supermaterialer bloghoved

Forskerspirer - der er håb forude!

Hvordan får man en superstresset forsker til at hjælpe svare på ens mange spørgsmål? Det kan man blandt andet lære i denne blog om Forskerspirer, et landsdækkende program for at hjælpe forskningsinteresserede gymnasieelever på vej.

Her på den lukkede nanoafdeling, er der ikke mere lukket end at vi ind imellem tager erhvervspraktikanter, gymnasieelever og andre værdigt videnstrængende ind under vores vinger. Det plejer at være sjovt og spændende, også for os. Der er intet som en spørgelysten knægt eller knægtinde der kan minde en om hvad rå nysgerrighed kan udrette - når den endnu ikke er forplumret af et tonstykt tæppe af "det kan man ikke", "det er prøvet" eller "det har vi ikke råd til".

Jeg har været med alskens formidlings aktiviteter - naturvidenskabsfestival, "ring-en-forsker", populære foredrag og så videre, men praktikanter - dem der kommer her personligt - tager vi ikke så mange af, og vi tager kun de allermest motiverede (ikke nødvendigvis de dygtigste :).

Så er det jo EKSTRA sjovt når man bliver udsat for nogen der både er dygtig, motiveret og piv-stædig.

Jeg fik besked om at en pige, Dorte Danielsen, som havde været i erhvervspraktik i 2012, nu som gymnasieelev var blevet indrulleret i et program kaldet Forskerspirer. Det er noget så dansk som et ikke-elitært eliteprogram: "projekt forskerspirer giver elever med stort fagligt overskud, mulighed for at fordybe sig i et selvvalgt emne og udarbejde forslag til et forskningsprojekt". Det vil sige en "erhvervspraktikant" der SELV skal komme på banen og skrive et forslag til noget sej og ny forskning.

Det gjorde *mig *nysgerrig, så vi (mig, PhD studerende Lisa Tschammer og min kollega Tim Booth) inviterede denne forskerspirende Dorte til møde for at se hvad det var en. Rimeligt hurtigt blev vi enige om et projekt om ... grafen. Suk, men det er altså det min gruppe beskæftiger sig med. Ikke nok med det - Dorte kastede sig over kvanteelektronik (!). Det skal lige siges at netop kvante- og nanoelektronik ikke er typiske førstevalg (for at sige det mildt) for piger, så det er stærkt opmuntrende at netop dette temmelig barske felt ikke kunne afskrække den unge spire.

Her blev jeg bekendt med Dortes innovative og særdeles effektive email teknik. Når de artikler vi havde stukket hende ud gav anledning til spørgsmål (og det gjorde de vist hele tiden), tikkede der emails ind i mailboksen, med sværere og sværere spørgsmål. Et par gange (må jeg tilstå), så jeg hendes email (sorry, Dorte) men kunne ikke tage mig sammen til at svare med det samme. No problemos - emailen blev bare sendt igen og igen, med stadigt kortere mellemrum - lige høfligt formuleret hver gang. Her kan man se hvor frygtindgydende godt det virker:

Illustration: Privatfoto

*Havde jeg vist ikke selv sagt til første møde at man skulle køre på og ikke acceptere ikke at blive lyttet til eller lagt mærke til når man havde med fortravlede forskere at gøre? *Hmrpf.

Jeg synes sgu det er friskt gået til sagen, og sikke nogen dejlige sjove spørgsmål. Jeg har klistret et (langt) eksempel på en sådan Q&A udveksling i halen på denne blog. Inden jeg starter på den smøre, skal jeg da lige berette hvordan det gik videre i Forskerspirer 2014. De 130 kandidater skrev en synopsis, hvoraf 12 blev nomineret til prisen - 3 inden for hver af naturvidenskab, samfundsfag, humaniora og sundhedsvidenskab. Og minsandten om ikke vores Dorte var blandt de 12. Jeg må da indrømme at jeg havde lidt sommerfugle i maven da Christina Antorini læste vinderne op i går mandag kl 14... Dorte blev slået på stregen af en anden meget dygtig gymnasieelev, Asbjørn Bækgaard Lauritsen.

Flot næsten vundet! Trods "nederlaget", viser Dortes synopsis at hun har formået at beskrive en rimelig heftig nanoteknolisk projektide med stort fagligt overskud. Det går ud på at få blok kopolymerer til at ensrette i et elektrisk felt, så man kan bruge dem til at lave ekstremt fine mønstre i grafen - og dermed åbne et fint lille elektronisk bandgap, som er nødvendig for at kunne lave højfrekvens analog elektronik (no gap no gain). Lisa Tschammer (PhD studerende) har aftalt med Dorte at det skal prøves under alle omstændigheder (også selvom hun ikke kommer med 20000 kr vinderpris til at udføre projektet), og det skal Dorte da være med til.

Og måske - man kan jo håbe - er der lagt i ovnen til næste spireprojekt. Vergil - vores erhvervspraktikant i denne uge - har travlt med at lære at syntetisere grafen på metaller, så vi kan lave bedre anti-korrosive coatings af stål i harske miljøer: olieboringer, pumper og sprøjtestøbeudstyr. Måske får vi ham at se som "spire" om et par år - det ville være herligt.

Nå men jeg beklager at jeg skriver sådan en overpositiv og ukritisk blog, i stedet for at brokke mig over et eller andet. Hvis det endelig er, så savner jeg email-terroristen Dortes mange gode og skræmmende dybdeborende spørgsmål. Man kan jo så tænke over om hun stiller alle de spørgsmål fordi hun er megadygtig, eller er megadygtig fordi hun stiller alle de spørgsmål.

HER KOMMER som lovet en af Dortes lange spørge emails, gengivet med tilladelse fra Studenten:

*DORTE: Thanks for answering :) -I’m hope I’m not exhausting you too much or making you regret, you agreed to help me. I think block copolymer lithography sounds interesting. But why is it harder to transfer the patterns to graphene than with this method, and what are doing in order to improve it?

PETER The density of the pattern, the very fine details, and the Chemical similarity to the carbon lattice (which is very thin) and the polymer, makes it difficult to etch the carbon without harming the mask - which will ruin the pattern.

DORTE Can you manipulate the morphology of the microdomains in the block copolymer once it phase-separates?

PETER not after it phase-separates, but Google graphoepitaxy and block copolymer and you can see that putting guides on the surface can help the patterns to organise in more ordered structures. ALso try directed self-assembly..

DORTE I was wondering since RGO (red. reduceret grafen oxid) already has holes and defects, can you use some sort of patterning technique to enlarge those holes to make a GNM?
Or is it perhaps more difficult to locate and enlarge already existing holes than to create new ones?

PETER Yes, but they are not ordered or spread out evenly - in fact they tend to form weird islands, so you will just get ruined graphene

DORTE When it comes to nanomeshes, I was also wondering, is there a reason why nanomeshes most of time have a "net”-structure?

PETER Firstly, this was the structure the theorists picked to calculate: a triangular lattice. Secondly, this structure is "closed packed" - the block copolymer can squeeze in more dots in same area by closed packing... This makes it a natural energy minimum shape for 2D crystallisation (Gibbs energy and all that). ---

DORTE Is it the easiest or most stable way to make them or does it affect the electronic properties or something like that?

PETER Cubic patterns (rectangular ) should be just as relevant for Electronics... and some people try to make those - however, this is very hard with BCP.

DORTE Have other possible networks of GNRs (red: Graphene Nanoribbons) been fabricated? -like aligned GNRs, connected at both ends?

PETER Yes. IBM published a paper on that. We are also making GNR arrays. Finally, a few Groups attempted making GNR arrays with electron beam lithography.

DORTE Is it possible at the moment to make gates that are short enough for the velocity overshoot to kick in?

PETER Yes. Let us just for my sake distinguish between a "gate" - the conductiing object you use to adjust the number of carriers in the channel, and the "channel", where the electrons flow as much as the "gate" allows. Then there is "source" and "drain" contacts or terminals, where the current flow in and out of the "channel". These are characterised by contact resistance mainly. The channel is characterised by "mobility" and "charge density" and the gate is characterised by the capacitance (as in the plate capacitance formula,) and leakage current (which should be as low as possible) which depends on the "gate dielectric" - an insulating layer whose dielectric constant and thickness is affecting the capacitance of the gate, and the number of traps and quality of the gate affects the leakage current as well as scattering effects caused by the traps.

DORTE: If you used GNRs as the gate, is it then possible to make a GNR which is short enough to be in the ballistic regime and narrow enough to open a sizable bandgap?

PETER if it is very short, i.e. in the ballistic regime, i guess electrons can tunnel from source to drain despite the bandgap - which is why it is difficult to turn the current off.

DORTE I read in the Frank Schwierz review article that edge roughness could be a problem for narrow GNRs and that it could affect both the bandgap and the carrier transport. Do you know what’s being done in order to smoothen GNR edges? - Do you e.g. use high temperature annealing?

PETER People have shown that, but i am not aware that transport measurements have been done before and after. That being said, current annealing (passing a high current through the structures) has been used to increase carrier mobility (transport properties) - but that is more likely to be an issue of cleaning the graphene from defects/contamination, and Thus removing scatterers.

DORTE Does the edge smooth- or roughness also affect the size of the mean free path?

PETER yes. If there is no edge-roughness -- you can have specular (mirror like) scattering in a long ballistic channel, and it would be like a waveguide - i.e just like in billiard the bouncing on the edges does not backscatter the electrons ( send them back where it came from), and Thus does not increase the measured resistance.

DORTE I was thinking my project could be about making narrow, short, connected aligned GNRs, if this is desirable.

PETER That would be very interesting. Perhaps we have a project that you could participate in... related to making aligning of BCP for nanopatterning. I dont think making linear BCP is realistic - it seems to be harder. But i wont lie to you: we have tried for years to makke BCP electrical devices and have had very few electrical Measurements - it is very difficult.

DORTE I’ve read some of the article, 'Epitaxial Graphene Nanoribbon Array Fabrication Using BCP-Assisted Nanolithography' http://ndl.ee.ucr.edu/IBM-NDL-Graphene-Ribbons.pdf. They combine graphoepitaxy with block copolymer lithography, using EBL to write parallel, straight lines of a polymer on top of the graphene with 170nm and 270nm spacings. The polymer has a higher affinity for one of the BCP blocks so when the copolymer phase-separate it forms lamellae micro-domains parallel to the straight lines, instead of patterns of curved lammellae.

PETER We have done something very similar to that.

DORTE My idea was you could use this method for making both, straight, aligned GNRs as they do, and short by limiting the length using graphoepitaxy too.

PETER Theoretically, making epitaxial GNR arrays is intereresting as GNR with different chirality (orientation) have different properties - but so far i dont think of any trnasport Measurements showing difference between e.g. armchair and zigzag. I guess in the article they Refer to the "epitaxial" Growth method of SiC... so its for free. Perhaps it matters, experiments dont show this yet. This is the IBM paper i refered to before. But as I said, it’s probably mostly interesting if it’s desirable to create short and narrow GNRs. I also don’t really understand why they make epitaxial GNR arrays -is it favorable in this case or is it because it’s quite difficult to obtain free-standing GNR arrays using this method?

DORTE The authors talk about epitaxy as in "we made graphene with SiC, so its epitaxial" and graphoepitaxi "we use surface structures to align BCP crystals"... these are two different Things. Did they align the surface structures to underlying graphene lattice? Perhaps :) i cannot remember. Otherwise, I was wondering if you think it could be possible to write about something else concerning Nanopatterning and GNRs or GNMs and bandgap and carrier transport and possibly exhibiting ballistic transport and the importance of edge smoothness?

PETER Yes, that sounds like a good plan. But i think you need a stronger motivation than just "its fun".. .also you need to have a clear vision of how you are going to achieve smooth edges

DORTE I would like to read these articles, if you have time to get them:

http://www.nature.com/ncomms/2014/140217/ncomms4305/full/ncomms4305.html... http://pubs.acs.org/doi/abs/10.1021/ar3001487

PETER Sure, and BY THE way, i am writing a technology blog about nanotechnology in Ingeniøren (ing.dk/nanoblogien) ... would you mind if i tell the story about our communication? I can do it without referring to you by name. I think this programme, forskerspirer, is interesting and inspiring. Its totally up to you - but i would like to show some of the Q&A between us. I think it is motivating to see that young people can play along with researchers in this way. * og sådan blev ideen om denne blog født.

Peter Bøggild er professor i nanoteknologi på DTU. På bloggen Supermaterialer skriver han om stort, småt og tusind gange mindre.
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