A typical radio has four main components: an antenna, a tuner, an amplifier and a demodulator. The antenna picks up radio signals; information like a voice or song is encoded into these signals by modulating the frequencies (FM radio) or amplitudes (AM radio) of the so-called carrier waves. When you change the dial on your car radio to tune in a specific station, the tuner is detecting a specific frequency of the carrier wave. Once that is accomplished, the radio’s amplifier increases the strength of the signal, making the details more apparent. The demodulator then decodes the information.
The carbon nanotube in Zettl’s radio, which is able to detect both AM and FM signals, takes on all four of these roles. To do so, a nanotube typically less than 0.005 of a millimetre long (and one millionth as thin as that) is anchored to a metal electrode that functions as a negative pole. A short distance away from it sits a positively charged metal electrode. Connecting a battery between the electrodes shoots current through the conductive nanotube to its tip. But the electrons won’t make the jump to the positively charged electrode about a micrometre away unless the conditions are just right. Instead they stop at the tip, creating an electric field. If the nanotube is exposed to an electromagnetic radio wave in a specific wavelength, the tip will begin to vibrate up and down rapidly.
Here, the nanotube is acting as an antenna. Changing the length of the nanotube tunes the nanoradio — the shorter the tube, the higher the frequency at which it will vibrate. As the nanotube antenna vibrates, the distance between the tip and the positive electrode changes. When voltage is applied, the tip of the nanotube ejects electrons in a phenomenon called field emission. This ejection is easier when the nanotube is closest to the nanoradio’s positive electrode because the electrons are more likely to jump across this shorter distance. Changing this distance affects how many electrons make it to the other side; the current flowing through the circuit represents the information coded in the carrier wave, essentially decoding it.
The carbon nanotube thus functions as an antenna, tuner and demodulator. But it’s also the final necessary component of any radio: the amplifier. The strength of the current sent to the speaker corresponds to the voltage the battery sends to the nanotube. Simply hooking up a larger battery, or more of them, to the nanoradio can generate a stronger signal.
The electrons will jump from the tube’s tip only in a vacuum, so the entire apparatus must be built into a an airtight capsule for it to work. For now, millimetre-size batteries power the radio, but in the future, Zettl predicts, “the radio will be housed in a nanoscale boron-nitride vacuum chamber, which can be inserted into a cell. The biochemistry of the cell itself will be tapped for the energy source, so no battery will be needed.”
From nanoscale to nanoradio
Zettl and his colleagues at COINS stumbled on the nanoradio while devising extremely sensitive scales from nanotubes. Scales and radios may not seem to have much in common, but both work according to the same principles at a nano level. Zettl’s nanoscale is physically almost identical to his radio. Like the radio, the scale’s nanotube vibrates to an electromagnetic radio signal. The difference is that the scale vibrates to a constant carrier wave without any extra information.
