Scanned Probe Microscopy

(SxM)

Kyle McElroy

Outline

• Optical microscopes

• Basic principles STM – Piezoelectrics

– Tunneling

– Feedback

– Some results

• S(insert measurement)M – SQM/SHM

– AFM

– NSOM

– SHM

• How do optical microscopes measure?

• What do they measure?

• Limitations of this?

Optical Microscopes and nano-

scale

Lengthscale, l ~ 300-800nm

Measure changes in e

• Many interesting physics at shorter lengthscales

• Much interesting physics doesn’t directly effect e

• How do you get smaller and what else can you measure? – Need control of nm distances

– Turn distance into a Voltage we like Voltages!

Optical Microscopes and nano-

scale

Piezoelectricity

• Electric dipole

linked to lattice

– Applications in sensors, lighters, actuators

– Length-scales involved can be ~nm/V

• Ordered domains

(like ferro-

magnetism)

• Electric field ≈

strain ≈

displacement

Scanning Tunneling

Microscopy

Tunneling

• QM and wave-functions

• Transmission through

barrier ~

• WKB approximation gives:

z

YI YB YT f

)(20

Emz

f

0/ zze

Exponential in barrier thickness

Lengthscale ~1Å

Scanning Tunneling Microscopy

•Sharp tip vacuum

sample tunneling

•Piezo actuator (high

Voltage)

•Measure tunneling

current (S/N)

•Computer acquisition

Feedback

•1Å is small

-1% of that is 10 fm

•Vibration kills

•Feedback protects

-Large gain can be

thrown out to gain

stability and linearity

1V~1nm ~10Å ~20,000 increase in current

STM results

• Atomic lattices and surface structure

Si(111) 7x7 reconstruction

Trenary: UIC

Gold surface

B. Barker: LPS

0

300 Å

STM results

• Atomic lattices and surface structure

Charge Density Wave

on TaSe2

Jixia Dai Colorado

0

300 Å

0

2

00 Å

Can selectively “write” on surface

~10nm lengths

STM results

• Electrons are waves

Fe on Cu(111)

D. Eigler: IBM Almaden

? on Cu(111)

Spin polarized Tunneling

• Aligned spin states means

more DOS for up than

down spins in both.

– Tunneling is easier!

• Misaligned spin states?

Less DOS for tunneling

electron

– Less tunneling.

z

z

Nano magnetics

• Spin polarized

Wiesendanger: Hamburg

Tunneling does not change spin. If tip like a

particular spin you van image the spin order

in a surface

Other scanned probes

S(X)M

Atomic Force Microscopy

Small tip

Force on tip bends

cantilever and deflects

laser

Floppy cantilever

k small

Atomic Force Microscopy

lambda-phage DNA in buffer

human lymphocyte chromosomes

Atomic resolution mica

JPK Instruments

Force curves

Sacrificial bonds in bone

Hansma, UCSB

MFM

• Manipulation of vortices

K Moler: Stanford

CoPt Wiesendanger: Hamburg

• Magnetic Domain

growth

SQM and SHM

• Magnetic field and vortices

K Moler: Stanford

NSOM

• Back to e, both read and write

Before and After Images of Spatial Hole-Burning of

PIC/PVS Molecular Yarn

Topography of a Small PIC Dye Crystal; (K) NSOM Fluorescene

Image of Crystal in Fig. 1J; (L) NSOM Image of 620 nm Emission

from Crystal in Fig. 1J.

Paul F. Barbara, UMM

Nanoscale manipulation

• If you can measure it you can interact with

it

Conclusion

• If you can get a small signal out of it

• If you can make it small and put it on the

end of a piezo

You can use it to measure things on nm

lengthscales.

What else can you measure

with an STM?

STM and LDOS and measuring

|wavefunctions|2

•Apply Vb

lower EF of sample

by eVb

•Measure I(Vb)

0z

zeV

0

eLDOS(E)dEI(V)

LDOS(E)dI/dV

Our STMs

•5-150 K

•Cryogenic, UHV.

•Low noise or high bandwidth capabilities

•Unique controller architecture allows more data

taking options

•Homemade sound room and vibration isolation

Capabilities at Colorado

91114I02

5.5” sq at

1.5”, 2.75”

0 560 Å

0

5

60

Å

Bi2Sr2CaCu2O8+d

Topography 0 256 Å

0

2

56

Å

0

64 Å

0

6

4 Å

Each bright spot

is a Bi atom --

Cu atom is about

5Å below

Why measure wavefunctions?

• Impurity states

Bi2Sr2Ca(Cu1-xNix)2O8+d : x 0.5%

256 Å, 4.2 K

100 pA, -100 mV

LDOS +9meV

2

)9~,( meVEr +Y

91118a00 -9mV 91118a00 +9mV

91117h14 a

b

• Impurity states

+9 mV -9 mV

3nm field of view,

structure of impurity

state seen

2

)9~,( meVEr Y

Why measure wavefunctions?

Typical FOV

0.4 nS

0.7 nS

400 Å

KM et al Science 309, 1048 (2005)

Can also image dopants that cause superconductivity

LDOS(r,E=-900 meV)