Nanoelectromechanical systems
Encyclopedia
Nanoelectromechanical systems (NEMS) are devices integrating electrical and mechanical functionality on the nanoscale. NEMS form the logical next miniaturization step from so-called microelectromechanical systems
, or MEMS devices. NEMS typically integrate transistor-like nanoelectronics
with mechanical actuators, pumps, or motors, and may thereby form physical, biological, and chemical sensors. The name derives from typical device dimensions in the nanometer range, leading to low mass, high mechanical resonance frequencies, potentially large quantum mechanical effects such as zero point motion
, and a high surface-to-volume ratio useful for surface-based sensing mechanisms. Uses include accelerometer
s, or detectors of chemical substance
s in the air.
. As noted by Richard Feynman
in his famous talk in 1959, There's Plenty of Room at the Bottom
, there are a lot of potential applications of machines at smaller and smaller sizes; by building and controlling devices at smaller scales, all technology benefits. Among the expected benefits include greater efficiencies and reduced size, decreased power consumption and lower costs of production in electromechanical systems.
In 2000, the first very-large-scale integration
(VLSI) NEMS device was demonstrated by researchers from IBM. Its premise was an array of AFM tips which can heat/sense a deformable substrate in order to function as a memory device. In 2007, the International Technical Roadmap for Semiconductors (ITRS) contains NEMS Memory as a new entry for the Emerging Research Devices section.
and electron beam lithography
, to manufacture devices. While being limited by the resolution of these methods, it allows a large degree of control over the resulting structures. Typically, devices are fabricated from metallic thin films or etched semiconductor
layers.
Bottom-up approaches, in contrast, use the chemical properties of single molecules to cause single-molecule components to (a) self-organize or self-assemble into some useful conformation, or (b) rely on positional assembly. These approaches utilize the concepts of molecular self-assembly
and/or molecular recognition
.
This allows fabrication of much smaller structures, albeit often at the cost of limited control of the fabrication process.
A combination of these approaches may also be used, in which nanoscale molecules are integrated into a top-down framework. One such example is the carbon Nanotube nanomotor
.
based, specifically carbon nanotube
s and graphene
. This is mainly because of the useful properties of carbon based materials which directly meet the needs of NEMS. The mechanical properties of carbon (such as large Young's modulus
) are fundamental to the stability of NEMS while the metallic and semiconductor
conductivities of carbon based materials allow them to function as transistor
s.
Both graphene and carbon exhibit high Young's modulus, excessively low density, low friction and large surface area. The low friction of CNTs, allow practically frictionless bearings and has thus been a huge motivation towards practical applications of CNTs as constitutive elements in NEMS, such as nanomotor
s, switch
es, and high-frequency oscillators Carbon nanotubes and graphene’s physical strength allows carbon based materials to meet higher stress demands, when common materials would normally fail and thus further support their use as a major materials in NEMS technological development.
Along with the mechanical benefits of carbon based materials, the electrical properties of carbon nanotubes and graphene allow it to be used in many electrical components of NEMS. Nanotransistors have been developed for both carbon nanotubes as well as graphene. Transistor
s are one of the basic building blocks for all electronic devices, so by effectively developing usable transistors, carbon nanotubes and graphene are both very crucial to NEMS.
Metallic carbon nanotubes have also been proposed for nanoelectronic interconnects since they can carry high current densities. This is a very useful property as wires to transfer current are another basic building block of any electrical system. Carbon nanotubes have specifically found so much use in NEMS that methods have already been discovered to connect suspended carbon nanotubes to other nanostructures. This allows carbon nanotubes to be structurally set up to make complicated nanoelectric systems. Because carbon based products can be properly controlled and act as interconnects as well as transistors, they serve as a fundamental material in the electrical components of NEMS.
. Similarly, other changes to the electronic and mechanical attributes of carbon based materials must fully be explored before their implementation, especially because of their high surface area which can easily react with surrounding environments. Carbon Nanotubes were also found to have varying conductivities, being either metallic or semiconducting depending on their helicity
when processed. Because of this, very special treatment must be given to the nanotubes during processing, in order to assure that all of the nanotubes have appropriate conductivities. Graphene also has very complicated electric conductivity properties compared to traditional semiconductors as it lacks an energy band gap
and essentially changes all the rules for how electrons move through a graphene based device. This means that traditional constructions of electronic devices will likely not work and completely new architectures must be designed for these new electronic devices.
and molecular dynamics
(MD), important behaviors of NEMS devices can be predicted via computational modeling before engaging in experiments. Additionally, combining continuum and MD techniques enables engineers to efficiently analyze the stability of NEMS devices without resorting to ultra-fine meshes and time-intensive simulations. Simulations have other advantages as well: they do not require the time and expertise associated with fabricating NEMS devices; they can effectively predict the interrelated roles of various electromechanical effects; and parametric studies can be conducted fairly readily as compared with experimental approaches. For example, computational studies have predicted the charge distributions and “pull-in” electromechanical responses of NEMS devices. Using simulations to predict mechanical and electrical behavior of these devices can help optimize NEMS device design parameters.
NEMS devices, if implemented into everyday technologies, could further reduce the size of modern devices and allow for better performing sensors. Carbon based materials have served as prime materials for NEMS use, because of their highlighted mechanical and electrical properties. Once NEMS interactions with outside environments are integrated with effective designs, they will likely become useful products to everyday technologies.
A graphene varactor has been fabricated which operates passively for radiation dosimetry applications .
Microelectromechanical systems
Microelectromechanical systems is the technology of very small mechanical devices driven by electricity; it merges at the nano-scale into nanoelectromechanical systems and nanotechnology...
, or MEMS devices. NEMS typically integrate transistor-like nanoelectronics
Nanoelectronics
Nanoelectronics refer to the use of nanotechnology on electronic components, especially transistors. Although the term nanotechnology is generally defined as utilizing technology less than 100 nm in size, nanoelectronics often refer to transistor devices that are so small that inter-atomic...
with mechanical actuators, pumps, or motors, and may thereby form physical, biological, and chemical sensors. The name derives from typical device dimensions in the nanometer range, leading to low mass, high mechanical resonance frequencies, potentially large quantum mechanical effects such as zero point motion
Quantum harmonic oscillator
The quantum harmonic oscillator is the quantum-mechanical analog of the classical harmonic oscillator. Because an arbitrary potential can be approximated as a harmonic potential at the vicinity of a stable equilibrium point, it is one of the most important model systems in quantum mechanics...
, and a high surface-to-volume ratio useful for surface-based sensing mechanisms. Uses include accelerometer
Accelerometer
An accelerometer is a device that measures proper acceleration, also called the four-acceleration. This is not necessarily the same as the coordinate acceleration , but is rather the type of acceleration associated with the phenomenon of weight experienced by a test mass that resides in the frame...
s, or detectors of chemical substance
Chemical substance
In chemistry, a chemical substance is a form of matter that has constant chemical composition and characteristic properties. It cannot be separated into components by physical separation methods, i.e. without breaking chemical bonds. They can be solids, liquids or gases.Chemical substances are...
s in the air.
Overview
Because of the scale on which they can function, NEMS are expected to significantly impact many areas of technology and science and eventually replace MEMSMicroelectromechanical systems
Microelectromechanical systems is the technology of very small mechanical devices driven by electricity; it merges at the nano-scale into nanoelectromechanical systems and nanotechnology...
. As noted by Richard Feynman
Richard Feynman
Richard Phillips Feynman was an American physicist known for his work in the path integral formulation of quantum mechanics, the theory of quantum electrodynamics and the physics of the superfluidity of supercooled liquid helium, as well as in particle physics...
in his famous talk in 1959, There's Plenty of Room at the Bottom
There's Plenty of Room at the Bottom
There's Plenty of Room at the Bottom is the title of a lecture given by physicist Richard Feynman at an American Physical Society meeting at Caltech on December 29, 1959...
, there are a lot of potential applications of machines at smaller and smaller sizes; by building and controlling devices at smaller scales, all technology benefits. Among the expected benefits include greater efficiencies and reduced size, decreased power consumption and lower costs of production in electromechanical systems.
In 2000, the first very-large-scale integration
Very-large-scale integration
Very-large-scale integration is the process of creating integrated circuits by combining thousands of transistors into a single chip. VLSI began in the 1970s when complex semiconductor and communication technologies were being developed. The microprocessor is a VLSI device.The first semiconductor...
(VLSI) NEMS device was demonstrated by researchers from IBM. Its premise was an array of AFM tips which can heat/sense a deformable substrate in order to function as a memory device. In 2007, the International Technical Roadmap for Semiconductors (ITRS) contains NEMS Memory as a new entry for the Emerging Research Devices section.
Importance for AFM
A key application of NEMS is atomic force microscope tips. The increased sensitivity achieved by NEMS leads to smaller and more efficient sensors to detect stresses, vibrations, forces at the atomic level, and chemical signals. AFM tips and other detection at the nanoscale rely heavily on NEMS. If implementation of better scanning devices becomes available, all of nanoscience could benefit from AFM tips.Approaches to miniaturization
Two complementary approaches to fabrication of NEMS systems can be found. The top-down approach uses the traditional microfabrication methods, i.e. opticalPhotolithography
Photolithography is a process used in microfabrication to selectively remove parts of a thin film or the bulk of a substrate. It uses light to transfer a geometric pattern from a photomask to a light-sensitive chemical "photoresist", or simply "resist," on the substrate...
and electron beam lithography
Electron beam lithography
Electron beam lithography is the practice of emitting a beam of electrons in a patterned fashion across a surface covered with a film , and of selectively removing either exposed or non-exposed regions of the resist...
, to manufacture devices. While being limited by the resolution of these methods, it allows a large degree of control over the resulting structures. Typically, devices are fabricated from metallic thin films or etched semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
layers.
Bottom-up approaches, in contrast, use the chemical properties of single molecules to cause single-molecule components to (a) self-organize or self-assemble into some useful conformation, or (b) rely on positional assembly. These approaches utilize the concepts of molecular self-assembly
Self-assembly
Self-assembly is a term used to describe processes in which a disordered system of pre-existing components forms an organized structure or pattern as a consequence of specific, local interactions among the components themselves, without external direction...
and/or molecular recognition
Molecular recognition
The term molecular recognition refers to the specific interaction between two or more molecules through noncovalent bonding such as hydrogen bonding, metal coordination, hydrophobic forces, van der Waals forces, π-π interactions, electrostatic and/or electromagnetic effects...
.
This allows fabrication of much smaller structures, albeit often at the cost of limited control of the fabrication process.
A combination of these approaches may also be used, in which nanoscale molecules are integrated into a top-down framework. One such example is the carbon Nanotube nanomotor
Nanotube nanomotor
A device generating linear or rotational motion using carbon nanotube as the primary component, is termed a nanotube nanomotor. Nature already has some of the most efficient and powerful kinds of nanomotors. Some of these natural biological nanomotors have been re-engineered to serve desired purposes...
.
Carbon allotropes
Many of the commonly used materials for NEMS technology have been carbonCarbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
based, specifically carbon nanotube
Carbon nanotube
Carbon nanotubes are allotropes of carbon with a cylindrical nanostructure. Nanotubes have been constructed with length-to-diameter ratio of up to 132,000,000:1, significantly larger than for any other material...
s and graphene
Graphene
Graphene is an allotrope of carbon, whose structure is one-atom-thick planar sheets of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice. The term graphene was coined as a combination of graphite and the suffix -ene by Hanns-Peter Boehm, who described single-layer...
. This is mainly because of the useful properties of carbon based materials which directly meet the needs of NEMS. The mechanical properties of carbon (such as large Young's modulus
Young's modulus
Young's modulus is a measure of the stiffness of an elastic material and is a quantity used to characterize materials. It is defined as the ratio of the uniaxial stress over the uniaxial strain in the range of stress in which Hooke's Law holds. In solid mechanics, the slope of the stress-strain...
) are fundamental to the stability of NEMS while the metallic and semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
conductivities of carbon based materials allow them to function as transistor
Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...
s.
Both graphene and carbon exhibit high Young's modulus, excessively low density, low friction and large surface area. The low friction of CNTs, allow practically frictionless bearings and has thus been a huge motivation towards practical applications of CNTs as constitutive elements in NEMS, such as nanomotor
Nanomotor
A nanomotor is a molecular device capable of converting energy into movement. It can typically generate forces on the order of piconewtons.A proposed branch of research is the integration of molecular motor proteins found in living cells into molecular motors implanted in artificial devices...
s, switch
Switch
In electronics, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another....
es, and high-frequency oscillators Carbon nanotubes and graphene’s physical strength allows carbon based materials to meet higher stress demands, when common materials would normally fail and thus further support their use as a major materials in NEMS technological development.
Along with the mechanical benefits of carbon based materials, the electrical properties of carbon nanotubes and graphene allow it to be used in many electrical components of NEMS. Nanotransistors have been developed for both carbon nanotubes as well as graphene. Transistor
Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...
s are one of the basic building blocks for all electronic devices, so by effectively developing usable transistors, carbon nanotubes and graphene are both very crucial to NEMS.
Metallic Carbon Nanotubes
Metallic carbon nanotubes have also been proposed for nanoelectronic interconnects since they can carry high current densities. This is a very useful property as wires to transfer current are another basic building block of any electrical system. Carbon nanotubes have specifically found so much use in NEMS that methods have already been discovered to connect suspended carbon nanotubes to other nanostructures. This allows carbon nanotubes to be structurally set up to make complicated nanoelectric systems. Because carbon based products can be properly controlled and act as interconnects as well as transistors, they serve as a fundamental material in the electrical components of NEMS.
Difficulties
Despite all of the useful properties of carbon nanotubes and graphene for NEMS technology, both of these products face several hindrances to their implementation. One of the main problems is carbon’s response to real life environments. Carbon nanotubes exhibit a large change in electronic properties when exposed to oxygenOxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
. Similarly, other changes to the electronic and mechanical attributes of carbon based materials must fully be explored before their implementation, especially because of their high surface area which can easily react with surrounding environments. Carbon Nanotubes were also found to have varying conductivities, being either metallic or semiconducting depending on their helicity
Helicity
The term helicity has several meanings. In physics, all referring to a phenomenon that resembles a helix. See:*helicity , the extent to which corkscrew-like motion occurs...
when processed. Because of this, very special treatment must be given to the nanotubes during processing, in order to assure that all of the nanotubes have appropriate conductivities. Graphene also has very complicated electric conductivity properties compared to traditional semiconductors as it lacks an energy band gap
Band gap
In solid state physics, a band gap, also called an energy gap or bandgap, is an energy range in a solid where no electron states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference between the top of the valence band and the...
and essentially changes all the rules for how electrons move through a graphene based device. This means that traditional constructions of electronic devices will likely not work and completely new architectures must be designed for these new electronic devices.
Simulations
Computer simulations have long been important counterparts to experimental studies of NEMS devices. Through continuum mechanicsContinuum mechanics
Continuum mechanics is a branch of mechanics that deals with the analysis of the kinematics and the mechanical behavior of materials modelled as a continuous mass rather than as discrete particles...
and molecular dynamics
Molecular dynamics
Molecular dynamics is a computer simulation of physical movements of atoms and molecules. The atoms and molecules are allowed to interact for a period of time, giving a view of the motion of the atoms...
(MD), important behaviors of NEMS devices can be predicted via computational modeling before engaging in experiments. Additionally, combining continuum and MD techniques enables engineers to efficiently analyze the stability of NEMS devices without resorting to ultra-fine meshes and time-intensive simulations. Simulations have other advantages as well: they do not require the time and expertise associated with fabricating NEMS devices; they can effectively predict the interrelated roles of various electromechanical effects; and parametric studies can be conducted fairly readily as compared with experimental approaches. For example, computational studies have predicted the charge distributions and “pull-in” electromechanical responses of NEMS devices. Using simulations to predict mechanical and electrical behavior of these devices can help optimize NEMS device design parameters.
Future of NEMS
Before NEMS devices can actually be implemented, reasonable integrations of carbon based products must be created. The focus is currently shifting from experimental work towards practical applications and device structures that will implement and profit from the use of carbon nanotubes. At this point in NEMS research, there is a general understanding of the properties of carbon nanotubes and graphene. The next challenge to overcome involves understanding all of the properties of these carbon based tools, and using the properties to make efficient and durable NEMS with low failure rates.NEMS devices, if implemented into everyday technologies, could further reduce the size of modern devices and allow for better performing sensors. Carbon based materials have served as prime materials for NEMS use, because of their highlighted mechanical and electrical properties. Once NEMS interactions with outside environments are integrated with effective designs, they will likely become useful products to everyday technologies.
A graphene varactor has been fabricated which operates passively for radiation dosimetry applications .