Light-Emitting Heterostructures with Transition-Metal Dichalcogenides

Imagine a sandwich but instead of bread, you have layers of special materials that can do amazing things with light and electricity. These layered materials are called van der Waals heterostructures, and scientists love them because they have unique properties that can be used in gadgets like smartphones and other devices. Today, we're going to talk about a specific type of these materials called transition-metal dichalcogenides (TMDs).

#What Are TMDs?

TMDs are like a superhero team of materials, each with its own special power. Some of them can emit light when excited, and that makes them super cool for optoelectronic applications. There are different kinds of TMDs, and they can be classified into two categories: bright and darkish.

• Bright TMDs like MoSe and MoTe are ready to shine because they have an optically active state that emits light easily.
• Darkish TMDs like WS and WSe, on the other hand, are a bit more shy. They have Excitons that don’t emit light as easily, but they can make a bunch of different light-emitting complexes when things get hot.

#What We Did

We decided to take a closer look at a special kind of light-emitting structure made from a WSe2 monolayer. Think of it as the star of our show! We added some layers of HBN (which is short for hexagonal boron nitride) to create a cozy environment for our WSe2. We also snuggled it between some graphene layers, which act like the bread of our sandwich.

To figure out how well our setup works, we performed experiments using two tricks: Photoluminescence (PL) and Electroluminescence (EL). PL is when we shine a laser on our sample and see what light it gives off. EL, however, is like turning on a light bulb by sending electricity through the material. We conducted these experiments at a really low temperature of 5 K to keep our materials calm and collected.

#What We Found

When we applied a bias voltage (think of it as giving our materials a little push), we noticed something interesting. The number of free carriers, which are like energetic little particles that can help create light, changed both in amount and type. This caused different excitonic complexes to pop up in our PL spectra.

Speaking of popping up, we also detected the EL signal, which was like seeing fireworks light up the sky. The PL and EL mechanisms behaved differently, which helped us see a range of emissions in both experiments.

#The Good Stuff: Optical Responses

Layered materials like our TMDs are a big deal. They have these cool properties that allow them to respond to light in unique ways. When we looked closely at the WSe2 monolayer, we were able to identify a variety of light emission peaks in the PL spectra at different bias voltages.

We could see that some excitonic complexes, like charged excitons, appeared prominently in the PL spectra. This suggested that the WSe2 monolayer was in great shape, ready to shine and dazzle!

#Excitons and Their Friends

Now let's have some fun with excitons-the little buddies that help our material emit light. In our case, we observed some interesting excitonic friends:

• Negative Biexcitons (XX): These guys were pretty popular and dominated the PL spectra when we didn’t apply any voltage.
• Negative Trions (T): They come in two flavors: spin-singlet and spin-triplet, and they also showed up when we turned the voltage on.

As we switched the voltage around, we saw new friends arrive on the scene. A dark intravalley spin-forbidden exciton (D) started to show its face, and we saw the intensity of our negative biexciton drop.

#The Search for Charge Neutrality

When we applied positive voltage, we were on a mission to find the charge neutrality point. This point is where the number of positive and negative charges in our material balances out. We hit that sweet spot at about 1.04 V.

Once we found it, we noticed the neutral biexciton make an appearance again. As we cranked the voltage further, we saw excitons change from negatively to positively charged as we introduced free holes into the mix.

#The Magic of Electroluminescence

Now, let’s switch gears and talk about the EL signal. When we cranked up the voltage to about 4 V and above, the magic happened. The EL signal flickered to life! We found that the EL spectra showed broad emission bands and looked surprisingly similar to previous studies.

With so many free carriers in the mix, we hypothesized that these emissions were linked to many-body complexes made up of charged particles and their accompanying “sea” of free carriers. This thing was really getting exciting!

#Voltage and Current: A Tale of Two Behaviors

As we increased the voltage, we noticed that things were behaving differently for positive and negative voltages. The IV (current-voltage) curve showed distinct features based on the signed voltage. For positive voltages, we saw an apparent onset around 0.8 V compared to a more gradual change at -1 V for negative voltages.

This got us thinking about how the thickness of the hBN barriers affected the tunneling of these free carriers. We imagined it like sipping a thick milkshake through a straw; it’s different if the straw is thin or thick.

#The Three-Tiered Tunneling Theory

From our observations, we came up with a three-step scenario for how carriers might be tunneling in our device:

1. First Step: The onset at 0.8 V and -1 V corresponds to holes and electrons making their way into the WSe2 monolayer.

2. Second Step: When we hit around 3 V, we thought these particles were forming excitons, which are pairs of electrons and holes that can emit light.

3. Third Step: At approximately 4.5 V, we suspected new species were emerging, thanks to the high levels of holes. This could lead to a collective response of the electron and holes coming together.

#Heating Things Up

As we applied high electrical bias, we noticed that our device was heating up. You know how when you run a marathon, your body warms up? It’s similar here. The heating of the device affects the emission spectra, making them broader-we didn’t expect to host a sauna session!

#Voltage Thresholds and Their Secrets

We were curious about why we needed higher voltage levels to see our EL signal. It turns out that this threshold depends on the material properties and the contacts used. We found out that we might need around two times larger voltages due to imperfections in our electrical contacts and the thickness of those hBN barriers.

#Conclusion: A Bright Future for TMDs

In summary, we’ve learned that our light-emitting tunneling structure made from WSe2 is a high-quality device that shows promise for future applications. Through PL and EL experiments, we confirmed that the mechanisms of light emission are different, leading to various results based on how we excite the material.

We’ve only scratched the surface of what these materials can do, and there’s much more to explore. This journey will be like peeling an onion-layer by layer, each revealing something new. We can't wait to see what fascinating discoveries lie ahead in the world of TMDs and their applications in the tech we love.