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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