(Nanowerk Spotlight) Water molecules make up the essence of life on Earth, playing a crucial role in countless biological and chemical processes. Each water molecule, with its positively charged hydrogen atoms and negatively charged oxygen, acts as a tiny magnet that can interact with and transport other materials. At the same time, advances in nanotechnology have produced quantum dots – semiconductor particles mere billionths of a meter in size that exhibit unique electrical and optical properties. Despite their promise for next-generation electronics, quantum dots face a fundamental limitation: the electrical charges they generate tend to stay trapped within individual particles instead of flowing freely where needed.
Now, researchers at Zhejiang University in China have combined these two elements in an innovative way. They created electronic light sensors that use water’s molecular properties to enhance the performance of quantum dots. This approach solves a key problem that has limited quantum dot devices – the difficulty of moving electrical charges between dots efficiently.
“The interactions between solid quantum dots are weak as the excitons in quantum dots are difficult to be dissolved into electrons and holes, which limits the performance of quantum dots based photodetector,” the researchers explain in their paper published in Advanced Functional Materials (“Liquid Water Molecular Connected Quantum Dots for Self-Driven Photodetector”). Their solution suspends quantum dots in water, allowing water molecules to help separate and transport the electrical charges that quantum dots generate when exposed to light.
Schematic diagrams of the device structure. (Image: Adapted with permission by Wiley-VCH Verlag)
The research team built a device that places this quantum dot solution between two electrical contacts – a single-atom-thick layer of carbon called graphene on one side and a semiconductor called N-type gallium arsenide on the other. When light strikes quantum dots in the device, it creates pairs of negative electrons and positive holes (known as excitons). The surrounding water molecules help split these pairs apart and create pathways for the charges to flow to the electrical contacts.
Using quantum dots made of molybdenum disulfide suspended in water, the team’s light sensor showed remarkable sensitivity to light. The device achieved what scientists call “responsivity” of 188.1 milliamps per watt – meaning that for each watt of light power hitting the sensor, it produces 188.1 milliamps of electrical current. This measurement tells us how efficiently the device converts light into electrical signals. The sensor also scored well on “detectivity,” a measure of its ability to detect very faint light signals while ignoring background noise, reaching 1.164 × 1010 Jones (the standard unit for this measurement).
To put these specifications in practical terms, such sensitivity could enable improved medical imaging devices that need less intense light to produce clear images, reducing patient exposure to radiation. The sensor could also enhance the performance of fiber optic communications, where detecting faint light signals accurately is crucial for transmitting data over long distances.
The researchers also found they could tune which wavelengths of light the device responds to most strongly by using different types of quantum dots. With cadmium selenide quantum dots, for example, they could create sensors that react particularly strongly to specific colors of light. This tunability could prove valuable in applications like environmental monitoring, where sensors need to detect very specific wavelengths of light to identify particular chemicals or pollutants.
The most striking results came from tests with cadmium selenide quantum dots. These dots can be precisely manufactured to absorb specific wavelengths of light. The water-based device showed especially strong performance improvements exactly at these target wavelengths, suggesting that manufacturers could customize such sensors for specific applications by selecting appropriate quantum dots.
The mechanism behind this performance relies on several complementary effects. Water molecules not only help separate the initial electron-hole pairs but also form organized channels that help transport these charges. The quantum dots act as relay stations along these channels, maintaining efficient charge transport over longer distances than previously possible in solid devices.
The researchers investigated how various factors affect device performance. They found that increasing the concentration of quantum dots improved sensitivity up to a point before leveling off. Similar behavior occurred with increasing light intensity, helping define the practical operating range of such devices.
This work opens new possibilities for quantum dot technology by treating water as an active component rather than just a carrier medium. The ability to tune the spectral response while maintaining high sensitivity could prove valuable for applications ranging from optical communications to medical imaging. The research also demonstrates a new approach to creating efficient pathways for charge transport in quantum dot devices – a fundamental challenge that has limited their practical use.
Get our Nanotechnology Spotlight updates to your inbox!
Thank you!
You have successfully joined our subscriber list.
Become a Spotlight guest author! Join our large and growing group of guest contributors. Have you just published a scientific paper or have other exciting developments to share with the nanotechnology community? Here is how to publish on nanowerk.com.
