Nanotechnology

Nanotechnology studies and controls matter whose important dimensions are on the nanoscale, conventionally about $1$–$100$ nanometres. A nanometre is $10^{-9}$ metres. The scientific interest comes from changes in behaviour at these dimensions, including the increasing role of surfaces and, in suitable systems, quantum confinement.^[ref]

For example, subdividing a fixed volume into smaller nanoparticles increases the exposed surface area, changing how much material can participate in a surface reaction. Nanostructures can also alter electrical conductivity, interaction with light and mechanical properties by controlling geometry rather than changing only chemical chemical composition. Useful nanotechnology therefore combines fabrication with measurements of structure and properties, such as electron microscopy or atomic force microscopy.^[ref]

A nanomaterial has constituent particles or structural features on the nanoscale, conventionally around $1$–$100\mathrm{nm}$. A collection of nanoparticles is one example; a bulk piece containing nanoscale internal structure can also be relevant. The scientific interest lies in how dimensions and interfaces affect behavior, rather than in treating $100\mathrm{nm}$ as a universal physical threshold.^[ref]

A simple geometric calculation explains one important effect. Suppose a fixed volume $V$ of solid is divided into identical spherical nanoparticles of radius $r$. Ignoring contact between particles, their number $N$ and total surface area $A$ areReducing $r$ from $50\mathrm{nm}$ to $5\mathrm{nm}$ makes $1000$ times as many particles and ten times as much surface area, at the same total volume. More material is consequently close to an interface. Surface reactions and interactions with the surrounding medium can become much more influential; this geometric result alone does not guarantee a faster useful reaction or a more stable material.^[ref]

Size can also affect electronic behavior through quantum confinement. Which effect matters depends on the material, shape and environment. A useful comparison therefore holds composition fixed, measures particle dimensions with electron microscopy, and compares a chosen property across a size series. Such a comparison separates a size-dependent change from a change caused merely by using a different substance.^[ref]

A nanohybrid material combines distinguishable material components with at least one component or important structural feature on the nanoscale. Examples include inorganic nanoparticles dispersed in an organic polymer, or nanoparticles carrying a functional molecular coating. The term alone does not specify a unique composition or structure.

The interface is often central: particles may provide an optical or catalytic response, while the surrounding material supplies mechanical support or processability. Performance depends on particle dispersion, interfacial bonding and the properties of each component. A mixed chemical composition is insufficient evidence for a designed hybrid structure; measurements must establish which components exist and how they are arranged. Electron microscopy, spectroscopy and bulk-property measurements can provide complementary evidence.

The nanoscale is a range of dimensions conventionally around $1$–$100$ nanometres, where a nanometre is $10^{-9}$ metres. It describes a length scale, not a distinct substance. A film can have nanoscale thickness while extending over a macroscopic area.^[ref]

Properties change when a relevant dimension approaches a physical length controlling the behaviour, such as an electronic confinement length or a characteristic surface region. For a sphere, surface area divided by volume is $3/r$, so reducing its radius increases the relative importance of its surface. Nanoscale geometry can consequently influence chemical reactions and interaction with light even without changing the constituent chemical element.^[ref]

Semiconductor device fabrication builds electronic or optoelectronic structures by controlling the geometry, composition and interfaces of semiconductor materials. Typical steps include deposition, photolithography, etching and controlled semiconductor doping. Repeating patterned steps creates devices and their interconnections.

For a simple patterned layer, deposit material, coat it with photoresist, expose through a photomask, develop the resist and transfer the pattern by etching. Later layers require alignment to earlier ones. A useful device depends on more than resolving a small feature: defects, contamination, interface quality and electrical contact resistance also affect performance. Fabrication therefore couples materials science with measurement and process control.^[ref]

Fabrication of small structures from deposited, patterned surface films. A structural layer can be built on a sacrificial layer and released by selective removal of that support while retaining anchors. Film thickness sets a vertical dimension; residual stress, access to the sacrificial material and release-induced stiction require separate control.

A temporary material supporting another structure during fabrication, then removed selectively. In an oxide-supported polysilicon beam, selective oxide removal makes an air gap while protected substrate contacts retain anchors. Removal must reach beneath the structure without destroying it; a purely vertical etch may not release a lateral overhang.

Selective removal of a sacrificial layer using gaseous reactants rather than a bulk liquid bath. A compatible vapor-phase oxide etch can avoid the liquid drying step responsible for one form of stiction. Selectivity, byproducts, humidity and access to the undercut region still matter.

Replication of a patterned master into a deformable or curable material to define nanoscale features. A subsequent filling, etching or transfer step can turn the replicated geometry into a functional structure. Master defects, residual layers, shrinkage and release damage must be controlled in repeated large-area fabrication.

Organization of interacting building blocks into a structure through their own dynamics under specified environmental conditions. A useful description states the components, interactions, environment and reversible transport/rearrangement mechanisms. A favourable thermodynamic equilibrium structure can still fail to form if aggregation or glassy arrest prevents defect correction.

A molecular layer formed through adsorption and lateral organization at a substrate. Anchoring groups, chain packing and exposed terminal chemistry jointly determine its properties. Hydroxyl-terminated alkanethiol films on gold provide a nonbiological example with controllable water wetting.

A nanostructure has structural features on the nanoscale. The relevant dimension may be the thickness of a thin film, the diameter of a nanowire, or the overall size of a nanoparticle. A material can therefore be macroscopically large while containing nanostructures internally.

The geometry matters because it changes the relative importance of surfaces, the distances over which charge carriers move, and sometimes the allowed quantum states. A thin film and a nanoparticle made from the same chemical material need not have the same properties: their quantum confinement dimensions and exposed surface areas differ. Characterizing a nanostructure requires specifying which dimensions are small, its shape and its chemical chemical composition, rather than reporting a single size alone.^[ref]

A nanoparticle is a particle whose external dimensions lie on the nanoscale. Its behaviour can depend strongly on its size, shape and surface chemistry, even when its overall chemical chemical composition is fixed.

For a sphere of radius $r$, the ratio of surface area to volume isHalving $r$ therefore doubles the surface area exposed per unit volume. At fixed total volume, making many smaller particles exposes more material to the surrounding medium. This helps explain why surface reactions and attached molecules can have a large effect on nanoparticles.^[ref]

The size of an individual nanoparticle also differs from the size of an particle aggregate containing many particles. Reports of particle size consequently need to identify what was measured. Gold nanoparticles provide a visible example: their interaction with light can differ markedly from bulk gold.

A particle aggregate is a cluster made from multiple primary particles. Its overall dimensions and internal arrangement differ from the size and shape of the individual nanoparticles. A measurement reporting particle size must consequently identify whether it resolves individual particles, a connected cluster or an effective size inferred from its motion.

Aggregation changes accessible surface area, fluid transport and interaction with light. A compact cluster and an open, branched cluster containing the same number of particles can therefore behave differently. Some measurement conventions reserve aggregate for strongly bonded or fused particles and use agglomerate for more weakly associated particles; experimental reports should state their convention and how sample preparation affects the clusters.

Deposition of clusters produced in a gas-phase aggregation source. Source conditions control the size distribution, and optional mass selection narrows it. Landing energy and substrate interactions can change the structure delivered to a surface.

A nanoparticle formed from several crystallographic domains joined by twin boundaries. Low-energy external facets can compensate the internal boundary and elastic-strain cost; fivefold motifs cannot extend as an unstrained periodic bulk crystal.

A multiply twinned particle containing twenty approximately tetrahedral domains around a common centre. Its idealized exterior has twenty triangular facets; surface-energy reduction competes with strain and twin-boundary costs.

A fivefold-twinned particle with five approximately tetrahedral domains sharing an axis. Five ideal fcc tetrahedral wedge angles leave an angular deficit of about $7.36^{\circ }$, accommodated by strain and defects.

Formation of particles or films by delivering precursor droplets into a heated environment where solvent evaporation and precursor decomposition occur. Drying and reaction within droplets can produce dense or porous particles.

A nanoscale particle containing two metallic elements. Spatial arrangements include disordered alloys, chemically ordered alloys, core-shell structures and segregated domains; composition alone does not specify the arrangement.

A processable dispersion of nanoparticles with a solvent and any stabilizers or binders. For a metallic printed conductor, drying, ligand removal and consolidation must turn the deposited particles into a connected low-resistance structure without damaging the substrate.

Patterned material removal by chemically reactive plasma species and directional ion bombardment.

Directional plasma etching used for deep, high-aspect-ratio features. Alternating fluorine-based silicon etching and sidewall passivation is one implementation; transport, mask erosion, charging, scallops and aspect-ratio-dependent etch rate constrain the achievable structure.

An elongated structure with nanoscale transverse dimensions. Crystal structure, growth direction, sidewall facets and surface chemistry determine its properties and morphology.

A solution route in which a polyol serves as solvent and reducing medium for metal nucleation and growth. For silver wires, controlled seed structure and capping chemistry can favour axial growth. Temperature, reduction rate, seed population, halides, oxygen and ligand coverage act together; the method is not defined by a single universal recipe.

A high-aspect-ratio silver structure with nanoscale cross section. Low intrinsic metal resistivity and sparse network connectivity make such wires useful for flexible transparent conductors, but nanowire junction resistance, insulating ligands, tarnish and mechanical connectivity can dominate device performance.

A silver wire composed of five twinned fcc domains around its axis, commonly with a pentagonal cross section. Sidewall passivation and atom addition at ends can favour one-dimensional growth from a multiply twinned seed. Cross-sectional microscopy and diffraction are stronger evidence for this structure than the outer wire outline alone.^[ref]

Growth by direct incorporation of vapour-derived species into a solid, without the defining liquid reservoir of vapor–liquid–solid growth. It can thicken a nanowire sidewall during tip growth.

Growth in which vapour supplies a liquid catalyst or reservoir and solid material precipitates at its interface. A cooled solid catalyst residue need not reproduce the liquid growth state.

A crystalline particle with nanoscale dimensions. Its shape and growth kinetics depend on exposed interfaces, defects and material supply, not just its bulk crystal structure.

A bimetallic nanocrystal is a crystalline particle containing two metallic chemical elements, with important dimensions on the nanoscale. Its atoms may form an alloy, chemically ordered regions, a core surrounded by a shell or spatially separated domains. Composition alone does not determine which arrangement is present.

For example, a gold–silver particle can expose a surface composition different from its bulk average. That affects surface chemistry and interaction with light. Establishing its structure requires complementary evidence: electron microscopy can reveal morphology, elemental mapping can locate the elements, and diffraction can constrain the crystal structure. These measurements answer different questions rather than providing interchangeable particle descriptions.

Addition or rearrangement of material in a nanocrystal. Solute addition, Ostwald ripening and oriented attachment are distinct mechanisms that can coexist.

Preferential adsorption of charged species onto particular crystal faces, reducing their normal incorporation rates. Both solution speciation and actual surface charge matter; crystallographic polarity alone does not fix adsorption selectivity.

Selection of unequal growth along otherwise equivalent directions, through defects, selective adsorption, kinetics or external asymmetry. A lower-symmetry shape need not imply a different bulk lattice.

Joining of crystallites after compatible crystallographic alignment. Elimination of exposed interface drives the process; incomplete rotation and healing can leave defects.

Growth by incorporation of dissolved individual atoms, ions or molecular units into a crystal, driven by supersaturation and limited by transport or attachment kinetics.

Removal of material with a focused ion beam, for example to prepare a thin transmission electron microscopy specimen. Implantation, amorphization and redeposition can require a low-energy finishing step.

Stripe-like thickness variations caused by nonuniform milling through regions with different sputtering response.

A nanoscale, approximately spherical carbon structure. Surface oxidation and grafting can change dispersion without requiring any biological processing.

A gold nanoparticle is a small particle of gold whose optical and surface chemistry depend on its geometry and surroundings. A dispersion of such particles can appear red or purple rather than resembling a piece of bulk gold.^[ref]

In 1857, Michael Faraday described ruby-coloured fluids containing finely divided gold. He compared the transmitted colour of these fluids with coloured glass and observed changes toward blue or violet under conditions that altered the dispersed particles. These observations established a striking connection between the physical state of gold and its interaction with light.^[ref]

A modern explanation involves localized surface plasmon resonance: the collective response of conduction electrons to an incident electromagnetic field changes absorption and scattering. The resulting spectrum depends on particle shape and the surrounding dielectric, as well as size. Thus colour alone is not a universal measure of nanoparticle diameter.

Experiments on gold nanorings show how geometry changes the response: varying the ratio of ring thickness to radius shifts the localized surface plasmon resonance. This is evidence for control through particle shape rather than a change of chemical element.^[ref]

A gold nanoring is a ring-shaped gold nanostructure whose central opening and wall dimensions alter its response to light. Charge oscillations associated with its inner and outer surfaces couple, changing its localized surface plasmon resonance relative to that of a compact particle.^[ref]

The 2003 experiments and modelling showed that varying the ring geometry shifts the optical resonance. Comparing rings with different wall-thickness-to-radius ratios thus tests a geometric mechanism while retaining the same elemental material. A measured colour or spectral peak must be interpreted together with geometry and the surrounding dielectric, not as a unique measure of overall particle diameter.^[ref]

Surface-bound molecules surrounding a nanoparticle. Solvation and interactions between outer groups influence dispersibility and aggregation.

Deposition through physical release and transport of material, including evaporation and sputtering.

Thickness deposited on a feature sidewall or bottom relative to a reference planar surface. Geometry, arrival directions, sticking and surface mobility all influence it.

Exclusion of arriving material from a region by an intervening feature. A broad incident angular distribution can reduce shadowing but does not guarantee conformality.

Deposition from a heated or electron-beam-evaporated source. High-vacuum transport is strongly directional and can give poor recessed-feature coverage.

Film deposition through chemical reactions of gas-phase precursors at a surface. Surface kinetics and delivery through a boundary layer can separately limit growth.

A chemical vapor deposition route in which catalyst particles form or travel in the gas stream rather than remaining on a macroscopic support. For carbon nanotubes, particle nucleation, tube growth and aggregation can occur in one flowing reactor. Product withdrawal becomes possible when the growing network is sufficiently connected.

Continuous withdrawal and consolidation of a connected carbon nanotube aerogel from a synthesis reactor. It differs from later spinning of a prepared liquid dispersion and from drawing a pre-grown substrate-supported forest.^[ref]

Cyclic film deposition using separated, ideally self-limiting surface reactions. Precursor saturation and purge steps help control conformality and thickness.

Deposition in which a plasma activates gaseous precursors, permitting film growth at reduced substrate temperature. Plasma damage, composition and surface chemistry still need control.

Growth limited principally by surface reaction kinetics. Uniform substrate temperature and controlled reactant concentrations help obtain uniform thickness.

Growth limited principally by transport of reactants to a surface, rather than their surface reaction rate.

Photolithography transfers a pattern defined by light into a photosensitive photoresist coating. After exposure and development, the remaining photoresist forms a relief pattern that can protect selected regions during etching or another material-transfer step.^[ref]

A photomask can place bright and dark regions in the projected optical image. A positive photoresist develops more rapidly where exposed, so sufficiently exposed regions are removed; a negative photoresist retains the exposed regions instead. The final geometry depends jointly on the optical image and the material response. In particular, diffraction limits the image delivered by the optics, while photoresist contrast determines how a gradual variation of exposure is converted into a material boundary. Photolithography therefore connects wave optics, photochemistry and semiconductor device fabrication.

Aerial-image modulation measures the variation of optical intensity in the image delivered to a photoresist before its chemical and development response. A common contrast measure is $M=(I_{\max }-I_{\min })/(I_{\max }+I_{\min })$, for nonnegative intensities with nonzero denominator. Equal bright and dark intensities give $M=0$; a dark minimum with a positive maximum gives $M=1$.

For example, intensities $9$ and $1$ give $M=0.8$. Multiplying both by the same exposure factor leaves $M$ unchanged, although it changes the deposited dose and can change the printed pattern. Image modulation and photoresist contrast therefore describe different stages of photolithography: the optical signal and the material's conversion of that signal into a developed profile.^[ref]

A photomask is a patterned optical component used to define the illumination reaching a photosensitive layer in photolithography. Its pattern controls transmission, reflection or optical phase, depending on the imaging system. In projection lithography, optics form a reduced image of the mask on the substrate.^[ref]

The printed boundary depends on diffraction, illumination and photoresist response as well as the drawn mask edge. Optical proximity correction deliberately changes mask geometry to compensate for those effects. A mask feature is therefore part of an imaging-and-material process, rather than a guarantee that an identical outline will appear on the wafer.

Controlled removal of a material by a chemical, plasma or physical process. etch selectivity compares removal rates for different materials; directionality compares removal rates along different geometric directions. High selectivity does not imply a vertical profile, and high directionality does not guarantee an undamaged surface.

A durable patterned layer used to protect underlying material during an etch. Its thickness and etch selectivity must survive the intended depth; erosion and lateral bias can change final feature dimensions even when the starting pattern is correct.

Chemical stripping of organic material, commonly photoresist, by reactive species produced in a plasma. oxygen radicals convert suitable organic material into volatile oxidation products. A remote source reduces direct energetic ion bombardment, but exposed materials can still oxidize and inorganic residue can remain.

An etch that removes a material in different directions without a strongly preferred geometrical direction. It can undercut an overhang and release a sacrificial layer, but can also widen a patterned opening. Access and transport can make the realized undercut nonuniform even with locally isotropic chemistry.

Creating a dense final pattern through several less-dense exposures or through additional pattern-transfer and pitch-division steps. Overlay and etch control replace part of the burden that would otherwise fall on single-exposure optical resolution.

Mask-based optical pattern transfer across a small noncontact gap. Fresnel diffraction across the gap blurs edges; a common resolution estimate is $w=k\sqrt{\lambda g}$ for wavelength $\lambda$, gap $g$ and process factor $k$.

Imaging a patterned mask onto a resist-coated substrate using projection optics.

Splitting a final pattern between separate lithographic steps to reduce the feature density that each exposure must resolve. The steps need a suitable intervening fixation, transfer or spacer process. Minimum single-feature width, pattern decomposition and overlay remain limitations.

A sequence that exposes and transfers one sparse pattern, then applies fresh resist and transfers a second aligned pattern. Their union defines a denser final mask or etched layer. Pattern transfer between exposures prevents simple addition of optical dose from replacing two independently resolved exposures.

Pattern-dependent modification of mask edges or assist features to compensate the finite imaging and pattern-transfer process. It is assessed by the resulting printed dimensions, not by similarity of the mask to the desired layout.

Illumination using noncentral pupil directions to place useful mask diffraction orders inside a projection lens. Grating orientation, coherence and process window affect the benefit.

A lithographic mask that controls optical phase as well as transmission to improve useful interference contrast. Benefits and phase conflicts depend on the pattern.

Pattern projection using extreme-ultraviolet radiation, multilayer reflective optics and a reflective mask.

Direct pattern writing by a scanned electron beam in an electron-sensitive photoresist.

Unintended exposure near a written feature caused by electron scattering in the resist and substrate. The deposited-energy distribution is broader than the incident probe, requiring dose or geometry correction for dense and isolated patterns.

Small-angle scattering that broadens an incident electron probe as it travels through resist.

Adjustment of exposure dose or shape to compensate for energy deposited outside the intended write location.

Projection lithography using a liquid between the final lens and wafer to increase numerical aperture through its refractive index, with stringent optical and chemical compatibility requirements.

The ratio of etch rates of a target and a protected or underlying material. Endpoint control and surface damage matter as well as this ratio.

Patterning a deposited film by dissolving an underlying patterned sacrificial resist. Film in exposed openings remains while film on the resist is removed.

The axial image-side range over which a specified image criterion remains satisfied. Lithographic scaling is often written $\mathrm{DOF}=k_2\lambda /{\mathrm{NA}}^2$; specify whether a quoted range is full width or one-sided.

A radiation-sensitive patterning film. Positive resist becomes more soluble after suitable exposure; processing sets its dose response.

Selective dissolution of resist after exposure to reveal the written pattern.

A photoresist whose exposed regions dissolve faster in the developer than unexposed regions.

A photoresist whose exposed regions become less soluble in developer over the relevant process window, so adequately exposed regions remain after development.

Radiant energy per exposed area. For steady intensity $I$ and exposure duration $\tau$, $D=I\tau$; the developed photoresist profile also depends on chemical processing.

Photoresist contrast measures how sharply a photoresist changes its developed thickness as the exposure dose changes. For a positive photoresist, let $h(D)$ be the remaining thickness after a specified development process and $h_0$ the initial thickness. A contrast curve plots $h(D)/h_0$ against ${\log }_{10}D$. The measured photoresist contrast is the magnitude of the slope near clearing; it need not describe the entire curve.^[ref]

In an idealized model, the thickness decreases linearly with ${\log }_{10}D$ between a retention exposure dose $D_0$ and a clearing exposure dose $D_c$, where $0<D_0<D_c$. ThenOutside that interval the idealized thickness is clipped to $1$ or $0$. A large $\gamma$ means a smaller exposure dose ratio suffices to change from retention to clearing. This is why photoresist contrast influences the critical modulation for a photoresist; the inverse-logarithm formula assumes this particular contrast-curve model.

The critical modulation for a photoresist is the smallest aerial-image modulation that permits a common exposure time to clear bright regions while retaining full thickness in dark regions. Consider a positive photoresist with retention exposure dose $D_0>0$ and clearing exposure dose $D_c>D_0$. Write the extreme image intensities asFor exposure time $t$, the requirements are $t{I}_{\max }\ge D_c$ and $t{I}_{\min }\le D_0$. A feasible $t$ exists exactly whenwith $M=1$ handled directly since the dark-region exposure dose is zero. Rearrangement givesThis derivation assumes exposure dose is intensity times exposure time and uses fixed retention and clearing thresholds. In the idealized linear contrast-curve model of photoresist contrast, $D_c/D_0=10^{1/\gamma }$, soFor example, $D_c=4D_0$ gives $M_c=3/5$. Greater photoresist contrast lowers the required aerial-image modulation in this model.^[ref]