Designing Nanoscale Materials
Lecture Series by 2004 Debye Institute Professor
Christopher B. Murray
IBM Research
Ornstein Laboratory 166
Office phone 253 2227
cbmurray@alum.mit.edu
Lecture Series: Designing Nanoscale Materials.
(1) Why smaller, is different; finite size effects & implications for Tech scaling.
(2) General nanoparticle production.
(3) Semiconductor nanocrystals (Quantum Dots)Part 1:
(4) Semiconductor nanocrystals (Quantum Dots), Part 2:
(5) Nanowires
(6) Nanostructured magnetic materials for IT
(7) Nanomagnetics for biotech & beyond.
(8) Self-assembled nanocrystal superlattices:
(9) Binary nanocrystal assembly a route to multifunctional nanomaterials.
(10) Nanoporous materials:
(11) Ethics, issues and emerging trends for nanomaterials research.
Designing Nanoscale Materials
1+2
3+4
5
6+7
8+9
10+11
Wed.
Wed.
Mon.
Wed.
Wed.
Wed.
Sept
Sept.
Sept.
Sept.
Oct.
Oct.
08
15
20
29
06
13
10.00-12.30
10.00-12.30
11.00-12.30
10.00-12.30
10.00-12.30
10.00-12.30
Why smaller, really is different:
Finite size effects in nanomaterials their
implications for scaling in conventional technology.
Christopher B. Murray
Manager Nanoscale Materials & Devices
IBM Research
Balancing the investments in Nanotechnology:
Extending the
Established
Technologies
Immediate
Impact
Exploring
Alternative/Disruptive
Technologies
Basic/Strategic
Research
Long-Term
Impact
Three Questions
ƒ What is nanotechnology?
ƒ Why is nanotechnology the future of information
technology?
ƒ How will we manufacture at the nanoscale?
Nanotechnology is …
… research and technology development at the
atomic, molecular or macromolecular levels, in the
length scale of approximately 1 – 100 nm …
National Science Foundation
Minimum machined dimension (microns)
A Unique Period in History
1000
100
Moore’s Law (1965)
10
1
90 nm Manufacturing
(2004)
193 nm immersion
EUV
industry roadmap
0.1
0.01
2004 commercial
niche lithographies
0.001
2004 best lab practice
0.0001
1800
after C. Ausschnitt, Microelectronic Engineering, 41/42 (1998) 41- 46
1850
1900
1950
2000
2050
2100
C o m p u tin g : W h ere D o W e G o F ro m H ere?
N a n o s c a le
S c ie n c e
and
Te c h n o lo g y
i10 m
i100 nm
Decreasing Costs of Computation
Source: Kurzweil 1999 – Moravec 1998
Constant Field Scaling
Voltage, V
WIRING
tox
Voltage, V / α
tox /α
W
GATE
n+
source
n+
source
n+
drain
L
p substrate, doping NA
SILICON
WAFER
GAT
E
WIRIN
G
W/α
n+
drain
L/α
xd
xd/α
p substrate, doping α*NA
SILICON
WAFER
RESULTS:
Higher Density:
Higher Speed:
Lower Power:
per circuit
Power Density:
α2
α
1/α2
Constant
Silicon Transistor - Already A Nanodevice
Logic
Gate
Source
Drain
TSi=7nm
Lgate=6nm
• Power4 Chip
• Gate length = 6 nm
• 174 million transistors
B. Doris et al., IEDM , paper 10.6, 2002.
J. Warnock et al., IBM J. R&D, p. 27, 2002
Size dependent phase
transformation
The transition from
c49 to c54 TiSi2
Result in a
Phases formed by heating of a
10nm Ti film. As-deposited, the
grain size is ~10nm (annealing
at low Temp small would yield
TiSi. The TiSi2 C49 phase
appears at 700°C while C54
forms at 850°C.
Silicon Transistor - Already A Nanodevice
Memory
• 512Mb DRAM prototype for
1Gb and beyond
• 110 nm DRAM, 8F2
S. Wuensche et al., Symp. VLSI Technology, 2002
H. Akatsu et al., Symp. VLSI Technology, p. 52, 2002
The conventional silicon field-effect transistor is still rapidly
advancing, with potential materials innovations such as …
ƒ Metal gate electrodes
ƒ High-dielectric constant
Gate
gate insulators
ƒ Ultra-thin (2 – 20 nm)
Si or Ge on insulator
insulator
Source
channel
insulator
ƒ High-electron-mobility
substrates (strain or
orientation)
silicon substrate
Drain
Si epitaxy on oxide
Epitaxial Growth of
Semiconductors on
Crystalline Oxides
0
10
x-ray reflectivity
-1
reflectivity
10
simulation
-2
10
........ experiment
-3
10
Ge epitaxy on oxide
-4
10
-5
10
-6
10
0.00
0.01
0.02
-1
0.03
qz (nm )
Bojarczuk, Guha et al.,
Appl. Phys. Lett. V83, 5443-5, (2003)
Double-gate Transistor (FinFET)
• Scalable to the smallest channel length
• World-record double-gate FET device performance
“gate delay” = 0.92 ps
current-carrying
surfaces
TEOS
Poly-Si
TEM
Tsi=20nm
Tox=1.6nm
H=65nm
Tox
H
Tsi
BOX
Cross-section of 60 nm channel length FET
Coulomb Blockade Effects:
Chuck Black, Bob Sandstrom, Chris Murray, Shouheng Sun
single-electron charging energy
r
C ~ 2πε0εr[ln(r/d)]
~ 1.3 aF
2d
r
d<<r
2-d hcp lattice, each nanocrystal has 6 nearest neighbors (nn):
Cnn = 7.8 aF
to charge nanocrystal with a single extra electron:
Ec = e2 ~ 10 meV
2Cnn
Coulomb energy dominates below ~ kBT=Ec/2
T~ 60K
o
Electronic Properties of Semiconductor and Metal Nanoparticles
ε
a
Charge not completely solvated
as in infinite solid
Nanoparticle capacitance C = 4πε oεa
Charging Energy
e2
Ec =
2C (a )
10 nm Al NC
Courtesy of C. T. Black, Thesis, Harvard U.
Coulomb blockade at
kBT<Ec
Structure from discrete electronic states of
metal NC
STM Measurements on Single QDs
InAs QDs
U. Banin et al. Nature 400, 542 (1999).
Parameters in NC arrays
ƒ Transport
– explained from Middleton-Wingreen(M-W) model
transport in linear and square arrays (how ideal?)
ƒ Achieving high operational T.
Fabrication method affects
– Size of particles
– Monodispersity
– Number of particles responsible for transport
– Dimensionality
– Homogeneity of array
Self assembly:
single layer 2-D array of Au crystals
ƒ
ƒ
ƒ
ƒ
ƒ
NC size : 2.2 - 2.9 nm,
Interparticle distance
s1-2=0.85nm, 1.2–0.1nm.
(Threshold voltage)VT ~ 10V
T independence! (12, 48, 77K)
e2/Cmax>kbT
ƒ Global structural disorder (topology)
ƒ Local structural disorder
(voids,interparticle distance)
ƒ Local charge disorder
(e.g
substrate,..)
R. Parthasarathy et al., Phys. Rev. Lett.
87, 186807
Spin-dependent tunneling in Nanocrystal arrays
Chuck Black, Bob Sandstrom, Chris Murray, Shouheng Sun
I (pA)
400
T = 70 K
T=2K
200
0
-200
-400
-0.4
shortest current path ~ 8 nanocrystals
-0.2
0.0
V (V)
0.2
0.4
10
2
10
1
10
0
1.00
-1
10
-2
10
-3
10
20
40
60
80x10
-3
-1
1/T (K )
data fit by:
ln(GV=0) = const. - Ec/kBT
from fit to data, measure EC~ 10 meV
for all devices measured, 10 meV < EC < 14 meV
R/RH=0
GV=0 (1/GΩ)
GV=0 follows simple thermal-activation
H
H
0.98
0.96
0.94
0.92
-0.4
-0.2
0.0
0.2
applied field (T)
0.4
Directed Self-assembly
Experimental Silicon Memory Device
Phase separation of
block copolymers
to form columnar
arrays
The process is then
used in fabricating
an exploratory
silicon memory
device
Source: C. Black, K. Guarini, IBM
Goal: Incorporate Nanoscale Components in IT Systems
Poromer
(dendritic polymer)
Porous Dielectric
for On-Chip Wiring
Ultra Low K Dielectrics
Basic Physics of Semiconductor Quantum Dots
C. R. Kagan, IBM T. J. Watson Research Center, Yorktown Heights, NY
Lowest
Unoccupied
Molecular
Orbital
Conduction
Band
Energy
Gap
Highest
Occupied
Molecular
Orbital
Valence
Band
Bulk Semiconductor
Quantum Dot
Like a
Molecule
Quantum Confinement
Low Dimensional Structures
ρ c (E ) ∝
(E − EC )
ρ c (E ) = cons tan t
ρ c (E ) ∝
1
(E − En )
ρ c (E ) ∝ δ (E − E n )
Particle-in-a-Sphere
j l (k n,l r )Yl m (θ ,φ )
Φ (r ,θ ,φ ) = C
r
a
Yl m (θ ,φ ) is a spherical harmonic
Potential V
∞
2s
1s
0
j l (k n,l r ) is the lth order spherical Bessel function
k n,l =
α n,l
a
r
solutions give
hydrogen-like orbitals with
quantum numbers
n (1, 2, 3 …)
l (s, p, d …)
m
E n,l
2 2
2 2
O
O
α n,l
k n,l
=
=
2mo
2mo a 2
Discrete energy levels
size-dependence
Size Dependent Absorption
Example: CdSe
150 Å
Absorbance (arbitrary units)
Absorbance (arbitrary units)
150Å
90 Å
72 Å
55 Å
45 Å
33 Å
29 Å
21 Å
17Å
17 Å
1.5 2.0 2.5 3.0 3.5
1.5 2.0 2.5 3.0 3.5
Energy (eV)
Energy (eV)
Real Band Structure
Example: CdSe
E
Cd 5s orbitals
2-fold degenerate at k=0
J=1/2
Eg
∆cf
crystal field splitting
k
hh
Se 4p orbitals
6-fold degenerate at k=0
Introduces splitting of bands
J=L+S
∆so
where
heavy hole
J=3/2
lh
light hole
so
spin-orbit splitoff
L=orbital angular momentum
S=spin angular momentum
J good quantum number due to strong spin-orbit coupling
J=1/2
Metal Nanoparticles
- - - - --
-- Metal
- Particle
-
--
-- - - --
Surface Plasmon Resonance
• dipolar, collective excitation between
negatively charge free electrons and
positively charged core
• energy depends on free electron density
and dielectric surroundings
Au nanoparticle
absorption
• resonance sharpens with increasing
particle size as scattering distance to
surface increases
Antiferromagnetically-Coupled media
Three-atom-thick layer of Ru sandwiched between two magnetic layers
Expected to increase current areal density limits to surpass 100
gigabits/inch2
Magnetic thickness
Magnetic thickness = Mr t
= (Mr t)top - (Mr t)bottom
AFC ( cont. AFC
)
Media
Magnetoresistance (%)
MRAM Technology
Write
Word
Line
MTJ
Bit
Line
MT
MT
M2
M1
Applied Field (Oe)
Magnetic tunnel junction device
and electrical characteristic
Schematic of two MRAM cells
128kb test chip
MRAM cell cross-section
in 0.18 µm technology
MRAM potentially has attributes of a universal memory:
Î fast, dense, nonvolatile, radiation hard
Catalysis
Au nanoparticles supported on TiO2 substrates show high activity for oxidation of
CO at room temperature and below.
Reaction proceeds at corner,
step, and edge sites of Au
CO adsorption
(on Au)
3.5 nm Au nanoparticle
12 Atoms in length
Oxygen
Adsorption
(on TiO2)
2-3 Atoms
high
TiO2 Support
Haruta, M.; Date, M. Applied Catalysis A: General 2001, 222, 427-437.
Bimetallic Catalysis
CH2=CH-CN + H2O
CH2=CH-CONH2
CH3-CH-CN
OH
Geometric effects lead to
higher activity and selectivity
for certain reactions.
Reaction proceeds most favorably with
Pd-Cu particles, and is 100% selective
when using a 3:1 Cu:Pd ratio.
Figure taken from: Toshima, N.; Yonezawa, T. New Journal of Chemistry 1998, 1179-1201 and references therein.
Two visions of nanofabrication…
ƒ “Old”
ƒ “New”
ƒ Top down
ƒ Bottom up
ƒ Lithography
ƒ Chemical Synthesis
ƒ Digital
ƒ Analog
ƒ Depend on Low Error Rates
ƒ Tolerate High Error Rates
ƒ Molecular Assemblers
ƒ Self-Assembly
Allowing a few components to approach equilibrium will
produce only simple structures …
Synthesis
Reagents
“Guiding” or “directing” the assembly process:
Semiconductor Nanocrystals
Synthesis
Reagents
Size Processing
Film Growth:
Self-Assembly
Nanocrystal
Superlattice
Multi-Component Nanocrystal Superlattices
F. X. Redl, K. S. Cho and C. B. Murray
Silicon Nanowires: In-situ Observation of Growth
viewing
direction
heated
substrate
Si2H6
110
111
112
110
111
112
Frances Ross, IBM Research
dark-field
image
Si nanowire growth
showing wire and drop
geometry, facet
formation and tapering
to termination
Frances Ross, IBM Research
Beyond the next transistor: Exploratory Memory
SL
m+1
SL
m
SL
m-1
WL
n-1
WL
n
WL
n+1
ƒ Everyone is looking for a dense (cheap) crosspoint memory.
ƒ It is relatively easy to identify materials that show bistable hysteretic
behavior (easily distinguishable, stable on/off states).
Technology Champions
(Companies)
Relative Maturity of Nonvolatile Memory Technologies
20
18
16
14
12
10
8
6
4
2
0
Product
Sampling
Development
Single Cell
Demo
Charts, No
Parts
Smaller projects are also exploring non-volatile memory based
on …
ƒ Perovskites
ƒ Chalcogenides
ƒ Organic materials
Beyond the Next Transistor
“Millipede” Storage
How will we manufacture at the nanoscale?
Carbon Nanotubes?
STM Image
Carbon Nanotube Transistor
dox=15nm
Ti
Ti Ti
Al Al
Appl. Phys. Lett. 80, 3817 (2002)
Comparison with silicon
p-MOSFET a)
p-CNFET
channel length
50nm
260nm
gate oxide thickness
1.5nm
~15nm
transconductance
650mS/mm
2300mS/mm
drive current (Vg-Vt=-1.0V)
650mA/mm
2100mA/mm
threshold voltage
IOn/IOff
subthreshold slope
a)
-0.2V
-0.5V
106 - 107
~106
70mV/dec
130mV/dec
R. Chau et al. Proceedings of IEDM 2001, p.621
Nanotube Infrared Emitter
VD
VS
VG
VS<VG<VD
Plenty of room for
improvement !
Nanotube Technology ?
How do you get from here to there?
100µm
Au CNT
No new architecture !
Establishing a Technology
Understanding:
ƒ Electrostatics, electrodynamics
ƒ Scalability (ballistic? contact-dominated transport ?)
ƒ Contacts, doping
ƒ Gate insulator, interface traps?
ƒ High yield, selective growth/synthesis of nanotubes with correct electrical
properties (single-wall, diameter, chirality)
Engineering:
ƒ Device structure with minimized parasitic resistance and capacitance
ƒ Fabrication processes leading to high device density (e.g. size of contacts
commensurate with gate length, means to connect one device to another)
ƒ Demonstrate device/circuits which satisfies ALL performance metrics (not
just some metrics)
ƒ Manufacturing tools and infrastructure, integration with silicon
ƒ Reliability
ƒ ...
Molecules = Small ?
Si FET
Molecular Device
Gate
Source
Drain
TSi=7nm
Lgate=6nm
B. Doris et al., IEDM , 2002.
ƒ All devices are governed by electrostatics and
- difficult to be much smaller than 2 - 3 nm
L >2.5 – 3 nm
eventually limited by tunneling
Building Molecular Structures to Study the Science
Pt coating
limit assembly to electrode
sidewall
Al
Au
SiO2
Ti
n+ Si
electrolyte
solution
pipette or
needle
also working
electrode
- - - + + + +
SiO2
Si
Drain
Source
Gate
ƒ
R'
R'
R'
R'
R'
R'
ƒ
ƒ
R
R
R
R
R
R
Lipid-like membrane
–
Self-assembled at air-water interface
–
Langmuir-Schaeffer transfer
In-situ polymerization
–
conjugated chain (schematic)
–
wide band conductor
–
end-to-end channel
Hydrophobic binding - gate insulator
Electrochemically Gate
Molecular Junction
Vg
Chemistry to Covalently Bind Molecules to Substrates
OH
OH OH OH OH OH OH OH
O C O CO C O CO C O CO C O C
oxidation
Si
O
O
Si
Si
O
O
O
Si
Si
O
O
O
O
O
Si
Si
O
O
O
O
Si
O
O
Si
O
O
Si
Si
O
O
O
O
Si
Si
O
O
O
O
Si
Si
O
O
O
O
Si
O
O
O
SiO2
SiO2
Si (intrinsic or doped)
Si (intrinsic or doped)
OH
CH2
OR
OH
OH
CH2 CH2
OH
OH
CH2 CH2
OH
O
OH
OH
CH2 CH2
CH2
O
Si
Si
O
O
O
Si
Si
O
O
O
O
O
Si
Si
O
O
O
O
having alcohol
functionality
esterifaction
with molecule
reduction
Si
O
Si
O
O
SiO2
Si (intrinsic or doped)
= molecule of
interest
esterifaction
with molecule
O
O
O
O
O
O
O
O
O C O CO C O CO C O CO C O C
Si
O
O
Si
Si
O
O
O
O
Si
Si
O
O
O
O
Si
Si
O
O
O
O
Si
O
O
O
SiO2
Si (intrinsic or doped)
O
Demonstrating O OSi
now
Si
Si
O
O
O
O
Si
Si
O
O
O
O
Si
Si
O
O
O
O
Si
O
O
SiO2
O
having carboxylic
acid functionality
Si (intrinsic or doped)
Choose Length of alkyl chain
Depending on desired function
"backbone"
(1)
(2)
(3)
UV
Substrate
Substrate
Substrate
O
O
H
O
O
H
O
H
O
O
H
O
O
H
O
H
Water Subphase
O
C
C
C
O
O
H
OH
O
OH
O
O
C
C
C
OH
OH
O
OH
O
OH
Water Subphase
“Layer-by-Layer” Growth of Conjugated Molecules
S
Au
Br
S
S
S
S
S
S
S
Br
S
S
+
Au
R3Sn
S
Au
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
Br
S
NBS
Au
S
S
S
S
S
S
S
S
S
S
S
S
Br
S
+
Br
S
S
Tailor end functionality
to assemble on oxide,
metal, or semiconductor
surface
S
S
S
S
Br
S
S
• Grow long conjugated molecules that would otherwise be insoluble
to span gap between electrodes
• Combine different molecules or oligomers for functionality
S
O
S
O
S
O
S
O
S
O
S
O
O
S
O
S
O
S
O
S
O
S
Br
O
S
N
Br
Au
S
S
S
S
S
N
Br
Zn
N
N
Br
Electron-donating/Electron-accepting
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
grow
metal
SiO2
n+ Si
Reduction-oxidation active centers
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
“Double FET”
with floating electrode
Leq
Leq
Leq
Layer-By-Layer Growth of Metal-Metal Bonded Compounds
Si
N
O O O O
N Rh Rh N
N N N N
O O O O
N Rh Rh N
N N N N
Ligand to
bind to
desired
substrate
surface
Add
metalmetal
Add
metalmetal
S
O O O O
N Rh Rh N
N N N N
N
• Choose M-M bond
M = V, Nb, Cr, Mo, W, Tc, Re, Fe, Ru,
Os, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag …
Add
ligand
N N N N
N Rh Rh N
O O O O
N
Au
N
N
=
MeO
N
O O O O
N Rh Rh N
N N N N
H
C
Leq
Layer 2
Layer 1
S
Leq
• Tailor end group to template
metal-metal bonded unit
Add
ligand
N N N N
N Rh Rh N
O O O O
Leq
• Tailor head group of ligand to bind
to particular substrate surface
Si
Si
M
Leq
N N N N
N Rh Rh N
O O O O
N N N N
N Rh Rh N
O O O O
Lax
Leq
M
Lax
OMe
N
N,N'-di(p-anisyl)formamidinate
N
• Choose ligand to bridge M-M bonded
units to tailor:
• Electronic coupling between
dimetal units
• Electrochemistry
• Solubility
• Structure ….
Beyond the Next Transistor
Molecular Cascade Logic
A.J. Heinrich, C.P. Lutz, J.A. Gupta, D.M. Eigler Science 2002
We are Just Getting Started!
Nanotechnology Definition Revised
The ability to design and control the structure of an
object on all length scales – from the atomic to the
macroscopic – reliably and repeatedly in a
manufacturing environment.