Observation of growth of metal nanoparticles

Abstract

An understanding of nanocrystal growth mechanisms is of significant importance for the design of novel materials. The development of liquid cells for transmission electron microscopy (TEM) has enabled direct observation of nanoparticle growth in a liquid phase. By tracking single particle growth trajectories with high spatial resolution, novel growth mechanisms have been revealed. In recent years, there has been an increasing interest in liquid cell TEM and its applications include real time imaging of nanoparticles, biological materials, liquids, and so on. This paper reviews the development of liquid cell TEM and the progress made in using such a wonderful tool to study the growth of nanoparticles (mostly metal nanoparticles). Achievements in the understanding of coalescence, shape control mechanisms, surfactant effects, etc. are highlighted. Other studies relevant to metal precipitation in liquids, such as electrochemical deposition, nanoparticle motion and electron beam effects, are also included. At the end, our perspectives on future challenges and opportunities in liquid cell TEM are provided.

Full text

11720 Chem. Commun., 2013, 49, 11720--11727 This journal is cThe Royal Society of Chemistry 2013
Cite this: Chem. Commun., 2013,
49, 11720
Observation of growth of metal nanoparticles
Hong-Gang Liao,
a
Kaiyang Niu
a
and Haimei Zheng*
ab
An understanding of nanocrystal growth mechanisms is of significant importance for the design of
novel materials. The development of liquid cells for transmission electron microscopy (TEM) has enabled
direct observation of nanoparticle growth in a liquid phase. By tracking single particle growth
trajectories with high spatial resolution, novel growth mechanisms have been revealed. In recent years,
there has been an increasing interest in liquid cell TEM and its applications include real time imaging of
nanoparticles, biological materials, liquids, and so on. This paper reviews the development of liquid cell
TEM and the progress made in using such a wonderful tool to study the growth of nanoparticles
(mostly metal nanoparticles). Achievements in the understanding of coalescence, shape control
mechanisms, surfactant effects, etc. are highlighted. Other studies relevant to metal precipitation in
liquids, such as electrochemical deposition, nanoparticle motion and electron beam effects, are also
included. At the end, our perspectives on future challenges and opportunities in liquid cell TEM are
provided.
1. Introduction
At the nanoscale, the quantum confinement of a nanocrystal
provides the most powerful means to direct the optical, electronic
and magnetic properties of the solid material.
1,2
Metal nanocrystals
possess many fascinating properties, and they have been applied in
a variety of areas including energy technology, life science and
environment, such as catalysis,
3
energy conversion and storage,
4,5
electronics,
6,7
information storage,
8
medicine
9,10
and so on. In
most of these applications, it requires that nanoparticles are of
well controlled size, shape and surface structure.
11,12
For instance,
tetrahedral, cubic, and spherical Pt nanocrystals show different
activities in the catalytic reactions between hexacyanoferrate(III)
and thiosulfate ions.
13
{331} facets exhibit much higher catalytic
activities towards oxidation of H
2
O
2
than the nanoparticles
bound by {111} facets.
14
These examples clearly illustrate the
importance of size, shape and structure for efficient utilization of
nanoparticles. Most metal nanocrystals share a similar cubic
close packed (ccp) structure with a face-centered cubic (fcc)
lattice. The properties of a nanoparticle are determined by a
set of physical parameters including its composition, size, shape,
surface modification, and environment. There has been significant
interest in the synthesis of nanocrystals with controlled size, shape
and composition.
Synthesis of nanocrystals by colloidal methods has been
advanced significantly. Solution based synthesis of nanoparticles
with tailored properties has thrived starting from 1990’s,
although growth of nanoparticles in solution phases can be
traced back to 1850’s when Michael Faraday prepared his ruby
gold by reducing gold chloride with phosphorous in water.
15
In the past two decades, nanocrystals with a variety of shapes
including sphere, cube, cuboctahedron, octahedron, tetra-
hedron, decahedron, icosahedron, thin plate, rod or wire, etc.
have been achieved. However, the mechanisms of nucleation
and growth especially the shape control mechanisms have not
been well understood. Thus, practical applications with optimized
performance are significantly affected by the poor predictability of
the size and morphology of nanoparticles during synthesis. In the
absence of a hard template, solution-based methods require precise
tuning of the growth conditions to achieve shape control. Factors,
such as the reduction potential, temperature, precursor concen-
tration, diffusivity, etc. are all important for shape-controlled
synthesis.
16
Our understanding of the nucleation and complex
growth steps involved in achieving a hierarchical functional
structure is limited. The primary barrier for obtaining enough
knowledge of nanocrystal formation arises from the difficulty of
‘‘seeing through’’ the liquids to probe chemical and physical
events in solvents during nanocrystal growth. This also leads to
challenges in understanding the relationship between structure
and functionality during material applications.
With the technical advances in electron microscopy and
nanofabrication, a new experimental platform, so called liquid
cell TEM has emerged, which has made it possible to observe
nanocrystal growth in real time. Liquid cell TEM has now been
applied to many different nanoparticle systems, where the
trajectories of the nanoparticle growth can be obtained. Novel
growth mechanisms have been identified although most of
a
Materials Sciences Division, Lawrence Berkeley National Laboratory,
1 Cyclotron Road, Berkeley, CA 94720, USA. E-mail: hmzheng@lbl.gov
b
Department of Materials Science and Engineering, University of California,
Berkeley, CA 94720, USA
Received 30th September 2013,
Accepted 25th October 2013
DOI: 10.1039/c3cc47473a
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these studies are limited to metal nanoparticle systems. In this
review, we show in Section 2.2 the development of liquid cell
TEM and discuss the advantages of this technique as compared
to other in situ methods. In Section 2.3, progress and achievements
in the study of metal nanoparticle growth using liquid cell TEM are
discussed. Several studies that contributed to the understanding of
nanocrystal shape control mechanisms are highlighted. At the end,
we provide an outlook of the future opportunities and challenges in
the study of nanoparticles or other materials in liquids using liquid
cell TEM.
2. Liquid cell TEM study of nanoparticle
growth
2.1 The development of liquid cell TEM
The concept of using window confined materials in TEM
characterization can be traced back to nearly the beginning of
electron microscopy when biological samples were first sand-
wiched between two thin aluminum foils by Marton in 1934.
17
However, there were limited activities
18–24
in imaging liquid
samples using TEM in the next several decades, which is
probably because of the incompatibilities of liquids with the
high vacuum environment under TEM. Conventional electron
microscopes require high vacuum (10
6
Torr or higher) within
the microscope column, both to allow the operation of the
electron source and to minimize scattering other than from
the sample. For many decades, the standard characterization
protocol for studying kinetics of a reaction process by TEM has
been used to image the specimens ex situ by stopping the
reaction periodically. Unfortunately, many dynamic processes
of materials during reactions cannot be obtained using ex situ
experiments, for example, the growth pathways of many nano-
crystals can only be achieved by in situ observations.
The recent advances in Micro-Electro-Mechanical Systems
(MEMS) have attracted renewed interest and significant progress
has been made in liquid environmental TEM. In 2003, Williamson
et al.
25
reported the development of electrochemical liquid cells for
TEM using silicon wafers with the electron transparent silicon
nitride membrane window. The gold electrodes were deposited on
the patterned bottom chip and it was glued together with the top
chip with a glass spacer in between to form an electrochemical
liquid cell. Additional containers for liquid electrolyte were
assembled in the electrochemical cell for the electric biasing
experiments (Fig. 1A). The electrochemical deposition of copper
clusters on the gold electrode was studied in situ.Sincethetotal
thickness of mass that the electron beam penetrating was large
(100 nm of each silicon nitride membrane; 50 nm of the gold
electrode and liquid thickness over 1 mm), the best resolution of
5 nm was achieved.
26,27
However, in order to study colloidal
nanocrystal growth under TEM, a liquid cell needs to offer better
resolution. Zheng et al. reported a self-contained liquid cell in 2009
and the window membrane thickness was pushed to 25 nm or
thinner, which enabled the study of single Pt nanoparticle growth
trajectories with sub-nanometer resolution.
28
This work has
attracted a lot of attention in the field of colloidal chemistry since
it opened the unprecedented opportunity to study nanocrystal
growth mechanisms by observation of nanoparticle growth in situ.
In 2009, de Jonge et al.
29
described the use of flow cell to image
whole cells in liquids with a constant flow of a buffer solution
(Fig. 1C). The liquid flow capability is appealing to researchers who
are interested in the study of reactions involving mixing solvents
instantaneously or injecting reactant agents. However, critical
issues still need to be addressed, such as sample drift introduced
by liquid flow, membrane rupture, potential contamination, etc.
There has been an increasing interest in liquid cell TEM in
recent years. Many other home-made liquid cells have been
reported. In general, a windowed environmental liquid cell
offers controllable liquid film thickness based upon the spacer
height between two thin film membranes, which can be
adjusted ranging from tens of nanometers to micrometers.
Electrodes with various geometries can be incorporated. Either
aqueous or organic solvents can beusedinliquidcellexperiments
to mimic synthetic conditions for nanomaterials. The liquid cell
TEM has found a wide range of applications from nanocrystal
synthesis to imaging of biological materials, soft materials, the
study of electrochemical reactions in situ, nanoparticle interaction
in liquids and so on. Fig. 2 shows a statistical plot of the
published papers using liquid cell methodology over the years
and the drastic increase in the number of publications in the last
few years is illustrated.
17–74
2.2 Advantages as compared to other in situ methods
For the observation of nanoparticle growth, many methods
have been applied, such as the in situ optical spectroscopy
method,
75
in situ X-ray diffraction
76–78
including synchrotron
based techniques,
79,80
in situ AFM and STM
81–83
and so forth.
Oezaslan et al.
84
reported the in situ measurements of the alloying
process of bimetallic Pt–Cu nanoparticles using high-temperature
Fig. 1 Schematic of an assembled liquid cell composed of two windows each
with the silicon substrate, silicon nitride membrane, and a spacer. (A) Biasing
liquid cell.
25
(B) Regular self-contained liquid cell, window: 1 50 mm;
28
(C) a
microfluidic chamber formed between two microchips in a flow cell.
29
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11722 Chem. Commun., 2013, 49, 11720--11727 This journal is cThe Royal Society of Chemistry 2013
X-ray diffraction. Polte et al.
77
investigated the growth process
of nanoparticles from an average radius of 0.8 nm to about
2 nm by time resolved in situ small angle X-ray scattering at
millisecond time resolution. Simm et al.
82
used in situ atomic
force microscopy (AFM) to study the growth of cobalt nuclei on
a boron doped diamond electrode under potentiostatic control.
The rate of growth of the nuclei at the electrode surface is
monitored using AFM as a function of time at different deposition
potentials. Using these in situ methods, critical information on
nanoparticle nucleation and growth has been achieved. However,
these methods have their limitations, for example, in situ spectro-
scopy methods lack the morphological information; in situ AFM
and STM can only image samples on a substrate and the temporal
resolution is also limited, thus, their applications in colloidal
synthesis are restricted.
Compared with other in situ methods, liquid cell TEM has its
unique advantages that researchers can directly observe the
structural and morphological changes of nanoparticles in
liquids during the reactions with high spatial resolution. Liquid
cell TEM allows atomic resolution imaging to be combined with
spectroscopic techniques for chemical identification, such as
electron energy loss spectroscopy (EELS), energy dispersive
spectroscopy (EDS), etc.
49,64,65
The setup is also applicable to a
wide range of chemical reactions and soft materials in a liquid
phase. It provides a unique platform for the study of nano-
particle formation that alternative analysis methods do not
offer. The major criticisms come from the electron beam
radiation damage and the challenges in temperature control
during reactions. Discussions on these issues will be provided
in the later section of this review.
2.3 Progress in metal nanoparticles synthesis by liquid
cell TEM
2.3.1 Coalescence vs. monomer attachment. Growth through
coalescence of nanoparticles (or aggregated growth) has been
frequently reported as an alternative to growth by monomer
attachment. Nanoparticle coalescence is especially common
in metal nanoparticle synthesis and it has been a major
concern in size control of nanoparticles. It is generally con-
sidered that the coalescence leads to a large nanoparticle size
distribution. There have been many studies on the growth
kinetics of metal nanoparticles by measuring ensemble particle
size distribution at a function of time.
85–90
However, there is no
consensus on the role of coalescence during growth. Through real
time observation of platinum nanoparticle growth trajectories
using liquid cell TEM, Zheng et al.
28
compared growth by nano-
particle coalescence with that by monomer attachment side-by-
side within the same field of view. It is very interesting that two
typesofgrowthreachedthesameparticlesize(Fig.3A).Itshowed
that the coalesced nanoparticle experienced recrystallization and
shape re-arrangement, which prevented additional platinum
atoms from attaching to the nanoparticle. So, a pause during
Fig. 2 Publications related to liquid cell electron microscopy since 1934.
Fig. 3 (A) Video images showing simple growth by monomer addition (left
column) or by coalescence (right column).
28
(B) The top row shows growth of the
dendrite, the bottom panel shows the velocity and tip radius versus time.
66
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growth was observed after a coalescence event, which allowed
the nanoparticle by monomer attachment to catch up. Studying
single particle growth trajectories makes it possible to obtain
such information without ambiguity. Recently, studies by
in situ X-ray assemble measurements have also confirmed that
although there are significant coalescence events during metal
nanoparticle growth, monodisperse nanoparticle can be achieved
eventually.
76,91
It has been reported that coalescence as an alternative to
simple growth by attachment of monomeric species plays an
important role in the synthesis of many other nanocrystals with
more complex shapes.
67,89,92–94
However, there is no doubt that
conventional growth by monomer attachment is still commonly
observed. Tracking the growth of nanostructures in situ, such as
dendritic growth of gold, was reported by Kraus et al.
66
(Fig. 3B). Dendrite nucleation was induced by the electron
beam which led to an initial burst of growth. During the
subsequent growth process gold nanoparticles were covered
with a thin liquid layer of precursor solution. The tip growth
velocity between 0.1 and 2.0 nm s
1
was observed. Tip velocity
fluctuations were captured among different dendrite geometries
grown from the tips. Those dendrites showing granularities in their
structure experienced the largest growth speed. By comparison of
the observed velocities with diffusion-limited growth rates, they
concluded that the dendrite growth at this scale was limited by
diffusion.
2.3.2 Shape control mechanisms and surfactant effects.
Synthesis of nanocrystals with different shape and architecture
has been a topic of significant interest.
95–100
Many factors, such
as precursor concentration, temperature, surfactants, etc. can
alter the chemical potential of crystallization in liquids, thus
the shape of the nanocrystals differs under different growth
conditions.
95,101–103
Using Pt–Fe nanocrystals as a model system, Liao et al.
systematically studied the effects of oleylamine concentration
on the shape evolution of platinum iron nanocrystals
67
(Fig. 4A). A self-contained liquid cell with ultra thin silicon
nitride membranes was used. Superb spatial resolution with
atomic level elemental information through high angle annular
dark field (HAADF) scanning TEM (STEM) was achieved
although they appear to be ex situ snap shot images
50
(Fig. 4B). The direct observation revealed that with a relatively
low concentration of oleylamine (20%), there were three stages
of growth: (I) nucleation and growth of platinum iron nano-
particles in the precursor solution; (II) end-to-end nanoparticle
attachment to form nanowires; (III) breakdown or shrinkage of
the nanowires into individual nanoparticles with large size
distribution. With 30% oleylamine, platinum iron nanowires
were obtained through shape directed nanoparticle attach-
ment, which was similar to that with 20% oleylamine. However,
nanowires were stabilized in the growth solution with 30%
oleylamine and they did not break down or shrink. As the
concentration of oleylamine increased to 50%, individual nano-
particles were stable in solution and merging between nano-
particles at the later stage was avoided. Visualization of Pt–Fe
nanorod growth in 30% oleylamine clearly showed that nano-
particles preferred to attach in one dimension, which suggests
the prominence of dipolar interactions between nanoparticles.
Such interactions were quantified for the first time by measuring
motions of nanocrystals and rearrangement of the nanocrystal
grains to accommodate the lattice mismatch of the nanocrystals.
Their studies also demonstrated that shape evolution of a
nanocrystal was strongly influenced by neighboring particles
due to the stereo hindrance effect, andtheoleylamine(surfactant)
effect on shape evolution of a nanoparticle was secondary to the
effects of neighboring nanoparticles.
Jungjohann et al.
49
investigated Pd growth in a dilute aqueous
Pd salt solutions containing Au nanoparticle seeds (Fig. 4C). Au–Pd
core–shell nanostructures were formed via deposition of Pd
0
,
generated by the reduction of chloropalladate complexes by
radicals, such as hydrated electrons in the solution induced by
electron beam. They showed that size and shape of the Au seeds
Fig. 4 (A) Schematic showing the surfactant (oleylamine) effect and stereo-
hindrance effects during growth. High-resolution STEM images of Pt
3
Fe nanorods
and the dynamic shape and orientation changes during structural relaxation.
(B) HAADF STEM image of a polycrystalline Pt
3
Fe nanorod, dimers, and nano-
particles obtained in a liquid cell. The dark spots (highlighted by arrows) indicate
the iron-rich regions (left). Sequential HRTEM images (I to IV) show both crystal
orientation and shape changes during the straightening of a twisted nano-
particle chain (right). (C) Comparison of Pd growth on 5 and 15 nm Au seeds.
(a–d) Dark-field STEM images of the two sized nanoparticles grown in a 10 mM
aqueous PdCl
2
solution. (b, e) The same two particles after Pd deposition.
(c, f) Schematic illustration of the Pd morphologies grown from two different
sized Au seeds. Note: images are from the original publications.
49,50,67
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determine the morphology of the Pd shells, via preferential Pd
incorporation in low-coordination sites and avoidance of
extended facets. Analysis of the Pd incorporation on Au particles
at different distances from a focused electron beam provided a
quantitative picture of the growth process. The growth was
limited by the ion diffusion in the solution.
2.3.3 High resolution imaging using graphene liquid cells.
Recently, graphene liquid cells have been emerged (Fig. 5A),
which use two layers of graphene to trap thin liquids for in situ
TEM. The graphene liquid cell has enabled the study of
colloidal nanocrystal growth with excellent high resolution
imaging (Fig. 4B). This is because electron scattering due to
the membrane window can be minimized. Yuk et al.
62
used this
new type of liquid cell to explore the mechanism of platinum
nanocrystal growth and discovered site-selective coalescence,
structural reshaping after coalescence, and surface faceting
along the growth trajectories. Recently, graphene liquid cells
have also been applied to the study of 3D motion of DNA linked
gold nanoparticles in liquids
63
(Fig. 5C). Although only pockets
of liquids can be trapped in such graphene liquid cells, the
high resolution imaging and the ease of fabrication open many
opportunities for future studies of nanoparticle growth or other
systems.
2.3.4 Electrochemical liquid cells and electrodeposition.
The electrochemical deposition of metal clusters or dendritic
structures
25,47
has attracted a lot of attention due to its relevance
to batteries or other energy storage devices. Using an electro-
chemical cell similar to the earlier work, White et al.
59
reported
the electrochemical deposition of lead from an aqueous solution
of lead(II) nitrate. Both the lead deposits and the local Pb
2+
concentration were visualized. Depending on the rate of potential
change and the potential history, lead deposited on the cathode
was either a compact layer or dendrites. In both cases the deposits
were removed when the electric bias was reversed. Asperities that
persist through many plating and stripping cycles consistently
nucleated larger dendrites. Quantitative image analysis showed
an excellent correlation between changes in the Pb
2+
concen-
tration or the rate of lead deposition and the current passed
through the electrochemical cell. These real-time observations
of nanomaterial growth and transformations using liquid cell
TEM provided a lot of critical information on the electro-
chemical processes important for the development of energy
storage technologies.
2.3.5 Nanoparticle motion. During nanoparticle synthesis,
nanoparticles may move around rigorously in solution. Motion
of nanoparticles in the growth solution may come from Brownian
motion, chemical reaction induced local environment changes,
liquid flow, electron beam effects, etc. An understanding of the
physics and the origin of nanoparticle motion is important for
many other studies using liquid cell TEM.
So far, there have been many reports on nanoparticle motion
imaged by liquid cell TEM. Both self-contained liquid cells
33
and flow cells
36,46
have been used. Zheng et al.
33
studied the
diffusion of spherical and rod-shape gold nanoparticles in
water with 15% glycerol. It was observed that nanoparticles
show random Brownian motion plus jumps. Long distance
motion due to liquid drag was also recorded. de Jonge et al.
34
used STEM to image gold nanoparticles in several micrometers
thickness water. White et al.
58
studied the charged Pt nanoparticle
dynamics in water. Mueller et al.
70
captured motion of gold
nanorods in a flow cell. Liu et al.
69
studied the self-assembly of
charged gold nanoparticles in liquid. Park et al.
55
directly observed
nanoparticle super lattice formation. There are other reports on
imaging nanoparticle motion, but only to prove their liquid cell
device is functional under TEM.
2.3.6 Reduction mechanisms and electron beam effects.
Most studies on nanoparticle synthesis by liquid cell TEM use
electron beam as the reducing agent to reduce metal precursors
in the growth solution. As to an electrochemical deposition,
the electric potential is the driving force for metal structure
growth, which often introduces large clusters or dendritic
structures.
25,47
Xin et al.
61
reported the first observation of
oscillatory growth of Bi at an elevated temperature of 180 1C
using liquid cell TEM (Fig. 6B). An electron beam has perceptible
effects on the liquid solution. Besides the reduction of the
metal precursor, it can produce bubbles in liquids, generate
solvated ions or electrons and rupture the liquid film.
58,72
Thus, the reactions under electron beam irradiation can be
complex. Although electron beam effects are pronounced in
many reports, the study of nanoparticle growth using liquid
cell TEM is of great interest to many researchers. It is well
known that the synthetic approach to colloidal nanocrystals is
extremely diverse. Besides the common thermal heating,
growth can be initiated by an electrochemical potential,
99
microwave,
104,105
solar light,
106
biomineralization,
88,107,108
electron beam irradiation
109,110
and so forth. Electron beam
induced nanoparticle growth using liquid cell TEM falls into
the above wide spectra of growth. Efforts to elucidate nano-
particle growth mechanisms under electron beam irradiation
are highly valuable for the understanding of nanocrystal growth
in general.
Fig. 5 (A) Illustration of graphene liquid cell (GLC) encapsulating the growth
solution. (B) Still snapshots of Pt nanocrystal growth via coalescence and crystal-
structure evolution observed with atomic resolution in a GLC. (C) Illustration of
DNA–Au nanoconjugates in a graphene cell. All images can be found in the
original publication.
62
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Woehl et al.
60
studied silver particle growth by using a flow
cell and found that electron beam current played a major role
in controlling the morphology of silver nanocrystals (Fig. 6A). It
was demonstrated that under a low beam current reaction
limited growth was preferred and nanocrystals with faceted
structures were achieved. Under a higher beam current, diffusion
limited growth was dominant and nanocrystals with more complex
shape were achieved. Isolation of these two growth regimes showed
a new level of control over nanocrystal growth under electron beam
irradiation. In the future, more quantitative study of reaction
mechanisms will certainly be highly beneficial to the under-
standing of electron induced growth and to the growth of in situ
electron microscopy field in general.
3. Conclusion and outlook
In situ liquid cell TEM is one of the recent advances in
characterization of solution-phase reactions. It allows us to
track morphological, structural and chemical changes of solids
in liquids in real time. Nucleation and growth of nanoparticles
occur dynamically and they are surrounded by solvent and
other molecules, which present challenges to in situ measure-
ments. Hence, there are only a few techniques that can address
issues on the mechanisms of nanoparticle growth through
in situ studies. In situ liquid cell TEM with a combination of
temporal and spatial resolution shows many advantages over
many other characterization techniques. This has enabled the
understanding of nanocrystal growth with an unprecedented
level of information. The growing interest in liquid cell TEM
has been demonstrated by the drastic increase of the number of
publications. This review has provided an overview of this
exciting technique and its applications in real-time observation
of colloidal nanoparticle growth.
Real time observation of the dynamic growth process sheds
light on the nanocrystal growth mechanisms, and points the
way towards the synthesis of nanomaterials with desired prop-
erties. However, due to the short time of development of the
modern liquid cell TEM, many important issues still need to be
addressed. For example, nanoparticles in fluids normally move,
which can lead to them becoming out of focus, thus affecting
the imaging quality especially at higher magnification or at an
elevated temperature. Liquids with high vapour pressure
including pure water are hard to handle since they can easily
dry out at sample loading or vanish under electron beam
irradiation, which increases the experimental challenge and
limits the application of liquid cell TEM techniques. Most
colloidal synthesis proceeds by hot injection (mixing precursor
solution together at a certain temperature for reactions), reac-
tions by solvent mixing have not been achieved by the current
liquid cell setup. Although ultra-thin silicon nitride membranes
andgraphenehavebeenincorporatedinthenewliquidcell
design, resolution still needs to be improved in many cases
where electron beam scattering is largely from a thick liquid
layer, therefore controlled thinning of liquid layer thickness is
required. Other future developments include low dose imaging
to reduce electron beam damage, faster imaging to capture fast
reaction dynamics, weak contrast imaging to enhance the con-
trast of soft materials, etc. The ultimate goal of liquid cell TEM is
to resolve single molecule interactions during a liquid phase
reaction. In order to reach such a level, there is still large room
for development. There is no doubt that the liquid cell TEM
technique will play an increasingly important role in discovering
nanocrystal growth mechanisms and a wide range of other
material transformations in materials science, chemistry and
biological science.
Acknowledgements
We thank the funding support of DOE Office of Science Early
Career Research Program.
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