For many years, scientists from all over the world have been doing two things - inventing and improving. And sometimes it is not clear which of these is more difficult. Take, for example, ordinary LEDs, which seem so simple and ordinary to us that we do not pay attention to them. But if you add some excitons, a pinch of polaritons and tungsten disulfide to taste, the LEDs will no longer be so prosaic. All these abstruse terms are the names of extremely unusual components, the combination of which allowed scientists from the City College of New York to create a new system that can transmit information extremely quickly using light. This development will help improve Li-Fi technology. What exactly were the ingredients of the new technology used, what is the recipe for this "dish" and what is the efficiency of the new exciton-polariton LED? The report of scientists will tell us about this. Go.
Research basis
If everything is simplified to one word, then this technology is light and everything connected with it. First, polaritons, which arise when photons interact with medium excitations (phonons, excitons, plasmons, magnons, etc.). Secondly, excitons are electronic excitation in a dielectric, semiconductor or metal, migrating through the crystal and not associated with the transfer of electric charge and mass.
It is important to note that these quasiparticles are very fond of cold; their activity can be observed only at extremely low temperatures, which severely limits their practical application. But that was before. In this work, scientists were able to overcome the temperature limitation and use them at room temperatures.
The main feature of polaritons is the ability to bind photons to each other. Photons colliding with rubidium atoms acquire mass. In the process of multiple collisions, photons bounce off each other, but in rare cases they form pairs and triplets, while losing the atomic component represented by the rubidium atom.
But in order to do something with the light, it must be caught. For this, an optical resonator is needed, which is a combination of reflective elements that form a standing light wave.
In this study, even more unusual quasiparticles, exciton-polaritons, which are formed due to the strong coupling of excitons and photons trapped in an optical cavity, play a crucial role.
However, this is not enough, because a material basis is necessary, so to speak. And who, if not transition metal dichalcogenide (TDM), will play this role better than others. To be more precise, a monolayer of WS2 (tungsten disulfide) was used as the emitting material, which has impressive exciton binding energies, which became one of the main criteria for choosing a material base.
The combination of all the elements described above made it possible to create an electrically controlled polariton LED operating at room temperature.
To implement this device, the WS2 monolayer is located between thin hexagonal boron nitride (hBN) tunneling barriers with graphene layers acting as electrodes.
Results of the study
WS2, being a transition metal dichalcogenide, is also an atomically thin van der Waals (vdW) material. This indicates its unique electrical, optical, mechanical and thermal properties.
In combination with other vdW materials, such as graphene (as a conductor) and hexagonal boron nitride (hBN, as an insulator), a whole variety of electrically controlled semiconductor devices, which include LEDs, can be realized. Similar combinations of van der Waals materials and polaritons have already been realized before, as the researchers candidly state. However, in previous writings, the resulting systems were complex and imperfect, and did not reveal the full potential of each of the components.
One of the ideas inspired by the predecessors was the use of a two-dimensional material platform. In this case, it is possible to implement devices with atomically thin emission layers that can be integrated with other vdW materials acting as contacts (graphene) and tunneling barriers (hBN). In addition, this two-dimensionality makes it possible to combine polariton LEDs with vdW materials that have unusual magnetic properties, strong superconductivity, and/or nonstandard topological transfers. As a result of such a combination, you can get a completely new type of device, the properties of which can be very unusual. But, as scientists say, this is a topic for another study.
Image #1
On the image 1a shows a three-dimensional model of a device that resembles a layer cake. The upper mirror of the optical resonator is a silver layer, and the lower one is a 12-layer distributed Bragg reflector*. There is a tunnel zone in the active region.
Distributed Bragg Reflector* - a structure of several layers, in which the refractive index of the material periodically changes perpendicular to the layers.
The tunnel zone consists of a vdW heterostructure consisting of a WS2 monolayer (light emitter), thin hBN layers on both sides of the monolayer (tunnel barrier) and graphene (transparent electrodes for the introduction of electrons and holes).
Two more WS2 layers were added to increase the overall strength of the oscillator and hence to have a more pronounced Rabi splitting of the polariton states.
The operating mode of the resonator is tuned by changing the thickness of the PMMA layer (polymethyl methacrylate, i.e. plexiglass).
Picture 1b this is a snapshot of a vdW heterostructure on the surface of a distributed Bragg reflector. Due to the high reflectivity of the distributed Bragg reflector, which is the bottom layer, the tunnel zone in the image has a very low reflection contrast, as a result of which only the upper thick layer of hBN is observed.
Timetable 1s represents the zone diagram of the vdW heterostructure in the tunnel geometry under displacement. Electroluminescence (EL) is observed above the threshold voltage when the Fermi level of the upper (lower) graphene is shifted above (below) the WS2 conduction (valence) band, allowing an electron (hole) to tunnel into the WS2 conduction (valence) band. This creates favorable conditions for the formation of excitons in the WS2 layer followed by radiative (radiative) electron-hole recombination.
Unlike light emitters based on pn junctions, which require doping to operate, the EL from tunnel devices depends solely on the tunnel current, which avoids optical losses and any changes in resistivity caused by temperature changes. At the same time, the tunnel architecture allows a much larger radiation area compared to dichalcogenide devices based on pn junctions.
Picture 1d demonstrates the electrical characteristics of the tunneling current density (J) as a function of bias voltage (V) between graphene electrodes. A sharp increase in current for both positive and negative voltage indicates the occurrence of a tunneling current through the structure. At the optimal thickness of hBN layers (~2 nm), a significant tunneling current and an increase in the lifetime of implanted carriers for radiative recombination are observed.
Prior to the electroluminescence experiment, the device was characterized by white light reflectivity with angular resolution to confirm the presence of strong exciton binding.
Image #2
On the image 2a angle-resolved reflectance spectra from the active region of the device are shown, showing anti-crossing behavior. Photoluminescence (PL) was also observed with non-resonant excitation (460 nm), showing intense emission from the lower branch of the polariton and weaker emission from the upper branch of the polariton (2b).
On the 2s the dispersion of the electroluminescence of a polariton is shown for an insertion of 0.1 ΞΌA/ΞΌm2. The Rabi splitting and resonator detuning obtained by fitting the oscillator modes (solid and dotted white line) to the electroluminescence experiment are ο½33 meV and ο½-13 meV, respectively. The resonator detuning is defined as Ξ΄ = Ec β Ex, where Ex is the exciton energy and Ec is the resonator photon energy with zero in-plane momentum. Schedule 2d it is a cut at different angles from the electroluminescent dispersion. Here one can clearly see the dispersion of the upper and lower polariton modes with anticrossing occurring in the exciton resonance zone.
Image #3
As the tunneling current increases, the total EL intensity increases. Weak EL from polaritons is observed near the threshold bias (3a), while at a sufficiently large shift above the threshold, polariton emission becomes distinct (3b).
On the image 3s shows a polar graph of the EL intensity as a function of angle, depicting a narrow emission cone of Β± 15Β°. The radiation pattern remains practically unchanged for both the minimum (green curve) and maximum (orange curve) excitation current. On 3d the integrated intensity is shown for various moving tunnel currents, which, as can be seen from the graph, is quite linear. Therefore, increasing the current to high values ββcan lead to successful scattering of polaritons along the lower branch and create an extremely narrow radiation pattern due to the generation of polaritons. However, in this experiment, this was not possible due to the limitation associated with the dielectric breakdown of the hBN tunneling barrier.
red dots on 3d show measurements of another indicator - external quantum efficiency*.
Quantum Efficiency* is the ratio of the number of photons whose absorption caused the formation of quasiparticles to the total number of absorbed photons.
The observed quantum efficiency is comparable to that in other polariton LEDs (based on organic materials, carbon tubes, etc.). It should be noted that the thickness of the light-emitting layer in the device under study is only 0.7 nm, while in other devices this value is much higher. Scientists do not hide the fact that the quantum efficiency index of their device is not the highest, but it can be increased by placing a larger number of monolayers inside the tunnel zone, separated by thin layers of hBN.
The researchers also tested the influence of the resonator detuning on the polariton EL by making another device, but with a stronger detuning (-43 meV).
Image #4
On the image 4a EL spectra are shown with an angular resolution of such a device at a current density of 0.2 ΞΌA/ΞΌm2. Due to the strong detuning, the device exhibits a pronounced bottleneck effect in the EL with an emission maximum occurring at a large angle. This is further confirmed in the image. 4b, where the polar plots of this device are compared with the first (2s).
For a more detailed acquaintance with the nuances of the study, I recommend looking at .
Finale
Thus, all the above observations and measurements confirm the presence of polariton electroluminescence in a vdW heterostructure embedded in an optical microcavity. The tunnel architecture of the device under study ensures the introduction of electrons/holes and recombination in the WS2 monolayer, which serves as a light emitter. It is important that the tunnel mechanism of the device does not require alloying of components, which minimizes losses and various temperature-related changes.
It was found that the EL has a high directivity due to the dispersion of the resonator. Therefore, improving the quality factor of the resonator and a higher current supply will improve the efficiency of microcavity LEDs, as well as electrically controlled microcavity polaritons and photon lasers.
This work once again confirmed that transition metal dichalcogenides have truly unique properties and a very wide range of applications.
Such research and innovative inventions can greatly influence the development and dissemination of data transmission technologies through LEDs and light itself. Such futuristic technologies include Li-Fi, which can provide significantly faster speeds than currently available Wi-Fi.
Thank you for your attention, stay curious and have a great week everyone! π
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Source: habr.com