www.sciencemag.org/content/346/6208/445/suppl/DC1

Supplementary Materials for

A photoinduced metal-like phase of monoclinic VO2 revealed by ultrafast

electron diffraction

Vance R. Morrison, Robert. P. Chatelain, Kunal L. Tiwari, Ali Hendaoui, Andrew Bruhács, Mohamed Chaker, Bradley J. Siwick*

*Corresponding author. E-mail: bradley.siwick@mcgill.ca

Published 24 October 2014, Science 346, 445 (2014) DOI: 10.1126/science.1253779

This PDF file includes: Materials and Methods

Figs. S1 to S4

References

1 Materials and Methods

1.1 Ultrafast electron diffraction with RF compressed electron pulses

The measurements presented here were made on a home built ultrafast electron diffractometer

(Fig. S1) employing radio-frequency pulse compression techniques (16). This approach has

very recently been shown to offer dramatic improvements in instrument performance (17, 18).

These techniques allow us to produce compressed electron pulses with ∼106 electrons per pulse

with ∼300 fs time resolution (17).

In these experiments, VO2 specimens initially in the M1 phase (∼310 K) are photoexcited

with 35 fs, 1.55 eV (800 nm) laser pulses at near normal incidence (∼10 degrees) with a range

of excitation fluences; the evolution of the SMT was then probed after a variable delay time

with an ultrashort electron pulse with ∼500,000 electrons per pulse. These electron pulses were

generated through photoemission in a DC, high voltage electron gun with 266 nm laser pulses

and accelerating voltages up to 95 kV. Although the diffractometer is capable of operating with

a repetition rate of 1 kHz, a delay of 20 ms between pump laser pulses was required before

reinitiating the transition in order to allow the samples to completely relax back to the insulating

phase. The diffraction pattern (Fig. S2) produced at each delay is the result of approximately

50 individual fifteen second exposures and was detected on a Gatan Ultrascan 1000, a phosphor

coated CCD with near single electron detection capabilities.

In addition to the ultrafast electron diffraction measurements performed, ultrafast infrared

transmittance measurements were conducted under identical pump-probe conditions. The IR

pulses (5 µm wavelength) were generated using difference frequency generation from the signal

and idler beams of an IR optical parametric amplifier (38). The ultrafast mid-IR transmission

measurements were made using the femtosecond pulse acquisition spectrometer from Infrared

Systems Development, based on 64 pixel Mercury Cadmuim Telluride IR detector arrays. The

VO2 samples were held at atmospheric pressure as opposed to the 10−7 Torr used for the electron

1

diffraction experiments.

1.2 Pulse laser deposition grown VO2 specimens

The high-quality stoichiometric polycrystalline vanadium dioxide films used in these measure-

ments were synthesized at the Laboratory of Micro- and Nanofabrication facility at INRS by

means of reactive pulsed laser deposition process. A pure (99.95%) vanadium target was used

with a KrF excimer laser (λ = 248 nm) in an oxygen environment with a pressure of 15 mTorr

and substrate temperature of 500 ◦C (23, 39). The samples were 70 nm thick and deposited on

50 nm thick amorphous silicon nitride windows. Temperature dependent resistivity curves are

shown in Fig. S3 and display the characteristic hysteretic behaviour of VO2.

2

Figure S1: Schematic diagram of the ultrafast electron diffractometer including the synchro-

nization electronics and the chirped pulse amplification laser system. The sample is pumped

with 35 fs, 800 nm laser pulses, while third harmonic generation is used to produce 266 nm,

UV pulses. A fast photo-diode is used to detect the timing of the pulses from the ultrafast os-

cillator; a phase-locked loop (PLL) is then used to produce synchronized RF pulses which are

then amplified by an RF amplifier. These pulses then drive the RF compression cavity. Two

magnetic lenses, S1 and S2, are used to collimate the electron beam as it exits the electron gun,

and then to focus the beam at the CCD detector.

3

Figure S2: Diffraction patterns of low temperature (A) and high temperature (B) phases of VO2.

The red arrows indicate the disappearance of diffraction features which are present in the M1

phase, but not allowed by symmetry in the R phase. The shadow in the center of the images is

a beam block used to prevent the main electron beam from saturating the CCD.

4

40 50 60 70 80 90 100

10−2

10−1

100

101

Temperature (ºC)

Res

istiv

ity (Ω

•cm

)

Figure S3: Temperature dependent resistivity curve with increasing temperature shown in red

and decreasing temperature shown in blue.

5

0

0.2

0.4

0.6

0.8

1

1.2

0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65

−0.02

0

0.02

0.04

0.06

0 5 101

1.01

1.02

1.03

1.04

Delay (ps)

-1 ps 10 ps

s (Å-1)

Inte

nsity

(arb

. uni

ts)

ΔI

(arb

. uni

ts)

A

B

220

302 313

Rel

ativ

e In

tens

ity

Figure S4: A) Time resolved, background subtracted, diffraction data from 0 ps to 10 ps and

an excitation fluence of 6.1 mJ/cm2. Inset) Time resolved intensity of the 220 reflection. B)

Time resolved change in diffracted intensity shown in A). No decrease is seen in the 302 and

313 reflections, indicating that the PLD remains intact. In addition, no changes are observed in

the s > 0.5 A−1 region.

6

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