Nano: Practically Invisible

This video, shared by a Twitter follower studying biomedical communication, is one of my new-found favorites for the simple communication of the concept of ‘nano’. The animation journeys from the macroscopic world of a human hair, through the micro-scopic world of the blood cells that carry oxygen through our veins, down to the world of nanomaterials, objects smaller even than the wavelength of the very light by which we can ‘see’ the world around us.

QD_S.jpg Nano-objects, typically classified as measuring between 1 and 100 nanometers, or 1/1,000,000,000 meters, in at least one dimension (height, width, or depth), are smaller than the wavelength of visible light. This means, as the video points out, that nanoscale objects are practically invisible. But this doesn’t mean that nano-objects don’t leave their traces in the world around us…

Many nanomaterials, despite being much smaller, Engineering at Cambridge.jpg interact very strongly with light as we know it. Noble metal nanomaterials, like those made out of silver or gold, do so in a phenomenon known as surface plasmon resonance, or in the case of nano-sized spheres or nanoparticles of gold or silver, localized surface plasmon resonance. Essentially, small noble metal particles or sharp noble metal tips have free electrons at their surfaces that resonate_: they oscillate togetherbench/2011/04/15/big-physics-for-small-science when met with light of a particular frequency, or as we notice it, color. These plasma oscillations are what give gold nanoparticle solutions their brilliant colors, and what allow light to be channeled through metal nano-wires much smaller than the wavelength of that light, where the wavelength is the width of the traveling light wave. Through surface plasmons, light can be concentrated down to a sharp tip, for example, that is an order of magnitude smaller than the wavelength of the light.

Lauwrence Berkeley Nat Lab Silicon Nanowires.jpg So, are nano-objects truly invisible_? Although we cannot see them with our naked eye, and even cannot resolve them with optical microscopes (precisely because they are smaller than the wavelength of light used in the microscope), nano-objects of various materials leave strong signs of their presence in the world as we see it. Because nanoparticles made of metal or semi-conductor materials display such amazing and unique interactions with light, they are being used as biosensors, light-capturing materials for solar panels, probes that guide the way to sites of disease in the human body, interconnects in ultra-fast computersbench/2011/07/22/light-logic-for-light-ning-fast-computers, and maybe even as key materials in invisibility cloaks!

integrated circuit.jpg


• Do You Know What Nano Means? Artist Daniel Gies, Science Alberta Foundation.
Quantum dots, Wiki Commons. “Colloidal quantum dots irradiated with a UV light. Different sized quantum dots emit different color light due to quantum confinement.”
Probe for surface-plasmon-enhanced spectroscopy, Flickr Engineering at Cambridge
• Nanowires of silicon and titanium oxide, Flickr Lawrence Berkeley National Laboratory: Highly dense vertical arrays of nanowires made from silicon and titanium oxide and measuring 20 microns in height show promise for the efficient production of hydrogen through solar power water splitting.
• Integrated Nanowire Sensor Circuitry, Flickr Lawrence Berkeley National Laboratory: Scientists at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory and the University of California at Berkeley have created the world’s first all-integrated sensor circuit based on nanowire arrays, combining light sensors and electronics made of different crystalline materials.
Baile Zhang, Yuan Luo, Xiaogang Liu, & George Barbastathis (2010). Macroscopic Invisibility Cloak for Visible Light Physical Review Letters 106, 033901 (2011) arXiv: 1012.2238v3