Markus Raschke

Imaging 2D Materials At a Smaller Scale

Steven Burrows — Mon, 05 Jun 2023 17:50:52 +0000

Imaging 2D Materials At a Smaller Scale Steven Burrows Mon, 06/05/2023 - 11:50

Kenna Hughes-Castleberry / JILA Science Communicator

The tip of an Atomic Force Microscope (AFM) measuring the differences in surfaces of a 2D material. Image credit: Steven Burrows / JILA

Two-dimensional materials, like graphene and 2D semiconductors, are an area of physics that has been growing tremendously in the last decade. According to JILA graduate student Ben Whetten, “That’s because they exhibit new spin and electronic physical phenomena and have much promise to build new miniaturized photonic or semiconductor nanoscale devices.” Researchers like Whetten, and his advisor, JILA Fellow, and University of Colorado Boulder professor Markus Raschke, develop methods to image these materials, giving a better understanding of their inner workings. In a new paper in NanoLetters, Raschke, and his team extended their ultrafast microscope to see nanometer-sized imperfection(s) within a 2D semiconductor sample that created some surprising nonlinear optical effects.

The Promise of 2D Materials

2D materials are an exciting field to study as they exhibit remarkable electronic, optical, and mechanical properties that differ significantly from their 3D counterparts. For example, graphene, a single layer of carbon atoms, has exceptionally high electrical conductivity, mechanical strength, and flexibility, making it an ideal candidate for electronic and mechanical applications. Yet, imaging these materials can be complex, as the small spatial scales of their features are beyond the resolution of conventional diffraction-limited optical microscopes.

To overcome these limitations, many researchers utilize AFM (atomic force microscopy) to provide information on topography, mechanical properties, and electrical conductivity of 2D materials with nanometer-scale resolution. However, AFM by itself limits what the researcher can study, constraining how much of the quantum interactions within the materials can be observed. So Raschke and his team devised a technique to use metallic AFM tips to focus laser light down to the 10-nm scale needed to be able to image optical and dynamic properties with the same resolution with which a typical AFM can image the static mechanical properties of a material.

Exciting Excitons

The specific material Raschke and his team studied is a monolayer of tungsten diselenide (WSe2), a transition metal dichalcogenide that possesses unique electronic and photonic dynamics. “We are looking at the elementary processes of light-matter interaction in these systems,” elaborated postdoctoral researcher, and first author, Wenjin Luo. “We then use ultrafast femtosecond laser pulses that we focus to the nanoscale to locally excite excitons.” A femtosecond is 10-15, or one quadrillionths of a second, which is astonishingly fast. The researchers then observed the excitons, which are a type of elementary quantum excitation of bound electrons specific to semiconductors. One of the long-unsolved puzzles in the field of 2D semiconductors has been how these excitons react to imperfections in the semiconductor material.

To study both the dynamic behavior of these excitons and how they respond to nanoscale defects in the material, the researchers utilized a nonlinear optical process known as four-wave mixing. As Luo explained: “It is a nonlinear optical effect in which three photons of light interact with the exciton coherently and generate a fourth signal photon which we detect. This process only occurs when we use short, coherent, and intense laser pulses.” Coherence occurs when things move in sync, such as the excitons when driven by the laser field. When studying a large ensemble of excitons in conventional spectroscopy, the excitons rapidly lose their coherence due to scattering on the defects. However, “what we observe is that the coherence time of the excitons can be more than an order of magnitude longer when probing on the nanoscale,” Raschke added.

Creating an Ultrafast Image

Besides the ultrafast laser system, the key component of the imaging system was the tiny tip of the AFM which the laser had to hit. As Whetten explained: “We fabricated these optical scanning probe tips ourselves. Those are unique optical devices in their own right. The tips are first etched electrochemically from a gold wire in a multi-step process. Then we used focused ion beam milling to write a grating onto the tip shaft to couple the femtosecond pulse [to the imaging]. Only once all that's prepared can we bring in the femtosecond pulse. We then illuminate this grating on the tip shaft, and the light pulse gets focused down to the apex. The image is then created when we measure the sample point by point.”

With the point-by-point coherent imaging process, the researchers created a high-resolution image of the exciton coherence and how this coherence varies with the imperfections in the sample’s surface. This process could give other scientists the information needed to develop more efficient 2D materials. “To measure their optical properties has conventionally been limited by the diffraction limit of light capping the resolution at about 500 nanometers or so,” Whetten added. “This is insufficient to resolve [differences in the surface in] the optical properties associated with defects and grain boundaries. Our method can image with up to 100 times higher spatial resolution than was previously possible.”

This “coherent nanoscope” allowed for better spatial resolution imaging of the sample, and thanks to the ultrafast lasers, it can measure excitons at the extremely fast time scales of the elementary processes of the motion of electrons. This has significant implications for imaging samples even at room temperature, where historically experiments are carried out at low temperatures to slow the motion of excitons down. With the coherent nanoscope’s extremely high time resolution of just a few femtoseconds, Raschke, Luo, and Whetten could image the exciton dynamics even under conditions where 2D materials would typically be used in real-world applications.

Looking at Imperfections

The researchers found that the dynamics of excitons varied spatially within the material, losing the coherence fast in some areas, and with longer coherence time in others. “To get a visual picture of that, you can imagine a lawn, where the grass does not grow evenly, fast in some but slow in other areas,” Raschke explained.

Because different surfaces in a 2D material can affect its performance, visualizing where these imperfections are can help scientists develop materials with fewer heterogeneities. “This is a burgeoning field where there are all these promises of amazing technologies and new semiconductor devices, and we can look at exactly what limits we can push them to,” elaborated Whetten. “We can ask: How clean do they need to be, and what happens if they’re not perfect? How does that affect the electronic and optical behavior of a semiconductor? And imaging the exciton coherence is the most elementary process allowing us to answer these questions.”
When the team used their imaging system to study these imperfections, they discovered something surprising. “Another big takeaway was that we also saw a completely novel and nonintuitive behavior of the nonlinear optical signal itself associated with the defects,” Whetten explained. “Normally, for sample regions with long [exciton] coherence, we would have expected the strongest signal because the longer the electrons oscillate in phase, the more coherent light they would radiate. But we saw the exact opposite. With the help of our theory collaborator from Texas A&M, Alexey Belyanin, we could explain this new effect. It has to do with spatial coherence, i.e., not just how the electrons start to oscillate out of sync as time progresses, but how their spatial correlation is modified due to defects and grain boundaries.

Incorporating Belyanin’s theory, the researchers found new models to describe what affects the coherence time between excitons. “So, our work not only shows how defects limit coherence, leading to the desire to have samples with low defect densities,” added Raschke. “But it also shows that through specific defect densities, we could engineer the interplay between coherence time and signal intensity in new ways as desirable for specific nano-photonic applications.” Raschke and his team found that they could exploit the material’s imperfections to tweak the coherency times between excitons. Raschke added: “Unfortunately, we do not yet know the exact nature of the specific defects, and how different types of defect or disorder would influence the spatio-temporal coherence as it is called.” This nature is what the team will try to discover in their future work while imaging different materials, semiconductors, and quantum devices to better understand the detrimental and beneficial effects of the different defects for improved materials' function and device performance.

Written by Kenna Hughes-Castleberry, JILA Science Communicator

Two-dimensional materials, like graphene and 2D semiconductors, are an area of physics that has been growing tremendously in the last decade. According to JILA graduate student Ben Whetten, “That’s because they exhibit new spin and electronic physical phenomena and have much promise to build new miniaturized photonic or semiconductor nanoscale devices.” Researchers like Whetten, and his advisor, JILA Fellow, and University of Colorado Boulder professor Markus Raschke, develop methods to image these materials, giving a better understanding of their inner workings. In a new paper in NanoLetters, Raschke, and his team extended their ultrafast microscope to see nanometer-sized imperfection(s) within a 2D semiconductor sample that created some surprising nonlinear optical effects.

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Ripples in Space-Time: Nano-Imaging Functional Materials at their Elementary Scales

Steven Burrows — Mon, 25 Apr 2022 17:04:26 +0000

Ripples in Space-Time: Nano-Imaging Functional Materials at their Elementary Scales Steven Burrows Mon, 04/25/2022 - 11:04

Kenna Hughes-Castleberry / JILA Science Communicator

Ultrafast infrared nano-imaging can improve characterization of electron and vibration dynamics with long-lived excitation states. Image credit: Steven Burrows / JILA

Functional materials—like molecular electronics, biomaterials, light-emitting diodes, or new photovoltaic materials—gain their electronic or photonic properties from complex and multifaceted interactions occurring at the elementary scales of their atomic or molecular constituents. In addition, the ability to control the functions of these materials through external stimuli , e.g., in the form of strong optical excitations, enables new properties in the materials, making them appealing for new technological applications. However, a major obstacle to overcome is the combination of the very fast time (billionths of a second) scales and the very small spatial (nanometer) scales which define the many-body interactions of the elementary excitations in the material which define its function. The extremely high time and spatial resolutions needed have been extremely difficult to achieve simultaneously. Many physicists have, therefore, struggled to visualize the interactions within these materials. In a paper recently published in Nature Communications, JILA Fellow Markus Raschke and his team report on a new ultrafast imaging technique that could solve this issue.

A New Microscope for Quantum Interactions

The technique is called ultrafast infrared pump-probe nano-imaging and builds on years of developments within the Raschke group. According to first author, and former postdoctoral researcher, Jun Nishida: “The technique we develop here is a new type of microscope. We have better, and now simultaneous, time resolution, spatial resolution, and also frequency resolution. We also made this very sensitive by combining different ways of modulating the laser light and how we detect it. This way we can selectively detect the tiny change in the already-small signal that comes from the nano volume [nanosized volume measurements]. With this development, we can study new phenomena that could not be probed with the previous existing microscope of this kind.”

The new ultrafast nano-imaging technique can probe functional material by exciting it with visible light and then imaging it in the infrared spectrum. “The materials’ electronic or optical functions arise from fairly low energy excitations and interactions within the atomic and molecular lattice,” explained Raschke. “That's why the infrared functional probing is really important. Whether it's a photovoltaic response or a quantum phase transition, many of the underlying many-body interactions all are low-energy excitations in the infrared energy range.” To achieve this goal, the team paired intense femtosecond laser pulses with a scanning probe microscope, where a tiny super-sharp metallic tip localizes the laser light resulting in an image with nanometer spatial resolution. By changing the timing between the laser pulses, they could take sequences of images and literally make movies of the motion of electrons and the coupled atomic lattice in the sample. Excited by their new method, the researchers knew the next step was to test it on a couple of functional materials to determine what useful information they would be able to get.

Rippling Electrons

The first functional material the team examined was vanadium oxide, a material that has intrigued physicists for decades. It can transition from an insulator into a metal by application of heat or light. “Vanadium oxide has potential applications for photochromic mirrors, for example,” Raschke said. “Imagine your window, transparent at low temperatures, becoming reflective at high temperatures. For example, a building may have entire windows made out of such types of material, where it will reflect later in the day when it gets hot, or converts the light into electricity, saving energy and money.”

“The material shows an insulator-to-metal phase transition with very complex interactions among electrons and atoms,” Nishida stated. “This is so complex that, even after half a century of intense debates, there are still ongoing discussions on the exact mechanism of the transition.” Raschke’s previous nano-imaging of nano-wires of vanadium dioxide revealed a high degree of disorder and heterogeneity (a material comprised of different ingredients) even in single crystals of the material, with regions within the wire more likely to transition to its metallic state than others. Because the material is made of differing ingredients, this difference can affect how the material transitions. “We really wanted to understand the nanoscale heterogeneity in this transition to have a full picture,” Nishida added.

The researchers used visible light excitation to induce the vanadium oxide to transition to metal, and then used infrared light to image the metallic state with their method. Raschke explained that their method mimicked ripples in a pond, as they perturbed the electrons in the material and monitored how it went back to its ground state. According to Nishida: “We used laser pulses to induce this transition, then image the transition with 100 femtoseconds (one millionth of one billionth of a second) and tens of nanometers (less than 1/1000 the thickness of a hair) resolution.” With their new technique, the team was able to witness how the heterogeneity affected the material's transition from an insulator to a metal. “Previously established mechanisms, such as strain, did not account for the heterogeneity we observed,” said Nishida.

Electron-Lattice Interactions in Solar Cells

The second material the researchers probed was lead halide perovskite. “This class of materials is a strong candidate for new and more efficient solar cells, which many people are looking into,” Nishida said. “It's competing with silicon because it's much cheaper. It's simpler to prepare and not as delicate, even if the surface quality is not very good, it still works very well.” Raschke explained the unique properties of the lead halide perovskite: “Light shines on the lead halide perovskite film, the electrons are excited by the light’s energy, and the electron is 'shielded' by the crystal lattice bending to stabilize the electron. So the electron is less likely to lose its energy before being converted to the desirable current the solar cell is supposed to produce.”

The researchers found that their new microscope can look into the intricate interactions between the excited electrons and a lattice. “Based on molecular vibrations of the lattice, we can look into the coupling between an electron and its surrounding lattice and molecules,” Nishida stated. “In this system, when a molecule ‘sees’ an electron in its vicinity, its vibrational frequency blue shifts. We find that this extent of the blue shift in the spectra of the material is different from point to point. This is really the first step to explain why perovskites are heterogeneous.” And adds Raschke, “Most importantly, it shows that perovskite solar cells have, by far, not yet reached their physical limit in terms of potential performance.” With their technique, the team of researchers was better able to understand how the interactions between electrons and atoms worked at the quantum level in a functional material, and therefore, the method can be used to guide materials science towards the targeted optimization of materials synthesis.

A Technique for Everything

Raschke, Nishida, Sam Johnson (a graduate student participating in the project), and their team are quite excited by their new technique. “Through the demonstration of the performance at [sic] these two different classes of materials, we were able to show how versatile our technique is, with its advances in multiple metrics of spatial resolution, time resolution, and sensitivity,” Johnson said. The researchers believe their ultrafast infrared pump-probe nano-imaging technique will be influential in studying more functional materials, leading to new discoveries in how they work.

Functional materials—like molecular electronics, biomaterials, light-emitting diodes, or new photovoltaic materials—gain their electronic or photonic properties from complex and multifaceted interactions occurring at the elementary scales of their atomic or molecular constituents. In addition, the ability to control the functions of these materials through external stimuli , e.g., in the form of strong optical excitations, enables new properties in the materials, making them appealing for new technological applications. However, a major obstacle to overcome is the combination of the very fast time (billionths of a second) scales and the very small spatial (nanometer) scales which define the many-body interactions of the elementary excitations in the material which define its function. The extremely high time and spatial resolutions needed have been extremely difficult to achieve simultaneously. Many physicists have, therefore, struggled to visualize the interactions within these materials. In a paper recently published in Nature Communications, JILA Fellow Markus Raschke and his team report on a new ultrafast imaging technique that could solve this issue.

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Seeing with the “Nano” Eye

Steven Burrows — Mon, 04 Oct 2021 18:32:17 +0000

Seeing with the “Nano” Eye Steven Burrows Mon, 10/04/2021 - 12:32

Kenna Hughes-Castleberry / JILA Science Communicator

The molecular monolayer of 4-nitrothiophenol being pierced by an atomic force microscope (AFM). Image credit: Steven Burrows

Understanding the chemical and physical properties of surfaces at the molecular level has become increasingly relevant in the fields of medicine, semiconductors, rechargeable batteries, etc. For example, when developing new medications, determining the chemical properties of a pill's coating can help to better control how the pill is digested or dissolved. In semiconductors, precise atomic level control of interfaces determines performance of computer chips. And in batteries, capacity and lifetime is often limited by electrode surface degradation. These are just three examples of the many applications in which the understanding of surface coatings and molecular interactions are important.
The imaging of molecular surfaces has long been a complicated process within the field of physics. The images are often fuzzy, with limited spatial resolution, and researchers may not be able to distinguish different types of molecules, let alone how the molecules interact with each other. But it is precisely this–molecular interactions–which control the function and performance of molecular materials and surfaces.

In a new paper published in Nano Letters, JILA Fellow Markus Raschke and graduate student Thomas Gray describe how they developed a way to image and visualize how surface molecules couple and interact with quantum precision. The team believes that their nanospectroscopy method could be used for molecular engineering to develop better molecular surfaces, with controlled properties for molecular electronic, photonic, or biomedical applications.

Imaging Monolayers with an Infrared Nano-eye

In order to test their new nanospectroscopic imaging method, the researchers used a so called self-assembled monolayer of small organic molecules of 4-nitrothiophenol. The monolayer was then placed under the tip of an atomic force microscope (AFM). Gray explained the process-: "We used infrared light, which has a very long wavelength, limiting spatial resolution to the order of microns, or thousands of molecules across. The way we get the nanometer spatial resolution is using the extremely sharp tip of an atomic force microscope, which is only tens of molecules across. It acts as a lightning nod, just for light, and can focus it to the nanoscale. This allows us to image and perform spectroscopy on the nanoscale with sensitivity as high as just a few molecules." Gray emphasized that because of its low energy, the infrared light directly probes molecular structure, as it could indicate if the 4-nitrothiophenol molecules interacted or coupled with each other.

Molecular Interactions and Quantum Sensors

Principal investigator Raschke was excited about seeing these molecular interactions, and postulated that these interactions could be used for quantum sensing. In order to test their hypothesis of successful quantum sensing, the team looked at the coupling to determine the size of the surface domains. Gray categorized the type of coupling as "a vibrational exciton delocalized across many molecules.” He added that: “When people hear the term exciton they think of electronic excitations. Our vibrational exciton is the conceptual analogue just for molecular vibrations." Using their new imaging systems, the team could see these vibrational excitons on their natural length scales extending across just a handful of molecules. Raschke explained that their nanolocalized infrared light of their imaging system improved their view of these excitons because "the tip itself already provides localization and spatial resolution down to a few tens of nanometers–that is already one ten-thousandth [the width of]of a human hair. The vibrational exciton quantum sensor provides another improvement of spatial resolution by a factor of ten into the true molecular scale. And with a sensitivity where we can distinguish if two, three, or four molecules would be interacting, meaning sharing their wavefunction, or colloquially, speaking to each other in a quantum sense." This new nanospectroscopic approach of vibrational exciton nanoscopy could be used to improve and engineer molecular materials from the from the start and better predict their properties.

Should the vibrational excitons become a successful quantum sensor, this might have implications for quantum technology more generally. "Some people have theorized quantum state transfer for quantum information applications based on these vibrational excitons," Gray stated. "And the reason it would be nice is because a vibrational exciton is potentially stable at room temperature and you wouldn't have to go down to these low temperatures typically required for quantum sensing or computing." Taking quantum technology out of the cold temperatures usually required for most quantum devices would make them more affordable and more widely accessible.

Having found success with their new imaging system, by taking advantage of vibrational excitons as a super-resolution imaging quantum sensor, Gray and Raschke are expanding their research focus. "We are now extending this work from molecular monolayers to molecular crystals used, for example, in molecular electronics or in light-emitting diodes," said Gray. While this new imaging system is helping to improve quantum technology it is also expanding knowledge about the interactions of molecules at the quantum level thatwill help in designing, improving, and controlling molecular materials in general.

In a new paper published in Nano Letters, JILA Fellow Markus Raschke and graduate student Thomas Gray describe how they developed a way to image and visualize how surface molecules couple and interact with quantum precision. The team believes that their nanospectroscopy method could be used for molecular engineering to develop better molecular surfaces, with controlled properties for molecular electronic, photonic, or biomedical applications.

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