Applications of Ultrafast Electron Diffraction:

Ultrafast Meets Ultrasmall?

1

UESDM Workshop, UCLA, 2012

Chong-Yu Ruan

Department of Physics & Astronomy

Michigan State University

1

Strength of ultrafast electron diffraction

• Structural dynamics

• Diffraction limit

• Surface sensitivity

• Single particle

diffraction

• Table-top system

2

vs. Ultrafast Optical and X-ray techniques

100μm 1μm 1 nm 1 Å

• High voltage

• Space charge effects

• Short coherence length

• Short penetration depth

• Conversion jitter

Pro: Con:

Smarter, smaller, and faster

3

Pro:

Con:

Low penetration (strong scatterer) Sensitive to surfaces and molecules less damaging to material

Deep penetration (weak scatterer) High coherence length. Long-range order

X-ray

electron

Engineering serves function

4

Complex materials

Key areas

Laser machining

Water splitting

Photo-catalysis

Nano-electronics

Photo-voltaics

Protein-folding

CY Ruan, UESDM 2012

(Ruan, Tomanek, MSU)

UEC vacuum system

Dual focussing lenses 2D scanning

Variable positioning in time

Sample manipulation

Single electron detection Femtosecond laser

beam (pump)

Probe size: 5 –30 um

Femtosecond electron

beam (probe)

Sample on a

TEM holder

Microscopy & microanalysis 15, 323 (2009) 6

Diffraction from nanocrystals

GND state gold nanoparticles (2nm)

Cuboctahedron

Decahedron

Icosahedron

Decahedron

Cuboctahedron Icosahedron Decahedron

Various models for 2nm gold

nanoparticles

Fourier Transform Nano Lett. 7, 1290 (2007)

7

Nanoparticle preparation

Sample Preparation

SEM image Diffraction Pattern Rocking curve

Yoshie Murooka

8

Cuboctahedral Au

i ri (Å) Shell constituents description

1 2.88 nearer face centers, same cell

2 4.08 along cube edges, same cell

3 5.00 farther face centers, same cell

4 5.78 face diagonals, same cell

5 6.46 nearer face center, 1st adjacent cells

6 7.07 body diagonal, same cell

7 7.64 farther face center, 1st adjacent cells

8 8.17 along cube edges, 1st adjacent cells

9 8.67 father face center, 1st adjacent cells

10 9.14 face diagonal, 1st adjacent cells

Pattern Repeats

Experimental RDF Data from Au NP

a =

4.0

8Å

9 CY Ruan, UESDM 2012

Reverse Monte Carlo (RMC) modeling

Microscopy & microanalysis 15, 323 (2009) 10 CY Ruan, UESDM 2012

Structural evolution of photoexcited Au NP

Breaking and making bonds

Microscopy & microanalysis 15, 323 (2009) 11 CY Ruan, UESDM 2012

Thermal vs. potential evolution of

photoexcited 2nm Au NP

-1.5 -1.0 -0.5 0.00.985

0.990

0.995

1.000

1.005

1.010

R/R

0

Vs(Volt)

R/R0

Lattice Temperature

Potential Correlation?

Si

e Au

230 ps

500 ps

*Temperature extracted from the thermal expansion coefficient from bulk Au crystal.

Thermal expansion coefficient should depend on the size of the nanocrystal (anharmonicity)

, but such data is currently not available at 2nm.

*

13

Transient surface voltage (TSV) effect in SiO2/Si interface

14

Si

4±2nm - SiO2

hv

800 nm _

Photoinduced transient refraction effects

15

Photoexcitation causes interfacial

charge redistributions that create

transient electric fields modifying the

diffraction patterns.

(1) Surface Dember field (ambipolar

diffusion. Slow, and weak effect.)

(2) Interface dipolar field. Charge

transfer across interface and

transiently get trapped at interface

states.

(3) Photoemission.

Common traits:

(1) Collective (non-reciprocal) shift of

diffraction pattern that cause the

diffraction pattern to deviate from

the reciprocal symmetry w.r.t. the

lattice (sine for powder, cosine for

single crystal)

(2) The phenomena are transient.

(3) Generally high-order Bragg peaks

shift less than the low-order ones

– opposite to structure-induced

shift pattern.

For details: see K. Chang, R.A. Murdick, Z. Tao, T.-R. T. Han,

CYR, (Review) Mod. Phys. Lett. B 25, 2099 (2011)

24

Structural transformation via electronic excitation

Meguro et al. Appl. Phys. Lett. 79, 3866 (2001)

• Shoot highly charged ion (Ar8+) at HOPG

• e- injection by STM tip forms Nano-diamond

Untreated

HOPG

Final product

Raman shift (cm-1)

Nakayama and Katayama-Yoshida

J.Phys.: Condens. Matter 15, R1077-R1091 (2003)

Hole doping Lowering of barrier

Destabilize internal coulomb field –

Trigger lattice rearrangement

Crystalline structure of graphite

26

(in-plane)

(out-of-plane)

(in-plane)

(out-of-plane)

In-plane vibration amplitude is x2 smaller

than out-of-plane (s vs. p bonds) 27

Atomic fluctuational (Debye-Waller) analysis

Raman et al., Phys. Rev, Lett. 101, 077401 (2008)

28

Transient sp3 bond formation

Raman et al., Phys. Rev, Lett. 101, 077401 (2008)

29

Molecular dynamics modeling of graphite-diamond transition

PRL 74, 4015(1995)

Network buckling Inlayer sliding Forming interlayer

Bonds (sp3) Diamonization

Sequences of Graphite-to-Diamond Transition

‘Structural refinement’ in powder diffraction mode

30 CY Ruan, UESDM 2012 Microscopy & microanalysis 15, 323 (2009)

Phys. Rev. B 81, 134104 (2010)

Structural refinement (RMC)

RMC Refinement, Supercell 15x15x15

31

Raman et al. Phys. Rev. Lett. 104, 123401(2010)

CY Ruan, UESDM 2012

Complex materials

32

Anisotropic electron-phonon interaction in electron materials

with reduced dimensionality

REW>~ 100Å-1

CY Ruan, UESDM 2012

What can we learn from ultrafast probes?

33

•Ultrafast photodoping provides

access to different phases than

thermal equilibrium

•Recovery to electronic ground

state disclose mechanistic

information about the gap and

the structure.

•Time-resolved probes

(diffraction, reflectance, ARPES)

directly assess coupling

hierarchy between different

degrees of freedom

hv

K. E. Wagner et al., PRB 78, 104520 (2008).

2D charge density waves in CeTe3

36

N. Ru et al. & I. R. Fisher, Phys. Rev. B 77, 035114 (2008).

Transport

X-ray diffraction

Prototypical quasi-2D CDW (RETe3)

37

RE

BCS-like

2nd order Phase

transition

Weak-coupling

High-anisotropy

in out-of-plane /

in-plane

conductance

Quasi-2D metal

S

E. DiMasi, M.C. Aronson, J.F. Mansfield, B. Foran, S. Lee, PRB 52, 14516 (1995).

SmTe3 TEM Study

CDW satellites

CY Ruan, UESDM 2012

Characterize CDW order parameter(s)

Phase

Amplitude

For electronic and optical studies: charge gap (DEc)

For structural studies (X-ray, TEM, STM): periodic lattice distortion (dc)

Order parameter

dc ~~CDWI~

Diffraction

Angle-Resolved

Photo-Emission

Spectroscopy (ARPES)

Optical

Reflectance

d d

38

CY Ruan, UESDM 2012

CDW mechanisms

39

Fermi Surface

Nesting

el-ph

Coupling

el-el

Correlation

Lattice

Deformation

Mott

Peierls

CY Ruan, UESDM 2012

Open questions on CDW coupling dynamics

40

TiSe2 (excitonic CDW) Mohr-Vorobeva et al, PRL 107, 036403 (2011)

K0.3MoO3 (Peierls CDW) Tomeljak et al, PRL 102, 066404 (2011)

1T-TaS2 (Mott CDW) Perfetti et al, PRL 97, 067402 (2006)

Earlier ultrafast spectroscopic studies of CDW materials

have identified a sub-ps partial recovery of electronic

ordering following optical quenching – seemingly

independent of the perceived underpinning mechanism.

CY Ruan, UESDM 2012

Photo-induced structural dynamics of 2D charge density waves

41

UEC examine the symmetry

breaking mechanism of an

uniaxial 2D CDW (CeTe3) with

complementary view from ionic

degree of freedom.

UED study on 1T-TaS2

Sample from M. Kanatizidis

at Northwestern University

T.R.T. Han et al., Phys. Rev. B 86, 075145 (2012)

CDW satellites

• Satellite exhibits a two-step suppression, suggesting that

the melting of CDW is driven by a fast (< 1ps) and a slow

(~ 3.3 ps) processes.

42 T.R.T. Han et al., Phys. Rev. B 86, 075145 (2012)

Bragg reflection

• Symmetry breaking

in the suppression of

the Bragg reflections

from the square

lattice.

• CDW-dynamics

cause an elevated

fluctuation along the

c-axis.

43

CDW structure factor

• n=0: Bragg reflection

• n=1: CDW satellite

44

Bessel function of first kind.

Phase mode

Amplitude mode

Phonons

J0 and J1 anti-correlate with each other.

CY Ruan, UESDM 2012

Excited states of CDW

45

Phase mode

Amplitude mode

dc

CY Ruan, UESDM 2012

Order parameter dynamics

• Electronic melting appears on the

sub-ps timescale. It induces phase

fluctuation in the ionic modulation

of CDW, but the local CDW

distortion remains largely intact.

• Phase mode drives the fast (and

limited, max ~0.3) suppression of

satellite.

• Amplitude mode drives the melting

of CDW on the ps timescale.

• The recovery of fast component

coincides with the recovery of

electronic CDW condensate.

46 CY Ruan, UESDM 2012

Critical energy density for CDW melting

47

Critical energy density for electronic

melting:

dL =20nm (penetration depth),

R=0.7 (reflectivity)

Fc=1.9 mJ/cm2 (at z=0)

Ec=0.9 ±0.2 eV/(unit cell)

Peierls CDW condensation energy

n(F)=1.48 state/eV/(unit cell)

F=3.25 eV. D(CDW gap)=0.4 eV

Eel=0.8 eV/(unit cell)

Fast process is mainly

electronically driven and involves

little lattice involvement !! CY Ruan, UESDM 2012

Lattice fluctuation dynamics

• CDW-related c-axis fluctuation

(dc) is nearly linear w.r.t the

excitation fluence.

• The amplitude fluctuation drives

the melting of CDW

phononically driven scenario.

48

Three-temperature modeling of CDW melting

49

In the CDW collective state,

the electron dynamics and

the ion dynamics are

decoupled at the short time.

CY Ruan, UESDM 2012

VO2 microbeam

• J Cao .. J Wu, Nature Nanotechnology 4, 732 (2009).

• J Wei, Z Wang, W Chen, DH Cobden, 4, 420 (2009).

54

Phase transitions of VO2 nanobeam gently placed on TEM grid

55

Metal-insulator transition (MIT) is tracked by the change of optical reflectance.

Structural phase transition (SPT) is tracked by the disappearance of dimer reflection.

Monoclinic to rutile transition Symmetry recovery

Optical microscopy

TEM Tao et al., PRL 109, 166406 (2012)

Sample from J. Wu

at UC Berkeley

Phase transitions on different surfaces

• The structural and electronic phase transitions are strongly first

order.

• TcSPT upshifts from Tc

MIT by different amounts on different metallic

grids, whereas on Si the SPT and MIT critical temperatures are

approximately equal.

56 Tao et al., PRL 109, 166406 (2012)

Decoupling of MIT and SPT are reproducible in different samples

57

A new monoclinic metal (M3) state induced by interfacial charge doping

• lnsulator-metal transition decouples from monoclinic-rutile

transition on metal substrate.

• MIT and SPT might be driven by two separate mechanisms. 58

Metal

Insulator

Cooperative regime: Si surface supported VO2 beam

• Full scale SPT (M1 to R) can be

induced by fs laser pulses. The

critical fluence (Fc) is linear w.r.t

the base temperature (TB).

• Critical energy density for SPT

Eph = Fc / d

d=127 nm (M1 penetration depth)

Eph = -Cv (TB-Tc)

From fitting, Cv =3.2±0.2 JK-1cm-3

[ M1 Cv =3.1 JK-1cm-3 ]

Since entropy of VO2 is dominated

by lattice component, this

agreement suggests that SPT in

the cooperative regime has a

strong Peierls character !

59 Tao et al., PRL 109, 166406 (2012)

Noncooperative regime: VO2 on gold surface

• No SPT is observed up to

7 mJ/cm2 >> Fc in the

cooperative regime.

• Phonon softening is

observed in the metallic

state along the zig-zag

axis (perpendicular to b-c

plane).

• Atomic fluctuations

suggest that the optical

absorbance is reduced.

60 Tao et al., PRL 109, 166406 (2012)

Shaping space-charge-limited electron bunch

Controlling photo-emission

Laser – RF synchronization

Key areas :

Martin Berz (Beam physics)

Marc Doleans (RF buncher)

Marty Crimp (Electron microscopy)

Phil Duxbury (Molecular dynamics)

Marcos Dantus (Laser pulse

shaping)

Chong-Yu Ruan (Ultrafast electron

diffraction)

Development of a high-brightness ultrafast electron microscope

for single-shot, single-particle diffraction

Electron optics and RF system development

Collaborators

61 CY Ruan, UESDM 2012

FMM simulation of high-intensity photoemission

63

He Zhang

64

Space-Time focusing

Spatial focusing (going through a magnetic lens)

He Zhang

Jenni Portman

Zhensheng Tao

Simulated space-time resolution in

RF-enabled UEM

65

Z. Tao, H. Zhang, P.M. Duxbury, M. Berz, CYR,

J. Appl. Phys. 111, 044316 (2012).

NSF MRI project

Kiseok Chang

Acknowledgements

Collaborators

MSU

Martin Berz, Kyoko Makino, He Zhang

Phil Duxbury, Jenni Portman

Bhanu Mahanty

Marty Crimp

Marcos Dantus

Columbia U

Simon Billinge, Chris Farrow

Northwestern

Mercouri Kanatzidis

Christos Malliakas

SOLEIL

Ti Ruan

National Superconducting Cyclotron Laboratory

Marc Doleans

UC-Berkeley

Jianquo Wu

Students & Postdocs*

Ramani Raman

Yoshie Murooka*

Ryan Murdick

Aric Pell

Ki Hyun Kim*

Richard Worhatch

Terry Han

Zhensheng Tao

Fei Yuan

Kiseok Chang

Austin Lo

Thiago Szymann

67