Optical Rectennas Get an Efficiency Boost

https://www.photonics.com/Article.aspx?AID=63054

ATLANTA, Jan. 29, 2018 — The research team that announced the first optical rectenna in 2015 reports that it has improved multiwall carbon nanotube (MWCNT) rectennas by creating and optimizing new diode structures that allow optical rectification with air-stable devices. The incorporation of double-insulator layer tunnel diodes, fabricated on MWCNT arrays, enables the use of air-stable top metals with excellent asymmetry for rectification applications. 

To provide ease of electron flow and thus a low work function, researchers from Georgia Institute of Technology (Georgia Tech) initially used calcium as the metal in the oxide insulator-metal diode junction of the rectenna. Because calcium breaks down rapidly when exposed to air, the device had to be encapsulated during operation and fabricated in a glovebox, making it difficult to fabricate and impractical for most applications. 

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Georgia Tech researchers have developed a new higher efficiency rectenna design. Here, the device’s ability to convert blue light to electricity is tested. Courtesy of Christopher Moore, Georgia Tech.

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Researchers replaced the calcium with aluminum and experimented with a variety of oxide materials before selecting a bilayer material composed of alumina (AL₂O₃) and hafnium dioxide (HfO₂). They created the combination coating for the carbon nanotube junction using an atomic deposition process. 

By engineering the oxide electronic properties instead of the metals, researchers were able to develop a coating that could provide the electron tunneling properties required to allow the use of air stable metals with higher work functions than calcium. Rectennas fabricated with the new combination have remained functional for as long as a year. 

The researchers also engineered the slope of the hill down which the electrons fall in the tunneling process, increasing the efficiency of the device and allowing a variety of oxide materials to be used. 

The new design increased the asymmetry of the diodes, which boosted efficiency. For the most asymmetric device structure, AL₂O₃-HfO₂ (4/4 nm), optical rectification at a frequency of 470 THz (638 nm) was demonstrated. 

“By working with the oxide electron affinity, we were able to increase the asymmetry by more than ten-fold, making this diode design more attractive,” said professor Baratunde Cola. “That’s really where we got the efficiency gain in this new version of the device.” 

Optical rectennas operate by coupling the light’s electromagnetic field to an array of MWCNTs whose ends have been opened. The array serves as an antenna. The electromagnetic field creates an oscillation in the antenna, producing an alternating flow of electrons. When the electron flow reaches a peak at one end of the antenna, the diode closes, trapping the electrons. It then reopens to capture the next oscillation, creating a current flow. 

The switching must occur at terahertz frequencies to match the light. The junction between the antenna and diode must provide minimal resistance to electrons flowing through it while it is open, yet prevent leakage while closed. 

“The name of the game is maximizing the number of electrons that get excited in the carbon nanotube, and then having a switch that is fast enough to capture them at their peak. The faster you switch, the more electrons you can catch on one side of the oscillation,” Cola said. 

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NSF Graduate Research Fellow Erik Anderson tests the conversion of blue light to electricity with a new higher efficiency rectenna design. Courtesy of Christopher Moore, Georgia Tech.

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The early version of the rectenna produced power at microvolt levels. The new version of the rectenna produces power in the millivolt range. Conversion efficiency has gone from 10^-5 to 10^-3 — still low, but a significant gain. 

“Though there still is room for significant improvement, this puts the voltage in the range where you could see optical rectennas operating low-power sensors,” Cola said. “There are a lot of device geometry steps you could take to do something useful with the optical rectenna today in voltage-driven devices that don’t require significant current.” 

Cola believes the rectennas could be useful for powering internet of things devices, especially if they can be used to produce electricity from scavenged thermal energy. The researchers believe their device design could eventually compete with conventional photovoltaic technologies for producing electricity from sunlight and other sources. 

The team plans to improve its understanding of how the rectenna works, allowing further optimization. One day, Cola hopes the devices will help accelerate space travel, producing power for electric thrusters that will boost spacecraft. 

“Our end game is to see carbon nanotube optical rectennas working on Mars and in the spacecraft that takes us to Mars,” he said. 

The research was published in Advanced Electronic Materials (doi: 10.1002/aelm.201700446).

Abstract-An Energy Conserving Routing Scheme for Wireless Body Sensor Nanonetwork Communication

Fariha Afsana, Md. Asif-Ur-Rahman, Muhammad R. Ahmed, Mufti Mahmud, M. Shamim Kaiser

https://arxiv.org/abs/1801.02621

Current developments in nanotechnology make electromagnetic communication (EC) possible at the nanoscale for applications involving Wireless [Body] Sensor Networks (W[B]SNs). This specialized branch of WSN has emerged as an important research area contributing to medical treatment, social welfare, and sports. The concept is based on the interaction of integrated nanoscale machines by means of wireless communications. One key hurdle for advancing nanocommunications is the lack of an apposite networking protocol to address the upcoming needs of the nanonetworks. Recently, some key challenges have been identified, such as nanonodes with extreme energy constraints, limited computational capabilities, Terahertz frequency bands with limited transmission range, etc., in designing protocols for wireless nanosensor networks (WNN). This work proposes an improved performance scheme of nanocommunication over Terahertz bands for wireless BSNs making it suitable for smart e-health applications. The scheme contains -- a new energy-efficient forwarding routine for EC in WNN consisting of hybrid clusters with centralized scheduling, a model designed for channel behavior taking into account the aggregated impact of molecular absorption, spreading loss, and shadowing, and an energy model for energy harvesting and consumption. The outage probability is derived for both single and multilinks and extended to determine the outage capacity. The outage probability for a multilink is derived using a cooperative fusion technique at a predefined fusion node. Simulated using a Nano-Sim simulator, performance of the proposed model has been evaluated for energy efficiency, outage capacity, and outage probability. The results demonstrate the efficiency of the proposed scheme through maximized energy utilization in both single and multihop communication, multisensor fusion enhances the link quality of the transmission.

Abstract- EOC: Energy Optimization Coding for Wireless Nanosensor Networks in the Terahertz Band

Long-Jun Huang, Xin-Wei Yao,  Wan-Liang Wang,  Shi-Genshen,

TS-OOK-based Coding scheme.

Long-Jun Huang, Xin-Wei Yao,  Wan-Liang Wang,  Shi-Genshen, 

http://ieeexplore.ieee.org/document/7847323/

Wireless nanosensor networks (WNSNs), which consist of numerous nanosensors, offer a number of unprecedented and promising applications in the biomedical, environmental, industrial, and military fields. However, a single nanosensor in WNSNs has very limited capability as a result of nanoscale components, especially the extremely small nanobatteries. Therefore, energy efficiency has become an essential issue for WNSNs. In this paper, by considering the scenarios of transmitting binary source symbols in WNSNs, an energy optimization coding (EOC) for communication in WNSNs is proposed, and the energy model by jointly accounting for the energy consumption of both a transmitter and a receiver is presented. Based on the optimal source-word length and the optimal code-word length by solving an energy optimization problem, an energy-efficient coding scheme and the corresponding coding algorithm are presented. Simulation results show that EOC performs better energy efficiency than the existing nanonetwork minimum energy coding while requiring a smaller source-word length. Moreover, the proposed coding algorithm is more suitable for the scenarios of transmitting long binary source symbols.

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.

Graphene radios could unlock ‘Internet of Nano-Things’

By Jack Loughran
https://eandt.theiet.org/content/articles/2016/11/graphene-radios-could-unlock-internet-of-nano-things/

Miniature radios made from graphene that broadcast on the little-used terahertz band could enable an ‘Internet of Nano-Things’, according to a team at the University of Buffalo.

Their work centres on development of extremely small radios made of graphene and semiconducting materials that enable short-range, high-speed communication. 

The technology could ultimately reduce the time it takes to complete complex tasks, such as migrating files from one computer to another, from hours to seconds.

Other potential applications include implantable body nanosensors that monitor sick or at-risk people, and nanosensors placed on ageing bridges, in polluted waterways and other public locations, to provide ultra-high-definition streaming.

Although technological advancements have made wireless data transmission more efficient, bandwidth issues persist as wireless devices proliferate and the demand for data grows.

The solution could be found by using the terahertz band, which is sandwiched on the spectrum between radio waves (part of the electromagnetic spectrum that includes AM radio, radar and smartphones) and light waves (remote controls, fibre-optic cables and more).

The Buffalo team believes that graphene-based radios could help overcome the main problem with terahertz waves: they do not retain their power density over long distances.

Graphene is a two-dimensional sheet of carbon that, in addition to being incredibly strong, thin and light, has tantalising electronic properties. For example, electrons move 50 to 500 times faster in graphene compared to silicon.

In previous studies, researchers showed that tiny graphene antenna strips 10-100 nanometres wide and one micrometre long (pictured above), combined with semiconducting materials such as indium gallium arsenide, can transmit and receive terahertz waves at wireless speeds greater than one terabit per second.

However, to make these radios viable outside the laboratory, the antennas need other electronic components, such as generators and detectors that work in the same environment.

Josep Jornet, assistant professor at the University at Buffalo, is attempting to develop these components to make graphene radios a reality.

Jornet says thousands of these arrayed radios working together could allow terahertz waves to travel greater distances.

The nanosenors could be embedded into physical objects, such as walls and street signs, as well as chips and other electronic components, to create an 'Internet of Nano-Things'.

“For wireless communication, the terahertz band is like an express lane. But there’s a problem: there are no entrance ramps,” Jornet said.

“We’ll be able to create highly accurate, detailed and timely maps of what’s happening within a given system. The technology has applications in health care, agriculture, energy efficiency—basically anything you want more data on.”

In September, a team demonstrated how baking graphene in a microwave oven imbued it with properties that make it perfect for next-generation electronic and energy devices. 

Making massive MIMOs for high speed short range comms

By:Graham Pitcher
http://www.newelectronics.co.uk/electronics-technology/making-massive-mimos-for-high-speed-short-range-comms/111039/

The communications issues related to the Internet of Things have been discussed over the last few years and a range of solutions is available, although some remain proprietary. But a new set of challenges is emerging as designers look to enable communications between devices taking advantage of nanotechnology.

In the words of Ian Akyildiz, professor of telecommunications at Georgia Tech in the US: “We’re now talking about the Internet of Nanothings.” And the research is being enabled by graphene.

“I first had the idea about nanoscale communications in about 2006,” he said, “but I thought the only way nanoscale machines could communicate would be through biology. But my PhD student Josep Jornet said we should look at electromagnetic means and we realised we could enable this in the THz band – and we found graphene.”

Transmitting at more than 1Tbit/s

Jornet started to explore how graphene could be used to create antennas and, between them, the team had success in its first year. “We validated performance, presented the concept at a conference and applied for a patent,” Prof Akyildiz noted.

What graphene enabled was an antenna that supported data rates in excess of 1Tbit/s, but only over distances of up to 1m. “That’s impractical for many applications,” he added.

Jornet, now an assistant professor at the State University of New York (SUNY), said that graphene has extraordinary properties when it comes to its use in antennas. “The most important thing is that it supports the propagation of surface plasmon polaritons and this is the key property that enables the development of small, efficient antennas.

“Plasmons – surface confined waves – exist in other materials,” he continued, “but generally at optical frequencies. This is the first time it has been achieved at the low end of the terahertz spectrum.”

“When electrons in graphene are excited by an incoming electromagnetic wave, they start moving back and forth,” Prof Akyildiz explained. “Because of graphene’s properties, this global oscillation of electrical charge results in a confined electromagnetic wave on top of the graphene layer.”

It would be possible to take advantage of plasmons in metals such as silver and gold, but this would mean devices operating at hundreds of THz. “While those frequencies might offer advantages in communication speed,” Prof Akyildiz, pointed out, “their range would be limited by propagation losses to just a few microns.” And copper is ruled out because it doesn’t support plasmons.

The nano antenna developed by the Georgia Tech team, working with researchers from SUNY, comprises a layer of graphene on a dielectric and a ground plane. “The graphene must be on top of a dielectric, such as gallium arsenide,” Prof Akyildiz said. “Metallic antennas don’t need this extra layer. It’s not like we have taken a classical design and used graphene; there’s a lot of IP involved in our antenna.”

While the team is working on single graphene antennas, their research holds out the prospect of something far more exciting – ultra massive MIMO antennas. Prof Akyildiz noted that the concept of MIMO antennas – many inputs, many outputs – emerged about 10 years ago.

“The first such devices were 2 x 2; now, it’s up to 64 x 64 and the approach is in the plans for 5G. But it has to be limited to small numbers – perhaps 100 x 100 – because you need a certain spacing between each antenna to avoid interference problems. But how much space is there in a small phone for a 100 x 100 MIMO?” he asked

Ultra massive MIMO antennas

Jornet added: “There has been much talk about massive MIMOs, but they are more likely to be used in basestations; we’re talking about using them in a mobile phone. When we work with graphene, we can make things smaller and put them closer together. We may be able to create a 1k x 1k MIMO and put it anywhere.”

Prof Akyildiz says 1024 graphene nano antennas can be created in an area of about 1mm2. “Plasmonic nano antenna arrays exhibit high gain, which can help us to increase the communication distance at THz frequencies,” he said. “For example, a 1k x 1k beamforming set up can provide a gain of about 80dB; enough to establish a 2Tbit/s link at 10m when transmitting at 1THz – more than two orders of magnitude better than any existing standard.”

Transmission distance, however, remains a challenge. “The atmosphere affects signal propagation at higher frequencies,” Jornet noted. “However, there are windows that allow longer distance transmission. We have ideas for distance aware modulation techniques and may be able to transmit over 50m, but we’re still looking for more.”

But the ultra massive MIMO array is, for the moment, a concept. “We have developed analytical and simulation models,” Prof Akyildiz continued. “Fabrication and experimental validation will follow in the near future.”

“Our project shows the concept of graphene based nano antennas is feasible, especially when taking into account very accurate models of electron transport in graphene,” he concluded. “Many challenges remain, but this is a first step toward creating advanced nanomachines with many applications in the biomedical, environmental, industrial and military fields.

“It may take another couple of years, but but they could change the entire communications paradigm because they’re so tiny.”

However, one other problem remains to be solved: the cost of graphene. Jornet said:“We can use small samples of graphene in the lab; enough to make an antenna and transceiver. We’re hoping the materials people can reduce the production cost so our antennas can be mass produced.”

Printable antennas may bring low cost and flexibility to a range of applications Researchers from the University of Manchester have used compressed graphene ink to print an RF antenna measuring 14cm x 3.5mm onto a piece of paper. According to the team, the antenna performed well enough to make it practical for use in RFID tags and wireless sensors. Graphene ink is usually made by mixing graphene flakes with a solvent, and sometimes a binder. Graphene ink with binders usually conducts electricity better, but only after the binder – an insulator – is broken down by annealing. But this high temperature process limits the surfaces onto which graphene ink can be printed. The team found that by printing and drying the ink, then compressing it with a roller, graphene’s conductivity was increased by more than 50 times. Researcher Dr Zhirun Hu said: “What makes printed graphene attractive for antenna applications is its ultra low cost and flexibility and the fact that it can be printed on any substrate without needing a high temperature process. We can use screen printing to produce graphene antennas, which suits low cost mass production.” Expanding, Dr Hu noted: “Being able to print antennas on any substrate means we could see a disruptive technology for low cost, wearable communications products. In addition, we’ll be able to print a complete RF transceiver in the near future.”

Tunable graphene antenna Europe funded Project Nano RF has demonstrated a graphene antenna that operates in the microwave spectrum and which can be tuned using an external voltage. The antenna is less than 1mm thick, with a diameter of 100mm, which makes it one of the smallest such devices. According to the researchers, the main application for the antenna will be in RF communications, where its tunability will allow switching of communication channels.

- See more at: http://www.newelectronics.co.uk/electronics-technology/making-massive-mimos-for-high-speed-short-range-comms/111039/#sthash.TxCeyxul.dpuf