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Nanochemistry

NAN 601

Instructor:

Dr. Marinella Sandros

Quantum Dots

 Polymers, either in solution for coating thin films or as particles for construction  Metal nanoparticles and nanowires • Semiconductor nanospheres, rods, wires, tetrapods • Carbon nanotubes 4

 Quantum dots are semiconductors whose excitons are confined in all three dimensions of space.

 Quantum dots have properties combined between ◦ Those of bulk semiconductors ◦ Those of atoms 5

 A material with electrical conductivity due to electron flow.

 A substance whose resistivity lies between that of a conductor and an insulator. The resistance of a semiconductor decreases as the temperature increases.

 As the temp. of the semiconductor is increased, these electrons gain more energy. Some gain enough energy to break free of their bonds, and wander through the piece of material.  Once an electron moves out of a bond, it leaves behind a ‘hole’ in that bond 8

  This hole is positive, and so can attract nearby electrons which then move out of their bond etc.

Thus, as electrons move in one direction, holes effectively move in the other direction Electron moves to fill hole As electron moves in one direction hole effectively moves in other 9

 There is a difference between conduction in metals and semiconductors, in metals conduction is due solely to movement of electrons, in semiconductors it is due to movement of negative electrons and positive holes.

   States in a bulk semiconductor are so closely spaced that the conduction and valence bands appear to be continua. The number of these states per unit volume in an energy band is described by the density of states.

Because no available states exist within the bandgap, the density of states is measured from the bottom of the conduction band and the top of the valence band from which it is, for a bulk semiconductor, a continuously increasing function with carrier energy . 11

  Electrons in conduction band (and holes in the valence band) are free to move in all three dimensions of space.

So their energy spectrum is almost continuous, and the density of allowed electron states-per-unit energy increases as the square root of the energy B.E.A. Saleh, M.C. Teich. Fundamentals of Photonics. fig. 16.1-10 a smooth square root dependence and 16.1-29.

  Electrons in conduction band (and holes in the valence band) are free to move in two dimensions.

Confined in one dimension by a potential well.

◦ ◦ Potential well created due to a larger bandgap of the semiconductors on either side of the thin film.

Thinner films lead to higher energy levels.

• Quantum wells are formed in semiconductors by having a material, like gallium arsenide sandwiched between two layers of a material with a wider bandgap, like aluminium arsenide.

B.E.A. Saleh, M.C. Teich. Fundamentals of Photonics. fig. 13.1-11 and 16.1-29.

Distinct Steps: with steps occurring at the energy of each quantized level

 Thin semiconductor wire surrounded by a material with a larger bandgap.

◦ Surrounding material confines electrons and holes in two dimensions (carriers can only move in one dimension) due to its larger bandgap.

B.E.A. Saleh, M.C. Teich. Fundamentals of Photonics. fig. 16.1-29.

   Electrons and holes are confined in all three dimensions Discrete energy levels (artificial atom) Like bulk semiconductor, electrons tend to make transitions near the edges of the bandgap in quantum dots.

3D 2D 1D 0D 15 B.E.A. Saleh, M.C. Teich. Fundamentals of Photonics. fig. 16.1-29.

Nontraditional Semiconductors Quantum Dots, also known as nanocrystals, are a non-traditional type of semiconductor with limitless applications as an enabling material across many industries. Quantum dots have a specified, unique composition and size that give them novel quantum properties.

Shortcomings of Traditional Semiconductors Traditional semiconductors have a shortcoming- they lack versatility. Their optical and electronic qualities are costly to adjust, because their bandgap cannot be easily changed. Their emission frequencies cannot be easily manipulated by engineering. 17

• • There exists a forbidden range of energy levels in any material called the band gap.

The electrons in bulk (much bigger than 10 nm) semiconductor material have a range of energies. 18

• One electron with a different energy than a second electron is described as being in a different energy level, and it is established that only two electrons can fit in any given level. • In bulk, energy levels are very close together, so close that they are described as continuous, meaning there is almost no energy difference between them. 19

• It is also established that some energy levels are simply off limits to electrons; this region of forbidden electron energies is called the bandgap, and it is different for each bulk material. • Electrons occupying energy levels below the bandgap are described as being in the valence band. • Electrons occupying energy levels above the bandgap are described as being in the conduction band 20

1. In natural bulk semiconductor material, an extremely small percentage of electrons occupy the conduction band the overwhelming majority of electrons occupy the valence band, filling it almost completely. 2. The only way for an electron in the valence band to jump to the conduction band is to acquire enough energy to cross the bandgap, and most electrons in bulk simply do not have enough energy to do so. 3. Applying a stimulus such as heat, voltage, or photon flux can induce some electrons to jump the forbidden gap to the conduction band. 4. The valence location they vacate is referred to as a hole since it leaves a temporary "hole" in the valence band electron structure.

1. Electrons in natural semiconductor bulk that have been raised into the conduction band will stay there only momentarily before falling back across the bandgap to their natural, valence energy levels. 2. As the electron falls back down across the bandgap, electromagnetic radiation with a wavelength corresponding to the energy it loses in the transition is emitted. 3. The great majority of electrons, when falling from the conduction band back to the valence band, tend to jump from near the bottom of the conduction band to the top of the valence band in other words, they travel from one edge of the bandgap to the other. 4. Because the bandgap of the bulk is fixed, this transition results in fixed emission frequencies. 5. Quantum dots offer the unnatural ability to tune the bandgap and hence the emission wavelength.

• The average distance between an electron and a hole in a exciton is called the Excited Bohr Radius.

• When the size of the semiconductor falls below the Bohr Radius, the semiconductor is called a quantum dot.

 ◦ ◦ ◦ Very small semiconductor particles with a size comparable to the Bohr radius of the excitons (separation of electron and hole).

Typical dimensions: 1 – 10 nm Can be as large as several μm.

Different shapes (cubes, spheres, pyramids, etc.) 24

  The energy levels depend on the size, and also the shape, of the quantum dot.

Smaller quantum dot: ◦   Higher energy required to confine excitons to a smaller volume.

Energy levels increase in energy and spread out more.

Higher band gap energy.

Figures are from “Quantum Dots Explained.

” Evident Technologies. 2008.

Because quantum dots' electron energy levels are discrete rather than continuous, the addition or subtraction of just a few atoms to the quantum dot has the effect of altering the boundaries of the bandgap. Changing the geometry of the surface of the quantum dot also changes the bandgap energy, owing again to the small size of the dot, and the effects of quantum confinement. 26

3D Nanocomposites, cellular materials, porous materials, nanocrystal arrays, block co-polymers 2D Quantum wells, superlattices, membranes 1D Nanotubes, nanowires, nanorods 0D Nano dots from the gas phase Colloids and nanoparticles by other methods 27

Semiconductors

derive their great importance from the fact that their electrical conductivity can be greatly altered via an external stimulus (voltage, photon flux, etc), making semiconductors critical parts of many different kinds of electrical circuits and optical applications.

Quantum dots

are a special class of materials known as semiconductors, which are crystals composed of periodic groups of II-VI, III-V, or IV-VI materials. Quantum dots are unique class of semiconductor because they are so small, ranging from 2-10 nanometers (10-50 atoms) in diameter. At these small sizes materials behave differently, giving quantum dots unprecedented tunability and enabling never before seen applications to science and technology.

 Quantum dots are ordered collections of hundreds to thousands of semiconductor- type atoms.

 The electrons associated with a dot are confined to this small set of atoms.

 On the nano-scale, when the electron is confined, the change in energy levels becomes distinctly discrete – a condition known as “quantum confinement”.

 Therefore the band gap is a function of size.

 When there are lots of atoms, such as in a traditional semiconductor, the bandgap is not a function of size.

 The significance of this nanoproperty is that different size quantum dots will fluoresce in different colors.

The goal is to elaborate a method of synthesis, which a) is reproducible b) yields monodisperse nanoparticles c) produces „perfect“ particles d) may control the shape of the particles e) is easy, cheap The chemical methods are either based on the kinetic control of nucleation and growth of the particles, on electrostatic stabilization in (aqueous) suspension, or on the introduction of spatial constraints. The latter include particle formation within or at the interface of micelles, vesicles, or bilayer lipid membranes, within the channels of zeolites, in interlayers of clay, in peptides, or in biological cells.

The stages of nucleation and growth for the preparation of monodisperse NCs in the framework of the La Mer model. As NCs grow with time, a size series of NCs may be isolated by periodically removing aliquots from the reaction vessel.

An organometallic method is used for the fabrication of highly monodisperse cadmium selenide nanocrystal quantum dots.

Nucleation and subsequent growth of QDs occurs after a quick injection of metal and chalcogenide precursors into the hot, strongly coordinating solvent such as a mixture of trioctylphosphine (TOP) and trioctylphosphine oxide (TOPO) in the case shown. After a fixed period, removing the heat source stops the reaction. As a result, NQDs of a particular size form.

Size- and material-dependent emission spectra of several surfactant-coated semiconductor nanocrystals in a variety of sizes. The blue series represents different sizes of CdSe nanocrystals with diameters of 2.1, 2.4, 3.1, 3.6, and 4.6 nm (from right to left). The green series is of InP nanocrystals with diameters of 3.0, 3.5, and 4.6 nm. The red series is of InAs nanocrystals with diameters of 2.8, 3.6, 4.6, and 6.0 nm.

 http://www.youtube.com/watch?v=VjznErmcLnU 41

 Name a difference between bulk and QDs semiconductors in terms of their energy band level?

 In bulk, energy levels are very close together, so close that they are described as continuous, meaning there is almost no energy difference between them.  