Self-assembling nanoparticle arrays can switch between a mirror and a window

By finely tuning the distance between nanoparticles in a single layer, researchers have made a filter that can change between a mirror and a window.

The development could help scientists create special materials whose optical properties can be changed in real time. These materials could then be used for applications from tuneable optical filters to miniature chemical sensors.

Creating a 'tuneable' material - one which can be accurately controlled - has been a challenge because of the tiny scales involved. In order to tune the optical properties of a single layer of nanoparticles - which are only tens of nanometres in size each - the space between them needs to be set precisely and uniformly.

To form the layer, the team of researchers from Imperial College London created conditions for gold nanoparticles to localise at the interface between two liquids that do not mix. By applying a small voltage across the interface, the team have been able to demonstrate a tuneable nanoparticle layer that can be dense or sparse, allowing for switching between a reflective mirror and a transparent surface. The research is published today in Nature Materials.

Study co-author Professor Joshua Edel, from the Department of Chemistry at Imperial, said: "It's a really fine balance - for a long time we could only get the nanoparticles to clump together when they assembled, rather than being accurately spaced out. But many models and experiments have brought us to the point where we can create a truly tuneable layer."

The distance between the nanoparticles determines whether the layer permits or reflects different wavelengths of light. At one extreme, all the wavelengths are reflected, and the layer acts as a mirror. At the other extreme, where the nanoparticles are dispersed, all wavelengths are permitted through the interface and it acts as a window.

In contrast to previous nanoscopic systems that used chemical means to change the optical properties, the team's electrical system is reversible.

Study co-author Professor Alexei Kornyshev, from the Department of Chemistry at Imperial, said: "Finding the correct conditions to achieve reversibility required fine theory; otherwise it would have been like searching for a needle in a haystack. It was remarkable how closely the theory matched experimental results."

Co-author Professor Anthony Kucernak, also from the Department of Chemistry, commented: "Putting theory into practice can be difficult, as one always has to be aware of material stability limits, so finding the correct electrochemical conditions under which the effect could occur was challenging."

Professor Kornyshev added: "The whole project was only made possible by the unique knowhow and abilities and enthusiasm of the young team members, including Dr Yunuen Montelongo and Dr Debarata Sikdar, amongst others who all have diverse expertise and backgrounds."

Electrotunable nanoplasmonic liquid mirror
Yunuen Montelongo, Debabrata Sikdar, Ye Ma, Alastair J. S. McIntosh, Leonora Velleman, Anthony R. Kucernak,    Joshua B. Edel & Alexei A. Kornyshev
Nature Materials (2017) doi:10.1038/nmat4969

Imperial College London

Nanostructured metal coatings let the light through for electrical devices

An array of nanopillars etched by thin layer of grate-patterned metal creates a nonreflective yet conductive surface that could improve electronic device performance. Image courtesy of Daniel Wasserman

Light and electricity dance a complicated tango in devices like LEDs, solar cells and sensors. A new anti-reflection coating developed by engineers at the University of Illinois at Urbana Champaign, in collaboration with researchers at the University of Massachusetts at Lowell, lets light through without hampering the flow of electricity, a step that could increase efficiency in such devices.

The coating is a specially engraved, nanostructured thin film that allows more light through than a flat surface, yet also provides electrical access to the underlying material – a crucial combination for optoelectronics, devices that convert electricity to light or vice versa. The researchers, led by U. of I. electrical and computer engineering professor Daniel Wasserman, published their findings in the journal Advanced Materials.

“The ability to improve both electrical and optical access to a material is an important step towards higher-efficiency optoelectronic devices,” said Wasserman, a member of the Micro and Nano Technology Laboratory at Illinois.

At the interface between two materials, such as a semiconductor and air, some light is always reflected, Wasserman said. This limits the efficiency of optoelectronic devices. If light is emitted in a semiconductor, some fraction of this light will never escape the semiconductor material.

Alternatively, for a sensor or solar cell, some fraction of light will never make it to the detector to be collected and turned into an electrical signal. Researchers use a model called Fresnel’s equations to describe the reflection and transmission at the interface between two materials.

“It has been long known that structuring the surface of a material can increase light transmission,” said study co-author Viktor Podolskiy, a professor at the University of Massachusetts at Lowell.

“Among such structures, one of the more interesting is similar to structures found in nature, and is referred to as a ‘moth-eye’ pattern: tiny nanopillars which can ‘beat’ the Fresnel equations at certain wavelengths and angles.”

Although such patterned surfaces aid in light transmission, they hinder electrical transmission, creating a barrier to the underlying electrical material.

“In most cases, the addition of a conducting material to the surface results in absorption and reflection, both of which will degrade device performance,” Wasserman said.

The Illinois and Massachusetts team used a patented method of metal-assisted chemical etching, MacEtch, developed at Illinois by Xiuling Li, U. of I. professor of electrical and computer engineering and co-author of the new paper. The researchers used MacEtch to engrave a patterned metal film into a semiconductor to create an array of tiny nanopillars rising above the metal film. The combination of these “moth-eye” nanopillars and the metal film created a partially coated material that outperformed the untreated semiconductor.    

“The nanopillars enhance the optical transmission while the metal film offers electrical contact. Remarkably, we can improve our optical transmission and electrical access simultaneously,” said Runyu Liu, a graduate researcher at Illinois and a co-lead author of the work along with Illinois graduate researcher Xiang Zhao and Massachusetts graduate researcher Christopher Roberts.

The researchers demonstrated that their technique, which results in metal covering roughly half of the surface, can transmit about 90 percent of light to or from the surface. For comparison, the bare, unpatterned surface with no metal can only transmit 70 percent of the light and has no electrical contact.

The researchers also demonstrated their ability to tune the material’s optical properties by adjusting the metal film’s dimensions and how deeply it etches into the semiconductor.

“We are looking to integrate these nanostructured films with optoelectronic devices to demonstrate that we can simultaneously improve both the optical and electronic properties of devices operating at wavelengths from the visible all the way to the far infrared,” Wasserman said.

The National Science Foundation and Lam Research supported this work.

University of Illinois at Urbana Champaign

Twente researchers develop flexo-electric nanomaterial

Researchers at the University of Twente's MESA+ research institute, together with researchers from several other knowledge institutions, have developed a ‘flexo-electric’ nanomaterial.

The material has built-in mechanical tension that changes shape when you apply electrical voltage, or that generates electricity if you change its shape. In an article published in the leading scientific journalNature Nanotechnology, the researchers also show that the thinner you make the material, the stronger this flexo-electric effect becomes. Professor Guus Rijnders, who was involved in the research, describes this as a completely new field of knowledge with some interesting applications. You could use the material to recharge a pacemaker inside the human body, for example, or to make highly sensitive sensors. 

Piezoelectric materials are widely used in electronic applications. In specific terms, these are crystalline materials that can convert electrical power into pressure and vice versa. The disadvantage of these materials is that they contain lead - which has environmental and health risks - and that the piezoelectric effect decreases when you make the material thinner. 

The thinner the material, the stronger the effect

 

Ever since the 1960s physicists have been arguing that the flexo-electric effect could exist. This would enable non-piezoelectric materials to be given piezoelectric properties. At that time, however, manufacturing methods were inadequate for the production of such materials.

Now, researchers from the University of Twente, the Catalan Institute of Nanoscience and Nanotechnology and Cornell University have succeeded in developing a flexo-electric nano system just 70 nanometres thick. It turns out that even though the flexo-electric effect is very weak, the thinner you make the material, the stronger the effect becomes.

Ultrasensitive sensors

According to Professor Guus Rijnders, who was involved in the research, it will eventually be possible to create flexo-electric materials with a thickness of just a few atomic layers. This discovery could have all kinds of interesting applications. ‘You could make sensors that can detect a single molecule, for example. A molecule would land on a vibrating sensor, making it just fractionally heavier, slowing the vibration just slightly.

The reduction in frequency could then easily be measured using the flexo-electric effect.’ In addition to ultra-sensitive sensors, flexo-electric materials could also be useful in applications that require a limited amount of power, but which are difficult to reach, such as in pacemakers or cochlear implants inside the human body.

University of Twente

The Antimatter puzzle: Searching for clues with a highly integrated particle sensor

Researchers in Munich have presented a highly sensitive sensor for precise measurement of particle tracks. This is the first module for the Vertex Detector of the Belle II experiment at the Japanese accelerator center KEK. The detector is expected to start operation in 2017, recording collisions between electrons and their antiparticles, positrons.

With this experiment, scientists are pursuing the question of why there is no antimatter to speak of in today’s universe.

The sensor was developed by the MPG Halbleiterlabor (HLL), the semiconductor laboratory of the Max Planck Society (MPG). The Belle II Vertex Detector is being created in an international collaboration led by the Max Planck Institute for Physics.

In the experiment, scientists bring matter and antimatter particles into collision and analyze the decay patterns of the mesons and corresponding antiparticles produced. “We are searching for infinitesimal differences. For that, precisely spotting the decay location – also known as the vertex – is crucial,” explains Prof. Christian Kiesling, a researcher at the Max Planck Institute for Physics.

"These measurements are performed by the recently finished sensor, with characteristics that make it unrivaled worldwide.” "Made from silicon a thousand times more pure than conventional transistors or memory chips, the module integrates 200,000 DEPFET pixel cells on a surface area of eight square centimeters. (DEPFET stands for “depleted p-channel field-effect transistor.”)

It was invented at the MPG HLL and is fabricated there exclusively.The DEPFET component enables the detection of photons – or, as in this case, of highly energetic particles – with the utmost efficiency and precision. “The fundamental process is very similar to what goes on in a conventional photo or video camera,” explains Dr. Jelena Ninkovic, head of the HLL.

“However, the primary signal upon detection of individual photons or particles is very much smaller.”Self-amplifying sensorThis is where the major advantage of the DEPFET comes into play: The tiny primary signal is amplified within the sensor itself. Thus the DEPFET is the sensor material and the first stage of amplification rolled into one. Arranging many DEPFETs in a matrix produces an image sensor with which a particle’s point of origin can be precisely determined. “In our case,” Ninkovic continues, “this can be done with an accuracy of around one-hundredth of a millimeter.

”Control of the pixels in a matrix and rapid processing of the DEPFET signals require additional electronics, which have been produced in collaboration with German universities.

These electronics, in the form of application-specific integrated circuits (ASICs), are placed directly on the sensor substrate. The ASICs allow digitization of signals from the pixel matrix, as well as lossless data compression, to transmit them at the highest speed (50,000 images per second).

Complex electronics on a hair-thin filmThus the DEPFET matrix becomes a very complex module with maximal integration density, which despite all its complexity is extremely thin and light, so that the measurement of particle tracks cannot be corrupted by the sensor material itself.

The HLL has developed a unique technology for this purpose that makes it possible to fabricate extremely thin and highly integrated sensor modules. In the process, the sensitive part of the module, the DEPFET matrix, is thinned by a customized etching technique to 75 micrometers, roughly the thickness of a human hair.

These bendable silicon films are supported by a monolithically integrated framework, on which the readout and control electronics are mounted. The power supply and data lines run through a flexible ribbon cable, which is attached to the end of the module.The HLL technology makes it possible to arrange the thin DEPFET matrices in a cylindrical form, without any further support, around the interaction point of the experiment. With that, the highly precise measurement of particle tracks is becoming reality.

Max Planck Institute for Physics

How to spawn an “exceptional ring”

A schematic drawing of how a ring of exceptional points (shown in white) can be spawned from a Dirac point (a dot), and thus change the dispersion from the normal, widely known conical shape into an exotic lantern-like shape
Courtesy of the researchers

Researchers create exotic states that could lead to new kinds of sensors and optical devices.

The Dirac cone, named after British physicist Paul Dirac, started as a concept in particle and high-energy physics and has recently became important in research in condensed matter physics and material science. It has since been found to describe aspects of graphene, a two dimensional form of carbon, suggesting the possibility of applications across various fields.

Now physicists at MIT have found another unusual phenomenon produced by the Dirac cone: It can spawn a phenomenon described as a “ring of exceptional points.” This connects two fields of research in physics and may have applications in building powerful lasers, precise optical sensors, and other devices.

The results are published this week in the journal Nature by MIT postdoc Bo Zhen, Yale University postdoc Chia Wei Hsu, MIT physics professors Marin Soljačić and John Joannopoulos, and five others.

This work represents “the first experimental demonstration of a ring of exceptional points,” Zhen says, and is the first study that relates research in exceptional points with the physical concepts of parity-time symmetry and Dirac cones.

Individual exceptional points are a peculiar phenomenon unique to an unusual class of physical systems that can lead to counterintuitive phenomena. For example, around these points, opaque materials may seem more transparent, and light may be transmitted only in one direction. However, the practical usefulness of these properties is limited by absorption loss introduced in the materials.

A schematic picture showing the conical dispersion of a Dirac cone being deformed into a new hour-glass-like shape due to radiation. Courtesy of the researchers

The MIT team used a nanoengineered material called a photonic crystal to produce the exceptional ring. This new ring of exceptional points is different from those studied by other groups, making it potentially more practical, the researchers say.

“Instead of absorption loss, we adopt a different loss mechanism — radiation loss — which does not affect the device performance,” Zhen says. “In fact, radiation loss is useful and is necessary in devices like lasers.”

This phenomenon could enable creation of new kinds of optical systems with novel features, the MIT team says.

“One important possible application of this work is in creating a more powerful laser system than existing technologies allow,” Soljačić says. To build a more powerful laser requires a bigger lasing area, but that introduces more unwanted “modes” for light, which compete for power, limiting the final output.

“Photonic crystal surface-emitting lasers are a very promising candidate for the next generation of high-quality, high-power compact laser systems,” Soljačić says, “and we estimate we can improve the output power limit of such lasers by a factor of at least 10.”

“Our system could also be used for high-precision detectors for biological or chemical materials, because of its extreme sensitivity,” Hsu says. This improved sensitivity is due to another exotic property of the exceptional points: Their response to perturbations is not linear to the perturbation strength.

Normally, Hsu says, it becomes very difficult to detect a substance when its concentration is low. When the concentration of the target substance is reduced by a million times, the overall signal also decreases by a million times, which can make it too small to detect.

“But at an exceptional point, it’s not linear anymore,” Hsu says, “and the signal goes down by only 1,000 times, providing a much bigger response that can now be detected.”

Demetrios Christodoulides, a professor of optics and photonics at the University of Central Florida who was not involved in this work, says, “This represents the first observation of an exceptional ring in a 2-D crystal associated with a two-dimensional band. The MIT work opens up a number of opportunities … in particular, around exceptional points where systems are known on many occasions to behave in a peculiar fashion.”

The research team also included Yuichi Igarashi of NEC Corp. in Japan and MIT research scientist Ling Lu, postdoc Ido Kaminer, Harvard University graduate student Adi Pick, and Song-Liang Chua at DSO National Laboratory in Singapore. The work was supported, in part, by the Army Research Office through MIT’s Institute for Soldier Nanotechnologies, the National Science Foundation, and the Department of Energy.

MIT

Measuring the smallest vibration

absolute zero. This required building a sensor capable of resolving the smallest vibration allowed by quantum mechanics.

Quantum mechanics predicts that a mechanical object, even when cooled to absolute zero, produces small vibrations, called “zero-point fluctuations”. The reason we don’t observe these vibrations in everyday life is that for a tangibly sized object at room temperature, they are much smaller than the vibrations caused by thermal motion of atoms. EPFL researchers have overcome this challenge by coupling a micrometer-sized glass string to a very precise, optical displacement sensor. The sensor is so precise that it can, in principle, resolve the string’s zero-point fluctuations before they are obscured by thermal vibrations. Combining this low-noise readout with a feedback force, the scientists were able to suppress the string’s thermal vibrations to a magnitude only 10 times larger than their zero-point value – in effect realizing an extreme version of a noise cancellation headphone. The work is published in Nature.

Feedback at the quantum limit

Feedback is a ubiquitous tool in modern engineering, used in applications ranging from cruise control to atomic clocks. The basic paradigm uses a sensor to monitor the state of a system (e.g. a car's velocity) and an actuator (e.g. an engine throttle) to steer the system along a desired path (e.g. within the ed limit).

As sensor technology advances, it has been proposed to use such "feedback control" to prepare and stabilize delicate quantum states – for instance, the celebrated half-living, half-dead state of Schrodinger's Cat. Aside from their fundamental interest, the ability to cultivate quantum states is expected to play a crucial role in future technologies such as quantum computers.

The main challenge to quantum feedback control is something called "decoherence", which dictates that the behavior of a system in a quantum state is rapidly destroyed by its interaction with the thermal environment. This places stringent requirements on the speed and precision of the sensor. Successful demonstrations have therefore been limited to a small subset of well-isolated systems, like individual trapped atoms, photons, and superconducting circuits.

Shedding light on the problem

The lab of Tobias J. Kippenberg at EPFL has fabricated an extremely precise optical position sensor that may – surprisingly – extend quantum feedback control to engineered mechanical devices . In the "blink of an eye" (0.3 - 0.4 seconds) the sensor is capable of resolving a displacement 100 times smaller than the size of a proton. Making use of such a high-speed sensor, it is possible to capture an image in which the blur (or uncertainty) of an object's position is smaller than the uncertainty caused by the thermal motion of its constituent atoms.

Using a continuous stream of such "freeze frames", the researchers have used feedback to reduce the motion of a mechanical device – in this case the vibration of a micron-sized, glass string – to the value it would have if the device were cooled 0.001 degrees above absolute zero. The residual vibration of the string is only 10 times bigger than the minimum (``zero-point”) value allowed by quantum mechanics. This means that the string spends 10% of its time in its quantum “ground state.”

Kippenberg’s lab specializes in the field of “cavity optomechanics”. In optomechanics, much like in high-speed photography, the motion of a mechanical device is imaged using an intense beam of light. The challenge in this case was to focus the light into a very small spot, in order to maximize its interaction with the tiny string.

The researchers achieved this by confining the light and the string to a miniature “hall of mirrors” called an “optical microcavity”. Developed in the Center of MicroTechnology at EPFL, the optical microcavity is a small, disk-shaped piece of glass, above which the string is suspended by 50 nm. Laser light coupled into the disk circulates along its periphery ~10,000 times, each time reflecting off the string and incurring a small delay in proportion to the string’s vibration amplitude. This delay is measured using a technique called interferometry.

To cool the string’s vibration, the researchers took advantage of a well-known side-effect of optical measurement: namely, that each reflection of the circulating field also imparts a small force, called “radiation pressure”, on the string. Using a sequence of electronics, the researchers imprinted a measurement of the string’s vibration onto the intensity of a second laser field. As lead author Dal Wilson explains, “The radiation pressure applied by the second field, when appropriately delayed, exactly opposes the thermal motion of the string, like a noise-cancellation headphone”.

http://www.nanotechnologyworld.org/#!Measuring-the-smallest-vibration/c89r/55c8cde20cf2244af607fa2e

A graphene‐based sensor that is tunable and highly sensitive

Researchers at EPFL and ICFO have developed a sensor made from graphene to detect molecules such as proteins and drugs. This is one of the first devices exploiting the unique electronic and optical properties of graphene for a practical application. The work is published in Science. 

Many areas of fundamental research are interested in graphene owing to its exceptional characteristics. It is made of one layer of carbon atoms, which makes it light and sturdy, and it is an excellent thermal and electrical conductor. Despite its apparently limitless potential, however, few applications have been demonstrated to date. Scientists at EPFL’s Bionanophotonic Systems Laboratory (BIOS) together with researchers from the Institute of Photonic Sciences (ICFO, Spain) have now added another one. They have harnessed graphene’s unique optical and electronic properties to develop a reconfigurable highly sensitive molecule sensor. The results are described in an article appearing in the latest edition of the journalScience.

Focussing light to improve sensing 

The researchers used graphene to improve on a well-­‐known molecule-­‐detection method: infrared absorption spectroscopy. In the standard method, light is used to excite the molecules, which vibrate differently depending on their nature. It can be compared to a guitar string, which makes different sounds depending on its length. By virtue of this vibration, the molecules reveal their presence and even their identity. This “signature” can be “read” in the reflected light.

This method is not effective, however, in detecting nanometrically-­‐sized molecules. The wavelength of the infrared photon directed at a molecule is around 6 microns (6,000 nanometres – 0.006 millimeters), while the target measures only a few nanometres (about 0.000001 mm). It is very challenging to detect the vibration of such a small molecule in reflected light.

There is where graphene comes in. If given the correct geometry, the graphene is able to focus the light on a precise spot on its surface and “hear” the vibration of a nanometric molecule that is attached to it. “We first pattern nanostructures on the graphene surface by bombarding it with electron beams and etching it with oxygen ions,” said Daniel Rodrigo, co‐author of the publication. “When the light arrives, the electrons in graphene nanostructures begin to oscillate. This phenomenon, known as ‘localized surface plasmon resonance,’ serves to concentrate light into tiny spots, which are comparable with the dimensions of the target molecules. It is then possible to detect nanometric structures.”

Reconfiguring graphene in real time to see the molecule’s structure

There is more to it. In addition to identifying the presence of nanometric molecules, this process can also reveal the nature of the bonds connecting the atoms that the molecule is composed of.

When a molecule vibrates, it does not give off only one type of "sound." It produces a whole range of vibrations, which are generated by the bonds connecting the different atoms. Returning to the example of the guitar: each string vibrates differently and together they form one musical instrument. These nuances provide information on the nature of each bond and on the health of the entire molecule. “These vibrations act as a fingerprint that allow us to identify the molecule; such as proteins, and can even tell their health status” said Odeta Limaj, another co-­author of the publication.

In order to pick up the sound given off by each of the strings, it has to be possible to identify a whole range of frequencies. And that is something graphene can do. The researchers “tuned” the graphene to different frequencies by applying voltage, which is not possible with current sensors. Making graphene's electrons oscillate in different ways makes it possible to “read” all the vibrations of the molecule on its surface. “We tested this method on proteins that we attached to the graphene. It gave us a full picture of the molecule,” said Hatice Altug.

A big step closer to using graphene for molecule sensing

The new graphene-­‐based process represents a major step forward for the researchers, for several reasons. First, this simple method shows that it is possible to conduct a complex analysis using only one device, while it normally requires many different ones. And all this without stressing or modifying the biological sample. Second, it shows graphene’s incredible potential in the area of detection. “There are many possible applications,” said Altug. “We focussed on biomolecules, but the method should also work for polymers, and many other substances,” she added.

http://www.nanotechnologyworld.org/#!A-graphene‐based-sensor-that-is-tunable-and-highly-sensitive/c89r/55b64f500cf25507a8bff844