Ultrafast Terahertz Nanoscopy: Social Distancing on the Nanoscale

 

Interlayer excitons (glossy ellipsoids), which can form after electrons and holes (red and blue spheres) are separated between optically excited, atomically thin layers (top and bottom sheet). Credit: Markus Plankl (2021)

https://scitechdaily.com/ultrafast-terahertz-nanoscopy-social-distancing-on-the-nanoscale/

A team of physicists from Germany, the US and the UK managed to observe the motion of electrons from one atomically thin layer into an adjacent one with nanoscale spatial resolution. The new contact-free nanoscopy concept, which shows great potential for investigations into conducting, nonconducting, and superconducting materials, will be introduced in the new volume of the science journal Nature Photonics.

Nanotechnology sometimes still sounds like science fiction, but is already an integral part of modern electronics in our computers, smart phones, and cars. The size of electronic components, like transistors and diodes has reached the nanoscale, corresponding to only one-millionth of a millimeter. This makes conventional optical microscopes no longer sufficient for inspecting these nanostructures. To develop innovative future nanotechnology, scientists have replaced the optical microscope with much more sophisticated concepts, such as electron or scanning tunneling microscopy. However, these techniques use electrons instead of light, which can influence the properties of nanoscale devices. Furthermore, these important measurement techniques are limited to electrically conducting samples.

A team of physicists around Rupert Huber and Jaroslav Fabian at the Regensburg Center for Ultrafast Nanoscopy (RUN) at Universität Regensburg together with colleagues Tyler Cocker from Michigan State University, USA, and Jessica Boland from the University of Manchester, UK, have introduced a new technique, which can resolve electron motion on the nanoscale without needing to be electrically contacted. Better still, the new method also reaches unbelievable time resolution as good as one quadrillionth of a second (the femtosecond timescale). Combining these extreme spatial and temporal resolutions makes the recording of slow-motion movies of ultrafast electron dynamics on the nanoscale possible.

The concept behind the technique works similar to contactless payment (Chipcard, Phone, Scanner), which has become an increasingly common component in our lives since the start of the pandemic. These payment methods are based on established frequencies and protocols on the macroscale such as Near Field Communication (NFC). Here, the scientists transferred this idea down to the nanoscale by using a sharp metallic tip as a nano-antenna, which is brought close to the investigated sample.

In contrast to the aforementioned established techniques, where tips are used to drive a current through the sample, the new concept uses a weak alternating electric field to scan the sample contactlessly. The frequency used in the experiments is boosted to the terahertz spectral range, approximately 100,000 times higher than the one used in NFC scanners. Minute changes in these weak electric fields allow for precise conclusions about the local electron motion within the material.

Combining the measurements with a realistic quantum theory shows that the concept even allows for quantitative results. In order to achieve high temporal resolution additionally, the physicists used extremely short light pulses to record crisp snapshots of the movement of electrons over nanometer distances.

The team chose a sample of a new material class called transition metal dichalcogenides, which can be produced in atomically thin layers, as their first test sample. When these sheets are stacked under freely chosen angles, new artificial solids emerge with novel material properties, which are prominently investigated in the Collaborative Research Center 1277 in Regensburg.

The sample under study was made from two different atomically thin dichalcogenides to test the centerpiece of a futuristic solar cell. When shining green light onto the structure, charge carriers emerge that will move in one or the other direction depending on their polarity – the basic principle of a solar cell, which converts light into electricity. The ultrafast charge separation was observed by the scientists in time as well as in space with nanometer precision. To their surprise, the charge separation even works reliably when the dichalcogenide layers lay over tiny impurities like a mini carpet – important insights to optimize these new materials for future use in solar cells or computer chips.

The researchers are exhilarated by their insightful results. “We can’t wait to videotape further fascinating charge transfer processes in insulating, conducting and superconducting materials,” explains Markus Plankl, first author of the publication. Postdoctoral colleague and co-author Thomas Siday adds: ”Insights on the ultrafast transport on the relevant length and timescales will help us to understand how tunneling shapes the functionalities in a wide range of condensed matter systems.”

Besides nanostructures in physics, previously elusive quantum processes in biological systems can now be accessed. These results reflect the increasing focus of researchers from biology, chemistry, and physics at the University of Regensburg towards ultrafast nanoscopy, which led to the approval of the new Regensburg Center for Ultrafast Nanoscopy (RUN). The RUN building, which is currently under construction on the University campus, should provide the optimal environment for such an interdisciplinary exploration of the nanocosm.

Reference: “Subcycle contact-free nanoscopy of ultrafast interlayer transport in atomically thin heterostructures” by M. Plankl, P. E. Faria Junior, F. Mooshammer, T. Siday, M. Zizlsperger, F. Sandner, F. Schiegl, S. Maier, M. A. Huber, M. Gmitra, J. Fabian, J. L. Boland, T. L. Cocker and R. Huber, 13 May 2021, Nature Photonics.
DOI: 10.1038/s41566-021-00813-y

Abstract-Convergence of terahertz radiation and nanotechnology

 

Nicholas B. Lawler, Diwei Ho, Cameron W. Evans,  Vincent P. Wallace,   K. Swaminathan Iyer, 

https://pubs.rsc.org/en/content/articlelanding/2020/tc/d0tc01716g#!divAbstract

There is a growing number of advanced applications that integrate nanotechnology and terahertz radiation. The development of these niche applications has relied on convergent multidisciplinary techniques in conjunction with developments in advanced nanomaterials. The synergistic application of these fields has historically been difficult to realise as the wavelength of terahertz radiation is orders of magnitude larger than the nanoscale. In this review we explore recent developments and unique nanomaterial implementations that have facilitated advancements beyond this size mismatch, resulting in novel techniques offering alternatives to established processes or new avenues of exploration into the study of nanoscale materials and structures.

Abstract-Enhanced spread in time on-off keying technique for dense Terahertz nanonetworks

Hakim Mabed,

http://ieeexplore.ieee.org/document/8024611/

Nanotechnology becomes reality paving the way for many new applications. In nanonetwork system, each nanosized device is equipped with limited capabilities and is dedicated to a basic task but the combination of the numerous devices actions results in high-level functions. In this context, large number of devices concentrated in a limited area must exchange data using wireless links. Spread in Time On-Off Keying (TS-OOK) protocol was proposed as a technique to share the radio channel over the different terahertz nano-devices. TS-OOK is based on a Femtosecond-Long pulse modulation where communication data are sent using a sequence of pulses interleaved by a constant duration randomly selected. In this paper we provide a critical analysis of the TS-OOK approach. We prove that the TS-OOK is not adaptive against the traffic load variation and induces an imbalance between the active nodes. This inequity is due to the dependency of a communication quality on the randomly chosen symbol rate. We propose a dynamic TS-OOK modulation approach, called SRH-TSOOK (Symbol rate Hopping TSOOK), where the duration between two consecutive pulses of the same transmission follows a pseudo-random sequence. We show that this approach performs better than the standard protocol when the number of active nodes increases and guarantees a better distribution of the channel capacity over the active communications. For instance, while the throughput of a communication within a TS-OOK protocol may falls bellow 10⁵ frames/s with 300 active nodes, the throughput in the SRH-TSOOK protocol stabilizes around 20⁷ frames/s for all the active communications. The comparison is made on the basis of probabilistic analysis allowing a numerical and accurate evaluation of the protocols performance

What is the internet of things at nanoscale?

https://internetmedicine.com/2017/04/01/65348/

What is the internet of nanoscale things?

Nanoscale technology is enabling the development of devices as small as one to a few hundred nanometers (10^-9 meters). To give a sense of scale, a strand of human DNA is roughly 2.5 nanometers in diameter. At this scale, a nanomachine is defined as the most basic functional unit and able to perform simple tasks such as sensing or actuation.

The U.S.’ National Nanotechnology Initiative requested $1.5 billion in federal funding in fiscal year 2016 and has been awarded over $22 billion since FY 2001.

Coordination and information sharing among several nanomachines will expand the potential applications of individual devices both in terms of complexity and range of operation, according to the Georgia Institute of Technology. The resulting nano-networks will be able to cover larger areas, and reach hard-to-reach locations. Moreover, the interconnection of nanoscale devices with classical networks and the internet defines a new networking paradigm, to which Georgia Institute of Technology refers to as the “internet of nano-things.”

Use cases

Some potential applications include:

In-body networks monitoring real-time blood, sickness and breath tests;

Use in public locations to monitor the spread of viruses and diseases; and

Hooked up to wearable health and environmental trackers.

When it arrives, the internet of nanoscale things could provide much more detailed, inexpensive and up-to-date pictures of our cities, homes, factories – even our bodies. Today traffic lights, wearables or surveillance cameras are getting connected to the internet with  billions of expected nanosensors harvesting huge amounts of real-time information and beaming it up to the cloud, according to Scientific American.

Methods of communication

It is still not clear how nanomachines are going to communicate. Georgia Tech presents two main alternatives for communication in the nanoscale, namely molecular communication and nano-electromagnetic communication:

Molecular communication:

This is defined as the transmission and reception of information encoded in molecules. Molecular transceivers are expected to be easily integrated in nano-devices due to their size and domain of operation. These transceivers are able to react to specific molecules, and to release others as a response to an internal command or after performing some type of processing.

Nano-electromagnetic communication:

This is defined as the transmission and reception of electromagnetic radiation from components based on novel nanomaterials.

The unique properties observed in these materials will decide the specific bandwidth for emission of electromagnetic radiation, the time lag of the emission and the magnitude of the emitted power for a given input energy.

Network architecture for IoNT

Georgia Tech proposes the study of the terahertz band for nano-electromagnetic communication and provides a network architecture for nano devices.

In intrabody networks, nanomachines such as nanosensors and nanoactuators deployed inside the human body are remotely controlled from the macroscale and over the internet by an external user such as a health care provider. The nanoscale is the natural domain of molecules, proteins, DNA, organelles and the major components of cells. Amongst others, existing biological nanosensors and nanoactuators provide an interface between biological phenomena and electronic nano-devices, which can be exploited through this new networking paradigm.

In the interconnected office, every single element normally found in an office and even its internal components are provided of a nanotransceiver which allows them to be permanently connected to the internet.

The use cases in these different environments shows that nanotechnology has the ability to create new applications in the biomedical, industrial and military fields as well as in consumer and industrial goods.

Demands for nano IoT

These are factors, according to Georgia Tech, that will increase demand for nano devices:

Convenience and almost seamless deployment;

Tiny and nonobtrusive devices;

The possibility to harvest vibrational, mechanical or even electromagnetic energy from the environment;

Ultra-low power consumption; and

Reasonable computing capabilities.

Here are the physical components required for the internet of nano things architecture:

Nano-nodes

The smallest and simplest nanomachines, they are able to perform simple computation, have limited memory and can only transmit over very short distances, mainly because of their reduced energy and limited communication capabilities. Biological nanosensor nodes inside the human body and nanomachines with communication capabilities integrated in all types of things such as books, keys, or paper folders are good examples of nano-nodes.

Nano-routers

Comparatively larger computational resources than nano-nodes and are suitable for aggregating information coming from limited nanomachines. In addition, nano-routers also can control the behavior of nano-nodes by exchanging very simple control commands (on/off, sleep, read value, etc.). However, this increase in capabilities involves an increase in their size, and this makes their deployment more invasive. Nano-micro interface devices are able to aggregate the information coming from nanorouters, to convey it to the microscale, and vice versa.

Gateway

Enables the remote control of the entire system over the internet. For example, in an intrabody network scenario, an advanced cellphone can forward the information it receives from a nano-micro interface in our wrist to our health care provider. In the interconnected office, a modem-router can provided this functionality. Despite the interconnection of microscale devices, the development of gateways and the network management over the internet are still open research areas, in the remaining of this article we mainly focus on the communication challenges among nanomachines.

Technologies enabling smaller data collection

Scientists have started shrinking sensors from millimeters or microns in size to the nanometer scale, small enough to circulate within living bodies and to mix directly into construction materials. There are five new developments that are helping enable the shrinking of sensors and collection of data from nano devices, according to Computer Business Review.

Nanotubes

Carbon nanotubes are a nanotechnology constructed with a length-to-diameter ratio of up to 132,000,000:1.

Uses of the solution span from incorporation in portable electronics to help fighting cancer and creating artificial muscles.

Bleeding plastic

Scientists have also developed a bleeding plastic with self-healing capabilities that could put an end to nearly anything getting broken, including cars, airplanes or everyday devices.

Nano-nodes

Nano-nodes are nanomachines with the capability to perform simple computation, but could be used in the future to make nearly every object and person connected to the internet.

In a whitepaper from IEEE Wireless Communications, Ian Akyildiz and Josep Jornet from the Georgia Institute of Technology explained that nano-nodes have limited memory, and can only transmit over very short distances, mainly because of their reduced energy and limited communication capabilities.

Nanoantennas

Nanoantennas are a new emerging technology that could help power wearables, smarten up buildings or keep lights on.

Research center physicist recognized by his peers

By NIKKI FICKEN AMRDEC

http://www.theredstonerocket.com/military_scene/article_ffedf934-bc80-11e6-bf22-2b921fd3b353.html

An Aviation and Missile Research, Development and Engineering Center employee has been named a Fellow of the American Association for the Advancement of Science, a distinction that recognizes his outstanding contributions in scientific research.

Dr. Henry Everitt, an AMRDEC physicist, is among 391 AAAS members elected by their peers to the rank of Fellow this year, one of two researchers employed by the Army. AAAS is the world’s largest multidisciplinary scientific society, with more than 120,000 members, and a leading publisher of cutting-edge research through its science family of journals. The AAAS seeks to “advance science, engineering and innovation throughout the world for the benefit of all people.”

“It is a tremendously humbling honor to be recognized by your peers for the work you do,” Everitt said. “I’m blessed to get paid for doing research, something I so much enjoy doing. My career has been about doing research in a variety of ways, including mentoring outstanding students and researchers and sponsoring top scientists and engineers around the world. Any successes I’ve had derive from their successes.”

He is one of the Army’s three dozen chief scientists, a senior executive “scientific technologist” who has worked for 25 years as a program manager, researcher and adviser on subjects as wide ranging as nanotechnology, photonics, ultrafast spectroscopy of semiconductor physics, quantum information, molecular spectroscopy and terahertz frequency holography and radar. 

“Dr. Everitt has an extensive technical knowledge-base as represented in his numerous journal publications, book chapters, manuscripts and numerous technical reports,” Juanita Harris, AMRDEC Weapons Development and Integration Directorate director, said. “His guidance and strategic direction in the quantum networking activities allows the Army to lay out its strategic path in response to this very critical OSD initiative on quantum research. Dr. Everitt has proven himself time and time again to utilize his technical strengths to provide real world solutions to critical missile-based problems.”

In announcing the honor, AAAS recognized him “for pioneering spectroscopic investigations of wide bandgap semiconductors and ultraviolet plasmonic nanostructures, terahertz holography and nascent federal programs in quantum information, photonic crystals and nanotechnology.”

Because of his pioneering work in these fields, Everitt was previously selected as a Fellow of the Optical Society of America and the American Physical Society. He also holds adjunct faculty positions at several universities including Duke University, Rice University and the University of Alabama in Huntsville where he has taught classes and personally mentored more than 75 undergraduate researchers, graduate students and research scientists.

He and the other new Fellows will be presented with an official certificate and rosette pin on Feb. 18 at the AAAS Fellows forum during the 2017 AAAS annual meeting in Boston. 

“As an Army scientist, my job and the purpose of my research is ultimately about service, service to my colleagues, service to the nation and especially service to the brave men and women in uniform who protect our freedoms,” Everitt said. “Selfless service is one of the Army’s core values, so it means a lot that AAAS recognizes Fellows for their ‘service to society.’ I hope I’ve done that.”

Terahertz wave manipulation using graphene cut-wires

http://iopscience.iop.org/0957-4484/labtalk-article/63437

Integrated phase modulation devices are critical to the application of terahertz technology, such as imaging and communication. Reporting in Nanotechnology, researchers propose a graphene metasurface, consisting of nano-patterned graphene cut-wires. This metasurface can dynamically modulate the wave front of incident infrared and terahertz waves to enable new functionality, such as anomalously reflecting and focusing with a relatively high efficiency of about 60%.

When circularly polarised light impinges on graphene cut-wires supported by metal-dielectric dual layers, the scattered electromagnetic field will acquire a geometric phase shift related to the orientations of cut-wires. So by presetting the distribution profiles of cut-wires direction angle Θ, the wave front of scatterred light can be regulated into any shape desired. Discrete micro- or nanoscale graphene patches can also be fabricated by one-step (electron beam) lithography and O2 plasma etching process, once transferred on the metal-dielectric substrate. All the patches are doped by a thin layer of high-capacitance ion-gel that can inject carriers up to Fermi-level 0.8 eV.

Graphene focusing mirror

With this mechanism, flat focusing mirror made by graphene can be designed. When tuning the Fermi-level to 0.8 eV, this one-atom-thick graphene pattern layer can converge the energy of incident terahertz wave (58 μm) into the focal spot (F = 290 μm) with about 5 times intensity enchancement, even if the numerical aperture is very small.

Potential applications

Despite metasurfaces anomalously reflecting light that does not obey the traditional Snell laws, more functional components for infrared and terahertz waves can be achieved based on the metal-dielectric-graphene metasurface. This includes vortex beam generation for terahertz communications.

More information about this research can be found in the journal Nanotechnology 26 505203.

OT-LUNA blog-Early and reliable detection of organ injury biomarkers using NANOenhanced SPRi technology

Jennifer Miller and Lance Li
Luna’s Nanomaterials Group

http://lunainc.com/early-reliable-detection-organ-injury-biomarkers-nanoenhanced-spri-technology/

What are biomarkers and why are they important? 

Biomarkers are measurable indicators of the presence and extent of a disease process in the body, reflecting an interaction between a biological system in the body and a potential hazard. These indicators can be a number of different substances including nucleic acids, proteins and small molecules (Figure 1). Testing for the presence of certain biomarkers can provide important information for identifying disease and the extent to which it has progressed.  This information can be critical in making timely and effective treatment decisions.

Figure 1. Biomarkers and their organ-specific release siteshttp://www.nature.com/nrcardio/journal/v9/n8/fig_tab/nrcardio.2012.60_F1.html

Biomarkers in detection and treatment of multiple organ injury
Major organ injuries such as liver, kidney, heart and lung injuries can lead to high morbidity and mortality in critically ill patients. Safety biomarkers have been suggested to have predictive values before tissue injury for specific organs becomes irreversible. Identification of such biomarkers in clinical samples would improve the early detection of organ injury, help identify appropriate preventive or curative strategies, preventing organ injury into organ failure, and improving the quality of life. It has been demonstrated that detection using a panel of biomarkers, provides greater sensitivity and specificity for assessing the presence and extent of a disease or injury process than with the use of a single biomarker. A highly accurate blood-based diagnostic panel can be developed from a reasonable number of individual serum biomarkers that are relatively weak classifiers when used singly.
NANOenhanced SPRi diagnostic platform
Luna is developing a nanotechnology empowered surface plasmon resonance imaging platform (NANOenhanced SPRi) for performing the ideal assay to simultaneously detect a panel of biomarkers for the diagnosis of organ injury. The novel NANOenhanced sensing array incorporates several important features that need to function in a concerted way leading to significantly improved diagnostic sensitivity and specificity. The assay minimizes non-specific protein, cell, and lipid binding reducing the background signal and permitting the direct testing of plasma, blood, urine, saliva and other body fluids. Another important advantage is the use of nanotechnology for signal amplification.  It provides high diagnostic accuracy, is non-invasive (tests body fluids), uses a small sample size (less than 100 μL), has a fast turnaround time (within 30 minutes) and can test for multiple biomarkers using one sample (Figure 2). The sensing chips are customizable to test tens to hundreds of biomarkers simultaneously that could be proteins, DNAs, microRNAs, small molecules and their combinations. The sensing chips are premade for the use with any SPRi platform (Figure 3).This platform can be further used to develop assays for the detection of a variety of biomarker panels known to be indicative of almost all diseases, providing the timely and reliable results critical for successful patient treatment plans.  The NANOenhanced SPRi technique is superior to current leading quantitative assay methods such as MALDI-MS, PCR, microarray, and ELISA.

Figure 2. NANOenhanced SPRi Diagnostic Platform

Figure 3. Operation diagram of the NANOenhanced SPRi diagnostic platform – sensing chips are customized for detecting selected panels of biomarkers and pre-prepared for use with any SPRi platforms.

As an example, Luna’s NANOenhanced SPRi Multiplex Biomarker Assay for Organ Injury demonstrates femtogram sensitivity and multiplex capabilities with the detection of three biomarkers, Mi1222, KIM-1 and HMGB-1 (protein biomarkers specific to liver and kidney injury) in a single sample (Figure 4). The sensing arrays were able to show distinct binding signals to corresponding biomarkers. The assay detected KIM-1 biomarker at fg/mL level. Detection requires a run time of less than 30 minutes, providing important clinical information rapidly.

Figure 4. NANOenhanced SPRi Assay demonstrates multiplex detection of three biomarkers, MiR122, KIM-1 and HMGB-1 in a single sample.

Luna is presenting (an oral presentation and a poster) at the 2015 NanoTechnology for Defense (NT4D) conference – the 13th Annual Nano for Defense Conference – Transitioning Emerging Technologies (NT4D), 16 – 19 November 2015 in City of Industry, CA. This event was initiated and is supported by the Air Force Research Laboratory, Army Research Laboratory, the Office of Naval Research, the Defense Threat Reduction Agency, and the Defense Advanced Research Projects Agency. NT4D is the premier event addressing emerging nanoenabled technologies for defense.

Nano-sized Chip "Sniffs Out" Explosives Far Better than Trained Dogs: TAU researcher's groundbreaking sensor detects miniscule concentrations of hazardous materials in the air

http://www.nanotech-now.com/news.cgi?story_id=49855

Nano-sized Chip "Sniffs Out" Explosives Far Better than Trained Dogs: TAU researcher's groundbreaking sensor detects miniscule concentrations of hazardous materials in the air

Abstract:
Security forces worldwide rely on sophisticated equipment, trained personnel, and detection dogs to safeguard airports and other public areas against terrorist attacks. A revolutionary new electronic chip with nano-sized chemical sensors is about to make their job much easier.

New York, NY | Posted on July 23rd, 2014

The groundbreaking nanotechnology-inspired sensor, devised by Prof. Fernando Patolsky of Tel Aviv University's School of Chemistry and Center for Nanoscience and Nanotechnology, and developed by the Herzliya company Tracense, picks up the scent of explosives molecules better than a detection dog's nose. Research on the sensor was recently published in the journal Nature Communications.

Existing explosives sensors are expensive, bulky and require expert interpretation of the findings. In contrast, the new sensor is mobile, inexpensive, and identifies in real time — and with great accuracy — explosives in the air at concentrations as low as a few molecules per 1,000 trillion.

A nano-nose to compete with a dog's

"Using a single tiny chip that consists of hundreds of supersensitive sensors, we can detect ultra low traces of extremely volatile explosives in air samples, and clearly fingerprint and differentiate them from other non-hazardous materials," said Prof. Patolsky, a top researcher in the field of nanotechnology. "In real time, it detects small molecular species in air down to concentrations of parts-per-quadrillion, which is four to five orders of magnitude more sensitive than any existing technological method, and two to three orders of magnitude more sensitive than a dog's nose.

"This chip can also detect improvised explosives, such as TATP (triacetone triperoxide), used in suicide bombing attacks in Israel and abroad," Prof. Patolsky added.

The clusters of nano-sized transistors used in the prototype are extremely sensitive to chemicals, which cause changes in the electrical conductance of the sensors upon surface contact. When just a single molecule of an explosive comes into contact with the sensors, it binds with them, triggering a rapid and accurate mathematical analysis of the material.

"Animals are influenced by mood, weather, state of health and working hours, the oversaturation of olfactory system, and much more," said Prof. Patolsky. "They also cannot tell us what they smell. Automatic sensing systems are superior candidates to dogs, working at least as well or better than nature. This is not an easy task, but was achieved through the development of novel technologies such as our sensor."

A technology for a safer world

The trace detector, still in prototype, identifies several different types of explosives several meters from the source in real time. It has been tested on the explosives TNT, RDX, and HMX, used in commercial blasting and military applications, as well as peroxide-based explosives like TATP and HMTD. The latter are commonly used in homemade bombs and are very difficult to detect using existing technology.

"Our breakthrough has the potential to change the way hazardous materials are detected, and of course should provide populations with more security," said Prof. Patolsky. "The faster, more sensitive detection of tiny amounts of explosives in the air will provide for a better and safer world."

Tracense has invested over $10M in research and development of the device since 2007, and expects to go to market next year. Prof.Patolsky and his team of researchers are currently performing multiple and extensive field tests of prototype devices of the sensor.

####

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Importance of Electron-Electron Interactions in Graphene

[This article is written by Lynn Yarris of Lawrence Berkeley National Laboratory]

Michael Crommie [Photo by Roy Kaltschmidt, Courtesy: Lawrence Berkeley National Laboratory]

 Perhaps no other material is generating as much excitement in the electronics world as graphene, sheets of pure carbon just one atom thick through which electrons can race at nearly the speed of light – 100 times faster than they move through silicon. Superthin, superstrong, superflexible and superfast as an electrical conductor, graphene has been touted as a potential wonder material for a host of electronic applications, starting with ultrafast transistors. For the vast potential of graphene to be fully realized, however, scientists must first learn more about what makes graphene so super. The latest step in this direction has been taken by researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) Berkeley.

Michael Crommie, a physicist who holds joint appointments with Berkeley Lab’s Materials Sciences Division and UC Berkeley’s Physics Department, led a study in which the first direct observations at microscopic lengths were recorded of how electrons and holes respond to a charged impurity – a single Coulomb potential – placed on a gated graphene device. The results provide experimental support to the theory that interactions between electrons are critical to graphene’s extraordinary properties. This work has been published online on July 29th in the journal 'Nature Physics'[1].

“We’ve shown that electrons in graphene behave very differently around charged impurities than electrons in other materials,” Crommie says. “Some researchers have held that electron-electron interactions are not important to intrinsic graphene properties while others have argued they are. Our first-time-ever pictures of how ultra-relativistic electrons re-arrange themselves in response to a Coulomb potential come down on the side of electron-electron interactions being an important factor.”

Graphene sheets are composed of carbon atoms arranged in a two-dimensional hexagonally patterned lattice, like a honeycomb. Electrons moving through this honeycomb lattice perfectly mimic the behavior expected of highly relativistic charged particles with no mass: think of a ray of light that is electrically charged. Because this is the same behavior displayed by highly relativistic free electrons, charge-carriers in graphene are referred to as “Dirac quasiparticles,” after Paul Dirac, the scientist who first described the behavior of relativistic fermions in 1928.

“In graphene, electrons behave as massless Dirac fermions,” Crommie says. “As such, the response of these electrons to a Coulomb potential is predicted to differ significantly from how non-relativistic electrons behave in traditional atomic and impurity systems. However, until now, many key theoretical predictions for this ultra-relativistic system had not been tested.”

Image 1: This zoom-in STM topograph shows one of the cobalt trimers placed on graphene for the creation of Coulomb potentials – charged impurities – to which electrons and holes could respond. (Image courtesy of Crommie group) 

Working with a specially equipped scanning tunneling microscope (STM)in ultra-high vacuum, Crommie and his colleagues probed gated devices consisting of a graphene layer deposited atop boron nitride flakes which were themselves placed on a silicon dioxide substrate, the most common of semiconductor substrates.

“The use of boron-nitride significantly reduced the charge inhomogeneity of graphene, thereby allowing us to probe the intrinsic graphene electronic response to individual charged impurities,” Crommie says. In this study, the charged impurities were cobalt trimers constructed on graphene by atomically manipulating cobalt monomers with the tip of an STM.”

Image 2: The response of ultrarelativistic electrons in graphene to Coulomb potentials created by cobalt trimers was observed to be signficantly different the response of non-relativistic electrons in traditional atomic and impurity systems. (Image courtesy of Crommie group) 

The STM used to fabricate the cobalt trimers was also used to map (through spatial variation in the electronic structure of the graphene) the response of Dirac quasiparticles – both electron-like and hole-like – to the Coulomb potential created by the trimers. Comparing the observed electron–hole asymmetry to theoretical simulations allowed the research team to not only test theoretical predictions for how Dirac fermions behave near a Coulomb potential, but also to extract graphene’s dielectric constant.

“Theorists have predicted that compared with other materials, electrons in graphene are pulled into a positively-charged impurity either too weakly, the subcritical regime; or too strongly, the supercritical regime,” Crommie says. “In our study, we verified the predictions for the subcritical regime and found the value for the dielectric to be small enough to indicate that electron–electron interactions contribute significantly to graphene properties. This information is fundamental to our understanding of how electrons move through graphene.”

Reference:
[1] Yang Wang, Victor W. Brar, Andrey V. Shytov, Qiong Wu, William Regan, Hsin-Zon Tsai, Alex Zettl, Leonid S. Levitov, Michael F. Crommie, "Mapping Dirac quasiparticles near a single Coulomb impurity on graphene", Nature Physics, doi:10.1038/nphys2379 (Published online July 29, 2012). Abstract.

Study Compares Techniques For Doping Graphene For Device And Interconnect Fabrication

MY NOTE: THE LATEST ON GRAPHENE, WHICH WILL HAVE PROFOUND SIGNIFICANCE FOR THz DEVELOPMENT IN THE FUTURE, AND IS A TOPIC WHICH READERS HERE, HAVE SHOWN  GREAT INTEREST IN.

Graduate research assistant Kevin Brenner holds a graphene sample under study in the Georgia Tech Nanotechnology Research Center. The cryogenic probe station shown behind him is used to study doping effects in the material. Credit: Georgia Tech Photo: Gary Meek


http://www.redorbit.com/news/technology/1112416733/study-compares-techniques-for-doping-graphene-for-device-and-interconnect-fabrication/index.html
Nanotechnology researchers at the Georgia Institute of Technology have conducted the first direct comparison of two fundamental techniques that could be used for chemically doping sheets of two-dimensional graphene for the fabrication of devices and interconnects.
Chemical doping is routinely used in conventional three-dimensional semiconductors to control the density of electron carriers that are essential to the operation of devices such as transistors. But graphene, a semi-metal available in sheets just one atom thick, has properties very different from traditional materials such as silicon – though researchers say doping will still be needed for producing electronic devices.
The bad news is that electronic designers working with graphene won’t be able to simply apply what they’ve been doing with three-dimensional semiconductors – which would translate to vastly degraded material quality for graphene. The good news, according to the study, is that graphene doping can be combined with other processes – and need be applied only to the edges of nanoscale structures being fabricated.
“We are learning how to manipulate these two-dimensional sheets of carbon atoms to get some very unusual results that aren’t available with any other material,” said James Meindl, director of Georgia Tech’s Nanotechnology Research Center, where the research was conducted. “Doping graphene to try to influence its properties is important to being able to use it effectively.”
Details of the research were published online in the journal Carbon on Oct. 29. The research was supported by the Semiconductor Research Corporation (SRC), the Defense Advanced Research Projects Agency (DARPA) through the Interconnect Focus Center, and the National Science Foundation (NSF).
Because graphene sheets contain so few atoms by area, the substitution of elements such as oxygen or nitrogen for carbon atoms in the lattice – as in conventional doping – detracts from the high electron mobility and other properties that make the material interesting. So the researchers are rethinking the doping process to take advantage of graphene’s unique properties.
“When we work with a three-dimensional semiconductor, we embed the dopant species in the bulk material and then fabricate it into a device,” said Kevin Brenner, a graduate research assistant in the Georgia Tech School of Electrical and Computer Engineering. “With graphene, we will dope the material as we process it and fabricate it into devices or interconnects. Doping may be done as part of other fabrication steps such as plasma etching, and that will require us to reinvent the whole process.”
Using sheets of exfoliated graphene, Brenner and collaborators Raghu Murali and Yinxiao Yang evaluated the effectiveness of two different techniques: edge passivation by coupling electron-beam lithography with a common resist material, and adsorption from coating the surface of the material. They found that the edge treatment, which chemically reacts with defects created when the material is cut, was a thousand times more efficient at producing carriers in the graphene sheets than the surface treatment.
“We will only be working with the edges of the material,” Brenner explained. “That will allow us to leave the center pristine and free of defects. Using this approach, we can maintain very high mobilities and the special properties of graphene while creating very high carrier densities.”
Because of the two-dimensional nature of the graphene, controlling the edge chemistry can provide control over the bulk properties of the sheet. “At nanoscale dimensions, the edge atoms tend to dominate over surface absorption techniques,” he added. “With a seven nanometer by seven nanometer graphene device, passivating just one edge C-atom provides the doping equivalent of covering the entire surface.”
For doping the edge of a graphene structure, the team applied a thin film of hydrogen silsesquioxane (HSQ), a chemical normally used as a resist for etching, then used electron beam lithography to cross-link the material, which added oxygen atoms to the edges to create p-type doping. The resist and electron beam system combined to provide nanometer-scale control over where the chemical changes took place.
Doping treatment could also be applied using plasma etching, Brenner said. Controlling the specific atoms used in the plasma, or conducting the etching process in an environment containing specific atoms, could drive those atoms into the edges where they would serve as dopants.
“Anytime you create an edge, you have created a location where you can passivate using a dopant,” he added. “Instead of needing to embed it in the surface, you can just take the edge that is already there and passivate it with oxygen, nitrogen, hydrogen or other dopant. It could be almost an effortless process because the doping can be done as part of another step.”
Beyond fabricating electronic devices, Nanotechnology Research Center scientists are interested in using graphene for interconnects, potentially as a replacement for copper. As interconnect structures become smaller and smaller, the resistivity of copper increases. Edge-doped graphene sheets exhibit a trend of increasing coping with reduced dimensions, possibly becoming more conductive as their size shrinks below 50 nanometers, making them attractive for nanoscale interconnects.
Armed with basic information about graphene doping, the researchers hope to now begin producing devices to study how graphene actually performs.
“Now that we have made a start at understanding how to dope the material, the next step is to begin putting this into nanoscale devices,” Brenner said. “We want to see what kind of performance we can get. That may tell us where graphene’s niche could be as an electronic material.”
Meindl, who has worked with silicon since the dawn of integrated circuits, says it’s too early to predict where graphene will ultimately find commercial applications. But he says the material’s properties are too interesting not to explore.
“The chances are that something very interesting and unique will develop from the use of graphene,” he said. “But we don’t yet have the ability to predict what we will be able to do with this new material.
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Using Nanotechnology and Metamaterials to bridge the Terahertz gap

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MY NOTE: I'M NOT CERTAIN HOW CURRENT THIS STORY IS, BUT IT'S ANOTHER GOOD PIECE ON THE USE OF NANOTECHNOLOGY AND METAMATERIALS IN TERAHERTZ 
GENERATION

http://www.scientistlive.com/European-Science-News/Nanotechnology/Bridging_the_terahertz_gap/20227/
A frequency-agile metamaterial that for the first time can be tuned over a range of frequencies in the so-called "terahertz gap" has been engineered by a team of researchers from Boston College, Los Alamos National Laboratory and Boston University.
The team incorporated semiconducting materials in critical regions of tiny elements - in this case metallic split-ring resonators - that interact with light in order to tune metamaterials beyond their fixed point on the electromagnetic spectrum, an advance that opens these novel devices to a broader array of uses, according to findings published in the on-line version of the journal Nature Photonics.
"Metamaterials no longer need to be constructed only out of metallic components," said Boston College Physicist Willie J. Padilla, the project leader. "What we've shown is that one can take the exotic properties of metamaterials and combine them with the unique prosperities of natural materials to form a hybrid that yields superior performance."
Padilla and BC graduate student David Shrekenhamer, along with Hou-Tong Chen, John F. O'Hara, Abul K. Azad and Antoinette J. Taylor of Los Alamos National Laboratory, and Boston University's Richard D. Averitt assembled a single layer of metamaterial and semiconductor that allowed the team to tune terahertz resonance across a range of frequencies in the far-infrared spectrum.
The team's first-generation device achieved 20 percent tuning of the terahertz resonance to lower frequencies - those in the far-infrared region -addressing the critical issue of narrow band response typical of all metamaterial designs to date.
Constructed on the micron-scale, metamaterials are composites that use unique metallic contours in order to produce responses to light waves, giving each metamaterial its own unique properties beyond the elements of the actual materials in use.
Within the past decade, researchers have sought ways to significantly expand the range of material responses to waves of electromagnetic radiation - classified by increasing frequency as radio waves, microwaves, terahertz radiation, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays. Numerous novel effects have been demonstrated that defy accepted principles.
"Metamaterials demonstrated negative refractive index and up until that point the commonly held belief was that only a positive index was possible," said Padilla. "Metamaterials gave us access to new regimes of electromagnetic response that you could not get from normal materials."
Prior research has shown that because they rely on light-driven resonance, metamaterials experience frequency dispersion and narrow bandwidth operation where the centre frequency is fixed based on the geometry and dimensions of the elements comprising the metamaterial composite. The team believes that the creation of a material that addresses the narrow bandwidth limitations can advance the use of metamaterials.
Enormous efforts have focused on the search for materials that could respond to terahertz radiation, a scientific quest to find the building blocks for devices that could take advantage of the frequency for imaging and other applications.
Potential applications could lie in medical imaging or security screening, said Padilla. Materials undetectable through x-ray scans - such as chemicals, biological agents, and certain explosives - can provide a unique "fingerprint" when struck by radiation in the far-infrared spectrum. Metamaterials like the one developed by the research team will facilitate future devices operating at the terahertz frequency of the electromagnetic spectrum.
In addition to imaging and screening, researchers and high-tech companies are probing the use of terahertz in switches, modulators, lenses, detectors, high bit-rate communications, secure communications, the detection of chemical and biological agents and characterisation of explosives, according to Los Alamos National Laboratory.

MIT researchers have developed a new way to tune the frequency of lasers

that operate in the terahertz spectrum. The result is an important step toward airport scanners that could tell whether a vial in a closed suitcase contains aspirin, methamphetamines or an explosive. Tucked between microwaves and infrared rays on the electromagnetic spectrum, terahertz rays can penetrate clothing, plastic, and human tissue, but they’re thought to be safer than x-rays. Since they’re absorbed to different degrees by different molecules, they can also tell chemicals apart. But accurately gauging an object’s chemical composition requires exposing it to a continuous range of terahertz frequencies, which interact with it in different ways. In a paper appearing in the most recent issue of Nature Photonics, Qing Hu, a professor of electrical engineering at MIT’s Research Laboratory of Electronics, and his colleagues describe the first practical method for tuning terahertz quantum cascade lasers, a type of laser considered the most promising source of terahertz radation. What’s more, the method is a fundamentally new approach to laser tuning that could have implications for other emerging technologies. How they did it: Tuning a laser usually requires either changing the length of its light-emitting cavity or changing its temperature. Hu compares these two approaches to changing the pitch of a guitar string by pressing down on it — changing its length — or screwing its tuning peg — changing its tension. Neither approach, however, works very well with terahertz quantum cascade lasers. A third way to change the pitch of a guitar string, however, is to change its diameter: the lower-pitched strings on a guitar are thicker than the higher-pitched ones. And Hu’s tuning technique is, roughly speaking, to change the diameter of the light beam. A light beam confined in, say, an optical fiber or a long, thin, quantum cascade laser exhibits an electromagnetic-field pattern called a “transverse mode.” The transverse mode is kind of like another electromagnetic wave that’s perpendicular to the light beam, except that unlike a light beam, it dies off very quickly. Hu’s new tuning technique requires a particular type of quantum cascade laser called a wire laser, where the wavelength of the transverse mode is actually greater than the width of the laser itself. Bringing a block of another material close enough to the laser deforms the transverse mode, which in turn changes the wavelength of the emitted light. In experiments, Hu and his colleagues found that a metal block shortened the wavelength of the light, while a silicon block lengthened it. Varying the proximity of the blocks also varies the extent of the shift. Next steps: In its experiments, Hu’s group used a mechanical lever to bring a block of either silicon or metal close to a quantum cascade laser from a single direction. But they’ve designed and are now building chips that would use electronically controlled microelectromechanical devices to bring the silicon and metal blocks in from different directions, giving the laser a precise and continuous tuning range from short to long wavelengths. Posted December 7th, 2009

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