1. Nanocharacterization
나노구조의 특성
SPM의 원리 및 응용
전자분광학의 원리 및 응용

2. 1. 나노구조의 특성 Quantum confinement Quantum size effect
Energy bands and electronic transition
Charge quantization
Thermodynamic properties

3. Nanostructures

4. Comparison: Microstructure vs. Nanostructure
Microstructure Nanostructure
Physics Semi-classical Quantum mechanical
Electron Particle–like Wave-like
nature
E or k-space Continuous Discrete
Current Continuous Quantized
Decision Deterministic Probabilistic
Fabrication Micro-fabrication Nano-fabrication
Surface area/ Small Very large
volume
Packing Low Very high

5. What is new phenomena ? Length Scale: 10-9~10-7 M (1~100 nm)
Quantum size confinement
Wave-like transport
Dominant interfacial phenomena
Electrical, optical, thermal, magnetic, chemical, mechanical, and biological properties ??

6. Electron Nature in Smaller Sizes
Energy quantization
d ~ Fermi wave length of electron in a metal (lF)
or exciton diameter in a semionductor
Charge quantization
Charging energy (Ec) >> Thermal energy (kT)
Ballistic
d<mean free path (l)
Free electron case: see Kittel
Y = exp(ikr), k =2pn/L
E= h2k2/2m
N = 2x (4pkF3/3)/(2p/L)3 = VkF3/3p2
Let electron concentration N = N/V
EF= (h2/2m) kF2 = (h2/2m) (3p2 N) 2/3
kF = (3p2 N)1/3
lF= 2p/kF= 2p (3p2 N)-1/3

7. Exciton : e-h pair bounded by attractive electrostatic interaction
H atom-like state E E
Conduction
band
Exciton
levels Eg
Exciton binding
energy: Eex
Eg-Eex
n =2
n=1 Eg
Valence
band
Binding energy: Eex =me4/2eh2n2
Bohr (exciton) radius: r = n2eh2/me2; 1/m=1/me +1/mh
Si Ge GaAs CdSe KCl
Eex (meV)
r (nm)

8. Quantum Confinement Exciton r radius
Energy for the lowest excited state
relative to Egap
E(R) = h2p2/2mR2 – 1.8e2/2eR …
dot R
Particle in a box problem
R<< r: Strong Confinement
- 1st term (localization) dominant
- Electron and hole are quantized
- Energy gap ~1/R2
eg) Si<4.3 nm, Ge<11.5 nm, GaAs<12.4
R>> r: Weak confinement
- 2nd term (coulomb attraction) dominant
- Exciton confinement character
L.E. Brus, J. Chem. Phys. 80, 4403(1984)

9. Density of State: # of states per unit energy range
1D D D
DOS
DOS
DOS
N = 2n/L
N =2pn2/L2
N =8pn3/3L3
dN /dE ~ E-1/2
dN /dE = const
dN /dE ~ E 1/2
k=2pn/L
E = hk2/2m
k =(2mE)-1/2/h
N = 2xn/L= k/p
= (1/ph)(2mE) 1/2
dN /dE = ((2m)1/2/2ph)(E)-1/2

10. Size Effect: Energy Levels and DOS
Semiconductor
Bulk Nano atom
particle
3d d d d CB VB
LUMO
HOMO
DOS
Band
gap EF
Energy
Size controlled band gap tuning
Discrete Energy levels
A.P. Alivisatos, Science 271, 933 (1996)

11. Size Effect:1D-Quantum well states
F.J. Himpsel et al, Adv. Phys. 47, 511 (1998)

12. Size Effect: Optical Spectra
Shift to higher energy in smaller size
Discrete structure of spectra
Increased absorption intensity
A.P.Alivisatos, J. Phys. Chem. 100, (1996)

13. Size effect: Tunable Band Gap
Bulk Si = 1.14 eV
GaAs =1.5 eV
Optical excitation is significantly enhanced, both, in frequency and intensity, in smaller sizes.
S. Ogut et al, Phys. Rev. Lett. 79, 1770 (1997)

14. Energy Bands

15. Energy Band Structure: Energy vs. k
Y = Cnfn
V = Cn V n
(h2/2m)2Y + V Y = E Y
Ej= a +2b cos 2pj/N
index j = 0, 1,  2 …
Define a new index k = 2pj/Na: wave vector
E(k) = a +2b coska,
Yk = eiknafn : Bloch wave function
(symmetry adapted LCAO) a 2 1 E …. ….
a -2b ….
= 2 p/k = 2a
= ∞ a
a +2b ….
p/a k=0 p/a

16. Electronic Transition
Electric Transition dipole moment mif = <ff |er| fi>
Direct transition (Dk=0)
In phase
Added transition dipole
Electronically
allowed transition E ff
mif fi ff
Indirect transition (Dk ≠ 0)
Out of phase
Cancelled transition dipole
Electronically forbidden
but vibronically allowed
mif fi
p/a k=0 p/a
Band width: overap of wave functions
Slope dE/hdk = hk/m = vg: group velocity of electron

17. Absorption spectra: Direct and Indirect Transition
Electronic absorption spectra for three sizes of CdSe nanocrystals, in the wurtzite (direct) and rock salt (indirect) structures. In each instance the direct gap spectrum is structured and intense, while the indirect gap one is featureless and relatively weaker. The relative absorption efficiencies do not change, despite the concentration of oscillator strength due to quantum confinement.

18. Fermi Golden rule Transition rate
W = ∫(2p/h 2) | mif |2d(Ef -Ei  hv) r(Efk) r(Eik)dE
mif : Transition dipole moment
r(Efk); r(Eik): density of states of final and initial states
mif = <ff |er| fi>< ff,vib | fi,vib ><eikfna | eikina>
Phase factor or
envelope function
Electric dipole
Transition moment
Frank-Condon factor

19. Size Effect: Enhanced Absorption
For quantum dot,
Energy levels: discrete
DOS: delta function
N(E) E k E
DxDp ~ h
x: well defined
p=hk: Not well-defined
k is not an exact quantum
number for QD
Envelope functions sample larger k-space
Overlap of wave functions
- Increased absorption intensity
M.S. Hybersten, Phys. Rev. Lett. 72, 1514 (1994)

20. Photon absorption: Direct vs. Indirect Transition
phonon q Eg hv k
Selection rule k’ = k (Dk = 0) k’ = k + q (Dk ≠ 0)
Energy relationship hv = Eg hv = Eg + hv(q)
Interaction electronic: two body vibronic: three body
Transition rate fast ~ sec slow ~ 10-2 sec
Radiative efficiency high low
Example GaAs (Eg (dir.) =1.4 eV) Si (Eg (ind.) = 1.1 eV)
(Eg (dir.) = 3.37 eV)

21. Size Effect: Radiative Transition Rate
Radiative transition rate (1/t) increases with smaller sizes.
For the size >2nm, phonon assisted transitions dominate,
while zero-phonon transitions allowed for smaller sizes
1.5 nm Si dots: quasi direct transition

22. Approaches for Light Emission from Si
Quantum confinement
Zone folding: quasi-direct gap
Er doped Si
Direct gap formation by alloying

24. Property:Polarized PL from Nanowire InP
J. Wang et al, Science 293, 1455 (2001)
Polarization ratio:
r= (I⊥- I∥)/(I⊥+ I∥) = 0.96
Large dielectric contrast between
nanowires and surrounding
Polarization sensitive Nanoscale
photodetector

25. Charge Quantization e e d Charging energy: Ec = e2/2C >> kT3 2 1 e e
N=0 d
Charging energy: Ec = e2/2C >> kT
At T =300K
kT = 26 meV
C<< 3.1x10-19 F
C = 4pe d
4pe = 1.1x10-10 J -1 C2m-1
For charge quantization,
the diameter of dot (d) must be << 28 nm

26. Tunneling Spectroscopy of InAs QD
STM
T=4.2K
Optical
d = 32A
S-like
P-like
U. Banin et al, Nature,
400, 926 (2000)
Ec=0.11 eV: single electron charging energy
Eg=1.02 eV: nanocrystal band gap

27. Property: Crystallization of Opals from Polydisperse Nanoparticles
Driving force for size selective crystallization and phase separation is the size dependence of the dispersional interactions.
Dispersional interactions are strong enough to dominate over entropic effects, but weak enough to allow the nanocrystals to anneal into low energy, equilibrium configuaration.
Ohara et al, Phys. Rev. Lett. 75, 3466 (1995)

28. Property: Melting Temperature of Nanocrystal
A.P.Alivisatos, J. Phys. Chem. 100, (1996)

29. Property: Thermodynamic Behaviors of Metal Clusters
As the cluster size decreases, the melting
temperature (Tm) monotonically decreases,
However, when the cluster size is small enough, Tm does not vary monotonically with cluster size.
The absence of a premelting peak in heat capacity curves for some clusers.
Premelting: surface melting, partial melting, orientational melting, and isomerization
Y.J. Lee et al, J. Comp. Chem 21, 380 (2000), Phys. Rev. Lett. 86, 999 (2001)

30. 2. SPM의 원리 및 응용 References:
1. Nanotechnology Research Directions: IWGN report (1999):
2. Scanning Tunneling Microscopy and related methods edited by R.J. Behm, N. Garcia, and H. Rohrer (1990)
3. R.J. Hamers, Scanning Probe Microscopy in Chemistry, J. Phys. Chem.100, (1996)
4. G.S. McCarty and P.S.Weiss, Scanning Probe Studies of Single Nanostructures,
Chem. Rev. 99, 1983 (1999)
5. PSIA,
6. ThermoMicroscope,
7. Digital Instrument,
8. H.-Y. Nie, publish.uwo.ca/~hnie/mmo/all.html
IWGN: Interagency working group on nanoscience , engineering, technology

31. 강의 내용 나노시스템의 특성 개요 기존 현미경과 SPM의 분해능 비교 STM/STS의 원리 및 응용 AFM의 원리 및 응용

32. Scientific Issues in Nanoscience
(Nanotech.Res. Directions: IWGN report,1999)
Fundamental properties of isolated nanostructures
Fundamental properties of ensemble of isolated nanostructures
Assemblies of nanoscale building blocks
Evaluation of concepts for devices and systems
Nanomanufacturing
Connecting nanoscience and biology
Molecular electronics
Local probes with nm spatial resolution for characterization

33. Tools for Characterization
Structural analysis: SEM, TEM, XRD, SAM, SPM, PEEM, LEEM,
STXM, SXPEM
Chemical analysis: AES, XPS, TPD, SIMS, EDX, SPM
Electronic, optical analysis: UV/VIS, UPS, SPM
Magnetic analysis: SQUID, SMOKE, SEMPA, SPM
Vibrational analysis: IR, HREELS, Raman, SPM
Local physico-chemical probe: SPM
e, hv, ion
tip
e, hv, ion

34. 현미경들의 분해능 비교 106 SEM OM Vertical scale TEM 104 102 STM (A) 1
Lateral scale (A)

35. Characteristics of Microscopies
OM SEM/TEM SPM
Operation air,liquid vacuum air,liquid,UHV
Depth of field small large medium
Lateral resolution mm nm:SEM nm: AFM
0.1nm:TEM nm: STM
Vertical resolution N/A N/A nm: AFM
0.01nm: STM
Magnification X-2x103X X-106X x102X- 108X
Sample not completely un-chargeable surface height
transparent vacuum <10 mm
compatible
thin film: TEM
Contrast absorption scattering tunneling
reflection diffraction

36. 분해능의 한계 HM (High Resolution Optical Microscope)
diffraction limit ~ 150 nm
PCM (Phase Contrast Microscope)
subwave length 배율 가능
X-ray Microscope
l ~1A; Synchrotron radiation and X-ray optics
SEM (Scanning Electron Microscopy)
Focused e-beam size > 3nm
TEM (Transmission Electron Microscopy)
~1 A; 얇은 박막시료
LEEM (Low Electron Emission Microscopy)
low energy e-beam; 20nm
PEEM (Photoemission electron Microscopy)
lateral resolution~100nm; chemical analysis
FIM (Field Ion Microscopy)
tip 형태 시료

37. Interaction of Electrons with Sample
When 20 KV e-beam is used for Ni(Z=28).
Auger electron :10-30A
Secondary electron: 100A
Backscattering electron
: 1-2 mm
X-ray ~ 5 mm
Penetrated electron: 5 mm
Atomic Number
Accelerating voltage
Incidence angle
Interaction
Volume

38. Principle: Scanning Electron Microscopy (SEM)
Beam size: a few – 30 A
Beam Voltage: 20-40kV
Resolution: A
Magnification: 20x-650,000x
Imaging radiations: Secondary electrons,
backscattering electrons
Topographic contrast: Inclination effect,
shadowing, edge contrast,
Composition contrast: backscattering
yield ~ bulk composition
Detections:
- Secondary electrons: topography
- Backsactering electrons:
atomic # and topography
- X-ray fluorescence: composition
E-gun
Lens
Detectors
Secondary
electron
Sample

39. Instrumentation: SEM

40. Principle: Transmission Electron Microscopy (TEM)
Beam size: a few – 30 A
Beam Voltage: 40kV- 1MV
Resolution: 1-2A
Imaging radiations: transmitted electrons,
Imaging contrast: Scattering effect
Magnification: 60x-15,000,000x
Image Contrast:
1) Amplitude (scattering) contrast
- transmitted beam only (bright field image)
- diffraction beam only (dark field image)
2) Phase (interference) contrast
- combination of transmitted and diffraction
beam
- multi-beam lattice image: atomic
resolution (HRTEM)
E-gun
Condenser
lens
Thin
sample
Objective
lens
Projector
Screen

41. Example: TEM images of Co-nanoparticles
60 nm
Self-assembled
2D-monolayer
Surface
Modification A A
Co nanoparticles
평균 지름 = 7.8 ±1 nm B B
Self-assembled
3D-multilayer
From J.I. Park and J. Cheon

42. Interactions used for Imaging in SPM
Tunneling
I~exp(kd)
(b) Forces
F(d) ~ various
(c) Optical near fields
E ~1/d4
(d) Capacitance
C(d) ~ 1/d
(e) Thermal gradient
(f) Ion flow
f(d) ?
Resolution limits
The property probed
The probe size

43. Scanning Probe Microcopy
Scanning Tunneling Microscopy(STM): topography, local DOS
Atomic Force Microscopy (AFM): topography, force measurement
Lateral Force Microscopy (LFM): friction
Magnetic Force Microscopy (MFM): magnetism
Electrostatic Force Microscopy (EFM): charge distribution
Nearfield Scanning Optical Microscopy (NSOM): optical properties
Scanning Capacitance Microscopy (SCM): dielectric constant, doping
Scanning Thermal Microscopy (SThM): temperature
Ballistic Electron Emission Microscopy (BEEM): interface structure
Spin-polarized STM (SP-STM): spin structure
Scanning Electro-chemical Microscopy (SECM): electrochmistry
Scanning Tunneling Potentiometry (SPM): potential surface
Photon Emission STM (PESTM): chemical identification

44. Scanning Tunneling Microscope
Coarse positioning
device
Scanning Modes:
1. Constant current
2. Constant height
Piezo tube scanner
X,Y,Z
Tip
Current Feedback Computer
amplifier controller
Sample
구성: piezo tube scnner, tip, feedback control system
Voltage between tip and sample, tunneling current is measured, feed back controller compare with set current,
Vary the voltage applied to PZT scanner to maintain the set current
Sample bias
voltage
Real space imaging with atomic resolution

45. Constant current STM image corresponds to
Theory of STM
J. Tersoff and D.R. Hamann, Phys. Rev. lett. 50, 1988 (1983)
Figure From J. Wintterlin 
sample
tip Ef
eVbias Ef d
For meta surface, LDOS at a surface follows the corrugation of the surface atoms
For semiconductors, the suface state tend to be localized on some atoms or bonds
It ~  tip2 sample2 e-2d
~  sample2 d(E-Ef) for low voltage limit; a point tip
Constant current STM image corresponds to
a surface of constant state density.

46. Tunneling Spectroscopy
d: fixed
It ~  r tip (E) r sample (E-eV)T(E,V) dE
(Z)IV- x,y : Topography
di/dV~ rsample (E): LDOS
d2i/dV2: local vibrational spectra A
Si(111)-2x1 Si
DOS features seen inI/V curves are obscured by the fact that It depend on separation and aplied volatge.
This dependence can be partially removed by plotting the ration of differential to total conductivity dI/dV(I/V) dI/dV(I/V)
The normalized conductivity =surface DOS at least for a metal Ni
Theory
I vs V
(di/dV)/(I/V) vs V
J. A. Stroscio et al. Phys. Rev. Lett. 57, 2579 (1986)

47. Applications of STM Surface geometry Molecular structure
Local electronic structure
Local spin structure
Single molecular vibration
Electronic transport
Nano-fabrication
Atom manipulation
Nano-chemical reaction

48. Atom-resolved Surface Structure
2x1
c(4x2)
p(2x2)
Buckled 2x1
p(2x2)
c(4x2)
Various Reconstructions of Ge(100)-2x1
J.Y. Maeng et al (2001)

49. Si(100) and Ge(100) 2x1 surface  - * Clean Surface Structure Si(001)
at 80 K
Clean Surface Structure
Lattice Constant
Dimer Length
Tilt Angle
 - *
Buckled-Symmetric
Si(100)
5.43 Å
2.32 Å 70
0.66 eV
25 meV
Ge(100)
5.658 Å
2.41 Å
140
1.11 eV
60 meV

50. Molecular Orbitals sample tip - + Occupied state (HOMO) Ef Ef d
sample
tip
Occupied state (HOMO) Ef Ef d
Unoccupied state (LUMO)
STM image: Spatial location of the molecular orbitals, rather than, geometrical position of atoms
-sample bais image: occupied orbitals
+biais image: unoccupied orbitals Ef Ef d
J.J.Boland, Adv. Phys. 42, 129(1993)

51. STM Topograph of Quantum Dot
Ge pyramid containing
~2000 Ge atoms on Si(100)
Ge dome grown by PVD
on a 600 C Si(100)
R.S. Williams et al, Acc. Chem. Res. 32, 425 (1999)

52. STM Images of Ni Clusters at Different Sample Bias Voltages
Spectroscopic information can be obtained by recording multiple images at different voltages
Or spectra can be acquired at one selected point by holding the the probe still and sweeping the bias voltage

53. STS and STM of Self-assembled Co-nanoparticles
Tip on self-assembled monolayer
Tip on a Co nanoparticle
I vs V
dI/dV vs V
Pileni et al, Appl. Surf. Sci. 162/163, 519 (2000)

54. Tunneling Spectroscopy of InAs QD
STM
T=4.2K
Optical
d = 32A
S-like
P-like
Fig 1 a : double barrier junction configuration
Fig 1b shows Tunneling conductance spectrum and a series of single electron tunneling peaks
DT: hexane dithiol
Chrging energy(addtion spectrum) and discrete energy level spacings( excitation spectrum) of the QD
S-like feature: doublet
P-like feature: 6 peaks; p orbital can take 6 electrons
Ec=0.11 eV: single electron charging energy
Eg=1.02 eV: nanocrystal band gap
U. Banin et al, Nature,
400, 926 (2000)

55. Electronic Properties of Carbon Nanotubes
Constant LDOS
Metallic SWNT
Semiconducting
SWNT

56. Single Molecule Vibrational Spectroscopy
Vibration excitation of the molecule occurs when tunneling electrons have enough energy to excite a quantized vibrational level
Inelastic tunneling channel
B.C. Stipe. et. al., Science 280, 1732 (1998)

57. Real Space Imaging of Two-Dimensional Antiferromagnetism On the Atomic Scale Weisendanger et al, Science 288, 1805 (2000)
Nonmagnetic W tip
Magnetic W tip

58. Atomic Manipulation by STM
Iron on Copper (111):
Circular corral
radius= 71.3 A
48 Fe atoms
Quantum-mechanical
interference patterns
M.F. Crommie, C.P. Lutz, D.M. Eigler. Science 262, (1993).

59. Molecular Adsorption: 2+2 cycloaddition reaction
Fabrication of molecularly ordered organic films
Anisotropic optical and electrical properties
A Possible way to orient molecular devices
R.J. Hamers et al, J. Phys. Chem., 101, 1489 (1997)

60. In-situ Monitoring of Self-assembly
Self-directed growth of Nanostructures
Wolkow et al, Nature, 406, 48 (2000)

61. Single-Bond Formation by STM
H.J. Lee and W. Ho, Science, 286, 1719(1999)