QDs can be moved with “optical tweezers” – a system that sets up a gradient of forces from multiple laser beams – or by electrophoresis, in which a microscopic object with a surface charge can be pushed through a fluid or gel by applying a constant electric field. (Electrophoresis provided an early method of separating DNA for analysis.) To date, however, results from both methods have been insufficiently exact for anticipated applications, and typically move multiple particles at once.
But now a research team headed by Waks and Benjamin Shapiro of UMD’s Fischell Department of Bio-Engineering has invented a fully automated apparatus that controls the position of a single QD by manipulating the fluid in which the dots are immersed. The system exploits a phenomenon called electroosmosis, in which liquids with polar molecules such as water are pulled in specific directions by applied electrical fields.
At the ends of each channel are cylindrical fluid reservoirs, each of which contains a platinum electrode. By altering the voltage between pairs of electrodes, the scientists control the motion of the fluid in two dimensions.
To position a dot, the researchers first identify a single QD with a microscope. This is possible because when a dot is struck by a green laser beam (532 nm), it begins to blink, emitting red photons at 655 nm. The blinking light is detected and the individual dot’s motion is tracked by a microscope. When the dot goes “dark,” the tracking pauses until the next blink. When the QD blinks, its position relative to the target location is re-detected.
The researchers programmed their device to calculate how much voltage will be required to shove the dot by the right amount in the desired direction. At each blink, an appropriate voltage is applied to the electrodes. [See diagram at bottom.] The QD thus proceeds to its intended destination by a series of nudges. Once there, the system can keep it in place for more than an hour.
One potential problem the group faced is that the dots not only move in two dimensions, but also rise and fall within the fluid. As a result, the imaging microscope loses its sharp focus and the dot can be lost. The scientists compensated for this effect by programming their microscope to detect the onset of fuzziness in the image and automatically adjust its distance to the dot accordingly.
The microfluidic array, including reservoirs and electrodes, is about the size of a postage stamp. It fits easily atop a microscope slide which is placed on a small platform with a circular hole in the middle.
The microscope is located beneath the slide, and refocuses using a piezoelectric transducer – a device that allows extremely small displacements of special materials when exposed to electric potential. The microscope image is routed to a digitalcamera that records 20 frames per second. The entire system operates at room temperature and pressure.
Using the device, the scientists were able to move a single dot very accurately along a planned trajectory at an average rate of about two micrometers per second, pausing at intervals between blinks.
This “ability to individually select, characterize and position single nanoscopic objects with nanometer precision,” the team writes, “could enable integration of single quantum dots, or other visualizable nanoscale objects, with photonic structures and enable the development of novel nanophotonic devices and sensors.”
Curt Suplee | Newswise Science News
APEX takes a glimpse into the heart of darkness
25.05.2018 | Max-Planck-Institut für Radioastronomie
First chip-scale broadband optical system that can sense molecules in the mid-IR
24.05.2018 | Columbia University School of Engineering and Applied Science
The more electronics steer, accelerate and brake cars, the more important it is to protect them against cyber-attacks. That is why 15 partners from industry and academia will work together over the next three years on new approaches to IT security in self-driving cars. The joint project goes by the name Security For Connected, Autonomous Cars (SecForCARs) and has funding of €7.2 million from the German Federal Ministry of Education and Research. Infineon is leading the project.
Vehicles already offer diverse communication interfaces and more and more automated functions, such as distance and lane-keeping assist systems. At the same...
A research team led by physicists at the Technical University of Munich (TUM) has developed molecular nanoswitches that can be toggled between two structurally different states using an applied voltage. They can serve as the basis for a pioneering class of devices that could replace silicon-based components with organic molecules.
The development of new electronic technologies drives the incessant reduction of functional component sizes. In the context of an international collaborative...
At the LASYS 2018, from June 5th to 7th, the Laser Zentrum Hannover e.V. (LZH) will be showcasing processes for the laser material processing of tomorrow in hall 4 at stand 4E75. With blown bomb shells the LZH will present first results of a research project on civil security.
At this year's LASYS, the LZH will exhibit light-based processes such as cutting, welding, ablation and structuring as well as additive manufacturing for...
There are videos on the internet that can make one marvel at technology. For example, a smartphone is casually bent around the arm or a thin-film display is rolled in all directions and with almost every diameter. From the user's point of view, this looks fantastic. From a professional point of view, however, the question arises: Is that already possible?
At Display Week 2018, scientists from the Fraunhofer Institute for Applied Polymer Research IAP will be demonstrating today’s technological possibilities and...
So-called quantum many-body scars allow quantum systems to stay out of equilibrium much longer, explaining experiment | Study published in Nature Physics
Recently, researchers from Harvard and MIT succeeded in trapping a record 53 atoms and individually controlling their quantum state, realizing what is called a...
25.05.2018 | Event News
02.05.2018 | Event News
13.04.2018 | Event News
25.05.2018 | Event News
25.05.2018 | Machine Engineering
25.05.2018 | Life Sciences