Author Archives: eric easthope

Looking Behind the Display: Copper as a Low-Cost Emissive Material

Posted on April 4, 2019 | Leave a comment

Display technologies have come a long way over the past 100 years. What is it that makes many of our modern smartphone and TV displays look so amazing? Behind them are the efforts and breakthroughs of both chemists and material scientists alike. One such breakthrough, the invention of OLED (Organic Light-Emitting Diode) technology in 1987, is seeing increasing popularity for its low power demands and physical flexibility.

However, the materials needed for OLED displays are costly, and environmentally unsustainable. For example, the green pixels in many smartphone displays are made from iridium. As a rare earth metal, iridium is very expensive (approximately 45000 USD per kilogram). With this in mind, the discovery of new low-cost materials could open economically favourable avenues for OLED technologies.

Chemists at the University of British Columbia are steadily researching new materials to meet the economic and environmental demands of industry. We had the opportunity to sit down with Dr. Christopher Brown, a post-doctoral researcher in UBC’s Wolf Research Group, to discuss his recent discovery of a tuneable light-emitting copper-based compound. This compound exhibits a property known as thermochromic emission: it emits different colours of light in response to changes in temperature.

The liquid colour-changing crystal in a “mood ring” is one such example of a thermochromic material (source, available under public domain).

Dr. Brown observed that some copper compounds display noticeable changes in colour in response to temperature when exposed to UV light (like that produced by the sun). Moreover, he hypothesized that the effect is the result of a change in the compound’s geometry. We have produced a video, and a podcast, to showcase Dr. Brown’s work (we recommend setting the video quality to 1080p).

Dr. Brown emphasizes that his discovery does not have immediate applications to OLED technology, as the emergence of thermochromism requires supercool temperatures (approximately -196 °C). Nevertheless, his contributions suggest that copper, as a low-cost material, may play a role in future OLED applications.

– Nelson Bulaun, Angela Wei, Sarah Choi, Eric Easthope

We are deeply grateful to Dr. Christopher Brown for sharing his work with us.

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Posted in Outreach Project, Science Communication

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Glowing Pickles and OLEDs

Posted on March 18, 2019 | Leave a comment

Did you know that a pickle will glow if you pass electrical current through it?

“A pickle glowing due to electrical current” (source: Wikimedia Commons, available under CC BY 3.0)

This phenomenon, while it is peculiar and at first seemingly inapplicable, is a simple example of the same physical principles that underlie the beauty of our modern smartphone displays. An electrical current heats water in the pickle. The pickle rapidly dries out near the electrodes (here the electrodes are the forks at the ends of the pickle), causing sparks to leap between drier and wetter regions of the pickle. Sodium atoms throughout the pickle are then excited by these sparks to emit a characteristically yellow-orange light.

The same effect occurs in smartphone displays made from Organic Light-Emitting Diodes (OLEDs). However, instead of sodium, a film of some organic compound situated between two electrodes (of which one or more is transparent) is excited by electrical current to emit visible light. This approach to producing light differs from previous LED technologies that relied on a “backlight” (a fixed arrangement of LEDs) to produce visible light from electrically excited compounds.

These organic compounds are rarely simple molecules. Take, for example, an iridium-based chemical complex known as Ir(mppy)3, shown here.

“A diagram of Ir(mppy)3” (source: Wikimedia Commons, available as part of the public domain)

We will not discuss the structure or nomenclature of this compound, but it is worth mentioning that the compound is phosphorescent (it emits light without heat nor combustion) and will emit green light in response to electrical current. Other compounds similar to Ir(mppy)3 have been discovered to produce red and blue light. In application, these three colours (red, green, blue) may be added in different proportions to produce the many visible colours that we see (known as the RGB colour model).

It is noteworthy that the discovery of a blue LED was awarded the 2014 Nobel Prize in Physics, emphasizing the modern and increasing relevance of OLED research.

New research seeks to overcome limits on the efficiency, lifespan, and cost of OLEDs. For example, while OLEDs are often a low-power alternative to the former backlight-based LED technologies, displaying images with white backgrounds (such as most web pages) using OLEDs require as much as three times the power of common LEDs. The metals used in OLEDs (such as iridium) are also rare and often expensive, meaning that consumer technologies derived from OLEDs come at a greater cost to both their users and the environment.

With cost and efficiency in mind, the Wolf Research Group at UBC has explored the use of copper (which is abundant on Earth) and other elements in place of iridium to produce candidate compounds for OLED technologies. Breakthroughs like these push us towards new innovations to benefit consumer technologies, and materials science overall.

– Eric Easthope

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Posted in Issues in Science, Science Communication

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No Resistance: An Introduction to Superconductivity

Technologies like magnetometry (the measure of magnetism) and magnetic resonance imaging rely on the strength of magnetic fields. With increasing need for experimental precision and control, new physics are sought after to develop stronger electromagnets. One way is to eliminate a material’s electrical resistance. This is known as superconductivity. Herein we discuss what superconductivity is.

In physics we learn that electrons may be excited into higher energy states. We can excite electrons using energy carried by light (known as photoexcitation).

Semiconductors respond to photoexcitations. These are materials that exhibit an electrical resistance unlike that of insulators or conductors. Their electronic behaviour, the way that electrons “move” through the material, is often temperature dependent. All light carries energy. If this energy is absorbed by matter, the matter heats up. It is this heat that excites electrons, so we can use light to change the electronic behaviour of semiconductors. This makes them useful objects of study.

For simplicity, we treat these semiconductors as patterned arrangements of atoms (called a lattice). Where there are “gaps” in the arrangement, for not every lattice is densely packed, we imagine varying densities of electrons. It is reasonable to imagine these electrons as a cloud that pervades the atomic arrangement. If we excite the semiconductor with light, this electron cloud may change, thereby changing the semiconductor’s electronic behaviour.

Where does superconductivity arise? As mentioned, semiconductors are temperature dependent, so they respond to heating (in our example, by way of photoexcitation). What if we cool a semiconductor instead? As a general rule, the electrical resistance of a semiconductor increases as its temperature decreases. However, when cooled to a temperature near absolute zero (the semiconductor’s critical temperature), its electrical resistance vanishes. All magnetic field lines, like those seen with iron filings dropped around a bar magnet, are expelled from the interior of the semiconductor.

“Levitation of a magnet on a superconductor” (source: Wikimedia Commons, available under CC BY-SA 3.0)

This bizarre property of matter, while it is not universal, is the direct consequence of a lack of electron excitations. Recall that excited electrons (like those in an electronic circuit) emit energy as they relax. This energy is absorbed by matter, so energy is lost as heat. Electrons also need somewhere (i.e. a higher energy state) to be excited towards. This “somewhere” is unique to the material, so the material determines how and where electrons are excited.

“Overview of superconducting critical temperatures for a variety of superconducting materials since the first discovery in 1911” (source: Wikimedia Commons, available under CC BY-SA 4.0)

Supercooled materials lose many pathways to these higher energy states, so their electrons are never excited unwillingly, so no energy is lost as heat. Since electrical resistance in semiconductors is temperature dependent, a supercooled and thereby “heat-less” semiconductor has zero electrical resistance. It is superconductive.

– Eric Easthope

The systematic study of candidate superconductors (like bismuth selenide) is ongoing at UBC’s Stewart Blusson Quantum Matter Institute.

Source: General Chemistry: Principles, Patterns, and Applications (2012). Saylor Academy. Available here under CC BY-NC-SA 3.0.

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Posted on January 28, 2019 in Science Communication

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