Interactions between Sample and Tipin Force Microscopy

vdW force magnetic or adhesion elastic and attractive electrostatic bonding plastic propertiesionic repulsion forces friction forces

f(r) ~ -1/r6 +1/r12

van der Waals forceContact

Noncontact ~ 10-7 ~10-11 N

Intermittant contact

f

r

Force vs. Distance

touching

Pull off

untouch

Slope: elastic modulus of soft sample or force constant k of cantilever

Vacuum

Air

Contamnated in air

Capillaryforce

Atomic Force Microscope (AFM)Atomic Force Microscope (AFM)

•Sample: conductor, nonconductor, etc•Force sensor: cantilever•Deflection detection: photodiode interferometry (10-4A)

Contact strong (repulsive): constant force or constant distance

Noncontact weak(attractive): vibrating probe Intermittent contact strong(repulsive): vibrating probe

Lateral force frictional forces exert a torque on the scanning cantilever 

Magnetic force the magnetic field of the surface

Thermal scanning the distribution of thermal conductivity

Mode of Operation Force of interaction

Force Sensor: Force Sensor: CantileverCantilever

•Material: Si , Si3N4

•Dimension:

•Stiffness - soft: contact mode - stiff: vibrating (dynamic force) mode•Spring constant (k): 0.1-10N/m•Resonance frequency: 10~100kHz

From DI

100-200 m1-4 m

10 m

20nm

Scanning Modes

Polymer latex particle on mica

Contact Noncontact Vibrating (tapping)

Cantilever soft hard hardForce 1-10nN 0.1-0.01nNFriction large small smallDistance <0.2nm ~ 1nm >10nmDamage large small small

From DI

Vibrating Cantilever (tapping) Mode

Resonance Frequency keff = k0 - dF/dz feff = 2keff /m)1/2

•Amplitude imaging•Phase imaging•Removal of lateral force contribution

•Vibration of cantilever around its resonance frequency•Change of frequency (fo) due to interaction between sample and cantilever

H.-Y. Nie, http://publish.uwo.ca/~hnie/mmo/all.html

Image Artifacts

•Tip imaging•Feedback artifacts•Convolution with other physics•Imaging processing

How to check ?•Repeat the scan•Change the direction•Change the scan size•Rotate the sample•Change the scan speed

www.psia.co.kr/appnotes/apps.htm

AFM image of random noise. Left image is raw data. Right image is a false image, produced by applying a narrow bandpass filter to the raw data

Applications of SPMApplications of SPM• Surface morphology, especially for insulators

• Lateral, adhesion, molecular and magnetic forces

• Temperature, capacitance, and surface potential

mapping

• Nano-lithography

• Surface modification and manipulation

• Imaging of biomolecules, and polymers

STM and AFM Images of Co-NanoparticlesSTM and AFM Images of Co-Nanoparticles

AFM imageUHV-STM image (200 nm x 200 nm)

after annealing under 500 C

Co-nanoparticle stripes prepared

By Mirco Contact Printing

STM: on H/Si(111) AFM: on HOPG TEM: on Carbon grid

S.S.Bae et al (2001)

Force Measurement of DNA strands

Baselt et al, J. Vac. Sci. Tech. B14, 789 (1996)

Manipulation of Nanoparticles by AFM

Ref 15 in G.S. McCarth and P.S.Weiss, Chem. Rev. 99, 1983 (1999)

AFM-tip-induced Local Oxidation on Si

Ph. Avouris et al, App. Phys. A 66, S659 (1998)

Lateral Force Microscopy

High resolution topography (top) and Lateral Force mode (bottom) images of a commercially avail-able PET film. The silicate fillers show increased friction in the lateral force image.

Laterally twisted due to friction

www.tmmicro.com/tech/modes/lfm.htm

Force Modulation Micrscopy

PSIA, www.psia.co.kr/appnotes/apps.htm

•Contact AFM mode•Periodic signal is applied to either to tip or sample•Simultaneous imaging of topography and material properties•Amplitude change due to elastic properties of the sample

Phase Detection Microscopy

•Intermittant-contact AFM mode•Monitoring of phase lag between the signal drives the cantilever and the catilever oscillation signal•Simultaneous imaging of topography and material properties•Change in mechanical properties of sample surface

Chemical Force MicroscopyChemical Force Microscopy

A. Noy et al, Ann. Rev. Mater. Sci. 27, 381 (1997)

(A) topography (B) friction force using a tip modified with a COOH-terminated SAM,(C) frinction force using a tip modified with a methyl-terminated SAM. Light regions in (B) and (C) indicate high friction; dark regions indicate low fr

inction.

COOHCH3

Magnetic Force Microscopy

Glass Hard Disk Sample

AFMimage

MFMimage

http://www.tmmicro.com/tech/index.htm

•Ferromagnetic tip: Co, Cr•Noncontact mode•vdW force: short range force •Magnetic force: long range force; small force gradient•Close imaging: topography•Distant imaging: magnetic properties

Electrostatic Force Microscopy(EFM)

• Ferroelectric materials

• Charge distribution on surfaces

• Failure analysis on the device

http://www.tmmicro.com/tech/index.htm

Metallic tipBias Voltage

Scanning Kelvin Probe Microscopy (SKPM)Scanning Tunneling Potentiometry (STP)Scanning Maxwell Microscopy (SMM)

Electrostatic force bewteen sample and tip

F = Fcapacitance + F coulomb (Fcapacitance = dEc /dz , Ec = CV2/2) = (1/2)V2(dC/dz) + (1/4z2)QsQt

= (1/2) (dC/dz) (Vdc +Vac cos t)

- (1/4z2)Qs (Qs +CVdc +CVac cos t) = (1/2) (dC/dV) (V dc + (1/2) Vac ) - (1/4z2)Qs (Qs +CVdc) + (2Vdc dC/dz - Qs C /4z)Vac cos t + (1/4)dC/dzV ac cos 2t

•Ist harmonic term: Surface potential (Vdc ), surface charge (Qs)•2nd harmonic term: dC/dz doping concentration

Theory of EFM

Scanning Capacitance Microscopy (SCM)

Transistor Oxide Thickness

Topograph SCM

• LC capacitance circuit• Doping concentration• Local dielectric constant

Metallic tipContact

http://www.psia.co.kr/appnotes/apps.htm

Scanning Thermal MicroscopyScanning Thermal Microscopy

• Subsurface defect review• Semiconductor failure analysis • Measure conductivity differences in copolymers, surface coatings etc

Hybrid Tools: AFM inside SEM

M.F. Yu et al, Nanotechnology 10, 244 (1999)

Nearfield Scanning Optical MicroscopyNearfield Scanning Optical Microscopy

hv

Al

Glass

•Near field: d<<•Resolution ~ d and •Approach: shear force

=60-200nm

Detector

Topography and Optical properties

d

Fluorescence image of a single DiIC molecule. Image courtesy of P. Barabara/Dan Higgins

http://www.psia.co.kr/data/nsom.htm

Scanning Electrochemical Microscopy

ElectrochemistrySurface structureElectrochemical depositionCorrosion

From J. Kwak

A

A

FEEDBACK

Photon Emission STM

G.S. McCarty and P.S. Weiss, Chem. Rev. 99, 1983 (1999)

Local chemical environment

Photon detector

BEEM (Ballistic Electron Emission Microscopy)

A

A

FEEDBACK

Ballistic Electron

Tip Base Collector(Si)

Efgap

STP (Scanning Tunneling Potentiometry)

..

FEEDBACK

A

..

• SPM has eyes to see the geometry and properties of nanostructures and fingers to manipulate and build nanostructures

Summary

3. 전자 분광학의 원리 및 응용1. IMFP of electron2. Auger electron spectroscopy3. Photoelectron spectroscopy

- Koopmann’s theorem - Chemical shift - Spin-orbit coupling etc4. Microscopies: PEEM, LEEM, Imaging

XPS

References:1. Electron spectroscopy, theory, techniques, and applications I-IV, edited by C.R. Brundle (Academic, NY, 1977)2. G. Ertl and J. Kuppers, Low energy electrons and surface chemistry (VCH, 1985) 3. Practical surface analysis, I,II, edited by D. Briggs and M.P.Seah (Salle & Sauerlander)4. S. Hufers, Photoelectron spectroscopy, in Springer series in sol. state phys. v 82

5. Internet lecture: http://www.chem.qmw.ac.uk/surfaces/#teach

• Electron Spectroscopies

• Auger Electron Spectroscopy (AES)

• X-ray Photoelectron Spectroscopy (ESCA)

• Ultraviolet Photoelectron Spectroscopy (UPS)

• Electron Energy Loss Spectroscopy (EELS)

• High Resolution EELS

• Electron Microscopies

• Scanning Auger Microscopy (SAM)

• Photoemission Electron Microscopy (PEEM)

• Low Energy Electron Microscopy (LEEM)

• Scanning X-ray Photoelectron Microscopy

(SXPEM)

• Secondary Electron Microscopy with

Polarization Analysis (SEMPA)

Acronyms

100

10

1

01 10 100 1000 10000 Energy (eV)

IMFP

(nm

)

The inelastic mean free path (IMFP) of electrons isless than 1 nm for electron energies with 10~1000 eV.

Why are electron spectroscopies surface sensitive ?

Experimental Set Up

Auger Electron Spectroscopy

Auger electron

KE = EK – EL1 – EL23

L23

L1

KKinetic Energy of Auger Electrons for KLL Transition

Element Specific

ehv or e

e

Auger Electron Energies

Auger Spectra

Applicatons• Chemical identification: 1% monolayer

• Quantitative analysis

• Auger depth profiling

• Scanning Auger Microscopy (SAM)

- Spatially-resolved compositional

information

SEM and SAM

Focused electron gun

Energy Analyser

Detector

SEM topograph of Au-SiC codeposits

SAM image of Ag particles (d=1nm)

Secondary electrons

Auger electrons

SiC grain size= 0.04 m

Photoelectron SpectroscopyX-ray Photoelectron Spectroscopy (XPS): hv=200~2000 eVUltraviolet Photoelectron Spectroscopy (UPS): hv =10~50 eV

Ev

Ef

KE

BE

photoelectron

h

KE = h – BE -

KE: kinetic energyBE: binding energy: work function

hE,p e(E,)

Photoemission peak intensity

I(E, hv) ~ Nv(E) Nc(E) (E,hv): UPS limit

~ Nv(E)(E,hv): XPS limit,

where Nv(E): densities of initial states(i) Nc(E): densities of final states(f) (E,hv): photoionization cross section ~ |<f|AP|i>|2

The XPS spectra represent the total density-of-statesmodulated by the cross-section for photoemission

Koopman’s Theorem: frozen orbital approximation

A(N) + hv A+(N-1) + e

hv + e(KE)

initial state final state

Ei(N) + hv = Ef(N-1) + KEBE = hv – KE = Ef(N-1) - Ei(N) = -i

HF - relax + correl

BE of core level ≈ -iHF( ith orbital energy)

e

Chemical Shift of Binding Energy

Valence shell Electron charge q

Core electron e

The core electron feels an alteration in the chemical environment when a change in the charge of the valence shell occurs.

r

A change in q, q, gives a potential change E = e q/r

• the oxidation state of the atom • the chemical environment

Core level Spectra of SiON

J.W. Kim et al (2001)

Example of Chemical Shift

• The chemical shift: ~4.6 eV• Metals: an asymmetric line shape (Doniach-Sunjic)• Insulating oxides: more symmetric peak

Photoemission features

• Spin-orbit splitting

• Shake-up and shake off

• Multiplet splitting

• Plasmon losses

Ref: Electron spectroscopy I-IV edited by C.R. Brundle and A.D. baker Photoelectron spectroscopy by S. Hufner

Spin-orbit Coupling

Pd: (3d)10 + hv (3d)9 + e L =2 , S = ½, JL+S,…, L-S = 2D 5/2 g J = 2x{5/2}+1 = 6 2D 3/2 g J = 2x{3/2}+1 = 4

Shake up and shake off: Final state effect

•Multi-electronic transitions after creation of Ne 1s core hole - Excitation of electron to higher bound state: shake up - Excitation to continuum state: shake off

Shakeup and shake off (continued)

Multiplet Splitting

Fe 3+(3s23d5) + hv Fe 4+(3s13d5) + e

Initial state Final state

3d

3s

Terms: 6S 7S 5S

Applications

• Determination of energy levels

• Chemical bonding

• Oxidation states

• Density-of-states of valence bands

• Energy bands

• Quantum well states

• Quantitative analysis

XPS spectra

XPS spectra of CdSe-Nanoparticles

TOPO: n-trioctylphosphine oxide

Se 3d •Unstable to photoxidation•Chalcogenide at the surface is oxidized to sulfate or selenate and evaporate as molecular species

Angle-Dependent Analysis of a Silicon Wafer with a Native Oxide

Si 2p peak of the oxide (BE ~ 103 eV) increases at grazing emission angles

d

x

X= dcos

Imaging XPS

Si SiOx

• Scan Analyzer• Parallel Direct Imaging • X-ray microprobe/Zone plate

Present: Submicron resolution

Low Energy Electron Microscopies

Step decoration of Si(111)

LEEM: E Bauer, Rep. Prog. Phys. 57, 895 (1994)

•Strong elastic backward scattering of slow electrons (10~100eV)•Several monolayer sensitivity•Resolution: ~ 20 nm

Photoelectron Emission Microscopy

A threshold excitation lamp. g-Arc lamp, operating at 4.9 eV Due to the presence of oxide at the silicon surface,Si appears DARK (W >> 4.9 eV) and Pd appears BRIGHT (W = 5.12 eV ~ 4.9 eV)

hv

Ground

e

CathodeLens

Array detector

Resolution : 20~100nm

Scanning Transmission X-ray Microscopy

X-ray microscope image, and protein and DNA map, of air-dried bull sperm. Images were taken at six x-ray absorption resonance wavelengths, and were used to derive the quantitative maps. Zhang et al., J. Struct. Biol. 116, 335 (1996).

hv =10-1000 eV= 0.1-10 nm

Future Directions for Characterization of Nanosystems

• Instrument for analysis of biomolecules, and polymers• 3-D structure determination• Nanostructure chemical determination• Functional parallel probe arrays• Standardization and metrology• New nano-manipulator• Non-SPM probes that use ions or electrons• In-situ nondestructive monitoring techniques