We live in a digital age…but what does that mean? I don’t believe people are aware of the full extent that the coined term implies. And hey, that’s okay–there’s enough stuff out there to worry about in practical daily life before asking a pointed What is Fire? type of question. People are happy to use their tiny mobile phones and without even needing to ponder how central those magical boxes are to so many varieties of energy transfer. Energy transferrals which all achieve a common goal of information exchange. We can propose that “digital” implies microchips processing electrical pulses of a set voltage, antennae passing along microwave bursts, and fiber optics passing along pulses of light. All three passing along discrete “bits” of information just like Morse code. But what happens if we expand our sphere of thought, and change our goal from information exchange to something else? Hmm…first let’s settle on what it means to be thinking digital.
From classic studies of information transfer and exhange two terms emerged: analog and digital.[1] In an analog system, you vary the intensity of the signal along with its position in space/time continuously. Imagine that you have an amplifier and speaker system hooked up to a record player (phonograph) and a compact disc player. Imagine the zig-zag grooves in a vinyl record (I’m imagining Zappa in particular). With the aid of a needle on a phonograph and the right speed of rotation (in the right direction too–don’t want any subversive messages from those albums just yet), the signal translates as an analog stream of information that you hear directly as sound. But in a (binary) digital system, you form the signal into discrete ON/OFF elements and place them at discrete periodic places in space/time. Now imagine the dots and dashes (“pits”) etched into a compact disc. With the aid of a laser and a microchip (converting the bit coding given a known rate of data sampling), the signal translates as audio pulses of information that you recognize as sound.
But why don’t we hear the pulses? Because, if you squeeze the pulses together close enough–the two signals are equivalent. “Peaches en Regalia” will have the same information content for both players (admittedly, without the romance of vinyl white noise). Your ears hearing the pulses will tolerate the steps in exactly the same way that your eyes tolerate the watching rapidly changing frames in the movie theatre as continuous motion. In other words, by reducing the spacing of discrete pulses beyond a certain threshold, the aural neurons in your ear cannot tell the difference between a digital source and a continuous analog source.
But almost more important than that good quality sound or picture is the fact that the dots on a compact disc are not truly discrete! Whoa, take a minute to breathe as I just changed the perspective. Are the raised areas (“pits”) on a compact disc really square, with sharp changes to ON and OFF? Well, no–but the laser reading the pits will tolerate the fuzzy edges of the steps as an acceptable level of “noise”. By making the intensity of the CD pulses above a certain threshold (deep pits), and by keeping the size and spacing of dots larger than a second threshold (broad, well spaced pits) to separate the fuzzy edges, the photodiode in your compact disc player also cannot tell the difference between a continuous analog and a true digital source either. This process even has a name and a formula, the Nyquist-Shannon sampling theorem: for a band-limited signal the sampling frequency must be greater than twice the input signal bandwidth in order to reconstruct the original perfectly from the sampled version. In other words, we can make perfect systems out of imperfect parts as long as we scale down to a certain threshold.
And that brings us to considering another form of “digital”. Digital materials assembly; used in the sense that discrete units of a compound material (primary clusters of atoms as “bits”, to extend the analogy) are self-assembled to form a new product having unique collective attributes distinct from those of the original “bit”. Something like this is being investigated on macroscopic scales at SQUID labs, and it would be really groovy if one could assemble nanoscale digital materials. The reason being, you can pack a whole lot of signal into an itty-bitty volume. The main contribution to the signal in many nanoscale materials is at the surface, and when you shrink something down to the size of molecules (<5nm), the material is almost all surface.
But wait a second, nanoscale digital materials? That rings a bell–what about DNA? Yes, DNA is an old-school nanoscale digital material (like prehistoric…like Precambrian) using four bits (the molecules Adenine and Thymine, Guanine and Cytosine) that link together in two types of pairings (A with T and G with C) in a double chain of bit-pairs with periodic spacings. Okay, so we have a “proof of concept” for nanoscale digital materials that has been road-tested for around 3.5 billion years. I think we can step up to the plate and work around this concept in research now.
This change in perspective from “analog” to a “bit-wise” assembly of materials allows the researcher to apply concepts from the Digital Revolution to materials design. From this perspective, the researcher is aware that all of the nanomaterial parts being used to assemble a new structure are imperfect and “analog” in form (just like the pits in a compact disc, the primary clusters have unfulfilled surface bonds, and distorted structures). But the nanomaterial parts are also very much digital, and the goal would be for those particles to self-assemble into a new material with unique properties different from the individual unit.
As it turns out, we have another tangible example of that as well. These materials are called photonic crystals, and the most prevalent example is the naturally occurring gemstone: opal (although researchers are actively pursuing other synthetic materials and routes of assembly). Opal is a naturally occurring colloidal crystal composed of SiO2 (silica) spheres (hundreds of nm in diameter) packed together in an ordered 3-D array, and surrounded by molecular water. By itself, silica is optically transparent and is the basis for ordinary window glass. But packed together in hydrated spheres, the path of light is affected and distorted to reveal an unusual pastel rainbow play-of-colors. Even a thin film of this nanostructured material can be used to change the way that light propagates–and it’s nothing like glass.
So let’s return to expanding our sphere of thought about digital applications. What other goals can we achieve from nanoscale digital materials other than information transfer? Perhaps we can assemble a material of nanoscale proportions to efficiently extract electrons and holes from a light absorbing matrix and effectively shuttle them to ohmic contacts, as in a photovoltaic device? We could envision getting several-times the proverbial bang for an invested buck in these systems. Ideally, one can foresee increasing the surface-charge area, increasing the efficiency of charge separation, while also reducing the volume of material that we need to use to construct a solar cell. Maybe the same principles could be translated into building better energy storage devices (e.g. batteries and ultracapacitors) as well. It often strikes me that those would be some pretty cool advanced photovoltaics and battery technologies–kind of like switching from a coiled-filament light bulb in a flashligt to using an intensely bright laser in a pointer. To put things in perspective, would you have thought 50 years ago when the first coherent light systems were being made in research labs, that the results would lead to the pocket laser diodes today? What are the possibilities, and where could “thinking digital” take us in the next 50 years of solar cell and battery research?
Note: many of the links provided come from the HyperPhysics site at the Dept. of Physics and Astronomy at Georgia State University. HyperPhysics is a robust science education source for physical phenomena and their applications in our lives.
[1] Claude Shannon. A mathematical theory of communication. The Bell System Technical Journal, 27:370–423, 623–656, July, October 1948.
