Semiconductor Nanocrystals Aka Quantum Dots Rachel Wooten February 28, 2006 Fat Tuesday! Transmission Electron Microscope image of CdSe nanocrystal

Outline What are semiconductor nanocrystals? How do we make them? How do they work? Applications: Why do we care?

Semiconductor Nanocrystals As the name implies, Semiconductor nanocrystals are tiny (generally fewer than 100 Angstroms in diameter) crystals of a semiconductor material.

How to make them Vapor deposition Ion implantation Sol-gel methods Micelle methods Organometallic synthesis

Why they work - Bohr exciton radius Roughly follows particle in a box model In bulk CdSe, absorption of a photon creates an electron hole pair Electron and hole maintain a characteristic distance known as the bulk Bohr exciton radius. This value depends on the material properties. For CdSe, this radius ~ 56 Å Time scale ~ microseconds

Bohr exciton radius When electron gets “kicked up” into the band gap, it physically separates from the hole it leaves behind. However, they don’t get any farther away from each other than the Bohr exciton radius due to Coulomb attraction Bohr radius is larger for semiconductors with higher dielectric constants (Coulomb field is hampered more by larger dielectric constant, so electron and hole “feel” each other less.)

Quantum confinement When nanocrystal has a diameter of less than 112 Å, electron and hole cannot achieve their desired separation.

Particles in a box Quantum Confinement Electron and hole become “particles in a box” because they cannot escape the outer walls of the crystal. The outer wall presents an infinite potential for practical purposes.

Tunable infinte spherical well Qualitatively same behavior as particle in an infinite square well As the crystal gets smaller, the energy of the first excited state increases; as is gets larger, the energy decreases. Small 1.7nm <<---------->>Large 7.0nm

The Brus Model Three assumptions 1. The nanocrystal is spherical. 2. The interior of the nanocrystal is a uniform medium; there are no point charges or occupied spaces other than the excited electron and hole 3. The potential energy outside the nanocrystal is infinite; thus the electron and hole are always found within the nanocrystal

An imperfect model Unfortunately, model only works well for larger nanocrystals The smaller nanocrystals have transition energies much higher than predicted in Brus’s model. * There are better models, but they are much more complicated

Applications Photovoltaics Single-electron transistors Fluorescent tags for biological imaging Surface paints and coatings Tunable Light Emitting Diodes (LEDs) The new lightbulb?

Photovoltaics Sunlight incident on semiconductor produces electron-hole pairs: move electrons laterally using a bias voltage. Relatively cheap, easy to produce However, currently very inefficient due to electron-hole recombination losses.

Single-elelctron transistors Because nanocrystals are tiny semiconductors, could be used in microscopic circuits Might be made by vapor deposition to lay them out regularly on a substrate (e.g. a circuit)

Fluorescent tags for biological imaging Attach bioactive molecule to single color nanocrystals and apply light to system. Nanocrystals fluoresce at their specific  making microscopic chemical processes visible. Simultaneous monitoring of multiple processes due to narrow, size dependent spectra. Fluorescent tags for biological imaging

Qdots beat organic molecules. Resistance to photodegradation, improved brightness, nontoxicity Size-dependent, narrow emission spectra allows simultaneous observation of multiple processes using different colors. The absorption spectra are continuous above band gap, so one light source excites different sized nanocrystals. (Extra energy absorbed into thermal motion!)

Surface Paints and Coatings Nanocrystals produced by organometallic synthesis have tri-octane chains attached to exterior cadmiums Dissolve in oil or plastics (or any non-polar liquid) Could be used as a fluorescent paint, especially for raves. Again, excellent color tuning.

Tunable LEDs Dots absorb high energy light and reemit light at a lower frequency that can be controlled by crystal size. They can also emit light when electron-hole pair produced by electrical stimulation Currently not producing true quantum dot LEDs, and coatings are cheaper, but true LEDs may yet be produced.

White light LEDs? Recently discovered that very tiny crystals (1.7nm-7nm) emit broad-spectrum white light rather than the expected narrow band very blue light. Huh? Thought to be due in part to high surface to volume ratio. Large crystals have many areas that see similar local potentials. In small crystals, all areas are different. +, smallest ones have high energy of containment

HERE IT IS !!!!! Three extra features seen. Maybe one peak is from surface effects, one from interior effects? The other? Needs some study