A quantum dot is -

Correct — option (4), A semiconductor nanostructure.

Wikipedia's Quantum dot article (September 2021 revision) opens: "Quantum dots (QDs) are semiconductor particles a few nanometres in size, having optical and electronic properties that differ from larger particles due to quantum mechanics."

The article calls them zero-dimensional: electrons or holes are confined in all three dimensions. That confinement is why a dot's colour of emitted light depends on its size.

So a quantum dot is a semiconductor nanostructure.

The idea to remember: a quantum dot is a semiconductor crystal a few nanometres across, confined in all three dimensions.

Why the others are wrong

• (1)
  Electron microscopy image of nanostructures smaller than 1 nanometre — This describes a picture, not an object. A quantum dot is a physical semiconductor particle, used in devices such as displays.

  The size is also off. Wikipedia describes quantum dots as a few nanometres across and gives examples of 2–3 nm and 5–6 nm dots, larger than 1 nanometre.

• (2)
  Nanoscale analog of radio antennas — This describes a nantenna (nano-antenna). Wikipedia's Optical rectenna article (September 2021 revision) says the term is sometimes used for either an optical rectenna or an optical antenna by itself.

  A rectenna combines an antenna and a diode to turn electromagnetic waves into direct current; an optical rectenna would do this with visible or infrared light. A quantum dot is a semiconductor particle, not an antenna.

• (3)
  A fictional nanorobot — Quantum dots are real materials, not fiction and not robots. Wikipedia says they were first synthesised in a glass matrix by Alexey Ekimov in 1981.

  The same article records a commercial use: the first was Sony's XBR X900A series of flat-panel televisions, released in 2013.

When a semiconductor crystal is made small enough, its electrons are squeezed into a tiny space. Wikipedia calls this quantum confinement: the allowed energy levels then depend on the crystal's size.

Structures are named by how many dimensions are confined. A quantum well confines electrons or holes in one dimension, a quantum wire in two, and a quantum dot in all three.

Because of this, quantum dots have discrete, atom-like energy states and are sometimes called artificial atoms.

Key facts

• Quantum dots are semiconductor particles a few nanometres in size (Wikipedia, September 2021 revision).
• A quantum dot confines electrons or holes in all three dimensions; a quantum wire confines them in two, a quantum well in one.
• Larger dots (5–6 nm) emit longer wavelengths such as orange or red; smaller dots (2–3 nm) emit blue or green.
• Wikipedia says quantum dots were first synthesised in a glass matrix by Alexey Ekimov in 1981; the term was coined in 1986.
• Wikipedia names Sony's XBR X900A televisions, released in 2013, as the first commercial application of quantum dots.

Common traps

• An image is not the object: option (1) describes a microscope picture, while a quantum dot is a physical semiconductor particle.
• Dot, wire, well: count the confined dimensions — three for a dot, two for a wire, one for a well.
• Bigger dot, redder light: a larger quantum dot emits a longer wavelength, not a shorter one.

• practice — not a real PYQ

  In a quantum wire, electrons are confined in how many spatial dimensions?

  • (a)One
  • (b)Two
  • (c)Three
  • (d)None
  Answer(2) — Wikipedia says quantum wires confine electrons or holes in two spatial dimensions and allow free movement in the third. Option (1) describes a quantum well, option (3) a quantum dot, and option (4) a bulk material with no confinement.
• practice — not a real PYQ

  Compared with a 2–3 nm quantum dot of the same material, a 5–6 nm quantum dot emits light of –

  • (a)shorter wavelength, such as blue
  • (b)longer wavelength, such as red
  • (c)the same wavelength
  • (d)no visible wavelength at all
  Answer(2) — Wikipedia says larger 5–6 nm dots emit longer wavelengths such as orange or red, and smaller 2–3 nm dots emit shorter wavelengths such as blue and green. Option (1) reverses this, and options (3) and (4) ignore the size effect.