New materials: semiconductors, superconductors and nanomaterials

The first principle

What a material does depends on how its electrons are allowed to move. In a solid, electrons can occupy only certain bands of energy, and the gap between the filled band and the next empty one decides whether the solid conducts like copper, blocks current like glass or sits in between like silicon. Change the arrangement of atoms, cool the material until its electrons pair up, or shrink it until only a few hundred atoms remain, and the rules for those electrons change, which is why semiconductors, superconductors and nanomaterials behave so differently from ordinary matter.

The core

Semiconductors

• Bands and the band gap. In a solid the energy levels of countless atoms merge into bands. The highest filled band is the valence band and the next one up is the conduction band, and only electrons in the conduction band, or vacancies in the valence band, carry current. In a metal the highest occupied band is only partly filled, or overlaps the next one, in an insulator such as diamond the gap is wide (about 5.5 electronvolts), and in a semiconductor such as silicon it is small (about 1.1 electronvolts), so heat or light can lift some electrons across.
• Heat works the other way round. A heated metal conducts less as vibrating atoms scatter electrons, while a pure semiconductor conducts more as heat lifts electrons across the gap.
• Doping makes semiconductors useful. Adding a few atoms of phosphorus, which has one more outer electron than silicon, gives free electrons and an n type semiconductor; adding boron, with one fewer, leaves vacancies called holes and gives a p type semiconductor. Doping changes conductivity by many orders of magnitude and lets engineers place exactly where current can flow.
• The p–n junction forms where p type and n type regions meet. Electrons and holes cross and cancel near the boundary, leaving a thin depletion layer with a built in electric field, so current flows easily one way and hardly at all the other. This makes a diode; light falling on the junction drives a current, which is a solar cell; electrons and holes meeting across the junction give out light, which is a light emitting diode (LED); and the same doping, with a voltage on a gate opening a channel between doped regions, makes the transistor, as the chips note explains.
• Wide band gap semiconductors such as gallium nitride (about 3.4 electronvolts) and silicon carbide (about 3.3 electronvolts) withstand higher voltages and temperatures than silicon and switch faster. They are used in fast chargers, electric vehicle inverters, 5G base stations and radar, and gallium nitride made the blue LED possible, for which Isamu Akasaki, Hiroshi Amano and Shuji Nakamura received the Nobel Prize in Physics in 2014.

Superconductors

• Zero resistance. In 1911 Heike Kamerlingh Onnes found that mercury cooled below about 4.2 kelvin (K) loses all electrical resistance, so a current set going in a superconducting ring flows for years without a power source. The temperature below which this happens is the critical temperature.
• The Meissner effect. A superconductor also pushes a magnetic field out of its interior, which is why a magnet floats above a cooled superconductor. This expulsion, not zero resistance alone, marks true superconductivity, and a strong enough field or current destroys the state.
• Why it happens. In conventional superconductors, explained by the Bardeen–Cooper–Schrieffer theory of 1957, electrons bind into Cooper pairs through tiny distortions of the crystal lattice, and the pairs move together as a single quantum state that the lattice cannot scatter.
• High temperature superconductors. In 1986 Georg Bednorz and Alex Müller found superconductivity in a copper oxide ceramic, and within a year yttrium barium copper oxide was superconducting at about 93 K, above the 77 K of cheap liquid nitrogen. No material has been confirmed to superconduct at room temperature and ordinary pressure; the claim for a material called LK-99 in 2023 failed when other laboratories tested it.

Nanomaterials

• The nanoscale is roughly 1 to 100 nanometres, where a nanometre is a billionth of a metre, about ten atoms across. A human hair is tens of thousands of nanometres wide and the double helix of deoxyribonucleic acid (DNA) about 2.
• Surface takes over. As a particle shrinks, a larger share of its atoms sit on the surface, so nanoparticles react, dissolve and catalyse far faster than the same mass in bulk; this is why nanosilver kills microbes and why nano catalysts need less metal.
• Quantum confinement changes properties with size. When a particle is smaller than the natural spread of its electrons, its energy levels are squeezed apart, so a cadmium selenide quantum dot glows a different colour at each size; gold nanoparticles look red for a related reason, the collective oscillation of their surface electrons. Moungi Bawendi, Louis Brus and Alexei Ekimov received the Nobel Prize in Chemistry in 2023 for discovering and making quantum dots, now used in displays and medical imaging.
• Carbon at the nanoscale. Fullerenes, cage molecules such as C60, were found in 1985; carbon nanotubes are rolled sheets of carbon, very strong and, depending on how they are rolled, metallic or semiconducting; and graphene, a single layer of carbon atoms in a honeycomb, isolated in 2004 by Andre Geim and Konstantin Novoselov (Nobel Prize in Physics 2010), is among the strongest materials measured, conducts heat and electricity extremely well, and is almost transparent, but it has no band gap, which limits its use in transistors.

Dimensions

• Economic. Semiconductors underpin electronics, solar power and electric vehicles, and the materials that go into them, such as high purity silicon, gallium and germanium, are now traded and controlled like strategic goods.
• Strategic. China's export controls on gallium and germanium from 2023 showed how a few materials can become chokepoints, a theme the economy notes take up under critical minerals.
• Energy and climate. Wide band gap chips cut losses in power conversion, and superconducting cables and magnets promise lossless transmission and fusion.
• Health. Nanoparticles can carry drugs to tumours and improve diagnostics, but their small size also lets them pass into the lungs, blood and cells in ways larger particles cannot, which is the concern of nanotoxicology.
• Environmental. Nanoparticles from sunscreens, textiles and fertilisers reach soil and water, where their long term behaviour is poorly understood.
• Regulatory. Rules judge a chemical by its composition, but a nanomaterial of the same composition can act very differently, so bulk chemical rules miss its risks.

In practice

Uses

• Semiconductors are in every phone, computer and LED lamp, and in solar cells, where silicon remains dominant and perovskites are the leading new material, as the renewable energy note explains. India's chip programme sits in the chips note.
• Superconductors make the strong magnets of magnetic resonance imaging (MRI) scanners, usually niobium–titanium wire cooled by liquid helium, as well as particle accelerators such as the Large Hadron Collider and the magnets of tokamaks such as India's Steady State Superconducting Tokamak (SST-1). Japan's superconducting maglev trains and the qubits of many quantum computers use them too.
• Nanotechnology is used in sunscreens (titanium dioxide and zinc oxide), antibacterial coatings (silver), water filters, stronger composites for sports goods and aircraft, cancer drugs carried inside tiny fat bubbles called liposomes, and the lipid nanoparticles that carry messenger RNA (mRNA) vaccines into cells.

India's programmes and rules

• The Nano Mission of the Department of Science and Technology (DST), approved in 2007 with Rs 1,000 crore for five years, funded research centres, training and the Institute of Nano Science and Technology (INST) at Mohali. The Ministry of Electronics and Information Technology (MeitY) supports nanofabrication facilities at the Indian Institute of Science (IISc) Bengaluru and the Indian Institute of Technology (IIT) Bombay, opened to researchers across the country through the Indian Nanoelectronics Users Programme.
• Nano fertilisers. The Indian Farmers Fertiliser Cooperative (IFFCO) launched liquid nano urea in 2021 to replace part of granular urea with a spray. A two year field trial at Punjab Agricultural University found that, compared with conventional nitrogen, it lowered wheat yield by about 21.6 per cent and rice yield by about 13 per cent and reduced grain nitrogen, which shows why such products need independent, multi season trials before they are scaled up.
• Regulation. The Guidelines for Evaluation of Nanopharmaceuticals in India were issued in 2019 by the Department of Biotechnology (DBT), the Indian Council of Medical Research (ICMR) and the Central Drugs Standard Control Organisation (CDSCO), but there is no general law or regulator for nanomaterials in consumer goods, food or the environment.

Issues and way forward

• Cooling cost. Most practical superconductors need scarce, costly liquid helium, so wider use depends on high temperature materials made into reliable wire and on cheaper cooling.
• Lab to market. Graphene has been studied for two decades but few mass products use it, because large, defect free sheets are hard to make cheaply; India's research output in nanoscience is strong, but its manufacturing of nanomaterials is small, so pilot plants and industry partnerships are needed.
• Material chokepoints. Gallium, germanium and rare earths come mostly from one country, so recycling, stockpiles and domestic refining are part of technology security.
• Unknown risks. Toxicity tests for nanoparticles are still being standardised and products carry little labelling, so India needs nano specific safety tests, labels and monitoring in soil and water.
• Hype and evidence. Claims such as room temperature superconductivity or a highly efficient nano fertiliser should be judged by independent replication before promotion or subsidy.

Common confusions

• Semiconductor and superconductor. A semiconductor conducts a little and can be controlled by doping and voltage, while a superconductor conducts with zero resistance below a critical temperature; one is the basis of electronics, the other of strong magnets and lossless current.
• Perfect conductor and superconductor. A material with zero resistance alone would trap whatever magnetic field it had, while a superconductor actively expels the field, the Meissner effect.
• High temperature superconductor. "High" means above about 30 to 77 K, still far below room temperature, so these materials still need liquid nitrogen cooling.
• Nanotechnology and miniaturisation. Making things smaller is not by itself nanotechnology; the point is that below about 100 nanometres new properties appear from surface and quantum effects.
• Graphene and graphite. Graphite in a pencil is millions of graphene layers stacked; a single layer has properties that the stack does not.

For the exam

• Keyword bank: band gap, valence and conduction bands, doping, p–n junction, wide band gap semiconductor, critical temperature, Meissner effect, Cooper pairs, liquid nitrogen cooling, nanoscale, surface area to volume ratio, quantum confinement, graphene, nanotoxicology, Nano Mission.
• Previous year questions: Practise the previous year questions on this topic →
• Use in an answer:
  • Introduction with data: Silicon's band gap of about 1.1 electronvolts, small enough for heat and light to cross yet large enough to switch off, is why one element carries almost all of modern electronics.
  • Introduction with a source: The Nobel Prize in Physics 2010 for graphene and the Nobel Prize in Chemistry 2023 for quantum dots both rewarded the same idea, that shrinking a material to a few atoms changes what it can do.
  • Conclusion: India has strong research in new materials but small manufacturing and weak regulation, so the next step is to turn laboratory results into products while testing nanomaterials for safety before they are scaled up.
  • Conclusion: Control over materials, from gallium for chips to helium for magnets, is now part of technological sovereignty, so materials science deserves the same mission mode support as space or semiconductors.