verify this, and nature gave us a pleasant surprise: the presence of a new magnetic ion turned out not to destroy the properties of the quantum dot," says Jakub Kobak, doctoral student at the University of Warsaw.

Researchers from the University of Warsaw have presented two new systems with single magnetic ions: CdTe quantum dots with a cobalt atom, and cadmium selenide (CdSe) dots with a manganese atom.

As already stated, manganese atoms exhibit the most powerful magnetic properties. Unfortunately, they are caused by the atomic nucleus as well as the electrons, which means that quantum dots containing manganese ions are complex quantum systems. The discovery made by physicists at the University of Warsaw demonstrates that other magnetic elements – such as chromium, iron and nickel – can be used in place of manganese. These elements do not have nuclear spin, which should make quantum dots that contain them easier to manipulate.

In quantum dots where tellurium is replaced by the lighter selenium, researchers observed that the duration for which information was remembered increased by an order of magnitude. This finding suggests that using lighter elements should prolong the time quantum dots containing single magnetic ions store information, perhaps even by several orders of magnitude.

"We have demonstrated that two quantum systems that were believed not to be viable in fact worked very effectively. This opens up a broad field in our search for other, previously rejected combinations of materials for quantum dots and magnetic ions," concludes Dr. Wojciech Pacuski (FUW).

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The research into quantum dots containing single magnetic ions was funded with grants from the Polish National Science Centre and the Polish National Centre for Research and Development, as well as project funds from the Centre for Preclinical Research and Technology.

Physics and Astronomy were first taught at the University of Warsaw (UW) in 1816, at what was then the Faculty of Philosophy. The UW Astronomical Observatory was founded in 1825. Currently, the UW Faculty of Physics consists of the Institute of Experimental Physics, the Institute of Theoretical Physics, the Institute of Geophysics, the Faculty of Mathematical Methods, and the Astronomical Observatory. Research is conducted into most fields of modern physics, on scales ranging from the quantum to the cosmological. The UW Faculty of Physics has over 200 research and teaching staff, including 80 with the title of professor. The Faculty is attended by approx. 1,000 undergraduates and more than 140 doctoral students.

SCIENTIFIC PAPERS:

"Designing quantum dots for solotronics"; J. Kobak, T. Smoleński, M. Goryca, M. Papaj, K. Gietka, A. Bogucki, M. Koperski, J.-G. Rousset, J. Suffczyński, E. Janik, M. Nawrocki, A. Golnik, P. Kossacki & W. Pacuski; Nature Communications5:3191, 27 January 2014; DOI: 10.1038/ncomms4191

CONTACTS:

Prof. Piotr Kossacki
The Institute of Experimental Physics, Faculty of Physics, University of Warsaw
tel. +48 22 5532217, +48 22 5503232
email: piotr.kossacki@fuw.edu.pl

Dr Wojciech Pacuski
The Institute of Experimental Physics, Faculty of Physics, University of Warsaw
tel. +48 22 5532217, +48 22 5532329
email: wojciech.pacuski@fuw.edu.pl

LINKS:

http://www.fuw.edu.pl/

Faculty of Physics at the University of Warsaw website.

http://www.fuw.edu.pl/informacje-prasowe.html

Press Office for the Faculty of Physics at the University of Warsaw.

IMAGES:

FUW140127b_fot01s.jpg

HR: http://www.fuw.edu.pl/press/images/2014/FUW140127b_fot01.jpg

A cross-section of the quantum dots developed, constructed and tested by the Institute of Experimental Physics at the Faculty of Physics at the University of Warsaw. The color red marks an ion (cobalt or manganese) with magnetic properties (symbolized by the arrow). Yellow represents a quantum dot (cadmium telluride or indium arsenide, respectively). Blue shows the semiconductor layer securing the quantum dot. (Source: Faculty of Physics, University of Warsaw)

FUW140127b_fot02s.jpg

HR: http://www.fuw.edu.pl/press/images/2014/FUW140127b_fot02.jpg

Researchers from the Institute of Experimental Physics at the Faculty of Physics at the University of Warsaw have developed, constructed and tested groundbreaking new quantum dots containing single cobalt ions. Here Wojciech Pacuski, PhD, is shown with the molecular beam epitaxy device used to construct the quantum dots. (Source: Faculty of Physics, University of Warsaw)

News Relese Source: New quantum dots herald a new era of electronics operating on a single-atom level