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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
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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
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Atomic Force Microscope (AFM)Atomic Force Microscope (AFM)
•Sample: conductor, nonconductor, etc•Force sensor: cantilever•Deflection detection: photodiode interferometry (10-4A)
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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
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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
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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
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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
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H.-Y. Nie, http://publish.uwo.ca/~hnie/mmo/all.html
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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
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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
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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)
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Force Measurement of DNA strands
Baselt et al, J. Vac. Sci. Tech. B14, 789 (1996)
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Manipulation of Nanoparticles by AFM
Ref 15 in G.S. McCarth and P.S.Weiss, Chem. Rev. 99, 1983 (1999)
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AFM-tip-induced Local Oxidation on Si
Ph. Avouris et al, App. Phys. A 66, S659 (1998)
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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
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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
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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
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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
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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
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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)
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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
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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
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Scanning Thermal MicroscopyScanning Thermal Microscopy
• Subsurface defect review• Semiconductor failure analysis • Measure conductivity differences in copolymers, surface coatings etc
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Hybrid Tools: AFM inside SEM
M.F. Yu et al, Nanotechnology 10, 244 (1999)
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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
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Scanning Electrochemical Microscopy
ElectrochemistrySurface structureElectrochemical depositionCorrosion
From J. Kwak
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A
A
FEEDBACK
Photon Emission STM
G.S. McCarty and P.S. Weiss, Chem. Rev. 99, 1983 (1999)
Local chemical environment
Photon detector
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BEEM (Ballistic Electron Emission Microscopy)
A
A
FEEDBACK
Ballistic Electron
Tip Base Collector(Si)
Efgap
STP (Scanning Tunneling Potentiometry)
..
FEEDBACK
A
..
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• SPM has eyes to see the geometry and properties of nanostructures and fingers to manipulate and build nanostructures
Summary
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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
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• 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
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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 ?
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Experimental Set Up
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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
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Auger Electron Energies
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Auger Spectra
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Applicatons• Chemical identification: 1% monolayer
• Quantitative analysis
• Auger depth profiling
• Scanning Auger Microscopy (SAM)
- Spatially-resolved compositional
information
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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
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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,)
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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
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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
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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
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Core level Spectra of SiON
J.W. Kim et al (2001)
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Example of Chemical Shift
• The chemical shift: ~4.6 eV• Metals: an asymmetric line shape (Doniach-Sunjic)• Insulating oxides: more symmetric peak
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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
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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
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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
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Shakeup and shake off (continued)
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Multiplet Splitting
Fe 3+(3s23d5) + hv Fe 4+(3s13d5) + e
Initial state Final state
3d
3s
Terms: 6S 7S 5S
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Applications
• Determination of energy levels
• Chemical bonding
• Oxidation states
• Density-of-states of valence bands
• Energy bands
• Quantum well states
• Quantitative analysis
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XPS spectra
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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
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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
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Imaging XPS
Si SiOx
• Scan Analyzer• Parallel Direct Imaging • X-ray microprobe/Zone plate
Present: Submicron resolution
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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
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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
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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
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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