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Selected article 1

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Typical atomic force microscope setup: The deflection of a microfabricated cantilever with a sharp tip is measured by reflecting a laser beam off the backside of the cantilever while it is scanning over the surface of the sample.

Atomic force microscopy

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Atomic force microscopy (AFM) or scanning force microscopy (SFM) is a very high-resolution type of scanning probe microscopy, with demonstrated resolution on the order of fractions of a nanometer, more than 1000 times better than the optical diffraction limit. The precursor to the AFM, the scanning tunneling microscope, was developed by Gerd Binnig and Heinrich Rohrer in the early 1980s at IBM Research - Zurich, a development that earned them the Nobel Prize for Physics in 1986. Binnig, Quate and Gerber invented the first atomic force microscope (also abbreviated as AFM) in 1986. The first commercially available atomic force microscope was introduced in 1989. The AFM is one of the foremost tools for imaging, measuring, and manipulating matter at the nanoscale. The information is gathered by "feeling" the surface with a mechanical probe. Piezoelectric elements that facilitate tiny but accurate and precise movements on (electronic) command enable the very precise scanning. In some variations, electric potentials can also be scanned using conducting cantilevers. In newer more advanced versions, currents can even be passed through the tip to probe the electrical conductivity or transport of the underlying surface, but this is much more challenging with very few research groups reporting consistent data.

Selected article 2

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Diagram of a single-walled carbon nanotube

Carbon nanotube

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Carbon nanotubes (CNT) 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. These cylindrical carbon molecules have unusual properties, which are valuable for nanotechnology, electronics, optics and other fields of materials science and technology. In particular, owing to their extraordinary thermal conductivity and mechanical and electrical properties, carbon nanotubes find applications as additives to various structural materials. For instance, in (primarily carbon fiber) "baseball bats, car parts" and even "golf clubs", where nanotubes form only a tiny portion of the material(s).

Nanotubes are members of the fullerene structural family, which also includes the spherical buckyballs, and the ends of a nanotube may be capped with a hemisphere of the buckyball structure. Their name is derived from their long, hollow structure with the walls formed by one-atom-thick sheets of carbon, called graphene. These sheets are rolled at specific and discrete ("chiral") angles, and the combination of the rolling angle and radius decides the nanotube properties; for example, whether the individual nanotube shell is a metal or semiconductor. Nanotubes are categorized as single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs). Individual nanotubes naturally align themselves into "ropes" held together by van der Waals forces, more specifically, pi-stacking.

Selected article 3

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A representation of the classic DPN mechanism where alkane thiols are deposited through a water meniscus

Dip-pen nanolithography

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Dip-pen nanolithography (DPN) is the nanotechnology analog of the dip pen (also called the quill pen), where the tip of an atomic force microscope cantilever acts as a "pen," which is coated with a chemical compound or mixture acting as an "ink," and put in contact with a substrate, the "paper." This technique allows surface patterning on scales of under 100 nanometers, and recent advances have demonstrated massively parallel patterning using two-dimensional arrays of 55,000 tips. Applications of this technology currently range through chemistry, materials science, and the life sciences, and include such work as ultra high density biological nanoarrays, and additive photomask repair.

The uncontrollable transfer of a molecular 'ink' from a coated AFM tip to a substrate was first reported by Jaschke and Butt in 1995, but they erroneously concluded that alkanethiols could not be transferred to gold substrates to form stable nanostructures. A research group at Northwestern University led by Chad Mirkin studied the process and determined that under the appropriate conditions, molecules could be transferred to a wide variety of surfaces to create stable chemically-adsorbed monolayers. Mirkin and his coworkers hold the patents on this process, and the patterning technique has expanded to include liquid "inks", which are governed by a very different deposition mechanism when compared to "molecular inks".

Selected article 4

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A model of a DNA tetrahedron. Each edge of the tetrahedron is a 20bp DNA duplex, and each vertex is a three-arm junction. In this model each basepair is represented by five pseudo-atoms, representing the two sugars, the two phosphates, and the major groove.
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DNA nanotechnology

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DNA nanotechnology is the design and manufacture of artificial nucleic acid structures for technological uses. In this field, nucleic acids are used as non-biological engineering materials for nanotechnology rather than as the carriers of genetic information in living cells. Researchers in the field have created static structures such as two- and three-dimensional crystal lattices, nanotubes, polyhedra, and arbitrary shapes, as well as functional devices such as molecular machines and DNA computers. The field is beginning to be used as a tool to solve basic science problems in structural biology and biophysics, including applications in crystallography and spectroscopy for protein structure determination. Potential applications in molecular scale electronics and nanomedicine are also being investigated.

The conceptual foundation for DNA nanotechnology was first laid out by Nadrian Seeman in the early 1980s, and the field began to attract widespread interest in the mid-2000s. This use of nucleic acids is enabled by their strict base pairing rules, which cause only portions of strands with complementary base sequences to bind together to form strong, rigid double helix structures. A number of assembly methods are used to make these structures, including tile-based structures that assemble from smaller structures, folding structures using the DNA origami method, and dynamically reconfigurable structures using strand displacement techniques.

Selected article 5

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K. Eric Drexler (left) and Richard E. Smalley (right)
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Drexler–Smalley debate on molecular nanotechnology

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The Drexler–Smalley debate on molecular nanotechnology was a public dispute between K. Eric Drexler, the originator of the conceptual basis of molecular nanotechnology, and Richard Smalley, a recipient of the 1996 Nobel prize in Chemistry for the discovery of the nanomaterial buckminsterfullerene. The dispute was about the feasibility of constructing molecular assemblers, which are molecular machines which could robotically assemble molecular materials and devices by manipulating individual atoms or molecules. The two also traded accusations that the other's conception of nanotechnology was harmful to public perception of the field and threatened continued public support for nanotechnology research.

The debate was carried out from 2001 to 2003 through a series of published articles and open letters. It is often cited in the history of nanotechnology due to the fame of its participants and its commentary on both the technical and social aspects of nanotechnology. It has also been widely criticized for its adversarial tone, with Drexler accusing Smalley of publicly misrepresenting his work, and Smalley accusing Drexler of failing to understand basic science, causing commentators to go so far as to characterize the tone of the debate as similar to "a pissing match" and "reminiscent of [a] Saturday Night Live sketch".

Selected article 6

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C540, one member of the class of fullerenes

Fullerene

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A fullerene is any molecule composed entirely of carbon, in the form of a hollow sphere, ellipsoid, or tube. Spherical fullerenes are also called buckyballs, and they resemble the balls used in association football. Cylindrical ones are called carbon nanotubes or buckytubes. Fullerenes are similar in structure to graphite, which is composed of stacked graphene sheets of linked hexagonal rings; but they may also contain pentagonal (or sometimes heptagonal) rings.

The first fullerene to be discovered, and the family's namesake, buckminsterfullerene (C60), was prepared in 1985 by Richard Smalley, Robert Curl, James Heath, Sean O'Brien, and Harold Kroto at Rice University. The name was an homage to Buckminster Fuller, whose geodesic domes it resembles. The structure was also identified some five years earlier by Sumio Iijima, from an electron microscope image, where it formed the core of a "bucky onion." Fullerenes have since been found to occur in nature. More recently, fullerenes have been detected in outer space. According to astronomer Letizia Stanghellini, "It’s possible that buckyballs from outer space provided seeds for life on Earth.”

The discovery of fullerenes greatly expanded the number of known carbon allotropes, which until recently were limited to graphite, diamond, and amorphous carbon such as soot and charcoal. Buckyballs and buckytubes have been the subject of intense research, both for their unique chemistry and for their technological applications, especially in materials science, electronics, and nanotechnology.

Selected article 7

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Schematic diagram of a scanning tunneling microscope.

Scanning tunneling microscope

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A scanning tunneling microscope (STM) is an instrument for imaging surfaces at the atomic level. Its development in 1981 earned its inventors, Gerd Binnig and Heinrich Rohrer (at IBM Zürich), the Nobel Prize in Physics in 1986. For an STM, good resolution is considered to be 0.1 nm lateral resolution and 0.01 nm depth resolution.With this resolution, individual atoms within materials are routinely imaged and manipulated. The STM can be used not only in ultra-high vacuum but also in air, water, and various other liquid or gas ambients, and at temperatures ranging from near zero kelvin to a few hundred degrees Celsius.

The STM is based on the concept of quantum tunneling. When a conducting tip is brought very near to the surface to be examined, a bias (voltage difference) applied between the two can allow electrons to tunnel through the vacuum between them. The resulting tunneling current is a function of tip position, applied voltage, and the local density of states (LDOS) of the sample. Information is acquired by monitoring the current as the tip's position scans across the surface, and is usually displayed in image form. STM can be a challenging technique, as it requires extremely clean and stable surfaces, sharp tips, excellent vibration control, and sophisticated electronics. The components of an STM include scanning tip, piezoelectric controlled height and x,y scanner, coarse sample-to-tip control, vibration isolation system, and computer. The tip is often made of tungsten or platinum-iridium, though gold is also used.

Selected article 8

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Solutions of gold nanoparticles of different sizes, which appear as different colors

Colloidal gold

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Colloidal gold is a colloid (a suspension in a fluid) of sub-micrometer-sized particles of gold. Due to the unique optical, electronic, and molecular-recognition properties of gold nanoparticles, they are the subject of substantial research, with applications in a wide variety of areas, including electron microscopy, electronics, nanotechnology, and materials science.

Known since ancient times, the synthesis of colloidal gold was originally used as a method of staining glass. Modern scientific evaluation of colloidal gold did not begin until Michael Faraday's work of the 1850s, who was the first to recognize that the color was due to the minute size of the gold particles. In 1898 Richard Adolf Zsigmondy prepared the first colloidal gold in diluted solution. Gold nanoparticles are currently being investigated for modern applications. Colloidal gold and various derivatives have long been among the most widely-used contrast agents for biological electron microscopy. Gold nanoparticles are being investigated as carriers for hydrophobic drugs, which require molecular encapsulation. In cancer research, colloidal gold can be used to target tumors and provide detection using SERS in vivo. Small diameter gold nanorods are being used as photothermal converters of near infrared light, which transmits readily through human skin and tissue.

Selected article 9

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DNA arrays that display a representation of the Sierpinski gasket on their surfaces

DNA computing

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DNA computing is a form of computing which uses DNA, biochemistry and molecular biology, instead of the traditional silicon-based computer technologies. DNA computing, or, more generally, biomolecular computing, is a fast developing interdisciplinary area. DNA computing is fundamentally similar to parallel computing in that it takes advantage of the many different molecules of DNA to try many different possibilities at once.

This field was initially developed by Leonard Adleman of the University of Southern California, in 1994. Adleman demonstrated a proof-of-concept use of DNA as a form of computation which solved the seven-point Hamiltonian path problem. Since the initial Adleman experiments, advances have been made and various Turing machines have been proven to be constructible. While the initial interest was in using this novel approach to tackle NP-hard problems, it was soon realized that they may not be best suited for this type of computation, and several proposal have been made to find a "killer application" for this approach.

Selected article 10

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Richard Feynman, whose 1959 talk "There's plenty of room at the bottom" many years later inspired the conceptual foundations of nanotechnology

History of nanotechnology

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The history of nanotechnology traces the development of the concepts and experimental work falling under the broad category of nanotechnology. Although nanotechnology is a relatively recent development in scientific research, the development of its central concepts happened over a longer period of time. The emergence of nanotechnology in the 1980s was caused by the convergence of experimental advances such as the invention of the scanning tunneling microscope in 1981 and the discovery of fullerenes in 1985, with the elucidation and popularization of a conceptual framework for the goals of nanotechnology beginning with the 1986 publication of the book Engines of Creation. The field was subject to growing public awareness and controversy in the early 2000s, with prominent debates about both its potential implications as well as the feasibility of the applications envisioned by advocates of molecular nanotechnology, and with governments moving to promote and fund research into nanotechnology. The early 2000s also saw the beginnings of commercial applications of nanotechnology, although these were limited to bulk applications of nanomaterials rather than the transformative applications envisioned by the field.

Selected article 11

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A reverse-biased nanofluidic diode

Nanofluidic circuitry

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Nanofluidic circuitry is a nanotechnology aiming for control of fluids in nanometer scale. Due to the effect of an electrical double layer within the fluid channel, the behavior of nanofluid is observed to be significantly different compared with its microfluidic counterparts. Its typical characteristic dimensions fall within the range of 1–100 nm. At least one dimension of the structure is in nanoscopic scale. Phenomena of fluids in nano-scale structure are discovered to be of different properties in electrochemistry and fluid dynamics. With the development of microfabrication and nanotechnology, the study of microfluidics and nanofluidics is drawing more attention. Research on microfluidic found its advantages in DNA analysis, lab-on-a-chip, and micro-TAS. Devices in a microfluidic system include channels, valves, mixers, and pumps. Integration of these microfluidic devices enables sorting, transporting, and mixing of substances within fluids. However, the failure of moving parts in these systems is usually the critical issue and the main drawback. Mechanisms to control flow without using mechanical parts are always desired for reliability and lifetime.

Selected article 12

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A schematic illustration showing how nanoparticles or other cancer drugs might be used to treat cancer

Nanomedicine

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Nanomedicine is the medical application of nanotechnology. Nanomedicine seeks to deliver a valuable set of research tools and clinically useful devices in the near future, and the National Nanotechnology Initiative expects new commercial applications in the pharmaceutical industry that may include advanced drug delivery systems, new therapies, and in vivo imaging. Tools being developed include medical applications of nanomaterials, to nanoelectronic biosensors, neuro-electronic interfaces, and even possible future applications of the speculative field of molecular nanotechnology including cell repair machines. Current problems for nanomedicine involve understanding the issues related to toxicity and environmental impact of nanoscale materials.

Nanomedical approaches to drug delivery center on developing nanoscale particles or molecules to improve drug bioavailability, whichis the presence of drug molecules where they are needed in the body and where they will do the most good. Drug delivery focuses on maximizing bioavailability both at specific places in the body and over a period of time. This can potentially be achieved by molecular targeting by nanoengineered devices. In vivo imaging is another area where tools and devices are being developed. Using nanoparticle contrast agents, images such as ultrasound and MRI have a favorable distribution and improved contrast. The new methods of nanoengineered materials that are being developed might be effective in treating illnesses and diseases such as cancer.

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Nanometrology

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Nanometrology is a subfield of metrology, concerned with the science of measurement at the nanoscale level. Nanometrology has a crucial role in order to produce nanomaterials and devices with a high degree of accuracy and reliability in nanomanufacturing. A challenge in this field is to develop or create new measurement techniques and standards to meet the needs of next-generation advanced manufacturing, which will rely on nanometer scale materials and technologies.

At nanoscale due to the small dimensions various new physical phenomena can be observed. It becomes important to measure the physical parameters so as to apply these phenomena into engineering of nanosystems and manufacturing them. The measurement of length or size, force, mass, electrical and other properties is included in Nanometrology. The problem is how to measure these with reliability and accuracy. The measurement techniques used for macro systems cannot be directly used for measurement of parameters in nanosystems. Various techniques based on physical phenomena have been developed which can be used for measure or determine the parameters for nanostructures and nanomaterials. Some of the popular ones are X-Ray Diffraction, Transmission Electron Microscopy, High Resolution Transmission Electron Microscopy, Atomic Force Microscopy, Scanning Electron Microscopy, Field Emission Scanning Electron Microscopy and Brunauer, Emmett, Teller method to determine specific surface.

Selected article 14

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Graphical representation of a rotaxane, useful as a molecular switch.

Molecular scale electronics

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Molecular scale electronics, also called single molecule electronics, is a branch of nanotechnology that uses single molecules, or nanoscale collections of single molecules, as electronic components. Because single molecules constitute the smallest stable structures imaginable this miniaturization is the ultimate goal for shrinking electrical circuits. The performance and complexity of conventional integrated circuits have been growing exponentially (a trend also known as Moore’s law) and has forced the feature sizes of the embedded components to shrink accordingly.

In single molecule electronics, the bulk material is replaced by single molecules. The molecules utilized have properties that resemble traditional electronic components such as a wire, transistor or rectifier. Single molecule electronics is an emerging field, and entire electronic circuits consisting exclusively of molecular sized compounds are still very far from being realized. However, the continuous demand for more computing power together with the inherent limitations of the present day lithographic methods make the transition seem unavoidable. Currently, the focus is on discovering molecules with interesting properties and on finding ways to obtaining reliable and reproducible contacts between the molecular components and the bulk material of the electrodes.

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Nanostars of vanadium(IV) oxide

Nanoparticle

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Nanoparticles, a particle is defined as a small object that behaves as a whole unit in terms of its transport and properties. Particles are further classified according to size : in terms of diameter, coarse particles cover a range between 10,000 and 2,500 nanometers. Fine particles are sized between 2,500 and 100 nanometers. Ultrafine particles, or nanoparticles are sized between 100 and 1 nanometers. The reason for this double name of the same object is that, during the 1970-80's, when the first thorough fundamental studies were running with "nanoparticles" in the USA (by Granqvist and Buhrman) and Japan, (within an ERATO Project) they were called "ultrafine particles" (UFP). However, during the 1990s before the National Nanotechnology Initiative was launched in the USA, the new name, "nanoparticle" had become fashionable (see, for example the same senior author's paper 20 years later addressing the same issue, lognormal distribution of sizes). Nanoparticles may or may not exhibit size-related properties that differ significantly from those observed in fine particles or bulk materials. Although the size of most molecules would fit into the above outline, individual molecules are usually not referred to as nanoparticles.

Nanoclusters have at least one dimension between 1 and 10 nanometers and a narrow size distribution. Nanopowders are agglomerates of ultrafine particles, nanoparticles, or nanoclusters. Nanometer-sized single crystals, or single-domain ultrafine particles, are often referred to as nanocrystals. Nanoparticle research is currently an area of intense scientific interest due to a wide variety of potential applications in biomedical, optical and electronic fields.

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A putative molecular planetary gear system

High-resolution transmission electron microscopy

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High-resolution transmission electron microscopy (HRTEM) is an imaging mode of the transmission electron microscope that allows for direct imaging of the atomic structure of the sample. HRTEM is a powerful tool to study properties of materials on the atomic scale, such as semiconductors, metals, nanoparticles and sp2-bonded carbon (e.g. graphene, C nanotubes). At present, the highest point resolution realised in phase contrast TEM is around 0.5 ångströms (0.050 nm). At these small scales, individual atoms of a crystal and its defects can be resolved. For 3-dimensional crystals, it may be necessary to combine several views, taken from different angles, into a 3D map. This technique is called electron crystallography.

One of the difficulties with HRTEM is that image formation relies on phase contrast. In phase-contrast imaging, contrast is not necessarily intuitively interpretable, as the image is influenced by aberrations of the imaging lenses in the microscope. The largest contributions for uncorrected instruments typically come from defocus and astigmatism. The latter can be estimated from the so-called Thon ring pattern appearing in the Fourier transform modulus of an image of a thin amorphous film.

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Typical densities of states as a function of energy for metallic and semiconducting single-wall carbon nanotubes
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Optical properties of carbon nanotubes

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Within materials science, the optical properties of carbon nanotubes refer specifically to the absorption, photoluminescence, and Raman spectroscopy of carbon nanotubes. Spectroscopic methods offer the possibility of quick and non-destructive characterization of relatively large amounts of carbon nanotubes. There is a strong demand for such characterization from the industrial point of view: numerous parameters of the nanotube synthesis can be changed, intentionally or unintentionally, to alter the nanotube quality. Optical absorption, photoluminescence and Raman spectroscopies allow quick and reliable characterization of this "nanotube quality" in terms of non-tubular carbon content, structure (chirality) of the produced nanotubes, and structural defects.

Whereas mechanical, electrical and electrochemical (supercapacitor) properties of the carbon nanotubes are well established and have immediate applications, the practical use of optical properties is yet unclear. The aforementioned tunability of properties is potentially useful in optics and photonics. In particular, light-emitting diodes (LEDs) based on a single nanotube have been produced in the lab. Their unique feature is not the efficiency, which is yet relatively low, but the narrow selectivity in the wavelength of emission and detection of light and the possibility of its fine tuning through the nanotube structure. In addition, bolometer and optoelectronic memory devices have been realised on ensembles of single-walled carbon nanotubes.

Selected article 18

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Colloidal quantum dots irradiated with a UV light. Different sized quantum dots emit different color light due to quantum confinement.

Quantum dot

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A quantum dot is a portion of matter (e.g., semiconductor) whose excitons are confined in all three spatial dimensions. Consequently, such materials have electronic properties intermediate between those of bulk semiconductors and those of discrete molecules. Stated simply, quantum dots are semiconductors whose electronic characteristics are closely related to the size and shape of the individual crystal. For example, in fluorescent dye applications, this equates to higher frequencies of light emitted after excitation of the dot as the crystal size grows smaller, resulting in a color shift from red to blue in the light emitted. In addition to such tuning, a main advantage with quantum dots is that, because of the high level of control possible over the size of the crystals produced, it is possible to have very precise control over the conductive properties of the material.

Researchers have studied quantum dots in transistors, solar cells, LEDs, and diode lasers. They have also investigated quantum dots as agents for medical imaging and hope to use them as qubits in quantum computing. Quantum dots of different sizes can be assembled into a gradient multi-layer nanofilm. They were discovered at the beginning of the 1980s by Alexei Ekimov in a glass matrix and by Louis E. Brus in colloidal solutions. The term "quantum dot" was coined by Mark Reed.

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A diagram illustrating near-field optics, with the diffraction of light coming from NSOM fiber probe, showing the wavelength of light as compared to the near-field region

Near-field scanning optical microscopy

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Near-field scanning optical microscopy (NSOM or SNOM) is a form of scanning probe microscopy that breaks the far field optical resolution limit by exploiting the properties of evanescent waves. This is done by placing the detector very close (distance much smaller than wavelength λ) to the specimen surface. This allows for the surface inspection with high spatial, spectral and temporal resolving power. With this technique, the resolution of the image is limited by the size of the detector aperture and not by the wavelength of the illuminating light. In particular, lateral resolution of 20 nm and vertical resolution of 2–5 nm have been demonstrated. As in optical microscopy, the contrast mechanism can be easily adapted to study different properties, such as refractive index, chemical structure and local stress. Dynamic properties can also be studied at a sub-wavelength scale using this technique.

Edward Hutchinson Synge, a scientist, is given credit for conceiving and developing the idea for an imaging instrument that would image by exciting and collecting diffraction in the near field in 1928. It was Ash and Nicholls who, in 1972, first broke the Abbe’s diffraction limit using radiation with wavelength of 3 cm. It was twelve more years before the first papers that used visible radiation for near field scanning were published by Pohl et al., and Lewis et al.

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A diagram of a wrap-around gate CNTFET, where the entire circumference of the nanotube is gated, which ideally improves the electrical performance of the CNTFET

Carbon nanotube field-effect transistor

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A carbon nanotube field-effect transistor (CNTFET) refers to a field-effect transistor that utilizes a single carbon nanotube or an array of carbon nanotubes as the channel material instead of bulk silicon in the traditional MOSFET structure. First demonstrated in 1998, there have been major developments in CNTFETs.

The exceptional electrical properties of carbon nanotubes arise from the unique electronic structure of graphene itself that can roll up and form a hollow cylinder. A carbon nanotube’s bandgap is directly affected by its chirality and diameter. If those properties can be controlled, CNTs would be a promising candidate for future nano-scale transistor devices. Moreover, because of the lack of boundaries in the perfect and hollow cylinder structure of CNTs, there is no boundary scattering. CNTs are also quasi-1D materials in which only forward scattering and back scattering are allowed, and elastic scattering mean free paths in carbon nanotubes are long, typically on the order of micrometers. As a result, quasi-ballistic transport can be observed in nanotubes at relatively long lengths and low fields. Because of the strong covalent carbon-carbon bonding in the sp2 configuration, carbon nanotubes are chemically inert and are able to transport large amounts of electric current. In theory, carbon nanotubes are also able to conduct heat nearly as well as diamond or sapphire, and because of their miniaturized dimensions, the CNTFET should switch reliably using much less power than a silicon-based device.

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Self-assembled monolayer schematic

Self-assembled monolayer

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A self-assembled monolayer (SAM) is an organized layer of amphiphilic molecules in which one end of the molecule, the "head group" shows a specific, reversible affinity for a substrate. SAMs also consist of a tail with a functional group at the terminal end. SAMs are created by the chemisorption of hydrophilic head groups onto a substrate from either the vapor or liquid phase followed by a slow two-dimensional organization of hydrophobic tail groups. Initially, adsorbate molecules form either a disordered mass of molecules or form a "lying down phase", and over a period of minutes to hours, begin to form crystalline or semicrystalline structures on the substrate surface. Areas of close-packed molecules nucleate and grow until the surface of the substrate is covered in a single monolayer.

Selecting the type of head group depends on the application of the SAM. Typically, head groups are connected to an alkyl chain in which the terminal end can be functionalized (i.e. adding –OH, –NH3, or –COOH groups) to vary the wetting and interfacial properties. Substrates can be planar surfaces, such as silicon and metals, or curved surfaces, such as nanoparticles. Alkanethiols are the most commonly used molecules for SAMs. They are used on noble metal substrates because of the strong affinity of sulfur for these metals. Silanes are generally used on nonmetallic oxide surfaces. Metal substrates for use in SAMs can be produced through physical vapor deposition techniques, electrodeposition or electroless deposition.

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Richard Feynman circa 1965

There's Plenty of Room at the Bottom

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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 the California Institute of Technology on December 29, 1959. The talk is considered to be a seminal event in the history of nanotechnology, as it inspired the conceptual beginnings of the field decades later. Feynman considered the possibility of direct manipulation of individual atoms as a more powerful form of synthetic chemistry than those used at the time. He was particularly interested in the possibilities of denser computer circuitry, and microscopes which could see things much smaller than is possible with scanning electron microscopes. At the meeting, Feynman concluded his talk with two challenges, and he offered a prize of $1000 for the first individuals to solve each one.

The published versions of Feynman’s talk had a negligible influence in the twenty years after it was first published, as measured by citations in the scientific literature. Subsequently, interest in "Plenty of Room" in the scientific literature greatly increased in the early 1990s. This is probably because the term "nanotechnology" gained serious attention just before that time, following its use by K. Eric Drexler who took the Feynman concept of a billion tiny factories and added the idea that they could make more copies of themselves, via computer control instead of control by a human operator, in his 1986 book Engines of Creation: The Coming Era of Nanotechnology.

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A 3D representation of the local oxidation nanolithography process.  A voltage pulse applied between the AFM tip and the scanned surface causes a nanometric oxidation reaction confined to a liquid meniscus between the tip and the surface.

Local oxidation nanolithography

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Local oxidation nanolithography (LON) is a tip-based nanofabrication method. It is based on the spatial confinement on an oxidation reaction under the sharp tip of an atomic force microscope. LON allows to create a large variety of motives like dots, lines and letters with nanometer accuracy. The features created with this technique can be used for the template growth and preferential deposition of different molecules like single molecule magnets, biomolecules and conjugated organic molecules. By using local oxidation nanolithography as tool for the fabrication of etch-resistant nanomasks, it is possible to fabricate nanoscale electronic devices, such as field effect transistors, single electron transistors, Josephson junctions, quantum rings or SQUIDs. LON also allows to fabricate silicon nanowires (SiNWs) in a top-down fashion starting from silicon on insulator (SOI) wafers.

Currently, local oxidation experiments are performed with an atomic force microscope (AFM) operated in contact or noncontact mode with additional circuits to apply voltage pulses between tip and sample. The local oxidation process is mediated by the formation of a water meniscus. An applied voltage induces the formation of a water bridge between tip and sample whenever the amplitude of the voltage pulse is above a certain threshold voltage, breaking the covalent bonds in the water molecules. The liquid bridge provides the oxyanions (OH-,O-) needed to form the oxide and confines the lateral extension of the region to be oxidized.

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The Prato reaction, a reaction of a small molecule with a buckminterfullerene molecule

Fullerene chemistry

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Fullerene chemistry is a field of organic chemistry devoted to the chemical properties of fullerenes. Research in this field is driven by the need to functionalize fullerenes and tune their properties. For example fullerene is notoriously insoluble and adding a suitable group can enhance solubility. By adding a polymerizable group, a fullerene polymer can be obtained. Functionalized fullerenes are divided into two classes: exohedral fullerenes with substituents outside the cage and endohedral fullerenes with trapped molecules inside the cage.

Fullerene or C60 is soccer-ball-shaped or Ih with 12 pentagons and 20 hexagons. Because of the molecule's spherical shape the carbon atoms are highly pyramidalized, which has far-reaching consequences for reactivity. It is estimated that strain energy constitutes 80% of the heat of formation. The conjugated carbon atoms respond to deviation from planarity by orbital rehybridization of the sp² orbitals and pi orbitals to a sp2.27 orbital with a gain in p-character. The p lobes extend further outside the surface than they do into the interior of the sphere and this is one of the reasons a fullerene is electronegative.

The double bonds in fullerene are not all the same. Two groups can be identified: 30 so-called [6,6] double bonds connect two hexagons and 60 [5,6] bonds connect a hexagon and a pentagon. Of the two the [6,6] bonds are shorter with more double-bond character and therefore a hexagon is often represented as a cyclohexatriene and a pentagon as a pentalene or [5]radialene. In other words, although the carbon atoms in fullerene are all conjugated the superstructure is not a super aromatic compound.

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Thermodynamics of nanostructures

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As the devices continue to shrink further into the sub-100 nm range following the trend predicted by Moore’s law, the topic of thermal properties and transport in such nanoscale devices becomes increasingly important. Display of great potential by nanostructures for thermoelectric applications also motivates the studies of thermal transport in such devices. These fields, however, generate two contradictory demands: high thermal conductivity to deal with heating issues in sub-100 nm devices and low thermal conductivity for thermoelectric applications. These issues can be addressed with phonon engineering, once nanoscale thermal behaviors have been studied and understood.

In general two carrier types can contribute to thermal conductivity - electrons and phonons. In nanostructures phonons usually dominate and the phonon properties of the structure become of a particular importance for thermal conductivity. Unlike bulk materials, nanoscale devices have thermal properties which are complicated by boundary effects due to small size. It has been shown that in some cases phonon-boundary scattering effects dominate the thermal conduction processes, reducing thermal conductivity.

Depending on the nanostructure size, the phonon mean free path values may be comparable or larger than the object size, L. When L is larger than the phonon mean free path, Umklapp scattering process limits thermal conductivity. When L is comparable to or smaller than the mean free path, the continuous energy model used for bulk materials no longer applies and nonlocal and nonequilibrium aspects to heat transfer also need to be considered. More severe changes in thermal behavior are observed when the feature size L shrinks further down to the wavelength of phonons.

Selected article 26

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Free standing copper metal replica of etched ion tracks in polycarbonate

Nanowire

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A nanowire is a nanostructure with the diameter of the order of a nanometer (10−9 meters). At these scales, quantum mechanical effects are important — which coined the term "quantum wires". Many different types of nanowires exist, including metallic, semiconducting, and insulating. The nanowires could be used, in the near future, to link tiny components into extremely small circuits. There are two basic approaches to synthesizing nanowires: top-down and bottom-up. A top-down approach reduces a large piece of material to small pieces, by various means such as lithography or electrophoresis. A bottom-up approach synthesizes the nanowire by combining constituent adatoms. Most synthesis techniques use a bottom-up approach.

Several physical reasons predict that the conductivity of a nanowire will be much less than that of the corresponding bulk material. Nanowires also show other peculiar electrical properties due to their size. Unlike carbon nanotubes, whose motion of electrons can fall under the regime of ballistic transport (meaning the electrons can travel freely from one electrode to the other), nanowire conductivity is strongly influenced by edge effects. Furthermore, the conductivity can undergo a quantization in energy: i.e. the energy of the electrons going through a nanowire can assume only discrete values, which are multiples of the Von Klitzing constant.

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