The spectrometer is a powerful measuring tool that is essential in all branches of science. Astronomers afix them to satellites so they can scan the depths of space, doctors use them to detect problems like intracranial bleeding and farmers even use them to determine the quality of their harvests. This versatile and essential tool has now been tweaked and modified using nanotechnology. The result? A spectrometer so small it could be used over your phone camera, so light it can be sent into space without spending more on fuel, and so inexpensive that it only requires a few dollars for the raw materials.
Designed to measure the properties of light, the spectrometer has gone through lots of changes over the years. The earliest models were prisms that would separate beams of light into its constituent wavelengths. Now, the MIT team have replaced the prisms with quantum dots. The new spectrometer is the smallest of its kind; it is currently about the size of a quarter.
Quantum dots are tiny, nanometer-sized crystals made from combining metals like lead or cadmium with other elements, such as arsenic or sulfur. The dots can be given the ability to 'observe' a unique wavelength by changing the ratio of the starting elements, as well as the temperature and reaction times. These variations change a property in each dot known as 'bandgap.'
Bandgap measures how much energy it takes for an electron to jump from one electron shell to a shell with higher energy. The electrons jump between shells by absorbing a light particle, or photon. The wavelength of the photon determines how much energy is fed into the quantum dot, thus dictating whether an electron can jump its bandgap or not. A quantum dot with a large bandgap can absorb a photon with a tiny wavelength, like UV or X-rays. A quantum dot with a small bandgap, however, can absorb light with long wavelengths, such as infrared. So each different quantum dot can absorb a different wavelength of light while letting all other wavelengths pass through without a fuss.
Hundreds of these quantum dots, each with a different bandgap value to absorb a different wavelength of light, are assembled onto a thin film and placed atop a photodetector. This photodetector could, for example, be the camera on your phone.
There is a saying, "the more, the merrier," and this certainly applies to quantum dots. Cramming more types of dots into the spectrometer means that a larger number of wavelengths can be detected. At the moment, the researchers have used around 200 different types of dot, and they are spread over a teeny space of 300 nanometers. This is about three times as large as the human immunodeficiency virus (HIV).
The idea is that this tiny film could be placed over a phone's camera. When filtered light is detected by the camera, a program would analyze the percentage of photons absorbed by each filter. The program could then trace backwards to the wavelength of the original rays of light.
The potential uses for this tiny spectrometer are vast, especially in remote locations. It could be used to analyze urine or diagnose skin conditions like cancer, for example.