Tag: optoelectronics

Robust supercrystals for the LEDs of the future

Robust supercrystals for the LEDs of the future

Nanometre-sized crystals of perovskite offer great potential for applications in the field of light-emitting diodes, solar cells and optical switching elements. A particularly interesting arrangement occurs when a large number of these nanocrystals join to form a larger structure – a supercrystal. Researchers at the University of Tübingen have now found a novel way of achieving this. They resorted to a clever technique to produce perovskite supercrystals that are particularly stable and therefore useful. Some important analyses were carried out at DESY: Using its X-ray source PETRA III, the team managed to determine the precise structure of the supercrystals. They are now presenting their findings in the journal ACS Nano.

Perovskites, named after the Russian mineralogist Lev Perovski, are a class of crystalline materials with a characteristic lattice structure. Lead halide perovskites are particularly promising, as they can be produced relatively easily by chemical methods and have remarkable optoelectronic properties – which is why these materials are the subject of intensive research. The first practical applications now appear to be within reach: Experts are working on high-performance solar cells based on perovskites, as well as on a new generation of highly efficient LEDs and laser chips.

Perovskite nanocrystals are a particularly exciting area of research. “Their optoelectronic properties depend heavily on their size – which is typical of quantum behaviour,” explains Jonas Hiller from the University of Tübingen, one of the authors of the study. “Because of this, their properties can be specifically customised. The energy of the light they absorb or emit varies depending on their composition and their size.”

Under certain conditions, these perovskite nanocrystals can form larger structures, creating a supercrystal. “This is a crystal made up of crystals,” Hiller explains. “You can compare it to a Rubik’s Cube which is made up of several smaller cubes.” The exciting thing is that while the individual nanocrystals retain their desired quantum properties, they can be handled as a macroscopic unit and thus deployed in practical applications.

Forming a supercrystal in two phases

Until now, supercrystals like this have been created by allowing a solvent containing the perovskite to slowly evaporate. The resulting structures form very gradually on the substrate. “However, the supercrystals are produced at random sites around the substrate,” explains the project manager Ivan Zaluzhnyy from Tübingen. “Also, the individual nanocrystals are surrounded by a protective layer of organic molecules which makes the entire supercrystal very soft.” As a result, they break very easily when you try to move them around mechanically. This poses a real obstacle for applications in which the positioning of the materials is crucial, such as between two electrodes in an electrical component.

To solve this problem, the team opted for an alternative approach: two-phase diffusion. A solution containing the nanocrystals is layered on top of a second liquid: acetonitrile. This acts as an anti-solvent for the perovskite crystals. As it slowly penetrates the solution containing the nanocrystals, it gradually reduces their solubility. “This results in crystal growth beginning at the boundary surface between the two phases,” explains Jonas Hiller. The acetonitrile displaces the organic molecules coating the crystals, resulting in a firmer, more stable structure.

In order to examine the structure of these supercrystals more closely, the team used the narrow X-ray beam at GINIX, an instrument installed at the PETRA III beamline P10. “The beam diameter of just 300 nanometres makes it possible to examine different regions within a supercrystal with high precision,” explains DESY physicist Wojciech Roseker. And Jonas Hiller adds: “The extremely high quality of the diffraction data was a key element of this study. It enabled us to analyse the structure of the supercrystals in great detail.”

The team found that the supercrystals produced, typically had an area of 10 by 10 square micrometres but were significantly thicker than the comparatively flat structures that could be achieved using the old method. Their height was more than five micrometres which improves their stability. This makes the supercrystals robust enough to be gripped with micromanipulators and moved to other locations – a first for perovskite structures.

Read more on DESY website

Image: Like a crystal rubik’s cube: The research team has found a way to create ordered perovskite supercrystals.

Credit: University of Tübingen, figure from original publication

Finetuning perovskites for new applications in solar cells, LEDs and semiconductors

Finetuning perovskites for new applications in solar cells, LEDs and semiconductors

Perovskite is a rising star in the field of materials science. The mineral is a cheaper, more efficient alternative to existing photovoltaic materials like silicon, a semiconductor used in solar cells. Now, new research has shown that applying pressure to the material can alter and fine-tune its structures — and thus properties — for a variety of applications.

Using the Canadian Light Source (CLS) at the University of Saskatchewan, a team of researchers observed in real time what happened when they “squeezed” a special type of perovskite between two diamonds. 2D hybrid perovskite is made up of alternating organic and inorganic layers. It’s the interaction between these layers, says Dr. Yang Song, professor of chemistry at Western University, that determines how the material absorbs, emits, or controls light.

The research team found that applying pressure significantly increased the material’s photoluminescence, making it brighter, which Song says hints at potential applications in LED lighting. The team also observed a continuous change in its colour from green to yellow to red. “So you can tune the colour.” Being able to observe changes to the material as they happen using ultrabright synchrotron light was critical to their research, said Song.

One of the biggest changes in the material came when the researchers applied a very large amount of pressure to the perovskite: It started glowing differently, signaling that its ability to handle light had improved. They also found the material squished more in one direction than others and that its internal structure became less twisted. Most similar materials become more twisted when they’re squeezed. The findings of the research, which also involved the Advanced Photon Source (APS)  at Argonne National Laboratory in Chicago, were published recently in the journal Advanced Optical Materials.

Read more on CLS website

Stabilising fleeting quantum states with light

Stabilising fleeting quantum states with light

Quantum materials exhibit remarkable emergent properties when they are excited by external sources. However, these excited states decay rapidly once the excitation is removed, limiting their practical applications. A team of researchers from Harvard University and the Paul Scherrer Institute PSI have now demonstrated an approach to stabilise these fleeting states and probe their quantum behaviour using bright X-ray flashes from the X-ray free electron laser SwissFEL at PSI. The findings are published in the journal Nature Materials.

Some materials exhibit fascinating quantum properties that can lead to transformative technologies, from lossless electronics to high-capacity batteries. However, when these materials are in their natural state, these properties remain hidden, and scientists need to gently ask for them to pop up. One way they can do this is by using ultrashort pulses of light to alter the microscopic structure and electronic interactions in these materials so that these functional properties emerge. But good things do not last forever – these light-induced states are transient, typically persisting only a few picoseconds, making them difficult to harness in practical applications. In rare cases, light-induced states become long-lived. Yet our understanding of these phenomena remains limited, and no general framework exists for designing excited states that last.

A team of scientists from Harvard University together with PSI colleagues overcame this challenge by manipulating the symmetry of electronic states in a copper oxide compound. Using the X-ray free electron laser SwissFEL at PSI, they demonstrated that tailored optical excitation can induce a ‘metastable’ non-equilibrium electronic state persisting for several nanoseconds – about a thousand times longer than they usually last for. 

Steering electrons with light

The compound under study, Sr14Cu24O41 – a so-called cuprate ladder – is nearly one-dimensional. It is composed of two distinct structural units, the ladders and chains, representing the shape in which copper and oxygen atoms organise. This one-dimensional structure offers a simplified platform to understand complex physical phenomena that also show up in higher-dimensional systems. “This material is like our fruit fly. It is the idealised platform that we can use to study general quantum phenomena,” comments experimental condensed matter physicist Matteo Mitrano from Harvard University, who lead the study. 

One way to achieve a long-lived (‘metastable’) non-equilibrium state is to trap it in an energy well from which it does not have enough energy to escape. However, this technique risks inducing structural phase transitions that change the material’s molecular arrangement, and that is something Mitrano and his team wanted to avoid. “We wanted to figure out whether there was another way to lock the material in a non-equilibrium state through purely electronic methods,” explains Mitrano. For that reason, an alternative approach was proposed.

In this compound, the chain units hold a high density of electronic charge, while the ladders are relatively empty. At equilibrium, the symmetry of the electronic states prevents any movement of charges between the two units. A precisely engineered laser pulse breaks this symmetry, allowing charges to quantum tunnel from the chains to the ladders. “It’s like switching on and off a valve,” explains Mitrano. Once the laser excitation is turned off, the tunnel connecting ladders and chains shuts down, cutting off the communication between these two units and trapping the system in a new long-lived state for some time that allows scientists to measure its properties.

Cutting-edge fast X-ray probes

The ultra-bright femtosecond X-ray pulses generated at the SwissFEL allowed the ultrafast electronic processes governing the formation and subsequent stabilisation of the metastable state to be caught in action. Using a technique known as time-resolved Resonant Inelastic X-ray scattering (tr-RIXS) at the SwissFEL Furka endstation, researchers can gain unique insight into magnetic, electric, and orbital excitations – and their evolution over time – revealing properties that often remain hidden to other probes. 

“We can specifically target those atoms that determine the physical properties of the system,” comments Elia Razzoli, group leader of the Furka endstation and responsible for the experimental setup. 

This capability was key to dissecting the light-induced electronic motion that gave rise to the metastable state. “With this technique, we could observe how the electrons moved at their intrinsic ultrafast timescale and hence reveal electronic metastability,” adds Hari Padma, postdoctoral scholar at Harvard and lead author of the paper.

Read more on PSI website

Image: Laser pulses trigger electronic changes in a cuprate ladder, creating long-lived quantum states that persist for about a thousand times longer than usual.

Credit: Brad Baxley/Part to Whole

Transition metal insulators: The origin of colour

Transition metal insulators: The origin of colour

In a theoretical study, researchers have explained the vibrant colours of two compounds whose electronic properties seemingly prohibit such colouring. The hues exhibited by the two insulators originate from transitions in the spins of the electrons, which modify the way the materials absorb and reflect light in such a way as to create the bright colours. The theoretical framework employed by the team promises new insights in fields such as optoelectronics or in the study of qubits, the quantum bits used in quantum computers. 

Although colour is a familiar phenomenon, it is sometimes challenging to explain how the hues of certain materials come about. This is the case with insulators that contain transition metals. In these compounds, the energy gap between the valence band, in which the electrons are tightly bound to the atoms, and the conduction band, in which the electrons can move freely, is larger than the highest energy of photons of visible light—meaning that these materials should not absorb visible light. As the colour of a compound is complementary to the wavelengths it absorbs, we should thus perceive these insulators as being transparent instead of coloured. 

A team of researchers including the head of the European XFEL Theory group, Alexander Lichtenstein, now used two complementary theoretical methods to study the origin of colour in two typical transition metal insulators: nickel(II) oxide (NiO)—a green compound used in the production of ceramics and nickel steel as well as in thin-film solar cells, nickel–iron batteries, and fuel cells—and manganese(II) fluoride (MnF2), a pink material employed in the manufacture of special kinds of glass and lasers.

Read more on XFEL website

Image: Visualization of the orbital character of low-laying excitons in NiO, corresponding to a local ‘Frenkel’ exciton at an energy of 1.6 eV and a weakly bound, bright ‘Wannier-Mottâ’ exciton at an energy of 3.6 eV