Dogs are often used to sniff out the presence of explosives and drugs by the police. Now, at the Electrical Engineering department of Indian Institute of Technology, Bombay, research is on to develop an “electronic nose” which can avoid this altogether. Based on the process of inelastic quantum tunnelling, the “nose” can differentiate between molecules based on their vibrational energies.
How do we smell?
There are two competing theories that try to explain olfaction, or the action of smelling – the shape theory and the vibration theory. According to the shape theory, the shape of a particular molecule acts as a “key” and fits into the “lock” in the nose, thereby stimulating it to perceive a smell. The vibration theory, on the other hand, proposes that the nose senses not the shape but the vibrational energy of the odorant molecule. Perhaps the actual mechanism is even a combination of the two processes, While the argument goes on, the IIT Bombay researchers have showed that, using inelastic quantum mechanical tunneling, it is possible to construct an artificial “nose” that differentiates between different odorant molecules by sensing their vibrational energies. Quantum effects can get washed out at room temperature, but they propose a way that is sturdy even at room temperature.
The nose they have proposed is actually a one-dimensional “resonant tunnelling diode” – a quantum device that conducts only when it is excited by special discrete values of electrical energy. Essentially this is a modification of a quantum wire – a linear array in which the electrons can move in one dimension only. “Our device takes a quantum wire a little further in a modification without which we do not see room-temperature operation. It adds a double barrier structure — with a [potential] well in the middle — in the transport direction as well,” says Swaroop Ganguly, one of the authors of the paper published in Scientific Reports.
A double potential barrier (like two barricades) is created on the quantum wire, which confines the electrons further to be enclosed in a small region. It is known in quantum mechanics that this allows the confined electrons to possess discrete values of energy. This device selectively allows electrons with specific resonant incident energies to tunnel through the barriers.
“The resonant tunneling diode shows a peak in the current but does not shut down completely away from the peak. The way the odorant molecule information enters is through the creation of a vibrational mode that modulates the current, specifically by opening up an inelastic channel for tunnelling wherein the electron loses energy in the form of vibration,” explains Prof. Ganguly. Normally such vibrational signatures are washed out at higher temperatures, but the paper shows that for the proposed device they would persist at room temperature.
Making the device involves fabricating a one-dimensional semiconductor device with a double barrier, using standard semiconductor fabrication technology. “This is the approach we are embarking on now, with my colleague Dipankar Saha, also a co-author on this paper, who fabricates these really beautiful one-dimensional structures here in IIT Bombay’s Nanofabrication Facility,” Prof. Ganguly adds.
