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
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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