GRANET: Graphene-enabled Nanonetworks in the Terahertz Band

Project Description

Nanonetworks and Their Applications

In 1959, the Nobel laureate physicist Richard Feynman, in his famous speech entitled "There's Plenty of Room at the Bottom", described for the first time how the manipulation of individual atoms and molecules would give rise to more functional and powerful man-made devices. In his talk, he noted that several scaling issues would arise when reaching the nanoscale, which would require the engineering community to totally rethink the way in which nano-devices are conceived. More than half century later, nanotechnology is providing a new set of tools to the engineering community to design and manufacture devices just a few hundred nanometers in size, which are able to perform only very simple tasks.

For example, one of the early applications of these nano-devices is in the field of nanosensors. A nanosensor is not just a tiny sensor, but a device that makes use of the novel properties of nanomaterials to identify and measure new types of events in the nanoscale. Amongst others, nanosensors can detect and measure physical characteristics of structures just a few nanometers in size, chemical compounds in concentrations as low as one part per billion, or the presence of biological agents such as virus, bacteria or cancerous cells. However, the sensing range of a single nanosensor is limited to its close nano-environment and, thus, many nanosensors are needed to cover significant regions or volumes. Moreover, an external device and the user interaction are necessary to read the actual measurement from the nanosensor.

Similarly to the way in which communication among computers enabled revolutionary applications such as the Internet, we believe that, by means of communication, nano-devices will be able to overcome their limitations and to expand their potential applications. In our vision, nanonetworks will be able to cover larger areas, to reach unprecedented locations in a non-invasive way, and to perform additional in-network processing. Nanonetworks have a vast amount of applications in which classical wireless networks cannot be used. These are classified in four main groups:

  • Military and defense applications: Advanced nuclear, biological and chemical (NBC) defenses, and sophisticated damage detection systems for civil structures, soldiers' armor and military vehicles, are two examples of the military applications enabled by nanonetworks. For example, a network of nanosensors can be used to detect harmful chemicals and biological weapons with unprecedented accuracy and in very different scenarios, from the battle-field (e.g., deployed from an unmanned vehicle and imperceptible by the human eye) to airport lobbies or a conference room (e.g., contained within the wall paint). We can defeat attacks made at the nanoscale only if we detect and act at the nanoscale.
     
  • Biomedical applications: The nanoscale is the natural domain of molecules, proteins, DNA, organelles and the major components of cells. As a result, a very large number of applications of nanonetworks is in the biomedical field. For example, nanomaterial-based biological nanosensors can be deployed over (e.g., tattoo-like) or even inside the human body (e.g., a pill or intramuscular injection) to monitor glucose, sodium, and cholesterol, to detect the presence of infectious agents, or to identify specific types of cancer. A wireless interface between these nano-devices and a micro-device, such as a cellphone or medical equipment, could be used to collect data and to forward it to a healthcare provider.
     
  • Environmental applications: Trees, herbs, or bushes, release several chemical composites to the air in order to attract the natural predators of the insects that are attacking them, or to regulate their blooming among different plantations, amongst other. Chemical nanosensors could be used to detect the chemical compounds that are being released and exchanged between plants. Nanonetworks can be build around classical sensor devices which are already deployed in agriculture fields. Other environmental applications include biodiversity control, biodegradation assistance, or air pollution control.
     
  • Industrial and consumer goods applications: The applications of nanotechnology in the development of new industrial and consumer goods range from flexible and stretchable electronic devices to new functionalized nanomaterials for self-cleaning anti-microbial textiles. For the specific case of nanonet- works, the integration of nano-devices with communication capabilities in every single object will allow the interconnection of almost everything in our daily life, from cooking utensils to every element in our working place, or also the components of every device, enabling what we define as the Internet of Nano-Things.
Figure 1 - The Internet of Nano-Things.
 
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Nano-device Hardware Architecture

Many nano-device components have already been prototyped and tested. However, there are still several challenges from the device perspective that need to be addressed in order to turn existing nano-devices into autonomous machines. In our vision (Fig. 2), several nano-components such as a nano-processor, a nano- memory and a power nano-system, have to be integrated into a device with a total size a few hundred square micrometers at most. To date, several solutions for these nano-components have been proposed:

  • Sensing Unit: Several physical, chemical and biological nanosensors have been developed by using novel nanomaterials such as graphene (the Nobel Prize in Physics 2010 was awarded to its discoverers) [48] and its derivatives, namely, graphene nanoribbons (GNRs) and carbon nanotubes (CNTs). They provide an output in terms of a change in electric current or resistance when an event is sensed. Their accuracy and resolution is much higher than existing sensors.
     
  • Processing Unit: Nanoscale processors are being enabled by the development of tinier FET transistors in different forms. The smallest transistor that has been experimentally tested to date is based on a thin GNR, made of just 10 by 1 carbon atoms. These transistors are not only smaller, but also able to operate at higher switching frequencies. However, the number of transistors in a single nano-processor is limited, and this restricts the complexity of the operations doable by the nano-processor.
     
  • Storage Unit: Nanomaterials and new manufacturing processes are enabling single-atom nano-memories, in which the storage of 1 bit of information requires only one atom. For example, in a magnetic memory, atoms are placed over a surface by means of magnetic forces. For the time being, these memories are still not ready for nano-devices, but they serve as a starting point. The total amount of information storable in a nano-memory will depend on the maximum size of the nano-device.
     
  • Power Unit: Powering nano-devices requires new types of nano-batteries as well as nanoscale power generating systems. One of the most promising techniques relies on the piezoelectric effect seen in zinc oxide nanowires, which are used to harvest mechanical, vibrational and hydraulic energy and convert it into electricity. This energy can then be stored in a nano-battery or a super capacitor.
     
  • Communication Unit: Nanomaterials such as CNTs and GNRs have also been proposed for the de- velopment of novel nano-antennas. Amongst others, in we determined that a 1 micrometer long graphene-based nano-antenna can only efficiently radiate electromagnetic (EM) waves in the Terahertz range (0.1-10 THz). This frequency band matches the predictions for the operation frequency of graphene- based radio-frequency (RF) transistors.
In addition, there are also major challenges in the integration of the different components into a single device. New methods to position and contact different nano-components are currently being developed. Amongst others, DNA scaffolding is one of the most promising techniques. So far, procedures to arrange DNA synthesized strands on surfaces made of materials compatible with semiconductor manufacturing equipment have been demonstrated. The positioned DNA nano-structures can serve as scaffolds, or miniature circuit boards, for the precise assembly of components such as carbon nanotubes, nanowires, nanoribbons and nanoparticles.
 
Figure 2 - Nano-Device Hardware Architecture.

 
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Graphene-based Nano-Antennas for Nano-Devices

Reducing the size of a metallic antenna down to a few hundreds of nanometers would impose the use of very high resonant frequencies. In particular, a one-micrometer long dipole antenna would resonate at approximately 150 THz. Due to the very limited power of nanosensors, the low mobility of electrons in conventional metals when nanometer scale structures are considered, and the challenges in implementing a nano-transceiver able to operate at this extremely high frequency, the feasibility of communication among nano-devices and nanonetworks would be compromised if this approach was followed.

Alternatively, the use of nanomaterials to fabricate miniaturized antennas can help overcome the aforementioned limitations. A few nano-antenna designs built with carbon-based nanomaterials have been already investigated. For example, CNTs can be used to develop nano-dipole antennas (Fig. 3, right). Using a transmission line model of CNTs it can be shown that, due to the atomic structure of CNT, there exists a kinetic inductance in nano-antennas that dominates over the common magnetic inductance between the nano-antenna and the ground plane. Because of this, the wave propagation speed in a CNT-based nano-antennas can be up to 100 times lower than the propagation speed in the free-space. Resulting from the retarded propagation of EM waves in CNTs, the resonant frequency of nano-antennas can be up to two orders of magnitude below that of a conventional metallic antenna. The possibility to operate at much lower frequencies relaxes the energy and power requirements for the nano-devices.

When it comes to graphene, the propagation of EM waves on infinitely large 2D planes of this nanomaterial has been theoretically analyzed recent works. However, little research has been conducted on EM propagation in GNRs, which is what is mainly needed for the design of a nano-antenna. Amongst others, it has been recently shown that the very large kinetic inductance that has been observed in CNTs can be drastically reduced in GNRs by increasing their width. As a result, both the contact resistance of GNRs and the EM wave propagation speed can be tuned by modifying the dimensions of the GNR. For this, we have recently proposed for the first time a nano-patch antenna based on a GNR (Fig. 3, left) and we have analyzed and compared its performance to that of a nano-dipole antenna based on a CNT. Amongst others, we showed that a 1 micrometer long graphene-based nano-antenna can radiate in the Terahertz Band (0.1-10 THz).

Figure 3 - GNR-based nano-patch antenna (left) and CNT-based nano-dipole antenna (right).
In this project, we will further investigate the radiation characteristics of planar nano-antennas based on GNRs and larger graphene sheets. Consistently with the trends in RF nano-transistors research, we advocate for the use of GNRs rather than CNTs for the development of nano-antennas. For this, we will initially focus on the radiation characteristics of our recently proposed nano-patch antenna and will further characterize the nano-antenna in terms of frequency response, radiation effiency and radiation pattern. In addition, to the nano-antenna, it is necessary to develop the circuit to drive it. This driving circuit needs to operate at the same frequency as the antenna itself, i.e., the Terahertz Band according to recent related works and our preliminary works. [Back to top]

Terahertz Channel Modeling for Nanonetworks

Based on our preliminary results, the envisioned nano-devices will be able to communicate in the Terahertz Band (0.1-10 THz). After nano-device design and manufacturing, the Terahertz channel is one of the main aspects that makes the realization of nanonetworks a challenge. The few Terahertz channel models existing to date are aimed at characterizing the communication between devices that are several meters far. In particular, due to the very high attenuation created by molecular absorption (hundreds of dB/m), current efforts both on device development and channel characterization are focused on the communication in the absorption-defined window at 300 GHz. However, thinking of the short transmission range of nano-devices, there is a need to understand and model the entire Terahertz band from 0.1 to 10 THz for distances much below one meter.

In this project, we think of the Terahertz Band (0.1-10 THz) as a single transmission window almost 10 Terahertz wide and develop a new channel model for Terahertz band communications. In particular, we will use radiative transfer theory to analyze the several phenomena affecting the propagation of EM waves in the Terahertz band, by starting from the absorption by molecules found along the path. Stemming from our preliminary results, we will obtain formulations for the:

  • Path-loss: The total path-loss for a travelling wave in the Terahertz band is defined as the addition of the spreading loss and the molecular absorption loss. The spreading loss accounts for the attenuation due to the expansion of the wave as it propagates through the medium, and it depends only on the signal frequency and the transmission distance. The absorption loss accounts for the attenuation that a propagating wave suffers because of molecular absorption, i.e., the process by which part of the wave energy is converted into internal kinetic energy of some of the molecules that are found in the medium. Different types of molecules have different resonant frequencies and, in addition, the absorption at each resonance is spread over a wide range of frequencies. As a result, the Terahertz channel is very frequency selective.
     
  • Noise: The ambient noise in the Terahertz channel is mainly contributed by the molecular absorption noise. The absorption from molecules does not only attenuate the transmitted signal, but it also introduces noise. The equivalent noise temperature at the receiver is determined by the number and the particular mixture of molecules found along the path. In addition, the molecular absorption noise is not white. Indeed, because of the different resonant frequencies of each type of molecule, the power spectral density of noise is not flat. Moreover, this type of noise appears only when the channel is in use, i.e., there will be no noise unless the molecules are excited.
     
  • Channel Capacity: Molecular absorption determines the usable bandwidth of the Terahertz channel. Therefore, the available bandwidth depends on the molecular composition of the channel and the transmission distance. For the very short range, this is almost the entire band. As a result, the predicted channel capacity of the Terahertz band is promisingly very large, in the order of a few terabits per second. This very large capacity enables both, the transmission at very high bit-rates, and the development of novel simple communication schemes suited for the computational and energy constraints of autonomous Nano-Things.
In addition, the model will be extended to take into account multi-path propagation effects and scattering from rough surfaces. Moreover, the origen and uniqueness of molecular absorption noise will be analyzed and optimal power allocation schemes that try to avoid the effects of molecular absorption will be investigated. [Back to top]

Communication Mechanisms and Protocols for Nanonetworks

The expected capabilities of the nano-devices and the peculiarities of the Terahertz Band require the development of novel information modulation and encoding techniques as well as protocols for nanonetworks.

First, existing modulations, which usually consist in encoding the information into one or more parameters of a periodic waveform, are not applicable in nanonetworks because of their relatively complex encoding and decoding processes and their high energy requirements. On the contrary, nano-devices require new modulations able to exploit the huge bandwidth provided by the terahertz channel, while still remaining feasible for their hardware limitations. We will investigate and propose modulation schemes specifically suited for nanonetworks.

Second, MAC protocols used in traditional networks, usually based on carrier sensing (e.g., CSMA and all its variations) need to be revised. One the one hand, information can be transmitted very fast, and, thus, there is “not much to sense”. Within this project, we will propose and design MAC protocols specifically suited for nanonetworks. For example, if the transmitted packets are very short, the probability of collision between nanosensor motes trying to access the channel at the same time will be much lower than in traditional wireless networks. In some scenarios, it is even possible that the probability of collision may be neglected, leading to the integration of MAC protocols in the physical layer. We will investigate novel MAC protocols taking into account the particularities of nanonetworks and the terahertz channel.

Third, the tiny dimensions of nano-devices make them suitable for exploring unreachable locations with unprecedented resolution with respect to traditional devices. However, they also make them vulnerable in front of any kind of external force. For example, nanosensor motes randomly deployed in an open field can be easily taken away by an air flow, or biological nanosensors can be swept away by a body fluid, for example. This may be seen as a shortcoming but can also be exploited in a beneficial way. This random movement of nano-devices will enable novel applications in which mobility of nanosensors can make the difference. Therefore, we will investigate novel routing mechanisms that consider the particular energy requirements and mobility pattern of nano-devices. [Back to top]

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