The Art of Quantum Dots and Flat Material

Scope Correspondent

Think of chemistry and you probably imagine a lab filled with beakers and flasks, bubbling with colorful liquids in a variety of hues.  There’s an element of this in Will Tisdale’s chemical engineering lab, which overlooks a courtyard on the MIT campus dominated by a huge red modernist sculpture.  Here, his team mixes together chemicals and boils them in oxygen-free environments to make a substance called quantum dots.

But in the Tisdale lab, these dots are often just a means to an end.  On a dreary December afternoon, two lab members, Ferry Prins and Aaron Goodman, shined lasers on colorful quantum dots to explore how they share energy and electrons with a two-dimensional chemical called molybdenum disulfide.

In the best partnerships, the strengths of each contributor make for a sum that is greater than its parts.  Tisdale’s lab is exploring whether what’s true of people might also be true of these two popular materials.  If they’re right, quantum dots and molybdenum disulfide could enhance each other’s ability to conduct electricity and energy, possibly leading to better light-emitting diodes or light detectors.

Quantum dots are nanometer-scale particles (we’re talking billionths of a meter long) made of semiconductor materials.  But unlike bulk semiconductors—a slab of silicon, say—each quantum dot is very good at capturing and emitting a very narrow set of wavelengths of light, allowing for extremely precise color.  A fluorescent rainbow can be made from the same reaction pot; different size dots will emit different wavelengths of light, meaning that creating a wide variety of colors is fairly easy.  Because their fine-tuned wavelengths allow for an ever-larger number of individual colors, quantum dots are popping up in a variety of commercial products, such as extra-sharp color televisions.

Once the members of the Tisdale lab make a thin layer of dots on top of a slab of a different material, they shine pulses of laser light on the quantum dot layer, which gives the dots a jolt of energy.  Next, they follow up with a second laser aimed at the other material.  That second laser will change color if it picks up the energy given off by the quantum dots.  This laser technique, called ultrafast spectroscopy, can thus measure whether energy has been transferred from the quantum dots to the other material.  Although these experiments probably won’t lead directly to the newest light detector or brightest TV, a basic scientific understanding of how quantum dots move energy and charge around is important for designing more efficient devices.

The second member of the duo under investigation by the Tisdale lab is molybdenum disulfide (MoS2), a rare breed of material.  Unlike most solids, which have a three-dimensional structure, MoS2 is perfectly happy to exist as a two-dimensional sheet.  Two-dimensional materials are so unusual that graphene, a honeycomb of carbon molecules that is the best known example of a flat structure, won its discoverers the 2010 Nobel Prize in Physics.  Whereas graphene is good enough at conducting electricity to act like a metal, MoS2 is a semiconductor and can even replace silicon in transistors.  MoS2 has another useful property: its electrons can exist in different energy states.  Providing energy to those electrons by shining light on them can change how well the electrons flow in the material, something that doesn’t happen in graphene.  But MoS2 and graphene do share a couple of characteristics besides their flat structure.  Both are strong and can absorb light.

It turns out, though, that MoS2 isn’t very good at absorbing light. Thus, despite its neat properties of being thin, transparent, and good at conducting electricity, it’s not a shoo-in for use in devices like light detectors.  So now the Tisdale lab is investigating what happens when MoS2 is combined with quantum dots, which are extremely good at capturing and emitting a very narrow set of wavelengths of light.  The hope is that the strength and flatness of MoS2 and the good light properties of quantum dots would not only make MoS2 better at capturing and sending out light, but also make quantum dots themselves more efficient.  Currently, quantum dot-only light emitting diodes drop in efficiency with higher current running through them.

To test this idea, Ferry Prins, a postdoctoral fellow in the Tisdale lab, looked under the microscope.  When MoS2 flakes, which fluoresce naturally, are layered over quantum dots, they emit much less light.  This suggests that either electrons or energy are transferred between the flat molybdenum layer and the quantum dots.  The question is:  which is it?

Prins and graduate student Aaron Goodman hoped that a different laser technique, ultrafast microscopy, might help them solve the puzzle.  In the lab’s black-lined laser room, there are two large air tables, specially built to reduce vibrations and stabilize the lasers and other sensitive equipment resting on the surface.  Normally, separate experiments run on the two tables.  One of them supports a complex system of lasers, mirrors and lenses for ultrafast spectroscopy, and the other holds microscopes. For this experiment, Prins and Goodman used both tables, shining the laser’s bluish light from one table to the other in order to try a combination of the two techniques.  While ultrafast spectroscopy can resolve fast events in time, adding microscopy allows for a higher spatial resolution to see what is going on at exactly the points where MoS2 and quantum dots intersect.  Unfortunately, the fluorescence of the MoS2 is so weak compared to that of the quantum dots that it decays too quickly to be measured by this technique.

Since that day, Goodman has found a workaround, by exploring what happens to MoS2 when it is under different types of quantum dots—those that can transfer energy to MoS2, and those whose energies are too low to excite it.  These new experiments suggest that MoS2 and quantum dots might be cooperating after all.  So there’s still a chance that someday a material made out of a MoS2/quantum dot union might find its way into actual devices in the real world.