Posts

Fluorescence and phosphorescence, Lighting up the world!

 

The magical glow of glow-in-the-dark toys and trinkets has always lit up children’s rooms for many years, from glowing ghosts to star decorations plastered across the walls. They only need some exposure to light before being charged and emitting their own light. All materials with this property, toys or otherwise, are known as phosphors. Besides looking mesmerising, understanding the underlying mechanisms allows scientists to research and apply these materials to various fields, or at the very least, have a newfound appreciation for the glowing dino sitting on the bedside table.

 

Glow In The Dark Stars and Moon – OddGifts.com

Image credit: oddgifts.com

 

When light, or photons, are absorbed by electrons in a material, the electrons are said to be in an excited state. Systems in nature always prefer to have the lowest energy possible, so the excited electrons will try to relieve this extra energy by dropping to a lower energy level and emitting energy in a process known as relaxation. While this explanation is simple, it doesn’t explain the whole story. These states that an electron can occupy are split into sublevels based on vibrational modes or classified based on the spin of the electron, which have different observable effects when the electron relaxes. That is great, but what does this have to do with a glow-in-the-dark toy you can find at the store? To better understand all this, we can look at Jablonski’s diagram that covers the basics of electronic transitions:

 

 

1. Jablonski diagram explaining the occurrence of fluorescence and | Download Scientific Diagram

Jablonski diagram. Image courtesy of Michael Eck. (2)

 

Let’s review all the types of transitions to make better sense of all this. The first step is excitation/absorption, which describes adding energy to our system; in our case, let’s say light. Suppose the energy of the light matches the energy of the first excited state. In that case, our electron is promoted from our ground state S0 to our first excited state S1. What is the time scale of these transitions? 10-15 s, or in the femtosecond range, which is remarkably fast! Next are the many ways the electron can fix this energy imbalance. Vibrational relaxation sees the electron dropping from one vibrational mode, or the specific way the atoms of the molecule move, to a lower vibrational mode within the same electronic energy state. This process is classified as non-radiative, so no light is produced as the outputted energy is kinetic energy. A similar process is intersystem crossing, where vibration states from one excited state overlap with vibrational states of another, giving electrons more paths to reduce their energy. The time scale for vibrational relaxation is 10-14 s – 10-12 s, another fast process! While the time scale for intersystem crossing is 10-8 s – 10-3 s, which, in this scale, is considered an eternity… Mainly because it must compete with the two relaxation pathways of interest, fluorescence and phosphorescence!

The phosphors are classified by whether the light emitted is fluorescent or phosphorescent, which is tied to the spin before the electron relaxes to the ground state. Fluorescence is the process of an electron dropping to its ground state from the excited state of the same spin. However, phosphorescence is less straightforward, in which the electron’s spin flips in a process known as intersystem crossing before it can relax to the ground state. But in both cases, light is produced! The time scales of both processes are relatively long and can depend on the structure of the specific phosphor, which is why the glimmering shine of those plastic stars lasts a while!

 

As such, scientists find great uses for the phosphor’s signature glows, primarily for marking and staining bacteria and cells! One team in Germany used a fluorescent Terbium complex for gram-negative bacterial staining, which can help identify different types of bacteria based on what glows. (3) . Another team used a phosphorescent iridium complex to stain the cytoplasm of living cells. (4) . Besides giving scientists valuable information on these microscopic systems using various imaging techniques, the pictures produced are quite mesmerising and go to show the benefit of lighting up our world, one cell (or dino!) at a time.

 

Cationic iridium(iii) complexes for phosphorescence staining in the cytoplasm of living cells - Chemical Communications (RSC Publishing)

Phosphorescent iridium complex is used to stain living cells. (4)

 

Dual-sensitized Eu( iii )/Tb( iii ) complexes exhibiting tunable luminescence emission and their application in cellular-imaging - Dalton Transactions (RSC Publishing) DOI:10.1039/D2DT00051B

Bioimaging of cells with phosphor complexes! (5)

 

 

 

References:

1 Eck, Michael. (2014). Performance enhancement of hybrid nanocrystal-polymer bulk heterojunction solar cells : aspects of device efficiency, reproducibility, and stability.

2 So, P. T., & Dong, C. Y. (2001). Fluorescence spectrophotometry. e LS.

3 Ulrich Kynast, Marina Lezhnina Muenster University of Applied Sciences, Institute for Optical Technologies, Stegerwaldstr. 39, 48565 Steinfurt, Germany

4 Yu, Mengxiao & Zhao, Qiang & Shi, Linxi & Li, Fuyou & Zhou, Zhiguo & Yang, Hong & Yi, Tao & Huang, Chunhui. (2008). Cationic iridium(III) complexes for phosphorescence staining in the cytoplasm of living cells. Chemical communications (Cambridge, England). 2115-7. 10.1039/b800939b.

5  Dasari, Srikanth & Singh, Swati & Sivakumar, Sri & Patra, Ashis. (2016). Dual-Sensitized Luminescent Europium(ΙΙΙ) and Terbium(ΙΙΙ) Complexes as Biomaging and Light-Responsive Therapeutic Agents. Chemistry (Weinheim an der Bergstrasse, Germany). 22. 10.1002/chem.201603453.

/0 Comments/by

Strange matter

You may have heard that Tungsten is a really dense metal however what is truly the most dense thing throughout the universe? In room temperature comparing chemical bond lengths and crystal shapes could help with calculating which material can take the most dense shape. However when it come to truly the most dense materials, they are formed forcefully through gravitational forces.

When the lifecycle of a star comes to an end, meaning when the star has run out of all the fuel it possessed, the energy created by the star to keep the gravitational forces from imploding the star ceases to exist. This results in the star imploding in on itself and may result in one of two things. If the mass of the star is big enough, around 5 times the mass of our sun or 5 solar masses, it will collapse into a black hole. If it is not as massive it, around 2.5 solar masses, it will collapse into a neutron star. (Lea, 2023)(Sachev, et al., 2020)

Life Cycle of a Star | The Schools' Observatory

image 1. Lifecycle of a star (The Schools’ Observatory)

In neutron stars the outer core continues the fusion process to form even heavier metals then iron which where fusion stops for normal stars, while the core is so dense that the electrons merge with protons which turns them into neutrons. This is why it is called a neutron star.  Theoreticality it is possible that the implosion of the star may result in an even denser form of a neutron star. In this case, not only the electrons and protons merge but also the neutrons that are formed and that were already their also merge into one big sea of quarks. (Cooper, 2022)s

Quarks are the current fundamental building blocks of the universe and come in  6 different types. these types are based on their chare mass and spin. However unlike the protons and neutrons they form, the charge that they posses is a fractional number. These quarks always come in aa group and they are hard to separate due to the forces that hold them together. The binding forces that gluons carry that bind the quarks are relatively week, and at rest they are nearly non existent, however once you start giving it energy to pull them apart they use the energy to create more gluons that increase the binding forces making it really hard to separate.(Britannica, 2024)

A four-by-four table of particles. Columns are three generations of matter (fermions) and one of forces (bosons). In the first three columns, two rows contain quarks and two leptons. The top two rows' columns contain up (u) and down (d) quarks, charm (c) and strange (s) quarks, top (t) and bottom (b) quarks, and photon (γ) and gluon (g), respectively. The bottom two rows' columns contain electron neutrino (ν sub e) and electron (e), muon neutrino (ν sub μ) and muon (μ), and tau neutrino (ν sub τ) and tau (τ), and Z sup 0 and W sup ± weak force. Mass, charge, and spin are listed for each particle.

image 2. Quarks (Wikipedia, 2024)

In case of the aforementioned stars, the gravitational forces are so strong that the quarks can separate causing them to flow freely withing the core of the star. Since, the star is formed of quarks it is called a quark star. Quark stars are seemingly the same as neutron stars from the outside yet on the inside it is a lot more dense and is made out of quarks.(Cooper, 2022)

the extreme forces at the core might cause the up and down quarks to turn into strange quarks, which are a lot more dense yet also more stable. it is so stable in fact that after one forms it turns the other quarks in its vicinity into strange quarks as well. through this occurrence all the quarks in the star turn into strange quarks forming the collection of quarks named strange matter.(Klähn, Blaschke, 2017)

In science in general, having a lower potential energy means that that thing is more stable. The materials and reactions always tend to move from high energy to low energy. (Hunt)

Under high pressure, the up and down quarks have a higher mass, which corresponds to energy with the relation E=mc^2, under high pressures the strange quark with the higher mass is more stable and after it is formed into strange matter it does not decompose back into up and down quarks. This means that the strange matter is a stable and possible the densest material in the universe, potentially second to the singularity at the centre of a black hole.(Workman, 2022)

These aspects of strange matter make it one of the most dangerous materials throughout the universe. Since it is stable enough to continue to exist on its own and the fact that when it touches other materials the energy released causes everything else it touches to turn into strange matter itself means that even the smallest piece of strange matter, which is named strangelets, to have the ability to destroy any planet or star it touches. Although it won’t just fly away on its own merging with a different neutron star could cause all the strange matter to spew out into space and a particle of that size and speed would mean that we wouldn’t even see it coming if one was making its way towards earth. And potentially the only way to get rid of it might be to just throw it into a black hole and hope that it never escapes.(Jaffea, et al., 2000)

 

References:

Lea, R.(2023). What are neutron stars? space.com https://www.space.com/22180-neutron-stars.html

Sachdev, S., Hanna C., Sathyaprakash, B.S., Sholtis, S.(2020). Black hole or neutron star? LIGO-Virgo scientists find mystery object in ‘mass gap’. PennState https://www.psu.edu/news/research/story/black-hole-or-neutron-star/#:~:text=—%20When%20the%20most%20massive%20stars,of%20stars%20called%20neutron%20stars.

Cooper, K.(2022)Quarks. What are they? space.com https://www.space.com/quarks-explained

Britannica, T. Editors of Encyclopaedia (2024). quark. Encyclopedia Britannica. https://www.britannica.com/science/quark

Klähn, T., . Blaschke, D.B. (2017). Strange matter in compact stars. https://arxiv.org/pdf/1711.11260

Hunt, I.(). Thermodynamics and Stability. University of Calgary, School of Chemistry https://www.chem.ucalgary.ca/courses/350/Carey5th/useful/thermo.html#:~:text=The%20lower%20the%20potential%20energy,less%20stable%20to%20more%20stable.

Workman, R.L, Particle data group (2022). Quarks. Prog.Theor.Exp.Phys. https://pdg.lbl.gov/2022/tables/rpp2022-sum-quarks.pdf

Jaffea, R.L., Buszaa, W., Sandweissb, J., Wilczek, F. (2000). Review of Speculative “Disaster Scenarios” at RHIC. https://arxiv.org/pdf/hep-ph/9910333

Images:

The Schools’ Observatory (). Lifecycle of a star. https://www.schoolsobservatory.org/learn/astro/stars/cycle

Wikipedia (2024), Quark. https://en.wikipedia.org/wiki/Quark

/0 Comments/by

Nanostructures: Finding a happy medium between physical and chemical techniques for advances in technology. #NA

Colour has captivated the human imagination for centuries, inspiring artists, poets, and scientists alike. While most of the colours we encounter in everyday life are produced by pigments, there exists a fascinating alternative known as structural coloration. In this blog, we embark on a journey through the captivating realm of structural coloration within the field of nanoscience, uncovering the intricate mechanisms behind nature’s vibrant hues. 

  

Understanding Structural Coloration: 

Unlike pigments, which absorb certain wavelengths of light and reflect others to produce colour, structural coloration arises from the interaction of light with nanostructured materials. At the heart of this phenomenon lies the manipulation of light waves through precise control of nanoscale structures. These structures can be found in diverse organisms. Most famously seen in the Morpho butterfly but these organic nanostructures also occur in birds to beetles and even some plants. Scanning Electron Microscopy (SEM) images of the wing of a morpho butterfly are shown [below] [1]. Where colour is typically seen from a material absorbing certain regions of light within the visible spectrum, thus reflecting the rest and this reflection being what we perceive as colour, this is not present in these wings.   

As seen in the figure above, the striking blue hue of the Morpho butterfly is due the arrangement of nanostructures on its wings as opposed to a blue pigment. Thus structural colour must be a result of the interference of light being scattered and reflected off these structures. Comparing the SEM images of the butterfly wings above to a diffraction grating, a tool commonly used to demonstrate how white light can be separated into a spectrum via these same principles of diffraction and interference shows this. [2] 

In butterfly wings, these ‘organic diffraction gratings’ are formed from tree shaped arrays of the polymer ‘chitin’, an amide derivative of glucose – a sugar found in most living organisms. These structures are so small in fact that when the first scientists investigating the iridescent nature of butterfly wings and peacock feathers, two rather famous blokes – Newton and Hooke – could not resolve them with simple light microscopes. This property of these structures is not only the reason we need SEM to gain a good understanding of their physical and optical properties but will also be one of the main challenges faced by scientists in mimicking these architectures.

So how do we currently make these patterned nanostructures??

For years different lithography methods have been used, with photolithography being the most common by far due to its scalability. This method works by laying a thin layer of a polymeric photoresist on top of a wafer (typically silicon). A mask is then placed atop this photoresist in the shape of the desired structure, or the negative of this structure depending on the photoresist’s nature. Exposing this to light results in the unmasked photoresist undergoing a reaction which either results in the polymers comprising the photoresist becoming stronger or weaker depending on its nature. This allows for selective removal of either the photoresist protected by the mask or that which was exposed to light. Naturally this method is very scalable as the only limiting factor is the size of the wafer on which you can print your photoresist.

 

Due to the limited resolution which photolithography can provide, other methods for forming nanostructures have needed to be developed. Minimum feature size is proportional to wavelength given by the Rayleigh criterion:       1.22(λ/D)

 

Where D is the diameter of the aperture of the lens used to focus the light on the sample. Visible light, having a large wavelength (yes in this case a few hundred nanometres is large) in the best-case scenario can only be used to form structures approximately 100 nm in size. To combat this electron (E) beam lithography was developed, taking advantage of the wave-particle duality of electrons discovered by Louis de Broglie. Electrons, having a miniscule wavelength in comparison to visible light can be used to make features on length scales unfathomably small with reference to what was possible with photolithography. These wavelengths operate within a range depending on the voltage at which the electrons are accelerated with but for example at 100 keV the electron will have a wavelength of 3.88 pm. [3] Each picometre is 1 thousandth of a nanometre. To show how small this is, the average diameter of a human hair is 50 micrometres, this is equivalent to approximately 500 million picometres.

Perfect right? We are now able to make things as small as, and even smaller than the nanostructures we are trying to mimic, all problems sorted. Unfortunately, no. Where advances in E beam lithography have allowed us to create nanoscale architectures with sub 10 nm resolution, this method also has its flaws. Despite being very slow as you need to direct each and every electron individually, like writing with a pen, and hence very expensive we also cannot make structures as complex and intricate as billions of years of evolution have allowed the Morpho butterfly. Another huge problem is the fact that these methods are only capable of working on flat surfaces as incident light/electron beams need to be orthogonal to the photoresist they intend to activate.

Negative resist (portion exposed to light remains) and positive resist (portion exposed to light is removed)

This inability to make these nanostructures on curved surfaces was a large problem faced by lithographic techniques. Without extending our ability to form these nanoscale architectures into three dimensions we could never hope to achieve the same control over light which flora and fauna have so gracefully managed through millennia of evolution.

An entirely new, collaborative approach was needed. Enter the humble physical chemist. As mentioned, the Morpho butterfly forms its nanostructures with tree like architectures from the polymer chitin and in the immortal words of Aristotle – “If you can’t beat them, join them”. By gaining control of Block Copolymers, [4] a method in which two polymers with wildly different chemical properties are linked via a covalent bond. Polymer chemistry allows the formation of chains with almost any different structure desired. Choosing the right one may allow structures as seen for the Morpho butterfly to be made, but why stop there? The limitation is simply your imagination. Once the polymer is chosen, the two polymers can be easily phase separated into different domains but essentially remain linked via their covalent bond. This is known as microphase separation. This bond stops the two polymers from fully avoiding each other once in solvent and thus forms region after region of phases with opposite properties. These phases can be as big or as little as one likes (within reason i.e. 5-100 nm). Now with the phases separated, one can take either a chemical approach to remove the undesired polymer and leave the desired one or return to lithography if the undesired polymer has been designed to separate in a reaction with light. This process is shown below, where the first step has the chains of polymers tangled as much as your headphones are when you take them out of your pocket.

This method has allowed for many different styles of nanostructure be formed, mimicking many different architectures found in nature such as the nanowires shown above which are akin to the structures found in peacock feathers or the nanorods found in the eyeballs of moths which are used for light absorption to avoid predators/prey seeing them at night. These structures have many prospective uses in tech and optics industries. Think about how hard it is to see your phone screen on a bright day. An antireflective coating with nanostructures like those found in moths would render this problem null and void.

Further applications include the superhydrophic nature of the nanostructures found in cicada wings. [5] These structures have patterns so small that they go beyond the Rayleigh criterion for distinguishable features with natural light.  This essentially means that they are transparent to the human eye, a wonderful feature for laying a coating on top of screens and anything else we wish to see behind. As mentioned, this was an area where E-beam lithography excelled. These tiny structures repel water and thus all the germs and bacteria present in water. These structures are shown in the Atomic Force Microscopy image below.

For this reason, when a cicada dies its body decays as any typical organism would, but its wings do not. It’s wings last unaffected by decay for hundreds of years due to these nanostructures. While I said above that E-beam lithography is expensive and difficult to scale, in the future if it is possible to scale this technology up, or if further advances are made in the block copolymer process, this technology could be used in the healthcare sector. Coating floors, walls and screens in hospitals, areas which are constantly being touched by both sanitized and unsanitized hands and feet may aid in the quelling of disease spread.

Links and references

  1. https://www.smartmaterialsolutions.com/blog/tag/Blog
  2. https://www.google.com/url?sa=i&url=https%3A%2F%2Fwww.plymouthgrating.com%2Fproduct%2Ftransmission-diffraction-gratings%2F&psig=AOvVaw1GrGzVqljOxIWKV357pwS5&ust=1713006595193000&source=images&cd=vfe&opi=89978449&ved=0CBIQjRxqFwoTCIDbhZDFvIUDFQAAAAAdAAAAABAO
  3. https://advanced-microscopy.utah.edu/education/electron-micro/index.html
  4. Mokarian, Parvenah. “Chemistry of Polymers and Macromolecules”, JS NPCAM course, Trinity College Dublin.

/0 Comments/by

What is the Bulk Photovoltaic effect?

Over the past few decades there has been a great interest in the field of photovoltaics for their use in sustainable energy generation. However, as the Shokley-Queisser limit, that dictates the maximum theoretical efficiency of a solar cell, has been met there has been a need to research alternatives. One such phenomena is the Bulk photovoltaic effect something that I hope to give you a brief insight into below.

Unlike the traditional photovoltaic effect, where a p-n junction is used to create electron hole pairs, the bulk photovoltaic effect is seen in materials with broken inversion symmetry, thus displaying non-linear optical properties. A non-linear optical process is one where the response of the medium depends non-linearly on the electric field of the beam. When exposed to high energy photons, the electrons and holes will become spatially separated leading to the generation of a built-in electric field and thus electrical polarisation, producing a d.c shift current. This is complimented with the ballistic current which results from the electrons being able to pass through the material in a direction determined by the symmetry of the crystal without losing energy. Both of these currents will result in the production of a voltage when connected to an external circuit.

The effectiveness of the Bulk Photovoltaic effect in being able to harness a photovoltage that is greater than the material’s band gap stems from the fact that it can use a wide range of wavelengths unlike the traditional photovoltaic effect which only allows the absorption of specific wavelengths. Certain materials are required for this phenomenon to be observed, including transition metal dichalcogenides and certain perovskite oxides with broken inversion symmetry. The aim is to find materials that have an optimal band structure, crystal symmetry, charge transport properties and materials that display minimal scattering to enhance the ballistic current. The problem with many of these materials is the difficulty in making them, especially on a large scale. Methods such as mechanical exfoliation are incredibly slow and offer very low yields, only allowing small flakes of a few microns wide to be made. These flakes can often crack rendering them unusable only to have to start the long process again, something which I have frustratingly experienced many times before.

With the effects of climate change driving the production of sustainable energy, I think it is clear to see the importance of this rather unheard-of phenomenon and I hope I have been successful in giving you a brief insight into the complicated world of physics wherein it lies!

References:
Panoiu N . ‘Chapter 1 – Introduction to nonlinear optics at the nanoscale’ in ‘Fundamentals and Applications of Nonlinear Nanophotonics’, 2024
Chan Y-H . ‘Giant exciton-enhanced shift currents and direct current conduction with subbandgap photo excitations produced by many-electron interactions’ 2021
Zhenbang Dai . ‘Phonon-Assisted Ballistic Current from First-Principles Calculations’ 2021
Zhenbang Dai . ‘Recent progress in the theory of bulk photovoltaic effect’ 2023

/0 Comments/by

About | Blog | Contact | Terms