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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. 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