MRS Meetings Blog

In situ characterization techniques are in!

As materials science advances lead to fascinating new materials and clever ways of processing them, it becomes more and more necessary to see these things in action. That's where in situ techniques come in and there was an entire symposia dedicated to these at the Fall meeting.

To quote an example, in situ Transmission Electron Microscope (TEM) was used by Dr. Dresselhaus' team at MIT to smoothen out the edges of carbon nanotubes, to better control their electrical properties. Using Joule heating, the reserachers were actually able to iron out these creases and wrinkles, in much the same way one would iron a shirt. Some of these experiments were also geared towards practical applications, like studying the structure of platinum nanoparticles and other types of catalysts used in fuel cells. Vendors like JEOL, Protochips etc. demonstrated some of the specialized sample holders used to conduct such experiments.

I find it cool that we are at a point where we can move beyond theories and speculation to actually see what's happening in our new materials.

 

Solution-Based Quartz Crystal Microbalance Measurements!

Ok, so this post is in the "Why didn't anybody think of this sooner?" category…

Aram Amassian, of the recently opened King Abdullah University of Science and Technology (KAUST) is looking into studying early-stage thin film (TF) growth from solution. Unlike in TF growth from the vapour-phase, we can't use scanning probe techniques to follow the process. In fact, solution-growth systems are fairly complicated when you consider how fast solution parameters (like concentration) are changing.

So, how do they do it? If you've ever grown a thin film, chances are that you're aware of the utility of the Quartz Crystal Microbalance (QCM). Indeed, QCMs give rapid access to information about mass, thickness but also stiffness of a growing layer. In his research, Amassian takes the utility of the QCM and applies it to the solution phase. By drop-casting a solution onto a QCM and encasing the whole setup into an nitrogen gas flow cell, Amassian can monitor growth over time and vary N2 flow to increase or decrease solvent evaporation rate. Combined with some optical microscopy and time-resolved XRD, it makes for some very cool experiments!


Since the University was only built a couple of years ago, you can imagine that the research is still at a very junior stage. While the surface of the QCM they use is composed of amorphous-SiO2, they've got preliminary results for SAM-coated QCM surfaces… which should give some really awesome data about solution phase growth in the near future! Can't wait for a publication from these guys!

Australia to the Rescue

There was a lot of discussion on TIPS-Pentacene today, which is the soluble derivative of pentacene developed by the John Anthony group. The major conclusion of the presentations today was that TIPS-pentacene is a great innovation for solution-processing the semiconductor, but that its chemical instability is severely limiting... especially for OPV applications.

Gavin Collis, of the little known Australian National Laboratory CSIRO, presented an alternative to TIPS-pentacene this afternoon. The compound family he talked about was the TIPS-dibenzon[b,def]chrysenes (TIPS-DBC), which can be synthesized from a commercially available DBC derivative (a cheap starting material at 30$/kg). Collis and co-workers have prepared a large number of these compounds, so many in fact that device fabrication has become too onerous and they have moved to pre-screening their compounds using PhotoCELIV! They showed some pretty interesting structural findings, not least of which was that gentle variation of the TIPS (triisopropylsilyl) group to a TES (triethylsilyl) group results in dramatic crystal structure differences… going from slipped pi-stack to a herringbone structure.

Remarkably, devices based on TES-DBC have shown a 2.25% power conversion efficiency, which is a very competitive value for small molecule polyaromatic hydrocarbons.

Looking forward to more results in the near future from CSIRO!

Transparent conducting oxides (TCO)

I spend a lot of time in symposia that deal with organic photovoltaics. Almost every new technical advancement in this field hits a commercial bottleneck when it comes to the transparent conductive oxides (TCOs). The current workhorse TCO is Indium Tin Oxide (ITO). In spite of its limitations; both technical (surface roughness, high temperature instability) and commercial (Indium reserves running out ?), I feel ITO sets some pretty high standards for alternative TCOs to live upto. 

Graphene, carbon nanotubes (CNT), conducting polymers,  Al doped Zinc Oxide (ZnO) are all promising contenders. There were talks on all these at the conference this year and it was indeed promising to see the conductivity values steadily approaching the levels set by ITO. I saw an interesting poster from North Western University which talked about using spinel oxides like ZnCoO4-Co3O4 and ZnRh2O4 as TCOs. Their reasoning is that ligand field splitting in these materials depend on the cation type and hence allows tremendous flexibility in tuning band gaps. Well, I hope so too.

 

Unexpected Bonuses

With all of the talks and posters being presented this week, there is a lot of learning going on.  Obviously this is expected at a meeting of so many scientific individuals.  People attend talks to learn about improved techniques, better materials and the advancements of whatever field you may be studying.  But, of course, there is more!  There are small bits of "bonus knowledge" sprinkled throughout the talks and posters that make you sit up and think, "Huh, I did not know that, " or maybe just, "that's pretty neat."

These are not earth shattering discoveries or anything, but small bits of information that might help in designing a future experiment, or just a fact that may win you a bet at the bar (talking about science at a bar is completely normal right?).  For me, I saw a few techniques that I can use teach my undergrad to help him complete his project.  I also learned about staining in electron microscopy which can make the organic surface of nanoparticles appear as a bright halo. 

There are also the clever names and ackronyms that people come up with for their particles.  One from today was the ackronym COINs (composite organic-inorganic nanoparticles)!

Let me know if you have found any knowledge "easter eggs" in the talks this week, either in the comments or on twitter (I am @theStudentT or you can use hashtag #MRSbonus).

It's Thursday and Things are Still Humming

The conference is almost over, but there is still great science being reported, from novel ways to make PMMA in Symposium OO to Nanowires for CMOS Technology in Symposium W. Further, the poster session tonight is FULL of great work and ideas. I'm looking forward to another informative evening. 

Bio-inspired

As a strong proponent of the theory of evolution, I find the emerging field of bio-inspired science and engineering extremely smart (I'm hoping to pursue a PhD in a bioinspired project myself). To see what is out there, I've decided to camp out in Symposium OO on bio-inspired materials for the rest of the afternoon.

The talk by Timothy Merkel from UNC that just ended was trying to address the issue of limitations caused by small capillary sizes when designing particles for the human body. Instead of trying to fit everything into a smaller particle, they were inspired by red blood cells, which are highly deformable and can easily pass through small capillaries even though they are much larger. Therefore, they have designed particles that mimic red blood cells and tested their ability to move around.

This is what I love about bio-inspired engineering. Some times the solution actually is quite simple and elegant! Excited for the upcoming talks.

MRS Sleepers!

One of the reasons MRS is so great is that it brings together people from all parts of the world. An unintended consequence of this internationality, though, it that there is a huge variation in attendees' virtual time zones. Nowhere is this more obvious than in the symposium rooms themselves.

Coming from Montreal, which is in the same time zone as Boston, I really shouldn't be poking fun at the poor sleep-deprived attendees that I've seen this week… but I'm going to anyway. You just can't help but chuckle when you see someone passing out in the front row of a talk. Yesterday I actually saw a guy drooling. Yes... drooling!

I figure I can't be the only one who finds this funny, so I've started to make a little collection of MRS sleeper pictures (one of which is below). If you have any good ones yourself, post them up!

Thursday

My morning was spent in Symposium A again, learning about new ways to dope, grow, or utilize diamond.  My favorite was the talk about using kitchen wrap to grow diamond films.  Very interesting.

Keeping on today's theme of "Carbon", I also went to the Symposium X talk about epitaxial graphene.  I now have a better understanding of the challenges associated with graphene, and where we are now as far as getting to the point where the promise of graphene becomes a reality.

More Symposium E

I'm in Symposium E again this morning and the topic of discussion is Materials Design in Thin-Film Transistors (TFTs), which is right up my alley.

The talk that I was looking forward to most was to be given by Dr. Antonio Fachetti, a PDF-for-life with Tobin Marks at Northwestern University. The Marks group is a big name in the field of Organic Semiconductors (along with Zhenan Bao and a couple of other notables), and they have especially strong expertise in the field of n-channel materials. However, as often happens with big names, a replacement was sent in to give the presentation. 

While he did a very good job pinch-hitting, the replacement presenter spent about 15 minutes eternalizing on the merits of the Polyera corporation (a spin-off company of the Marks group)… and while it is true that Polyera produces probably the best n-type organic semiconductors in the world, being repeatedly reminded of their product "portfolios" made me feel like I was in a sales session rather than at an academic conference. That being said, I did learn something cool from the experience: Polyera is getting into OPV bulk heterojunctions (which I blogged about yesterday) and have developed a product with an NREL-certified power conversion efficiency of 7.26%, with results of >8% expected by next year!

So whining aside, the second part of the talk more than made up for its questionable beginnings…work incorporating carbonyl and dicyanovinylene moieties into ladder-type polymers was demonstrated and the presentation followed the familiar pattern of B3LYP/6/31G** DFT Calculations --> Synthesis --> Characterization --> Device Fabrication.



In the characterization portion of the talk, the presenter showed that when carbonyl functionalities are replaced by dicyanovinylene moieties, the conjugation length in the molecule is preserved and yet there is huge change in melting point (~80-100 deg. C). He contrasted this to other very similar structures, showing that in the latter case, introduction of the dicyanovinylene moiety leads to a much smaller changes in MP. The reason? Steric disruption which indicates poor porspects for mobility. I was pretty amused to see how they had used ye olde melting point apparatus to predict performance characteristics in resulting compounds… it's undeniably awesome that an analytical technique with a fixed cost of ~$100 can provide structural information as useful as >$100 000 machines. 

Bottom line of this work was that the findings for this new family of compounds are consistent with previous stuff published by the Fachetti group… namely that it is possible to modulate the charge carrier in a material family (from p-type to p/n ambipolar type to n-type) by varying molecular LUMO energy... and that LUMO energy dictates ambient air stability of n-channel operation.