Consider the following statements for Nano-technology : 1. It is the technology of creating materials and devices atom-by-atom. 2. Physical properties change at the nanometer scale. 3. Chemical properties change at the nanometer scale. Which of the above statements are correct ?

Correct — D, (d) 1, 2 and 3.

ALL THREE STATEMENTS ARE CORRECT, and they are not three separate facts. They are the definition of nanotechnology followed by the two reasons the nanoscale is worth having a technology about at all.

STATEMENT 1 — 'It is the technology of creating materials and devices atom-by-atom.' This is the standard definition. A nanometre is one billionth of a metre, and the nanoscale is conventionally taken as roughly 1 to 100 nanometres — a range in which a structure is built of a countable number of atoms rather than of bulk matter. The field has two routes to that scale. The TOP-DOWN route carves structures out of a larger block, as photolithography does when it patterns a silicon wafer. The BOTTOM-UP route assembles them from atoms and molecules upward, through chemical self-assembly or by direct manipulation. Statement 1 describes the bottom-up ambition, and it is the description examinations use, because it is what distinguishes nanotechnology from ordinary miniaturisation: the aim is control at the level of the individual atom. That control became real when the scanning tunnelling microscope arrived in 1981, which could not only image individual atoms but push them about.

STATEMENT 2 — 'Physical properties change at the nanometer scale.' They change dramatically, and this is not a small correction to bulk behaviour. Melting point falls as particles shrink, so gold nanoparticles a few nanometres across melt hundreds of degrees below the 1064 degrees Celsius of bulk gold. Optical behaviour changes: a suspension of gold nanoparticles is red or purple rather than gold-coloured, which is why medieval glassmakers could get ruby glass from gold without knowing why. Mechanical strength rises. Magnetic behaviour changes, with particles below a certain size becoming superparamagnetic. In semiconductor quantum dots the band gap itself depends on the size of the dot, so the colour a dot emits can be tuned by making it bigger or smaller.

STATEMENT 3 — 'Chemical properties change at the nanometer scale.' The reason is geometry. As a particle shrinks, its surface area falls as the square of its size while its volume falls as the cube, so the SURFACE-AREA-TO-VOLUME RATIO rises steeply and an ever larger fraction of the atoms sit on the surface with unsatisfied bonds. Surface atoms are the reactive ones, so reactivity and catalytic activity rise sharply. The standard illustration is gold again: bulk gold is a noble metal, chemically inert, which is why it does not tarnish — yet gold particles a few nanometres across are active catalysts for the oxidation of carbon monoxide at low temperature. Aluminium, harmless as foil, is used as a nanoparticle additive in rocket propellant because at that size it burns readily.

Statements 2 and 3 together are the whole point of the field. The nanoscale matters because it is where a material stops behaving like the bulk substance and starts behaving in a way that depends on its SIZE.

The stem prints 'Nano-technology' hyphenated with a capital N while statements 2 and 3 print 'nanometer' as one lower-case word; that inconsistency is the paper's own and the text is reproduced as set. The three statements are followed straight away by the question sentence, with no codes line printed.

Why the others are wrong

• (a)1 and 2 only — This accepts the definition and the change in physical properties but drops statement 3, the change in CHEMICAL properties — and statement 3 is true for a reason that can be stated in one line. When a particle shrinks, its surface area falls as the square of the linear size while its volume falls as the cube, so the proportion of its atoms lying on the surface rises steeply. Surface atoms have unsatisfied bonds and are the chemically active ones, so a nanoparticle is far more reactive than the same mass of bulk material. The most quoted case is gold: inert enough as bulk metal to be the standard of chemical nobility, yet an effective low-temperature catalyst when divided to a few nanometres. Nanoscale titanium dioxide, nanoscale silver used for its antimicrobial action and nanoscale aluminium in propellants are all in use precisely because their chemistry changed with their size. A candidate who thinks only of strength, melting point and colour has remembered the physics of the nanoscale and forgotten that the chemistry moves with it.
• (b)1 and 3 only — The mirror image of the previous option: it accepts the definition and the chemical change but drops statement 2, the change in PHYSICAL properties. That change is if anything the more spectacular of the two. Melting point falls sharply with particle size, so nanoscale gold melts hundreds of degrees below bulk gold. Optical properties change, which is why colloidal gold is red rather than gold-coloured and why the same metal can colour glass. Magnetic behaviour changes, with sufficiently small particles losing their permanent magnetisation and becoming superparamagnetic. In semiconductor quantum dots, quantum confinement makes the band gap depend on the dot's diameter, so the wavelength of light emitted is set by the size of the particle and not only by what it is made of. Electrical and thermal conduction, hardness and tensile strength all shift too. Any of these on its own establishes statement 2.
• (c)2 and 3 only — This one accepts both property changes and rejects statement 1, the definition itself, which is the hardest of the three to argue against. Creating materials and devices atom by atom is exactly how the field describes its own aim, from Richard Feynman's 1959 lecture on manipulating matter at the smallest scale to the coining of the word 'nanotechnology' by Norio Taniguchi in 1974 and the demonstration in 1989 that individual xenon atoms could be positioned deliberately with a scanning tunnelling microscope. The only quibble available is that a great deal of practical nanotechnology is TOP-DOWN — lithography cutting features into a wafer rather than building them up from atoms — but even top-down work is aimed at atom-level control, and the bottom-up description is the one every syllabus uses. Rejecting statement 1 also leaves the item incoherent, since statements 2 and 3 are about a scale that statement 1 is what defines.

NANOTECHNOLOGY is the understanding and control of matter at dimensions of roughly 1 to 100 nanometres, where size-dependent phenomena make a material behave unlike its bulk form. A NANOMETRE is 10 to the power minus 9 metres — a billionth of a metre. For scale, a human hair is about 80,000 nanometres across, a DNA double helix about 2 nanometres wide and a single gold atom about a third of a nanometre.

THE TWO ROUTES TO THE NANOSCALE. TOP-DOWN — start from bulk material and remove or pattern it down to nanoscale features. Photolithography and etching in semiconductor manufacture are the industrial example. BOTTOM-UP — start from atoms and molecules and build upward, through chemical synthesis, self-assembly, or the direct positioning of atoms with a probe. This is what statement 1 of the question describes.

WHY PROPERTIES CHANGE AT THIS SCALE — two mechanisms, and it is worth keeping them apart. SURFACE-AREA-TO-VOLUME RATIO. Area scales as the square of a length and volume as the cube, so halving a particle's size doubles its surface area per unit mass. Below about 100 nanometres a large fraction of all the atoms present are surface atoms, and surface atoms carry unsatisfied bonds. This drives the change in CHEMICAL behaviour: higher reactivity, higher catalytic activity, higher adsorption. QUANTUM CONFINEMENT. When a particle becomes comparable in size to the wavelength associated with its electrons, the electron energy levels are no longer effectively continuous, and the band gap of a semiconductor comes to depend on the size of the particle. This drives much of the change in PHYSICAL behaviour: optical, electronic and magnetic properties that vary with diameter.

FAMILIAR NANOMATERIALS. Carbon nanotubes — rolled sheets of graphene, with tensile strength far above steel at a fraction of the weight. Graphene — a single atomic layer of carbon in a hexagonal lattice, isolated in 2004 by Geim and Novoselov, who received the Nobel Prize in Physics in 2010. Fullerenes, of which the sixty-carbon buckminsterfullerene is the best known. Quantum dots — semiconductor nanocrystals whose emission colour is set by their size. Nanoscale silver, used for its antimicrobial action, and nanoscale titanium dioxide and zinc oxide, used in sunscreens because at that size they stop scattering visible light and become transparent while still blocking ultraviolet.

THE INSTRUMENTS THAT MADE THE FIELD POSSIBLE. The SCANNING TUNNELLING MICROSCOPE, built by Gerd Binnig and Heinrich Rohrer at IBM Zurich in 1981, which images and moves individual atoms on a conducting surface and won its inventors the Nobel Prize in Physics in 1986. The ATOMIC FORCE MICROSCOPE, which extended the technique to non-conducting surfaces. Without instruments that can see and touch single atoms, nanotechnology would have remained a thought experiment.

APPLICATIONS ACROSS SECTORS. Medicine — targeted drug delivery, contrast agents, diagnostics. Energy — better catalysts, electrodes and photovoltaics. Water — nanofiltration membranes and arsenic and fluoride removal. Electronics — transistor features now measured in nanometres. Materials — stronger composites, self-cleaning and scratch-resistant coatings, stain-resistant textiles.

THE CONCERNS ARE REAL AND EXAMINABLE. The same high reactivity and small size that make nanoparticles useful also let them cross biological barriers, so nanotoxicology and questions of occupational exposure, environmental release and regulation travel with the technology.

IN INDIA, the Department of Science and Technology runs the NANO MISSION, launched in 2007 as an umbrella capacity-building programme covering basic research, infrastructure, human resources and application-oriented projects. It followed an earlier Nanoscience and Technology Initiative begun in 2001.

Key facts

• A nanometre is one billionth of a metre; the nanoscale is conventionally 1 to 100 nanometres.
• Nanotechnology is the design and creation of materials and devices with atomic or molecular control, by top-down patterning or bottom-up assembly.
• Physical properties change at the nanoscale: melting point falls, optical and magnetic behaviour changes, and strength rises.
• Chemical properties change because the surface-area-to-volume ratio rises steeply, putting a large fraction of atoms on the reactive surface.
• Bulk gold is chemically inert, but gold nanoparticles are active catalysts for carbon monoxide oxidation — the standard demonstration that chemistry changes with size.
• Quantum confinement makes the band gap of a semiconductor quantum dot depend on its diameter, so its emission colour is set by size.
• The scanning tunnelling microscope, built by Binnig and Rohrer at IBM Zurich in 1981, made imaging and moving single atoms possible; they shared the 1986 Nobel Prize in Physics.
• The word 'nanotechnology' was coined by Norio Taniguchi in 1974; Richard Feynman's 1959 lecture is treated as the field's conceptual starting point.
• Graphene, a single atomic layer of carbon, was isolated in 2004 by Geim and Novoselov, who received the 2010 Nobel Prize in Physics.
• India's Nano Mission was launched by the Department of Science and Technology in 2007 as an umbrella capacity-building programme.

Common traps

• Assuming that in an all-true statement list at least one item must be wrong. Here every statement is correct and the answer is the option naming all three.
• Remembering the physical changes at the nanoscale and forgetting that the chemistry changes too, which is what option (a) is built to catch.
• Treating nanotechnology as a synonym for miniaturisation. Making something small is not the point; the point is that behaviour changes at that scale.
• Confusing the nanoscale with the microscale. A micrometre is a thousand nanometres, and bacteria are micrometre-sized objects, not nanoscale ones.
• Assuming a nanoparticle behaves like the bulk material it is made of. Inert bulk gold and catalytic nanoscale gold are the same element.
• Reading the absence of a 'select using the codes given below' line as a printing error. About a dozen items on this booklet are set that way.

• practice — not a real PYQ

  Nanoparticles are chemically far more reactive than the same mass of bulk material principally because

  • (a)they carry a permanent electric charge
  • (b)a much larger fraction of their atoms lie on the surface
  • (c)their atoms are smaller than the atoms of bulk material
  • (d)they are always made of noble metals

  Answer(b) a much larger fraction of their atoms lie on the surface — surface area falls as the square of the linear size while volume falls as the cube, so the surface-area-to-volume ratio rises steeply as a particle shrinks, and surface atoms with unsatisfied bonds are the reactive ones. The atoms themselves are unchanged in size, which is why option (c) is impossible.

• practice — not a real PYQ

  India's Nano Mission, an umbrella capacity-building programme for nanoscience and nanotechnology, is run by which one of the following ?

  • (a)Ministry of Electronics and Information Technology
  • (b)Council of Scientific and Industrial Research
  • (c)Department of Science and Technology
  • (d)Indian Council of Medical Research

  Answer(c) Department of Science and Technology — the Nano Mission was launched under the Department of Science and Technology in 2007, following the earlier Nanoscience and Technology Initiative of 2001.