The Wisconsin Institutes for Discovery (Nano + Bio + Info) Nanoscience and Nanotechnology Architect’s view of this new high-tech building, which opened last fall on the UW-Madison campus between University Ave. and Johnson St, surrounded by the buildings for Biochemistry, Medical Sciences Engineering, Materials Science, Physics, and Computer Sciences – a truly interdisciplinary environment that is characteristic of nanoscience. The Wisconsin Institutes for Discovery (Nano + Bio + Info)

How small is a Nanometer ? Universe (largest length) 1027 10–35 Planck Length (smallest length) 10–3 m m 10–6  m 10–9 n m Taking a human as reference, the Sun is a billion times larger, a nanometer a billion times smaller. How small is a Nanometer ? A nanometer is hard to imagine, because it is so small. This slide gives a comparison to astronomy, where the distances become incredibly large. Here is another way to get a feeling how small a nanometer is: While you are reading this sentence (in a few seconds), your fingernails grew by a nanometer. This can be verified by a simple back-of-the-envelope calculation: How often do you cut your fingernails? By how much? Say every 3 weeks by 1 mm: Then the nails grew with the velocity of 1 mm / 3 weeks = 1 mm / 3x7x24x60x60 sec = 1 mm / 1.8 million sec ~ 1 nanometer / 2 sec

How small is a Nanometer ? Each panel 10x smaller Each time something different Two more steps to reach one nanometer A molecular memory project at Hewlett Packard (Palo Alto) demonstrates the many orders of magnitude in size between a nanostructure and a macroscopic device. One has to do this nine times to go from a meter to a nanometer, and every time something new appears. 125 nm Hewlett-Packard molecular memory

Fundamental Length Scales Consider devices operating at room temperature. Their energy levels have to be separated by more than the thermal energy kBT = 25 meV . Convert this energy scale to a length scale by fundamental constants. Quantum Electric Magnetic Quantum Well: Quantum Well Laser Capacitor: Single Electron Transistor Magnetic Particle: Data Storage Media l E1 E0 Physicists like to search for fundamental laws that involve fundamental length scales generated by fundamental constants (h, e, me,…). In nanotechnology, they ask: What’s so special about dimensions of a few nanometers, as opposed to a few micrometers or millimeters? In fact, there are three fundamental length scales that can be obtained from fundamental constants, such as the mass m and the charge e of the electron. These originate from three different parts of physics and all converge onto the single digit nanometer regime: Quantum physics becomes important at room temperature in nanometer-sized objects (such as colorful nanocrystals and the quantum well laser). Single electron transistors start operating at room temperature if one makes the smaller than about 10 nanometers (we are not quite there yet). Magnetic storage media lose their memory at room temperature if one makes the magnetic particles smaller than about 5 nanometers (“Arizona mailbox test” for hard disks). The only part that is not fundamental about these length scales is the requirements of room temperature operation, with a thermal energy kBT  25 meV. But that is important for us. a = V1/3 d Energy Levels 3h2/8m l2 Charging Energy 2e2/ d Spin Flip Barrier ½ M2a3 l < 7 nm d < 9 nm a > 3 nm

Quantum Length Scale: The Quantum Well Laser The quantum well laser and LED (light emitting diode) are ubiquitous, from CD/DVD readers to laser pointers and traffic lights. A quantum well laser traps electrons and holes in a material with smaller band gap, which gives them more time to find each other and recombine into a photon. The optimum thickness of this layer is only 6 nanometers. 6 nm optimum thickness, comparable to the electron wavelength

Confinement in Nanocrystals Nanocrystals demonstrate how the properties of a material change, when the electrons and their wave functions are boxed into a region comparable to their wavelength. The band gap changes, and with it the color of the emitted light. Smaller particles have a larger band gap and emit blue light, while large particles emit red light, which is characteristic of normal CdSe. Quantum confinement : Crystal size determines the color (blue-shifted when smaller)

When Does Silicon Cease to be Silicon When Does Silicon Cease to be Silicon? The band gap of silicon nanoclusters GaAs The band gap between the occupied valence band and the empty conduction band of a semiconductor determines not only the color (as in the previous example), it also is crucial for the performance of a transistor. In principle, one could increase the band gap of silicon to that of gallium arsenide (a high-performance, but expensive semiconductor) by making it small enough. However, this does not happen at the dimensions of today’s silicon transistors, which are built with 40 nanometer technology. One would have to get down to 3 nanometers. This possibility of changing the behavior of a material by just changing its size bring back the dream of the medieval alchemists: If we can make silicon to behave like GaAs, why not make lead behave like gold? In principle, this can be done by manipulating the wave function of the electron with nanotechnology, but in practice it has not been achieved yet. However, it has been possible to make gold behave like platinum the element right next to it in the Periodic Table. While gold is inert in bulk form, it becomes a catalyst like platinum when dispersed into clusters of a few atoms onto an oxide surface. It could be that the oxygen removes an electron from the gold and thereby changes the number of electrons to that of platinum, but that remains to be confirmed. A goal of the modern alchemist is the ability to use abundant, inexpensive 3d transition metals (such as iron) as catalysts instead of expensive 5d and 4d transition metals (such as platinum, rhodium, and ruthenium). This would be a breakthrough not only for catalytic converters in cars, but also for splitting water to create fuel from solar energy or for dye-sensitized solar cells. Bulk Silicon 3 nm : Gap begins to change

Electric Length Scale: Single Electron Transistor dot Vg  e/Cg N-½ N+½ Cg e- Vg N N-1 Electrons on the dot To add one extra electron to the dot requires the voltage Vg  e/Cg (Cg= Qg/Vg = gate/dot capacitance). This costs the charging energy UC = eVg = e2/Cg . The smaller the dot, the smaller the capacitance, the larger the charging energy per electron. This effect can be observed when the charging energy becomes larger than the thermal energy: UC > kBT For room temperature operation (kBT = 25 meV) this happens when the dot is smaller than  10 nm . This picture is for a small negative source voltage (and an equally small positive drain voltage) which draws electrons from source to drain. Both voltages are assumed to be negligible compared to the gate voltage.

Magnetic Length Scale: Superparamagnetic Size Limit Barrier E Energy When ferromagnetic particles become smaller than about 10 nm , they lose their magnetism over time (a few years at room temperature). The thermal energy can flip all the spins in a particle (see Lect. 24, Slide 10). If one measures magnetic hysteresis loops (Lect. 23, Slide 4) at various temperatures, they switch rather suddenly from ferromagnetic loops to paramagnetic lines at a temperature called the blocking temperature. This looks like the magnetic phase transition at the Curie temperature, but it depends on how fast the measurement is performed. The spin flip rate is given by an Arrhenius law which describes thermally-activated hopping across an energy barrier E. Contrary to other thermally-activated processes, such as diffusion, the attempt frequency is not related to vibrations, but to spin rotation (Larmor frequency in the internal magnetic field).