Imagine a future in which your iPhone 20 charges itself as you walk down the street and your house is powered by photoelectrochemical (PEC) cells installed in your backyard, using the power of the sun to convert captured rainwater into clean hydrogen. If we have any hope of realizing this future, you can be certain that nanowires will play a significant role. So what are nanowires, and why are they so important?
My previous blog post  was dedicated to examining a class of objects known as topological materials, with an emphasis on the bizarre realm of topological insulators (TIs). Implicit in the definition of topological insulators in the previous post was the assumption that the bulk of this insulator behaves as one would expect from its name: preventing the flow of electric current. Real-world TIs are not quite so simple, however. The bulk of TIs is often somewhat conductive due to the presence of material defects, negating the purpose of the insulator (to prevent the flow of electric current). As a result, one principal objective of topological materials research is to minimize the influence of the bulk. There are various ways to accomplish this, one of which being to shrink everything down to minuscule scales. The incentive to reduce bulk influence has given rise to nanowires, one of the most promising geometric configurations of topological materials. Nanowires are structures that have diameters on the order of nanometers (a nanometer is 0.000000001 meters, which is very small), with variable lengths. These minuscule wires are known for possessing a diversity of fascinating properties, making them exceptionally versatile and, thus, extremely useful for industrial, electronic, and other technological applications, from dramatically increasing transistor efficiency to creating a synthetic tooth enamel that mimics the real thing exceptionally well . Not all nanowires are topological materials, however. Topological and non-topological nanowires alike have a wide range of applications. During my time at IU, our group was focused on “growing” nanowires and testing their electrical/magnetic/thermal properties in order to look for interesting physics that occurs on such scales. For the purpose of this post, I will focus on three of these properties and how each endows nanowires with certain advantages when compared to bulk materials, enabling them to have a wide range of applications in the real world.
Maximize Surface Area to Volume Ratio
In order to minimize bulk, one must maximize the surface area to volume ratio of the topological material (the nanowire, in this instance). By doing so, one also maximizes the effects of the topological surface states. Consider a cylinder, a shape approximating that of a nanowire: the ratio of surface area to volume is inversely proportional to the diameter. In other words, to minimize bulk, it is best to minimize diameter. One application for which the enhanced surface properties of nanowires are well-suited is in the production of environmentally-friendly energy via a range of processes known as solar water splitting. One specific example of this process in which nanowires play a key role is photoelectrochemical (PEC) water splitting. PEC cells use certain materials to convert solar energy into chemical energy by splitting water into oxygen and, crucially, hydrogen. This process, however, can be very inefficient. The exceptionally large surface area to volume ratio provided by a quasi-one-dimensional nanowire dramatically increases the available space for surface reactions; namely, the reactions necessary for water separating the oxygen from the hydrogen. More space on which these reactions can occur translates to greater efficiency in the water splitting process.
Oftentimes, unique nanowire properties arise out of an effect known as quantum confinement. Quantum confinement describes a shift in certain properties of a material, namely electronic and optical, when the dimensions of size are sufficiently squished down to the nanometer scale. Generally, quantum confinement effects are auxiliary to electronic nanowire usage; nanowires that are already in use as a result of their unique surface states, flexibility, etc., gain an additional favorable “boost” from confinement. This is the case for certain nanowire biosensing devices. Biosensing is the process by which biomolecules are detected using some sort of analytical device. Nanowires have been shown to be effective biosensors for detecting small amounts of pathogenic microorganisms and other organic compounds . Due to confinement effects present in a certain species of these wires, they show a “strong enhancement of performance characteristics such as mobility, drive current, and current density…as a result of increased quantum confinement” . A disposable electrode device containing polyaniline nanowires was recently shown to be effective in the rapid detection of a highly lethal shrimp virus, known as White Spot Syndrome . This tool has the potential to dramatically reduce the spread of the virus by quickly identifying infected shrimp, preventing it from quickly wrecking entire farming operations.
Induced Strain Capabilities
When a nanowire is flexed, the lattice structure of the material is changed. This flexibility can be used for harvesting electricity from random mechanical motion accomplished by certain nanowires, such as piezoelectrics (materials that generate internal electric charge from an induced strain). Materials such as these have the potential to be used for powering mobile electronic devices, among other things. Nanowires, in particular, are excellent candidates for the production of so-called “nanogenerators” due to “their high mechanical robustness and responsiveness to tiny random mechanical disturbances/stimulation” . The process works as follows: a compressive force is applied to the nanowires, creating a piezoelectric field and producing a momentary flow of free electrons that naturally accumulate. Once the compressive force terminates, the accumulated electrons are released and the process is free to repeat. In short, an electrical potential is created through the application and subsequent relinquishment of a form of mechanical stress imposed on the nanowires, resulting in an alternating current in an external circuit.
Recently, tech giant Amazon partnered with the piezoelectric microphone startup Vesper , making it a distinct possibility that the Alexa of tomorrow will use their extremely durable, voice recognition microphones powered by piezoelectric zinc oxide nanowire arrays.
I hope it is evident by now that nanowires are an exceedingly interesting nanomaterial configuration that are only going to be increasingly integrated into our technologically developing world. The market size for nanowires is expected to double by 2027 . Due to their unique shape, minuscule size, and robust mechanical properties, one thing seems certain: nanowires are extraordinarily cool.
This post was written with suggestions and guidance from Dr. Shixiong Zhang. The author acknowledges the National Science Foundation for funding support.
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