Friday, May 16, 2014

Laser enhancement - A path to defect free diamond growth?

In this post I'll take a look at an article titled "Control of crystallographic orientation in diamond synthesis through laser resonant vibrational excitation of precursor molecules" published recently in Scientific Reports.  In the article, Zhi Qiang Xie and coworkers describe the potential benefits of using a laser to introduce resonant vibrations of molecules during the growth of diamond films. How can laser light improve the growth process, and does this technique hold promise for improving diamond growth in general?

File:Laser play.jpg
Lasers! (Image from Wikipedia)  
You may be aware that it is possible to create diamond using lab equipment instead of mining it where it occurs naturally in the earth, but it may surprise you to learn how many ways there actually are to grow diamond in a lab. The one that is most interesting to me, as someone interested in diamond electronics, is a method called Chemical Vapor Deposition, or CVD.  The basic idea is to take molecules with carbon and hydrogen in them (which are not so surprisingly called hydrocarbons), and then excite them with some form of energy to form more reactive species.  Those excited molecules then react with a surface, adding carbons one at a time to an existing piece of diamond.  If that existing diamond is already a single crystal, the new diamond grown on top (with the exception of some defects that may grow) will be a part of that same single crystal.  Growth can also be done on existing diamond that is in the form of tiny nanodiamond crystals (in case you missed it, nanodiamonds were introduced in the last Blingtronics post).  Using nanodiamonds spread onto a surface, a diamond film can be grown as the individual nanodiamonds grow into larger (but still small) diamond crystals, bonded together strongly at the interfaces between the crystals. When the tiny crystals grow large enough, this kind of diamond is called polycrystalline diamond (which means "many crystals") and the interfaces between the crystal "grains" are called grain boundaries.

File:Apollo synthetic diamond.jpg
This single crystal diamond was grown by CVD (from Wikipedia)  

There are a number of different kinds of reactors that can perform CVD reactions. The ones I am most familiar with, from using them at Michigan State, are microwave plasma reactors.  In a future post I hope to describe the MSU reactors better, but for now I'll just briefly mention some of the most common types of microwave reactors, just noting here that there are other kinds, and the one I have used is not one of the ones pictured below.

This Figure,  from [2], shows four common types of CVD reactor: (a) Hot Filament Reactor  (b) 'NIRIM' type Microwave Plasma Reactor (c) 'ASTEX' type Microwave Plasma Reactor (d) DC Arc Jet Plasma Torch Reactor 

The main distinction between these reactors is the method they use for splitting gasses into more reactive molecules that can then grow into diamond.  For example, in a hot filament reactor, like the one with the cross section up there labeled "a," the hydrocarbons are split into more reactive molecules by a filament, which is like the glowing thread you see inside an incandescent bulb.  The next two reactors shown use microwaves (just like in a microwave oven) for the splitting.  The last reactor shown is a bit different from the first three.  The first three reactors use a pump (like a strong vacuum) to remove most of the air from a chamber, leaving only hydrogen and the hydrocarbons gas, (like methane or acetylene) which are allowed into the reactor at a controlled rate.  On the other hand, the reactor in the fourth image, labeled (d), doesn't use a strong vacuum pump to make sure that the reactive diamond growing molecules are the only gasses around the sample.  It is a DC Arc Jet Plasma torch style reactor, and in this style the gasses at the point of growth are controlled by using a much higher flow rate of gasses, as a "jet" onto the sample as it's being grown.  Because the jet is strong, it doesn't need to be in an ultra pure environment pumped free of other air molecules like the previous reactors, and can even be operated outside of a chamber.  In this week's article, that's exactly the kind of system the authors used to grow their diamond, a plasma jet operated in air, and in this setup, they've modified it with a laser.

Diagram of the setup of the Arc Jet diamond deposition system, with a CO2 laser added to enhance the growth process, Figure from [1]  
So what does the laser do? Well, remember how I said that the reactor works by splitting the gasses into more reactive molecules? Basically the energy knocks some atoms off of a molecule, which gives the molecule new sites to bond to other things.  Molecules can also behave in different ways if they are excited by a resonant frequency.  It's kind of like that trick where a resonant sound frequency can break a wine glass.  Here's a nice video and explanation of how that works, (which I found here):

By using exactly the right frequency, the glass moves in sync with the sound waves.  In a similar way, molecules can move in resonant modes when they are hit with light waves of exactly the right frequency. In fact, there are a number of different types of these modes, where the hydrogens on a hydrocarbon move in a patterned way with each other: 
Vibrational modes of the methylene group, from the UC Davis Chem Wiki
So how do you find these exact light frequencies that make the hydrogens go into resonances like those? Remember  in the glass breaking video how he found the resonant frequency of the glass by hitting it and listening to it's natural vibration pitch? Something similar works for light waves and molecules.   Xie and coworkers shined laser light of different frequencies through the arc jet, and saw how much or how little was absorbed by the molecules.  If a lot was absorbed, it meant they were resonating with it.  The two peaks they found (10.532 μm and 10.22 μm) both correspond to a wagging mode (it's the fourth one up there!) of one of the hydrocarbons that is made in the splitting process in the diamond deposition reactor (the CH2- molecule).

The absorption for different laser wavelengths, from [1]
Xie and coworkers described in depth how these resonances might affect the diamond growth (which you should totally read about in their paper for more info if you're interested), and they looked to see if shining laser light of the resonant frequencies they found into the arc jet during deposition would change how the diamond grew.   They tried four different frequencies: a control with no laser, 10.22 μm (one of the wagging modes), 10.333 μm (no particular resonance) and 10.532 μm (the other wagging mode).  They grew polycrystalline diamond, which again means that lots of little diamond crystals will grow together into a rough film of connected individual crystals.  So did it work? Did the laser make the reactive molecules spin around more and give better crystals in the diamond as they grew?

polycrystalline diamond films grown with different enhancement laser wavelengths, from [1]
The short answer is, yes!  Both the 10.333 μm enhanced and the "no laser" films look more or less the same, showing that any change in the other two diamond films isn't just due to shining a laser into the jet.  The other two films do look different, especially in the bottom row, which were grown for 60 minutes instead of the 15 minutes in the top row.  The crystals in the  10.532 μm film look much bigger.  More interestingly, the crystals in the film in the 10.22 μm enhanced film look like they are all flat on top.  This means that the atoms in that film are all aligned in such a way that they show a particular crystal face.  That face is named the (100) face by crystallographers (totally a real job), and although I'm not going to get in depth about crystallography in this post, it's probably enough to know that the flat top surfaces are named (100), and that the fact that they are the (100) ones is a good thing.  For example, polycrystalline diamond with (100) faces on the surface is better for optics than other types of polycrystalline diamond, and so the ability to control the surface orientation to produce (100) polycrystalline diamond is a pretty neat advance in this field.  They also did the trick with the 10.22 μm laser to enhance single crystal growth, and got a single crystal diamond with virtually no defects on the surface.  A neat trick for sure!  Although these authors credit a lot of the improvement they saw from the laser enhancement to molecules that contain oxygen (which is not as possible in other diamond reactor designs), the increased energy of the right molecules by laser enhancement could, in theory, hold the key for defect free growth in other types of reactors as well.

So, as for our original questions, I think I've shown how laser light could improve the growth process, and that maybe this technique could have some potential for improving diamond growth in general! [1] Xie, Z., Bai, J., Zhou, Y., Gao, Y., Park, J., Guillemet, T., Jiang, L., Zeng, X., & Lu, Y. (2014). Control of crystallographic orientation in diamond synthesis through laser resonant vibrational excitation of precursor molecules Scientific Reports, 4 DOI: 10.1038/srep0458
[2] May, P. (2000). Diamond thin films: a 21st-century material Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 358 (1766), 473-495 DOI: 10.1098/rsta.2000.0542

What did you think of the post? Love it or hate it? I got it all wrong?  At last something's been explained so you understand it? What would you like to see more or less of? Please feel free to leave a comment!

Monday, March 31, 2014

How to Incorporate Elusive Atoms into Nanodiamond

In an article published last week in Nature Communications [1], Andrew Magyar and coworkers described a method for getting europium with a particular electronic structure into diamond.  Before we examine the how let's look for a second at the why, as in, "why would you want to put europium into nanodiamond?"

Europium is an atom of a class called lanthanides, which have some pretty amazing properties.

Periodic Table Lanthanides

Here they are!  The Lanthanides are all in this row of the periodic table, named for the first in the series, lanthanum.
Lanthanides form colored compounds, and have very long lived  luminescence, making them useful in all kinds of applications, from lighting, like in fluorescent bulbs, to tagging of materials and molecules to see how they move and change at a very small scale, to future applications including quantum computing.

Europium Assorted glow-in-the-dark paints
It glows!  The green and aqua contain europium, and are the brightest available phosphorescent powders 
And what about nanodiamonds?  Nanodiamonds are very small crystals of diamond, only several nanometers in size.  A huge advantage for nanodiamond is that it can be functionalized, that is modified by adding things to the surface, in a way that other forms of diamond (like bigger crystals) often can't.  They are also very small, and biocompatible.  They can get into cells, and deliver drugs, like for cancer treatment, in a well controlled way.  They can also be used as the seeds for the growth of continuous films of diamond. These films are called ultrananocrystalline diamond (UNCD) or nanocrystalline diamond (NCD) (it depends on how big the crystals of diamond are when they grow together), and they have lots of potential applications [3].

Structure of a single nanodiamond particle.
A nanodiamond  - This figure, from [2], shows the structure of a single nanodiamond particle

So back to the original question, why do we want to get these lanthanides into these diamond particles? Diamond is seen as the material of the future for quantum computing, it has a couple of interesting defects, particularly the NV center, which I discussed briefly  in my last post. Incorporating europium into diamond could open up exciting applications for quantum memory, and by incorporating the luminescent europium into nanodiamonds, they can be used as biomarkers, allowing researchers to track things that are happening inside cells.

The problem is that it has been hard to get these atoms into the diamond and having them stay active the way they should in order to preserve these luminescence properties.  Magyar mentions that in some previous work they tried ion implantation, without success.  Ion implantation is the process of bombarding a surface, in this case a bunch of nanodiamonds, with very high energy atoms, for example europium atoms, and then hoping they will land inside the material, without too much damage to material. For most materials you can repair some of the damage you cause in the process by heating it afterward, but as I mentioned briefly before, diamond doesn't heal unless you put it under a lot of pressure while you heat it.  As a result, getting europium atoms into the nanodiamonds was a bit tricky, and so required a trick.  And that trick is... (cue epic music) electrostatic assembly.

Electrostatic assembly is by no means a new trick, it is the method, for example, that printers and copiers use to position toner on paper.  There isn't just one way to do it either, a number of different sources of the charging, from large electrodes down to the tip of a nanometer sized probe can be used to guide the assembly.  Proving that europium can be put into diamond this way could mean that researchers may now be able to effectively "print" not just europium, but a wide variety of atoms onto a growth surface, and incorporate them into grown diamond films.

The trick here is that they can't just put the europium on the diamond surface and then grow and get the desired results.  Europium forms oxides (bonds with oxygen) easily, and the oxides make it really unlikely that the europium will incorporate itself into the diamond when they grow on it.  To do the electrostatic assembly, it's necessary to have an oxidized surface, since the oxygen acts as the negative charge of the electrostatic attraction.  To keep the europium safe from the oxidized surface, Magyar and coworkers used a polymer (polymer is essentially a fancy way of saying plastic) as a layer between the europium and the oxidized surface.

They found the best results when they formed an oxidized surface on a wafer of silicon dioxide (it's a common substrate, and really easily formed in modern silicon fabrication processes like what are used to make computer chips).  This is shown in the figure below where the blue circles with the negative charges in them are oxygen.  Just to be clear, in the figure below they seem to be saying that the surface is diamond, but in their most successful method it was actually not, it was silicon dioxide.  (The nanodiamonds will be added in the last step). So, after they oxidized the silicon dioxide, they used electrostatic assembly to put down the polymer layer on top.  Next comes a layer of europium, which is modified so that it is able to be used this way by surrounding it with carbon atoms (the pink dot in the figure below is europium, the grey material around it represents carbon atoms).  They then spread nanodiamonds over the surface, and put it into a diamond growth reactor.  I won't get too into the details of diamond growth in this post (but I definitely plan to in a later post), essentially they grew the diamond long enough that the individual nanodiamonds got larger, but stayed individual nanodiamonds without growing all together into a single film.

The method for assembling europium on the surface before growth (from [1])

Magyar and coworkers also tried this method for chromate, a source of another well known luminescent defect in diamond, and were successful with the chromate as well.  As a result, they believe it can be extended to a bunch of other atoms, essentially anything that can be electrostatically assembled.

So what does it all mean? In addition to luminescence related applications, like tagging of nanodiamonds for biological visualization, there are also other potentially interesting ways that patterning diamond with lanthanides and other interesting atoms could unlock new technologies.  Patterning is always a useful technique for electronics, and having a way to localize interesting atoms with techniques engineered based on this method using electrostatic assembly could unlock a whole host of new, cool diamond electronic and (and optoelectronic!) devices.

[1] Magyar, A., Hu, W., Shanley, T., Flatté, M., Hu, E., & Aharonovich, I. (2014). Synthesis of luminescent europium defects in diamond Nature Communications, 5 DOI: 10.1038/ncomms4523
[2] Mochalin, V., Shenderova, O., Ho, D., & Gogotsi, Y. (2011). The properties and applications of nanodiamonds Nature Nanotechnology, 7 (1), 11-23 DOI: 10.1038/nnano.2011.209
[3] Williams, O., Nesladek, M., Daenen, M., Michaelson, S., Hoffman, A., Osawa, E., Haenen, K., & Jackman, R. (2008). Growth, electronic properties and applications of nanodiamond Diamond and Related Materials, 17 (7-10), 1080-1088 DOI: 10.1016/j.diamond.2008.01.103

Saturday, March 29, 2014

What is "Blingtronics?"

The term "Blingtronics" seems to have first appeared in the New Scientist article Geek chic: The rise of blingtronics from April 2010.  'Bling' comes from the noise we imagine light makes when it glints off of something shiny and expensive, diamonds, gold or platinum for example.  Diamonds aren't just great at being shiny though, they have a whole host of properties that would make them really great for a surprisingly wide range of electronic applications.  So diamond electronics (especially if we make electrical contacts to them out of gold - extra shiny!) is what I am calling here "blingtronics," in the hope that it will attract some well deserved attention to this potential electronic juggernaut of a material.

GIA certified diamonds
As if being really sparkly weren't enough! (

In the Blingtronics article, author Jon Cartwright tours a lab in the Centre for Nanoscience and Quantum Information at the University of Bristol, UK, and gives a nice overview of both some very interesting potential applications for diamond, as well as some of the problems researchers face when dealing with this extreme material.

In discussing an application for turning heat into power (by thermionic emission), Cartwright notes one of the difficulties is that it is notoriously difficult to introduce other materials into diamond.  The atoms are extremely densely packed, and if you damage diamond badly, for example by bombarding it with atoms, you can't easily repair it.  In silicon, on the other hand, it's a fairly simple process to introduce non-silicon atoms into the material, and then heat it to repair the damage.  Heating diamond, unless it's under an exceptional amount of pressure, will only turn it into graphite, which is the state that carbon would naturally form itself into under normal atmospheric conditions.  This is a huge problem for electronics applications, since virtually all of the applications for electronics involve carefully patterning regions where atoms are inserted with one more or one less electron, to make circuits and switches.  This difficulty with just getting the other atoms, called dopants, into diamond, is an ongoing issue in diamond electronics, which I will certainly discuss more in this blog.

Cartwright also discusses several other promising applications for diamond.  Quantum computing and optical circuitry are connected fields that both see diamond as the material of the future.  Diamond has a particular type of defect, where a Nitrogen atom, and an empty place where an electron should be, called a vacancy, form a Nitrogen-Vacancy, or NV center.  The NV center, when hit with certain kinds of light, emit bright red light.  These emitting centers could be used as the basic units of a quantum computer, or for imaging, for example, NV centers in very tiny diamond particles called nanodiamonds, which allow tracking of biological processes taking place, even in living cells.

Diamond is also biocompatible, a substantial advantage over other electronic materials, in that elaborate encapsulation isn't required to interface diamond electronics with living tissues, for example in retinal implants for the blind.   Nanodiamonds can also be used for other biological applications as well, like drug delivery for cancer treatment.

Diamond is an amazing material, but is not without challenges.  I look forward to describing some of the major advances in this field, and hope you look forward to learning more about the exciting field of blingtronics!

An Introduction

Welcome to my brand new research blog!  I decided to try writing a research blog after a recent discussion with another graduate student here at Michigan State University, Katy Meyers, author of the hugely successful Bones Don't Lie blog.  She makes a very convincing case that research blogging is a nice way to stay caught up on the literature in your field (so important and so difficult to do!), and can even be seen as a form of publication.  I think it appeals to me as well because I feel passionate in general about conveying the importance and excitement of recent research findings to the general public in a way that is accessible.

On a personal level, I also hope to find a source of motivation here. For all its difficulties, I love being involved in the slow (often frustratingly so) march forward that is scientific progress.  I think it's really important to find perspective from time to time, to keep from being too bogged down by the minutiae that can sometimes dominate the experience of science.  I find it really motivational to take the time to look at the recent progress that has been made by other researchers, and to reflect on what others have accomplished.  I think when we see how interconnected all of our progress is, that each of us are able to do more as we collectively push open the door into the realm of the possible a little bit farther, that it can be invigorating and lead to even more new progress.  It is in this spirit of hope that  I will now begin my research blogging, so welcome and enjoy!