nanoscience and technology---class lectures (part iv) [compatibility mode]

8/8/2019 Nanoscience and Technology---Class Lectures (Part IV) [Compatibility Mode]

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Part-IV


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Emerging interfaces of nanotechnology

Nanoelectronics Nanophotonics Nanosensors Molecular nanomachines Nanotechnology for energy harvesting Nanobiology Nanomedicines Nanotribology, etc.


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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 interactions andquantum mechanical properties need to be studied extensively. As a result, present transistors (such as in

recent Intel Core i7 processors) do not fall under this category, even though these devices are

manufactured under 65 nm or 45 nm technology.

Nanoelectronics are sometimes considered as disruptive technology because present candidates are

significantly different from traditional transistors. Some of these candidates include: hybrid

molecular/semiconductor electronics, one dimensional nanotubes/nanowires or advanced molecular

electronics. The sub-voltage and deep-sub-voltage nanoelectronics are specific and important fields of

R&D, and the appearance of new ICs operating almost near theoretical limit (fundamental, technological,design methodological, architectural, algorithmic) on energy consumption per 1 bit processing is

inevitable. The important case of fundamental ultimate limit for logic operation is reversible computing.

Fundamental concepts : The volume of an object decreases as the third power of its linear dimensions, but

the surface area only decreases as its second power. This somewhat subtle and unavoidable principle has

huge ramifications. For example the power of a drill (or any other machine) is proportional to the volume,

while the friction of the drill's bearings and gears is proportional to their surface area. For a normal-sized

drill, the power of the device is enough to handily overcome any friction. However, scaling its length down

by a factor of 1000, for example, decreases its power by 10003 (a factor of a billion) while reducing thefriction by only 10002 (a factor of "only" a million). Proportionally it has 1000 times less power per unit

friction than the original drill. If the original friction-to-power ratio was, say, 1%, that implies the smaller

drill will have 10 times as much friction as power. The drill is useless.


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For this reason, while super-miniature electronic integrated circuits are fully functional, the same

technology cannot be used to make working mechanical devices beyond the scales where frictional forces

start to exceed the available power. So even though you may see microphotographs of delicately etched

silicon gears, such devices are currently little more than curiosities with limited real world applications,

for example, in moving mirrors and shutters. Surface tension increases in much the same way, thus

magnifying the tendency for very small objects to stick together. This could possibly make any kind of

"micro factory" impractical: even if robotic arms and hands could be scaled down, anything they pick up

will tend to be impossible to put down. The above being said, molecular evolution has resulted in working

cilia, flagella, muscle fibers and rotary motors in aqueous environments, all on the nanoscale. These

machines exploit the increased frictional forces found at the micro or nanoscale. Unlike a paddle or a

propeller which depends on normal frictional forces (the frictional forces perpendicular to the surface) to

achieve propulsion, cilia develop motion from the exaggerated drag or laminar forces (frictional forces

parallel to the surface) present at micro and nano dimensions. To build meaningful "machines" at the

nanoscale, the relevant forces need to be considered. We are faced with the development and design ofintrinsically pertinent machines rather than the simple reproductions of macroscopic ones.


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Approaches to nanoelectronics

Nanofabrication : For example, single electron transistors, which involve transistor operation based

on a single electron. Nanoelectromechanical systems also falls under this category. Nanofabrication

can be used to construct ultra-dense parallel arrays of nanowires, as an alternative to synthesizingnanowires individually.

Nanomaterials electronics : Besides being small and allowing more transistors to be packed into a

single chip, the uniform and symmetrical structure of nanotubes allows a higher electron mobility

(faster electron movement in the material), a higher dielectric constant (faster frequency), and a

symmetrical electron/hole characteristic. Also, nanoparticles can be used as quantum dots.

Molecular electronics : Single molecule devices are another possibility. These schemes would make

heavy use of molecular self-assembly, designing the device components to construct a largerstructure or even a complete system on their own. This can be very useful for reconfigurable

computing, and may even completely replace present FPGA technology.

Molecular electronics is a new technology which is still in its infancy, but also brings hope for truly

atomic scale electronic systems in the future. One of the more promising applications of molecular

electronics was proposed by the IBM researcher Ari Aviram and the theoretical chemist Mark

Ratner in their 1974 and 1988 papers Molecules for Memory, Logic and Amplification. This is one of

many possible ways in which a molecular level diode / transistor might be synthesized by organic

chemistry. A model system was proposed with a spiro carbon structure giving a molecular diode

about half a nanometre across which could be connected by polythiophene molecular wires.

Theoretical calculations showed the design to be sound in principle and there is still hope that such

a system can be made to work.


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Carbon nanotube field effect transistors (CNFETs)

Silicon-based CMOS is the dominant technology choice for high-performance digital circuits. While silicon technology

continues to scale, researchers are investigating other novel materials, structures, and devices to introduce into future

technology generations, if necessary, to extend Moore's law. Carbon nanotubes (CNTs) have been explored as a possibility

due to their excellent carrier mobility. Studies on different carbon-nanotube-based field-effect transistors (CNFETs) have

been reported including Schottky-barrier (SB) CNFETs, MOS CNFETs, and state-of-the-art Si MOSFETs systematicallyfrom a circuit/system design perspective. Parasitics play a major role in the performance of CNT-based circuits. In

addition, CNFET design's performance is limited by t he gate overlap capacitance and the quality of nanocontacts to these

promising transistors. Transient analysis of high-performing single-tube SB CNFET transistors and circuits revealed that

1-1.5 nm is the optimum CNT diameter resulting in best power-performance tradeoff for high-speed digital applications.

Presently, a considerable amount of research is focused on optimal spacing and layout of CNT arrays, an architecture that

is most likely required for driving capacitive loads and interconnects in digital applications. CNTs have an intrinsic

capability to improve performance, but many serious technological and experimental challenges remain that require more

research to harvest their potential.


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Carbon Nanotube Field Effect Transistors (CNFETs), consisting of semiconducting single walled Carbon

Nanotubes (CNTs), have several promising applications such as extensions to silicon VLSI and large area

electronics. Ideal CNFETs can provide significant energy and performance benefits over silicon-CMOS, and

CNFET processing is compatible with existing silicon-CMOS processing. While there has been significant

progress at a single-device level, a major gap exists between such results and their transformation into VLSICNFET technologies. Future gigascale systems cannot rely solely on existing chemical synthesis for guaranteed

ideal devices. VLSI-scale logic circuits using CNFETs must overcome major challenges posed by :

1) Mis-positioned CNTs 2) Metallic CNTs and 3) Wafer-scale Integration.

A suitable combination of design and processing techniques, can enable VLSI-scale CNFET logic circuits that

are immune to high rates of inherent imperfections. These techniques must be inexpensive compared to traditional

defect- and fault tolerance, do not impose major changes in VLSI design flows, and are compatible with VLSI

processing because they do not require special customization on chip-by-chip basis.

Mis-positioned CNTs can result in incorrect logic functionality of CNFET circuits. New layout design techniquesproduce CNFET circuits for arbitrary logic functions that are immune to a large number of mis-positioned CNTs.

These techniques are significantly more efficient compared to traditional defect- and fault-tolerance techniques.

Furthermore, the techniques are VLSI-compatible and do not require changes to existing VLSI design and

manufacturing flows.

A CNT can be semiconducting or metallic depending upon the arrangement of carbon atoms. Typical CNT

synthesis techniques yield 10-50% metallic CNTs. Metallic CNTs create source-drain shorts in CNFETs resulting

in excessive leakage (Ion/Ioff< 10, i.e., orders of magnitude lower than silicon CMOS) and highly-degraded noise

margins. New techniques overcome challenges posed by metallic CNTs by combining layout design with CNFET

processing.


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Magnetic tunnel junctions (MTJs) consist of thin insulating barrier layers sandwiched

between ferromagnetic layers. The large tunnel magnetoresistance (TMR) found in thesestructures has led to an intensive investigation of their properties over the last ten years.Most recently, there have been some exciting developments with the realization of a 2-3fold increase in the values of TMR compared to those previously obtained, hence makingthese structures even more promising for applications in areas such as spintronics andhigh density magnetic recording. Currently there are several theoretical modelsproposing alternative suggestions for the mechanisms underlying the spin-polarizedtunneling. Whilst models for MTJs consisting of crystalline MgO barriers consider the

symmetry of the wave functions to be the key factor, alternative models for Co/aluminabased MTJs suggest that chemical interfacial bonding plays a dominant role. These lattermodels also predict a dependence of the interfacial magnetism on the chemical bondingbetween ferromagnet and barrier. Thus measurements of the interfacial magnetism canhelp to assess the validity of these models.

Magnetic tunnel junctions are manufactured by thin film technology. On an industrialscale the film deposition is done by magnetron sputter deposition, on a laboratory scale

molecular beam epitaxy, pulsed laser deposition and electron beam physical vapordeposition are also utilized. The junctions are prepared by photolithography.

Magnetic tunnel junctions (MTJs)


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If no voltage is applied to the junction, electrons tunnel in both directions with equal rates. With a bias

voltage U, electrons tunnel preferentially to the positive electrode. With the assumption that spin is

conserved during tunneling, the current can be described in a two-current model. The total current issplit in two partial currents, one for the spin-up electrons and another for the spin-down electrons.

These vary depending on the magnetic state of the junctions.

There are two possibilities to obtain a defined anti-parallel state. First, one can use ferromagnets with

different coercivities (by using different materials or different film thicknesses). And second, one of

the ferromagnets can be coupled with an antiferromagnet (exchange bias). In this case the

magnetization of the uncoupled electrode remains "free".

The TMR decreases with both increasing temperature and increasing bias voltage. Both can be

understood in principle by magnon excitations and interactions with magnons.It is obvious that the TMR becomes infinite if P1 and P2 equal 1, i.e. if both electrodes have 100% spin

polarization. In this case the magnetic tunnel junction becomes a switch, that switches magnetically

between low resistance and infinite resistance. Materials that come into consideration for this are

called ferromagnetic half-metals. Their conduction electrons are fully spin polarized. This property is

theoretically predicted for a number of materials (e.g. CrO2, various Heusler alloys) but could not be

experimentally confirmed to date.

TMR = 2P1P2 /(1- P1P2)


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Antiparallelmagnetizations

Parallelmagnetizations

Ferromagnet

Ferromagnet

Insulator

Resistance: R

R

RRTMR=

Tunnelingcurrent

Resistance: R

A magnetic tunnel junction (MTJ) consists of two layers of magnetic metal, such as cobalt-iron, separated byan ultrathin layer of insulator, typically aluminum oxide with a thickness of about 1 nm. The insulating layer is

so thin that electrons can tunnel through the barrier if a bias voltage is applied between the two metal

electrodes. In MTJs the tunneling current depends on the relative orientation of magnetizations of the twoferromagnetic layers, which can be changed by an applied magnetic field. This phenomenon is called

tunneling magnetoresistance (TMR).

Nowadays, MTJs that are based on transition-metal ferromagnets and Al2O3 barriers can be fabricated withreproducible characteristics and with TMR values up to 50% at room temperature. Recently large values of

TMR observed in crystalline MTJs with MgO barriers further boosted interest in spin dependent tunneling.

MTJs are promising for applications in magnetic storage and sensor industry.

The read-heads of modern hard disk drives work on the basis of magnetic tunnel junctions. TMR, or more

specifically the MTJ, is also the basis of MRAM, a new type of non-volatile memory. The 1st generation

technologies relied on creating cross-point magnetic fields on each bit to write the data on it, although thisapproach has a scaling limit at around 90-130nm. There are two 2nd generation techniques currently being

developed: Thermal Assisted Switching (TAS) which is being developed by Crocus Technology, and SpinTorque Transfer (STT) on which Crocus, Hynix, IBM, and several other companies are working.


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Magnetic random access memories (MRAM)Magnetoresistive Random Access Memory is a non-volatile computer memory (NVRAM) technology,

which has been under development since the 1990s. Continued increases in density of existing

memory technologies notably Flash RAM and DRAM kept it in a niche role in the market, but its

proponents believe that the advantages are so overwhelming that Magnetoresistive RAM willeventually become dominant for all types of memory, becoming a true universal memory.

Simplified structure of a MRAM cell


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Unlike conventional RAM chip technologies, in MRAM data is not stored as electriccharge or current flows, but by magnetic storage elements. The elements are formedfrom two ferromagnetic plates, each of which can hold a magnetic field, separated by athin insulating layer. One of the two plates is a permanent magnet set to a particularpolarity, the other's field will change to match that of an external field. A memory device

is built from a grid of such "cells".

Reading is accomplished by measuring the electrical resistance of the cell. A particularcell is (typically) selected by powering an associated transistor which switches currentfrom a supply line through the cell to ground. Due to the magnetic tunnel effect, theelectrical resistance of the cell changes due to the orientation of the fields in the twoplates. By measuring the resulting current, the resistance inside any particular cell canbe determined, and from this the polarity of the writable plate. Typically if the two plates

have the same polarity this is considered to mean "0", while if the two plates are ofopposite polarity the resistance will be higher and this means "1".

Data is written to the cells using a variety of means. In the simplest, each cell liesbetween a pair of write lines arranged at right angles to each other, above and below thecell. When current is passed through them, an induced magnetic field is created at thejunction, which the writable plate picks up. This pattern of operation is similar to corememory. This approach requires a fairly substantial current to generate the field,however, which makes it less interesting for low-power uses, one of MRAM's primary

disadvantages. Additionally, as the device is scaled down in size, there comes a timewhen the induced field overlaps adjacent cells over a small area, leading to potentialfalse writes. This problem, the half-select (or write disturb) problem, appears to set afairly large size for this type of cell. One experimental solution to this problem was to usecircular domains written and read using the giant magnetoresistive effect.


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Another approach, the toggle mode, uses a multi-step write with a modified multi-layercell. The cell is modified to contain an "artificial antiferromagnet" where the magneticorientation alternates back and forth across the surface, with both the pinned and freelayers consisting of multi-layer stacks isolated by a thin "coupling layer". The resultinglayers have only two stable states, which can be toggled from one to the other by timingthe write current in the two lines so one is slightly delayed, thereby "rotating" the field.Any voltage less than the full write level actually increases its resistance to flipping. Thatmeans that other cells located along one of the write lines will not suffer from the half-select problem, allowing for smaller cell sizes.


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MRAM array and cell cross section (NEC Corporation)

A newer technique, spin-torque-transfer (STT) or Spin Transfer Switching, uses spin-aligned("polarized") electrons to directly torque the domains. Specifically, if the electrons flowing intoa layer have to change their spin, this will develop a torque that will be transferred to thenearby layer. This lowers the amount of current needed to write the cells, making it about thesame as the read process. There are concerns that the "classic" type of MRAM cell will have

difficulty at high densities due to the amount of current needed during writes, a problem STTavoids. For this reason, the STT proponents expect the technique to be used for devices of 65nm and smaller. The downside is the need to maintain the spin coherence. Overall, the STTrequires much less write current than conventional or toggle MRAM. However, higher speedoperation still requires higher current.

Other potential arrangements include Thermal Assisted Switching" (TAS-MRAM) whichbriefly heats up (reminiscent of phase-change memory) the magnetic tunnel junctions duringthe write process and keeps the MTJ's stable at a colder temperature the rest of the time; and

"vertical transport MRAM" (VMRAM), which uses current through a vertical column to changemagnetic orientation, a geometric arrangement that reduces the write disturb problem and socan be used at higher density.


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Spin batteries

The secret behind this technology is the use of nano-magnets to induce an electromotive force. It uses the

same principles as those in a conventional battery, except in a more direct fashion. The energy stored in abattery, be it in an iPod or an electric car, is in the form of chemical energy. When something is turned "on"

there is a chemical reaction which occurs and produces an electric current. The new technology converts themagnetic energy directly into electrical energy, without a chemical reaction. The electrical current made in

this process is called a spin polarized current.

The new discovery advances our understanding of the way magnets work and its immediate application is to

use the MTJs as electronic elements which work in different ways to conventional transistors. Although theactual device has a diameter about that of a human hair and cannot even light up a LED, the energy that

might be stored in this way could potentially run a car for miles.

The electromotive force predicted by Faraday's law reflects the forces acting on the charge, e, of an electronmoving through a device or circuit, and is proportional to the time derivative of the magnetic field. This

conventional e.m.f. is usually absent for stationary circuits and static magnetic fields. There are also forcesthat act on the spin of an electron; it has been recently predicted that, for circuits that are in part composedof ferromagnetic materials, there arises an e.m.f. of spin origin even for a static magnetic field. This e.m.f. can

be attributed to a time-varying magnetization of the host material, such as the motion of magnetic domains ina static magnetic field, and reflects the conversion of magnetic to electrical energy. It has been shown that

such an e.m.f. can indeed be induced by a static magnetic field in magnetic tunnel junctions containing zinc-blende-structured MnAs quantum nanomagnets. The observed e.m.f. operates on a timescale of

approximately 102103 seconds and results from the conversion of the magnetic energy of thesuperparamagnetic MnAs nanomagnets into electrical energy when these magnets undergo magnetic

quantum tunneling. As a consequence, a huge magnetoresistance of up to 100,000 per cent is observed forcertain bias voltages. The results strongly support the contention that, in magnetic nanostructures, Faraday'slaw of induction must be generalized to account for forces of purely spin origin. The huge magnetoresistance

and e.m.f. may find potential applications in high sensitivity magnetic sensors, as well as in new activedevices such as 'spin batteries'.


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Phase change random access memories (PRAM)Phase-change memory (also known as PCM, PRAM, PCRAM, Ovonic Unified Memory, Chalcogenide

RAM and C-RAM) is a type of non-volatile computer memory. PRAM uses the unique behavior of

chalcogenide glass, which can be "switched" between two states, crystalline and amorphous, with the

application of heat. Recent versions can achieve two additional distinct states, effectively doubling its

storage capacity. PRAM is one of a number of new memory technologies that are attempting to

compete in the non-volatile role with the almost universal Flash memory, which has a number of

practical problems these replacements hope to address.

A cross-section of two PRAM memory cells. One cell is in low resistance crystalline state, the other in high resistanceamorphous state.

GeSbTe, AgInSbTe.


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The crystalline and amorphous states of chalcogenide glass have dramatically different electrical

resistivity, and this forms the basis by which data are stored. The amorphous, high resistance state is

used to represent a binary 0, and the crystalline, low resistance state represents a 1. Chalcogenide is

the same material used in re-writable optical media (such as CD-RW and DVD-RW). In those instances,

the material's optical properties are manipulated, rather than its electrical resistivity, as

chalcogenide's refractive index also changes with the state of the material.

Although PRAM has not yet reached the commercialization stage for consumer electronic devices,nearly all prototype devices make use of a chalcogenide alloy of germanium, antimony and tellurium

(GeSbTe) called GST. The stoichiometry or Ge:Sb:Te element ratio is 2:2:5. When GST is heated to a

high temperature (over 600C), its chalcogenide crystallinity is lost. Once cooled, it is frozen into an

amorphous glass-like state and its electrical resistance is high. By heating the chalcogenide to a

temperature above its crystallization point, but below the melting point, it will transform into a

crystalline state with a much lower resistance. The time to complete this phase transition is

temperature-dependent. Cooler portions of the chalcogenide take longer to crystallize, and overheated

portions may be remelted. Commonly, a crystallization time scale on the order of 100 ns is used. This

is longer than conventional volatile memory devices like modern DRAM, which have a switching time

on the order of two nanoseconds. However, a January 2006 Samsung Electronics patent application

indicates PRAM may achieve switching times as fast as five nanoseconds.

A more recent advance pioneered by Intel and ST Microelectronics allows the material state to be

more carefully controlled, allowing it to be transformed into one of four distinct states; the previous

amorphic or crystalline states, along with two new partially crystalline ones. Each of these states has

different electrical properties that can be measured during reads, allowing a single cell to represent

two bits, doubling memory density.


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Millipede drive

Millipede is a non-volatile computer memory stored on nanoscopic pits burned into the surface of a

thin polymer layer, read and written by a MEMS-based probe. It promises a data density of more than 1

terabit per square inch (1 gigabit per square millimeter), about 4 times the density of magnetic storage

available today.Millipede storage technology is being pursued as a potential replacement for magnetic recording in

hard drives, at the same time reducing the form-factor to that of Flash media. IBM demonstrated a

prototype millipede storage device at CeBIT 2005, and was trying to make the technology

commercially available by the end of 2010. At launch, it would probably be more expensive per-

megabyte than prevailing technologies, but this disadvantage is hoped to be offset by the sheer

storage capacity that Millipede technology would offer.


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The main memory of modern computers is constructed from one of a number of DRAM-related devices.DRAM basically consists of a series of capacitors, which store data as the presence or absence of electrical

charge. Each capacitor and its associated control circuitry, referred to as a cell, holds one bit, and bits can be

read or written in large blocks at the same time.In contrast, hard drives store data on a metal disk that is covered with a magnetic material; data is

represented as local magnetisation of this material. Reading and writing are accomplished by a single "head",which waits for the requested memory location to pass under the head while the disk spins. As a result, the

drive's performance is limited by the mechanical speed of the motor, and is generally hundreds of thousandsof times slower than DRAM. However, since the "cells" in a hard drive are much smaller, the storage density

is much higher than DRAM.Millipede storage attempts to combine the best features of both. Like the hard drive, millipede stores data in a

"dumb" medium that is simpler and smaller than any cell used in an electronic medium. It accesses the data

by moving the medium under the "head" as well. However, millipede uses many nanoscopic heads that canread and write in parallel, thereby dramatically increasing the throughput to the point where it can compete

with some forms of electronic memory. Additionally, millipede's physical medium stores a bit in a very smallarea, leading to densities even higher than current hard drives.

Mechanically, millipede uses numerous atomic force probes, each of which is responsible for reading andwriting a large number of bits associated with it. Bits are stored as a pit, or the absence of one, in the surface

of a thermo-active polymer deposited as a thin film on a carrier known as the sled.

Any one probe can only read or write a fairly small area of the sled available to it, a storage field. Normally thesled is moved to position the selected bits under the probe using electromechanical actuators similar to

those that position the read/write head in a typical hard drive, although the actual distance moved is tiny. Thesled is moved in a scanning pattern to bring the requested bits under the probe, a process known as x/y

scan.The amount of memory serviced by any one field/probe pair is fairly small, but so is its physical size. Many

such field/probe pairs are used to make up a memory device. Data reads and writes can be spread acrossmany fields in parallel, increasing the throughput and improving the access times. For instance, a single 32-

bit value would normally be written as a set of single bits sent to 32 different fields. In the initial experimental

devices, the probes were mounted in a 32x32 grid for a total of 1,024 probes. Their layout looked like the legson a millipede, and the name stuck.

The design of the cantilever array is the trickiest part, as it involves making numerous mechanicalcantilevers, on which a probe has to be mounted. All the cantilevers are made entirely out of silicon, using

surface micromachining at the wafer surface.


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Holographic data storage system (HDSS)Holographic storage uses two laser beams, a reference and a data beam to create an interference

pattern at a medium where the two beams intersect. This intersection causes a stable physical or

chemical change which is stored in the medium. This is the write sequence. During reading, the action

of the reference beam and the stored interference pattern in the medium recreates this data beam

which may be sensed by a detector array. The medium may be a rotating disk containing a polymeric

material, or an optically sensitive single crystal. The key to making the holographic data storage

system work is the second laser beam which is fired at the crystal to retrieve a page of data. It must

match the original reference beam angle exactly. A difference of just a thousandth of a millimeter will

result in failure to retrieve the data. Holography is expected to be of value in archival or library storage

applications where large quantities of data are required to be retained at the very lowest costs

possible.


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Since it involves no moving parts, holographic data storage will be far more reliable than existing hard

disk technologies. IBM has already demonstrated the possibility of holding 1 TB of data in a crystal the

size of a sugar cube and of data access rates of one trillion bits per second. The major challenge ahead

is expected to be the development of a rewritable form of holographic storage.

Laser,

Beam splitter : object, reference beamsSpatial light modulator,

Scanner assembly : A mechanical scanner that

changes the angle of the reference beam. Changing

this angle allows the slices of information to be

layered on the cube.Cube : A light sensitive crystal or a photopolymer.

Hologram reader : CCD camera


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NanophotonicsIt is considered as a branch of optical engineering which deals with optics, or the interaction of light with

particles or substances, at deeply sub wavelength length scales. Technologies in the realm of nano-

optics include near-field scanning optical microscopy (NSOM), photoassisted scanning tunneling

microscopy, and surface plasmon optics.

Traditional microscopy makes use of diffractive elements to focus light tightly in order to increase

resolution. But because of the diffraction limit (Rayleigh Criterion), propagating light may be focused to

a spot with a minimum diameter of roughly half the wavelength of the light. Thus, even with diffraction-

limited confocal microscopy, the maximum resolution obtainable is on the order of a couple of hundred

nanometers. The scientific and industrial communities are becoming more interested in the

characterization of materials and phenomena on the scale of a few nanometers, so alternative

techniques must be utilized.

The study of nanophotonics involves two broad themes (i) studying the novel properties of light at the

nanometer scale (ii) enabling highly power efficient devices for engineering applications.

The study has the potential to revolutionize the telecommunications industry by providing low power,

high speed, interference-free devices such as electrooptic and all-optical switches on a chip.

Basics : The term nanophotonics typically refers to phenomena of ultraviolet, visible and near IR light,

with a wavelength of approximately 300 nm to 1.2 m.

The interaction of light with these nanoscale features leads to confinement of the electromagnetic field

to the surface or tip of the nanostructure resulting in a region referred to as the optical near field. This

effect is to some extent analogous to a lightning rod, where the field concentrates at the tip. In this

region, the field may need to adjust to the topography of the nanostructure (boundary conditions of

Maxwell's equations). This means that the electromagnetic field will be dependent on the size and shape

of the nanostructure that the light is interacting with.


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This optical near field can also be described as a surface bound optical oscillation which can

vary on length scale of tens or hundreds of nanometers - a length scale smaller than the

wavelength of the incoming light. This can provide higher spatial resolution beyond the

limitations imposed by the law of diffraction in conventional far-field microscopy. The

technique derived from this effect is known as near-field microscopy, and opens up many new

possibilities for imaging and spectroscopy on the nanoscale. A novel embodiment which haspicometer resolution in the vertical plane above the waveguide surface is dual polarisation

interferometry.

Novel optical properties of materials can result from their extremely small size. A typical

example of this type of effect is the color change associated with colloidal gold. In contrast to

bulk gold, known for its yellow color, gold particles of 10 to 100 nm in size exhibit a rich red

color. The critical size where these and related effects take place are correlated with the mean

free path of the conduction electrons of the metal.

In addition to these extrinsic size effects that determine a material's optical response toincoming light, the intrinsic properties of the material can change. These size effects occur as

particles become even smaller. At this stage some of the intrinsic electronic properties of the

medium itself change. One example of this phenomenon is in semiconductor nanostructures

where the extremely small particle size confines the quantum mechanical wavefunction,

leading to discrete optical transitions, e.g., fluorescence colors that depend on the size of the

particle. The changing bandgap of the semiconductor is the reason for this color change. This

effect, however, since not directly correlated with optical wavelength, is not unanimously

included when referring to nano-optics.


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Nanosensors

Nanosensors are any biological, chemical, or electronic sensory points used to convey information about

nanoparticles to the macroscopic world. Their use may include various medicinal purposes as well as gatewaysto building other nanoproducts, such as computer chips and nanorobots. Presently, there are several ways

proposed to make nanosensors, including top-down and bottom-up design, top-down lithography, bottom-up

assembly, and molecular self-assembly.

Medicinal uses of nanosensors mainly revolve around the potential of nanosensors to accurately identifyparticular cells or places in the body in need. By measuring changes in volume, concentration, displacement

and velocity, gravitational, electrical, and magnetic forces, pressure, or temperature of cells in a body,nanosensors may be able to distinguish between and recognize certain cells, most notably those of cancer, at

the molecular level in order to deliver medicine or monitor development to specific places in the body. Inaddition, they may be able to detect macroscopic variations from outside the body and communicate these

changes to other nanoproducts working within the body.

One example of nanosensors involves using the fluorescence properties of CdSe quantum dots as sensors to

uncover tumors within the body. By injecting a body with these quantum dots, a doctor could see where a tumoror cancer cell was by finding the injected quantum dots, an easy process because of their fluorescence.

Developed nanosensor quantum dots would be specifically constructed to find only the particular cell for whichthe body was at risk. A downside to the CdSe dots, however, is that they are highly toxic to the body. As a result,

researchers are working on developing alternate dots made out of a different, less toxic material while stillretaining some of the fluorescence properties. In particular, they have been investigating the particular benefits

of ZnS quantum dots which, though they are not quite as fluorescent as CdSe, can be augmented with other

metals including manganese and various lanthanide elements. In addition, these newer quantum dots becomemore fluorescent when they bond to their target cells. (Quantum) Potential predicted functions may also include

sensors used to detect specific DNA in order to recognize explicit genetic defects, especially for individuals athigh-risk and implanted sensors that can automatically detect glucose levels for diabetic subjects more simply

than current detectors. DNA can also serve as sacrificial layer for manufacturing CMOS IC, integrating ananodevice with sensing capabilities. Therefore, using proteomic patterns and new hybrid materials,

nanobiosensors can also be used to enable components configured into a hybrid semiconductor substrate as

part of the circuit assembly. The development and miniaturization of nanobiosensors should provide interestingnew opportunities.


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A nanosensor probe carrying a laser beam (blue) penetrates a living cell to detect the presence of a product indicatingthat the cell has been exposed to a cancer-causing substance.

Self-Assembled DNA Nanostructures. (A) DNA tile structure consisting of four branched junctions oriented at 90

intervals. These tiles serve as the primary building block for the assembly of the DNA nanogrids shown in (B). Eachtile consists of nine DNA oligonucleotides as shown. (B) An atomic force microscope image of a self-assembled DNAnanogrid. Individual DNA tiles self-assemble into a highly ordered periodic two-dimensional DNA nanogrid.


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One of the first working examples of a synthetic nanosensor was built by researchers at the Georgia

Institute of Technology in 1999. It involved attaching a single particle onto the end of a carbon

nanotube and measuring the vibrational frequency of the nanotube both with and without the particle.

The discrepancy between the two frequencies allowed the researchers to measure the mass of the

attached particle.

Chemical sensors, too, have been built using nanotubes to detect various properties of gaseous

molecules. Carbon nanotubes have been used to sense ionization of gaseous molecules whilenanotubes made out of titanium have been employed to detect atmospheric concentrations of

hydrogen at the molecular level. Many of these involve a system by which nanosensors are built to

have a specific pocket for another molecule. When that particular molecule, and only that specific

molecule, fits into the nanosensor, and light is shone upon the nanosensor, it will reflect different

wavelengths of light and, thus, be a different color.

There are currently several hypothesized ways to produce nanosensors. Top-down lithography is the

manner in which most integrated circuits are now made. It involves starting out with a larger block of

some material and carving out the desired form. These carved out devices, notably put to use inspecific MEMS used as microsensors, generally only reach the micro size, but the most recent of these

have begun to incorporate nanosized components.

Another way to produce nanosensors is through the bottom-up method, which involves assembling the

sensors out of even more minuscule components, most likely individual atoms or molecules. This

would involve moving atoms of a particular substance one by one into particular positions which,

though it has been achieved in laboratory tests using tools such as AFMs, is still a significant difficulty,

especially to do en masse, both for logistic reasons as well as economic ones. Most likely, this process

would be used mainly for building starter molecules for self-assembling sensors.


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Molecular nanomachines

The concept of molecular nanomachines has become a reality in the past few years in organic and

supramolecular chemistry, in biochemistry and in atom-scale manipulation with the scanning tunneling

microscope (STM).

In chemistry, molecules can be designed and synthesized to have specific electrical, mechanical,optical or reactive properties. In biochemistry, single natural biomolecules can be isolated and

activated as nanomachines. In atom-scale manipulation, the STM can be used to power and to control

the operation of individual molecules as molecular nanomachines.

Azobenzene is an incredible small organic molecule that has the potential to perform as a

photoswitchable nanoscale piston on a surface. Azobenzene exists in two different shapes (isomers,

labeled trans and cis) and can be reversibly and reliably driven between these two shapes by shining

UV and blue light on it (photoisomerization). The lengthening and contracting of azobenzene as it

changes shape allows it to do mechanical work.


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Nano solar cells

Conventional solar cells have two main drawbacks: they can only achieve efficiencies around tenpercent and they are expensive to manufacture. The first drawback, inefficiency, is almost unavoidable

with silicon cells. This is because the incoming photons, or light, must have the right energy, called the

band gap energy, to knock out an electron. If the photon has less energy than the band gap energy then

it will pass through. If it has more energy than the band gap, then that extra energy will be wasted as

heat. These two effects alone account for the loss of around 70 percent of the radiation energy incident

on the cell. Consequently, according to the Lawrence Berkeley National Laboratory, the maximum

efficiency achieved today is only around 25 percent . Mass-produced solar cells are much less efficient

than this, and usually achieve only ten percent efficiency.

Nanotechnology might be able to increase the efficiency of solar cells, but the most promisingapplication of nanotechnology is the reduction of manufacturing cost. Chemists at the University of

California, Berkeley, have discovered a way to make cheap plastic solar cells that could be painted on

almost any surface. These new plastic solar cells achieve efficiencies of only 1.7 percent. These new

plastic solar cells utilize tiny nanorods dispersed within in a polymer. The nanorods behave as wires

because when they absorb light of a specific wavelength they generate electrons. These electrons flow


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through the nanorods until they reach the aluminum electrode where they are combined to form a

current and are used as electricity. This type of cell is cheaper to manufacture than conventional

ones for two main reasons. First, these plastic cells are not made from silicon, which can be very

expensive. Second, manufacturing of these cells does not require expensive equipment such as

clean rooms or vacuum chambers like conventional silicon based solar cells. Instead, these plastic

cells can be manufactured in a beaker.Another potential feature of these solar cells is that the nanorods could be tuned to absorb

various wavelengths of light. This could significantly increase the efficiency of the solar cell

because more of the incident light could be utilized.


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Nano fuel cellsA fuel cell produces electricity using fuels. Normally, the fuels are mainly H2 and O2.Chemicals such as methane and methanol also can be used for fuel.

The two electrodes are separated by an electrolyte, a material that allows chargedmolecules or ions to move through it.


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Motorola

NEC

Toshiba


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Catalysts are used with fuels such as hydrogen or methanol to produce hydrogen ions.Platinum, which is very expensive, is the catalyst typically used in this process.Companies are using nanoparticles of platinum to reduce the amount of platinumneeded, or using nanoparticles of other materials to replace platinum entirely andthereby lower costs.

Fuel cells contain membranes that allow hydrogen ions to pass through the cell but donot allow other atoms or ions, such as oxygen, to pass through. Companies are usingnanotechnology to create more efficient membranes; this will allow them to build lighterweight and longer lasting fuel cells.

Small fuel cells are being developed that can be used to replace batteries in handhelddevices such as PDAs or laptop computers. Most companies working on this type offuel cell are using methanol as a fuel and are calling them DMFC's, which stands for

direct methanol fuel cell. DMFC's are designed to last longer than conventionalbatteries. In addition, rather than plugging your device into an electrical outlet andwaiting for the battery to recharge, with a DMFC you simply insert a new cartridge ofmethanol into the device and you're ready to go.

Fuel cells that can replace batteries in electric cars are also under development.Hydrogen is the fuel most researchers propose for use in fuel cell powered cars. Inaddition to the improvements to catalysts and membranes discussed above, it isnecessary to develop a lightweight and safe hydrogen fuel tank to hold the fuel andbuild a network of refueling stations. To build these tanks, researchers are trying todevelop lightweight nanomaterials that will absorb the hydrogen and only release itwhen needed.


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A scanning electron microscopy image of platinum nanostructures synthesized via asimple wet chemical method, at room temperature, using neither template norsurfactant. The nanostructures consisted of numerous single-crystal Pt nanowireswith diameters of 4 nm and lengths that may reach hundred nanometers. Right isdetail view of left image.


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Nano medicine

Nanomedicine is the medical application of nanotechnology. The approaches to nanomedicine range

from the medical use of nanomaterials, to nanoelectronic biosensors, and even possible future

applications of molecular nanotechnology. Current problems for nanomedicine involve understanding

the issues related to toxicity and environmental impact of nanoscale materials.

Medical use of nanomaterials :

Drug delivery

Protein and peptide delivery Cancer treatment Visualization/Imaging Surgery Cell repair machines


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Nano biotechnology

Nanobiotechnology is the branch of nanotechnology with biological and biochemical applications or

uses. Nanobiotechnology often studies existing elements of nature in order to fabricate new devices.

The term bionanotechnology is often used interchangeably with nanobiotechnology, though adistinction is sometimes drawn between the two. If the two are distinguished, nanobiotechnology

usually refers to the use of nanotechnology to further the goals of biotechnology, while

bionanotechnology might refer to any overlap between biology and nanotechnology, including the

use of biomolecules as part of or as an inspiration for nanotechnological devices.

Nanobiotechnology is that branch of one, which deals with the study and application of biological

and biochemical activities from elements of nature to fabricate new devices like biosensors.

Nanobiotechnology is often used to describe the overlapping multidisciplinary activities associated

with biosensors particularly where photonics, chemistry, biology, biophysics nanomedicine andengineering converge. Measurement in biology using for example, waveguide techniques such as

dual polarisation interferometry are another example.


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nanoscience and technology---class lectures (part iv) [compatibility mode]

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