Alex's Blog - semiconductors and crystals and and pretty stuff

semiconductors and crystals and and pretty stuff

hullo!

I'm Alex, rising senior (whoa!) - I'll be sharing my experiences at Rutgers in this blog. I'm currently a part of Dr. Jing Li's group. Updates will be whenever I feel like :)

10.7.2017 (1)

Up till now I've been busy synthesizing a series of phosphorescent crystals which will be sent out for analysis this month. A bit of background; this group has already produced a method for solid-state lighting (LEDs and stuff, as opposed to incandescent and fluorescent lighting) which is completely free of rare-earth metals. Instead of REMs, they have created and patented an inorganic-organic hybrid semiconductor. These semiconductors are metal complexes which have an ionic backbone and organic ligands.

cool stuff huh.

lesson 1!

A metal complex is a chemical compound which has several coordinate bonds. A typical chemical bond is composed of two electrons shared between two atoms. Each atom contributes a single electron. In a coordinate covalent bond, both electrons in the bond originate from one atom. One of the simplest examples is the ammonium ion:

one of the N-H bonds consists of electrons solely from the nitrogen atom

Polar compounds with lone electron pairs (including water!) tend to form coordinate bonds with metal ions in solution. The complexes I'm currently working with involve a CuBr backbone and a series of organic ligands.

12.7.2017 (2)

okay being truthful first post was kinda me being lazy.

Complexes are very much related to what the group is doing; the whole project is based on forming semiconductors with an inorganic background and organic ligands (if you're interested, go on PubMed and read up on "hybrid inorganic-organic semiconductors"). Knowing about complexes, however, was really just a convenience that let me know what was going on ¯\_(ツ)_/¯

Over the past two weeks I've followed a pretty standard synthesis developed in the group's previous papers. Here's a link to the ACS page; I'm not legally allowed to post the full paper here, but message me if you're interested! Much of the previous work is centered around CuI phosphors. At the first group meeting I attended, one of the senior researchers gave a work update detailing some trends he found with CuBr and CuCl compounds. Without going into too much detail (I want this blog to be about something else!), the data was quite contrary to what one might expect. My supervisor had given me a bit of leeway in what I was to accomplish this summer, so I took it upon myself to see if the published methods could be reproduced with CuBr and CuCl.

making a CuCl stock solution matcha lookin ass

Rutgers, being Rutgers, is not super rich. The equipment on our floor for crystal analysis is quite limited (we have a PXRD device and a fluorometer and a few others), so crystals are sent out in batches to other locations for analysis and the numbers come back in a few days.

ghEttO a$$ lAb

Meaning I've had nothing to really do aside from reading up, performing syntheses, and playing Pokemon Go.

1. reading up

Naturally I had to get myself acquainted with crystals and if you do a quick Google search you'll find your Bravais lattices and your -monoclinic- -orthorhombic- -pew pew- stuff; general chemistry! Reading papers is a different story; all crystals are described by not only lattice constants, they are described by space groups. Space groups are super cool! They're sorta the next step from wallpaper groups.

wallpapers :)

2. syntheses

Most of the syntheses I've performed thus for have been parallel reactions; that is, varying only a single component of the reaction per every trial. In my case, I've spent most of the past weeks on synthesizing CuBr crystals via a three layer process. The first layer is a saturated solution of CuBr.

But Alex, most halides are insoluble! CuBr is, in fact, soluble in a saturated solution of Br^- anions. The dissolution is driven by the formation of a CuBr₂^- complex. More on that here. I supersaturated a solution of KBr to get the job done (hydrobromic acid would have created acidic conditions and messed with the reaction mechanism).

here's a blue-fluorescing CuBr compound

The second layer is 2 mL aceotonitrile, and the final layer is an organic ligand dissolved in some solvent, usually dimethyl-formaldehyde or an alcohol. Of the 30 or so unique syntheses I've performed thus far, 9 appear to have a promising quantum yield.

a collection of the hybrid compounds i've synthesized which produce visible emission

3. mannn i almost caught a machamp today too :/

17/07/2017 (3)

Most of my work will be done on crystals, so i wanted to include a short background; for further learning I recommend Frank Hoffmann's online YouTube course on crystallography.

So to start, what is a crystal? You may start with an image of something like this:

an amethyst crystal

And you'd be right! Amethyst is a variation of quartz (SiO₂) which has been doped with iron impurities, giving it a purple hue. What about this generic image made you think of a crystal? Was it the geometric patterns? Sharp edges? Jagged points sticking out of a rock? These are all common visual properties of the crystals we find in nature. But what truly defines a crystal?

From Atkins inorganic: "A crystal of an element or compound can be regarded as constructed from regularly repeating structural elements, which may be atoms, molecules, or ions."

This is not a bad definition. It is missing something important, however: all crystals are anisotropic. More on that later.

We can now explore crystals with the following questions:

1. "From which structural elements (atoms, molecules, ions) is the crystal formed?"

2. "How can the regularity of the crystal be described?"

From the naked eye, crystals may be jagged or smooth, large or small, and a whole range of colors, but they are all constructed of regularly repeating building blocks. No, not atoms; if all crystals were characterized by their atoms, then even a simple crystal like quartz, which is composed of silicon dioxide, would have to be characterized by the arrangement of silicon and the arrangement of oxygen. For more complex crystals containing many elements, this is not practical. What is meant by "building blocks" is a repeating structure, which we call the unit cell.

Consider the figure on the right. Is the concept more intuitive? There are countless crystals which share such a configuration; the elements which compose the cube may vary from crystal to crystal, but the crystals share the property that the cube is regularly repeated along three dimensions, forming a larger structure. For a more clear cut and formal exploration, click here.

Of course, a cube is not the only form a unit cell can take. Notice the term "lattice points" in the above figure. We can draw a set of axes along them to make them seem a little more familiar:

Notice how the vertices of the cube (our lattice points!) can be seen to form our friendly xyz plane, with everything at right angles.

But some unit cells are slanted! Like this one:

In this case, drawing the crystal on our standard, orthogonal xyz plane would cause a lot of trouble; it wouldn't line up! So instead we've just shifted the entire set of lattice points. In total, there are only 7 general lattice point structures, which we call crystal systems.

The "cell constants" refer to the distance between each lattice point in each direction, and the "cell angles" refer to the angle between the axes created by the lattice points. Notice that there is a column for "maximum symmetry"; it serves to differentiate the trigonal and hexagonal systems. There is one key thing I'd like to point out from this table; the crystal system is determined by symmetry, not cell constants and angles. A triclinic crystal may coincidentally have orthogonal angles, but will never have the same maximum symmetry as a cubic, orthorhombic or tetragonal system.

Things are starting to get a little bit technical, so I'll cut it off here. There's a lot more to crystallography than these basics, and I encourage you to explore the field more extensively!

31/7/2017 (4)

hullo again!

I've been using a lot more of the available equipment over the past two weeks or so; unfortunately all of the files are super weird and I can't open them without the appropriate software :(

Out of all the phosphors I compiled within the first few weeks, one CuBr crystal showed promise as a candidate for further analysis. Just one! The other phosphors were produced in the powder phase, indicating poor stability. On the other hand, I may be publishing a paper in Royal Society of Chemistry sometime, so stay on the lookout for that!

Here's a quick look at the crystal:

rod-shaped crystals via 3-layer synthesis

The material shows strong luminescence at 573 nm under 365 nm light, making it a strong candidate for warm-white light applications. Lightbulb shopping can get pretty intense; kitchen and household settings might prefer warm white light with higher intensity red and orange components, while cold white light is preferred for labs and hospitals.

spectra for warm/cool light LEDs

Here's some funky equipment I've been working with:

our TGA device

This device, bought from our beloved Texas Instruments, is a means of measuring thermostability. A small amount of some sample (literally tenths of milligrams) is loaded onto a small platinum pan. The sample is then heated under certain conditions and the mass is tracked throughout the process. The device can be set to measure isothermal conditions, say, 500K for two hours, or increasing conditions, usually up 10K every minute up to 973K. These measurements can all be classified as thermogravimetric analysis (TGA).

TGA for CuBr compound

The graph above shows a dip in weight percentage at around 250C, indicating evaporation of the organic component around that temperature (recall that these crystals are inorganic-organic hybrids).

Here's another thing I use :)

they're called bombs lol

While their official name is "steel autoclaves", these containers are usually just called bombs by the people in my group. A small container filled with reactants is put inside each of them, and they're screwed shut via wrench and vice. Putting these bombs into ovens allows for high pressure and high temperature conditions.

In other news, I'm trying to design 3-D templates for a makeshift upconversion device as well as my spectrophotometer from STEM day. More on those coming soon :)