Ken BURCH (U. Toronto): Tuning Materials with Mechanical Exfoliation

In order to tune properties - how to tune carrier densities without affecting purity, homogeneity etc.

OUTLINE:

• Introduction
  
• Method: exfoliation. A very expensive glove-box is (almost) all one needs.

• Materials: BSCCO, Bi2Se3. Using spectroscopy (Raman) in order to characterize the tickness of the layers.

E.g. in BSCCO. Characteristic features in the Raman spectrum: a low and a higher energy peaks. Changing the doping level enhances the intensity of the higher-energy (3-phonon) peak.

• Future directions

I. Introduction.
Disorder vs. doping.
E.g. STM work in BSCCO-2212 (McElroy, Science 309, 1048 (2005)) shows a lot of disorder on the surface (and presumably the bulk too). One would like to tune the doping without affecting the disorder. Exfoliation technique is promising.

Spectroscopy is a powerful tool to study materials, e.g.:
- ps-gap in the cuprates. Optical studies (Basov and Timusk, RMP 77, 721 (2005)) were one of the first to show the existence of the ps-gap. Also Fermi-arcs were observed in ARPES (Tanaka, Science 314, 1910 (2006)). Raman studies (Blumberg, Science'1997).
- Fe-pnictides - a new exciting direction.

Interfaces and surfaces (epitaxially grown)
- Ohmoto et al, thin films of BaTiO3/ Nature ()
- ferroelectrics

Field-effect transistor.
Consider SiO2 grown on top of Si:P.
There are few examples of combined thin layer materials (e.g. LaAlO3/LaVO3 etc), but material and growth issues are daunting. Lattice mismatch is important and prevents one from growing arbitrary combination of the film/substrate.
The exfoliation technique, on the other hand, is free from this drawback - the same thin film can be exfoliated onto a number of different substrates.

II. A new way: exfoliation technique
Pioneered by Andrew Geim for graphene. A.K.A. "skotch-tape" technique

- graphene
E.g. Tony Heinz's group showed that if grown on MICA, the graphene layer is very flat, unlike if grown on e.g. silica.
(The blogger did not know what MICA was - as far as I understood (please correct in incorrect) it is a substrate typically used in AFM for calibration. The surface of it is covered with Na aroms, leaving an atomically flat surface underneeth it)

- topological insulators
See Peng, Nat. Mater 9, 225 (2009); D. Hsieh, Science (2009)
e.g. Bi2Se3, Bi2Te3
Since MICA is transparent, the speaker used it for exfoliation, and then measured Raman on it.

- NbSe2
Raman A1g mode shifts depending on the thickness of the layers.

- MoS2
A. Splendiani.., Nano Lett. 4, 1271 (2010)

III. Results on BSCCO-2212
Bi2Sr2Ca0.6Dy0.4Cu2O(8+x)
x = 0.3, 0.4, in the ps-gap regime

Depending on the film thickness, the colour of the crystal is changing. Where does it come from? Chemistry is unchanged! Turns out - interference effect by reflection from different layers. Plot so-called contrast depending on the layer thickness - convincing.

Lab Tour: 9 T magnet, ellipsometer, Raman scattering ...

History of Raman: sunlight from a telescope, passed through a polarizer, filters to a single wave-length, and scatters it from the sample. The shift in frequency (Raman shift) is what measured.

Sharp features in Raman - 2-phonon joint density of states.
Broad features: two-magnon excitations. Review: T. Deveraux and R. Hackl RMP 79, 175 (2007)
E(1 magnon) = 2J;
E(2 magnon)=3J - most pronounced signal, since one breaks 6 spin-spin bonds by flipping two adjacent spins, paying energy cost J/2 for each.
Note: Other frequencies are possible (e.g. E(2 magnon) = 4.5 J).

The 2-magnon excitations have definite selection rules. E.g. B1g excitation corresponds to XX or X'X' polarization. The intensity of the Raman signal from thin films (exfoliated direcltly on the SiO2 substrate) greatly enhances compared to the bulk (See data in: Sandilands, PRB (2010)).

NOTE: Raman signal, even from the bulk, always has interference correction from reflection off different layers. (See Y. Wang, APL 92, 043121 (2008) ).

So what is the reason for much enhanced intensity of the B1g 2-magnon peak?
It turns out - this is the effect of the changed doping level. In fact, the bulk data show that reducing concentration of holes (under-doping) hardens the 2-magnon peak (i.e. shifts it to higher frequency). This is precisely what we observe in our exfoliated thin film flakes.

SUMMARY:

• using exfoliation technique allows to create very thin films without worrying about the interaction with the substrate, lattice mismatch etc.
• optical studies are a very powerful method
  
• thinning out the BSCCO films seems to result in increasing under-doping, judging from the 2-magnon Raman peak frequency.

Questions:
Q: P. Hirschfeld. Presumably, the reason for under-doping in exfoliated films is that you lose O atoms from the surface. However, don't you think that only few top layers would be affected?
A: It may well be true. Our data show however that the effective doping level, as measured by Raman 2-magnon peak position, reduces. We are not quite sure at present, due to what microscopic mechanism.

Q: H. Alloul. Are data taken at room temperature
A: Yes. The low-temperature data can be different

Q: A. Chubukov. Were all the measurements done for one doping level?
A: Yes, so far we started from overdoped O-concentrations. We would like in future to do exfoliation starting from optimally doped samples. We have also experimented with different Dy doping (not shown in this talk).

Q: G. Blumberg.
What was the laser power
A: a few miliwatts.
Blumberg's comment: concentrating this much power on a tiny surface area would lead to heating of the sample, and hence oxygen diffusion off the surface.

Q: P. Coleman: what do you know about the dependence of exfoliation on temperature?
A: we heat the substrate before exfoliation. We haven't yet done the exfoliation at low temperatures.
Q: Is the size of the flakes sufficient to attach leads?
A: The flakes are too small. It would be great to be able to make them larger.

Q: A. Schofield. What are the prospects of this exfoliation technique?
A: Exploring the phase diagram of e.g. cuprates, without affecting the purity of the crystals.

Q: Y. Grin. What do you know about the real structure of the spacer? Can you do TEM?
A: TEM proves hard. We do AFM, showing very flat surfaces. We'd like to do other structural probes in the future.

Q: [someone]. Can one see a gradual change in the Raman B1g position from bulk to thin film, by gradually increasing the film thinkness?
A: No, it's very hard to control the film thickness during exfoliation process. Somewhat of a black art.

Q: P. Hirschfeld. How important is lattice mismatch with the substrate? E.g. other studies on epitaxially grown films?
A: We hope that we are not stretching the film, but we cannot really be sure. We tried different substrates to try to answer this question.