Developing Gold Nanoparticle-Embedded Dielectric Thin Films

research on noble metal nanoparticles has always remained interesting because of their optical and electronic properties. Gold nanoparticles (AuNPs), in particular, have been intensively studied for their fascinating localized surface plasmon resonance (LSPR) peak in the visible region of electromagnetic spectrum. The tunable nature of LSPR of AuNPs leads to a large number of applications of AuNPs in the fields of plasmonics and bioscience.

The LSPR peaks of AuNPs can be tuned from the visible to the near-infrared region by controlling the shape, size, and structure of the particles. Another way of changing the optical properties is to change the surrounding of AuNPs as the LSPR peak is also very sensitive to the dielectric properties of the surrounding medium. The later method becomes an easy and superior one when the nanoparticles are embedded in a dielectric matrix. If the surrounding matrix is a transparent one, at least in the region of the spectrum where LSPR occurs, it becomes an added advantage for many photonic and plasmonic applications.

In order to explore and control the effects of shape, size, structure and dielectric media on LSPR properties, several preparation methods are used to form hybrid structures of nanoparticles and dielectric interfaces. In the present work, different ways of tuning LSPR positions of metal-dielectric nanocomposite thin films have been demonstrated with AuNPs embedded indium oxide (Au:IO) thin films as a case study. Au:IO thin films have been prepared by a sequence-specific sandwich method. The films were characterized by glancing angle x-ray diffraction (GXRD), optical absorption, high-resolution transmission electron microscopy (HRTEM) and Rutherford backscattering spectrometry (RBS). The advantages of the sandwich method have been shown by comparing the optical properties of Au:IO thin films so formed with that of Au:IO films formed by the most commonly used co-sputtering method.

Importance of the work

The work in the publication titled “Tailoring plasmonic properties of metal nanoparticles embedded dielectric thin films: The sandwich method of preparation” by Ranjit Laha et al. reports the experimental ways of tuning plasmonic positions (following a derived working formula) of AuNP embedded metal oxide thin films. The novelty lies in followings

• A specific sequence of the sandwich method has been suggested that enables the many independent ways of tuning the plasmonic positions. The different steps involved such as thermal evaporation, sputtering and intermediate annealing are common techniques used in industry as well as research & development laboratories. Therefore, it is not very difficult to adopt the method.
• Controlled agglomeration-cum-self assembling process has been used as an advantage in tuning the plasmonic properties against the common belief that “agglomeration of small particles into big ones is a disadvantage in many nanoparticles research problems”.
• The shape of the embedded AuNPs has been characterized to be oblates by a combination of HRTEM and RBS studies. Normally, characterizing shape of buried particles is a difficult proposition.
• An empirical working formula has been derived for tailoring the plasmonic position of any pair of the metal-dielectric nanocomposite. Thus, by knowing the shape and volume fraction of the embedded nanoparticles, plasmonic position can be worked out.
• To the best of our belief, the work will have impact among the researchers working on nurturing the nanoscale properties in photonic and plasmonic applications.

This study, Tailoring plasmonic properties of metal nanoparticle-embedded dielectric thin films: the sandwich method of preparation was recently published in the Journal of Nanoparticle Research.

Nano aluminium offers fuel cells on demand – just add water

The accidental discovery of a novel aluminium alloy that reacts with water in a highly unusual way may be the first step to reviving the struggling hydrogen economy. It could offer a convenient and portable source of hydrogen for fuel cells and other applications, potentially transforming the energy market and providing an alternative to batteries and liquid fuels.

“The important aspect of the approach is that it lets you make very compact systems,” says Anthony Kucernak, who studies fuel cells at Imperial College London and wasn’t involved with the research. “That would be very useful for systems which need to be very light or operate for long periods on hydrogen, where the use of hydrogen stored in a cylinder is prohibitive.”

The discovery came in January, when researchers at the US Army Research Laboratory at Aberdeen Proving Ground, Maryland, were working on a new, high-strength alloy, says physicist Anit Giri. When they poured water on it during routine testing, it started bubbling as it gave off hydrogen.

That doesn’t normally happen to aluminium. Usually, when exposed to water, it quickly oxidises, forming a protective barrier that puts a stop to any further reaction. But this alloy just kept reacting. The team had stumbled across the solution to a decades-old problem.

Hydrogen has long been touted as a clean, green fuel, but it is difficult to store and move around because of its bulk. “The problem with hydrogen is always transportation and pressurisation,” says Giri.

Slow reaction

If aluminium could be made to effectively react with water, it would mean hydrogen on demand. Unlike hydrogen, aluminium and water are easy to carry – and both are stable. But previous attempts to drive the reaction required high temperatures or catalysts, and were slow: obtaining the hydrogen took hours and was around 50 per cent efficient.

The new alloy, which the team is in the process of patenting, is made of a dense powder of micron-scale grains of aluminum and one or more other metals arranged in a particular nanostructure. Adding water to the mix produces aluminium oxide or hydroxide and hydrogen – lots of it. “Ours does it to nearly 100 per cent efficiency in less than 3 minutes,” says team leader Scott Grendahl. Moreover, the new material offers at least an order of magnitude more energy than lithium batteries of the same weight. And unlike batteries, it can remain stable and ready for use indefinitely.

The army team has used the material to power a small, radio-controlled tank. Grendahl doesn’t see any practical issues with scaling up production to produce hundreds of tonnes of the stuff as it can be made from scrap aluminium, which is relatively cheap. The new material could power everything from laptops to buses and cars.

“In principle, the process should work,” says Robert Steinberger-Wilckens, who directs a fuel cell programme at the University of Birmingham, UK.

But he cautions that a repeat experiment is needed to show that the reaction works the way it should. “There’s a lot of stuff that works in the laboratory but not in the field.”

If it does pan out, the powder could also be used as the raw material for 3D printing. The researchers have put forward proposals – now being considered by the army – for small air or ground robots that use their own structure as fuel. These self-cannibalising machines would be useful for one-way intelligence-gathering missions, burning themselves up at the end to leave no trace.

more information: https://www.newscientist.com/article/2142693-nano-aluminium-offers-fuel-cells-on-demand-just-add-water/

Hybrid graphene–quantum dot phototransistors with ultrahigh gain

Graphene is an attractive material for optoelectronics1 and
photodetection applications2–6 because it offers a broad spectral
bandwidth and fast response times. However, weak light
absorption and the absence of a gain mechanism that can generate
multiple charge carriers from one incident photon have
limited the responsivity of graphene-based photodetectors to
∼1022 AW21. Here, this group demonstrate a gain of ∼108 electrons
per photon and a responsivity of∼107 AW21 in a hybrid photodetector
that consists of monolayer or bilayer graphene covered
with a thin film of colloidal quantum dots. Strong and tunable
light absorption in the quantum-dot layer creates electric
charges that are transferred to the graphene, where they
recirculate many times due to the high charge mobility of
graphene and long trapped-charge lifetimes in the quantumdot
layer. The device, with a specific detectivity of 731013
Jones, benefits from gate-tunable sensitivity and speed,
spectral selectivity from the short-wavelength infrared to the
visible, and compatibility with current circuit technologies.

more information: Gerasimos Konstantatos et al.  nature nanotechnology, DOI: 10.1038/NNANO.2012.60.

Our New paper in journal of magnetism and magnetic materials

Congratulations for the publication of paper “Enhanced Faraday rotation in one dimensional magneto-plasmonic structure due to Fano resonance” by S. Sadeghi and S. M. Hamidi.

Abstract
Enhanced Faraday rotation in a new type of Magneto-plasmonic structure with the capability of Fano resonance, has been reported theoretically. A magneto-plasmonic structure composed of a Gold corrugated layer deposited on a magneto-optically active layer was studied by means of Lumerical software based on finite-difference time-domain. In our proposed structure, plasmonic Fano resonance and localized surface plasmon induced enhancement in magneto-optical Faraday rotation. It is shown that the influence of geometrical parameters in Au layer offers a desirable platform for engineering spectral position of Fano resonance and enhancement of Faraday rotation.

Quantitative Evaluation of Nanosecond Pulsed Laser-Induced Photomodification of Plasmonic Gold Nanoparticles

Biophotonic application of plasmonic gold nanoparticles has become a highly active field of research in recent years due to their unique chemical and physical properties, such as high absorption cross sections and spectral tunability¹. Many of the unique properties of gold nanoparticles are governed by the surface plasmon resonance (SPR) effect, a collective oscillation of electrons on the nanoparticle surface that occurs when excited with light at an appropriate wavelength. The SPR results in a strongly enhanced electromagnetic field near the particle surface, which causes unique, shape- and material-dependent spectral variations in light absorption and scattering. These properties allow nanoparticles to be used not only as therapeutic agents^2,3,4, but also for diagnostic imaging^5,6. The therapeutic effects produced by laser-nanoparticle interaction can occur through a variety of mechanisms. Photothermal transduction causes rapid heating in a localized area around the irradiated nanoparticles and has been used for the treatment of solid tumors in vivo, with both continuous-wave and nanosecond-pulsed lasers^7,8,9. Photomechanical effects, such as cavitation, can occur when exposing plasmonic nanoparticles to pulsed laser light (nanosecond-to-femtosecond), and the resultant bubbles are capable of disrupting cancer cell membranes^10,11,12. Photochemical effects, like the production of reactive oxygen species, have also been observed from laser irradiation of gold nanoparticles, with pulsed lasers having a greater effect than continuous-wave lasers^13,14. The properties that make gold nanoparticles so effective for laser-based therapeutics can also lead to unintended side-effects in diagnostic procedures.

One of the most promising diagnostic techniques that may employ gold nanoparticle contrast agents is photoacoustic imaging (PAI), a rapidly maturing biomedical modality capable of macro- and micro-scale imaging⁵. In PAI, tissue is illuminated with a nanosecond pulsed laser, typically a Q-switched Nd:YAG with pulse duration in the range of 5–10 ns, at exposures below standard safety limits¹⁵. This exposure causes rapid optical absorption and thermal expansion that produces acoustic waves, which can be detected using an ultrasonic transducer. The use of gold nanoparticles as contrast agents for PAI has the potential to generate photothermal, photomechanical, or photochemical effects that result in tissue injury. Furthermore, the spectral changes brought about by nanoparticle photomodification can cause spectral shifts in absorption that degrade PAI-nanoparticle product performance¹⁶.

While there are well-established laser safety standards for the skin and eye, these standards are insufficient when exogenous chromophores such as nanoparticles are present. Additionally, no standards exist for evaluating the performance of laser-nanoparticle combination products for medicine. In recent years, a number of studies have contributed knowledge on the laser-nanoparticle interaction processes and resultant bioeffects, as well as test methods that could form the basis of new standards. One key step in elucidating laser-nanoparticle interactions is to understand the photostability of nanoparticles and characterize laser induced photomodification processes, including photothermally-induced melting/reshaping effects that have been documented for different types of metallic nanoparticles^{16,17,18,19,20,21,22}. they chose to study spherical gold nanoparticles as their surface plasmon resonance is close to the wavelength (532 nm) of the second-harmonic Q-switched Nd:YAG laser, which is broadly employed in biophotonics and photomedicine^23,24,25, and because these nanoparticles possess well-defined morphological properties and their spherical shape is well-suited for theoretical investigation.

Several prior studies have evaluated the effect of pulsed laser irradiation on gold nanospheres – more accurately gold “pseudospheres” since most of the particles studied, as well as commercially available versions, have faceted rather than smooth edges. One such study by Takami et al. looked at the size reduction of approximately 50 nm gold nanospheres at different pulse energies¹⁸. Werner et al. investigated the effect of irradiation wavelength on 55 nm gold nanospheres and was mostly concerned with modeling the difference between interband and intraband excitation²². Another study used dynamic light scattering (DLS), electron microscopy and UV-Vis spectroscopy to examine the impact of a single laser pulse on particles of different sizes²⁶. While results were presented for a range of energy levels, specific damage thresholds were not determined and comparison with theoretical results was not presented. These prior studies have provided significant insights, yet are deficient in terms of generating a standardized test methodology as they either only used a single nanoparticle size or provided a qualitative description of the damage with no well-defined threshold.

Our overall objective was to facilitate the optimization of safety and effectiveness in emerging biophotonic products incorporating nanoparticles. Previous works have not established a common approach for assessing laser damage thresholds in nanoparticles, and information on the role of particle size in nanosphere damage is typically spread across multiple reports with inconsistent experimental conditions. Therefore, the goals of this proof-of-concept study were to implement and assess methodologies to quantitatively determine nanoparticle photomodification thresholds, and to generate data on the melting process and damage thresholds that can be used to improve understanding of this process. Specifically, we have conducted quantitative experimental and analytical investigation of the interaction between nanosecond laser pulses at 532 nm and plasmonic gold nanoparticles with diameters from 20 to 100 nm over a wide range of laser radiant exposures.

More information: Andrew M. Fales et al. Quantitative evalution of nanosecond pulsed laser-induced photomodification of plasmonic gold nanoparticles, Scientific Reports(2017). 10.1038/s41598-017-16052-7.

Optoelectronics without glass

Microscopic image of a chip. Top left: functional modulator with electrical contacts; right: test modulator without electrical contact; below: test components.

Researchers at ETH Zurich have developed the first opto-electronic circuit component that works without glass and is instead made of metal. The component, referred to as a modulator, converts electrical data signals into optical signals. It is smaller and faster than current modulators, and much easier and cheaper to make.

Optical components for microelectronics must be made of glass. Metals are not suitable for this purpose, since optical data can propagate only across roughly a distance of 100 micrometres. This was the general view of scientists until recently. A team of researchers headed by Juerg Leuthold, professor in the Department of Information Technology and Electrical Engineering, has now succeeded in doing what was thought to be impossible and developed a light-processing component made of metal. The researchers have presented their findings in the latest issue of the journal Science.

They accomplished this feat by building a small enough component: at just 3 x 36 micrometres, it is within a size range in which both optical and electrical information can propagate in metals.

Component for fibre optic networks

The component is a modulator: modulators convert electrical data signals into optical signals. They are installed in modern internet routers used for fibre optic networks and enable fibre optic data connections between computer units in data centres. However, the standard components used today function differently than the new modulators.

The new component works by aiming the light from a fibre optic source at the modulator, causing the electrons on its surface to oscillate. Experts refer to this as a surface plasmon oscillation. This oscillation can be changed indirectly by electrical data pulses. When the oscillation of the electrons is converted back into light, the electrical information is now encoded onto the optical signal. This means that the information is converted from an electrical into an optical data pulse that can be transmitted via fibre optics.

Schematic representation of the metallic modulator: Left: a continuous beam of light strikes a metallic lattice that deflects the light onto the chip. Right: an optical data pulse exits the component.

Faster and smaller

Two years ago, Leuthold and his colleagues developed one of these plasmonic modulators. At the time, it was the smallest and fastest modulator ever built, but the semiconductor chip still had various glass components.

By replacing all the glass components with metallic ones, the scientists have succeeded in building an even smaller modulator that works up to highest speed. “In metals, electrons can move at practically any speed, whereas the speed in glass is limited due to its physical properties,” says Masafumi Ayata, a doctoral student in Leuthold’s group and lead author of the study. In the experiment, the researchers succeeded in transmitting data at 116 gigabits per second. They are convinced that with further improvements, even higher data transfer rates will be possible.

Etched from a gold layer

The modulator prototype tested by the ETH researchers is made of a gold layer that lies on a glass surface. The scientists emphasised that the glass has no function. “Instead of the glasslayer, we could also use other suitable smooth surfaces,” says Leuthold. It might also be possible to use less expensive copper instead of gold for industrial applications. The important point is that only one metallic coating is required for the new modulators. “This makes them much easier and cheaper to fabricate,” says Leuthold.

The researchers are already working with an industrial partner in order to put the new modulator into practice, and talks with other partners are in progress. However, Leuthold believes that further development may be required before the technology is ready for the market; for example, he expects that the current loss of signal strength during modulation can be reduced further.

For computers and autonomous vehicles

The new modulator could one day be used not only for telecommunications applications, but for computers as well. “The computer industry is considering using fibre optics to transfer data between the individual chips inside computers,” says Leuthold. However, this would require tiny modulators – such as Leuthold and his team have developed.

Ultimately, it is also conceivable that the modulators could be used in displays – including bendable ones – and optical sensors, such as those in the Lidar system for distance measurement that are used in (semi-) autonomous cars.

More information: Masafumi Ayata et al. High-speed plasmonic modulator in a single metal layer, Science (2017). DOI: 10.1126/science.aan5953

Switching light with a silver atom

The switch is based on the voltage-induced displacement of one or more silver atoms in the narrow gap between a silver and a platinum plate.

Researchers working under Juerg Leuthold, Professor of Photonics and Communications, have created the world’s smallest integrated optical switch. Applying a small voltage causes an atom to relocate, turning the switch on or off.

The quantity of data exchanged via communications networks around the globe is growing at a breathtaking rate. The volume of data for wired and mobile communications is currently increasing by 23% and 57% respectively every year. It is impossible to predict when this growth will end. This also means that all network components must constantly be made more efficient.

These components include so-called modulators, which convert the information that is originally available in electrical form into optical signals. Modulators are therefore nothing more than fast electrical switches that turn a laser signal on or off at the frequency of the incoming electrical signals. Modulators are installed in data centres in their thousands. However, they all have the disadvantage of being quite large. Measuring a few centimetres across, they take up a great deal of space when used in large numbers.

From micromodulators to nanomodulators

Six months ago, a working group led by Jürg Leuthold, Professor of Photonics and Communications already succeeded in proving that the technology could be made smaller and more energy-efficient. As part of that work, the researchers presented a micromodulator measuring just 10 micrometres across – or 10,000 times smaller than modulators in commercial use.

Tiny plates made of silver (light grey) and platinum (mint) are placed on an optical waveguide (blue)

Leuthold and his colleagues have now taken this to the next level by developing the world’s smallest optical modulator. And this is probably as small as it can get: the component operates at the level of individual atoms. The footprint has therefore been further reduced by a factor of 1,000 if you include the switch together with the light guides. However, the switch itself is even smaller, with a size measured on the atomic scale. The team’s latest development was recently presented in the journal Nano Letters.

In fact, the modulator is significantly smaller than the wavelength of light used in the system. In telecommunications, optical signals are transmitted using laser light with a wavelength of 1.55 micrometres. Normally, an optical device can not be smaller than the wavelength it should process. “Until recently, even I thought it was impossible for us to undercut this limit,” stresses Leuthold.

New structure

But his senior scientist Alexandros Emboras proved the laws of optics wrong by successfully reconfiguring the construction of a modulator. This construction made it possible to penetrate the order of magnitude of individual atoms, even though the researchers were using light with a “standard wavelength”.

Emboras’s modulator consists of two tiny pads, one made of silver and the other of platinum, on top of an optical waveguide made of silicon. The two pads are arranged alongside each other at a distance of just a few nanometres, with a small bulge on the silver pad protruding into the gap and almost touching the platinum pad.

Set-up used in the lab to test the new type of switches.

Short circuit thanks to a silver atom

And here’s how the modulator works: light entering from an optical fibre is guided to the entrance of the gap by the optical waveguide. Above the metallic surface, the light turns into a surface plasmon. A plasmon occurs when light transfers energy to electrons in the outermost atomic layer of the metal surface, causing the electrons to oscillate at the frequency of the incident light. These electron oscillations have a far smaller diameter than the ray of light itself. This allows them to enter the gap and pass through the bottleneck. On the other side of the gap, the electron oscillations can be converted back into optical signals.

If a voltage is now applied to the silver pad, a single silver atom or, at most, a few silver atoms move towards the tip of the point and position themselves at the end of it. This creates a short circuit between the silver and platinum pads, so that electrical current flows between them. This closes the loophole for the plasmon; the switch flips and the state changes from “on” to “off” or vice versa. As soon as the voltage falls below a certain threshold again, a silver atom moves back. The gap opens, the plasmon flows, and the switch is “on” again. This process can be repeated millions of times.

ETH Professor Mathieu Luisier, who participated in this study, simulated the system using a high-performance computer at the CSCS in Lugano. This allowed him to confirm that the short circuit at the tip of the silver point is brought about by a single atom.

A truly digital signal

As the plasmon has no other options than to pass through the bottleneck either completely or not at all, this produces a truly digital signal – a one or a zero. “This allows us to create a digital switch, as with a transistor. We have been looking for a solution like this for a long time,” summarises Leuthold.

As yet, the modulator is not ready for series production. Although it has the advantage of operating at room temperature, unlike other devices that work using quantum effects at this order of magnitude, it still remains very slow for a modulator: so far, it only works for switching frequencies in the megahertz range or below. The ETH researchers want to fine-tune it for frequencies in the gigahertz to terahertz range.

Improving the lithography process

The researchers also want to further improve the lithography method, which was redeveloped by Emboras from scratch to build the parts, so that components like this can be produced reliably in future. At present, fabrication is only successful in one out of every six attempts. Nevertheless, the researchers consider this a success, as lithography processes on the atomic scale remain uncharted territory.

In order to continue his research into the nanomodulator, Leuthold has strengthened his team. However, he points out that greater resources would be required to develop a commercially available solution. Despite this, the ETH professor is confident that he and his team will be able to present a practicable solution within the next few years.

More information: Alexandros Emboras et al. Atomic Scale Plasmonic Switch, Nano Letters (2016). DOI: 10.1021/acs.nanolett.5b04537

Our New paper in Photonics and Nanostructures: Fundamentals and applications

Congratulations for the publication of paper”All-optical photonic crystal logic gates using nonlinear directional coupler” in Journal of Photonics and Nanostructures – Fundamentals and Applications, by H. Sharifi,  S. M. Hamidi and K. Navi.

Assembly of nanoparticles proceeds like a zipper

According to scientists from the Biohybrid Materials Group, led by Prof. Mauri Kostiainen, nature’s own charged nanoparticles – protein cages and viruses – can be utilized to determine the structure of composite nanomaterials.

Viruses and proteins are ideal model particles to be used in materials science, as they are genetically encoded and have an atomically precise structure. These well-defined biological particles can be used to guide the arrangement of other nanoparticles in an aqueous solution. In the present study, the researchers show that combining native Tobacco Mosaic Virus with gold nanoparticles in a controlled manner leads to metal-protein superlattice wires.

“They initially studied geometrical aspects of nanoparticle superlattice engineering. We hypothesized that the size-ratio of oppositely charged nanorods (TMV viruses) and nanospheres (gold nanoparticles) could efficiently be used to control the two-dimensional superlattice geometry. They were actually able to demonstrate this. Even more interestingly, our structural characterization revealed details about the cooperative assembly mechanisms that proceeds in a zipper-like manner, leading to high-aspect-ratio superlattice wires,” Kostiainen says. “Controlling the macroscopic habit of self-assembled nanomaterials is far from trivial,” he adds.

Superlattice wires potential to form new materials

The results showed that nanoscale interactions really controls the macroscopic habit of the formed superlattice wires. The researchers observed that the formed macroscopic wires undergo a right-handed helical twist that was explained by the electrostatic attraction between the asymmetrically patterned TMV virus and the oppositely charged spherical nanoparticles. As plasmonic nanostructures efficiently affect the propagation of light, the helical twisting resulted in asymmetric optical properties (plasmonic circular dichroism) of the material.

“This result is ground breaking in the sense that it demonstrates that macroscopic structures and physical properties can be determined by the detailed nanostructure, i.e. the amino acid sequence of the virus particles. Genetical engineering routinely deals with designing the amino acid sequence of proteins, and it is a matter of time when similar or even more sophisticated macroscopic habit and structure-function properties are demonstrated for ab-initio designed protein cages,” explains Dr. Ville Liljeström who worked on the project during three years of his doctoral studies.

The research group demonstrated a proof-of-concept showing that the superlattice wires can be used to form materials with physical properties controlled by external fields. By functionalizing the superlattice wires with magnetic nanoparticles, the wires could be aligned by a magnetic field. In this manner they produced plasmonic polarizing films. The purpose of the demonstration was to show that electrostatic self-assembly of nanoparticles can potentially be used to form processable materials for future applications.

 Explore further: Self-assembled nanostructures can be selectively controlled

More information: Ville Liljeström et al. Cooperative colloidal self-assembly of metal-protein superlattice wires, Nature Communications (2017). DOI: 10.1038/s41467-017-00697-z

Read more at: https://phys.org/news/2017-09-nanoparticles-proceeds-zipper.html#jCp

our new paper in IEEE Transaction on magnetism

Congratulations for the publication of paper” characterization of Au/Co/Au Magneto-plasmonic multilayer as an ethanol vapor sensor” in Journal of IEEE transaction on magnetism by M.Afsharia and S. M. Hamidi.