Controlled nanoscale mechanical phenomena discovered with ultrafast electron microscopy

DOI: 10.1002/anie.200704147

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Angewandte Chemie - International Edition p. 9206 - 9210 (2007)

Update date:2022-08-22

Topics:

Authors:

Flannigan, David J.

Lobastov, Vladimir A.

Zewail, Ahmed H.

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Article abstract of DOI:10.1002/anie.200704147

On again, off again: The reversible expansion and contraction of single crystals of [Cu(TCNQ)] induced by near-infrared laser pulses was studied with ultrafast electron microscopy (TCNQ = 7,7,8,8-tetracyanoquinodimethane). The crystal expands along the σ-

Full text of DOI:10.1002/anie.200704147

Communications

DOI: 10.1002/anie.200704147

Ultrafast Electron Microscopy

Controlled Nanoscale Mechanical Phenomena Discovered with

Ultrafast Electron Microscopy**

David J. Flannigan, Vladimir A. Lobastov, and Ahmed H. Zewail*

Dedicated to Professor Sir John Meurig Thomas on the occasion of his 75th birthday

For more than a century, the synthesis and control of

nanoscale molecular structures has been at the forefront of

The strong electron acceptor (p acid) TCNQ undergoes a

facile redox reaction at room temperature with metals such as

silver and copper. In single crystals of the resulting [Cu-

[

1,2]

many fields, such as organic/inorganic synthesis,

chemical/

[3,4]

[5]

ꢀ

biological catalysis,

and crystallography. With light,

(TCNQ)] charge-transfer complex, Cu and TCNQ C form

discrete columnar stacks in a face-to-face configuration with

strong overlap in the p system. Further, the copper atoms are

bound in a four-coordinate highly distorted tetrahedral

geometry to the nitrogen atoms on the cyano groups of the

TCNQ molecules. The strong through-space interactions

between the p electrons and the resulting quasi-one-dimen-

sional structure of the material in the solid state impart

especially with lasers, there exists another frontier in control

whereby changes of population in or phases of the quantum

states can be explored, and some intellectually interesting

[6]

demonstrations have been reported. Recently, in the lexicon

of nanoscience and nanotechnology, the interest has primarily

focused on the development of nanoscale “devices” with

[

7–10]

functional purposes,

tronics and possibly the making of mimics of biomachines,

such as the potential for nanoelec-

interesting structural and electronic properties in the field of

[11]

[17–19]

which operate with impressive molecular scale precision.

low-dimensional organic solids,

and in the exploration of

[20]

Uncovering function of nanoscale phenomena requires direct

imaging at sufficiently high spatial and temporal resolutions.

As importantly, such visualization provides understanding of

the fundamental nature of the physical forces, which derive

the directed function in these complex systems.

one-dimensional semiconducting nanostructures.

One property intrinsic to this material is the change in

conductivity by the application of an external electric field.

This change between two states has also been reported to

occur by using light, and the effect was studied mainly by

[

21–23]

Herein, we report the discovery of a mechanical nanoscale

molecular phenomenon, a switchable channel or gate,

observed with the newly developed ultrafast electron micro-

scope, which is capable of imaging with the combined atomic-

absorption and Raman spectroscopic probes.

Photoabla-

tion of thin films was examined and shown, after ex situ

[

24,25]

radiation, to produce metal particles.

Numerous reports

have been made because of their unique properties, structure

[

12–16]

[26–28]

scale spatial and ultrafast temporal resolutions.

The

and synthesis,

media.

and potential applications as memory

[

29–30]

control is made using near infrared laser pulses, and the

material is the crystalline quasi-one-dimensional semicon-

ductor [Cu(TCNQ)] (TCNQ = 7,7,8,8-tetracyanoquinodime-

thane, C H N ). Remarkably, the switching, after a shock, not

Intrinsic phenomena that arise solely from, for

example, motion in space–time, could not be observed

directly, thus leaving a whole parameter space unexplored

in these and other materials.

only is reversible with the pulses being on or off, but also

returns the material in space to the original structure. The

functional behavior is robust in the relatively low-fluence

regime. At significantly higher fluences, we observe, in the

microscope, the internal dilation and the reduction of the

copper ions to form islands of neutral copper metal structures.

The power of ultrafast electron microscopy (UEM) is in the

ability to visualize in situ these spatiotemporal behaviors,

which are otherwise inferred from spectroscopic and post-

event probing of films, as discussed below.

For UEM studies of [Cu(TCNQ)], we examined single

crystals (not films) synthesized directly on Si N membrane

grids by a solid/liquid reaction. Initial mapping of the sample

by UEM showed that the synthetic methodology employed

resulted in the growth of hundreds of [Cu(TCNQ)] crystalline

rods ranging in width and length from several micrometers

down to tens of nanometers. The crystallinity of the specimen

was confirmed by selected-area electron diffraction in UEM,

as shown below. [Cu(TCNQ)] is known to exist in both a

kinetically favored state (phase I; needle crystal habit; Pn,

a = 3.8878, b = c = 11.266 , a = g = 90, b = 90.00(3)8) and a

thermodynamically favored state (phase II; platelet crystal

habit; P2/n, a = 5.3337, b = 5.3312, c = 18.875 , a = g = 90,

[

*] Dr. D. J. Flannigan, Dr. V. A. Lobastov, Prof. Dr. A. H. Zewail

Physical Biology Center for Ultrafast Science and Technology

California Institute of Technology

[

26]

b = 94.0368). The material studied herein is that of phase I,

as determined by the crystal habit and confirmed by indexing

of the diffraction pattern. Importantly, during the initial

characterization by UEM, no charging of the crystals was

observed; no defocusing of the electron beam or motion of

the specimen in the field of viewwas observed in all studies

made. Details of the apparatus UEM1 can be found in

references [13] and [16]. Herein, the excitation pulses were

Pasadena, CA 91125 (USA)

Fax: (+1)626-792-8456

E-mail: zewail@caltech.edu

[

**] This work was supported by the Gordon and Betty Moore

Foundation, the Air Force Office of Scientific Research, and the

National Science Foundation. We wish to thank the referees for

insightful comments.

206

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Angewandte

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used at 776 nm, whereas the electron pulses were generated

by frequency doubling to provide the needed ultraviolet

radiation. In the microscope, the two beams coincide spatially

and temporally, and both real-space images and Fourier-space

diffraction patterns were recorded and processed digitally. A

mechanical shutter was placed in the excitation beam path to

control a window of time when trains of optical pulses are on

or off.

relative value of 0.5 over the course of the recording time of

1 s, but the sharpness of the channel edges is maintained.

From these experiments the timescale of the channel switch-

ing was estimated to be less than 10 ms. The expansion/

contraction was also observed in intact single crystals (i.e., not

photoshocked) as changes at the ends of the material.

Four typical frames of the images obtained when the laser

pulses are on or off are given in Figure 1a–d. The higher-

resolution image in Figure 1e shows the nanoscale channel

that is formed, in this case with a width of 140 nm. The

reversibility of gating is demonstrated

Figure 1 shows a [Cu(TCNQ)] single crystal that has been

shocked by initially applying a burst of the laser light,

in Figure 1 f by measuring the actual

magnitude (distance) of the channel

opening/closing while varying the

number of frames recorded. For quan-

tification of the data, we present in

Figure 2 the diffraction patterns that

were obtained in the microscope by the

selected-area method, together with

changes of the lattice spacing and

channel width at different fluences.

The diffraction pattern viewed along

the [011] zone axis perpendicular to the

stacking a axis ([100] direction) shows

two distinct separations in Fourier

space, S₁₀₀and S , whereby S is

011

related to the corresponding planar

spacing by the order of diffraction.

Because the order is one, S and the

interplanar distances are directly

related. Thus, the values of 3.89 and

.97 , respectively, were obtained for

the distance between the planes, which

agree well with those obtained by X-

ray analysis of [Cu(TCNQ)] phase I.

Figure 1. UEM images of channel gating: a–d) Sequential views (scale bar=500 nm) of the

already shocked [Cu(TCNQ)] single crystal in the “off” structure (a, c; no pulsed-laser irradiation)

and in the “on” structure (b, d; pulsed-laser irradiation). e) Higher-resolution image of the crystal

in the “off” structure illustrating the section within which quantification of the reproducibility of

the expansion/contraction was performed. f) Plot showing the results of a sequence of “on” and

[26]

The change in separation of these

stacking planes (100) and those per-

“

off” cycles. The channel width W varied from 0 (pulsed-laser irradiation) to 140ꢁ5 nm (no

pendicular (011) as a function of the

pulsed-laser irradiation) over a series of 50 frames. The bar at the top is to illustrate the

modulation in a memory recording. The difference in contrast along the edges is due to crystal

shape and position on the substrate (crystal habit with thickness projected differently).

energy deposited (fluence) is plotted in

Figure 2c, together with real-space

values. For expansion along the a axis,

the change is nearly linear until a

relatively high fluence is reached, and

ꢀ

ꢀ1

typically for 100 ms at a fluence of 0.5 mJcm per pulse for

20-fs pulses with 80 MHz repetition rate. Following this

the rate of change is almost 1 mJ cm . For the perpendic-

ular direction, there is no observable change within error.

These “microscopic”, sub-angstrom, changes at the atomic

scale were checked against the “macroscopic”, nanometer,

channel formation. For comparison, the channel width as a

function of fluence, shown in Figure 2d, has a large rate of

shock, the crystal, which was micrometers in length, was

directly observed in UEM1 to fracture and separate by tens of

nanometers depending upon the fluence of the initial pulse

sequence. When the shutter was open (i.e., the sample was

irradiated with laser pulses) the crystal expanded along the

long axis and closed the gap completely; that is, the two

exposed faces of the crystal came into full contact. When the

shutter was closed (i.e., the specimen was not irradiated with

laser pulses) the material returned to its original structure in

which the channel was again clearly observed. We then

repeated these experiments by switching between the on and

off modes. The crystal was observed to expand and contract

along the long axis in synchrony with the shutter. However,

the contrast of the image in the open zone changes to give a

ꢀ1

change of 220 nmmJ cm . These results illustrate the signifi-

cant change of the channel width with energy and the clear

anisotropy along different directions, whereby the dominant

change is along the stacking axis.

Conceptually, the macroscopic gating process can be

understood from the microscopic structural dynamics. As

seen in the crystal structure in Figure 3, the a axis is unique for

stacking. Hence, the large anisotropy of gating must reflect

the unique changes along the stacking axis [100] and not

perpendicular to it. The modulation (i.e., closing and open-

Angew. Chem. Int. Ed. 2007, 46, 9206 –9210

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Communications

length scale of the material used.

The agreement supports the

notion that the macroscopic phe-

nomenon is driven by the molec-

ular interactions. The forward

motion is not caused by equilibri-

um thermal heating because of the

observed lack of expansion in the

direction perpendicular to stack-

ing (compare with Figure 2).

Moreover, the thermal expansion

coefficient for stacks of TCNQ

molecules, although different by a

factor of about 2 (a

= 12

stacking

ꢀ5

ꢀ

ꢀ1

ꢀ1 [32]

K ; a = 5.8 10 K ),

predict a rise of at least 100 K at

equilibrium, a rise which is not

observed even when the pulse

fluence is orders of magnitude

[

33]

larger than used here.

It is

possible, however, that a nonequi-

librium heating on the nanoscale

can drive the reversible change

[

25]

[

see Eq. (1)].

In the linear regime of expan-

sion, the channel width changes at

ꢀ1

the rate of 220 nmmJ cm in the

crystals studied. The fluence

ꢀ

varied from 0.05 to 0.60 mJcm

per

pulse

steps

Figure 2. UEM diffraction patterns and effect of fluence on lattice spacings and channel width. Panels

a) and (c) illustrate the effects of increased laser fluence on the lattice structure of [Cu(TCNQ)] single

ꢀ2

.007 mJcm , and the UEM

(

image was acquired at each step.

The gap spanning the two parts of

the previously fractured crystal

was quantified for each image at

crystals. The spacings in reciprocal space between the (100) and (011) planes were measured in the

diffraction pattern (a) as a function of fluence and the results are plotted in (c). The real-space

expansion that corresponds to the observed contraction in reciprocal space is labeled for two

corresponding points in (c). b) Typical diffraction pattern obtained for a [Cu(TCNQ)] single crystal in

UEM in which the electron beam is along the [011] zone axis. d) Plot

of the channel width in real space as a function of laser fluence. The

rates of expansion for each case are discussed in the text. Note the

large anisotropy in the expansion along the stacking axis.

ing) of the channel is a modulation of the p-electron

interactions of the stacked TCNQ molecules. Before expo-

ꢀ

sure to laser pulses, the crystal is in the [Cu TCNQ C]

electronic structure. During the “on” period, the excitation

creates a mixed-valence structure [Eq. (1)], as our crystals

display absorption bands similar to the known charge-transfer

[

22,31]

band (600 to 1200 nm) in films.

The mixed-valence

structure leads to transport of charges and to a newlattice

along the stacking direction.

Figure 3. Conceptual representation of the molecular structure and the

p stacking. The diagram on the left in conjunction with the diagrams

of the single TCNQ molecules (lower center) illustrate how the

molecules in neighboring stacks are rotated 908 from one another. The

ball-and-stick diagram in the upper right of the figure illustrates this

point further with a view of the crystal down the stacking a axis. The

unit cell axes (a, b, c) are labeled in the diagram on the left, and the

colors of the axes are carried over into the ball-and-stick diagrams on

the right. The additional lines in the diagrams show the dimensions of

the unit cell, which is essentially tetragonal. The stacked layered

structure is responsible for the highly anisotropic properties of this

material.

fs pulse

ꢀC

ðCu TCNQ Þ !Cu þTCNQ þ½ðCu TCNQ Þxꢀy

ð1Þ

heat

The change in the a axis before melting of the crystal

occurs calculated from the diffraction data in Figure 2 is

approximately 0.2 . This change in intermolecular planes

suggests that for a channel width of 40 nm, the macroscopic

effective length over which the propagation is active should

ꢀ2

be around 2 mm (at 0.1 mJcm per pulse), which is indeed the

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Angew. Chem. Int. Ed. 2007, 46, 9206 –9210

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Angewandte

Chemie

each laser fluence. It was found that the channel width

decreased linearly as a function of fluence (i.e., the crystals

smaller clusters to coalesce (Ostwald ripening). This behavior

greatly enhances the formation of larger copper clusters and

further reduces the ordered structure of the [Cu(TCNQ)]

material until finally, at relatively high fluence, the crystal

separates into its constituent atomic and molecular compo-

nents. In the channel-gating regime, it is of interest to

compare the threshold behavior at the fluences used here

ꢀ

expanded linearly along the a axis) up to 0.28 mJcm per

pulse, at which point the rate of crystal expansion became ill-

defined owing to loss of crystallinity and formation of discrete

domains corresponding to the individual atomic and molec-

ular components of the material.

[

22,25]

ꢀ2

At higher fluences, the [Cu(TCNQ)] crystals were

observed to lose all crystallinity and undergo a laser-induced

reductive formation of copper metal domains along with

strands of neutral TCNQ lying approximately perpendicular

to the long axis of the crystal. It is interesting that even with

UV excimer lasers, photoablation of polycrystalline films can

with those obtained using electric fields.

At 0.1 mJcm

per pulse and using approximately 10 pulses in the window,

the calculated electric field is about 10 Vcm , a value that is

within an order of magnitude of that used with electric

We also note that previously reported damage of

films with electrons did not occur in our UEM image and

diffraction studies because of the very lownumber of

ꢀ1

[

22,25]

fields.

[

25]

[

24]

produce metal particles.

As shown in Figure 4, a single

[12–16]

electrons in each pulse.

We see no deterioration of

diffraction in UEM images, but it was severe when the

microscope was operated in the conventional TEM mode

with a thermally generated continuous beam.

Some final comments regarding the timescales involved:

If the initial fracture, which results from the homogeneous

[

34–36]

illumination of the crystal, is driven by dislocations,

would occur at the speed of sound, which in this material is

ꢀ

estimated to be 1.5 kms , suggesting a timescale of 70 ps for

the fracture to occur locally. However, if, under homogeneous

illumination, fracture occurs by propagation in the material,

the timescale would be nanoseconds. Similarly, if the speed of

sound is the controlling velocity of the channel dynamics, then

over a micrometer scale the time for closing and opening

should be about one nanosecond. However, the charge

transport in the mixed-valence state may in fact accelerate

the process considerably. Knowing our fluence and the

absorption cross section at the wavelength used, we estimate

that the number of photons per copper atom is on the order of

ꢀ

Figure 4. UEM images and diffraction at high fluences. a) A crystal of

Cu(TCNQ)] that has been irradiated at relatively low fluence. Some

10 , suggesting that the complete reduction of copper over

[

the entire crystal will require about 10 of our pulses. Future

melting of the crystal is evident, although some order is still present.

b) Nearly complete transformation of the [Cu(TCNQ)] crystal at high

fluence showing discrete copper crystalline domains (c–e) along with

strands that are likely charge-neutral TCNQ. The scale bar is the same

in (a–d). e) Selected-area diffraction of the area in (c) shows the

material to be crystalline copper metal. The diffraction is analyzed to

show the characteristic pattern of copper metal.

experiments will utilize the excitation-probe scheme of UEM

to resolve these different timescales.

In conclusion, it was possible to observe the reported

phenomena of nanoscale structural changes because of the

in situ capability of direct imaging and diffraction with UEM.

Our initial interest in the TCNQ systems was to study the

ultrafast dynamics, but in the process we realized a new

dimension of UEM, namely, the study of macroscopic–

microscopic transformations and the possible control with

light or electrons of such transformations. The results

discussed herein for channel formation and reductive metal

clustering open the door to numerous further studies, includ-

ing those of nucleation, charge/energy transport, and ultrafast

dynamics of dislocations and coherent nuclear motions. The

findings may be of value in applications involving molecular

nanoswitches and channels, as well as optical pulse memory.

crystal of [Cu(TCNQ)] can be exposed to laser pulses of

increasing power, and when the fluence is high, isolated

domains of copper metal are formed. Selected-area electron

diffraction of these domains in UEM shows that they are

composed of crystalline copper metal; thus the Cu reduction

process can be imaged at high fluences. In addition to these

isolated copper metal domains, much smaller copper metal

particles are also formed from a thin amorphous film of

material, presumably [Cu(TCNQ)], underlying the single

crystals (see selected area of the background in Figure 4d).

The controlling excitation at 1.6 eV is not only the key to

gating of the channel at lower fluences, but also it is sufficient

to facilitate charge transfer and hence weakening of copper

bonding and enhancement of its mobility at higher fluences.

The surface energy of metal clusters composed of a fewatoms

is quite large, and there is a strong driving force for the

Received: September 8, 2007

Revised: October 8, 2007

Published online: November 6, 2007

Keywords: charge transfer · copper · electron diffraction ·

electron microscopy · quinodimethanes

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ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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210 www.angewandte.org

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