NANOSTRUCTURES IN SOLID STATE SCIENCES
H.-U. Habermeier
Max-Planck-Institute for Solid State Research Stuttgart Germany
OUTLINE
1. NANOPHYSICS AND NANOCHEMISTRY
Some Basics
2. SEMICONDUCTOR NANOSTRUCTURES
3. NANOCHEMISTRY AT INTERFACES IN IONIC
CRYSTALS AND THIN FILMS
4. TAILORING THE FIGURE OF MERIT FOR THERMO-
ELECTRIC MATERIALS BY NANOTECHNOLOGY
5. NOVEL PHENOMENA AT INTERFACES IN COMPLEX
OXIDES
6. CONCLUSIONS
1. NANOPHYSICS AND NANOCHEMISTRYSOME BASICS
WHAT IS NANO ?
Greek:
nano = dwarf
Science:
prefix 10-9
SIZE RELATIONS
~109~109 ~109
NANOMATERIALSMatter in a size range of 1-100 nm is called nanomatter:
•large clusters / particles (3-D)
•thin films and layers (2-D)
•rods and wires (1-D)
•small clusters: quantum dots (0-D)
Size dependent properties
Properties which for macroscopic amounts do not depend on particle size become size dependent
•melting point and other phase transition temperatures•colour•conductivity: how many atoms make a metal?•ionisation potential and electron affinity: chemical properties•magnetic properties: when gold and platinum become magnetic•capillary forces: pore confined matter is different•break-down of the concept of phase: can a cluster of 5 atoms melt? after E. Rodumer
TWO TYPES OF NANO - SIZE EFFECTS
*Smoothly scalable effects:origin: surface atoms are different from bulk atoms surface-to-volume ratio:~1/R ~ 1/N1/3
*Quantum effects:origin: electronic wave function extends over the entire particlecompare: particle in a box
a cluster is a pseudo-atom
after E. Rodumer
SMOOTHLY SCALABLE EFFECTSDISPERSION [fraction of atoms exposed to surface ]
after E. Rodumer
COORDINATION NUMBER[ # of nearest neighbours ]
Large coordination number:
= large number of bonds
= large stabili-zation energy
Order of stability
bulk > surface > edge > corner
CONSEQUENCES FOR CHEMICAL
REACTIVITYA. I. Frenkel et al. J. Phys. Chem. B 105 ( 2001 ) 12703after E. Rodumer
QUANTUM EFFECTSEVOLUTION OF BANDGAP AND DOS WITH NUMBER
OF ATOMS IN A CLUSTER
after E. Rodumer
TAKE HOME MESSAGES
1. Physical properties and chemical reactivity are
system size dependent.
2. Quasi - continuum approximation does not hold
anymore.
3. Quantum effects dominate the physical system
properties
IN SYSTEMS AT THE NANOSCALE
2. SEMICONDUCTOR NANOSTRUCTURES
2.1 THE 2-D ELECTRON GAS
MINIATURIZATION OF SEMICONDUCTOR DEVICES
J. Kilby B. - Noyce
SEMICONDUCTOR SUPERLATTICES
Formation of a 2 D Electron Gas
High Electron Mobility Transistor
High Electron MobilityTransistor
Solid State Laser
QUANTUM HALL EFFECT QHE - FQHE
2.2 QUANTUM WIRES ( 1- D ) e.g. CNTs
Carbon Structures
diamond (3D)
single-walled carbon nanotube (1D)
C60 (0D)
Graphene ( 2D )
CARBON NANOTUBES AS 1 – DIMENSIONAL
OBJECTS
NT Quantized Resistance
MWNT on piezo-controlled Tip
Contact to liquid metal
→ quantized conductivity
n·G0=n·2e2/hBallistic Electrontransport
• Resistance independent of length
• > 1 mA per NT ! Frank et al., Science 280 (1998) 1744
(n [25.8 kΩ] −1)
Electron Transport in SWNT
metallic SWNT
Pt SiO2 Pt
50 nmTans et al., Nature 386 (1997) 474
Vgate
small voltage → no current (Coulomb-blockade)
larger voltages → quantized increase of conductivity
→ electronentransport via discrete elektron. States
→ metallic SWNT 1-dim quantum wire
IV-CURVE @ 50 mK
2.3 QUANTUM DOTS ( 0 D )
From G. Costantini
From G. Costantini
after G. Costantini
SIZE DEPENDENCE
Photoluminescence spectroscopy
From G. Costantini
TAKE HOME MESSAGES
SEMICONDUCTOR PHYSICS IS “THE“ LABORATORY TO STUDY THE PROPERTIES OF THE LOW
DIMENSIONAL ELECTRON GAS
Quantum effects: weak localization
integer QHE
fractional QHE
Dimensioality effects in heterostructures:
new electronic devices
( HEMT‘s Lasers )
3. NANOCHEMISTRY AT INTERFACES IN IONIC CRYSTALS AND THIN FILMS
DEFECTCHEMISTRY
WHY DEFECT CHEMISTRY ?
1. Intrinsic defects
unavoidable: Thermodynamics requires defects
System minimizes Gibbs Free Energy G = H - T.S
Greal = Gideal + ΔDG + Gconfig Gconfig = - T.Sconfig
(Gibbs Free Energy for Defect Formation > 0)
S = kln(number of possible configurations )
after J. Fleig
after J. Fleig
after J. Fleig
TAKE HOME MESSAGES
DEFECT CHEMISTRY IN NANOSYSTEMS DESEREVES MUCH MORE ATTENTION
- Intrinsic defects, extrinsic defects and coupled ionicand electronic defects can change materialsproperties drastically
- Grain boundaries are a sink of vacancies – formation of a space charge region
- Oxygen vacancies in complex oxides are an additional parameter to be considered
4. TAILORING THE FIGURE OF MERIT FOR THERMOELECTRIC MATERIALS
BY NANOTECHNOLOGY
U.S. Energy Flow, 2002 (Quads = 10U.S. Energy Flow, 2002 (Quads = 10 1515 BTU) BTU) 61.5% of energy is wasted61.5% of energy is wasted
1‰ of waste energy = 109€
P. Dehmer
DoE
MRS Fall 2006
1Quad ~ 3.1011
kWh
MOTIVATION AND SOME BASICS (1)
The key parameter is Z (thermoelectric figure of merit)
ZT = S2σT/κ = PT/κ
S = Seebeck coefficient σ = electrical conductivityκ = thermal conductivityT = absolute temperatureP = power factor
SOME BASICS (2)
PbTe/PbSeTe
S2σ (μW/cmK2) 32 28k (W/mK) 0.6 2.5 ZT (T=300K) 1.6 0.3
BulkNano
Harman et al., Science, 2003
Bi2Te3/Sb2Te3
S2σ (μW/cmK2) 40 50.9k (W/mK) 0.6 1.45 ZT (T=300K) 2.4 1.0
BulkNano
Venkatasubramanian et al., Nature, 2002.
0.0
0.5
1.0
1.5
2.0
2.5
3.0
1940 1960 1980 2000 2020
FIG
UR
E O
F M
ERIT
(ZT)
max
YEAR
Bi2Te3 alloy
PbTe alloy
Si0.8Ge0.2 alloy
Skutterudites
PbSeTe/PbTeQuantum-dotSuperlattices(Lincoln Lab)
Bi2Te3/Sb2Te3Superlattices(RTI)
AgPbmSbTe2+m(Kanatzidis)
State-of-the-Art (SOA) in Thermoelectrics
SOME BASICS (3)
WHAT DO WE NEED ???
Optimize ZT = S2σT/(κe + κph )
NO WAY TO INDEPENENTLY OPTIMIZE S, σ and κ
e.g. in metals thermal and electrical conductivity are
related by Wiedemann-Franz law
λ / σ = LT ( L = Lorenz number = 2.44.10-8 WΩK-2 )
APPROACH: SMART MATERIALS DESIGN
SOME BASICS (4)
Two important developments
I. 1993 – Hicks & DresselhausLower dimensional structure:
Large and asymmetric density of states at the Fermi level.Enhanced phonon scattering on nanometer-size features.
Spectacular ZT values for Bi2Te3/Sb2Te3 superlattices (ZT ~ 2.2)
PbTe/PbSe QD superlattices (ZT ~ 3.0)
Venkatasubramanian (2001)
Harman et al. (2005)
II. 1995 – G. A. SlackPhonon-Glass-Electron-Crystal paradigm (PGEC)
Decoupling of electronic and vibrational degrees of freedom. Best chance to realize in materials with an open structure.Dramatic impact on the search for novel bulk TE.
Bulk semicond. Quantum Well Quantum Wire Quantum Dot
Find ways to incorporate low-dimensional features in bulk matrices.
THE OXIDE CHALLENGE
PREPARE THERMALLY STABLE MATERIALS WITH HIGH S and σ
byELECTRONIC AND ORBITAL ENGINEERING
PREPARE THERMALLY STABLE MATERIALS AS
ELECTRON CRYSTAL AND PHONON GLASS
MATERIALS BY DESIGN –THE THIN FILM OPPORTUNITY ( 1 )
PATHWAYS TO ACCOMPLISH THE OXIDE CHALLENGESa.) electronic engineering by DOPING
Maignan et al Eur. Phys. J. B 39, 145–148 (2004)
MATERIALS BY DESIGN –THE THIN FILM OPPORTUNITY ( 2 )
PATHWAYS TO ACCOMPLISH THE OXIDE CHALLENGES
b.) orbital engineering by epi strain
MATERIALS BY DESIGN –THE THIN FILM OPPORTUNITY ( 3 )
PATHWAYS TO ACCOMPLISH THE OXIDE CHALLENGES
c.) orbital engineering by interface design
YBCO-LCMO-
Λ = 17 nm
Y. Kuru 2009
MATERIALS BY DESIGN –THE THIN FILM OPPORTUNITY (4)
PATHWAYS TO ACCOMPLISH THE OXIDE CHALLENGES
d.) phonon scattering engineering by superlattice designand interface control
superlattice approach
good conductorbad cond /insulator
good conductor
good conductorbad cond /insulator
bad cond /insulator
MATERIALS BY DESIGN –THE THIN FILM OPPORTUNITY (5)
PATHWAYS TO ACCOMPLISH THE OXIDE CHALLENGES
e.) phonon scattering engineering by nanoparticles and grain boundary design
SOME EXAMPLES ( 3 )
LASER – INDUCED GENERATION OF ΔT
LASER- INDUCED THERMOELECTRIC VOLTAGE LITV
[ ]
[ ] 0
2sin2
100
010
=
⋅Δ⋅Δ=
U
TSt
lU θ
SOME EXAMPLES ( 4 )
YBa2Cu3O7 @ RT
Testardi et al APL 64 (1994) 2347, Lengfellner et al Europhys. Lett 25 (1994): 375, Habermeier et al. Sol. State Comm.110 (1999) 473
SINGLE LAYERS
DEPENDENCE ON NUMBER OF PERIODS
Φ = 50 mJ/cm2
L = 3 mm
DRASTIC ENHANCEMENT OF
SIGNAL
ENHANCED ANISOTROPY OR
SOMETHING INTRINSICALLY NEW
??
SUPERLATTICES YBa2Cu3O7 / La0.6Pb0.4MnO3
Φ = 50 mJ/cm2
L = 3 mmYBCO
LPMO
SOME EXAMPLES ( 6 )
4.2 ENHANCEMENT OF ZT IN SUPERLATTICESTHERMOELECTRICITY IN STO BASED SUPERLATTICES
ZT value is 0.37 at 1000 K, highest among the reported n-type oxides
bulk vs. film
4. SOME EXAMPLES ( 7 )
H Ohta et al Nature Mat 6 (2007) 129
SOME EXAMPLES ( 8 )
First MPI – TE Results
LCMO 2000 ADIMENSIONALITY or INTERFACE EFFECTS ???
S. Heinze et al. 2009
TAKE HOME MESSAGES
NANO - ENGINEERING AS A TOOL TO IMPROVE THERMOELECTRIC PROPERTIES
- Separate control of electronic and phononicproperties (electron crystal - phonon glass )
- Novel interface related modifications of Seebeck coefficient
5. NOVEL PHENOMENA AT INTERFACES IN COMPLEX OXIDES
PHYSICS AT THE NANOSCALE
QUASI - CONTINUUM
APPROXIMATION DOES
NOT HOLD ANYMORE
MATERIALS WITHSTRONG CHARGE CARRIER
CORRELATION
SINGLE PARTICLE
APPROXIMATION DOES
NOT HOLD ANYMORE
INTERFACES
COMPLEX OXIDES WITH PEROVSKITE STRUCTURE
ABO3
INSULATORS METALS SUPERCONDUCTORS
FERROMAGNETS ANTIFERROMAGNETS FERROELECTRICS
MULTIFEROICS
INTERPLAY OF CHARGE SPIN ORBITAL DEGREE OF FREEDOM + LATTICE INTERACTION
PHYSICS ( idealized picture ) - III
crystal structure e.g. perovskite
* atomically sharp, * no interaction, interdiffusion etc.* no defects etc.
5.2 PHENOMENOLOGY COMMENTS ON OXIDE HETEROSTRUCTURES
Nature 427 (2004) 423
10-4 Torr
SrTiO3-LaAlO3
field effect transistorThiel et al., Science 2006
ZnO-Mg1-xZnxO
quantum Hall effectTsukazaki et al., Science 2007
RECENT PHENOMENOLOGICAL ACHIEVEMENTS
Nature 427 (2004) 423
LRO-1
LRO-2
LRO-2LRO-1
LRO-1
SUBSTRATE
QUESTIONS:
a.) crosstalk ??
b.) interfaces??
c.) artificial multiferroics ??
d.) construct new materials ??
COMPLEX OXIDE SUPERLATTICESCOMBINING MATERIALS WITH DIFFERENT
FUNCTIONALITIES
FILM PREPARATION
PULSED LASER DEPOSITION
Pyrometric temperature control
180nm YBCO dep. rate ~15 nm/min
Total high T exposure < 20 min
KrF - Excimerlaser 248 nm Oxygen: .5 mbarComputer - controlled target exchange
STRUCTURAL CHARACTERIZATION
X-ray diffraction
Y. Kuru 2008
CuO2
LaCaO
CuO
BaOCuO2Y
MnO
BaO
CuO
Ba
MnYLa(Ca)
A
B
MIRROR INTERFACES
A - B vs B – A
ARE NOT IDENTICAL
But
BOTH INCLUDE CuO2
PLANES
Z. Zhang, U. Kaiser, Uni Ulm (7 uc LCMO in between of 50 nm YBCO)
HIGH RESOLUTION TEM
LCMO
YBCO
YBCO
H.-U. H., G. Christiani et al. Physica C 364 ( 2001 ) 298 H.-U. H. and G. Christiani J. of Supercond. 15 (2002) 425
• depression of TCurie and Tc
• magnetism matters
• “superconductivity“ matters- regime of pseudogap ?? -
bilayers
T. Holden, C. Bernhard, H.-U. H. et al., Phys. Rev. B 69, (2004) 064505
Tc=85 K; Tmag=245 K
Tc=73 K; Tmag=215 K
Tc=60 K; Tmag=120 K
SPECTROSCOPICELLIPSOMETRY
strange metal (SC) + strangemetal (FM) strangeinsulator
YBCO
LCMO
YBCO
LCMO
YBCO
LCMO
STO
Charge Transfer Santamaria, Varela et al.
Diffusion of spinpolarized quasiparticlesSoltan et al.
Proximity effect coupling via YBCO Bulaevski, Efetov …. Sa de Melo
Magnetic proximity effect Bergeret, Efetov
Coupling via FM spacer
5.3 OXIDE FERROMAGNET- SUPERCONDUCTOR INTERACTIONS
STRUCTURAL – ELECTRONIC – MAGNETIC
CHARGE REDISTRIBUTION AT INTERFACE
Distance for movement of charges is given byThomas Fermi screening length
λTF =12
a0
n1/ 3
Bohr radius
Charge carrier density
Typically 1019-1022 cm-3 in complex oxides
λTF = 2 – 6 Å (1–2 unit cells)
Chakhalian, Keimer, HUH,…- Science October 2007
YBaCu2O7-x
YB
CO
LCM
O
1/3
PHASE DIAGRAMSCharge Transfer across the interface
IS THAT THE WHOLE STORY ???
Interface structure
Two different atomic plane stacking sequences at the up and down interfaces
MAGNETIC CORRELATIONS AT INTERFACE AS REVEALED BY NEUTRON REFLECTOMETRY
specular neutron reflectivity
→ Bragg reflections due to structural and magnetic periodicity
second Bragg peak forbidden if structural and magnetic denisty profiles are equal
J. Stahnet al. Phys. Rev. B
71 (05) 140509
R
N S
c
A
La Ca MnO2/3 1/3 3
YBa Cu O2 3 7
SrTiO3
X-raybeam
reflected light
absorbed light
Soft x-ray range -100 eV -1.5 KeV(3d generation facilities - ESRF, APS, ALS, SLS and CLS)
H
Total electronyield
Resonant soft X-ray absorption (XAS)ELEMENT SPECIFIC PROBE OF MAGNETIC MOMENT
taken from J. Chakhalian
X-ray
µsµs µlµl
µµ
e-
LCMOYBCO
H
•• Magnetic moment on Cu below Magnetic moment on Cu below TTscsc !!•• Cu and Cu and MnMn magnetic moments are magnetic moments are antianti--aligned.aligned.
Ba
Y
Ba
La
3.89A
Cu
O
Mn
3.82A
LCMO
YBCO
Mn-O-Cu
3.87A
interface
X-ray magnetic dichroism in YBCO/LCMO SL
x2-y2 3z2-r2
x2-y2
→ antiferromagneticcoupling, as observed
orbital reconstruction at interface
YBCO:
LCMO:3z2-r2 orbital depleted
assume bulk orbital occupancy is maintained at interface
metallic LCMO: fluctuating orbital occupancy
metallic YBCO: x2-y2 orbital occupied
→ferromagnetic exchange coupling acrossinterface inconsistent with experiment
Superexchangeinteraction across
interface
Chakhalian et al. Nature Physics 2(2006) 244
Chaloupka & Khaliullin, PRL 100, 016404 (2008)
CAN WE GENERATE THE CUPRATE SITUATION IN OTHER TRANSITION METAL OXIDES ?
Key elements in cuprate physics:
– no orbital degeneracy– spin 1/2– strong AF coupling– two-dimensionality
Candidates: RTiO3, Sr2VO4, Sr2CoO4, NaNiO2, RNiO3
5.4 .OXIDE HETEROSTRUCTURES AS A LABORATORY FOR MANY BODY PHYSICS -
NICKELATE PROJECT
x2-y2
yz xz
3z2-r2
xy
La2CuO4
• spin-1/2• 2D bond network• orbitally
non-degenerate• Mott insulator
Cu2+ (3d9)
O2-(2p6) Ni3+ (3d7)
O2-(2p6)
yz xz xy
x2-y2 3z2-r2
LaNiO3
• spin-1/2• 3D bond network• orbitally
degenerate• metal
TASKS
Cut 3D network to a 2D one
prevent c-axis conduction
Lift orbital degeneracy
orbital engineering
5 nm
LaNiO3 – LaAlO3 SUPERLATTICES
[(LaNiO3)3 /(LaAlO3)3]42
LaNiO3
LaAlO3
A. Boris 2008/2009: stabilization of in-plane charge ordering
SrTiO3 tensile strain LaSrAlO4 compressive strain
SLs100 nm
films
ε1 at 0.85 eV vs. temperature
TAKE HOME MESSAGES
COMPLEX OXIDE INTERFACES AS A LABORATORY FOR MANY-BODY PHYSICS
- Generate novel quantum states at interfaces
( metallicity, superconductivity (?) ferromagnetism (?))
- Charge transfer and orbital reconstruction at interfaces
- Superconductivity at higher Tc‘s ???
6. CONCLUSIONS1. Solid State Nanophysics and Nanochemistry are the
parents of nanoresearch
2. Semiconductor physics and technology offer thepossibility to investigate electronic properties at thesystem nanoscale - new devices
3. Nanochemistry at interfaces and defect chemistry is a research area indispensible to improve solid statebatteries and solid oxide fuel cells
4. Nanotechnology enables the improvement of thermoelectric materials
5. Complex oxide interfaces are a new research areawith the potential for unpredictable novelbreakthrough discoveries