Synchrotron X-Ray Scanning Tunneling Microscopy

Nathaniel BeaverPHYS-570

Outline

Scanning tunneling microscopy (STM)– What it is.– How it works.– Why you would want to enhance it with X-rays.Specific papers on synchrotron x-ray enhanced STM.– Challenges and recent progress

What is scanning tunneling microscopy?

http://researcher.ibm.com/researcher/files/us-flinte/stm15.jpg

http://researcher.ibm.com/researcher/view_project_subpage.php?id=4252

Iron on copper. Michael Crommie, Chris Lutz and Don Eigler, IBM.

1986 Nobel Prize

Gerd Binnig and Heinrich Rohrer

European Press photo Agency

http://www.nytimes.com/2013/05/22/science/heinrich-rohrer-physicist-who-won-nobel-dies-at-79.html

What does it do?

Tunneling: measure electrons near Fermi energy without contact.

Scanning: move the measuring tip around on the sample.

Source: Michael Schmid, TU Wien; adapted from the IAP/TU Wien STM Gallery

Process

Electrons tunnel into tip.

A piezoelectric actuator moves the tip around.

Process

Two modes:

1. Constant current (feedback loop with piezeo to move it up and down).

2. Constant height (faster).

Q: How do you get a tip that sharp?

A:

www.csun.edu/~hpostma/2013-1-466/stm-introduction.pdf

Henk Postma

Q: Doesn't it tunnel from the tip to the sample as well?A: Yes. This is why they use a bias voltage and a tip material with a stable (“flat”) density of states for the energy range of the sample's Fermi surface.

http://hoffman.physics.harvard.edu/research/STMtechnical.php

Typical orders of magnitude for STM

Separation distance: a few Å.

Tunneling current: pA to nA

Bias voltage: a few hundred mV

Spatial resolution (depth): <Å.

Spatial resolution (side to side): ~Å.

http://depts.washington.edu/nanolab/NUE_UNIQUE/.../5_Lab_Unit_STM.pdf

Requirements

1. Vacuum to avoid oxidation of tip and sample (not strictly necessary).

2. Flat surfaces.

3. High precision tips.

4. Vibration isolation.

5. Reliable, precise electronics.

Tip materials

Noble metals (platinum, iridium) because any oxidation would ruin the conductivity

In vacuum, you can use e.g. tungsten.

What STM does well:

Spatial resolution

I = C exp(-k z)

Individual atoms!

What STM does well:

http://researcher.ibm.com/researcher/view_project_subpage.php?id=4251

Xenon on nickel.

What STM does well:

http://www.physics.purdue.edu/nanophys/stm.html

What STM does poorly:

Chemical and magnetic contrast.

At the Fermi level, you can distinguish conductors from insulators, but generally not much more than that.

Energy resolution is highly dependent on temperature, sample cleanness, vibrations, etc.

Synchrotron X-ray techniques

Beam spot size is on the order of ~10 nm with zone plates (not very good compared to STM).

But, it answers questions like:

Does this sample contain iron?Is this sample ferromagnetic?Is the iron in this sample oxidized?And much more!

Rose, Volker, and John W. Freeland. “Nanoscale Chemical Imaging Using Synchrotron X-Ray Enhanced Scanning Tunneling Microscopy.” In AIP Conference Proceedings, 1234:445, 2010. http://link.aip.org/link/?APCPCS/1234/445/1.

Let's combine it with STM!

http://www.protectamerica.com/home-security-blog/home-defense-advice/the-deadliest-doombas-the-internet-has-ever-produced_2388/attachment/doomba1

Let's combine it with STM! (2010)

Rose, Volker, and John W. Freeland. “Nanoscale Chemical Imaging Using Synchrotron X-Ray Enhanced Scanning Tunneling Microscopy.” In AIP Conference Proceedings, 1234:445, 2010. http://link.aip.org/link/?APCPCS/1234/445/1.

2010 prototype

Let's combine it with STM! (2011)

Cummings, M.L., T.Y. Chien, C. Preissner, V. Madhavan, D. Diesing, M. Bode, J.W. Freeland, and V. Rose. “Combining Scanning Tunneling Microscopy and Synchrotron Radiation for High-Resolution Imaging and Spectroscopy with Chemical, Electronic, and Magnetic Contrast.” Ultramicroscopy 112, no. 1 (January 2012): 22–31. doi:10.1016/j.ultramic.2011.09.018.

(2013)

Rose, Volker, Kangkang Wang, TeYu Chien, Jon Hiller, Daniel Rosenmann, John W. Freeland, Curt Preissner, and Saw-Wai Hla. “Synchrotron X-Ray Scanning Tunneling Microscopy: Fingerprinting Near to Far Field Transitions on Cu(111) Induced by Synchrotron Radiation.” Advanced Functional Materials 23, no. 20 (May 28, 2013): 2646–2652. doi:10.1002/adfm.201203431.

How does it work?

http://www.aps.anl.gov/Xray_Science_Division/Sxspm/Research/Physical%20Concept/

How does it work?

http://www.aps.anl.gov/Xray_Science_Division/Sxspm/Research/Physical%20Concept/

How does it work?

– primary photoelectrons(from the photoabsorption of theincident X-rays)– primary Auger electrons(from the de-excitation afterphotoionization)– secondary photoelectrons(due to photoabsorption of fluorescentradiation in the sample)– secondary Auger electrons(from the relaxation of secondaryexcited atoms).

Partial solution:insulated tips.

Rose, Volker, Kangkang Wang, TeYu Chien, Jon Hiller, Daniel Rosenmann, John W. Freeland, Curt Preissner, and Saw-Wai Hla. “Synchrotron X-Ray Scanning Tunneling Microscopy: Fingerprinting Near to Far Field Transitions on Cu(111) Induced by Synchrotron Radiation.” Advanced Functional Materials 23, no. 20 (May 28, 2013): 2646–2652. doi:10.1002/adfm.201203431.

Rose, V., T. Y. Chien, J. W. Freeland, D. Rosenmann, J. Hiller, and V. Metlushko. “Spin-Dependent Synchrotron X-Ray Excitations Studied by Scanning Tunneling Microscopy.” Journal of Applied Physics 111, no. 7 (2012): 07E304–07E304.

See oxidation,even withouttunneling!

Rose, Volker, Kangkang Wang, TeYu Chien, Jon Hiller, Daniel Rosenmann, John W. Freeland, Curt Preissner, and Saw-Wai Hla. “Synchrotron X-Ray Scanning Tunneling Microscopy: Fingerprinting Near to Far Field Transitions on Cu(111) Induced by Synchrotron Radiation.” Advanced Functional Materials 23, no. 20 (May 28, 2013): 2646–2652. doi:10.1002/adfm.201203431.

But thereare problems.

It has to keepon tunnelingfor good spatialresolution.

Rose, Volker, Kangkang Wang, TeYu Chien, Jon Hiller, Daniel Rosenmann, John W. Freeland, Curt Preissner, and Saw-Wai Hla. “Synchrotron X-Ray Scanning Tunneling Microscopy: Fingerprinting Near to Far Field Transitions on Cu(111) Induced by Synchrotron Radiation.” Advanced Functional Materials 23, no. 20 (May 28, 2013): 2646–2652. doi:10.1002/adfm.201203431.

Rose, Volker, Kangkang Wang, TeYu Chien, Jon Hiller, Daniel Rosenmann, John W. Freeland, Curt Preissner, and Saw-Wai Hla. “Synchrotron X-Ray Scanning Tunneling Microscopy: Fingerprinting Near to Far Field Transitions on Cu(111) Induced by Synchrotron Radiation.” Advanced Functional Materials 23, no. 20 (May 28, 2013): 2646–2652. doi:10.1002/adfm.201203431.

Rose, Volker, Kangkang Wang, TeYu Chien, Jon Hiller, Daniel Rosenmann, John W. Freeland, Curt Preissner, and Saw-Wai Hla. “Synchrotron X-Ray Scanning Tunneling Microscopy: Fingerprinting Near to Far Field Transitions on Cu(111) Induced by Synchrotron Radiation.” Advanced Functional Materials 23, no. 20 (May 28, 2013): 2646–2652. doi:10.1002/adfm.201203431.

Extrapolating photo-ejected electrons(the ones we don't care about)

Questions?

Rose, V., T. Y. Chien, J. W. Freeland, D. Rosenmann, J. Hiller, and V. Metlushko. “Spin-Dependent Synchrotron X-Ray Excitations Studied by Scanning Tunneling Microscopy.” Journal of Applied Physics 111, no. 7 (2012): 07E304–07E304.

Example: Spin-dependent x-ray

excitations (2012)

Aside: AFMSTM: 1981.Atomic force microscopy (AFM): 1986.(Inspired by STM)

http://www.nanoscience.gatech.edu/zlwang/research/afm.html

Aside: not STM

Scanning electron microscope (SEM):emits electron beam, and spatial resoution worse by an order of magnitude.

Aside: not STM

https://commons.wikimedia.org/wiki/File:SEM_image_of_a_Peacock_Head,_slant_view.JPG