New Strategies for Designing Efficient Electroluminescent Materials

A collaborative team of materials scientists and theoretical chemists provide hybrid perovskite nanoparticles that are high-efficiency light emitters by using a comprehensive defect-suppression strategy.

New research details how a class of electroluminescent materials, key components of devices such as LED lights and solar cells, can be designed to work more efficiently. Published in Nature Photonics, the combined efforts of experimental and theoretical researchers provides insights into how these and other similar materials could be used for novel applications in the future.

This work was the result of a collaboration between Penn, Seoul National University, the Korea Advanced Institute of Science and Technology, the Ecole Polytechnique Fédérale de Lausanne, the University of Tennessee, the University of Cambridge, the Universitat de Valencia, the Harbin Institute of Technology, and the University of Oxford.

Two years ago, Penn theoretical chemist Andrew M. Rappe visited the lab of Tae-Woo Lee at Seoul National University, and the discussion soon turned to whether they could develop a theory to help explain some of their experimental results.

The material they were studying was formamidinium lead bromide, a type of metal-halide perovskite nanocrystal (PNC). Results collected by the Lee group seemed to indicate that green LEDs made with this material were working more efficiently than expected. “As soon as I saw their data, I was amazed by the correlation between the structural, optical, and light-efficiency results. Something special had to be going on,” says Rappe.

PNCs like formamidinium lead bromide are used in photovoltaic devices, where they can store energy as electricity or convert electric current into light in light-emitting devices (LEDs).

In LEDs, electrons are carried from an electron-rich (n-type) region to a high-energy level in an electron-poor (p-type) region, where they find an empty lower-energy state, or “hole,” to drop down into and emit light.

A material’s efficiency is determined by how well it can convert light into electricity (or vice versa), which depends on how easily an excited electron can find a hole and how much of that energy is lost to heat.

To make sense of the Lee group’s results, Penn postdoc Arvin Kakekhani began working with Young-Hoon Kim and Sungjin Kim of Seoul National University to develop a computational model of the material’s unexpected efficiency and to design targeted follow-up experiments to confirm these new theories. “We spent a lot of time cross linking experiment and theory to rationalize every single experimental observation that we have,” says Kakekhani about the research process.

After months of exchanging ideas and narrowing down potential theories, the researchers developed a theoretical model using a method known as density functional theory, a modeling approach that relies on mathematical theories from quantum mechanics.

While DFT has been used in the field for many years, the implementations of this theory can now efficiently incorporate the impacts of small, delocalized quantum mechanical interactions, known as van der Waals forces, which are known to play a major role in the behavior of soft materials that are similar to the PNCs used in this study.

Using their new model, the researchers found that the PNCs were more efficient if the size of the quantum dots were smaller, since the probability of an electron finding a hole was much greater.

But because reducing a particle’s size also means increasing its surface-to-volume ratio, this also means that there are more places along the material’s surface that are prone to defects, where energy from electrons can easily be lost.

To address both challenges, the researchers found that a simple chemical substitution, replacing formamidinium with a larger organic cation called guanidinium, made the particles smaller while also preserving the structural integrity of the material by allowing more hydrogen bonds to form.

Building on this alloying approach, the researchers found additional strategies to improve efficiency, including the addition of long-chain acids and amines to stabilize surface ions and the addition of defect-healing groups to “heal” any vacancies that might form.

As a theoretical chemist, one thing that stood out to Kakekhani was how well the model’s predictions and experimental data aligned, which he attributes in part to using a theory that incorporates van der Waals forces. “You don’t fit parameters that make the theory specific to the experiment,” he says. “It’s more like first principles, and the only knowledge that we have is what type of atoms the materials have.

The fact that we predicted the results based on almost pure mathematical operations and quantum mechanical theories in our computers, in close correspondence to what our experimental colleagues found in their labs, was exciting.”

While the current study provides specific strategies for materials that have the potential for widespread use as solar cells and LEDs, this strategy is also something that could be adopted more generally in the field of material science.

“Advancement of the Internet of Things and the drive toward optoelectronic computing both demand efficient light sources, and these novel perovskite-based LEDs can lead the way,” Rappe says.

For Kakekhani, this work also highlights the importance of detailed, theory-driven insights for gaining a thorough understanding of a complex material. “If you don’t fundamentally know what is going on and what is the underlying reason, then it is not really extendable to other materials,” says Kakekhani. “In this study, having that long period of trying to rule out theories that didn’t actually work was useful. At the end, we found a really deep reason that was self-consistent. It took a lot of time, but I think it was worth it.”

No More Needles for Diagnostic Tests? Engineers Develop Nearly Pain-Free Microneedle Patch

Nearly pain-free microneedle patch can test for antibodies and more in the fluid between cells.

Blood draws are no fun.

They hurt. Veins can burst, or even roll — like they’re trying to avoid the needle, too.

Oftentimes, doctors use blood samples to check for biomarkers of disease: antibodies that signal a viral or bacterial infection, such as SARS-CoV-2, the virus responsible for COVID-19, or cytokines indicative of inflammation seen in conditions such as rheumatoid arthritis and sepsis.

These biomarkers aren’t just in blood, though. They can also be found in the dense liquid medium that surrounds our cells, but in a low abundance that makes it difficult to be detected.

Until now.

Engineers at the McKelvey School of Engineering at Washington University in St. Louis have developed a microneedle patch that can be applied to the skin, capture a biomarker of interest and, thanks to its unprecedented sensitivity, allow clinicians to detect its presence.

The technology is low cost, easy for clinicians or patients themselves to use, and could eliminate the need for a trip to the hospital just for a blood draw.

The research, from the lab of Srikanth Singamaneni, the Lilyan & E. Lisle Hughes Professor in the Department of Mechanical Engineering & Material Sciences, was published online January 22, 2021, in the journal Nature Biomedical Engineering.

In addition to the low cost and ease of use, these microneedle patches have another advantage over blood draws, perhaps the most important feature for some: “They are nearly pain-free,” Singamaneni said.

Finding a biomarker using these microneedle patches is similar to blood testing. But instead of using a solution to find and quantify the biomarker in blood, the microneedles directly capture it from the liquid that surrounds our cells in skin, which is called dermal interstitial fluid (ISF). Once the biomarkers have been captured, they’re detected in the same way — using fluorescence to indicate their presence and quantity.

ISF is a rich source of biomolecules, densely packed with everything from neurotransmitters to cellular waste. However, to analyze biomarkers in ISF, conventional method generally requires extraction of ISF from skin. This method is difficult and usually the amount of ISF that can be obtained is not sufficient for analysis. That has been a major hurdle for developing microneedle-based biosensing technology.

Another method involves direct capture of the biomarker in ISF without having to extract ISF. Like showing up to a packed concert and trying to make your way up front, the biomarker has to maneuver through a crowded, dynamic soup of ISF before reaching the microneedle in the skin tissue. Under such conditions, being able to capture enough of the biomarker to see using the traditional assay isn’t easy.

But the team has a secret weapon of sorts: “plasmonic-fluors,” an ultrabright fluorescence nanolabel. Compared with traditional fluorescent labels, when an assay was done on a microneedle patch using plasmonic-fluor, the signal of target protein biomarkers shined about 1,400 times as bright and became detectable even when present at low concentrations.

“Previously, concentrations of a biomarker had to be on the order of a few micrograms per milliliter of fluid,” said Zheyu (Ryan) Wang, a graduate student in the Singamaneni lab and one of the lead authors of the paper. That’s far beyond the real-world physiological range. But using plasmonic-fluor, the research team was able to detect biomarkers on the order of picograms per milliliter.

“That’s orders of magnitude more sensitive,” Wang said.

These patches have a host of qualities that can make a real impact on medicine, patient care and research.

They would allow providers to monitor biomarkers over time, particularly important when it comes to understanding how immunity plays out in new diseases.

For example, researchers working on COVID-19 vaccines need to know if people are producing the right antibodies and for how long. “Let’s put a patch on,” Singamaneni said, “and let’s see whether the person has antibodies against COVID-19 and at what level.”

Or, in an emergency, “When someone complains of chest pain and they are being taken to the hospital in an ambulance, we’re hoping right then and there, the patch can be applied,” said Jingyi Luan, a student who recently graduated from the Singamaneni lab and one of the lead authors of the paper. Instead of having to get to the hospital and have blood drawn, EMTs could use a microneedle patch to test for troponin, the biomarker that indicates myocardial infarction.

For people with chronic conditions that require regular monitoring, microneedle patches could eliminate unnecessary trips to the hospital, saving money, time and discomfort — a lot of discomfort.

The patches are almost pain-free. “They go about 400 microns deep into the dermal tissue,” Singamaneni said. “They don’t even touch sensory nerves.”

In the lab, using this technology could limit the number of animals needed for research. Sometimes research necessitates several measurements in succession to capture the ebb and flow of biomarkers — for example, to monitor the progression of sepsis. Sometimes, that means a lot of small animals.

“We could significantly lower the number of animals required for such studies,” Singamaneni said.

The implications are vast — and Singamaneni’s lab wants to make sure they are all explored.

There is a lot of work to do, he said: “We’ll have to determine clinical cutoffs,” that is, the range of biomarker in ISF that corresponds to a normal vs. abnormal level. “We’ll have to determine what levels of biomarker are normal, what levels are pathological.” And his research group is working on delivery methods for long distances and harsh conditions, providing options for improving rural healthcare.

“But we don’t have to do all of this ourselves,” Singamaneni said. Instead, the technology will be available to experts in different areas of medicine.

“We have created a platform technology that anyone can use,” he said. “And they can use it to find their own biomarker of interest.”

C-STAR Subjective Evaluation

Each group participating in the C-STAR III evaluation this year has been assigned a series of translations for human grading.

The grading assignments for each group are split into 5 files, listed under the group name. In order to keep the overall file length manageable for a single grader in a single grading session, each file contains around 100 sentences.

To start grading a file, simply click on the filename in this list. The grading page will open automatically, after prompting you for a group user name and password.

A sample grading turn is shown below. For each translation, the grader must supply grades for fluency and adequacy. These terms are explained in the instructions section of each of the grading pages.

IMPORTANT NOTE: Each of the files contains 100 translations that must be graded in a single grading session. Graders should complete the entire file and submit the grades before moving on to another file or closing the browser.

Special Characters and Null Translations: NO_TRANSLATION_FOUND indicates that the translation engine could not produce any output for the current input sentence. Non-Latin characters in the output indicate that some words from the input could not be translated.

Instructions:

On this page you will be presented with a series of translations to evaluate. Each translation segment appears underneath a Reference sentence.

Compare the two sentences and then make a decision about the quality of the translation, which is labelled “Evaluate this segment.” You will be asked to judge the quality of the translation based on two criteria: Adequacy and Fluency

Adequacy indicates how much of the information from the Reference sentence is also in the sentence below it. Please select one of “All”, “Most”, “Much”, “Little”, or “None”. 
Fluency indicates how the evaluation segment sounds to a native speaker of English.

Please select the phrase that best describes the level of English used in the translation: “Flawless English”, “Good English”, “Non-native English”, “Disfluent English”, or “Incomprehensible”. 

An area for comments is also provided. You may choose to leave this field blank. 

A rule of thumb for grading is to spend no more than 30 seconds on each sentence. When you are finished, be sure to click on the “Submit” button at the bottom of the page.

A result should be returned to you displaying the average scores that you assigned to this document. You may wish to save the result page in order to verify that your results were stored. 

CSTAR Workplan

At the  C-STAR consortium  meeting held in Trento on December 2002, 
the decision was taken to organize, on a regular basis, speech translation 
evaluation campaigns and workshops, mainly focusing on  speech translation 
research and evaluation. 

ctivities within C-STAR will as well  include the 
development of a large multilingual parallel corpus to be used for common 
evaluations.

Evaluation Campaign 2003

The first evaluation campaign and workshop will be in May 2003 and Sept. 2003, 
respectively.

This year, both events will be restricted to CSTAR members only, 
and  the evaluation will be limited  to written texts.

In particular, training and 
testing data will be based on the BTEC  corpus developed by ATR and extended 
by the partners to their respective languages.

Specifications

– The first evaluation campaign will concentrate on assessing text translation algorithms in the tourism domain.  Translation directions will be from Chinese, Italian,  Japanese, and Korean into English, for the primary  condition, and any 
   other direction for the secondary condition.

– Training  data will  consist of  a fixed  amount of  English sentences provided 
    with   translations  into the respective source language. Participants will be allowed 
    to use any additional monolingual resources, e.g.  text corpora,  grammars,  word lists, 
    segmentation tools.

– Test data of the primary condition will consist of  English sentences taken from phrase-books not included in the training data.   Test data  for the 
    the secondary condition will consist of manual translations of the English sentences 
    into all the considered source languages.

–  The  primary condition will  be mandatory for all  participants.  Participants will 
    be  invited   to  submit  more  runs  for   each  condition,  possibly corresponding to 
    different translation directions.

Evaluation Protocol

– Automatic scoring will be carried out with the  NIST/BLEU software. In particular, a 
   server will be set-up that will permit participants to remotely score the output of their system.  Hence,  for each translation direction, multiple translations will be used as references.

– Subjective evaluation on the primary condition will be distributed across the participant sites.  English native speakers will evaluate the output of each system against one gold-standard reference. The evaluation will follow guidelines similar to those applied by LDC 
   in the NIST MT evaluation campaigns.

– While  automatic evaluation  will be  applied to  all  submitted runs, subjective evaluation will 
    be applied to only one run per participant, namely the first run submitted  under the primary 
    condition.

– Finally, participants are allowed to discuss their results without restriction. Disclosure of the 
   results of other participants are not allowed without their permission.