Nanotechnology is, in many respects, already a key player in Information and Communication Technologies (ICT) research and development, both in academia and industry. Computer microprocessors and memory storage devices have followed a path of miniaturization over the last 20 years that has ‘naturally’ brought transistors to have dimensions lower than 100 nm. There are now challenges to meet to continue this miniaturization: as the materials of semiconductors, metals and insulators are reduced to nano-size, quantum effects start to predominate and to determine their properties and this is resulting in a number of issues. Nanotechnology offers the opportunity to exploit, rather than avoid, quantum effects in the development of the next generation of integrated circuits. As miniaturization cannot proceed forever with the methods and tools that have been used so far, new approaches will be needed. Nanomaterials, precisely for their quantum properties, and nanotechnology tools allow for the creation of new data storage and processing methods. These developments are discussed in the sections on integrated circuits and data storage. The section on photonics looks in detail at this emerging technology for Opto-communication. Another essential contribution of nanotechnology is in the field of displays, which are becoming thinner, lighter and less energy-consuming. The contribution of nanotechnology in this area is covered in the section on displays.
But the evolution of the ICT sector will most likely go beyond what is considered ‘electronics’ today (i.e. devices that perform a task). There are visions of electronics embedded in our clothing and in the environment around us in what is conceived as a network of devices that create ‘ambient intelligence’. Although still more a vision than reality, there is intense research to bring to fruition the tools required to realize ‘ambient intelligence’.
This chapter looks at these various areas of applications in the ICT sector and considers what the impact of nanotechnologies could be in each.
What You Will Learn
- 1 Integrated circuits
- 2 Photonics
In the late 1960s, Gordon E. Moore, co-founder of Intel Corporation, made a memorable observation that later became known as Moore’s Law. He observed that the number of transistors on a chip roughly doubled every 18 months. Originally, this was only an observation but was later adopted as an industry goal. The progression in transistor density on a chip over the last 40 years has, indeed, followed Moore’s law (Figure 1). In order to keep up with this law, transistors have become smaller and smaller. The first transistor was about 1 cm high and made of two gold wires 0.02 inches apart on a germanium crystal. The latest transistors from Intel® (Penryn and Nehalem) have 45 nm feature sizes and in a quad-core configuration have over 731 million transistors! These numbers give an idea of the enormous miniaturization efforts that have been achieved by the semiconductor industry.
The scaling-down of transistors has been the driving force for a number of reasons. Transistors are the basic building blocks of the elements in an integrated circuit (microprocessor, mass memory, logic gates, etc.). Integrated circuits are the core of the information and memory storage devices that are an essential part of all electronics used: from computers to refrigerators, cars, mobile phones, etc. As the size of the transistor is reduced, its density on a chip increases, which increases the speed, the amount of memory stored per area, and the number of functions that can be integrated on a single device.
TIP FOR TEACHERS: To illustrate this, ask students to describe the size and function of the first mobile phone they can remember. In the early 1990s, it was just a phone, quite bulky and heavy. Later it became smaller, lighter, and with many more functions. Nowadays, there are all-in-one devices that are phones, digital music players, radios, video cameras, Internet platforms, and more!
The enormous advances in computing that have characterized the last 15 years (including the advent of the Internet and the ‘Information Age’) have come about as a result of the miniaturization of all the elements in computer chips. In the future, high-performance computing is expected to deliver tools that not yet available, such as a real-time language translator or ubiquitous sensing.
The scaling down of transistors cannot go on indefinitely with the tools and materials now available. Experts say that Moore’s law will last until the year 2015, perhaps a few years beyond. Eventually, a point will be reached where the transistor is so small that quantum effects start to predominate. In a transistor the ‘on’ and ‘off’ state is determined by the flow (or not) of electrons through the n-p junction (gate). At very low dimensions (e.g. 1 nm) electrons will be able to pass through the gate via tunneling, which is a fundamental quantum effect (for details see Module 1, Chapter 4: Fundamental ‘nano-effects’.
Another issue is power consumption and heat generation. This is an issue that everyone using modern mobile phone experiences daily. As the number of functions integrated into a mobile phone has increased, so has the amount of power it consumes (the type and battery life has become determinant to the quality and performance of the device and its cost). This also leads to heat generation, which, at smaller scales, becomes even more important and is considered a research priority. Reducing power consumption is also critical, not just for the performance of the device but also for environmental considerations (energy saving).
In simple terms, by continuing to use the current technology, a stage of development will be reached where further scaling down will create effects opposite to those intended such as growing energy consumption in standby mode and performance restrictions in active mode. New technologies will be needed to allow the miniaturization of electronic devices along Moore’s curve.
WHAT CAN NANOTECHNOLOGIES DO? Current transistors are already below 100 nm, so technically the semiconductor industry already makes use of nanotechnologies. Continued miniaturization of transistors using the complementary metal-oxide-semiconductor (CMOS) platform (More Moore technology) has already produced transistors smaller than 100 nm. This threshold was crossed around the year 2000, and today transistors are 45 nm. Although the More Moore approach does involve working with materials with nanoscale sizes (and related fabrication methods), it has the task of scaling existing CMOS processes and reaching the ‘CMOS limit’ (i.e. the size limit obtainable with this technology). Some consider this not to be ‘true nanotechnology’, since it does not make explicit use of the unique properties of nanomaterials (quantum effects etc.). This development course (i.e. developing fundamentally new approaches to information processing and data storage, beyond that which used today, such as self-assembly, or totally new materials such as molecular electronics) belongs to the domain called Beyond CMOS. In addition to these two domains, is the More than Moore domain, meaning technologies that provide functionality beyond traditional computing (such as intelligent systems that can interface with the user and the environment). Without going into the technical details too deeply, the next sections outline the impact of nanotechnologies in these domains of technology innovation.
Semiconductor components (microprocessors, mass memories, logic gates, etc.) will need to follow the miniaturization trend described by Moore’s law to keep up with the industry demand (More Moore). This means introducing new materials and new device architectures. One of the most critical elements in current transistors is the insulating layer between the two gate electrodes. This layer is currently made of silicon oxide: as the transistor is made smaller, the thickness of this layer is reduced to a point where leakage (the passage of electrons between the two electrodes when voltage is off (i.e. tunnel leakage current)) becomes a problem. In the latest processors, silicon dioxide is replaced with a high- permittivity material (high-k) for lower power consumption. Hafnium compounds are the most promising high-k materials and are currently used in Intel® 45 nm technology generation (node).
As the materials used in transistors change and smaller features are needed, the fabrication methods used in CMOS also need to evolve. In general, the limit of the smallest size that can be created depends on the tool used. Features about 500 nm in size could be fabricated using optical lithography but, in order to create smaller features, extreme-UV lithography is needed. Advances in CMOS technology have always implied enormous investments for the development of fabrication techniques able to deliver the required transistor size. As new materials are introduced into CMOS technology, new fabrication technologies also need to be integrated. For example, the 45 nm node is made using a fabrication process different from the conventional top-down lithography approach. Hafnium materials are normally deposited using the atomic layer deposition method (ALD) which is a bottom-up fabrication method (for a review of fabrication methods used in nanotechnologies, see Module 1, Chapter 7: Fabrication methods). The method is believed to be used in the latest generation of Intel® processors. Another nanofabrication method that is being considered to pursue Moore’s law of miniaturization is nanoimprint lithography. Although there are numerous fabrication techniques that can be used to make nanoscale features (of which the scanning tunneling microscope is the most powerful), in the context of the semiconductor industry only a few are feasible candidates. Any new fabrication technique (different to conventional lithography) must satisfy a number of stringent requirements: resolution, defect rate, throughput (i.e. being capable of producing many features quickly), and must be easily integrated into current fabrication facilities (otherwise massive financial investments would be needed). Nanoimprint lithography seems to be the technology that, at the moment, can best satisfy these needs and will most likely be the technology used for the future 32 nm and 22 nm nodes.
Another key element in integrated circuits starting to create problems is the copper connectors between the transistors. The effective resistivity of copper increases at smaller dimensions and the integrity of the signal as it travels along the connector is becoming a major issue. Nanotubes and nanowires are now being considered as alternatives to conventional copper. Carbon nanotubes are considered an ideal material since they can be outstanding conductors (with almost zero resistance and heat dissipation), and they are extremely strong mechanically and inert to chemicals. The current problem is fabrication (pure carbon nanotubes are hard to produce) and alignment.
More than Moore
The ‘More than Moore’ domain refers to the development of functional components beyond the traditional computing ones (microprocessors and memory). Future electronics will integrate an interface with the real word (so-called ambient intelligence) and will need to integrate not just a processor and memory, but also sensors, actuators, RF interfaces, etc. They will also need to satisfy a number of requirements, such as low power consumption, flexibility, thermal management, and last but not least, cost. Nanotechnologies are going to have an essential role in all those systems. In the last part of this chapter, some ideas of what ‘ambient intelligence’ might do for us and what this might imply in terms of social and ethical consequences are discussed.
A detailed analysis of the technical elements that will be integrated into these systems is not covered here as it would be too advanced for secondary school level; however, the new materials and fabrication methods that will be needed in order to integrate these smart multifunctional systems and connect them to the outside world are outlined. Various nanodevices based on different technologies and processes will need to coexist within the same package. This is conceptually very different from the current concept of ‘multifunctionality’ were different ‘packages’ with specific functionalities are separated and added together only the end of the manufacturing process. In the future, there will be ‘system in a package’ solutions where the different functionalities are integrated into one 3D architecture as the package is constructed (nanosensors, nano actuators, processors, etc.). This requires the heterogeneous integration of electrical and non-electric components, which, in turn, requires the development of new materials (e.g. plastic electronics). In the short term, nanotechnologies will probably contribute to improving the properties of current materials, for example with the addition of nanoparticles to adjust parameters such as electrical resistance, thermal conductivity, coefficient of thermal expansion, etc. In the long term, nanotechnologies are expected to be used to develop nanoscale interconnectors and self-assembly technologies that to go beyond current architectures.
It has been mentioned already that the miniaturization of microprocessors and memory cells cannot continue indefinitely, and Moore’s law will necessarily come to a halt, if the same CMOS technology is used. On the other hand, miniaturization and system integration will surely continue to be fundamental in our communication society, even more than today. Ubiquitous sensing, ambient sensing, ‘constant’ networking and communication in an ‘always connected’ state will most likely be part of our future.
For this to be possible, new materials and processing methods will be required once the dimensions of transistors become so small that quantum effects start to predominate. Nanotechnologies will allow us to exploit these effects and realize the future generation of electronics to store and process information where quantum effects and other nano-effects determine the property and functionality of the device. Without going into too much detail, there now follows some information on the fundamental developments that researchers are working on to ensure that miniaturization can continue beyond the intrinsic limits of the CMOS technology.
New data storage and processing nanotechnologies
Microprocessors work by processing information through the passage of an electrical signal. In the future, information processing will be done using a state variable other than electric charges, such as spin (spintronics), molecular states (molecular electronics), photons (photonics), mechanical state, resistance, quantum state (including phase) and magnetic flux. These new state variables will require new materials to be used (e.g. relying on spin implies using magnetic materials rather than a semiconductor) and new functional organization of the device (architecture).
The following is a list of some new concepts that are being developed for processing and data storage that make use of state variables other than the ‘conventional’ used in CMOS (electric charge).
- Spintronics: This is a new type of technology that exploits the spin of the electron (rather than only its charge) to store and process information. It requires thin layers of magnetic materials. The giant magnetoresistance effect (GMR) represents the first type of this new technology and is used in the new-generation memory devices such as the Magnetic Random Access Memory or MRAM used, for example, in the Apple iPod. The GMR and data magnetic storage devices based on this effect are discussed in the next section, Data
- Photonics: Another possibility is the use of photons (rather than electrons) in the visible or IR range to transmit and process data (optical communication). In this case, semiconductors (e.g. silicon) can no longer be used since they cannot emit and transport light efficiently. Other materials (photonic crystals) engineered at the nanoscale need to be Photonic crystals and their use in optical communication is discussed in the section on photonics in this chapter.
- Quantum electronics: This term means the explicit use of the tunneling effect to transport electrons from the source to the drain of the One such transistor exists and is called the single-electron transistor (SET). In this type of transistor, three is a quantum dot between the source and the drain: electrons must tunnel through this dot in order to get from the source to the drain. In simple terms, this is like having two electrodes (source and drain) separated by a thin insulating barrier with a third electrode (the quantum dot, or quantum island) in the middle of this insulating gap. The transport of charge from the source to the quantum island and then from the quantum island to the drain occurs via the quantum mechanical process of tunneling.
- Carbon nanotube transistors: In many ways, carbon nanotubes are considered a ‘dream’ material when it comes to electronics: they can be conductive or insulating depending on their chirality (and when they are conductive, they are extremely good electronic conductors, with little resistance and, consequently, little heat dissipation); they are strong (mechanically) and chemically inert; they are resistant to high temperatures, and they can be functionalized with specific molecules to act as anchoring Carbon nanotubes are 1–2 nm in width and they are as narrow as the double-stranded DNA molecule (the molecule that carries our genetic information). So arranging nanotubes into electronic circuitry could allow miniaturization by a factor of about 100 over the current limit.
- Molecular electronics: molecules in natural systems (plants, animals) are arranged in macromolecular systems (nanostructures) that perform numerous tasks which involve the transmission of charges, photons, These macromolecular systems have developed over millions of years of evolution and are an example of excellence when it comes to efficiency! Examples included the electric flow in nerve signaling, the control of changes in the ionic pump, and the absorption of light and transmission of photons and charge in plant chlorophyll, and many more. Molecular electronics is a branch of nanoscience that aims to make explicit use of molecular assemblies for the transmission and storage of data. The field includes molecular wires, molecular switchers, molecular sensors and other ‘hardware’ components of electronics. The idea is to assemble molecules in nanostructures that can perform a specific function (e.g. the transport of charge) depending on its configuration. An example is a two-terminal transistor having three benzene rings that act as the charge transfer site. The central benzene ring is functionalized with two groups (NH2 and NO2) that make the overall molecule very susceptible to an electronic field. The electric field can induce the distortion (twisting) of the molecule. In simple terms, this gives rise to an electronic device where if a voltage is applied, the molecule twists in such a way that the current flow is stopped; when the voltage is turned off, the molecule springs back to its native conformation and current flows again. This is just one of many molecular electronic devices under research. In order to fabricate and test such small devices, nanotools such as the STM are needed. These types of devices are still ata very early stage of development because of the time required to fabricate and test them. Furthermore, in order to make any useful molecular circuit, a vast number of devices need to be orderly arranged and securely fixed to a solid support to prevent them from interacting randomly with one another. In the future, molecular wires (e.g. made of conductive polymers such as polypyrrole) or carbon nanotubes could be incorporated into integrated circuits, which would notably reduce the size of computer chips (wires used today use about 70 % of the real space on a chip!).
Data storage is a key component of many devices in use every day: computers (which contain both a hard drive and RAM), mobile devices (in which the memory medium is normally a memory card such as digital cameras), and portable media players (e.g. iPods and mobile phones that use Flash memory).
Data storage technologies include two main groups, hard disk drives (having a mechanical component) and solid-state data storage devices, which can be further divided in volatile and non- volatile. ‘Volatile’ means that memory is lost once the power is turned off; ‘non-volatile’ means that memory is retained when the power of the device is switched off.
- Volatile memory storage devices today include mainly static random access memory (SRAM) and dynamic random access memory (DRAM).
- Non-volatile memory storage devices, such as Flash memory, are used in mobile devices and portable media
Each type of memory has its advantages and disadvantages: DRAM has a smaller memory cell size than SRAM and can, therefore, store more data but needs to be refreshed periodically, consuming power and lowering speed; SRAM can store less data but is faster; flash memory is non-volatile, making it ideal for devices where the power is often turned off.
Memory storage devices are described by their capacity (amount of data in MB), memory density (a function of the capacity and the size of the memory cell), lifetime (how many read/write cycles it can perform before it degrades), read/write speed and cost.
The concept of a universal memory
The need for memory storage devices with an ever-increasing density capacity has been the driving force in this industry. The motivation has been the need to keep pace with Moore’s law (and therefore reduce the size of the memory devices), and the proliferation of mobile devices that demand low power operation and batteries that have a low consumption in standby mode. These two trends have catalyzed the search for a universal memory, meaning a memory storage device that addresses the major technical challenges of existing memory technology (DRAM, Flash memory, etc.), while combining the valuable properties of each — speed, density and non-volatility. Although a universal memory is still a concept, research towards its realization is intense and new technologies that have been developed are trying to go in this direction. Nanotechnology is an essential part of all of these.
Nanotechnology developments in data storage
In the last few decades, the dimension of memory storage cells has decreased, following Moore’s law. This continuing size reduction has, however, led to memory cells with extremely small transistors. At the current 90 nm process node, SRAM and DRAM are beginning to suffer from a number of scaling issues. As the semiconductor is reduced in size, quantum effects come into play, and electrons can ‘jump’ from the source to the drain by tunneling or just by thermal motion. Moreover, other elements become critical, such as charge leakage due to silicon substrate crystalline defects. In Flash memory, an insulating layer of silicon oxide ‘wraps’ the gate architecture and serves as a barrier for storing the change. As this layer becomes thinner, the charge can start to leak out of the device. These are only a few of the problems facing memory storage development today.
To deal with these challenges, new concepts that make use of fundamental nano-concepts and nanotechnology tools have recently been introduced:
- magnetoresistive random access memory (MRAM), where each memory cell is made of two ferromagnetic thin layers separated by an insulating layer;
- ferroelectric RAM (FeRAM), similar in architecture to MRAM, but where a ferroelectric layer replaces the dielectric layer;
- resistive RAM (RRAM), in which a conduction path is created through a dielectric material;
- phase transition memory, also known as phase-change memory (PCM), which uses the phase transition of a material from crystalline to amorphous;
- nanotube RAM, a trademark of the company Nantero (NRAMTM), which uses carbon nanotubes to determine memory
Each of these technologies is at a different level of maturity and time will be needed before they can compete with Flash memory. However, development in this sector is very rapid and it is projected that by 2012, nanotechnology-enabled storage devices will account for 40 % of the total memory market.
Magnetic materials are used in magnetoresistive random access memory (MRAM), in which each memory cell consists of two magnetic thin-film materials separated by an isolating layer.
Magnetic multilayer nanocomposites (very thin films made of magnetic materials) display magnetoresistance properties. Magnetoresistance is a phenomenon whereby the application of a DC magnetic field changes the resistance of a material. In metals, this effect occurs only at very high magnetic fields and low temperatures. In 1988, it was discovered that a very pronounced effect, now known as the giant magnetoresistance effect (GMR), could be obtained in materials made of alternating layers of nanometre thick ferromagnetic material and non-magnetic but conducting material (Figure 2). The magnetization vector can be aligned (parallel) or not (anti-parallel)in the magnetic layers forming the multilayer, as shown in Figure 2. Polarity is persistent, therefore the device is non-volatile. If current flows through the device and the layers have a parallel configuration, the resistance is lower than if the layers are anti-parallel. In the MRAM memory cell, the two thin layers of metals are separated by a thin insulating material. This gives rise to another effect called the tunneling magnetoresistance effect (TMR). In contrast to GMR, TMR uses ultra-thin non-conducting spacer layers, which renders the ferromagnetic layers electrically isolated. Electrons can thus tunnel through this barrier (13). As in the GMR effect, the passage of a current changes the polarity of the magnetic material causing either a high or a low resistance across the insulating barrier, which can be read as a ‘1’ or a ‘0’. To write data in such memory cells, a current is passed through the cell, which alters the polarity of the magnetic layers.
Some materials have been discovered having even larger magnetoresistive effects than layered materials and this phenomenon is called colossal magnetoresistance (CMR). These materials (e.g. LaSr- and LaCa-manganites) have a peculiar crystal structure that is responsible for the CMR effect and could have a number of applications in memory storage devices.
The company Nantero Inc. has introduced a new type of non-volatile random access memory, NRAMTM, where the N stands for ‘nanotube’ since it uses carbon nanotubes. The nanotubes are assembled as fibers and are suspended perpendicularly across trenches of etched silicon wafers. At the end of the silicon trench, there is a contact electrode. When a potential is applied between the fabric and the electrode, the fabric bends and touches the electrodes. The interaction is based on van der Waals forces and remains even after the power is turned off. When the fiber is far away from the electrode, the junction has a high resistance and this is read as a ‘0’ state. As current is passed, and the fiber touches the electrode, the junction resistance is lowered, and this is read as a ‘1’ state. Carbon nanotubes are produced using an alternative to the conventional method (chemical vapor deposition, CVD) and which does not require high temperatures. The process developed by Nantero, Inc. allows the production of pure carbon nanotubes at room temperature. This is an added advantage to the technology as CVD requires temperatures so high as to damage the other constituents of the memory device. The NRAM™ is considered a very important development towards the concept of universal memory, and it has the advantage that it is fabricated using current CMOS technology platforms: this ensures immediate manufacturability.
Phase-change memory (PCM) (also called phase random access memory, PRAM) is another new and promising concept in non-volatile memory storage. It uses a material, chalcogenide glass, which can be ‘switched’ between two states, crystalline and amorphous, through the application of heat. The amorphous state has high resistance and is used to represent ‘0’, whilst the crystalline state has lower resistance and is associated with the state ‘1’.
Overall, PRAM (or PCB) is considered the most promising advancement in non-volatile memory solutions. In February 2008, Nymonyx started shipping the first PCM prototypes for customer evaluation. The product, Alverstone, is a 128 MB PRAM produced on a 90 nm CMOS fabric. Numonyx is the new company formed by STMicroelectronics and Intel®, producing non-volatile memory. Progress in this technology is fast and, in December 2009, Nymonyx announced a paper showing the scaling of PCM to the 45 nm lithography node for the first time on a 1 GB product with an effective cell size of 0.015 µm2. Numonyx researchers report good electrical properties and reliability results, confirming that PCM has reached the maturity to become a mainstream technology for high-density non-volatile memory applications.
Photonics is the study of the interaction of light with matter. The field was opened up in the 1960s with the invention of the laser. Ten years later, the invention of the optical fiber as a means of transmitting information via light formed the basis for optical communication. The field is now enormous and consists of many sub-disciplines and applications, such as laser technology, biological and chemical sensing, display technology, optical computing, fiber optics, photonic crystals and more.
In 1987, Eli Yablonovitch at Bell Communications Research Centre created an array of 1 mm holes in a material with a refractive index of 3.6. It was found that the array prevented microwave radiation from propagating in any direction. This discovery launched the research on photonic crystals, but it took more than a decade to fabricate photonic crystals that do the same in the near-IR and visible range. Nowadays, photonic crystals are an important nanomaterial being investigated for numerous applications including, in particular, optical communication.
A photonic crystal consists of a periodic structure made of dielectric materials that affect the propagation of light. Essentially, photonic crystals contain regularly repeating internal regions of high and low dielectric constant. Photons (behaving as waves) propagate through this structure — or not — depending on their wavelength. The periodicity of the photonic crystal structure has to be of the same length scale as half the wavelength of the electromagnetic waves (i.e. approximately 200 nm (blue) to 350 nm (red) for photonic crystals operating in the visible part of the spectrum. Such crystals have to be artificially fabricated by methods such as electron-beam lithography and X-ray lithography.
Photonic crystals exist in nature. For example, in Module 1, Chapter 2: Nanoscience in nature it was shown how the beautiful blue wings of some butterflies owe their color to their internal nanostructure, which is in fact a photonic crystal structure: another example is opals.
Applications in communication
Photonic crystals are now receiving much attention because of their potential particularly in the optical communication industry. The current explosion in information technology has been enabled by semiconductor technology and the ability to fabricate materials where the flow of electrons can be controlled in the most intricate ways. Photonic crystals promise to give us similar control over photons — with even greater flexibility because scientists have far more control over the properties of photonic crystals than they do over the electronic properties of semiconductors. The goal of putting more transistors on a chip (to make smaller and faster integrated electronic circuits) requires further miniaturization. This, unfortunately, leads to higher resistance and more energy dissipation, putting a limit on Moore’s law. Researchers are considering using light and photonic crystals (as an alternative to electrons traveling in wires) for the new generation of integrated circuits. Light can travel much faster in a dielectric medium than an electron in a wire, and it can carry a greater amount of information per second. Given the impact that semiconductor materials have had on every sector of society, photonic crystals could play an even greater role in the 21st century.
Properties of photonic crystals
Photonic crystals are designed to confine, manipulate and control the propagation of photons in three dimensions. The properties of a photonic crystal are determined by the radius of the holes (or other features such as dielectric rods) forming the array, the periodicity of the holes (or rods), the lattice structure, the thickness of the material and the refractive index.
The properties of photonic crystals can be understood by considering an analogy with semiconductors. In general, in a semiconductor, the valence or conduction electrons can move in a periodic potential arising from the positively charged ion cores (the nucleus). The potential is characterized by two areas of allowed energy, which are separated by a region of forbidden energy, called the energy gap, which means that there are certain wavelengths that will not propagate in the lattice (i.e. no electrons will be found in the bandgap region). This is true for a perfect silicon crystal. However, in real materials, the situation is different: electrons can have energy within the bandgap if the periodicity of the lattice is broken. This can be the result of a missing silicon atom or the presence of an impurity atom occupying a silicon site, or if the material contains interstitial impurities (additional atoms located at non-lattice sites). The doping of semiconductors is intentionally done in microelectronics.
Now consider photons moving through a block of transparent dielectric material containing a number of tiny air holes arranged in a lattice pattern. The photons will pass through regions of the high refractive index the dielectric alternating with regions of the low refractive index the air holes. This is analogous to the periodic potential that an electron experiences when traveling through a silicon crystal. If there is a large difference in the refractive index between the two regions, then most of the light will be confined either within the dielectric material or the air holes. This confinement results in the formation of allowed energy regions separated by a forbidden region the so-called photonic bandgap. Since the wavelength of the photons is inversely proportional to their energy, the patterned dielectric material will block light with wavelengths in the photonic bandgap, while allowing other wavelengths to pass freely.
In a semiconductor, it is possible to break the perfect periodicity of a silicon-crystal lattice by introducing defects (doping), and to have electrons in the forbidden energy gap region. Similarly, it is possible to create energy levels in the photonic bandgap by changing the size of the holes (or rods) forming the photonic crystals. This introduces an ‘allowed frequency’ (i.e. an allowed wavelength) in the photonic bandgap, and the photonic crystal then acts as a waveguide.
Therefore, in simple terms, semiconductors consist of periodic arrays on the atomic scale that control the flow of electrons; photonic crystals consist of periodic arrays on the scale of the wavelength of dielectric material that controls the propagation of light waves. Periodic structures made of materials having different dielectric properties (high and low refractive index materials) serve as waveguides. Optical properties can be kept absolutely under control by controlling the structure and material composition of the photonic crystal.
Fabrication of photonic crystals
There are different approaches to building a photonic crystal. The first researchers to report this idea were Yablonivich and John in 1987, who fabricated a crystal at microwave wavelengths. The fabrication consisted of covering a block of dielectric material with a mask consisting of an ordered array of holes and drilling through these holes in the block on three perpendicular facets. The material, which became known as ‘Yablonovite’, prevented microwaves from propagating in any direction — in other words it exhibited a 3D photonic bandgap. Despite this early success, it has taken over a decade to fabricate photonic crystals that work in the near-infrared and visible regions of the spectrum. For example, for visible light, a lattice dimension of approximately 500 nm would be required, which means fabricating a material with holes separated by 500 nm. This is an extremely difficult task that requires suitable materials and processing techniques.
Instead of drilling holes on a surface, another approach to creating a photonic crystal is to build the lattice out of isolated dielectric materials that are not in contact, as illustrated in Figure 3.
If an entire line defect is introduced in the lattice in Figure 3 by removing an entire row of rods, the regions where the rods have been removed act like a waveguide, and an allowed energy level is created in the photon gap. A waveguide is like a pipe that confines electromagnetic energy, enabling it to flow in only one direction. One interesting characteristic of such a waveguide is that light passing through it can turn very sharp corners, which is not true in an optical fiber.
If only one rod is eliminated in the lattice in Figure 3 (or if its diameter is changed), a resonant cavity is formed, which also puts an energy level into the gap. As already mentioned, this energy gap depends on the radius of the rod, which provides an opportunity to tune the frequency of the cavity. This ability to tune the light and concentrate it in small spaces makes photonic crystals useful for filters and couplers in lasers.
In theory, photonic crystal applications reach across the entire electromagnetic spectrum, from UV to radio waves. In practice, the challenge is the nanofabrication of these devices. Novel nanofabrication methods, including self-assembly approaches, will play an essential role in the development of this field.
Display technology has progressed enormously in the last decade. Until a few years ago, there were bulky televisions with cathode ray tube (CTR) technology and mobile phones with black and white displays that could only show text. Nowadays, LCD televisions are becoming the norm and mobile phones that can show photos and movies are the norm, many even with touch displays. This progress has been enabled by intense research in the field, driven by a billion-dollar industry and the constant need for more functional devices combining image and video quality, low power consumption and low cost. Thickness and flexibility are also becoming an important requirement. Here, some of the latest advances in display technology and the impact that nanotechnologies have had in their development are reviewed. Organic light-emitting diodes (OLEDs), quantum dot light-emitting diodes (QD- LEDs) and electronic paper (e-paper) are described in detail. Another area of development is field emission display (FED), which, in a sense, is the progression of the old cathode ray technology to the nanoscale. Field emission display uses an element, such as carbon nanotubes, as a source of electrons that strike a colored phosphor. This technology is not commercial yet and remains at the prototype level.
Current displays rely mainly on two approaches: liquid crystal displays (LCD) and plasma display panels (PDP). These two technologies have basically replaced the old cathode tube technology which is being phased out. Plasma display panels require much more energy than liquid crystal displays to operate and are currently not as successful as LCDs, especially at a time when energy consumption is a major concern. LCD technology, on the other hand, is becoming extremely common due to a considerable price drop in the last few years.
Liquid crystal displays offer a number of advantages compared to the old cathode tube technology: first of all, displays are much thinner and less bulky due to their inherent structure and assembly, and they provide a much better image quality. However, LCDs have two main problems connected to their inherent composition: first, LCD displays can be difficult to see at oblique angles, as laptop computer users are well aware; second, they require a backlight, which consumes power.
The need for backlight explains why, in an LCD, black looks more like deep grey than true black.
Organic light-emitting diodes have emerged as a new technology offering a number of advantages over LCDs (Figure 4). In this technology, the display is formed of an emissive and conductive layer sandwiched between an anode and a cathode. A voltage is applied across the OLED so that the anode is positive with respect to the cathode. This creates a flow of electrons from the anode to the cathode: therefore, electrons are removed from the conductive layer and added to the emissive layer. The conductive layer becomes positively charged and the emissive layer negatively charged. This causes electron holes to appear at the boundary between the two layers: electrostatic forces induce electrons and holes to get closer and they recombine, emitting energy as a photon. Emission is in the visible region and this is what generates the colors in the display.
Molecules used in OLEDs as the name suggests — are organic molecules such as organo-metallic chelates and conjugated dendrimers (two types of macromolecules that can reach the nanoscale). The molecules can be directly evaporated or printed on the polymer substrate, which is a notable advantage and can lead to very complex multilayer structures. The molecules are deposited in rows and columns onto the flat carrier (simply by printing) and the resulting matrix of pixels can emit different colors.
Therefore, the color generation in an OLED is fundamentally different from that in an LCD, which is powered by a fluorescent lamp (backlight) color-filtered to produce red, green and blue pixels. Thus, when an LCD screen displays full white color, two-thirds of the light is absorbed by the filters. This explains some fundamental advantages of OLEDs over LCD.
- OLED pixels directly emit light, they do not require a backlight, thus consuming less power than When in the OFF mode, the OLED elements produce no light and consume no power.
- OLEDs have a much better picture quality because they naturally achieve a higher contrast ratio. For a viewer, the most noticeable effect is that the black areas in an OLED appear true
- OLEDs can be much thinner and lighter than LCD In addition, OLED can be printed onto any suitable substrate using an inkjet printer so they can, theoretically, be incorporated into flexible substrates, which opens up numerous new display possibilities (roll-up displays or displays included in fabrics).
- Unlike LCDs, OLED panels can be seen well under sunlight and at different OLED technology is currently used in small displays (OLEDS are used in numerous colour mobile phones) but it is not yet established in television panels due to some challenges presented by the OLED technology. Many of these challenges can be addressed with the use of nanomaterials and nanotechnologies.First, the organic molecules used in the OLED have an inherent tendency to degrade in time. Increasing the lifetime of OLEDs is a research priority. This is particularly true for the color blue (e.g. the lifetime of a blue OLED is 14 000 hours, whereas a typical LCD panel has a lifetime of 60 000 hours). Much research is being dedicated to the synthesis and testing of new organic molecules that can increase the lifetime of OLEDs. The problem of OLED lifetime is particularly important in those electronics that require a long lifetime (e.g. televisions). In devices that are used intermittently (mobile phones etc.), lifetime is less important. This explains why OLED technology is already fairly well diffused in mobile devices.
A second problem is in the nature of the top cathode layer of the OLED, which needs to be a transparent electrode. Currently, ITO (indium tin oxide) is used since it offers a combination of conductivity and transparency. The problem is the availability of ITO, which is declining, and in the way it is processed, which is by chemical vapor deposition (CVD). This deposition method requires very high temperatures which limit the type of substrate that can be used. ITO is slightly opaque and this affects image quality. A very active field of research is the search for suitable nanomaterials to replace ITO in OLED cathodes. One possibility is carbon nanotubes. Currently, there are numerous methods to fabricate CNTs (alternatives to CVD) such as spray coating and roll-to-roll printing. Furthermore, numerous companies are emerging that produce CNTs and the price is dropping.
Another issue in OLEDs is that the organic molecules contained within them are very sensitive to water and other substances. Therefore, the coating and packing of the OLED components is of fundamental importance. One of the problems is that current packaging materials are either too brittle (which is a problem in flexible screens) or require too high a temperature during the production process, which would destroy the device. Currently, alternating layers of organic and inorganic materials are used. This gives some degree of flexibility and blocks the entry of foreign substances (although not totally). Nanomaterials could help solve this challenge: for example, a method was recently developed whereby nanoparticles are used to fill the pores in these organic and inorganic layers and this markedly improved the packaging of these devices.
Application of OLEDs
Organic light-emitting diode technology can be used for two main applications: displays and light sources. The possibility of using OLEDs instead of traditional light bulbs is very attractive in terms of energy consumption (conventional incandescent lights lose most of their energy as heat). OLED technology is thus being considered for use in lights and signs. The potential of OLED technology in the energy sector is discussed in this module, Chapter 3: Energy.
The second mainstream application is in the display industry. Currently, they are used as small screens for mobile phones, portable digital audio players (MP3), car radios, digital cameras, etc. OLED technology in TV is just appearing on the market. The first commercial OLED TV was the Sony XEL-1, commercialized in 2007 (at a cost of about USD 2 500 in the United States). In October 2008, Samsung announced the world’s largest OLED Television (40 inches) with a full HD resolution of 1 920 x 1 080 pixels. The same company showcased the thinnest OLED display, only 0.05 mm (i.e. thinner than paper). This opens up numerous opportunities. Clearly, all major display companies are investing hugely in this new technology.
The fact that OLEDs can be fabricated on flexible substrates opens the door to OLEDs in textiles, labels and other flexible materials.
Quantum dot light-emitting diodes (QD-LED)
Quantum dots (QD) are another class of nanomaterials under investigation in the manufacture of more efficient displays and light sources (QD-LEDs). Quantum dots are nanoscale semiconductor particles characterized by emitting a specific color based on the size of the nanoparticle. A minute change in particle size results in a totally different color being emitted; for instance, a 6 nm-diameter particle would glow red, while another of the same material but only 2 nm wide would glow blue. Light emission from a QD is monochromatic, therefore it is very pure. As a consequence, their use in displays would lead to images of exceptional quality. The most exciting property of QD-LEDs, however, is that they use much less power than the currently used LCDs where light is filtered by numerous polarisers. Like OLEDs, QD-LEDs emit light, rather than filtering it, so, for this reason, QD-LEDs are expected to be more energy-efficient. In June 2006, QD Vision announced the first proof of concept of a quantum dot display.
Electronic paper (e-paper)
e-paper is a display technology that aims to mimick the appearance of ordinary ink on paper. There are various technologies to create e-paper, among which electrophoretic paper is the most established. Unlike conventional flat panel displays, electrophoretic displays rely on reflected ambient light, rather than the emission of light from the display, and can retain text or images without constant refreshing, thereby requiring much less power. This also means that they can be used under sunlight without the image fading. The e-paper is also very light and flexible. e-paper has numerous commercial applications, such as e-books (i.e. e-readers that can display the digital version of a book), e-papers, electronic labels, general signage, timetables at bus stations, etc.
The first electronic paper was invented in the 1970s by Nick Sheridon (Xeron) and was called Gyricon (Figure 5). The technology behind the electrophoretic paper is fundamentally different to that behind other displays. In this case, the information display is formed by the rearrangement of charged pigment particles in an applied electric field: titanium oxide (TiO2) particles about 1 µmin diameter are dispersed in oil with a dark-colored dye together with surfactants and charging agents. The mixture is placed between two parallel conductive plates and a voltage applied. The charged particles move electrophoretically towards the plate with their opposing charges. When the particles are on the front side of the display, they scatter light and appear white; if they are at the back of the display, the display looks black because the incident light is absorbed by the dye. The back electrode is divided into a number of small picture elements, the pixels, so the image is formed by delivering the appropriate voltage to the pixel (resulting in a white or black pixel). Therefore, the image is created by a repeated pattern of reflecting (white) and absorbing (black) regions.
Developers imagine that e-paper will be used as a support (imagine a roll to keep in your pocket that opens up on command) on which to download information (from an overhead satellite, a mobile phone network or an internal memory chip) and then as reading support which does not require power and can be seen under sunlight etc. (Figure 6). Once the reading is done, the device can be rolled up again, placed in the pocket and reused.
At present, the technology is limited to black and white but numerous companies are investing resources to develop a colored e-paper solution. Another issue is the low refresh rate, making electrophoretic paper unsuitable for displaying animation or video. Some newspapers are already offering their subscriber’s e-paper versions of their print editions.
Nanotechnologies for e-paper
Some new materials are emerging as elements of e-paper. In May 2008, Unidym and Samsung produced the first active-matrix e-paper device incorporating carbon nanotubes as the transparent electrodes. Other approaches use photonic crystals, liquid crystals and other nanomaterials.
Another development uses a thermochromic display (Figure 7) which can easily be fabricated using soft lithography. The thermochromic material is a microencapsulated powder mixed inside a polymer (PDMS). The thermochromic material is dark green at room temperature but turns white when heated above 60 °C. Once the two components have been mixed, they form a polymer composite that can be spin-coated over a substrate and, once it is cured, this forms a thin polymeric sheet that has the characteristic (inherent to PDMS) of being very flexible but also resistant to stretching etc. The thermochromic display is basically made of a single layer of the thermochromic sheet in which a conductive wire, shaped in the logo desired, is embedded. When a voltage is applied, an electric current is generated in the wire, which creates a localized heat. Above 60 °C, the color of the thermochromic composite in the area corresponding to where the wire becomes white, and the image of the logo appear. The main limitation of this technology is that the image is created through a wire, and not pixels.
Information storage devices
Numerous miniaturized devices now exist that can store and transmit data. Two examples are ‘smart cards’ and ‘smart tags’. These devices have been created to meet the need to collect and transmit data using less space (chips) and wirelessly. The application of these devices covers personal data cards (e-patents, e-health cards, credit cards, etc.), tags for package protection and tracking, etc. The trend in the development of these devices has been opened up by miniaturization: integration of more functions in a smaller space. In the future, this trend will continue and nanotechnology will most likely be the enabling technology.
Products are most commonly identified and tracked using a label, which is also a code: the one still commonly in use is the barcode. In the last years, the emergence of another labeling and tracking system has been seen radio frequency identification (RFID). An RFID is a small, wireless integrated circuit (IC) chip (Figure 8) with a radio circuit and an identification code embedded in it. The advantages of the RFID tag over other scannable tags (e.g. barcodes) are that the RFID tag is small enough to be embed- ded in the product itself (not just on its package); it can hold much more information; it can be scanned at a distance (and through materials, such as boxes or other packaging); and many tags can be scanned at the same time. RFID tags are already being used in many ways, for example for livestock tracking (attached to the ear or injected into the animal) or in the latest e-passports. They are used in libraries, schools, transport systems (toll roads), tickets (parking tickets), sports (race timing), etc. Developers of the technology envision a world where they can ‘identify any object anywhere automatically.’
The current size of the RFID chip is about the size of a dust mite, the smallest is about 50 x 50 µm. There are already some developments that make use of nanotechnology, in particular tags directed towards the authentication and tracking of valuable products such as drugs. The goal is to avoid counterfeiting and to make sure that the drug is not released into the wrong market (e.g. black market). One example is a technology developed by NanoInk®, the company that owns the patent of the Dip Pen Nanolithography® (DPN®) (14) instrument, which was invented by the company owner. The company had developed a technique for encrypting pills using the DPN. Each pill can be encrypted with information about place and day of manufacture, target market and expiry date. To make this encrypting technology fully workable, the same encrypting should be placed on the package of the drugs. Another method developed at the National Physics Laboratory in the UnitedKingdom makes use of electron beam lithography to encrypt pills. This way, the pill carries information in a very secure way: a special reader is needed for it to be decoded. Furthermore, the information is so tiny that it cannot be seen with the naked eye, making it ideal for covertly marking things.
Another strategy for authentication, developed particularly for packing, has been advanced by Nano- plex Technologies, Inc. (USA) in the form of nano barcodes. These are made of nanoparticles of gold, silver and platinum, which are grown into stripes. Each stripe has a different metal combination which leads to a different reflectivity of the stripe. A special microscope reader is needed to decode the information. Nanoplex can create different codes by alternating the stripe order so billions of different codes can be made. Each unique code can be associated with an item, which allows the company to track where the item is or has been.
Wireless sensing and communication
At present, ‘electronics’ basically means devices pieces of equipment that offer a service (computation, information, entertainment and communication). The development path over the last years has been adding performance and complexity to these devices. One of the visions of the ICT industry is the concept of ambient intelligence: computation and communication always available and ready to serve the user in an intelligent way (i.e. satisfying certain requirements). The vision is that electronics will be embedded in the natural surroundings of life (clothes, books, doors, etc.), present whenever needed, enabled by simple and effortless actions, attuned by users’ senses, adaptive to users’ needs and actions, and totally autonomous. The concept of ambient intelligence is partly still a science-fiction vision, and technologies are not yet developed that allow this vision to be realized. However, the vision of actively interfacing humans and electronics is not totally abstract: consider, for example, the Wii system, which allows a person to command (and play with) software simply by moving at a distance, or virtual reality games and communication tools. The vision of ambient intelligence, however, is even more ambitious. Scientific progress and industrial investment are likely to make ambient intelligence (or at least some of its concepts) a reality in the future.
Electronic devices in the ‘intelligent ambient world’ will become a gateway between the user and the environment. A fundamental requirement is ubiquitous sensing and computing: devices must be highly miniaturized, integrated into the environment, autonomous, robust, and require low power consumption. They should be created easily and survive without particular management. All of these requirements are likely to be met with the use of nanotechnologies: some key elements are now listed.
- Miniaturization and system integration: Numerous different functions (logic, memory, radio frequency, sensors and software) will need to be integrated in a single This implies new and severe demands on the microelectronic, optoelectronic and microsystem components that are the ‘building blocks’ of the Information Society technologies. It means developing new manufacturing technologies and materials that can make it possible to reach this advanced system integration at feasible costs. Nanotechnologies allow the use of nano-sized material (carbon nanotubes, molecular electronics, etc.) and realize nanosystem transistors hundreds of times smaller than the current ones. Miniaturization is the gateway to a number of key elements: mobility, low power consumption, more performance, small sizes and low weight, low cost, high reliability, more flexibility and ubiquity.
- Embedded sensors: One of the key enablers of ambient intelligence is the presence of sensors embedded in the user and in the environment with which it needs to communicate to gather information and communicate data Nanotechnologies may render this possible by enhancing the sensory skills of humans based on wearable or embedded sensors (e.g. in clothing) and the ability to process this enormous amount of sensory data through powerful computers. In order to achieve a true integration between the sensor element and the physical object, the device should adapt to the environment that surrounds it: so those devices should become ‘intelligent’ in the sense that learning should be a key property of their systems, similar to the way systems grow and adapt in the biological world. Ambient sensors should also be robust, survive in harsh environments or, in the case of wearable electronics, survive washing, be inexpensive and be ecologically sustainable.
- Integrated power sources. The power supply is crucial for all embedded electronics, whetherFrom ‘chips’ to embedded ‘soft’ electronics: Presently, electronics are in the form of solid, rigid ‘wafers’ or ‘chips’. To realize the concept of ambient intelligence, a key requirement is the embedding of electronics in many different types of materials, from plastics to Organic electronics (i.e. meaning naturally conducting molecules, such as polypyrrole) will have an important role. At the moment, organic materials are used in OLEDs (see the OLED section in this chapter), organic solar cells (discussed in this module, Chapter 3: Energy), and in organic transistors (still at a proof-of-concept stage). Although all these systems are still in development, research is very intense and it is likely that organic electronics will become a key component in our future electronic devices. The reason is that organic electronics are produced by depositing and patterning thin films of organic conductors, semiconductors or insulators. Organic films can be deposited in a vacuum, but also inexpensively from solution or via high- resolution inkjet printing. Processing temperature is always low (below 200 °C) meaning that organic thin films can be integrated with soft materials such as plastics, opening the way to flexible organic electronics. wearable (e.g. electronics in clothing, shoes) or embedded in objects surrounding the user. For example, in the case of wearable electronics, where low power is required, the electrical power could either be body heat or body movement. Although the field is in its infancy, research is very intense and some systems have been demonstrated in laboratories in the form of miniaturized Thermo generators or miniaturized energy scavenger systems (these are discussed in Module 2, Chapter 3: Energy).
Wearable sensing textiles
Sensors could be inserted inside clothes to gather information about the wearer’s location and other environmental conditions around them, such as temperature, pressure, etc. This would enormously facilitate the task of locating missing people. For example, a jacket (or other wearable textiles) with an integrated GPS sensor could become a standard part of children’s clothing to ensure their safety. Other electronics could become integrated as well, such as phones, music devices, etc. With the advancement of nanoelectronics, these types of clothing could become a reality.
Intelligent sensors are already a reality in the form of wearable sensing textiles that can monitor fundamental physiological parameters such as heart rate, temperature, respiratory rate, etc., with applications in monitoring and prevention of cardiovascular risk. For example, the Italian company Smartex has developed a prototype that measures all of these parameters, including posture. The information registered by the sensing textile is sent to a computer via Bluetooth. The company is also part of a new European project, Biotex, which aims to create wearable textiles with even more sophisticated sensing capabilities. The aim is to create biosensing clothes that remotely monitor physiological and metabolic functions in order to improve early diagnoses. The approach aims to develop sensing patches adapted to various targeted body fluids and biological species to be monitored, where the textile itself is the sensor.